Novel Near‐Infrared Squaraine Sensitizers for Stable and Efficient Dye‐Sensitized Solar Cells

Abstract

A new molecular design strategy for tuning the energy levels of cis‐configured squaraine sensitizers for dye‐sensitized solar cells is described. The Hammett substituent constant and the π‐conjugation length are used as quantitative indicators to modify the central squarate moiety of the sensitizer dyes; specifically, novel near‐infrared squaraine dyes HSQ3 and HSQ4 are synthesized by incorporation of an electron‐withdrawing and π‐extending ethyl cyanoacetate unit on the central squarate moiety. The solution absorption maximum of HSQ4 occurs at 703 nm, and the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital are in the ideal range for energetically efficient electron injection and regeneration of the oxidized dye. A solar cell sensitized with HSQ4 exhibits a broad incident photo­n‐to‐current conversion efficiency spectrum, extending into the near‐infrared region with a maximum value of 80% at 720 nm, which is is the highest value reported for a squaraine dye–based dye‐sensitized solar cell. The HSQ4‐sensitized solar cell also exhibits excellent durability during light soaking, owing to the double anchors attaching the dye to the TiO2 surface and to the long alkyl chains extending outward from the surface.

Dicyanovinyl and Cyano-Ester Benzoindolenine Squaraine Dyes: The Effect of the Central Functionalization on Dye-Sensitized Solar Cell Performance

“…This suggests that S2 transition is more efficient at converting photons to electrons than the so-called S1 transition. More in detail, the central functionalization results in a 50 nm shift, comparing the non-functionalized squaraines (VG1 and VG10) with the dicyanovinyl substituted ones (VG2 and VG11), in agreement with the results reported by Maeda's and Han's groups for non-symmetrical squaraines [13,16]. On the contrary, replacing the dicyanovinyl group with a cyano-butyl-ester central unit (VG2 Ñ VG3) slightly influences the absorption wavelength, but doubles the incident photon-to-electron conversion efficiency (from 23% at 710 nm to 41% at 715 nm).…”

“…Moreover, it is worth mentioning that, as previously reported by Tatarets et al [21], the condensation reaction for the squaraine formation is performed in a toluene/butanol mixture using a cyano-butyl-ester. When performing the reaction using a cyano-ethyl-ester in butanol, a mixture of the cyano-ethyl and cyano-butyl compounds was obtained by us, in contrast to what Han's group has previously reported [16,17].…”

“…A greater bathochromic shift has been reported by Han's group on a new series of squaraines centrally functionalized with a cyano-ester moiety [16,17]. Accordingly to these strategies, better performances are obtained with symmetrical dianchored squaraines centrally substituted with an electron withdrawing group as reported by Maeda et al [13] (η = 3.6%) and Qin et al [16] (η = 5.66%).…”

Squaric Acid Core Substituted Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Effect of Electron Acceptors on Their Photovoltaic Performance

“…We have earlier reported the dependence of V oc on alkyl chain length, with a minimum chain length of four carbons giving a reasonable V oc and the optimum V oc achieved with an alkyl chain length of twelve carbons [63]. However, even when incorporating a shorter alkyl chain (butyl) on one of the indole donor moieties of the core-substituted unsymmetrical squaraine dyes, the V oc values obtained were comparable to or somewhat better than those reported in the literature for core-substituted squaraine dyes with a similar molecular framework [40,41,72,73]. In addition to this, the J sc of the DSSCs is also influenced by the charge recombination kinetics at the interface of TiO 2 /dye/electrolyte, as discussed earlier.…”

“…The low FF value as a result of higher FTO resistance of 23 Ω/cm 2 (when compared to a resistance of 15 Ω/cm 2 for the FTO substrate used by Yum et al) for the DSSCs fabricated in this work might be the reason for this observed difference [56]. Similarly, SQ-222, consisting of butyl cyanoacetate as an electron acceptor in the squaraine core, exhibited an efficiency of 4.12%, which was slightly lower than the PCE (4.6%) obtained by Qin et al using a dye with similar molecular framework with ethyl cyanoacetate as an electron acceptor [40]. This slightly lower value may be attributed to the inability of the shorter butyl chain to prevent recombination effectively [63].…”