•New carbazole based chromophores, N1–3 were co-sensitized with NCSU-10 dye in DSSCs.•Photovoltaic performances of co-sensitized DSSC were evaluated by a standard method.•N1 and N3 dyes showed better PCE than NCSU-10 dye alone in co-sensitized devices.•As per EIS studies, estimated lifetime is in good agreement with observed VOC values.Herein, we report photovoltaic performance studies of three carbazole based dyes (N1–3) derived from (Z)-3-(9-hexyl-9H-carbazol-3-yl)-2-(thiophen-2-yl)acrylonitrile scaffold as effective co-sensitizers in Ru (II) complex, i.e. NCSU-10 sensitized DSSCs. From the results it is evident that, the device fabricated using co-sensitizer N3 with 0.2 mM of NCSU-10 exhibited improved photon conversion efficiency (PCE) of 8.73% with JSC of 19.87 mA·cm− 2, VOC of 0.655 V and FF of 67.0%, while N1 displayed PCE of 8.29% with JSC of 19.75 mA·cm− 2, VOC of 0.671 V and FF of 62.6%, whereas NCSU-10 (0.2 mM) alone displayed PCE of 8.25% with JSC of 20.41 mA·cm− 2, VOC of 0.667 V and FF of 60.6%. However, their EIS studies confirm that, N1, showing higher VOC is efficient in suppressing the undesired charge recombination in DSSCs through enhanced surface coverage on TiO2 and thereby resulting in longer electron lifetime than that of NCSU-10 dye alone. Here, the higher PCE of N3 can be attributed to its improved light harvesting efficiency, which is due to the presence of highly electron withdrawing barbituric acid in its structure. Conclusively, the results showcase the potential of simple carbazole based dyes as co-sensitizers in improving efficiency of DSSCs.Download high-res image (166KB)Download full-size image

In this study, tris-heteroleptic Ru(II) sensitizers denoted as SD-15 and SD-16 with a 4,4′,5,5′-tetramethyl-1H,1′H-2,2′-bis-imidazole moiety as the ancillary ligand were designed, synthesized and characterized using FTIR-ATR, 1H-NMR and UV-vis spectrophotometry. Structural modulations resulted in a red shift in the low-energy metal-to-ligand charge transfer (MLCT) absorption peak. The novel bis-imidazole based Ru(II) sensitizer was adsorbed on a TiO2 surface via –COOH anchoring groups. The band gap, and the ground and excited state oxidation potentials were measured. The photovoltaic performance of the as-fabricated solar cells was measured and showed that the presence of bis-imidazole provided more negative free energy for dye regeneration (−ΔGregeneration) and less driving force for electron injection (−ΔGinjection) into the conduction band edge of TiO2. Therefore, the use of SD-15 and SD-16 resulted in lower JSC values of 10.20 and 3.52 mA cm−2, respectively, than N719. The open-circuit photovoltage (VOC) of SD-15 and SD-16 was 0.58 and 0.52 V, respectively, less than N719 under the same experimental conditions due to the decreased eTiO2 lifetime of 0.71 ms compared to 8.8 ms of N719 for the injected electrons that was calculated using impedance measurements. Our efforts in this research program led to the development of more stable Ru(II) complexes compared to N719 NCS-based complexes; however, the conversion efficiency was lower. Hence, the fine-tuning of bi-imidazole is needed to identify more efficient NCS-free Ru(II) sensitizers for higher efficiency DSSCs.

In this work, we report synthesis and device fabrication studies of four metal-free D–A-type dyes (A1–A4) based on structurally simple N,N-dimethyl-4-vinyl aniline carrying four different acceptor/anchoring groups, as sensitizers for sensitizing photoanode (TiO2). In the sensitizers, N,N-dimethylaniline ring acts as an electron donor, while barbituric acid, N,N-dimethyl barbituric acid, thiobarbituric acid and N,N-diethyl thiobarbituric acid function as electron acceptor/anchoring units. They were synthesized in good yield via Knoevenagel protocol in neutral condition without any catalyst. Further, they were subjected to structural, electrochemical and optical characterization in order to evaluate their structure, band gap and absorption/emission behavior. The studies reveal that all the four dyes have thermodynamic feasibility of electron injection as well as electron recombination; their optical band gaps were found to be in the range of 2.35–2.56 eV. High-quality crystals of A2 and A4 were grown by slow evaporation technique using its solution with 1:1 pet ether (60–80 °C)/ethyl acetate solvent mixture at room temperature. Their SC-XRD studies disclose that the crystals are in the triclinic system with space group P-1. Further, DFT studies were performed using Turbomole V7.1 software package to evaluate their optimized geometry and HOMO and LUMO levels. Finally, DSSC device fabricated with the dye A1 showed relatively good efficiency when compared to other dyes mainly due to the effective binding of barbituric acid on the surface of TiO2 through NH or OH functional group.

•Designed and synthesized four new metal-free carbazole based chromophores with D-π-A architecture for DSSC application.•The dyes P1-2 possess higher molar extinction coefficient, though they show lower absorption maxima, when compared to P3-4.•The negative free energy for electron injection as well as photovoltaic performance of dyes P1-4 follows the same order.•As per DFT simulations, P1 carrying cyanoacetic acid as anchoring unit possesses better charger separation in its FMO levels.Herein, we report design, synthesis and photovoltaic performance of four new metal-free heteroaromatic dyes (P1-4) with D-π-A architecture carrying electron donating carbazole core connected to four different electron withdrawing/anchoring groups, viz. cyanoacetic acid, rhodanine-3-acetic acid, barbituric acid and thiobarbituric acid and phenylene ring as a π-spacer. The newly designed P1-4 were synthesized from carbazole derivative using Suzuki cross coupling approach followed by Knoevenagel condensation reaction. Their structures were confirmed by FTIR, NMR, Mass spectral and elemental analyses. The dyes were subjected to optical and electrochemical studies in order to investigate their absorption/emission behavior as well as HOMO/LUMO energies. The UV–vis spectral studies reveal that the P1-4 showed λmax at 412, 439, 458 and 489 nm, respectively. Their optical band-gap is in the range of 2.17 to 2.61 eV and fluorescence quantum yield is in the order of 44–70%. From energy level diagram, it is clear that all the dyes possess good thermodynamic feasibility for electron injection into CB edge of TiO2 as well as their regeneration from electrolyte system. The photovoltaic performance studies indicate that among the tested dyes, P1 anchored with cyanoacetic acid displayed the highest IPCE (32%), resulting in improved PCE (1.94%), JSC (4.68 mA cm−2), VOC (0.588 V) and FF (70.3%) values, when compared to other dyes. Finally, DFT studies were performed using Turbomole 7.1 V software to investigate their electron cloud delocalization in HOMO/LUMO levels and theoretical absorption spectral data. The results reveal that the dye P1 showed effective charge separation in its FMO levels, which has reflected in its ICT behavior and hence P1 displayed the improved photovoltaic performance.Download high-res image (230KB)Download full-size image

