Improved efficiency of DTGe(FBTTh2)2-based solar cells by using macromolecular additives: How macromolecular additives versus small additives influence nanoscale morphology and photovoltaic performance

In order to address the question of how macromolecular additive processing in recent years, sparsely used, affects structural organization and photovoltaic performance, various macromolecular additives (PS, PDMS, and PMMA) vis-a-vis small DIO additive are applied to a model bulk-heterojunction (BHJ) system based on DTGe(FBTTh2)(2). These additives have been shown to exhibit a strong influence on film morphology as a function of additives type, with significant changes in packing orientation, film roughness, and feature size observed, leading to improved power-conversion efficiency (PCE). Interestingly, an unexpectedly large increase in PCEs, from 6.03% to 7.07%, is demonstrated upon adding 1% (w/w) PMMA to DTGe(FBTTh2)(2):PC61BM devices, possessing an even greater PCE than DIO. In addition, when 0.2% (v/v) DIO is added to DTGe(FBTTh2)(2):PC71BM, several key device metrics increase, producing the highest PCE of 7.55% for DTGe(FBTTh2)(2)-based devices to date. In the present work, PMMA is confirmed to provide a highly effective means for controlling morphology and significantly enhancing PCEs. Not only do the results from this study will aid in understanding the mechanisms underlying the impact of macromolecular additive processing, but they will also be key in leading to future studies that will elucidate the driving forces behind and exact points of interaction of macromolecular additives. (C) 2016 Elsevier Ltd. All rights reserved.