Perovskite Solar Cells: Influence of Hole ... - Wiley Online Library

DOI: 10.1002/cssc.201501228

Reviews

Perovskite Solar Cells: Influence of Hole Transporting Materials on Power Conversion Efficiency Sadia Ameen,[a] Malik Abdul Rub,[b] Samia A. Kosa,[b] Khalid A. Alamry,[b] M. Shaheer Akhtar,[c] Hyung-Shik Shin,[a] Hyung-Kee Seo,[a] Abdullah M. Asiri,[b] and Mohammad Khaja Nazeeruddin*[d]

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Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Reviews The recent advances in perovskite solar cells (PSCs) created a tsunami effect in the photovoltaic community. PSCs are newfangled high-performance photovoltaic devices with low cost that are solution processable for large-scale energy production. The power conversion efficiency (PCE) of such devices experienced an unprecedented increase from 3.8 % to a certified value exceeding 20 %, demonstrating exceptional properties of perovskites as solar cell materials. A key advancement in perovskite solar cells, compared with dye-sensitized solar cells, occurred with the replacement of liquid electrolytes with solidstate hole-transporting materials (HTMs) such as 2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene (SpiroOMeTAD), which contributed to enhanced PCE values and im-

proved the cell stability. Following improvements in the perovskite crystallinity to produce a smooth, uniform morphology, the selective and efficient extraction of positive and negative charges in the device dictated the PCE of PSCs. In this Review, we focus mainly on the HTMs responsible for hole transport and extraction in PSCs, which is one of the essential components for efficient devices. Here, we describe the current stateof-the-art in molecular engineering of hole-transporting materials that are used in PSCs and highlight the requisites for market-viability of this technology. Finally, we include an outlook on molecular engineering of new functional HTMs for high efficiency PSCs.

1. Introduction

was sandwiched between two conducting electrodes (2) a mesoporous semiconductor to accept electrons from the photoexcited perovskite, and (3) an organic hole-transporting semiconductor. Improved PCEs using this basic design were achieved by Snaith et al.[5] and Grtzel et al.[6] Nowadays, the typical device architectures of PSCs are analogous to solid-state DSSCs, which generally include a compact TiO2 blocking layer of 20–50 nm deposited on a transparent conductive oxide (TCO) substrate, such as fluorine-doped tin oxide (FTO), followed by 100–400 nm of mesoporous TiO2 (mpTiO2) as the electron-transporting material (ETM). An important alteration to the discussed architecture is the replacement of the mp-TiO2 layer by an insulating scaffold, such as Al2O3, as demonstrated by Snaith et al.[6] The perovskite film is then deposited by spin-coating a solution of the perovskite precursors on the mp-TiO2 layer using solvents, such as N,N-dimethyl formamide (DMF), g-butyrolactone (GBL), or dimethyl sulfoxide (DMSO), followed by the deposition of 150–200 nm of a holetransport material (HTM). Finally, a metal electrode, such as Au or Ag, is thermally evaporated on top of the HTM (Figure 1). On the other hand, the inverted structure is obtained by simply inverting the positions of the ETM and HTM.[7, 8] For example, poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) or NiO are commonly used HTMs, while phenyl-C61-butyric acid methyl ester (PCBM) is an often used ETM in inverted cells.[9] The thicknesses of the HTMs are typically in the range of 150–200 nm to minimize charge recombination and improve the direct contacts between the perovskites and metal electrodes. The variation in device architectures now occurs in the nature of the positive (p) and negative (n) contact materials, with the optional insertion of a thin interlayer of mesoporous scaffold to either aid film formation or charge extraction. The solar cells could thus be considered n-i-p and p-i-n structures, where the perovskite is an intrinsic semiconductor, and light enters through the n-type or the p-type layer. The engineering of new ETMs and HTMs is driven by the desire for improved performances of PSCs. The working principle of a perovskite solar cell involves (1) electron and hole generation upon irradiation, (2) transfer of the excited electrons into the conduction band of the semi-

Excessive greenhouse gas emission demands the use of carbon-free energy sources, such as solar energy. In this respect, photovoltaic cells, which can be used to convert solar energy into electrical energy, are promising renewable alternatives to fossil fuels. The current photovoltaic market is dominated by single crystalline silicon-based solar cells; however, novel low-cost approaches are progressing to compete/coexist with the silicon-based devices. In particular, perovskite solar cells (PSCs) using methylammonium lead halide perovskites (CH3NH3PbX3, X = Cl, Br, I) are promising and initially exhibited power conversion efficiencies (PCE) of 3.8–6.5 % in liquid-based dye-sensitized solar cells (DSSCs).[1–3] The PCE of the liquid-based CH3NH3PbI3 solar cell was improved by modifying the TiO2 surface and the processing method for the perovskite deposition, leading to a PCE of 6.5 %.[4] However, these devices were highly unstable and degraded rapidly as a result of the liquid electrolytes. This instability was eventually resolved by replacing the liquid electrolyte with solid-state hole conductors. The modified devices consisted of three main components: (1) the perovskite layer to absorb light, which [a] Dr. S. Ameen, Prof. H.-S. Shin, Prof. H.-K. Seo Energy Materials & Surface Science Laboratory Solar Energy Research Center School of Chemical Engineering Chonbuk National University Jeonju, 561-756 (Republic of Korea) [b] M. A. Rub, S. A. Kosa, Dr. K. A. Alamry, Prof. A. M. Asiri Center of Excellence for Advanced Materials Research (CEAMR) King Abdulaziz University Jeddah (Saudi Arabia) [c] Prof. M. S. Akhtar New & Renewable Energy Material Development Center (NewREC) Chonbuk National University Jeonbuk (Republic of Korea) [d] Dr. M. K. Nazeeruddin Group for Molecular Engineering of Functional Materials Institute of Chemical Science and Engineering Êcole Polytechnique f¦d¦rale de Lausanne Station 6 CH-1015 Lausanne (Switzerland) E-mail: [email protected]

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Reviews conductor, (3) transfer of the hole on the perovskite into the hole conductor, and (4) injection of the hole into the gold contact electrode. Noticeably, undesirable charge-transfer (CT) processes also occur resulting from recombination of photogenerated electrons at the TiO2/perovskite/HTM interfaces. In addition to extracting photogenerated positive charges from the perovskite and transporting these charges to the back contact metal electrode, the HTM helps to minimize recombination losses at the TiO2/perovskite/HTM interface and thus, leads to improved device performance. In less than three years, the overall PCE of PSCs have quickly jumped to a certified value of 20.1 %.[10] Significant research efforts were devoted to the developments of PSCs, including the device architectures,[11, 12] applications of various ETMs and nanostructures,[13, 14] chemical optimization of perovskite compositions,[15, 16] deposition techniques for high-quality perovskite films,[7, 17] and engineering of HTMs. In this work, we have reviewed the recent progresses in PSCs using different HTMs. Their structural and opto-electronic properties, synthetic developments and challenges, and applications in devices are addressed. At the end, an outlook on the development of more efficient HTMs for highly efficient PSCs is addressed toward further improvements of PSCs.

