| Because of relatively high solar energy conversion efficiency and low cost, Dye-sensitized solar cells (DSCs) have attracted great attention. Most of the organic sensitizers are in the style of Donor-πSpacer-Acceptor (D-π-A). D-π-A organic dyes have some advantages, such as their excellent high extinction molar absorption coefficients, optimizing of molecular structures and easily designing, simple preparation, making them attractive as photosensitizers for DSCs. However, the organic dyes with D-π-A structures among others, mainly two problems remain challenging:new chromophores for sensitization in the near infrared region (NIR), and new anchoring groups non-coupled to the acceptor parts of the dye molecules.Several NIR dyes, such as phthalocyanine and perylene dyes, have been reported in literatures. However, a major problem with reported NIR dyes is difficulty of modifying the molecular structures for enhancing significantly the solar cell performance. D-π-A organic dyes with absorption in NIR region in DSCs will be an ideal choice. However, most D-π-A organic dyes have a common anchoring group cyanoacrylic acid. This limited the fine-tuning of HOMO-LUMO levels (extending the absorption spectra of dyes) for the difficulty in changing the structure of anchoring group and the acceptor part.In this paper, we designed 18 novel D-π-A organic dyes and 4 referenced dyes. In this series of novel dyes, the electron donor is different substituted tetrahydroquinoline unit, different lateral chain carboxylic acid acts as the anchoring group for attachment on TiO2 surface, kinds of strong electron withdrawing groups are adopted as the electron acceptors and differentπ-conjugation moieties are introduced to the molecules and serve as electron spacers. Comparing to the conventional D-π-A dyes (containing cyanoacrylic acid as acceptor and the anchoring group), the anchoring group in our new lateral dyes has been separated from the acceptor units. This change allows us to tune the HOMO-LUMO level (absorption spectra) in an easier way by modifying the structure of the acceptor units. Mass spectra (MS) and proton nuclear magnetic resonance ('H NMR) technology were used to character the structures of the dyes.The molecular structures of the dyes make important influence on dyes' photophysical and electrochemical properties. The introductions of kinds of strong electron withdrawing groups are adopted as the electron acceptors instead of using cyanoacrylic acid as acceptor and the anchoring group can enlarge push-pull strength, red-shift absorptions and enhance light-absorbing abilities of the dyes. The HOMO and LUMO levels of all the dyes match well with potential of I-/I3- and the conduction band of TiO2, respectively.The introductions of different strong electron withdrawing groups and different length of the lateral anchoring group can adjust electron transfer yields in TiO2, IPCE values and solar energy conversion efficiencies of DSCs. And the introductions of long carbon chain into donor part can increase the opencircut-voltages of DSCs to a large extent, which can be ascribed to inhibit dye molecular aggregation on the surface of TiO2 showing dark current. Differentπ-conjugation moieties (thienyl, thienylvinyl,3,4-ethylenedioxythiophene (EDOT), etc) serve as electron spacers, which can red shifted the absorption of dyes to 1000 nm. Through optimization of the dyes and fine-tuning planar configurations of molecular structures, under AM 1.5,100 mW/cm2, solar energy conversion efficiencies of 5.1% were achieved with DSCs sensitized by the dye HY113 with a short-circuit current (Jsc) of 13.35 mA cm-2, an open-circuit voltage (Voc) of 519 mV and a fill factor (ff) of 0.73. Up to now, this is the highest solar energy conversion efficiency of NIR dye-sensitized solar cells reported.
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