We are challenging to develop high efficiency plastic solar cells by combination of originally developed materials and methods, i.e., nanostructured porous crystalline ZnO photoelectrodes prepared by electrodeposition and metal-free organic photosensitizer dyes. Modification of the ZnO nanostructure combined with indoline dyes specially developed for ZnO continue to improve the cell efficiency as well as the stability. The highest efficiency was pushed up to 5.84%. Full plastic submodule of 10×10 cm size also achieved a promising efficiency of 3.2% (Both under AM 1.5, 100 mW cm^-2).
Scaling down the geometry of the ZnO nanowire structure was found to improve the electron transport to improve the fill factor. Electrodeposition of compact ZnO bottom layer improved adhesion of porous layer thus achieving maximum IPCE reaching 90% and good fill factor. On the other hand, D149 dye was found to form multilayer under excessive soaking in dye solution, leading to decrease of the fill factor. At the same time, however, the same dye is desorbed in contact with the electrolyte solution causing problems with cell stability. Such problems were significantly reduced by chemical modification of the dyes. Various new dyes such as asymmetric squarilium dyes were also synthesized to harvest photons in the near-IR region.
Stability tests of the plastic solar cells with ZnO and organic dye have indicated encouraging results to confirm “usable” stability already at the present level, although not yet to guarantee over 10 years life for outdoor use.
This work was supported by NEDO.