Temperature-Independent Photoconductivity in Thin Films of Semiconducting Polymers: Photocarrier Sweep-Out Prior to Deep Trapping

Abstract

In thin films, the steady-state photoconductivity is temperature $\left(T\right)$ independent in agreement with transient photoconductivity in the subnanosecond regime. As thickness is increased, an activated $T$ dependence emerges. The $T$ independence of the product of the carrier density $\left(n\right)$ times the mobility $\left(\mu \right)$ for thin films results from carrier sweep-out prior to trapping; the crossover from $T$-independent $\mu$ to activated $\mu$ occurs when the transit time across the film is comparable to the time required for deep trapping. Thus, steady-state photoconductivity confirms that, in semiconducting polymers, the carrier generation mechanism is $T$ independent, inconsistent with the Onsager model of photocarrier generation.