Tracking Photoexcited Carriers in Hybrid Perovskite Semiconductors: Trap-Dominated Spatial Heterogeneity and Diffusion

Abstract

We use correlated confocal and wide-field fluorescence microscopy to probe the interplay between local variations in charge carrier recombination and charge carrier transport in methylammonium lead triiodide perovskite thin films. We find that local photoluminescence variations present in confocal imaging are also observed in wide-field imaging, while intensity-dependent confocal measurements show that the heterogeneity in nonradiative losses observed at low excitation powers becomes less pronounced at higher excitation powers. Both confocal and wide-field images show that carriers undergo anisotropic diffusion due to differences in intergrain connectivity. These data are all qualitatively consistent with trap-dominated variations in local photoluminescence intensity and with grain boundaries that exhibit varying degrees of opacity to carrier transport. We use a two-dimensional kinetic model to simulate and compare confocal time-resolved photoluminescence decay traces with experimental data. The simulations further support the assignment of local variations in nonradiative recombination as the primary cause of photoluminescence heterogeneity in the films studied herein. These results point to surface passivation and intergrain connectivity as areas that could yield improvements in perovskite solar cells and optoelectronic device performance.

Local Crystal Misorientation Influences Non-radiative Recombination in Halide Perovskites

“…As stated earlier, understanding grain boundary properties has been critical to the development of many other photovoltaic semiconductor technologies such as CdTe, GaAs, InP, and Cu(In1-xGax)Se2, [22][23][24][25][26][27][28][29] and indeed other materials, such as high temperature polycrystalline superconductors. 45,46 We anticipate that understanding the nature of the different grain boundaries identified by EBSD will be critical to understanding the properties of halide perovskite photovoltaics, 21 and we speculate that properties such as anisotropic charge carrier transport across grain boundaries observed in halide perovskites 13,[47][48][49] could depend on grain boundary properties such as misorientation angle, type of boundary, and the boundary interface energy.…”

“…Our group has previously demonstrated that confocal PL maps, when measured at excitation densities below the local trap density (~10 15 cm -3 to ~10 17 cm -3 ), 12,[53][54][55] can reflect the local trap distribution in the film. 12,13 Although carrier diffusion is still important, 13,56 under such conditions, high PL intensities correspond to regions with lower trap density, and low PL intensities to regions with higher trap density. 13…”

Imaging and Mapping Characterization Tools for Perovskite Solar Cells

“…The findings revealed variations in local nonradiative decay rates. [139] PL intensity heterogeneity was shown when comparing confocal and widefield fluorescence microscopy methods. [132] Also, a relationship between grain boundaries and PL quenching was evidenced contradicting positive results about grain boundaries.…”

“…Specifically, local steady-state and time-resolved measurements show that darker regions (lower PL intensity) have greater nonradiative loss. [139] From this method, diffusion of carriers was observed outside the confocal site and showed there are intergrain transport variations. [146,147] PL intensity decreased around 65% in these regions.…”

“…Furthermore, mapping of the PL has also been applied to characterize the homogeneity/heterogeneity of crystalline quality of semiconductor wafers, [130] to PSCs. [139] Micro photoluminescence (µ-PL) mapping has diverse advantages due to its high-spatial resolution and simple setup that does not need a special environment such as low temperature, high vacuum or controlled atmosphere to perform the measurement. [139] Micro photoluminescence (µ-PL) mapping has diverse advantages due to its high-spatial resolution and simple setup that does not need a special environment such as low temperature, high vacuum or controlled atmosphere to perform the measurement.…”