Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually 'High'? (1510.08927v1)

Abstract: We present an experimental and theoretical viewpoint on the electronic carrier mobilities of typical hybrid organic-inorganic perovskites (HOIPs). While these mobilities are often quoted as high, a review of them shows that although otherwise the semiconducting properties of HOIPs are impressively good, mobilities of HOIPs used in most solar cells are actually not that high. This is especially apparent if they are compared to those of inorganic semiconductors used in other high efficiency solar cells. We critically examine possible causes and focus on electron-lattice coupling mechanisms that are active at room temperature, and can lead to carrier scattering. From this, we propose scattering due to acoustic phonons or polarons as possible causes, but also point out the difficulties with each of these in view of additional experimental and theoretical findings in the literature. Further research in this direction will contribute to making HOIP solar cells even more efficient than they already are.

Charge Carrier Mobilities in Hybrid Organic-Inorganic Perovskites: A Critical Evaluation

The paper "Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually 'High'?" presents a nuanced examination of charge carrier mobilities within hybrid organic-inorganic perovskites (HOIPs), with a focus on their comparison to traditional inorganic semiconductors. The paper raises significant questions regarding the perceived "high" mobilities in HOIPs, particularly highlighting a disparity when juxtaposed against standard inorganic materials like silicon (Si) and gallium arsenide (GaAs).

Charge Transport Properties of HOIPs

HOIPs have garnered considerable interest due to their superior performance in photovoltaic applications, attributed largely to long charge-carrier diffusion lengths and significant carrier lifetimes. These metrics are usually seen as indicative of excellent semiconducting properties. However, the authors argue that the carrier mobilities, a critical parameter for efficient electronic transport, warrant a more critical comparison with traditional inorganic semiconductors.

Comparative Analysis

The charge carrier mobilities in HOIPs, particularly electron and hole mobilities, are measured to be substantially lower than those in notable inorganic photovoltaic materials. For instance, the electron mobility in GaAs is approximately 8000 cm²/Vs and in Si about 1500 cm²/Vs, whereas typical HOIPs exhibit an order of magnitude lower in mobilities. The effective masses of carriers in HOIPs, similar to Si and GaAs, point towards scattering mechanisms rather than intrinsic material limitations influencing these mobility values.

Electron-Phonon Coupling and Mobility

Through an examination of material properties, the authors hypothesize that scattering due to acoustic phonons could primarily account for the observed mobility constraints. The mechanical softness of HOIPs, as evidenced by their low bulk and Young’s moduli, supports the presence of significant electron-phonon interactions. This is elucidated by temperature-dependent mobility studies, indicating T^-1.3 to T^-1.6 dependence typical of phonon scattering.

Polaronic Effects and Further Investigations

There is also a consideration for polaronic phenomena, which might contribute to charge-carrier scattering. Such effects could potentially reconcile the long lifetimes observed with these modest mobilities by increasing effective masses through polar lattice fluctuations. However, these aspects remain under-explored, with the need for more direct experimental validation under varying conditions, such as illumination.

Implications and Future Work

The paper suggests that existing discrepancies between theoretical predictions and experimental findings underscore a complex interaction of various scattering mechanisms. The authors propose further extensive experimental work and refined theoretical models to unravel these mechanisms, potentially leading to enhancements in mobility and photovoltaic efficiency of HOIPs.

In summary, while HOIPs exhibit some exceptional properties for photovoltaic applications, understanding and improving their charge transport properties require a deeper investigation into their phononic and polaronic behavior. This paper challenges the prevailing assumptions of high mobility in HOIPs compared to traditional semiconductors, setting the stage for future research aimed at optimizing these promising materials.