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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Hierarchical Double‐Shell Nanostructures of TiO2 Nanosheets on SnO2 Hollow Spheres for High‐Efficiency,Solid‐State,Dye‐Sensitized Solar Cells

A high‐energy conversion efficiency of 8.2% at 100 mW cm^?2 is reported, one of the highest values for N719‐based, solid‐state, dye‐sensitized solar cells (ssDSSCs). The solar cells are based on hierarchical double‐shell nanostructures consisting of inner SnO₂ hollow spheres (SHS) surrounded by outer TiO₂ nanosheets (TNSs). Deposition of the TNS on the SHS outer surface is performed via solvothermal reactions in order to generate a double‐shell SHS@TNS nanostructure that provides a large surface area and suppresses recombination of photogenerated electrons. An organized mesoporous (OM)‐TiO₂ film with high porosity, large pores, and good interconnectivity is also prepared via a sol‐gel process using a poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer template. This film is utilized as a matrix to disperse the double‐shell nanostructures. Such nanostructures provide good pore‐filling for solid polymer electrolytes, faster electron transfer, and enhanced light scattering, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and intensity‐modulated photocurrent spectroscopy (IMPS)/intensity‐modulated photovoltage spectroscopy (IMVS).     

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.     

Dependence of Dye Regeneration and Charge Collection on the Pore‐Filling Fraction in Solid‐State Dye‐Sensitized Solar Cells

Solid‐state dye‐sensitized solar cells rely on effective infiltration of a solid‐state hole‐transporting material into the pores of a nanoporous TiO₂ network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole‐transfer yield from the dye to the hole‐transporting material 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9'‐spirobifluorene (spiro‐OMeTAD) is shown to rise rapidly with higher pore‐filling fractions as the dye‐coated pore surface is increasingly covered with hole‐transporting material. Once a pore‐filling fraction of ≈30% is reached, further increases do not significantly change the hole‐transfer yield. Using simple models of infiltration of spiro‐OMeTAD into the TiO₂ porous network, it is shown that this pore‐filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole‐transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole‐transporting material. Comparison of these results with device parameters shows that improvements of the power‐conversion efficiency beyond ≈30% pore filling are not caused by a higher hole‐transfer yield, but by a higher charge‐collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power‐conversion efficiencies with increasing pore‐filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro‐OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO₂ with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore‐filling fraction.     

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.     

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.     

Transparent,Double‐Sided,ITO‐Free,Flexible Dye‐Sensitized Solar Cells Based on Metal Wire/ZnO Nanowire Arrays

Transparent, double‐sided, flexible, ITO‐free dye‐sensitized solar cells (DSSCs) are fabricated in a simple, facile, and controllable way. Highly ordered, high‐crystal‐quality, high‐density ZnO nanowire arrays are radially grown on stainless steel, Au, Ag, and Cu microwires, which serve as working electrodes. Pt wires serve as the counter electrodes. Two metal wires are encased in electrolyte between two poly(ethylene terephthalate) (PET) films (or polydimethylsiloxane (PDMS) films) to render the device both flexible and highly transparent. The effect of the dye thickness on the photovoltaic performance of the DSSCs as a function of dye‐loading time is investigated systematically. Shorter dye‐loading times lead to thinner dye layers and better device performance. A dye‐loading time of 20 min results in the best device performance. An oxidation treatment of the metal wires is developed effectively to avoid the galvanic‐battery effect found in the experiment, which is crucial for real applications of double‐metal‐wire DSSC configurations. The device shows very good transparency and can increase sunlight use efficiency through two‐sided illumination. The double‐wire DSSCs remain stable for a long period of time and can be bent at large angles, up to 107°, reversibly, without any loss of performance. The double‐wire‐PET, planar solar‐cell configuration can be used as window stickers and can be readily realized for large‐area‐weave roll‐to‐roll processing.     

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.     

Hybrid Templated Synthesis of Crack‐Free,Organized Mesoporous TiO2 Electrodes for High Efficiency Solid‐State Dye‐Sensitized Solar Cells

Organic/inorganic hybrid templates, i.e., aluminium oxide (Al₂O₃) nanoparticles grafted with poly(oxyethylene) methacrylate, Al₂O₃‐POEM, are synthesized via surface‐initiated atom transfer radical polymerization (ATRP), as confirmed by Fourier transform‐infrared spectroscopy (FT‐IR) and thermogravimetric analysis (TGA). Upon combining the Al₂O₃‐POEM with titanium(IV) isopropoxide (TTIP), hydrophilic TTIP is selectively confined in the hydrophilic POEM chains through hydrogen bonding interactions. Following the calcination at 450 °C and the selective etching of Al₂O₃ with NaOH, the OM‐TiO₂ films with high surface areas, good interconnectivity, and anatase phase are obtained. The solid‐state dye‐sensitized solar cells (ssDSSCs) fabricated with OM‐TiO₂ photoelectrodes and a polymerized ionic liquid (PIL) show a high energy conversion efficiency of 7.3% at 100 mW cm^?2, which is one of the highest values for ssDSSCs. The high cell performance is due to the well‐organized structure, resulting in improved dye loading, excellent pore filling of electrolyte, enhanced light harvesting, and reduced charge recombination.