The Influence of Absorber Thickness on Cu(In,Ga)Se$_{\bf 2}$ Solar Cells With Different Buffer Layers

This study investigates the interplay between the absorber layer of Cu(In,Ga)Se ₂ solar cells and the other layers of these devices. Cu(In,Ga)Se ₂ devices with absorbers of different thicknesses and different buffer layers are fabricated. Absorber layers and finished devices are characterized. Good efficiencies are obtained, also for devices of substandard thickness down to 0.3 μm. Best open-circuit voltages and fill factors are found for cells with half the standard absorber thickness, but the highest efficiencies are found for cells with the standard thickness of 1.6 μm due to their higher short-circuit current density. Cu(In,Ga)Se ₂ cells with Zn(O,S) buffer layers are more efficient than CdS reference devices for the same absorber thickness due to a higher short-circuit current. For cells with thin absorber layers, a part of the higher current is caused by higher quantum efficiency at long wavelengths. Electrical simulations indicate that the loss in the open-circuit voltage for the thinnest devices is due to recombination in the back contact region. The difference in long-wavelength quantum efficiency between the buffer layers is attributed to a difference in the CIGS band bending. Acceptors at the Cu(In,Ga)Se ₂ -CdS interface are proposed as an explanation for this difference. A low-quality back contact region enhances the effect.