Tingting Shi

Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber

Thin-film solar cells based on Methylammonium triiodideplumbate (CH3NH3PbI3) halide perovskites have recently shown remarkable performance. First-principle calculations show that CH3NH3PbI3 has unusual defect physics: (i) Different from common p-type thin-film solar cell absorbers, it exhibits flexible conductivity from good p-type, intrinsic to good n-type depending on the growth conditions; (ii) Dominant intrinsic defects create only shallow levels, which partially explain the long electron-hole diffusion length and high open-circuit voltage in solar cell. The unusual defect properties can be attributed to the strong Pb lone-pair s orbital and I p orbital antibonding coupling and the high ionicity of CH3NH3PbI3.

Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance

Halide perovskites solar cells have the potential to exhibit higher energy conversion efficiencies with ultrathin films than conventional thin-film solar cells based on CdTe, CuInSe2 , and Cu2 ZnSnSe4 . The superior solar-cell performance of halide perovskites may originate from its high optical absorption, comparable electron and hole effective mass, and electrically clean defect properties, including point defects and grain boundaries.

Unipolar self-doping behavior in perovskite CH3NH3PbBr3

Recent theoretical and experimental reports have shown that the perovskite CH3NH3PbI3 exhibits unique ambipolar self-doping properties. Here, we show by density-functional theory calculation that its sister perovskite, CH3NH3PbBr3, exhibits a unipolar self-doping behavior—CH3NH3PbBr3 presents only good p-type conductivity under thermal equilibrium growth conditions. We further show that despite a large bandgap of 2.2 eV, all dominant defects in CH3NH3PbBr3 create shallow levels, which partially explains the ultra-high open-circuit voltages achieved by CH3NH3PbBr3-based thin-film solar cells. Our results suggest that the perovskite CH3NH3PbBr3 can be both an excellent solar cell absorber and a promising low-cost hole-transport material for lead halide perovskite solar cells.

Physics of grain boundaries in polycrystalline photovoltaic semiconductors

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

Structural, electronic, and optical properties of Cu3-V-VI4 compound semiconductors

Cu-V-VII chalcogenide semiconductors have recently been considered promising earth-abundant solar cell materials. Using first-principles density-functional theory with hybrid functional, we have studied the structural, electronic, and optical properties of Cu3-V-VI4 compounds. We find that Cu3PS4 and Cu3PSe4 prefer energetically the enargite structure, whereas other compounds favor the famatinite structure. The Cu3-V-VI4 family exhibits bandgaps ranging from 0.88 eV to 2.51 eV, revealing the potentials for both single junction and multijunction solar cell applications. The calculated bandgaps for Cu3-V-VI4 compounds are in good agreement with the available theoretical and experimental results.

The structure and properties of (aluminum, oxygen) defect complexes in silicon

The atomic structure and electronic properties of aluminum (Al)-related defect complexes in silicon (Si) are investigated using first-principles calculations. Individual substitutional Al (AlSi), interstitial Al (Ali) and their possible complex configurations with oxygen (O) atoms are studied. We find a unique stable complex configuration consisting of an Ali and an oxygen dimer, Ali-2Oi, which introduces deep levels in the band gap of Si. The formation energies of the Ali-2Oi complexes could be lower than that of individual Ali atoms under oxygen-rich conditions. The formation of Ali-2Oi complexes may explain the experimental observation that the coexistence of Al and O results in reduced carrier lifetime in Si wafers.

Defect Physics of CH3NH3PbX3 (X = I, Br, Cl) Perovskites

The properties of defects, including point defects, grain boundaries, and surfaces in photovoltaic absorbers play critical roles in determining the nonradiative recombination behavior, and, consequently, the performance of solar cells made of these absorbers. Here, we review our theoretical understanding of the defect properties of organic–inorganic methylammounium lead halide perovskites, CH3NH3PbX3 (X = I, Br, Cl), using density-functional theory calculations. We show that CH3NH3PbI3 perovskites exhibit unique defect properties—point defects with low formation energy values only create shallow levels, whereas point defects with deep levels have high formation energies. Surfaces and grain boundaries do not produce deep levels. These unique defect properties are attributed to the antibonding coupling between Pb lone-pair s and I p orbitals, the high ionicity, and the large lattice constants. We further show that CH3NH3PbI3 exhibits an intrinsic ambipolar self-doping behavior with electrical conductivity tunable from p-type to n-type via controlling the growth conditions. However, CH3NH3PbBr3 exhibits unipolar self-doping behavior. It demonstrates a preference for p-type conductivity if synthesized under thermal equilibrium growth conditions. CH3NH3PbCl3 may exhibit a compensated self-doping behavior due to its large bandgap. Doping CH3NH3PbI3 using external dopants may improve the p-type conductivity, but not the n-type conductivity due to compensation from intrinsic defects.

Spectroscopic ellipsometry studies of CH3NH3PbX3 thin films and their growth evolution

CH3NH3PbX3 (X=I, Cl) perovskite films of interest for photovoltaics (PV) devices have been deposited by (i) evaporation followed by vapor annealing and (ii) co-evaporation. Near infrared to ultraviolet spectroscopic ellipsometry has been used to determine the spectroscopic optical response for vapor-annealed material ex situ and co-evaporated material in situ during growth. Real time spectroscopic ellipsometry (RTSE) applied during co-evaporation has tracked the formation and time evolution of perovskite and phase segregated layers at the substrate and surface interfaces. Good agreement between density functional theory (DFT) calculations of optical response and experimental measurements has also been demonstrated.

Chapter 6:Structural, Electronic, and Optical Properties of Lead Halide Perovskites

Organic–inorganic methylammonium lead halide perovskites have recently emerged as superior solar photovoltaic absorbers. In this chapter, we present our recent theoretical studies on the structural, electronic, and optical properties of metal-halide perovskites, including crystal structures, electronic structures, and the optical absorption coefficient of bulk, point defect, and grain boundaries. The passivation effect of Cl on grain boundaries is proposed. The relation of our theoretical results with the existing experiments as well as current challenges are discussed.

First principles study of aluminum-oxygen complexes in silicon

The atomic structure and electronic properties of aluminum-related defect complexes in silicon are investigated using first-principles calculations. Various configurations of Al-O complexes containing interstitial and substitutional Al atoms and interstitial O atoms are studied. We find that interstitial Al atoms induce deep gap states. The formation energies of interstitial AlO complexes could be much lower than that of interstitial Al under oxygen-rich conditions. We propose that the formation of Al-O complexes may explain the experimental observation that the coexistence of Al and O results in reduced lifetime in Si wafers.