2. Role of HTMs in Perovskite Solar Cells The ideal positive charge extraction layer should have a HOMO energy level that is compatible with the valence band of the perovskite. In addition, good hole mobility, minimal absorption in the visible and near-IR region of the solar spectrum, as well as excellent thermal and photochemical stability are required.[18] The huge interest of PSCs does not only lie in the high efficiencies, but also in stability for commercial applications. So far, a large number of HTMs were developed and incorporated in PSCs, which are composed of organic and inorganic materials. Very recently, Delgado and co-workers reviewed the most promising organic materials, which are able to transport electrons or holes from a structural point of view. The organic materials are selected owing to their ease of prepMd. K. Nazeeruddin is a Professor at the Êcole polytechnique f¦d¦rale de Lausanne, head of the group for molecular engineering of functional materials. He has been appointed as World Class University (WCU) professor by the Korea University, Jochiwon, Korea, Adjunct Professor by the King Abdulaziz University, Jeddah, Saudi Arabia and Eminent Visiting Professor at University of Brunei, DARUSSALAM. He has published over 500 papers, 10 reviews/book chapters and is inventor or co-inventor of 55 patents with an h-index of 104. His current research is focused on the dyesensitized solar cells, perovskite solar cells, and organic light emitting diodes.

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Figure 1. (a) Structure of a typical perovskite, (b) schematic diagram of a PSC device, and (c) schematic diagram of energy levels and electron transfer processes in a PSC device.

aration and manipulation, and the ability to tune their properties through chemical synthesis.[19] In the context of organic materials, 2,2’,7,7’-tetrakis-(N,N-di4-methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) is widely used as an HTM, exhibiting impressive photovoltaic performance when used in PSC with a PCE of over 15 % by using a sequential deposition[20] or a dual source thermal evaporation of the perovskite layer.[21] Although, it was demonstrated that Spiro-OMeTAD is an excellent small molecule HTM, its high cost accompanied by multi-step synthesis and difficult purifica12


Reviews tion steps curtail its commercial viability.[22] In particular, SpiroOMeTAD performs efficiently when it is doped with a metal complex and/or additives;[23] however, the use of dopants/additives induces device instability, and the oxidized form of the Spiro-OMeTAD acts as a filter around 520 nm owing to increased absorption in this region.[24] The lack of reproducibility often observed in PSCs stems partly from the low intrinsic hole mobility and conductivity of Spiro-OMeTAD, which requires doping to improve its conductivity. Pristine Spiro-OMeTAD is essentially a poorly conducting insulator, causing devices to suffer from high series resistance (RS) owing to poor hole transport through the device. This leads to poor charge collection efficiencies as charge recombination dominates over charge transport.[25, 26] Unlike chemical oxidants, the common SpiroOMeTAD additive, lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI), does not directly oxidize Spiro-OMeTAD, but promotes the oxidative reaction between Spiro-OMeTAD and oxygen in the presence of either light or thermal excitation. Recently, McGehee and co-workers reported the hole conductivity of Spiro-OMeTAD without oxygen or lithium salts by using a dicationic salt of Spiro-OMeTAD, abbreviated as Spiro(TFSI)2, in PSC and DSSC.[27] Herein, Spiro(TFSI)2 is unique and no chemical additives, such as lithium salts, cobalt complexes, or ionic liquids are needed to oxidize Spiro-OMeTAD in the HTM since Spiro(TFSI)2 is in itself simply Spiro-OMeTAD that has been pre-oxidized. Thus, for commercial purposes, several researchers are working on replacing Spiro-OMeTAD with alternative low-cost and efficient HTMs that yield PSC devices with high efficiency and stability. Many approaches were taken towards Spiro-OMeTAD alternatives; As a result, small molecule-based HTMs, such as 3,4-ethylenedioxythiophene,[28] pyrene,[29] and other linear pconjugated structures,[30] are reported to provide high PCEs of 12–13 %. Generally, a thick layer of HTM is required to avoid short circuits; however, thick HTM layers can also lead to high RS. Thus, it is essential to find the optimum HTM thickness that has desirable conductivity and charge mobility to reduce RS and simultaneously create a pinhole-free layer. In the quest for promising HTMs, a large number of organic molecules,[29] inorganic materials,[31, 32] and conducting polymers[33, 34] were developed and tested in PSCs. The best performing polymer-based HTM is poly(tertiary arylamine) (PTAA), which yields a PCE of over 20 % when used in a PSC device.[35] Owing to the wide varieties of HTMs, it is important to identify the pertinent HTMs governing the performances of PSCs. A summary of different organic small molecule,[36–55] oligomeric,[56–57] spiro-based,[58–63] and polymeric[33, 34, 64–74] HTMs, along with inorganic[31, 32, 75–83] HTMs and their corresponding device properties in PSCs are discussed below and tabulated in Table 1.

ed cation over a planar amine, mimicking the structural framework of N,N,N’,N’-tetrakis(4-methoxyphenyl) benzidine (MeOTPD). The UV/Vis spectra of OMeTPA-FA and OMeTPA-TPA in chlorobenzene are displayed as an inset of Figure 2 b. The absorption spectrum of OMeTPA-TPA exhibits an intense peak at 374 nm whereas, the OMeTPA-FA shows an absorption band at 383 nm that is red-shifted relative to that of OMeTPA. This red-shift is attributed to a more planar configuration resulting from the small torsion angle of the biphenyl unit. Figure 2 b shows the absorption spectra of the various HTMs processed on the perovskite-coated TiO2 films, which absorb a broad range of light from visible to the near-infrared. The HTMcoated photoactive films exhibit two enhanced absorption bands at 395 and 490 nm owing to the superimposed absorption characteristics of their constituents and the energy level diagram of the respective components is shown in Figure 2 c. The device structure of the PSC is displayed in Figure 2 d and the formation of a well-defined hybrid structure with visible interfaces is revealed by the cross-sectional SEM image (Figure 2 e). The SEM confirms the layer thicknesses of the TiO2, perovskite, and HTM at approximately 300, 200, and 180 nm, respectively. The device comprising mp-TiO2/MAPbI3/Au without a HTM exhibited a short circuit photocurrent density (JSC) of 14.3 mA cm¢2, an open circuit voltage (VOC) of 0.78 V, and a fill factor (FF) of 0.61, affording an overall PCE of 6.9 %. These values are comparable to those reported by Etgar et al.[37] On the other hand, the devices in the presence of the HTMs exhibited remarkably improved photovoltaic performances. The device comprising mp-TiO2/MAPbI3/OMeTPA-FA/Au without any additives in the HTM showed JSC of 19.4 mA cm¢2, VOC of 0.91 V, and a FF of 0.63, corresponding to a PCE of 11.7 %. Burschka et al.[38] reported that the addition of a cobalt(III) complex along with the additives 4-tert-butylpiridine (tBP) and LiTFSI to Spiro-OMeTAD could enhance the conductivity of Spiro-OMeTAD and the photovoltaic performances. The device prepared with each of these three additives/dopants and the OMeTPA-FA HTM yielded an average PCE of 12.7 % and the highest PCE of 13.6 %. The best-performing device demonstrated JSC of 21.0 mA cm¢2, VOC of 0.97 V, and FF of 0.67, affording a PCE of 13.6 %. Under the same conditions, the OMeTPA-TPAand Spiro-OMeTAD-based cells gave a JSC of 20.9 and 21.6 mA cm¢2, VOC of 0.95 and 0.99 V, and a FF of 0.62 and 0.68, corresponding to PCEs of 12.3 and 14.7 %, respectively. Ko and co-workers[39] designed two additional HTMs, coded as Triazine-Th-OMeTPA and Triazine-Ph-OMeTPA with donor–p– acceptor (D–p–A) systems by incorporating an electron-deficient 1,3,5-triazine core and an electron-rich diphenylamino unit, as shown in Figure 3 a. The selection of 1,3,5-triazine unit was based on the fact that the radical anion formed during irradiation was stabilized owing to the electron deficiency of the core.[40, 41] Another strategy for introducing the triarylamine derivatives was that the star-shaped organic materials are widely and successfully used as HTMs in optoelectronic devices.[42–44] The HOMO levels of Triazine-Th-OMeTPA and Triazine-PhOMeTPA were measured as ¢5.04 and ¢5.11 eV, respectively, which are well above the perovskite CB level (CH3NH3PbI3,

2.1. Organic small molecules Ko and co-workers[36] developed new types of HTMs containing a planar amine or star-shape triphenylamine derivative, coded as OMeTPA-FA and OMeTPA-TPA (Figure 2 a). The design of such molecules was based on the criteria of increasing the lifetime of the charge-separated state by delocalizing the generatChemSusChem 2016, 9, 10 – 27

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Reviews Table 1. Summary of the highest-performing HTMs. Device[a]

Perovskite

Dopants

JSC [mA cm¢2]

VOC [V]

FF

PCE [%]

Refs.

