Engineering of Push‐Pull Thiophene Dyes to Enhance Light Absorption and Modulate Charge Recombination in Mesoscopic Solar Cells

The elaborate selection of diverse π‐conjugated segments which bridge the electron donors and acceptors in organic push‐pull dyes can not only tune the molecular energy‐levels but also impact the interfacial energetics and kinetics of dye‐sensitized solar cells (DSCs). In this paper, a series of triphenylamine‐cyanoacrylic acid photosensitizers is reported with TT, EDOT‐BT, EDOT‐CPDT, and CPDT‐EDOT (herein TT, EDOT, BT, and CPDT denote terthiophene, ethylenedioxythiophene, bithiophene, and cyclopentadithiophene, respectively) as the π‐linkers, and the dye‐structure correlated photocurrent and photovoltage features of DSCs based on a cobalt electrolyte are scrutinized via analyzing light absorption and multichannel charge transfer kinetics. Both stepwise incorporation of more electron‐rich blocks and rational modulation of connection order of dissimilar segments can result in a negative movement of ground‐state redox potential and a red‐shift of the absorption peak. While these styles of reducing energy‐gap do not exert too much influence on the electron injection from photoexcited dye molecules to titania, the dyestuff employing the EDOT‐BT linker presents a faster interfacial charge recombination and a slower dye regeneration, accounting for its inferior cell efficiency of 5.3% compared to that of 9.4% at the AM1.5G conditions achieved by the CPDT‐EDOT dye.     

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.     

Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO2 Nanocrystalline Films

An optimized unsymmetrical squaraine dye 5‐carboxy‐2‐[[3‐[(2,3‐dihydro‐1, 1‐dimethyl‐3‐ethyl‐1H‐benzo[e]indol‐2‐ylidene)methyl]‐2‐hydroxy‐4‐oxo‐2‐cyclobuten‐1‐ylidene]methyl]‐3,3‐dimethyl‐1‐octyl‐3H‐indolium ( SQ02 ) with carboxylic acid as anchoring group is synthesized for dye‐sensitized solar cells (DSCs). Although the π‐framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01 , photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency η of 5.40% which is improved by 20% when compared to SQ01 . UV‐vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02 . A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the π–π* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO₂ and hence a fast electron transfer.     

Significant Improvement of Dye‐Sensitized Solar Cell Performance by Small Structural Modification in π‐Conjugated Donor–Acceptor Dyes

Two donor‐π‐acceptor (D‐π‐A) dyes are synthesized for application in dye‐sensitized solar cells (DSSC). These D‐π‐A sensitizers use triphenylamine as donor, oligothiophene as both donor and π‐bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO₂ surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid‐electrolyte‐based dye‐sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO₂‐photoanode.     

Electrodeposition of Inorganic/Organic Hybrid Thin Films

Electrodeposition of inorganic compound thin films in the presence of certain organic molecules results in self‐assembly of various hybrid thin films with new properties. Examples of new discoveries by the authors are reviewed, taking cathodic formation of a ZnO/dye hybrid as the leading example. Hybridization of eosinY leads to the formation of highly oriented porous crystalline ZnO as the consequence of dye loading. The hybrid formation is a highly complicated process involving complex chemistry of many molecular and ionic constituents. However, electrochemical analyses of the relevant phenomena indicate the possibility of reaching a comprehensive understanding of the mechanism, giving us the chance to further develop them into industrial technologies. The porous crystals are ideal for photoelectrodes in dye‐sensitized solar cells. As the process also permits the use of non‐heat‐resistant substrates, the technology can be applied for the development of colorful and light‐weight plastic solar cells.     

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.     

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.     

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.     

High‐Performance Metal‐Free Solar Cells Using Stamp Transfer Printed Vapor Phase Polymerized Poly(3,4‐Ethylenedioxythiophene) Top Anodes

The use of vapor phase polymerized poly(3,4‐ethylenedioxythiophene) (VPP‐PEDOT) as a metal‐replacement top anode for inverted solar cells is reported. Devices with both i) standard bulk heterojunction blends of poly(3‐hexylthiophene) (P3HT) donor and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₀ (PCBM) soluble fullerene acceptor and ii) hybrid inorganic/organic TiO₂/P3HT acceptor/donor active layers are studied. Stamp transfer printing methods are used to deposit both the VPP‐PEDOT top anode and a work function enhancing PEDOT:polystyrenesulphonate (PEDOT:PSS) interlayer. The metal‐free devices perform comparably to conventional devices with an evaporated metal top anode, yielding power conversion efficiencies of 3% for bulk heterojunction blend and 0.6% for organic/inorganic hybrid structures. These encouraging results suggest that stamp transfer printed VPP‐PEDOT provides a useful addition to the electrode materials tool‐box available for low temperature and non‐vacuum solar cell fabrication.     

