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Dye-sensitized solar cells (DSSCs) have emerged as promising alternatives to traditional silicon-based solar cells due to their relatively high conversion efficiency, low cost, flexibility, and environmentally benign fabrication processes. In DSSCs, platinum (Pt)-based materials used as the counter electrode (CE) exhibit the superior catalytic ability toward the reduction reaction of triiodide ions, which are attributed to their excellent catalytic activity and high electrical conductivity. However, Pt-based materials with high cost and limited supply hinder them from mass production. Developing highly active and stable CE materials without noble metals has been a persistent challenge for the practical application in DSSCs. Recently, a number of earth-abundant catalysts, especially carbon-based materials, display high activity, low cost, and good stability that render them attractive candidates to replace Pt in DSSCs. Herein, we will briefly review recent progress on carbon-based electrocatalysts as CEs in DSSC applications. The strategies of improving the catalytic activity of carbon-based materials such as structural engineering and/or heteroatom doping will be introduced. The active sites toward the reduction reaction of triiodide ions summarized from experimental results or theoretical calculation will also be discussed. Finally, the futuristic prospects and challenges of carbon-based electrocatalysts as CEs in DSSCs will be briefly mentioned.

Materials with different nanostructures can have diverse physical properties, and they exhibit unusual properties as compared to their bulk counterparts. Therefore, the structural control of desired nanomaterials is intensely attractive to many scientific applications. In this brief review, we mainly focus on reviewing our recent reports based on the materials of graphene and the transition metal chalcogenide, which have various low-dimensional nanostructures, in relation to the use of electrocatalysts in electrochemical energy applications; moreover, related literatures were also partially selected for discussion. In addition, future aspects of the nanostructure design related to the further enhancement of the performance of pertinent electrochemical energy devices will also be mentioned.

Boron nitride (BN) is newly introduced as a non-metal electro-catalyst for the counter electrode of a dye-sensitized solar cell (DSSC). By applying a conductive binder of sulfonated poly(thiophene-...

A cobalt oxide (Co3O4)-decorated silicon carbide (SiC) nano-tree array (denoted as Co3O4/SiC NTA) electrode is synthesized, and it is investigated for use in micro-supercapacitor applications. Firstly, the well-standing SiC nanowires (NWs) are prepared by nickel (Ni)-catalyzed chemical vapor deposition (CVD) method, and then the thin layer of Co3O4 and the hierarchical Co3O4 nano-flower-clusters are, respectively, fabricated on the side-walls and the top side of the SiC NWs via electrodeposition. The deposition of Co3O4 on the SiC NWs benefits the charge transfer at the electrode/aqueous electrolyte interface due to its extremely hydrophilic surface characteristic after Co3O4 decoration. Furthermore, the Co3O4/SiC NTA electrode provides a directional charge transport route along the length of SiC nanowires owing to their well-standing architecture. By using the Co3O4/SiC NTA electrode for micro-supercapacitor application, the areal capacitance obtained from cyclic voltammetry measurement reaches 845 mF cm−2 at a 10 mV s−1 scan rate. Finally, the capacitance durability is also evaluated by the cycling test of cyclic voltammetry at a high scan rate of 150 mV s−1 for 2000 cycles, exhibiting excellent stability.

The production of hydrogen via the water splitting process is one of the most promising technologies for future clean energy requirements, and one of the best related challenges is the choice of the most highly efficient and cost effective electrocatalyst. Conventional electrocatalysts based on precious metals are rare and very-expensive for large-scale production of hydrogen, demanding the exploration for low-cost earth abundant alternatives. In this context, extensive works from both theoretical and experimental investigations have shown that two-dimensional (2D) layered materials have gained considerable attention as highly effective electrocatalytic materials for electrical-driven hydrogen production because of their unique layered structure and exciting electrical properties. This review highlights recent advancements on 2D layered materials, including graphene, transitional metal dichalcogenides (TMDs), layered double hydroxides (LDHs), MXene, and graphitic carbon nitride (g-C3N4) as cost-effective and highly efficient electrocatalysts for hydrogen production. In addition, some fundamental aspects of the hydrogen evolution reaction (HER) process and a wide ranging overview on several strategies to design and synthesize 2D layered material as HER electrocatalysts for commercial applications are introduced. Finally, the conclusion and futuristic prospects and challenges of the advancement of 2D layered materials as non-precious HER electrocatalysts are briefly discussed.

Dye-sensitized solar cell (DSSC) is one of the third generation solar cells converting solar light into electricity that attracted a remarkable research interest globally due to their simple device fabrication processes in ambient conditions and reasonable cell efficiency for commercialization; moreover, the advantages on colorfulness, possible plasticity, and high conversion efficiencies under indoor illumination also render DSSC to play a crucial role in energy-harvesting systems for green buildings. To eliminate the electrolyte leakage issues in traditional DSSCs (i.e., cells with organic solvent-based electrolytes), new class of ionic liquids, namely poly(ionic liquid)s, are recently investigated and used as the quasi/all–solid state electrolyte to improve the cell durability. Herein, we give a short review on the poly(ionic liquid)s to be used as the quasi/all–solid state electrolytes in DSSCs. Finally, the future perspectives for the development of poly(ionic liquid)s for DSSCs are also briefly discussed.

Nitrogen-doped graphene hollow nanoballs (N-GHBs) were synthesized in chemical vapor deposition (CVD) reaction using melamine as a chemical precursor via an in situ nitrogen-doping approach. In the CVD reaction, N-GHBs were deposited directly on carbon cloth (CC) to be used as an efficient metal-free electrocatalyst for dye-sensitized solar cell (DSSC) applications. The highly curved N-GHBs could avoid the self-assembly restacking of planar graphene sheets, which usually occurred during the film preparation. Keeping oxygen contaminations from N-GHBs, the characteristic electrical conductivity of graphene was preserved in the as-synthesized N-GHBs. By controlling the evaporation temperature of melamine, the nitrogen-doping content of 8.7–14.0% and different nitrogen-doped configurations in N-GHBs could be adjusted. The catalytic activities of different nitrogen-doped states in N-GHBs toward the triiodide (I 3− ) reduction in DSSCs were investigated, revealing that the pyridinic and quaternary nitrogens, rather than the total nitrogen doping level, in N-GHBs are mainly responsible for their catalytic activities in DSSCs. For solar cell applications, the high surface area and heteroatomic nitrogens of GHBs can remarkably improve the catalytic activity toward the triiodide reduction, lower the charge-transfer resistance, and enhance the corresponding photovoltaic performance (7.53%), which is comparable to that (7.70%) of a standard sputtered Pt counter electrode-based cell. These exceptional properties allow N-GHBs/CC to act as a promising electrocatalytic electrode for DSSC and other electrochemical energy applications.

