A Thermoplastic Gel Electrolyte for Stable Quasi‐Solid‐State Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) are receiving considerable attention as low‐cost alternatives to conventional solar cells. In DSSCs based on liquid electrolytes, a photoelectric efficiency of 11 % has been achieved, but potential problems in sealing the cells and the low long‐term stability of these systems have impeded their practical use. Here, we present a thermoplastic gel electrolyte (TPGE) as an alternative to the liquid electrolytes used in DSSCs. The TPGE exhibits a thermoplastic character, high conductivity, long‐term stability, and can be prepared by a simple and convenient protocol. The viscosity, conductivity, and phase state of the TPGE can be controlled by tuning the composition. Using 40 wt % poly(ethylene glycol) (PEG) as the polymeric host, 60 wt % propylene carbonate (PC) as the solvent, and 0.65 M KI and 0.065 M I₂ as the ionic conductors, a TPGE with a conductivity of 2.61 mS cm^–2 is prepared. Based on this TPGE, a DSSC is fabricated with an overall light‐to‐electrical‐energy conversion efficiency of 7.22 % under 100 mW cm^–2 irradiation. The present findings should accelerate the widespread use of DSSCs.     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

New Ruthenium Complexes Containing Oligoalkylthiophene‐Substituted 1,10‐Phenanthroline for Nanocrystalline Dye‐Sensitized Solar Cells

Two new ruthenium complexes [Ru(dcbpy)(L)(NCS)₂], where dcbpy is 4,4′‐dicarboxylic acid‐2,2′‐bipyridine and L is 3,8‐bis(4‐octylthiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P1) or 3,8‐bis(4‐octyl‐5‐(4‐octylthiophen‐2‐yl)thiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P2), are synthesized, characterized by physicochemical and semiempirical computational methods, and used as photosensitizers in nanocrystalline dye‐sensitized solar cells. It was found that the difference in light‐harvesting ability between CYC‐P1 and CYC‐P2 is associated mainly with the location of the frontier orbitals, in particular the highest occupied molecular orbital (HOMO). Increasing the conjugation length of the ancillary ligand decreases the energy of the metal‐to‐ligand charge transfer (MLCT) transition, but at the same time reduces the molar absorption coefficient, owing to the HOMO located partially on the ancillary ligand of the ruthenium complex. The incident photon‐to‐current conversion efficiency curves of the devices are consistent with the MLCT band of the complexes. Therefore, the overall efficiencies of CYC‐P1 and CYC‐P2 sensitized cells are 6.01 and 3.42 %, respectively, compared to a cis‐di(thiocyanato)‐bis(2,2′‐bipyridyl)‐4,4′‐dicarboxylate ruthenium(II)‐sensitized device, which is 7.70 % using the same device‐fabrication process and measuring parameters.     

High Molar Extinction Coefficient Ion‐Coordinating Ruthenium Sensitizer for Efficient and Stable Mesoscopic Dye‐Sensitized Solar Cells

Ru(4,4‐dicarboxylic acid‐2,2′‐bipyridine) (4,4′‐bis(2‐(4‐(1,4,7,10‐tetraoxyundecyl)phenyl)ethenyl)‐2,2′‐bipyridine) (NCS)₂, a new high molar extinction coefficient ion‐coordinating ruthenium sensitizer was synthesized and characterized using ¹H NMR, Fourier transform IR (FTIR), and UV/vis spectroscopies and cyclic voltammetry. Using this sensitizer in combination with a nonvolatile organic‐solvent‐based electrolyte, we obtain a photovoltaic efficiency of 8.4 % under standard global AM 1.5 sunlight. These devices exhibit excellent stability when subjected to continuous thermal stress at 80 °C or light soaking at 60 °C for 1000 h. An electrochemical impedance spectroscopy study revealed that device stability is maintained by stabilizing the TiO₂/dye/electrolyte and Pt/electrolyte interface during the aging process. The influence of Li⁺ present in the electrolyte on the device photovoltaic parameters was studied, and the FTIR spectral and photovoltage transient study showed that Li⁺ coordinates to the triethyleneoxide methylether side chains on the K60 sensitizer molecules.     