•Three novel organic dyes coded DE-1, DE-2 and DE-3 were characterized for DSSCs.•DE-1, DE-2 and DE-3 contain one, two and three anchoring groups, respectively.•As the number of anchoring group increased, Voc, ε, & hyperpolarizability increased.•DE-3 showed the greatest light harvesting, IPCE and photovoltaic properties.Three novel organic sensitizers were molecularly designed and synthesized based on triphenylamine (TPA), as the core donor (DTPA), linked to one, two or three carbazole (DCarb) moieties with different number of anchoring groups (AG) to generate: DTPA-π-DCarb-A (DE1), DTPA(π-DCarb-A)2 (DE-2) and DTPA(π-DCarb-A)3 (DE-3). The primary goal of this study was to investigate the influence of bridging different number of DCarb and AG, through π-spacer, with TPA on the photovoltaic performance in DSSCs. The molar extinction coefficient increased with increasing the number of DCarb and A. DE-3 showed the greatest light harvesting in solution and when anchored onto TiO2. Hence, the increase in the number of DCarb and AG led to better light harvesting and stronger electronic coupling with TiO2, which reflected positively on the photovoltaic properties, and this is the first systematic study of its kind involving TPA-π-carbazole motif. DE-3 achieved the best efficiency of 5.44% and showed IPCE of 74% at 470 nm. The Voc increased with increasing the number of anchoring groups. Equilibrium molecular geometries and frontier molecular orbitals studied by DFT and TD-DFT calculations were in excellent agreement with experiments, which showed enhanced electronic coupling with TiO2 and photovoltaic performance as the number of anchoring groups increased.

•Designed and synthesized three new metal-free organic chromophores with D-π-A-π-A architecture.•The DFT calculations showed better charge separation between the HOMO and LUMO energy levels of the dyes and their simulated absorption spectra are well in agreement with the experimental data.•The sensitizer N1 showed maximum PCE of 3.55% with JSC value of 9.08 mA cm−2.•The co-sensitized device fabricated using N1 and NCSU-10 dyes displayed better light harvesting ability when compared to NCSU-10 alone.Herein we report the design, synthesis and photovoltaic performance studies of three new D-π-A-π-A architectured organic chromophores (N1-3) derived from (Z)-3-(9-hexyl-9H-carbazol-3-yl)-2-(thiophen-2-yl) acrylonitrile scaffold. In the new design, the electron rich carbazole unit is connected to three different electron withdrawing/anchoring species, viz. cyano acetic acid, rhodanine-3-acetic acid and barbituric acid via cyano vinyl thiophene as π-spacer. Newly synthesized dyes were characterized by spectral, photophysical and electrochemical analyses. Their optical band-gap, GSOP and ESOP values, as calculated from the optical and CV studies were found to be in the range of 2.12–2.21, −5.52 to −5.43 and −5.40 to −3.25 eV respectively. The DFT and TD-DFT studies were performed using Turbomole 7.1V software and the results indicated the existence of proper charge separation between HOMO and LUMO levels of the dyes. Also, the results revealed good matching of theoretically generated optical spectral data with the experimental values. Finally, DSSC devices were fabricated using these three dyes and the dye N1 containing cyanoacetic acid as an acceptor unit showed better photo conversion efficiency (η) of 3.55% than the other two dyes. It’s JSC, VOC, and IPCE parameters were shown to be 9.06 mA cm−2, 0.577 V and 48%, respectively. The obtained EIS data and electron lifetimes of N1–3 sensitized devices are well in accordance with experimental photovoltaic parameters.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%.Download high-res image (135KB)Download full-size image

The objective of this work was to increase color yield of direct dyes on cotton with the aid of grafted quaternary ammonium monomers. Plasma-induced graft polymerization of diallyldimethylammonium chloride (DADMAC) and [2-(acryloyloxy)ethyl]trimethylammonium chloride (AOETMAC) on cotton followed by dyeing with direct dyes was studied using different concentrations of the monomers and plasma conditions, and the color yield was evaluated using K/S measurements. Colorfastness and staining after laundering were evaluated using the standard grayscale. A significant increase of 149% in color yield was achieved when 30 g/L of DADMAC was used. 100–300% increase in K/S was achieved when ∼20 g/L of AOETMAC was used. To enhance the wash and lightfastness of the direct dyes on cotton, plasma-induced graft polymerization of 1,1,2,2-tetrahydroperfluorododecyl acrylate was achieved on the dyed cotton and showed considerable enhancement in both the wash and lightfastness.

•BODIPY dyes with carbazole electron donating groups are characterized.•Photophysics is discussed based on transient and steady state spectroscopy results.•Impedance spectroscopy found huge charge transport related losses on TiO2.In this study, three boron dipyrromethenes (BODIPY) dyes with extended conjugation and electron donating carbazole groups with different alkyl chain lengths tethered to N-carbazole were synthesized and characterized for dye-sensitized solar cells. The goal was to study the effect of different alkyl chain lengths on dye aggregation at TiO2 surface. The proposed molecular strategy resulted in BODIPY dyes which showed interesting electronic absorption and fluorescence properties. It was observed that intramolecular energy transfer decreases with the increase in alkyl chain length possibly due to induced changes in molecular geometry caused by long alkyl chains. Additionally, interface analysis by impedance spectroscopy in comparison to N719 sensitized TiO2 solar cell showed significant charge transport related losses (Nyquist plot) most likely due to impedance resulted from aggregated BODIPY dye on TiO2 surface. Femtosecond transient absorption studies showed the loss of excited electrons by recombination with oxidized ground state of the sensitizers.

•Two novel heteroleptic Ru(II) terpyridyl complexes (DH-1 and DH-2) were developed.•Both DH-1 (4.68%) and DH-2 (3.82%) were less efficient than N749 for DSSCs.•HOMO was mainly delocalized on three NCS in the case of N749.•HOMO was mainly delocalized on one NCS in DH-1 and on Cl in DH-2.Two novel terpyridyl-based Ru(II) complexes, DH-1 and DH-2, were synthesized and characterized as photosensitizers for dye-sensitized solar cells. To enhance the thermal and photostability, a strong electron donor bidentate carbazole-based ancillary ligand was molecularly engineered and used to replace two monodentate labile isothiocyanates. Photovoltaic characterizations, I–V curve and IPCE, of DH-1 and DH-2 were carried out in presence of deoxycholic acid as an additive to determine the best device working conditions. DH-1 showed IPCE of 45% at 540 nm, short circuit photocurrent density (Jsc) of 11.69 mA cm−2, open circuit photovoltage (Voc) of 0.64 V, fill factor of 0.63, producing an overall conversion efficiency (η) of 4.68%, where DH-2 showed IPCE of 45% at 540 nm, Jsc of 10.90 mA cm−2, Voc of 0.59 V and FF of 0.59, which translated into η of 3.82%. DFT calculations rationalized the light harvesting efficiency and charge separation of why DH-1 > DH-2.

