Improving the Performance of P3HT-Fullerene Solar Cells with Side-Chain-Functionalized Poly(thiophene) Additives: A New Paradigm for Polymer Design

Abstract

The motivation of this study is to determine if small amounts of designer additives placed at the polymer–fullerene interface in bulk heterojunction (BHJ) solar cells can influence their performance. A series of AB-alternating side-chain-functionalized poly(thiophene) analogues, P1–6, are designed to selectively localize at the interface between regioregular poly(3-hexylthiophene) (rr-P3HT) and PCnBM (n = 61, 71). The side chains of every other repeat unit in P1–6 contain various terminal aromatic moieties. BHJ solar cells containing ternary mixtures of rr-P3HT, PCnBM, and varying weight ratios of additives P1–6 are fabricated and studied. At low loadings, the presence of P1–6 consistently increases the short circuit current and decreases the series resistance of the corresponding devices, leading to an increase in power conversion efficiency (PCE) compared to reference P3HT/PC61BM cells. Higher additive loadings (>5 wt %) lead to detrimental nanoscale phase separation within the active layer blend and produce solar cells with high series resistances and low overall PCEs. Small-perturbation transient open circuit voltage decay measurements reveal that, at 0.25 wt % incorporation, additives P1–6 increase charge carrier lifetimes in P3HT/PC61BM solar cells. Pentafluorophenoxy-containing polymer P6 is the most effective side-chain-functionalized additive and yields a 28% increase in PCE when incorporated into a 75 nm thick rr-P3HT/PC61BM BHJ at a 0.25 wt % loading. Moreover, devices with 220 nm thick BHJs containing 0.25 wt % P6 display PCE values of up to 5.3% (30% PCE increase over a control device lacking P6). We propose that additives P1–6 selectively localize at the interface between rr-P3HT and PCnBM phases and that aromatic moieties at side-chain termini introduce a dipole at the polymer–fullerene interface, which decreases the rate of bimolecular recombination and, therefore, improves charge collection across the active layer.