SMT Perspectives & Prospects: What I Hope the Next Generation Will Carry Forward
Knocking Down the Bone Pile: Interposer Rework and Precision Solder Ball Attachment
Amorphous silicon
Amorphous silicon (a-Si) is the noncrystalline allotropic form of silicon. It has a higher bandgap (1.7 eV) than crystalline silicon (c-Si = 1.1 eV), which means it absorbs the visible and ultraviolet portion of the spectrum. It can be deposited as a thin film by chemical vapor deposition, typically plasma-enhanced chemical vapor deposition (PE-CVD), from silane gas and hydrogen gas. PVs made this way tend to have a lower energy conversion efficiency of around 10%.
Thin-film types
Organic photovoltaic cells (OPV) are usually a thin-film type. OPVs are finally becoming competitive with crystalline silicon modules in terms of efficiency, stability and cost. The efficiency of OPV has gone from 1% to 2% to about 9%. Recently, a project funded by FlexTech Alliance boosted efficiency to nearly 12% using high-efficiency donor polymer materials developed by Solarmer Energy Inc. These flexible cells are approaching the sub-$0.50/watt level cost needed to outperform traditional power production.
Copper indium gallium selenide and copper indium diselenide
Copper indium gallium selenide (CIGS) and copper indium diselenide (CIS) These materials from the periodic table from I - III - VI elements are of interest because of their high optical absorption coefficients and electrical/optical properties. These materials are mixed in chalcopyrite thin films of CIS, CIGS, and copper gallium selenide (CGS) and have achieved greater than 12% energy efficiency.
Currently, their manufacturing costs are higher than amorphous silicon solar cells, but ongoing work is leading to more cost-effective processes. The best efficiency of a thin-film solar cell was 19.9% reported for CIGS in March 2008. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the solar light. The world’s largest CIGS gigawatt-scale factory has been turned on in Kunitomi, Japan by Solar Frontier.
In Germany, a six-layer system is coated on polyimide film that is 300mm wide and 25mm thick and has achieved 10.2% efficiency. The six layers are:
- Molybdenum back electrical contact (back)
- Copper
- Indium
- Gallium
- Diselenide
- Zinc oxide electrical contact window with optional cover protection of ORMOCER hybrid polymer (front)
- Cadmium Telluride
Cadmium telluride (CdTe) is an efficient, light-absorbing material for thin-film solar cells. CdTe is easier to deposit and suitable for roll-to-roll production. Its efficiency is now up to 16% and it is currently the most popular thin-film PV material. Although cadmium is toxic, it is not released into the air and does not leach. The value of the telluride makes recycling very practical.
Others
Light-absorbing dyes and organic/polymers solar cells
Typically, a ruthenium (Ru) metal-organic dye is used as a monolayer light-absorbing material over a mesoporous layer of nanoparticulate titanium dioxide. A redox couple in the electrolyte, which can be a liquid or solid, completes the circuit.
Organic/polymer solar cells are built from thin-film compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerene. Energy conversion efficiencies achieved to date are low at 6%. However, these cells could benefit from their mechanical flexibility and the ability to screen or offset print their structures into large formats.
In the final part of this article series, Happy Holden will discuss the opportunities for printed electronics as well the future of the industry.
You can read the Part 1 of this article series here; Part 2 here; and Part 3 here.
Editor's Note: This paper has been published in the proceedings of SMTA International.
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