Fabrication of GaAs and GaP Inetrmediate Band Solar Cells by Liquid Phase Epitaxy and the Incorporation of Transition Metals

The intermediate band solar cell (IBSC) concept was proposed in 1997 by Professor A. Luque in Spain. The addition of an intermediate band (IB) has been proposed as a method for creating a single-junction cell with a theoretical efficiency better than that of a double-junction solar cell. The basic idea is to introduce an IB of energy levels within the normally forbidden energy gap of the host semiconductor to enable the absorption of sub-bandgap photons. The output voltage is given by the difference between electron and hole quasi-Fermi levels and is limited by the total bandgap. In this way, the ability of the IBSC to increase the photogenerated current without voltage degradation leads to a very high efficiency. The insertion of transition elements into the semiconductor host is expected to introduce deep states that would act as non-radiative recombination centers. At low impurity concentrations, it would therefore be detrimental to the performance of a solar cell due to non-radiative recombination centers. However, non-radiative recombination can be inhibited through the formation of a band if the density of these impurities exceeds the limit established by the Mott transition (＞ 6×1019 cm-3). From the theoretical simulation, it is known that the value of bandgap GaP is the better IBSC candidate material and its limiting efficiency of 61.8% is closer to the absolute limiting efficiency of 63.2% for an IBSC, compared with 59.5% for GaAs. In this project, we will study the incorporation of two transition elements (Ti or Fe) into GaAs and GaP grown by liquid phase epitaxy (LPE) and will produce the IBSCs of GaAs and GaP. The LPE is a thermal equilibrium growth technique and has the advantages of low cost and high epi-layer quality. In past, it was widely used to fabricate high quality light-emitting diodes and already extremely was mature. Many papers have been investigated in the incorporation of various transition metals into GaAs or GaP to form deep energy levels in bandgap. They focused on the defect energy level or the getting effects. However, no discussion about the amounts of incorporated impurity relates to the IB formation and solar cell performance. In this project, we will extensively investigate the amount of incorporated transition metals correlating to the IB formation and solar cell performance. The fabrication process and performance characterization will be deeply examined. It is believed that the implementation of high efficiency IBSCs of GaAs and GaP will be achievable.

Project ID:PB9807-2591
External Project ID:NSC98-2221-E182-006