Hwang, Sehyun2019-03-132019-03-132018-12https://hdl.handle.net/11299/202189University of Minnesota Ph.D. dissertation.December 2018. Major: Electrical/Computer Engineering. Advisor: Stephen Campbell. 1 computer file (PDF); xii, 133 pages.In this study, we present the development of copper-indium-aluminum-gallium-selenium (Cu(In1-x-yAlyGax)Se2, or CIAGS) as a wide bandgap top cell absorber for tandem photovoltaic (PV) applications. Realizing a tandem PV structure could lead to a breakthrough for high efficiency solar cells. CIAGS absorbers were grown in a single-step process using a custom-designed co-evaporation system under an ultra-high vacuum. The material properties of CIAGS thin films were analyzed in terms of grain morphology, elemental composition, and energy bandgap. The bandgap of CIAGS is tuned by controlling the elemental composition of group III elements. The relation between energy bandgap and elemental composition was empirically established for CIAGS absorbers with varying bandgaps. The CIAGS grown here targeted a bandgap of ~1.65 eV which is optimal for a tandem top cell. CIAGS solar cell devices were fabricated and characterized electrically by J-V measurements. The highest efficiency obtained was 12.8%, although the efficiency tends to decrease as the bandgap increases. Poor film adhesion or delamination is a major problem in wide bandgap CIAGS solar cells. Delamination occurs at the interface between the CIAGS absorber and the Mo back contact layer. We suggest two possible delamination mechanisms caused by interfacial molybdenum diselenide (MoSe2) in the wide bandgap CIAGS. The CIAGS/Mo interface was characterized mechanically (adhesion) and electrically (contact resistance). A TiN diffusion barrier to selenization improves the CIAGS/Mo interfacial adhesion and provides a potential solution to the delamination problem in the wide bandgap absorbers such as CIAGS.enChalcopyriteDevicePhotovoltaicSolar cellThin filmWide bandgapThesis or Dissertation