Key aspects of individual layers in solid-state dye-sensitized solar cells and novel concepts to improve their performance

Abstract

In this feature article, we discuss the key aspects of solid-state dye-sensitized solar cells (SDSC) and propose different concepts based on extensive studies carried out in our group to improve their performance. The influence of compact TiO 2 layer, novel donor-antenna sensitizing dyes, nature of nanocrystalline-TiO 2 layers and solid-state organic hole conductors on the performance of SDSC is discussed in this article. Both preparation and thickness of the compact TiO 2 layer were optimized using spray pyrolysis. The studies revealed that an optimum film thickness of 120–150 nm of compact TiO 2 yielded the best rectifying behavior and SDSC performance. The influence of three different mesoporous titania films, obtained from three different titania nanocrystals, prepared by sol–gel, thermal, and colloidal-microwave process, was also studied and discussed here. The TiO 2 layer with the optimum pore volume and pore diameter (∼44 nm) displayed the highest efficiency and IPCE in SDSC. The importance of pore size rather than high surface area for filling the mesoporous layer with solid-state hole conductor became evident from this study. A series of heteroleptic Ru(II) complexes carrying donor antenna moieties, namely, triphenylamine (TPA) or N, N′-bis(phenyl)- N, N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), were synthesized and applied in SDSC. These novel donor-antenna dyes revealed spectacular performances of power conversion efficiencies in the range 1.5–3.4%, as measured under AM 1.5 spectral conditions. This was attributed to highly efficient light harvesting of these novel dyes and the improved charge-transfer dynamics at TiO 2-dye and dye-hole conductor interfaces. Different low molecular weight and polymeric triphenyldiamines were synthesized and utilized as hole-transporting layers (HTL) in SDSC. Different studies showed that low molecular TPDs displayed better efficiency than polymeric counterparts due to their improved filling into the pores of nc-TiO 2 layer. Another interesting study revealed that an optimum driving force in terms of HOMO-level difference between the dye and HTL decides charge carrier generation efficiency. Recently, novel hole conductors with spiro-bifluorene-triphenylamine core for transporting holes and tetraethylene glycol side chains for binding lithium ions were synthesized and applied in SDSC. This work clearly emphasizes that Li +-salt is required at the TiO 2/dye interface as well as in the bulk of HTL. It was also found that the addition of about 5–20% of these Li +-binding hole conductors and higher Li-salt ( N-lithiotrifluoromethane sulfonamide) concentrations improved the SDSC performance. An improvement of about 120% in the solar cell efficiency as compared to the reference cells was achieved with an optimum composition of Li +-binding hole conductor and Li-salt.