Perovskite Solar Cells

Perovskite solar cells (PSCs) are third-generation photovoltaic devices that utilize perovskite-structured materials, typically hybrid organic-inorganic lead or tin halides (chemical formula ABX₃), as light-absorbing layers. First developed in 2009 with an initial efficiency of 3.8%, PSCs have achieved certified efficiencies exceeding 26% by 2025. These cells are characterized by high absorption coefficients, tunable bandgaps, and low-temperature solution-based manufacturability. Research focuses on improving stability, scalability, and mitigating environmental risks associated with lead content. Global deployment initiatives include building-integrated photovoltaics (BIPV) and tandem configurations with silicon.

PSCs originated from dye-sensitized solar cells (DSSCs), with Miyasaka et al. first demonstrating methylammonium lead iodide (CH₃NH₃PbI₃) as a sensitizer in 2009 . Liquid electrolytes initially caused perovskite dissolution, limiting stability. Breakthroughs in 2012 replaced liquid electrolytes with solid hole-transport materials (e.g., spiro-OMeTAD), enabling efficiencies near 10% and enhanced durability. By 2015, formamidinium lead iodide (H₂NCHNH₂PbI₃) emerged as a superior absorber due to its narrower bandgap (1.48 eV), closer to the Shockley-Queisser optimal limit of 1.34 eV.

• 2016: Tin-based perovskites (e.g., CH₃NH₃SnI₃) explored as lead-free alternatives, though instability from Sn²⁺ oxidation limited efficiencies to <10%.
• 2021: Single-junction PSCs reached 25.8% efficiency using formamidinium-rich compositions.
• 2024: Strain-regulation techniques using phenylselenenyl chloride improved operational stability under thermal cycling, achieving certified efficiencies of 26.3%.

The ABX₃ lattice comprises:

• A-site: Organic cations (methylammonium, formamidinium) or cesium (Cs⁺).
• B-site: Divalent metals (Pb²⁺, Sn²⁺).
• X-site: Halogens (I⁻, Br⁻, Cl⁻).
• Goldschmidt’s tolerance factor (t) and octahedral factor (μ) determine structural stability, with optimal ranges of 0.8–1.0 and 0.44–0.90, respectively.

Photoexcitation generates electron-hole pairs (excitons) with low binding energy (<50 meV), enabling efficient separation at interfaces. Electrons inject into electron transport layers (ETLs) like SnO₂ or TiO₂, while holes move through hole transport layers (HTLs) such as spiro-OMeTAD. Long carrier diffusion lengths (>1 μm) reduce recombination losses.

Tandem cells combining perovskite with silicon or CIGS absorbers capture broader solar spectra:

• Perovskite/silicon tandems: 29.8% efficiency (2021).
• All-perovskite triple-junction cells: 33.1% efficiency (2024).

• Lead-based PSCs: Degrade into water-soluble PbI₂ (solubility: 0.044 g/100 mL). Zebrafish studies (Danio rerio) show PbI₂ exposure causes lethal concentrations (LC₅₀) at 0.83 mM, inducing developmental defects like heart edema.
• Tin-based PSCs: Form SnI₄, which hydrolyzes to hydroiodic acid (HI), acidifying aquatic environments. LC₅₀ for SnI₂ is lower (0.09 mM) than PbI₂, challenging its "eco-friendly" label.
• Encapsulation reduces leakage, but module damage during extreme weather or fires releases toxins into soil and groundwater.

• Humidity: Induces reversible hydration or irreversible decomposition into PbI₂ and organic salts.
• Heat: Lattice strain from thermal cycling (25°C–55°C) accumulates deep traps, accelerating efficiency decay. Continuous illumination stabilizes strain, while day/night cycling degrades devices 1.5× faster.
• UV Light: Generates reactive oxygen species, degrading organic HTLs.

• Absorber Layers: Integrate lead-binding materials (e.g., mesoporous TiO₂) to capture 99% of lead upon module breakage.
• Aqueous Recycling: Dissolve degraded perovskites in water with additives (sodium acetate, NaI, hypophosphorous acid). Acetate ions solubilize PbI₂, while iodide ions control perovskite crystal reprecipitation. Achieves 99% material recovery and 23.4% efficiency in recycled devices.

Phenylselenenyl chloride additives anchor grain boundaries, reducing lattice distortion during thermal cycling. This extends T80 (time to 80% efficiency retention) tenfold under natural operating conditions.

• Inorganic Perovskites: CsPbI₃ avoids organic cation instability but requires phase stabilization (α-phase to δ-phase transition).
• 2D Perovskites: Ruddlesden-Popper structures (e.g., BA₂MAₙ₋₁PbₙI₃ₙ₊₁) improve moisture resistance via hydrophobic spacers.