Coral‐shaped ZnO nanostructures for dye‐sensitized solar cell photoanodes

A highly efficient ZnO photoanode for dye‐sensitized solar cells was successfully grown by a simple, low cost, and scalable method. A nanostructured coral‐shaped Zn layer was deposited by sputtering onto fluorine‐doped tin oxide/glass slices at room temperature and then thermally oxidized in ambient atmosphere. Stoichiometry, crystalline phase, quality, and morphology of the film were investigated, evidencing the formation of a highly porous branched nanostructure, with a pure wurtzite crystalline structure. ZnO‐based dye‐sensitized solar cells were fabricated with customized microfluidic architecture. Dye loading on the oxide surface was analyzed with ultraviolet‐visible spectroscopy, and the dependence of the cell efficiency on sensitizer incubation time and film thickness was studied by current‐voltage electrical characterization, incident photon‐to‐electron conversion efficiency, and impedance spectroscopy measurements, showing the promising properties of this material for the fabrication of dye‐sensitized solar cell photoanodes with a solar conversion efficiency up to 4.58%. Copyright © 2012 John Wiley & Sons, Ltd.     

PtCoFe Nanowire Cathodes Boost Short‐Circuit Currents of Ru(II)‐Based Dye‐Sensitized Solar Cells to a Power Conversion Efficiency of 12.29%

PtCoFe nanowires with different alloying compositions are chemically prepared and acted as counter electrodes (CEs) in dye‐sensitized solar cells (DSSCs) with Ru(II)‐based dyes. Due to their superior reduction activity, PtCoFe nanowires with rich (111) facets enhance the performance of DSSCs. Hence, N719 DSSCs with PtCoFe nanowires, respectively, produce better power conversion efficiency (PCE) of 8.10% for Pt₃₃Co₂₄Fe₄₃ nanowire, 8.33% for Pt₇₄Co₁₂Fe₁₄ nanowire, and 9.26% for Pt₄₉Co₂₃Fe₂₈ nanowire in comparison to the PCE of Pt CE (7.32%). Further, the PRT‐22 DSSC with Pt₄₉Co₂₃Fe₂₈ nanowire exhibits a maximum PCE of 12.29% with a certificated value of 12.0%, which surpass the previous PCE record of the DSSCs with Ru(II)‐based dyes. The photovoltaic and electrochemical results reveal the composition‐dependent activity along with a volcano‐shaped trend in the I^?/ redox reaction. Theoretical work on the adsorption energies of I₂, the desorption energies of I^?, and the corresponding absolute energy demonstrates that the reduction activity followed in the order of Pt₄₉Co₂₃Fe₂₈(111) plane > Pt₇₄Co₁₂Fe₁₄(111) plane > Pt₃₃Co₂₄Fe₄₃(111) plane, proving Pt₄₉Co₂₃Fe₂₈ nanowire to be a superior cathode material for DSSCs.     

Edge‐Fluorinated Graphene Nanoplatelets as High Performance Electrodes for Dye‐Sensitized Solar Cells and Lithium Ion Batteries

Edge‐selectively fluorinated graphene nanoplatelets (FGnPs) are prepared by mechanochemically driven reaction between fluorine gas (20 vol% in argon) and activated carbon species from graphitic C–C bonds unzipped by high‐speed stainless steel balls with a high kinetic energy. The fluorination at edges of the unzipped graphene nanoplatelets (GnPs) is confirmed by various analytical techniques while the content of fluorine in FGnPs is determined to be 3.0 and 3.4 at% by X‐ray photoelectron spectroscopy and energy‐dispersive X‐ray spectroscopy, respectively. Because of the large difference in electronegativity between carbon (χ = 2.55) and fluorine (χ = 3.98) and the strong C–F bond, the edge‐fluorination of GnPs can provide the maximized charge polarization with an enhanced chemical stability. Thus, electrodes based on the resultant FGnPs demonstrate superb electrochemical performance with excellent stability/cycle life in dye‐sensitized solar cells (FF: 71.5%; J_sc: 14.44 mA cm^?2; V_oc: 970 mV; PCE: 10.01%) and lithium ion batteries (650.3 mA h g^?1 at 0.5 C, charge retention of 76.6% after 500 cycles).     

