Xiaofei Han

Non-vacuum electroplated al for n-side electrode in Si solar cells

This paper reports Al electroplating on a Si substrate using a room-temperature ionic liquid for the metallization of Si solar cells. The ionic liquid electrolyte was prepared by mixing anhydrous AlCl3 and 1-ethyl-3-methylimidazolium tetrachloroaluminate ([EMIM]AlCl4). The plating process was carried out in a dry nitrogen box. A sacrificial Al anode was employed, making the electrolyte reusable for many deposition runs. The sheet resistance of the Al deposits was investigated to reveal the effects of pre-bake conditions, deposition temperature, and post-deposition annealing conditions. It was found that dense and adherent Al deposits with low electrical resistivity can be obtained directly on Si substrates over a wide range of temperatures using galvanostic deposition. The resistivity of the Al deposits was in the high 10-6 Ω-cm range, similar to that of screen-printed Ag. The maximum process temperature for electroplated Al was 350°C. An all-Al Si solar cell, with an electroplated Al front electrode and a screen-printed Al back electrode, has been demonstrated, and its optimization and characterization will be reported soon.

Al and F codoped ZnO by a novel co-spray deposition technique for solar cells applications

ZnO was successfully synthesized by a novel co-spray deposition technique. Two solutions were prepared and sprayed through two spray heads. In one solution, Zn(O2CCH3)2 (Zn precursor) and AlCl3 (Al precursor) were dissolved in a mixture of deionized water and ethanol. In another solution, NH4F (F precursor) was dissolved in the same mixture. The deposition was carried out at 500°C on soda-lime glass in air. Compared to Al-doped or F-doped ZnO, Al and F codoped ZnO resulted in much lower sheet resistance. The lowest sheet resistance obtained so far is below 55 Ω/sq from solutions with 0.6 at.% Al and 55 at.% F, respectively, after vacuum annealing at 400°C for 1 h. The average transmittance of Al and F codoped ZnO is above 80% in the visible range. The effects of dopant concentrations in the starting solutions on the electrical, structural, and optical properties of Al and F codoped ZnO were also investigated.

Zr0.05Ti0.95O2-Nanowire-Based Ultraviolet Photodetector with Self-Powered Properties

Zr0.05Ti0.95O2 solid solution nanowires were synthesized directly on a SnO2:F-coated glass via a low-temperature hydrothermal method, and a high-sensitivity self-powered ultraviolet detector based on SnO2:F/Zr0.05Ti0.95O2 heterojunction was fabricated subsequently. Zr doping not only improves the detectors photoresponse but also adjusts the response range to the short-wavelength direction. At 0 V bias, a highly responsivity of 1.03 A/W and an internal gain of 3.6 were obtained, together with favorable visible-blind characteristics and short response time. This high-sensitivity self-powered ultraviolet detector provides potential applications in wireless environmental monitoring, biological detection, and ultraviolet astronomy.

Earth-abundant iron oxysulfide (FeS x O y ) for bandgap optimization

Transition metal sulfides with small bandgap are attractive for solar cell applications since most transition metals are abundant and many can be deposited in solution. Pyrite FeS₂, with a bandgap of 0.95 eV, is one of the most desirable for solar cell applications. The problem with FeS₂ is that its bandgap is ~0.45 eV smaller than the optimum bandgap for maximum efficiency, which should be ~1.4 eV. In this paper, we propose the concept of metal oxysulfide as the approach to a low-cost Earth-abundant semiconductor with a direct bandgap of ~1.4 eV for terawatt-scale solar cells. This is because the bandgap of Fe₂O₃ is 2.2 eV. By introducing O into FeS₂, the bandgap should increase. In our experiments, we use oxidation of electrodeposited FeS_x for this purpose. FeS_x films were electrodeposited on FTO-coated glass. Post-deposition annealing was carried out in vacuum to make FeS_x films denser and more stable in air. SEM and EDX confirm that the as-grown FeS_x film is amorphous and the S/Fe ratio in the film is slightly above 1. Oxidation of the FeS_x films was performed either in a tube furnace under air or electrochemically in an electrolyte. After oxidation, the bandgap in the resultant FeS_xO_y is ~1.3 eV by electrochemical oxidation and ~1.1 eV by thermal oxidation. Further optimization is expected to produce a FeS_xO_y with a ~1.4 eV bandgap.

Green electrodeposition of ZnO as a TCO in terawatt solar cells

Electrodeposition of ZnO can be performed in an aqueous solution using a greener recipe, where the solution can be reused for multiple deposition runs. The solution in this greener recipe has only one function, i.e. to provide electrical conductivity for the deposition reactions. A Zn sheet serves as the anode, which dissolves during the deposition as the Zn source. O2 is bubbled into the solution and reduced to OH- ions as the O source. This recipe minimizes concentration changes in the solution as deposition proceeds, making the solution reusable. An initial Zn2+ concentration of a few mM in the solution is required, not to serve as a Zn source but to facilitate the deposition and prevent precipitation of ZnO in the solution. Multiple deposition runs for ZnO films in the same solution have been demonstrated. X-ray diffraction, optical transmittance and absorption spectra reveal that all the ZnO films have similar structural and optical properties. They all display high transmittance of ~80% and low absorbance of ~10%.

Zn as the protective layer for Cu electrode in wafer-Si solar cells

Zn is proposed as the protective layer for the Cu electrode in wafer-Si solar cells to replace todays Ag front electrode. Zn provides a lower material cost, a lower resistivity and more abundant material reserve than Sn. The thermal stability of the Zn/Cu/Ni stack is examined by annealing it in air and the Zn/Cu/Ni stack is advantageous over the Sn/Cu/Ni stack in thermal stability. This is attributed to the better coverage of electroplated Zn on Cu and the higher melting point of Zn over Sn. The sheet resistance of the Zn/Cu/Ni stack is also lower than the Sn/Cu/Ni stack due to the lower resistivity of Zn. XRD measurements before and after annealing confirms that the increased sheet resistance upon annealing is due to oxidation and alloying of the Cu layer. For solderability, a Sn/Zn/Cu/Ni stack with reduced Sn thickness is demonstrated by sequential electroplating.

Low-cost ultrapure (>99.9999%) solar-grade Si by electrorefining

The current SiHCl3-based processes to produce ultrapure Si are one of the major bottlenecks to lower-cost wafer-Si solar cells. Molten-salt electrorefining of Si is an alternative process to ultrapure Si, with a significantly lower energy input of ~15 kWh/kg. It is demonstrated that controlling the potential applied to the working electrode is critical in reducing impurity concentrations. A more positive potential on the working results in purer Si deposited on the counter electrode. With margin of error, the amount of Si deposited on the cathode is almost the same as the amount dissolved from the anode. A nonequilibrium factor which affects impurity concentrations is identified, i.e. contamination.