Hall effect is a valuable and widely used semiconductor characterization technique in the microelectronic industry, giving access to information on charge carrier type (n- or p-), density and mobility. Photo-Hall effect was further developed over the last few years to extend the pool of parameters accessible and probe minority carrier properties. Combined knowledge of majority and minority carrier mobility, density, lifetime and diffusion lengths are particularly sought after for the development of solar and photodetection technologies. Yet, the use of Hall and photo-Hall effect with organic semiconductors is faced with important challenges, including the need for heterojunctions to dissociate tightly bound excitons and, therefore, generate a photo-Hall signal.

In this study, we develop a Hall device merged with a solar cell architecture to probe photocarrier properties in the organic semiconductor of interest. Polycrystalline rubrene is selected for this study, benefiting from a band-like transport facilitating the interpretation of the Hall signal. Moreover, crystalline rubrene exhibits photocurrent generation in its pristine form, providing an ideal test bed for photo-Hall comparison with and without heterostructures. The ‘gated’ heterostructure is compared with simple mono- and bi-layer Hall devices, demonstrating a two-order of magnitude increase in photoconductivity upon appropriate source and gate bias. While the use of a gated heterostructure was aimed at facilitating exciton dissociation, our photo-Hall data reveal that the increase in photoconductivity is primarily associated with an impact of the device structure on charge carrier mobility. We observe a decrease in mobility with increasing light intensity in pristine rubrene, potentially associated with Coulomb scattering with nearby negative charges in a low dielectric environment. Spatial separation of holes and electrons enabled by the gated bilayer heterostructure however maintains hole mobility at its highest level throughout the light intensity range studied (up to ~100 mW/cm², equivalent to 1 Sun AM 1.5). Further analysis of the photocarrier density evolution with light intensity reveals strong deviations with Langevin theory, as expected for high mobility semiconductors and strong mobility imbalance between holes and electrons.

In summary, through this work we demonstrate that photo-Hall measurements can be performed in organic semiconductors, giving access to valuable information on charge carrier transport, generation and recombination. Importantly, our photo-Hall results reveal that strong spatial separation of holes and electrons is crucial for efficient photocarrier transport and extraction, suggesting the need to further explore crystalline bilayer heterostructures. Contrary to traditional bulk heterojunctions, such device would combine spatial separation of charges with high exciton and charge carrier diffusion lengths.