[Show abstract][Hide abstract] ABSTRACT: In an attempt to develop small organic molecules with potential applications as donors in organic photovoltaic (OPV) devices, we have synthesized and characterized four novel benzothiadiazole (A) core structured D-π-A-π-D dyes featuring carbazole and benzocarbazole as donors (D) and fluorene and thiophene as spacers (π). The effects of the π-spacer units and variations in donor strength on the photophysical, electrochemical and thermal properties of the molecules have been investigated in detail. The replacement of fluorene by thiophene as a π-spacer promotes planarity, resulting in a larger bathochromic absorption shift, enhanced emission profiles and an enhanced intramolecular charge transfer (ICT) transition. The introduction of the benzocarbazole unit creates a low-lying HOMO level, as inferred from cyclic voltammetry studies. All the dyes exhibit remarkable thermal robustness. Theoretical calculations have been carried out to understand the structure–property relationships of the synthesized materials. The results obtained from the characterization methods reveal that the dyes with thiophene π-spacers show better optoelectronic properties compared to their fluorene counterparts. Solution-processable bulk-heterojunction devices with a structure of ITO/PEDOT:PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm) were fabricated using the materials investigated in this study as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC61BM) as an acceptor. A power conversion efficiency of 1.62% for the molecule with thiophene as a spacer and carbazole as donor/PC61BM was achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm−2).
[Show abstract][Hide abstract] ABSTRACT: We report the improvement of the light-harvesting property in the bulk heterojunction organic polymer solar cell based on poly(3-hexylthiophene) (P3HT) and PC70BM, with the incorporation of a nearinfrared absorbing squaraine (SQ) dye, bis[4-(2,6-di-tert-butyl)vinylpyrylium] squaraine (TBU-SQ). With the incorporation of TBU-SQ dye (2.5% by wt) in a P3HT:PC70BM (1:1 wt ratio) blend, the power conversion efficiency (PCE) has been enhanced up to 4.55% as compared to 3.47% for the device based on the P3HT:PC70BM binary blend. The
improvement in the photovoltaic performance with the incorporation of TBU-SQ attributed to the improvement in the light-harvesting efficiency in the near-infrared region of solar spectrum and increased exciton dissociation into free charge carriers in the ternary blended film. The PCE has been further enhanced to 5.15% when the thermally annealed P3HT:TBUSQ:PC70BM blend was used as the photoactive layer. It was observed that the absorption profile of the active layer was broadened upon thermal treatment as a result of the red shift as well as widening of the P3HT absorption band and the slight red shift of the TBU-SQ absorption peak in the blended film. The improved light-harvesting property of the thermally annealed film
and balanced charge transport in the device were attributed to the improvement in the PCE. These results show that TBU-SQ is a promising molecular sensitizer for increasing the PCE of P3HT:PC70BM-based polymer solar cells.
[Show abstract][Hide abstract] ABSTRACT: We have designed and synthesized four new metal free D–A–π–A type dyes (9–12) with variations in their acceptor/anchor groups. The four dyes carry tert-butyl substituted triphenylamine as donor, thiadiazole as acceptor and bithiophene as π-spacer. Cyanoacetic acid, rhodanine-3-acetic acid, 2-(4-methoxyphenyl)acetic acid and 2-phenylacetic acid are used as acceptor/anchor groups, respectively in the dyes 9–12. The acceptor/anchor effect on their photophysical, electrochemical and photovoltaic properties was investigated. The dyes exhibited good power conversion efficiency ranging from 1.95–4.12%. Among the four dyes, 9 showed the best photovoltaic performance: short-circuit current density (Jsc) of 8.50 mA cm−2, open-circuit voltage (Voc) of 645 mV and fill factor (FF) of 0.75, corresponding to an overall conversion efficiency of 4.12% under standard global AM 1.5 solar light conditions.