Cobalt diselenide/cobalt selenite (CoSe2/CoSeO3 ) composite crystals with nanocube (NC), nanorod (NR), and nanoparticle (NP) structures were obtained through a microemulsion-assisted hydrothermal synthesis by incorporating the anionic surfactant docusate sodium salt (AOT), the cationic surfactant cetyltrimethyl ammonium bromide (CTAB), and the non-ionic surfactant Triton ® X-100, respectively. The as-prepared CoSe2/CoSeO3 crystals were utilized as the electrocatalysts in dye-sensitized solar cells (DSSCs). Among all the crystals, the one obtained through Triton ® X-100 enables its DSSC to achieve the best power conversion efficiency (η ) of 9.27%, which is even better than that of the DSSC with platinum (Pt) (7.91%). The catalytic abilities of the counter electrodes were analyzed by cyclic voltammetry (CV), Tafel polarization plots, and electrochemical impedance spectra (EIS). The photovoltaic behaviors are substantiated by incident photon-to-electron conversion efficiency (IPCE) spectra. The DSSC with CoSe 2/CoSeO3-NP exhibited an η of 9.27% at 100 mW cm−2, a better η of 9.31% at 50 mW cm−2, and the best η of 9.41% at 10 mW cm−2, thereby extending the scope of application of this DSSC from outdoor to indoor regions. The earth abundant, low-cost material of CoSe2/CoSeO3-NP can be a promising electrocatalytic material to replace the expensive Pt in a DSSC.

An ionic liquid (IL), 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)amino]ethyl}-1H-imidazol-3-ium tetrafluoroborate (TILBF4), containing a stable radical, 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO), was synthesized successfully. The TILBF4 shows two distinctive properties having the characteristics of ion-storage and supporting electrolyte materials. In this study, a hybrid electrochromic device (ECD) with a high coloration efficiency based on this TILBF4 and a poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) thin film was proposed. The maximum absorbance of the PProDOT-Et2/TILBF4 ECD was noticed at 590 nm upon applying the cell potential between −1.0 and 1.0 V. The device exhibits a transmittance change (ΔT) of 62.2% at 590 nm when being switched in the first transition, with short switching times of 4.0 s for bleaching and 3.6 s for coloring. A relatively high coloration efficiency (η) of 983.0 cm2/C was achieved at 590 nm. Furthermore, the ECD maintained 98.0% of its initial ΔT after consecutive operation for 1000 cycles between −1.0 and 1.0 V.

A composite thin film of nitrogen-doped graphene and molybdenum disulfide (NGr/MoS2) was prepared via a simple drop-coating method, and used as an electrocatalytic film for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The NGr was intended for increasing the conductivity and serving as the skeleton of the composite film, while the MoS2 with remarkable electrocatalytic ability increased the effective electrocatalytic sites in the composite film. The NGr acts as the architecture framework for MoS2 sheet, which could expose the active sites on its edge, as demonstrated in scanning electron microscopy (SEM). The weight percent (wt %) of the NGr in the composite film was optimized for obtaining the best performance to the pertinent DSSC. The NGr/MoS2 composite film shows much higher electrocatalytic ability for the reduction of I−/I3−, compared to the films of bare NGr and bare MoS2. Electrocatalytic abilities of the films were estimated by cyclic voltammetry (CV), rotating disc electrode (RDE), Tafel polarization plot, and electrochemical impedance spectroscopy (EIS) analyses. The DSSC with the NGr/MoS2 CE exhibits a solar-to-electricity power conversion efficiency of 7.82%, close to 8.25% of the DSSC with a platinum CE film. The low-cost NGr/MoS2 composite film is a potential alternative to replace the expensive platinum film for use as the catalytic film on the counter electrode of a DSSC.

A composite film of molybdenum diselenide nanosheets and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (MoSe 2 NS/PEDOT:PSS) was prepared for the counter electrode (CE) of a dye-sensitized solar cell (DSSC), via a low-cost drop-coating method. The two-dimensional (2D) nanosheets of MoSe 2 (MoSe 2 NS) were aimed to provide the composite film not only a large amount of electrocatalytic active sites but also orientational pathways for electron transfer, from the conducting substrate of the CE to the interface at the CE electrolyte. The PEDOT:PSS acts as the conductive matrix for the composite film, and also enables multiple interfacial electron transfer pathways between the MoSe 2 NS and the substrate. Owing to the combination of the advantages of both MoSe 2 NS and PEDOT:PSS, the DSSC with the composite film of MoSe 2 NS/PEDOT:PSS on its CE exhibited a power conversion efficiency ( η ) of 7.58 ± 0.05%, which is comparable to that of the cell with a Pt CE (7.81 ± 0.03%). The counter electrode films were characterized by scanning electron microscopy. Various electrochemical analyses, including those with cyclic voltammetry, rotating disk electrode, Tafel polarization curves, and electrochemical impedance spectroscopy, were performed intending to quantify the electrocatalytic ability of the counter electrode films; these techniques were able to precisely distinguish the functions of MoSe 2 NS and PEDOT:PSS in the composite films; the former worked as an electrocatalyst and the latter as a conductive binder. Incident photon-to-current conversion efficiency (IPCE) was used to substantiate the photovoltaic parameters. When the MoSe 2 NS/PEDOT:PSS composite film was coated on a flexible titanium (Ti) foil, the pertinent DSSC showed even a higher η of 8.51 ± 0.05%, as compared to a lower η of 8.21 ± 0.02% using the Pt-coated Ti foil CE. The results indicate the promising potential of MoSe 2 NS/PEDOT:PSS composite film to replace the expensive Pt.

A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN(-), which is aimed to form a SeCN(-)/(SeCN)3(-) redox couple with a more positive redox potential than that of traditional I(-)/I3(-), (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)3(-) ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of b-d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h.

