Estimating the Maximum Attainable Efficiency in Dye‐Sensitized Solar Cells

For an ideal solar cell, a maximum solar‐to‐electrical power conversion efficiency of just over 30% is achievable by harvesting UV to near IR photons up to 1.1 eV. Dye‐sensitized solar cells (DSCs) are, however, not ideal. Here, the electrical and optical losses in the dye‐sensitized system are reviewed, and the main losses in potential from the conversion of an absorbed photon at the optical bandgap of the sensitizer to the open‐circuit voltage generated by the solar cell are specifically highlighted. In the first instance, the maximum power conversion efficiency attainable as a function of optical bandgap of the sensitizer and the “loss‐in‐potential” from the optical bandgap to the open‐circuit voltage is estimated. For the best performing DSCs with current technology, the loss‐in‐potential is ～0.75 eV, which leads to a maximum power‐conversion efficiency of 13.4% with an optical bandgap of 1.48 eV (840 nm absorption onset). Means by which the loss‐in‐potential could be reduced to 0.4 eV are discussed; a maximum efficiency of 20.25% with an optical bandgap of 1.31 eV (940 nm) is possible if this is achieved.     

Optimization of a Quasi‐Solid‐State Dye‐Sensitized Photoelectrochemical Solar Cell Employing a Ureasil/Sulfolane Gel Electrolyte

A quasi‐solid‐state, dye‐sensitized photoelectrochemical solar cell employing a gel electrolyte obtained by sol–gel chemistry is described. The gel electrolyte is based on a ureasil precursor (i.e., a poly(propylene oxide) oligomer end‐capped by triethoxysilane groups through urea bridges) and sulfolane and it incorporates the I₃^–/I^– redox couple. It is shown that the combination of these two reagents prevents crystallization of KI, thus ensuring a long life for the cell and a satisfactory overall efficiency that surpasses 5 %. Cell efficiency increases with temperature. Optimization of gel‐electrolyte performance has been obtained by studying mobility with fluorescence‐quenching techniques complemented by direct‐current conductivity measurements.     

Charge Generation and Photovoltaic Operation of Solid‐State Dye‐Sensitized Solar Cells Incorporating a High Extinction Coefficient Indolene‐Based Sensitizer

An investigation of the function of an indolene‐based organic dye, termed D149, incorporated in to solid‐state dye‐sensitized solar cells using 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxypheny‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) as the hole transport material is reported. Solar cell performance characteristics are unprecedented under low light levels, with the solar cells delivering up to 70% incident photon‐to‐current efficiency (IPCE) and over 6% power conversion efficiency, as measured under simulated air mass (AM) 1.5 sun light at 1 and 10 mW cm^?2. However, a considerable nonlinearity in the photocurrent as intensities approach “full sun” conditions is observed and the devices deliver up to 4.2% power conversion efficiency under simulated sun light of 100 mW cm^?2. The influence of dye‐loading upon solar cell operation is investigated and the thin films are probed via photoinduced absorption (PIA) spectroscopy, time‐correlated single‐photon counting (TCSPC), and photoluminescence quantum efficiency (PLQE) measurements in order to deduce the cause for the non ideal solar cell performance. The data suggest that electron transfer from the photoexcited sensitizer into the TiO₂ is only between 10 to 50% efficient and that ionization of the photo excited dye via hole transfer directly to spiro‐OMeTAD dominates the charge generation process. A persistent dye bleaching signal is also observed, and assigned to a remarkably high density of electrons “trapped” within the dye phase, equivalent to 1.8 × 10¹⁷ cm^?3 under full sun illumination. it is believed that this localized space charge build‐up upon the sensitizer is responsible for the non‐linearity of photocurrent with intensity and nonoptimum solar cell performance under full sun conditions.     

Room‐Temperature Synthesis of Porous Nanoparticulate TiO2 Films for Flexible Dye‐Sensitized Solar Cells

A novel room‐temperature method for the preparation of porous TiO₂ films with high performance in dye‐sensitized solar cells (DSSCs) has been developed. In this method a small amount of Ti^IV tetraisopropoxide (TTIP) is added to an ethanolic paste of TiO₂ nanoparticles, where it hydrolyzes in situ and connects the TiO₂ particles to form a homogenous and mechanically stable film of up to 10 μm thickness without crack formation. Residual organics originating from the TTIP were removed by UV–ozone treatment of the films, leading to a remarkable improvement of the cell efficiency. Intensity‐modulated photocurrent/voltage spectroscopy (IMPS/IMVS) showed that the main effect of the UV–ozone treatment is to suppress the recombination of photogenerated electrons, thereby extending their lifetime. The efficiency was further increased by preheating the TiO₂ nanoparticles before the paste preparation to remove contaminants originating from the preparation process of the particles. Solar‐to‐electric energy conversion efficiencies of 4.00 and 3.27 % have been achieved for cells with conductive glass and plastic film substrates, respectively, under illumination with AM 1.5 (100 mW cm^–2) simulated sunlight.     

