Titanium dioxide (TiO₂) is a wide bandgap semiconductor material that has been used in many areas, including photocatalysis, photovoltaics, sensors, electrochromic devices, pigments, cosmetics and protective coatings, related to its advantageous properties such as non-toxicity, chemical stability, low-cost and wide variety of controlled synthesis methods. The photocatalytic and photovoltaic activity of titania highly depends on its physical properties such as crystal structure, surface area, particle size and particle shape. TiO₂ has three crystallographic phases: anatase, brookite and rutile. For dye-sensitized solar cells (DSSCs), anatase is the most studied phase with the highest efficiency. Although rutile is the most stable bulk phase at higher temperature, as a nanomaterial it is less efficient for a variety of reasons. Brookite is a more promising alternative to anatase, however, there are not many reports on brookite DSSCs due to the challenges in synthesis of phase-pure brookite nanomaterials.  

In this work, two methods for the synthesis of brookite nanoparticles are reported. We use amorphous titania as starting material, which is converted to brookite using a hydrothermal treatment in either acidic or basic conditions. Laboratory scale DSSCs were fabricated with brookite and anatase, and the performance of the cells were compared using electrochemical characterization methods such as current-voltage curves, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). 

Solar cells with efficiencies of 4.0% and 0.016% have been obtained for cells of 0.5 cm^₂ active area under 1 sun illumination for brookite synthesized in acidic and basic media, respectively. The very low efficiency for the cells based on basic brookite is related to the surface chemistry of the TiO₂ and the interaction with the dye, which can be controlled using surface treatment by immersion in dilute HCl. The efficiency of the solar cells based on basic brookite after acid treatment increased up to 1.6%, a factor 100 higher than for the untreated basic brookite-based cells. The acid treatment did not result in any change in the TiO₂ phase or morphology. The influence of surface chemistry on dye adsorption and injection efficiency is discussed in detail.