R. R. King

Studies of diffused phosphorus emitters: saturation current, surface recombination velocity, and quantum efficiency

The surface recombination velocity s for silicon surfaces passivated with thermal oxide was experimentally determined as a function of surface phosphorus concentration for a variety of oxidation, anneal, and surface conditions. This was accomplished by measuring the emitter saturation current density J/sub 0/ of transparent diffusions for which the J/sub 0/ is strongly dependent on s. At the lowest doping levels, the value of s was confirmed by measurements of s on substrates with uniform phosphorus doping. The impact of these measurements on solar cell design is discussed. >

Studies of diffused boron emitters: saturation current, bandgap narrowing, and surface recombination velocity

The emitter saturation current density, J/sub 0/ was measured on diffused boron emitters in silicon for the case in which the emitter surface is passivated by a thermal oxide and for the case in which Al/Si is deposited on the emitter surface. The oxide-passivated emitters have a surface recombination velocity, s, which is near its lowest technologically achievable value. In contrast, the emitters with Al/Si on the surface have surface recombination velocities which approach the maximum possible value of s. From the J/sub 0/ measurements, the apparent bandgap narrowing as a function of boron doping was found. Using this bandgap narrowing data, the surface recombination velocity at the Si/SiO/sub 2/ interface was extracted for surface boron concentrations from 3*10/sup 17/ to 3*10/sup 19/ cm/sup -3/. >

Doped surfaces in one sun, point‐contact solar cells

This letter reports two new types of large‐area (>8.5 cm2), backside, point‐contact solar cells with doped surfaces, designed for use in unconcentrated sunlight. One type was fabricated on an intrinsic substrate with an optimized phosphorus diffusion on the sunward surface. The apertured‐area efficiency was independently measured to be 22.3% at 1 sun (0.100 W/cm2), 25 °C, the highest reported for a silicon solar cell. The other type is constructed on a doped substrate, and has an apertured‐area efficiency of 20.9%, the highest reported for a point‐contact solar cell with a base in low‐level injection. Both cells have record open‐circuit voltages above 700 mV.

Front and back surface fields for point-contact solar cells

The authors discuss the use of planar dopant diffusions to reduce surface recombination in point-contact solar cells. These noncurrent collecting diffusions can boost the efficiency of point-contact cells significantly for incident intensities below about 5 suns (0.500 W/cm/sup 2/). At these low power levels, the surface recombination is the dominant recombination mechanism. Measured values of the emitter saturation current density, J/sub o/, of phosphorus diffusions at the oxidized silicon surface are presented for a range of surface concentrations and furnace conditions on untexturized

Photoinjected hot‐electron damage in silicon point‐contact solar cells

Initial designs of single‐crystal silicon point‐contact solar cells have shown a degradation in their efficiency after being exposed to concentrated sunlight. The main mechanism appears to be an increase in recombination centers at the Si/SiO2 interface due to ultraviolet light photoinjecting electrons from the silicon conduction band into the silicon dioxide that passivates the cell’s front surface. The instability of the interface is significantly worse if the oxide is grown in the presence of trichloroethane. Texturization of the surface also leads to more instability. A reasonably good resistance to ultraviolet can be created by putting a phosphorus diffusion at the surface, and can be improved further by stripping off the deposited oxide after the diffusion and regrowing a dry thermal oxide.

Stable passivations for high-efficiency silicon solar cells

Initial designs of single-crystal silicon point-contact solar cells have shown a degradation in their efficiency after being exposed to concentrated sunlight. The main mechanism appears to be an increase in recombination centers at the Si/SiO/sub 2/ interface due to ultraviolet light photoinjecting electrons from the silicon conduction band into the silicon dioxide that passivates the cells front surface. Trichloroethane, texturization, and aluminum during the forming gas anneal all contribute to the instability of the interface. A reasonably good resistance to UV light can be obtained by putting a phosphorus diffusion at the surface and can be improved further by stripping off the deposited oxide after the diffusion and regrowing a dry thermal oxide. A second technique, which utilizes ultrathin oxides and thin polysilicon films and can yield stable point-contact solar cells that are more efficient at higher concentrations, is also described.<<ETX>>

Passivated emitters in silicon solar cells

In high-efficiency silicon solar cells with low metal contact coverage fractions and high bulk lifetimes, cell performance is often dominated by recombination in the oxide-passivated diffusions on the cell surface. Measurements of the emitter saturation current density, J/sub o/, of oxide-passivated, boron and phosphorus diffusions are presented, and from these measurements, the dependence of surface recombination velocity on dopant concentration is extracted. The lowest observed values of J/sub o/ which are stable under UV light are given for both boron- and phosphorus-doped, oxide-passivated diffusions, for both textured and untextured surfaces. Contour plots which incorporate the above data were applied to two types of backside-contact solar cells with large area (37.5 cm/sup 2/) and one-sun efficiencies up to 22.7%.<<ETX>>

Developments in module ready SI backside-contact solar cells

Moderate quantities of high-efficiency Si backside-contact solar cells were fabricated. Compromises were made at the design level in order to investigate the possibilities for obtaining a high fabrication yield. The cells were tested in a module-ready state, soldered down to copper cell mounts. The efficiencies for the best cells peak at 26% for a range of 5-12 W/cm/sup 2/ (50-120 suns). For a design point of 36 W/cm/sup 2/, a majority of the 283 cells from two successive runs have efficiencies in a tight distribution around 23%. The performance difference between these cells and the best laboratory cells demonstrated to date (26% versus 28.3%) is primarily due to the sunward-side dopant diffusion, not present in the previous cells but incorporated in the current designs in order to make them stable to degradation in ultraviolet light. Accordingly, improved performance is expected to come primarily from improvements in stable front-surface passivations.<<ETX>>

A Technology-Based Comparison between Two-Sided and Back-Contact Silicon Solar Cells

A modeling comparison between Two-Side Contacted and Back-Contact cells is made to determine which cell structure offers the best performance potential for concentrated light. The comparison includes both low-resistivity and nearly intrinsic silicon substrates. The model parameters are carefully selected to reflect the state-of-the-art of crystalline silicon technology. Optical and ohmic losses due to the front metal grid constitute the most significant difference between the Front-Contact and Back-Contact cell designs. On the other hand, recombination at the front surface emitter is the main factor that limits the internal cell performance. If the fabrication process is kept reasonably simple, the Back-Contact cell is clearly advantageous, with an achievable efficiency of 26% in the range of 100 to 300 suns. Possible advancements such as polysilicon emitters are also discussed and predicted to give an efficiency of 27.5%.