Organic small molecules Py-A ¢5.41/¢2.78 Py-B ¢5.25/¢2.82 Triazine-Th-OMeTPA ¢5.04/¢2.53 Triazine-Ph-MeTPA ¢5.11/¢2.45 OMeTPA-FA ¢5.14/¢2.21 OMeTPA-TPA ¢5.13/¢2.19 FA-MeOPh ¢5.15/¢2.45 TPA-MeOPh ¢5.29/¢2.59 MeO-DATPA ¢5.02/¢2.29 Me2N-DATPA ¢4.40/¢1.79 X19 ¢5.00/¢2.34 X51 ¢5.23/¢2.29 M1 ¢5.29/¢3.45

M M M M M M M M M (Al2O3) M (Al2O3) M M P

MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3¢xClx MAPbI3¢xClx MAPbI3

LiTFSI, tBP, FK209 LiTFSI, tBP, FK209 – – LiTFSI, tBP, FK209 LiTFSI, tBP LiTFSI, tBP, FK209 LiTFSI, tBP H-TFSI, Et4N-TFSI[b] H-TFSI, Et4N-TFSI[b] LiTFSI, tBP LiTFSI, tBP –

10.8 20.40 20.7 19.1 21.0 20.9 18.4 17.3 16.4 18.8 17.1 16.8 19.1

0.89 0.95 0.92 0.93 0.97 0.95 0.92 0.99 0.96 0.87 0.76 0.88 1.02

0.35 0.64 0.66 0.61 0.67 0.62 0.70 0.63 0.56 0.50 0.58 0.66 0.68

3.3 12.3 12.5 10.9 13.6 12.3 11.9 10.8 8.8 8.0 7.6 9.8 13.2

[29] [29] [39] [39] [36] [36] [45] [45] [84] [84] [85] [85] [86]

Organic spiro-based molecules Spiro-OMeTAD ¢5.22/¢2.28 pm-Spiro-OMeTAD ¢5.22/¢2.28 po-Spiro-OMeTAD ¢5.31/¢2.31 KTM3 ¢5.29/¢2.42

M M M M

MAPbI3 MAPbI3 MAPbI3 MAPbI3

LiTFSI, LiTFSI, LiTFSI, LiTFSI,

20.0 21.1 21.2 13.0

0.99 1.01 1.02 1.08

0.73 0.65 0.78 0.78

15.0 13.9 16.7 11.0

[20] [59] [59] [61]

Organic polymers P3HT P3HT PCBTDPP PCBTDPP PDPPDBTE PCPDTBT PCDTBT PTAA PTAA PFB PANI

¢5.20/– ¢5.20/– ¢5.40/– ¢5.40/– ¢5.40/– ¢5.30/– ¢5.45/– ¢5.20/– – ¢5.10/– ¢5.27/–

P P M M M M M M M M M

MAPbI3¢xClx MAPbI3¢xClx MAPbBr3 MAPbI3 MAPbI3 MAPbI3 MAPbI3 MAPbI3¢xBrx (FAPbI3)1¢x(MAPbBr3)x MAPbI3 MAPbI3

– LiTFSI, – – LiTFSI, LiTFSI, LiTFSI, LiTFSI, LiTFSI, LiTFSI, LiTFSI,

20.8 19.1 4.5 13.9 14.4 10.3 10.5 19.5 22.5 13.8 18.0

0.92 0.98 1.16 0.83 0.86 0.77 0.92 1.09 1.11 0.91 0.88

0.54 0.66 0.59 0.48 0.75 0.67 0.44 0.76 0.73 0.64 0.40

10.4 12.4 3.0 5.6 9.2 5.3 4.2 16.2 18.4 8.0 6.3

[87] [88] [64] [64] [65] [33] [33] [7] [35] [73] [74]

Inorganic materials CuPc CuI CuSCN

¢5.20/(N)[c] ¢5.20/(N)[c] ¢5.30/(N)[c]

M M M

MAPbI3¢xClx MAPbI3 MAPbI3

– – –

16.3 17.8 19.7

0.75 0.55 1.02

0.40 0.62 0.62

5.0 6.0 12.4

[89] [31] [32]

HTM

HOMO/LUMO [eV]

tBP, FK209 tBP tBP tBP, FK269

tBP

tBP tBP tBP tBP tBP tBP tBP

[a] M and P represent mesoscopic and planar configuration, respectively. [b] H-TFSI = bis(trifluoromethanesulfonyl)imide; Et4N-TFSI = tetraethyl bis-(trifluoromethane)sulfonamide. [c] Not mentioned.

¢5.43 eV) and the cross-sectional SEM image confirmed the formation of a well-defined hybrid structure with clear interfaces. Figure 3 b and c shows photocurrent density–voltage (J–V) curves and the corresponding incident photon to current efficiency (IPCE) spectra of three devices (PSC devices using Triazine-Th-OMeTPA, Triazine-Ph-OMeTPA, or Spiro-OMeTAD as the HTM). An impressive enhancement in PCE was achieved by increasing the conductivity through doping with additives, such as tBP, LiTFSI, and tris(1-(pyridin-2-yl)-1H-pyrazol)cobalt(III) tris(hexafluorophosphate) (FK102), into Triazine-Th-OMeTPA or Triazine-Ph-OMeTPA. The Triazine-Ph-OMeTPA based device exhibits JSC of 19.1 mA cm¢2, VOC of 0.93 V, and FF 0.61, affording a PCE of 10.9 %. Under similar conditions, Triazine-Th-OMeTPAand Spiro-OMeTAD-based cells show JSC of 20.7 and 21.4 mA cm¢2, VOC of 0.92 and 0.94 V, and FF of 0.66 and 0.67, corresponding to PCEs of 12.5 and 13.5 %, respectively. The high photocurrent density of the Triazine-Th-OMeTPA based ChemSusChem 2016, 9, 10 – 27

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cell relative to Triazine-Ph-OMeTPA based is responsible for the broad and red-shifted absorption of the mp-TiO2/CH3NH3PbI3/ Triazine-Th-OMeTPA device. In addition, the high FF of the Triazine-Th-OMeTPA based cell is interpreted in terms of its low RS and high mobility. Furthermore, the photocurrent action spectra of the three devices exhibits the photocurrent density of Triazine-Th-OMeTPA and Triazine-Ph-OMeTPA as 20.1 and 18.6 mA cm¢2, respectively, which is in good agreement with the measured photocurrent density of 20.7 and 19.1 mA cm¢2. The other star-shaped HTMs shown in Figure 4 a, coded as TPA-MeOPh and FA-MeOPh, were also developed and used in PSCs.[45] The UV/Vis spectra of TPA-MeOPh and FA-MeOPh measured in chlorobenzene , shown as an inset in Figure 4 b, exhibits an intense peak at 396 nm. The absorption peak maximum (lmax) of FA-MeOPh (414 nm) is red-shifted by about 18 nm compared with TPA-MeOPh, which is attributed to the more planar configuration in FA-MeOPh, as compared with a larger twist angle (48.98) between the core nitrogen and 14


