MOVPE of GaAsP/GaAs heterostructures for fabrication of 1.7 eV cell junctions in 4-terminal tandem III- V/Si solar cells

Multi-junction (tandem) solar cells in the form of monolithically stacked III-V cells, each absorbing a different interval of the solar spectrum allow to reach external quantum efficiencies well beyond the Schockley-Qeisser limit for single-junction solar cells.1 Tandem solar cells based on a Silicon bottom junction are very attractive due to the relative low cost of Silicon substrates; a dual-junction cell with a 1.7 eV top junction based on III-V semiconductors (e.g. GaAsP) and a Silicon (1.12 eV) bottom cell has a theoretical efficiency of ?38%. However, serious structural constrains limit the monolithic growth of III-V compounds onto Si and performances of as-fabricated tandem solar cells remains far from theoretical figures. Four-terminal tandem solar cells composed of a thin GaAs film mechanically stacked onto interdigitated back contact Silicon solar cell with a glass interlayer have shown efficiency up to 32.6%.2 The main advantage of such approach is that high quality III-V top cells could be monolithically grown on a GaAs substrate. Despite using costly GaAs wafers in the epitaxy of the III-V top cells increases the production costs, detach (by chemical lift-off) of the cells from the underlying substrates and multiple re-utilization of the latter have been demonstrated in the literature,3 as viable strategies to keep production costs low. We present a study on the metalorganic vapor phase epitaxy (MOVPE) growth and structural-optical properties of GaAsP-based heterostructures on (100)GaAs, with the aim to fabricate a high efficiency 1.7 eV top cell for utilization in a stacked 4-terminal tandem III-V/Si solar cells. Despite GaAsP epilayers grown by MOVPE are commonly used in the fabrication of (In)GaAs-based heterostructures for applications to solar cells and laser diodes, not so much as been reported to date on growth details and related structural (strain, pleastic relaxation) and radiative (lumines- cence) properties of tensile-strained GaAsP epilayers on GaAs. In this work P incorporation into GaAsP alloys has been determined along with the solid-vapor distribution curve as function of growth temperature by employing tertiary- buthylarsine and tertiarybuthyl-phosphine as As and P precursors respectively, in combination with trimethylgallium. Analysis of as-grown samples by high-resolution X-ray diffraction evidenced the elastic deformation state of the ma- terial and the onset of plastic deformation, which turned out to agree well with what expected from People-Bean relaxation model (values of critical thickness turned out to range up to few-hundreds nanometer).4 Low tempera- ture photoluminescence spectra further showed a near band-gap emission for most GaAsP samples. Examples of high quality (pseudomorphic) step-graded GaAsP buffer layers on (100)GaAs will be finally reported. Acknowledgements. This work has been funded through the MUR-PON Project "Bifacial Efficient Solar cell Tech- nology with 4 terminal architecture for Utility scale PV (BEST4U)". References 1. F. Dimroth, S. Kurtz, High-efficiency multijunction solar cells, MRS bulletin 32 (3) (2007) 230-235. 2. R. C. Whitehead, K. T. VanSant, E. L. Warren, J. Buencuerpo, M. Rienäcker, R. Peibst, J. F. Geisz, A. C. Tamboli, Optimization of four terminal rear heterojunction GaAs on si interdigitated back contact tandem solar cells, Applied Physics Letters 118 (18) (2021) 183902. 3. C.-W. Cheng, K.-T. Shiu, N. Li, S.-J. Han, L. Shi, D. K. Sadana, Epitaxial lift-off process for gallium arsenide substrate reuse and flexible electronics, Nature communications 4 (1) (2013) 1-7. 4. R. People, J. Bean, Calculation of critical layer thickness versus lattice mismatch for ge x si1- x/si strained-layer heterostructures, Applied Physics Letters 47 (3) (1985) 322-324.