Parameters Influencing Charge Separation in Solid‐State Dye‐Sensitized Solar Cells Using Novel Hole Conductors

Solid‐state dye‐sensitized solar cells employing a solid organic hole‐transport material (HTM) are currently under intensive investigation, since they offer a number of practical advantages over liquid‐electrolyte junction devices. Of particular importance to the design of such devices is the control of interfacial charge transfer. In this paper, the factors that determine the yield of hole transfer at the dye/HTM interface and its correlation with solid‐state‐cell performance are identified. To this end, a series of novel triarylamine type oligomers, varying in molecular weight and mobility, are studied. Transient absorption spectroscopy is used to determine hole‐transfer yields and pore‐penetration characteristics. No correlation between hole mobility and cell performance is observed. However, it is found that the photocurrent is directly proportional to the hole‐transfer yield. This hole‐transfer yield depends on the extent of pore penetration in the dye‐sensitized film as well as on the thermodynamic driving force ΔG_dye–HTM for interfacial charge transfer. Future design of alternative solid‐state HTMs should focus on the optimization of pore‐filling properties and the control of interfacial energetics rather than on increasing material hole mobilities.     

Room‐Temperature Synthesis of Porous Nanoparticulate TiO2 Films for Flexible Dye‐Sensitized Solar Cells

A novel room‐temperature method for the preparation of porous TiO₂ films with high performance in dye‐sensitized solar cells (DSSCs) has been developed. In this method a small amount of Ti^IV tetraisopropoxide (TTIP) is added to an ethanolic paste of TiO₂ nanoparticles, where it hydrolyzes in situ and connects the TiO₂ particles to form a homogenous and mechanically stable film of up to 10 μm thickness without crack formation. Residual organics originating from the TTIP were removed by UV–ozone treatment of the films, leading to a remarkable improvement of the cell efficiency. Intensity‐modulated photocurrent/voltage spectroscopy (IMPS/IMVS) showed that the main effect of the UV–ozone treatment is to suppress the recombination of photogenerated electrons, thereby extending their lifetime. The efficiency was further increased by preheating the TiO₂ nanoparticles before the paste preparation to remove contaminants originating from the preparation process of the particles. Solar‐to‐electric energy conversion efficiencies of 4.00 and 3.27 % have been achieved for cells with conductive glass and plastic film substrates, respectively, under illumination with AM 1.5 (100 mW cm^–2) simulated sunlight.     

Efficient Dye‐Sensitized Solar Cells with Potential‐Tunable Organic Sulfide Mediators and Graphene‐Modified Carbon Counter Electrodes

A new class of organic sulfide mediators with programmable redox properties is designed via density functional theory calculations and synthesized for efficient dye‐sensitized solar cells (DSCs). Photophysical and electrochemical properties of these mediators derived from systematical functionalization of the framework with electron donating and withdrawing groups (MeO, Me, H, Cl, CF₃, and NO₂) are investigated. With this new class of organic mediators, the redox potential can be fine‐tuned over a 170 mV range, overlapping the conventional I^?/I₃^?couple. Due to the suitable interplay of physical properties and electrochemical characteristics of the mediator involving electron‐donating MeO group, the DSCs based on this mediator behave excellently in various kinetic processes such as dye regeneration, electron recombination, and mass transport. Thus, the MeO derivative of the mediator is identified as having the best performance of this series of redox shuttles. As inferred from electrochemical impedance spectroscopy and cyclic voltammetry measurements, the addition of graphene into the normal carbon counter electrode material dramatically improves the apparent catalytic activity of the counter electrode towards the MeO derivative of mediator, resulting in N719 based DSCs showing a promising conversion efficiency of 6.53% under 100 mW·cm^?2 simulated sunlight illumination.     

Non‐Corrosive,Non‐Absorbing Organic Redox Couple for Dye‐Sensitized Solar Cells

A new colorless electrolyte containing an organic redox couple, tetramethylthiourea (TMTU) and its oxidized dimer tetramethylformaminium disulfide dication ([TMFDS]²⁺), is applied to dye‐sensitized solar cells (DSCs). Advantages of this redox couple include its non‐corrosive nature, low cost, and easy handling. More impressively, it operates well with carbon electrodes. The DSCs fabricated with a lab‐made HCS‐CB carbon counter‐electrode can present up to 3.1% power conversion efficiency under AM 1.5 illumination of 100 mW·cm^?2 and 4.5% under weaker light intensities. This result distinctly outperforms the identical DSCs with a Pt electrode. Corrosive experiments reveal that Al and stainless steel (SS) sheets are stable in the [TMFDS]²⁺/TMTU‐based electrolyte. Its electrochemical impedance spectrum (EIS) is used to evaluate the influence of different counter‐electrodes on the cell performance, and preliminary investigations reveal that carbon electrodes with large surface areas and ideal corrosion‐inertness toward the sulfur‐containing [TMFDS]²⁺/TMTU redox couple exhibit promise for application in iodine‐free DSCs.     

