Metal-Free Tetrathienoacene Sensitizers for High-Performance Dye-Sensitized Solar Cells

Abstract

A new series of metal-free organic chromophores (TPA-TTAR-A (<b>1</b>), TPA-T-TTAR-A (<b>2</b>), TPA-TTAR-T-A (<b>3</b>), and TPA-T-TTAR-T-A (<b>4</b>)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D−π–A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5–5.2 × 10⁴ M^–1 cm^–1). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO₂ surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye <b>3</b>) promotes more vertical dye orientation and denser packing on TiO₂ (molecular footprint = 79 Ų), thus enabling optimal dye loading. Using dye <b>3</b>, a DSSC power conversion efficiency (PCE) of 10.1% with <i>V</i>_oc = 0.833 V, <i>J</i>_sc = 16.5 mA/cm², and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I^–/I₃^– redox shuttle. Photophysical measurements on dye-grafted TiO₂ films reveal that the additional thiophene unit in dye <b>3</b> enhances the electron injection efficiency, in agreement with the high quantum efficiency