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Figure 2. (a) Chemical structures of OMeTPA-FA and OMeTPA-TPA, (b) UV/Vis absorption spectra of HTMs coated on mp-TiO2 and mp-TiO2/CH3NH3PbI3 films. Insert: absorption spectra of the HTMs in chlorobenzene. (c) Energy level diagram of the HTMs. (d) Depiction of the whole device. (e) SEM image of the cross section of the mp-TiO2/CH3NH3PbI3/HTMs/Au. Reproduced from Ref. [36] with permission of Wiley.

phenyl unit in TPA-MeOPh. The fluorescence spectrum of FAMeOPh exhibits a maximum emission at 488 nm with a small Stokes shift of 74 nm relative to 86 nm for TPA-MeOPh, demonstrating that a small structure change in the excited state occurs in FA-MeOPh and TPA-MeOPh owing to their rigid configuration. Figure 4 c shows the energy level diagram of the corresponding components in the device. The HOMO levels of TPA-MeOPh and FA-MeOPh are measured at ¢5.29 and ¢5.15 eV, respectively, displaying suitable energy level with the CH3NH3PbI3 (¢5.43 eV). The device structure of the PSCs where FTO is deposited with a thin compact layer, followed by deposition of 260 nm thick mp-TiO2 is displayed as Figure 4 d; the cross-sectional SEM image (Figure 4 e) shows the formation of a well-defined hybrid structure with clear interfaces. The TPAMeOPh-based cell showed JSC of 17.3 mA cm¢2, VOC of 0.99 V, ChemSusChem 2016, 9, 10 – 27

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and a FF of 0.63, leading to a PCE of 10.8 %. Under the similar conditions, the FA-MeOPh and Spiro-OMeTAD-based cells presented JSC of 18.4 and 19.9 mA cm¢2, VOC of 0.92 and 0.89 V, and a FF of 0.70 and 0.72, affording a PCE of 11.9 and 12.8 %, respectively. As VOC is determined by the difference between the quasi-Fermi levels of the TiO2 and the HOMO of HTM, the higher VOC in the TPA-MeOPh-based cell was in good agreement with the relative difference in the HOMO levels of two HTMs,[46] whereas the high JSC of the FA-MeOPh-based cell relative to TPA-MeOPh was attributed to the enhanced and redshifted absorption of the mp-TiO2/CH3NH3PbI3/FA-MeOPhbased cell. Nazeeruddin and co-workers[47] reported the photovoltaic application of a new low band gap (Eg) small molecule HTM based on quinolizino acridine. The functionalized, colored qui15


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Figure 3. (a) Chemical structures of Triazine-Th-OMeTPA and Triazine-Ph-OMeTPA. (b) J–V characteristics of the solar cells with Triazine-Th-OMeTPA, TriazinePh-OMeTPA, and Spiro-OMeTAD as the HTM and the (c) corresponding IPCE spectra. Reproduced from Ref. [39] with permission of the Royal Society of Chemistry.

nolizino acridine-based compound worked as an effective HTM in PSCs; the molecular structure of the HTM (Fused-F) is shown in Figure 5 a. The triarylamine-based donor was flattened and functionalized with dihexyl-dithienosilole/benzothiadiazole/terthiophene arms to obtain a low Eg, highly absorbing starshaped HTM. The flattened star-shaped organic HTM was expected to enhance the intermolecular p–p stacking interactions, resulting in efficient hole transport and increasing the lifetime of the charge-separated state. The UV/Vis absorption and emission spectra of Fused-F, as shown in Figure 5 b, shows intensive CT absorption bands in the visible region with peaks at 421 nm (molar extinction coefficient, e = 108 000 L mol¢1 cm¢1) and 580 nm (e = 135 000 L mol¢1 cm¢1). The fluorescence spectrum shows a maximum emission at 746 nm, with a large Stokes shift of more than 150 nm. To further investigate the contribution of light absorption from Fused-F, the UV/Vis spectra of CH3NH3PbI3 films with and without Fused-F were recorded (Figure 5 c); a significant enhancement in absorption is observed in the whole visible region for the perovskite film, which confirms the possibility of its additional contribution for light harvesting. The best device with Fused-F displayed JSC of 17.9 mA cm¢2, VOC of 1.04 V, and FF of 0.68, leading to a PCE of 12.8 %. Under similar conditions, the device without a HTM exhibited relatively low JSC and VOC, yielding a PCE of 7.7 %. Arguably, the high JSC obtained from the device with Fused-F was a result of effective charge extraction as well as the improved light harvesting. ChemSusChem 2016, 9, 10 – 27

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It is interesting to note that the studies performed with FusedF were without any additives/dopants. To further enhance the stability of the HTMs, symmetrically substituted zinc(II)octa(2,6-diphenylphenoxy) phthalocyanine, namely TT80 (as shown in Figure 6 a), was explored in PSC.[48] The UV/Vis spectrum of TT80 in CHCl3 solution exhibited an intense Q band at 696 nm. For the photovoltaic studies, when TT80 was employed without any additive, the measured efficiency remains low (PCE = 2.57 %). However, with the additives LiTFSI and tBP, a considerable improvement of all the photovoltaic parameters was observed, as shown in Figure 6 b. The champion cell displays a remarkable enhancement, yielding a JSC of 16.4 mA cm¢2, VOC of ~ 0.80 V, a FF of 0.50, and an overall PCE of 6.7 %. Recently, Ahmad and co-workers[49] reported the synthesis of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) as a HTM in PSC. The chemical structure of TIPS-pentacene and the corresponding energy level diagram are depicted in Figure 7 a and b. The two TIPS side groups at the C-6 and C-13 positions of the pentacene core stabilized the HOMO level, facilitated stability and solubility in various organic solvents, and resulted in a 2 D lamellar structure with a brick wall style in the solid state.[50–53] Further, the co-facial p–p stacking of TIPS-pentacene allowed for improved charge-transport properties over unsubstituted pentacene.[54] Substitution of the solubilizing TIPS group facilitated fine-tuning of electronic properties, making it suitable for use as a HTM with perovskite. The devi16


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Figure 4. (a) Chemical structures of TPA-MeOPh and FA-MeOPh. (b) UV/Vis absorption spectra of HTMs coated on mp-TiO2 and mp-TiO2/CH3NH3PbI3 films. Inset shows absorption and emission spectra of the HTMs in chlorobenzene. (c) Energy level diagram of each component. (d) Depiction of the whole device. (e) SEM image of the cross section of the mp-TiO2/CH3NH3PbI3/HTMs/Au. Reproduced from Ref. [45] with permission of the Royal Society of Chemistry.

ces fabricated with TIPS-pentacene (Figure 7 c and d) in pristine form provided a competitive PCE (11.5 %), whereas the device using TIPS-pentacene doped with LiTFSI and tBP as additives obtained an average PCE of 8.2 %. Recently, a new HTM (V886) was reported, based on methoxydiphenylamine-substituted carbazole, with performance very similar to that of Spiro-OMeTAD.[55] The high solubility of V886 in organic solvents (> 1000 mg mL¢1 in chlorobenzene) and simple two-step synthesis made this HTM very appealing for the commercial prospects of PSCs. A cross-section SEM of the best V886 device is shown in Figure 8 a. In its pristine form, V886 absorbs light only in the UV region below ChemSusChem 2016, 9, 10 – 27

