Although organic photovoltaic cells (OPVCs) fabricated from semiconducting polymer have been demonstrated with performance comparable to or in some cases even better than their inorganic counterparts, the typically short lifetime of the OPVCs must be overcome before the launch of the large scale organic photovoltaics will be realized.
Most conventional OPVCs are degraded when exposed to water vapor and/or oxygen in the air. The top electrode in conventional OPVCs is air sensitive and thereby become the central cause of the instability of OPVCs. To overcome this issue, a feasible strategy by means of reversing the polarity of the devices are therefore adopted, and thereby inverted OPVCs have been investigate recently. The inverted OPVCs have the anti-oxidant and high stability characteristics which outperform the conventional OPVCs. The anodes (i.e. top electrode) of the inverted organic OPVCs are made of metals with high work-function, and the cathodes of those are composed of metal oxide.
Although such inverted OPVCs are provided with better stability, the inefficient electronic coupling of the inorganic metal oxide layer, such as the zinc oxide layer or the titania layer, to the polymer active layer is detrimental to the electron extraction.
The electron collection mechanisms at metal oxide layer are relatively less studied and less understood. It is believed that the electron collection loss at the interfaces is also a major contributing factor to the low efficiency of current inverted OPVCs. Electrons can not be efficiently captured from the blending system by the metal oxide, causing a space-charge buildup and the photoelectric conversion efficiency of the inverted OPVCs generally is worse than the conventional OPVCs thereby.
The inefficient electronic coupling of the underlying metal oxide layer to the upper blend film can be overcome or at least altered to acceptable levels by several methods. Attempts to create a self-assembly monolayer of fullerene derivatives have been reported with promising results. However, the drawbacks for the self-assembly monolayer of the fullerene derivatives are incomplete coverage on the metal oxide layer and have the probable desorption during the follow-up process (Hau, S. K.; Yip, H.-L.; Acton, O.; Baek, N. S.; Ma, H.; Jen, A. K. Y. J. Mater. Chem. 2008, 18, 5113; and Hau, S. K.; Yip, H.-L.; Ma, H.; Jen, A. K. Y. Appl. Phys. Lett. 2008, 93, 233304/1).
In other literatures, the metal oxide nanotubes filled with fullerene derivatives are tried to be a selected layer to overcome the drawbacks of poor electron capture efficiency. However, based on the wet etching process, the mutual erosion between layers could not be prevented, such that for establishing the multilayer OPVCs with high efficiency, it is still unable to solve at the present stage (Huang, J.-S.; Chou, C.-Y.; Lin, C.-F. Sol. Energy Mater. Sol. Cells 2010, 94, 182).
Accordingly, it is an object of the present invention to provide an improved, high efficiency OPVCs.
Another object of the present invention is to provide an OPVC which reduces or eliminates losses found in prior art (White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Appl. Phys. Lett. 2006, 89, 143517/1).