Vanadium oxide as a passivating-carrier selective contact for silicon heterojunction solar cell application

The potential of thermal evaporation-grown vanadium pentoxide (V₂O₅) as a passivating-carrier selective contact material for high-performance heterojunction crystalline silicon solar cells was examined in this work, with particular emphasis on the effects of film thickness from 1.5 nm to 4.5 nm on the characteristics of vanadium sub-oxide (V₂O_x) thin films and their passivation characteristics on n-type c-Si (111) Cz wafers. V₂O_x thin films were made by thermal evaporation at a chamber pressure of 5 × 10^–5 mbar, with different film thicknesses. The findings give an insight into the effect of film thickness on the electrical and surface passivation capabilities of V₂O_x films. With increasing layer thickness, electrical resistance was observed to rise while carrier concentration decreased, but a slight improvement in carrier mobility. The minority carrier lifetime values ranged from 64.07 ± 3.39 µs to 230 ± 4.30 µs and thicker films showed enhanced passivation. It is hypothesised that the formation of a sub-stoichiometric SiO_x interlayer containing fixed charges adds to the field-effect passivation mechanism. The analysis revealed that a 4.5 nm thick V₂O_x film had the best optoelectronic characteristics, an optical transmittance of above 90%, an electron concentration of 2.62 × 10¹³ cm⁻³ and a mobility of 40 cm²/V-s. This 4.5 nm thick V₂O_x film as a passivation layer gives a minority carrier lifetime of 230 ± 4.30 µs and an implied open-circuit voltage (iV_oc) of 652 ± 1 mV for a commercial n-type silicon wafer. A passivating contact solar cell has been fabricated for validating the passivation studies with an open-circuit voltage (V_oc) of 627 mV. These results are important for developing heterojunction silicon solar cell technology.