TiO2 Antireflection Coating Deposited by Electro-Beam Evaporation: Thin Film Thickness Effect on Weighted Reflectance and Surface Passivation of Silicon Solar Cells

Abstract

Titanium dioxide was extensively used in solar cell industry and currently has been studied to produce passivated contacts in PERC/PERT and TOPCon solar cells. The aim of this paper was to analyze the impact of the thickness of TiO 2 thin films deposited by electro-beam evaporation on the weighted reflectance and the surface passivation on silicon solar cells. Thin films with different thicknesses were deposited to produce PERT solar cells, varying from 50 to 90 nm. The surface passivation was enhanced as the thickness was increased. For instance, at 400 nm, the internal quantum efficiency increased from 71% to 76% when the thickness of the TiO 2 was augmented from 50 nm to 90 nm. The lowest weighted reflectance was obtained in samples with 80 nm thick TiO 2 films. Considering the compromise between antireflection properties and surface passivation, the highest efficiency solar cells were produced with 80 nm thick TiO 2 .

TiO2 Thin Films by Atmospheric Pressure Chemical Vapor Deposition for Rear Surface Passivation of p-PERT Solar Cells

“…In a second processed sample, a film thickness of (75 ± 6) nm and (59 ± 3) nm was estimated, before and after firing, respectively. For TiO 2 film deposited by electron beam evaporation and firing process at 860 ºC, Model et al 12 observed a wavelength shift of the minimum spectral reflectance (Δλ min ) of around 40 nm, value similar to that reported by Ly et al 19 . On the contrary, an average Δλ min of 130 nm was observed for the TiO 2 films deposited by APCVD in this work.…”

“…The solar cells with the front and rear faces coated with TiO 2 deposited by APCVD reached an efficiency of 15.6% (with V OC = 589.8 mV and J SC = 34.9 mA/cm 2 ), a value 0.3% above than that achieved in previous studies with TiO 2 films deposited by electron beam evaporation 12 . With this process, it is necessary to use high vacuum equipment, requiring longer running times and, thus, resulting in lower throughput.…”

“…Solar-grade silicon wafers grown by the Czochralski technique, p-type, boron doped with a diameter of 100 mm were used. Baseline process to produce n + pp + PERT solar cells was presented elsewhere 12 . The steps of the solar cell fabrication process were: texture etching, RCA cleaning, boron spin-coating, boron diffusion and oxide growth, RCA cleaning, phosphorus diffusion (POCl 3 as source of phosphorus), RCA cleaning, deposition of TiO 2 on the front and rear faces, metal paste screen-printing (Ag/Al and Ag on the rear and front face, respectively), metallization paste firing and laser edge isolation.…”

“…Clean dry air was used in the process. The peak temperature was set to 860 ºC 12 , and belt speed was 300 cm/min.…”

“…However, it requires a long processing time and high-cost equipment 11 . High vacuum evaporation, used to obtain antireflection TiO 2 films and slight surface passivation of the n + emitter, was reported by Model et al 12 . The atmospheric pressure chemical vapor deposition (APCVD) was also used to obtain antireflection TiO 2 films and to passivate boron doped (p + ) surfaces 13 .…”