Efficiency Improvement Mechanism by Light and Dark Soaking for CdSe_xTe_1-x Thin-Film Solar Cells through Novel In-Situ Dopant Profiling

Cadmium Telluride (CdTe) thin-film solar cells have shown remarkable efficiency and durability in the past decade, reaching over 22.1 % in laboratory-scale tests, close to the theoretical Shockley-Queisser limit of ∼30 %. Furthermore, they have also been widely deployed in commercial applications, with more than 30 GWp of modules installed worldwide. CdTe is the leading thin-film technology for cost-effective solar energy production due to its low fabrication costs, high-power conversion efficiency, reasonable long-term performance stability, and short energy-payback time. Recently, researchers have explored the use of Se to create band grading either in addition to or as a replacement for CdS. CdSe_xTe_1-x is a promising material with bandgap lowering below 1.4 eV, which allows the short-circuit-current (Jsc) to approach its theoretical limit. For a long period, CdS was regarded as a crucial component for achieving high performance. Cells made without CdS (i.e., a direct CdTe junction to the transparent conducting oxide electrode) exhibit very low open circuit voltages (Voc and fill factors (FF), implying that the CdTe/oxide interfaces were of low quality. Although CdS has a benefit in that intermixing eases the lattice mismatch at the interface, this approach has a drawback: it also reduces the photocurrent by absorbing light in the 300 - 525 nm range. This means less light reaches the active layer of the solar cell, resulting in efficiency loss. Hence, recent efficiency improvement has focused on introducing Se to create band grading either in addition to or as a replacement for CdS. CdSe_xTe_1-x is promising, with bandgap lowering below 1.4 eV, enabling Jsc to be close to its theoretical limit.<br/>In this study, we have fabricated CdSe_xTe_1-x devices by vapor transport technology (VTD) and investigated the detailed improvement mechanism of efficiency improvement through in-situ Cu dopant profiling under various light and dark soak conditions. Moreover, devices were stressed at elevated temperatures simultaneously under various bias conditions, both with illumination and in the dark. During light soaking, different intensities of light and wavelengths were tested to characterize devices. In addition, we modeled the result with our in-house MATLAB modeling suites, which we developed in our group to understand the efficiency improvement mechanism. Connected to the external TCAD simulators (Synopsys Sentaurus), we modeled and explained the results with different input parameters based on the dopant in-situ profiling. The results indicate that CdSe_xTe_1-x devices have shallow donor and acceptor energy states near the main front junction interface, and acceptor activation energy (Ea) is approximately 0.3 – 0.4 eV based on light and voltage-biased quantum efficiency at different temperatures. The Cu dopant concentration is approximately 5 x 10¹⁴ cm³ under no bias conditions. However, a peak doping concentration evolves toward the front CdSe_xTe_1-xjunction during in-situ measurements. The weak blue light (0.01 sun at 450 nm) can dramatically improve the carrier collection as compared to the same intensity of red light (660 nm). Interestingly, the stability of CdSe_xTe_1-x solar cells was found to be bias-dependent and device-specific during light and dark soaking. CdSe_xTe_1-x devices without Cu dopant demonstrated a reduction of depletion width under voltage and light-biased conditions. The depletion width of CdSe_xTe_1-x devices without Cu is reduced to approximately 24 % under applied voltage biases and dark soaking conditions as compared to no bias condition, while efficiency, Jsc, and Voc decreased. Under light soaking conditions at 85 C, the increase in Voc and efficiency depends on the thickness of CdSe deposition thickness.