463080 Highly Efficient Solar Cells Made with Cu1-XKxInSe2 Alloys: A Foundation for Engineering K in Cu(In,Ga)Se2

Monday, November 14, 2016: 9:58 AM
Golden Gate 5 (Hilton San Francisco Union Square)
Christopher P. Muzzillo1,2, Jian V. Li3 and Timothy J. Anderson1, (1)Chemical Engineering, University of Florida, Gainesville, FL, (2)National Renewable Energy Laboratory, Golden, CO, (3)Physics, Texas State University, San Marcos, TX

For the first time, Cu-KF-In-Se co-deposition was used to form Cu1-xKxInSe2 (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 device—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 ≥ 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.

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See more of this Session: Nanostructured/Thin Film Photovoltaics
See more of this Group/Topical: Materials Engineering and Sciences Division