Abstract
Organo-lead halide perovskite solar cells represent a revolutionary shift in solar photovoltaics, introducing relatively soft defect containing semiconductors as materials with excellent charge collection for both electrons and holes. Although they are based on the nominally simple cubic perovskite structure, these compounds are in fact very complex. For example, in (CH3NH3)PbI3 the dynamics and ensuing structural fluctuations associated with the (CH3NH3)+ ions and the interplay with the electronic properties are still not fully understood, despite extensive study. Here, using ab-initio calculations, we show that at room and higher temperature, the rotation of CH3NH3 molecules can be viewed as effectively giving local structures that are cubic and tetragonal like from the point of view of the PbI3 framework, though in fact having lower symmetry. Both of these structures are locally polar, with sizable polarization, ~10 μC/cm2 due to the dipoles on the organic. They become energetically degenerate in the volume range, V ~ 250 Å3/f.u–265 Å3/f.u. We also find very significant dependence of the band gap on the local structure. This type of transition is analogous to a transition between two ferroelectric structures, where in-spite of strong electron phonon coupling, there is strong screening of charged defects which can lead to enhanced mobility and charge collection. The results provide insights into the enhanced light absorption near the band edge and good charge collection in this material.
Highlights
The organic-inorganic hybrid perovskites (OIHP) have emerged as an important new class of photovoltaics, exemplified by methylammonium lead iodide CH3NH3PbI3 (MAPbI3), Laboratory devices based on these have reported efficiency exceeding 21%1 which is comparable to or even higher than the performance of existing solar cell technologies
We have studied the impact of correlated orientations of MA molecules on the evolution of crystal structures, energy band structures and energy band splitting of MAPbI3 focusing on Pm-3m and P4mm structure
The energy band gap of I4/mcm structure is 1.55 eV which is in perfect agreement with experimental report but it is about 1.2 eV for Pm-3m and P4mm structure when MA molecule is in the [001] and [110] direction
Summary
The organic-inorganic hybrid perovskites (OIHP) have emerged as an important new class of photovoltaics, exemplified by methylammonium lead iodide CH3NH3PbI3 (MAPbI3), Laboratory devices based on these have reported efficiency exceeding 21%1 which is comparable to or even higher than the performance of existing solar cell technologies. By using the Van der Waals corrected Density Functional Theory, Motta et al. revealed that the rotation of orientation of CH3NH3 molecule in cubic MAPbI3 from [111] direction to [110] direction distorts the PbI6 octahedral cage and results in indirect band gap Such an effect has been investigated by Gao, et al.. Our results show that the MA units rotate in PbI6 cuboctahedral, consistent with experiment, and these rotations are coupled significantly to the PbI3 lattice inducing structural changes from cubic Pm-3m to tetragonal P4mm and vice versa This causes a momentum dependent splitting of energy band by Rashba effect due to the spin-orbit coupling, prevents the electron recombination, and induces a multi band gap electronic structure
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
