Radical enhanced intersystem crossing mechanism, electron spin dynamics of high spin states and their applications in the design of heavy atom-free triplet photosensitizers

Heavy atom-free triplet photosensitizers (PSs) can overcome the high cost and biological toxicity of traditional molecular systems containing heavy atoms (such as Pt(II), Ir(III), Ru(II), Pd(II), Lu(III), I, or Br atoms) and, therefore, are developing rapidly. Connecting a stable free radical to the chromophore can promote the intersystem crossing (ISC) process through electron spin exchange interaction to produce the triplet state of the chromophore or the doublet (D) and quartet (Q) states when taking the whole spin system into account. These molecular systems based on the radical enhanced ISC (REISC) mechanism are important in the field of heavy atom-free triplet PSs. The REISC system has a simple molecular structure and good biocompatibility, and it is especially helpful for building high-spin quantum states (D and Q states) that have the potential to be developed as qubits in quantum information science. This review introduces the molecular structure design for the purpose of high-spin states. Time-resolved electron paramagnetic resonance (TREPR) is the most important characterization method to reveal the properties of these molecular systems, generation mechanism and electron spin polarization (ESP) of the high spin states. The spin polarization manipulation of high spin states and potential application in the field of quantum information engineering are also summarized. Moreover, molecular design principles of the REISC system to obtain long absorption wavelength, high triplet state quantum yield and long triplet state lifetime are introduced, as well as applications of the compounds in triplet-triplet annihilation upconversion, photodynamic therapy and bioimaging. This review is useful for the design of heavy atom-free triplet PSs based on the radical-chromophore molecular structure motif and the study of the photophysics of the compounds, as well as the electron spin dynamics of the multi electron system upon photoexcitation.

Steric Effects on the Photovoltaic Performance of Panchromatic Ruthenium Sensitizers for Dye-Sensitized Solar Cells

Three new heteroleptic Ru complexes, CYC-B22, CYC-B23C, and CYC-B23T, were prepared as sensitizers for coadsorbent-free, panchromatic, and efficient dye-sensitized solar cells. They are simultaneously functionalized with highly conjugated anchoring and ancillary ligands to explore the electronic and steric effects on their photovoltaic characteristics. The coadsorbent-free device based on CYC-B22 achieved the best power conversion efficiency (PCE) of 8.63% and a panchromatic response extending to 850 nm. The two stereoisomers, CYC-B23C and CYC-B23T coordinated with an unsymmetrical anchoring ligand, display similar absorption properties and the same driving forces for electron injection as well as dye regeneration. Nevertheless, the devices show not only the remarkably distinct PCE (6.64% vs 8.38%) but also discernible stability. The molecular simulation for the two stereoisomers adsorbed on TiO2 clarifies the distinguishable distances (16.9 Å vs 19.0 Å) between the sulfur atoms in the NCS ligands and the surface of the TiO2, dominating the charge recombination dynamics and iodine binding and therefore the PCE and stability of the devices. This study on the steric effects caused by the highly conjugated and unsymmetrical anchoring ligand on the adsorption geometry and photovoltaic performance of the dyes paves a new way for advancing the molecular design of polypyridyl metal complex sensitizers.

Advancements and Prospects in Perovskite Solar Cells: From Hybrid to All-Inorganic Materials

Hybrid perovskites, materials composed of metals and organic substances in their structure, have emerged as potential materials for the new generation of photovoltaic cells due to a unique combination of optical, excitonic and electrical properties. Inspired by sensitization techniques on TiO2 substrates (DSSC), CH3NH3PbBr3 and CH3NH3PbI3 perovskites were studied as a light-absorbing layer as well as an electron-hole pair generator. Photovoltaic cells based on per-ovskites have electron and hole transport layers (ETL and HTL, respectively), separated by an ac-tive layer composed of perovskite itself. Major advances subsequently came in the preparation methods of these devices and the development of different architectures, which resulted in an efficiency exceeding 23% in less than 10 years. Problems with stability are the main barrier to the large-scale production of hybrid perovskites. Partially or fully inorganic perovskites appear promising to circumvent the instability problem, among which the black perovskite phase CsPbI3 (α-CsPbI3) can be highlighted. In more advanced studies, a partial or total substitution of Pb by Ge, Sn, Sb, Bi, Cu or Ti is proposed to mitigate potential toxicity problems and maintain device efficiency.

