Donor/Acceptor Indenoperylene Dye for Highly Efficient Organic Dye-Sensitized Solar Cells

Far-red active unsymmetrical squaraine dyes containing N-arylated indoline donors for dye sensitized solar cells

Squaraine dyes possess sharp far-red active transition with high extinction coefficient and form aggregates on TiO2 surface. Aggregation of dyes on TiO2 has been considered as a detrimental factor for DSSC device performance, which can be controlled by appending alkyl groups to the dye structures. Hence by integrating alkylated (alkyl groups with both in-plane and out-of-plane) aryl group with indoline moiety to make it compatible with other electrolytes and for controlling the dye-aggregation, a series of squaraine acceptor-based dyes SQA4-6 have been designed and synthesized. SQA4-6 dyes showed absorption between 642 and 653 nm (λmax), photophysical and electrochemical studies indicated that the HOMO energy levels of this sets of dyes are well aligned with the potentials of I-/I 3 - and [Co(bpy)3]2+/3+ redox shuttles for better dye regeneration process. DSSC device efficiency of 3% has been achieved for SQA5 dye with iodolyte (I-/I 3 - ) electrolyte in the presence of 0.3 mM of chenodeoxycholic acid (CDCA). The IPCE profile of DSSC device fabricated with SQA4-6 dyes indicated the contribution of aggregated structures for the photocurrent generation.

Effect of Position of Donor Units and Alkyl Groups on Dye-Sensitized Solar Cell Device Performance: Indoline-Aniline Donor-Based Visible Light Active Unsymmetrical Squaraine Dyes

Indoline (In) and aniline (An) donor-based visible light active unsymmetrical squaraine (SQ) dyes were synthesized for dye-sensitized solar cells (DSSCs), where the position of An and In units was changed with respect to the anchoring group (carboxylic acid) to have In-SQ-An-CO2H and An-SQ-In-CO2H sensitizers, AS1-AS5. Linear or branched alkyl groups were functionalized with the N atom of either In or An units to control the aggregation of the dyes on TiO2. AS1-AS5 exhibit an isomeric π-framework where the squaric acid unit is placed in the middle, where AS2 and AS5 dyes possess the anchoring group connected with the An donor, and AS1, AS3, and AS4 dyes having the anchoring group connected with the In donor. Hence, the conjugation between the middle squaric acid acceptor unit and the anchoring -CO2H group is short for AS2, AS5, and AK2 and longer for AS1, AS3, and AS4 dyes. AS dyes showed absorption between 501 and 535 nm with extinction coefficients of 1.46-1.61 × 105 M-1 cm-1. Further, the isomeric π-framework of An-SQ-In-CO2H and In-SQ-An-CO2H exhibited by means of changing the position of In and An units a bathochromic shift in the absorption properties of AS2 and AS5 compared to the AS1, AS3, and AS4 dyes. The DSSC device fabricated with the dyes contains short acceptor-anchoring group distance (AS2 and AS5) showed high photovoltaic performances compared to the dyes having longer distance (AS1, AS3, and AS4) with the iodolyte (I-/I3-) electrolyte. DSSC device efficiencies of 5.49, 6.34, 6.16, and 5.57% have been achieved for AS1, AS2, AS3, and AS4 dyes, respectively; without chenodeoxycholic acid (CDCA), small changes have been observed in the device performance of the AS dyes with CDCA. Significant changes have been noted in the DSSC parameters (open-circuit voltage VOC, short-circuit current JSC, fill factor ff, and efficiency η) for the AS5 dye while sensitized with CDCA and showed highest DSSC efficiency of 8.01% in the AS dye series. This study revealed the potential of shorter SQ acceptor-anchoring group distance over the longer one and the importance of alkyl groups on the overall DSSC device performance for the unsymmetrical squaraine dyes.

Screening novel candidates of ZL003-based organic dyes for dye-sensitized solar cells by modifying auxiliary electron acceptors: A theoretical study

