Bridging the Gap Between Steady-State and Transient Characterization of Carrier Cooling for Hot-Carrier Solar Cells

Exploring the harnessing of excess electron energy through the suppression of thermalization represents a promising way for achieving single-junction devices that surpass the Shockley-Queisser limit. This strategy necessitates slow cooling processes to allow the extraction of carriers before a substantial energy loss occurs. Currently, these mechanisms are extensively investigated optically using time-resolved characterization techniques, particularly transient absorption spectroscopy, with a focus on perovskite semiconductors. The application of this spectroscopic method aims to derive a cooling time, serving as a metric to assess the material's potential as a hot-carrier absorber. However, the interpretation of this parameter raises questions, given its variability with injection conditions and its determination in a transient regime, distinct from the steady-state conditions characteristic of solar cell operation. To address this uncertainty, we scrutinize the appropriateness of the cooling time as a metric for quantifying reductions in thermalization losses and whether it accurately reflects an effective carrier temperature in a steady-state scenario. Our investigation involves a comparative analysis of time-resolved and steady-state experiments conducted on thin films of gallium arsenide. Gallium arsenide is selected due to its robustness, stability, and extensive prior research, making it an ideal reference material. The transient absorption spectroscopy employs a high temporal resolution of 250 fs, and the extracted characteristics are systematically contrasted with steady-state properties obtained through photoluminescence experiments. We delve into the impact of various parameters such as injection level and excitation energy. To bolster our experimental findings, we undertake a modeling approach grounded in phonon rate equations. This integrated experimental and modeling investigation aims to provide an in-depth understanding of the relationship between transient and steady-state characteristics, shedding light on the properties to use as a metric for thermalization losses in hot-carrier absorbers.