Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process

Abstract

Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley–Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck–Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.

Hybrid Halide Perovskites: Fundamental Theory and Materials Design

“…, where m * e , m * h and µ * are the electron, hole and reduced effective masses respectively. The exciton binding energy is calculated using the expression E b = µ * / 2 ∞ × ERy eV, with ERy = 13.60565 eV, the reduced effective mass µ * = 0.11 me and the high-frequency dielectric constant ∞ = 5.8. a b c (Davies et al, 2017) d e (Pérez-Osorio et al, 2015) f (Hirasawa et al, 1994) g (Poglitsch and Weber, 1987) h (Miyata et al, 2015) conduction band edges exhibit a parabolic profile over a range of 300 meV (FIG. 4b).…”

“…The parabolicity of the conduction and valence band edges has important implications in the understanding of both absorption and recombination mechanisms in CH 3 NH 3 PbI 3 . Using a combined experimental and theoretical analysis of the optical absorption lineshape, (Davies et al, 2017) showed that the optical absorption spectrum of CH 3 NH 3 PbI 3 can be modelled using the Elliot's theory (Elliott, 1957) up to 100 meV above the absorption onset. Within this premise, (Davies et al, 2017) decoupled the excitonic and continuum part of the absorption spectrum, and calculated the bimolecular recombination rate using the van Roosbroek-Shockley formalism (van Roosbroek and Shockley, 1954).…”

“…Within this premise, (Davies et al, 2017) decoupled the excitonic and continuum part of the absorption spectrum, and calculated the bimolecular recombination rate using the van Roosbroek-Shockley formalism (van Roosbroek and Shockley, 1954). By comparing this model to direct transient spectroscopic measurements, (Davies et al, 2017) demonstrated that bimolecular recombination in CH 3 NH 3 PbI 3 , is an inverse absorption process. This result reinforces the conclusion that CH 3 NH 3 PbI 3 is in many respects very similar to conventional semiconductors, such as GaAs (Herz, 2016).…”

“…The quasiparticle band structure is represented by the blue lines, the grey background is the DFT/LDA band structure. The quasiparticle band structure was obtained by performing a Wannier interpolation of the quasiparticle eigenvalues calculated for a 4 × 4 × 4 Γ-centered grid, using the GW and the self-consistent scissor scheme (Davies et al, 2017;Filip et al, 2015). b.…”

“…The summation over the crystal momenta is performed by discretizing the Brillouin zone on a Γcentered grid of 40×40×40 points. The DFT and GW eigenvalues are obtained on this grid from Wannier interpolation (Davies et al, 2017;Filip et al, 2015). The model joint densities of states (light blue line for GW and grey for DFT) are calculated using the assumption that the bands are parabolic using, J DFT/GW (ω) = 1/4π 2 (2µ DFT/GW / 2 ) 3/2 (ω − Eg) 1/2 , where µ is the reduced effective mass calculated from DFT (0.06 me) or GW (0.11 me).…”