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.     

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.     

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.     

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.     

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.     

A New Ionic Liquid for a Redox Electrolyte of Dye‐Sensitized Solar Cells

A new ionic liquid, 1‐vinyl‐3‐heptylimidazolium iodide (VHpII), was synthesized and applied as a redox electrolyte for dye‐sensitized solar cells. The chemical structure of the synthesized VHpII was confirmed using ¹H NMR. Thermogravimetric analysis showed that the VHpII was stable for thermal stress of up to 250°C. The energy conversion efficiencies of the VHpII‐based dye‐sensitized solar cells were investigated in terms of the effect of a lithium iodide addition. A solar cell containing the redox couple of VHpII and iodine showed a conversion efficiency of 2.63% under 1 sun light intensity at AM 1.5. Adding 0.4 M LiI results in a conversion efficiency of 3.63%, which was an improvement of about 40%. The increased conversion efficiency was ascribed to an increase in external quantum efficiency.     

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.     

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.     

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.     

Novel Near‐Infrared Squaraine Sensitizers for Stable and Efficient Dye‐Sensitized Solar Cells

A new molecular design strategy for tuning the energy levels of cis‐configured squaraine sensitizers for dye‐sensitized solar cells is described. The Hammett substituent constant and the π‐conjugation length are used as quantitative indicators to modify the central squarate moiety of the sensitizer dyes; specifically, novel near‐infrared squaraine dyes HSQ3 and HSQ4 are synthesized by incorporation of an electron‐withdrawing and π‐extending ethyl cyanoacetate unit on the central squarate moiety. The solution absorption maximum of HSQ4 occurs at 703 nm, and the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital are in the ideal range for energetically efficient electron injection and regeneration of the oxidized dye. A solar cell sensitized with HSQ4 exhibits a broad incident photo­n‐to‐current conversion efficiency spectrum, extending into the near‐infrared region with a maximum value of 80% at 720 nm, which is is the highest value reported for a squaraine dye–based dye‐sensitized solar cell. The HSQ4‐sensitized solar cell also exhibits excellent durability during light soaking, owing to the double anchors attaching the dye to the TiO₂ surface and to the long alkyl chains extending outward from the surface.     

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.     

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.     

Solvent‐Free Ionic Liquid Electrolytes for Mesoscopic Dye‐Sensitized Solar Cells

Ionic liquids have been identified as a new class of solvent that offers opportunities to move away from the traditional solvents. The physical‐chemical properties of ionic liquids can be tuned and controlled by tailoring their structures. The typical properties of ionic liquids, such as non‐volatility, electrochemical stability and high conductivity, render them attractive as electrolytes for dye‐sensitized solar cells. However, the high viscosity of ionic liquids leads to mass transport limitations on the photocurrents in the solar cells at full sunlight intensity, but the contribution of a Grotthous‐type exchange mechanism in these viscous electrolytes helps to alleviate these diffusion problems. This article discusses recent developments in the field of high‐performance dye‐sensitized solar cells with ionic liquid‐based electrolytes and their characterization by electrochemical impedance analysis.     

p–n‐Junction‐Based Flexible Dye‐Sensitized Solar Cells

In this paper, a new type of flexible working electrode, TiO₂/CuI/Cu, is reported, in which the p–n junction of TiO₂–CuI is introduced into dye‐sensitized solar cells (DSSCs) for the first time. The devices give a high conversion efficiency of up to 4.73% under 1 sun illumination. The excellent performance is ascribed to the existence of the p–n junction, which forms a single directional pathway for electron transport which benefits the charge separation, and improves the efficiency of the flexible solar cells as a result.     

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.     

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.     

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.     

Structure–Property Relations in All‐Organic Dye‐Sensitized Solar Cells

A structure–property relationship in all‐organic dye solar cells is revealed by first‐principles molecular dynamics and real‐time time‐dependent density functional theory simulations, accompanied with experimental confirmation. An important structural feature at the interface, Ti–N anchoring, for a broad group of all‐organic dyes on TiO₂ is inferred from energetics, vibrational recognition, and electronic data. This fact is contrary to the usual assumption; however, it optimizes electronic level alignment and photoelectron injection dynamics, greatly contributing to the observed efficiency improvement in all‐organic cyanoacrylate dye sensitized solar cells.     

Band‐Edge Engineered Hybrid Structures for Dye‐Sensitized Solar Cells Based on SnO2 Nanowires

In this report, we show for the first time that SnO₂ nanowire based dye sensitized solar cells exhibit an open circuit voltage of 560 mV, which is 200 mV higher than that using SnO₂ nanoparticle based cells. This is attributed to the more negative flat band potential of nanowires compared to the nanoparticles as determined by open circuit photo voltage measurements made at high light intensities. The nanowires were employed in hybrid structures consisting of highly interconnected SnO₂ nanowire matrix coated with TiO₂ nanoparticles, which showed an open circuit voltage of 720 mV and an efficiency of 4.1% compared to 2.1% obtained with pure SnO₂ nanowire matrix. The electron transport time constants for SnO₂ nanowire matrix were an order of magnitude lower and the recombination time constants are about 100 times higher than that of TiO₂ nanoparticles. The higher efficiency observed for DSSCs based on hybrid structure is attributed to the band edge positions of SnO₂ relative to that of TiO₂ and faster electron transport in SnO₂ nanowires.