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Electronic and optoelectronic materials and devices inspired by nature

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Rep. Prog. Phys. 76 034501 (http://iopscience.iop.org/0034-4885/76/3/034501) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

REPORTS ON PROGRESS IN PHYSICS

Rep. Prog. Phys. 76 (2013) 034501 (36pp)

doi:10.1088/0034-4885/76/3/034501

Electronic and optoelectronic materials and devices inspired by nature P Meredith1 , C J Bettinger2 , M Irimia-Vladu3,4,5 , A B Mostert6 and P E Schwenn1 1 Centre for Organic Photonics and Electronics, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland, Australia 2 Biomedical Engineering Group, Department of Materials Science and Engineering, Carnegie Melon University, Pittsburgh, PA, USA 3 Linz Institute for Organic Cells (LIOS), Department of Physical Chemistry, Johannes Kepler University, Linz, Austria 4 Department of Soft Matter Physics (SoMaP), Johannes Kepler University, Linz, Austria 5 Institute for Surface Technologies and Photonics, Joanneum Research, Weiz, Austria 6 Nanotechnology Group, Department of Physics, University of Lancaster, Lancaster, Lancashire, UK

E-mail: [email protected]

Received 31 August 2012, in final form 19 November 2012 Published 14 February 2013 Online at stacks.iop.org/RoPP/76/034501 Abstract Inorganic semiconductors permeate virtually every sphere of modern human existence. Micro-fabricated memory elements, processors, sensors, circuit elements, lasers, displays, detectors, etc are ubiquitous. However, the dawn of the 21st century has brought with it immense new challenges, and indeed opportunities—some of which require a paradigm shift in the way we think about resource use and disposal, which in turn directly impacts our ongoing relationship with inorganic semiconductors such as silicon and gallium arsenide. Furthermore, advances in fields such as nano-medicine and bioelectronics, and the impending revolution of the ‘ubiquitous sensor network’, all require new functional materials which are bio-compatible, cheap, have minimal embedded manufacturing energy plus extremely low power consumption, and are mechanically robust and flexible for integration with tissues, building structures, fabrics and all manner of hosts. In this short review article we summarize current progress in creating materials with such properties. We focus primarily on organic and bio-organic electronic and optoelectronic systems derived from or inspired by nature, and outline the complex charge transport and photo-physics which control their behaviour. We also introduce the concept of electrical devices based upon ion or proton flow (‘ionics and protonics’) and focus particularly on their role as a signal interface with biological systems. Finally, we highlight recent advances in creating working devices, some of which have bio-inspired architectures, and summarize the current issues, challenges and potential solutions. This is a rich new playground for the modern materials physicist. (Some figures may appear in colour only in the online journal) This article was invited by Athene M Donald.

Contents 1. Introduction 2. Physics of electrical conduction in organic and bio-organic conductors

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2.1. Electrons and holes

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2.2. Ions and protons

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0034-4885/13/034501+36$88.00

2.3. A hybrid ionic–electronic conductor (problem case study) 3. Photon harvesting and light interactions 3.1. The excitonic nature of organic chromophores 3.2. Synthetic molecular heterojunctions for light harvesting and photo-detection

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© 2013 IOP Publishing Ltd

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Printed in the UK & the USA

Rep. Prog. Phys. 76 (2013) 034501

4. Applications 4.1. Electrical circuit elements—electronics, ionics and nanoprotonics 4.2. Optical and optoelectronic interfacing with biological systems 4.3. Light harvesting devices

P Meredith et al

4.4. Natural materials for bio-inspired devices: substrates, insulators, semiconductors 5. Future perspectives: opportunities and challenges Acknowledgments References

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a complex biological system with control circuitry in order to ‘read or write’ an electrical signal [2–4]? Biology is dominated by ion gradients and currents, membrane electric potential gradients and local chemical reactions. As well as being hostile to biological tissues, semiconductors are electronic. As such, new concepts such as high fidelity ionto-electron transduction and ‘ionics’ (electrical circuits and elements based on the flow of ions) become relevant and necessary [5, 6]. The recent demonstration by the Rolandi group (to be discussed in detail later in the review) of a biologically inspired ‘bioprotonic field-effect transistor’ made from a proton conducting polysaccharide is one of the first examples of this new kind of electrical device, and a major advance [6]. Nature, of course, through millions of years of evolution, presents an almost limitless supply of materials, concepts and architectures to address these critical challenges. A desire to understand and then mimic the natural world is part of our ‘scientific-DNA’. For example, artificial photosynthesis is a popular but hugely challenging concept as a potential solution for future clean energy (trees can do it, so why can’t we?), powerful computational systems based upon neural networks are established and effective ‘artificial decision makers’, and synthetic nano-and-micro engines can mimic the elegant functionality of cellular motors. In the semiconductor space, organic electronic and optoelectronic materials are coming of age, their evolution driven by the imperatives described above in applications such as displays, lighting, plastic electronics and sensors [7]. Metallic and semiconducting polymers, organic small molecules and dendrimers contain delocalized π -electron systems, akin to proteins and other functional biological macromolecules, which confer electrical conductivity, and optical and transport gaps of order a few eV. These materials are often photo-conductive and can be engineered to have high photoluminescent and electroluminescent quantum yields. Photo-excitations in organic semiconductors are ‘excitonic’ in nature—characteristically low dielectric constants mean weak screening of the electron–hole (e–h) electrostatic interactions, and the pair remain Coulombically bound (the exciton) with a typical binding energy of 100s of meV. In an inorganic semiconductor photo-excitation leads to the spontaneous creation of a free electron and hole. The excitonic nature of organic semiconductors is a defining feature of their physics and an essential functional element of natural chromophores and proteins. Does the exciton limit or enhance our ability to make use of naturally inspired photo-active materials? Furthermore, organic conductors and semiconductors can display ‘hybrid’ electrical physics, i.e. current flow via

1. Introduction Inorganic semiconductors such as silicon and gallium arsenide have fuelled the high technology, photonics and computer revolutions of the past five decades. Devices containing semiconductor processors, memory elements, detectors, sensors, circuitry, light emitting diodes, photodiodes, etc. control or assist virtually every facet of our modern life: healthcare, energy, entertainment, transport, communications, even thinking! In the developed world particularly, there is a whole generation that cannot conceive of life without a mobile phone or computer. Many of these modern-day essentials are deliberately designed to be obsolete within a few years of purchase, and a very small proportion are recycled or even capable of being recycled. This is leading to a number of extremely unfortunate and undesirable outcomes: for example, massive amounts of often toxic electronics waste, and rapid depletion of already scarce elements such as gallium and indium (‘endangered elements’). This is simply unsustainable. Furthermore, the processing of inorganic semiconductors is an energy intensive business. If you think about the manufacturing chain of events that turns a humble pile of sand into a GHz silicon processor, the problem becomes all too apparent. The energy expended in manufacturing just one chip processor exceeds the total energy used by a modern laptop over its typical 3 year lifespan. In an energy-constrained, carbon sensitive world, this will become unacceptable. There is no doubt that the ‘silicon age’ has delivered a discontinuous advance in humankind’s technological ability, but how much longer can it last, and what is next? Additionally, concepts such as the ‘ubiquitous sensor network’ [1] with visions of stand-alone sensing and control systems deployed to manage energy consumption, health and medical interventions, internal and external environments and communications, will require components of extremely low power consumption and which can be integrated into a range of hosts—building fabric, biological tissue, natural environments, etc. Bio-compatibility, low embedded energy, low power consumption (or even self-powered), minimal environmental footprint and mechanical flexibility are all key requirements of the functional materials making up these devices and indeed their architectures. It is not clear how, or even if, our current toolkit of inorganic semiconductors can meet these requirements. The revolutions in socalled ‘bioelectronics’ and ‘nanomedicine’ which promise splendours such as in situ real-time health monitoring with integrated biosensors, tissue repair, targeted drug delivery, cellular-level control and stimulation, pose even greater materials challenges—for example, how does one interface 2

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Figure 1. The diverse elements of science and engineering that intersect with or utilize advanced functional electronic and optoelectronic materials inspired by nature. Many of the sub-fields listed are mature—such as organic optoelectronics, whilst others are newly emerging, such as bioelectronics. The classical fields of physics, chemistry, engineering, materials science and biology feed these multi-disciplinary endeavours, and physics in particular has a pivotal role to play.

conversion and light detection, electronic and computing components, sensors and particularly bioelectronics and bionanoelectronics for interfacing the biological world with conventional control systems. This is not an exhaustive list, but gives a flavour of the diversity and opportunity. The report will cover a broad range of topics from materials to device architectures and pay particular attention to the underlying electrical and photo-physics as we examine the specific challenges of achieving pre-requisite basic device characteristics such as low power consumption, fast response, high carrier mobilities and high photoluminescent quantum yields. The field is by its very nature highly multidisciplinary, and is emerging as a key frontier in advanced functional materials. The principles of optoelectronics, condensed matter, quantum and solid-state physics, along with synthetic organic and physical chemistry, materials science and engineering, biomaterials and applied biotechnology are all essential elements of this challenging endeavour. The interactions between these diverse fields and various application areas are shown in figure 1. Not all of the subelements of the science are at the same stage of maturity, and so naturally there will be some degree of ‘imbalance’ between subjects covered in the review. Furthermore, we have taken the approach of providing more detail where the physics is better understood—and also highlighted those areas where more attention is needed. In summary, this report seeks to provide a brief overview of the current status, highlight some of the key issues and summarize the early adoption application areas. Language and nomenclature are diverse as befits a modern

ions/protons and electrons/holes. This is not a common feature in either synthetic or natural compounds, the classic synthetic example being poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT : PSS) [4]. Mostert and coworkers recently reported hybrid behaviour in the natural pigment melanin—for over four decades considered to be one of the first examples of an amorphous organic semiconductor [8]. It appears that the complex electrical physics of these important bio-macromolecules is dominated by the transport of protons (also to be discussed later in the review in detail). It is true to say that the underlying physics of organic (and bioorganic) semiconductors is significantly more complex and less well understood than their inorganic counterparts. This inherent complexity is in part due to the molecular nature of organic systems (providing strong correlations at the electronic level) and the high levels of structural disorder which renders traditional condensed matter physics approaches such as band theory somewhat ineffective, especially when considering room temperature properties. However, the rationale design of new, high performance materials requires a deep understanding of the underlying structure–property relationships, and this is driving a ‘renaissance’ in the basic physics of natural and bioinspired-synthetic functional systems. This Report on Progress in Physics examines the current state-of-the-art of advanced functional electronic and optoelectronic materials and devices inspired by, or indeed derived from, nature. In particular, we will focus upon organic and bio-organic systems with semiconducting or photo-active properties. Application areas of interest include light energy 3

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Figure 2. (a) An idealized electronic band structure for a crystalline material, where E is the energy, k the reciprocal lattice vector for the Brillouin zone (crystal unit cell), EF the Fermi energy (depicted here for both an insulator and metal) and Eg is the energy gap between the valence and conductive bands (for an insulator). Electrons fill up the lowest bands first to EF (which is the electronic chemical potential at T = 0). (b) The density of states for an amorphous semiconductor. There are several energy levels defined as: EV (EC ) the valence (conduction) mobility edge, EB (EA ) is the valence (conduction) level, EF is the Fermi energy, E0 is the optical gap, EV (EC ) is the energy gap between the valence (conduction) level and mobility edges, and Eg is the mobility gap. (c) An Idealized electronic picture for the ethene molecule using MO theory. The pz orbitals from each carbon atom are combined to form a new electronic ‘Molecular Orbital’. The HOMO is separated from the LUMO by an energy gap, Eg which is completely analogous to the traditional band gap.

electronic charges. An idealized energy spectrum for a banded solid is shown in figure 2(a) and from this emerge the natural concepts of the band gap and conduction and valence bands. The position of the Fermi energy EF , determines whether a material is a metal or an insulator. In a metal EF slices through an energy band, and in an insulator (or semiconductor), EF lies in-between bands. Thus, in order to excite an electron into an unoccupied (conductive) band, one needs to cross the energy gap Eg . In this model, the difference between an insulator and a semiconductor is merely a matter of degree, where Eg for a semiconductor is considered to be 1 to 3 eV, and for an insulator higher. Thus, the key to understanding σ is to examine how Eg impacts the carrier density available for conduction. In the first instance, the relationship between Eg and n is given by a Boltzmann description:

frontier subject where traditional physics interacts with, and contributes to, an emerging field.

