Polymer solar cells with open-circuit voltage of 1.3 V using polymer electron acceptor with high LUMO level

Abstract

Open-circuit voltage (VOC) of polymer solar cells (PSCs) is always in the range of 0.6–0.9V and rarely exceeds 1.0V. The common strategy to enhance VOC of PSCs is to use polymer donors with low-lying highest occupied molecular orbital energy level (EHOMO). In this manuscript, we report high VOC of 1.3V for PSCs by further using a polymer acceptor with high-lying lowest unoccupied molecular orbital energy level (ELUMO). With poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDTBT) as the donor and poly[(N,N′-bis(2-hexyldecyl)-diamine-bis(difluoro-borane)−2,2′-bipyridine)-alt-(2,5-thiophene)] (P-BNBP-T) as the acceptor, the PSC devices show the VOC of 1.30V and power conversion efficiency (PCE) of 3.20%. This VOC is higher than that of the conventional acceptor materials by 0.4V owing to the high-lying ELUMO of P-BNBP-T. We have also investigated the effect of processing solvents on the blend morphology and the device performance. Compared with high boiling-point solvent o-dichlorobenzene, low boiling-point solvent chloroform gives finer phase separation size, improved exciton dissociation and consequently better photovoltaic device performance. The demonstration of high VOC in this work indicates that there is large room for PCE enhancement of PSCs.