How far can a minority charge carrier of an organic semiconductor walk? An in situ observation by scanning photocurrent microscopy

Abstract

The direct observation of intrinsic minority charge carrier diffusion length in organic semiconductors remains scarce, for not only a prevailing perception of their low mobility and inherently short diffusion lengths but also experimental light interference and charge transfer in electrodes. Our recent experimental endeavor, however, has unveiled a remarkable finding: a classic organic semiconductor, TIPs-pentacene-based organic field-effect transistor with PMMA dielectric, exhibits an exceptional photocurrent decay process with a charge carrier diffusion length of 2.2 micrometers. Careful evaluation of confocal resolution, optical interference, waveguide action and electrode influence with experimental conditions confirms that the observed attenuation comes from the intrinsic minority carrier diffusion length in situ. Furthermore, we posit that the protracted lifetime of intrinsic charge recombination, stemming from the structural relaxation of anionic polarons into local energy minima and metastable configurations, acts as a pivotal factor in enabling this extraordinary micron-level diffusion length. These findings not only shed light on the exceptional performance of organic semiconductors in thick-film and long-channel devices but also stimulate fresh perspectives in the design of organic semiconductor device architectures, broadening the horizons of their potential applications.