APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 9
1 MARCH 2004
Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenesÕ fullerene derivative Abay Gadisa Department of Physics (IFM), Linko¨ping University, S-581 83 Linko¨ping, Sweden
Mattias Svensson and Mats R. Andersson Department of Organic Chemistry and Polymer Technology, Chalmers University of Technology, S-41296 Go¨teborg, Sweden
Olle Ingana¨sa) Department of Physics, Linko¨ping University, S-581 83 Linko¨ping, Sweden
共Received 24 November 2003; accepted 5 January 2004兲 The photovoltaic parameters of donor/acceptor blend organic solar cells are highly influenced by several parameters, such as the strength of the acceptor species, the morphology of the film due to the solvent, and the mobility of the free charge carriers. In this work, the open-circuit voltage (V oc) of solar cells based on series of conjugated polythiophene polymers were measured and compared. In every cell, the donor polymer was blended with an electron acceptor fullerene molecule. The devices were constructed in a sandwich structure with indium tin oxide 共ITO兲/metallic polymer 共PEDOT:PSS兲 acting as an anode and Al or LiF/Al acting as a cathode. Comparing the V oc of all the cells shows that this important photovoltaic parameter is systematically varying with the polymer. The variation of photovoltage is attributed to the variation of the oxidation potential of the donor conjugated polymers after due consideration of the different injection conditions in the varying polymers. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1650878兴
Unlike conventional inorganic solar cells, light absorption in organic solar cells leads to the production of excited bound electron–hole pairs commonly known as excitons. To achieve a substantial photovoltaic effect, the excited charge pairs thus produced need to be dissociated into free charge carriers through the assistance of an electric field, bulk trap sites, or interface of materials with different electron affinities. Several approaches have already been adopted to achieve efficient exciton dissociation.1– 4 So far, the method of blending conjugated polymers with high electron affinity molecules like C60 has turned out to be the most efficient way for rapid exciton dissociation resulting in solar cells with high-power conversion efficiencies.4 The polymer/C60 interpenetrating networks give ultrafast 共less than 100 fs兲 electron transfer from the optically excited polymer to C60 , resulting in improvements of photocurrent.5 One of the basic photovoltaic parameters that influence the overall performance of organic solar cells is the opencircuit voltage (V oc). In thin-film solar cells of the classical geometry, metal–insulator–metal sandwich structure, V oc is, in principle, equal to the work function difference of the two metal electrodes. For blend organic solar cells, this simple physical principle seems to fail,6 and an alternative explanation for the origin of V oc needs to be given. Brabec et al. have argued that the V oc of polymer/fullerene-based solar cells is independent of device geometry and film thickness but strongly depends on the reduction potential of the fullerene.6 It has also been demonstrated that V oc for polymer/fullerene solar cells is affected by the morphology a兲
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of the active layer.7 In bilayer polymer/polymer solar cells, V oc is influenced by a dipole created by photoinduced charge transfer at the interface of the two polymers.8 Consequently, as the composite bulk heterojunction polymer/molecule film cannot be described with a simple metal–insulator–metal model, a complete explanation of the origin of the OC voltage of organic solar cells has not yet been reached. Although device morphology,7 and the reduction potential of the acceptor molecule6 have a definite effect on the V oc of blend solar cells, the effect of the intrinsic property of the donor polymer has not been fully investigated. The implicit expectation is that the V oc scales with the difference of the lowest unoccupied molecular orbital 共LUMO兲 of the acceptor, and the highest occupied molecular orbital 共HOMO兲 of the donor; these being the orbitals where photoinduced charge transfer deposits charges. The LUMO and HOMO can, in principle, be measured by the reduction and oxidation processes followed in electrochemical experiments. We have, in other studies, systematically compared the electrochemical processes in a range of polythiophene materials, an issue not without complications, but at the very least giving the basis for relative comparison of the oxidation potentials, directly related to the HOMO orbitals of the polymers.9 The polymers studied in this work span a considerable variation in electronic structure, as evident in optical absorption and photoluminescence, as well as in electrochemical properties. With decreasing bandgap, and extension of the optical absorption into lower energies, it is expected that a higher photocurrent can be generated from the solar spectrum. On the other hand, with decreasing bandgap, there is a systematic change of the HOMO and LUMO positions of the polymer.