•Two novel heteroleptic Ru (II) bipyridyl complexes (HL-41 and HL-42) were developed.•HL-41 outperformed N719, but HL-42 was comparable to N719 for DSSCs.•Presence of OC2H5ortho to the CHCH spacer produced more steric effect in HL-42.•DFT calculations revealed that ancillary ligand in HL-42 was more twisted.Here we report structure–property relationships of naphthalene-based ancillary ligands for DSCs employing Ru (II) bipyridyl complexes containing methoxy at 4,7-positions (HL-41) and ethoxy at 2-position (HL-42) on the naphthalene moiety respectively. 2-ethoxy naphthalene stilbazole-based ligand was strategically chosen as an ancillary ligand to ascertain the influence of steric effect of alkoxy group ortho to the spacer group (CHCH) of stilbazole on the photovoltaic properties. It was found that under similar conditions photocurrent response was in the following order HL-41 > HL-42 > N719. Both HL-41 and HL-42 showed similar redox potential. The overall conversion efficiency for devices employing HL-41, HL-42 and N719 were 7.8%, 7.6% and 7.6%, respectively. DFT calculations showed that the torsion angles between the naphthyl moiety and CHCH spacer was 34.81° and 39° for HL-41 and HL-42, respectively. Thus, for HL-42 ethoxy ortho to CHCH produces more twisted naphthyl which precludes efficient charge transfer from the ancillary ligand to the metal.

In this study, an intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands in Ru(II) sensitizers for dye sensitized solar cells (DSCs) has been investigated using femtosecond transient absorption spectroscopy. Both anthracene and phenanthrene based sensitizers HD-7 and HD-8, respectively, resulted in a similar extinction coefficient, photophysical and thermodynamic free energy of electron injection and dye regeneration as measured by UV-Vis, excited state lifetime and cyclic voltammetry measurements, respectively. However, TiO2 adsorbed HD-7 resulted in up to 45% less photocurrent density than HD-8 although photovoltage was similar owing to comparable thermodynamic characteristics. It was obvious from the measurement of incident photon to current conversion efficiency (IPCE) that excited electrons in HD-7 are prone to internal energy loss before injection into the TiO2 conduction band. Analysis of photo-induced spectral features measured by femtosecond transient absorption spectroscopy showed that excited electrons in HD-7 are prone to ISC (intersystem crossing) much more than HD-8 and those triplet electrons are not injected into TiO2 efficiently. Interestingly, from impedance measurements, HD-7 showed higher recombination resistance than HD-8 and N719, but a shorter lifetime for electrons injected into the TiO2 conduction band.

Three halogen-free, phosphorus-based flame retardant monomers were synthesized with good yields and characterized using FT-IR, 1H NMR, and 31P NMR. Two of the monomers were novel phosphoramides and the third was derived from cyclotriphosphazene. Each monomer was coated onto cotton substrates with the aid of a UV flood curing system. The impacts of monomer concentration, photoinitiator concentration, UV exposure time and proximity of the specimen to the UV lamp on coating yield were evaluated by experiments designed with JMP Pro 10. Of the three monomers, the cyclotriphosphazene derivative was polymerized into a coating that was durable to Soxhlet extraction with acetone. Vertical burn testing showed that all three monomers are valuable flame retardants. These results agreed with thermogravimetric analysis findings that demonstrated quantitatively the effectiveness of each monomer at promoting char formation. For the cyclotriphosphazene derivative, the coating was easily visualized covering significant portions of the fabric using scanning electron microscopy.

High-density, non-thermal, glow-discharge atmospheric-pressure plasma was used for graft polymerization of a vapor deposited fluorocarbon mixture of 1,1,2,2-tetrahydroperfluorodecyl acrylate and 1,1,2,2-tetrahydroperfluorododecyl acrylate on undyed cotton fabrics, which furnished a highly durable nanolayer water and oil repellent finish. In this study, monomer vapor was deposited onto cotton fabrics on single and double sides of the fabrics. The influence of monomer flow rate and plasma preactivation was studied. The surface of the cotton treated with fluorocarbons is evaluated using the standard AATCC Test Methods. Surface chemistry and morphology of the treated cotton were characterized using FTIR, XPS, SEM, and TOF–SIMS. Plasma-assisted graft polymerization of fluorocarbon in the presence of the crosslinker pentaerythritol triacrylate (10:1 molar ratio of monomer: crosslinker) resulted in a polyfluorocarbon nanolayer on cotton, which was hydrophobic and durable to one accelerated laundering, which is equivalent to 10 home launderings.

A novel heteroleptic Ru(II) bipyridyl complex (HD-1-mono) was molecularly designed with a mono-carbazole ancillary ligand, synthesized and characterized for DSCs. The aim was to systematically study the influence of mono (HD-1-mono) versus bis-carbazole ancillary ligand (NCSU-10) on molar absorptivity, light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), electron injection from the first excited singlet and triplet states, short-circuit photocurrent density (Jsc), and total solar-to-electric conversion efficiency (η) for DSCs. This study showed that HD-1-mono exhibited slightly lower Voc but greater Jsc compared to NCSU-10. Though HD-1-mono showed lower extinction coefficient than NCSU-10, interestingly, it was found that the decrease in molar extinction coefficient of HD-1-mono is not directly related to the short-circuit photocurrent density (Jsc). For example, HD-1-mono showed a higher Jsc of 21.4 mA cm−2 without the presence of any additives. However, under optimized conditions, HD-1-mono showed a Jsc of 19.76 mA cm−2, Voc of 0.68 V, and (%η) of 9.33 compared to a Jsc of 19.58 mA cm−2, Voc of 0.71 and (%η) of 10.19 for NCSU-10, where N719 achieved a Jsc of 16.85 mA cm−2, Voc of 0.749 V and (%η) of 9.33 under the same experimental device conditions. Impedance results for HD-1-mono showed a shorter recombination time as compared to N719 and NCSU-10, which justify its lower Voc. Femtosecond transient absorption spectroscopy results elucidated that electron injection from the first triplet state is 63% more efficient for HD-1-mono than that of NCSU-10.

