In standard dye-sensitized solar cells (DSSCs), platinum is typically used as a cathode material. Pt, however, has a few disadvantages: it is rare, expensive, and not transparent. Furthermore, Pt cathodes are passive cathodes, i.e., the electrode does not actively contribute to the light-to-energy conversion. A photoactive cathode could additionally contribute to the efficiency of the solar cell.[1] NiO is one of the most promising materials for application in p-type DSSCs. It is photostable, cheap, and shows higher photocurrents than many materials tested for replacing NiO as a photocathode.[2] Recently, there has been a renascent interest toward photoactive cathodes for DSSCs based on NiO, as relatively high IPCEs could be achieved, and the open-circuit voltage could successfully be increased. Li et al. reported a photocathode for DSSCs based on p-type nanostructured NiO and a pushand-pull-designed p-type organic dye P1, a triarylamine-based sensitizer, where an incident occurred photoconversion efficiency (IPCE) of 64% (in the visible range) was achieved.[3] The optimum thickness of NiO photocathodes is reported to be in the range of 2−3 μm due to the effective diffusion length of the hole in NiO.[2] The oxide morphology is vitally essential for good efficiency due to several reasons. On the one hand, the surface area needs to be maximized to allow enhanced dye adsorption and consequently increased light absorption. Also, small dimensions limit the photogenerated electron−hole pairs' recombination, as the charge carriers only have to diffuse short distances to reach the surface and react with the electrolyte.[4] The charge separation is crucial for non-sensitized photoelectrode materials; in dye-sensitized devices, the charge injection into the semiconductor material needs to be optimized. One possibility to simultaneously increase the surface area and minimize the diffusion paths for the charge carriers is to generate nanostructured photoelectrodes—nanoparticle-based NiO photocathodes [2].

Here, we report on the growth of crack-free anodic NiO with a nanoporous geometry and tunable thickness up to several microns. The novel nanosponge structure shows superior properties for application in p-type DSSCs compared to commonly used nanoparticle architectures and to previously reported NiO nanostructures prepared by anodization and is a promising material for improving the overall efficiency of p-type and tandem DSSCs. Dye-sensitized photocathodes have the potential to significantly contribute to the efficiency of the solar light-to-current conversion in tandem dye-sensitized solar cells (DSSCs). A novel, highly porous nanoarchitecture of NiO is developed in this project. The sponge-like material is grown by anodization, shows a virtually crack-free morphology, firmly adheres to the substrate, and can be grown with a controllable thickness of at least up to 6.5 μm. The oxide composition is NiO, and the nanosponge exhibits p-type semiconductive behavior. A doubling of the maximum reported p-type DSSC efficiency compared to coumarin C343-sensitized NiO nanoparticle photocathodes can be achieved with the NiO nanosponge. The developed dye-sensitized p-type NiO-based cathodes are promising for application as photocathodes in tandem DSSC devices.

[1] U. Sultan, F. Ahmadloo, G. Cha, B. Gökcan, S. Hejazi, J.-E. Yoo, N.T. Nguyen, M. Altomare, P. Schmuki, M.S. Killian, ACS Appl. Energy Mater. 3 (2020) 7865–7872.

[2] J. He, H. Lindström, A. Hagfeldt, S.-E. Lindquist, Sol. Energy Mater. Sol. Cells 62 (2000) 265–273.

[3] F. Odobel, Y. Pellegrin, J. Phys. Chem. Lett. 4 (2013) 2551–2564.

[4] I. Paramasivam, H. Jha, N. Liu, P. Schmuki, Small 8 (2012) 3073–3103.