(99g) Highly Efficient Solar Cells Made with Cu1-XKxInSe2 Alloys: A Foundation for Engineering K in Cu(In,Ga)Se2

For the first time, Cu-KF-In-Se co-deposition was used to form Cu_1-xK_xInSe₂ (CKIS) films over the full composition range (0 < x < 1). Compared to K-free CuInSe₂, the CKIS films exhibited increased majority carrier concentrations and wider band gaps, while 0 < x < 0.22 compositions showed increased minority carrier lifetimes. Thin film solar cells were fabricated using soda-lime glass/Mo/CKIS/CdS/intrinsic-ZnO/Al:ZnO/Ni/Al stacks, where a Ni/Al finger grid with partial shading was used for the top contact. Photovoltaic (PV) performance was optimal at x ~ 0.07, which resulted in an officially-measured 15.0%-efficient deviceâ??equal to the world record CuInSe₂ efficiency (grown at 575 °C), despite being grown at 500 °C, without the three stage process, and without device stack optimization. The devices were characterized with quantum efficiency, temperature- and light-intensity-dependent current-voltage, capacitance-voltage, and admittance spectroscopy. These data were then used to calculate charge carrier recombination rates in the bulk absorber, the depletion region, and at the interface. Bulk recombination was reduced for x > 0. Interface recombination was reduced at x ~ 0.07, while x â?¥ 0.22 compositions had poorer performance due to interface recombination, relative to x ~ 0. These trends are consistent with both surface and bulk recombination affecting apparent lifetimes. The results showed that large K compositions deteriorated PV performance in CKIS alloys, while nevertheless, Cu(In,Ga)Se₂ solar cells with record efficiencies have exhibited large K compositions at the absorber/buffer interface. Processing techniques were employed to address this apparent discrepancy in material quality at high K content. Increasing the substrate's Na composition was found to increase the extent of CKIS alloy formation, relative to CuInSe₂ + KInSe₂ formation. Similarly, decreased substrate temperature was found to favor CKIS formation, relative to CuInSe₂ + KInSe₂. Substrate Na and temperature can therefore be used to engineer K bonding in Cu(In,Ga)Se₂ absorbers to enhance both initial and long-term PV power generation. Material growth methods such as these may also be used to investigate the differences between CKIS alloys and Cu-K-In-Se films with very low KInSe₂ content.