Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt‐Electrolyte‐Based Dye‐Sensitized Solar Cells

Novel phenoxazine dyes are successfully introduced as sensitizers into dye‐sensitized solar cells (DSCs) with cobalt‐based electrolyte. In sensitizers with triphenylamine (TPA) groups recombination from electrons in the TiO₂ conduction band to the cobalt(III) species is suppressed. The effect of the steric properties of the phenoxazine sensitizers on the overall device performance and on recombination and regeneration processes is compared. Optimized DSCs sensitized with IB2 having two TPA groups in combination with tris(2,2′‐bipyridyl) cobalt(II/III) yield efficiencies of 6.3 %, similar to that of IB3 , which is equipped with mutiple alkoxy groups. TH310 with only one TPA group gives lower efficiency and open circuit voltage, while IB1 without TPA groups performs even worse. These results demonstrate that both TPA groups on the IB2 are needed for an efficient blocking effect. These results reveal a possible new role for TPA units in DSC sensitizer design.     

Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

The use of renewable energy is essential for the future of the Earth, and solar photons are the ultimate source of energy to satisfy the ever-increasing global energy demands. Photoconversion using dye-sensitized solar cells (DSCs) is becoming an established technology to contribute to the sustainable energy market, and among state-of-the art DSCs are those which rely on ruthenium(ii) sensitizers and the triiodide/iodide (I₃⁻/I⁻) redox mediator. Ruthenium is a critical raw material, and in this review, we focus on the use of coordination complexes of the more abundant first row d-block metals, in particular copper, iron and zinc, as dyes in DSCs. A major challenge in these DSCs is an enhancement of their photoconversion efficiencies (PCEs) which currently lag significantly behind those containing ruthenium-based dyes. The redox mediator in a DSC is responsible for regenerating the ground state of the dye. Although the I₃⁻/I⁻ couple has become an established redox shuttle, it has disadvantages: its redox potential limits the values of the open-circuit voltage (V_OC) in the DSC and its use creates a corrosive chemical environment within the DSC which impacts upon the long-term stability of the cells. First row d-block metal coordination compounds, especially those containing cobalt, and copper, have come to the fore in the development of alternative redox mediators and we detail the progress in this field over the last decade, with particular attention to Cu²⁺/Cu⁺ redox mediators which, when coupled with appropriate dyes, have achieved V_OC values in excess of 1000 mV. We also draw attention to aspects of the recyclability of DSCs.

The progress over the last decade in the applications of first row d-block metal, especially iron, cobalt, copper and zinc, coordination compounds in redox shuttles and sensitizers in dye-sensitized solar cells is reviewed.     

Substituted polypyridine complexes of cobalt(II/III) as efficient electron-transfer mediators in dye-sensitized solar cells

A number of cobalt complexes of substituted polypyridine ligands were synthesized and investigated as possible alternatives to the volatile and corrosive iodide/triiodide redox couple commonly used as an electron-transfer mediator in dye-sensitized solar cells (DSSCs). The extinction coefficients in the visible spectrum are on the order of 10(2) M(-1) cm(-1) for the majority of these complexes, diminishing competition with the light-harvesting dye. Cyclic voltammetric studies revealed a dramatic surface dependence of the heterogeneous electron-transfer rate, which is surprisingly different for gold, carbon, and platinum electrodes. DSSCs were assembled using a mediator that consisted of a mixture of Co(II) and Co(III) complexes in a 10:1 ratio. DSSCs containing these mediators were used to characterize incident photon-to-current conversion efficiency and photoelectrochemical responses. The best performing of these mediators were identified and subjected to further study. As suggested by electrochemical results, gold and carbon are superior cathode materials to platinum, and no evidence of corrosion on any cathode material was observed. Addition of lithium salts to the mediator solution resulted in a dramatic improvement in cell performance. The observed Li(+) effect is explained in terms of the recombination of injected electrons in the photoanode with the oxidized mediator. The best mediator, based on tris(4,4'-di-tert-butyl-2,2'-dipyridyl)cobalt(II/III) perchlorate, resulted in DSSCs exhibiting efficiencies within 80% of that of a comparable iodide/triiodide-mediated DSSC. Due to the commercial availability of the ligand and the simplicity with which the complex can be made, this new mediator represents a nonvolatile, noncorrosive, and practical alternative as an efficient electron-transfer mediator in DSSCs.     