2. Physics of electrical conduction in organic and bio-organic conductors In this section we briefly review the relevant physics that underpins electrical conduction in organic and bio-organic conductors. From the most fundamental of perspectives, the charge transport properties of any material are described by the simple conductivity (σ ) equation: σ = enµ,

(1)

where e is the fundamental charge, n is the conductive charge density and µ is the mobility of the conductive charge carrier. The study of transport ultimately is based upon understanding σ in terms of n and µ and their interplay. In the case of organic and bio-organic conductors one must frame this analysis in the context of the defining physics, namely, the ‘molecular nature’ of organic solids, the interactions of strongly correlated π electrons in organic semiconductors, high levels of structural and energetic disorder, and the possibility of ions/protons contributing to or even dominating conduction. We consider these concepts in the foregoing discussion.

n ∼ exp(−Eg /2kB T ),

(2)

where kB is Boltzmann’s constant and T is temperature. Note here that the activation energy is Eg /2 since for every electron excited to the conduction band, a hole is left behind in the valence band which acts as the counter charge carrier [9]. This process can be enhanced by photo-conduction with light of energy >Eg . The mobility µ is controlled by the shape of the bands (band shape affects the group velocity of the charges [9]) and n can be manipulated by doping. However, the salient point is that the basic characteristic of a crystalline semiconductor is a dependence of σ on T in a Boltzmann fashion with an Eg of 1–3 eV.

2.1. Electrons and holes 2.1.1. Band theory of electron transport. A logical place to begin this discussion is with the classical view of electron/hole conduction in a standard semiconductor. This is historically what has defined the language and nomenclature of transport physics. Crystalline materials are usually modelled with an electronic Hamiltonian that reflects the underlying periodicity of the crystal [9], which leads to the concept of delocalized

2.1.2. Modification of band theory, Mott–Davis model. When solids are amorphous, disorder forces localization of the electronic states. Anderson famously described the formation of these localized states in a periodic system [10] and a simple 4

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(a)

(b)

Figure 3. (a) An idealized conception of a crystalline solid in one dimension. This periodic arrangement with crystal spacing a (in the tight binding approximation [9]) leads to delocalized electrons within a narrow band width. (b) In amorphous conductors, a between atoms remain the same but the depth of the energy well randomly changes with a distribution variance of V0 , thus broadening the bandwidth [11]. This randomness is referred to as vertical disorder [11] to distinguish it from horizontal disorder (e.g. normal liquids).

schematic of the underlying problem is depicted in figure 3. Essentially, Anderson demonstrated that at a specific value for V0 /B a transition occurs between delocalized and localized transport (the Anderson transition), leading to a major change in µ. This model was exemplified for the case of amorphous semiconductors—most famously by Mott and Davis [11], which lead to the concept of an extended density of states as shown in figure 2(b). The key to understanding the electronic structure of an amorphous semiconductor is to realize that electrons in the gap between the so called ‘mobility edges’ (shaded area in figure 2(b)) occupy localized states. In order for an electron to move from a fully occupied localized state to an unoccupied localized state requires an activation energy resulting in a conductivity described by: 
B (3) σ = A exp − n , T

to progressive depopulation of the occupied, localized states below the mobility edge to unoccupied ‘band’ states above: another ‘quirk’ of amorphous semiconductors of relevance to the organic/bio-organic semiconductor discussion. 2.1.3. Relevance to organic and bio-organic conductors. An obvious question arises: can the above solid-state models explain the transport behaviour in organic semiconductors and particularly biological semiconductors that are usually wet and disordered? Early pioneering work [15, 16] on proteins suggested that band theory could indeed be applied as the appropriate model. Haemoglobin, for example, appeared to conduct electrons exclusively even after doping with water [17]. Furthermore, temperature dependent studies on haemoglobin crystals suggested a Boltzmann dependent σ with an Eg of ∼2.3 eV [18]. Essentially, these protein crystals seemed to act as classic semiconductors! However, subsequent analysis of these experiments (and a little hindsight) casts doubt upon this interpretation, notably:

where A is the usual pre-exponential factor (related to µ), B is the activation energy depending on the wavefunction shape and density of states of the material, and n is determined by the temperature range. At ‘reasonable’ temperatures n = 1, but at low temperatures n = 1/4. When the mobility edge (EC ) is crossed by the application of high temperatures or strong electric fields, the conductivity changes character to:
 (EC − EF ) , (4) σ = σmin exp − kB T

(1) The experiments only covered two orders of magnitude in temperature because of the limitations of biological molecule stability—this is not a sufficient range to formally distinguish between exponential and power law dependence; (2) Water has a large effect on σ —the experiments [18] established that Eg changed from 2.3 eV in the dry state to 1.45 eV in the wet state which translates to a change in the capacitance of orders of magnitude. This water ‘doping’ effect was ascribed to a modification of the dielectric constant leading to a shift in Eg . Clearly, the environment was not merely a perturbation even in such an artificial bio-system as a crystal. (3) Temperature and hydration are linked—the haemoglobin experiments essentially changed two first order parameters simultaneously, making it impossible to truly assign functional temperature dependence to the conductivity.

where σmin is the minimum metallic energy and encapsulates µ. This quantity is important since it is substantially higher than A, and as the name suggests metallic (band like) transport becomes the main mechanism. Thus, in amorphous conducting solids, by changing the temperature (or electric field in electrical switching experiments (see [12] for an original example)) one sees fundamentally different physics. This was an argument relied upon as we shall see later by McGinness et al in categorizing the natural pigment melanin as an amorphous semiconductor [13]. The density of states as shown in figure 2(b) also explains the optical ‘tail’ observed in amorphous semiconductors. In very broad terms, in crystalline semiconductor systems (figure 2(a)), absorption of the light occurs if the incoming electromagnetic radiation is of energy equal to or greater than the energy gap Eg . At this point a very sudden and large increase in the conductivity is seen. In an amorphous semiconductor there is a gradual increase in conductivity with the energy of the photo-excitation [14]. This is due

One would have to conclude that a combination of these experimental and material factors likely led to an erroneous or at best inaccurate outcome which has proven incapable of repetition. The direct application of amorphous semiconductor theory to a biological system is almost unheard of—the case of melanin being an important exception [19]. Specifically, McGinness et al [13, 20] suggested that Mott– Davis theory was an appropriate description of the materials’ solid-state physics, since it appeared to possess an optical tail [13] and also demonstrated electrical switching [20]. 5

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Like the haemoglobin example, the interaction of temperature and hydration, compounded by the particular challenges of electrical measurements in these systems, delivered what now appears to be another erroneous outcome, the details of which will be discussed later in this section.

hopping (long length-scales) do not dominate; and, (iii) the work-functions of the injecting and collecting electrodes must lie in the HOMO–LUMO gap. A famous experiment by Porath et al [27] appeared to have demonstrated this for short (and apparently reasonably dry) DNA strands (both in air and vacuum) across 2 orders of magnitude temperature range. The effect of water was not discussed except to note that ionic conduction could be excluded. However, it is interesting to note that a sample subjected to heat-cycling showed a pronounced change in the slope of its temperature dependent voltage gap (reminiscent of the protein example). Again, there appears to be no systematic studies seeking to decouple temperature and hydration effects in DNA. In keeping with the bulk transport discussion, the final mechanism postulated for conduction in single bio-molecules is hopping (analogous to Mott–Davis). As molecules get longer, the environment (either through vibrational [28] or conformational changes induced by steric hindrance or solvation [29]) reduce stiffness and delocalization. This drives charge localization (or polaron formation [11]) and incoherent hopping between localized electronic states dominate. Thus, the environment plays a key role in changing the mechanism of charge transport. This is an interesting concept (not unexpected in biological systems, it must be said) in contrast to solid-state amorphous materials, where changes in transport are intrinsic to the material. For localized hopping the conduction (in general) becomes length independent [23–25] and the mobility is given by: 
−EH , (6) µ = µ0 exp kB T

2.1.4. Single molecule conduction, molecular orbitals and polarons. An alternative and popular lens through which to interpret the electronic landscape of conducting organic molecules is molecular orbital (MO) theory [21]. MO theory is in large part an interpretative framework in which one constructs an electronic wave function for an entire molecule from electron ‘molecular orbitals’ (eMOs). The eMOs are themselves derived from a linear combination of atomic orbitals (LCAOs) across the molecule. In effect, the eMO assumes an electron to be delocalized across a molecule (similar to electrons in a crystal). Indeed, much of the early enthusiasm for DNA research was based on the principle that base pairs of DNA had electron orbitals that would overlap to form a delocalized MO [22]. The Molecular Orbital construct continues to dominate the organic semiconductor community as we shall also see in section 3.1. A simple example of an energy level landscape derived from MO is shown in figure 2(c) for the case of ethene. The diagram is very reminiscent of the crystalline model where two energy levels are separated by an energy gap. Given that MO models look like crystalline models (or indeed localized amorphous materials, as will become apparent), similar conclusions may be drawn in relation to transport. Again the idea is to excite an electron across the HOMO–LUMO gap (highest occupied and lowest unoccupied molecular orbital) and then consider how the free carrier density and mobility are thus affected. For single conducting bio-molecules (predominantly DNA), there appears to be 3 different transport mechanisms dependent on the length of the molecule [23– 26]. For short molecular lengths, the rate of electronic charge transfer (CT) is given by the tunnelling equation: ket = k0 exp(−βRDA ),

where µ0 is the pre-exponential factor and EH is the average height of an energy barrier for a hopping event. There appears to be no exp(−C/T 1/4 ) behaviour at low temperatures as seen in amorphous semiconductor solids (see equation (3)). In this regard localized hopping transport is somewhat different between bulk and single molecule materials. However, the hopping-polaronic model is basically the accepted view to explain the transport physics of most organic semiconductors in the bulk, with the exception of potentially band-like processes in smaller molecules, which is controversial to say the least.