Recent synthetic strategies of small heterocyclic organic molecules with optoelectronic applications: a review

Over the past few years, there have been tremendous developments in the design and synthesis of organic optoelectronic materials with appealing applications in device fabrication of organic light-emitting diodes, superconductors, organic lasers, organic field-effect transistors, clean energy-producing organic solar cells, etc. There is an increasing demand for the synthesis of green, highly efficient organic optoelectronic materials to cope with the issue of efficiency roll-off in organic semiconductor-based devices. This review systematically summarized the recent progress in the design and synthesis of small organic molecules having promising optoelectronic properties for their potential applications in optoelectronic devices during the last 10-year range (2010-early 2021).

Efficient D-π-π-A-Type Dye Sensitizer Based on a Benzothiadiazole Moiety: A Computational Study

The design of highly efficient sensitizers is one of the most significant areas in dye-sensitized solar cell (DSSC) research. We studied a series of benzothiadiazole-based D-π-π-A organic dyes, putting emphasis on the influence of the donor moiety on the DSSC's efficiency. Using (linear-response time-dependent) density functional theory ((TD)DFT)) with the CAM-B3LYP functional, different donor groups were characterized in terms of electronic absorption spectra and key photovoltaic parameters. As a reference, a dye was considered that had a benzothiadiazole fragment linked via thiophene rings to a diphenylamine donor and a cyanoacrylic-acid acceptor. The different systems were first studied in terms of individual performance parameters, which eventually aggregated into power conversion efficiency. Only the amino-substituted species showed a modest increase, whereas the dimethylamino case showed a decrease.

Novel D-A-π-A1 Type Organic Sensitizers from 4,7-Dibromobenzo[d][1,2,3]thiadiazole and Indoline Donors for Dye-Sensitized Solar Cells

Two novel D-A-π-A1 metal-free organic dyes of the KEA series containing benzo[d][1,2,3]thiadiazole (isoBT) internal acceptor, indoline donors fused with cyclopentane or cyclohexane rings (D), a thiophene as a π-spacer, and a cyanoacrylate as an anchor part were synthesized. Monoarylation of 4,7-dibromobenzo[d][1,2,3]thiadiazole by Suzuki-Miyamura cross-coupling reaction showed that in the case of indoline and carbazole donors, the reaction was non-selective, i.e., two monosubstituted derivatives were isolated in each case, whereas only one mono-isomer was formed with phenyl- and 2-thienylboronic acids. This was explained by the fact that heterocyclic indoline and carbazole fragments are much stronger donor groups compared to thiophene and benzene, as confirmed by cyclic voltammetry measurements and calculation of HOMO energies of indoline, carbazole, thiophene and benzene molecules. The structure of monoaryl(hetaryl) derivatives was strictly proven by NMR spectroscopy and X-ray diffraction. The optical and photovoltaic properties observed for the KEA dyes showed that these compounds are promising for the creation of solar cells. A comparison with symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 showed that the asymmetric nature of benzo[d][1,2,3]thiadiazole KEA dyes leads to a hypsochromic shift of the ICT band in comparison with the corresponding benzo[c][1,2,5]thiadiazole isomers. KEA dyes have a narrow HOMO-LUMO gap of 1.5–1.6 eV. Amongst these dyes, KEA321 recorded the best power efficiency (PCE), i.e., 5.17%, which is superior to the corresponding symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 (5.07 and 4.90%).

Two multifunctional benzoquinone derivatives as small molecule organic semiconductors for bulk heterojunction and perovskite solar cells

Two novel quinone derivatives (NN3 and NN4) were synthesized in this work and they were characterized to be used as small organic semiconductor molecules in different types of photovoltaic applications. To make accessible compounds, three simple steps were followed to prepare NN3 and NN4 compounds. Furthermore, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined for the computationally optimized models of the investigated compounds. The obtained optical and electrochemical results of this work indicated that NN3 and NN4 compounds were good candidates for application in the fields of bulk heterojunction (BHJ) and perovskite solar cells. Indeed, investigating new energy resources has been seen an important topic of research for producing clean energies and portable storage systems.

