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

Abstract

a low-cost, industry-scalable photovoltaic (PV) technology. Single-junction PSCs have reached a light-to-electricity conversion efficiency of 25.2%. [1] Concurrently, perovskite/silicon tandem structures are attracting significant attention from the PV community as one of the most promising pathways to overcome the fundamental limit of single-junction solar cells. Monolithic perovskite/silicon tandem cells have achieved a record efficiency of 28% which is higher than that of crystalline silicon (c-Si) solar cells. [1] However, a major challenge for PSCs is their sensitivity to the surrounding environment and operating conditions. Light, oxygen, temperature, humidity, electric field, and more can all induce degradation in PSCs. [2][3][4][5][6] Therefore, an insightful understanding of the degradation mechanisms of PSCs is critical for the development of stable perovskite and perovskite/silicon tandem solar cells.The most direct way to observe degradation is to monitor finished solar cell performance through standard PV parameters including external/internal quantum yields, short circuit currents, open circuit voltages, fill factors, and efficiencies. [7][8][9][10] However, these are global parameters and thus cannot give information about how different degradation mechanisms evolve across the solar cells. Also, this approach requires a complete cell structure. Another common approach is to monitor photoluminescence (PL) or electroluminescence (EL) intensities of the devices via both mapping and imaging tools. [11][12][13][14][15] This approach yields spatial information and does not require a complete device (in case of PL measurements). However, luminescence intensity can be confounded by several competing mechanisms. Under excitation, there are two phenomena simultaneously happening and affecting the luminescence intensityion migration and light-induced trap deactivation. [16][17][18][19] The response rate to each phenomenon varies depending on initial qualities of the samples and also during the degradation process. [16][17][18][19] These two phenomena often obscure the detected luminescence signal during the intended degradation tests, Instability in perovskite solar cells is the main challenge for the commercialization of this solar technology. Here, a contactless, nondestructive approach is reported to study degradation across perovskite and perovskite/silicon tandem solar cells. The technique employs spectrally and spatially resolved absorptivity at sub-bandgap wavelengths of perovskite materials, extracted from their luminescence spectra. Parasitic absorption in other layers, carrier diffusion, and photon smearing phenomena are all demonstrated to have negligible effects on the extracted absorptivity. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap, and intensity. The technique is applied to study various common factors which induce and accelerate degradat...

Revealing the internal luminescence quantum efficiency of perovskite films via accurate quantification of photon recycling

“…These results corroborate four important aspects: (1) the proposed conception of PL emission (Figure 2A) indeed results in a variety of different PL spectra (red-shifted/ broadened/asymmetric) depending on the magnitude of scattering, which is set by the inverse scattering length (l À1 s ) in the simulations (see further examples in Figure S20 and Table S4); (2) in line with intensity-dependent measurements of E exp tot ðlÞ (Figure S18), 58 the spectral shape of E sim tot ðlÞ is independent of Q lum i and thus is the value of p e (see discussion in Note S5); (3) E exp tot ðlÞ can be fitted with Equation 17to accurately determine Q lum i and p e and to give a good approximation for p eÀd and p eÀs ; and (4) the fitted value of z av is inversely proportional to l s and thereby, considering scattering mainly takes place at interfaces, allows comparison of the magnitude of scattering for different samples by relating it to the film thickness and defining the dimensionless variable M s = d zav .…”

“…Considering the fundamental interdependence of absorptance and emissivity, 52,53 here we summarize a comprehensive procedure detailing how to extract the absorption coefficient aðl; TÞ from the confocal PL spectra that allows determination of the internal PL spectrum Iðl; TÞ considering the generalized Planck's law. Similar procedures have previously been employed for Si [54][55][56] and perovskite films [57][58][59][60] and more details are provided in Note S4. For ease of comparison with externally observed PL spectra, we define Iðl; TÞ as the shape of the internal PL spectrum that would be observed in case the refractive index is nðlÞ = 1 52,53 : Iðl; TÞzb,4 bb ðl; TÞ,aðl; TÞ;…”

“…We stress that Equation 6 is only valid if there is no emission from other layers in the layer stack and the shape of Eðl; TÞ is unaffected by carrier diffusion and scattering of initially trapped PL (see further discussion in Note S4). 58 At the high-energy side of the spectrum, aðl; TÞ can be considered approximately constant 46,54,58 and, therefore, a fit to the confocal PL spectra with Eðl; TÞ = b,l À4 , À expð À hc=ðl$kTÞÞ À 1 Á À1 (with b and temperature T as free parameters) allows extracting T as shown in Figure S11a. At low energies, aðl; TÞ is governed by the temperature-dependent Urbach energy E U ðTÞ (i.e., the slope of the exponential decrease in absorption below the bandgap) where in good approximation aðl; TÞfaðl; TÞfexp À ðhc=ðl$E U ðTÞÞ Þ.…”

Mechanisms and Applications of Steady-State Photoluminescence Spectroscopy in Two-Dimensional Transition-Metal Dichalcogenides

“…In fact, the applications of steady-state PL spectroscopy to predict the performance of solar cells have been widely known and well developed for the conventional (silicon − and CIGS , ) and emerging (perovskites − ) photovoltaic materials. Combining spectrally resolved absorption, reflection, and PL emission from the materials, one can calculate both the voltage and internal/external QY potentially achieved.…”

Contactless Series Resistance Imaging of Perovskite Solar Cells via Inhomogeneous Illumination

“…ELi and PLi have been used to study charge‐carrier recombination and resistive losses, [ 36 ] detect pinholes, [ 37 ] assess the quality and uniformity of the perovskite layer, [ 14,38 ] and so on. [ 39,40 ] They have been also used to image i V OC , [ 41–45 ] spatial variation of the optical bandgap, [ 46 ] current transportation, [ 12,44 ] and R s . [ 47,48 ]…”