A composite film based on carbon black nanoparticles and sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) (CB-NPs/s-PT) is formed on a flexible titanium foil for the use as the electro-catalytic counter electrode (CE) of dye-sensitized solar cells (DSSCs). The CB-NPs provide the large amount of electro-catalytic active sites for the composite film, and the s-PT polymer serves as a conductive binder to enhance the inter-particle linkage among CB-NPs and to improve the adhesion between the composite film and the flexible substrate. The flexible CB-NPs/s-PT composite film is designed to possess good electro-catalytic ability for I−/ I 3 − redox couple by providing large active sites and rapid reduction kinetic rate constant of I 3 − . The cell with a CB-NPs/s-PT CE exhibits a good cell efficiency (η) of 9.02 ± 0.01% at 100 mW cm−2, while the cell with a platinum CE shows an η of only 8.36 ± 0.02% under the same conditions. At weak light illuminations (20–80 mW cm−2), a DSSC with CB-NPs/s-PT CE still exhibits η's of 7.20 ± 0.04–9.08 ± 0.02%. The low-cost CB-NPs/s-PT CE not only renders high cell efficiency to its DSSC but also shows a great potential to replace the expensive platinum; moreover it is suitable for large-scale production or for indoor applications.

A series of new organic dyes containing fluoreneamine-based donor and carbazole π-spacer are reported. These dyes displayed longer wavelength charge transfer absorption when compared to that of the corresponding phenylamine analogs. Fluoreneamine-based donors shifted the lowest unoccupied molecular orbital upwardly which is beneficial for the injection of electrons from the excited dye into the conduction band of TiO 2 and lowered the oxidation potential of the dyes. The electronic perturbations caused by the fluorene unit have been confirmed by the density functional theoretical calculations. A device fabricated using a dye featuring difluorenylaminocarbazole donor and thiophene in the π-spacer exhibited promising power conversion efficiency (6.4%). The interfacial electron transfer kinetics has been elucidated by electrochemical impedance spectroscopy which showed the beneficial role of difluorenylamine unit in suppressing the electron recombination.

A hierarchical cobalt oxide (Co3O4)-functionalized silicon carbide nanowire (SiC NW) array electrode is synthesized on 4H-SiC(0001) substrates and investigated for use in the oxygen evolution reaction (OER) of water splitting. Ni-catalyzed chemical vapor deposition is employed to obtain vertically-aligned SiC NWs, and electrodeposition is used to functionalize the nanowires with hierarchical Co3O4 nano-clusters and a thin Co3O4 layer on the top side and the side-walls of the NWs, respectively. By using the Co3O4-functionalized SiC NW electrode for OER, the overpotential required to drive an anodic current density (J(anodic)) of 10 mA cm(-2) is 0.22 V, a value much lower than those of Co3O4 nano-clusters (0.42 V), of bare SiC NWs (1.27 V) electrodes and of Pt foil (0.56 V). The overpotential value of 0.22 V is among the most active for the noble metal-free electro-catalytic electrode to produce J(anodic) of 10 mA cm(-2). The deposition of Co3O4 on the SiC NW substrate produces an extremely hydrophilic surface, which improves the charge transfer characteristics at the interface between the electrode and the aqueous electrolyte. Moreover, the vertically-aligned SiC NWs provide a directional charge transport route along the nanowire length, as seen from a lower charge transfer resistance (R-ct) in the Co3O4-functionalized SiC NW electrode compared to the Co3O4-functionalized SiC electrode under the same OER operating conditions. Finally, the Co3O4-functionalized SiC NW electrode maintains stable performance in an alkaline electrolyte (i.e., 1 M KOH aqueous solution) during continuous electrolysis testing over 16 h. The outstanding electro-catalytic performance and stability make the Co3O4-functionalized SiC NW electrode a promising noble metal-free electro-catalytic electrode for the OER and other renewable energy applications. (C) 2017 Elsevier Ltd. All rights reserved.

Polypyrrole (PPy) with various morphologies were synthesized by chemical oxidative polymerization and further used as the counter electrode (CE) in dye–sensitized solar cells (DSSCs). The conventional anionic surfactant, docusate sodium salt (AOT), cationic surfactant, cetylmethyl ammonium bromide (CTAB), and the newly developed polymeric dispersant, poly(oxyethylene)–imide (POEM), were employed in the PPy synthesis. Scanning electron microscopy images (SEM) revealed diversified morphologies of the synthesized PPy nanoparticles with irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), corresponded to the choice of the commercial surfactants AOT and CTAB, as well as the home–made POEM, respectively. Fourier transform spectroscopy (FT–IR) and X–ray diffraction (XRD) analysis were used to confirm the PPy structures and crystalline properties. When fabricated into films and used as CE in DSSCs, the PPy–HNS demonstrated the superior cell efficiency of 6.71 ± 0.16% to those of PPy–IS (5.46 ± 0.31) and PPy–NS (6.31 ± 0.24), respectively. The excellent electrocatalytic ability of PPy–HNS was essentially attributed to its high electrochemical surface area ( A e ), which was quantitatively calculated through a rotating disk electrode system by using the Koutecky–Levich equation. Brunauer–Emmett–Teller (BET) surface area measurement, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were also used to substantiate the explanation for the DSSC performances.

A series of arylamine-free organic dyes that have a pyrenoimidazole donor, a cyanoacrylic acid acceptor, and different π-linkers were synthesized and characterized by optical, electrochemical, and theoretical investigations. The nature of the π-spacer exerts a significant effect on electronic properties of these dyes. The absorption maxima red-shifts on progressive addition of thiophene units in the conjugation pathway. All the dyes displayed negative solvatochromism indicative of effective solvation of the dyes by polar solvents in the ground state. The oxidation potential of the dyes shifted negatively on introduction of electron rich spacer. TDDFT computations were also performed to explain the optical and electrochemical properties of the dyes. They were demonstrated as sensitizers in dye-sensitized solar cells, the efficiency of which is promising.

Mesoporous TiO2 spheres (hereafter, MSs) were synthesized and incorporated into the photoanode of a quasi-solid state dye-sensitized solar cell (QSS-DSSC) containing an air stable ionic liquid electrolyte, namely 1-propyl-3-methylimidazolium iodide (PMII) along with triethylmethylammonium methyl sulfate (TEMAMS) (65 : 35 = v/v), in order to improve its performance. The presence of large pores in the MS, as confirmed using a high resolution scanning electron microscope (HR-SEM) and a mercury porosimeter, facilitates the penetration of QSS-electrolytes into the thin film, whereas their high surface area (108.1 m2 g−1) helps for high dye loading. Further, the large particle size increases the scattering ability of incident light leading to an excellent increment in the number of photons. The performance of DSSCs containing bi-ionic liquid (bi-IL) based electrolytes, namely, PMII/TEMAMS and PMII/1-ethyl-3-methylimidazolium tetrafluoborate (EMIBF4), with the addition of 0.2 M iodine, 0.4 M N-methylbenzimidazole (NMBI), and 0.15 M guanidiniumthiocyanate (GuSCN), was compared, where the DSSC based on PMII/TEMAMS bi-ILs demonstrates superior efficiency (6.18%) than that of PMII/EMIBF4 based one (4.53%). The DSSC with PMII/TEMAMS shows extraordinary durability and unfailing stability for 1200 h even though it was stored in the dark at 50 °C. Further, the PMII/TEMAMS bi-IL electrolyte shows a gel-state at room temperature naturally without adding any gelators or polymers.