Two novel heteroleptic Ru(II) bipyridyl complexes, HD-2 and HD-2-mono, were molecularly engineered, synthesized and characterized for dye-sensitized solar cells (DSCs). The influences of mono versus bis electron-donor benzodioxan ancillary ligands on optical, dye packing, electrochemical and photovoltaic properties were examined and compared to the benchmark N719. HD-2 and HD-2-mono achieved solar-to-power conversion efficiencies (%η) of 9.64 and 9.50, respectively, compared to 9.32 for N719 under the same experimental device conditions. Optical results showed that HD-2 and HD-2-mono have much higher molar extinction coefficients, longer excited state lifetimes and narrower HOMO–LUMO gaps compared to N719. Although the molar extinction coefficient of HD-2-mono was 27% less than that of HD-2, it outperformed HD-2 in photovoltaic performance when anchored on TiO2, owing to better dye packing and loading of the former. Charge recombination at the dye/TiO2 interface by impedance spectroscopy analysis showed that the recombination resistance and the lifetime of injected electron in TiO2 conduction band is directly proportional to the open-circuit voltage (Voc) observed. Furthermore, compared to HD-2 and HD-2-mono, the greater Voc of N719 can be attributed to the greater negative free energy for dye regeneration. Both HD-2 and HD-mono have almost the same negative free energy, which explains why they achieved almost the same Voc. Decay dynamic analysis for solar devices fabricated from the named dyes, by time correlated single photon counting (TCSPC), elucidated that the lowest excited state decay lifetime for HD-2-mono, HD-2 and N719 are 3, 10 and 20 ps, respectively. The shorter the decay lifetime, the less kinetic redundancy, which leads to better photocurrent, and that explanation is consistent with the measured photocurrent and total solar-to-power conversion efficiency of the named dyes in the order of HD-2-mono > HD-2 > N719.Keywords: dye-sensitized solar cells; IPCE; mono versus bis-electron-donor ancillary ligands; Ru(II) bipyridyl complexes; solar-to-electric conversion

Here we report two novel amphiphilic Ru(II) heteroleptic bipyridyl complexes, HD-14 and HD-15, compared to previously reported NCSU-10. We have combined the strong electron donor characteristics of carbazole and the hydrophobic nature of different long alkyl chains, C7, C18 and C2 (NCSU-10), tethered to N-carbazole to study their influence on photocurrent, photovoltage and long term stability for dye-sensitized solar cells. Photon harvesting efficiency and electron donating characteristics of carbazole-based ancillary ligands were found to be unaffected by different alkyl chain lengths. However, a slight drop in the Voc of HD-14 and HD-15 was observed compared to that of NCSU-10. It was found by nanosecond flash photolysis transient absorption (TA) measurements that the faster the dye regeneration the higher the photocurrent density response, and the dye regeneration time was found to be 2.6, 3.6, and 3.7 μs for HD-14, HD-15, and N719 dyes, respectively. The difference in the amplitude of the transient absorption (TA) signal of the oxidized dye as measured by femtosecond TA studies is in excellent agreement with the photocurrent generated, which was in the following order HD-14 > HD-15 > N719. Under 1000 h light soaking conditions, HD-15 maintained up to 98% (only 2% loss) of the initial power conversion efficiency compared to 8% loss for HD-14 and 22% loss in the power conversion efficiency for NCSU-10. HD-15 was strikingly stable to light soaking conditions when employed in the presence of an ionic liquid electrolyte, which paves the way for widespread applications of dye-sensitized solar cells with long term stability. The total power conversion efficiency (η) was 9.27% for HD-14 and 9.17% for HD-15 compared to 8.92% for N719.

Four novel Ru(II) bipyridyl complexes MH12–15 were synthesized and characterized for dye-sensitized solar cells (DSSCs). Their photovoltaic performance including incident photon-to-current conversion efficiency (IPCE), total solar-to-power conversion efficiency (η%) and ground and excited state oxidation potentials and photoelectrochemical properties were evaluated on mesoporous nanocrystalline TiO2 and compared with the benchmark N719-dye under the same experimental conditions. MH12–15 showed stronger MLCT with significantly higher molar extinction coefficient for the lower energy absorption bands at 553 nm (27500 M−1 cm−1), 554 nm (34605 M−1 cm−1), 577 nm (23300 M−1 cm−1), and 582 nm (39000 M−1 cm−1), respectively, than that of N719 (14200 M−1 cm−1). The introduction of a cyclometallated ligand in dyes MH14 and 15 improved the optical properties and red-shifts of 24 nm and 28 nm, respectively, compared to the non-cyclometallated analogs MH12 and 13. The red shift in the UV-Vis spectra of MH14 and 15 can be attributed to the destabilization of the HOMO t2g of Ru(II). However, the destabilization of the HOMO furnished an upward shift of the ground state oxidation potentials (GSOPs) of MH14 and 15 at −5.44 eV and −5.36 eV against vacuum, respectively, which resulted in a driving force of only 0.22 and 0.16 eV for regeneration of dyes MH14 and 15, respectively. In the case of NCS analogs, MH12 and 13, the GSOPs, however, were −5.56 and −5.51 eV, respectively, which produced a driving force of more than 0.25 eV for dye regeneration. The nanosecond transient absorbance measurements showed that the time needed for the oxidized forms of MH12–MH15 to regenerate the neutral dye is 6 μs, 4 μs, 13 μs and 18 μs, respectively, compared to N719 (2.3 μs). These kinetic data confirmed that the weak thermodynamic force, small negative free energy (−ΔG), for regeneration of MH14 and 15 neutral dyes makes the dye regeneration process kinetically sluggish, which contributed significantly to the loss of both photocurrent and photovoltage. This study clearly elucidated that although cyclometallation may produce significantly better light harvesting, the driving force of less than 0.25 eV is not sufficiently enough for effective dye regeneration.

•A easy processing and quasi-uniform nanostructured counter electrode with triple features was proposed in DSSCs.•Enhancement of light reflection by nanostructured counter electrode was studied.•Improvement of charge transfer characteristics was presented.•Local Surface Plasmon effect was developed in cathode process to increase the performance of DSSCs.A nanostructure-based Pt counter electrode for dye-sensitized solar cells (DSSCs) is fabricated by assembly of silver nanoparticles on glass substrate and deposition of a thin Pt layer. This typical counter electrode has several unique behaviors such as good conductivity, quasi-uniform undulating morphology and high surface area. Studies indicate that the application of the FTO-free nanostructure-based Pt counter electrode in DSSCs can decrease the charge-transfer resistance of the Pt/electrolyte interface, enlarge the light pathway and enhance the light reabsorption superior to the devices with planar Pt counter electrode. In addition, theoretical analysis and experimental study demonstrate that the hot electrons injection effect caused by Localized Surface Plasmon Resonance effect of silver nanoparticles enhances the charge transport characteristic at the Pt/electrolyte interface, and this SPR effect makes the certain contributions on the enhancement of the photovoltaic performance of DSSCs. Compared to the DSSC with traditional planar counter electrode, the incident photon-to-current conversion efficiency, short-circuit current, and power conversion efficiency of DSSCs with nanoparticulate structure are increased by 1.117 times, 1.156 times, and 1.145 times, respectively; and the final power conversion efficiency (PCE) increases from 6.95% to 7.96%.Graphical abstract

