J
jeremy
7 minutes
Heterojunction Solar Cells: Amorphous Silicon and ITO at Low Temperature
Heterojunction solar cells join two kinds of silicon. The core is a crystal n-type wafer. On each face, the factory adds a few nanometres of amorphous silicon. Amorphous means its atoms have no regular pattern. This layer holds a lot of hydrogen, which ties up broken bonds and heals the surface very well. It is grown by PECVD from silane and hydrogen, below about 200 C. Doped layers come from adding phosphine or diborane gas. Both gases are highly toxic, so leak sensors and exhaust burners control them. Next, a thin layer of ITO is sputtered on each side. ITO is indium tin oxide, a see-through film that carries current. Silver fingers go on top. Because amorphous silicon breaks down above about 250 C, the cell cannot be fired. It uses a paste cured at low heat instead, which needs more silver per watt. The process has fewer steps and no hot diffusion furnace, so it saves energy. The cells reach high voltage and lose less power on hot days. The costs are silver and indium. Indium is rare and comes mostly as a by-product of zinc refining. Plants reclaim spent ITO targets, and some test indium-free oxides. Chamber cleaning uses fluorine gases, which have a strong climate effect. After this Concept you can explain why a heterojunction cell is made cold, and weigh its energy savings against its silver and indium use.