Molecular Control of the Band Edge Movement and the Recombination Process in Donor–Acceptor Hemicyanine-Sensitized Solar Cells

Abstract

The presence of downward shift in the band edge and the recombination reactions in the hemicyanine-sensitized solar cell reduces the open-circuit potential (V OC ) and the short-circuit current density (J SC ), which in turn decreases the dye cell performance. Choosing either an electrolyte possessing minimum overpotentials or a systematic dye design which can efficiently suppress the diffusion of charged species toward the TiO 2 can improve the overall power conversion efficiency (PCE). Here, a series of donor−acceptor (D-A) hemicyanine dyes were synthesized utilizing a planar heterotriangulene (HT) or triphenylamine (TPA) donor and alkyl-functionalized indolium carboxylic acid acceptor unit. By introducing strong HT donor instead of TPA, the photophysical, and electrochemical properties of D-A dyes are significantly modulated. The strong donor nature of HT and effective passivation of surface by hydrophobic alkyl chains close to the anchoring group for NC3 dye exhibits an average PCE of 4.34% with a V OC of 0.416 V, J SC of 20.04 mA cm −2 , and fill factor (ff) of 52.03% under simulated AM 1.5G illumination (100 mW cm −2 ) without 3α,7αdihydroxy-5β-cholic acid coadsorbent (CDCA). The intrinsic dipole of the hemicyanine dye and the presence of Li + ions in iodide/triiodide redox couple without tert-butylpyridine (TBP) additive induces a downward shift in conduction band edge (E CB ) of TiO 2 . By rational molecular design, the extend of shift in E CB is controlled and enhanced the V OC . Electrochemical impedance spectroscopy (EIS) studies revealed the high charge transfer resistance (R ct ) and long lifetime (τ) of injected electrons in HT-based dyes than that of TPA derivatives, which provide insight into the passivation of Li + and I − ions by current D-A dye design possessing alkyl functionalities to increase both the J SC and V OC .

Visible, Far-Red, and Near-Infrared Active Unsymmetrical Squaraine Dyes Based on Extended Conjugation within the Polymethine Framework for Dye-Sensitized Solar Cells

“…9 To date, a maximum photovoltaic efficiency of 13.6% 10 has been achieved by metal-free organic dye, 14.2% 11 and 15.2% 12 by cosensitizing with optically complementary dyes, and 37.07% 13 device efficiency under the ambient light condition. Further, polymethine dyes such as cyanine, 14 hemicyanine (ionic dyes), 15 squaraine, 16 and croconaine dyes 17 of its sharp absorption in visible and far-red regions with high extinction coefficient and high DSSC performance. 18,19 Suitably functionalized squaraine dyes have been explored in the area of sensor, 20 DSSC, 9,21,22 photodynamic therapy, 23 protein labeling, 24 and fluorescent probe.…”

“…50,51,55 Squaraine dyes showed high DSSC efficiency with I − /I 3 − redox electrolyte by achieving the high V OC and J SC of DSSC devices in the visible and far-red regions. 15,56,57 By considering the controlled aggregation of dyes on TiO 2 and extending the conjugation of polymethine dyes, a series of visible to NIR-light active dyes (AM4-AM7, Figure 1c) have been designed, synthesized, and tested for DSSC applications. To control the aggregation of dyes on the TiO 2 surface, one of the indoline moieties of this dye series is wrapped with symmetrical alkyl groups.…”

“…To date, a maximum photovoltaic efficiency of 13.6% has been achieved by metal-free organic dye, 14.2% and 15.2% by cosensitizing with optically complementary dyes, and 37.07% device efficiency under the ambient light condition. Further, polymethine dyes such as cyanine, hemicyanine (ionic dyes), squaraine, and croconaine dyes (zwitterionic) absorb visible, far-red, and NIR light with high extinction coefficients. Among the polymethine dyes, squaraine dyes are very impressive because of its sharp absorption in visible and far-red regions with high extinction coefficient and high DSSC performance. , Suitably functionalized squaraine dyes have been explored in the area of sensor, DSSC, ,, photodynamic therapy, protein labeling, and fluorescent probe. , Further, these chromophores form H- and J- type aggregates in solution as well as on thin film and such feature has been utilized in DSSC and OPV. − To date, maximum photovoltaic efficiency achieved by the unsymmetrical squaraine dye is 7.93% with I – /I 3 – redox shuttle and 8.05% with Cu­(I/II) electrolyte has been achieved in the absence of CDCA.…”

“…The photovoltaic parameters of DSSC devices such as J SC , V OC , and ff can be effectively controlled and modulated by addition of lithium salts (Li + ), − 4- t -butylpyridine (TBP), and ionic liquids. − Such additives are responsible to change the E CB position of the TiO 2 , which modulate DSSC parameters directly. Variation in the concentrations of LiI modulates the Coulombic interaction of Li ions with the metal oxide TiO 2 surface, which changes the flat band potential and affects the J SC . ,, Squaraine dyes showed high DSSC efficiency with I – /I 3 – redox electrolyte by achieving the high V OC and J SC of DSSC devices in the visible and far-red regions. ,, By considering the controlled aggregation of dyes on TiO 2 and extending the conjugation of polymethine dyes, a series of visible to NIR-light active dyes ( AM4 - AM7 , Figure c) have been designed, synthesized, and tested for DSSC applications. To control the aggregation of dyes on the TiO 2 surface, one of the indoline moieties of this dye series is wrapped with symmetrical alkyl groups.…”