Effects of additive-solvents on the mobility and recombination of a solar cell based on PTB7-Th:PC_(71)BM

摘要

We have performed a study on the effect of the 1,8-diiodooctane (DIO) additive on the performance of an organic bulk-heterojunction solar cell made with PTB7-Th:PC71BM nanostructured blend. The devices were fabricated using either pure chlorobenzene as solvent or mixed with 1,8-diiodooctane. Current-voltage measurements carried in dark, at different temperatures, were analyzed by the Mott-Gurney equation, in which the charge carrier mobility was obtained as fitting parameter. On the sequence, current-voltage curves recorded under 1 Sun illumination, also at different temperatures, were fitted by an analytical equation for the photocurrent, which took into account second-order kinetics for the bimolecular recombination. It is already known that DIO additive selectively dissolves the fullerene and reduces the domain sizes of PC71BM forming a donor-acceptor bicontinuous interpenetrating network, resulting in an increase of the device external quantum efficiency. From the adjustments obtained by the measurements in dark, and that of the photocurrent equation on the photovoltaic responses, we analyzed the effect of temperature on the charge carrier mobility mu and on the recombination reduction factor zeta. It was evident that the effect of DIO on the morphology of the active layer improves the conduction process by hopping, and decreases the recombination coefficient. This improvement of the photocurrent response is most probably due to the fragmentation of the PC71BM aggregates and their better permeation in the polymer matrix of PTB7-Th, which facilitates the dissociation of charge transfer states at PTB7-Th:PC71BM interfaces.