Charge carrier mobility in hybrid halide perovskites (1411.7904v2)

Abstract: The charge transport properties of hybrid halide perovskites are investigated with a combination of density functional theory including van der Waals interaction and the Boltzmann theory for diffusive transport in the relaxation time approximation. We find the mobility of electrons to be in the range 5-10 cm$^2$V$^{-1}$s$^{-1}$ and that for holes within 1-5 cm$^2$V$^{-1}$s$^{-1}$, where the variations depend on the crystal structure investigated and the level of doping. Such results, in good agreement with recent experiments, set the relaxation time to about 1 ps, which is the time-scale for the molecular rotation at room temperature. For the room temperature tetragonal phase we explore two possible orientations of the organic cations and find that the mobility has a significant asymmetry depending on the direction of the current with respect to the molecular axis. This is due mostly to the way the PbI$_3$ octahedral symmetry is broken. Interestingly we find that substituting I with Cl has minor effects on the mobilities. Our analysis suggests that the carrier mobility is probably not a key factor in determining the high solar-harvesting efficiency of this class of materials.

• The paper shows that electron mobility spans 5–10 cm²/Vs and hole mobility 1–5 cm²/Vs, aligning closely with experimental observations.
• It utilizes DFT with van der Waals interactions and Boltzmann transport theory across multiple crystal phases to uncover significant anisotropy effects.
• The study finds that Cl doping minimally affects mobility, suggesting that extended carrier lifetimes rather than high mobility drive photovoltaic efficiency.

Charge Carrier Mobility in Hybrid Halide Perovskites

Hybrid halide perovskites (HHPs), notably including methyl-ammonium lead iodide (CH₃NH₃PbI₃), have garnered significant interest in the field of photovoltaic technology due to their exceptional energy-harvesting efficiency and comparatively low production costs. Despite ongoing research, the precise mechanisms underpinning their high performance continue to be debated, particularly concerning charge transport properties. This paper investigates the charge carrier mobility in these materials through density functional theory (DFT) integrated with the Boltzmann theory for diffusive transport.

Theoretical Framework

The paper employs DFT calculations, incorporating van der Waals interactions, to explore the electronic structure of HHPs. In conjunction with the Boltzmann transport theory under the relaxation time approximation, the analysis focuses on the temperature- and doping-dependent mobility for both electrons and holes across different crystallographic phases including cubic, tetragonal, and orthorhombic structures.

Key Findings

The analysis establishes that:

• Electron mobility in HHPs spans 5-10 cm²/Vs, while hole mobility ranges between 1-5 cm²/Vs. These findings align closely with recent experimental results.
• Crystal phase and organic cation orientation significantly influence mobility. The paper reveals a pronounced anisotropy in mobility within the tetragonal phase, attributed to the structural symmetry breaking of the PbI₃ octahedral network.
• Substitution of iodine with chlorine (Cl doping) exerts negligible influence on the mobility, contradicting some prior assumptions regarding its potential to enhance charge transport.

Computational Details

The computations confirm that organic cations intimately affect the inorganic matrix stabilization. Calculations performed without spin-orbit coupling were found to be aligned with those that included it, except for a notable gap renormalization. Effective mass fitting along key crystallographic directions further supports these findings.

Implications and Future Directions

This research suggests that the high photovoltaic efficiency of HHPs is not primarily due to high charge carrier mobility; instead, other factors, such as extended carrier lifetimes, are likely contributors. In terms of practical application, this implies that mobility enhancements will not necessarily translate to significantly improved solar cell efficiencies. The paper encourages further theoretical and experimental efforts aimed at elucidating the complex dynamics between lattice vibration, impurity presence, and charge carrier interaction, potentially via detailed electron-phonon coupling analyses.

Given the implications of this research, the understanding of carrier mobility in HHPs will continue to evolve, potentially informed by advancements in theoretical modeling techniques that can more accurately account for factors such as dynamic lattice effects and disorder. This paper provides a solid foundation for future inquiries into the fundamental processes governing the charge transport in perovskite-based devices.