Theory of doping and defects in complex semiconductor materials
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Understanding the properties of defects and the ability to control their concentration in semiconductors are crucial to apply semiconductor materials in modern electronic and optoelectronic technologies, such as field effect transistors (FET), light emitting diodes (LED), lasers, and solar cells. It is usually difficult to directly characterize and identify defects and impurities in experiments. Over the past few decades, first-principles calculations have emerged as powerful techniques that complement experiments and have become quite reliable to predict defect properties in solids. In this work, we use first-principles calculations based on density functional theory (DFT) to investigate doping and defect properties of some complex semiconductor materials. We propose a model to explain the extremely long persistent photoconductivity in strontium titanate based on its defect physics. Halide vacancies, doping possibilities and layered materials of hybrid perovskites are studied with a focus on their potential application as photovoltaic materials. ☐ It has been recently revealed that strontium titanate (SrTiO3) displays persistent photoconductivity with unique characteristics: it occurs at room temperature and lasts over a very long period of time. Illumination of SrTiO3 crystals at room temperature with sub-band-gap light reduces the electrical resistance by three orders of magnitude and persists for weeks or longer [Tarun et al., Phys. Rev. Lett. 111, 187403 (2013)]. Experiments indicate that oxygen vacancy and hydrogen play important roles, yet the microscopic mechanism responsible for this remarkable effect has remained unidentified. Our DFT calculations with hybrid functional show that this phenomenon can be explained by the instability associated with the HO defect in SrTiO3. The singly ionized HO+ defect is introduced during annealing process and then turned into HO2+ under illumination, releasing an electron to the conduction band. The metastable HO2+ then turns into an interstitial hydrogen and an oxygen vacancy, further increasing free carriers in the conduction band. The model is then extended to other oxides like TiO2 and BaTiO3, for which long lasting persistent photoconductivity is also predicted to occur. Interestingly, this phenomenon represents an elegant way of proving the existence of hydrogen substituting on an oxygen site (HO), forming an interesting, and rarely observed, type of three-center two-electron bond, where H is bonded to two Ti atoms instead of the commonly expected O-H configuration. ☐ Extensive research on hybrid halide perovskites for solar cells in the past few years has led to rapid increase in its power conversion efficiency, reaching 25%. Further progress is likely to come from improving our understanding of the role played by defects. It is yet unclear whether some of the native defects induce deep or shallow levels in the band gap, thus, impacting device efficiency. In this work we use hybrid DFT calculations to systematically study the behavior of halide vacancies in the perovskites CH3NH3PbX3 (MAPbX3, X= I, Cl, Br). We show that although the halide vacancies are shallow donors, upon optical excitation with photon energies above band gap, these vacancies trap an electron to become electrically neutral, followed by a large local lattice relaxation. Once in the neutral charge state, these vacancies can capture a hole in a radiative or non-radiative process, returning to the positively charged state, possibly contributing to the efficiency loss in devices. ☐ Current solar-cell designs use semi-insulating or low carrier-density layers coupled to hole and electron transport contact materials. Controlled doping, both n and p-type, would allow for tuning electrical conductivity and fabrication of p-n homojunctions, opening great opportunities in device applications that go beyond solar cells. In this work we discuss possible approaches to doping of MAPbX3 by substituting the organic molecule MA with other organic molecules. We explore different possible configurations of the various dopants, analyzing formation energies and thermodynamic transition levels. ☐ Finally we study several Am'(MA)n-1PbnI3n+1-type 2D halide perovskites with different organic spacers A', in collaboration with the experimental group of Professors Klaus Theopold and Micheal Crawford at University of Delaware. 2D halide perovskites consist of much more diversified elements and structures, with properties that can be fine tuned for a wide range of electronic or optoelectronic applications. We investigate the stable crystal structures and compare the electronic properties of the 2D perovskites with different number of inter layers and organic spacers.
Description
Keywords
Density functional theory, Doping and defects, First-principles calculations, Perovskite, Persistent photoconductivity, Strontium titanate