[Show abstract][Hide abstract] ABSTRACT: A series of organic dyes, CSORG6–CSORG9 were synthesized and successfully applied for dye-sensitized solar cells. All the dyes exhibited high open circuit voltage (VOC) compared to N719. Among all phenoxazine (CSORG7 and CSORG9) based sensitizers showed high PCE (η) of 6% under standard AM 1.5G solar irradiation. Under similar fabrication and evolution conditions N719 reached PCE (η) of 6.4%.
[Show abstract][Hide abstract] ABSTRACT: Molecular first hyperpolarizabilities (nonlinear optical responses) of selected s-triazine based heteroaromatic molecules are determined using experimental methods. A large enhancement in nonlinear optical response, in spite of a relatively weak donor–acceptor system, is observed. We have carried out a detailed analysis using computational chemistry techniques to account for this behavior.
Journal of Molecular Structure. 01/2014; 1075:118–123.
[Show abstract][Hide abstract] ABSTRACT: Six novel anthracene-oxadiazole derivatives, (2-(4-(anthracen-9-yl)phenyl)-5-p-tolyl-1,3,4-oxadiazole), (2-(4-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole), (2-(4-(anthracen-9-yl)phenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole), (2-(4-(anthracen-9-yl)phenyl)-5-m-tolyl-1,3,4-oxadiazole), (2-(3-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) and (2-(3-(anthracen-9-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole) have been synthesized and characterized for use as emitters in organic light emitting devices (OLEDs). They show good thermal stability (Td, 297-364 °C) and glass transition temperatures (Tg) in the range of 82-98 °C, as seen from the thermo gravimetric analysis and differential scanning calorimetric studies. The solvatochromism phenomenon and electrochemical properties have been studied in detail using UV-Vis absorption, fluorescence spectroscopy and cyclic voltammetry. TD-DFT calculations have been carried out to understand the electrochemical and photophysical properties. The spatial structures of and are further confirmed by X-ray diffraction analysis. Un-optimized non-doped electroluminescent devices were fabricated using these anthracene derivatives as emitters with the following device configuration: ITO (120 nm)/α-NPD (30 nm)/ or (35 nm)/BCP (6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al (150 nm). Among all the six compounds, displays the maximum brightness of 1728 cd m(-2) and current efficiency 0.89 cd A(-1). Furthermore, as an electron transporter, exhibited superior performance (current efficiency is 11.7 cd A(-1)) than the device using standard Alq3 (current efficiency is 8.69 cd A(-1)), demonstrating its high potential for employment in OLEDs. These results indicate that the new anthracene-oxadiazole derivatives could play an important role in the development of OLEDs.
Photochemical and Photobiological Sciences 12/2013; · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A series of five new ruthenium [3 + 2 + 1] complexes coded as MC107–MC111, with novel unsymmetrical bipyridines as ancillary ligands and terpyridine tricarboxylic acid as an anchoring ligand have been successfully synthesized and characterized by 1 H NMR, 13 C NMR and UV-Visible spectrometry. Improvement in the molar extinction coefficient of all these sensitizers was observed compared with reference standard N749 dye under comparable conditions. Among all the new sensitizers MC108 exhibited a maximum solar to electrical conversion efficiency of 5.573% (J sc ¼ 16.81 mA cm À2 , V oc ¼ 0.50 V. FF ¼ 0.65) under standard global AM 1.5 G solar condition, when compared to the N749 dye with an efficiency of 6.29% (J sc ¼ 13.74 mA cm À2 , V oc ¼ 0.67 V. FF ¼ 0.68) under similar fabrication and evaluation conditions. Density functional theory (DFT) and time-dependent DFT calculations are carried out for MC107–MC111 to understand their structural, electronic and photophysical properties.
