Highly efficient narrow bandgap Cu(In,Ga)Se₂ solar cells with enhanced open circuit voltage for tandem application

Although an ideal bandgap matching with 0.96 eV and 1.62 eV for a double-junction tandem is hard to realize practically, among all mature photovoltaic systems, Cu(In,Ga)Se₂ (CIGSe) can provide the closest bandgap of 1.00 eV for the bottom sub-cell by adjusting its composition. However, pure CuInSe₂ (CISe) solar cell suffers strong interfacial carrier recombination. We hereby present approaches to introduce appropriate Ga gradients in both the back and front parts of absorber while maintaining the absorption spectrum close to CISe. With an appropriate front Ga gradient, the open circuit voltage can be enhanced by ~30 mV. With a pre-deposited CIGSe layer and a high copper excess deposition during absorber growth, the Ga diffusion can be well suppressed and a wide U-shaped Ga grading with a minimum bandgap of 1.01 eV has been created. Our optimized narrow-bandgap CIGSe solar cell has achieved a certified record PCE of 20.26%, with a record-low open circuit voltage deficit of 368 mV and a record-high contribution of 10% absolute efficiency to a four-terminal tandem. This work demonstrates the potential of controlling gallium diffusion to improve the performance of narrow bandgap CIGSe solar cells for tandem applications.