| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
N4, Thin Film III-V Photovoltaics on Flexible Metal Substrates and Defect Mitigation Strategies |
| Author(s) |
Senthil Nathan Sambandam, Aarthi Sundaram, Akhil Mehrotra, Alex Freundlich, Venkat Selvamanickam |
| On-Site Speaker (Planned) |
Senthil Nathan Sambandam |
| Abstract Scope |
We have recently established a program to develop technology that combines the low-cost advantages of thin film PV with the high efficiencies only achieved with single crystalline photovoltaics. III-V photovoltaics have exhibited efficiencies above 40%, but have found only a limited use because of the high cost of single crystal substrates. At the other end of the spectrum, polycrystalline and amorphous thin film solar cells offer the advantage of low-cost fabrication, but have not yielded high efficiencies. Our program is based on single-crystalline-like thin film photovoltaics on polycrystalline substrates using biaxially-textured templates made by Ion Beam-Assisted Deposition (IBAD). MgO templates made by IBAD on flexible metal substrate have been successfully used for epitaxial growth of germanium films. In spite of a 4.5% lattice mismatch, heteroepitaxial growth of Ge was achieved on CeO2 that was grown on IBAD MgO template. Refraction index and extinction coefficient values of the Ge films were found to match well with that measured from a reference Ge single crystal. GaAs has been successfully grown epitaxially on Ge on metal substrate by molecular beam epitaxy. RHEED patterns indicate self annihilation of antiphase boundaries and the growth of a single domain GaAs. The GaAs is found to exhibit strong photoluminescence signal and, an existence of a relatively narrow (FWHM~20 meV) band-edge excitons measured in this film indicates a good optoelectronic quality of deposited GaAs. While excellent epitaxial growth has been achieved in GaAs on flexible metal substrates, the defect density of the films as measured by High Resolution X-ray Diffraction and etch pit experiments showed a high value of 5 * 10^8 per cm^2. Cross sectional transmission electron microscopy of the multilayer architecture showed concentration of threading dislocations near the germanium-ceria interface. The defect density was found decrease as the Ge films were made thicker. The defects appear to originate from the MgO layer presumably because of large lattice mismatches between the various layers. The defect density in GaAs was reduced by a factor of five by adding a step of in-situ deposition of Ge by MBE on the sputtered Ge prior to GaAs growth. A simple non-intrusive photoresist based lithographic process has been used to define high quality patterns on GaAs coated on flexible substrates in a single mesa etch and metal evaporation/resist lift-off. Patterns resolution of few microns with well-defined grid line of 30 µm has been realized on flexible templates. Solar cells of GaAs have been fabricated and defects caused by various mechanisms including diffusion of germanium have been observed. Outcomes of various strategies being pursued to mitigate defects in the III-V photovoltaic films on flexible substrates will be discussed in this presentation. |
| Proceedings Inclusion? |
Undecided |