Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells

Abstract

their optical and electrical properties. A common optical-based approach to estimate bandgaps of direct-gap semiconductors is to employ absorption spectroscopy. In this approach, one captures absorption spectra of materials and obtains absorption thresholds from Tauc plots. [3] The Tauc analysis provides a consistent way to compare among materials. However, as mentioned by Green et al., [4] these thresholds represent optical bandgaps, not electronic bandgaps, as the Tauc analysis underestimates the presence of excitonic transitions and broadening factors such as temperatures, defect densities, and material disorders. [4,5] Alternatively, one can capture the so-called band-to-band luminescence spectra from materials, which are results of radiative recombination between free electrons and holes in the conduction and valence bands, respectively. The peak wavelengths (or energies) of the luminescence spectra have been reported to be close to the absorption thresholds at room temperature by numerous authors [6][7][8][9][10] (also see Figure S1, Supporting Information). Thus, the two optical methods can be used to estimate the material optical bandgap although they often underestimate the true electronic bandgap. However, both of these approaches measure the spatially average optical bandgap over a certain area. Therefore, capturing spatially resolved optical bandgaps of the entire sample area requires point-by-point mapping in both X and Y directions, and thus it is time consuming and impractical for large-area device imaging or high-throughput production-line environments.In PV research and manufacturing, luminescence imaging, including both electroluminescence (EL) and photoluminescence (PL), is a mainstream characterization tool for crystalline silicon solar cells. [11][12][13][14][15][16][17] This camera-based method can provide fast diagnostics of PV materials and devices with micrometerscale spatial resolution. Recently, luminescence imaging has increased in significance for perovskite device and material characterization. Many works have extracted various device parameters of PSCs such as correlations between luminescence intensities and open-circuit voltages, [18][19][20] spatial distributions of nonradiative recombination centers, [21,22] series resistances from EL images, [23] and lateral inhomogeneities on each subcell in monolithic perovskite-silicon tandem structures. [24] It is also a powerful tool for monitoring long-term performance and A fast, nondestructive, camera-based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer-scale spatial resolution is developed. This imaging technique utilizes well-defined and relatively symmetrical band-to-band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same d...

Spatially and Spectrally Resolved Absorptivity: New Approach for Degradation Studies in Perovskite and Perovskite/Silicon Tandem Solar Cells

“…The PL intensity is determined by several mechanisms-light-induced trap deactivation, ion migration under light excitation, and actual degradation due to air exposure. [31] Thus, the spectral peak location and A are not affected. [30] Thus, location A (affected by air exposure initially) and location B (representative for areas unaffected by air exposure initially) have different PL intensity trends (Figure 6a).…”

“…Depending on the material and device quality at each location, its response rate to each mechanism varies. [31] However, under the air exposure, the material degrades and its PL spectrum shows a red shift and a reduction in intensity (see Figure S6, Supporting Information, for the data on bare perovskite films). Under the lightinduced trap deactivation and ion migration, the PL intensity is altered whereas the PL spectral shape is identical.…”

Loss Analysis in Perovskite Photovoltaic Modules

“…Perovskite modules are investigated for layer inhomogeneities using the described five-step characterization procedure. Two of the five steps in the process are based on perovskite luminescence images, where due to temporal changes in device luminescence, [37,[39][40][41][42][43] we used the same measurement method for all analyzed samples (see Supporting Information). The characterization procedure was used to detect and analyze inhomogeneities with the highest resolution of 10 μm.…”

“…In perovskite-based PV, there are multiple studies where photoluminescence (PL) mapping has been used to analyze heterogeneity in perovskite films on grain-size scales, including bandgap variations, charge-carrier dynamics, stability, and so forth. [37][38][39] Recently, Mundt et al [36] demonstrated a spatially resolved analysis of efficiency loss in perovskite solar cells (PSCs) based on dark lock-in thermography (DLIT) and multiwavelength light-beam induced current (LBIC). Local layer inhomogeneities were revealed to contribute major loss in cells with an active area of 1.1 cm 2 .…”

“…Thermography and luminescence‐based imaging techniques are well‐established and in‐line compatible characterization techniques, widely applied in commercialized PV technologies among solar cell production lines and module manufactures. In perovskite‐based PV, there are multiple studies where photoluminescence (PL) mapping has been used to analyze heterogeneity in perovskite films on grain‐size scales, including bandgap variations, charge‐carrier dynamics, stability, and so forth . Recently, Mundt et al demonstrated a spatially resolved analysis of efficiency loss in perovskite solar cells (PSCs) based on dark lock‐in thermography (DLIT) and multiwavelength light‐beam induced current (LBIC).…”