Efficient titanium nitride/titanium oxide (TiN/TiO2) composite photoanodes have been proposed for the use not only in dye-sensitized solar cells (DSSCs) but also for water splitting. When the TiN precursor films were sintered at 500 °C for 0.5 to 4 h, they were partially converted to crystalline TiO2 containing both rutile and anatase phases. For the DSSCs, higher TiN content in the photoanodes resulted in a more negative flat-band potential and higher conductivity but lower surface area for the dye adsorption; therefore, the increase in VOC and FF but the decrease in JSC value were observed. The best DSSC, with TiN/TiO2 composite photoanode annealed for 1 h, exhibited a power conversion efficiency of 7.27%, while the cell without TiN, i.e., the cell with a standard P25 photoanode, showed an efficiency of 7.02%. For the water splitting, higher TiN content in the photoanodes resulted in better triggering of the H2O electrolysis, but less photo-induced current response at UV light illumination. Considering the water splitting performance measured at AM 1.5G, the TiN/TiO2 composite photoanode annealed for 1 h showed the best photo-induced current density (Jpho) of 0.12 mA cm−2, as compared with that of the standard P25 film. With the TiN/TiO2 composite photoanode annealed for 1 h, both DSSCs and water splitting electrochemical devices achieved their best performance independently.

Dye-sensitized solar cells (DSSCs) are fabricated using low-cost materials and a simple fabrication process; these advantages make them attractive candidates for research on next generation solar cells. In this type of solar cell, dye-sensitized metal oxide electrodes play an important role for achieving high performance since the porous metal oxide films provide large specific surface area for dye loading and the dye molecule possesses broad absorption covering the visible region or even part of the near-infrared (NIR). Recently, metal-free sensitizers have made great progress and become the most potential alternatives. This review mainly focuses on recent progress in metal-free sensitizers for applications in DSSCs. Besides, we also briefly report DSSCs with near-infrared (NIR) organic sensitizers, which provide the possibility to extend the absorption threshold of the sensitizers in the NIR region. Finally, special consideration has been paid to panchromatic engineering, co-sensitization, a key technique to achieve whole light absorption for improving the performance of DSSCs.

Dye-sensitized solar cells (DSSCs) have been extensively investigated as a potential renewable power source because they do not rely on expensive or energy-intensive processing methods. Within a DSSC, the electrolyte is very important as it provides the ionic conductivity for charge transport between the photoanode and the cathode, as well as sets the potential barrier necessary for energy conversion. Considering the durability of DSSCs in practical applications, ionic liquids (ILs) have become the most promising electrolyte materials due to their advantages of negligible vapor pressure, high thermal stability, wide electrochemical window, and high ionic conductivity. Recently, great progress has been made in the development of solid-state ILs to avoid the fluidity and potential leakage of IL-based electrolytes during long-term operation under solar light illumination. This chapter reviews some of the key research on the topic of solid-state ILs using polymeric ILs or IL crystals for DSSCs, and some of the relevant literature has also been briefly introduced.

Organic dyes containing 2-diphenylaminocarbazole donors and decorated with auxiliary chromophores at the diphenylamine moiety have been synthesized and characterized. The nature of the chromophores on diphenylamine unit alters the light-harvesting properties and HOMO/LUMO energies of the dyes. Butoxy substitution raises the HOMO and lowers the LUMO energies. But the incorporation of fluorenyl units fine-tunes the LUMO upwardly, which improves the thermodynamic driving force for electron injection into the conduction band of TiO2 and hikes the incident photon-to-current conversion efficiency. Consequently, a dye with fluorene in the donor unit and thiophene in the π-conjugation pathway is the most efficient (5.76 %) and has the highest photocurrent density (14.60 mA cm−2) in the series. Electrochemical impedance analysis of the devices showed the importance of fluorene units in suppressing electron recombination.

A novel polymeric ionic liquid (PIL), poly(oxyethylene)-imide-imidazole complex coupled with iodide anions (coded as POEI-II), was synthesized for preparing the gel electrolyte for quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). Herein, POEI-II, which acts simultaneously as a redox mediator in the electrolyte and a polymer for the gelation of an organic solvent-based electrolyte, was used to improve cell durability. In the structure of POEI-II, the presence of the POE segment can chelate lithium cations (Li+) within the electrolyte to improve the open-circuit voltage (VOC) of a QSS-DSSC, and enable a strong dipole–dipole lone-pair electron interaction with iodide ions (I−) of electrolyte composition rendering the high ionic conductivity and the diffusivity of a redox couple within the gel electrolyte. Consequently, the QSS-DSSC with the POEI-II gel electrolyte reaches a high cell efficiency of 7.19%. In addition, 5 wt% of multi-wall carbon nanotubes (MWCNTs) are incorporated into the POEI-II gel electrolyte as the extended electron transfer material (EETM) to facilitate charge transfer from the counter electrode to the redox mediator, which benefits the dye regeneration more efficiently. Meanwhile, the POE segments in POEI-II can prevent the MWCNTs from aggregation, which makes the well dispersed MWCNTs largely exposed to redox mediators. The highest cell efficiency of 7.65% was achieved by using the POEI-II/MWCNT gel electrolyte and it showed an unfailing durability of greater than 1000 h under 50 °C. This properly designed PIL paves a promising way for developing highly efficient and durable QSS-DSSCs.

