Indoline-Based Donor-π-Acceptor Visible-Light Responsive Organic Dyes for Dye-Sensitized Solar Cells: Co-sensitization with Squaraine Dye for Panchromatic IPCE Response

Abstract

A series of metal-free indoline-based donor-π-acceptor (D-π-A) visible-light active organic dyes, where an indoline unit wrapped with alkyl or alkyl and glycolic (TEG) groups is used as a donor, a mono- or bi-thiophene unit is used as a π-spacer, and cyanoacrylic acid acts as an acceptor unit, have been designed and synthesized for dye-sensitized solar cell (DSSC) application. These dyes showed three absorption bands in a dichloromethane solution: the first and second transitions are observed in the UV (337–385 nm) and visible (477–514 nm) regions and correspond to π–π* and intramolecular charge transfer (ICT) transition, respectively, whereas the third absorption band obtained in the near-IR (629–758 nm) region has been observed for the first time for a D-π-A type dye architecture. Furthermore, the ICT transition band for AKT dyes showed negative solvatochromism with increasing polarity of solvents. Photovoltaic characterizations for AKT dyes have been systematically studied with a cobalt electrolyte to evaluate the effect of thiophene incorporation and alkyl group substitution by the TEG group on the DSSC device performance. Here, AKT2 showed the highest DSSC efficiency of 3.16% with V OC of 0.680 V and J SC of 6.24 mA/cm2 without chenodeoxycholic acid (CDCA) in the AKT dye series, whereas DSSC efficiencies for AKT dyes were not affected much upon further addition of CDCA. Furthermore, AKT dyes were co-sensitized with a complementary optical responsive indoline-based SQS4 dye and showed enhanced efficiency. The combination of AKT2/SQS4 dyes with a ratio of 1:1 showed a maximum of 5.20% co-sensitized solar cell efficiency compared to other dye combinations with the cobalt electrolyte.

D‐A‐D‐based Unsymmetrical Thiosquaraine Dye for the Dye‐Sensitized Solar Cells†

“…SQ5 and SQ5S dyes have been synthesized by following the procedures reported in the literature, and the required chemicals for the synthesis were purchased from commercial resources. Bruker NMR spectrometers with AV 400 MHz frequency were used to record 1 H and 13 C NMR in CDCl 3 solvent. SQ5S dye mass was analyzed with Applied Bio system 4800 PLUS MALDI TOF/TOF mass spectrometer.…”

“…Additional supporting information may be found online in the Supporting Information section at the end of the article: Figure S1. 1 H and 13 C NMR (CDCl 3 ) spectra of unsymmetrical thiasquaraine dye, SQ5S.…”

“…Furthermore, modulation of various device components such as anode, cathode, electrolyte and light‐absorbing sensitizers make the dye‐sensitized solar cell (DSSC) (2–7) very attractive, besides other third‐generation photovoltaic technologies. Organic sensitizers offer different architectures to enhance the light‐harvesting properties of the dyes, such as D‐A (8–10), D‐π‐A (11–13), D‐A‐π‐A (14,15), D‐D‐π‐A (16,17), and D‐A‐D (18–23) types. Furthermore, dyes with a broad spectral response is vital besides the NIR‐light absorption and co‐sensitization for the enhanced photocurrent generation (24–29).…”

Visible‐Light‐Active Unsymmetrical Squaraine Dyes with 1 V of Open‐Circuit Voltage for Dye‐Sensitized Solar Cells

“…[47][48][49][50] Further, the photo-physical and electrochemical properties of the DÀ AÀ D based unsymmetrical squaraine dyes can be modulated by extending the conjugation between the donor and acceptor moieties. All the above characteristics of squaraine dyes make it as a potential sensitizer for the DSSC device fabrication with both I À /I 3 À electrolyte and Co(II/III) [26,51] electrolytes. Recently, by modulating the conjugation, structural optimization for indoline and aniline donors based visible active squaraine dye was reported for DSSC device with I À /I 3 À electrolyte.…”

“…Dyes wrapped with the auxiliary donor [22] and acceptor [23] moieties have been utilized for DSSC which gave good device efficiency by broadening the spectral responses associated with the visible and NIR regions [24,25] due to the intramolecular charge transfer between the donor and acceptor moieties. Further, structural modulations in the donor and acceptor opens the opportunity for tuning the HOMO and LUMO electronic levels, which facilitate to use the other electrolytes to maximize the photovoltaic parameters such as V OC and J SC [15,26] beside side chain engineering [27–30] . The HOMO energy level of the dyes based on the D−A, D−π−A, and D−A−π−A with more positive potential vs NHE have been used with the Cu(I/II) electrolytes which exhibited excellent V OC and DSSC device efficiencies [31–38] .…”