Improvement of polymer:fullerene bulk heterojunction morphology via temperature and anti-solvent effect

Abstract

Morphology of polymer:fullerene bulk heterojunctions (BHJ) displays beside suitable energy level alignment between donor and acceptor as well as a sufficient absorption one of the most crucial impacts on organic photovoltaic device efficiency and stability. In particular, the structural order of the polymer phase impacts the charge separation efficiency and transport. In this work we present a comprehensive study on the control of the polymeric order initiated already in solution by using fractions of anti-solvent isopropanol (IPA), and via adjusting the processing temperature for common solutions of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) bearing statistically either linear (octyloxy) or branched (2-ethylhexyloxy) side-chains (AnE-PVstat) and phenyl-C61-butyric acid methyl ester (PCBM). Detailed analysis of photophysical data, absorption, and photoluminescence (PL) spectra, of deposited AnE-PVstat:PCBM thin films suggest the formation of J-aggregates for the copolymer to be in strong correlation with the processing formulation (presence of non-solvent IPA) and post-treatment parameters (annealing temperature). The findings for our system indicate that the addition of IPA increases the intermolecular interaction between the polymer chains, consistent with enhanced charge carrier (hole) mobility (μh) in the final film reaching values as high as 10−3 cm2 V−1 s−1. The subtle control of polymer aggregation led to an overall variation in the solar cell device efficiency by about 50%, with optimal processing settings of IPA additive and a solution temperature of 15 °C.