Plasma-deposited aluminum oxide coatings improve dopant-free silicon solar cells by enforcing interfacial chemical purity instead of relying on hydrogen content. Engineers long believed that packing extra hydrogen atoms into protective coatings was the primary way to seal silicon surface flaws. Instead, plasma removes residual carbon fragments and adds excess oxygen atoms to build a negative charge that keeps electrical carriers from vanishing into surface traps.
Standard thermal heating leaves behind unburned methyl groups and incomplete oxide structures that spoil the silicon boundary. Plasma deposition burns away these organic residues like a clean flame clearing away soot. This thorough combustion allows extra oxygen to bind across the contact zone, driving the negative fixed charge density to -6.6 × 10 12 per square centimeter. The resulting electric field repels electrons away from boundary defects, keeping energy flowing freely through the silicon.
Researchers analyzed the atomic layers using standardless ion-beam quantification and photo-induced force microscopy. Thermal coatings contained 5.0 atomic percent hydrogen while the superior plasma coatings contained only 3.7 atomic percent, disproving the hydrogen rule. The plasma process achieved a minority carrier lifetime of 2780 microseconds, an interface trap density of 4.3 × 10 9 electron-volts per square centimeter, and a planar cell efficiency of 9.15 percent.
The team established complete interfacial oxidation as a guiding design principle for building carrier-selective contacts in dopant-free photovoltaics. Computer simulations indicate that applying these pure coatings to three-dimensional microstructures could raise solar cell conversion efficiencies toward 20 percent.