| Research Abstract |
Generation of high output current from a photovoltaic cell require a low band gap to maximize absorption in the semiconductor material, whereas high output voltage requires a high band gap to maximize Fermi level separation. The optimal output is therefore a particular band gap, which is a compromise between conflicting requirements upon current and voltage. The aim of the multi Quantum Well (MQW) Solar Cell was to eliminated this compromise by allowing the optimum current and voltage to be determined separately. Although these parameters cannot in practice be made independent, it is hoped that an MQW cell can be made to be more efficient than a traditional cell made of material of a single band gap. However, the quality of the active layrs in MQW cells is degraded by the lattice mismatch between the GaAs and InGaAs layrs. Optimizing the stress in this configuration, the conversion efficiency has been improved to 18% at the 1sun AM1.5 conditions. MQW GaAs/In_<0.19>Ga_<0.81>As solar cel
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ls have been measured under low concentration levels (1-10suns). An efficiency of 22% has been obtained at a ratio of 4suns as opposed to 18% under 1sun AM1.5 conditions. We explain the results in terms of an enhancement in minority-carrier lifetime under concentration. Even when the concentration ratio is low, the high-injection regime can be achieved since the carrier concentration in the intrinsic layr is very low. The existence of a high concentration of defects in the base layr has been observed by the DLTS analysis. Enhancement of the minority-carrier lifetime under concentration is thought to be due to recombination probability saturation of recombination centers with high-injection of minority carriers.
In order to transfer the high efficiency cell design for 1sun operation into a concentrator design, the photovoltaic properties of InGaP/GaAs tandem solar cells under concentration have been investigated. A high efficiency of 31.2% has been obtained for the InGaP/GaAs tandem solar cell under 5.1suns of AM1.5 illumination (Jsc=70.28mA/cm^2, Voc=2.59V,FF=0.874). However, the efficiency of the tandem cell is limited due to the reduction in the fill factor as the light intensity increases. The results show that very high peak current, Jp of the tunnel diode is not only needed but also the valley current, Jv, of the tunnel diode is necessary for high concentration, which should be greater than the short current, Jsc under concentration. The efficiency predications of the InGaP/GaAs tandem cell for concentration applications have been carried out, the results shows that to reduce the series resistance to lower than 0.01OMEGA/cm^2 for use under high concentratios (100-500suns). A conversion efficiency of over 33% will be obtained at 500X if this can be achieved. The temperature properties of the InGaP/GaAs tandem cell are very good even at relatively high temperatures 1sun condition. However, it is expected that at a high temperatures, current mismatch will give a significant effect on the efficiency under concentration, which should be considered in the design of concentrator cells. Less
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