Solar Cells最新文献

In order to increase the efficiency of large-area solar cells made from thin (less than or equal to 200 μm) crystal wafers, without eliminating the thick film process sequence, an innovative back junction cell with a front contact grid on the lit side was developed. The major advantages of the reverse cell over the conventional cell are as follows:

• 1.

  (1) the reverse cell is suitable for the collection of long-wavelength light reflected on the back surface and at the same time of the short wavelengths owing to the opportunity of forming a shallow front surface p + layer without the risk of leakages from the contact metallization.

• 2.

  (2) the series resistance of the back and front contacts can be greatly reduced.

In this work we carried out a process sequence for reverse cell production: up to now our best fill factor was 78% which is probably the highest value obtained using thick film technology.

In addition, using numerical modelling, we analysed the spectral responsivity, the short-circuit current and the dependence of the efficiency on the front surface field depth. The behaviour of the short-circuit current with increasing lifetime was also investigated.

The sensitivities of selected single-junction and multijunction cells to variations in solar irradiance are presented. The one-sun spectral irradiance is varied as a function of air mass, optical aerosol depth (turbidity) and amount of precipitable water vapor for direct-normal and global-normal geometries. Several devices, including one-, two- and three-junction devices with low and high bandgaps and either series- or independent-connection schemes, were investigated. The effects of air mass and turbidity on the consistency of high-bandgap device performance are shown to be greater than the effect of precipitable water vapor. Low-bandgap devices are less affected by variations in air mass and turbidity, but are more sensitive to high water-vapor conditions. The efficiency gained by redesigning a multijunction device for the latitude at which it is expected to be used is shown to be less than about 3% (relative).

The high efficiencies lately achieved with solar cells are partly based on the use of novel light-cell-coupling schemes that are described. From some of these which are not yet used very much, we expect further achievements in the near future.

More specifically, new cell structures that avoid the front metal grid, and micro-concentrators that avoid rejection of light falling on the metal fingers are described. Special attention is paid to the study of the confinement of light inside the solar cells and outside the solar cells in external cavities, presenting here their theoretical limits which are not very well known. New types of external cavity with a large entry aperture are also reviewed as is their use in spectrum-splitting schemes.

A detailed theoretical analysis was made of a new experimental practice for determining the minority carrier lifetime τ_n and the back surface recombination velocity $\text{S}{}_{\mathrm{<mtext>B</mtext>}}$ in silicon solar cells by the short circuit current decay method. The measurements were taken by monitoring both the current-voltage characteristics and a single transient of an operating cell at any level of injection when no power supply is required. The complete continuity differential equation for minority carrier transport including the generation rate of carriers is considered. A practicable analytical expression of the current response derived and the analysis includes both thick and thin base cells. Good precision and sensitivity are obtained for actual high quality solar cells and a lower precision appears for high values of both τ_n and $\text{S}{}_{\mathrm{<mtext>B</mtext>}}$ (τ_n greater than the extrinsic lifetime value as defined by the Shockley-Read-Hall recombination process, $\text{S}{}_{\mathrm{<mtext>B</mtext>}}$ greater than 10⁴ cm/s). Experimental results are presented to support the mathematical analysis.

The theory for the general case of solar cells operating inside integrating cavity receivers is established. This is applied to the particular case of different configurations of silicon and GaAs cells. The results of the analysis show that a composite system of silicon and GaAs cells manufactured using relatively simple technology could reach an efficiency of 34%. The optimal configuration is that in which the GaAs cells are placed in the directly illuminated area of the receiver and the silicon cells are placed in the indirectly illuminated area of the receiver.

Experimental results show that the transient behaviour of the open-circuit voltage is the same for red, yellow and green monochromatic light. In contrast, the transient behaviour of the short-circuit current is strongly dependent on the wavelength of the illuminating light in the time range $\text{(1–10) × 10}{}^{\mathrm{-4}}\text{s}$. An explanation is given for these results in terms of the localized character of the electronic states.

Leakage current from glass/photovoltaic (PV) thin film/ethylene vinyl acetate/glass photovoltaic modules can be reduced to acceptable levels by removing the PV thin films at the border of the module. Sandblasting is a workable method to accomplish this. However, the resulting increase in the surface conductivity of the glass in the sandblasted region increases the width of the border required.

Polycrystalline CdTe solar cells with efficiencies of approximately 10% were achieved by metal organic chemical vapor deposition growth of CdTe on glass/SnO₂/CdS substrates. An in situ pre-heat treatment of the CdS substrate at 450 °C in an H₂ ambient was found to be essential for high performance devices because it removes oxygen-related defects on the CdS surface. This heat treatment also results in a cadmium-deficient CdS surface which may, in part, limit the CdTe cell efficiency to 10% owing to cadmium vacancy related interface defects. The CdCl₂ treatment used in CdTe cell processing was found to promote grain growth, reduce series resistance and interface state density, and change to dominant current transport mechanism from thermally assisted tunneling and recombination via interface states to recombination in the depletion region. These beneficial effects resulted in an increase in the CdTe/CdS cell efficiency from around 2% to approximately 9%. In addition to the CdTe cells, polycrystalline 1.7 eV CdZnTe films were grown by molecular beam epitaxy for tandem cell applications. CdZnTe/CdS cells processed using the standard CdTe cell fabrication procedure resulted in 4.4% efficiency, high series resistance, and a band gap shift to 1.55 eV. Formation of ZnO at and near the CdZnTe surface was found to be the source of high contact resistance. A saturated dichromate etch instead of the Br₂:CH₃OH etch prior to contact deposition was found to solve the contact resistance problem. The CdCl₂ treatment was identified to be the cause of the observed band gap shift owing to the preferred formation of ZnCl₂. A model for the band gap shift along with a possible solution using an overpressure of ZnCl₂ in the annealing ambient is proposed. Development of a sintering aid which promotes grain growth and preserves the optimum 1.7 eV band gap is shown to be the key successful wide gap CdZnTe cells.