Abstract
We investigate the possible formation of polarons in four organic perovskites (CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, and CH3NH3PbI2Cl1) using a density functional theory (DFT) calculations with local potentials and hybrid functionals. We show that DFT+U method with U = 8 eV predicts a correct band-gap and matches the forces on ions from hybrid calculations. We then use the DFT + U approach to study the effect of polarons, i.e. to search the configuration space and locate the lowest energy localized band gap state self-trapped hole (STH). STH configurations were found for three pure halides and one mixed halide system. Spin orbit coupling (SOC) was also taken into account and the results may be found in the supplementary material. This study focuses on the +U method; however, SOC corrections added to the DFT+U calculations also resulted in STH states in all four systems.
Highlights
Organohalide lead perovskites offer a tantalizing prospect for novel, high-efficiency, low-cost solar cells.[1,2,3,4,5,6] Despite the rapid advancement of optoelectronic applications of organohalide lead perovskites, the fundamental understanding of the transport properties in these materials is still missing
It is unclear as to: i) why these polycrystalline materials show remarkably long minority carrier diffusion length,[7,8] and ii) why do theoretical calculations predict charge carrier mobility much higher[9,10] than what is measured experimentally?11–13 If the mobility of carriers is low, it is unclear why the recombination rates are below the Langevin recombination limit by several orders of magnitude.[14]
To account for the discrepancies seen with the ground state properties of complex perovskites, more adequate exchange-correlation functionals may be used such as Green’s functions (GW),[21] B3LYP,[22] or HSE23 hybrid functionals
Summary
Organohalide lead perovskites offer a tantalizing prospect for novel, high-efficiency, low-cost solar cells.[1,2,3,4,5,6] Despite the rapid advancement of optoelectronic applications of organohalide lead perovskites, the fundamental understanding of the transport properties in these materials is still missing. (Received 22 October 2016; accepted 4 December 2016; published online 16 December 2016)
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