Estimation of front and back junctions of CZTSe:Ge solar cells by combined modulus and impedance spectroscopy

We have fabricated and investigated CTSe:Ge bi-layer devices by modulus spectroscopy combined with impedance spectroscopy to understand the electrical properties of hetero- and back-contact junctions. The equivalent circuit model is developed to decouple the properties of each junction at different frequency ranges. Modeling and numerical simulations with the in-house MATLAB modeling suites connected to external simulators, such as Sentaurus TCAD and LEVM/LEVMW software are conducted to estimate the impact of each junction component to modulus and impedance spectroscopy with a parallel equivalent circuit combination of resistance and capacitance components. The fabricated devices show doping concentration of 6.4 × 10¹⁵/cm³, and the conversion efficiency is 8.1% based on capacitance–voltage and current–voltage characterization. From the characterization and modeling, the built-in potential of the hetero-junction is estimated as 801 meV, which is ∼10% smaller than the ideal case from TCAD, 894 meV. Conversely, the built-in potential of the back contact junction is 428 meV. The total apparent built-in potential is estimated to be 373 meV from the Mott–Schottky equation. (paper)