Three novel heteroleptic amphiphilic polypyridyl Ru-complexes, MH01, MH03, and MH05, with oxygen-containing-electron-donor stilbazole-based ancillary ligands were synthesized to study the influence of the cyclic-electron-donor (MH01), the presence of the cyclic electron donor coupled to acyclic electron-donor auxochromes (MH03) ortho to the CHCH bridge of stilbazole, and the presence of only acyclic electron-donor methoxy group (MH05) on molar extinction coefficient, light harvesting efficiency (LHE), ground and excited state oxidation potentials, and photovoltaic performance for DSSCs. Although MH05 has three electron donor methoxy groups, it achieved the lowest molar extinction coefficient of 18250 M−1 cm−1 and exhibited the lowest photocurrent. The highest photocurrent density (JSC) was observed for the longest interatomic distance between the CHCH bridge of the stilbazole moiety and cyclic-electron-donor auxochromes (MH01). It was also shown that while incorporation of acyclic electron-donor auxochromes ortho to the CHCH bridge (MH03) has little effect on the ground and excited state oxidation potentials, λmax of the low energy MLCT, and molar absorptivity, the lowest photovoltage and %η were observed. When compared under the same experimental device conditions using 0.3 M tert-butylpyridine (TBP), only MH01-TBA achieved 18% more in JSC and 8.6% greater in η than the benchmark dye N719. To probe the interrelationship among the cyclic-vs.-acyclic oxygen-containing electron donors of the ancillary ligands, photocurrent and photovoltage of these dyes, the equilibrium molecular geometries of the ancillary ligands were calculated using DFT. The HOMO distribution on cyclic-vs.-acyclic electron donors and the position of OMe in the ancillary ligands rationalized the fundamental science behind the photovoltaic performance and photostability of these dyes.

Two new heteroleptic Ru(II) bipyridyl complexes MH06 and MH11 were designed, synthesized and characterized for DSSCs. While the ancillary ligand of MH06 was molecularly engineered with a strong electron donating group coupled with an extended π-conjugated system, the ancillary ligand of MH11 contained a longer π-conjugated system only. Molecular modeling, photophysical, and photovoltaic properties were compared under the same experimental conditions against the benchmark N719. In an effort to understand the structure–property relationship, their photovoltaic and photoelectrochemical properties including Jsc, Voc, ground and excited state oxidation potentials, UV-Vis absorption, and molar extinction coefficients were studied. The UV-Vis results showed intense MLCT absorption peaks of MH06 and MH11 in the visible region with a red shift of 12 and 18 nm, respectively, with significantly higher molar extinction coefficients compared to N719. Tetrabutylammonium (TBA) substituted MH11-TBA demonstrated the most efficient IPCE of over 90% in the plateau region covering the entire visible spectrum and extending into the near IR region (ca. 890 nm), which showed a solar-to-power conversion efficiency (η) of 10.06%, significantly higher than that of the benchmark N719 dye (9.32%). The superior performance in terms of the IPCE and Jsc of MH11 can be attributed to the bulky and highly hydrophobic nature of the pyrene-based ancillary ligand, which behaves as a shielding barrier for hole-transport recombination between TiO2 and the electrolyte. In addition, the IMPS results showed that the contribution of dyes to the conduction band shift of the TiO2 level is almost similar, regardless of different substitutions on the bipy-moiety. This implies that the open-circuit photovoltage (Voc) increases with reduced charge recombination in the presence of a thick layer of tetrabutyl ammonium ions (TBA) of the dye anchored on the surface of TiO2.

This paper reports preparation of a dual functional 50/50 nylon/cotton blend fabric (NyCo) with an anti-bacterial bulk and repellent front surface using atmospheric pressure glow discharge plasma. In this report, a C6 fluorocarbon monomer, 2-(perfluorohexyl)ethyl acrylate was graft polymerized using plasma on the front surface of a nylon–cotton fabric which was already grafted with polyDADMAC for anti-bacterial properties. The surface was characterized by Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The presence of fluorine on the surface was mapped and confirmed by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Scanning electron microscopy (SEM) images showed a uniform layer of fluorocarbon polymer on the fiber surface. High water contact angle of 144° was obtained on the surface. The surface also achieved a high American Association of Textile Chemists and Colorists (AATCC) Test Method 193 rating of 8 and AATCC Test Method 118 rating of 5 indicating that the surface could repel a fluid with surface tension as low as 25 dyn/cm.Highlights► Plasma graft-polymerization of a C6 FC onto 50/50 NyCo military fabric was achieved. ► A dual functional surface with repellent front and absorbent back surface was achieved. ► C8 FC was successfully replaced with benign C6 FC yet maintaining the performance.

•Plasma-induced graft polymerization of DADMAC onto PP was successfully achieved.•6 log reduction in gram-positive and gram-negative bacteria colonies was achieved.•Plasma key parameters were identified by applying a design of experiment.Spunbond nonwoven polypropylene (SB-PP), commonly used for hygienic products, was treated with diallyldimethylammonium chloride (DADMAC) to confer antimicrobial properties. Atmospheric pressure glow discharge plasma was used to induce free radical chain growth polymerization of DADMAC monomer onto the SB-PP in the presence of a crosslinker, which produced a graft polymerized network on the fabric with durable antimicrobial properties. The effects of different DADMAC concentrations and plasma operating condition parameters including the RF power (400–800 W) and the time of plasma exposure (0–120 s) were studied and the optimum treatment conditions were identified by determining the surface charge density on the treated fabrics. The presence of grafted poly-DADMAC on the polypropylene surface was confirmed using SEM, FT-IR and TOF-SIMS techniques. Antibacterial performance was investigated using the AATCC Test Method 100 for both gram positive and gram negative bacteria, and the antimicrobial test results showed 6 log reductions in the number of bacterial activities of Klebsiella pneumoniae and Staphylococcus aureus on the SB-PP samples, which is unprecedented.