Unusual Alternating Crystallization-Induced Emission Enhancement Behavior in Nonconjugated ω-Phenylalkyl Tropylium Salts

The alternating physical properties, especially melting points, of α,ω-disubstituted n-alkanes and their parent n-alkanes had been known since Baeyer's report in 1877. There is, however, no general and comprehensive explanation for such a phenomenon. Herein, we report the synthesis and examination of a series of novel ω-phenyl n-alkyl tropylium tetrafluoroborates, which also display alternation in their physicochemical characters. Despite being organic salts, the compounds with odd numbers of carbons in the alkyl bridge exist as room temperature ionic liquids. In stark contrast to this, the analogues with even numbers of carbons in the linker are crystalline solids. These solid nonconjugated molecules exhibit curious photoluminescent properties, which can be attributed to their ability to form through-space charge-transfer complexes to cause crystallization-induced emission enhancement. Most notably, the compound with the highest photoluminescent quantum yield in this series showed an unusual arrangement of carbocationic dimer in the solid state. A combination of XRD analysis and ab initio calculations revealed interesting insights into these systems.

Fluorescent Chitosan Modified with Heterocyclic Aromatic Dyes

Chitosan is a valuable, functional, and biodegradable polysaccharide that can be modified to expand its applications. This work aimed to obtain chitosan derivatives with fluorescent properties. Three heterocyclic aromatic dyes (based on benzimidazole, benzoxazole, and benzothiazole) were synthesized and used for the chemical modification of chitosan. Emission spectroscopy revealed the strong fluorescent properties of the obtained chitosan derivatives even at a low N-substitution degree of the dye. The effect of high-energy ultraviolet radiation (UV–C) on modified chitosan samples was studied in solution with UV–Vis spectroscopy and in the solid state with FTIR spectroscopy. Moreover, cytotoxicity towards three different cell types was evaluated to estimate the possibilities of biomedical applications of such fluorescent chitosan-based materials. It was found that the three new derivatives of chitosan were characterized by good resistance to UV–C, which suggests the possibility of using these materials in medicine and various industrial sectors.

Recent Advances on Pt-Free Electro-Catalysts for Dye-Sensitized Solar Cells

Since Prof. Grätzel and co-workers achieved breakthrough progress on dye-sensitized solar cells (DSSCs) in 1991, DSSCs have been extensively investigated and wildly developed as a potential renewable power source in the last two decades due to their low cost, low energy-intensive processing, and high roll-to-roll compatibility. During this period, the highest efficiency recorded for DSSC under ideal solar light (AM 1.5G, 100 mW cm-2) has increased from ~7% to ~14.3%. For the practical use of solar cells, the performance of photovoltaic devices in several conditions with weak light irradiation (e.g., indoor) or various light incident angles are also an important item. Accordingly, DSSCs exhibit high competitiveness in solar cell markets because their performances are less affected by the light intensity and are less sensitive to the light incident angle. However, the most used catalyst in the counter electrode (CE) of a typical DSSC is platinum (Pt), which is an expensive noble metal and is rare on earth. To further reduce the cost of the fabrication of DSSCs on the industrial scale, it is better to develop Pt-free electro-catalysts for the CEs of DSSCs, such as transition metallic compounds, conducting polymers, carbonaceous materials, and their composites. In this article, we will provide a short review on the Pt-free electro-catalyst CEs of DSSCs with superior cell compared to Pt CEs; additionally, those selected reports were published within the past 5 years.

Tuning the Properties of Azadipyrromethene-Based Near-Infrared Dyes Using Intramolecular BO Chelation and Peripheral Substitutions

Tetraphenylazadipyrromethenes (ADPs) are attractive near-infrared (NIR) dyes because of their simple synthesis and exceptional optical and electronic properties. The typical BF₂ and less explored intramolecular BO coordination planarize the molecule, making them promising π-conjugated materials for organic electronic applications. However, their use has been mostly limited to vacuum-deposited devices. To improve the properties, we synthesized and characterized a series of ADP complexes and used density functional theory calculations to further explain the properties. Hexyloxy solubilizing groups increase the complexes' solubility in organic solvents and enable film formation from solution. Phenylethynyls at the pyrrolic positions extend π conjugation, red-shift absorption and emission peaks, and increase the ionization potential (IP) and electron affinity. When the properties of complexes with hexyloxy and phenyethynyl substitutions are compared, the BO complex is more planar and has a smaller IP than the corresponding BF₂ complex because of increased electron density on the proximal phenyls. The BO complex has an unusual combination of properties: a solution λ_max of 781 nm, emission at 805 nm, a small Stokes shift, and a quantum yield of 6%. It forms transparent films with a low optical gap of 1.22 eV. This new complex is a promising candidate for transparent solar cells and NIR photodetectors.

