Impact of Cation Multiplicity on Halide Perovskite Defect Densities and Solar Cell Voltages

Abstract

Metal-halide perovskites feature very low deep-defect densities, enabling thereby high operating voltages on solar cell level. Here, by precise extraction of their absorption spectra, we find that the low deep-defect density is unaffected when Cs + and Rb + are added during the perovskite synthesis. By comparing single-crystals and polycrystalline thin-films of methyl ammonium lead iodide/bromide, we find these defects to be predominantly localized at surfaces and grain boundaries. Furthermore, for the most important photovoltaic materials, we demonstrate a strong correlation between their Urbach energy and open-circuit voltage deficiency on the solar cell level. Via external quantum yield photoluminescence efficiency measurements, we explain these results as a consequence of non-radiative open-circuit voltage losses in the solar cell.Finally, we define practical power conversion efficiency limits of solar cells by taking into account the Urbach energy.

Are Defects in Lead-Halide Perovskites Healed, Tolerated, or Both?

“…12 The difference with the single-crystal values can be attributed to surface defects (including those at interfaces and grain boundaries). While this rationalization has been questioned (see below), it is consistent with results from passivation experiments, 61 especially for high surface area nm-scale particles.…”

Nanoscale Structural Heterogeneity and Efficient Intergrain Charge Diffusion in a Series of Mixed MA/FA Halide Perovskite Films

“…These developments represent a new phase in perovskite solar cell research. Currently, the promising material appears to be the triple-cation (containing Cs, MA, and FA, with high FA ratio) perovskite with mixed anions (I and Br). , The goal of systematically mixing the ratios , of these materials is to improve both the performance and ambient stability, but there are still several major drawbacks − to their practical use. These materials are largely limited by the grain boundaries , and by the presence of defects ,,, that can trap photogenerated charge carriers ,, and lead to their nonradiative recombination ,− and structural heterogeneity. ,, …”

“…2,3 The goal of systematically mixing the ratios 5,6 of these materials is to improve both the performance and ambient stability, but there are still several major drawbacks 7−22 to their practical use. These materials are largely limited by the grain boundaries 13,14 and by the presence of defects 8,9,15,22 that can trap photogenerated charge carriers 10,11,16 and lead to their nonradiative recombination 12,17−20 and structural heterogeneity. 15,16,21 Fluorescence microscopy in combination with other techniques has served as a powerful tool to find nanoscale structure−function relationships in a variety of materials, 23 including perovskite thin films, 24,25 and has provided insight into potential sources of nonradiative loss 20,21 and heterogeneity in perovskites.…”

Unique Curve for the Radiative Photovoltage Deficit Caused by the Urbach Tail

“…Recently the influence of the Urbach tail on the conversion efficiency has been amply discussed for different photovoltaic technologies ,− and particularly for the metal halide perovskites. − The effect of the Urbach tail impacts mainly the voltage, since the generation of current from the sub-band gap levels is negligible. It is observed that the voltage deficit increases with increasing Urbach energy for a variety of technologies, as shown in Figure and in ref ; see also refs and .…”