多吡啶钌染料和有机染料共染色的钴基敏化太阳电池

具有低费米能级的外球电子媒介体的开发带来了染料敏化太阳电池性能的重大进展.针对这种快复合器件,通过精细的调控二氧化钛表面的染料包覆层结构来有效抑制界面电荷复合是目前该领域的一个重要研究主题.在本文中,利用高吸收系数的多吡啶钌染料与具有三维立体结构的有机给受体染料对二氧化钛薄膜进行共染色.基于邻菲罗啉钴氧化还原电对,相对于纯钌基染料染色的器件,瞬态吸收与瞬态光电压衰减测试表明具有三维立体结构的有机染料的引入不仅提高了电子注入效率,还同时减慢了二氧化钛中的电子与氧化态染料及电解质中的电子受体之间的复合反应速率,使器件开路电压从808 mV提升到883 mV.这种界面光活性层微结构变化诱导的电子注入效率的改善和电荷复合的减慢还过补偿了因薄膜光吸收减弱带来的不利影响,获得了更大的光电流输出,在模拟AM1.5太阳光辐照条件下器件功率转换效率从8.5%提升到10.3%.     

Controlling Interfacial Recombination in Aqueous Dye‐Sensitized Solar Cells by Octadecyltrichlorosilane Surface Treatment

A general and convenient strategy is proposed for enhancing photovoltaic performance of aqueous dye‐sensitized solar cells (DSCs) through the surface modification of titania using an organic alkyl silane. Introduction of octadecyltrichlorosilane on the surface of dyed titania photoanode as an organic barrier layer leads to the efficient suppression of electron recombination with oxidized cobalt species by restricting access of the cobalt redox couple to the titania surface. The champion ODTS‐treated aqueous DSCs (0.25 mM ODTS in hexane for 5 min) exhibit a V_oc of 821±4 mV and J_sc of 10.17±0.21 mA cm^?2, yielding a record PCE of 5.64±0.10 %. This surface treatment thus serves as a promising post‐dye strategy for improving the photovoltaic performance of other aqueous DSCs.     

Molecular Engineering of Pyrido[3,4‐b]pyrazine‐Based Donor–Acceptor–π‐Acceptor Organic Sensitizers: Effect of Auxiliary Acceptor in Cobalt‐ and Iodine‐Based Electrolytes

Due to the ease of tuning its redox potential, the cobalt‐based redox couple has been extensively applied for highly efficient dye‐sensitized solar cells (DSSCs) with extraordinarily high photovoltages. However, a cobalt electrolyte needs particular structural changes in the organic dye components to obtain such high photovoltages. To achieve high device performance, specific requirements in the molecular tailoring of organic sensitizers still need to be met. Besides the need for large electron donors, studies of the auxiliary acceptor segment of donor–acceptor–π‐acceptor (D‐A‐π‐A) organic sensitizers are still rare in molecular optimization in the context of cobalt electrolytes. In this work, two novel organic D‐A‐π‐A‐type sensitizers ( IQ13 and IQ17 ) have been developed and exploited in cobalt‐ and iodine‐based redox electrolyte DSSCs, specifically to provide insight into the effect of π‐bridge modification in different electrolytes. The investigation has been focused on the additional electron‐withdrawing acceptor capability with grafted long alkoxy chains. Optoelectronic transient measurements have indicated that IQ17 containing a pyrido[3,4‐b]pyrazine moiety bearing long alkoxyphenyl chains is more suitable for application in cobalt‐based DSSCs.     

Synthesis and Characterization of Organic Dyes with Various Electron‐Accepting Substituents for p‐Type Dye‐Sensitized Solar Cells

Four new donor‐π‐acceptor dyes differing in their acceptor group have been synthesized and employed as model systems to study the influence of the acceptor groups on the photophysical properties and in NiO‐based p‐type dye‐sensitized solar cells. UV/Vis absorption spectra showed a broad range of absorption coverage with maxima between 331 and 653 nm. Redox potentials as well as HOMO and LUMO energies of the dyes were determined from cyclic voltammetry measurements and evaluated concerning their potential use as sensitizers in p‐type dye‐sensitized solar cells (p‐DSCs). Quantum‐chemical density functional theory calculations gave further insight into the frontier orbital distributions, which are relevant for the electronic processes in p‐DSCs. In p‐DSCs using an iodide/triiodide‐based electrolyte, the polycyclic 9,10‐dicyano‐acenaphtho[1,2‐b]quinoxaline (DCANQ) acceptor‐containing dye gave the highest power conversion efficiency of 0.08 %, which is comparable to that obtained with the perylenemonoimide (PMI)‐containing dye. Interestingly, devices containing the DCANQ‐based dye achieve a higher V_OC of 163 mV compared to 158 mV for the PMI‐containing dye. The result was further confirmed by impedance spectroscopic analysis showing higher recombination resistance and thus a lower recombination rate for devices containing the DCANQ dye than for PMI dye‐based devices. However, the use of the strong electron‐accepting tricyanofurane (TCF) group played a negative role in the device performance, yielding an efficiency of only 0.01 % due to a low‐lying LUMO energy level, thus resulting in an insufficient driving force for efficient dye regeneration. The results demonstrate that a careful molecular design with a proper choice of the acceptor unit is essential for development of sensitizers for p‐DSCs.     

