Matching Items (2)
Description
III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells.
Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states.
The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented.
Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states.
The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented.
ContributorsMaros, Aymeric (Author) / King, Richard R. (Thesis advisor) / Honsberg, C. (Christiana B.) (Committee member) / Goodnick, Stephen M. (Committee member) / Ponce, Fernando A. (Committee member) / Arizona State University (Publisher)
Created2017
Description
Silicon photovoltaics is the dominant contribution to the global solar energy production. As increasing conversion efficiency has become one of the most important factors to lower the cost of photovoltaic systems, the idea of making a multijunction solar cell based on a silicon bottom cell has attracted broad interest. Here the potential of using dilute nitride GaNPAs alloys for a lattice-matched 3-terminal 2-junction Si-based tandem solar cell through multiscale modeling is investigated. To calculate the electronic band structure of dilute nitride alloys with relatively low computational cost, the sp^3 d^5 s^* s_N tight-binding model is chosen, as it has been demonstrated to obtain quantitatively correct trends for the lowest conduction band near Γ, L, and X for dilute-N GaNAs. A genetic algorithm is used to optimize the sp^3 d^5 s^* tight-binding model for pure GaP and GaAs for their optical properties. Then the optimized sp^3 d^5 s^* s_N parametrizations are obtained for GaNP and GaNAs by fitting to experimental bandgap values. After that, a virtual crystal approach gives the Hamiltonian for GaNPAs alloys. From their tight-binding Hamiltonian, the first-order optical response functions of dilute nitride GaNAs, GaNP, and GaNPAs are calculated. As the N mole fraction varies, the calculated critical optical features vary with the correct trends, and agree well with experiment. The calculated optical properties are then used as input for the solar device simulations based on Silvaco ATLAS. For device simulation, a bottom cell model is first constructed to generate performance results that agree well with a demonstrated high-efficiency Si heterojunction interdigitated back contact (IBC) solar cell reported by Kaneka. The front a-Si/c-Si interface is then replaced by a GaP/Si interface for the investigation of the sensitivity of the GaP/Si interface to interface defects in terms of degradation of the IBC cell performance, where we find that an electric field that induces strong band bending can significantly mitigate the impact of the interfacial traps. Finally, a lattice-matched 3-terminal 2-junction tandem model is built for performance simulation by stacking a dilute nitride GaNP(As) cell on the Si IBC cell connected through a GaP/Si interface. The two subcells operate quasi-independently. In this 3-terminal tandem model, traps at the GaP/Si interface still significantly impact the performance of the Si subcell, but their effects on the GaNP subcell are relatively small. Assuming the interfacial traps are well passivated, the tandem efficiency surpasses that of a single-junction Si cell, with values close to 33% based on realistic parameters.
ContributorsZou, Yongjie (Author) / Goodnick, Stephen M. (Thesis advisor) / Honsberg, C. (Christiana B.) (Committee member) / King, Richard R. (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2019