Band‐Edge Engineered Hybrid Structures for Dye‐Sensitized Solar Cells Based on SnO2 Nanowires

In this report, we show for the first time that SnO₂ nanowire based dye sensitized solar cells exhibit an open circuit voltage of 560 mV, which is 200 mV higher than that using SnO₂ nanoparticle based cells. This is attributed to the more negative flat band potential of nanowires compared to the nanoparticles as determined by open circuit photo voltage measurements made at high light intensities. The nanowires were employed in hybrid structures consisting of highly interconnected SnO₂ nanowire matrix coated with TiO₂ nanoparticles, which showed an open circuit voltage of 720 mV and an efficiency of 4.1% compared to 2.1% obtained with pure SnO₂ nanowire matrix. The electron transport time constants for SnO₂ nanowire matrix were an order of magnitude lower and the recombination time constants are about 100 times higher than that of TiO₂ nanoparticles. The higher efficiency observed for DSSCs based on hybrid structure is attributed to the band edge positions of SnO₂ relative to that of TiO₂ and faster electron transport in SnO₂ nanowires.     

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.     

Structure–Property Relations in All‐Organic Dye‐Sensitized Solar Cells

A structure–property relationship in all‐organic dye solar cells is revealed by first‐principles molecular dynamics and real‐time time‐dependent density functional theory simulations, accompanied with experimental confirmation. An important structural feature at the interface, Ti–N anchoring, for a broad group of all‐organic dyes on TiO₂ is inferred from energetics, vibrational recognition, and electronic data. This fact is contrary to the usual assumption; however, it optimizes electronic level alignment and photoelectron injection dynamics, greatly contributing to the observed efficiency improvement in all‐organic cyanoacrylate dye sensitized solar cells.     

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.     

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.     

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.     

New Ruthenium Complexes Containing Oligoalkylthiophene‐Substituted 1,10‐Phenanthroline for Nanocrystalline Dye‐Sensitized Solar Cells

Two new ruthenium complexes [Ru(dcbpy)(L)(NCS)₂], where dcbpy is 4,4′‐dicarboxylic acid‐2,2′‐bipyridine and L is 3,8‐bis(4‐octylthiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P1) or 3,8‐bis(4‐octyl‐5‐(4‐octylthiophen‐2‐yl)thiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P2), are synthesized, characterized by physicochemical and semiempirical computational methods, and used as photosensitizers in nanocrystalline dye‐sensitized solar cells. It was found that the difference in light‐harvesting ability between CYC‐P1 and CYC‐P2 is associated mainly with the location of the frontier orbitals, in particular the highest occupied molecular orbital (HOMO). Increasing the conjugation length of the ancillary ligand decreases the energy of the metal‐to‐ligand charge transfer (MLCT) transition, but at the same time reduces the molar absorption coefficient, owing to the HOMO located partially on the ancillary ligand of the ruthenium complex. The incident photon‐to‐current conversion efficiency curves of the devices are consistent with the MLCT band of the complexes. Therefore, the overall efficiencies of CYC‐P1 and CYC‐P2 sensitized cells are 6.01 and 3.42 %, respectively, compared to a cis‐di(thiocyanato)‐bis(2,2′‐bipyridyl)‐4,4′‐dicarboxylate ruthenium(II)‐sensitized device, which is 7.70 % using the same device‐fabrication process and measuring parameters.     

High Molar Extinction Coefficient Ion‐Coordinating Ruthenium Sensitizer for Efficient and Stable Mesoscopic Dye‐Sensitized Solar Cells

Ru(4,4‐dicarboxylic acid‐2,2′‐bipyridine) (4,4′‐bis(2‐(4‐(1,4,7,10‐tetraoxyundecyl)phenyl)ethenyl)‐2,2′‐bipyridine) (NCS)₂, a new high molar extinction coefficient ion‐coordinating ruthenium sensitizer was synthesized and characterized using ¹H NMR, Fourier transform IR (FTIR), and UV/vis spectroscopies and cyclic voltammetry. Using this sensitizer in combination with a nonvolatile organic‐solvent‐based electrolyte, we obtain a photovoltaic efficiency of 8.4 % under standard global AM 1.5 sunlight. These devices exhibit excellent stability when subjected to continuous thermal stress at 80 °C or light soaking at 60 °C for 1000 h. An electrochemical impedance spectroscopy study revealed that device stability is maintained by stabilizing the TiO₂/dye/electrolyte and Pt/electrolyte interface during the aging process. The influence of Li⁺ present in the electrolyte on the device photovoltaic parameters was studied, and the FTIR spectral and photovoltage transient study showed that Li⁺ coordinates to the triethyleneoxide methylether side chains on the K60 sensitizer molecules.     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

Solution‐Processed Organic Tandem Solar Cells

A solution‐processed polymer tandem cell fabricated by stacking two single cells in series is demonstrated. The two bulk‐heterojunction subcells have complementary absorption maxima at λ_max ～ 850 nm and λ_max ～ 550 nm, respectively. A composite middle electrode is applied that serves both as a charge‐recombination center and as a protecting layer for the first cell during spin‐coating of the second cell. The subcells are electronically coupled in series, which leads to a high open‐circuit voltage of 1.4 V, equal to the sum of each subcell. The layer thickness of the first (bottom) cell is tuned to maximize the optical absorption of the second (top) cell. The performance of the tandem cell is presently limited by the relatively low photocurrent generation in the small‐bandgap polymer of the top cell. The combination of our tandem architecture with more efficient small‐bandgap materials will enable the realization of highly efficient organic solar cells in the near future.     

