Ronald A. Sinton

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.

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. >

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.

Point-contact silicon solar cells

A new type of silicon photovoltaic cell designed for high-concentration applications is presented. The device is called the point-contact-cell and shows potential for achieving energy conversion efficiencies in the neighborhood of 28 percent at the design operating point of 500× geometric concentration and 60°C cell temperature. This cell has alternating n and p regions that form a polkadot array on the bottom surface. A two-layermetallization on the bottom provides contact. Initial experimental results have yielded a cell with 20-percent efficiency at a concentration of 88.

Simplified backside-contact solar cells

Experimental and modeling results for a family of simplified backside-contact designs are presented. This simplification hinges upon the demonstration of a self-aligned metalization technique as well as an optimization of heavily doped, compensated regions in the solar cell. The resulting devices can be fabricated without mask alignment and require as few as one mask. A 10.5 cm/sup 2/ one-sun cell that achieves 21.9% efficiency is presented as an example of the potential for these device designs. Prospective performance for concentrator cells of this type is also discussed. >

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.

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.

26-percent efficient point-junction concentrator solar cells with a front metal grid

Silicon concentrator cells with point diffusion and metal contacts on both the front and backsides are discussed. The design minimizes reflection losses by forming an inverted pyramid topography on the front surface and by shaping the metal grid lines in the form of a triangular ridge. A short-circuit current density of 39.6 mA/cm/sup 2/ has been achieved, even though the front grid covers 16% of the cells active area of 1.56 cm/sup 2/. This, together with an open-circuit voltage of 700 mV, has led to an efficiency of 22% at one sun, AM1.5 global spectrum. Under direct-spectrum 8.8-W/cm/sup 2/ concentrated light, the efficiency is 26%. This is the highest ever reported for a silicon cell having a front metal grid.<<ETX>>

Multilevel metallization for large area point-contact solar cells

An analysis of the series resistances of different metallization schemes for large-area backside-contact (BC) solar cells is presented. The need for developing a multilevel metallization technology for such cells is demonstrated. The authors propose a new design for the metallization of BC cells that present a series resistance independent of the cell size. The particular features required for such a multilevel interconnection are studied, and a process using anodic oxidation of aluminum is presented. BC silicon solar cells of 0.64 cm/sup 2/ have been processed in this technology, resulting in 26.2% efficiencies at 10 W/cm/sup 2/ (100 suns AM1.5, 25.5 degrees C). Subsequent runs with a simplified process and a new cell design have given 27.3% efficiency cells. The cells have been soldered on alumina mounts. Results of thermal cycling are given.<<ETX>>

Light‐induced degradation at the silicon/silicon dioxide interface

Single‐crystal silicon point‐contact solar cells show a degradation in their efficiency after being exposed to concentrated sunlight. This change has been linked to an increase in the surface recombination velocity. A similar effect is produced by carrier injection under forward bias. The annealing kinetics, the role of ultraviolet light, and possible causes for the creation of surface states are discussed.