Efficient tuning of zinc phthalocyanine-based dyes for dye-sensitized solar cells: a detailed DFT study

The growing energy demand speed up the designing of competent photovoltaic materials. Herein, five zinc phthalocyanine-based donor materials T1–T5 are designed by substituting various groups (isopropoxy, cyano, fluoro, methoxycarbonyl, and dicyanomethyl) around zinc phthalocyanine. B3LYP/6-31G (d,p) level density functional theory (DFT) was used to investigate the optoelectronic properties of five zinc phthalocyanine-based dyes T1–T5 for dye-sensitized solar cells. The designed molecule T1 shows maximum absorption wavelength (λ_max) in the absorption spectrum at 708.89 and 751.88 nm both in gaseous state and in THF (tetrahydrofuran) solvent. The E_g value of T1 (1.86 eV) is less than reference R, indicating a greater charge transfer rate for T1 among the molecules. The values of open-circuit voltages achieved with acceptor polymer PC₇₁BM are higher than R except for T1 and are 0.69 V, 1.95 V, 1.20 V, 1.44 V, and 1.84 V for T1, T2, T3, T4, and T5, respectively. The lower the reorganization energy, the higher the charge transfer for T1 due to its lower hole mobility (0.06297 eV) than R. Thus, the designed T1–T5 molecules are expected to exhibit superior performance in dye-sensitized solar cells.

We used a quantum chemical approach to investigate the optoelectronic properties of dyes T1–T5 for dye-sensitized solar cells using DFT and TD-DFT computation. The newly designed molecules exhibited outstanding photovoltaic and optoelectronic properties.

Structural Engineering of Organic D-A-π-A Dyes Incorporated with a Dibutyl-Fluorene Moiety for High-Performance Dye-Sensitized Solar Cells

Structural engineering of the light-harvesting dyes employed in DSSCs (dye-sensitized solar cells) with a systematic choice of the electron-donating and -accepting groups as well as the π-bridge allows the (photo)physical properties of dyes to match the criteria needed for improving the DSSC efficiency. Herein, we report an effective approach of molecular engineering of DSSC sensitizers, aiming to gain insights on the configurational impact of the fluorenyl unit on the optoelectronic properties and photovoltaic performance of DSSCs. Five new organic dyes (GZ116, GZ126, GZ129, MA1116, and MA1118) with a D-A-π-A framework integrated with a fluorenyl moiety were designed and synthesized for DSSCs. The fluorenyl unit is configured as part of the π-spacer for the GZ series, whereas it connected on the electron-deficient quinoxaline motif for the MA series. The devices fabricated from the MA1116 sensitizer produced the best performance under standard AM 1.5 G solar conditions as well as dim-light (300-6000 lx) illumination. The devices fabricated from MA1116 displayed a PCE of 8.68% (J_sc = 15.00 mA cm^-2, V_oc = 0.82 V, and FF = 0.71) under 1 sun and 26.81% (J_sc = 0.93 mA cm^-2, V_oc = 0.68 V, and FF = 0.76) under 6000 lx illumination. The device efficiency based on dye MA1116 under 1 sun outperformed that based on the standard N719 dye, whereas a comparable performance between devices based on MA1116 and N719 was achieved under dim-light conditions. A combination of enhancing the charge separation, suppressing dye aggregation, and providing better insulation that prevents the oxidized redox mediator from approaching the TiO₂ surface all contribute to the superior performance of DSSCs fabricated based on these light-harvesting dyes. The judicious integration of the fluorenyl unit in a D-A-π-A-based DSSC would be a promising strategy to boost the device performance.

Structural modulation of phenothiazine and coumarin based derivatives for high performance dye sensitized solar cells: a theoretical study

