载流子复合及能量无序对聚合物太阳电池开路电压的影响

聚合物太阳电池中载流子的复合与能量无序对器件的开路电压有着深刻的影响.本文同时研究了基于传统富勒烯(PC71BM)和非富勒烯(O-IDTBR)电子受体的聚合物太阳电池.通过交流阻抗谱、低温电流密度-电压谱、瞬态光电压以及电致发光光谱等手段重点研究了载流子复合及能量无序对电池器件开路电压的影响.具体地,交流阻抗谱和瞬态光电压测试结果表明,富勒烯体系载流子复合损失较为严重.电致发光光谱研究显示,PC71BM器件的发光峰随着注入电流的增加不断向短波长处移动,而O-IDTBR体系发光峰位置基本不变,该结果证明PC71BM体系中能量无序度更高.载流子复合严重及能量无序度更高共同作用导致了富勒烯器件开路电压的降低.

Impact of charge carrier recombination and energy disorder on the open-circuit voltage of polymer solar cells

Charge carrier recombination and energy disorder in organic solar cells both have a profound impact on the open-circuit voltage of the device.In this paper,both traditional fullerene-(PC71BM)and nonfullerene-(OIDTBR)based solar cells were fabricated using the same electron donor material(PTB7-Th).The roomtemperature current density–voltage characteristics showed that despite the values of their power conversion efficiencies were very close,there was a huge open circuit voltage(Voc)difference between the PC71BM and OIDTBR devices.To understand the sources of the Voc variation,characterization techniques such as impedance spectra,low temperature electrical characterization method,transient photovoltage,and electroluminescent spectra were carried out.Temperature-dependent Voc of the devices were measured in a large temperature range between 120 K and 300 K.The charge transfer state energy(ECT)of the fullerene and the nonfullerene cells were determined to be 1.13 V and 1.34 V,respectively.Furthermore,the Mott-Schottky equation was applied to analyze the capacitance-voltage curves of the fabricated devices.Results showed that the built-in voltage(Vbi)of the O-IDTBR based cell(1.38 V)was much higher than that of the PC71BM cell(1.15 V).By analyzing the above data,it was easy to speculate that charge carrier recombination loss in the PC71BM device was more serious since the net electric field was relatively weak.Impedance spectra were used to measure the charge carrier recombination process in both devices.Fitting results through the equivalent circuit stated clearly that values of the recombination resistance in the O-IDTBR device were much higher in the test range,indicating that the charge carrier was less easy to recombine in the nonfullerene device.This speculation could be verified by the transient photovoltage(TPV)measurements since the carrier lifetime in the O-IDTBR device was much longer.The excited states in the devices were investigated by the electroluminescence spectra.Since the full width at half maximum(FWHM)of the O-IDTBR emission spectrum was narrower,the excited state energy distribution in the O-IDTBR device was more uniform.Based on the above analyses,the higher Voc in the OIDTBR device was attributed to the mild charge carrier recombination and low energy disorder.