Abstract
Tunnel silicon oxides form a critical component for passivated contacts for silicon solar cells. They need to be sufficiently thin to allow carriers to tunnel through and to be uniform both in thickness and stoichiometry across the silicon wafer surface, to ensure uniform and low recombination velocities if high conversion efficiencies are to be achieved. This paper reports on the formation of ultra-thin silicon oxide layers by field-induced anodisation (FIA), a process that ensures uniform oxide thickness by passing the anodisation current perpendicularly through the wafer to the silicon surface that is anodised. Spectroscopical analyses show that the FIA oxides contain a lower fraction of Si-rich sub-oxides compared to wet-chemical oxides, resulting in lower recombination velocities at the silicon and oxide interface. This property along with its low temperature formation highlights the potential for FIA to be used to form low-cost tunnel oxide layers for passivated contacts of silicon solar cells.
Highlights
IntroductionUltra-thin and uniform silicon oxide (SiOx ) layers have played a key role as an interfacial layer in a number of high efficiency silicon solar cell designs, including acting as intermediate layers in metal-insulator-semiconductor (MIS) solar cells [1], enhancing passivation at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction interfaces [2,3,4] and passivating thin metal-oxide passivated contacts [5,6] and poly-silicon (poly-Si) contacts [7,8,9,10,11,12,13,14]
We have demonstrated the potential of using anodic silicon oxide (SiOx) formed by field-induced anodisation (FIA) as a tunneling oxide layer for passivated contacts for Si solar cells
It was shown that the fraction of Si-rich sub-oxides present in the FIA oxide layer was higher than detected in the thermally-oxidised film, but lower than that observed in the wet-chemical oxide, of a comparable thickness
Summary
Ultra-thin and uniform silicon oxide (SiOx ) layers have played a key role as an interfacial layer in a number of high efficiency silicon solar cell designs, including acting as intermediate layers in metal-insulator-semiconductor (MIS) solar cells [1], enhancing passivation at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction interfaces [2,3,4] and passivating thin metal-oxide passivated contacts [5,6] and poly-silicon (poly-Si) contacts [7,8,9,10,11,12,13,14]. The reported method was based on a traditional clipping arrangement where the applied bias generates a potential gradient laterally across the wafer, resulting in non-uniform oxidation To address this issue, processes have been developed that use illumination or an electric field to induce a current flowing through the wafer in a direction that is perpendicular to the surface that is anodised [25,26,27]. It should be noted that when forward biasing a p-n junction in the FIA process, if the surfaces are not sufficiently doped, Schottky diodes can form with the depletion region extending further into the wafer with increasing anodisation bias and rectifying current flow through the wafer [30] This can be addressed through the use of illumination to generate a photocurrent across the depletion regions at the silicon surfaces so that generated carriers can be swept out of the regions by the applied electric field, allowing current to flow through the wafer. FIA may be a promising alternative when forming a uniform and controllable tunneling SiOx at room temperature
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