American Chemical Society
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Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells

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journal contribution
posted on 2018-03-23, 00:00 authored by Jin Fang, Dan Deng, Zaiyu Wang, Muhammad Abdullah Adil, Tong Xiao, Yuheng Wang, Guanghao Lu, Yajie Zhang, Jianqi Zhang, Wei Ma, Zhixiang Wei
An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage (Voc) and fill factor (FF) in the inverted device were studied. The reduced Voc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C71-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs.