| Summary: | 碩士 === 長庚大學 === 電子工程學系 === 99 === 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 GaAs is the better IBSC candidate material and its limiting efficiency of 59.5%. In this thesis, we will study the incorporation of the transition elements (Fe) into GaAs grown by liquid phase epitaxy (LPE). Experimental results indicated that the carrier concentration and mobility of Fe-doped GaAs samples are decreased and increased, respectively, in comparison with un-doped GaAs samples. In photoluminescence measurement, a trap of 0.5eV from valence band is observed and increased with increasing Fe-doping. It can be attributed to intermediate band formation or lattice relaxation. Otherwise, the band-to-band peak intensity decreases and half width at full maximum increased. It indicates that the quality of Fe-doped samples is not good. For the fabricating solar cells, the soar efficiency is reduced from 9.88% to 2.15%, short circuit current is increased from 1.07 to 1.51mA and open circuit voltage is reduced from 0.82 to 0.24V.
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