Roberto Faria1,Daniel Roger Amorim1,Francineide Araújo1,Douglas Coutinho2
Univ of Sao Paulo1,Technical Federal University of Paraná2
We have derived an analytical expression for the photocurrent of organic bulk-heterojunction solar cells, taking into account second-order kinetics for bimolecular recombination. Concomitantly, we have carried out current-voltage measurements (<i>J-V</i>), in dark and under 1 Sun illumination, at different temperatures, in devices made with PTB7-Th:PC<sub>71</sub>BM blends, processed either from a solution of pure chlorobenzene or from a mixture of chlorobenzene and the co-solvent 1,8-diiodooctane (DIO). It is already known that DIO additive selectively dissolves the fullerene and reduces the domain sizes of PC<sub>71</sub>BM forming a donor-acceptor bicontinuous interpenetrating network, resulting in an increase of the device external quantum efficiency. From the adjustments obtained by the Mott-Gurney equations on the measurements in dark, and that of the photocurrent equation on the photovoltaic responses, we analyzed the effect of temperature on the charge carriers mobility and on the Langevin reduction factor. 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 PCBM aggregates and their better permeation in the polymer matrix of PTB7, which facilitates the dissociation of charge transfer states at PTB7-Th-PC<sub>71</sub>BM interfaces.