(5)

where k0 is the kinetic prefactor, β is the tunnelling factor (which MO can help determine) and RDA is the tunnelling distance between two parts of the molecule, an electron (hole) donating part and electron (hole) accepting part. This mechanism has been demonstrated for DNA molecules, with guanine bases at the ends of the transport pathway and adeninethymine bases acting as the connecting wires [25]. Again, water was required to create and stabilize charge carriers, emphasizing the first order effect of the environment. There is also some evidence that single proteins behave similarly and can be modelled through a simple tunnelling equation [26]. The second mechanism of conduction in single conducting bio-molecules involves band-like transport as similarly discussed for ‘bulk’ conduction in crystals of haemoglobin. For band-like transport, the molecule must satisfy three conditions [27]: (i) they have to be stiff to facilitate delocalization; (ii) the molecules have to be of ‘intermediate’ length to ensure that tunnelling (short length-scales) and

2.2. Ions and protons Biological systems are wet and ionic. It is thus natural to expect that ionic and particularly protonic species play a role in transporting charge, and furthermore that the local environment is part of the ‘transporting’ system. It should be acknowledged that ionic charge transport is a vast subject and includes materials from ceramics to organics [30], with many different ionic entities found in water and dissociated salts [31]. Here, we will focus on water containing systems (and proton transport), but the physics is broadly applicable. There are two basic mechanisms by which protons can be transported through water: The first is simple centre of mass diffusion, also known as the ‘vehicle mechanism’ [31, 32] 6

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Figure 4. Top: The Grotthuss mechanism whereby an ionic charge defect is shuffled through a hydrogen-bonded network. Bottom: The comproportionation equilibrium reaction in melanin: a hydroquinone reacts with a quinone and water to yield hydronium and semiquinone free radicals. Adapted from Mostert et al [8, 38]. This reaction dominates the local chemical behaviour of melanin and also has a profound effect upon the macroscopic transport physics.

conductivity regime. Proton transport research relies on a convergent experimental philosophy of ‘demonstrating within reasonable doubt’ via a multitude of different observations that the carrier type is ionic and then making derivative conclusions accordingly [31]. As we shall see in section 4.1.1, extracting and injecting protonic carriers during a solid-state electrical measurement is a significant hurdle. So called ‘blocking’ and ‘un-blocking’ contacts in ionic circuits are in some senses analogous to the ohmic or Schottky concept in electronics—although a strongly blocking electrode is completely debilitating from a measurement perspective and particularly so with large static electric fields. At small-tomoderate fields, injection and extraction can just about keep pace and the system can appear to be ‘ohmic’ (linear current– voltage behaviour). However, at high fields blocking contacts lead to build up of space charge and strong capacitive effects [35]. Thus, the dc field dependence of the current becomes highly non-ohmic, looking more like a Child’s Law relation:

and described by the Einstein–Stokes equation for charged particles: Dq µ= , (7) kB T where D is the diffusion constant and q charge. The second mechanism is proton tunnelling or hopping [31–33] and is usually connected with co-operative proton transport (also called the Grotthuss Mechanism [31]). This mechanism is shown schematically in figure 4 and involves the transfer of an ionic defect through a hydrogen-bonded network from one side to the other. However, there is still doubt as to the extent of this co-operative behaviour, and it would appear that the orientation and polarization of the water molecules (hydrogen bonding) in the chain is key in facilitating this faster form of transport [31, 33]. In general, the conductivity in proton transport can be described by [31]:
 A −EA σ = exp , (8) T kB T

i ∝ V α,

where the first part of the equation captures centre of mass diffusion and the exponential captures the tunnelling, heat activated process [32]. It has been suggested that transitions can be driven between the two transport regimes within hydrated lysozyme powders using a percolation argument [34]. At low hydration, conductivity is limited to diffusion, implying that µ remains constant with water content. At high hydrations, enough water molecules are present such that a continuous chain of hydrogen bonded molecules across the conduction pathway is formed. This leads to a rapid change in µ (and thus σ ) with: σ ∝ Hd, (9)

(10)

where i denotes current, V is the voltage and α is a power ranging from 1 (ohmic) to 2 (pure blocking) [35]. Distressingly, electronic systems can also exhibit Child’s Law behaviour which means blocking is not sufficient to confirm carrier type. An exception to this general rule is the protein crystal work cited above [17], where electronic transport was deduced from the use of a proton blocking electrode. A possible way to overcome blocking issues in the study of bulk processes is to use ac fields (for example impedance spectroscopy [36]). In principle this technique can isolate different transport processes (migration, hopping, etc) assuming that each has a unique time constant. However, since both ionic and electronic transport processes in disordered materials are dominated by hopping (equations (6) and (8) respectively), differentiation between charged entities is to say the least, problematic. It is possible to model the ac conduction contribution from both ionic and electronic species but it is by no means a precise science (mixed ionic electronic conductors, see [37] for a discussion). Furthermore, the usual tools for studying temperature dependence are not likely to provide valuable insight, given the similarities between equations (6) and (8), the limited temperature range available for organics (particularly bio-organics), and the afore-mentioned hydration problems. More suitable approaches are based on water

where H is the water content of the sample and d is the critical exponent. It should be noted that to demonstrate percolation using equation (9) one requires detailed knowledge of the critical exponent and several orders of magnitude in data around the critical point—virtually impossible for hydration of an organic or bio-organic conductor. So, the theory remains largely untested. In isolation, it is extremely difficult from conductivity measurements to distinguish between electronic or ionic transport [31]. Polarons (p-type in particular) exhibit many of the features of protons as far as macroscopic electrical behaviour is concerned, particularly in the low 7

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content, pH dependence, calorimetry, etc (see [31] for a comprehensive overview). 2.3. A hybrid ionic–electronic conductor (problem case study) To exemplify many of the issues raised in sections 2.1 and 2.2 we now turn to a recently published body of work seeking to elucidate the transport physics of a bio-organic conductor [8, 19, 38, 39]. The material in question is the pigmentary macromolecule melanin found throughout the biosphere, with particular importance to humans as our primary photo-protectant, and which also colours our skin, hair and eyes [19]. Eumelanin is the ‘archetypal melanin’ and is composed of randomly cross-linked sheets of 5,6dihydroxindole (DHI), 5,6-dihydroxindole,2-carboxylic acid (DHICA) and their various redox forms (see figure 4). Melanins have an intriguing collection of physico-chemical properties including broad, monotonic absorption at optical wavelengths (they are black), a persistent and stable free radical population, UV and visible photo-conductivity, and dc and ac electrical conductivity (strongly hydration dependent). This collection of properties led to speculation in the early 1970s that melanin may be a natural amorphous semiconductor and this was apparently confirmed in a series of landmark experiments by McGinness and co-workers who demonstrated the material could undergo bistable electrical switching [20] (section 2.1). All subsequent electrical studies were interpreted within this framework and the paradigm became enshrined [40, 41]. However, and in keeping with the discussion of section 2.2, McGinness et al [20] observed a number of unusual features in the switching response—most notably the need for the melanin sample (a pressed powder pellet) to be hydrated. This was explained by invoking the modified dielectric constant argument (section 2.1.3) whereby the absorbed water essentially compressed the transport gap. Given that melanin powders can absorb up to 35% by weight of water [40], and the clear impact of hydration on electrical properties, it is somewhat surprising that until recently no adsorption isotherms on electrically relevant morphologies have been reported [38]. These isotherms are essential for elucidating the equilibrium kinetics of hydration, and subsequently led to a clear understanding of how inappropriate electrode geometries can produce erroneous conductivity versus hydration profiles [38–40]. Critically, Mostert et al [39] found that a two contact sandwich geometry of the type universally applied in previous studies (including McGinness et al) imposed non-equilibrium behaviour of a functional form similar to that expected from the modified dielectric model. The actual equilibrium behaviour as measured through a surface contact geometry was radically different and actually very similar to the percolation model described above (see figure 5(a)). Mostert et al [8] also deployed a range of spin spectroscopy techniques (including muon spin relaxation (µSR) [42] and various electron paramagnetic resonance (EPR) methods) to examine the microscopic nature of the carriers [8]. In particular, they found that the muon relaxation

Figure 5. (a) Conductivity of melanin as a function of water content. The measured humidity was converted into weight per cent gained using a previously reported adsorption isotherm [38]. The line shows the best fit to the modified dielectric theory (sections 2.1.2 and 2.1.3). (b) The paramagnetic and diamagnetic µSR relaxation rates for melanin pellets as a function of water content. The muon hopping rate, ν, was found to be constant at 0.28 ± 0.03 µs−1 (S.E.). This shows that proton mobility does not change in the same way as conductivity, but carrier concentration does, reflected for protons and electrons by  and λ respectively. Reproduced with permission from Mostert et al [8]. Copyright 2012 National Academy of Sciences.

parameters associated with both the proton and electron free spin densities followed the conductivity versus humidity dependence (figure 5(b)). They also demonstrated that the solution EPR, pH titration dependence, derived from the creation of semiquinone free spins in the comproportionation equilibrium reaction as per figure 4, had a similar dependence. By combining macroscopic measurements of electrical parameters with local microscopic measurements of spin dynamics, Mostert et al [8] were finally able to show that melanin’s conductivity is not associated with amorphous semiconductivity. In a process they termed ‘chemical self doping’ absorbed water titrates the comproportionation equilibrium generating semiquinone spins (electrons) and protons which are transported through the hydration matrix most likely via the Grotthuss mechanism. Hence, it was established that melanin is a hybrid conducting material. This 8

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example serves as a clear demonstration of how environmental factors extrinsic to the material itself can play a major role in the electrical properties of organic and bio-organic conductors. It also serves as a salutary reminder of the complexity and richness of transport physics in organic conductors.

conventional solid-state analogy would be to describe these transitions as occurring between states in the valence and conduction bands. Once again, given the ‘molecular nature’ of organic semiconductors and chromophores, this conventional solid-state description is inadequate in capturing the true nature of the physics at play. Hence, the more appropriate, but still not totally accurate, HOMO–LUMO nomenclature has become standard. The molecular nature of these materials has a another important consequence: by changing the chemical constituents and level of conjugation, the optical gap (Eopt or Eg ) can be ‘tuned’ over a range of 1–3 eV [45]. This range covers the peak in the solar spectrum thus offering a toolbox with which to develop new optoelectronic materials as diverse as life itself. Organic optoelectronic and semiconducting materials can be broadly classed as polymeric, dendritic or small molecule. As highlighted in section 1 and discussed in detail in section 2, spatial and energetic disorder in organic conducting solids limits the extent of delocalized bands. Band-like transport is only likely in highly ordered single crystal organic semiconductors, even then, strong coupling to low energy phonon modes can give the appearance of an inverse temperature dependence to the mobility which is characteristic of a hopping mechanism between localized states [46, 47]. Disorder can be introduced by impurities, or in polymeric systems as defects in the chain that disrupts the conjugation creating an ‘ensemble of conjugation lengths’. Each continuous conjugated system defines a chromophore with each chromophore absorbing photons of a particular wavelength. Larger segments may have smaller energy gaps, while shorter segments may have larger energy gaps. This ensemble results in an inhomogeneous broadening of the peaks in the optical spectrum. Broadening may also occur from coupling of the excitation to the vibration of the nuclei or neighbouring molecules [48]. The case of melanin is an extreme example of this ensemble effect—the material in solution or solid state has a broad monotonic absorption which Meredith et al have shown to arise from extreme ‘chemical disorder’ [49]. Most organic semiconductors are characterized by a low relative static permittivity (εr ) of between 2 and 6. This results in poor screening of charge carriers. For an εr = 3 the Coulomb interaction [V = q 2 /(4π εr ε0 r)] is around 0.5 eV for an e–h separation of 1 nm. This is much larger than the thermal energy at room temperature (kb T ≈ 26 meV, kb is Boltzmann’s constant). Therefore, optical excitations on a single chromophore do not typically result in spontaneous free carrier generation as happens with inorganic semiconductors. Instead, localized charges on the same molecule are created in the form of a neutral strongly bound (0.1–1 eV) [50] excitation often referred to as a Frenkel-type exciton [48]. Depending on the local electronic landscape, an excited electron can be transferred to a neighbouring chromophore but still remain correlated with the parent hole forming a CT exciton. The exciton has a finite lifetime since a number of processes can occur, namely: e–h radiative recombination (photon emission through fluorescence); non-radiative recombination (phonon emission); annihilation with a free or trapped charge carrier; or exciton–exciton annihilation. The

