Transparent electrodes on silicon are today mostly used in crystalline HIT (Heterojunction with Intrinsic Thin layer) and amorphous solar cells, where a transparent conductive oxide (TCO) is forming a contact with a highly doped amorphous silicon layer. For HIT cells, Sn-doped In2O3 (ITO) provides highest efficiencies. The efficiency is however limited by the necessity of a highly doped amorphous silicon layer. Transparent contacts to silicon,in combination with suitable electro-catalysts, will also become important for solar fuel generation from integrated photovoltaic (PV)-photoelectrochemical (PEC) devices.
The current project will be devoted to the preparation of advanced transparent contacts to silicon, complemented with ultrathin passivating and/or electrocatalytic oxides. So far, no systematic correlation between the electrical behavior of the overall PV or PEC device and the interface characteristics has been performed for such contacts. The project will start with an analysis of the direct contacts between Si and various low-cost TCO materials, based on earth-abundant metals. The latter are uptill now typically characterized by high interface state densities and staggered energy band alignment, resulting in inferior properties for most applications.
In a second step, interface passivation for improving the electrical properties will be attempted by various means. Passivation effects are identified using Fermi level measurements by XPS, in combination with electrical and solar cell or photoelectrochemical properties. Various passivation layers, including ultrathin atomic layer deposited Al2O3 and TiOx will be studied. The passivation layers will not only be selected to reduce the density of interface states, but also to modify the energy band alignment for a variation of barrier heights.
Finally contacts between various TCO materials and earth-abundant electrocatalysts will be analyzed. The electrical and interface properties of these contacts are crucial as they govern the transfer of charges into photovoltaic-photoelectrochemical devices for solar fuel generation. Variations of (reactive) sputter deposition parameters, TCO composition and doping will be analyzed with respect to their potential for optimizing electrical interface properties.
Project Partners and their roles
The overall research approach as described above will systematically rely on the complementary and recognised expertise already available at the partners:
Division of Materials and Process Engineering, Université catholique de Louvain, Belgium
Silicon process technology; photo-electrochemical device preparation and testing;
supervisor: Prof. Dr. Joris Proost
Surface Science Unit, Department of Materials Science, Technical University Darmstadt
TCO and other oxide deposition with in-situ analysis of both strain and interfacial effects on electronic properties;
supervisor: Prof. Dr. Andreas Klein
Industry Partner: AGC Glass R&D Europe, Gosselies, Belgium (http://www.agc-solar.com/)
Host Country (employment): Belgium
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