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450 nm (Figure 8 b), whereas chemically oxidized V886 exhibits visible absorption bands at 628 and 814 nm, showing the formation of the oxidized species. The performance of V886 was tested in CH3NH3PbI3-based solar cells using a mp-TiO2 photoanode and an Au cathode (Figure 8 c). The perovskite device with V886 shows a maximum PCE of 16.9 % under AM 1.5 G illumination, whereas PCE values exceeding 14 % are routinely observed. The measured JSC is 21.4 mA cm¢2, VOC is 1.09 V, and FF is 0.73. The best device from the same batch of solar cells prepared following the same device fabrication procedure, but using Spiro-OMeTAD as hole-extracting layer, displayed a PCE of 18.4 %. 17


Reviews 2.2. Organic oligomers To fabricate stable, cost effective, high-efficiency solid-state PSCs, a new approach was adopted by using oligomers as HTMs.[56] For example, the novel 2,4-dimethoxyphenyl-substituted triarylamine oligomer (S197) was used as the HTM in CH3NH3PbI3-based devices (Figure 9 a); the corresponding energy level diagram of the TiO2/CH3NH3PbI3/S197/Au heterojunction solar cell is depicted as Figure 9 b. The energy levels of TiO2, CH3NH3PbI3, and S197 are well aligned, which could allow for efficient charge extraction. The HOMO energy level of S197 is estimated at 5.12 eV, which is higher than that of CH3NH3PbI3, allowing for efficient hole transfer from CH3NH3PbI3 to S197, and subsequently to the Au counter electrode. UV/Vis spectra were recorded for the TiO2/PbI2 films before and after converting to perovskite. The PbI2 loading was found to decrease systematically with increasing spinspeed; thus, it was determined that the PbI2 was not completely converted to CH3NH3PbI3 under the given conditions if the layer was too dense/thick. The formation of a perovskite overlayer with homogeneous thickness throughout the dimension of the substrate is shown in (Figure 9 c). The J–V characteristics of the champion devices based on the structure: FTO/ compact TiO2/mp-TiO2/CH3NH3PbI3/S197/Au, as shown in Figure 9 d, displays the best photovoltaic performance with JSC of 17.6 mA cm¢2, VOC of 0.97 V, FF of 0.70, and an overall PCE of 12.0 %, using 6500 rpm spin-speed for PbI2 deposition. The PbI2 deposition at 5500 rpm yields the formation of the most uniform CH3NH3PbI3 overlayer on the TiO2 surface; however, the obtained JSC is lower than that of a device obtained at 6500 rpm. Such a decrease in the JSC is attributed to the presence of unconverted PbI2 as established from absorption studies. Grtzel and co-workers[57] reported acceptor–donor–acceptor (A–D–A)-type HTMs incorporating a rigid S,N-heteropentacene central unit, shown in Figure 10 a, as HTMs for efficient PSCs. These synthesized HTMs are good hole transporters, which simultaneously absorb light in the visible and near-infrared regions. The HOMO and LUMO energy levels of the two HTMs 1 and 2 (Figure 10 b) are determined from the onset of the first oxidation and reduction waves. In comparison with 2, the decrease in HOMO energy level of 1 is anticipated to lead a higher VOC, because the VOC is dependent on the difference between the HOMO level of the HTM and the quasi-Fermi level of TiO2. The cross-sectional SEM, showed that the HTM penetrated into the remaining space of the pores in the TiO2/perovskite layer and additionally formed a thin capping layer. Finally, the devices were completed by the evaporation of a thin Au layer as the back contact. These soluble HTMs allowed finetuning of the frontier orbital energies by variation of the linkers between the central thiophene–pyrrole based S,N-heteropentacene and the terminal dicyanovinylene (DCV) acceptor groups, contributing to the effective charge transport and the photocurrent enhancement in the device. The UV/vis absorption spectra (Figure 10 c–d) of 1 and 2 in dichloromethane solution shows an intensive CT absorption band with a maximum at 655 nm and a high molar extinction coefficient

Figure 5. (a) Molecular structure of the HTM Fused-F. Optical characterization: (b) UV/Vis absorption and fluorescence spectra of Fused-F in chloroform solution; and (c) UV/Vis absorption spectra of Fused-F coated on mpTiO2 and TiO2/CH3NH3PbI3 films (the TiO2/CH3NH3PbI3 film without HTM is shown for comparison). Films used for the UV measurement are made under exactly the same conditions as the devices. Reproduced from Ref. [47] with permission of the American Chemical Society.

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Figure 6. (a) Molecular structure of TT80. (b) J–V curves under simulated full sun illumination (AM 1.5 G) for the three cells made with TT80 as HTM using different solvents and additives (PhCl = chlorobenzene; PhCH3 = toluene; tBP = 4-tert-butylpyridine). Reproduced from Ref. [48] with permission of the Royal Society of Chemistry.

(117 600 L mol¢1 cm¢1) whereas, the CT-band of the longer oligomer 2 is blue-shifted to 630 nm and less intense (86 300 L mol¢1 cm¢1) owing to the more flexible bithiophene units. The optical band gaps (Egopt) for 1 and 2 measured in the solid-state are about 0.16 and 0.18 eV lower than the Egopt obtained from solution spectra. In the cyclic voltammetry measurements, two 1 electron oxidation waves were observed for oligomers 1 and 2, which were assigned to the formation of stable radical cations and dications, typical for an oligothiophene backbone. The estimated HOMO energies of both oligomers 1 and 2 were suitable for their use as HTMs in PSCs containing CH3NH3PbI3 as a light harvester and mp-TiO2 as an electron transport layer. The J–V characteristics of solar cells based on the structure FTO/compact-TiO2/mp-TiO2/CH3NH3PbI3/oligomer 1 or 2/Au were measured and a reference cell without any HTM displayed a JSC of 14.0 mA cm¢2, VOC of 0.79 V, and a FF of 0.69, which resulted in a PCE of 7.6 %. The PCEs of the most efficient devices were 10.5 % for 1 and 9.5 % for 2 under standard global AM 1.5 G illumination.

