• Energy Research

Laser chemical processing (LCP) is an attractive doping technique for thin films due to its process simplicity, high achievable doping concentrations, and relatively shallow doping depths. During LCP processing, an infinite supply of dopants is available from the pressurized doping medium. In this paper, LCP is employed for n-type doping of poly-silicon thin films on glass. We achieved a peak doping concentration of 6 × 1018 to 1 × 1019 cm-3 and a junction depth up to 350 nm, as determined by electrochemical capacitance-voltage profiling and secondary ion mass spectrometry. We evaluate the structural quality of the LCP-doped layers with cross-sectional transmission electron microscopy (XTEM), as well as ultraviolet reflectance measurements. The LCP-doped regions are of suitable material quality for device fabrication. The resulting sheet resistance and doping levels are promising for a back surface field for poly-silicon thin-film solar cells on glass (e.g., an n+/ n-/ p+/glass layer structure in superstrate configuration).

AbstractPresently, large-area high-efficiency (> 19%) screen printed p-type silicon solar cells are dominated by the aluminium local back surface field (Al-LBSF) technology. However, all those cells were fabricated with tube diffused emitters. Inline diffusion, using phosphoric acid as the dopant source, offers potentially low-cost emitter formation for p-type silicon wafer solar cells. To achieve higher efficiencies for these solar cells, the authors have applied a new Si etch solution to remove the dead layer of the inline diffused emitter. Efficiencies up to 18.3% were obtained for standard Al back surface field (Al-BSF) solar cells. In this work, the same etch-back process was applied to Al-LBSF devices. We report a maximum efficiency of 19.0%, an average batch efficiency of 18.9% (± 0.1% StDev), and a maximum open-circuit voltage of 640mV for the cells, using industry-grade p-type 6 inch wide pseudosquare Cz mono-Si wafers. These results indicate that inline-diffused emitters can be used in high-efficiency silicon wafer solar cells.

In this study, we propose a geometric optical model to represent alkaline saw-damage-etched (SDE) surfaces of monocrystalline silicon wafers. An experimental study is carried out to characterize the optical properties of alkaline SDE surfaces on monocrystalline silicon wafers. Based on the surface characteristics measured by goniometry and height profiling, a geometric optical model is developed to describe the SDE surface with two parameters: characteristic angle and planar fraction. Using the path-tracing method, spectral reflectance simulations are carried out for four different types of samples. With the measured characteristic angle of 22° and planar fraction of 0.25 or 0.36, we find that this representation of SDE surface can predict the reflection and transmission with a root-mean-square error (RMSE) of the equivalent current density from 0.19 to 0.57 mA/cm 2 . The developed model is also applied to the optical loss analysis of aluminum local back surface field (Al-LBSF) solar cells with an SDE rear surface. We find that SDE rear surfaces provide better light trapping than planar surfaces. As a consequence, Al-LBSF solar cells with pyramids on the front and an SDE rear are predicted to produce 0.6 mA/cm 2 more photocurrent than similar cells with a planar rear surface.

We report an outstanding level of surface passivation for both n+ and p+ silicon by AlOx/SiNx dielectric stacks deposited in an inline plasma-enhanced chemical vapor deposition (PECVD) reactor for a wide range of sheet resistances. Extremely low emitter saturation current densities (J0e) of 12 and 200 fA/cm2 are obtained on 165 and 25 Ω/sq n+ emitters, respectively, and 8 and 45 fA/cm2 on 170 and 30 Ω/sq p+ emitters, respectively. Using contactless corona-voltage measurements and device simulations, we demonstrate that the surface passivation mechanism on both n+ and p + silicon is primarily due to a relatively low interface defect density of <;1011 eV-1cm-2 in combination with a moderate fixed negative charge density of (1-2) × 1012 cm-2. From advanced modeling, the fundamental surface recombination velocity parameter is shown to be in the order of 104 cm/s for PECVD AlOx/SiNx passivated heavily doped n+ and p+ silicon surfaces.

State-of-the-art surface passivation results are obtained on undiffused p-type commercial-grade Czochralski Si wafers with effective surface recombination velocity S eff values of ∼8 cm/s and implied open-circuit voltage iV oc values of up to 715 mV with an industrially fired dielectric stack of silicon oxide and silicon nitride (SiO ${}_x$ /SiN ${}_x$ ) deposited in an industrial inline plasma-enhanced chemical vapor deposition reactor. We are able to controllably vary the total positive charge density Q total in the stack by more than one order of magnitude (1011–10 12 cm−2) with no impact on midgap interface state density D it,midgap (5 × 1011 eV−1·cm−2) by altering the deposition temperature of the SiO ${}_x$ layer in the stack. We show experimentally that, for inversion conditions, S eff scales with the inverse square of the charge density $1/Q_{{\rm total}}^2$ , which is in good agreement with theory. Based on the measured injection-level–dependent minority carrier lifetimes and the total positive charge densities, it is shown that films with higher positive charge density have higher 1-sun V oc and fill factor ( FF ) potential. Large-area alloyed aluminum local back surface field solar cells confirmed this by showing higher conversion efficiency by 0.17% absolute due to improved cell V oc and FF of the solar cells featuring a SiO ${}_x$ /SiN ${}_x$ stack with a higher Q total.

Cd-Free CZTS solar cell with above 10% efficiency was achieved by an Al2O3passivation layer prepared by ALD.

In this work the mechanism of c-Si surface passivation by Al 2O3 films is studied in detail by means of spatially resolved electron energy loss spectroscopy (EELS). The bonding configuration of Al and O is studied in as-deposited and annealed Al2O3 films grown on c-Si substrates by plasma-assisted and thermal atomic layer deposition (ALD). The ratio of tetrahedrally and octahedrally coordinated Al is found to increase after annealing, especially for the plasmaassisted ALD sample. The increase is strongest close to the c-Si/Al2O3 interface and thus these results strongly support tetrahedrally coordinated Al as the origin for the negative fixed charge in Al2O3.