Synthesis Of Organic Dyes With Novel Electron Acceptors And Study On Their Photovoltaic Properties

Since the discovery of dye-sensitized solar cells, it has become an effective way to achieve photovoltaic conversion due to its high efficiency, low cost, and facile fabrication. It is well known that photosensitizer is the key part of dye-sensitized solar cells, and it directly determines the photovoltaic properties of the cell. DSSCs based on metal complex photosensitizers have achieved remarkable conversion efficiency, however, the use of noble metal leads to high cost and environmental issue. Recent attentions have focused on metal-free organic dyes as sensitizers for DSSCs because of their high molar absorption coefficient, ease of structure modification, and low material cost.Generally, organic dyes for DSSCs are the linking of electron donor/electron acceptor (D-A) systems through π-conjugated bridge, which possess photoinduced intramolecular charge transfer properties. In recent years, the research has focused on the donor (D) and π-conjugated bridge, but the acceptor (A) has been rarely studied. A commonly D-π-A photosensitizer mostly take cyanoacetic acid as electron acceptor. In order to study whether cyanoacetic acid is the best electron acceptor, six kinds of D-π-A dyes with different electron acceptors have been designed and synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The effects of different electron acceptors on the photovoltaic properties of the dyes were studied. It was found that the dyes’absorption spectra in dichloromethane solution mainly ranged from300to500nn, and their molar extinction coefficients were high (more than10000M-1cm-1). The electrochemical data indicated that all the six dyes could meet the requirements of photosensitizers for DSSCs. Among the six dyes, the dye (TPC) with cyanoacetic acid as electron acceptor achieved the highest conversion efficiency to4.93%(short-circuit photocurrent density Jsc=8.59mA·cm-2, open-circuit photovoltage Voc=0.77V and fill factor ff=0.75). Compared with other simple electron acceptors, cyanoacetic acid is the best one.The dye (TPNC) containing nitro group as acceptor has very good UV-visible absorption performance according to density functional theory calculation. Unfortunately,2-nitroacrylic acid moiety was found to be unstable in air. Thus, two new stable dyes S and L which used nitrobenzene unit instead of nitro group were designed and synthesized. The photovoltaic properties of these two dyes were studied. It was found that both of these two dyes had strong absorption region from400to500nm in dichloromethane solution, and their molar extinction coefficients were high (more than10000M-1cm-1). The electrochemical data indicated that both of these two dyes’energy levels could meet the requirements of photosensitizers for DSSCs. The dye L with longer conjugated chain had higher efficiency than the dye S due to its more negative LUMO energy level which leads to greater driving force for the electron injection from the dye to the semiconductor TiO2conduction band. It demonstrated that the energy level of dyes, especially the LUMO energy level, was one of the essential factor which determined the cell’s photovoltaic properties.