3. Photon harvesting and light interactions Organic chromophores within biological proteins are the cornerstone of terrestrial life on Earth and the inspiration for some remarkable progress in synthetic bio-inspired organic light-harvesting systems over the last few decades. In natural photosynthetic systems the excitations of the photoactive chromophores (encapsulated within complex proteins) undergo long-range energy transfer processes to ‘reaction centres’, where multiple CT reactions lead to water splitting and solar energy storage in the form of chemical bonds in carbohydrates (fuel production). Artificial photocatalytic systems attempt to replicate this complex processes to split water into H2 and O2 , reduce CO2 to produce fuels such as methanol or methane, or directly convert photons to electrons in photovoltaic devices. Under naturally occurring low light conditions, excitation energy transfer processes are extremely optimized in biology. However, despite quantum efficiencies of near unity [43], the energy conversion efficiency is typically 99% biodegradable content have been demonstrated which are robust and stable even in aqueous environments [101]. The basic concepts and methods for creating bioelectronic circuit implants with engineered lifetimes, i.e. which can be broken down by the biological environment, were recently described in ground-breaking work by Hwang et al [102] in which they describe a ‘Physically Transient Form of Silicon Electronics’. Bridging the ‘Biotic–Abiotic Signal Gap’ as described earlier in this section is really only now beginning to impact the agenda in artificial skin. True interfacing of the electronic sensing element to the natural signal carrying pathways is an enormous challenge and will require refinement and integration of high signal-to-noise transduction via robust, bio-compatible electrical connections—maybe using bioprotonics! Specific challenges associated with neural interfaces in the brain and peripheral and central nervous systems were recently summarized in a series of articles edited by Bellamkonda et al [103]—in particular the opportunities for using optical processes, a theme we will pick up on in section 4.2.

4.2. Optical and optoelectronic interfacing with biological systems In section 4.1 we dealt in the main with electrical or electrochemical devices both bio-inspired, and for interfacing with biological systems. The complexity and technical challenges associated with these ‘electrical tissue-device interfaces’ may be circumvented in some scenarios through alternative signal transduction mechanisms. A potentially novel direction in this regard is the use of light as a means for indirectly modulating cellular function. The application of this technology is especially prudent for selective control of neuronal function, touched upon in section 4.1.6, and so in this section we will provide a very brief overview of the current state-of-the-art and challenges in this embryonic field. 4.2.1. General principles of optical interfaces. The underlying premise for optical-cell interfaces is based on the phenomenon of so-called ‘optogenetics’, which has been extensively reviewed elsewhere [104]. The intrinsic advantages of light-activated control of neuronal function are numerous—in particular precise control afforded by the ability to accurately manipulate the intensity, spatial position and timing of optical stimulation and subsequent neuronal signalling. Adequate spatiotemporal control is much more challenging than electrical approaches due to the omnidirectional and diffuse nature of signal generation. Initial advances in optogenetics focused on the use of neurons that are genetically modified with genes from light sensitive rhodopsin channel proteins. These transmembrane proteins serve as light-gated ion channels that are able to transduce exogenous light cues into electrical depolarization events. The challenges of delivering effective photo-therapies can also be considered in the same context as cellular optical addressing, and in both cases dynamic control of the ‘optical dose’ in space and time is a critical feature. Photo-therapy for neuro-modulation requires additional considerations because of the device-based nature of the delivery system. A prerequisite for this technology is the ability to efficiently and stably interface viable photo-sensitive cell populations with novel optical devices [105]. For example, devices must be able to interface with neurons incorporating additional considerations such as protein optical absorption, matching the mechanical properties at the cell/tissue-device interface, and generally ensuring long-term biocompatibility. 4.2.2. Optical-to-electrical transduction: a familiar example. A potential cornerstone technology that may aid in the pursuit of reliable delivery of cues for optical stimulation is once again plastic, flexible electronics based on organic semiconductors. One notable, recent demonstration of this concept utilizes a hybrid optical bio-organic interface fabricated from blends of poly(3-hexylthiophene-2,5-diyl) [P3HT] and phenyl-C61butyric-acid-methyl ester [PCBM], two of the materials described in the molecular heterojunction discussion of section 3.2 (see also figure 11) [105]. P3HT : PCBM photo-conversion devices can be fabricated in an almost identical manner to OSC bulk heterojunctions (section 4.3). However, the governing performance requirements for optical 17

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Figure 11. Optical stimulation of neurons cultured onto an organic photovoltaic device. (a) Schematic representation of the optical stimulation paradigm including the localization of the stimulus in a region surrounding the patched neuron (scale bar, 10 µm). (b) Scheme of the photosensing interface, with the neuronal network grown on top of the polymer active layer during patch-clamp recordings. (c) Online monitoring of pH of the extracellular solution during the experiments in the presence (black) or absence (red) of photostimulation (means ± s.e.m). (d) Action-potential generation in response to a photostimulation pulse (50 ms). (e) Example of spike train generated with 20 ms pulses repeated at 1 Hz (upper panel). Peristimulus time histogram (PSTH) count was computed and normalized by considering spike trains in all recorded neurons (bottom panel). The right plot shows (means ± s.e.m.) the latency to the spike peak with respect to the light onset computed by averaging all spikes in the train obtained from all recorded neurons and the jitter calculated as the s.d. of spike latencies measured across all recorded neurons. (f ) Spatial properties of the photostimulating interface. A grid of nine spots (diameter 20 µm, spacing 30 µm) was overlaid to a patched neuron and spikes were counted. PSTHs, arranged in a similar grid, represent the spike counts normalized for the total number of sweeps in all recorded neurons. Each histogram represents the count of the spikes recorded at the soma by the corresponding stimulation spot. (g) Model of the polymer/electrolyte and electrolyte/neuron interface, where Ci and Ri represent the capacitance and the resistance of the double layer at the interface of electrolyte and polymer, respectively. Vb represents the bias voltage traditionally applied for the cell stimulation where Vb = 0, Rs the electrolyte resistance, and Vm the measured membrane potential. Green bars and traces represent light pulses in all panels. Adapted and reprinted by permission from Macmillan Publishers Ltd: Nature Communications [105], copyright (2011). 18

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Figure 12. Photoexcitation of neurons using infrared irradiation occurs due to thermally induced changes in membrane electrical capacitance. (a), (b) Simplified equivalent circuit diagram and current equation for a passive membrane. The membrane current (Im (t)) depends on the membrane voltage (Vm ), the Thevenin conductance (gT ) and potential (VT ), the bilayer surface charges (Vs ), and the temperature-dependent membrane capacitance (Cm (T (t))), which is highlighted in red. (c) Simulated (red) and experimental (black) current responses for a lipid bilayer in symmetric NaCl solution at a holding potential of +200 mV, based on the temporal profile of temperature response to a 10 ms (7.3 mJ) infrared pulse. (d) Simulated current–voltage response to a 10 ms (7.3 mJ) pulse at voltages ranging from −200 mV (bottom curve) to +200 mV (top curve). (e) Predicted voltage-dependent charge induction for bilayers in different ‘outer’ solutions: symmetric NaCl solution (Ctrl); 14mM MgCl2 (Mg2+ ); 1 mM GdCl3 (Gd3+ ). (f ) Simulated current–voltage response across bilayers for the Mg2+ condition described in (e). (g) Charge–voltage curves measured experimentally in solutions matching the conditions modelled in (e). (g) A representative current–voltage response for the Mg2+ condition in (g). Adapted and reprinted by permission from Macmillan Publishers Ltd: Nature Communications [106], copyright (2012).

for the development of brain-machine interfaces based on organic photodetectors (section 4.3). Another logical clinical application for flexible and conformal photo-conversion devices is in retinal prostheses—the target structure being an inherently two-dimensional curvilinear geometry. One grand challenge for this specific application is ensuring that sufficient light intensities can be harvested and transduced to trigger local cellular depolarization.

interfacing applications are substantially different compared to the traditional metrics for photovoltaics. Rather than focusing exclusively on power conversion efficiency (PCE) and facile processing of large scale devices, organic semiconductorbased phototherapy devices require stabile operation in aqueous environments with high salinity. There can be rapid degradation of the photocurrent generated during continuous operation of the device in electrolyte solutions, as observed by Ghezzi et al [105]. Additionally, maintaining low opencircuit voltages (∼1 V) is essential to minimize electrolysis of water and preserve the viability of integrated cell populations. Additional stipulations include: promoting cell adhesion for a robust interface; and development of micro-scale structures to both support neuronal integration and match the characteristic length scale of viable cell structures. In the example cited here, in vitro photo-stimulation of primary neuronal cultures using P3HT : PCBM heterojunctions proceeds by exposing cells to visible light (λ = 532 nm) at intensities of ∼10 mW cm−2 for durations of 50 ms. Efficient charge generation in the junction (as described in section 3.2) produces currents that induce electrical depolarization events in the cells. These events can subsequently be measured indirectly through patch clamp techniques. This approach promises an intriguing level of utility to study in vitro models for disease progression. Indeed, the concept may eventually lead to novel neuronal communication and photo-manipulation techniques, thus paving the way

4.2.3. Direct optical stimulation. As we saw in section 4.2.1, excitation with visible light can be achieved through genetic insertion of rhodopsin proteins. Depolarization events can also be triggered with infrared irradiation. Photo-stimulation in this part of the spectrum is based on local heating and is therefore particularly attractive in the context of obviating the technical complications associated with stable gene delivery strategies. The mechanism of this action varies significantly compared to optical stimulation in the visible regime. Namely, depolarization of lipid membranes can be achieved through the temperature-dependent changes in capacitance (figure 12) [106]. This mechanism is most accurately modelled through the Gouy–Chapman–Stern theory of double-layer capacitance [107]. This theoretical construct can be used to predict the capacitive charge across the membrane as a function of temperature [108]. The following relationships can be used to accurately solve the potentials on either size of the membrane (0 and 0 − Vm ) as a function of temperature and hence 19

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Figure 13. Common OPV device architectures. (a) Bilayer device architecture with the donor and acceptor material as separate layers, usually formed by vacuum sublimation. (b) Bulk heterojunction (BHJ) device architecture with the donor and acceptor materials blended together in the same layer. Some degree of asymmetry is needed to separate and collect the free electrons and holes. This asymmetry is provided by depositing charge selective contacts between the electrodes; an electron blocking layer between the transparent ITO electrode and the active layer, and a hole blocking layer between the low work function metal and the active layer.

cousins the OPD and the dye-sensitized solar cell (DSSC). This will not be an exhaustive discussion of the topic, but together with the photo-physics described in section 3.2 should provide an indication of progress and of the challenges. For a recent, excellent review on OSCs, the interested reader is directed towards Deibel and Dyakonov [109], for photodiodes Gong et al [110] and for DSSCs Hardin et al [111]. Artificial photosynthetic systems that can generate fuels directly from light, as in an artificial leaf, will not be covered as they fall somewhat outside the remit of this review. The interested reader is referred to a publication by Nocera in this regard [112].

calculate the transmembrane current: 2
n  εb cjo (∞) σo + (i − o ) = 2εTsol RT δb j =1   i   zj F o × exp − (19) (o − s ) − 1 RT
2 n  εb σi + (i − o ) = 2εTsol RT cko (−∞) δb k=1  

i zk F i (20) ((i − Vm ) − s ) − 1 , × exp − RT where σo and σi are the intrinsic charge density of the outer and inner bilayers, respectively; o and i are the potential at the outer and inner bilayer surface, respectively; εb is the permittivity of the lipid bilayer; δb is the width of the lipid bilayer; εTsol is the permittivity of bulk aqueous solution at temperature T ; cjo (∞) and cko (−∞) are the concentrations of the j th ionic species in the outer buffer and the kth ionic species in the inner buffer, respectively; zji and zki F are the valences of the j th ionic species in the outer buffer and the kth ionic species in the inner buffer, respectively; os and is are the potential drops at the outer and inner bilayers, respectively, due to the Stern layer; Vm is the potential between inner and outer bulk solutions with the outer solution set at zero. Finally, the use of infrared stimulation as a method for neuronal excitation is attractive from the perspective of clinical translation for several reasons. Harnessing infrared-based excitation strategies will circumvent technical and clinical challenges related to the stable transfection of primary neurons. Furthermore, direct stimulation of neuronal cells with light is a strategy that is amenable to spatially-precise two-photon excitation approaches at tissue-transparent wavelengths.

