Solar Cells最新文献

We present experimental data on the voltage and temperature $\text{(80<T<300}\text{K}\text{)}$ dependence of electroluminescence and forward bias current in hydrogenated amorphous silicon (a-Si:H) p-i-n structures. Since electrons and holes are injected from opposite sides of the sample, we are able to probe non-geminate radiative and non-radiative recombination processes in the intrinsic layer of actual device structures. We find that the effective generation rate in the electroluminescence experiment is proportional to the square of the applied voltage because the radiative recombination rate is proportional to the double-injection electron density. A simple model of electron recombination rates explains the data. The non-radiative recombination rate was found to be temperature dependent, but the radiative recombination rate is temperature independent.

In previous work, we have investigated the implications of projected advances in thin film solar cell technology for producing electrolytic hydrogen from photovoltaic (PV) electricity. These studies indicate that if year 2000 cost and efficiency goals for thin film solar cells are achieved, PV hydrogen produced in the Southwestern U.S. could become roughly cost competitive with other synthetic fuels for applications such as automotive transport and residential heating, if efficient energy use is stressed. This suggests that PV hydrogen could potentially play a significant role in future energy supply.

However, the estimated production cost of PV hydrogen depends on the cost and performance parameters assumed for the PV hydrogen system. In this paper we investigate the sensitivity of PV hydrogen production costs to changes in the system parameters and identify key conditions for low cost PV hydrogen production.

In the last two years, rapid progress has been made in the energy conversion efficiencies of GaAs solar cells fabricated from molecular beam epitaxy (MBE) material. The efficiencies of cells fabricated from MBE material are now comparable with those fabricated from metal-organic chemical vapor deposition material, even for cells of dimensin $\text{2}\text{cm}\text{× 4}\text{cm}$. This paper reviews the progress in MBE cell efficiencies. Also discussed is the role oval defects play in GaAs diode and solar cell performance.

Hydrogenated amorphous silicon (a-Si:H), 1–10 μm thick, was deposited onto stainless steel and molybdenum sheets using catholic d.c. glow discharge in a gradient field and by plasma-enhanced chemical vapor deposition. The films were subsequently crystallized by isothermal heating in N₂, rapid thermal processing, isothermal annealing in vacuum (IAV) or isothermal annealing after vycor encapsulation (IAE). All techniques led to crystallization as revealed by X-ray diffraction. Annealing by IAV at 1000 °C for 7 h or IAE at 700 °C for 8 h gave the most intense (111) silicon diffraction peaks. Auger electron spectroscopy showed significant diffusion of iron into the silicon for stainless steel substrates. Energy recoil detection of as-deposited a-Si:H showed good uniformity of both silicon and hydrogen.

Amorphous silicon (a-Si) photovoltaic (PV) modules are generally manufactured in a single-junction p-i-n configuration and in sites ranging from a few square centimeters to about 4000 cm². These modules are being used in a number of both indoor and outdoor low wattage (less than 20 W_p (peak watt)) applications, but have not found widespread use in most higher wattage power applications owing to relatively low stabilized conversion efficiencies (approximately 4%–5%). The recent improvements in the performance and stability of a-Si based multijunction modules indicates that these modules should soon start to appear in the higher wattage outdoor applications. When multijunction modules are manufactured in totally automated facilities, the manufacturing costs should fall below $1 per W_p, and these modules should then start penetrating the grid-connected power generation markets.

Efficiencies of up to 12.3% have been achieved on small devices. It is expected that 14% efficiency will be exceeded on small devices by improving the fill factors on the present devices in the reasonably near future. Efficiencies in the range 16%–18% are expected to be achieved in the longer term.

Modules of 6 W, approximately 929 cm² in area with an active area efficiency of over 8% (aperture efficiency of 7.3%) have been achieved.

The feasibility of producing 4 ft² modules of CdS/CdTe has been shown and requires further efforts in order to realize the overall potentials.

The structural integrity of the encapsulation design has been studied by thermal cycling and outdoor life testing. Submodules have been life tested for over 270 days with no observable degradation by the SERI Outdoor Reliability and Life Testing Laboratory.

In addition to further optimization of materials and device structure, module output in the future will be increased by an improvement in the uniformity of the deposition process, and by minimizing the loss of active area due to cell division interconnections. Module output is expected to attain 135 W m⁻² in the mid 1990s and over 150 W m⁻² in the long term.

A downstream near afterglow plasma was used to deposit epitaxial GaAs at substrate temperatures as low as 300 °C. Feedstock organometallics of trimethylgallium and trimethylarsenic were employed at a ratio of 1:2, respectively. The observed growth rate varies with the substrate temperature, but no growth occurs without the plasma. Scanning electron microscopy electron backscattering was used to probe the single crystal quality of the deposited layers.

Solarex began using ethylene vinyl acetate (EVA) as an encapsulant for photovoltaic modules during the Jet Propulsion Laboratory sponsored Block IV Program in 1979. Experience was gained in the processing and use of EVA during a number of Department of Energy sponsored projects through the early 1980s. In 1982 Solarex began using EVA in commercial modules and has continued to use it up to the present time. EVA has proven to be a highly reliable encapsulant, with no reported instances of Solarex module field failures being attributed to failure of the EVA encapsulant. The EVA encapsulation system is complex, requiring well controlled manufacture of the film itself and the correct lamination procedure to assure adequate cure and bonding to the glass, cell and backsheet surfaces. The initial Springborn work on EVA included accelerated testing, which indicated that at temperatures considerably higher than experienced during normal module operation, the EVA system will suffer thermally induced degradation. However, no major degradation was experienced under normal operating conditions during either Springborn's testing or Solarex's 10 years of field experience.

We report a comparison study of high-deposition-rate (approximately 2 nm s⁻¹) amorphous hydrogenated silicon (a-Si:H) solar cells using silane (SiH₄) and disilane (Si₂H₆) source gases. Our results show that under optimized deposition conditions, films deposited from silane or disilane at the same deposition rate have similar properties. Efficiencies higher than 10% have been achieved in both cases for 1 cm² area single-junction solar cells. The key for achieving high efficiency, high-deposition-rate, solar cells using SiH₄ source gas is the p-i interface.

Electron spin resonance was used to characterize concentrations of thermal equilibrium defects from room temperature to 280 °C in a 60 μm thick hydrogenated amorphous silicon film. A defect formation energy of 0.35 eV was found in material with $\text{1×10}{}^{15}\text{cm}{}^{\mathrm{-3}}$ spins at 190 °C. Annealing of defects quenched in from 250 °C revealed an activation energy of 2.2 eV. Annealings at 150 °C of defects quenched in at 250 °C and 190 °C were compared; the additional defects introduced at the higher temperature annealed 10 times faster, supporting a model in which metastable states with higher formation energies have smaller annealing activation energies. Light-induced defects are described in terms of a very “high-temperature” distribution similar to that which might be quenched in as a result of $\text{kT ≈ 0.5}\text{eV}$.