Studies on high-efficiency and low-cost compound semiconductor/Si tandem solar cell

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Compound semiconductor/Si tandem solar cell has attracted attention for high efficiency solar cell more than 30%. The important technology to obtain a high efficiency solar cell is to reduce the dislocation and the stress of GaAs on Si. We reduced the dislocation density and the stress by thermal cycle annealing (TCA) and YAG laser irradiation, respectively. The growth was performed by rf-heated metalorganic chemical vapor deposition. The dislocation density is reduced with increasing the TCA temperature during the growth. The minority carrier is improved to 3.36 ns with utilzing the 1000ﾟC TCA.The buffer layr of InGaAs/GaAs strained layr superlattice is also effective to improve the minority carrier lifetime. The bending of the dislocation was confirmed at the interfaces by the transmission electron microscopy. The dislocation density of GaAs on Si with 1000ﾟC TCA is 9*10^6 cm^<-2>. The stress-free GaAs was obtained on Si substrate with optimizing the YAG laser pulse irradiation conditions such as laser power, pulse width, number of irradiation, etc.AlGaAs/Si and GaAs/Si tandem solar cell were fabricated using the high quality cystal. The conversion efficiency was improved with utilizing the compositionally step graded emitter layr. The current matching was realized by the Al_<0.15>Ga_<0.85>As/Si tandem solar cell, and the efficiency was 21.2 % under two-terminal configuration. In the case of GaAs/Si tandem solar cell, 22.1% was obtained (top cell : 17.7%, bottom cell : 4.4%) under three-terminal configuration. This is the highest efficiency for monolithic for compound semiconductor/Si tandem solar cell as ever reported.