4.3.1. Organic solar cells. OSCs are a direct application of the molecular heterojunction concept. The first organic photovoltaic junctions utilized the so-called ‘planar heterojunction’ (figure 13(a)) in which thin films of acceptor and donor are sequentially deposited in much the same way as a conventional inorganic thin film solar cell. Given the limitations of exciton diffusion and carrier mobility in organic semiconductors, this has a number of implications: (i) the individual acceptor–donor layers must be thin (10s of nms); (ii) only excitons generated within a diffusion length of the interface have a reasonable probability of creating free carrier pairs; and, (iii) the interfacial area of the planar heterojunction limits charge separation. In practice, only small molecule, crystalline organic semiconductors are capable of generating high efficiencies in this configuration—indeed, the multi-junction vacuum evaporated devices developed by the Dresden group, and now being commercialized by Heliatek, are an excellent example of how to fully optimize and utilize sequential structures such as these [113]. Figure 13(b) shows an alternative configuration called the ‘bulk heterojunction’ or BHJ which has more of a ‘natural’ architecture wherein the acceptor and donor are intimately blended at the nanoscale. The BHJ is more suited to all-solution processing and presents a higher

4.3. Light harvesting devices In this section we will address three direct applications of the molecular heterojunction for light harvesting: the OSC, its 20

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surface area for exciton separation. Active layers can be thicker (and hence absorb more light) than the planar junction equivalents and non-crystalline or semi-crystalline morphologies can be tolerated. Furthermore, this ‘single active layer’ configuration means that both channel I and channel II processes can be utilized simultaneously to generate photocurrent in a complementary absorption strategy [51, 114]. However, sequential processing of organic semiconductors is significantly easier than deposition of multiple components from a co-solvent, and achieving the required nanoscale phase separation is somewhat of a ‘black art’ requiring post deposition thermal treatments and/or controlled drying. Recently, numerous groups have shown [115] that certain polymer donor: fullerene acceptor combinations can be processed sequentially from solution using orthogonal solvents and then post processed to develop the bulk heterojunction blend—the best of both worlds it would seem. Irrespective of the active layer architecture, all thin film OSCs have a number of common features: (i) a low and high work function electrode pair (one of which must be transparent and conducting) forming the cathode and anode respectively; (ii) a supporting substrate (which can be a plastic); and (iii) hole and electron blocking layers at the cathode and anode respectively to facilitate charge extraction, minimize contact resistance and reduce dark current. All OSCs are characterized as ‘thin film’ being in total normally no more than several hundred nms thick. Recent state-of-the-art efficiencies have exceeded 10% in both evaporated planar multijunctions and solution processed BHJs at the laboratory-scale (8% [45]. However, such performance metrics are yet to be translated to large areas due to a number of factors, not least of which is the relatively high sheet resistance (>10 /sq) of the current transparent conducting anode material of choice—indium tin oxide (ITO). Jin et al recently reported a 3.2% 5 cm × 5 cm ‘monolithic’ (i.e. single active layer area) sub-module using the PCDTBT : PC70BM acceptor–donor combination ([poly[N-9 -heptadecanyl-2,7-carbazole-alt-5,5-(4 ,7 -di2-thienyl-2 ,1 ,3 -benzothiadiazole)] and methanofullerene phenyl-C71-butyric-acid-methyl-ester) [116]. They achieved this by replacing the ITO with a new lower sheet resistance, high optical transparency anode made from a carefully tuned stack of molybdenum oxide and silver in a so-called inverted illumination geometry where the cell was built upwards from the metallic cathode. There are many other challenges to achieving high efficiencies at the scale needed for viable OSCs and these are well covered in multiple recent reviews and papers (see for example [109, 116]). The amount of current that can be extracted and the voltage that can be generated are intrinsic properties of the materials that are used in the active layer, electrodes and transport layers. For a multiple junction device, these parameters are also dependent upon the type of interconnection between the subcells (parallel or tandem). As shown by the current density– voltage plot in figure 14, an OSC is a photo-diode and the maximum extractable current density occurs at short circuit (JSC ), whilst the maximum voltage occurs at open circuit.

Figure 14. Current density–voltage characteristics for a typical OSC made from the classic P3HT : PCBM combination. Also pictured are the directions of current flow and the field in the device. From left to right in the diagram: in reverse bias, the field is such that photo-generated carriers are extracted; at short-circuit the electrodes are at the same potential but the field inside the device is strong enough to cause carriers to drift to the contacts; at open circuit the photo-generated current is cancelled out by the reverse saturation current density and the rate of generation equals recombination. However, the zero field condition inside the device is at a point known as the compensation voltage V0 slightly higher than VOC [217] (forming the flat ‘band’ condition with nomenclature borrowed from the inorganic community); under forward bias conditions charges are injected into the device.

From a fundamental perspective, VOC is defined by the smallest of: (i) the energy difference between the donor and acceptor DA ); or (ii) the difference in electrode material charge states (VOC WF ), as depicted in figure 15 [117]. work functions (VOC DA VOC = P−A − P+D

(21)

WF VOC = cathode − anode .

(22)

However in practice, VOC is often reduced from the maximum due to recombination of charge carriers [118]. Recombination can take place before the exciton is fully separated (geminate recombination) or between charges generated from different excitons (bimolecular recombination) [109, 119]. These recombination processes are shown as current reducing pathways in figure 7 (section 3.2.1) and will be discussed further when we address free carrier extraction. The short-circuit current density is given by the convolution of the EQE as defined in section 3.2.1 with the incident spectral photon flux, typically defined by the AM1.5G (Air Mass Index 1.5 Global) reference solar spectral irradiance EAM1.5 (λ) [120].  λ2 eλ JSC = ηEQE (λ)EAM1.5 (λ) dλ. (23) λ1 hc Clearly, a close correspondence between the optical absorption of the solar cell and the solar spectrum is desirable to generate maximum photocurrent. As described in section 3.2.1, the absorption in a single-or-multiplejunction device (irrespective of the architecture) is defined 21

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Figure 16. Absorption spectrum of P3HT : PCBM blends (dark line) together with the AM1.5G photon flux (light line and shaded) and the maximum possible photocurrent (dotted line). The absorption spectrum of the polymer P3HT and PCBM do not overlap with the maximum in the photon flux, limiting the efficiency to 1 V) and JSC (>15 mA cm−2 ) have yet to be realized. These advances, mostly in donor polymer molecular design have meant that a PCE over 7% is now considered the new standard when combined with fullerene derivatives such as PC70 BM [133]. The fullerenes are by far the most successful acceptor moiety in high performance OSCs. But in an attempt to bring further flexibility to the design of synthetic light harvesting systems, recent efforts have been focused on alternative non-fullerene acceptor platforms [141] such as small molecules [142–145], polymers [146, 147] and 3D

Figure 15. Origin of the open circuit voltage arising from the DA Donor/Acceptor interface (VOC ) determined by the charge separated WF states, and difference in electrode work function () to give (VOC ) depending on which is smaller. P+A,+D represents the energy of the relaxed cation in the solid (positive polaron level) of the donor and acceptor respectively, and P−D,−A represents the energy of the relaxed anion in the solid (negative polaron level). Eopt is the optical gap.

by the optical gaps of the respective donor (channel I) and acceptor (channel II). Figure 16 shows this overlap for the P3HT : PC60 BM combination and it is clear how many photons are ‘wasted’. The overall PCE of the device is given by the product of VOC and JSC and the fill factor (FF): PCE = VOC × JSC × FF.

(24)

The FF is a measure of how close to the maximum power point the device is to the imaginary power maximum VOC × JSC (imaginary because at VOC , JSC is zero and vice versa). Simultaneous optimization of the VOC and JSC is the focus of efficiency improvement efforts in OSC research. In the main, ‘materials-centric strategies’ have dominated. This is not surprising given the vast molecular design space afforded by organic semiconductors. Numerous ‘architectural strategies’ such as junction structuring [121] and optical field enhancement by various means are also now beginning to emerge largely aimed at increasing the in-coupling and utilization of the solar spectrum [116, 122–128]. Several of the more successful approaches are summarized below: (i) Molecular design: various approaches have been successful in pushing the absorption of acceptor and donor materials into the NIR region of the spectrum [129, 130]. These can be broadly classed as: (i) increasing the effective conjugation length (and planarity) [131]; and (ii) reduction in bond length alternation (reduction of the Peierls effect) through the 22

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Figure 17. Chemical structures for some high performance OSC materials.

in order to reclaim some of the energy outside the absorption spectrum of the active material [126, 127, 152]. Triplet upconversion yields as high as 40% have been observed in solution [127] and it is hoped that such high yields are possible in the solid state. Triplet–triplet annihilation has been employed as a means to improve the red harvesting capability of inorganic solar cells [128] and DSSCs [152] and may prove to be a valuable tool to boost light harvesting in future OPVs.

dendrimers [148]. Non-fullerene acceptors generally award greater flexibility in the tuning of absorption and energy level properties. For example, a VOC > 1.3 V has been reported using small molecule acceptors [149, 150] in a single junction device. However, non-fullerene acceptors have so far been limited to relatively low PCEs of less than 4% [144, 151]. (ii) Optical field enhancement: another method to increase JSC is to enhance the optical field by surface plasmon resonance. Optical field enhancement has been achieved by including metal nanoparticles and photonic crystals in the blend. An increase in the broadband absorption of >20% has been predicted for silver gratings and one-dimensional photonic crystals [122] while gold nanowires have been used to increase JSC by 23% [123]. In a similar manner, the inclusion of Ag nano-prisms has resulted in a three-fold increase in the free carrier polaron yield attributed to increased absorption [124]. Patterning of the active material has been shown to result in a 10% increase in JSC by both coherent light scattering and improved charge collection [121]. An alternative approach to increase photon absorption is to reduce reflection losses at the device surface. The electrode-air dielectric interface can be tuned to maximize the optical field in the active material. As highlighted earlier, an MoOx layer has been used in this fashion to great effect in large area (5 cm × 5 cm) PCDTBT : PC70BM sub-modules delivering a PCE >3% in an inverted illumination structure without ITO [116]. Recently Kim et al [125] demonstrated a novel method of extending the spectral response of OSCs by introducing wrinkles and deep folds in the device. The folds and wrinkles help trap and guide the light, increasing the path length through the active material. The effect was most striking in the near-infrared where the material is only weekly absorbing, boosting the EQE by more than 600% and extending the spectral response by 200 nm. The folds and wrinkles were also shown to benefit the device robustness under repeated mechanical stress. (iii) Spectral re-shaping: photon up-conversion in organic systems, using low intensity incoherent light, has been used

