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PV Q&A

Q: What is photovoltaic (PV)?
A: Photovoltaic = Photon + Voltaic, which means a phenomenon of generating electricity under the influence of light. Solar cell is a semiconductor device which is designed to absorb and convert light energy from the sun light into electricity.
Q: What is typical structure for amorphous Si solar cells?
A: In amorphous thin-film silicon solar cells, the p-i-n sandwich structure is always used. Doping of a-Si:H inevitably leads to the creation of dangling bonds, causing the very small diffusion length of charge carriers in doped a-Si:H. Therefore, a-Si:H solar cells can NOT work successfully as a p-n junction usually used in crystalline wafer silicon solar cells; however, a relatively defect free intrinsic layer has to be inserted to enhance the collection of photo-generated carriers.
Q: What are the possible applications?
A: Amorphous solar roof tiles, large surface modules for metal roofs, or solar synthetic roofing membrane - the combination of lightweight roofing and solar power generation develops potential for application ranging from private homes to industrial buildings.

Thin Film Silicon PV

Photovoltaic devices absorb sunlight and transform light energy into electricity. The sunlight-to-electricity phenomenon, so called photovoltaic effect, was found in the early 20th century. In 1954, the first PV cell (photovoltaic cell) producing sufficient electrical power came to the earth by Dr. Pearson et al. at Bell Lab.

A simple concept to understand photovoltaic effect is explained with the figure, shown above, of a PV device structure. Thin film silicon solar cells are generally fabricated with a p-i-n design where an undoped absorber material (i.e. intrinsic layer) is sandwiched between negatively (n-layer) doped and positively (p-layer) doped layers, thus creating a built-in electric field in between. With absorbing the incident photons, the generated electrons and holes drift toward the electrodes at the n-type side and the p-type side, respectively. Those electrons and holes reaching the electrodes further flow through external loads and deliver output power.

The device structure of a thin film silicon cell differs from a P-N junction commonly used in a wafer-based silicon PV cell. Since a-Si is extremely sensitive to visible spectrum in nature, a thin a-Si-layer is enough for effective photo-absorption. Therefore, cost reduction can be readily reached due to less material consumption. In theory, a good PV cell is the one that is able to convert sunlight into electricity as much as possible. During exposure to sunlight, photons penetrate into top layer and are mainly absorbed within i layer. Photon-generated electron-hole pairs are separated by built-in electric field within i layer. Conceptually the i-layer has to be thick enough so that most of the light can be absorbed but a too thick i-layer will impede transport of electron hole pairs generated. In spite of the trade-off, the thickness of the i-layer needed is still much less than that required for conventional wafer-based silicon PV cells.

Module Assembly Step by Step

Front-End Processes

The module production processes in front-end part are described as follows:
 1. A glass substrate is cleaned up by detergent.
 2. Zinc oxide (ZnO), a transparent conductive oxide (TCO) layer, is deposited on the front side of the glass substrate. The TCO layer serves as front contact (i.e. positive electrode) of the cell in module.
 3. A laser scribing system is utilized to pattern the TCO layer for defining the electrode of individual cell.
 4. Various silicon films p-, i-, and n- layers are subsequently deposited on the front side of the glass substrate by plasma enhanced chemical vapor deposition (PECVD) system.
 5. Another laser scribing system is utilized to remove silicon films from the substrate for defining the active layer of individual cell.
 6. TCO layer is deposited upon the silicon layers for being back contact (i.e. negative electrode).
 7. Laser scribing system is again used to scribe the TCO and silicon layers for defining another electrode of individual cell. After this scribing all cells deposited upon the glass substrate are connected in series, and thus a monolithic module is formed on front glass.
 8. A quality check by photo-voltage system is performed prior to back-end processes.

Back-End Processes

The detailed module production processes in back-end part are described as follows:
 1. Silver paste is dispersed onto two electrode parts of the monolithic module.
 2. Two tin-coated copper ribbons are attached to the silver paste mentioned in (1) to be the electrical leads of the whole module.
 3. Oven is utilized to dry dispersed silver paste.
 4. For enhancement of light absorption , white ink is screen printed as a reflection layer exploit the long wavelength response in a better way. In addition, by using a reflection layer, the product is opaque. For see-through products, simply the white ink printing process is ignored.
 5. Oven is utilized to dry printed white ink.
 6. Sand blasting is utilized to remove those films coated at the edges for good electrical isolation.
 7. Flasher is utilized for preliminary I-V performance test.
 8. Two cross contacting ribbons for electrical connection of junction box are soldered onto electrical leads mentioned in (2).
 9. Lamination foil is covered onto front glass.
10. Back glass is posited onto lamination foil which covers front glass.
11. For encapsulation, a laminator is utilized to press heated stack of front glass/lamination foil/back glass in vacuum.
12. After lamination, the remaining lamination foil at the edges is trimmed.
13. Junction box is fixed onto the rear side of the laminated panel.
14. Flasher is utilized for I-V performance test of final product.

PRODUCTION

Proven technology for amorph and microcrystalline PV absorbers

High speed laser scribing for all three patterns

Device contacting and contact curing