ABSTRACT
The magnetoconductance (MC) effect was investigated for two types of organic solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor (A). Three components with different half-field-at-half-maximums (B 1/2) of 4 ± 1, 20 ± 15 and >400 mT, hereafter referred to MCS,M,B in a sequence, were observed in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCS,M,B components is sensitive to not only the junction structure of the cell but also the presence or absence of incident light. The bias voltage (V) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (V ON) of the dark current, where a flat band condition of the active layer is achieved. The B 1/2 for the MCM component of the SJ-cell increases with V beyond V ON. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCS,M components for the BJ-cell in light increase with the incident light power. The transient MCS,M components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all of the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)-hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B 1/2 values for MCS,M,B are, respectively, understood by the spin conversion due to the hyperfine interaction, the spin relaxation, and the g-factor difference for e (PCBM-) and h (P3HT+). Kinetic simulations of the MCS,M components for the BJ-cell observed at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.
