Physics / mechanism
Tandem solar cells stack two absorber layers with complementary bandgaps to capture a broader slice of the solar spectrum than any single junction permits. Silicon bottom cells (~1.1 eV bandgap) absorb red/NIR photons; perovskite top cells (~1.6–1.8 eV) absorb blue/green photons. Each sub-cell operates near its own Shockley-Queisser limit, and the stack exceeds single-junction theoretical efficiency (~33%) with a combined ceiling ~47%. Practical two-terminal monolithic devices require current-matching between sub-cells, a critical engineering constraint. Lab records now sit at ~33.9% PCE (LONGi, 2023); commercial modules are targeting >30% with 25-year stability still unproven. Key technical challenges: perovskite lead toxicity, moisture/thermal degradation, and scalable deposition (perovskite via slot-die or vapour on textured Si).
Competitive landscape
Competing approaches include III-V multijunctions (GaAs/InGaP), which reach 47%+ under concentration but cost 100× more—irrelevant for utility-scale PV. All-perovskite tandems (wider top + narrower bottom perovskite) eliminate silicon but face worse stability. Organic PV and CIGS remain single-junction. The real competitive tension is against high-efficiency monocrystalline silicon (TOPCon, HJT) hitting 26–27% at scale; tandem must justify its complexity premium.
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Frontier (open questions)
- To be added.