Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
ACS Nano. 2017 Nov 28;11(11):11542-11549. doi: 10.1021/acsnano.7b06413. Epub 2017 Nov 7.
This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance-voltage (CV) signal is observed as a broad "shoulder" in the depletion region from -0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA with difficult-rotational formamidinium (FA) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.
这篇文章报道了在无滞后的甲基铵（MA）卤化铅钙钛矿太阳能电池[ITO/PEDOT：PSS/钙钛矿/PCBM/PEI/Ag]中光诱导体相极化的实验演示。基于电容-电压（CV）测量，在光激发下观察到一个异常的电容-电压（CV）信号，这是一个在-0.5 到+0.5 V 的耗尽区中的宽“肩”。在 CV 测量中，通过逐渐扫描直流偏压来连续漂移可移动离子，以在低交流偏压（50 mV，5 kHz）下检测局部极化。从本质上讲，逐渐扫描直流偏压并施加低交流偏压可以分别从光激发下的局部极化产生连续漂移的离子和体相 CV 信号。特别是，当在氯掺入后器件效率从 12.41%提高到 18.19%时，这个异常的 CV 信号可以增强 3 倍。通过与界面电荷积累相关的表面极化相区别，这个异常的 CV 信号可以被分配为光诱导体相极化的特征。同时，用不易旋转的甲脒（FA）阳离子代替易旋转的 MA 阳离子可以大大降低这种异常的 CV 信号，这表明光诱导的体相极化依赖于偶极有机阳离子的取向自由度。此外，凯氏探针力显微镜研究表明，氯掺入可以通过减少来自带电缺陷的屏蔽效应来抑制载流子复合，从而增强光诱导的体相极化，因为带电缺陷的密度降低了。一个偏压依赖的光致发光研究表明，通过库仑屏蔽效应降低电荷俘获概率，增加体相极化可以抑制载流子复合。显然，我们的研究为深入了解钙钛矿太阳能电池中的光诱导体相极化及其对光伏作用的影响提供了有价值的认识。
