Electro-optical studies on MDMO-PPV:PCBM bulk-heterojunction solar cells on the millisecond time scale: Trapped carriers

The influence of a periodically modulated electric field on the concentration of photogenerated charges in bulk-heterojunction solar cells of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 (PCBM) is studied at 80 K by near-steady-state photoinduced absorption (PIA) spectroscopy. Measurements are complemented with recording the photocurrent. The results show that the lifetime of extractable charge carriers under conditions where the built-in electrical field is largely compensated, is in the sub-millisecond time domain. PIA is dominated by the contribution from trapped carriers and application of a −4 V bias increases the number of trap sites available to the carriers in comparison with a +1 V bias voltage. The reverse bias voltage also leads to an enhancement in the generation rate of carriers that can be trapped and/or the lifetime of the trapped carriers. The experiment indicates that the lifetime of the trapped carriers at −4 V bias voltage remains in the sub-millisecond time domain.