Two novel high molar extinction coefficient heteroleptic Ru(II) isomers, NCSU-10 and NCSU-10′, based on carbazole antenna were synthesized with the aid of Knoevenagel reaction, to study the influence of the carbazole antenna and anchoring group (COOH) isomerization on the light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), short-circuit photocurrent density (JSC), and total solar-to-electric conversion efficiency (η) for DSSCs, and their device performances were compared to the benchmark dye N719. The photophysical and photoelectrochemical properties discussed herein addressed the significant impact of the carbazole antenna and the position of the anchoring group on JSC and η in DSSCs. Tetrabutylammonium (TBA) substituted NCSU-10 achieved efficient sensitization of nanocrystalline TiO2 over the whole visible range, extending into the near IR region (ca. 870 nm) with an excellent power conversion efficiency (η) of 9.37% under an irradiation of full sunlight (100 mW cm−2) with mask compared to 8.17% of N719 under optimized conditions. NCSU-10 outperformed N719 by 45% in molar absorptivity, 18.8% in JSC, and 14.6% in the total conversion efficiency. Molecular modeling studies (DFT/TD-DFT) of NCSU-10 and NCSU-10′ showed that the HOMO is delocalized not only on Ru and NCS but also on the carbazole with a large coefficient, indicating that the second charge generation transfer in the visible region at ∼400 nm is a mixture of metal-to-ligand charge transfer (MLCT) and strong ligand–ligand charge transfer (LLCT) with a significant HOMO coefficient originating from the carbazole antenna (π) to the bipyridyl electron acceptor (π*). Moreover, DFT calculations showed that the 4,4′-isomer (NCSU-10) is a significantly stronger electron acceptor than the 5,5′-isomer (NCSU-10′), which explained the inferior electron injection and significantly lower JSC of the 5,5′-isomer.

Four new donor–π–acceptor organic dyes (YF01–04), containing naphthalene-substituted amines as an electron donor and cyanoacrylic acid as an electron acceptor, were designed and synthesized, and their photophysical properties and dye-sensitized solar cells (DSCs) performances were characterized. Dyes YF02 and YF04, with 2,6-disubstituted naphthalene frameworks, were superior than their analog dyes YF03 and YF01, having 1,2-disubstituted naphthalene moiety, in incident-photo-to-current conversion efficiency (IPCE) and total solar-to-electric conversion efficiency (η). The DSCs based on YF02, comprised of diphenylamine moiety as the donor, produced the highest η of 5.29% compared to 4.03% of the analog dye YF04, which has pyrrolidine as the donor. Remarkably, a high open-circuit photovoltage (Voc) of 0.799–0.807 V was achieved in the cases of YF02–03, which have diphenylamine-donors. To better understand the structure–property relationship for DSCs application, molecular modelling was performed on YF01–04 and vertical electronic excitations were calculated using long-range corrected energy functional WB97XD and CAM-B3LYP at the basis set level DGDZVP, which were in excellent agreement with the experimental results. Moreover, the equilibrium molecular geometries of dyes YF01–04 were calculated at the density function theory (DFT) level using the hybrid energy functional B3LYP and basis set DGDZVP. The torsion angles (θ) between the naphthalene moiety and diphenylamine donor in YF02 and YF03 were more twisted than that of the pyrrolidine-donor dyes YF01 and YF04, precluding efficient intermolecular π–π charge transfer, which translated into high Voc. Compared to the reference dye TA-St-CA, which is based on diphenylamine as an electron donor linked to a phenyl ring, YF02 achieved higher Voc, which indicated that naphthalene substituted with diphenylamine is more efficient in retarding charge recombinations.

In this study, diallyldimethylammonium chloride (DADMAC), a quaternary ammonium salt monomer, is graft polymerized on 50–50 Nylon-Cotton (NyCo) standard military fabric using atmospheric pressure glow discharge plasma to impart self-detoxification capability. Atmospheric pressure plasma is used to induce free radical chain polymerization of the DADMAC monomer to introduce a graft polymerized network on the fabric with durable antimicrobial properties. Pentaerythritol tetraacrylate (PETA) is used as a cross-linking agent to obtain a highly cross-linked, durable polymer network. The presence of polyDADMAC on the fabric surface is confirmed using acid dye staining, SEM, and TOF–SIMS. Antibacterial performance is investigated using standard AATCC Test Method 100 for both gram positive and gram negative bacteria. Results show 99.9 % reduction in the bacterial activities of Klebsiella pneumoniae and Staphylococcus aureus.

The synthesis, in vivo and in vitro antitumor evaluation, and QSAR studies of some novel pyrazole analogs against Ehrlich Ascites Carcinoma (EAC) cells were described. In vitro results revealed that compounds 10, 6 and 4 were the most potent analogs against EAC, respectively. Moreover, in vivo evaluation of compounds 6 and 10 proved their capability to normalize the blood picture in comparison to 5-FU, a well known anticancer drug. These novel pyrazole analogs were molecularly designed with the goal of having significant potent cytotoxic effect against EAC cells. To develop a QSAR model capable of identifying the key molecular descriptors associated with the biological activity of the novel pyrazole analogs and predicting the cytotoxic effect for other novel pyrazole analogs against EAC cells, different QSAR models, using different physicochemical and topological molecular descriptors, were developed. Different molecular descriptors were predicted solely from the chemical structures of 16 pyrazolo-diazine and triazine analogs following the prediction of the equilibrium molecular geometry of each analog at the DFT level using B88-LYP functional energy and double zeta valence polarized (DZVP) basis set. It was found that dipole moment, excitation energy, the energy value of LUMO, solvent accessible surface area, and heat of formation were the key molecular descriptors in descriping the cytotoxic effect of those compounds against EAC.

A series of novel unsymmetrical 1-hetaryl-4-(2-chloroquinolin-3-yl) azines 4–9 was selectively synthesized in a three-step procedure starting from acetanilide (1). The molecular structures of 4–9 were established by elemental analyses, spectral data, hybrid density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Molecular conformation analysis for compounds 4–9, performed using DFT calculations utilizing the energy functional 3-Parameter (Exchange), Lee, Yang and Parr (B3LYP) and the full-electron basis set Density Gauss double-zeta with polarization functions (DGDZVP), on the synthesized azines considering the torsion angles (θ1, θ2, θ3) revealed 8 plausible conformers for each compound. Electronic and thermodynamic properties including the dipole moment and the thermodynamic energy values of the Frontier occupied and virtual molecular orbitals, HOMO and LUMO, respectively, were calculated for the most stable conformer for each compound. Furthermore, TD-DFT calculations coupled with the polarizable conductor calculation model (PCM), performed on the most stable conformers in DMF to account for the solvent effect, revealed that the optical properties including λmax and oscillator strength performed on the most stable conformers were in excellent agreement with the experimental λmax and molar extinction coefficient, which clearly validate the most stable molecular conformers identified for compounds 4–9. Comparison of the biological results to the calculated electronic and thermodynamic properties showed that the cytotoxicity is dependent on the low-lying ELUMO because compound 8 has the lowest ELUMO value and exhibited the greatest antitumor activity.

