Nanostructured III-V Photovoltaics
Nanomaterials and Device Epitaxy
The enabling feature of many of these next generation PV technologies is not only the ability to grow the PV device, but also the ability for synthesis of the nanostructures. In both the research and industrial field, organomatallic vapor phase epitaxy (OMVPE) has developed into a viable technique for production of semiconductor device layers. OMVPE is the preferred epitaxial growth method for large scale production companies due to reduced system downtime and low overall cost of ownership. Nanostructured devices produced by OMVPE would be easily transferable to the larger industrial scale OMVPE systems.
Development of quantum dot (QD) epitaxy has been rapid in recent years as many improvements in device performance are anticipated. The application of QD’s to laser diodes , have enabled modest improvement in many device characteristics. The application of QD’s to semiconductor devices is relatively new and continued effort is required to fulfill all the theoretically anticipated benefits.
QD epitaxy by organometallic vapor phase epitaxy (OMVPE) occurs by the Stranski-Krastanov (SK) mechanism where a lattice mismatched material (ε ~ 1-10%) is deposited on a substrate. In initial stages, a thin wetting layer is formed epitaxially in a two-dimensional growth mode. After reaching a critical thickness, the surface strain energy is minimized by forming 3-D islands. Ideally these islands are strained and coherent with the surrounding material. The InAs/GaAs system satisfies the requirements for both SK epitaxy (strain ~ 7%), as well as bandgap (0.35eV), necessary for sub-bandgap absorption within a GaAs matrix.
QD nucleation, growth and uniformity becomes are seen to be more sensitive toward OMVPE growth parameters such as growth temperature, growth rate and V/III ratio. Fundamental characterization of growth mechanisms and physical behavior of quantum nanostuctures is important to advancing their use in commercial PV devices and OMVPE systems. Shown below are Atomic Force Micrographs of InAs quantum dots on a GaAs substrate grown using the OMVPE technique. These dots were subsequently used in high efficiency solar cells and resulted in substantial gains in quantum dot enhanced solar cell efficiency.
- Recent Publications:
- S.M. Hubbard, C.D. Cress, C.G. Bailey, R.P. Raffaelle, S.G. Bailey, D.M. Wilt, “Effect of strain compensation on quantum dot enhanced GaAs solar cells”, Appl. Phys. Lett. 92, 123512 (2008).
- S. M. Hubbard, C. Bailey, C. D. Cress, S. Polly, J. Clark, D. V. Forbes, R. P. Raffaelle, S. G. Bailey, D. M. Wilt, “Short circuit current enhancement of GaAs solar cells using strain compensated InAs quantum dots”, Proc. of 33rd IEEE Photovoltaic Specialists Conf. 1, pp. pending (2008).
- S.M. Hubbard, D. Wilt, S. Bailey, D. Byrnes, R. Raffaelle, “OMVPE Grown InAs Quantum Dots for Application in Nanostructured Photovoltaics”, Proc. of the IEEE World Conference on Photovoltaic Energy Conversion, 2006, pp. 118-121.