Charge Carrier Transport in Crystalline Organic and Hybrid Perovskite Semiconductors Investigated with a High-Resolution ac-Hall Technique

The importance of Hall effect measurements in evaluating the charge carrier mobility of novel materials is being increasingly recognized in materials science, semiconductor physics, and electronics. Hall measurements are advantageous over other mobility measurement techniques in that they: (<b>a</b>) allow a direct access to the mobile carrier concentration and mobility in a steady-state charge transport regime; (<b>b</b>) provide an alternative for mobility evaluation in systems where other techniques are hard to implement; (<b>c</b>) provide means of distinguishing between different transport mechanisms (e.g., hopping, band-like, or their combination); and (<b>d</b>) help disentangling the contributions of various carrier types or trap states. However, in many intrinsically low-mobility materials and devices, conventional (<i>dc</i>) Hall measurements are extremely hard to carry out due to typically weak Hall signals (and, thus, low signal-to-noise ratio).<br/>High-resolution <i>ac</i> Hall and photo-Hall measurements recently developed in our group have resulted in a significant progress in our understanding of the charge transport and photophysical properties of organic semiconductors and lead-halide perovskites.[1,2] In this talk, I will discuss a few examples of the transport physics studies using Hall-effect measurements. For instance, Hall measurements have led to elucidation of the role of grain boundaries in high-performance polycrystalline organic field-effect transistors (OFETs), where capacitively charged grain boundaries lead to an “underdeveloped” Hall effect.[3] In another example, the <i>intrinsic</i> mobility-strain relationship has been experimentally revealed in organic semiconductors for the first time, achieved by simultaneous measurements of FETs and Hall effect under a calibrated uniaxial strain in ultra-thin, flexible, single-crystal rubrene OFETs.[4] These measurements revealed an anisotropic and reversible modulation of charge mobility with strain. A photo-Hall effect has also been recently measured in organic semiconductors for the first time, with the developed model capable of correctly disentangling the mobilities and densities of the photogenerated mobile electrons and holes.[2] These photo-Hall effect measurements performed in high-mobility rubrene single crystals confirmed the older findings that photoconductivity in this material is due to the interaction of long-lived mobile triplet excitons with surface states [5,6]. Finally, high-resolution <i>ac</i>-Hall measurements have also proven very useful in the investigation of the intrinsic (trap-free) charge transport in epitaxial lead-halide perovskite FETs recently developed in our group.[7]<br/><br/><b>References: </b><br/><br/>1. Y. Chen, H. T. Yi and V. Podzorov, <b><i>Phys. Rev. Applied </i>5</b>, 034008 (2016).<br/>2. V. Bruevich, H. H. Choi and V. Podzorov, <b><i>Adv. Funct. Mater.</i></b>, DOI:10.1002adfm.202006178 (2020).<br/>3. H. H. Choi, A. F. Paterson, M. A. Fusella, J. Panidi, O. Solomeshch, N. Tessler, M. Heeney, K. Cho, T. D. Anthopoulos, B. P. Rand and V. Podzorov, <b><i>Adv. Funct. Mater.</i></b>, 1903617 (2019).<br/>4. H. H. Choi, H. T. Yi, J. Tsurumi, J. J. Kim, A. L. Briseno, S. Watanabe, J. Takeya, K. Cho and V. Podzorov, <b><i>Adv. Science</i></b>, 1901824 (2019).<br/>5. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, Observation of long-range exciton diffusion in highly ordered organic semiconductors, <b><i>Nature Mater.</i></b> <b>9</b>, 938 (2010).<br/>6. P. Irkhin, H. Najafov and V. Podzorov, Steady-state photoconductivity and multi-particle interactions in high-mobility organic semiconductors, <b><i>Sci. Reports </i>5</b>, 15323, DOI: 10.1038/srep15323 (2015).<br/>7. V. Bruevich, L. Kasaei, S. Rangan, H. Hijazi, Z. Zhang, T. Emge, E. Y. Andrei, R. A. Bartynski, L. C. Feldman, V. Podzorov, Intrinsic (Trap-Free) Transistors Based on Epitaxial Single-Crystal Perovskites. <b><i>Adv. Mater.</i></b> <b>34</b>, 2205055 (2022). https://doi.org/10.1002/adma.202205055.