Estimating the Maximum Attainable Efficiency in Dye‐Sensitized Solar Cells

For an ideal solar cell, a maximum solar‐to‐electrical power conversion efficiency of just over 30% is achievable by harvesting UV to near IR photons up to 1.1 eV. Dye‐sensitized solar cells (DSCs) are, however, not ideal. Here, the electrical and optical losses in the dye‐sensitized system are reviewed, and the main losses in potential from the conversion of an absorbed photon at the optical bandgap of the sensitizer to the open‐circuit voltage generated by the solar cell are specifically highlighted. In the first instance, the maximum power conversion efficiency attainable as a function of optical bandgap of the sensitizer and the “loss‐in‐potential” from the optical bandgap to the open‐circuit voltage is estimated. For the best performing DSCs with current technology, the loss‐in‐potential is ～0.75 eV, which leads to a maximum power‐conversion efficiency of 13.4% with an optical bandgap of 1.48 eV (840 nm absorption onset). Means by which the loss‐in‐potential could be reduced to 0.4 eV are discussed; a maximum efficiency of 20.25% with an optical bandgap of 1.31 eV (940 nm) is possible if this is achieved.     

Weakly Conjugated Hybrid Zinc Porphyrin Sensitizers for Solid‐State Dye‐Sensitized Solar Cells

Molecularly engineered weakly conjugated hybrid porphyrin systems are presented as efficient sensitizers for solid‐state dye‐sensitized solar cells. By incorporating the quinolizino acridine and triazatruxene based unit as the secondary light‐harvester as well as electron‐donating group at the meso‐position of the porphyrin core, the power conversion efficiencies of 4.5% and 5.1% are demonstrated in the solid‐state devices containing 2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spiro bifluorene as hole transporting material. The photovoltaic performance of the triazatruxene donor based porphyrin sensitizer is better than that of the previously published porphyrin molecules exhibiting strongly conjugated push–pull structure. The effect of molecular structure on the optical and electrochemical properties, the dynamics of charge extraction, as well as the photovoltaic performance are systematically investigated, which offers a new design strategy for further refinement of porphyrin molecules.     

Significant Improvement of Dye‐Sensitized Solar Cell Performance by Small Structural Modification in π‐Conjugated Donor–Acceptor Dyes

Two donor‐π‐acceptor (D‐π‐A) dyes are synthesized for application in dye‐sensitized solar cells (DSSC). These D‐π‐A sensitizers use triphenylamine as donor, oligothiophene as both donor and π‐bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO₂ surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid‐electrolyte‐based dye‐sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO₂‐photoanode.     

Carbon Counter Electrodes in Dye‐Sensitized and Perovskite Solar Cells

Developing highly effective and stable counter electrode (CE) materials to replace rare and expensive noble metals for dye‐sensitized and perovskite solar cells (DSC and PSC) is a research hotspot. Carbon materials are identified as the most qualified noble metal‐free CEs for the commercialization of the two photovoltaic devices due to their merits of low cost, excellent activity, and superior stability. Herein, carbonaceous CE materials are reviewed extensively with respect to the two devices. For DSC, a classified discussion according to the morphology is presented because electrode properties are closely related to the specific porosity or nanostructure of carbon materials. The pivotal factors influencing the catalytic behavior of carbon CEs are also discussed. For PSC, an overview of the new carbon CE materials is addressed comprehensively. Moreover, the modification techniques to improve the interfacial contact between the perovskite and carbon layers, aiming to enhance the photovoltaic performance, are also demonstrated. Finally, the development directions, main challenges, and coping approaches with respect to the carbon CE in DSC and PSC are stated.     

