Composition and electronic structure of $${\rm SiO}_{\rm x}$$/$${\rm TiO}_{\rm y}$$/Al passivating carrier selective contacts on n-type silicon solar cells

Abstract

Carrier-selective and passivating SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO$$_{\rm x}$$ x and TiO$$_{\rm y}$$ y layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO$$_{\rm x}$$ x . However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO$$_{\rm x}$$ x layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.

Optimization of a Solution-Processed TiOx/(n)c-Si Electron-Selective Interface by Pre- and Postdeposition Treatments

“…We surmise that annealing fractures C–O–Ti bonds, causing released O and Ti to diffuse to the interface, thereby modulating their chemical and electrical properties. Diffusion of Ti and O toward interface upon annealing has been reported in other works as well. ,,− The surface and depth profiling XPS results suggest that annealing superficially oxidizes TiO x , reducing the Ov concentration close to the surface region, whereas, in the vicinity of the Si/SiO x /TiO x interface, it leads to the appearance of Ti lower oxidation states and an augmentation in the Ov content. Consequently, annealing promotes the formation of a mixed oxide layer at the interface composed of substoichiometric SiO x and TiO x , while it causes near-stoichiometric TiO 2 composition at the surface.…”

“…The attained low contact resistivity is also owing to both chemical and field-effect mechanisms, which can be construed as follows: (i) Chemical passivation screens the dangling bonds and trap states at the interface hindering Fermi-level pinning, even in the absence of LiF x interlayer. (ii) It is well-known that TiO x with a higher concentration of Ov yields higher conductivity and thus lower ρ c . ,,, The overlaying metal can affect the TiO x composition, which indeed depends on the metal work function. , The redox reaction of TiO x enhances when it comes in contact with a low WF overlayer, leading to a reduction in ρ c . ,, (iii) Meanwhile, as probed by AFM (Figure S4b), longer annealing time coincides with the development of valleys or pinholes, which, first, promotes current flow through defect-assisted tunneling and, second, facilitates LiF x diffusion in the TiO x underlay, evoking energy states close to the conduction and/or valence bands of TiO x and thus exciting its conductivity . Because of the aforementioned reasons, we observed a lower ρ c as LiF x and/or a longer annealing duration was employed (see Figure c).…”

“…Prior to TiO x deposition, the insertion of an ultrathin tunnel oxide layer between the substrate and TiO x has been suggested to enhance the passivation quality by oxygen termination of dangling bonds on the silicon surface. In this respect, various wet-chemical (RCA2, H 2 O 2 , HNO 3 , ozonized DI-H 2 O), dry photo UV/O 3 , and thermal oxidation methods have been explored to synthesize such an oxide layer. ,− Moreover, the prominence of postdeposition treatments, such as hydrogen plasma, , light soaking, , and particularly thermal activation, , on the TiO x electrical properties has been extensively investigated. The solution-process-based methods hold advantages over other deposition techniques in terms of simplicity and fabrication cost, allowing thin films to be readily deposited at room temperature and atmospheric pressure.…”

Optimizing charge transport and band-offset in silicon heterojunction solar cells: impact of TiO2 contact deposition temperature

“…Improvement in the FF of photovoltaic devices is generally due to the decreased surface and interface recombination [50]. Moreover, creating higher oxygen vacancies enriches the electrical conductivity of TiO 2 layers, which alternatively helps improve the photovoltaic device parameter of heterojunction devices [10,51,52]. Thus, due to the combined effect of work function tuning, higher oxygen vacancies, higher effective carrier mobility, reduced trap states, and better diode quality, the PCE of T3 devices has been higher [53].…”