Influence of Triarylamine and Indoline as Donor on Photovoltaic Performance of Dye-Sensitized Solar Cells Employing Cobalt Redox Shuttle

Two organic dyes XS51 and XS52 derivated from triarylamine and indoline are synthesized for dye-sensitized solar cells (DSCs) employing cobalt and iodine redox shuttles. The effects of dye structure upon the photophysical, electro-chemical characteristics and cell performance are investigated. XS51 with four hexyloxyl groups on triarylamine performs better steric hindrance and an improvement of photovoltage. XS52 provides higher short-circuit photocurrent density due to the strong electron-donating capability of indoline unit. The results from the redox electrolyte on cell performances indicate that the synthesized dyes are more suitable for tris(1,10-phenanthroline)cobalt(II/III) redox couple than I^?/I₃^? redox couple in assembling DSCs. Application of XS52 in the cobalt electrolyte yields a DSC with an overall power conversion efficiency of 6.58% under AM 1.5 (100 mW/cm²) irradiation.     

An alternative efficient redox couple for the dye-sensitized solar cell system

A series of new cobalt complexes [Co(LLL)(2)X(2)] were synthesized and evaluated as redox mediators for dye-sensitized nanocrystalline TiO(2) solar cells. The structure of the ligand and the nature of the counterions were found to influence the photovoltaic performance. The one-electron-transfer redox mediator [Co(dbbip)(2)](ClO(4))(2) (dbbip = 2,6-bis(1'-butylbenzimidazol-2'-yl)pyridine) performed best among the compounds investigated. Photovoltaic cells incorporating this redox mediator yielded incident photon-to-current conversion efficiencies (IPCE) of up to 80%. The overall yield of light-to-electric power conversion reached 8 % under simulated AM1.5 sunlight at 100 W m(-2) intensity and more than 4% at 1000 W m(-2). Photoelectrodes coated with a 2 microm thick nanoporous layer and a 4 microm thick light-scattering layer, sensitized with a hydrophobic ruthenium dye, gave the best results.     

Organic redox couples and organic counter electrode for efficient organic dye-sensitized solar cells

A series of organic thiolate/disulfide redox couples have been synthesized and have been studied systematically in dye-sensitized solar cells (DSCs) on the basis of an organic dye (TH305). Photophysical, photoelectrochemical, and photovoltaic measurements were performed in order to get insights into the effects of different redox couples on the performance of DSCs. The polymeric, organic poly(3,4-ethylenedioxythiophene) (PEDOT) material has also been introduced as counter electrode in this kind of noniodine-containing DSCs showing a promising conversion efficiency of 6.0% under AM 1.5G, 100 mW·cm(-2) light illumination. Detailed studies using electrochemical impedance spectroscopy and linear-sweep voltammetry reveal that the reduction of disulfide species is more efficient on the PEDOT counter electrode surface than on the commonly used platinized conducting glass electrode. Both pure and solvated ionic-liquid electrolytes based on a thiolate anion have been studied in the DSCs. The pure and solvated ionic-liquid-based electrolytes containing an organic redox couple render efficiencies of 3.4% and 1.2% under 10 mW·cm(-2) light illumination, respectively.     

Electron-rich heteroaromatic conjugated polypyridine ruthenium sensitizers for dye-sensitized solar cells

Ru(II) heteroleptic complexes as photosensitizers for dye-sensitized solar cells (DSCs) are presented. The article outlines design strategies, synthetic routes, optical and photovoltaic properties of ruthenium dyes based on polypyridines as ancillary ligands containing π-conjugated electron-rich heteroaromatic groups. The integration of donor heteroaromatic substituents, typically thiophene-based moieties, strongly improves the optical properties of the sensitizers in terms of bathochromic and hyperchromic shift compared to prototypical dyes N3 and N719. These favorable properties in turn yield DSCs with superior light harvesting abilities, higher external quantum efficiencies, improved device photocurrents, and top-ranked power conversion efficiencies. In combination with excellent stabilities under thermal stress and light soaking, this class of DSC photosensitizer has great potential for practical applications.     

