Enhanced performance for organic bulk heterojunction solar cells by cooperative assembly of ter(ethylene oxide) pendants

Abstract

A ter(ethylene oxide) (TEO) functionalized copolymer donor and fullerene acceptor, namely poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-4,6-thieno[3,4-b]thiophene-2-carboxylic acid 2-[2-(2-methoxy-ethoxy)-ethoxy]-ethyl ester} (PBDTT-TT-TEO) and [6,6]-phenyl C₆₁ butyric acid 2-[2-(2-methoxy-ethoxy)-ethoxy]-ethyl ester (PCB-TEO), respectively, are explored to manipulate the self-assembly nanoscale morphology and enhance the stability of the photoactive layer in polymer solar cells. Compared with the copolymer PBDTT-TT, TEO side chains induce more ordered molecular packing in PBDTT-TT-TEO, leading to an improvement in the power conversion efficiency (PCE) of the device from 3.6% for PBDTT-TT:PCBM (PCBM is (6,6)-phenyl-C₆₁ butyric acid methyl ester) to 4.1% for PBDTT-TT-TEO:PCBM. Directly blending PBDTT-TT-TEO with PCB-TEO , for use as the active layer, does not afford high device performance due to the discontinuous morphology of blend film caused by the poor solubility of PCB-TEO. However, precisely controlling the loading of PCB-TEO in the PBDTT-TT-TEO:PCBM blend greatly promotes the PCE of the device, because the TEO modified fullerene can serve as an effective compatibilizer to manipulate the miscibility between the polymer donor and acceptor and achieve a favorable heterojunction morphology by a cooperative assembly effect. The device with 5% loading of PCB-TEO achieves the highest PCE of 4.8%, with a short circuit current (J_sc) of 12.18 mA cm⁻², an open circuit voltage (V_oc) of 0.722 V and a fill factor (FF) of 55.0%, approximately a 33% improvement in PCE over the PBDTT-TT:PCBM device. In addition, the PCB-TEO compatibilizer located at the interface of the donor and acceptor strengthens the interaction of PBDTT-TT-TEO with PCBM, consequently improving the morphological and device stabilities.