Temperature dependence of HOMO-LUMO levels and open circuit voltage for P3HT:PCBM organic solar cells
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Temperature dependence of HOMO-LUMO levels and open circuit voltage for P3HT:PCBM organic solar cells Yang Shen1, Louis Scudiero2* and Mool C. Gupta1* 1
Department of Electrical and Computer Engineering, University of Virginia, 351 McCormick Road, Charlottesville Virginia 22904, USA 2 Chemistry Department, Washington State University, Fulmer 261A, Pullman, Washington 99164, USA ABSTRACT In this study, the open circuit voltage (VOC) of poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells was measured at temperatures ranging from 300 K to 400 K. The temperature dependence of the vacuum shift and of the highest occupied molecular orbital (HOMO) energy level of P3HT and PCBM were measured by ultraviolet photoelectron spectroscopy (UPS) in the same temperature range. The temperature dependence of the absorption edge was also studied in the same temperature range to obtain the temperature variation of the optical band gap energy (Eg). The measured VOC of the devices showed a clear decreasing trend with increasing operating temperature and the total decrease was found to be about 0.1 V. Although the origin of VOC is still not fully understood it is generally believed that the energy level offset between the HOMO of the donor and the LUMO of the acceptor minus the exciton binding energy (0.3 eV) directly determines the value of VOC. However, by utilizing the measured values of the HOMO for the P3HT (donor) and of the LUMO for the PCBM (acceptor), we have found that the calculated values of VOC and its temperature dependence do not agree with the measured VOC values. This indicates that factors other than the offset between the HOMO of the donor and the LUMO of the acceptor materials are impacting VOC. * Corresponding authors: [email protected] (Mool C. Gupta), [email protected] (Louis Scudiero) INTRODUCTION Organic photovoltaics have been receiving increasing attention over the last few decades. Comparing with conventional photovoltaic materials such as Si, organic polymers offer many practical advantages due to the solution processing techniques. Organic polymers also have relatively high absorption coefficients, in the visible spectrum (usually > 105 cm-1); yielding high absorption in devices thinner than 100 nm [1-3].The promise of organic solar cells is limited by its low efficiency. To date, the maximum demonstrated efficiency of an organic BHJ solar cell is only ~ 8.3% as reported by Konarka Technologies, Inc. and verified by the National Energy Renewable Laboratory (NREL) [4]. Open circuit voltage (VOC) is one of the parameters which determine the power conversion efficiency of a solar cell. Although the factors influencing the VOC of organic solar cells are still not fully understood, it is generally believed that the energy level offset between the HOMO of the donor and the LUMO of the acceptor (EDHOMO - EALUMO) minus the exciton bonding energy directly determines the value of VOC. Studies on different donor/acceptor systems ha
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