Solvent‐Free Ionic Liquid Electrolytes for Mesoscopic Dye‐Sensitized Solar Cells

Ionic liquids have been identified as a new class of solvent that offers opportunities to move away from the traditional solvents. The physical‐chemical properties of ionic liquids can be tuned and controlled by tailoring their structures. The typical properties of ionic liquids, such as non‐volatility, electrochemical stability and high conductivity, render them attractive as electrolytes for dye‐sensitized solar cells. However, the high viscosity of ionic liquids leads to mass transport limitations on the photocurrents in the solar cells at full sunlight intensity, but the contribution of a Grotthous‐type exchange mechanism in these viscous electrolytes helps to alleviate these diffusion problems. This article discusses recent developments in the field of high‐performance dye‐sensitized solar cells with ionic liquid‐based electrolytes and their characterization by electrochemical impedance analysis.     

Conformal Nano‐Sized Inorganic Coatings on Mesoporous TiO2 Films for Low‐Temperature Dye‐Sensitized Solar Cell Fabrication

Here, a new method based on sol–gel electrophoretic deposition to produce uniform high‐quality inorganic conformal coatings on mesoporous nano‐particulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO₂ and/or MgO shell onto photoanodes used in dye‐sensitized solar cells (DSSCs) improves their light‐to‐electric‐power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO₂ coating improves the electronic inter‐particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low‐temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO₂ coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO₂/MgO coating.     

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.     

Organic Dye Design Tools for Efficient Photocurrent Generation in Dye‐Sensitized Solar Cells: Exciton Binding Energy and Electron Acceptors

The relationship between the exciton binding energies of several pure organic dyes and their chemical structures is explored using density functional theory calculations in order to optimize the molecular design in terms of the light‐to‐electric energy‐conversion efficiency in dye‐sensitized solar cell devices. Comparing calculations with measurements reveals that the exciton binding energy and quantum yield are inversely correlated, implying that dyes with lower exciton binding energy produce electric current from the absorbed photons more efficiently. When a strong electron‐accepting moiety is inserted in the middle of the dye framework, the light‐to‐electric energy‐conversion behavior significantly deteriorates. As verified by electronic‐structure calculations, this is likely due to electron localization near the electron‐deficient group. The combined computational and experimental design approach provides insight into the functioning of organic photosensitizing dyes for solar‐cell applications. This is exemplified by the development of a novel, all‐organic dye (EB‐01) exhibiting a power conversion efficiency of over 9%.     

A New Ionic Liquid for a Redox Electrolyte of Dye‐Sensitized Solar Cells

A new ionic liquid, 1‐vinyl‐3‐heptylimidazolium iodide (VHpII), was synthesized and applied as a redox electrolyte for dye‐sensitized solar cells. The chemical structure of the synthesized VHpII was confirmed using ¹H NMR. Thermogravimetric analysis showed that the VHpII was stable for thermal stress of up to 250°C. The energy conversion efficiencies of the VHpII‐based dye‐sensitized solar cells were investigated in terms of the effect of a lithium iodide addition. A solar cell containing the redox couple of VHpII and iodine showed a conversion efficiency of 2.63% under 1 sun light intensity at AM 1.5. Adding 0.4 M LiI results in a conversion efficiency of 3.63%, which was an improvement of about 40%. The increased conversion efficiency was ascribed to an increase in external quantum efficiency.     

Interface Engineering for Solid‐State Dye‐Sensitized Nanocrystalline Solar Cells: The Use of Ion‐Solvating Hole‐Transporting Polymers

The control of interfacial charge transfer is central to the design of photovoltaic devices. This charge transfer is strongly dependent upon the local chemical environment at each interface. In this paper we report a methodology for the fabrication of a novel nanostructured multicomponent film, employing a dual‐function supramolecular organic semiconductor to allow molecular‐level control of the local chemical composition at a nanostructured inorganic/organic semiconductor heterojunction. The multicomponent film comprises a lithium ion doped dual‐functional hole‐transporting material (Li⁺–DFHTM), sandwiched between a dye‐sensitized nanocrystalline TiO₂ film and a mono‐functional organic hole‐transporting material (MFHTM). The DFHTM consists of a conjugated organic semiconductor with ion supporting side chains, designed to allow both electronic and ionic charge transport properties. The Li⁺–DFHTM layers provide a new and versatile way to control the interface electrostatics, and consequently the charge transfer, at a nanostructured dye‐sensitized inorganic/organic semiconductor heterojunction.