In a molecular heterojunction (natural or artificial), the requirements for harvesting free carriers post interfacial charge separation are very application specific. In an OSC, as discussed in section 3.2.1, the architecture of the junction and the blending and morphology of the donor and acceptor play a central role in the efficiency of free carrier extraction. In particular, for a bulk heterojunction the formation of a nano-scale bi-continuous charge transport network is critical to performance. Within these phases, the charge carrier mobility (µ) is a defining parameter and is typically of the order of 10−3 cm2 V−1 s−1 for high performance materials due to disorder and localization onto transport sites rather than in delocalized transport bands. However, ambipolar mobilities greater than 1 cm2 V−1 s−1 have been reported for certain push– pull polymers, although this is not strictly necessary for thin film OSCs [153]. Macroscopically, the photocurrent from an OSC is given by the typical diode expression [154] 
 

Rp q(V − J Rs ) V J = Js exp −1 + Rs + R p nkB T Rp − Jph (V ),

(25)

where Rs is the series resistance and Rp is the parallel or shunt resistance. Under the assumption Rp  Rs , and at open circuit conditions (J = 0, V = VOC ) and ideality factor n = 2 typical for an OSC, the open circuit voltage can be derived 23

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from equation (25) in terms of the recombination rate krec [118]
 nkB T JSC VOC ≈ ln (26) q qkrec [D+ A− ]

There are some notable additional considerations for OPDs versus solar cells. For example, the minimum detectable optical power of a device is given by the noise-equivalent power (NEP, units W Hz−1/2 ) [163, 164]. This is defined by the incident power required to obtain a signal equal to the noise in a 1 Hz bandwidth, i.e. a signal (S) to noise (N ) ratio S/N = 1 = Ri Pin / in (0 dB) where the device responsivity (R, units A W−1 ) and incident spectral power (Pin ) determine the signal strength, and in is the noise spectral density [A W−1/2 ]. The NEP is therefore equal to

krec is of the form given by equation (16) (section 3.2.1), while [D+ A− ] represents the concentration of CT states at the donor–acceptor interface. Equation (26) with reference to equation (16) suggests that decoupling the charge separated state wave functions from the ground state wave functions is one way to reduce recombination, making the recombination kinetics highly material specific. Note also that for some systems G◦ for the recombination reaction D+ + A− → D + A may be much larger than the reorganization energy for the reaction, reducing the recombination rate (the so called Marcus–Hush inverted region) and is the likely origin for the reduced recombination in some systems (non-Langevin recombination [155]) while not in others [156]. Indeed the reduced e–h recombination reaction in natural photo-systems is believed to arise from a small reorganization energy (λ ∼ 0.25 eV) despite a large exothermic driving force (G◦ ∼ 1.1 eV) [157]. D’Souza et al found that the recombination rate could also be suppressed by the small reorganization energy in phthalocyanine [158] for a self-assembled zinc phthalocyanine electron donor and fullerene electron acceptor biomimetic supramolecular structure. A recent novel approach to engineering the interfacial recombination kinetics was reported by Lobez et al where they introduced additives that selectively interact between the donor and acceptor [159]. By adding small fractions of P3HT with modified side chains that selectively mix at the interface, they were able to form interface dipoles that reduce the bimolecular recombination rate, resulting in an increasing in the PCE of 30%. Finally, efficient extraction of carriers at the interface requires the fabrication of ohmic contacts to the electron (hole) transporting acceptor (donor) phase, effectively blocking extraction of holes (electrons) at that electrode. Ohmic contacts can be made by careful selection of electrode or interlayer materials [160]. Engineering the organic-electrode interlayer can not only benefit charge extraction but can also enhance the built-in potential across the device—thus eliminating space charge build up and improving transport through the bulk. Recently, interface modification using a water/alcohol soluble conjugated polymer layer at the cathode in conjunction with many other optimization techniques discussed in this section, was used to push the PCE >8.3% for PTB7 : PC70BM blends [161].

NEP =

in . Ri

(27)

The detector performance is inversely proportional to the NEP (a lower NEP correlates to greater detector sensitivity). The figure of merit for a photodetector is the normalized detectivity, defined as the signal-to-noise ratio of the detector for 1 W of power incident on a unit area of the device Ad = 1 cm2 in a noise-equivalent bandwidth of f = 1 Hz. √ Ad f 1 D∗ = = . (28) NEP NEP √ The normalized detectivity has units cm. Hz W−1 (or Jones) and is directly proportional to the detector performance. If the dark current is dominated by shot noise then the detectivity can be written in terms of the dark current according to D ∗ = (Jph /Llight )/(2eJd )1/2 where e is the electron charge, Jd is the dark current density and Llight is the incident power density. In practice, separate photodetector materials are used in traditional photodetectors to cover the UV (GaN, 250– 400 nm), visible (Si 450–900 nm) and NIR (InGaAs, 800– 1.7 µm) parts of the spectrum with typical detectivities ∼1012 Jones. For InGaAs NIR devices, higher detectivities require cooling to liquid helium temperatures ∼4.5 K. Recently, Gong et al reported an organic photodetector composed of a smalloptical-gap polymer blended with the fullerene derivative PC60 BM that exhibited a broad spectral response (300 to 1450 nm) [110]. By incorporating electron-blocking layers at the organic/electrode interface they were able to reduce the thermally generated dark current sufficiently to exhibit an extraordinary broadband detectivity >1012 Jones at room temperature and a linear dynamic range over 100 dB. This represents the current state-of-the-art. Stimulated by advances such as these, and also the significant progress in OSCs, the OPD field is emerging as the next frontier in organic optoelectronics. As such, a number of novel approaches are emerging. For example, Li et al have attempted to circumvent the ‘charge mobility bottleneck’ in organic semiconductors (low mobilities defining a slow response) by engineering a device that operates on displacement currents as opposed to conduction currents [165]. By sandwiching the photoactive layer between two transparent organic dielectric layers composed of PVDF, the device effectively operates as two capacitors in series, reducing the time constant for the signal fall-off time, while at the same time replacing the conduction current with a displacement current.

4.3.2. Organic and bio-inspired photodetectors (OPD). Many of the key principles outlined for an OSC, and all of the basic physics described for a molecular heterojunction apply also to organic photo-detectors (photo-diodes). Organic optoelectronic materials offer the potential for low cost photodetection systems for many applications including sensing and imaging. Furthermore, the intrinsic ability to create versatile, flexible form-factors with biocompatible materials has the potential to broaden applications to biometrics and bionics (e.g. artificial retinas) [105, 162]. 24

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The original design by Gr¨atzel and co-workers in 1991 [167] was based on a Ru complex/iodide redox couple system with a solar power efficiency of 7%. Immense research efforts have seen the efficiency of this system climb to over 11% [168]. However, it has been estimated that the Ru complex/iodide system has a maximum efficiency of ∼13%, limited by the large over-potential associated with the charge injection from the dye, and the multi-electron transfer processes of the iodide redox couple [169]. The use of rare earth metals such as Ru in the sensitizing dye limits the cost of DSSCs. Furthermore, Ru complexes generally have low molar absorption extinction coefficient (∼104 M−1 cm−1 [170]) and relatively weak absorption in the NIR part of the solar spectrum. Therefore, a vast array of alternative sensitizers have been explored ranging from naturally occurring pigments [171], all-organic metalfree dyes [172, 173] to earth abundant metal-organic complex alternatives [174]. In particular, porphyrin-based pigments, being ubiquitous in natural light harvesting systems, have been thought to hold great promise [175, 176]. Porphyrins are characterized by delocalized macrocyclic structures and strong absorption. Perhaps not surprisingly, to-date the most efficient DSSC—a PCE of 11.9% under 1 sun—was achieved by Gr¨atzel and co-workers [177] using a zinc porphyrin dye as the sensitizer in conjunction with a Co(II/III) tris(bipyridyl)-based redox shuttle better matched to the reduction potential of the dye; resulting in a VOC close to 1 V. By including a co-adsorbed all organic dye to broaden the spectral response, the efficiency was boosted slightly to 12.3%, suggesting that further gains may be possible by engineering porphyrin dyes with stronger NIR absorption. A PCE as high as 19% may be achievable in the porphyrin/Co(II/III) system with further reduction of the over-potential associated with dye regeneration, and a dye absorption onset extended to 920 nm [111]. However, reducing the optical gap of the sensitizer while holding the ionization energy of the excited dye constant (for efficient electron injection into TiO2 ) and maintaining the low recombination kinetics is not trivial. In an effort to extend the photon energy harvested in DSSCs, McGehee, Gr¨atzel and co-workers introduced the concept of an energy relay dye; a non-bonded molecule added to the electrolyte that transfers blue energy excitation to the sensitizing dye by excitation energy transfer processes [178– 180]. This concept was inspired by the energy transfer mechanisms in purple bacteria, where spatially separated pigments in the LH-II complex transfer its excitation via an intermediate LH-I complex with high efficiency to the reaction centre. More recently colloidal nanoparticles have been used as energy relays to boost the NIR response by utilizing upconversion nanoparticles [181, 182]. One of the engineering challenges of the DSSC is the long term encapsulation of the liquid electrolyte, which is often composed of volatile organic solvents to aid diffusion of the redox shuttles. Efforts have therefore been directed at replacing the liquid electrolyte with a solid state holeconductor (p-type materials). Recently Chung et al reported a solid-state DSSC with a solar PCE as high as 10.2% [183]. Their solution processed intrinsic p-type material

Figure 18. Morpho butterfly inspired MWIR detector (adapted from Pris et al) [166]. When the low thermal mass Morpho nanostructure absorbs MWIR, the nanostructured optical cavity expands, changing the reflected iridescence.