fluoromethylsulfonyl)imide](FK209), which was believed to create additional charge carriers (holes), supporting an increased charge carrier density. The J–V curves of PSCs utilizing PST1 as the HTM exhibited JSC of 17.63 mA cm¢2, VOC of 1.02 V, and FF of 0.73, yielding an overall PCE of 13.4 %. Under similar conditions, the device with Spiro-OMeTAD as HTM produced a comparable PCE of 12.2 %. A higher VOC measured for the PST1-based device was responsible for the slightly higher PCE than that of the Spiro-OMeTAD-based device, which was directly attributed to the difference in their HOMO energy levels. Kim et al.[5] achieved a high PCE of 9.7 % in a solid-state mesoscopic heterojunction solar cell using CH3NH3PbI3 nanoparticles and Spiro-OMeTAD (Figure 12 a). The femtosecond laser studies and photo-induced absorption measurements showed that the charge separation occurred through hole injection from the excited CH3NH3PbI3 nanoparticles to SpiroOMeTAD, followed by electron transfer to the mesoscopic TiO2. The PSC device with Spiro-OMeTAD exhibited a large photocurrent (JSC) of 17.0 mA cm¢2, VOC of 0.89 V, FF of 0.62, and a PCE of 9.7 %. Subsequently, Seok and co-workers[59] synthesized the three Spiro-OMeTAD derivatives (HTM1, HTM2, and HTM3, shown as Figure 12 b by considering the effect of the position of the ¢ OMe group in the aromatic ring.[60] The absorption spectra of these HTMs were recorded in chlorobenzene, with absorption maxima centered at 387, 387, and 381 nm, respectively. The Egopt of the material helped in identifying the effect of the substitution position. The absorption onset wavelengths of HTM1, HTM2, and HTM3 moved to shorter wavelengths (423, 413, and 409 nm, respectively), which implied that these HTMs had higher Egopt compared with the state-of-art Spiro-OMeTAD. The solar cell using the ortho-substituted derivative of SpiroOMeTAD (HTM3) showed better performance compared to the para- and meta-substituted derivatives (HTM1 and HTM2, respectively), each with a PCE of 16.7 %.[59] Mhaisalkar and co-workers[61] synthesized the swivel-cruciform 3,3’-bithiophene-based KTM3 (Figure 12 c), which showed optimum HOMO level and low recombination rates when used

2.3. Organic spiro-based molecules In the quest of new HTMs, Nazeeruddin and co-workers[58] reported a new class of spiro-type HTMs. For example, PST1 (Figure 11 a) is utilized for the fabrication of PSC. The champion PSC device reached 13.4 % PCE in the presence of a cobalt(III) dopant; however, the dopant-free devices achieved a very similar PCE of 12.7 %, which was commensurate with the performance of Spiro-OMeTAD in the presence of dopant. XRD analysis confirmed the presence of a tetrahedral sp3-hybridized carbon in between the two propylenedioxy thiophenes that adopted a spiro structure conformation. The UV/Vis absorption spectrum of PST1 (Figure 11 b and c) in dichloromethane depicts two absorption bands in the UV region at 300 and 396 nm, which are red shifted by 25 nm compared with the absorption band edge of Spiro-OMeTAD. The formation of a cation radical revealed the effective doping of PST1 with tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III) tris[bis(triChemSusChem 2016, 9, 10 – 27

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Figure 7. (a) Chemical structure of TIPS-pentacene and (b) the corresponding energy level diagram of the PSC device. (c) J–V characteristics of the CH3NH3PbI3 perovskite-based devices with pristine TIPS-pentacene (red), TIPS-pentacene with additives (blue), and Spiro-OMeTAD (black) as the HTM measured in the dark and under illumination at AM 1.5 G, 100 mW cm¢2 in a reverse bias scan. (d) J–V characteristics of the champion device with pristine TIPS-pentacene as HTM measured in a reverse bias scan. Reproduced from Ref. [49] with permission of the Royal Society of Chemistry.

Figure 8. (a) Cross-section SEM image of the best device containing a 100 nm thick layer of V886 as a hole conductor. (b) UV/Vis absorbance spectra of a solution of V886 in chlorobenzene (b) and V886 chemically oxidized by addition of 10 mol % FK209 (c). Upon chemical oxidation, peaks at 628 and 814 nm appear. (c) J–V characteristics of PSCs using SpiroOMeTAD (red) and V886 (black) as HTMs. Reproduced from Ref. [55] with permission of Wiley.

in PSCs. Swivel-cruciform thiophene-based moieties exhibited good solubility for further synthetic functionalization, allowing for tuning of the molecular properties by manipulating the pendant group.[62] The PSC devices with KTM3 showed a PCE of 7.3 %. Upon the addition of the cobalt(III) dopant FK102, a PCE of 8.7 % was achieved, which was further increased to 11.0 % by using bis[2,6-di(1H-pyrazol-1-yl)pyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide)] (FK269) that has a more stabilized HOMO level. The increase in the efficiency with FK269 was a result of improvement in the photovoltaic parameters like VOC and FF compared to the well-known SpiroChemSusChem 2016, 9, 10 – 27

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OMeTAD (JSC of 17.2 mA cm¢2, VOC of 1.06 V, FF of 0.63, and PCE of 11.4 %), which was confirmed by its lower HOMO level, transient photovoltage decay, and impedance spectroscopy. Very recently Palomares and co-workers[63] synthesized a new small molecule, that is, tetra{4-[N,N-(4,40-dimethoxydiphenylamino)]phenyl}ethene (TAE-1), as an efficient and robust HTM without the incorporation of additives/dopants; they applied this new HTM in CH3NH3PbI3 PSCs. The HOMO energy value was close to the value measured for the Spiro-OMeTAD. Although, the JSC was unexpectedly lower than that of the ref20


Reviews device stability. Never the less, TAE-1-based PSCs achieved PCEs well beyond 10 %. 2.3. Organic polymers In the context of HTMs, besides the state-of-the-art SpiroOMeTAD, the most efficient polymer showed maximum efficiencies of 20.1 %.[33, 34] In this case, the polymer-based HTMs displayed negligible absorption in the visible and near infrared regions. Low Eg donor–acceptor polymers with strong absorption in this spectral region were also tested as HTMs in PSCs and exhibited rather low efficiencies 4.2–9.2 %.[64, 65] Similar to colorless HTMs, the polymers only serve as hole transporter whereas the perovskite acts as the light harvester. Snaith and co-workers[66] replaced Spiro-OMeTAD with a much cheaper and highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) to achieve PCE of 14.5 %, with PEDOT casted from a toluene-based ink. To determine the suitability of PEDOT as an HTM processed on top of the perovskite layer in PSCs, the optical and photo-physical properties of Spiro-OMeTAD and PEDOT films isolated and in contact with perovskite films were investigated. The films coated with Spiro-OMeTAD and PEDOT showed similar levels of photoluminescence quenching with similar decay rates, which inferred that the hole transfer from the perovskite films to PEDOT was similar in efficiency to SpiroOMeTAD. The FF showed even more significant improvements with the PEDOT, leading to improved PCE values for the devices incorporating PEDOT (14.5 %) compared with those using Spiro-OMeTAD (12.4 %) as HTM. Pristine poly(3-hexylthiophene2,5-diyl) (P3HT, Figure 13 a) in conjunction with MAPbI3¢xClx in planar PSCs, exhibited overall PCE of ~ 10 % through the optimization of deposition parameters and precursor concentrations. Johansson and co-workers studied transient photovoltage decay measurements to compare the electron lifetimes of devices based on P3HT and Spiro-OMeTAD.[67] The recombination rate in devices with P3HT was found to be 10 times higher than Spiro-OMeTAD, which might arise from closer contact between the flat molecular structure of P3HT and the perovskite surface, providing a lower photovoltaic performance. Furthermore, a fullerene self-assembled monolayer (C60SAM)-functionalized mp-TiO2, a perovskite absorber (CH3NH3PbI3¢xClx), and P3HT were utilized for achieving a 6.7 % efficient hybrid solar cell, by Snaith and co-workers.[68] The presence of the C60SAM significantly enhanced the contribution to photocurrent by light absorbed in the P3HT polymer, but in this configuration the SAM also unexpectedly enhanced the VOC in comparison to the bare TiO2 PSC. The photoexcitations in both the perovskite and the polymer underwent very efficient electron transfer to the C60SAM. The C60SAM acted as an electron acceptor, but inhibited further electron transfer into the mp-TiO2 owing to energy level misalignment and poor electronic coupling. Thermalized electrons from the C60SAM were then transported through the perovskite phase. This strategy allowed a reduction of energy loss, while still employing a mesoporous electron acceptor, representing an exciting and versatile route forward for hybrid photovoltaics incorporating light-absorbing polymers.