[Show abstract][Hide abstract] ABSTRACT: New low bandgap small molecules based on a squaraine (SQ) chromophore, bis[4-(2,6-di-tert-butyl)vinylpyrylium]squaraine (), bis[2,6-di-tert-butyl-4-(prop-1-en-2-yl)pyrylium]squaraine () and bis[4-(but-1-en-2-yl)-2,6-di-tert-butylpyrylium]squaraine (), were synthesized and used as electron donors along with PC70BM for their application in solution processed organic bulk-heterojunction (OBHJ) solar cell (SC). The long wavelength of these SQ dyes are located in between 650-750 nm in thin films and the optical bandgaps are about 1.64, 1.52 and 1.48 eV, respectively. The electrochemical properties of these SQ dyes indicate that they are well suited for the fabrication of OBHJSCs as electron donors along with fullerene derivatives as electron acceptors. The OBHJ photovoltaic (PV) devices fabricated with the blend of :PC70BM, :PC70BM and :PC70BM cast from chloroform (CF) solvent exhibited a power conversion efficiency (PCE) of 1.71%, 2.15%, and 1.89%, respectively. The PCE of the OBHJSCs based on :PC70BM blends cast from DIO-THF (DIO = 1,8-diiodooctane) additive solvent and cast from DIO-THF with subsequent thermal annealing have been further improved up to 2.73% and 3.14%, respectively. This enhancement in the PCE is attributed to the improvement in the crystalline nature of the blend and more balanced charge transport resulting from the higher hole mobility. All these results have been supported by the quantum chemical calculations.
Photochemical and Photobiological Sciences 06/2013; · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The molecular structures of the ground state (S 0) and the first singlet excited state (S 1) of tris(5-(aryl donor/acceptor)-8-quinolinolate) Al(III) complexes, in which the aryl electron donating/electron withdrawing groups are substituted in the C5-position of the quinolinolate ligand have been optimized at B3LYP/6-31G(D) and CIS/6-31G(D) method respectively. The frontier molecular orbitals characteristics of these complexes have been analyzed systematically to understand the electronic transitions. From these results it is observed that the highest occupied molecular orbital (HOMO) is localized on the phenoxide side of A-ligand while lowest unoccupied molecular orbital (LUMO) is on the pyridyl side of B-ligand for S 0 states irrespective of the aryl electron donating/electron withdrawing groups at C5-position of the quinolinolate ligand. The absorption and emission wavelengths have been evaluated at the TD-PBE0 level using the B3LYP/6-31G(D) and CIS/6-31G(D) optimized geometries respectively and are comparable with the experiment. The reorganization energies have been calculated at B3LYP/6-31G(D) level and the results shows that the charge mobilities of these complexes are higher than the mer-Alq3.
[Show abstract][Hide abstract] ABSTRACT: Two novel D–A–π–A metal free dyes with triphenylamine as donor, dithiophene-diketo-pyrrolo-pyrrole as acceptor unit, thiophene and phenyl π-conjugated bridges and a cyanoacetic acid as electron acceptor (TDPP1 and TDPP2 were denoted for thiophene and phenyl π-conjugated bridge, respectively) have been designed and used as sensitizers for DSSCs. Incorporation of dithiophene-diketo-pyrrolo-pyrrole, reduces the band gap significantly. The influence of π-conjugated bridge on optical and electrochemical properties were investigated. Results demonstrated that the absorption band of TDPP with thiophene π-conjugated bridge has red shifted due to the enhancement of electron donating ability of π-conjugated bridge. The DSSC based on TDPP1 shows prominent power conversion efficiency about 4.81%, which is higher that for TDPP2 (3.42%). The electrochemical impedance spectroscopy analysis reveal that the charge recombination resistance at the TiO2/dye/electrolyte interface for the DSSC based on TDPP1 is higher than that for TDPP2, which improves both Jsc and Voc. The PCE of the DSSC based on TDPP1 is further improved up to 6.34%, when deoxycholic acid (DCA) was employed as coadsorbant.