Composite films of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with various functional ionic liquids were obtained by electro-polymerization process on indium-doped tin oxide (ITO) conducting glasses. The ITO glasses coated with the composite films were used as the counter electrodes in dye-sensitized solar cells (DSSCs). Various imidazolium cations with different alkyl chains (–C2H5, –C6H13, –C10H21) and anions ( BF 4 − , PF 6 − , SO 3 CF 3 − , TFSI − ) were used as the ionic liquids. The films of PEDOT doped with the ionic liquids render for their DSSCs enhanced power conversion efficiencies (η), compared to that of the cell with bare PEDOT counter electrode. The DSSC with 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMITFSI)-doped PEDOT counter electrode shows a high η of 8.87±0.11, while the DSSCs with bare PEDOT and platinum counter electrodes show the ηs of 6.19±0.09 and 8.09±0.06, respectively. X-ray photoelectron spectroscopy, scanning electron microscopy, and contact angle data were used to characterize the ionic liquid-doped PEDOT films. Cyclic voltammetry, rotating disk electrode, Tafel polarization plots, electrochemical impedance spectroscopy, and incident photon-to-current conversion efficiency curves were used to substantiate the photovoltaic performance.

The high performances of dye-sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron-deficient diphenylquinoxaline moiety integrated in the π-conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon-to-electron conversion efficiencies extends to the onset at the near-infrared region due to strong internal charge-transfer transition as well as the effect of electron-deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO2 to the electrolyte and boosts the photovoltage, comparable to their Ru(II) counterparts. Detailed spectroscopic studies have revealed the dye structure-cell performance correlations, to allow future design of efficient light-harvesting organic dyes.

New organic dyes containing fluorene functionalized with two imidazole chromophores as donors and cyanoacrylic acid acceptors have been synthesized and successfully demonstrated as sensitizers in nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs). The monoimidazole analogues were also synthesized for comparison. The Sommelet reaction of bromomethylated 2-bromo-9,9-diethyl-9H-fluorene produced the key precursor 7-bromo-9,9-diethyl-9H-fluorene-2,4-dicarbaldehyde required for the preparation of imidazole-functionalized fluorenes. Since the dyes possess weak donor segment, the electron-richness of the conjugation pathway dictated the optical, electrochemical, and photovoltaic properties of the dyes. The dyes served as sensitizers in DSSC and exhibited moderate efficiency up to 3.44%. The additional imidazole present on the fluorene has been found to retard the electron recombination due to the bulkier hydrophobic environment and led to high open-circuit voltage in the devices.

A series of new metal free organic dyes containing carbazole as donor and π-linker have been synthesized and characterized as effective sensitizers for dye sensitized solar cells (DSSCs). The carbazole functionalized at C-2 and C-7 served as electron-rich bridge. The donor property of the carbazole is substantially enhanced on introduction of tert-butyl groups at C-3 and C-6 positions and the oxidation propensity of the dyes increased on insertion of thiophene unit in the conjugation pathway. These structural modifications fine-tuned the optical and electrochemical properties of the dyes. Additionally, the presence of tert-butyl groups on the carbazole nucleus minimized the intermolecular interactions which benefited the performance of DSSCs. The dyes served as efficient sensitizers in DSSCs owing to their promising optical and electrochemical properties. The efficiency of DSSCs utilizing these dyes as sensitizers ranged from 4.22 to 6.04%. The tert-butyl groups were found to suppress the recombination of injected electrons which contributed to the increment in the photocurrent generation (JSC) and open circuit voltage (VOC). A dye with carbazole donor functionalized with tert-butyl groups and the conjugation bridge composed of 2,7-disubstituted carbazole and thiophene fragments exhibited higher VOC value. However, the best device efficiency was observed for a dye with unsubstituted carbazole donor and the π-linker featuring carbazole and bithiophene units due to the high photocurrent generation arising from the facile injection of photogenerated electrons into the conduction band of titanium dioxide (TiO2) facilitated by the low-lying LUMO.

Co-sensitization is a well-known method to broaden the light absorption wavelength. The co-sensitizer adsorption kinetics in dye-sensitized solar cells (DSSCs) is investigated by using simple UV–visible absorption spectra and images for co-sensitizers before and after TiO2 films soaking. The adsorption rate and surface coverage on TiO2 films of the co-sensitizers, i.e., a metal-free squaraine dye (SQ2) and a diarylaminofluorene-based organic dye (JD1), are also studied to explain the performance of DSSCs. The power conversion efficiency (η) of the co-sensitized DSSC is enhanced due to the cooperative interactions of the organic co-sensitizers. A wide absorption wavelength (400–750 nm) for light harvesting is found for the co-sensitizer at the molar ratio of 6 to 4 for JD1 to SQ2 dye, and thereby an η value of 6.36% is achieved for the pertinent DSSC, which is much higher than those of the cells sensitized with individual dyes absorbing narrow wavelength of light, i.e., η of 5.44% and 4.11% for the DSSC sensitized with JD1 (absorption wavelength from 400 to 550 nm) and SQ2 dye (absorption wavelength from 550 to 750 nm), respectively.

A coral-like film of nickel@nickel sulfide (Ni@NiS) was obtained on a conducting glass through an electrochemical method, in which the Ni functioned as a template. Three types of Ni thin films were electrodeposited on fluorine-doped tin oxide (FTO) substrates by a pulse current technique at the passed charge densities of 100, 200, and 300 mC cm−2, which rendered custard apple-like, coral-like, and cracked nanostructures, respectively. Subsequently, nickel sulfide films were coated on these Ni films by using a pulse potential technique. Due to the template effect of the Ni films, the composite films of Ni@NiS also assumed the same structures as those of their nickel templates. In each case of the films the particle of the film assumed a core–shell structure. The Ni@NiS coated FTO glasses were used as the counter electrodes for dye-sensitized solar cells (DSSCs). The DSSC with the coral-like Ni@NiS film on its counter electrode exhibits the highest power conversion efficiency (η) of 7.84%, while the DSSC with platinum film on its counter electrode shows an η of 8.11%. The coral-like Ni@NiS film exhibits multiple functions, i.e., large surface area, high conductivity, and great electrocatalytic ability for iodine/triiodine (I−/I3−) reduction. X-ray photoelectron spectroscopy (XPS), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and four-point probe technique were used to characterize the films. The photovoltaic parameters are substantiated using incident photon-to-current conversion efficiency (IPCE) curves, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization plots. The IPCE curves were further used to calculate theoretical short-current densities of the cells.

A new type of ionic liquid that contains a nitroxide radical (NOC) and iodide as two redox couples, JC-IL, has been suc- cessfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electro- lyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetra- methylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electro- lyte show promising cell efficiencies if coupled with dyes CR147 (8.12 %) or D149 (6.76 %). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electro- lyte alone.

