Roll-to-roll processing of compact and mesoporous TiO2 layers for third generation photovoltaic devices

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Poster, Cecile Charbonneau, 107
Publication date: 1st March 2014

Titanium dioxide is a ubiquitous material in the fabrication of third generation hybrid organic/inorganic photovoltaic (HOPV) devices. Throughout their development, it has played a number of important roles, namely; (i) as a mesoporous dye-hosting scaffold in dye-sensitized solar cells (DSCs), (ii) as a compact “blocking” layer to prevent shunting in solid-state devices (ss-DSC) [1] and (iii) in the more recent lead halide perovskite devices where a compact “electron acceptor” TiO₂ layer is used to optimize the collection and transport of electrons from the sensitizer to the substrate [2]. In the laboratory, compact TiO₂ layers are typically deposited using spin coating or spray pyrolysis, with their associated scaling and reproducibility challenges. For the commercialization of HOPV devices to hasten, the parallel and coordinated development of roll-to-roll compatible processes, adapted TiO₂ nanoparticulate coating media and stable Ti-based solution precursors is required.

This paper first presents our latest developments associated with low temperature solution processes for the deposition of TiO₂ layers. Compact thin films have been produced on FTO glass using an automated temperature controlled K-bar coating instrument and optimized for the fabrication of both dye-sensitized and lead halide perovskite devices. Clusters of TiO₂ nanoparticles were incorporated into the formulation of a Ti-based precursor to provide the resulting dry film with a controlled texture. The presence of these clusters (Figure 1A)  is shown to improve the wetting of the perovskite precursor on the TiO₂ compact layer and promote crystallization by acting as nucleation sites without impeding on device performance, for instance by the generation of resistive losses reported elsewhere [3].

Furthermore, early results are presented on the surface area enhancement and architecture control of TiO₂ scaffolds for ss-DSC devices. A chemical process is described whereby oxalic acid molecules are used to self-assemble small (~ 5 nm) and medium size (Aeroxide® P25, ~ 30 nm) TiO₂ nanoparticles. Unlike with conventional high temperature oven sintering, the application of ultra-fast near infrared (NIR) heat treatment to the TiO₂ carrier medium is shown to preserve the initial bimodal particle size distribution once sintered into a TiO₂ composite scaffold. The resulting film has many advantageous properties such as a large surface area (84.6 m²/g vs 48.9 m²/g for P25 alone) and a meso-macroporous structure due to the small and medium size particles, respectively (Figure 1B). Such development offers a new route for higher dye loading levels alongside with an improved infiltration of hole transport materials (HTMs). 

A) Electron microscopy image (top view) of a solution processed blocking layer provided with nucleation promoting TiO2 nanoparticle clusters (circled) on fluorine-doped tin oxide; B) Surface area and pore size distribution of TiO2 nano-structured films and electron microscope image of the composite P25/nanoparticles matrix.
[1] Howie, W. H.; Harris, J. E.; Jennings, J. R.; Peter, L. M. Solid-state dye-sensitized solar cells based on spiro-MeOTAD. Sol. Energy Mater. Sol. Cells, 91(5), 2007, 424-426. [2] Wang, J. T.-W. ; Ball, J. M.; Barea, E. M.; Abate, A.; Alexander-Webber, J. A.; Huang, J.; Saliba, M.; Mora-Sero, I.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Low-Temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano Letters, 2013 in press. [3] Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater., 24, 2014, 151-157.

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