Ultrathin Flexible Ge Solar Cells for Lattice-Matched Thin-Film InGaP/(In)GaAs/Ge Tandem Solar Cells

Abstract

Ultrathin Ge single-junction (1J) solar cells transferred onto a flexible substrate are envisioned to open up a novel lattice-matched thin-film InGaP/(In)GaAs/Ge tandem solar cell for enabling highly efficient, low-cost, and light-weight flexible devices. The ultrathin Ge 1J solar cell structures are epitaxially grown onto a GaAs substrate via a low-pressure metal–organic chemical vapor deposition system using an isobutylgermane metalorganic source as a Ge precursor. A simple and fast epitaxial lift-off method allows the epi structures to transfer onto the flexible substrates, by which 2 inch wafer-scale flexible ultrathin Ge 1J solar cells with the mechanical stability under bending test (R = 12.5 mm) are fabricated. Their maximum power conversion efficiency (5.40%) is achieved with the optimum thickness of Ge p-n junction as well as a delta-doping technique that utilizes the multiple cycles of Ga-dopant injection and halt during the growth of thick p-Ge base layer. The power-to-weight ratio value of the ultrathin Ge 1J solar cells is 56.65 times higher than that of bulk-type Ge solar cells, holding great potential to be used for the power sources of unmanned aerial vehicles as well as the portable and wearable devices.