Control of Phase Separation of Benzothienoisoindigo-Benzodithiophene Copolymer for Organic Photovoltaics

Abstract

A new low-bandgap polymer composed of benzothienoisoindigo (BTIDG) and benzodithiophene (BDT) was designed and synthesized. By virtue of unsymmetrical BTIDG moiety, BDT-BTIDG copolymer exhibits moderate electrochemical properties in both bandgap (1.5 eV) and the highest occupied molecular orbital (HOMO, -5.4 eV). They are 0.4 eV wider and 0.5 eV deeper than those of previous symmetric thienoisoindigo (TIDG)-BDT analogue, respectively. Although the BDT-BTIDG is highly soluble in common organic solvents like chlorobenzene, the blend film of phenyl-C61-butyric-acid-methyl ester (PCBM) caused a large phase separation over 200 nm spatial scale, leading to a structural color due to light interference. We have mitigated the phase separation to some extent using chloroform and solvent additive, giving the PCE of 0.83% for BDT-BTIDG:PCBM = 1 : 2. This work indicates that solubility of copolymer is not directly relevant to optimal bulk heterojunction morphology.

Organic photovoltaics of diketopyrrolopyrrole copolymers with unsymmetric and regiorandom configuration of the side units

“…13,14 Diketopyrrolopyrrole (DPP) [15][16][17][18][19][20][21][22][23] and isoindigo (IDG) [24][25][26][27][28][29][30][31][32][33][34] are relatively strong electron acceptors and have been polymerized into narrow-bandgap polymers for OPV. Based on modication of IDG, Ashraf et al, 35 Pruissen et al 36 and Koizumi et al 37 have reported the synthesis of thienoisoindigo (TIDG) and its copolymers as organic eld-effect transistors (OFETs), which is followed by applications of TIDG-based near-infrared (IR)-absorbing copolymers to OPV [38][39][40][41][42] and OFET. [43][44][45][46][47] IDG and TIDG have structural similarities to DPPs with symmetric side units of benzene (Ph) or thiophene (Th), respectively.…”