The impact of the linkage position on photovoltage performance of carbazole-based double-branched D–π–A sensitizers: Synthesis and DFT evaluation

In this work we rationally designed and synthesized three new carbazole-based sensitizers based on the traditional single-branched D–π–A system (CZ-1) and the double-branched D–π–A system (CZ-2 and CZ-3), featuring carbazole as the donor, difuropyridine (π-DFP) as the new heterocyclic π-bridge, and the carboxylic group as the anchoring unit. Frontier molecular orbitals (FMOs), energy levels, molecular electrostatic potential (ESP) maps, total density of states (TDOS), transition density maps (TDMs), and electron–hole overlap heat maps (EHMs) were theoretically investigated. UV–Vis spectroscopy, cyclic voltammetry (CV), and current-voltage (J-V) measurements were systematically launched to investigate the photophysical, electrochemical, and photovoltage properties, respectively. The results revealed a slight variation in the photophysical and electrochemical properties even with shifting the location of the linkage unit in CZ-2 and CZ-3. In contrast, photovoltage characterization showed that repositioning of the linkage from bridge-bridge (CZ-2) to donor-donor (CZ-3) leads to a pronounced increase in the short-current density (Jsc) from 11.18 to 17.67 mA/cm-2 and the open circuit voltage (Voc) from 0.654 to 0.716 V, respectively. Moreover, the power conversion efficiency (PCE) based on CZ-2 and CZ-3 was 4.78% and 8.40%, respectively. The lower PCE of CZ-2 is mostly attributed to the serious dye aggregation evidenced by the shorter electron lifetime (38.44 ms). Markedly, the PCE of CZ-2 significantly increased from 4.78% to 7.83% when 0.2 mM of CDCA was introduced, which is suppressing dye aggregation. These findings illustrate how the linkage position in the double-branched D–π–A system plays vital roles in developing high-performance and stable photosensitizers for DSSCs.

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