Abstract
The high open-circuit voltage and the slow recombination in lead-halide perovskite solar cells has been one of the main contributors to their success as photovoltaic materials. Here, we review the knowledge on recombination in perovskite-based solar cells, compare the situation with silicon solar cells, and introduce the parameters used to describe recombination and open-circuit voltage losses in solar cells. We first discuss the effect of lifetimes and surface recombination velocities on photovoltaic performance before we study the microscopic origin of charge-carrier lifetimes. The lifetimes depend on defect positions and densities and on the kinetic prefactors that control the phonon-assisted interaction between the extended states in the conduction and valence band and the localized defect states. We finally argue that the key to understand the long lifetimes and high open-circuit voltages is a combination of a low density of deep defects and a slow dissipation of energy via multiphonon processes due to the low phonon energies in the lead-halide perovskites.
Highlights
If we imagine radiative recombination to happen within the bulk of our solar cell absorber material, the photon created by that recombination event may either contribute to the externally measured luminescence that defines Jem or be reabsorbed within the device
Understanding the open-circuit voltage of a solar cell is a challenge that requires a deep understanding of the different recombination mechanisms that occur in the bulk and at surfaces of the absorber material
In the case of lead-halide perovskites, the non-radiative recombination is relatively slow compared to the radiative recombination. This is a desirable condition for a solar cell because it allows low thicknesses for full absorption due to the fast radiative recombination, while non-radiative decay does not add huge additional losses in the voltage at open circuit or the maximum power point
Summary
Lead-halide perovskites have recently received attention from various scientific communities due to their peculiar optoelectronic properties.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] In particular, in the case of photovoltaic technologies, efficiency has so far been strongly correlated with the energy, temperature, and cost invested into preparing wafers, films, or devices. Equation (4) assumes that radiative recombination is a bimolecular process proportional to the product of concentrations of free electrons and holes This assumption is certainly valid in all solar cells where luminescence originates from delocalized band like states that are typically far away from the quasi-Fermi levels at the relatively moderate injection conditions used for photovoltaic operation. While we have learned how to relate the external luminescence quantum efficiency Qelum to the open-circuit voltage V oc and the internal luminescence quantum efficiency Qilum to experimentally observable quantities like charge carrier lifetimes, τ, and recombination coefficients, k, we have not yet discussed a connection between the internal and external quantities. If we imagine radiative recombination to happen within the bulk of our solar cell absorber material, the photon created by that recombination event may either contribute to the externally measured luminescence that defines Jem or be reabsorbed within the device. We will use this terminology and apply it to the case of lead-halide perovskite based solar cells
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