Controlling Energy Levels and Blend Morphology for All-Polymer Solar Cells via Fluorination of a Naphthalene Diimide-Based Copolymer Acceptor

Abstract

We investigate the photovoltaic properties and charge dynamics of all polymer solar cells (all-PSCs) based on poly[(N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5'-(2,2'-bithiophene)] (P-(NDI2OD-T2)) and its fluorinated analogue, poly[(N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5'-(3,3'-difluoro-2,2'-bithiophene)] (P-(NDI2OD-T2F)), as the acceptor polymer and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione] (PBDTTTPD) as the donor polymer. The PBDTTTPD:P(NDI2OD-T2)-based device has a high open-circuit voltage (V-OC) of 1.03 V but suffers from low power conversion efficiency (PCE) of 2.02% with a short-circuit current density (J(SC)) and fill factor (FF) of 4.45 mA cm(2) and 0.44, respectively. In a stark contrast, the PCE of PBDTTTPD:P(NDI2OD-T2F)-based PSC dramatically increases to 6.09% (V-OC = 1.00 V, J(SC) = 11.68 mA cm(2), and FF = 0.52). These results are attributed to the fluorination, which removes the energetic barrier for hole transfer and promotes the formation of the donor/acceptor blend morphology with suppressed phase separation and enhanced intermixed phases. The detailed charge dynamics examined by femtosecond transient absorption spectroscopy suggests the significantly increased hole transfer efficiency and larger populations of long-lived polarons for PBDTTTPD:P(NDI2OD-T2F).