A series of dyes with the D-π-A architecture has been designed and studied for dye sensitized solar cells (DSSCs). We have used phenothiazine (PTZ) and coumarin (COU) derivatives as the donor unit and benzopyrrole (BTZ) and 2-methyl-2H-isoindole-1,3-(3aH,7aH)-diene (IND) as the acceptor unit along with the azomethine group and thiophene ring as the π-spacer unit. Three electron donating groups viz. -CH3, -NH2, and -OH and four electron withdrawing groups viz. -CF3, -COCl, -F and -NO2 have been attached at the donor and the acceptor units respectively of the four unsubstituted dyes COU-BTZ, PTZ-BTZ, COU-IND and PTZ-IND. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods have been employed to investigate the structural, electronic and photochemical properties of these dyes. The study reveals that the unsubstituted dye PTZ-BTZ possesses the lowest value of ΔH-L. Our study also reveals that attachment of the -NO2 group at the acceptor unit lowers the ΔH-L values of all the dye molecules. We have observed that the excited state oxidation potential (ESOP) of all the dyes lies above the conduction band of the TiO2 semiconducting surface. However, the ground state oxidation potential (GSOP) of most of the dyes belonging to the COU-BTZ and COU-IND groups lies below the redox potential of the I-/I3- redox couple. The total reorganization energy (λtot) values of the COU-BTZ and COU-IND groups of dyes are observed to be low compared to the other groups of dyes. The study of the charge transport properties of the dyes confirms that the designed dyes will act as electron transport materials. The absorption properties of the dyes show that the COU-BTZ group of dyes possesses the maximum values of the absorption wavelength (λmax values) and attaching the -NO2 group at the acceptor unit shifts the λmax values of all the dyes to the longer region. From the study of the electronic properties of the dye-TiO2 complexes it has been observed that the performance of the dyes has been enhanced compared to the isolated dye molecules.

Synthesis of Novel 3,6-Dithienyl Diketopyrrolopyrrole Dyes by Direct C‒H Arylation

Direct C-H arylation coupling is potentially a more economical and sustainable process than conventional cross-coupling. However, this method has found limited application in the synthesis of organic dyes for dye-sensitized solar cells. Although direct C-H arylation is not an universal solution to any cross-coupling reactions, it efficiently complements conventional sp²−sp² bond formation and can provide shorter and more efficient routes to diketopyrrolopyrrole dyes. Here, we have applied palladium catalyzed direct C-H arylation in the synthesis of five new 3,6-dithienyl diketopyrrolopyrrole dyes. All prepared sensitizers display broad absorption from 350 nm up to 800 nm with high molar extinction coefficients. The dye-sensitized solar cells based on these dyes exhibit a power conversion efficiency in the range of 2.9 to 3.4%.

DFT characterization and design of anthracene-based molecules for improving spectra and charge transfer

Four anthracene-based dyes (AN-3, AN-11, AN-12, AN-14) are investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) for dye-sensitized solar cells (DSSCs), involving the stable molecular geometries, the electronic structures, the absorption and fluorescence spectra, and the performance of photoelectric properties. For the simulation of the realistic environment, some important parameters, including energy levels, energy gaps, electron density, projected density of states (PDOS), absorption, vertical dipole moment, first hyperpolarizability, light-harvesting efficiency (LHE), evaluation on electron injection, are calculated for each dye molecule. The relevant electron transfer (ET) and dynamic processes were studied by using the charge different density (CDD) and Newns-Anderson model. The relationship between structure and performance are established. Furthermore, six dyes are designed and examined on the basis of AN-11 to improve optical response and electron injection. It is expected that this study will give theoretical guidance and ideas for finding potential solar cell materials.

Mapping the optoelectronic property space of small aromatic molecules

Small aromatic molecules and their quinone derivatives find use in organic transistors, solar-cells, thermoelectrics, batteries and photocatalysts. These applications exploit the optoelectronic properties of these molecules and the ease by which such properties can be tuned by the introduction of heteroatoms and/or the addition of functional groups. We perform a high-throughput virtual screening using the xTB family of density functional tight-binding methods to map the optoelectronic property space of ~250,000 molecules. The large volume of data generated allows for a broad understanding of how the presence of heteroatoms and functional groups affect the ionisation potential, electron affinity and optical gap values of these molecular semiconductors, and how the structural features - on their own or in combination with one another - allow access to particular regions of the optoelectronic property space. Finally, we identify the apparent boundaries of the optoelectronic property space for these molecules: regions of property space that appear off limits for any small aromatic molecule.

Stacked nanocarbon photosensitizer for efficient blue light excited Eu(III) emission

Photosensitizer design to allow effective use of low-energy light is important for developing photofunctional materials. Herein, we describe a rational photosensitizer design for effective use of low-energy light. The developed photosensitizer is a stacked nanocarbon based on a rigid polyaromatic framework, which allows efficient energy transfer from the low-energy T₁ level to the energy acceptor. We prepared an Eu(III) complex consisting of a luminescent center (Eu(III)) and stacked-coronene photosensitizer. The brightness of photosensitized Eu(III) excited using low-energy light (450 nm) is more than five times higher than the maximum brightness of previously reported Eu(III) complexes.

