Lateral homogeneity of porous silicon for large area transfer solar cells

Transfer solar cells are monocrystalline silicon thin film cells grown epitaxially on annealed double layer porous silicon then transferred from the host wafer onto a foreign substrate. Porous silicon forms by electrochemical etching of the silicon wafer in hydrofluoric acid. An upper low porosity layer forms at low etch current density and a buried high porosity layer forms by increasing the etch current density. After heat treatment, the upper layer serves as a seed for the epitaxy growth, where a buried high porosity layer transforms to the separation layer. The separation layer is mechanically weak and allows the transfer of the monocrystalline epitaxy layer to a foreign substrate by gluing it and applying a mechanical force. After the transfer, the processing temperature is limited to T = 220 °C due to the change of the optical properties of the epoxy glue. Moreover, the yield Y of the transfer process is about Y = 33% due to the lateral inhomogeneity of the etched porous silicon. Therefore, increasing the yield of the transfer process requires the increase of the lateral homogeneity of porous silicon formation. The present contribution introduces a new etching setup which improves the lateral homogeneity of porous silicon films by about 10% and hence enables the fabrication of a stand alone 47.4 m thin large area circular epitaxy membranes with diameter D ≈ 13 cm. The new etching setup has increased the transfer process yield Y from Y = 33% to Y = 65%. Solar cells with an area A = 2 × 2 cm2 are fabricated by epitaxy growth of both p-type absorber and n-type emitter prior to the transfer process to investigate the stability of the membranes. First experiment results in stand alone 47.4 µm thin solar cell with a maximum efficiency η = 12.3%. With the new etching process, it is possible to fabricate stand alone thin film solar cells with large area. Avoiding the epoxy glue enables a higher temperature limit T = 420 °C for the back side processing and an easier module connection.