Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free org. dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br-/Br3- and I-/I3-. The effects of additive Li+ ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addn. of Li+ ions in electrolytes can broaden the absorption spectra of the dyes on TiO2 films and shift both the LUMO levels of the dyes and the conduction band of TiO2, thus leading to the increase of Jsc and the decrease of Voc. Upon using Br-/Br3- instead of I-/I3-, a large increase of Voc is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO2, as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochem. impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φr) depend on the p.d. (the driving forces) between the oxidized dyes and the Br-/Br3- redox couple. For the dyes for which the HOMO levels are more pos. than the redox potential of Br-/Br3- sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br-/Br3-, insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.