Metal-Free
Tetrathienoacene Sensitizers for High-Performance
Dye-Sensitized Solar Cells
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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
× 10<sup>4</sup> M<sup>–1</sup> cm<sup>–1</sup>). 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 TiO<sub>2</sub> 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 TiO<sub>2</sub> (molecular footprint = 79 Å<sup>2</sup>), thus enabling optimal dye loading. Using dye <b>3</b>, a DSSC power conversion efficiency (PCE) of 10.1% with <i>V</i><sub>oc</sub> = 0.833 V, <i>J</i><sub>sc</sub> = 16.5 mA/cm<sup>2</sup>, and FF = 70.0% is achieved, among the
highest reported to date for metal-free organic DSSC sensitizers using
an I<sup>–</sup>/I<sub>3</sub><sup>–</sup> redox shuttle.
Photophysical measurements on dye-grafted TiO<sub>2</sub> films reveal
that the additional thiophene unit in dye <b>3</b> enhances
the electron injection efficiency, in agreement with the high quantum
efficiency