Physics

Dr. Dinesh Kabra
IIT Mumbai

Metal halide perovskite semiconductors have become a potential candidate for optoelectronic device community ^[1] and efforts from academic and industries laboratories are underway for deployment, in particular for photovoltaic applications ^[2]. Outside environment is different from laboratory condition and in-depth understanding of temperature dynamics of these emerging materials is very essential. In this talk, we show the interplay of structural dynamics to electronic states of perovskite semiconductors, for which temperature coefficient of bandgap is positive.^[3] The temperature dependence of the optical properties of methylammonium lead iodide (MAPbI₃ = CH₃NH₃PbI₃) from room temperature to 6K. In both, the tetragonal (T> 163K) and the orthorhombic (T<163K) phase of MAPbI3, the band gap (from both ab-sorption and photoluminescence (PL) measurements) de-creases with decrease in temperature - in contrast to what is normally seen for many inorganic semiconductors, like; Si, GaAs, GaN etc. We show that in the perovskites reported here, the temperature co-efficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton linewidth allows to distinguish between static and dynamic disorder.^[3] The low energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modelled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton linewidth at low tem-perature is small. Above 60 K, thermal disorder increases the linewidth. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

References:

1.     “Quantitative Correlation of Perovskite Film Morphology to Light Emitting Diodes Efficiency Parameters” N. K. Kumawat, N. Jain, A. Dey, K. L. Narasmhan, D. Kabra Adv. Func. Mater (2016) (DOI: 10.1002/adfm.201603219)

2.      “On the uniqueness of ideality factor and voltage exponent of perovskite-based solar cells” S. Agarwal, M. Seetharaman, N. K Kumawat, A. S Subbiah, S. K Sarkar, D. Kabra, M. AG Namboothiry, P. R Nair J. Phys. Chem. Lett. Vol. 5, pp- 4115 (2014)

3.      “Effect of Thermal and Structural Disorder on the Electronic Structure of Hybrid Perovskite Semiconductor CH₃NH₃PbI₃” S. Singh, C. Li, F. Panzer,K. L. Narasimhan, A. Graeser, T. P. Gujar, A. Köhler, M. Thelakkat, S. Huettner, and D. Kabra J. Phys. Chem. Lett. Vol. 7, pp- 3014 (2016)