Figure 9. (a) Chemical structure of oligomer S197. (b) Energy level diagram of the TiO2/CH3NH3PbI3/S197/Au heterojunction solar cell. (c) Cross-sectional SEM images of the device with 6500 rpm PbI2 deposition. (d) J–V characteristics of CH3NH3PbI3-based solar cells obtained using different spin-speeds for PbI2 deposition, measured in dark and under illumination. Reproduced from Ref. [56] with permission of Wiley.

erence solar cell using Spiro-OMeTAD, but the fact that the synthesis only required straightforward synthetic steps and obtained excellent final product yield (72 %) made TAE-1 a good candidate for top efficiency PSCs. Moreover, in contrast to other previously reported small organic molecules, the authors avoided the use of chemical oxidants that might lead to lower ChemSusChem 2016, 9, 10 – 27

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Figure 10. (a) Chemical structure of p-conjugated oligomers 1 and 2. (b) Energy level diagram of the components used in solar cells. (c) UV/Vis absorption spectra of 1 and 2 in dichloromethane solution and (d) 1 and 2 coated on mp-TiO2 and TiO2/perovskite films. A TiO2/perovskite film without HTMs is shown for comparison. Reproduced from Ref. [57] with permission of the Royal Society of Chemistry.

The C60SAM-functionalized mp-TiO2 device achieved PCE of 11.7 % using Spiro-OMeTAD including additives/dopants as the HTM. There was a notable increase in FF (0.65–0.72) with the addition of the C60SAM, a marginal increase in JSC (19.4– 19.6 mA cm¢2), and a slight increase in VOC (0.82–0.84 V). The PCE increased from 10.2 % to 11.7 % in the best devices. Additionally, the slight increase in VOC suggested that the addition of lithium additives to the HTMs enabled a certain degree of forward electron transfer to the TiO2,[69, 70] whereas the improved FF suggested reduced recombination under working conditions. Following their work of utilizing conducting polymers as HTMs for the fabrication of efficient PSCs, Snaith and co-workers[71] approached a way to mitigate thermal degradation by replacing the organic HTMs with polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix. With this composite structure, they achieved the PCEs of 15.3 % with an average efficiency of ~ 10 2 %. They have investigated the three commonly employed HTMs of Spiro-OMeTAD, P3HT, and PTAA for selective charge transport of photogenerated holes mediated by the functionalized SWNTs. Initially the devices were fabricated with an uncovered film of P3HT/SWNTs as the HTM. In the best-performing device, a remarkably high JSC of 20.8 mA cm¢2 indicated that efficient hole transfer could occur through the nanohybrids, unhindered by their relatively low contact area with the perovskite. The best-performing device delivered a scanned PCE of ChemSusChem 2016, 9, 10 – 27

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7.4 % and demonstrated that charge-selective transport was in fact supported by the polymer functionalized SWNTs. The performance however, limited by the low FF and low voltage that most likely occurred from recombination losses owing to direct contact of the metal electrode to the perovskite through the gaps between the P3HT/SWNT mesh. This also caused a very high failure rate of devices and thus, produced a poor average PCE with a very broad distribution (2.8 2.7 %). By depositing the poly(methyl methacrylate) (PMMA) matrix on top of the P3HT/SWNT layer, the gaps within the nanohybrid network were filled, preventing direct contact of the metal cathode with the perovskite absorber. The reduction of recombination losses and increase in shunt-resistance owing to the presence of the PMMA layer generated a significant increase in VOC and FF. The average PCE for 143 individual devices with this structure was 9.3 2.9 % with the best-performing device reaching a PCE of 14.2 %. Seok and co-workers[33] compared three thiophene-based polymeric HTMs, including P3HT (Figure 13 a), PCPDTBT (Figure 13 b), and PCDTBT (Figure 13 c), with poly(triarylamine) (PTAA, (Figure 13 d). The PTAA-based PSC presented the best performance with a maximum PCE up to ~ 12 %. The superior performance obtained with PTAA might a result of a stronger interaction between the perovskite with PTAA and higher hole mobility of the PTAA (~ 1 Õ 10¢2 to 1 Õ 10¢3 cm2 V¢1 s¢1) compared with the other polymers.

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Figure 11. (a) Chemical structure of PST1 and state-of-the-art HTM Spiro-OMeTAD. (b) Normalized absorption spectra of PST1, Spiro-OMeTAD in DCM, and PST1 in solid state. (c) Absorption spectra of PST1 in the presence of various doping ratio (5, 10, 15, 20, and 25 %) of FK209. Reproduced from Ref. [58] with permission of the Royal Society of Chemistry.

The thiophene-based conducting polymers were also explored as HTMs in PSCs by several researchers. Qiu and coworkers synthesized PCBTDPP (Figure 13 e) as a HTM in CH3NH3PbBr3- and CH3NH3PbI3-based solar cells.[64] The PSC fabricated with CH3NH3PbBr3 and PCBTDPP displayed a much higher VOC (1.16 V) than for P3HT (0.50 V), resulting from several factors, such as deeper HOMO level (¢5.4 eV), higher hole mobility (0.02 cm2 V¢1 s¢1) a large offset between the conduction band of CH3NH3PbBr3 and the quasi Fermi level of TiO2, as well as a possible interaction between the TiO2/perovskite/ PCBTDPP. The another HTM, PDPPDBTE (Figure 13 f), was also studied in mesoscopic CH3NH3PbI3 based PSCs.[65] The PSC based on PDPPDBTE displayed PCE of 9.2 %, higher than SpiroOMeTAD (7.6 %), owing to a deeper HOMO level (¢5.4 eV) and higher hole mobility of such polymer (10¢3 cm2 V¢1 s¢1), leading to lower RS, and yielding higher FF. Yan et al.[72] synthesized ultrathin polythiophene films and used them as HTMs to substitute conventional PEDOT:PSS in planar p-i-n CH3NH3PbI3 PSCs, affording a series of ITO/polythiophene/CH3NH3PbI3/C60/BCP/Ag (BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) devices. The ultrathin polythiophene film possessed good transmittance, high conductivity, smooth surface, high wettability, compatibility with PbI2–DMF solution for subsequent solution processing of the perovskite layer, and matched energy levels with CH3NH3PbI3 for efficient charge injection. A promising PCE of 15.4 % was achieved from

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this device architecture, featuring a high FF of 0.77, VOC of 0.99 V, and JSC of 20.3 mA cm¢2. Yang and co-workers[73] reported polyfluorene-based polymers that contain fluorine and arylamine groups, namely TFB (Figure 13 g) and PFB (Figure 13 h), as HTMs in mesoscopic PSCs using CH3NH3PbI3. TFB-based devices presented promising PCEs (10.9–12.8 %), which were comparable to the corresponding values for Spiro-OMeTAD (9.8–13.6 %). Finally, Ameen et al.[74] recently reported a distinguished morphology of polyaniline (Figure 13 i) nanoparticles (PANINPs) as an efficient HTM with CH3NH3PbI3 PSCs and obtained 6.29 % efficiency.

