Cu2ZnSnS4 thin film solar cells

An essential issue in developing semiconductor devices for photovoltaics and thermoelectrics is to design materials with appropriate band gaps plus the proper positioning of dopant levels relative to the bands. Local density (LDA) and generalized gradient approximation (GGA) density functionals generally underestimate band gaps for semiconductors and sometimes incorrectly predict a metal. Hybrid functionals that include some exact Hartree−Fock exchange are known to be better. We show here for CuInSe2, the parent compound of the promising CIGS Cu(InxGa1−x)Se2 solar devices, that LDA and GGA obtain gaps of 0.0−0.01 eV (experiment is 1.04 eV), while the historically first global hybrid functional, B3PW91, is surprisingly better than B3LYP with band gaps of 1.07 and 0.95 eV, respectively. Furthermore, we show that for 27 related binary and ternary semiconductors, B3PW91 predicts gaps with a mean average deviation (MAD) of only 0.09 eV, which is substantially better than all modern hybrid functionals.

Accurate Band Gaps for Semiconductors from Density Functional Theory

Sequential cation cross-substitution in zinc-blende chalcogenide semiconductors, from binary to ternary to quaternary compounds, is systematically studied using first-principles electronic structure calculations. Several universal trends are found for the ternary and two classes of quaternary chalcogenides, for example, the lowest-energy structure always has larger lattice constant $a$, smaller tetragonal distortion parameter $\ensuremath{\eta}=c/2a$, negative crystal-field splitting at the top of the valence band, and larger band gap compared to the metastable structures for common-row cation substitution. The band structure changes in the cation substitution are analyzed in terms of the band offsets and band character decomposition, showing that although the band gap decreases from binary II-VI to ternary ${\text{I-III-VI}}_{2}$ are mostly due to the $p\ensuremath{-}d$ repulsion in the valence band, the decreases from ternary ${\text{I-III-VI}}_{2}$ to quaternary ${\text{I}}_{2}{\text{-II-IV-VI}}_{4}$ chalcogenides are due to the downshift in the conduction band caused by the wave-function localization on the group IV cation site. We propose that common-row-cation ${\text{I}}_{2}{\text{-II-IV-VI}}_{4}$ compounds are more stable in the kesterite structure, whereas the widely assumed stannite structure reported in the literature for experimental samples is most likely due to partial disorder in the I-II (001) layer of the kesterite phase.

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Electronic structure and stability of quaternary chalcogenide semiconductors derived from cation cross-substitution of II-VI and I-III-VI2 compounds

This article describes the advances that have been made over the past ten years on the problem of fracton excitations in fractal structures. The relevant systems to this subject are so numerous that focus is limited to a specific structure, the percolating network. Recent progress has followed three directions: scaling, numerical simulations, and experiment. In a happy coincidence, large-scale computations, especially those involving array processors, have become possible in recent years. Experimental techniques such as light- and neutron-scattering experiments have also been developed. Together, they form the basis for a review article useful as a guide to understanding these developments and for charting future research directions. In addition, new numerical simulation results for the dynamical properties of diluted antiferromagnets are presented and interpreted in terms of scaling arguments. The authors hope this article will bring the major advances and future issues facing this field into clearer focus, and will stimulate further research on the dynamical properties of random systems.

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Dynamical properties of fractal networks: Scaling, numerical simulations, and physical realizations

We report CuInSe2/CdS p‐n heterojunction photovoltaic detectors which display uniform quantum efficiencies of up to ∼70% between 0.55 and 1.25 μ. Response times as short as 5 nsec have been observed. A weak electroluminescence (0.01% external quantum efficiency) peaking near 1.4 μ has also been observed at room temperature.

CuInSe2/CdS heterojunction photovoltaic detectors

Various optical and electrical properties of the I-III-V${\mathrm{I}}_{2}$ compounds CuGa${\mathrm{S}}_{2}$ and CuIn${\mathrm{S}}_{2}$ have been studied. From the results of low-temperature luminescence and reflectivity, both crystals are determined to have a direct band gap. The band gaps at 2 \ifmmode^\circ\else\textdegree\fi{}K are 2.53 eV for CuGa${\mathrm{S}}_{2}$ and 1.55 eV for CuIn${\mathrm{S}}_{2}$. CuIn${\mathrm{S}}_{2}$ has been made conducting both $n$ and $p$ type, while CuGa${\mathrm{S}}_{2}$ has been made $p$ type only. Electroreflectance measurements have been performed in an attempt to determine the band structure. The highest valence band appears to be a doublet with a large admixture of Cu $3d$ wave functions.