The result was a reduction of rise time and fall time by 10% and 86% respectively. Pris et al have recently demonstrated a novel mid-wave infrared (MWIR) detector inspired by the iridescence of Morpho butterfly scales [166]. The butterfly scales possess a hierarchical nanostructure (see figure 18) that forms an optical cavity which expands on absorption of incident MWIR (3–8 µm). Their innovative approach uses this thermal expansion and consequent change in the iridescence of the scales to effectively convert a modulation in MWIR to modulation in the visible-light iridescence. The absorption of MWIR was enhanced by doping the scales with single-walled carbon nanotubes, resulting in a remarkable noise-equivalent temperature difference detection of 18–62 mK at 35–40 Hz without the need for active- or passive-cooling. 4.3.3. Dye-sensitized solar cells. No discussion of bioinspired light harvesting systems would be complete without at least a brief mention of the now infamous DSSC. Some would argue that this device is as close to a working artificial photosynthetic system developed to date. This analogy is perhaps a little strong, but there are certainly a number of biomimetic features—for example, a photo-electrochemical driving force across a membrane, coupled reduction-oxidation reactions, and separation of the light absorption and carrier transport function in different components [111]. It is also worth noting that the basics of operation are quite different to an OSC as described above, although both devices rely upon excitonic photo-excitation. The structure and operating principles are shown in figure 19. A nanostructured wide bandgap semiconductor such as TiO2 forms the electron transporting material and a high-surface area onto which a monolayer of sensitizing dye molecules are chemically adsorbed. The high-surface area increases the light absorption in the device. A redox couple—typically I− /I3− in a liquid electrolyte—chemically reduces the oxidized dye back to its neutral state. The redox couple transports holes to the counterelectrode (typically nanostructured Pt), completing the circuit by being reduced back by electrons from the photoanode after they have passed through an external load. The open-circuit voltage is determined by the difference in TiO2 Fermi energy and electrolyte redox potential. 25

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Figure 19. Left: Common DSSC architecture. Light enters the transparent conducting oxide (TCO) where the dye adsorbed onto TiO2 absorbs light. An ultrafast (50 fs–150 ps) photoinduced electron transfer reaction injects an electron into the titania conduction band. The oxidized dye is regenerated (chemically reduced) back to its neutral state by an electron transfer reaction with the reduced redox species (R) of the redox mediator. The oxidized redox mediator (R+) then diffuses to the counter electrode (effectively transporting holes through the electrolyte) where it is chemically reduced by electrons that have passed through the circuit. Diffusion of the reduced species back to the dye ensures that the dye is continually regenerated. Right: A kinetic diagram showing the essential DSSC processes as solid arrows, and recombination losses as dashed lines. The photovoltage generated depends on difference between the redox potential of the redox mediator and the Fermi level of TiO2 .

was composed of CsSnI2.95 F0.05 doped with SnF2 and also contributed to photogeneration, enhancing the photo-response in the visible and red part of the solar spectrum. Other advances include progress towards water soluble/environmentally stable electrolytes [184]. 4.4. Natural materials for bio-inspired devices: substrates, insulators, semiconductors In this final section of the applications overview, we summarize recent progress aimed at identifying and demonstrating natural materials that could be used as substrates or active components in the range of devices described in sections 4.1 and 4.2. In the review so far we have encountered a number of such materials (for example melanin and maleic-chitosan) used to exemplify a particular application or basic physical principle. These will not be discussed any further. The recent surge in interest in organic optoelectronics and the drive towards cheap, bio-compatible and even bio-degradable components has reinvigorated the search for materials from natural, widely available and sustainable sources. This endeavour may seem like a ‘needle in a haystack’ problem but progress is being made and solutions are emerging from the most unexpected of sources!

Figure 20. (a) OFETs and integrated circuits on paper substrates. (Reprinted with permission from [185] Copyright 2004, American Institute of Physics). (b) A fully solution-processed reel-to-reel printed solar cell on paper. From H¨ubler et al [190] Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Traditionally, candidate materials for such substrates have originated from natural polymers, which are either polypeptides (proteins) or polysaccharides. One of the most familiar ‘substrate’ materials produced from natural products is paper. Produced in many grades and types, paper is an excellent choice for flexible, low-cost, ‘use-and-throw’ applications [185–189]. An early example of the use of paper in electronics is shown in figure 20(a) in which Eder et al employed hot-pressed cotton fibre paper as a substrate for the fabrication of field-effect transistors and circuits [185]. Using pentacene as the organic semiconductor channel, the group reported a p-type mobility of ∼0.3 cm2 V−1 s−1 and a

4.4.1. Substrates. Bio-compatible or naturally available substrates, preferably with mechanical flexibility, are an important primary element in any device application. 26

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A number of synthetic substrate materials have also been shown to possess acceptable biocompatibility. For example, polydimethylsiloxane (PDMS) and polyethylene (PE) are synthetic polymers emerging as preferable choices for interfacing organic electronics and silicon electronics with living tissue [194, 195, 203]. PDMS and polyvinyl alcohol (PVA) were also recently investigated as sacrificial substrates for biocompatible electronics—for example, as a multifunctional electronic circuit designed to be transferred and incorporated onto human skin [195, 199].

signal delay of 12 ms per stage in a ring oscillator. Paper substrates have also been used for flexible electro-wetting displays [189], as well as thermo-chromic displays targeted for disposable consumer products [187]. Furthermore, encouraging levels of performance have been demonstrated for flexible OSCs on paper substrates [190, 191]. Utilizing an inverted architecture with a printed ZnO/Zn back electrode and a conducting polymer transparent top electrode, these solar cells combine ultra low-cost materials and high-throughput low-temperature reel-to-reel printing, rendering a flexible final product. A photograph and a schematic inset of a paper solar cell are shown in figure 20(b). H¨ubler et al [190] employed a thin semitransparent sheet of paper as the primary substrate, a conducting polymer transparent electrode (once again PEDOT : PSS), a typical organic semiconductor blend active layer (P3HT : PCBM), and a reflective back electrode. The entire fabrication was carried out via low-temperature gravure and flexographic printing techniques. Another class of natural materials recently reported as biocompatible substrates for organic electronics are various polysaccharides made from potato or corn starch and polylactic acid [192, 193]. These polymers have been commercially mass-produced as biodegradable plastics for plastic bags and nursery foils. Additionally, leather and fabric [194], silk [195– 197], biocompatible polymers [101, 198, 199], hard gelatin and caramelized sugars [192] have all been demonstrated as viable substrate materials. Among these, silk is showing great promise—a natural polymer composed of two proteins: sericin and fibroin, the latter consisting of repeating units of amino acids glycine, serine, and alanine. The amino acids afford inter-chain hydrogen bonding, delivering the well know mechanical robustness of silk fibres. This material can be easily extracted from the spin of the silkworm, and subsequently processed into thin films of thicknesses down to 2.5 µm, with a roughness comparable to glass. Silk is biocompatible and ‘bio-absorbable’. It elicits no immune or inflammatory response when implanted into the body, and is fully biodegradable. Its dissolution can be adjusted from hours to days, a fact of paramount importance for applications involving drug storage, controlled delivery or temporary implants [200]. A number of groups have exploited these properties in a range of devices including OFETs, OLEDs and OSCs [195–197, 201, 202]. Gelatin (a protein) is another promising substrate candidate—particularly in the context of orally administered, short interrogation-time sensing or diagnostic electronics. In this context, OFETs produced directly onto hard gelatin capsules have been recently demonstrated [192, 193]. Likewise, caramelized glucose, with film forming characteristics rivalling those of standard synthetic polymers or even glass, has been explored as a truly exotic substrate for electronics [193]. Another natural candidate with a long history of use in film-forming applications is ‘Shellac’—a hard cross-linked polyester resin. Shellac is produced by female lac beetles, and is harvested from trees in India and Thailand. An alternative synthetic route has been recently reported yielding a high quality substrate material for high performance OFETs [198].

4.4.2. Dielectrics. Dielectrics represent a fundamental constituent in electronics, for example as the gate insulator in OFETs, passivation layers in transistors or intermediate layers in multi-junction solar cells. Nature is rich in biocompatible and biodegradable molecules that are easily processable into insulating thin films. One such example is DNA (readily available from many sources such as waste from the fishing industry) which has stimulated significant interest for several decades for its conducting as well as insulating properties [204–207]. Solution processed cross-linked DNA [207] as well as vacuum processed nucleobases (adenine, guanine, thymine and cytosine) [192, 193, 208] have been successfully implemented as gate dielectric layers in low operating voltage OFETs. An immediate advantage of using the constituents of DNA (i.e. nucleobases) rather than DNA strands rests in the high purity of nucleobases that are amenable to rigorous processing via train sublimation. An example of a ‘DNA OFET’ was reported by Irimia-Vladu et al [193, 208] where a thin film of adenine was used in combination with an electrochemically grown aluminium oxide dielectric and a C60 fullerene organic semiconductor channel. This device had a very low operating voltage of ∼0.5 V and high n-type carrier mobility (up to 5.5 cm2 V−1 s−1 ) [193, 208]. Silk in thin film form has also been recently shown to be an effective gate dielectric for OFETs and organic light emitting transistors (OLETs) [209] (as well as a viable substrate). Figure 21(a) shows an OFET device with a thin film of silk as the dielectric and pentacene as the semiconductor channel; the transfer characteristics of the device are shown in figure 21(b). The calculated field effect mobility, 23.2 cm2 V−1 s−1 of pentacene grown on silk is higher than the mobility observed in standard inorganic amorphous semiconductor devices such as those fabricated with IGZO (indium gallium zinc oxide) [202]. Dielectrics from the sugar family have also been employed in OFETs [192, 193]. Sucrose, glucose and lactose can be processed in aqueous solvents and have comparable filmforming dielectric characteristics to polyvinyl alcohol (PVA) in terms of smoothness and dielectric breakdown voltage. Finally, the protein albumin, from chicken egg whites, has been recently reported as an exotic natural dielectric, once again for OFETs. Solution processed albumin is cross-linkable and affords simple fabrication of high mobility devices [210]. 4.4.3. Semiconductors. Compared to the significant number of articles dealing with natural and biocompatible/biodegradable materials as insulators and substrates, the number of reports of natural semiconductors remains very 27

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Figure 21. (a) Photograph of an OFET with pentacene semiconductor channel and silk dielectric. (b) Transfer characteristic of the OFET showing field effect mobility of 23.2 cm2 V−1 s−1 . Reproduced with permission from [202]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

low indeed. Natural organic semiconductors have generally been considered as somewhat of a novelty, and have never really delivered anywhere near respectable performance (mobilities, photoluminescence quantum yield, etc). As we saw earlier in the melanin (and DNA) case, there are also complications associated with exotic hybrid conducting behaviour, which confuse and confound attempts to disentangle the transport physics, and hence build consistent structure–property relationships to deliver better design and performance. For example, modest mobilities of ∼1 × 10−4 cm2 V−1 s−1 were reported for β-carotene, a linear π-conjugated molecule that acts as a p-type semiconductor [193]. However, significant advancements have recently been obtained with nontoxic synthetic textile or cosmetic dyes such as natural anthraquinone, perylene diimide derivatives, as well as other semiconductors based on a thiophene core with mobilities in the range of 10−2 –10−1 cm2 V−1 s−1 [101, 192, 193]. Even more encouragingly, a real breakthrough in terms of natural semiconductor performance has been recently achieved with indigo and its derivatives [198]. OFETs from all natural materials with ambipolar charge transport and semiconductor mobilities in the range 10−2 –0.4 cm2 V−1 s−1 have been demonstrated with indigo and 6,6 -dibromoindigo (Tyrian purple) [198, 211, 212]. The unique feature of indigo and its derivatives is the presence of hydrogen bonded induced planarity which forces the molecules to adopt a preferential orientation in solid state evaporated films. Indigo molecules form sheets mediated by these H-bonds and tight π -staking with characteristic heteroaromatic ∼3.4 Å inter-planar spacing. Due to the unidirectionality of the π -stacking, the charge transport in indigoids is highly anisotropic. Hence, controlling the orientation in these materials is a critical design consideration to engineer high mobilities. Furthermore, it is also possible to control the ‘global direction’ of this stacking relative to the substrate and electrode planes. For example, when grown on aliphatic gate dielectrics, the indigoids adopt an end-on orientation ideally suited for electron and hole transport along the ‘horizontal plane’ between source and drain electrodes. Some of the indigoids are also air stable due to their relatively deep LUMO

levels. An example of an OFET with a Tyrian purple active channel has been recently reported by Glowacki et al [211]. This device delivered remarkably stable ambient environment performance (almost unheard of in traditional ‘synthetic’ semiconductor OFETs). In addition, complementary inverter circuits with Tyrian purple channels were created by connecting two OFETs fabricated on the same gate electrode, and with only one type of source–drain electrode (i.e. gold). Such an inverter has produced amongst the best recorded quasi-static gain of ∼250–290 for an OFET [211].

