The impact of adjusting auxiliary donors on the performance of dye-sensitized solar cells based on phenothiazine D-D-π-A sensitizers

Three new D-D-pi-A sensitizers ZHG5, ZHG6 and ZHG7 have been prepared by gradually improving the steric hindrance of auxiliary donors and the power conversion efficiencies (PCE) are 5.64%, 5.32% and 2.74%. UV-Vis absorption indicates that the molar extinction coefficients decrease with the increased steric hindrance of auxiliary donors. X-ray photoelectron spectroscopy (XPS) indicates that the tilt angles of ZHG5 and ZHG6 anchored on the TiO2 film are similar and ZHG7 is almost standing rather vertical on the TiO2 film with the smallest tilt angle. The results of dye desorption and XPS experiments indicate that the dye loading amount of ZHG6 with larger steric hindrance is lower than that of ZHG5, ZHG7 with largest auxiliary donor has the maximum loading amount probably due to its smallest tilt angle. Larger auxiliary donor of ZHG6 leads to higher open circuit voltage (V-OC = 734 mV) but lower shorter circuit current (J(SC) = 12.63 mA cm(-2)) compared with that of ZHG5 (V-OC = 730 mV, J(SC) = 12.06 mA cm(-2)). However, dense packing of dye ZHG7 anchored on the TiO2 leads to more serious intermolecular pi-pi aggregation effects. Perhaps this effect and lowest molar extinction coefficient are the reason that DSSC based on ZHG7 have the lowest PCE. So above results indicate that auxiliary donor with overlarge steric hindrance will have smaller tilt angle of dye anchored on TiO2 and may lead to more dye loading amount but serious intermolecular pi-pi aggregation effects. (C) 2017 Elsevier Ltd. All rights reserved.