Nanostructured III-V Photovoltaics
The recent demand for alternative energy production has propelled considerable interest in next-generation photovoltaics. The preliminary 2007 Department of Energy report on U.S. energy consumption states that renewable sources account for only 7% of the energy production, with solar energy representing a minuscule 1%. Solar energy is a particularly attractive renewable source given that the sun radiates more energy onto the earth in one hour than the entire human population uses in one whole year.
The Nanostructured Photovoltaics (Nano-PV) approach presents a highly transformative technology, with an overarching goal of making PV cost competitive. The nanomaterials approach to PV device development centers around the fact that the electrical, optical and even thermal properties of these materials can be controlled by changing the particle size. Our focus in this area is for both space and terrestrial applications.
- 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).
- C. Cress, S.M. Hubbard, B. Landi, D. Wilt, R. Raffaelle, “Quantum dot solar cell tolerance to alpha-particle irradiation”, Appl. Phys. Lett. 91, 183108 (2007).
Nanostructured Concentrator Photovoltaics
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 in the i-region of the IBSC design. The extended absorption spectrum (and thus enhanced short circuit current) is predicted to occurs 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).
Quantum Dot Space Solar Cells
Improving the efficiency of solar cells is the main focus of the space photovoltaic community. Design enhancements at the cell and module levels have steadily increased the efficiency over the years. Current state-of-the-art triple-junction space cells exceed 30% efficiency. The ultimate efficiency of high-efficiency cells is constrained by several factors. The available bandgaps for each junction is restricted by the lattice-matching requirement for high quality material. The current use of InGaP/GaAs/Ge, while enabling >30% efficiency, is not the most optimum bandgap combination for highest efficiency.
An additional limitation is the series nature of the multiple junctions with the high-efficiency solar cell. Each cell must be constructed to be current-matched with the others in series. This reduces currents and increases device complexity. The current-limiting junction in a conventional triple-junction cell is the GaAs middle cell. Theoretical calculations have shown that the overall efficiency of the triple-junction stack can be markedly improved by extending the spectral bandwidth of the cell and thus increasing the current. The lattice-matching requirement, however, preclude a straightforward extension of the composition of the middle cell without resorting to buffer layer methodologies.
Current enhancements can be obtained via an extended absorption spectrum of low bandgap, low-dimensional materials. In this approach, quantum-size features with lower bandgap within the current limiting junction absorb extra photons that would otherwise be transmitted. This approach mitigates the lattice-matching constraint by using nanoscale heterostructures. Modeling of an InGaP/GaAs/Ge triple junction cell, in which QDs are used to extend the middle junction absorption spectrum, have indicated that the efficiency could be improved to 47% under one sun AM0.
- Recent Publications:
- R.P. Raffaelle, S. Sinharoy, J. Andersen, D.M. Wilt, S.G. Bailey, “Multi-Junction Solar Cell Spectral Tuning with Quantum Dots”, Proc. of the IEEE World Conference on Photovoltaic Energy Conversion, 1, 162 (2006).
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.
