Influence of the donor size in panchromatic D–π-A–π-A dyes bearing 5-phenyl-5H-dibenzo-[b,f]azepine units for dye-sensitized solar cells

• Tow D-π-A-π-A dyes bearing 5-phenyl-5H-dibenzo-[b,f]azepine units were synthesized.
• The effect of donor size on the property of sensitizer was systematically studied.
• The dyes show panchromatic absorption between 300 and 800 nm in solution and neat film.
• PCE of 4.38% was achieved for DSSCs based on I3−/I− electrolyte without co-sensitizer.

Two D–π-A–π-A organic dyes (YC-1 and YC-2) with 5-phenyl-5H-dibenzo[b,f]azepine derivatives as donor, thiophene as π bridge, and isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a panchromatic absorption between 300 nm and 800 nm both in solution and neat film. The photovoltaic performances of both dyes were evaluated in dye-sensitized solar cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based device displays better device performance with open-circuit photocurrent density of 12.12 mA cm−2, open-circuit voltage of 0.53 V, and fill factor of 68.9%, corresponding to overall conversion efficiency (η) of 4.38%. The inferior performance of device based on YC-2 (η = 1.46%) is ascribed to short electron lifetime as evidenced from electrochemical impedance spectroscopy measurement. This research provided a potential promising donor unit for organic dyes and revealed the influence of donor size in organic dyes for photovoltaic performances.