Dye-sensitized solar cells (DSSCs) are under intensive investigation because of the cost-effectiveness and flexibility of devices. In 1999, such cells employing ruthenium complexes as sensitizers have achieved power conversion efficiency (η) over 11% under standard global air mass of 1.5.
Meanwhile, metal-free organic dyes, usually with an electron donor?π bridge?electron acceptor (D? π ?A) configuration, have been actively pursued due to low material costs, easy synthesis, high molar extinction coefficients, and environmental friendly materials with respect to classical ruthenium dyes.
In the past decade, many kinds of organic dyes with such configuration have been explored for DSSCs. It has been well known that one can improve device performance of DSSCs by facilitating electron injection and suppressing charge recombination. This can be done by controlling the assembly morphology of sensitizers on the TiO2 surface. Introduction of alkyl chains to the dye skeleton based on thiophene or its derivative has proved to be an effective method to improve the DSSC performance since they can not only prevent dye aggregation but also diminish the charge recombination between electrons and electron acceptors in the electrolyte.
Herein, we report the design and synthesis of two new organic dyes with n-hexyl (H28 and H29) substituted at the spacer part, and their use as sensitizers in DSSCs. Compared to H28 (with alkyl chains at the acceptor position of dye), the JSC of H29 (with alkyl chains at the donor position of dye) was enhanced significantly from 9.05 to 14.3 mA cm?2, and VOC also increased from 570 to 670 mV due to the position of the incorporated alkyl chains. Details on the difference in the device performance between H28 and H29 will be discussed from the structural viewpoint.