Photovoltaic Materials

Core Idea

Photovoltaic materials convert sunlight to electricity through the photovoltaic effect: photons with energy greater than the band gap generate electron-hole pairs that are separated by a built-in electric field (p-n junction or heterojunction) and collected as current. The Shockley-Queisser limit sets the maximum theoretical efficiency for a single-junction cell at about 33% (for a 1.34 eV band gap) — photons below the gap are not absorbed, and excess energy above the gap is lost as heat. Materials chemistry drives PV technology: crystalline silicon dominates (95%+ market share, ~26% record efficiency), but thin-film technologies (CdTe, CIGS, perovskites) and multi-junction cells offer routes to higher efficiency or lower cost.

Explainer

Photovoltaic technology converts the most abundant energy source on Earth — sunlight — into electricity, and the chemistry of the absorber material determines the efficiency, cost, and practicality of every solar cell. The fundamental physics is the same for all PV materials: a photon with energy above the band gap is absorbed, creating an electron-hole pair; a built-in electric field separates the carriers before they recombine; and external contacts collect the photocurrent. The materials chemistry challenge is finding absorbers that maximize this process while being manufacturable at scale.

Crystalline silicon dominates photovoltaics because of four decades of manufacturing optimization, not because silicon is the ideal PV material. Its indirect band gap requires thick wafers (~180 micrometers) and elaborate light-trapping texturing. Its surface must be passivated (typically with SiNx or Al2O3) to prevent carrier recombination at dangling bonds. The p-n junction is formed by diffusing phosphorus into a boron-doped wafer. Despite these complications, silicon cell efficiencies now exceed 26% in the lab and 24% in production, approaching the Shockley-Queisser limit. The manufacturing learning curve has driven module costs below $0.20/watt, making solar electricity cheaper than fossil fuel generation in most of the world.

Thin-film technologies use direct-gap materials that absorb sunlight in layers 100-1000 times thinner than silicon. CdTe (1.45 eV direct gap, close to the S-Q optimum) is the leading thin-film technology, manufactured by First Solar at GW scale. The chemistry challenge is controlling the CdTe/CdS heterojunction and managing the toxicity of cadmium. CIGS (Cu(In,Ga)Se2) offers band gap tunability through the In/Ga ratio but suffers from compositional complexity (four elements that must be precisely controlled). Halide perovskites (methylammonium or formamidinium lead halides) have achieved >26% efficiency from solution processing — a manufacturing paradigm-shift — but stability under real-world conditions remains the critical unsolved problem.

The frontier of PV materials chemistry is tandem cells that exceed the single-junction Shockley-Queisser limit. A perovskite top cell (1.7 eV) on a silicon bottom cell (1.1 eV) can theoretically reach ~43% efficiency by using each part of the solar spectrum more efficiently. Perovskite/silicon tandems have already exceeded 33% in the lab, surpassing the theoretical limit for silicon alone. The materials chemistry challenges are formidable: the perovskite must be stable for 25+ years, the intermediate recombination layer must be optically transparent and electrically conductive, and the processing of the perovskite must not damage the underlying silicon cell. Solving these challenges represents one of the highest-impact applications of materials chemistry to global energy problems.