[Show abstract][Hide abstract] ABSTRACT: In this study, using DFT methods, we analyze the electronic structures at both the molecular and solid-state level of the recently suggested three-dimensional semiconducting bis(hexylthieno)benzobisthiazole molecule (2), the unsubstituted benzobisthiazole ring (1), its 2,6-disubstituted derivatives (3 and 4), and its 2,6-dihexyl-4,8-dithiophene derivative (7). The uniqueness of 2, as per literature reports, is the close intermolecular contacts in the three directions (3D ordering) due to the crystal packing which should lead to more efficient charge transport. Gas-phase geometry optimization of the above molecules using the B3LYP functional is carried out and compared with the X-ray data. Reorganization energies to estimate the charge transfer, calculated using the same functional, do not indicate any particular bias toward hole transport or electron transport process. Transfer integrals (for estimation of intermolecular charge hopping pathways) between the HOMOs (for hole transport) are quite large in the π–π stacked direction in the 1–3 crystals, while in n-type 4 the transfer integrals between the LUMOs are large. In the case of 7, which has not been tested so far for its semiconducting properties, the calculated transfer integral is very small and reorganization energies are large, indicating that it may show poorer performance when compared to the other derivatives. Analysis of the intermolecular interactions (noncovalent) in the crystals has been carried out using dispersion-corrected functionals, namely B2PLYP-D, M06-2X, and B97-D. For the 2,6 derivatives, a maximum of 24 kcal/mol interaction energy (B2PLYP-D) is obtained in the π–π stacking direction but for the S–N contacts the binding energies are only around 5–6 kcal/mol. In 7, the binding energies obtained are much smaller and in the range of 2–9 kcal/mol only. Band structure calculations using the PBE functional of crystal 2 are also carried out. We conclude from this study that this fused heterocyclic ring at the molecular level by itself is not in the same class like semiconducting pentacene or rubrene but substituted benzobisthiazole rings pack in the crystal in some cases like 2, 3, and 4 to yield very large transfer integrals which can play a crucial role in charge transport. The present work will be helpful to understand and rationally design molecules which self-assemble in the solid state giving intermolecular close contacts in the crystal.
The Journal of Physical Chemistry C. 10/2012; 116(43):22663–22674.
[Show abstract][Hide abstract] ABSTRACT: Three porphyrin dyes, P1, P2 and P3, bearing one, two and four pyridyl groups, respectively, in the meso positions, acting as electron acceptor anchoring groups, were synthesized, characterized and investigated as sensitizers for the fabrication of dye sensitized solar cells (DSSCs). The overall power conversion efficiencies (PCEs) of DSSCs based on these dyes lay in the range 2.46–3.9% using a 12 μm thick TiO2 photoanode. Porphyrin P2 achieved the maximum performance, which can be rationalized by the high dye loading, efficient electron injection, dye regeneration process and longer electron lifetime, as demonstrated by the electrochemical impedance spectroscopy (EIS) measurements. The PCE of the DSSC based on the P2 sensitizer when the photoanode was treated with formic acid, showed an enhanced efficiency of 5.23%. This improvement, attributed to multifunctional properties such as higher dye uptake, reduced recombination process and enhanced charge collection efficiency. Deoxycholic acid (DCA) was also used as a coadsorbent in order to prevent dye aggregation and it was found that the PCE improved up to 6.12% for sensitizer P2 and the modified TiO2 photoanode, which can be attributed to further improvement in the electron injection efficiency and charge collection efficiency.