New metal free dianchoring organic dyes featuring A–π–D–π–D–π–A (acceptor – π bridge – donor – π bridge – donor – π bridge – acceptor) configuration have been designed incorporating fluorene and oligothiophene units and successfully synthesized. Elongating the conjugation pathway between the donor and acceptor units altered the distance between the anchoring sites besides the absorption and redox properties. These dyes exhibited broad and intense absorption when compared to the corresponding monoanchoring donor–π–acceptor congeners. Though the dye containing bithiophene unit exhibited comparatively low Voc due to low electron life time and facile back electron transfer, showed high power conversion efficiency arising from the good light-harvesting capability attributable to the intense absorption peak in the visible region and enhanced interfacial electron transfer rate. This work demonstrates that the smaller distance of separation between the anchoring units increases the insulating capacity of the molecular layer which retards the back electron transfer.

Seven ionic liquids (ILs) based on asymmetric trialkyl sulfonium and tetraalkylammonium cations with bis(trifluoromethanesulfonyl)imide anion were prepared and characterized. Physico-chemical and electrochemical properties of these ionic liquids including density, decomposition temperature, viscosity and conductivity were determined. The photovoltaic parameters of dye-sensitized solar cells (DSSCs) containing these sulfonium and ammonium ion-based ILs were compared. Among these two groups of ILs, the DSSCs with sulfonium cation-based ILs show higher cell efficiencies than that of ammonium counterpart due to lower charge transfer resistance exhibited by the former when compared to the latter, as confirmed by electrochemical impedance spectroscopy (EIS). The best cell efficiency achieved in this work was 4.61%. Moreover, long-term durability of the quasi-solid-state DSSCs at 50 °C was investigated.

The pure organic dyes are investigated recently because of its high extinction coefficient and relatively low cost. In order to enhance the performance of the organic dyes, co-sensitization strategy is applied. The co-sensitization by the absorption of SQ2/5c shows enhanced photovoltaic performance for the pertinent DSSCs, which leads to a broader light harvesting window (400–750 nm) and a higher overall efficiency of 6.24% while the performances of DSSCs with the individual dyes are 5.33% (5c) and 3.98% (SQ2), respectively. Furthermore, the overall efficiency can be further improved to 6.71% with the incorporation of chenodeoxycholic acid (CDCA). In addition, the incident photon-to-current conversion efficiency (IPCE) and UV–visible absorption spectra are also used to explain and support the behavior of the co-sensitized solar cells. The investigation of the long-term stability of the DSSC based on co-sensitization of SQ2/5c dyes with the use of binary ionic liquid is performed. The DSSC with the co-sensitization strategy possesses an unfailing durability up to 1000 h.

The chemical stability of ZnO against acidic dye molecules is an important issue to be improved in a dye-sensitized solar cell (DSSC). In this study, ZnO nanowires (ZnO-NWs) are prepared on a FTO glass through chemical bath deposition, and then the surface of ZnO-NWs are precisely coated with a thin TiO 2 layer step by step using a simple dipping–rinse–hydrolyzation (DRH) process to create the core–shell structure of ZnO@TiO 2 nanowires (ZnO@TiO 2 -NWs) for improving the chemical stability of ZnO. This core–shell structure of ZnO@TiO 2 -NWs on a FTO glass is further used as the photoanode of a DSSC with an acidic N3 dye as sensitizer. The TiO 2 shell acts not only as a protective layer to avoid the formation of Zn 2+ /dye complexes on the surface of ZnO-NWs, but also as a barrier against the recombination of injected electrons with the triiodide ions at the interface of ZnO-NWs and the electrolyte. A higher efficiency of 1.17% is achieved for the core–shell ZnO@TiO 2 -NWs based DSSC with optimized dye sensitization time and DRH cycles, when compared to the optimized cell with pristine ZnO-NWs (0.75%).

Titanium-based ceramic nanoparticles, i.e. , titanium nitride nanoparticles (TiN-NPs) and titanium carbide nanoparticles (TiC-NPs), are used on counter electrodes (CEs) for quantum-dot-sensitized solar cells (QDSSCs). The QDSSC with TiC-NPs-CE exhibits the highest solar-to-electricity conversion efficiency ( η ) of 2.11 ± 0.06%, which is higher than that of the cell with sputtered-Au-CE (1.64 ± 0.06%). This is due to the higher electrocatalytic ability of the TiC-NPs for reducing the polysulfide (S x 2− ) ions and the lower charge-transfer resistance at the interface of the CE and the electrolyte. Meanwhile, the three-dimensional structure of the Ti-based ceramic materials also acts as an extended charge-transfer surface, which facilitates electron transfer on the CE toward S x 2− ions. Explanations are substantiated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and incident photon-to-current conversion efficiency (IPCE) curves.

New organic dyes containing diphenylamine donor, 2,7-carbazolyl π-linker, and cyanoacrylic acid acceptor have been synthesized and characterized. They possess red-shifted absorption with high molar extinction coefficient when compared to the corresponding dyes with fluorene or phenyl units in the conjugation. The dye-sensitized solar cells fabricated using these dyes as sensitizers exhibited power conversion efficiencies as high as 6.8% which increased to 7.2% when chenodexoycholic acid (CDCA) was used to impede dye aggregation. The DSSCs with ionic liquid electrolytes displayed marked stability over 1000 h.

A quantum dot-sensitized solar cell (QDSSC) is fabricated using a composite layer, consisting of ZnO nanowires and ZnO nanoparticles (CL-ZnO) as its semiconductor film. The diameter and the thickness of each ZnO nanowire (ZNW) are ∼100 nm and ∼6 μm, respectively. The structure and morphology of the films with ZnO nanoparticles (ZNPs), ZNWs, and CL-ZnO are characterized by X-ray diffraction (XRD) patterns and scanning electron microscopic (SEM) images. Cadmium sulfide quantum dots (CdS QDs) are deposited on the ZNWs, ZNPs, and CL-ZnO by a successive ionic layer adsorption and reaction (SILAR) cycle. The highest efficiency (η) for the QDSSC sensitized by CdS (CdS-QDSSC) is obtained in the case of CL-ZnO photoanode, and this is attributed to significant improvement in the short-circuit photocurrent density (JSC) of the pertinent cell, which is 2.81- fold higher than that of the cell with ZNWs. Moreover, cadmium selenide quantum dots (CdSe QDs) are used as co-sensitizer along with the CdS QDs for the CL-ZnO semiconductor layer (film designated as CL-ZnO/CdS/CdSe), and the pertinent QDSSC showed much higher efficiency than the one obtained in the case of the cell using only CdS QDs as the sensitizer. Finally, a passivation layer of ZnS was deposited on the film of CL-ZnO/CdS/CdSe, and the cell with this film (CL-ZnO/CdS/CdSe/ZnS) showed an efficiency of 0.55%, the highest in this research. Electrochemical impedance spectra (EIS), UV–vis absorbance spectra, transmission electron microscopy (TEM), and incident photon-to-current conversion efficiency (IPCE) curves are used to substantiate the explanations.

