High open-circuit voltage solid-state dye-sensitized solar cells with organic dye

Tailoring of energy levels in D-π-A organic dyes via fluorination of acceptor units for efficient dye-sensitized solar cells

A molecular design is presented for tailoring the energy levels in D-π-A organic dyes through fluorination of their acceptor units, which is aimed at achieving efficient dye-sensitized solar cells (DSSCs). This is achieved by exploiting the chemical structure of common D-π-A organic dyes and incorporating one or two fluorine atoms at the ortho-positions of the cyanoacetic acid as additional acceptor units. As the number of incorporated fluorine atoms increases, the LUMO energy level of the organic dye is gradually lowered due to the electron-withdrawing effect of fluorine, which ultimately results in a gradual reduction of the HOMO-LUMO energy gap and an improvement in the spectral response. Systematic investigation of the effects of incorporating fluorine on the photovoltaic properties of DSSCs reveals an upshift in the conduction-band potential of the TiO₂ electrode during impedance analysis; however, the incorporation of fluorine also results in an increased electron recombination rate, leading to a decrease in the open-circuit voltage (V_oc). Despite this limitation, the conversion efficiency is gradually enhanced as the number of incorporated fluorine atoms is increased, which is attributed to the highly improved spectral response and photocurrent.

Rational modifications on ruthenium terpyridine sensitizers with large Jsc for dye-sensitized solar cells: combined DFT and relativistic TDDFT studies

A series of ruthenium sensitizers DX2–DX5 derived from a phosphine-coordinated ruthenium sensitizer DX1 for dye sensitized solar cells (DSSCs) were designed and calculated based on density functional theory (DFT) and relativistic time-dependent DFT calculations.

Rapid construction of TiO2 aggregates using microwave assisted synthesis and its application for dye-sensitized solar cells

Hierarchical TiO₂ nanocrystallite aggregates are obtained by microwave synthesis in a few minutes, and they show high light scattering and larger surface area as well as improve the performance of dye-sensitized solar cells.

Can silicon substituted metal-free organic dyes achieve better efficiency compared to silicon free organic dyes? A computational study

DFT and TD-DFT calculations performed using metal free organic dyes containing silicon substituted silole units and/or donor systems exhibit significantly improved optical properties compared to their corresponding silicon free dyes.

Engineering of Ru(II) dyes for interfacial and light-harvesting optimization

A new Ru(II) dye, Ru(L1)(L2) (NCS)2, L1 = (4-(5-hexylthiophen-2-yl)-4'(4-carboxyl-phenyl 2,2'-bipyridine) and L2 = (4-4'-dicarboxy-2,2'-bipyridine), labelled MC112, based on a dissymmetric bipyridine ligand for improved interfacial and optical properties, was synthesized and used in DSCs, yielding photovoltaic efficiencies of 7.6% under standard AM 1.5 sunlight and an excellent device stability. Increased light harvesting and IPCE maximum were observed with MC112 compared to the prototypical homoleptic N719 dye, due to the functionalized bipyridyne ligand acting as an antenna. In addition, the mixed bipyridyne ligand allowed MC112 binding to TiO2 to occur via three anchoring carboxylic groups, thus exhibiting similar interfacial properties to those of the N719 dye. DFT/TDDFT calculations were performed on the new dye, both in solution and adsorbed on a TiO2 surface model, revealing that the peculiar photovoltaic properties of the MC112 dye are related to its anchoring mode. The new design rule thus allows us to engineer both light-harvesting and interfacial properties in the same dye.

Adsorption of organic dyes on TiO2 surfaces in dye-sensitized solar cells: interplay of theory and experiment

First-principles computer simulations can contribute to a deeper understanding of the dye/semiconductor interface lying at the heart of Dye-sensitized Solar Cells (DSCs). Here, we present the results of simulation of dye adsorption onto TiO(2) surfaces, and of their implications for the functioning of the corresponding solar cells. We propose an integrated strategy which combines FT-IR measurements with DFT calculations to individuate the energetically favorable TiO(2) adsorption mode of acetic acid, as a meaningful model for realistic organic dyes. Although we found a sizable variability in the relative stability of the considered adsorption modes with the model system and the method, a bridged bidentate structure was found to closely match the FT-IR frequency pattern, also being calculated as the most stable adsorption mode by calculations in solution. This adsorption mode was found to be the most stable binding also for realistic organic dyes bearing cyanoacrylic anchoring groups, while for a rhodanine-3-acetic acid anchoring group, an undissociated monodentate adsorption mode was found to be of comparable stability. The structural differences induced by the different anchoring groups were related to the different electron injection/recombination with oxidized dye properties which were experimentally assessed for the two classes of dyes. A stronger coupling and a possibly faster electron injection were also calculated for the bridged bidentate mode. We then investigated the adsorption mode and I(2) binding of prototype organic dyes. Car-Parrinello molecular dynamics and geometry optimizations were performed for two coumarin dyes differing by the length of the π-bridge separating the donor and acceptor moieties. We related the decreasing distance of the carbonylic oxygen from the titania to an increased I(2) concentration in proximity of the oxide surface, which might account for the different observed photovoltaic performances. The interplay between theory/simulation and experiments appears to be the key to further DSCs progress, both concerning the design of new dye sensitizers and their interaction with the semiconductor and with the solution environment and/or an electrolyte upon adsorption onto the semiconductor.

