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Contrasting temperature dependence of band gap in CH3NH3PbX3 (X=I, Br, Cl) : insight from lattice dilation and electron-phonon coupling

According to the ITRPV (Eleventh Edition, April 2020) roadmap, Passivated Emitier Rear Contact (PERC) cell technology is expected to gain a market share > 70% in 2020 with Aluminium Back Surface Field (Al-BSF) cells technology trailing far behind at 20%. In September 2019, Canadian solar, one of the world’s largest solar cell manufacturing company set a world record of 22.80% conversion efficiency for p-type large area (246.7 cm2 ) multi-crystalline PERC solar cell.

Although hybrid halide perovskites (MAPbX3 , MA=CH3 NH3 , and X=I, Br, Cl) have been ubiquitously explored from the photovoltaic perspective, there are still few unanswered questions which require a more fundamental understanding. One such unsetiled issue is the puzzling behavior of band gap. Unlike conventional semiconductors, MAPbX3 (X=I, Br) is found to show a blue shiti (increase) in the band gap (Eg) with increasing temperature (T), while MAPbCl3 shows an unusual red shiti (decrease). In order to understand this, we performed a detailed T-dependent study of electronic, optical and structural properties of MAPbX3 combined with the state of the art first principles calculatikons. With increasing T, two dominant mechanisms which come into play are latice dilation and electron phonon coupling (EPC). The former (later) is responsible for increase (decrease) in Eg. We found that latice dilation effect dominates in MAPbX3 (X=I, Br), causing an enhancement in Eg. EPC involves various contributions, of which the interaction of charge carriers with the longitudinal optical (LO) phonon mode via Frohlich interaction is the most dominant one at room T. We quantify this contribution using Frohlichs theory of large polarons and show that the Eg correction due to this effect in MAPbCl3 is almost double as compared to MAPbBr3? and thus explain the reduction in Eg for the former.

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