Solar photovoltaics

solar_photovoltaics
 

ALD has applications in almost all branches of solar photovoltaics (PV). The most important application is surface passivation of crystalline silicon (c-Si) solar cells with a thin layer of Al2O3, which improves the cell efficiency 1-2 % and can increase the carrier lifetimes from the microsecond range to several milliseconds. Compared to the conventional passivation methods, for example thermal oxidation of silicon, ALD has several benefits:

  • Possibility to low(er) processing temperatures (lower temperatures less detrimental to bulk Si) and material savings. Because in ALD the deposition time per area is a constant for given process there is no need to increase reactant flux to coat larger specimens
  • Always 100% uniform, dense, pinhole-, defect- and crack-free, conformal coating even on large surfaces as well as challenging nanoscale structural details
  • Controlled film growth by sequential atomic layers
  • Gentler to the surface than bombardment with high energy ions (e. g. sputtering and other PVD-methods)
  • Al2O3 is a superior passivation layer material due to the strong field-effect by negative charges in it, its low interface defect density, transparency, and high UV stability. Al2O3 deposition is also one of the most well-known, simple and cost-efficient ALD processes

For thin film cells, e.g. CIGS, both transparent conductive oxide (TCO, which works as a current collector) and buffer layers can be deposited with ALD. Replacing the typical TCO materials, for example tin-doped indium oxide (ITO), with ALD-grown, aluminum-doped zinc oxide (ZnO:Al) there is a possibility to not only lower the cell price but also to get rid of the dependency of indium, which already scarce reserves have been predicted to end in only a few years with today's level of consumption. ALD-deposited zinc-magnesium oxide ((Zn,Mg)O) or zinc oxysulfide (Zn(O,S)) instead of toxic cadmium in the buffer layer, on the other hand, makes the cell more environmentally friendly and safe. ALD-deposited buffer layer has also been shown in the literature to yield even higher cell efficiencies than the conventional, Cd-containing one.

TCO-replacements are needed also in the new, so-called 3rd generation PV devices such as dye, organic, and nanostructured solar cells; also, recombination blocking layers (TiO2) and moisture/oxygen barrier layers (Al2O3) can be deposited with ALD. 

Moisture and oxygen barrier, i.e. encapsulation layer, is naturally a necessary component also in c-Si and thin film cells. Nowadays, the encapsulants are typically polymer films, for example EVA (ethyl-vinyl acetate) that are laminated on top of the complete panels and which barrier properties could be vastly improved with a thin layer of compact, conformal and pinhole-free ALD Al2O3.