Redox couple related influences of bulky electron donor as well as spacer in organic dye-sensitized mesoscopic solar cells

For dye-sensitized solar cells (DSCs), it is of great significance to understand the structure–performance relationship of photosensitizers. Herein, we scrutinize the influences of the arylamine electron donors as well as the fused thiophene spacer on the optoelectronic features of thin-film DSCs employing the tris(1,10-phenanthroline)cobalt(II/III) or the iodide/triiodide redox couple. Interestingly, the incorporation of bulky dihexyloxybenzene-substituted triphenylamine (DHOB-TPA) electron donor does not resulted in an improved electron lifetime, which is sharp contrast with the conventional concept on bulky electron donor. On the other hand, the introduction of the DHOB-TPA electron donor or the dithienopyrrole spacer significantly increases the molar absorption coefficients of dyes, which govern the performance of thin-film DSCs. This work demonstrates how organic dyes must be tailored carefully depending on the electrolyte red/ox couple used.     

Combination of Cobalt and Iron Polypyridine Complexes for Improving the Charge Separation and Collection in Ru(terpyridine)2‐Sensitised Solar Cells

Mixtures of polypyridine Fe^II and Co^II complexes are used as electron mediators in Ru–thienyltpy‐sensitised solar cells (tpy=terpyridine). The use of the metalorganic redox couples allows for improved charge‐collection efficiency with respect to the classical iodide/iodine couple which, when associated to Ru–tpy₂ dyes, usually produces poor performance. The improved charge collection is explained by a combination of effective dye regeneration and decreased recombination with the oxidised electrolyte on the basis of data obtained by transient spectroscopy and photoelectrochemical measurements. The efficiency of the regeneration cascade is also critically dependent upon the ability of the Co^II complex to intercept Fe^III centres, as clearly indicated by chronocoulometry experiments.     

Theoretical rationalization for reduced charge recombination in bulky carbazole‐based sensitizers in solar cells

The search for greater efficiency in organic dye‐sensitized solar cells (DSCs) and in their perovskite cousins is greatly aided by a more complete understanding of the spectral and morphological properties of the photoactive layer. This investigation resolves a discrepancy in the observed photoconversion efficiency (PCE) of two closely related DSCs based on carbazole‐containing D–π–A organic sensitizers. Detailed theoretical characterization of the absorption spectra, dye adsorption on TiO₂, and electronic couplings for charge separation and recombination permit a systematic determination of the origin of the difference in PCE. Although the two dyes produce similar spectral features, ground‐ and excited‐state density functional theory (DFT) simulations reveal that the dye with the bulkier donor group adsorbs more strongly to TiO₂, experiences limited π–π aggregation, and is more resistant to loss of excitation energy via charge recombination on the dye. The effects of conformational flexibility on absorption spectra and on the electronic coupling between the bright exciton and charge‐transfer states are revealed to be substantial and are characterized through density‐functional tight‐binding (DFTB) molecular dynamics sampling. These simulations offer a mechanistic explanation for the superior open‐circuit voltage and short‐circuit current of the bulky‐donor dye sensitizer and provide theoretical justification of an important design feature for the pursuit of greater photocurrent efficiency in DSCs. © 2017 Wiley Periodicals, Inc.     

Efficient iodine-free dye-sensitized solar cells employing truxene-based organic dyes

Two new truxene-based organic sensitizers (M15 and M16) featuring high extinction coefficients were synthesized for dye-sensitized solar cells employing cobalt electrolyte. The M16-sensitized device displays a 7.6% efficiency at an irradiation of AM1.5 full sunlight.     

Roles of electrolytes on charge recombination in dye-sensitized TiO(2) solar cells (2): the case of solar cells using cobalt complex redox couples

