Toward digital twins by one-dimensional simulation of thin-film solar cells: Cu(In,Ga)Se2 as an example

Thin-film solar cells demonstrate high power conversion efficiencies, but are complex in their structure. To fully understand their functionality and to devise further efficiency improvements, a physical cell model or digital twin is required. As the thin film's bulk and interface material parameters might change during subsequent device processing, this model needs to be inferred from characterization of the finished solar cell only. In this work, we derive one-dimensional optoelectronic models based on a variety of optoelectronic experimental results. As an example for thin-film devices, we take three Cu(In,Ga)Se2 solar cells, two with high efficiency (19% and 20%) and one with moderate efficiency at around 15%. The development of the models uses the simulation tool Synopsys TCAD as the subprogram of a routine that fits the manifold optoelectronic experiments simultaneously. By the newly developed iterative one-dimensional simulation, we are able to fit the measured time -resolved photoluminescence at varied excitation and forward bias, the current -voltage curves in dark and under illumination, the (biased) external quantum efficiency, the voltage -dependent capacitance, and the drive -level capacitance of the solar cell. As an outcome, we obtain a digital model comprising the effective values for the doping densities in all layers, the charge carriers' lifetimes and mobilities in the absorber, and the band alignment between the absorber and the buffer. Among these material parameters, the electron minority carrier lifetime appears to be of paramount importance. It exhibits values of 57 and 74 ns in the highly efficient cells and 2.5 ns in the moderately efficient cell. A hole mobility being substantially smaller compared to that of electrons is irremissible to explain the voltage dependence of the time -resolved photoluminescence of all investigated cells. Furthermore, a small positive conduction -band offset between 100 and 200 meV at the CdS/Cu(In,Ga)Se2 interface is the result of the model for all investigated cells. Finally, the digital twin of a highly efficient cell is used to assess the dominant recombination processes and to calculate a hypothetical device performance if these dominant processes could be inhibited by material optimization.