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Appl. Phys. Lett., Vol. 84, No. 9, 1 March 2004
TABLE I. V oc for various cells based on polymer/PCBM blends sandwiched between ITO/PEDOT:PSS anode and Al or LiF/Al cathode. The oxidation potential of the polymers are listed for comparison.
Donor polymer name I II III IV V VI
FIG. 1. The chemical structure of PCBM and the conjugated polymers.
Typically, a lower bandgap is associated with a smaller HOMO, thus leading to a lower photovoltage. In this work, we report and discuss the correlation between OC voltage and oxidation potential of a series of substituted polythiophenes with optical bandgaps spanning from 2.05 to 2.98 eV. We have chosen to focus exclusively on the photovoltage, as this is a property not similarly affected by the limitations to transport due to varying morphology in the devices. All the studies are based on solar cells constructed from different polythiophene conjugated polymers blended with an electron acceptor molecule 关6,6兴-phenyl-C61-butyric acid methyl ester 共PCBM兲. The chemical structures of the polymers and the acceptor molecule PCBM are depicted in Fig. 1. The electrochemical properties of all the polymers were studied based on their cyclic voltammograms, the details of which are given elsewhere.9 The solar cells were constructed in the form of the traditional sandwich structure through several steps. The metallic polymer 关poly 共3,4-ethylene dioxythiophene兲-poly 共styrene sulphonate兲兴 共PEDOT:PSS; from Bayer AG兲 was spin coated from an aqueous solution on a pre-cleaned indium tin oxide 共ITO兲/glass substrate giving a thickness of about 100 nm as measured by DEKTAK 3030 surface profilometer. The blend polymer/PCBM 共1:4 wt.兲 solution in chloroform was spin coated on top of the PEDOT:PSS after the later was annealed for 5 min at 120 °C to give a film of thickness ranging from 130 to 150 nm. This was followed by the evaporation of aluminum 共60 nm兲, which serves as a cathode, on top of the polymer/PCBM layer. In some of the devices a thin layer of LiF 共about 0.7 nm兲 was evaporated prior to evaporation of Al metal. The thermal evaporation of Al and LiF was done under a shadow mask in a base pressure of about 10⫺7 mbar. The active area of the devices thus constructed varies from 4 to 6 mm2 . Device preparation and characterization was done under ambient conditions except the evaporation of Al and LiF. The I – V measurements were
Maximum value of V oc 共mV兲
Oxidation potential 共V兲
With out LiF
With LiF
0.525 0.770 0.785 0.795 0.815 0.945
388 685 829 664 699 655
449 781 714 740 701 805
made with a Keithley 2400 electrometer. An ozone-free xenon lamp coupled with A.M. 1.5 solar spectrum simulating filters was used to provide a simulated light of intensity 100 mW/cm2 . It is expected that the OC voltage of devices made of pure polymer in the form of ITO/PEDOT:PSS/polymer/Al can be described mathematically as eV oc⫽ PEDOT⫺ Al , 10 where e is an electronic charge, PEDOT is the anode work function, and Al is the cathode work function. In such devices, the dissociation of the photogenerated electron–hole pairs and the subsequent collection of free charges solely depend on the internal built in potential created by the work function difference of the two electrodes. This rule is not valid for solar cells constructed from polythiophene/fullerene bulk heterojunctions. Table I shows the maximum value of V oc of devices whose active layer is composed of polythiophene/PCBM blends. The polymer/PCBM stoichiometry is identical 共1:4 wt.兲 in all the devices studied. In addition, the active layer of every device is a smooth film indicating phase separation at most on the small dimensions. As can be seen from the table, the considerable variation of V oc with different polymers but identical electrode materials is inconsistent with the expectation of work-functioncontrolled V oc . This leads us to conclude that the V oc is highly dependent on the intrinsic property of the polymer type used in the individual cell. This is expected since the electron transfer from the excited donor 共polymer兲 to the acceptor molecule 共PCBM兲 is sensitive to the oxidation potential of the donor polymer, in addition to the influence due to the strength of the reduction potential of PCBM. Figure 2共a兲 shows the correlation between the V oc and oxidation potential of the conjugated polymers for devices with Al cathode. As can be seen from the figure, the values of V oc are a function of the oxidation potential of the polymer used in the cell throughout the 0.4 V range of variation. The monotonic correlation, which has a slope close to 1 but with much scatter, shows the polymer HOMO to be a determining property. In some of the cells a thin film of LiF 共about 0.7 nm兲 was introduced below the cathode electrode. It has been demonstrated that the presence of LiF below the Al electrode gives better performance11 as it protects the active organic/molecule blend layer from the incoming hot Al atoms during thermal evaporation. It was also suggested that lowering of the work function of Al electrode is possible up on placing LiF beneath it. The measured photovoltaic values for such cells also follow the same trend as in the cells with out LiF 关Fig. 2共b兲兴.