New cyclic and acyclic imidazole-based sensitizers for achieving highly efficient photoanodes for dye-sensitized solar cells by a potential-assisted method

A series of systematically tailored aryl acyclic (biphenyl, dye 1 and 2), cyclic (phenanthrene, dye 3 and 4) and methyl-substituted (dye 5) imidazole derivatives were synthesized and characterized for application in dye-sensitized solar cells (DSSCs).

C-14 powered dye-sensitized betavoltaic cells

A dye-sensitized betavoltaic cell is developed for the first time, which utilizes radioisotopic carbon, composed of nano-sized quantum dots, and ruthenium-based dye sensitized TiO₂ as electrodes.

Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells

In recent years, copper(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper(I/II) complexes with 1,10-phenanthroline- or 2,2'-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6-11%) with a high open-circuit voltage (V_OC) over 1.0 V due to the positive potentials. Although copper(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp)₂]^1+/2+), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)₂]^1+/2+), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep)₂]^1+/2+), and bis(2,9-diphenyl-1,10-phenanthroline)copper(I/II) ([Cu(dpp)₂]^1+/2+). The more positive redox potentials of [Cu(emp)₂]^1+/2+ and [Cu(dep)₂]^1+/2+, compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)₂]^1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)₂]^1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu(bp)₂]^1+/2+ (5.90%) is almost comparable to the DSSC with [Cu(dmp)₂]^1+/2+ (6.29%). In contrast, the DSSCs with [Cu(emp)₂]^1+/2+ (3.25%), [Cu(dep)₂]^1+/2+ (2.56%), and [Cu(dpp)₂]^1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu(dmp)₂]^1+/2+ and [Cu(bp)₂]^1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)₂]^1+/2+ and [Cu(dmp)₂]^1+/2+, the use of copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high V_OC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.

Prediction of Absorption Spectrum Shifts in Dyes Adsorbed on Titania

Dye adsorption on metal-oxide films often results in small to substantial absorption shifts relative to the solution phase, with undesirable consequences for the performance of dye-sensitized solar cells and optical sensors. While density functional theory is frequently used to model such behaviour, it is too time-consuming for rapid assessment. In this paper, we explore the use of supervised machine learning to predict whether dye adsorption on titania is likely to induce a change in its absorption characteristics. The physicochemical features of each dye were encoded as a numeric vector whose elements are the counts of molecular fragments and topological indices. Various classification models were subsequently trained to predict the type of absorption shift i.e. blue, red or unchanged (|Δλ| ≤ 10 nm). The models were able to predict the nature of the shift with a good likelihood (~80%) of success when applied to unseen data.

Direct comparison of dithienosilole and dithienogermole as π-conjugated linkers in photosensitizers for dye-sensitized solar cells

Dithienosilole (DTS) and dithienogermole (DTG) are useful building units of π-conjugated organic materials. In the present work, donor-π-acceptor (D-π-A) dyes with bis(dihexyloxyphenyl)aminophenyl, DTS or DTG, and pyridine or cyanoacrylic acid as the donor (D), the π-conjugated linker (π), and the acceptor (A) units, respectively, were prepared and their optical properties were investigated. The D-π-A dyes exhibited strong absorption in the visible region, indicating efficient intramolecular donor-acceptor interaction. The addition of trifluoroacetic acid to solutions of pyridine-containing dyes led to red-shifts of the absorption bands as a result of pyridinium salt formation. Similar red-shifts were observed for cyanoacrylic acid dyes, which were due to the enhanced formation of neutral dyes relative to the separated ion pairs. The D-π-A dyes, however, showed similar absorption spectra when attached to the TiO2 surface, indicating that the dye-TiO2 electronic interaction was rather weak. In contrast to the finding that these dyes exhibited similar optical properties regardless of the π-linker (i.e., DTS or DTG), dye-sensitized solar cells (DSSCs) based on DTG-containing dyes exhibited superior performance compared to those based on DTS-containing dyes. Electrochemical impedance spectroscopy measurements supported the higher performance of the DSSCs with DTG-containing dyes.

