Unsymmetric Platinum(II) Bis(aryleneethynylene) Complexes as Photosensitizers for Dye‐Sensitized Solar Cells

Four new unsymmetric platinum(II) bis(aryleneethynylene) derivatives have been designed and synthesized, which showed good light‐harvesting capabilities for application as photosensitizers in dye‐sensitized solar cells (DSSCs). The absorption, electrochemical, time‐dependent density functional theory (TD‐DFT), impedance spectroscopic, and photovoltaic properties of these platinum(II)‐based sensitizers have been fully characterized. The optical and TD‐DFT studies show that the incorporation of a strongly electron‐donating group significantly enhances the absorption abilities of the complexes. The maximum absorption wavelength of these four organometallic dyes can be tuned by various structural modifications of the triphenylamine and/or thiophene electron donor, improving the light absorption range up to 650 nm. The photovoltaic performance of these dyes as photosensitizers in mesoporous TiO₂ solar cells was investigated, and a power conversion efficiency as high as 1.57 % was achieved, with an open‐circuit voltage of 0.59 V, short‐circuit current density of 3.63 mA cm^?2, and fill factor of 0.73 under simulated AM 1.5G solar illumination.     

Application of the Organic Photosensitizers Bearing Two Carboxylic Acid Groups to Dye‐Sensitized Solar Cells

Three electron donor‐?? bridge‐electron acceptor (D‐π‐A) organic dyes bearing two carboxylic acid groups were applied to dye‐sensitized solar cells (DSSC) as sensitizers, in which one triphenylamine or modified triphenylamine and two rhodanine‐3‐acetic acid fragments act as D and A, respectively. It was found that the introduction of t‐butyl or methoxy group in the triphenylamine subunit could lead to more efficient photoinduced intramolecular charge transfer, thus improving the overall photoelectric conversion efficiency of the resultant DSSC. Under global AM 1.5 solar irradiation (73 mW·cm^?2), the dye molecule based on methoxy‐substituted triphenylamine achieved the best photovoltaic performance: a short circuit photocurrent density (J_sc) of 12.63 mA·cm^?2, an open circuit voltage (V_oc) of 0.55 V, a fill factor (FF) of 0.62, corresponding to an overall efficiency (η) of 5.9%.     

Sonochemical Preparation of Hierarchical ZnO Hollow Spheres for Efficient Dye‐Sensitized Solar Cells

Hierarchical ZnO hollow spheres (400–500 nm in diameter) consisting of ZnO nanoparticles with a diameter of approximately 15 nm have been successfully prepared by a facile and rapid sonochemical process. The formation of hierarchical ZnO hollow spheres is attributed to the oriented attachment and subsequent Ostwald ripening process according to time‐dependent experiments. The as‐prepared ZnO hollow spheres are used as a photoanode in dye‐sensitized solar cells and exhibit a highly efficient power conversion efficiency of 4.33 %, with a short‐circuit current density of 9.56 mA cm^?2, an open‐circuit voltage of 730 mV, and a fill factor of 0.62 under AM 1.5 G one sun (100 mW cm^?2) illumination. Moreover, the photovoltaic performance (4.33 %) using the hierarchical ZnO hollow spheres is 38.8 % better than that of a ZnO nanoparticle photoelectrode (3.12 %), which is mainly attributed to the efficient light scattering for the former.     

Opp‐Dibenzoporphyrins as a Light‐Harvester for Dye‐Sensitized Solar Cells

New opp‐dibenzoporphyrins were prepared in a concise method that was based on a Pd⁰‐catalyzed cascade reaction. These porphyrins, which contained carboxylic‐acid linker groups on benzene rings that were fused to the porphyrin at their β,β′‐positions, were examined as sensitizers for dye‐sensitized solar cells for the first time. Whereas all of the porphyrins showed solar‐energy‐to‐electricity conversion, an opp‐dibenzoporphyrin with conjugated carboxylic‐acid linkers displayed the highest conversion efficiency and an exceptionally high J_sc value. Cyclic voltammetry of these porphyrins suggested that the fusion of two aromatic benzene rings onto the periphery of the porphyrin lowered the HOMO–LUMO energy gap; the incorporation of a conjugated carboxylic‐acid linker group decreased the HOMO–LUMO gap even further. These CV data are consistent with DFT calculations for these porphyrins and agree well with the UV/Vis absorption‐ and fluorescence spectra of these porphyrins.     

Platinum‐Free Binary Co‐Ni Alloy Counter Electrodes for Efficient Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) have attracted growing interest because of their application in renewable energy technologies in developing modern low‐carbon economies. However, the commercial application of DSSCs has been hindered by the high expenses of platinum (Pt) counter electrodes (CEs). Here we use Pt‐free binary Co‐Ni alloys synthesized by a mild hydrothermal strategy as CE materials in efficient DSSCs. As a result of the rapid charge transfer, good electrical conduction, and reasonable electrocatalysis, the power conversion efficiencies of Co‐Ni‐based DSSCs are higher than those of Pt‐only CEs, and the fabrication expense is markedly reduced. The DSSCs based on a CoNi_0.25 alloy CE displays an impressive power conversion efficiency of 8.39 %, fast start‐up, multiple start/stop cycling, and good stability under extended irradiation.     

