Quantification of spatial inhomogeneity in perovskite solar cells by hyperspectral luminescence imaging

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...

Section: Methods Descriptionmentioning

confidence: 99%

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

Nguyen

Gerritsen

Mahmud

et al. 2019

“…While the absolute locations of E F,e and E F,h are generally not accessible, the QFLS can be determined directly by means of absolute PL measurements . This methodology has been proven to be an efficient approach for quantifying recombination losses in the neat perovskite, multilayer assemblies or even complete perovskite solar cells …”

Section: Theorymentioning

confidence: 99%

On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells

Caprioglio

Stolterfoht

Wolff

et al. 2019

306

356

photoluminescence yields (>20%). [5] In principle, this would allow open-circuit voltages (V OC ) very close to the radiative limit (≈1.3 V for a bandgap of 1.6 eV) using already existing perovskites. However, despite the tremendous effort devoted by the scientific community on the improvement of this solar cell technology, the experimental efficiencies are still far from the Shockely-Queisser (S.Q.) theoretical predictions of power conversion efficiency (PCE) up to 30%. [6] Specifically, in order to further improve the PCE, the effort must be focused on increasing the V OC and the fill factor (FF) through the reduction of nonradiative recombination losses. Moreover, a better understanding on the predominant energy loss mechanisms in the working device has to be accomplished.Perovskite solar cells generally consist of a 300-500 nm layer of photoactive absorber, sandwiched between two charge transporting layers that have the function of selectively transporting the photogenerated electrons (holes) to the cathode (anode). In an ideal solar cell, all photons are absorbed in the perovskite films, generating electrons and holes with unity efficiency, and-under open-circuit conditions-the only recombination channel is the radiative recombination of free electrons and holes in the same layer where they are generated. Commonly, reported values for V OC are much lower due to unwanted nonradiative recombination. During the past years, many studies have evaluated recombination in perovskites layers and suggested that defects at the perovskite surface or at grain boundaries as possible reasons Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (V OC ) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the V OC on the very same devices, including pin-type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the V OC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the V OC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-V OC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the V OC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the V OC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-V OC relation is of great importance.       J J qV n k T radiative recombination current J rad...

“…Hyperspectral imaging is an alternative approach which compromises between luminescence imaging and spectroscopy. The technique utilizes one physical dimension of a camera chip ( X or Y ) as a physical dimension on the sample surface ( X or Y ), and the other physical dimension of the chip as a spectral dimension to provide a luminescence spectrum for each pixel on a sample . Thus, one, in principle, can extract an optical bandgap image, which is an image of peak locations of band‐to‐band luminescence spectra, from the sample using hyperspectral imaging tools.…”

Section: Introductionmentioning

confidence: 99%

Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells

Chen

Peng

Shen

et al. 2018

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...