Abstract
This thesis focuses on III-V/Si integration of single- and dual-junction solar cells by Metalorganic Vapor Phase Epitaxy (MOVPE). The photovoltaic devices were made with materials lattice matched to InP. The goal was to firstly obtain efficient devices on InP substrates and then to integrate them on a structure with a different lattice parameter, by evaluating the effects on the photovoltaic performances. This required the realization and the optimization of several manufacturing steps.Firstly, we realized an InP single junction device, which may correspond to the top cell of our tandem structure. This was also necessary to set up a manufacturing process for all the next cells. The growth conditions were optimized by taking advantage of material characterization techniques such as XRD, C-V profiling and SIMS. By optimizing thicknesses and doping levels of the various layers of the device, as well as the clean room process, we obtained a conversion efficiency of 12.9%, with a FF of 84.3%. We demonstrated that the use of an AlInAs window layer instead of InP may increase the efficiency to 13.5%, despite a slight reduction in FF (81.4%). The same procedure was then extended to the realization of an InGaAs solar cell as the bottom component of the tandem device. We obtained an efficiency of 11.4% and a FF of 74.5%.In parallel, tunnel junctions able to electrically connect the two subcells in a tandem device were studied. In particular, we focused our attention on the growth conditions of the junction anode, which was made in AlInAs and doped with CBr4 precursor. The particular chemical interactions that such a precursor has with Al precursor and In required a relevant reduction of growth temperature to 540 °C. By determining the effects of the flows on composition and doping levels of the compound, we obtained a high doping of +4x1019 cm-3. By obtaining an equivalent level for the InP:S cathode, we realized a device presenting a Jp of 1570 A/cm2, able to work under high solar concentration conditions. By finally combining the three presented devices in a tandem cell, we could obtain an overall conversion efficiency of 18.3%, with a FF of 83.9%.A proper template for III-V/Si integration was determined by testing several possibilities provided by different partners. XRD and AFM characterizations demonstrated that an InP/GaP/Si template provided by NAsP Company was the best option. This was confirmed by the growth of an InP single junction cell over the template. The processing over a Si substrate was made possible by shifting the rear contact of the cell on the front side of the device, which required the development of a proper set of photolithographic masks. The successful integration of the III-V solar cells on Si was confirmed by the relevant produced photocurrent. This corresponds to around 60% of the value obtained on InP substrates. Furthermore, the measured J-V characteristics show a diode-like trend, which demonstrates the validity of the proposed approach.