Three novel heteroleptic amphiphilic polypyridyl Ru-complexes, MH01, MH03, and MH05, with oxygen-containing-electron-donor stilbazole-based ancillary ligands were synthesized to study the influence of the cyclic-electron-donor (MH01), the presence of the cyclic electron donor coupled to acyclic electron-donor auxochromes (MH03) ortho to the CHCH bridge of stilbazole, and the presence of only acyclic electron-donor methoxy group (MH05) on molar extinction coefficient, light harvesting efficiency (LHE), ground and excited state oxidation potentials, and photovoltaic performance for DSSCs. Although MH05 has three electron donor methoxy groups, it achieved the lowest molar extinction coefficient of 18250 M−1 cm−1 and exhibited the lowest photocurrent. The highest photocurrent density (JSC) was observed for the longest interatomic distance between the CHCH bridge of the stilbazole moiety and cyclic-electron-donor auxochromes (MH01). It was also shown that while incorporation of acyclic electron-donor auxochromes ortho to the CHCH bridge (MH03) has little effect on the ground and excited state oxidation potentials, λmax of the low energy MLCT, and molar absorptivity, the lowest photovoltage and %η were observed. When compared under the same experimental device conditions using 0.3 M tert-butylpyridine (TBP), only MH01-TBA achieved 18% more in JSC and 8.6% greater in η than the benchmark dye N719. To probe the interrelationship among the cyclic-vs.-acyclic oxygen-containing electron donors of the ancillary ligands, photocurrent and photovoltage of these dyes, the equilibrium molecular geometries of the ancillary ligands were calculated using DFT. The HOMO distribution on cyclic-vs.-acyclic electron donors and the position of OMe in the ancillary ligands rationalized the fundamental science behind the photovoltaic performance and photostability of these dyes.

Here we report two novel amphiphilic Ru(II) heteroleptic bipyridyl complexes, HD-14 and HD-15, compared to previously reported NCSU-10. We have combined the strong electron donor characteristics of carbazole and the hydrophobic nature of different long alkyl chains, C7, C18 and C2 (NCSU-10), tethered to N-carbazole to study their influence on photocurrent, photovoltage and long term stability for dye-sensitized solar cells. Photon harvesting efficiency and electron donating characteristics of carbazole-based ancillary ligands were found to be unaffected by different alkyl chain lengths. However, a slight drop in the Voc of HD-14 and HD-15 was observed compared to that of NCSU-10. It was found by nanosecond flash photolysis transient absorption (TA) measurements that the faster the dye regeneration the higher the photocurrent density response, and the dye regeneration time was found to be 2.6, 3.6, and 3.7 μs for HD-14, HD-15, and N719 dyes, respectively. The difference in the amplitude of the transient absorption (TA) signal of the oxidized dye as measured by femtosecond TA studies is in excellent agreement with the photocurrent generated, which was in the following order HD-14 > HD-15 > N719. Under 1000 h light soaking conditions, HD-15 maintained up to 98% (only 2% loss) of the initial power conversion efficiency compared to 8% loss for HD-14 and 22% loss in the power conversion efficiency for NCSU-10. HD-15 was strikingly stable to light soaking conditions when employed in the presence of an ionic liquid electrolyte, which paves the way for widespread applications of dye-sensitized solar cells with long term stability. The total power conversion efficiency (η) was 9.27% for HD-14 and 9.17% for HD-15 compared to 8.92% for N719.

Four novel Ru(II) bipyridyl complexes MH12–15 were synthesized and characterized for dye-sensitized solar cells (DSSCs). Their photovoltaic performance including incident photon-to-current conversion efficiency (IPCE), total solar-to-power conversion efficiency (η%) and ground and excited state oxidation potentials and photoelectrochemical properties were evaluated on mesoporous nanocrystalline TiO2 and compared with the benchmark N719-dye under the same experimental conditions. MH12–15 showed stronger MLCT with significantly higher molar extinction coefficient for the lower energy absorption bands at 553 nm (27 500 M−1 cm−1), 554 nm (34 605 M−1 cm−1), 577 nm (23 300 M−1 cm−1), and 582 nm (39 000 M−1 cm−1), respectively, than that of N719 (14 200 M−1 cm−1). The introduction of a cyclometallated ligand in dyes MH14 and 15 improved the optical properties and red-shifts of 24 nm and 28 nm, respectively, compared to the non-cyclometallated analogs MH12 and 13. The red shift in the UV-Vis spectra of MH14 and 15 can be attributed to the destabilization of the HOMO t2g of Ru(II). However, the destabilization of the HOMO furnished an upward shift of the ground state oxidation potentials (GSOPs) of MH14 and 15 at −5.44 eV and −5.36 eV against vacuum, respectively, which resulted in a driving force of only 0.22 and 0.16 eV for regeneration of dyes MH14 and 15, respectively. In the case of NCS analogs, MH12 and 13, the GSOPs, however, were −5.56 and −5.51 eV, respectively, which produced a driving force of more than 0.25 eV for dye regeneration. The nanosecond transient absorbance measurements showed that the time needed for the oxidized forms of MH12–MH15 to regenerate the neutral dye is 6 μs, 4 μs, 13 μs and 18 μs, respectively, compared to N719 (2.3 μs). These kinetic data confirmed that the weak thermodynamic force, small negative free energy (−ΔG), for regeneration of MH14 and 15 neutral dyes makes the dye regeneration process kinetically sluggish, which contributed significantly to the loss of both photocurrent and photovoltage. This study clearly elucidated that although cyclometallation may produce significantly better light harvesting, the driving force of less than 0.25 eV is not sufficiently enough for effective dye regeneration.

In this study, an intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands in Ru(II) sensitizers for dye sensitized solar cells (DSCs) has been investigated using femtosecond transient absorption spectroscopy. Both anthracene and phenanthrene based sensitizers HD-7 and HD-8, respectively, resulted in a similar extinction coefficient, photophysical and thermodynamic free energy of electron injection and dye regeneration as measured by UV-Vis, excited state lifetime and cyclic voltammetry measurements, respectively. However, TiO2 adsorbed HD-7 resulted in up to 45% less photocurrent density than HD-8 although photovoltage was similar owing to comparable thermodynamic characteristics. It was obvious from the measurement of incident photon to current conversion efficiency (IPCE) that excited electrons in HD-7 are prone to internal energy loss before injection into the TiO2 conduction band. Analysis of photo-induced spectral features measured by femtosecond transient absorption spectroscopy showed that excited electrons in HD-7 are prone to ISC (intersystem crossing) much more than HD-8 and those triplet electrons are not injected into TiO2 efficiently. Interestingly, from impedance measurements, HD-7 showed higher recombination resistance than HD-8 and N719, but a shorter lifetime for electrons injected into the TiO2 conduction band.

