| 摘 要： | 1 Results The conversion efficiency of sunlight to electricity is limited around 25%,when we use single junction solar cells. In the single junction cells,the major energy losses arise from the spectrum mismatching. When the photons excite carriers with energy well in excess of the bandgap,these excess energies were converted to heat by the rapid thermalization. On the other hand,the light with lower energy than that of the bandgap cannot be absorbed by the semiconductor,resulting in the losses. One way of reducing these losses is fabricating tandem or stacked solar cells. Merely stacking the more number of cells with different bandgap can increase the conversion efficiency. III-V compound multi-junction solar cells,such as InGaP/GaAs/Ge,have the potential for achieving high conversion efficiencies that are promising for space and terrestrial applications. One of the important issues for realizing the high conversion efficiency is the optically and electrically low-loss interconnection of each sells. A degenerately impurity doped tunnel junction,which is thin and wide-bandgap,is an attractive one,and a double hetero structure is useful for preventing impurity diffusion during the overgrowth of the top cell. Another issue is a lattice matching. Since there is a slightly difference in the lattice constant between Ge substrate and GaAs,the misfit dislocations are generated in thick GaAs layers and the electrical properties degrade. To prevent this problem,InGaAs is applied as a middle cell material,which is lattice matching to the Ge substrate. So far,the conversion efficiency of InGaP/InGaAs/Ge has been improved up to 29%-30% (AM0) and 31.7% (AM1.5G). The concentrator cells achieved the higher conversion efficiency up to 40.7% under 240 suns. For realizing a future multi-junction solar cell with ultra-high performance,InGaAsN and related materials are investigated,because it can be grown lattice matched to GaAs with a band gap in the range of 0.9-1.4 eV. When we adopt the InGaP/GaAs/InGaAsN/Ge structure as a four-junction solar cell,the efficiency over 40% (AM1.5G) will be expected. However,the minority carrier diffusion length in the present InGaAsN crystal is too short to realize the tandem solar cells with the expected high performance. To solve this problem,we have been developing the chemical beam epitaxy (CBE) method for achieving InGaAsN with good quality. The films are grown using organic gas molecules as sources under a high vacuum condition (10-2 Pa). Because of the ultra low pressure,the reactions between the source gases in the gas phase are suppressed and the chemical reactions occur only on a growing surface,which allow using active source gases that decompose at low temperatures. GaAsN thin films are grown using monomethylhydrazine as a N source with narrow X-ray diffraction peaks at growth temperatures in the 380-420 ℃ range. In the present talk,we will review the progress of high conversion efficiency tandem-solar cells and discuss some of our recent results.
|