[Show abstract][Hide abstract] ABSTRACT: The reaction of the cyclometalated chloro-bridged iridium(III) dimers [(ppy)(2) Ir(μ-Cl)](2) (ppyH = 2-phenyl pyridine) and [(tpy)(2)Ir(μ-Cl)](2) (tpyH = 2-p-tolylpyridine) with 3,5-diphenylpyrazole (Ph(2)PzH) in the presence of sodium methoxide resulted in the formation of heterobridged dimers [(ppy)(2)Ir(μ-OH)(μ-Ph(2)Pz)Ir(ppy)(2)] (1) and [(tpy)(2)Ir(μ-OH)(μ-Ph(2)Pz)Ir(tpy)(2)] (2). Interestingly, the reaction of [(ppy)(2)Ir(μ-Cl)](2) with 3(5)-methyl-5(3)-phenylpyrazole (PhMePzH) afforded both a heterobridged dimer, [(ppy)(2)Ir(μ-OH)(μ-PhMePz)Ir(ppy)(2)] (3), and the monomer [(ppy)(2)Ir(PhMePz)Cl] (4). The compound [(ppy)(2)Ir(PhMePz)OH] (5) containing a terminal OH was obtained in a hydrolysis reaction involving 4, sodium methoxide, and PhMePzH. Complexes 1-5 were characterized by X-ray crystallography and electrospray ionization high-resolution mass spectrometry. All of the complexes are luminescent at room temperature in their dichloromethane solutions. The luminescence of the dinuclear complexes is characterized by a single structureless band centered at λ(max) = 550 nm (1 and 3) and 546 nm (2). The emission spectra of the mononuclear complexes 4 and 5 display vibronic structures with their λ(max) values at 497 nm (4) and 513 nm (5). In each case, the main emission bands are accompanied by shoulder bands at 526 (4) and 534 nm (5). The quantum yields, calculated with reference to [Ir(ppy)(2)(bpy)]PF(6) (Φ(CH(3)CN) = 0.0622), range from 0.11 to 0.17 for the dinuclear complexes and 0.045 to 0.048 for the mononuclear complexes. The lifetimes of the emission are in the microsecond region, suggesting the phosphorescent nature of the emission. Density functional theory (DFT) and time-dependent DFT calculations were performed on complexes 1 and 4 in the ground state to gain insight into the structural, electronic, and photophysical properties. Electrochemical studies on complexes 1-3 showed the presence of two consecutive one-electron-oxidation processes, assigned as the stepwise oxidation of the two Ir(III) centers, i.e., Ir(III)-Ir(III)/Ir(III)-Ir(IV) and Ir(III)-Ir(IV)/Ir(IV)-Ir(IV) couples, respectively. The monomers displayed single-oxidation peaks. No reduction process was observed within the solvent cathodic potential limit.
[Show abstract][Hide abstract] ABSTRACT: Four new ruthenium bipyridyl complexes denoted as MC103–MC106, with novel unsymmetrical bipyridines as ancillary ligands have been successfully synthesized as efficient sensitizers for application in dye-sensitized solar cells (DSC). The spectral, electrochemical and photovoltaic properties of the new sensitizers have been investigated. Alkyl thiophene/alkyl bithiophene and trialkyl phenyl as substituents on ancillary ligands improved the spectral properties and hence better efficiencies of DSCs are achieved. The higher efficiencies of the new sensitizers obtained are 9.56%, 9.58%, 8.34% and 8.32%, respectively, compared to 7.2% for the standard N719 sensitizer fabricated and evaluated under similar conditions.
Journal of Materials Chemistry 08/2012; 22(36). · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Semi-squaraines (SMSQ) are known as donor-acceptor (D-A) type molecules whereas squaraines (SQ), which differs from SMSQ by an extra donor group, are more or less biradicaloids in nature. The effect of the additional donor group in SQ, which changes the nature of the molecule, on geometrical and electronic structure are studied here and compared with the corresponding SMSQ. It is noticed from the geometrical parameters that, a strong resonance exists in SQ whereas disparity in carbon-carbon bond lengths of central C(4) ring is seen in SMSQ dyes. The increasing and decreasing of antibonding interactions between central C(4) ring and side donor groups cause destabilization of HOMO and stabilization of LUMO respectively in case of SQ compared to SMSQ molecules. This leads to decreasing the HOMO-LUMO gap and promotes biradicaloid character of SQ. The absorption maxima obtained by using TD-DFT method with BLYP, B3LYP, BHandHLYP, CAM-B3LYP and M06-2X functionals are not in good agreement with experimental results. On the other hand SAC-CI method gives better results for all the molecules. From this work we can evolve a design principle of these molecules which play a role as sensitizers in dye sensitized solar cells.
Journal of Molecular Modeling 08/2012; · 1.98 Impact Factor