An impact of higher fullerene on charge recombination in organic solar cell studied by magnetoconductance

In order to clarify roles of higher fullerenes on the charge recombination in organic solar cells based on the bulk heterojunction (BJ) of polymers as electron donors (Ds) and fullerenes as electron acceptors (As), we measured magnetoconductance (MC) effects for the BJ-cells consisted of a blend film of regioregular poly[3-hexylthiophene] (P3HT) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM), hereafter referred to as C-71-cell, and compared them with the results for P3HT:PC61BM-cell (C-61-cell) reported in our previous paper. As with C-61-cell, three components (MCS,M,B) with half-field-at-half-maximums of 4 +/- 1, 28 +/- 12 and > 400 mT were detected in the magnetic field dependence of the MC effects for the C-71-cell under dark and light conditions. From the incident light power and time dependences of the positive MCS and MCM components in light, it was assinged that the light MC effects observed for the C-71-cell is attributed to geminate recombination of the electron-hole (e-h) pair, whereas the C-61-cell shows positive light MC effects due to nongeminate recombination. This is explained by decrease of the possiblity to form nongeminate e-h pair because C-71-cells have wide D/A junction caused by uniform mixing of P3HT and PC71BM more than C-61-cells. The positive low-field MC effects due to the geminate e-h pair in the C-71-cell indicates that the recombination rate constant for the (3)e-h pair (k(T)) to the triplet exciton ((3)ex) is greater than that for the (1)e-h pair (k(S)) to the singlet ground state ((1)gr). The magnitude relationship of k(S) < k(T) is opposite to that for C-61-cell. Kinetic analysis on the low-field MC effects for the C-71-cell provided that the recombination yield of the geminate e-h pair is 0.5%. The inversion of magnitude relationship for the spin-dependence in recombination rate by replacing PC61BM with PC71BM are explained by a reorganization energy of PC71BM smaller than that of PC61BM. We concluded that the recombination to the (1)gr is in the inverted regime of the Marcus theroy, but that to the (3)ex is in the normal regime, so that decrease of the reorganization energy results in drastic decrease in k(S) more than k(T).