W.K. Schubert

Emitter wrap-through solar cell

The authors present a new solar cell concept (emitter wrap-through or EWT) for a back-contact cell. The cell has laser-drilled vias to wrap the emitter on the front surface to contacts on the back surface and uses a potentially low-cost process sequence. Modeling calculations show that efficiencies of 18 and 21% are possible with large-area solar-grade multi- and monocrystalline silicon EWT cells, respectively.<<ETX>>

The effect of encapsulation on the reflectance of photovoltaic modules using textured multicrystalline-silicon solar cells

Texturing multicrystalline-silicon solar cells is a promising technique for reducing reflectance losses. The authors investigated two methods for texturing multicrystalline-silicon solar cells: anisotropic chemical etch; and mechanical dicing saw. Their work emphasized reducing reflectance in the encapsulated module by using optical confinement in the module. They found that optical confinement in the module is very important in the optimization of texture geometries.

Characteristics of HEM silicon produced in a reusable crucible

A reusable crucible has been developed for directional solidification of multicrystalline silicon ingots by HEM. Up to five 40 kg, 33 cm square cross section ingots have been produced without visible degradation of the crucible. Characterization of the silicon shows large grain size, vertical orientation of grain boundaries, uniform resistivity, low defect density, diffusion lengths up to 272 /spl mu/m and less than 1 ppma oxygen concentration. Solar cells of 2 cm/spl times/2 cm size fabricated on 10 cm square wafers have shown good uniformity and efficiencies up to 15.1% and V/sub OC/ to 616 mV.<<ETX>>

Simplified processing for 23%-efficient silicon concentrator solar cells

The authors have developed a three-photomask process that requires only a single furnace step above 800/spl deg/C to fabricate silicon concentrator cells with efficiencies approaching 23% on float-zone (FZ) Si and, for the first time, an efficiency of 20% on solar-grade Czochralski (Cz) Si. Until now, high-efficiency silicon concentrator cells that have demonstrated conversion efficiencies greater than 22% have used FZ silicon and required multiple dopant diffusions and several high-temperature furnace steps. These complexities increase processing costs and preclude the use of lower cost CZ silicon, which is easily degraded by repeated high-temperature cycling. This simplified processing schedule retains most of the benefit available from high-performance solar cell designs while holding down processing costs, and it makes the use of lower cost CZ Si a viable option. The incorporation of a better optimized antireflection coating and a lower recombination back-surface field are projected to increase this performance even further.<<ETX>>

Phosphorus and aluminum gettering-investigation of synergistic effects in single-crystal and multicrystalline silicon [solar cells]

Synergistic effects from simultaneous phosphorus diffusion/aluminum-alloy gettering are investigated in three different crystalline-silicon substrate solar cells. The silicon materials, experimental design, characterization and analysis are presented. Some evidence for synergism is observed in the finished solar cells on all three substrate types. These results are combined with complementary observations of the effects of oxidation on bulk properties of previously gettered substrates to suggest a high-volume, low-cost process implementation which could give up to 9% relative increase in solar cell efficiency.

High-efficiency solar cells using HEM silicon

Developments in heat exchanger method (HEM) technology for production of multicrystalline silicon ingot production have led to growth of larger ingots (55 cm square cross section) with lower costs and reliability in production. A single reusable crucible has been used to produce 16 multicrystalline 33 cm square cross section 40 kg ingots, and capability to produce 44 cm ingots has been demonstrated. Large area solar cells of 16.3% (42 cm/sup 2/) and 15.3% (100 cm/sup 2/) efficiency have been produced without optimization of the material production and the solar cell processing.

A simple single photomask process for fabrication of high-efficiency multicrystalline-silicon solar cells

The authors have developed a simplified process sequence for the fabrication of high-efficiency multicrystalline-silicon (mc-Si) solar cells. Photolithography is required only to define the evaporated metal gridlines. The authors use this fast turn-around, high-yield baseline process to evaluate different mc-Si materials and new processing procedures. The process uses a one-step emitter diffusion/drive-in and an aluminum-alloyed back surface field to provide a well-passivated cell with excellent blue and red response. Laser-scribed cell-isolation grooves are used to define both moderate-area (4, 4.6, or 10.5 cm/sup 2/) and large-area (42 cm/sup 2/) cells. They have observed minority carrier diffusion lengths of around 300 /spl mu/m in 1.4-/spl Omega/cm mc-Si material and have achieved efficiencies of 16.8% in 4.6-cm/sup 2/ cells. Large-area cell efficiencies in the same material have reached 16.4%.

15%-Efficient multicrystalline-silicon photovoltaic modules: cell processing and characterization

Abstract A relatively simple single-photomask process was used to produce high-efficiency large-area solar cells on commercially available heat-exchanger-method (HEM) multicrystalline silicon (mc-Si). Large-area cell efficiencies up to 16.4% were achieved before lamination. These cells were built into two prototype one-sun modules whose performance has set a new standard for mc-Si photovoltaic modules (15% at standard reporting conditions). These results demonstrate that significant performance gains can be achieved in commercial mc-Si modules with currently available substrates through improved processing. Continued evolutionary developments in substrate and cell manufacturing promise to maintain, and likely improve mc-Sis PV market position.

Gettering in multicrystalline silicon-a design-of-experiments approach

Statistical methods were used to design and analyze the results of a gettering experiment on four industrial multicrystalline silicon solar cell materials. The experiment studied the effects of temperature and time in the POCl/sub 3/ diffusion process and the aluminum alloy process using simple diagnostic devices. The time and temperature ranges were restricted to maintain compatibility with commercial fabrication sequences. The design was capable of picking up second order interactions between the various processing factors. Statistically significant gettering effects were detected in only two of the four materials. The results for one of these materials were further tested using full solar cells. Strengths and weaknesses of this approach to gettering studies have become apparent in the present work and are discussed.