Amy Swartzlander

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin‐film solar cells

This short communication reports on achieving 18·8% total-area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se2/Mo polycrystalline thin-film solar cell. We also report a 15%-efficient, Cd-free device fabricated via physical vapor deposition methods. The Cd-free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se2 thin-film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m2, 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin-film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

High Tc superconductor/noble‐metal contacts with surface resistivities in the 10−10 Ω cm2 range

Contact surface resistivities (product of contact resistance and area) in the 10−10 Ω cm2 range have been obtained for both silver and gold contacts to high Tc superconductors. This is a reduction by about eight orders of magnitude from the contact resistivity of indium solder connections. The contact resistivity is low enough to be considered for both on‐chip and package interconnect applications. The contacts were formed by sputter depositing either silver or gold at low temperatures (<100 °C) on a clean surface of Y1 Ba2 Cu3 O7−δ (YBCO) and later annealing the contacts in oxygen. Annealing temperature characteristics show that for bulk‐sintered YBCO samples there is a sharp decrease in contact resistivity after annealing silver/YBCO contacts in oxygen for 1 h at temperatures above ∼500 °C and gold/YBCO contacts for 1 h above ∼600 °C. Oxygen annealing for longer times (8 h) did not reduce the contact resistivity of silver contacts as much as annealing for 1 h. Auger microprobe analysis shows that indium...

Phases, morphology, and diffusion in CuInxGa1−xSe2 thin films

CuInxGa1−xSe2 thin films, with various Ga/(Ga+In) ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a H2Se reactor in the temperature range of 400–500 °C. Cu/Ga/In and Cu/In/Ga precursors were obtained by sequential sputtering of the elemental layers. The Cu/Ga/In and Cu/In/Ga precursors, and the selenized films were characterized by scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The precursors contained only binary and elemental phases in the as-deposited condition and after annealing. The selenized films had a nonuniform distribution of Ga and In. The surface of the selenized films were In rich, while the Mo/film interface in these films was Ga rich. The selenized films with Ga/(Ga+In) ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with Ga/(Ga+In) ratios less than 0.6 contain a phase-separated mixture of CuInSe2 and CuGaSe2 with the CuInSe2 ne...

On the role of Na and modifications to Cu(In,Ga)Se/sub 2/ absorber materials using thin-MF (M=Na, K, Cs) precursor layers [solar cells]

The growth and characterization of Cu(In,Ga)Se/sub 2/ polycrystalline thin film solar cells under the presence of thin-MF (M=Na, K, Cs) precursor layers is presented. Some electrical, structural and electronic absorber properties due to the presence of such Group Ia impurities are quantified along with their influence in device performance. The authors present a growth model for the role of Na in Cu(In,Ga)Se/sub 2/ that attributes the enhancements in electrical conductivity and photovoltaic device performance to the extinction of a finite number of donor states (i.e., In/sub Cu/) at the bulk and grain-boundary regions.

Texture manipulation of CuInSe2 thin films

We present a growth method that allows tailoring of preferred orientation in CuInSe2 thin-films grown on Mo-coated soda-lime glass substrates. In particular, films exhibiting a (204) preferred orientation are demonstrated, in addition to already reported (112) and randomly oriented films. Effects of substrate composition, growth temperature, and final film composition on texture phenomena are presented. In general, we find that texture is highly dependent on growth temperature, substrate material and, in the case of Mo-coated soda-lime glass substrates, the structural properties of the Mo layer. We provide evidence for the attainment of such structural orientation, and we present a growth model to explain the mechanism allowing such modifications.

BGaInAs alloys lattice matched to GaAs

We report the epitaxial growth of zinc-blende BxGa1−x−yInyAs and BxGa1−xAs on GaAs substrates with boron concentrations (x) up to 2%–4% by atmospheric-pressure metalorganic chemical vapor deposition. The band gap of BxGa1−xAs increases by only 4–8 meV/%B with increasing boron concentration in this concentration range. We demonstrate an epitaxial BxGa1−x−yInyAs layer deposited on GaAs with a band gap of 1.34 eV that is significantly less strained than a corresponding Ga1−yInyAs layer with the same band gap.

Epitaxial growth of BGaAs and BGaInAs by MOCVD

Abstract The growth of epitaxial zinc-blende B x Ga 1− x As and B x Ga 1− x − y In y As alloys using diborane is complicated by a thermodynamic miscibility gap and complex gas-phase chemistry. We have characterized the growth behavior of these alloys when they are grown using trimethylgallium (TMG), triethylgallium (TEG), trimethylindium (TMI), and arsine by low-pressure and atmospheric-pressure metal–organic chemical-vapor-deposition. Boron incorporation into B x Ga 1− x As exhibits qualitatively different behavior using TEG and TMG, but the incorporation efficiency and maximum achievable boron concentration decrease dramatically at growth temperatures greater than 600°C using either Ga source.

Studies on sulfur diffusion into Cu(In,Ga)Se2 thin films

A systematic study was carried out to investigate the distribution of sulfur (S) in CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) absorbers which were exposed to an H2S atmosphere at elevated temperature. Results demonstrated that S diffusion into CIS layers was a strong function of the original stoichiometry of the absorber before sulfurization. Sulfur inclusion into Cu-rich CIS films was much more favorable compared to S diffusion in Cu-poor layers. The sulfur distribution profile was also strongly influenced by the micro-structure of the original CIS and CIGS layers, with sections of the films with smaller grains accommodating more S. Copyright

12.3% Efficient CuIn1 − x Ga x Se2‐Based Device from Electrodeposited Precursor

Of the emerging materials for solar cell applications, CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) is a leading candidate and has received considerable attention in recent years. Copper-indium-gallium-selenium (Cu-In-Ga-Se) precursor thin films have been prepared by electrodeposition techniques on molybdenum substrates. The films have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, x-ray diffraction, electron probe microanalysis, current-voltage characteristics, spectral response, and electron-beam-induced current. Additional In or Cu, Ga, and Se have been added to the electrodeposited precursor film by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}, and allowed to crystallize at 550 C. A ZnO/CdS/CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} device fabricated using electrodeposited Cu-In-Ga-Se precursor layers resulted in an efficiency of 12.3%.

Photoemission investigation of the electronic structure at polycrystalline CuInSe2 thin‐film interfaces

The surface versus bulk composition and electronic structure of polycrystalline CuInSe2 thin‐film interfaces were studied by synchrotron radiation soft‐x‐ray photoemission spectroscopy. An n‐type In2Se3/CuIn3Se5 surface layer forms on enhanced‐grain polycrystalline thin‐film p‐type CuInSe2 during fabrication. Enhanced‐grain CuInSe2 films were sputter etched (500 V Ar) and analyzed in situ to determine core‐level binding energies and Fermi‐level positions for the n‐type surface and the p‐type CuInSe2 bulk within ±0.1 eV. The transition between the n‐type surface and the p‐type bulk was experimentally observed by noting the change in the position of the valence‐band maximum relative to the Fermi level EF. From these measurements, the valence‐band offset ΔEv between the layers was determined to be 0.50 eV. Measurement of the work functions φ was also completed and reveals φ=4.75 eV for the In2Se3 (CuIn3Se5) surface layer and φ=4.04 eV for the bulk CuInSe2. Combining these results allows construction of a sur...