A block copolymer enhances the efficiency of small-molecule bulk-heterojunction photovoltaics

Abstract

Block copolymers can self-assemble into ordered structures having feature dimensions on the order of 10 to 100 nm; we took advantage of the different polarities of the blocks of a low-molecular-weight diblock copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO) that interact differentially with small molecules and fullerenes to tune the extent of phase separation in solution-processed small-molecule bulk-heterojunction (SMBHJ) solar cells. We incorporated small amounts of nanostructured PS-b-PEO to solar cells' active layers featuring 7,7′-{4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl}bis{6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole} (p-DTS(FBTTh₂)₂) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) for optimizing the morphology and thus enhancing the devices' power conversion efficiency. For understanding the effect of PS-b-PEO on the devices' performances, we used synchrotron grazing-incidence wide-angle X-ray scattering, atomic force microscopy and transmission electron microscopy to probe and to decipher the morphologies of the resulting SMBHJ thin films. Without undergoing any annealing process, a device with an active layer of p-DTS(FBTTh₂)₂:PC₇₁BM (1.5 : 1, w/w) that incorporated 0.5 wt% of PS-b-PEO and was processed with a 1,8-diiodooctane solvent additive displayed a power conversion efficiency (PCE) of 7.3%, a relative increase of 2.5 times as compared to the PCE of 2.1% for the control device featuring only p-DTS(FBTTh₂)₂ and PC₇₁BM. Thus, incorporating this nanostructured block copolymer in the active layer allowed effective tuning of the small molecule active layer morphology and resulted in enhanced device efficiency.