Potential and Present Status of III-V/Si Tandem Solar Cells

III-V compound and Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. However, problems to be solved are high density of dislocations and large residual strain in III-V compound on Si in the case of hetero epitaxy of III-V compound materials on Si substrates. There are other approaches such as epitaxial lift-off and direct bonding technologies in order to prevent such problems. This paper presents efficiency potential of III-V/Si tandem solar cells and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. The paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of 111-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells. We have demonstrated 20% efficiency (under I-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si and Ohio State University has also demonstrated 17.1 % (AMO) efficiency with GaAs single-j unction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding. Future prospects are also discussed.