The Photo-Hall Effect in High-Mobility Organic Semiconductors

High-mobility single crystal organic semiconductors are important for applications in advanced organic electronics and photonics. Photogeneration and transport of mobile photocarriers in these materials, although very important, remain underexplored. For sustained progress in the field, understanding the intrinsic (i.e., not limited by static disorder) charge transport properties of crystalline organic semiconductors is important. The Hall effect represents one of the most efficient experimental tools for assessing intrinsic charge transport, as Hall effect in single crystal materials probes band-like (i.e., delocalized) mobile charges. The photoconductivity-based <i>photo-Hall effect</i> can be used to address the fundamental charge transport properties of these functional molecular materials. Photocarrier generation by light allows circumventing the challenging problem of carrier injection from contacts (especially when measurements of both electrons and holes are pursued), avoiding carrier trapping and scattering associated with the interfacial charge transport as in OFETs, and preventing added disorder associated with chemical doping. Therefore, photo-Hall effect opens new opportunities in probing the intrinsic charge transport in organic semiconductors.<br/>The photo-Hall measurements in pristine, ungated organic semiconductors pose an even greater challenge than the Hall measurements in OFETs or chemically doped samples, because a much lower carrier density (and, thus, much higher sample's resistances) is typically generated by light. By taking advantage of a sensitive ac-Hall methodology previously developed in our group, we have performed reliable photo-Hall effect measurements in a benchmark organic semiconductor rubrene.<br/>We present the first clear demonstration of a photo-Hall effect in organic semiconductors, using a benchmark molecular crystal rubrene as an experimental platform. The concept of a photo-Hall effect allows probing the intrinsic transport properties of pristine semiconducting materials in stand-alone crystals, free from the problems associated with charge injection from contacts, interfacial transport, or additional disorder typically encountered in the conventional approaches based on FETs or chemically doped materials. In photo-Hall measurements, the steady-state carrier density and mobility are measured independently, with no assumptions regarding the photocarrier generation efficiency or lifetime, thus providing a direct experimental access to these important parameters. We find that under a <i>cw </i>illumination in the visible range, only mobile holes are generated in pristine rubrene crystals, leading to a substantial p-type surface photoconductivity with the hole mobility <i>μ</i>_photo–Hall = 11 ± 1 cm²V⁻¹s⁻¹ and the density that follows a power law <i>n</i>_photo–Hall ∝ <i>P^α</i>, with the power exponent α = 1/4–1/3. In addition, by intentionally inhibiting the surface transport via the gauge effect, we observed a small residual bulk photocarrier density governed by a bimolecular electron-hole recombination (α = 1/2) with a higher true-bulk hole mobility of <i>μ</i>_photo–Hall = 16 ± 1 cm²V⁻¹s⁻¹.<br/>These experiments significantly advance our fundamental understanding of charge transport and photoconductivity in rubrene, based on triplet exciton diffusion and surface dissociation, in which the latter process is shown to be governed by electron trapping with a release of mobile hole. Overall, our work demonstrates that the photo-Hall effect is a powerful tool for addressing the intrinsic charge transport in organic semiconductors and potentially other emergent semiconducting materials.<br/><b>Reference</b>:<br/>Bruevich, V., Choi, H. H., Podzorov, V., The Photo-Hall Effect in High-Mobility Organic Semiconductors. Adv. Funct. Mater. 2021, 31, 2006178. https://doi.org/10.1002/adfm.202006178