Novel D–A−π–A Organic Dyes with Phenoxazine as a Donor Unit for Dye-Sensitized Solar Cells: The Effect of an Ethynyl Group on Performance

We designed and synthesized two novel D–A−π–A organic dyes, POZ-4 and POZ-6, with phenoxazine (POZ) as the donor unit for application in dye-sensitized solar cells (DSSC). Our studies reveal that the introduction of the ethynyl group can optimize the planarity of the molecule and reduce the dihedral angle from 32° (POZ-4) to 2° (POZ-6), which promotes the intramolecular charge transfer (ICT) process. Hence, POZ-6 has a higher qCT value and more effective hole charge separation efficiency, ensuring that electrons are more effectively conducted from the donor (D) to the acceptor (A) unit, thereby yielding a higher molar extinction coefficient (ε) (εPOZ-4 = 1.00 × 104 M–1 cm–1 and εPOZ-6 = 1.70 × 104 M–1 cm–1). Furthermore, the POZ-6 device achieved a significantly higher Jsc (12.95 mA cm–2) and Voc (680 mV) than that of POZ-4 (10.36 mA cm–2, 650 mV), which is attributed to the higher IPCE and τ. Correspondingly, the DSSC based on POZ-6 obtained the highest power conversion efficiency (PCE) of 6.08% (N719, 8.45%).