We have been investigating fabrication of a crystallized Si (c-Si) film deposited on a glass substrate at low temperature, using a poly-yttria-stabilized zirconia (poly-YSZ) film as a seed layer. It can be expected that the crystallographic information of the YSZ layer transmits to the deposited Si film so that it stimulates its crystallization even at lower temperature. To reduce the crystallization temperature further, we focused on the surface treatment of the YSZ layer, in which it was dipped in the HF solution, followed by rinsing with deionized water (DIW) or with ethanol.

After depositing the poly-YSZ layer by reactive magnetron sputtering on the quartz glass substrate, the Si film was deposited directly on it by e-beam vacuum evaporation in <10-6 Pa. The Raman spectra of the Si films deposited at 430 °C on the ethanol-rinsed glass without YSZ layer and on the DIW-rinsed YSZ layer showed amorphous phase. However, on the ethanol-rinsed YSZ layer, they showed a strong peak of c-Si of the deposited film, even at 350 °C, and indicated the beginning of the crystallization at 320 °C. This means that the YSZ layer enhanced the Si film crystallization, and that the ethanol-rinse was effective for low-temperature crystallization. The crystalline fraction of the Si film varied with the yttria content and the surface treatment of the YSZ layer prior to the Si film deposition. From the XPS results, we found out that fluorine F were adsorbed on the surface and were bonded with the Y. The Y 3d and F 1s peaks observed from the ethanol-rinsed YSZ were higher than those of the DIW rinse, which suggests that the Y and adsorbed F on the surface were removed by the DIW rinse, but remained even after the ethanol rinse. Based on the Raman and XPS results, we can speculate that the excessive amount of F on the YSZ layer surface prior to the Si film deposition play an important role on the Si film crystallization. We observed the surface of the Secco-etched Si/YSZ films deposited at 320, 350, and 430 °C by SEM. The number of crystallized Si grains was denser at higher temperature than that of lower temperature. That is, the Si film deposited at 430 °C was crystallized thoroughly on the whole substrate, but some regions of the Si films deposited at 350 and 320 °Cwere amorphous and removed by the Secco etching. The grain sizes were in the range from 20 to 40 nm, and the size at the lower deposition temperature was more uniform than the higher temperature. From SIMS observation, it was found that Zr atoms diffuse from the interface with the YSZ layer.

Demand for high efficiency, low-cost solar cells has led to strong interest in post-deposition processing techniques that can improve the crystallinity of thick (1 to 40 μm) silicon films deposited at high growth rates. Here we describe a high temperature grain reorientation annealing process that enables the conversion of polycrystalline silicon (poly-Si) into a single crystal material having the orientation of an underlying single crystal Si seed layer. Poly-Si films of thickness 0.5 to 1.0 μm were deposited by low pressure chemical vapor deposition (LPCVD) on substrates comprising a surface thermal oxide or a 100-oriented single crystal silicon-on-insulator (SOI) layer. After annealing at 1300 °C for 1 hour, poly-Si on oxide shows very significant grain growth, as expected. In contrast, the poly-Si deposited on SOI showed no grain boundaries after annealing, transforming into a single crystal material with a fairly high density of stacking faults. Possible uses and drawbacks of this approach for solar cell applications will be discussed.

We present advanced light trapping concepts for thin film silicon solar cells. When an amorphous and a microcrystalline absorber layers are combined into a micromorph tandem cell, light trapping becomes a challenge because it should combine the spectral region from 600 to 750 nm for the amorphous top cell and from 800 to 1100 for the microcrystalline bottom cell. Because light trapping is typically achieved by growing on textured substrates, the effect of interface textures on the material and electric properties has to be taken into account, and importantly, how the surface textures evolve with the thickness of the overgrowing layers. We present different scenarios for the n-i-p configuration on flexible polymer substrates and p-i-n cells on glass substrate, and we present our latest stabilized efficiencies of 9.8% and 11.1%, respectively.

The short circuit current and conversion efficiency of the poly(multi)-crystalline solar cells are increased by the passivation process using hydrogen plasma. The passivation rate apparently increases at a reverse bias voltage near 0.6V during the hydrogenation process. The effects of the bias voltage on the passivation are large at the substrate temperatures between 200C and 250C. The phenomena are likely due to the existence of positively-ionized hydrogen, H+. The H+ ions can be accelerated from the surface into the bulk by the electric field with the negative bias. The possibility of the H+ ions in the bulk silicon has been predicted in the previous reports. The increase of the incorporated hydrogen is confirmed by IR absorption measurements. The enhanced diffusion of hydrogen induced by the reverse bias is supported by the results of spectral response characteristics of the hydrogenated solar cells.

The enhancement of optical transmittance at the air/glass interface of amorphous silicon thin film solar cells was shown by application of a nanoporous polymethyl methacrylate (PMMA) antireflection (AR) coating. The PMMA coating was prepared by spin coating of PMMA solution in chloroform in the presence of a small amount of nonane. Because of the difference of the vapor pressure of chloroform and nonane, phase separated structure formed after complete evaporation of both of them during spin coating process. The Corning 1737 glass with the AR coating has high transmittance near 95% from 450-1100nm wavelengths. The amorphous silicon solar cells with the nanoporous PMMA AR coating realize an improvement in quantum efficiency (QE) up to 4% in 450-650nm spectral regions.

Significant advances have been made in increasing the deposition rate of hydrogenated silicon germanium alloys (a-SiGe:H) using a modified VHF glow discharge deposition method while also maintaining good electronic properties important for its application in photovoltaic devices. We examine the electrical and optical properties of these alloys deposited either by RF (13.56MHz) or the modified VHF methods over deposition rates varying from 1 to 10 Å/s. The electronic properties of a series of 1.4 eV optical gap a-SiGe:H i-layers, in many cases in solar cell device configurations, were characterized. Drive-level capacitance profiling was used to determine the deep defect densities, and transient photocapacitance measurements allowed us to determine the Urbach energies. Results were obtained for both the annealed and light-soaked degraded states and these results were correlated to the cell performance parameters. In general the a-SiGe:H layers deposited using the modified VHF excitation exhibited improved electronic properties at higher growth rates than the RF deposited samples.