Effects of Zn diffusion in tunnel junction and its solution for high efficiency large area flexible GaInP/GaAs/InGaAs tandem solar cell

Abstract The zinc (Zn) diffusion effects on metal–organic chemical vapor deposition grown flexible inverted metamorphic (IMM) GaInP/GaAs/InGaAs triple junction solar cell performance have been investigated. By studying electrical and spectral properties of solar cells with different configuration, it was found out that the Zn diffusion from GaAs middle cell AlGaAs:Zn back surface field (BSF) layer into AlGaAs/GaAs tunnel junction is the main reason for I–V curve inflection and low fill factor. Besides, the thermal annealing effects caused by the long epitaxial growth time also has significant effect on Zn diffusion and degrades the overall cell performance. These adverse effects caused by Zn diffusion was suppressed by replacing Zn with carbon as the p-type dopant source in BSF layer, and the thermal annealing effects was solved by fine tuning InGaAs bottom subcell base layer thickness. A large area, 4 × 2 cm2, flexible IMM GaInP/GaAs/InGaAs triple junction solar cell with 34.52% efficiency under AM 1.5D solar spectrum has been successfully fabricated.