Silicon point contact concentrator solar cells

Experimental results are presented for thin high resistivity concentrator silicon solar cells which use a back-side point-contact geometry. Cells of 130 and 233 µm thickness were fabricated and characterized. The thin cells were found to have efficiencies greater than 22 percent for incident solar intensities of 3 to 30 W/cm²(30-300 "suns"). Efficiency peaked at 23 percent at 11 W/cm²measured at 22-25°C. Strategies for obtaining higher efficiencies with this solar cell design are discussed.     

The interdigitated back contact solar cell: A silicon solar cell for use in concentrated sunlight

The theoretical and experimental performance of an interdigitated back contact solar cell is described. This type of cell is shown to have significant advantages over a conventional solar cell design when used at high concentration levels, namely, reduced internal series resistance, nonsaturating open-circuit voltage, and an absence of shadowing by front surface contacting fingers. The results of a computer study are presented showing the effects of bulk lifetime, surface recombination velocity, device thickness, contact dimensions, and illumination intensity on the conversion efficiency and general device operation. Experimental results are presented for solar illumination intensities up to 28 W/cm².     

27.5-percent silicon concentrator solar cells

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm²(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm²are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm²) range.     

Performance of n+-p silicon solar cells in concentrated sunlight

Performance data for n⁺-p silicon solar cells operating at illuminations up to 90 suns (9 W/cm²) and temperatures up to 100°C are presented. Experimental results for 2-cm²cells with different base resistivities are compared to performances predicted by a numerical device analysis computer code. Excellent agreement between numerical simulation and experiment is observed. For the illumination levels considered, an optimum base resistivity of approximately 0.3 Ω. cm is predicted by the numerical analyses and verified experimentally. The 0.3-Ω. cm cells exhibit conversion efficiencies above 11.8 percent up to 90 suns with a peak efficiency of 14 percent at approximately 30 suns. Preliminary results for a large-area (15.2 cm²) circular cell design are also presented for illuminations up to 60 suns. A peak conversion efficiency of 13.5 percent is measured for this cell at ∼25 suns.     

Design criteria for Si point-contact concentrator solar cells

Design criteria for concentrator solar cells are presented for the highly three-dimensional case of backside point-contact solar cells. A recent new experimental result, a 28-percent efficient cell (25°C, 15-W/cm²incident power) is used as a case study of the dependences of the recombination components and the carrier density gradients on the geometrical design parameters. The optimum geometry is found to depend upon the intended design power density as well as the attainable physical parameters allowed by the fabrication techniques utilized. Modeling projections indicate that an ultimate efficiency of 30.6 percent (36 W/cm², 300 K) is achievable using the diffused emitters presently employed on these cells. Incorporation of results from the study of polycrystalline emitters could improve these efficiencies toward 31.7 percent.     

A GaAs solar cell with an efficiency of 26.2% at 1000 suns and25.0% at 2000 suns

A GaAs solar cell without prismatic covers, with the highest efficiency known to the authors in the range of 1000-2000 suns for a single junction, is presented. Low temperature liquid phase epitaxy is used for its growth. In addition to improvements such as the achievement of a good quality material or a low contact resistance, this solar cell exhibits specific enhanced aspects. Among the most noticeable are: (1) an innovative design; (2) a double and gradual emitter layer; (3) a small size: 1 mm², (4) a finger width of the front metal grid of 3 μm; and (5) a tailored ARC deposition based on a nondestructive and accurate AlGaAs window layer characterization. As a consequence, an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns AM1.5D (standard conditions) is achieved thanks mainly to a short-circuit current density at 1000 suns of 26.8 A/cm² (and 53.6 A/cm² at 2000 suns) with a simultaneous series resistance of 3 mΩ·cm²     

18-percent efficient terrestrial silicon solar cells

Silicon solar cells are described which operate at energy conversion efficiencies in excess of 18 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm², 28°C). These are believed to be the most efficient silicon cells reported to date. The high efficiency is a result of the combination of high open-circuit voltage due to the careful attention paid to passivation of the top surface of the cell; high fill factors due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current due to the use of shallow diffusions, a low grid coverage, and an optimized double-layer antireflection coating.     

The design and fabrication of thin-film CdS/Cu2S cells of 9.15-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm²have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu2S cell.     

