Excited-State Electron-Phonon Coupling in Pristine and Doped Iron Pyrite

Iron pyrite (FeS₂) is a promising photovoltaic due to its strong light absorption, low-toxicity constituents, and low cost, yet pyrite devices suffer from poor open-circuit voltage and efficiency. The role of excited-state electron-phonon coupling (EPC), which drives structural distortion and energy loss following photoexcitation, remains underexplored in pyrite. Here, we use resonance Raman intensity analysis (RRIA) to quantify excited-state EPC in pristine, electron-doped, and hole-doped pyrite single crystals by determining the Huang-Rhys factors for three phonon modes. We find exceptionally strong excited-state EPC in pristine pyrite, dominated by the 347 cm^-1 mode. Sulfur vacancies and phosphorus doping reduce the EPC strength for this mode, while cobalt doping significantly suppresses the EPC for all modes. Correlation analysis further reveals that higher doping systematically weakens EPC through electronic screening. These results demonstrate that excited-state EPC varies substantially with doping and impacts nonradiative energy loss, directly informing strategies to suppress vibrational losses in pyrite photovoltaics.