Universal Electron Injection Dynamics at Nanointerfaces in Dye‐Sensitized Solar Cells

Initial nanointerfacial electron transfer dynamics are studied in dye‐sensitized solar cells (DSSCs) in which the free energy and kinetics vary over a broad range. Surprisingly, it is found that the decay profiles, reflecting the electron transfer behavior, show a universal shape despite the different kinds of dye and semiconductor nanocrystalline films, even across different device types. This renews intuitive knowledge about the electron injection process in DSSCs. In order to quantitatively comprehend the universal behavior, a static inhomogeneous electronic coupling model with a Gaussian distribution of local injection energetics is proposed in which only the electron injection rate is a variant. It is confirmed that this model can be extended to CdSe quantum dot‐sensitized films. These unambiguous results indicate exactly the same physical distribution in electron injection process of different sensitization films, providing limited simple and important parameters describing the electron injection process including electronic coupling constant and reorganization energy. The results provide insight into photoconversion physics and the design of optimal metal‐free organic dye‐sensitized photovoltaic devices by molecular engineering.     

Functional Supramolecular Ruthenium Cyclodextrin Dyes for Nanocrystalline Solar Cells

A supramolecular complex [Ru(dcb)₂(α‐CD‐5‐bpy)]Cl₂ ( 1‐α‐CD ) (dcb = 4,4′‐dicarboxyl‐2,2′‐bipyridine, α‐CD‐5‐bpy = 6‐mono[5‐methyl(5′‐methyl‐2,2′‐bipyridyl)]‐permethylated α‐CD) (CD: cyclodextrin) based on a ruthenium tris‐bipyridyl core with an appended α‐CD cavity is designed and synthesised, in order to facilitate dye/redox couple interaction and dye regeneration in nanocrystalline TiO₂ solar cells. The luminescent complex is fully characterized and anchored on mesoporous titania electrodes showing increased power‐conversion efficiency in solid‐state dye‐sensitized solar cells using a composite polymer electrolyte. Direct comparison of the properties of the CD complex with an analogous ruthenium complex [Ru(dcb)₂(5,5′‐dmbpy)]Cl₂ ( 2 ) (5,5′‐dmbpy = 5,5′‐dimethylbipyridine) without the CD cavity reveals that the photovoltaic performance of 1‐α‐CD is enhanced by about 40 % compared to 2 . Independent studies have shown complexation of the iodide redox couple to the CD in 1‐α‐CD . These results indicate that the CD moiety is able to act as a mediator and fine tune the photoelectrode/electrolyte interface.     

A Hybrid Poly(ethylene oxide)/ Poly(vinylidene fluoride)/TiO2 Nanoparticle Solid‐State Redox Electrolyte for Dye‐Sensitized Nanocrystalline Solar Cells

High‐efficiency all‐solid‐state dye‐sensitized nanocrystalline solar cells have been fabricated using a poly(ethylene oxide)/poly(vinylidene fluoride) (PEO/PVDF)/TiO₂‐nanoparticle polymer redox electrolyte, which yields an overall energy‐conversion efficiency of about 4.8 % under irradiation by white light (65.2 mW cm^–2). The introduction of PVDF (which contains the highly electronegative element fluorine) and TiO₂ nanoparticles into the PEO electrolyte increases the ionic conductivity (by about two orders of magnitude) and effectively reduces the recombination rate at the interface of the TiO₂ and the solid‐state electrolyte, thus enhancing the performance of the solar cell.     

Nanoparticle/Dye Interface Optimization in Dye‐Sensitized Solar Cells

A critical component in the development of highly efficient dye‐sensitized solar cells is the interface between the ruthenium bipyridyl complex dye and the surface of the mesoporous titanium dioxide film. In spite of many studies aimed at examining the detailed anchoring mechanism of the dye on the titania surface, there is as yet no commonly accepted understanding. Furthermore, it is generally believed that a single monolayer of strongly attached molecules is required in order to maximize the efficiency of electron injection into the semiconductor. In this study, the amount of adsorbed dye on the mesoporous film is maximised, which in turn increases the light absorption and decreases carrier recombination, resulting in improved device performance. A process that increases the surface concentration of the dye molecules adsorbed on the TiO₂ surface by up to 20% is developed. This process is based on partial desorption of the dye after the initial adsorption, followed by readsorption. This desorption/adsorption cycling process can be repeated multiple times and yields a continual increase in dye uptake, up to a saturation limit. The effect on device performance is directly related and a 23% increase in power conversion efficiency is observed. Surface enhanced Raman spectroscopy, infrared spectroscopy, and electrochemical impedance analysis were used to elucidate the fundamental mechanisms behind this observation.     

