Paper TF+EM+NS+SS-ThM3
Engineering Effective Back Contact Barrier by interfacial MoSe₂ defect states for CZTSe: nanolayer Ge solar cells.

Thursday, October 24, 2019, 8:40 am, Room A122-123

The steadily emerging Cu₂ZnSnSSe₄ (CZTSSe) devices are alternative thin film solar cells with abundant elements in earth’s crust for the past several years. Despite several advantagessuch as high absorption coefficient (>10⁴ cm^-1) and a tunable direct band gap energy (1 to 1.4 eV), the improvement and understanding have been stagnant in the past several years. Recently, CZTSe: nanolayer Ge solar cells have shown significantly improved pseudo-mono grain toward the depth direction.

Due to the improvement and the similarity between CZTSe and Cu(In,Ga)Se₂ (CIGS) thin film solar cells, the CZTSe/ Molybdenum (Mo) back contact interface was often misinterpreted by expecting the similar back contact property to CIGS. However, unlike the stable CISe (CuInSe₂)/Mo interface, the CZTSe/Mo interface is thermodynamically unstable due to the higher oxidation states of Sn. Although the presence of an interfacial MoSe₂ layer at Mo/absorber is always confirmed, properties of the back contact-interface such as structure and electrical behaviors are convoluted.

Following our empirical results about the back contact barrier of CZTSe: nanolayer Ge devices, we perform analytical and numerical modeling to explain the back contact improvement theoretically. The device modeling are carried out with the simulator, developed at Indiana State University. The tool is run in MATLAB environment, connected to other external tools (Sentaurus TCAD). Based on our result, defects in MoSe₂ interfacial layer dominate the back contact property of CZTSe: nanolayer Ge devices by increasing of the effective back contact barrier, which consists of two different back contact barriers, thereby increasing series resistance as well. The reduction of MoSe₂ defect concentration from 1 x 10¹⁷ to 1 x 10¹⁵ cm^-3 decreases the effective barrier height by 51 meV, which results in approximately 34 % decrease in the series resistance (See supporting data). Conversely, as the defect concentration increases, the benefit from the back contact barrier lowering by the valence bands offset between MoSe₂ and CZTSe absorber is reduced and essentially eliminated. However, the back contact barrier between MoSe₂ and Mo metal contact remains the same even with increased MoSe₂ defect concentration. Incorporating thin Ge nanolayer at the interface between the absorber and MoSe₂ positively influences and possibly reduces the defect states, lowering the effective barrier. The exponential fitting of the effective barrier and series resistance agrees well with the experimental results. The improvement of the back contact barrier for CZTSe: nanolayer Ge devices is calculated as 23.8 meV than CZTSe without nanolayer Ge devices.