Two novel high molar extinction coefficient heteroleptic Ru(II) isomers, NCSU-10 and NCSU-10′, based on carbazole antenna were synthesized with the aid of Knoevenagel reaction, to study the influence of the carbazole antenna and anchoring group (COOH) isomerization on the light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), short-circuit photocurrent density (JSC), and total solar-to-electric conversion efficiency (η) for DSSCs, and their device performances were compared to the benchmark dye N719. The photophysical and photoelectrochemical properties discussed herein addressed the significant impact of the carbazole antenna and the position of the anchoring group on JSC and η in DSSCs. Tetrabutylammonium (TBA) substituted NCSU-10 achieved efficient sensitization of nanocrystalline TiO2 over the whole visible range, extending into the near IR region (ca. 870 nm) with an excellent power conversion efficiency (η) of 9.37% under an irradiation of full sunlight (100 mW cm−2) with mask compared to 8.17% of N719 under optimized conditions. NCSU-10 outperformed N719 by 45% in molar absorptivity, 18.8% in JSC, and 14.6% in the total conversion efficiency. Molecular modeling studies (DFT/TD-DFT) of NCSU-10 and NCSU-10′ showed that the HOMO is delocalized not only on Ru and NCS but also on the carbazole with a large coefficient, indicating that the second charge generation transfer in the visible region at ∼400 nm is a mixture of metal-to-ligand charge transfer (MLCT) and strong ligand–ligand charge transfer (LLCT) with a significant HOMO coefficient originating from the carbazole antenna (π) to the bipyridyl electron acceptor (π*). Moreover, DFT calculations showed that the 4,4′-isomer (NCSU-10) is a significantly stronger electron acceptor than the 5,5′-isomer (NCSU-10′), which explained the inferior electron injection and significantly lower JSC of the 5,5′-isomer.

Four new donor–π–acceptor organic dyes (YF01–04), containing naphthalene-substituted amines as an electron donor and cyanoacrylic acid as an electron acceptor, were designed and synthesized, and their photophysical properties and dye-sensitized solar cells (DSCs) performances were characterized. Dyes YF02 and YF04, with 2,6-disubstituted naphthalene frameworks, were superior than their analog dyes YF03 and YF01, having 1,2-disubstituted naphthalene moiety, in incident-photo-to-current conversion efficiency (IPCE) and total solar-to-electric conversion efficiency (η). The DSCs based on YF02, comprised of diphenylamine moiety as the donor, produced the highest η of 5.29% compared to 4.03% of the analog dye YF04, which has pyrrolidine as the donor. Remarkably, a high open-circuit photovoltage (Voc) of 0.799–0.807 V was achieved in the cases of YF02–03, which have diphenylamine-donors. To better understand the structure–property relationship for DSCs application, molecular modelling was performed on YF01–04 and vertical electronic excitations were calculated using long-range corrected energy functional WB97XD and CAM-B3LYP at the basis set level DGDZVP, which were in excellent agreement with the experimental results. Moreover, the equilibrium molecular geometries of dyes YF01–04 were calculated at the density function theory (DFT) level using the hybrid energy functional B3LYP and basis set DGDZVP. The torsion angles (θ) between the naphthalene moiety and diphenylamine donor in YF02 and YF03 were more twisted than that of the pyrrolidine-donor dyes YF01 and YF04, precluding efficient intermolecular π–π charge transfer, which translated into high Voc. Compared to the reference dye TA-St-CA, which is based on diphenylamine as an electron donor linked to a phenyl ring, YF02 achieved higher Voc, which indicated that naphthalene substituted with diphenylamine is more efficient in retarding charge recombinations.

A novel heteroleptic Ru(II) bipyridyl complex (HD-1-mono) was molecularly designed with a mono-carbazole ancillary ligand, synthesized and characterized for DSCs. The aim was to systematically study the influence of mono (HD-1-mono) versus bis-carbazole ancillary ligand (NCSU-10) on molar absorptivity, light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), electron injection from the first excited singlet and triplet states, short-circuit photocurrent density (Jsc), and total solar-to-electric conversion efficiency (η) for DSCs. This study showed that HD-1-mono exhibited slightly lower Voc but greater Jsc compared to NCSU-10. Though HD-1-mono showed lower extinction coefficient than NCSU-10, interestingly, it was found that the decrease in molar extinction coefficient of HD-1-mono is not directly related to the short-circuit photocurrent density (Jsc). For example, HD-1-mono showed a higher Jsc of 21.4 mA cm−2 without the presence of any additives. However, under optimized conditions, HD-1-mono showed a Jsc of 19.76 mA cm−2, Voc of 0.68 V, and (%η) of 9.33 compared to a Jsc of 19.58 mA cm−2, Voc of 0.71 and (%η) of 10.19 for NCSU-10, where N719 achieved a Jsc of 16.85 mA cm−2, Voc of 0.749 V and (%η) of 9.33 under the same experimental device conditions. Impedance results for HD-1-mono showed a shorter recombination time as compared to N719 and NCSU-10, which justify its lower Voc. Femtosecond transient absorption spectroscopy results elucidated that electron injection from the first triplet state is 63% more efficient for HD-1-mono than that of NCSU-10.

Two new heteroleptic Ru(II) bipyridyl complexes MH06 and MH11 were designed, synthesized and characterized for DSSCs. While the ancillary ligand of MH06 was molecularly engineered with a strong electron donating group coupled with an extended π-conjugated system, the ancillary ligand of MH11 contained a longer π-conjugated system only. Molecular modeling, photophysical, and photovoltaic properties were compared under the same experimental conditions against the benchmark N719. In an effort to understand the structure–property relationship, their photovoltaic and photoelectrochemical properties including Jsc, Voc, ground and excited state oxidation potentials, UV-Vis absorption, and molar extinction coefficients were studied. The UV-Vis results showed intense MLCT absorption peaks of MH06 and MH11 in the visible region with a red shift of 12 and 18 nm, respectively, with significantly higher molar extinction coefficients compared to N719. Tetrabutylammonium (TBA) substituted MH11-TBA demonstrated the most efficient IPCE of over 90% in the plateau region covering the entire visible spectrum and extending into the near IR region (ca. 890 nm), which showed a solar-to-power conversion efficiency (η) of 10.06%, significantly higher than that of the benchmark N719 dye (9.32%). The superior performance in terms of the IPCE and Jsc of MH11 can be attributed to the bulky and highly hydrophobic nature of the pyrene-based ancillary ligand, which behaves as a shielding barrier for hole-transport recombination between TiO2 and the electrolyte. In addition, the IMPS results showed that the contribution of dyes to the conduction band shift of the TiO2 level is almost similar, regardless of different substitutions on the bipy-moiety. This implies that the open-circuit photovoltage (Voc) increases with reduced charge recombination in the presence of a thick layer of tetrabutyl ammonium ions (TBA) of the dye anchored on the surface of TiO2.