Jeffrey Yang

Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies

We have achieved 14.6% initial and 13.0% stable conversion efficiencies using an amorphous silicon-based alloy in a spectrum-splitting, triple-junction structure. These efficiencies have been confirmed independently by the National Renewable Energy Laboratory. Key factors leading to this major advance include improvements made in the low band-gap amorphous silicon–germanium alloy cell, the pn tunnel junction between the component cells, and the top conducting oxide.

Effect of hydrogen dilution on the structure of amorphous silicon alloys

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Innovative dual function nc-SiOx:H layer leading to a >16% efficient multi-junction thin-film silicon solar cell

We present our development of n-type nano-structured hydrogenated silicon oxide (nc-SiOx:H) as a dual-function layer in multi-junction solar cells. We optimized nc-SiOx:H and attained a conductivity suitable for a doped layer and optical property suitable for an inter-reflection layer. We tested the effectiveness of the dual-function nc-SiOx:H layer by replacing the normal n layer between the middle and the bottom cells in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure. A significant gain in the middle cell current density of ∼1.0 mA/cm2 is achieved. We further optimized the component cells and the triple-junction structures and attained an initial active-area cell efficiency of 16.3%.

Enhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cells

We have developed a microcrystalline fluorinated p+ silicon alloy which has high dark conductivity and low optical loss. Incorporation of this material in single and tandem amorphous silicon alloy based solar cells has resulted in increased open circuit voltage and conversion efficiency.

Band-gap profiling for improving the efficiency of amorphous silicon alloy solar cells

We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

Optimization of back reflector for high efficiency hydrogenated nanocrystalline silicon solar cells

We have studied the effect of texture in Ag/ZnO back reflectors (BRs) on the performance of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells. While a larger texture provides superior light trapping, it also deteriorates the nc-Si:H quality. We have used total and diffused reflection and atomic force microscopy to evaluate the BR texture. A BR with textured Ag and thin ZnO layers has been found to give the best cell performance. Using the optimized BR, we have achieved an initial active-area efficiency of 10.2% in a nc-Si:H single-junction cell and a stable total-area efficiency of 12.5% in a hydrogenated amorphous silicon/nc-Si:H/nc-Si:H triple-junction cell.

Amorphous silicon based photovoltaics: from earth to the final frontier

We highlight the advances made in amorphous silicon alloy photovoltaic technology leading to large-scale commercial deployment. The paper discusses multijunction devices made on lightweight flexible substrates; various aspects of attaining high efficiency devices are described. The eminent role of the roll-to-roll continuous deposition technique in propelling the technology to global market is elucidated. The logical emergence of this technology as a lightweight solar-power generator for extraterrestrial application is discussed. Results of high specific power under space conditions are presented. The future of the technology in terms of both device efficiency and product efficacy are given.

Hydrogen dilution profiling for hydrogenated microcrystallinesilicon solar cells

The structural properties of hydrogenated microcrystalline silicon solar cells are investigated using Raman, x-ray diffraction, and atomic force microscopy. The experimental results showed a significant increase of microcrystalline volume fraction and grain size with increasing film thickness. The correlation between the cell performance and the microstructure suggests that the increase of grain size and microcrystalline volume fraction with thickness is the main reason for the deterioration of cell performance as the intrinsic layer thickness increases. By varying the hydrogen dilution in the gas mixture during deposition, microstructure evolution has been controlled and cell performance significantly improved.

Light-induced metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells has been studied under different light spectra. The nc-Si:H studied contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). We observe no light-induced degradation when the photon energy used is lower than the bandgap of a-Si:H, while degradation occurs when the photon energy is higher than the bandgap. We conclude that the light-induced defect generation occurs mainly in the amorphous phase. Light soaking experiments on a-Si:H∕a-SiGe:H∕nc-Si:H triple-junction solar cells show no light-induced degradation in the bottom cell, because the a-Si:H top and a a-SiGe:H middle cells absorb most of the high-energy photons.