Wan-Jian Yin

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.

Halide perovskite materials for solar cells: a theoretical review

Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to 19.3% in 2014, by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites. The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency, but also flexibility in material growth and architecture. The superior performance of the perovskite solar cells suggested that perovskite materials possess intrinsically unique properties. In this review, we summarize recent theoretical investigations into the structural, electrical, and optical properties of halide perovskite materials in relation to their applications in solar cells. We also discuss some current challenges of using perovskites in solar cells, along with possible theoretical solutions.

Anomalous Alloy Properties in Mixed Halide Perovskites.

Engineering halide perovskite through mixing halogen elements, such as CH3NH3PbI3-xClx and CH3NH3PbI3-xBrx, is a viable way to tune its electronic and optical properties. Despite many emerging experiments on mixed halide perovskites, the basic electronic and structural properties of the alloys have not been understood and some crucial questions remain, for example, how much Cl can be incorporated into CH3NH3PbI3 is still unclear. In this Letter, we chose CsPbX3 (X = I, Br, Cl) as an example and use a first-principle calculation together with cluster-expansion methods to systematically study the structural, electronic, and optical properties of mixed halide perovskites and find that unlike conventional semiconductor alloys, they exhibit many anomalous alloy properties such as small or even negative formation energies at some concentrations and negligible or even negative band gap bowing parameters at high temperature. We further show that mixed-(I,Cl) perovskite is hard to form at temperature below 625 K, whereas forming mixed-(Br,Cl) and (I,Br) alloys are easy at room temperature.

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.

Creating intermediate bands in ZnTe via co-alloying approach

We propose an effective co-alloying approach to creating an intermediate band (IB) in bulk ZnTe. First-principles calculations show that a donor–acceptor co-alloying scheme can produce an IB within the band gap of ZnTe and that the position of the IB can be tuned by choosing appropriate donor–acceptor pairs. We find that the position of the IB is governed by the atomic d-orbital energies of dopants, and also by the charge transfer between donor and acceptor atoms and their subsequent intra- and inter-Coulomb interactions. Therefore, the location of the IB can be manipulated by selecting donors and acceptors with desirable d-orbital energies and electronegativities.

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.

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.