2.4. Inorganic materials Inorganic semiconductors appear to be good candidates as alternative HTMs in PSCs owing to their high hole mobility, ease of synthesis, and low production costs. So far, only a few studies using inorganic HTMs in PSCs have been reported owing to limited choices of suitable materials. In a quest to explore new inorganic HTMs for PSCs, Kamat and co-workers[31] reported the use of CuI and obtained a PCE of ~ 6.0 %. The VOC, compared with the best Spiro-OMeTAD devices, remained low, which was attributed to higher charge recombination as determined by impedance spectroscopy. However, the impedance spectroscopy also revealed that CuI exhibited two orders of 23


Reviews on devices with CuSCN, which was the highest value reported for inorganic HTMs. Inorganic NiO thin films were employed successfully as a HTM in DSSC and OPV devices[76–80] and were previously explored in inverse architecture PSCs with very low efficiency (0.05 %).[81] Recently, using spin-coated NiO as the HTM produced PSCs with PCEs of up to 7.8 %.[82] Sarkar and co-workers[83] fabricated the hybrid organic–inorganic semiconducting perovskite photovoltaic cells using electrodeposited NiO as hole conductor, and obtained PCE of 7.3 %. The inverse device architecture consisted of glass/FTO (bottom of image), NiO HTM with CH3NH3PbI3–xClx, and spin-coated PCBM as the electron transport layer.

3. Outlook PSCs have rejuvenated the photovoltaic community and several groups are focusing to reduce the costs of solar cells and improve the stability for their potential applications. The high efficiency and cost-effective materials of PSCs make them economically viable for commercialization. The PCE of PSCs experienced an unprecedented increase to the certified value exceeding 20 %; nevertheless, there are still many areas that require research and improve on such high photovoltaic performances. In this Review, we summarized significant progress on the developments of alternative HTMs in PSCs and identified that there remain some key issues that must still be addressed to further enhance the performances of PSC devices. One of the key aspects is the interaction between the perovskite and HTM during fabrication. The conductivity of thicker HTMs should be sufficient enough to minimize RS, otherwise it might result in lower FF. Thus, finding a HTM with a high hole mobility in its pristine form is desirable. Conducting polymers like PTAA exhibit high hole mobility and appear to be suitable candidates to achieve better FFs. Moreover, the design of low Eg HTMs could offer the possibilities of additional absorption of photons at the longer wavelength in the NIR region, which might result in further enhancement of the photocurrents of PSCs. Etgar et al.[37] reported that PbS quantum dots used as additional light harvesters together with CH3NH3PbI3 in which PbS quantum dots provide absorption in NIR region (up to ~ 1000 nm) result in surprisingly high JSC of up to 24.6 mA cm¢2. It is believed that designing low Eg HTMs that can perform this light harvesting function will be promising strategies to further enhance the JSC of PSCs without any significant losses of VOC. A better understanding of the photoelectrical behavior of perovskite semiconductors (exciton or free-carrier model) would also provide some deep insights into designing device architectures to further enhance the performances of PSCs. Furthermore, improving the quality of the contact layers by advanced deposition techniques (spray deposition, thermal evaporation) among the photoanode, perovskite, and hole transporters might also enhance FF. It is anticipated that a tandem solar cell combining the perovskite solar cell with a crystalline silicon cell/CIGS/CZTSSe could further improve the efficiency over 30 %.

Figure 12. Chemical structures of (a) Spiro-OMeTAD, (b) HTM1, HTM2, and HTM3, and (c) KTM3. Reproduced from Refs. [5, 59, 61] with permission of Nature Publishing Group and American Chemical Society.

magnitude higher electrical conductivity than Spiro-OMeTAD, which allowed significantly higher FF. Another Cu-based inorganic hole conductor CuSCN was also actively studied as an HTM in PSCs. Ito et al. reported the fabrication of planar heterojunction CH3NH3PbI3 PSCs using CuSCN as the HTM. In the PbI2 layer fabricated by the spin-coating method, small amounts of CH3NH3I and DMSO were incorporated as the first-drip precursor layer on a flat TiO2 layer. However, the MAI dripping method resulted in a significant photovoltaic effect for planar TiO2/CH3NH3PbI3/CuSCN solar cells with JSC of 14.5 mA cm¢2, VOC of 0.63, and FF of 0.53, yielding a PCE of 4.9 % reported.[75] Based on the same inorganic HTM, Qin et al.[32] used the well-established sequential deposition method to fabricate the perovskite layer. The loading of the mp-TiO2 with PbI2 was carried out twice to maximizing the light harvesting. The deposition of CuSCN using a doctor-blading technique efficiently blocked contacts between the perovskite and Au. A maximum PCE of 12.4 % was achieved based ChemSusChem 2016, 9, 10 – 27

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Figure 13. Chemical structures of (a) P3HT, (b) PCPDTBT, (c) PCDTBT, (d) PTAA, (e) PCBTDPP, (f) PDPPDBTE, (g) TFB, (h) PFB, and (i) PANI.

This Review surveys the numerous HTMs with deeper HOMO levels that aim to improve the VOC of the PSC device; however, performances are only marginally improved in most of the cases. The other factors, such as the recombination losses at the perovskite/HTM interface, must be taken into consideration to sufficiently translate the deeper HOMO levels into higher VOC, and accordingly higher overall PCEs. HTMs, such as P3HT, Spiro-OMeTAD, or PTAA, possess comparable HOMO energy levels, but so far, Spiro-OMeTAD has exclusively dominated the best-performing devices. Thus, a deep understanding for the chemical interactions between perovskites and the structures of HTMs is crucial for the rational design of new HTMs for achieving the higher performance PSCs in the future. The utilization of the conductive inorganic hole conductors, such as CuI and CuSCN usually offer other superior photovoltaic properties (PCE, JSC, VOC), but exhibit low FFs. In this regards, the suppression of the charge recombination losses between the perovskites and hole conductors might allow these materials to work more efficiently in PSC devices. One could be optimistically confident that there is still potential room for further enhancements of the overall PCEs in PSCs through rational design of new HTMs. The perovskite materials are sensitive towards humidity, stability under heat and light conditions, and reliability under operating conditions; thus, the solutions to these drawbacks are feasible by molecular engineering of functionalized charge-transporting materials that not only function to enable efficient charge extraction, but also as a shield for humidity and UV-light-induced degradation of the underlying perovskite layer. The prospects for perovskite materials in various applications are extremely bright with novel functionalized materials for markets spanChemSusChem 2016, 9, 10 – 27

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ning from solar cells to sensing, imaging, environment, and optoelectronics. We believe that this Review will stimulate research in developing functionalized novel HTMs for enhancing power conversion efficiency beyond 20 % and improve stability for wide spread applications of the perovskite solar cells.

Acknowledgements This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (79130-35-HiCi). The authors, therefore, acknowledge with thanks DSR for technical and financial support. M.K.N. acknowledges funding from the European Union Seventh Framework Programme [FP7/2007–2013] under grant agreement no. 604032 of the MESO project, (FP7/2007–2013) ENERGY.2012.10.2.1; NANOMATCELL, grant agreement no. 308997. Keywords: device structure · hole transport · molecular engineering · perovskite · solar cells [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050 – 6051. [2] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, 210th ECS Meeting, Cancun, Mexico, Oct. 2006, 29-Nov. 3. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, 214th ECS Meeting, Honolulu, Hawaii, Oct. 2008, vol 12 – 17. [4] J. H. Im, C. R. Lee, J. W. Lee, S. W. Park, N. G. Park, Nanoscale 2011, 3, 4088 – 4093. [5] H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. H. Baker, J. H. Yum, J. E. Moser, M. Grtzel, N. G. Park, Sci. Rep. 2012, 2, 591 – 596.

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Received: September 10, 2015 Published online on December 21, 2015

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