Titanium dioxide-based electrode coatings for DSSCs at ANSTO

As part of the CRC-P research program, researchers at the ANSTO, led by Gerry Triani from the Institute of Material Engineering, are developing low-temperature colloidal titanium dioxide pastes to cast crack-free, continuous and adherent electrodes for flexible DSSCs. Titanium dioxide films for DSSC are usually prepared by coating with a viscous paste containing organic binders, followed by heat treatment to temperatures above 400°C to remove organic compounds and neck particles to form a porous layer. When the pastes are used on polymer, rather than glass, substrates to make flexible solar cells, temperatures that exceed 150°C cannot be tolerated. The preparation of titanium dioxide films coupled with low temperature post treatment (<150°C) typically results in poor particle connectivity and thus low energy conversion efficiency (<3%). Using aqueous titanium dioxide sols, binder-free formulations have been developed which produce uniform films with a typical thickness in the range 4 to 6 micron. After thermal treatment to 150°C, these electrodes have been recorded to yield energy conversion efficiencies of over 5%. Most importantly, these binder-free formulations can also be employed with high temperature processing steps on conductive glass substrates, yielding higher efficiencies (>6%) from a single as-cast film. This methodology offers significant advantages for formulating titanium dioxide pastes and creates a versatile printing material which can be adapted to the many of the DSSC architectures being considered for commercialisation.

Figures from left: Titanium dioxide particles, a depiction of a thin ALD coating applied to titanium dioxide particles, and the use of a titanium dioxide paste in cell construction.

In addition to developing n-type electrode materials, ANSTO has led CRC-P research which has resulted in an atomic layer deposition (ALD) process for improving the performance of DSSCs (US patent 8,440,908). The deposition of atomic layer films at the interfaces of DSSCs has become a key technology for bridging the gap between performance and process temperature limitations found when processing at temperatures below 150°C. Using chemistry similar to that employed in solution processing, titanium dioxide thin-film structures with controlled thickness and chemical uniformity are prepared by a step-wise vapour phase chemisorption process. Researchers have demonstrated improvements to short circuit current, and hence the efficiency of the low temperature processed solar cells, of over 20% with the conformal deposition of thin interfacial layers. This increase is attributed to improvements in electron lifetime which act to suppress electron recombination. The CRC-P is currently seeking opportunities to license for this recently granted US patent to solar module manufacturers.