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.     

Interface Engineering for Solid‐State Dye‐Sensitized Nanocrystalline Solar Cells: The Use of Ion‐Solvating Hole‐Transporting Polymers

The control of interfacial charge transfer is central to the design of photovoltaic devices. This charge transfer is strongly dependent upon the local chemical environment at each interface. In this paper we report a methodology for the fabrication of a novel nanostructured multicomponent film, employing a dual‐function supramolecular organic semiconductor to allow molecular‐level control of the local chemical composition at a nanostructured inorganic/organic semiconductor heterojunction. The multicomponent film comprises a lithium ion doped dual‐functional hole‐transporting material (Li⁺–DFHTM), sandwiched between a dye‐sensitized nanocrystalline TiO₂ film and a mono‐functional organic hole‐transporting material (MFHTM). The DFHTM consists of a conjugated organic semiconductor with ion supporting side chains, designed to allow both electronic and ionic charge transport properties. The Li⁺–DFHTM layers provide a new and versatile way to control the interface electrostatics, and consequently the charge transfer, at a nanostructured dye‐sensitized inorganic/organic semiconductor heterojunction.     

Novel Conjugated Organic Dyes for Efficient Dye‐Sensitized Solar Cells

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO₂ solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm^–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (J_sc) = 12.9 mA cm^–2, open‐circuit voltage (V_oc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO₂ occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO₂ surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.     

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.     

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.     

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.     

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

An iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO₂ interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO₂ (OM‐TiO₂) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO₂ film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO₂ film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO₂/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO₂ film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance ( Ω ), and enhanced electron lifetime ( τ ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO₂ film interface are crucial for obtaining high‐energy conversion efficiencies in I₂‐free ssDSSCs.     

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.     

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.     

High Efficiency Solid‐State Dye‐Sensitized Solar Cells Assembled with Hierarchical Anatase Pine Tree‐like TiO2 Nanotubes

A facile and effective method to prepare hierarchical pine tree‐like TiO₂ nanotube (PTT) arrays with an anatase phase directly grown on a transparent conducting oxide substrate via a one‐step hydrothermal reaction. The PTT arrays consist of a vertically oriented long nanotube (NT) stem and a large number of short nanorod (NR) branches. Various PTT morphologies are obtained by adjusting the water/diethylene glycol ratio. The diameter of the NTs and the size of the NR branches decreases from 300 to100 nm and from 430 to 230 nm, respectively, with increasing water content. The length of the PTT arrays could be increased up to 19 μm to significantly improve the charge transport and specific surface area. The solid‐state dye‐sensitized solar cells (ssDSSC) assembled with the 19 μm long PTT arrays exhibit an outstanding energy‐conversion efficiency of 8.0% at 100 mW/cm², which is two‐fold higher than that of commercially available paste (4.0%) and one of the highest values obtained for N719 dye‐based ssDSSCs. The high performance is attributed to the larger surface area, improved electron transport, and reduced electrolyte/electrode interfacial resistance, resulting from the one‐dimensional, well‐aligned structure with a high porosity and large pores.     

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.     

Enhancing the Performance of Solid‐State Dye‐Sensitized Solar Cells Using a Mesoporous Interfacial Titania Layer with a Bragg Stack

High efficiency dye‐sensitized solar cells (DSSCs) are fabricated with a heterostructured photoanode that consists of a 500‐nm‐thick organized mesoporous TiO₂ (om‐TiO₂) interfacial layer (IF layer), a 7 or 10‐μm thick nanocrystalline TiO₂ layer (NC layer), and a 2‐μm‐thick mesoporous Bragg stack (meso‐BS layer) as the bottom, middle and top layers, respectively. An om‐TiO₂ layer with a high porosity, transmittance, and interconnectivity is prepared via a sol‐gel process, in which a poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer is used as a structure‐directing agent. The meso‐BS layer with large pores is prepared via alternating deposition of om‐TiO₂ and colloidal SiO₂ (col‐SiO₂) layers. Structure and optical properties (refractive index) of the om‐TiO₂ and meso‐BS layers are studied and the morphology of the heterostructured photoanode is characterized. DSSCs fabricated with the heterostructured IF/NC/BS photoanode and combined with a polymerized ionic liquid (PIL) exhibit an energy conversion efficiencies of 6.6% at 100 mW/cm², one of the highest reported for solid‐state DSSCs and much larger than cells prepared with only a IF/NC layer (6.0%) or a NC layer (4.5%). Improvements in energy conversion efficiency are attributed to the combination of improved light harvesting, decreased resistance at the electrode/electrolyte interface, and excellent electrolyte infiltration.     

Honeycomb‐Like Organized TiO2 Photoanodes with Dual Pores for Solid‐State Dye‐Sensitized Solar Cells

A solid‐state dye‐sensitized solar cell (ssDSSC) with 7.4% efficiency at 100 mW/cm² is reported. This efficiency is one of the highest observed for N719 dye. High performance is achieved via a honeycomb‐like, organized mesoporous TiO₂ photoanode with dual pores, high porosity, good interconnectivity, and excellent light scattering properties. The TiO₂ photoanodes are prepared without any TiCl₄ treatment via a one‐step, direct self‐assembly of hydrophilically preformed TiO₂ nanocrystals and poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer as a titania source and a structure‐directing agent, respectively. Upon controlling the secondary forces between the polymer/TiO₂ hybrid and the solvent by varying the amounts of HCl/H₂O mixture or toluene, honeycomb‐like structures are generated to improve light scattering properties. Such multifunctional nanostructures with dual pores provide good pore‐filling of solid polymer electrolyte with large volume, enhanced light harvesting and reduced charge recombination, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) analysis.     

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.     

Influence of Ion Induced Local Coulomb Field and Polarity on Charge Generation and Efficiency in Poly(3‐Hexylthiophene)‐Based Solid‐State Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSC) are a realistic option for converting light to electrical energy. Hybrid architectures offer a vast materials library for device optimization, including a variety of metal oxides, organic and inorganic sensitizers, molecular, polymeric and electrolytic hole‐transporter materials. In order to further improve the efficiency of solid‐state dye‐sensitized solar cells, recent attention has focused on using light absorbing polymers such as poly(3‐hexylthiophene) (P3HT), to replace the more commonly used “transparent” 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)9,9′spiro‐bifluorene (spiro‐OMeTAD), in order to enhance the light absorption within thin films. As is the case with spiro‐OMeTAD based solid‐state DSSC, the P3HT‐based devices improve significantly with the addition of lithium bis(trifluoromethylsulfonyl)imide salts (Li‐TFSI), although the precise role of these additives has not yet been clarified in solid‐state DSCs. Here, we present a thorough study on the effect of Li‐TFSI in P3HT based solid‐state DSSC incorporating an indolene‐based organic sensitizer termed D102. Employing ultrafast transient absorption and cw‐emission spectroscopy together with electronic measurements, we demonstrate a fine tuning of the energetic landscape of the active cell components by the local Coulomb field induced by the ions. This increases the charge transfer nature of the excited state on the dye, significantly accelerating electron injection into the TiO₂. We demonstrate that this ionic influence on the excited state energy is the primary reason for enhanced charge generation with the addition of ionic additives. The deepening of the relative position of the TiO₂ conduction band, which has previously been thought to be the cause for enhanced charge generation in dye sensitized solar cells with the addition of lithium salts, appears to be of minor importance in this system.     

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.