Maximizing the power generated per unit area can expedite the deployment of photovoltaic (PV) systems, as the cost distribution of PV systems is now predominantly driven by system balance components (such as installation systems, wiring, labor, and inverters), rather than the cost of PV panels themselves. These system balance costs roughly scale with the installation area and favor PV technologies with a high power-to-panel area ratio. However, crystalline silicon (C-Si) solar cells have reached a maximum power conversion efficiency (PCE) of 26.8%, approaching the theoretical limit of 29.5%. Under sunlight, the only verified way to overcome this PCE limitation is by combining several complementary light-active materials (multiple junctions) within a single device. Among the various types of multi-junction designs reported to date, the combination of c-Si with metal halide perovskites has been a focus of research in tandem solar cells because of its potential for high PCE and low manufacturing cost.
Metal halide perovskites exhibit several key properties suitable for efficient tandem photovoltaics, including high absorption coefficients, sharp absorption edges, bipolar charge transport with long diffusion lengths, and tunable bandgaps (Eg). Thin-film perovskite solar cells can be directly deposited on the front of c-Si cells to reduce thermalization losses and expand the achievable PCE range to >30%. The performance potential of single-junction two-terminal tandem structures has been demonstrated with reported PCEs as high as 33.7% on a 1-square-centimeter irradiation area. Most high-efficiency tandem cells reported to date use a single Si wafer with its front surface mechanically or chemically polished or featuring an adaptive sub-micron texture, typically on the scale of 500 nanometers to 1 millimeter. This planar or nanotextured front-topological structure—often created by etching pyramids several micrometers in height, a common preparation in the PV industry—allows the deposition of pinhole-free perovskite films from solution on the substrate. However, this modification comes at the cost of optical performance, as the front of tandem cells is flat, and, when submicron Si textures are used, the perovskite film is flattened by nonuniform solvent processing due to the lack of capillary effect. As a result, these cell designs present more reflection losses at the front of the tandem device due to the absence of the bounce-back effect. Overall, tandem devices featuring pyramid-textured fronts can limit reflection losses because they can absorb light reflected back from nearby pyramids, while the Si chip on both sides is textured to enhance infrared light absorption.
We previously reported a hybrid two-step deposition method that combines thermal evaporation and spin-coating to enable perovskite layers to cover microscale Si pyramids, achieving coverage in perovskite/c-Si tandem cells with texture on both the rear and front surfaces. While these tandem cells have higher photocurrents due to the front pyramid texture, nonradiative recombination losses are quite significant. One challenge is that most reported top surface passivation methods to date are not directly applicable to microscale textures because they involve depositing nanoscale organic layers from liquid solutions. Moreover, these processing routes often yield nonuniform (incomplete) coatings on such textured surfaces.
Key Innovations
The core innovation of this study lies in achieving a high conversion efficiency of up to 31.25% in perovskite/C-Si tandem solar cells. This accomplishment was made possible by using silicon wafers with micro-scale texturing, optimizing the perovskite deposition process, and employing phosphate groups for interface passivation. These measures successfully mitigated non-radiative recombination losses.
Overview of Data
Key Insights
This study identifies and mitigates non-radiative recombination losses occurring at the interface of perovskite/c-Si tandem solar cells on silicon wafers with micrometer-scale texturing, an industry standard in c-Si photovoltaics. The use of Me-4PACz reduces voltage losses at the perovskite/HTL interface, while the addition of FBPAc in the perovskite deposition sequence reduces voltage losses at the perovskite/C60 ETL interface, resulting in a more favorable perovskite microstructure with larger crystalline domains. XPS and SIMS imaging reveal the presence of FBPAc on the top surface of the perovskite, where it coordinates with lead defects in the perovskite through its phosphate groups. Overall, the combination of micrometer-scale textured c-Si, a 1-millimeter-thick perovskite absorber layer uniformly deposited on this texture using a hybrid two-step method, and phosphate groups on both sides of the absorber layer, improving interface passivation, yields a independently certified 31.25% PCE tandem solar cell. These results demonstrate how to upgrade c-Si solar cells with standard industrial micrometer-scale texturing to achieve PCEs greater than 30%.
Original Article: Xin Yu Chin et al. Interface passivation for 31.25%-efficient perovskite/silicon tandem solar cells. Science 381, 59-63 (2023). DOI: 10.1126/science.adg0091.
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