| Abstract
| - Dye-sensitized nanoporous TiO2 solar cells (DSSCs) can be classified into two types, namely, Type-I andType-II. Type-I DSSCs are the DSSCs in which electrons are injected from the adsorbed dyes by photoexcitationof the dyes followed by electron injection from the excited dyes to TiO2 (pathway A). Type-II DSSCs are theDSSCs in which electrons are injected not only by pathway A but also by direct one-step electron injectionfrom the dyes to TiO2 by photoexcitation of the dye-to-TiO2 charge-transfer (DTCT) bands (pathway B). TheDSSCs employing catechol (Cat) or its derivatives as the sensitizers have been the typical examples of Type-II DSSCs. However, their solar energy-to-electricity conversion efficiencies (η) have never exceeded 0.7%,and the external quantum efficiencies (EQE) at the absorption maximums of the DTCT bands have neverexceeded 10%. We found that the attachment of electron-donating compounds such as (pyridin-4-yl)vinyland (quinolin-4-yl)vinyl, respectively, to Cat (designated as Cat−v-P and Cat−v-Q, respectively) leads to 2-and 2.7-fold increases, respectively, in η, driven by large increases in short circuit current (Jsc). The EQEincreased from 8.5 to 30% at 400 nm upon changing from Cat to Cat−v-P, at which only the DTCT bandabsorbs. In the case of the Cat−v-Q-sensitized DSSC, even the η obtained by exciting only the DTCT bandwas higher than 1%. Interestingly, the illumination of only the DTCT band resulted in the increase of fillfactor from 62.6% to 72.3%. This paper provides for the first time an insight into the strategy to increase theη values of Type-II DSSCs.
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