Engineering topics
Publications and source records attributed to Milnes, A. G..
Proposed multilayer, front-contact grid structure for solar cells optimizes collection of photogenerated current with minimum power losses. It is constructed of several layers of conducting grids. With multilayer concept, peak efficiency can occur at higher output-power levels. Because of this, higher solar concentrations can be applied to solar-cell arrays.
The paper presents deep-level transient spectroscopy studies of Ni- and Zn-diffused vapor-phase epitaxy n-GaAs. Nickel diffused into VPE n-GaAs reduces the hole diffusion length L sub p from 4.3 to 1.1 microns. Deep-level transient spectroscopy was used to identify energy levels in Ni-diffused GaAs; the as-grown VPE GaAs contains traces of these levels and an electron trap. Ni diffusion reduces the concentration of this level by an amount that matches the increase in concentration of each of the two Ni-related levels. A technique for measuring minority-carrier capture cross sections was developed, which indicates that L sub p in Ni-diffused VPE n-GaAs is controlled by the E sub c - 0.39 eV defect level.
Titanium inserted in silicon by diffusion or during Czochralski ingot growth is electrically active to a concentration level of about 4 x 10 to the 14th per cu cm. It is reported that Hall measurements after diffusion show conversion of lightly doped p-type Si to n-type due to a Ti donor level at E sub c -0.22 eV. In addition, in DLTS measurements of n(+)p structures this level shows as an electron (minority carrier) trap at E sub c -0.26 eV with an electron capture cross section of about 3 x 10 to the -15th per sq cm at 300 K. Finally, a Ti electrically active concentration of about 1.35 x 10 to the 13th per cu cm in p type Si results in a minority carrier (electron) lifetime of 50 nsec at 300 K.
This communication considers the design of the front grid contact of np GaAs solar cells for high efficiency concentrator applications. This design involves shadowing, contact resistance, and active layer sheet resistance losses, and at high concentrations, the power loss due to voltage drop on the resistance of the grid fingers should be considered. Analysis of the performance can be calculated as a function of junction depth and surface recombination velocity. The junction depth can be optimized by considering its effect on the collection efficiency of the dark current-voltage characteristics or the open circuit voltage, and on the series resistance loss or the fill factor for material parameters. The choice of the material parameters, calculation of the short circuit current, the selection of the n layer thickness, and the cell maximum power and efficiency are discussed. It is concluded that optimized multi-grid structures should allow the use of 10 by 10 sq cm cells with good efficiencies at high concentration ratios, and efficiencies of 22 to 25% should be obtainable from large area cells at concentrations of 40 AM1.
In a front-contact grid pattern for a solar cell there is a trade-off necessary between shadowing loss and excessive power loss due to voltage drop in the metalization itself. If the metalization is too little there may be excessive contact resistance to the underlying semiconductor and insufficient coverage to control losses in the thin front-surface layer of the solar cell. Optimization of grid pattern area and geometry is considered analytically to minimize total losses. Worthwhile performance advantages are shown to be possible, particularly in concentrator systems, if multi-layer grid patterns are used. The current carrying fingers should be approximately square in metal cross section and the main current feedout bars should not only be wider but also thicker than the primary collecting fingers. This is termed multi-level metalization. Effective use of multi-level grid metalization allows much greater concentration-to-loss ratio for a cell of large area and permits good performance from cells of high front-layer sheet resistance.
Multilayer grid patterns consist of fine closely spaced grid lines overlaid by coarser patterns of wider and thicker grid lines to collect the current from the finer grids with low series voltage drop and low active-layer sheet losses. An analytical approach leads to closed form solutions with simple relationships between the power losses in the active layer, in the grid and shadowing losses for optimum design proportions. The results show that multilayer grids, with line thickness equal to line width, greatly reduce losses in cell efficiency under concentration conditions of high current collection. (AlGa)As-pn GaAs cells of areas 1-25 sq cm and sheet resistance 40 ohms/square are considered. Also the performance of a n/p GaAs cell of dimensions 10 x 10 cm is studied. With optimized grid patterns high efficiencies are predicted for large area cells.
In scanning electron microscope (SEM) injection measurements of minority carrier diffusion lengths some uncertainties of interpretation exist when the response current is nonlinear with distance. This is significant in epitaxial layers where the layer thickness is not large in relation to the diffusion length, and where there are large surface recombination velocities on the incident and contact surfaces. An image method of analysis is presented for such specimens. A method of using the results to correct the observed response in a simple convenient way is presented. The technique is illustrated with reference to measurements in epitaxial layers of GaAs. Average beam penetration depth may also be estimated from the curve shape.
Zinc was diffused into 4 times 10 to the 17th per cu cm n-type Al(0.7)Ga(0.3)As grown by liquid-phase epitaxy and also into n-type 2 times 10 to the 17th per cu cm doped GaAs slices at 600, 650, and 750 C. The Zn diffusion coefficient in the Al(0.7)Ga(0.3)As was about one order of magnitude larger than in GaAs. The significance of this fact is that diffusion of Zn through a 0.5 micron Al(0.7)Ga(0.3)As layer appears to be possible with adequate control of the junction depth in the underlying GaAs.
A computer-assisted analysis of the Al(x)Ga(1-x)As-GaAs heteroface solar cell is done to find the dependence of cell efficiency on substrate doping level. Assumptions for carrier lifetime needed for the evaluation of efficiency are based on measurements of experimental Al(x)Ga(1-x)As-GaAs heteroface cells. The results show the doping range 10 to the 16th-17th power per cu cm to be the best for heteroface solar cells. Calculations based on a three-halves temperature dependence for lifetimes agree well with early experimental efficiency versus temperature measurements on GaAs cells, but conflict with earlier results for an Al(x)Ga(1-x)As-GaAs heteroface cell.
The presence of localized regions of poor current-voltage characteristic in heteroface solar cells made by liquid phase epitaxy of Al(x)Ga(1-x)As on GaAs was demonstrated by the formation of mesa diodes with better characteristics than the parent cell, and by direct observation with a flying spot scanner. The latter was also correlated with the characteristics of subsequently etched mesas. Visible features on the cells were found to be harmless or actually regions of enhanced collection in some cases, and responsible for cell performance degradation in others.
The development, fabrication, and failure modes of AlxGa(1-x)As-GaAs heteroface solar cells are described. Crystal growth, the diffusion of Zn into the GaAs layer to form the p-n junction, SEM studies of the diffusion length of GaAs, and procedures for making ohmic contacts are discussed.
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Experimentals characteristics of semiconductor heterojunction pairs ZnS/GaP and ZnSe/GaP
Collection efficiency and spectral response calculations for semiconductor heterojunction solar cells
Fabrication problems and device characteristics of ZnSe-GaAs and Ge-GaAs heterojunctions
Heterojunctions with II-VI compound semiconductors, evaluating electro-optical performance
Solar cell efficiencies computed for semiconductor heterojunction cells
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