High Efficiency, Inexpensive Thin Film III-V Photovoltaics using Single-Crystalline-Like, Flexible Substrates
- Univ. of Houston, TX (United States)
An innovative, non-mainstream approach has been developed to fabricate inexpensive GaAs solar cells directly on epi-ready flexible metal substrates. Using templates made by ion beam assisted deposition (IBAD) and epitaxial germanium films made by Plasma Enhanced Chemical Vapor Deposition (PECVD), epitaxial GaAs films with high opto-electronic and structural quality have been grown on metal substrates using metal organic chemical vapor deposition (MOCVD). Crystallographic out-of-plane and in-plane texture spread less than 1 degree full-width-at-half-maximum (FWHM) was achieved in undoped, n-type and p-type GaAs films as well as in AlGaAs films on metal substrates. Electron mobility of >1300 cm2/V-s was attained in GaAs:Si n-type films. Also, hole mobility of >100 cm2/V-s was measured in GaAs:Zn films. Consistent doping was obtained in GaAs:Si (1015-1018 cm-3), and GaAs:Zn films (1017-1019 cm-3), necessary for the active device layers of a GaAs solar cell. Defect reduction techniques (thermal cycling, superlattice structures) were implemented to reduce defect density in GaAs films from 1 × 109 cm-2 to 9 × 107 cm-2. Lifetime of 2.3 ns was obtained in AlGaAs/GaAs double-heterostructure device. Solar cells were fabricated based on designs using 1-dimensional and 2-dimensional simulations tailored to the specific properties of the GaAs films on metal substrates. All device fabrication processes (lithography, anti-reflection coating (ARC), device isolation and mesa formation and ohmic metal electrode formation) suitable for III-V solar cell fabrication on metal substrates were fully developed. A bi-layered ARC using ZnS and MgF2 was used to minimize the reflection over wide range of spectrum. When this ARC was applied after removal of the parasitic absorption layer, the conversion efficiencies were improved up to 11.1% with Voc = ~0.621 V, Jsc = ~24.5 mA/cm2, and FF = ~0.735. Yet another accomplishment in this project was the development of all electrically-conducting architecture for GaAs solar cells on metal substrates. Biaxial-texture was successfully achieved in all electrically-conducting buffer layer on metal substrate in structure based on IBAD TiN instead of IBAD MgO. The all electrically-conducting Ge/NiSi2/TiN/IBAD-TiN/Hastelloy substrate was used to fabricate back contacts for vertically conducting solar cell structures on metal substrates. A metallic resistivity as low as 4.17 μΩ•cm was obtained. A significant improvement of the texture of the Ge template was achieved using a Ag buffer layer between the TiN films. Epitaxial GaAs was successfully grown on the all-electrically-conducting buffer architecture. A Liquid Phase Epitaxy (LPE) method to drastically increase grain size of epitaxial Ge while maintaining the single-crystalline-like texture. Grain size up to 35 µm was achieved in LPE Ge grown on sputtered Ge with a grain size of ~ 1 µm on IBAD template on metal substrate. Epitaxial GaAs was grown on LPE Ge, with an out-of-plane texture of 0.34° compared to 0.98° in GaAs grown on sputter deposited Ge. A model was developed to simulate the effect of grain boundaries as a function of grain size of GaAs films on metal substrates. This model showed that even the very low angle grain boundaries (<1º misorientation) of our GaAs films led to decrease in Voc. Hydrogen plasma passivation treatment is being developed to improve open circuit voltage and the conversion efficiency of GaAs solar cells on flexible substrate.
- Research Organization:
- Univ. of Houston, TX (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- EE0006711
- OSTI ID:
- 1508926
- Report Number(s):
- DOE-UH-6711-1
- Country of Publication:
- United States
- Language:
- English
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