5. Future perspectives: opportunities and challenges In this Report on Progress in Physics we have reviewed in brief the status of bio-inspired and bio-compatible electronic and optoelectronic materials and devices. The focus has been largely on organic and bio-organic conductors and photoactive systems, although the subjects of natural dielectrics and substrates have also been addressed. Research efforts in this emerging field are motivated by a number of factors: the need for more sustainable high tech materials; the requirements of interfacing with biological systems in bionanomedicine and bioelectronics; and the drive towards cleaner energy solutions. Traditional organic electronics and optoelectronics provide much of the chemical and physical ‘foundation elements’ for the field which is highly multi-disciplinary. The physics of organic and bio-organic semiconductors is dominated by a number of intrinsic features: high degrees of energetic and structural disorder; the molecular nature of organic solids; strong electronic correlations; electrical conduction within extended π -conjugated networks; hybrid electronic– ionic (protonic) transport; and the excitonic nature of photoexcitations. Solid-state transport models based upon Band Theory and Mott–Davies amorphous semiconductivity have limited applicability to most organic or bio-organic systems, although the concept of conduction through localized states via hopping has generic utility in single molecule and bulk situations. Transport of free carriers through an organic or bio-organic 28

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of spectral selectivity and spatial precision at the cellular scale. Direct optical addressing has benefited from recent advances in photonics, and optical-to-electrical transduction is a real possibility with bio-compatible, low power consumption organic photo-detectors [105]. Organic solar cells mimic natural light harvesting processes and devices with solar power conversion efficiencies >10% are a reality [45]. The almost limitless ability to tune an organic chromophore means that light harvesting molecules and combinations of molecules can be synthesized to cover specific or extended spectral ranges. Solution processing of organic solar cell components over large areas promises to deliver dramatic cost reductions and low embedded energy photovoltaics. New strategies, often naturally inspired, for improving harvesting efficiency in solar cells are emerging and include spectral re-shaping, plasmon enhancement, light guiding, energy funnelling, and the utilization of channel I and channel II photo-currents in complementary absorbers [51, 114]. Dye-sensitized solar cells are maybe an even closer mimic of natural photosynthesis. Recent advances in all-solidstate architectures could mean this technology is nearing the point of commercial deployment with efficiencies rivalling inorganic thin film photovoltaics [183]. Naturally available substrate materials for electronics and optoelectronics are plentiful. Proteins such as silk or gelatin, celluloses (paper), sugars, and polyester resins such as Shellac have all been demonstrated to be suitable substrates for organic electronics. The advantages of these materials are numerous—including the ability to tune their dissolution and absorption for a temporary implant. Likewise, natural dielectrics amenable to processing into device quality thin films are also numerous. In particular, silk and DNA have been incorporated into organic thin film transistors and delivered high carrier mobilities in n and p-type channels. More scarce are naturally occurring semiconductors delivering anywhere near useful performance. A standout example is the common indigoid dye Tyrian purple which has recently delivered some quite remarkable and unexpected results—ambipolar field effect mobilities of 0.4 cm2 V−1 s−1 [211]—by far the highest yet reported for a naturally occurring organic material and extremely promising. This may well stimulate renewed interest in natural semiconductors. The range of opportunities and applications described above, and the complex transport and photo-physics of organic and bio-organic semiconductors present a large number of challenges. From the fundamental perspective, suitably accurate and predictive models to explain carrier transport and light interactions are lacking. This is particularly true of hybrid conductors where ionic and electronic effects mix and interact. A quasi-band model appears to be applicable to explain transport in a proton transistor. Is this a generic feature of solid-state protonics and ionics? The mechanisms of exciton separation at a molecular heterojunction interface are still a matter of some debate—for example, is the charge transfer state a necessary precursor to the charge separated state? This is generally thought to be true for known organic semiconductors, but may not be universal depending upon interfacial subtleties like the existence of strong interface

conductor may be electronic, ionic or facilitated by a hybrid process. Efficient protonic conduction with mobilities of order 10−3 cm2 V−1 s−1 can be achieved in percolated hydrated networks if the Grotthuss mechanism dominates. The first order effect of the environment (in both disordered electronic and ionic/protonic conductors) cannot be neglected and is often the dominating factor. This complexity has led historically to misinterpretation and incorrect assignment of both carrier type and transport model [8]. In particular, positive polaron and proton conduction share several common features. The Coulomb-bound nature of excitons and relatively low dielectric constant of organic materials means that the creation of free charges from a photo-excitation requires an extra driving force. The molecular heterojunction is a natural architecture that provides an appropriate chemical potential gradient and hence driving force. Synthetic acceptor– donor interfaces can achieve almost unity quantum yield in this regard, and the so called ‘bulk heterojunction’ is a high interfacial area morphology that allows for efficient exciton separation and transport of free carriers to extraction electrodes. Electron and hole conduction in networks such as these is dominated by bimolecular recombination—an important limiting factor in synthetic light harvesting devices such as solar cells and organic photodiodes. The opportunities for naturally inspired materials and device architectures are simply immense. They include applications such as energy, sensing, memory, circuit elements, detectors, displays, interactive medicine and integrated health diagnostics. The fields of bioelectronics and bionanomedicine promise a new era in disease detection and treatment—in which bridging the ‘biotic–abiotic’ gap is a central endeavour [2–5]. Signals in biology are often carried via ionic currents, and interfacing a biological entity with conventional electronics requires high fidelity ion-to-electron or electronto-ion transduction. In this regard, ‘ionics’ (or ‘protonics’) is a new concept and involves the creation of electrical circuit elements driven by ions or protons, not electrons and holes. An all-solid-state proton transistor has recently been reported— a major advance, and precursor to complementary elements and ionic logic [6]. Any direct interface with a biological system must be compatible with its host. Synthetic conducting polymers such as PEDOT : PSS appear to be bio-compatible and possess hybrid electronic–ionic electrical properties. Electrodes made from PEDOT : PSS have been successfully used to stimulate neurons and sophisticated ion bipolar junction transistors can deliver analytes whilst maintaining long term neuronal function [5]. The natural hybrid conductor melanin can be processed to create device quality thin films and incorporated into prototype solid state electrochemically gated transistors [8]. Lipid bilayers with integrated ion channels can gate conventional silicon transistors delivering a platform for manipulating and combining ion and electron currents [95]. These sorts of materials and approaches may be the answer to ‘seamless integration of natural and manmade structures’ [86] when combined with lithographic patterning and the exploitation of nanoscale physics. Furthermore, optical interfacing to stimulate or read a biological signal is making rapid progress. Light has the intrinsic advantage 29

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dipoles (c.f. the narrow optical gap acceptor–donor polymers). Koster et al [114] recently postulated that ‘high dielectric constant’ organic materials could lift efficiency restrictions and reduce voltage losses in organic solar cells. The challenge is to create such materials whilst maintaining all the benefits and versatility of organic chromophores. The question then arises as to whether such systems remain excitonic? Historically, we have largely considered light harvesting and carrier transport processes in natural systems as ‘classical’. Recent experimental observations of room-temperature quantum coherent (wavelike) excitation energy transfer in natural photosynthetic systems have sparked immense interest and debate [62, 63]. At room temperature, interactions with the thermal bath of the molecular environment cause rapid decoherence. However, it has been proposed that noiseassisted dephasing can actually speed up transport to an electronic sink (reaction centre or electron acceptor site) by collapsing suppressive pathways [213, 214]. Furthermore, recent theoretical results by Ringsmuth et al suggest that coherent transport up to the largest length scale accessible within the exciton lifetime may be possible in a system possessing a hierarchically clustered structure and small static disorder, as found in natural light harvesting systems [215]. If such a biomimetic system could be engineered it may overcome one of the greatest bottlenecks in mass production of organic solar cells—creation of a nanoscale morphology to accommodate the short exciton diffusion length in organic semiconductors. This represents a truly ‘over the horizon’ strategy. Low carrier mobilities and densities in organic conducting systems have to date inhibited applications requiring high speed responses or large frequency bandwidths. Highly purified organic small molecule semiconductors are capable of field effect mobilities in excess of 10 cm2 V−1 s−1 , but are not particularly practical for large scale integration and rapid, low cost processing. Recently, n and p-type polymers have been reported with mobilities in the same range, and this represents a major step forward [216]. However, there is still some way to go to match traditional inorganic semiconductors, and the stateof-the-art in (for example) organic photodiodes falls somewhat short of the speed, bandwidth and sensitivity requirements of all but the most basic applications [110]. Returning to bioelectronics and bionanomedicine, clearly the challenges of interfacing electronics with a biological system are immense. The complexity of even the simplest cellular component is staggering. Where does one begin? The signal transduction issue is a ‘must solve’ problem—high fidelity, low signal-to-noise connection with minimal local interference and negligible power consumption would be a basic statement of the electrical requirements. Layer upon this the mechanical device requirements (lightweight, flexible, robust) and chemical prerequisites (bio-compatibility, stability or controlled breakdown and absorption) and one emerges with a truly grand challenge. Furthermore, biological systems contain a mix of liquid and solid components. Hence, we must be able to address both, translate liquid-solid interfaces into solid–solid interfaces, and find ways to perform liquid processing and flow control at the nanoscale.

All of these opportunities and challenges require a broad portfolio of disciplines to be brought to bear. Condensed matter, optical, molecular and quantum physics in particular are central elements of this portfolio approach since they not only deliver the necessary underlying material structure– property relationships but also provide the technological base for device architectures and signal probes. Modern physics has much to contribute to this emergent, frontier subject.

Acknowledgments PM is a Vice Chancellors Senior Research Fellow at the University of Queensland and previously a Queensland Smart State Senior Fellow. Research detailed in this paper was part-funded by the Australian Research Council (Discovery Programme) and Queensland Smart State Programme. PM acknowledges valuable contributions to the melanin research programme by Professor Paul Burn, Associate Professor Ben Powell, Professor Graeme Hanson and Professor Ian Gentle at the University of Queensland and Professor Tadeusz Sarna at Jagiellonian University, Krakow, Poland. CJB would like to acknowledge funding provided by the following organizations: the Berkman Foundation; the American Chemical Society Petroleum Research Fund (ACS PRF #51980-DNI7); the Proctor & Gamble Education Grant Programme; and the Carnegie Mellon University School of Engineering. MIV would like to acknowledge funding support from the grant P20772-N20 as well as financial support from the city of Linz and Land Oberoesterreich, Austria. PS and BM received PhD Scholarships from the Australian Government through the Australian Postgraduate Award (APA) scheme. The Centre for Organic Photonics and Electronics is a strategic initiative funded by the University of Queensland. Finally, we wish to acknowledge the contribution of many colleagues in generously assisting with reproduced figures.

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