Photovoltaic Properties and Charge Dynamics in Nanophase-Separated F8T2/PCBM Blend Films

Abstract

We studied organic photovoltaics (OPVs) in bulk-heterojunction cells (BHJ) based on a liquid-crystalline fluorine copolymer (F8T2) with the capability of forming periodic nanostructures as the donor and a soluble fullerene (PC70BM) as the acceptor. The surface morphology of the blend film shows that the F8T2 and PC₇₀BM phases are separated into large islands, which grow in size with increasing fraction of PC70BM. The power conversion efficiencies (PCEs) of the cells exhibit a distinct maximum as a function of the PC₇₀BM concentration in the BHJ OPVs; the PCE has a maximum at F8T2:PC₇₀BM (1:2) and decreases at both lower and higher concentrations of PC₇₀BM. The OPV with the highest performance has a PCE of 2.28%. In order to investigate the charge separation that occurs at the interface between F8T2 and PC₇₀ BM, we also performed ultrafast transient absorption spectroscopy on F8T2:PC₇₀BM (1:2) blend films.