INSIGHT                                                         

The low material cost and large-area deposition capabilities of amorphous silicon (a-Si) alloys have made this material a very viable candidate for commercial application of Photovoltaic. Having high absorption coefficient of a-Si alloy which causes tailing of the bands that affect the transport of the carriers and further deterioration of transport caused by the presence of dangling and weak bonds, micro voids and other heterogeneities, light-induced creation of metastable defects, makes these cells poor light-to-electricity conversing devices than their crystalline counterparts. While engineering innovations like the multi-junction approach have led to substantial improvement of stable efficiency, there is a need to improve the intrinsic material quality of these alloys. a-Si alloy is usually deposited by glow discharge decomposition of silane. It was first shown by Guha et al. in 1981 that a-Si alloy films grown by decomposing a dilute mixture of silane in hydrogen show less light-induced degradation. The technique has been used successfully to make improved quality a-Si alloy, amorphous silicon germanium (a-SiGe) alloy and silicon-carbon alloy. In fact it is now widely used by the entire photovoltaic industry to obtain high quality materials and devices, and has made the largest contribution to the improvement of stabilized cell and module efficiency. The paper discusses production of triple-junction amorphous silicon (a-Si) photovoltaic products at United Solar Systems Corp by continuous roll-to-roll deposition process.

The logical emergence of this technology as a lightweight solar power generator has been discussed. Future of the technology in terms of both device efficiency and product efficacy are given. The last quarter century witnessed the birth, the adolescence, and the maturity of amorphous silicon (a-Si) based photovoltaic (PV) technology. As we usher in the twenty-first century, we must also meet the ever-increasing energy demand while addressing environmental issues and ecological concerns. a-Si PV technology has positioned itself as the low-cost solution to the challenge of energy, environmental, and ecology -- the e-tripos. As the a-Si PV industry still experiences unavoidable growing pains, large-volume manufacturing plants are ready to demonstrate their competitiveness in the rapidly growing global market.  

2. Highlights of Innovation

• Triple-junction Structure.

•Flexible Substrate.

•Back Reflector

•a-Si Alloys

•a-SiGe Alloys.

•Microcrystalline Window Layer

•Tunnel Junctions

•Roll-to-roll Semiconductor Processor  

3. Advantages of a-Si based triple junction roll-to-roll continuous deposition process

 a-Si based materials Vs. Others

Low Cost.

High Efficiency

Strong and Durable.

Easy to Handle Lightweight and easy to install

Shadow tolerant

4. NEW ADVANCEMENTS

 The 5 principal areas in this research and development program are:

(1) Improved web heating and temperature monitoring systems;

(2) New online diagnostic systems;

(3)Testing and implementing reactive ZnO sputtering from Zn metal targets;    

(4) Improved rf PECVD cathode designs; and

(5) The development of new pinch valves.

United Solar is now developing a new set of online diagnostic systems that measure PV device characteristics. These systems will allow immediate recognition of production problems and enable the operators, and eventually expert systems, to continuously optimize the process. We expect that these new capabilities will significantly reduce the commissioning time of the new 25 MW equipment. The systems being developed include a scatterometer for back reflector surface texture measurements, reflection spectrometers for film thickness measurements, and PV Capacitive.

Other Uses

1. For neutron imaging                 2. Structural Standing Seam (SSR) Panels

3. PV Shingles (SHR)                  4. LCD

5. Digital radiography                   6.In Digital X-Ray 

Now as we conclude, although many innovative methods are being used to improve the efficiency further, an alternate method is to use the material grown above the edge (i.e. microcrystalline silicon). Impressive progress has been made in this field as well, and thin-film silicon solar cells are expected to play a major role in the photovoltaic market. New applications in space and stratosphere are enhancing the opportunities for these products in those premium markets.