Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites (1512.05593v2)

Abstract: The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI$_3$ perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPb$X_3$ perovskites we report on relativistic $GW$-BSE calculations. This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported 'large' values. For MAPbI$_3$, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarisation in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature.

• The paper employs first-principles calculations with the BSE and GW methods to reconcile diverse experimental exciton binding energies in metal halide perovskites.
• It demonstrates that polar phonons and polaron formation, rather than ionic screening, drive the temperature-dependent excitonic properties in materials like MAPbI₃.
• The findings offer actionable insights for optimizing perovskite solar cells and pave the way for future research on advanced hybrid photovoltaic materials.

Role of Polar Phonons in the Photo-excited State of Metal Halide Perovskites

The research conducted by Bokdam et al. explores the complex excitonic phenomena observed in metal halide perovskites, specifically methylammonium lead iodide (MAPbI$_3$), and the role of polar phonons in their photo-excited state. Notably, this paper provides a theoretical framework through first-principles calculations to reconcile the wide range of experimentally reported exciton binding energies, which have significant implications for the material’s photovoltaic efficiency.

Numerical and Theoretical Analysis

The paper employs the Bethe-Salpeter Equation (BSE) alongside the $GW$ approximation to calculate the excitonic properties from a first-principles perspective. The model accurately predicts exciton binding energies in the lower temperature regime that align with higher reported experimental values. Specifically, for MAPbI$_3$, MAPbBr$_3$, and MAPbCl$_3$, the calculated binding energies are approximately 45, 71, and 106 meV, respectively. These results provide a detailed insight into the electron-hole interaction, emphasizing the robustness of these materials for photovoltaic applications due to their large exciton binding energies.

The theoretical analysis further distinguishes between low-temperature ionic screening and room-temperature polaron formation as mechanisms for the material's observed properties. The computations reveal that at low temperatures, phonon modes contributing to the ionic screening are negligible within the exciton binding energy range. Interestingly, at room temperature, the ionic screening remains largely unchanged. Hence, the observed decrease in exciton binding energy with increasing temperature cannot be attributed to changes in ionic screening.

Polarons as an Alternative Explanation

The paper posits that instead of ionic screening, polarons could account for the observed changes in exciton properties at room temperature. By forming individual electron and hole polarons through lattice interactions, these quasiparticles effectively lower the fundamental electronic band gap by approximately 42 meV. This suggests that MAPbI$_3$ maintains an efficient charge separation mechanism, which could be crucial for the material's superior photovoltaic performance.

Implications and Future Directions

The findings carry significant theoretical and practical implications for the development of more efficient solar cell materials. The robust nature of the polaron formation suggests a potential pathway for energy conversion efficiency enhancements, not only for MAPbI$_3$ but also for other related perovskites. As shown in the extensive analysis across various perovskites ($OMX_3$, with $O$ being organic cations and $M$ being metal cations), iodine-based variants emerge as promising due to their optimal band gaps and binding energies conducive for solar energy applications.

The paper opens several future research avenues, including experimental verification of polaron formation effects and exploration of polarons in other hybrid organic-inorganic perovskites. Additionally, further refinement of computational models to include more complex geometries or dynamic processes at operational temperatures could enhance our understanding of these materials.

Conclusion

Bokdam et al. provide a comprehensive theoretical investigation that challenges the prevailing notions of ionic screening in the efficiency of perovskite solar cells. By underlining the role of polarons, the paper not only resolves a significant scientific debate but also paves a strategic path forward for novel perovskite material designs in photovoltaic technology. As this field advances, integrating theoretical insights with experimental findings will be critical in achieving the full potential of perovskite-based solar technologies.