Thick wafer-silicon is the dominant solar cell technology. It is of great interest to develop ultra-thin solar cells that can reduce materials usage, but still achieve acceptable performance and high solar absorption. Accordingly, we developed a highly absorbing ultra-thin crystalline Si based solar cell architecture using periodically patterned front and rear dielectric nanocone arrays which provide enhanced light trapping. The rear nanocones are embedded in a silver back reflector. In contrast to previous approaches, we utilize dielectric photonic crystals with a completely flat silicon absorber layer, providing expected high electronic quality and low carrier recombination. This architecture creates a dense mesh of wave-guided modes at near-infrared wavelengths in the absorber layer, generating enhanced absorption. For thin silicon (<2 μm) and 750 nm pitch arrays, scattering matrix simulations predict enhancements exceeding 90%. Absorption approaches the Lambertian limit at small thicknesses (100 μm) cells. There is potential for 20 μm thick cells to provide 30 mA/cm² photo-current and >20% efficiency. This architecture has great promise for ultra-thin silicon solar panels with reduced material utilization and enhanced light-trapping.

We investigate the silicon surface passivation property of Plasma Atomic Layer Deposited (PALD) hafnium oxide thin films and study its dependence on silicon (Si) doping type, film thickness, and post-deposition annealing conditions. Our results demonstrate that as-deposited HfO_x films exhibit poor passivation quality that can be improved by performing post-deposition annealing at 450 °C in hydrogen ambient. We demonstrate that the films can effectively passivate p-Si surfaces as compared to n-Si, where the surface passivation quality of the films improves with increasing film thickness for both silicon doping types. The best performance with a minority carrier lifetime of 1.7 ms, corresponding surface recombination velocity (SRV) ∼10 cm s⁻¹, is achieved for HfO_x films thickness ∼23 nm deposited on the p-Si substrate. The Capacitance-Voltage (C–V) measurements give an insight into the passivation mechanism of the studied films. Field effect passivation is found to be an important passivation mechanism in PALD-deposited HfO_x films, as revealed by C–V measurements. The films are also characterized using Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS), which reveals the chemical passivation provided by hydrogen ambient annealing. Overall, the impact of hafnium oxide film thickness and hydrogen ambient annealing conditions on silicon surface passivation is investigated. Our findings will help in utilizing plasma ALD process based HfO_x films for silicon solar cell device application.

Excellent silicon surface passivation is achieved by atomic layer deposition (ALD) grown hafnium oxide (HfO _x ) films on silicon surfaces (both n-type and p-type). It is inferred from the study that the silicon surface passivation by HfO _x thin films is a film-thickness-dependent quality and a minimum film thickness of ∼2 nm is essential to passivate the silicon surface. A good level of surface passivation (surface recombination velocity, SRV< 20 cm/s) can be achieved for film thickness, d >5 nm. However, the best results are obtained for hydrogen-annealed, ∼8.5-nm-thin HfO _x films. SRV as low as 3.5 cm/s (effective minority carrier lifetime, τ _eff ∼5 ms) and 4.4 cm/s (τ _eff ∼4 ms) are realized on p-type and n-type silicon surfaces, respectively. The injection level dependence of τ _eff reveals that HfO _x films provide better passivation for n-type silicon compared to p-type silicon at low injection levels. Hydrogen present in the annealing ambient first saturates the dangling bonds at the film/silicon interface and then affects the oxide charge density. Effective oxide charge density is positive in HfO _x /n-Si samples and negative in HfO _x /p-Si samples. This creates an accumulation condition near the silicon surface for both substrate-type situations and is responsible for effective passivation in both types of substrates. Thus, our study demonstrates that ALD-grown HfO _x films offer excellent passivation for silicon surfaces.