Negative capacitance and its photo-inhibition in organic bulk heterojunction devices

We report the dynamic admittance, both in the dark and under illumination, of heterojunctions made of poly(3-hexyl thiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (P3HT:PCBM) blends, which are used in efficient organic solar cells. In the dark there appears a huge low frequency negative capacitance which we associate with slow electron hole bimolecular recombination at the heterojunction interfaces. Surprisingly, under photoexcitation the negative capacitance gradually disappears with increasing light intensity. We attribute this positive photoinduced capacitance to the combined effect of (1) long lived photogenerated charges at the P3HT:PCBM interfaces that increase electron-hole bimolecular recombination rate, which in turn renders the capacitance less negative and (2) trapped photogenerated charges that increase the capacitance upon re-emission.