Novel D-π-A organic dyes with thieno[3,2-b]thiophene-3,4-ethylenedioxythiophene unit as a π-bridge for highly efficient dye-sensitized solar cells with long-term stability

This paper reports on new D-π-A organic dyes for application in dye-sensitized solar cells (DSSCs), which were developed by incorporating thieno[3,2-b]thiophene-thiophene (M9) and thieno[3,2-b]thiophene-EDOT (M10) as π-bridges. These dyes exhibited relatively small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps in spite of the short π-conjugation lengths, resulting in broad spectral responses. As photosensitizers in DSSCs, M10 showed a broader spectral response than M9, leading to a greater short-circuit photocurrent (Jsc). In addition, M10 exhibited higher open-circuit voltage (Voc) compared to M9, because of the greater electron lifetime of the photoanode. The impedance analysis revealed that the greater electron lifetime of the photoanode with M10 was attributed to the lower electron recombination rate caused by the blocking effect of the bulky EDOT unit. As a result, M10 showed much higher conversion efficiency (η = 7.00%) than M9 (η = 5.67%) under one sun condition (AM 1.5 G, 100 mW/cm(2)). This conversion efficiency was comparable to that of the conventional Ru-based dye N719 (η = 7.24%) under the same condition. In addition, M10 exhibited a remarkable long-term stability, i.e., 95% of the initial conversion efficiency was maintained after light soaking for 45 days (1080 h).

Theoretical insight on novel donor-acceptor exTTF-based dyes for dye-sensitized solar cells

A thorough density functional theory study is performed for the three carboxyl-based derivatives of the exTTF-TCF chromophore, where the π-extended tetrathiafulvalene (exTTF) electron-donor is linked to the tricyanofuran (TCF) electron-acceptor through an ethylene bridge, as dyes for dye-sensitized solar cells. Calculations predict that the carboxyl group in the acceptor moiety adopts an adequate orientation for an efficient anchoring on the semiconductor TiO₂ surface. The carboxylic acid group holds a negative charge twice larger than the cyano moiety that favors the electron injection to the semiconductor. Time-dependent calculations allow for the assignment of the absorption bands in the UV-vis spectrum of exTTF-TCF and confirm the presence of two low-lying charge-transfer electronic transitions that account for the moderately-intense absorption in the 450-800 nm range. The striking optical absorption properties of exTTF-TCF are preserved for the carboxylic analogues. Finally, periodic calculations show relevant topological differences between the carboxylic derivatives anchored on the TiO₂ surface, which would notably influence in the power conversion efficiency of a dye-sensitized solar cell.

Theoretical investigation of triphenylamine-based sensitizers with different π-spacers for DSSC

The molecular geometries, electronic structures, and absorption spectra of two organic dyes, 3-(5-(4-(IDB)phenyl)thiophene-2-yl)-2-cyanoacrylic acid (IDB-1), and 3-(5-(4-(IDB)styryl) thiophene-2-yl)-2-cyanoacrylic acid (IDB-2), before and after binding to TiO2 cluster were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods to understand the effect of enhanced π-conjugation of organic dye on the energy-to-electricity conversion efficiency (η) of dye-sensitized solar cell (DSSC), where, IDB=10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl. The introduction of vinyl unit into the π-spacer enhances the coplanarity and subsequently red-shifts, intensifies, and broadens the absorption spectrum of IDB-2, resulting in the stronger electronic coupling between dye and TiO2 conduction band, thus the more efficient electron transfer. From the theoretical evaluation of electron injection driving force (D), light-harvesting efficiency (LHE), and shift of TiO2 conduction band (ΔEcb), we successfully interpreted the experimentally observed efficiency difference between IDB-1 and IDB-2. Under this theoretical procedure, several novel D-π-A dyes namely IDB-3, IDB-4, and IDB-5, were designed. Our calculated results reveal that IDB-5 has the improved Jsc and Voc compared with IDB-2 because it performs nicely on the three key parameters (D, LHE, and ΔEcb). This work highlight the importance of using dimethyl-substituted cyclopentadithiophene group as π-spacer in achieving more efficient dyes for DSSC. We hope these discussions can provide fundamental guidelines for the optimization of existing cell efficiency as well as the design of novel high-efficiency organic dyes.

