First-Principles and Multi-Scale Modeling of Dynamic Ionic and electronic Processes in Hybrid and Halide Perovskites.
Abstract
Hybrid organic-inorganic perovskites (HOIPs) have emerged as promising candidates forfuture inexpensive and efficient solar cells. Recent revolutions in the ability to fabricate highqualityHOIPs and in the design of prototype solar cells have led to rapid growt"h in photovoltaic(PV) power conversion efficiency (PCE), from 2% a few years ago to certified cell values over20%. Besides photovo""ltaics, CH3NH3PbI3 (MAPbI3, MA=methylammonium) has also shownpromise for light-emitting diodes, photodetectors, and nanowire lasers"".Its unprecedented potential notwithstanding, the practical use of MAPbI3 in solar cellapplications faces several challenges relat""ed to its chemical and structural stability. MAPbI3 canbe unstable depending on external factors, such as temperature, oxygen, humi""dity, and solarirradiation. In particular, exposure to H2O is considered particularly detrimental for the long-termstability of th""is material. The direct exposure of MAPbI3 to illumination also causes structuraldegradation, which, although reversible, leads to" loss in photovoltaic efficiency. Insufficientstability is therefore a critical issue that must be solved before the hybrid perovskites can be usedin widespread applications.We propose to examine water reactivity within bulk hybrid perovskites. Focus areas toi"nclude: interactions of water fragments with cations and anions; determination of signaturevibrational modes, in order to enable ra"pid determination of the extent of degradation as well asthe exact chemical type; description of approaches for reversing each particular chemical type ofwater-induced degradation; and understanding and exploration of how incorporated water affectsother ion mig"ration processes.We also propose to examine ion migration in a multi-scale model, using accurateelectronic-structure calculations" to parameterize molecular dynamics that can run for sufficientlylong times to examine degradation as an in silico experiment. We propose to extend the time scaleby studying molecular reorientation with Monte Carlo calculations. This includes examination oflarg"e-scale ionic motions related to local or global phase transitions, and the formation andelimination of defects.We postulate the e"xistence of calculable quantities that predict the ease of photodegradation.Even though the exact mechanism for photo-degradation m"ay depend on themolecular species present in each hybrid perovskite, this information should be encoded in theexcited state electr""onic structure of the system. As a working hypothesis, we will take the electronphononinteraction coupling matrix, or some transfor""mation thereof, to be a good descriptor. Inthis project, we will calculate electron-phonon couplings from DFT, or from an electron-"phononmodel parameterized by MD. We will then compare these couplings with the various degradationmechanisms in the literature. If" they are found to be successful predictors of photo-degradation,this will enable computational materials design of photo-stable hy"brid perovskites.First-principles modeling offers the ability to assess material properties prior to synthesis.We will propose A-s"ite cations that have not been tried, including those that can impact the bandgap. We propose B-site cations that replace Pb but st"ill provide strong spin-orbit coupling. And weexplore combinations of anions that can influence the enthalpy and entropy in ways that favor thecubic perovskite (black) phase.Some promising results have shown that organic ions with longer chains offer someprote"ction against water-induced damage. Intriguingly, these cations can lead to 2D hybrid halideperovskites with their own promising pr""operties. In addition, the high surface areas ofnanomaterials and epitaxial layers offer opportunities to stabilize desired phases" throughadsorption interactions. These phenomena will be studied with an eye toward developing materialsand nano-morphologies that offer enhanced stability.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 05, 2017
- Source ID
- N000141712574
Entities
People
- Andrew M Rappe
Organizations
- Office of Naval Research
- United States Navy
- University of Pennsylvania