Spectral Response of CuGaSe2/Cu(In,Ga)Se2 Monolithic Tandem Solar Cell With Open-Circuit Voltage Over 1 V

Thin-film multijunction solar cells are considered to be the most promising structure for next-generation photovoltaic devices. We fabricated CuGaSe2 (CGS)/Cu(In,Ga)Se2 (CIGS) monolithic tandem solar cells. The intermediate AZO film was used as a recombination layer between the top cell and the bottom cell, and its thickness was varied from 50 to 200 nm. The best tandem cell parameters with a 50 nm thick Al-doped ZnO (AZO) layer showed a V OC = 1.03 V, J SC = 10.24 mA/cm2, FF = 41.5%, and efficiency = 4.32%. We showed the V OC of monolithic tandem cell to be over 1 V under illumination. We also observed the current continuity between the CGS cell and the CIGS cell which were connected in series as subcells. As the AZO thickness increased, the spectral response of the top cell decreased and the bottom cell was not completely saturated. The 50 nm thick AZO layer leads the CIGS bottom cell to be current-limiting, whereas the 200 nm thick AZO layer shifts the limitation to the CGS top cell. The results also showed that the In element diffusion into the CGS top absorber enhanced the electrical and optical properties of the top cell, whereas the Zn element diffusion into CIGS bottom absorber tended to degrade the bottom cell simultaneously.