01 · Chemistry
The perovskite crystal structure
A perovskite has the general formula ABX₃ where: A is a large organic or inorganic cation (methylammonium CH₃NH₃⁺, formamidinium HC(NH₂)₂⁺, or caesium Cs⁺); B is a smaller metal cation (Pb²⁺ or Sn²⁺); and X is a halide (I⁻, Br⁻, or Cl⁻). The B cation sits at the centre of an octahedron of X anions.
What makes the ABX₃ structure exceptional for photovoltaics:
- Tuneable direct bandgap — by varying the halide ratio (I/Br), the absorption edge shifts from ~1.2 eV (infrared) to ~2.3 eV (blue), enabling optimisation for the solar spectrum.
- High absorption coefficient — ~10⁵ cm⁻¹, meaning only ~300 nm of material is needed to absorb 90 % of above-bandgap light (versus ~200 µm for silicon).
- Long carrier diffusion lengths — electrons and holes travel micrometres before recombining, despite structural defects that would be catastrophic in silicon.
- Simple deposition — perovskite layers can be deposited from solution by spin-coating or slot-die printing at temperatures below 150 °C.
The ionic character of the crystal — unlike covalent silicon — also means the lattice can tolerate a high density of point defects without dramatically reducing efficiency. But it also means ion migration under electric field is a major cause of hysteresis and degradation in operating devices.
02 · Efficiency
From 3.8 % to 33 %: fifteen years of progress
The tandem approach stacks a perovskite top cell (~1.6–1.7 eV bandgap, absorbs blue/green photons) over a silicon bottom cell (~1.1 eV bandgap, absorbs red/near-IR photons). Each layer harvests a different portion of the solar spectrum, circumventing the Shockley–Queisser single-junction limit of ~33 % for silicon.
03 · Challenges
Why perovskites haven't taken over — yet
The stability problem is chemical. Each degradation pathway involves a bond-breaking or ion-migration process:
- Moisture — water molecules coordinate to Pb²⁺, forming monohydrate and dihydrate phases that convert irreversibly to PbI₂. Encapsulation is essential.
- Oxygen — O₂ oxidises the iodide surface under illumination, creating superoxide radicals that decompose the organic cation.
- Heat — methylammonium (MA) has a low decomposition temperature (~85 °C); MA-free formamidinium/caesium compositions are more thermally stable.
- Ion migration — halide ions migrate under electric field and illumination, creating concentration gradients that reduce open-circuit voltage and cause hysteresis.
Lead toxicity is also a serious concern. Although the total lead content in a perovskite module (~0.5 g/m²) is small, it is water-soluble and mobile in soil. Regulatory pathways in the EU require demonstrated recyclability or lead immobilisation before widespread deployment.
The thermodynamics of these degradation reactions — equilibrium between crystal phases, Arrhenius activation energies for ion migration, and the thermodynamics of phase transitions — are active research areas connecting fundamental physical chemistry to device engineering.
Perovskite solar cells — common questions
- Perovskite solar cells use crystals with ABX₃ structure — typically methylammonium lead iodide (MAPbI₃). They have exceptional light-harvesting properties. Unlike silicon (indirect bandgap), perovskites have direct bandgaps tuneable by composition, making them more efficient absorbers. See the Atomic structure guide for bandgap context.
- Perovskite cells degrade under moisture, oxygen, heat, and UV light — lifetime in the field remains shorter than commercial silicon (25 years). Lead content raises environmental and regulatory concerns. Tin-based alternatives are less toxic but less efficient and more sensitive to oxidation. Manufacturing scale-up reduces efficiency from lab records to commercial products.
- Single-junction perovskites: 26.1 % (NREL chart, 2024). Perovskite-silicon tandems: 33.2 % (Helmholtz Berlin, 2024). The Shockley–Queisser limit for a single junction at 1.1 eV (silicon bandgap) is ~29 %. Tandems overcome this by stacking two bandgap-complementary absorbers. Commercial tandem modules (LONGi, REC) reached 28–30 % in 2026.
- Photons above the bandgap energy excite electrons across the band gap, creating electron-hole pairs. The p-n junction provides a built-in electric field that separates charges. Electrons flow to the electron-transport layer (ETL) and holes to the hole-transport layer (HTL). The chemistry of these interfaces — particularly ion migration in perovskites — critically affects device stability.