Increased photogeneration in thin silicon concentrator solar cells

Recent progress in silicon concentrator solar cells has resulted in several designs capable of 25-percent efficiency with one group reporting 28 percent under 14 W/cm²of incident power at 25°C. It has been shown that further improvement is possible by restricting the sunlight acceptance angle of the cell. In this letter, a practical implementation which is equivalent in its effect is proposed which results in an increased utilization of weakly absorbed near-bandgap light. This increased absorption is obtained by placing the cells in a cavity with a small entrance aperture. An analysis is given based upon work on the acceptance angle enhancements by Campbell and Green. The design is expected to improve the efficiencies of existing solar cells to 30 percent. If used in conjunction with previously proposed cell improvements, the efficiencies will be improved towards 33 percent, very near the limit efficiency of 36 percent. This design also has the effect of decreasing the differences in performance between the leading candidate concentrator cell designs and diminishing the dependence of the efficiencies on the cell texturization and bulk carrier lifetimes.     

High-Power Pulsed UHF and L Band p+-n-n+ Silicon TRAPATT Diode Lasers

The design and performance of both the lumped-element TRAPATT oscillator circuit and deep-diffused p+-n-n+ silicon TRAPATT diodes designed primarily for pulsed RADAR applications in the UHF and L band frequency ranges are discussed. Circuit conditions for optimum performance are described. Methods of optimizing diodes are presented. Diode performance capability is shown to depend on the relative position of the junction in the device depletion region. Peak powers close to 900 W and maximum conversion efficiencies of 40 percent have been achieved from diodes with large p-region width to total depletion region width ratios. RF leading-edge jitter of less than 1 ns has been obtained under optimum circuit and diode operating conditions.     

A p-i-n thermo-photovoltaic diode

The analysis, fabrication, and operation of a p-in photovoltaic cell that is suitable for use at the very high illumination intensities, which are encountered in a thermo-photovoltaic system, are presented. The device performance has been measured under pulse conditions to 790 W/cm²of incident radiation. The efficiency of operation is limited at very high illumination levels by an increase in the reflection coefficient. Open circuit voltages of 0.42 volts and short circuit currents in excess of 40 A/cm²of illuminated surface have been observed for germanium devices. Operating efficiencies at various incident light intensities are reported. The analysis includes an estimate of the effect of the electric field that occurs in the intrinsic region.     

Large-signal analysis of Lo-Hi-Lo double-drift silicon IMPATT diodes at 50 GHz

Large-signal analysis of a lo-hi-lo double-drift silicon IMPATT diode at 50 GHz shows that the device is capable of output power of 1.1 W and efficiency of 20 percent for a device area of 2 × 10^-5cm²at a dc biasing current density of 12 kA/cm²and ac voltage amplitude of 12 V. It is also found that, both output power values and efficiencies decrease with increasing enhanced leakage current.     

Design and Realization of Wide-Band-Gap ($sim$ 2.67 eV) InGaN p-n Junction Solar Cell

The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%, which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV, respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm² are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure.     

Improved performance thin solar cells

A silicon solar cell device structure was developed to improve performance from thin cells. The design has collecting junctions both on the illuminated and most of the dark sides. Since the two junctions function interdependently, the device is called a tandem junction cell (TJC). The photoresponseI-Vperformance of cells was measured in two modes, with either collection from the junctions on both sides, or collection from only the nonilluminated side. Efficient current collection from TJC cells was observed in both modes, and current collection improved as the cell thickness was reduced. At the calculated optimum thickness of 30-50 µm, collection for the two modes should be equivalent.     

An experimental study on improvement of performance for hemispherically shaped high-power IRED's with Ga1-xAlxAs grown junctions

This paper describes the structure and performance of a high-power infrared emitting diode (IRED) designed as a high speed optical beam source for optoelectronic applications. The heterostructured junction is formed on a thick Ga1-xAlxAs liquid phase epitaxy (LPE) grown layer which is used to shape hemispherical emitting surfaces. Dislocation density in recombination region was considerably decreased by the thick layer growth on a GaAs wafer used as a primary substrate. Under dc operations, external quantum efficiencies of around 45 percent at a current density of 0.6 kA/cm²and about 110 mW of optical output power at 200 mA (1 kA/cm²) have been obtained from the diodes with a 160-µm junction diameter. The tendency to reach power saturation with increased current has been decreased by means of reducing of thermal resistance of the mount, and the diodes with 240- µm junction diameter have shown about 180 mW at 600 mA dc and 1.4 W at a 4-A pulse (60 Hz, 50 µs). A large improvement in high frequency response has been obtained and the bandwidth at -3-dB intensity has reached above 120 MHz.     