A novel 2,7-diaminofluorene-based organic dye (coded as JD2) with the diarylaminofluorene unit as the electron donor and the cyanoacrylic acid as the acceptor as well as anchoring group in a donor-π–donor-π–acceptor architecture is used in a dye-sensitized solar cell (DSSC). The TiO 2 film thickness and the dye soaking time are optimized, and a coadsorbate of bis-(3,3-dimethyl-butyl)-phosphinic acid (DINHOP) is added to the dye solution to reform the cell performance. Effects of the additives, N -methylbenzimidazole (NMBI), guanidinium thiocyanate (GuSCN) or 4-tert-butylpyridine (TBP) in the electrolyte on the photovoltaic performance of the DSSC are studied. Effects of change of DINHOP concentration in JD2 dye solution, change of ratio of LiI to 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and change of concentration of I 2 in the electrolyte on the photovoltaic performance of the DSSC are studied. A light-to-electricity conversion efficiency ( η ) of 6.31% is achieved for the JD2 dye-based DSSC, which is one of the best efficiencies for a cell with an organic dye. Explanations are made with electrochemical impedance spectra (EIS), laser-induced photo–voltage transients, and incident photo-to-current conversion efficiency (IPCE) curves.

Carbon black (CB), multi-walled carbon nanotube (MWCNT), and graphene (GN) were examined as catalytic films on the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). GN exhibits the best performance among these materials for the corresponding cell. A composite film of GN/Nafion® was next used as the catalytic film on the CE of a DSSC. Nafion® is demonstrated to be an excellent dispersant for inhibiting the aggregation of GN. A solar-to-electricity conversion efficiency (η) of 8.19% was achieved for the DSSC sensitized by TBA(Ru[(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-bis(5-(hexylthio)-2,2′-bithien-5′-yl)-2,2′-bipyridine)(NCS)2]), i.e., CYC-B11 dye, which was synthesized by our group, after the optimization of the composition of GN/Nafion®, and an η of 8.89% was exhibited for the cell with a sputtered-Pt (s-Pt) film on its CE under the same conditions. The significance of the cell with the composite film of GN/Nafion® lies in the fact that the expensive Pt is avoided in this case, thereby the cost of the pertinent DSSC was greatly reduced, even though its η (8.19%) is slightly smaller than that of the cell with the s-Pt film (8.89%).

a b s t r a c t Conducting polymer materials, i.e., polythiophene (PT), polypyrrole (PPy), and poly(3,4- ethylenedioxythiophene) (PEDOT) were used to prepare counter electrodes (CEs) for quantum-dot-sensitized solar cells (QDSSCs). The QDSSC with PEDOT-CE exhibited the highest solar-to-electricity conversion efficiency (� ) of 1.35%, which is remarkably higher than those of the cells with PT-CE (0.09%) and PPy-CE (0.41%) and very slightly higher than that of the cell with sputtered- gold-CE (1.33%). Electrochemical impedance spectra (EIS) show that this highest conversion efficiency of the PEDOT-based cell is due to higher electrocatalytic activity and reduced charge transfer resistance at the interface of the CE and the electrolyte, compared to those in the case of the cells with other conducting polymers and bare Au. Furthermore, the influences of morphology of the PEDOT film and the charge passed for its electropolymerization on the performance of its QDSSC were also studied. The higher porosity and surface roughness of the PEDOT matrix, with reference to those of other polymers are understood to be the reason for PEDOT to possess higher electrocatalytic activity at its interface with electrolyte.

Monodispersed polystyrene (PS, ca. 300 nm) latex particles are incorporated into a TiO2 film. A polystyrene-modified TiO2 film (M-TiO2) with micro-cluster structure, containing micro/nano-composite pores is thus obtained after sintering. Cadmium sulfide (CdS) quantum dots (CdS-QDs) are accumulated over M-TiO2 and bare TiO2 films (B-TiO2) by successive ionic layer adsorption and reaction (SILAR); we designate these films as M-TiO2/CdS and B-TiO2/CdS, respectively. Influence of SILAR cycles used for depositing CdS on B-TiO2 and M-TiO2 films on the performance of the pertinent quantum dot-sensitized solar cells (QDSSCs) is studied. The QDSSC with 6 SILAR cycles of M-TiO2/CdS (M-TiO2/CdS6) exhibited a solar-to-electricity conversion efficiency (η) of 1.79%, while the cell with B-TiO2/CdS5 shows an η of 1.35%, under the illumination of one sun. Moreover, guanidine thiocyanate (GuSCN) is found to be a promising additive to the polysulfide electrolyte. The additive renders higher conversion efficiency (2.01%) to its QDSSC. Durability of the CdS-QDSSC is also tested. Scanning electron microscopy (SEM) is used to obtain the images of TiO2 films and energy-dispersive X-ray spectroscopy (EDX) is employed to study the stoichiometric ratios of M-TiO2/CdS and B-TiO2/CdS. Incident photon-to-current conversion efficiencies (IPCE) of the QDSSCs are obtained to confirm the JSC behaviors of the cells.

A novel class of organic dyes containing pyrenoimidazole donors, cyanoacrylic acid acceptors, and oligothiophene π-linkers has been synthesized and characterized. The electro-optical properties of these dyes can be tuned by changing the conjugation length of the π-linkers. A dye containing terthiophene in the conjugation pathway exhibited a solar energy-to-electricity conversion efficiency of 5.65%.

