Ternary and quaternary chalcogenide semiconductors have emerged as critical materials for thin-film solar cells due to their tunable bandgaps, high absorption coefficients, and favorable optoelectronic properties. These materials, typically composed of elements from groups I-III-VI in the periodic table, offer advantages over binary chalcogenides by enabling precise control over electronic and structural properties through stoichiometric engineering. Their defect-tolerant nature and ability to form stable phases under varying conditions make them particularly suitable for photovoltaic applications.

Crystal structures of ternary and quaternary chalcogenides often derive from tetrahedrally coordinated frameworks, with the most common being the chalcopyrite (CuFeS2) and stannite (Cu2FeSnS4) structures. Chalcopyrite compounds, such as Cu(In,Ga)(S,Se)2 (CIGS), exhibit a tetragonal unit cell with space group I-42d, where cations (Cu, In, Ga) occupy specific lattice sites while anions (S, Se) form the framework. Stannite structures, like Cu2ZnSn(S,Se)4 (CZTS), share similarities but feature ordered cation arrangements that influence carrier transport. The kesterite structure, a derivative of stannite, is another common configuration where cation disorder can lead to band tailing and recombination losses. These materials exhibit a high degree of flexibility in stoichiometry, allowing for bandgap tuning between 1.0 eV and 2.4 eV by adjusting elemental ratios, such as Ga/(In+Ga) in CIGS or S/(S+Se) in CZTS.

Defect tolerance is a defining characteristic of these semiconductors. Unlike conventional silicon, where point defects often act as recombination centers, ternary and quaternary chalcogenides exhibit benign or even beneficial defects. Cu vacancies in CIGS, for example, act as shallow acceptors, enabling p-type conductivity without intentional doping. Similarly, antisite defects like InCu in CIGS or ZnSn in CZTS can form defect clusters that passivate deep levels. The high dielectric constants of these materials further screen charged defects, reducing their impact on carrier mobility. However, secondary phases such as Cu2S, ZnS, or SnS2 can form due to non-stoichiometric growth, leading to shunting paths or recombination. Precise control over composition and processing conditions is necessary to suppress these detrimental phases.

Stoichiometric variations play a crucial role in optimizing device performance. In CIGS, a slight Cu deficiency (Cu/(In+Ga) ≈ 0.8–0.9) enhances hole concentration while maintaining crystallinity. Excess Cu can lead to conductive secondary phases, whereas Cu deficiency beyond optimal levels increases defect densities. For CZTS, maintaining Zn/Sn ratios close to unity is critical to avoid Zn-rich or Sn-rich secondary phases. Selenium incorporation in CZTS (forming CZTSSe) improves grain growth and reduces band tailing, but excessive Se can lead to phase segregation. The flexibility in composition allows for gradient structures, such as Ga grading in CIGS, which creates internal electric fields to enhance carrier collection.

Deposition techniques for these materials must ensure compositional uniformity and phase purity. Sputtering is widely used for CIGS due to its scalability and control over metal ratios. Precursors like Cu, In, and Ga are sputtered sequentially or simultaneously, followed by reactive annealing in chalcogen atmospheres. The process allows for large-area deposition but requires precise control over sputtering rates to maintain stoichiometry. Co-evaporation offers superior compositional control by independently heating elemental sources in vacuum. This technique enables in-situ formation of CIGS with graded bandgaps, as evaporation rates can be dynamically adjusted. For CZTS, solution-based methods like spin-coating or inkjet printing are also employed, where metal salts and chalcogen precursors are dissolved and annealed. While cost-effective, these methods often struggle with carbon residues and incomplete crystallization.

Thermal processing is critical for achieving high-quality films. Selenization or sulfurization steps convert metal precursors into the desired phases, typically at temperatures between 400°C and 600°C. Rapid thermal processing minimizes elemental loss while promoting grain growth. The choice of atmosphere (e.g., H2Se, Se vapor, or S vapor) influences defect chemistry and phase formation. Post-deposition treatments like KF or RbF surface modification further enhance performance by reducing interface recombination.

Device architectures for chalcogenide solar cells typically consist of a Mo back contact, the absorber layer, a CdS or Zn(O,S) buffer, and a transparent conductive oxide. The heterojunction between the p-type absorber and n-type buffer is critical for charge separation. Band alignment engineering, such as using ZnMgO buffers, can minimize conduction band offsets and improve open-circuit voltage. Back contact optimization, including MoSe2 interfacial layers, reduces series resistance and enhances adhesion.

Performance metrics for these materials have shown steady progress. CIGS-based solar cells have achieved efficiencies exceeding 23%, while CZTS devices lag slightly at around 13%. The efficiency gap arises from higher defect densities and narrower processing windows in CZTS. Voltage deficits, defined as the difference between bandgap and open-circuit voltage, remain a challenge, particularly for CZTS, where non-radiative recombination dominates. Strategies like alkali doping (Na, K) and alloying with Ge or Ag have shown promise in mitigating these losses.

Environmental and economic considerations also influence material selection. CIGS contains scarce elements like In, driving research into In-free alternatives like CZTS. However, Sn volatility and Zn disorder pose challenges. Quaternary systems like AgBiS2 or Cu2BaSnS4 are being explored for their earth-abundant constituents, though their efficiencies remain modest.

Future directions include advanced characterization techniques like atom probe tomography to map elemental distributions at atomic scales. Machine learning approaches are being employed to optimize composition and processing parameters. Integration with tandem architectures could further push efficiency limits by combining chalcogenides with wider bandgap materials.

In summary, ternary and quaternary chalcogenide semiconductors offer a versatile platform for thin-film photovoltaics. Their defect-tolerant nature, tunable bandgaps, and compatibility with scalable deposition methods make them strong candidates for next-generation solar cells. Continued research into stoichiometric control, defect engineering, and novel device architectures will be essential to unlock their full potential.