Ultra‐thin flexible Ge solar cells for lattice‐matched thin‐film InGaP/(In)GaAs/Ge tandem solar cells

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.