High-efficiency tandem perovskite solar cells
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Introduction Metal-halide perovskites have garnered much attention for their rapid increase in single-junction record efficiencies to exceed 20%1–3 (see the Introductory article in this issue). However, they deserve further serious consideration as the sole entry in a unique class of solar cell materials: solutionprocessable large-bandgap materials with small energetic losses. Previously, materials have met two of these criteria but not all three. Polymers are solution-processable large-bandgap materials, and indium-gallium-phosphide (InGaP) is a largebandgap material with a small energetic loss.4 This classification makes metal-halide perovskites ideal for double-junction tandem cells. Silicon is a market-leading photovoltaic technology and will probably continue in this role for the near future as the technology continues expanding while lowering its cost structure. However, silicon’s record efficiency has only increased from 25.0% to 25.6% in the previous 15 years,5 asymptotically approaching its efficiency potential. As the overall cost of solar power shifts from a module dominated cost to a balance-of-systems dominated cost, improving the efficiency of installed modules becomes increasingly important. With greater efficiency, installing fewer modules reaches the same power target, reducing the balance-of-systems cost. A potential solution for improving the efficiency of modules is to make tandems.
Tandems split the solar spectrum into parts. An absorber is most efficient when absorbing photons with energy equal to its bandgap. Photons with higher energy are absorbed but lose excess energy as heat, called thermalization. Tandems minimize the amount of thermalization with multiple absorbers responsible for sections of the solar spectrum rather than a single absorber responsible for the entire spectrum (Figure 1). Single-junction solar cells are fundamentally limited to 33.7% efficiency, while double-junction tandems have a theoretical efficiency potential of 46.1%. A promising candidate for tandems is to use metal-halide perovskites to upgrade the performance of a commercially available solar cell, such as a silicon-based one.6–8 The solution processability of metalhalide perovskites provides the potential for a low upgrade cost to an existing manufacturing plant. Metal-halide perovskites may also improve the commercial viability of a technology close to mass commercialization, such as copper indium gallium selenide (CIGS)6 or copper zinc tin sulfide (CZTS).9
Tandem architectures There are three main architectures to consider when designing perovskite tandems: mechanically stacked (Figure 2a), monolithically integrated (Figure 2b), and spectrally split (Figure 2c). Mechanical stacking means that the top and bottom cells are fabricated independently, then assembled together in the module. In monolithic integration, all layers are sequentially
Colin D. Bailie, Materials Science and Engineering Department, Stanford University, USA; [email protected] Michael D. McGehee, Materials Science and Engineering Department, Stan
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