Efficient Organic Devices Based on π-Electron Systems: Comparative Study of Fullerene Derivatives Blended with a High Efficiency Naphthobisthiadiazole-Based Polymer for Organic Photovoltaic Applications

Abstract

This study focuses on a polymer based on quaterthiophene and naphthobisthiadiazole units (PNTz4T). Three fullerene derivatives, namely, PC61BM, PC71BM, and IC60BA, were selected as potential electron acceptors in regular device architectures. The resulting average PCE are 7.52, 8.52, and 2.58% for PC61BM, PC71BM, and IC60BA, respectively. Through a careful and systematic study, we investigated the origins of the differences observed in devices’ performances. In particular, the higher Jsc and consequently higher PCE of the PC71BM devices (as compared to PC61BM devices) can be easily explained by the better light-harvesting properties in the visible range of the larger fullerene derivative. Furthermore, we demonstrate that the limiting factor in these devices is the electron collection which is closely related to the crystallinity of the fullerene derivative. The low crystallinity and resulting low electron-transporting properties of IC60BA is at the origins of the low performances of the IC60BA-based devices. Through this comparative study, we confirm that developing new materials is the key to remarkably increase the PCE of polymer solar cells. However, in order to obtain PCE over 10%, a particular attention should be given to material combination, process, and charge balance in the devices.