Highly Efficient Solid‐State Dye‐Sensitized Solar Cells Based on Triphenylamine Dyes

Two triphenylamine‐based metal‐free organic sensitizers, D35 with a single anchor group and M14 with two anchor groups, have been applied in dye‐sensitized solar cells (DSCs) with a solid hole transporting material or liquid iodide/triiodide based electrolyte. Using the molecular hole conductor 2,2',7,7'‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)9,9'‐spirobifluorene (spiro‐OMeTAD), good overall conversion efficiencies of 4.5% for D35 and 4.4% for M14 were obtained under standard AM 1.5G illumination (100 mW cm^?2). Although M14 has a higher molar extinction coefficient (by ～ 60%) and a slightly broader absorption spectrum compared to D35 , the latter performs slightly better due to longer lifetime of electrons in the TiO₂, which can be attributed to differences in the molecular structure. In iodide/triiodide electrolyte‐based DSCs, D35 outperforms M14 to a much greater extent, due to a very large increase in electron lifetime. This can be explained by both the greater blocking capability of the D35 monolayer and the smaller degree of interaction of triiodide (iodine) with D35 compared to M14 . The present work gives some insight into how the molecular structure of sensitizer affects the performance in solid‐state and iodide/triiodide‐based DSCs.     

Amplifying Charge‐Transfer Characteristics of Graphene for Triiodide Reduction in Dye‐Sensitized Solar Cells

The fabrication and functionalization of large‐area graphene and its electrocatalytic properties for iodine reduction in a dye‐sensitized solar cell are reported. The graphene film, grown by thermal chemical vapor deposition, contains three to five layers of monolayer graphene, as confirmed by Raman spectroscopy and high‐resolution transmission electron microscopy. Further, the graphene film is treated with CF₄ reactive‐ion plasma and fluorine ions are successfully doped into graphene as confirmed by X‐ray photoelectron spectroscopy and UV‐photoemission spectroscopy. The fluorinated graphene shows no structural deformations compared to the pristine graphene except an increase in surface roughness. Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction increases with increasing plasma treatment time, which is attributed to an increase in catalytic sites. Further, the fluorinated graphene is characterized in use as a counter‐electrode in a full dye‐sensitized solar cell and shows ca. 2.56% photon to electron conversion efficiency with ca. 11 mA cm^?2 current density. The shift in work function in F^? doped graphene is attributed to the shift in graphene redox potential which results in graphene's electrocatalytic‐activity enhancement.     

Compact Inverse‐Opal Electrode Using Non‐Aggregated TiO2 Nanoparticles for Dye‐Sensitized Solar Cells

Compact inverse‐opal structures are constructed using non‐aggregated TiO₂ nanoparticles in a three‐dimensional colloidal array template as the photoelectrode of a dye‐sensitized solar cell. Organic‐layer‐coated titania nanoparticles show an enhanced infiltration and a compact packing within the 3D array. Subsequent thermal decomposition to remove the organic template followed by impregnation with N‐719 dye results in excellent inverse‐opal photoelectrodes with a photo‐conversion efficiency as high as 3.47% under air mass 1.5 illumination. This colloidal‐template approach using non‐aggregated nanoparticles provides a simple and versatile way to produce efficient inverse‐opal structures with the ability to control parameters such as cavity diameter and film thickness.     

Metal‐Free Sensitizers Containing Hydantoin Acceptor as High Performance Anchoring Group for Dye‐Sensitized Solar Cells

The anchoring group in dye‐sensitized solar cells (DSSCs) profoundly affects the electron injection and durability on TiO₂ films interface. Here, the hydantoin acceptor is introduced as anchoring group for DSSCs. The hydantoin based sensitizer achieves a photovoltaic efficiency of 7.66%, compared to 4.90% for sensitizer containing the conventional cyanoacrylic acid as anchoring group. Remarkably, the hydantoin anchoring group significantly enhances the electron‐injection efficiency (Φ_inj) and photocurrent (J_sc). The time dependent adsorption and desorption data indicate the strong binding strength and the superiority of stability for hydantoin based sensitizers. The Fourier transform infrared measurements investigate the adsorption mechanism of hydantoin on TiO₂ interface. These results strongly corroborate the advantages of incorporating hydantoin as acceptor and anchoring group. As a consequence, the sensitizer HY‐4 with hydantoin approaches the photovoltaic efficiency of 8.32% under 0.1 sunlight illumination. These observations offer a new route to design and develop efficient sensitizers for DSSCs.