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Gadisa et al.
Appl. Phys. Lett., Vol. 84, No. 9, 1 March 2004
FIG. 2. Variation of V oc with oxidation potential of blend bulk heterojunction devices with 共a兲 Al, and 共b兲 LiF/Al cathode. The open circles represent the mean value of all the measurements.
The photovoltage is influenced by many effects in addition to the difference between HOMO of donor and LUMO of acceptor, including the morphology7 of the blend. The photogenerated charge created by this charge transfer will diffuse through the polymer blend layer to be discharged at electrodes under short-circuit conditions. At OC conditions, as used here, this photocurrent is exactly canceled by the injected charges, holes from anode, and electrons from cathode.12 Therefore, small modification of the photovoltage will be caused by the different injection conditions that are expected as the polymer HOMO is moved. These varying conditions will form part of the explanation of the residual scatter of photovoltage between the different materials. The absolute value of the photovoltage does not correspond to the difference between the HOMO of polymer and LUMO of PCBM, ⌬⫽E HOMOគdonor⫺E LUMOគacceptor . This was also observed in other studies in which the acceptor LUMO has been varied.6 A qualitative explanation for the variation of V oc can also be inferred from studies of the photoinduced charge transfer from conjugated oligomers to an electronaccepting molecules like C60 in a solution.13,14 Based on spectroscopic studies, it has been shown that the barrier to photoinduced charge transfer in such a medium can be explained quantitatively by a continuum model for charge separation that describes the change in free energy of the charge separated states as a sum of the gap ⌬ and other terms contributed from the coulombic energy and polarization of the medium.13,14 Although many more complex considerations must be done for the case of solid solutions of donor/ acceptor, it is of interest to estimate driving forces using the
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same model to describe the photoinduced process, now for polymer/molecule blends in a film. We used the Weller equation,15 with reasonable approximations, to estimate the polarization effect on photoinduced charge transfer in polythiophene/PCBM blends. These estimates have shown that polarization energies ⬇1 eV could occur in the polythiophene/PCBM blend films. These will assure a negative change of free energy for all the donor/acceptor couples investigated here, consistent with generation of charge carriers by photoinduced charge transfer. In this study, the short-circuit current densities (J sc) were also measured and compared. Unlike the V oc , the J sc has no defined dependence on the oxidation potential. This can be justified based on the fact that the current of donor/ acceptor organic solar cells is limited by the scale of the polymer/PCBM phase separation as well as the mobility of the charge carriers, both of which may vary in the different blends. The dark-current–voltage characteristics are rectifying in all the devices reported here. With polythiophenes of even lower bandgap and HOMOs, the J – V characteristic is no longer rectifying. This is consistent as the barriers to injection are decreased, but also complicates the evaluation. We have therefore limited our selection. In summary, we have demonstrated that the open circuit voltage of solar cells constructed from conjugated polythiophene/PCBM blends varies with the polymer type used in the cell. For such cells, which were constructed under the same conditions, the variation of photovoltage is attributed to the variation of the oxidation potential of the donor conjugated polymers. One of the authors 共A.G.兲 gratefully acknowledges the financial support from the International Program in the Physical Sciences 共IPPS兲 of Uppsala University, Sweden. We thank the The Swedish Research Council for funding. 1
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