Dye-sensitized solar cells (DSC) were prepared from nanoporous TiO(2) electrodes with two different cobalt complex redox couples, propylene-1,2-bis(o-iminobenzylideneaminato)cobalt(II) {Co(II)(abpn)} and tris(4,4'-di-tert-buthyl-2,2'-bipyridine)cobalt(II) diperchlorate {Co(II)(dtb-bpy)(3)(ClO(4))(2)}. The performances of the DSCs were examined with varying the concentrations of the redox couples and Li cations in methoxyacetonitrile. Under 1 sun conditions, short-circuit currents (J(sc)) increased with the increase of the redox couple concentration, and the maximum J(sc) was found at the Li(+) concentration of 100 mM. To rationalize the observed trends of J(sc), electron diffusion coefficients and lifetimes in the DSCs were measured. Electron diffusion coefficients in the DSCs using cobalt complexes were comparable to the previously reported values of nanoporous TiO(2). Electron lifetime was independent of the concentration of the redox couples when the concentration ratio of Co(II)(L) and Co(III)(L) was fixed. With the increase of Li(+) concentration, the electron lifetime increased. These results were interpreted as due to their slow charge-transfer kinetics and the cationic nature of Co complex redox couples, in contrast to the anionic redox couple of I(-)/I(3)(-). The increase of the lifetimes with Li(+) was interpreted with the decrease of the local concentration of Co(III) near the surface of TiO(2). The addition of 4-tert-butylpyridine (tBP) with the presence of Li(+) increased J(sc) significantly. The observed increase of the electron lifetime by tBP could not explain the large increase of J(sc), implying that tBP facilitates the charge transfer from Co(II)(L) to dye cation, with the association of the change of the reorganization energy between Co(II) and Co(III).     

Successful demonstration of an efficient I(-)/(SeCN)2 redox mediator for dye-sensitized solar cells

A new I(-)/(SeCN)(2) redox mediator has favorable properties for dye-sensitized solar cells (DSCs) such as less visible light absorption, higher ionic conductivity, and downward shift of redox potential than I(-)/I(3)(-). It was then applied for DSCs towards increasing energy conversion efficiency, giving a new potential for improving performance.     

Factors Affecting the Performance of Champion Silyl‐Anchor Carbazole Dye Revealed in the Femtosecond to Second Studies of Complete ADEKA‐1 Sensitized Solar Cells

Record laboratory efficiencies of dye‐sensitized solar cells have been recently reported using an alkoxysilyl‐anchor dye, ADEKA‐1 (over 14 %). In this work we use time‐resolved techniques to study the impact of key preparation factors (dye synthesis route, addition of co‐adsorbent, use of cobalt‐based electrolytes of different redox potential, creation of insulating Al₂O₃ layers and molecule capping passivation of the electrode) on the partial charge separation efficiencies in ADEKA‐1 solar cells. We have observed that unwanted fast recombination of electrons from titania to the dye, probably associated with the orientation of the dyes on the titania surface, plays a crucial role in the performance of the cells. This recombination, taking place on the sub‐ns and ns time scales, is suppressed in the optimized dye synthesis methods and upon addition of the co‐adsorbent. Capping treatment significantly reduces the charge recombination between titania and electrolyte, improving the electron lifetime from tens of ms to hundreds of ms, or even to single seconds. Similar increase in electron lifetime is observed for homogenous Al₂O₃ over‐layers on titania nanoparticles, however, in this case the total solar cells photocurrent is decreased due to smaller electron injection yield from the dye. Our studies should be important for a broader use of very promising silyl‐anchor dyes and the further optimization and development of dye‐sensitized solar cells.     

Dye molecular structure device open-circuit voltage correlation in Ru(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells

Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.     

Non‐classical Design of High‐Efficiency Sensitizers for Dye‐Sensitized Solar Cells

The dye regeneration step in a dye‐sensitized solar cell (DSSC) affects significantly the device efficiency. To be able to predict the dye regeneration efficiency by the electrolyte this paper provides a facile way to design high‐efficiency sensitizers for DSSC. This paper proposes, for the first time, a simple and ingenious way to identify the dye regeneration sites and their relative efficiencies when a specific electrolyte is used. Two steps are proposed to identify the dye regeneration sites and their relative regeneration efficiencies: (1) drawing all the resonance structures of the oxidized dye to determine the regeneration sites, and (2) choosing the most favored site for dye regeneration as the chemically softest (when the redox couple used is soft I⁻/I₃⁻ pair) and the least spatially hindered site. The regeneration sites identified by the resonance structures are consistent with the β‐LUSO (β lowest unoccupied spin orbital) distribution, which is generally used for identifying the dye regeneration sites, calculated with DT‐DFT theory. The relative dye regeneration efficiency and photovoltaic performance of both ruthenium and metal‐free organic dyes predicted by the method reported here are supported by experimental data and the proposed dye regeneration mechanism. Several types of dye molecules are used to demonstrate the correctness of this new tool. This non‐classical tool, which uses the well‐known chemical knowledge of the resonance structure and hard–soft acid–base principle, without any computer calculation or physicochemical measurement, provides a very simple and powerful tool to quickly conceive high‐efficiency sensitizers for DSSCs.