Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se₂ Solar Cells Fabricated at 400 °C

The classical high-temperature synthesis process of Cu(In,Ga)Se₂ (CIGS) solar cells limits their applications on high-temperature intolerant substrates. In this study, a novel low-temperature (400 °C) fabrication strategy of CIGS solar cells is reported using the bismuth (Bi)-doping method, and its growth-promoting mechanism is systematically studied. Different concentrations of Bi are incorporated into pure chalcopyrite quaternary target sputtered-CIGS films by controlling the thickness of the Bi layer. Bi induces considerable grain growth improvement, and an average of approximately 3% absolute efficiency enhancement is achieved for Bi-doped solar cells in comparison with the Bi-free samples. Solar cells doped with a 50 nm Bi layer yield the highest efficiency of 13.04% (without any antireflective coating) using the low-temperature technology. The copper-bismuth-selenium compounds (Cu-Bi-Se, mainly Cu_1.6Bi_4.8Se₈) are crucial in improving the crystallinity of absorbers during the annealing process. These Bi-containing compounds are conclusively observed at the grain boundaries and top and bottom interfaces of CIGS films. The growth promotion is found to be associated with the superior diffusion capacity of Cu-Bi-Se compounds in CIGS films, and these liquid compounds function as carriers to facilitate crystallization. Bi atoms do not enter the CIGS lattices, and the band gaps (E_g) of absorbers remain unchanged. Bi doping reduces the number of CIGS grain boundaries and increases the copper vacancy content in CIGS films, thereby boosting the carrier concentrations. Cu-Bi-Se compounds in grain boundaries significantly enhance the conductivity of grain boundaries and serve as channels for carrier transport. The valence band, Fermi energy level (E_F), and conduction band of Bi-doped CIGS films all move downward. This band shift strengthens the band bending of the CdS/CIGS heterojunction and eventually improves the open circuit voltage (V_oc) of solar cells. An effective doping method and a novel mechanism can facilitate the low-temperature preparation of CIGS solar cells.