The final installment of this article discusses screen-printed solar cells, ultra-thin batteries, RFID devices, electroluminescent displays, and blood-glucose sensors.
By Wim Zoomer
Solar or photovoltaic (PV) cells are devices that convert energy from the sun into electric power (Figure 1). A rigid solar cell consists of a ~0.25-mm silicon wafer. Several cells are connected to a panel, and the wafer’s surface is chemically treated. The white or grey lines at the front of the wafer are screen-printed silver power collectors. The widest two lines are bush bars, and the fine lines are fingers.
The silicon wafer’s surface should be as large as possible to collect the maximum amount of solar energy. This means that the fingers must be narrower than 100μm. The dried and fired solvent-based ink de-posit is approximately 10 μm.
Present requirements are fired lines measuring 50-60 μm wide and 15-25 μm high. Careful selection of mesh (usually 325- to 400-thread/in. stainless steel), direct emulsions or capillary films, and the silver particle size in the conductive inks is critical to the success of such demanding work. Ideally, the press operator wants the screen to empty completely, the mesh to snap-off properly to minimize image distortion, and the mesh to stand up to the rigors of the long print run—all while using a relatively hard squeegee to print the required fine-line definition.
Flexible solar cells are contemporary developments, though the manufacturing process is similar: Silver conductive bush bars and fingers are flatbed or rotary screen printed on top of the plasma-coated, thin, silicon layers on a flexible, polyester web. The resistance of the silver ink is low enough after thermal curing to allow practical use. The power of a 1-sq-m panel made of flexible solar cells is approximately 35 W, suitable for battery chargers.
Latest developments are organic-dye solar cells, with nano-particles converting solar energy into electricity. A possible application is to integrate such a module into building or office façades to power promotional displays, because the screen-printed, electricity generating film between two glass panes is ultra-thin and, therefore, almost invisible. The efficiency of the organic-dye solar cells is relatively low. Therefore, they have yet to threaten the viability of conventional silicon solar cells.
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There has to be more improvements in the development of solar cells in order for solar panel to reach its maximum energy conversion rate. Many people have not found solar energy to be that efficient and affordable, but with more innovations in that technology, that could change.
Matt - eezytrade