Continuous-wave photoinduced absorption study on trapped carriers in bulk-heterojunction solar cells connected to load

• Hole transport process in a working polymer-based solar cell was investigated.
• We used a continuous-wave photoinduced absorption spectroscopy.
• A detrapping time constant decreases with a resistance value of a connected resistor.
• Detrapping is accelerated by an internal electric field.

By using continuous-wave photoinduced absorption spectroscopy, we investigate hole-transport processes in polymer-based bulk-heterojunction solar cells at various operating points between the open- and short-circuit conditions. The results reveal that the detrapping time constant of holes decreases as the short-circuit condition is approached. This decrease may be explained by an acceleration of detrapping that is caused by an internal electric field. This interpretation is supported by a numerical simulation based on rate equations, considering a low density of a single trap level. Since the detrapping signal disappears at short-circuit, we estimate the associated trap levels to be relatively shallow.