New carbazole based metal-free organic dyes with D-π-A-π-A architecture for DSSCs: Synthesis, theoretical and cell performance studies
Graphical abstract
Three new metal-free organic dyes with D-π-A-π-A architecture were successfully designed and synthesized. The DSSC device fabricated with N1 displayed maximum photon conversion of 3.55% with highest IPCE value of 48%.
Introduction
Over the past two decades, the quest for green and cost-effective energy generation has led to enhanced attention towards research on organic photovoltaics. In the field of organic photovoltaics, dye-sensitized solar cells (DSSCs) are widely investigated due to their promising performance, ecofriendly and economically viable nature (Grätzel, 2001; Hagfeldt et al., 2010, Wu et al., 2015). Typically, DSSCs consist of four major components, viz. mesoporous photo active anode (TiO2), sensitizer (dye), electrolyte and passive cathode (Platinum electrode). Among them, the dyes play a crucial role as photosensitizer in the process of DSSCs, which include sunlight harvesting and electron injection from excited dye (LUMO level) into the Conduction band (CB) of Titanium dioxide (TiO2) (Hardin et al., 2012). It is well-established that the devices fabricated with dyes based on Ru (II) complexes (Ruthenium-II complexes) showed better conversion efficiency over that of metal-free organic dyes. However, metal-free organic dyes possess several advantageous over the former class, such as easy availability, design versatility, high molar extinction coefficients, and cost effectiveness in their synthesis. As a result, a wide variety of organic dyes were designed and synthesized as potential sensitizers in DSSCs (Mishra et al., 2009, Ooyama and Harima, 2009, Ning et al., 2010). Also, the effect of their structure on device characteristics was investigated in depth. While designing new dyes, D-π-A configuration, wherein electron rich donor (D) is attached to the electron acceptor/anchoring group (A) through a π-spacer, is the most commonly studied strategy for structural modifications (Fischer et al., 2010, Wu and Zhu, 2013). In such sensitizers, during photoexcitation, an intramolecular charge transfer takes place from D to A, followed by effective injection of electrons from the excited dye into the CB of the mesoporous TiO2. Recently, it has been reported that incorporation of additional π-linker leading to design of D-π-A-π-A architecture favors better charge separation on photo excitation and thus enhancing the overall efficiency (Khanasa et al., 2014, Namuangruk et al., 2012).
Several organic dyes, based on fused aromatic heterocyclic systems such as carbazole (Khanasa et al., 2014, Justin Thomas et al., 2015), indole (Li et al., 2009), phenothiazine (Irgashev et al., 2015) phenoxazine (Bae et al., 2015, Huang et al., 2016) and coumarine (Seo et al., 2011, Han et al., 2015) with “push-pull” like structural architecture have been reported as good sensitizers with decent photon conversion efficiency when used in DSSCs. Amongst the aforesaid heterocycles, the carbazole moiety being a dibenzo-fused system possesses good thermal as well as photochemical stability with excellent optical properties and is not easily attacked by either acids or bases. The molecule has been widely accepted as an electron donor mainly due to its excellent hole transporting property with wide energy gap and facile functionalization at different positions (Venkateswararao et al., 2013, Wang et al., 2011). In view of this, carbazoles can be regarded as important scaffolds for the construction of π-functional materials; subsequently many D-π-A configured carbazole-based DSSCs have been reported in the literature. Further, thiophene nucleus has been established as a potential entity in the largely growing chemical world of heterocyclic compounds, as it possesses good tunable spectroscopic and electrochemical characteristics. Typically, thiophene and its derivatives have been successfully utilized as π conjugation bridges in the molecular design of organic dyes due to their high polarizability, stability and excellent charge transporting capability (Mishra et al., 2009). Their presence in between the donor and acceptor groups not only enhances π conjugation but also increases the overall stability of resulting molecules. Interestingly, introduction of cyanovinylene group in between two conjugated donors is a good approach to further increase π-conjugation and thereby shift the absorption maxima of resulting molecule to higher wavelength with enhanced molar extinction coefficient. Further, the added group plays an active role of auxiliary acceptor and it serves as an effective charge transporter between donor and acceptor molecules. In the design of new sensitizers, the most commonly used electron acceptor units include cyanoacetic acid and rhodanine-3-acetic acid mainly because of their strong electron withdrawing ability (Ambrosio et al., 2012). Also, barbituric acid is shown to be an active acceptor/anchoring unit due to effective binding of it on the surface of TiO2 through NH or OH functional group (Hosseinzadeh et al., 2015, Salimi Beni et al., 2015).
Against this background, as a further step in our contribution of developing of novel sensitizers, we report here on design and synthesis of three new organic dyes with the D-π-A-π-A type configuration (N1–3), wherein carbazole moiety functions as an electron donor, cyano vinylene and thiophene groups serve as π-spacers, while cyanoacetic acid, rhodanine-3-acetic acid and barbituric acid act as electron acceptor/anchoring units. In the photovoltaic device, on photo-oxidation of these dyes, the electrons are expected to be delocalized from the carbazole to electron accepting unit conveniently. Then, the accumulated charge on electron acceptor moiety would be injected into the conduction band of TiO2 through OH binding followed by regeneration of the dye by electrolyte system. Thus, it is anticipated that the synthesized dyes would show better photon conversion efficiency in DSSCs due to promising structural features.