Electrical Properties, Optical Properties, and Band Structure of CuGa S 2 and CuIn S 2

The valence bands of I-III-${\mathrm{VI}}_{2}$ compounds result from a hybridization of the noble-metal $d$ levels with $p$ levels on the other atoms. This results in large downshifts in the energy gaps relative to the binary analogs, and the absence of spin-orbit splitting in the uppermost valance bands of several sulfides. We estimate that the uppermost valence bands are \ensuremath{\sim}40% $d$-like in the Cu compounds and \ensuremath{\sim}20% $d$-like in Ag compounds.

p − d Hybridization of the Valence Bands of I-III- VI 2 Compounds

We report electroreflectance and photoluminescence studies of the chalcopyrite compounds AgIn${\mathrm{Se}}_{2}$ and CuIn${\mathrm{Se}}_{2}$. Observation of photoluminescence at low temperatures at the same energy as the direct energy gaps located by electroreflectance measurements confirms that both compounds have direct band gaps. At 300 \ifmmode^\circ\else\textdegree\fi{}K, the values for the energy gaps are 1.24 and 0.96 eV, respectively. The spin-orbit splittings of the uppermost valence bands as observed in electroreflectance measurements are considerably less than expected for $p$ levels, a result which we attribute to \ensuremath{\sim} 17% hybridization of Ag $4d$ levels, and \ensuremath{\sim} 34% hybridization of Cu $3d$ levels, with the otherwise $p$-like valence bands. An ultraviolet electroreflectance structure observed in CuIn${\mathrm{Se}}_{2}$ may result from transitions from the $d$ levels themselves to the lowest conduction-band minimum. The crystal-field and spin-orbit parameters for the uppermost valence bands of CuIn${\mathrm{Se}}_{2}$ disagree with values found in a recent energy-band calculation ignoring $d$ bands, a calculation which also predicted that CuIn${\mathrm{Se}}_{2}$ has an indirect energy gap. We also observe an anomalous temperature dependence of the energy gap in AgIn${\mathrm{Se}}_{2}$. Whereas the energy gap in CdSe (the binary analog of AgIn${\mathrm{Se}}_{2}$) decreases by approximately 80 meV as the temperature increases from 77 to 300 \ifmmode^\circ\else\textdegree\fi{}K, the energy gap of AgIn${\mathrm{Se}}_{2}$ is independent of temperature over this range within experimental error (\ifmmode\pm\else\textpm\fi{} 5 meV).

Electronic Structure of AgIn Se 2 and CuIn Se 2

From the results of low-temperature luminescence and reflectivity, both AgGa${\mathrm{S}}_{2}$ and AgGa${\mathrm{Se}}_{2}$ are determined to have a direct energy band gap. The values are 2.727 and 1.830 eV at 2 \ifmmode^\circ\else\textdegree\fi{}K, respectively. The gap shifts to slightly higher energy at 77 \ifmmode^\circ\else\textdegree\fi{}K, which is opposite to that observed in most semiconductors. Both crystals appear to contain shallow impurities or defects. However, the crystals are semi-insulating as-grown, and various annealing and diffusion procedures have failed to produce useful conductivity.

Optical and Electrical Properties of AgGa S 2 and AgGa Se 2

The symmetries and splittings of the uppermost valence bands in orthorhombic and chalcopyrite AgIn${\mathrm{S}}_{2}$ have been determined from electroreflectance measurements on oriented crystals using polarized radiation. The principal features of the valence-band structures in both phases are dominated by the deviations of the lattice constants from ideal wurtzite and chalcopyrite, respectively. On the other hand, the small splitting of the lowest two valence bands in orthorhombic AgIn${\mathrm{S}}_{2}$ is attributed to the "wurtzite-like" potential. The lowest band gaps at 300 \ifmmode^\circ\else\textdegree\fi{}K in the orthorhombic and chalcopyrite phases are 1.98 and 1.86 eV, respectively.

B. Tell

Papers

Electrical Properties, Optical Properties, and Band Structure of CuGa S 2 and CuIn S 2

p − d Hybridization of the Valence Bands of I-III- VI 2 Compounds

Electronic Structure of AgIn Se 2 and CuIn Se 2

Optical and Electrical Properties of AgGa S 2 and AgGa Se 2

Energy bands of AgIn S 2 in the chalcopyrite and orthorhombic structures