Steady-state photoconductivity and multi-particle interactions in high-mobility organic semiconductors.

V Podzorov,P Irkhin,H Najafov

doi:10.1038/srep15323

Abstract

Fundamental understanding of photocarrier generation, transport and recombination under a steady-state photoexcitation has been an important goal of organic electronics and photonics, since these processes govern such electronic properties of organic semiconductors as, for instance, photoconductivity. Here, we discovered that photoconductivity of a highly ordered organic semiconductor rubrene exhibits several distinct regimes, in which photocurrent as a function of cw (continuous wave) excitation intensity is described by a power law with exponents sequentially taking values 1, 1/3 and ¼. We show that in pristine crystals this photocurrent is generated at the very surface of the crystals, while the bulk photocurrent is drastically smaller and follows a different sequence of exponents, 1 and ½. We describe a simple experimental procedure, based on an application of “gauge effect” in high vacuum, that allows to disentangle the surface and bulk contributions to photoconductivity. A model based on singlet exciton fission, triplet fusion and triplet-charge quenching that can describe these non-trivial effects in photoconductivity of highly ordered organic semiconductors is proposed. Observation of these effects in photoconductivity and modeling of the underlying microscopic mechanisms described in this work represent a significant step forward in our understanding of electronic properties of organic semiconductors.

Highlights

Another, and they become notable in systems, where excitons and charge carriers are sufficiently mobile and have long life-times, such as in crystalline organic semiconductors
We use high-purity vapor grown rubrene single crystals with silver or carbon contacts deposited at the top (a, b) facet in a coplanar geometry and typically separated by a macroscopic distance L = 1–3 mm (Fig. 1)
The (a, b) facet is uniformly illuminated at a normal incidence with a monochromatic light with the penetration length α−1, where α is the absorption coefficient of crystalline rubrene

Summary

Introduction

Another, and they become notable in systems, where excitons and charge carriers are sufficiently mobile and have long life-times, such as in crystalline organic semiconductors. 2 of Supplementary Information) and a bulk character of the photoexcitation used in our experiment (light penetration length of several μ m is much greater than the lattice constant of rubrene, α−1 ≫ c = 2 7 Å), this observation implies that most of the photocurrent under illumination is generated and flows at the surface of the crystal.

Results

Conclusion