We investigated hydrogenated aluminum oxide (a-Al1-xOx:H) as a high quality rear surface passivation layer of crystalline silicon solar cells. The a-Al1-xOx:H films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of trimethylaluminum (TMA), carbon dioxide (CO2), and hydrogen (H2) at a low substrate temperature of about 200 °C. The ratio of CO2 to TMA during deposition and thermal annealing after the film deposition are the key factors in achieving high quality passivation. A 28-nm-thick a-Al1-xOx:H film deposited by PECVD showed a low surface recombination velocity of about 10 cm/s.
We developed a highly transparent n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) emitter for crystalline silicon (c-Si) heterojunction solar cells. A low emitter saturation current density (J0e) of 1.4×101 fA/cm2 was obtained under optimal deposition conditions. A c-Si heterojunction solar cell fabricated on a p-type c-Si wafer without texturing showed an active area efficiency of 17.9% [open-circuit voltage (Voc)=0.668 V, short-circuit current density (Jsc)=36.7 mA/cm2, fill factor=0.731]. The high Jsc value is associated with excellent quantum efficiencies at short wavelengths (<500 nm).
We have developed a simulation model for a heterojunction crystalline silicon (HJ-c-Si) solar cell with an n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) emitter and a p-type hydrogenated microcrystalline silicon oxide back surface field layer. Analyses of experimentally obtained solar-cell performance using the simulation model indicate that the conversion efficiency of the solar cell is limited by the rear-surface recombination velocity (Sr) and acceptor concentration (NA) of the p-type c-Si base region. Simulation results indicate that a potential conversion efficiency of HJ-c-Si solar cells using n-type nc-3C-SiC:H emitters is approximately 23% when Sr, NA, and bulk lifetime of the p-type base are 10 cm/s, 2 × 1016 cm−3, and 1.0 × 10−3 s, respectively.
Heterojunction crystalline silicon solar cells using a nanocrystalline cubic silicon carbide (nc-3C-SiC) emitter were optimized by changing the deposition time of a buffer layer. The implied open circuit voltage (implied-Voc) estimated from quasi-steady state photoconductance measurements strongly depended on the buffer deposition time. The implied-Voc of 0.690 V was achieved with a buffer deposition time of 30 s. The optimized solar cell showed an active area efficiency of 19.1% (Voc=0.680 V, Jsc=36.6 mA/cm2, and FF=0.769). The excellent cell performance is a direct evidence of the potential of the nc-3C-SiC:H emitter.
The epitaxial growth of the i-layer of crystalline silicon heterojunction solar cells has been widely accepted as harmful to surface passivation. In our experiments, however, although a very rough epitaxial phase in the intrinsic a-Si 1%x O x :H passivation layer was confirmed by transmission electron microscopy and spectroscopic ellipsometry, a high effective lifetime and an implied-V OC of over 720 mV were achieved with lifetime samples. The high passivation quality was confirmed by the obtained open circuit voltages of 728 and 721 mV for n-and p-type solar cells, respectively, with an a-Si 1%x O x :H/p-µc-Si 1%x O x :H stack rear structure. These results indicate that, contrary to the common knowledge, high surface passivation quality can be achieved even when the epitaxial phase is present.
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