Solar-cell design based on a distributed diode analysis

The front surface of a p-n junction solar cell has resistive losses associated with the diffused layer, the metal-semiconductor contact, and the grid structure. These losses are analyzed by considering the spatially distributed nature of the p-n junction and the grid conductors. This distributed diode analysis is especially useful for solar cells operated under concentrated sunlight conditions. The results show the dependence of the V-I characteristics and the maximum power output per unit cell on the ratio of the diffused layer resistance to the junction dynamic resistance. This ratio can assist the designer in establishing proper grid structure geometries and should tpically be less than 0.1 if the power output per unit cell is to be within 3 percent of that for the lossless case. Experimental measurements are reported which confirm the theoretical calculations. An analysis of the grid conductor losses associated with multiple-connected unit cells shows the disastrous effect that the grid header resistance can have on the performance of a solar cell. The results indicate that the use of a tapered header conductor to decrease the metal coverage may actually worsen cell performance.     

Optimum Design of Dye‐Sensitized Solar Module for Building‐Integrated Photovoltaic Systems

This paper presents a method for determining the optimum active‐area width (OAW) of solar cells in a module architecture. The current density–voltage curve of a reference cell with a narrow active‐area width is used to reproduce the current density profile in the test cell whose active area width is to be optimized. We obtained self‐consistent current density and electric potential profiles from iterative calculations of both properties, considering the distributed resistance of the contact layers. Further, we determined the OAW that yields the maximum efficiency by calculating efficiency as a function of the active‐area width. The proposed method can be applied to the design of the active area of a dye‐sensitized solar cell in Z‐type series connection modules for indoor and building‐integrated photovoltaic systems. Our calculations predicted that OAW increases as the sheet resistances of the contact layers and the intensity of light decrease.     

A comparison between BIMOS device types

This paper deals with the design and comparison of three types of BIMOS transistors (cascade, cascode, and parallel combinations of MOSFET and bipolar transistors). The first section of the paper presents a technique for accurately determining the overall deviceI-Vrelations for the cascade (Darlington) combination based on the area ratio for the MOSFET and bipolar transistors. From this, one sees that the optimum area ratio varies from 1.5 for 700-V devices to 0.2 for 80- V devices. The second section of the paper deals with the design and comparison of the cascode and parallel device types. In these cases, no optimum area ratio based solely on device conduction exists and the device design and comparison is based on achieving the maximum current density limited by keeping the power dissipation to under 100 W/ cm². Included in this analysis are losses due to conduction as well as switching. The results show that a 0.5 area ratio (MOSFET to bipolar transistor) is optimum for the cascode (series) combination and a 0.3 ratio is a good compromise for the parallel combination. The last section of the paper shows an overall comparison of all of the BIMOS device types together with the MOSFET and the bipolar transistor.     

Recombination in highly injected silicon

Recent advances in solar cells designed to operate under high-level injection conditions have produced devices that are approaching some of the limits imposed by the fundamental band-to-band Auger recombination in Silicon. A device has been optimized to study this recombination by using the fabrication technology developed for point-contact solar cells. Using both steady-state and transient measurements, the recombination rates in high-resistivity Si in the injected carrier density range of 10¹⁵to 2 × 10¹⁷carriers / cm³were investigated. The coefficient of the recombination, which depends on the carrier density cubed, is found to be 1.66 × 10^-30cm⁶/s ± 15 percent. This result is four times higher than the ambipolar Auger coefficient commonly used in the modeling of devices that operate in this injected carrier density range and lowers the expected limit efficiencies for silicon solar cells.     

Graphical optimization of the doping profile of transit-time devices

The doping profile of transit-time avalanche diodes has evolved from a compromise of technological convenience and the original structure proposed by Read [1]. A technique by which the doping profile my be quickly optimized is described, and it is shown that a four-layer p-n-p-n structure should give the highest efficiency for a transit-time diode. This technique, which my be generalized to other types of device, predicts the maximum power density available from the optimum structure. For a Read-type diode, operating in X band, this is in the region of 40 kW/cm²at an efficiency of around 30 percent. Higher efficiencies are attainable at the expense of output power.