Structural Control of Hierarchically‐Ordered TiO2 Films by Water for Dye‐Sensitized Solar Cells

A facile way of controlling the structure of TiO₂ by changing the amount of water to improve the efficiency of dye‐sensitized solar cells (DSSCs) is reported. Hierarchically ordered TiO₂ films with high porosity and good interconnectivity are synthesized in a well‐defined morphological confinement arising from a one‐step self‐assembly of preformed TiO₂ (pre‐TiO₂) nanocrystals and a graft copolymer, namely poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate). The polymer–solvent interactions in solution, which are tuned by the amount of water, are shown to be a decisive factor in determining TiO₂ morphology and device performance. Systematic control of wall and pore size is achieved and enables the bifunctionality of excellent light scattering properties and easy electron transport through the film. These properties are characterized by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency, and electrochemical impedance spectroscopy analyses. The TiO₂ photoanode that is prepared with a higher water ratio, [pre‐TiO₂]:[H₂O]=1:0.3, shows a larger surface area, greater light scattering, and better electron transport, which result in a high efficiency (7.7 %) DSSC with a solid polymerized ionic liquid. This efficiency is much greater than that of commercially available TiO₂ paste (4.0 %).     

Highly Catalytic Carbon Nanotube/Pt Nanohybrid‐Based Transparent Counter Electrode for Efficient Dye‐Sensitized Solar Cells

Low‐cost transparent counter electrodes (CEs) for efficient dye‐sensitized solar cells (DSSCs) are prepared by using nanohybrids of carbon nanotube (CNT)‐supported platinum nanoparticles as highly active catalysts. The nanohybrids, synthesized by an ionic‐liquid‐assisted sonochemical method, are directly deposited on either rigid glass or flexible plastic substrates by a facile electrospray method for operation as CEs. Their electrochemical performances are examined by cyclic voltammetry, current density–voltage characteristics, and electrochemical impedance spectroscopy (EIS) measurements. The CNT/Pt hybrid films exhibit high electrocatalytic activity for I^?/I₃^? with a weak dependence on film thickness. A transparent CNT/Pt hybrid CE film about 100 nm thick with a transparency of about 70 % (at 550 nm) can result in a high power conversion efficiency (η) of over 8.5 %, which is comparable to that of pyrolysis platinum‐based DSSCs, but lower cost. Furthermore, DSSC based on flexible CNT/Pt hybrid CE using indium‐doped tin oxide‐coated polyethylene terephthalate as the substrate also exhibits η=8.43 % with J_sc=16.85 mA cm^?2, V_oc=780 mV, and FF=0.64, and this shows great potential in developing highly efficient flexible DSSCs.     

Photophysical and Electrochemical Properties,and Molecular Structures of Organic Dyes for Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO₂, ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident‐solar‐light‐to‐electricity conversion efficiency and low cost of production. To develop high‐performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light‐harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch‐making molecular design of organic dyes for high photovoltaic performance and long‐term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.     

Computational Design of Cyclic Nitroxides as Efficient Redox Mediators for Dye‐Sensitized Solar Cells

Cyclic nitroxide radicals represent promising alternatives to the iodine‐based redox mediator commonly used in dye‐sensitized solar cells (DSSCs). To date DSSCs with nitroxide‐based redox mediators have achieved energy conversion efficiencies of just over 5 % but efficiencies of over 15 % might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one‐electron oxidation and it must possess an oxidation potential in a range of 0.600–0.850 V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25 °C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N‐containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl‐, methoxy‐, amino‐, carboxy‐, etc.) to the ring. Standard oxidation potentials were calculated using high‐level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16 mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired ${E{{{\circ}\hfill \atop {\rm ox}\hfill}}}$ , and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs.     

Efficient Counter Electrode Manufactured from Ag2S Nanocrystal Ink for Dye‐Sensitized Solar Cells

It is generally believed that silver or silver‐based compounds are not suitable counter electrode (CE) materials for dye‐sensitized solar cells (DSSCs) due to the corrosion of the I^?/I₃^? redox couple in electrolytes. However, Ag₂S has potential applications in DSSCs for catalyzing I₃^? reduction reactions because of its high carrier concentration and tiny solubility product constant. In the present work, CE manufactured from Ag₂S nanocrystals ink exhibited efficient electrocatalytic activity in the reduction of I₃^? to I^? in DSSCs. The DSSC consisting of Ag₂S CE displayed a higher power conversion efficiency of 8.40 % than that of Pt CE (8.11 %). Moreover, the devices also showed the characteristics of fast activity onset, high multiple start/stop capability and good irradiated stability. The simple composition, easy preparation, stable chemical property, and good catalytic performance make the developed Ag₂S CE as a promising alternative to Pt CE in DSSCs.     

Thieno[3,4‐b]thiophene‐Based Organic Dyes for Dye‐Sensitized Solar Cells

New dipolar sensitizers containing an ethyl thieno[3,4‐b]thiophene‐2‐carboxylate (ETTC) entity in the conjugated spacer have been synthesized in two isomeric forms. These compounds were used as the sensitizers of n‐type dye‐sensitized solar cells (DSSCs). The best conversion efficiency (5.31 %) reaches approximately 70 % of the N719‐based (7.41 %) DSSC fabricated and measured under similar conditions. The ETTC‐containing compounds exhibit a bathochromic shift of the absorption compared to their thiophene congeners due to the quinoid effect, however, charge‐trapping at the ester group of ETTC was found to jeopardize the electron injection and lower the cell efficiency. Charge trapping is alleviated as the ester group of ETTC is replaced with a hydrogen atom, as evidenced from the theoretical computation.     

Metal‐Free Organic Dyes for Dye‐Sensitized Solar Cells: From Structure: Property Relationships to Design Rules

Works without ruthenium as well : Dye‐sensitized solar cells (DSSCs) incorporating metal‐free organic dyes have been considerably improved in recent years. Various design strategies have been established and are employed successfully in the synthesis of novel sensitizers. In this Review, structure–property–efficiency correlations are deduced from a vast number of dyes, which should help to design new and highly efficient sensitizers.