The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells

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  Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]-phenyl C61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive
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   Submitted to 11 DOI: 10.1002/adfm.200800056 The relation between open-circuit voltage and the onset of photocurrent generation by charge-transfer absorption in polymer:fullerene bulk heterojunction solar cells **   By Koen Vandewal *,  Abay Gadisa, Wibren D. Oosterbaan, Sabine Bertho, Fateme  Banishoeib, Ineke Van Severen, Laurence Lutsen, Thomas J. Cleij, Dirk Vanderzande and  Jean V. Manca  [*] K. Vandewal, Dr. W. D. Oosterbaan, S. Bertho, Dr. F. Banishoeib, Dr. I. Van Severen, Dr. L. Lutsen, Prof. Dr. T. J. Cleij, Prof. Dr. D. Vanderzande, Prof. Dr. J. V. Manca Institute for Materials Research Hasselt University Wetenschapspark 1, 3590 Diepenbeek (Belgium) E-mail: koen.vandewal@uhasselt.be Dr. A. Gadisa, Dr. L. Lutsen, Prof. Dr. D. Vanderzande, Prof. Dr. J. V. Manca IMEC-IMOMEC, vzw Wetenschapspark 1, 3590 Diepenbeek (Belgium) [**] This research was funded by the IWT-projects Nanosolar and Polyspec, the interreg  project OLED+ and the FWO projects G.0396.08 and G.0091.07. K. V. acknowledges the institute for the promotion of innovation through science and technology in Flanders (IWT-Vlaanderen) for funding. S. B. is a research assistant of the Fund for Scientific Research Flanders (FWO-Vlaanderen). Keywords: photovoltaic devices, organic, charge-transfer complex, Fourier-transform  photocurrent spectroscopy Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer:[6,6]-phenyl C 61  butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (  EQE  ) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground-state charge-transfer complexes in the studied polymer:fullerene blends. The effective band gap (  E  g ) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all   Submitted to 22 devices, under various preparation conditions, the open-circuit voltage ( V  oc ) scales linearly with  E  g . The redshift of the CT band upon thermal annealing of regioregular poly(3-hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM  photovoltaic devices correlates with the observed drop in open-circuit voltage of high-temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of  E  g , proportional with the observed changes in V  oc  for different PPV:PCBM ratios. As  E  g  corresponds with the effective bandgap of the material blends, a measurement of the  EQE   spectrum by FTPS allows us to measure this energy directly on  photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.   Submitted to 33 1. Introduction Currently, the best performing polymer-based solar cells comprise bulk heterojunction (BHJ)  polymer/acceptor interpenetrating networks characterized by a three dimensional, nanoscale morphology. The generation of photocurrent in such solar cells follows a multi-step process, namely, generation of excited electron-hole pairs (excitons), mainly in the polymer phase and the subsequent splitting of these excited states at the polymer/acceptor interface leaving free holes in the polymer phase while transferring the electrons to the electron accepting material. Such a transfer process eventually leads to two separate percolation paths for the free charge carriers. [1,2]  Successful BHJ devices have been fabricated by solution deposition of mixtures of a soluble conjugated polymer and the fullerene derivative [6,6]-phenyl C 61  butyric acid methyl ester (PCBM). Among the commonly used soluble conjugated polymers are  poly(phenylene vinylene) (PPV) [3,4] , polyfluorene [5,6] and polythiophene [7]  derivatives. By controlling the BHJ nanomorphology, power conversion efficiencies up to 4-5 % have been achieved. [7,8] However, further improvements are still needed in order to obtain higher efficiencies that make polymer solar cells competitive with their inorganic counterparts. Understanding the factors that limit photovoltaic parameters such as short-circuit current (  J  sc ) and open-circuit voltage ( V  oc ), helps to optimize material and device structures, leading to higher efficiencies. While  J  sc is determined by the creation and subsequent dissociation of excitons at the polymer/acceptor interface followed by transport of free charge carriers towards the collecting electrodes [9] , V  oc  is primarily determined by the effective band gap of the BHJ film. [10,11] As far as the exciton dissociation process is concerned, recent theories and experimental evidences indicate that an intermediate charge-transfer (CT) state exists between the excitons created upon light absorption in the polymer and the long-lived, free charge carriers. Recently, the existence of such an intermediate CT state in a BHJ consisting of a low band gap   Submitted to 44 semiconducting copolymer and PCBM was inferred from analysis of its carrier recombination dynamics. [12]  Quantum chemical calculations of charge transfer in the poly(3-hexylthiophene) (P3HT):PCBM material system also support the presence of an extended electronic state created upon material blending. [13]  It was shown that this state has a significant probability distribution across the donor/acceptor (D/A) interface in its lowest excited state. The existence of such intermediate bound electron-hole pairs at the D/A interface was already assumed to explain the compositional dependence of the photocurrent generation in PPV:PCBM solar cells. [14]  It was shown that the incomplete dissociation of these photo-generated bound electron-hole pairs under operating conditions is a main loss mechanism in this type of solar cells. Further optimization of polymer:fullerene based solar cells thus requires a thorough study of these interfacial intermediate states. Highly sensitive studies of the absorption spectra of P3HT:PCBM and PPV:PCBM blends by our group, have revealed the presence of a long wavelength absorption band characteristic for a weak ground state CT complex (CTC), formed by the interaction of the lowest unoccupied molecular orbital of the fullerene acceptor LUMO(A) with the highest occupied molecular orbital of the polymer donor HOMO(D). [15-16]  Illumination with wavelengths in this CT band results in the direct creation of bound electron-hole pairs or CT excitons, as described by Foster  [17]  for CTCs in solution. CTC formation is widely observed when combining donor materials with electron acceptors such as C 60  and C 70  (see for example reference [18]  and references therein). Recently, it was shown that a whole range of π -conjugated polymers and oligomers form CT absorption bands when mixed with electron acceptors in chloroform solution. [19]  Using Photothermal Deflection Spectroscopy and photoluminescence spectroscopy, the formation of CTCs was also evidenced for solid-state blends of different types of polyfluorenes with PCBM. [20-21]     Submitted to 55 In reference [21]  a model for the involvement of these CTCs in the photocurrent generation mechanism was proposed. It was shown that the energy of the excited CTC is critical for determining whether photocurrent is generated, or energy transfer and subsequent emission from the PCBM singlet state takes place. In previous works involving CTCs, however, no correlation between the open-circuit voltage and this CTC state has been reported. In order to do so, we have measured the external quantum efficiency (  EQE  ) spectra of  photovoltaic devices consisting of a variety of conjugated polymers blended with PCBM by means of Fourier-transform photocurrents spectroscopy (FTPS). Originally applied to investigate subgap absorption by defects in hydrogenated microcrystalline silicon for  photovoltaic devices, FTPS has a superior sensitivity [22]  as compared to monochromatic  photocurrent measurement techniques. When applied to organic BHJ devices, it can be used for the determination of the device’s absorption window. Moreover, FTPS has the required sensitivity to measure the low signal sub-band gap photocurrent produced by the direct creation of CT excitons upon long wavelength illumination of the CTCs. [16]  In this paper it will be shown that the effective bandgap (  E  g ), determined by the onset of  photocurrent generation by CT absorption, linearly correlates with the V  oc  of the studied devices. Regardless of device type and treatment conditions such as annealing and aging a difference between the e E  g  and V  oc  of about 0.43 V was observed for measurements  performed at room temperature and under 1 sun illumination conditions. This investigation  provides a deeper understanding of the srcin of the photovoltage of polymer:PCBM based solar cells and hence may help to engineer material structures that lead to improved  performance of BHJ solar cells.
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