Nanostructured III-V Photovoltaics
Nanostructured Concentrator Photovoltaic
Our terrestrial approach involves use of nanostructures and III-V materials in high concentration PV systems. Under high concentration (200X-2000X), the cell cost plays less of a role and maximizing the cell efficiency becomes crucial. Thus, in this area, one can benefit from the increased efficiency of III-V based solar cells and under concentration; the higher costs of III-V solar cells are mitigated and increased efficiency translates directly into lower system cost. Recent advances in concentrator III-V photovoltaic systems have demonstrated that III-V based photovoltaics can result in direct reduction in cost per Watt.
The single junction limiting efficiency of a solar cell is given by the detailed balance calculations of Shockley and Queisser. Shown here are calculations of maximum efficiency versus bandgap for various concentration levels under AM1.5d illumination. As can been seen, the maximum efficiency ranges from 33% to 44% for concentration from one sun up to 46,200x (maximum theoretical solar concentration). At the same time, the optimal bandgap energy varies from ~1.2eV to 1.0eV. Thus, obtaining maximum efficiency from a solar cell under concentration will require tuning the materials to match the optimal bandgap.
A first approach to nanostructured concentrator photovoltaics can make use of the fact that the electrical and optical properties of nanomaterials can be controlled by changing the particle size. Thus, insertion of QDs or QWs into a standard single junction GaAs solar cell (Eg=1.4eV) can be used to tune the cell for operation under various concentration levels. The figure above shows a simplified band diagram of a QD enhanced solar cell. The extended absorption spectrum (and thus enhanced short circuit current) is predicted to occur in two steps. First, incident photons with energy below the host (GaAs) bandgap result in absorption to the quantum confined region (IB), creating a separate thermal distribution of electrons. Secondly, promotion (and subsequent collection) of these carriers from the IB to the conduction band can occur by either thermal (thermal assisted extraction), tunneling, or optical (photon assisted extraction) means. This project seeks to provide new photovoltaic cells for HCPV systems with higher efficiency, more favorable temperature coefficients and less sensitivity to changes in spectral distribution.
- Recent Publications:
- 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).