The designed organic dyes have been synthesized from the simple carbazole unit with good yield following multi-step synthesis. The Structures of fabricated dyes N1–3 and NCSU-10 are depicted in Fig. 1. The target molecules and their intermediates were well-characterized using spectral techniques such as FTIR, NMR, MS and elemental analysis for percentage composition. The electronic properties and optical band gap were calculated from the UV–Vis (UV–Visible) absorption and Photoluminance (PL) emission studies. Further, electronic distribution of HOMO and LUMO energy levels were calculated from cyclic voltammetry. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed using Turbomole 7.1V software in ground state in order to understand their structural geometry and spectral behavior. Finally, dyes N1–3 were applied as sensitizers towards the fabrication of DSSC to investigate their photovoltaic performance.
Section snippets
Materials, methods and instruments
The starting materials such as carbazole, bromo-hexane, thiophene-2-acetonitrile, cyanoacetic acid, rhodanine-3-acetic acid and barbituric acid were procured from Sigma-Aldrich, Alfa Aesar and Spectrochem Companies. All the solvents were distilled and dried prior to use in the experiments. The reactions were performed under an inert atmosphere and completion of the reaction was confirmed by TLC (Thin layer chromatography) technique. The synthesized dyes and their intermediates were purified
Synthesis
The synthetic pathways of three new organic dyes N1-3 are depicted in Scheme 1. N-hexyl carbazole (2) was synthesized from simple 9H carbazole (1). Then intermediate 2 was formylated using the standard Vilsmeier-Hack reaction protocol to yield 9-hexyl-9H-carbazole-3-carbaldehyde (3). Further, the 9-hexyl-9H-carbazole-3-carbaldehyde (3) was condensed with thiophene-2-acetonitrile by Knoevenagel condensation to obtain intermediate 4. Furthermore, intermediate 4 was subjected to Vilsmeier-Hack
Conclusion
We have successfully synthesized and characterized three new metal-free D-π-A-π-A architectured organic chromophores, N1–3 wherein the electron rich carbazole system is linked to three different electron accepting/anchoring species, viz. cyano acetic acid, rhodanine-3-acetic acid and barbituric acid via cyano vinyl thiophene as π-spacer. The synthesized dyes were subjected to photophysical, electrochemical, theoretical and device fabrication studies. The results of photophysical
Acknowledgements
The authors are thankful to the National Institute of Technology Karnataka, Surathkal, India, for providing necessary laboratory facilities. The authors are also thankful to the Department of Textile Engineering, Chemistry and Science at North Carolina State University for the financial support.
References (44)
- et al.
Molecular engineering and theoretical investigation of novel metal-free organic chromophores for dye-sensitized solar cells
Electrochim. Acta
(2015) - et al.
New D-π-A type indole based chromogens for DSSC: design, synthesis and performance studies
Dyes Pigm.
(2015) - et al.
Synthesis and photovoltaic performance of a novel asymmetric dual-channel co-sensitizer for dye-sensitized solar cell beyond 10% efficiency
Dyes Pigm.
(2017) - et al.
Near-IR organic sensitizers containing squaraine and phenothiazine units for dye-sensitized solar cells
Dyes Pigm.
(2015) - et al.
Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy
Sol. Energy Mater. Sol. Cells
(2005) - et al.
Novel organic dyes with anchoring group of barbituric/thiobarbituric acid and their application in dye-sensitized solar cells
Synth. Met.
(2015) - et al.
A facile and convenient synthesis and photovoltaic characterization of novel thieno[2,3-b]indole dyes for dye-sensitized solar cells
Synth. Met.
(2015) - et al.
Organic dyes containing fluoreneamine donor and carbazole π-linker for dye-sensitized solar cells
Dyes Pigm.
(2015) - et al.
The design, synthesis, and characterization of D-π-A-π-A type organic dyes as sensitizers for dye-sensitized solar cells (DSSCs)
Tetrahedron Lett.
(2014) - et al.
Molecular engineering of DA-π-A dyes with 2-(1, 1-dicyanomethylene) rhodanine as an electron-accepting and anchoring group for dye-sensitized solar cells
Electrochim. Acta
(2015)
Synthesis and characterization of organic dyes bearing new electron-withdrawing group for dye-sensitized solar cells
Electrochim. Acta
Coumarin dyes containing low-band-gap chromophores for dye-sensitised solar cells
Dyes Pigm.
Synthesis and characterization of organic dyes containing 2,7-disubstituted carbazole π-linker
Tetrahedron Lett.
A co-sensitized approach to efficiently fill the absorption valley, avoid dye aggregation and reduce the charge recombination
Electrochim. Acta
Effect of the anchoring group on electron injection: theoretical study of phosphonated dyes for dye-sensitized solar cells
J. Phys. Chem. C
New indole based co-sensitizers for dye sensitized solar cells exceeding 10% efficiency
RSC Adv.
A new mixing of Hartree-Fock and local density-functional theories
J. Chem. Phys.
Chemical capacitance of nanostructured semiconductors: its origin and significance for nanocomposite solar cells
Phys. Chem. Chem. Phys.
A novel carbazole-based dye outperformed the benchmark dye N719 for high efficiency dye-sensitized solar cells (DSSCs)
J. Mater. Chem.
D-π-A sensitizers for dye-sensitized solar cells: linear vs branched oligothiophenes
Chem. Mater.
Photoelectrochemical cells
Nature
Dye-sensitized solar cells
Chem. Rev.
Cited by (87)
Amplifying performance through co-sensitization of Acrylamide/2-Pyridone dyes in DSSCs
2024, Journal of Photochemistry and Photobiology A: ChemistryEnhancing optoelectronic properties of phosphonic acid based dyes via donor unit variation: A DFT/TD-DFT investigation
2024, Materials Today CommunicationsImproving the efficiency of dye-sensitized solar cells based on BODIPY dye and its analogous: The synergistic effect of benzo fusion and phenyl substitution
2023, Journal of Photochemistry and Photobiology A: Chemistry