Highly efficient dye‐sensitized solar cells using phenothiazine derivative organic dyes

Two novel organic dyes have been synthesized using electron rich phenothiazine as electron donors and oligothiophene vinylene as conjugation spacers. The two dyes (2E)‐2‐cyano‐3‐(5‐(5‐((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐7‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)acrylic acid (PTZ‐1) and (2E)‐3‐(5‐(5‐(4,5‐bis((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐3‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)thiophen‐2‐yl)‐2‐cyanoacrylic acid (PTZ‐2) were fully characterized and employed in dye‐sensitized solar cells (DSCs) to explore the effect of disubstituted donors on photovoltaic (PV) performance. The solar cells sensitized by the PTZ1 dye have a high IPCE plateau of 80% and achieve a short‐circuit photocurrent density of 12.98 mA/cm², an open‐circuit voltage of 0.713 V, and a fill factor (ff) of 66.6%, corresponding to a conversion efficiency of 6.17% under AM 1.5 100 mW/cm² illumination. The different performance of the solar cells based on the two dyes can be understood from the studies of the electron kinetics by electrochemical impedance spectroscopy (EIS). These investigations reveal that disubstituted donors in the organic sensitizers of three or more conjugation units deteriorate the PV performance due to enhanced recombination. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Performance and Stability Enhancement of Dye‐Sensitized and Perovskite Solar Cells by Al Doping of TiO2

Reversible photo‐induced performance deterioration is observed in mesoporous TiO₂‐containing devices in an inert environment. This phenomenon is correlated with the activation of deep trap sites due to astoichiometry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO₂ with aluminium has a striking impact upon the density of sub‐gap states and enhances the conductivity by orders of magnitude. Dye‐sensitized and perovskite solar cells employing Al‐doped TiO₂ have increased device efficiencies and significantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitutional incorporation of Al in the anatase lattice, “permanently” passivating electronic trap sites in the bulk and at the surface of the TiO₂.     

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.     

Enhanced efficiency of phenothiazine derivative organic dye‐sensitized ionic liquid solar cells on aging

We have used electrochemical impedance to investigate the improvement in photovoltaic performance in aging of ionic liquid dye‐sensitized solar cells using a high‐absorption coefficient organic dye (2E)‐2‐cyano‐3‐(5‐(5‐((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐7‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)acrylic acid, which is in contrast to N719‐based devices. It was found that the enhancement is due to reduced recombination of the photoexcited electrons. The decreased recombination plausibly originates from molecular re‐orientation along with cation adsorption, with Fourier transform infrared spectra lending support to the former mechanism. After aging, the photovoltaic device using the organic dye outperforms the counterpart by the ruthenium complex dye and achieves an impressive efficiency of 5.6% under AM 1.5 100 mW/cm² illumination. Copyright © 2011 John Wiley & Sons, Ltd.     

An Effective Approach for High‐Efficiency Photoelectrochemical Solar Cells by Using Bifunctional DNA Molecules Modified Photoanode

This paper firstly reports the effect of deoxyribonucleic acid (DNA) molecules extracted from chickpea and wheat plants on the injection/recombination of photogenerated electrons and sensitizing ability of dye‐sensitized solar cells (DSSCs). These high‐yield DNA molecules are applied as both linker bridging unit as well as thin tunneling barrier (TTB) at titanium dioxide (TiO₂ )/dye interface, to build up high‐efficient DSSCs. With its favorable energy levels, effective linker bridging role, and double helix structure, bifunctional DNA modifier shows an efficient electron injection, suppressed charge recombination, longer electron lifetime, and higher light harvesting efficiency, which leads to higher photovoltaic performance. In particular, a photoconversion efficiency (PCE) of 9.23% is achieved by the binary chickpea and wheat DNA‐modified TiO₂ (CW@TiO₂) photoanode. Furthermore, time‐resolved fluorescence spectroscopy measurements confirm a better electron transfer kinetics for DNA‐modified TiO₂ photoanodes, implying a higher electron transfer rate (k_ET). This work highlights a great contribution for the photoanodes that are linked with DNA molecule, which act as both bridging unit and TTB to control the charge recombination and injection dynamics, and hence, boost the photovoltaic performance in the DSSCs.