Influence of the electron donor properties of hypericin on its sensitizing ability in DSSCs

Rising demands for renewable energy sources have led to the development of dye sensitized solar cells. It is a challenge to find a good and low cost sensitizer, which has a low environmental impact. In this work, we conducted spectroscopic and electrochemical experiments, as well as quantum-chemical calculations of the natural pigment hypericin, in order to provide insight into its sensitizing efficiency. To this end, three identical cells were made and characterized. Although this pigment exhibited good adsorption onto a semiconductor surface, a high molar absorption coefficient (43 700 L mol^-1 cm^-1) and favorable alignment of energy levels and provided a long lifetime of electrons (17.8 ms) in the TiO₂ photoanode, it was found that the efficiency of hypericin-sensitized solar cells was very low, only 0.0245%. We suggest that this inefficiency originated from a low injection of electrons into the conduction band of TiO_2. This conclusion is supported by the density functional theory calculations which revealed a low electron density in the anchoring groups of electronically excited hypericin. The results of this work could be valuable not only in the photovoltaic aspect, but also for application of hypericin in medicine in photodynamic therapy.

Effect of furan π-spacer and triethylene oxide methyl ether substituents on performance of phenothiazine sensitizers in dye-sensitized solar cells

Synthesis and characterization of three phenothiazine dyes inspired by literature dye EO3 improving absorption properties and photovoltaic performance in DSSCs.

A Review on the Advancement of Ternary Alloy Counter Electrodes for Use in Dye-Sensitised Solar Cells

A dye-sensitised solar cell (DSSC) counter electrode (CE) plays a vital role in catalysing the conversion of triiodide ( I 3 − ) to iodide ions ( I − ), thereby ensuring the completion of the repetitive cycle of electricity generation. The platinum CE, despite being the standard counter electrode in DSSCs, has drawbacks of platinum’s rarity and high cost. Platinum is an excellent redox catalyst, and consequently, it is the most sought-after metal for catalytic conversions. The huge demand for platinum in the automotive industry for vehicular catalytic converters, the pharmaceutical industry, and in oil refining, as well as other industries, has driven its price to unprecedented levels. The prohibitive price of platinum has caused newer thin film technologies, such as the DSSC which depends on the platinum CE, to be cost-ineffective, thus meaning they cannot compete with the better-established silicon-based solar cells. These problems have stagnated the development of the DSSC, which in turn has dampened larger commercialisation prospects for this thin film technology. With this in mind, this review paper focuses on recent progress in the research and development of alternative cost-effective materials to replace Pt-based CEs. Ternary alloys are amongst the possible alternatives that have been explored, yielding varied results. Alloys, especially ternary sulphides, selenides, and oxides, are attractive as alternatives as they are cheap and are easily fabricated. Ternary alloys also have a synergistic effect produced by the coexistence of two metal ions in a crystal structure, which is believed to induce greater catalytic capability, thus making them ideal cost-effective materials to replace the Pt CE in DSSCs. This review intends to highlight the performance of ternary alloy counter electrodes through the analysis of charge transfer resistance and power conversion efficiencies. Focus is also given to the restrictions and impediments to the attainment of higher power conversion efficiency in alternative CEs. The advances in fabrication of simple ternary alloys, as well as more advanced hierarchical nanostructured counter electrodes, are discussed here in detail. Results obtained to date indicate that the efficiencies of ternary alloy counter electrodes are still below that of the platinum counter electrode, and hence more research is required to enhance their efficiencies.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Stimuli-Responsive Organic Dyes with Tropylium Chromophore

Tropylium ions possess an interesting combination of structural stability and chemical reactivity due to its Hückel aromaticity and its positively charged polyene nature, respectively. Herein we exploit the chemical versatility and unique structural properties of the tropylium ion to derive a family of novel push-pull organic dyes with strong absorption in the visible range via simple and practical synthetic protocols. These stable organic dyes are highly stimuli-responsive, as demonstrated by their sensitivity towards solvent, pH change, redox reaction, Lewis base and counterion, which marks them as potentially useful compounds for opto-electronic applications.

Benzene-fused bis(acenaphthoBODIPY)s, stable near-infrared-selective dyes

Benzene-fused bis(acenaphthoBODIPY)s prepared by retro-Diels-Alder reaction of bicyclo[2.2.2]octadiene-fused precursors showed strong absorption bands in the near-infrared region and very weak absorptions in the visible region.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.