Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. As the Shockley-Queisser limit is approached, the recombination and mobility of charge-carriers will be limited only by intrinsic properties. Yet, the mechanisms for these parameters is still poorly understood.

We show that bimolecular (band-to-band) recombination of charge-carriers in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption [1]. The sharpening of photon, electron and hole distribution functions therefore significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are also strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant [2]. By investigating films whose thickness varies between 50 and 533 nm, we show that the bimolecular charge recombination rate indeed appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8×10⁻¹⁰cm³s⁻¹ is common to all samples irrespective of film thickness [2]. Using such models, we examine the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers.

Finally, we examine the prospect of such highly performing hybrid lead iodide perovskites in solar concentrator environments [3]. We theoretically predict solar cell performance parameters as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. We demonstrate that in the absence of degradation, perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages.

[1] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz, Nature Communications 9, 293 (2018).

[2] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz, Nano Lett. 17, 5782 (2017).

[3] Q. Lin, Z. Wang, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Science 5 (2018) DOI: 10.1002/advs.201700792.