Computational modeling of single- versus double-anchoring modes in di-branched organic sensitizers on TiO2 surfaces: structural and electronic properties

DFT calculations support the doubly anchored monodentate adsorption of a di-branched exTTF-based organic sensitizer onto titania and demonstrate its beneficial electronic effects for DSCs.

Theoretical investigation on structural and electronic properties of organic dye C258 on TiO2(101) surface in dye-sensitized solar cells

Electron gets directly transferred from the HOMO of C258 to the bottom conduction band of TiO₂ in bidentate bridging adsorption mode.

Array of solid-state dye-sensitized solar cells with micropatterned TiO2 nanoparticles for a high-voltage power source

We demonstrate an array of solid-state dye-sensitized solar cells (SS-DSSCs) for a high-voltage power source based on micropatterned titanium dioxide nanoparticles (TNPs) as photoanodes connected in series. The underlying concept of patterning the TNP of a few micrometers thick lies on the combination of the lift-off process of transfer-printed patterns of a sacrificial layer and the soft-cure treatment of the TNP for fixation. This sacrificial layer approach allows for high pattern fidelity and stability, and it enables to construct stable, micrometer-thick, and contamination-free TNP patterns for developing the SS-DSSC array for miniature high-voltage applications. The array of 20 SS-DSSCs integrated in series is found to show a voltage output of around 7 V.

Intermolecular Interactions in Dye-Sensitized Solar Cells: A Computational Modeling Perspective

We present a unified overview of our recent activity on the modeling of relevant intermolecular interactions occurring in dye-sensitized solar cells (DSCs). The DSC is an inherent complex system, whose efficiency is essentially determined by the interrelated phenomena occurring at the multiple molecular-semiconductor-electrolyte heterointerfaces. In this Perspective, we illustrate the basic methodology and selected applications of computational modeling of dye-dye and dye-coadsorbent intermolecular interactions taking place at the dye-sensitized interface. We show that the proposed methodology offers a realistic picture of aggregation phenomena among surface-adsorbed dyes and nicely describes semiconductor surfaces cosensitized by different dyes. The information acquired from this type of studies might constitute the basis for an integrated multiscale computational description of the device functioning, including all of the possible interdependencies among the device constituents, which may further boost the DSCs efficiency. We believe that this direction should be the target of future computational research in the DSC field.

Molecular design principle of all-organic dyes for dye-sensitized solar cells

All-organic dyes have shown promising potential as an effective sensitizer in dye-sensitized solar cells (DSSCs). The design concept of all-organic dyes to improve light-to-electric-energy conversion is discussed based on the absorption, electron injection, dye regeneration, and recombination. How the electron-donor-acceptor-type framework can provide better light harvesting through bandgap-tuning and why proper arrangement of acceptor/anchoring groups within a conjugated dye frame is important in suppressing improper charge recombination in DSSCs are discussed. Separating the electron acceptor from the anchoring unit in the donor-acceptor-type organic dye would be a promising strategy to reduce recombination and improve photocurrent generation.

First-principles modeling of dye-sensitized solar cells: challenges and perspectives

Since dye-sensitized solar cells (DSSCs) appeared as a promising inexpensive alternative to the traditional silicon-based solar cells, DSSCs have attracted a considerable amount of experimental and theoretical interest. In contrast with silicon-based solar cells, DSSCs use different components for the light-harvesting and transport functions, which allow researchers to fine-tune each material and, under ideal conditions, to optimize their overall performance in assembled devices. Because of the variety of elementary components present in these cells and their multiple possible combinations, this task presents experimental challenges. The photoconversion efficiencies obtained up to this point are still low, despite the significant experimental efforts spent in their optimization. The development of a low-cost and efficient computational protocol that could qualitatively (or even quantitatively) identify the promising semiconductors, dyes, and electrolytes, as well as their assembly, could save substantial experimental time and resources. In this Account, we describe our computational approach that allows us to understand and predict the different elementary mechanisms involved in DSSC working principles. We use this computational framework to propose an in silico route for the ab initio design of these materials. Our approach relies on a unique density functional theory (DFT) based model, which allows for an accurate and balanced treatment of electronic and spectroscopic properties in different phases (such as gas, solution, or interfaces) and avoids or minimizes spurious computational effects. Using this tool, we reproduced and predicted the properties of the isolated components of the DSSC assemblies. We accessed the microscopic measurable characteristics of the cells such as the short circuit current (J(sc)) or the open circuit voltage (V(oc)), which define the overall photoconversion efficiency of the cell. The absence of empirical or material-related parameters in our approach should allow for its wide application to the optimization of existing devices or the design of new ones.