Titanium nitride (TiN) or its thermally treated version is incorporated into the gel electrolyte of a dye-sensitized solar cell (DSSC) and the consequent effects are investigated in terms of photovoltaic performance of the cell. The gel electrolyte is essentially prepared by using poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP). With an addition of 3 wt% TiN, the solar-to-electricity conversion efficiency ( η ) of the DSSC reaches 5.33% from 4.15% of the cell without TiN. X-ray diffraction (XRD) spectra of thermally treated-TiN (tt-TiN) clearly shows the partial conversion of TiN into TiO 2 with both anatase and rutile crystal phases. The DSSC with the incorporation of 3 wt% of tt-TiN into its electrolyte shows a further improved efficiency of 5.68%, with reference to the efficiency of TiN-incorporated DSSC. The cell with 3 wt% of tt-TiN also shows unfailing at-rest stability after more than 1000 h. Electrochemical impedance spectroscopy (EIS) is used to obtain charge transfer properties of the cells. Crystallinity and crystalline phases are analyzed by X-ray diffraction (XRD) spectra. Surface morphologies are observed with scanning electron microscopy (SEM). IPCE curves substantiate the variations in short circuit photocurrents.

Various weight percentages of titanium nitride (TiN) and thermally-treated titanium nitride (tt-TiN) are incorporated into the binder-free TiO 2 films, prepared at low temperature (LT, 120 °C), and the corresponding dye-sensitized solar cells (DSSCs) are made. Effects of their incorporation on the photovoltaics of the DSSCs are studied. With the incorporation of 0.125 wt% of TiN in a TiO 2 film of only 5.5 μm-thickness, the corresponding solar-to-electricity conversion efficiency ( η ) has reached to 3.40% from 2.38% of a cell with a bare TiO 2 film of same thickness; this increase in efficiency is attributed to the higher electron conductivity and longer electron life time in the TiO 2 film with TiN. Incorporation of 1.0 wt% of tt-TiN into the TiO 2 film has further improved the conversion efficiency to 3.77%; this further enhancement in the efficiency is attributed to the partial conversion of TiN into TiO 2 due to the thermal treatment (450 °C). X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to study the partial conversion of TiN into TiO 2 . Other explanations are supported by the data of UV-absorption spectra, electrochemical impedance spectroscopy (EIS), and laser-induced photo-voltage transients.

Two unsymmetrical squaraines, where the electron-rich 3,4-ethylenedioxythiophene or bithiophene conjugated fragment was used to link unconventionally the squaraine core and the hexyloxyphenyl amino group, were applied for DSCs. The corresponding photovoltaic devices exhibit an attractively panchromatic response and also convert a portion of the near-infrared photons into electricity.

A quasi solid-state dye-sensitized solar cell (DSSC) is fabricated using 1-propyl-3-methylimidazolium iodide (PMII) and polyaniline-loaded carbon black (PACB) as the composite electrolyte. The electrolyte without added iodine is sandwiched between TiO 2 working electrode and platinum counter electrode (CE). A power conversion efficiency ( η ) of 5.81% is achieved with this type of cell. With the addition of 1-ethyl-3-methylimidazolium thiocyanate (EMISCN), a low-viscosity ionic liquid (IL), the cell with the binary ionic liquid (bi-IL) renders an efficiency of 6.15%, the best for any quasi solid-state DSSC without the addition of iodine. To fabricate a low cost DSSC using the bi-IL, the platinum layer of the counter electrode is replaced with a polymer layer, 3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4] dioxepine (PProdot-Et 2 ) through electrodeposition, and the corresponding DSSC shows an efficiency of 5.27%. At-rest stability of the quasi solid-state DSSC with bi-IL is compared with that of a liquid electrolyte DSSC at room temperature; the power conversion efficiency of the former shows a decrease of hardly 3% after 1000 h, while that of the latter shows a decrease of about 30%. The quasi solid-state cell shows unfailing durability at 70 °C.

We have fabricated organic photovoltaic devices with blends of poly[9,9′-dioctyl-fluorene- co -bithiophene] (F8T2) and fullerene as an electron donor and electron acceptor, respectively. A significant improvement of the photovoltaic efficiency was found in device by using active materials with complementary spectra. The different weight ratios of composite film were also performed to correlate between morphology and the device performance. A power conversion efficiency (PCE) up to 2.0% with an open-circuit voltage of 0.85 V and a short-circuit current ( J SC ) of 5.3 mA/cm 2 , was achieved by blending the F8T2 with [6,6]-phenyl-C71-butyric acid methyl ester (PC[70]BM).

In this study, binary ionic liquids (bi-IL) of imidazolium salts containing cations with different carbon side chain lengths (C = 2, 4, 6, 8) and anions such as iodide (I − ), tetrafluoroborate (BF 4 − ), hexafluorophosphate (PF 6 − ) and trifluoromethansulfonate (SO 3 CF 3 − ) were used as electrolytes in dye-sensitized solar cells (DSSCs). On increasing the side chain length of imidazolinium salts, the diffusion coefficients of I 3 − and the cell conversion efficiencies decreased; however, the electron lifetimes in TiO 2 electrode increased. As for different anions, the cell which contains 1-butyl-3-methyl imidazolium trifluoromethansulfonate (BMISO 3 CF 3 ) electrolyte has better performance than those containing BMIBF 4 and BMIPF 6 . From the impedance measurement, the cell containing BMISO 3 CF 3 electrolyte has a small charge transfer resistance ( R ct2 ) at the TiO 2 /dye/electrolyte interface. Moreover, the characteristic frequency peak for TiO 2 in the cell based on BMISO 3 CF 3 is less than that of BMIBF 4 and BMIPF 6 , indicating the cell with bi-IL electrolyte based on BMISO 3 CF 3 has higher electron lifetime in TiO 2 electrode. Finally, the solid-state composite was introduced to form solid-state electrolytes for highly efficient DSSCs with a conversion efficiency of 4.83% under illumination of 100 mW cm −2 . The long-term stability of DSSCs with a solidified bi-IL electrolyte containing SiO 2 nanoparticles, which is superior to that of a bi-IL electrolyte alone, was also presented.

Dye-sensitized solar cells (DSSCs) are fabricated using low-cost materials and a simple fabrication process; these advantages make them attractive candidates for research on next generation solar cells. In this type of solar cell, dye-sensitized metal oxide electrodes play an important role for achieving high performance since the porous metal oxide films provide large specific surface area for dye loading and the dye molecule possesses broad absorption covering the visible region or even part of the near-infrared (NIR). Recently, metal-free sensitizers have made great progress and become the most potential alternatives. This review mainly focuses on recent progress in metal-free sensitizers for applications in DSSCs. Besides, we also briefly report DSSCs with near-infrared (NIR) organic sensitizers, which provide the possibility to extend the absorption threshold of the sensitizers in the NIR region. Finally, special consideration has been paid to panchromatic engineering, co-sensitization, a key technique to achieve whole light absorption for improving the performance of DSSCs.