Progress on III-nitride/silicon hybrid multijunction solar cells

We report on the progress towards a high efficiency InGaN/Si tandem hybrid solar cell. The proof of principle has been demonstrated in a 5 × 5 mm III-nitride/Si dual junction solar cell, with p/n GaN junction grown by molecular beam epitaxy (MBE) functioning as the top cell and a standard n-type Si wafer with an Al doped p-type surface functioning as the bottom cell. An open circuit voltage (Voc) of 2.5 V was measured under 1× AM1.5G illumination conditions with additional UV laser illumination of the GaN junction. The quantum efficiency spectra show that both junctions are active and working in series. The 1× sun conversion efficiency of the GaN/Si tandem cell is limited to less than 1% due to the large band gap of GaN not being matched to the solar spectrum. Ongoing work is therefore focused on lowering the bandgap of the top cell to an optimum of about 1.8 eV by increasing the indium content of the top InGaN cell in order to match the current of the Si bottom cell under solar illumination. Very recently, we have achieved PV action in the first InGaN/Si hybrid cells. The remaining challenge lies in maintaining a high quality pn- junction in InGaN as the In fraction has to be increased towards 45%.