Layer-selective lithium incorporation enables 64.3% of the radiative-limit open-circuit voltage in solution-processed Cu2ZnSnS4 solar cells

Solution-processed Cu 2 ZnSnS 4 (CZTS) thin-film solar cells remain limited by significant voltage losses associated with defects, interfacial recombination, and non-ideal film morphology. Here, we demonstrate a simple layer-selective Li incorporation strategy, in which a Li-containing precursor solution is introduced during different coating cycles of a six-layer CZTS absorber. The Li introduction sequence strongly affects device performance. Li introduction during the first coating cycle reduces the fill factor, whereas introduction during the sixth (top, 6-Li) cycle yields a champion power conversion efficiency of 10.6% and a maximum open-circuit voltage of 797 mV. With a bandgap of 1.51 eV, the 6-Li device shows a Shockley–Queisser radiative-limit open-circuit voltage ( V OC /V OC SQ ) of 64.3%, among the highest values reported for solution-processed CZTS solar cells. Structural characterization shows that Li incorporation promotes grain growth and suppresses the fine-grained region near the Mo/CZTS interface while preserving the kesterite phase. Time-of-flight secondary ion mass spectrometry shows that Li introduced in the top coating cycle redistributes throughout the CZTS absorber during sulfurization. Room-temperature photoluminescence (PL) and temperature- and excitation-power-dependent PL measurements further indicate reduced non-radiative losses and modified defect-related recombination.