Exploring the heterogeneous interfaces in organic or ruthenium dye-sensitized liquid- and solid-state solar cells

The interfacial properties were systematically investigated using an organic sensitizer (3-(5'-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D)) and inorganic sensitizer (bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bipyridine-4,4'-dicarboxylato) ruthenium(II) (N719)) in a liquid-state and a solid-state dye-sensitized solar cell (DSC). For liquid-DSCs, the faster charge recombination for the surface of D-sensitized TiO2 resulted in shorter diffusion length (LD) of ∼3.9 μm than that of N719 (∼7.5 μm), limiting the solar cell performance at thicker films used in liquid-DSCs. On the other hand, for solid-DSCs using thin TiO2 films (∼ 2 μm), D-sensitized device outperforms the N719-sensitized device in an identical fabrication condition, mainly due to less perfect wetting ability of solid hole conductor into the porous TiO2 network, inducing the dye monolayer act as an insulation layer, while liquid electrolyte is able to fully wet the surface of TiO2. Such insulation effect was attributed to the fact that the significant increase in recombination resistance (from 865 to 4,400 Ω/cm(2)) but shorter electron lifetime (from 10.8 to 0.8 ms) when compared to liquid-DSCs. Higher recombination resistance for solid-DSCs induced the electron transport-limited situation, showing poor performance of N719-sensitized device which has shorter electron transport time and similar LD (2.9 μm) with D-sensitized device (3.0 μm).

Self-Organized One-DimensionalTiO2Nanotube/Nanowire Array Films for Use in Excitonic Solar Cells: A Review

We review the use of self-assembled, vertically oriented one-dimensional (1D) titania nanowire and nanotube geometries in several third-generation excitonic solar cell designs including those based upon bulk heterojunction, ordered heterojunction, Förster resonance energy transfer (FRET), and liquid-junction dye-sensitized solar cells (DSSCs).

Computational modelling of TiO2 surfaces sensitized by organic dyes with different anchoring groups: adsorption modes, electronic structure and implication for electron injection/recombination

We present a Density Functional Theory investigation aimed to model the possible adsorption modes to the TiO(2) surface of two representative TPA-based dyes, termed L0 and rh-L0, having the two mostly employed anchoring groups, namely the cyanoacrylic and rhodanine-3-acetic acids respectively. The bidentate coordination with proton transfer to a nearby surface oxygen is found to be the energetically favored anchoring mode for both dyes. The calculations show that the different dye anchoring groups give rise to a very different electronic coupling between the dye and the manifold of unoccupied semiconductor states, thus implying different electron injection mechanisms. The strongly coupled L0 dye possibly shows an adiabatic electron injection mechanism, while a non-adiabatic electron injection can be foreseen for the weakly coupled rh-L0 dye. The different orientation with respect to the TiO(2) surface for the two classes of dyes, implying different distances of the donor group from the oxide surface, together with the different electron injection mechanisms might account for the faster recombination reaction measured for the rhodanine-based dyes.

Voltage-enhancement mechanisms of an organic dye in high open-circuit voltage solid-state dye-sensitized solar cells

Sensitization of solid-state dye-sensitized solar cells (SSDSSCs) with a new, organic donor-π-acceptor dye with a large molar absorption coefficient led to an open-circuit voltage of over 1 V at AM1.5 solar irradiance (100 mW/cm(2)). Recombination of electrons in the TiO(2) film with the oxidized species in the hole-transfer material (HTM) was significantly slower with the organic dye than with a standard ruthenium complex dye. Density functional theory indicated that steric shielding of the electrons in the TiO(2) by the organic dye was important in reducing recombination. Preventing the loss of photoelectrons resulted in a significant voltage gain. There was no evidence that the organic dye contributed to the high voltage by shifting the band edges to more negative electrode potentials. Compared with an iodide-based liquid electrolyte, however, the more positive redox potential of the solid-state HTM used in the SSDSSCs favored higher voltages.