Resumen de Design, fabrication and characterisation of interdigitated back-contacted c-si solar cells based on transition metal oxides
The photovoltaic industry is mainly dominated by crystalline silicon (c-Si) solar cells, in which contact selectivity is usually achieved by doping the wafer surfaces with phosphorous (n+) and boron (p+) by means of high temperature oven-based diffusions, called pn-Junction (pnJ). This requires complex and energy consuming processes and lengthy cleaning protocols to avoid possible degradation of bulk lifetime, increasing the number of steps involved in the manufacturing and consequently the production cost.
In order to replace those high temperature diffusions, several approaches have been studied. The well-known silicon heterojunction (SHJ) structure using doped and intrinsic hydrogenated amorphous silicon (a-Si:H) films is probably the best known. Nevertheless, this option uses toxic and flammable gases as dopant precursors, which is not desirable from the point of view of process simplicity.
In parallel, several contact structures have been developed from the past to now. Among them, two cell architectures have pushed forward the efficiency of c-Si solar cells. On the one hand, the passivated emitter and rear locally-diffused (PERL) cell structure, which was designed by the group leader by Prof. Martin A. Green at the University of New South Wales, Sydney, achieving power conversion efficiencies of up to 25%. On the other hand, the interdigitated back-contacted (IBC) cell structure, which was firstly designed for photovoltaic concentration applications, but in recent years have been applied on non-concentration purposes. In fact, the world solar cell efficiency record on c-Si substrates (26.6%) is achieved with an IBC cell structure. This thesis deals with the research on c-Si solar cells using both an IBC structure and an alternative approach to the conventional pnJ and SHJ concepts by implementing novel selective contacts, which are those based on transition metal oxides (TMOs).
The first part of the Thesis is focused on the use of vanadium oxide (V2Ox) as hole-selective contact. It is demonstrated that the V2Ox has better selectivity behaviour when a nickel metal, which has a high work-function, is used as a capping layer. Finally, the selective contact based on Ni-capped V2Ox film is applied to an IBC c-Si solar cell yielding an efficiency of 19.7%. In this device the electron-selective contact is performed with the laser-doped technique, where the n+ region is formed by the laser-processing of a dielectric film, consisting of a phosphorous-doped amorphous silicon carbide layer.
The last part of the Thesis is related to the development of an electron-selective contact based on TMOs replacing the laser-doped contacts. In this way, a titanium oxide (TiOx) film is used in combination with a thin aluminium oxide (Al2Ox) passivating interlayer. Once again, the metal capping has an influence to the selectivity of the final contact, being the best option the use of a magnesium film, which has a low work-function. Finally, a novel IBC c-Si solar cell is developed, whcih combines the previously mentioned Ni-capped V2Ox as hole-selective contact and Mg-capped Al2Ox/TiOx as electron-selective contact, yielding a proof-of-concept device with a conversion efficiency of up to 19.1%. It is important to stress that both selective contacts are fabricated at temperatures lower than 100 ºC reducing the overall thermal budget of the fabrication process, as well as circumventing the use of toxic and flammable gasses as dopant precursors.
