Weak electron-phonon renormalization effect caused by the counteraction of the different phonon vibration modes in FeS2

Abstract

Fluctuations on operating temperatures of solar cells may change the electronic structures of absorption layer materials, which will have a profound influence on the photoelectric conversion efficiency. Based on the electron-phonon renormalization (EPR) method, we investigate the temperature dependence of the band gap of optoelectronic pyrite FeS2. The zero point renormalization (ZPR) on the band gap of FeS2 is less than 100 meV, while the vibration-induced band gap reduction is even less than ZPR within the temperature range 0 ∼ 600 K. The fitted Varshni coefficients have agreement with the experimental result. The relatively small reduction of the band gap by the vibrations can be rationalized by the counteraction of the different phonon modes with opposite influences on the EPR effect. By analyzing the mode-decomposed EPR, we reveal that the shortening of the S-S bond caused by the tilting of FeS6 octahedral (A g phonon mode) is responsible for the increase of the band gap. On the other hand, the change of the Fe-S bond length (T u phonon mode) reduces the band gap value. Our work reveals the theoretical understanding of the weak EPR effect in pyrite FeS2.