Development of solution processed, flexible, CuInSe₂ nanocrystal solar cells

Clean sources of energy, especially photovoltaics (PVs), are urgently needed to cope with global energy shortage and environmental pollution. For PVs to play a significant role in energy production, the current prices must be brought down. Thin film PVs made using layered Mo or Au/CuInGaSe₂(CIGS)/CdS/ZnO/ITO have already shown high efficiencies. Traditionally, most layers in CIGS solar cells are deposited using high-cost techniques requiring high temperatures and ultra-low pressures. By replacing the traditionally processed CIGS with a nanocrystal layer that can be deposited at mild processing conditions, the fabrication cost can be reduced. In this study, a high yielding synthesis method for CuInSe₂ nanocrystals has been developed which gives the best efficiency (3.1%), so far, for low-temperature processed CuInSe₂ nanocrystal PVs. An important challenge that nanocrystal solar cells currently face is low device efficiency, resulting in higher operating cost. CuInSe₂ nanocrystals can remain suspended in solution because of the long chain organic ligands attached to the surface. However, these ligands hinder charge transfer between nanocrystals causing low device efficiency. These ligands have been successfully replaced with smaller sulfide ions thereby improving the best efficiency of low-temperature processed CuInSe₂ nanocrystal solar cells from 3.1 % to 3.5%. Another approach to reducing the cost of CuInSe₂ PVs is by replacing the glass support medium with cheaper alternatives like paper. Flexible CuInSe₂ nanocrystal solar cells are successfully fabricated on paper with efficiencies reaching up to 2.25%. This is the first time a nanocrystal solar cell has been fabricated on paper. There is no significant loss in PV device performance after more than 100 flexes to 5 mm radius, and the devices continue to perform when folded into a crease. Apart from the absorber layer, the replacement of other high-temperature and vacuum processed device layers with ambient solution-processed layers lowers the manufacturing cost. This has been achieved by spin coating suitable nanomaterials as device layers. Lastly, for commercialization of CuInSe₂ nanocrystal solar cells, multiple devices need to be connected to achieve the desired current and voltage. A fabrication process has been developed for building multiple nanocrystal PVs on a single substrate using 3D printed masks