Constructing Organic D–A–π‐A‐Featured Sensitizers with a Quinoxaline Unit for High‐Efficiency Solar Cells: The Effect of an Auxiliary Acceptor on the Absorption and the Energy Level Alignment

Abstract

Four organic D–A–π‐A‐featured sensitizers (TQ1, TQ2, IQ1, and IQ2) have been studied for high‐efficiency dye‐sensitized solar cells (DSSCs). We employed an indoline or a triphenylamine unit as the donor, cyanoacetic acid as the acceptor/anchor, and a thiophene moiety as the conjugation bridge. Additionally, an electron‐withdrawing quinoxaline unit was incorporated between the donor and the π‐conjugation unit. These sensitizers show an additional absorption band covering the broad visible range in solution. The contribution from the incorporated quinoxaline was investigated theoretically by using DFT and time‐dependent DFT. The incorporated low‐band‐gap quinoxaline unit as an auxiliary acceptor has several merits, such as decreasing the band gap, optimizing the energy levels, and realizing a facile structural modification on several positions in the quinoxaline unit. As demonstrated, the observed additional absorption band is favorable to the photon‐to‐electron conversion because it corresponds to the efficient electron transitions to the LUMO orbital. Electrochemical impedance spectroscopy (EIS) Bode plots reveal that the replacement of a methoxy group with an octyloxy group can increase the injection electron lifetime by a factor of 2.4. IQ2 and TQ2 can perform well without any co‐adsorbent, successfully suppress the charge recombination from TiO 2 conduction band to I 3− in the electrolyte, and enhance the electron lifetime, resulting in a decreased dark current and enhanced open circuit voltage (Voc) values. By using a liquid electrolyte, DSSCs based on dye IQ2 exhibited a broad incident photon‐to‐current conversion efficiency (IPCE) action spectrum and high efficiency (η=8.50 %) with a short circuit current density (Jsc) of 15.65 mA cm−2, a Voc value of 776 mV, a fill factor (FF) of 0.70 under AM 1.5 illumination (100 mW cm−2). Moreover, the overall efficiency remained at 97 % of the initial value after 1000 h of visible‐light soaking.