In this work, a series of ZL003-based free-metal sensitizers with the donor-acceptor-π- conjugated spacer-acceptor (D-A-π-A) structure were designed by modifying auxiliary electron acceptors for the potential application in dye-sensitized solar cells. The energy levels of frontier molecular orbitals, absorption spectra, electronic transition, and photovoltaic parameters for all studied dyes were systematically evaluated using density functional theory (DFT)/time-dependent DFT calculations. Results illustrated that thienopyrazine (TPZ), selenadiazolopyridine (SDP), and thiadiazolopyridine (TDP) are excellent electron acceptors, and dye sensitizers functionalized by these acceptors have smaller HOMO-LUMO gaps, obviously red-shifted absorption bands and stronger light harvesting. The present study revealed that the photoelectric conversion efficiency (PCE) of ZL003 is around 13.42 % with a JSC of 20.21 mA·cm-2, VOC of 966 mV and FF of 0.688 under the AM 1.5G sun exposure, in good agreement with its experimental value (PCE = 13.6 ± 0.2 %, JSC = 20.73 ± 0.20 mA·cm-2, VOC = 956 ± 5 mV, and FF = 0.685 ± 0.005.). With the same procedure, the PCE values for M4, M6, and M7 were estimated to be as high as 19.93 %, 15.38 %, and 15.80 % respectively. Hence, these three dyes are expected to be highly efficient organic sensitizers applied in practical DSSCs.

A quantum chemical study: thoughtful exploration for optimal donors in Y-type dual donor-based dye sensitizers

This research explores the influence of different dual donors on the effectiveness of dye sensitizers. We selected 35 diverse donors to construct Y-type dual donor-based dyes, connecting them with thiophene as the π-spacer and cyanoacrylic acid as the acceptor. Density functional theory calculations indicate that these dual donor-based dyes exhibit superior optoelectronic properties compared to their single donor counterparts. Notably, significant variations in charge distribution among the different dual donors affect their donor capabilities. Our calculations specifically highlight the enhanced thermodynamic parameters, including light harvesting efficiency (LHE), the free energy of dye injection (ΔGinject), and regeneration (ΔGreg), for donor moieties containing nitrogen atoms, such as NS-3 (N,N-dimethylaniline), NS-5 (diphenylamine), NS-6 (triphenylamine), and NS-8 (4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline). These results suggest that nitrogen-containing donor moieties act as promising candidates for donors for efficient dye sensitizers. However, further experimental validation in the near future will be necessary to confirm our findings.

Organic Solar Cells: Physical Principle and Recent Advances

Organic solar cells (OSC) based on organic semiconductor materials that convert solar energy into electric energy have been constantly developing at present, and also an effective way to solve the energy crisis and reduce carbon emissions. In the past several decades, efforts have been made to improve the power conversion efficiency (PCE) of OSCs. During this period, a variety of structural and material forms of OSCs have evolved. Commercializing OSCs, extending their service life and exploring their future development are promising but challenging. In this review, we first briefly introduce the development of OSCs and then summarize and analyze the working principle, performance parameters, and structural features of OSCs. Finally, we highlight some breakthrough related to OSCs in detail.

Model systems for dye-sensitized solar cells: cyanidin-silver nanocluster hybrids at TiO2 support

Theoretical study of structural, optical, and photovoltaic properties of novel bio-nano hybrids (dye-nanocluster), as well as at TiO2 surface model support is presented in the context of the application for dye-sensitized solar cells (DSSC). A group of anthocyanidin dyes (pelargonidin, cyanidin, delphinidin, peonidin, petunidin, and malvidin) represented by cyanidin covalently bound to silver nanoclusters (NCs) with even or odd number of valence electrons have been investigated using DFT and TDDFT approach. The key role of nanoclusters as acceptors in hybrids cyanidin-NC has been shown. The nanoclusters with an even number of valence electrons are suitable as acceptors in hybrids. The interaction of bio-nano (cyanidin-NC) hybrid with the TiO2 surface model has been investigated in the context of absorption in near-infrared (NIR) and charge separation due to donor and acceptor subunits. Altogether, the theoretical concept serves to identify the key steps in the design of novel solar cells based on bio-nano hybrids at TiO2 surface for DSSC application.

A Molecularly Tailored Photosensitizer with an Efficiency of 13.2% for Dye-Sensitized Solar Cells

Photosensitizers yielding superior photocurrents are crucial for copper-electrolyte-based highly efficient dye-sensitized solar cells (DSCs). Herein, two molecularly tailored organic sensitizers are presented, coded ZS4 and ZS5, through judiciously employing dithieno[3,2-b:2″,3″-d]pyrrole (DTP) as the π-linker and hexyloxy-substituted diphenylquinoxaline (HPQ) or naphthalene-fused-quinoxaline (NFQ) as the auxiliary electron-accepting unit, respectively. Endowed with the HPQ acceptor, ZS4 shows more efficient electron injection and charge collection based on substantially reduced interfacial charge recombination as compared to ZS5. As a result, ZS4-based DSCs achieve a power conversion efficiency (PCE) of 13.2% under standard AM1.5G sunlight, with a high short-circuit photocurrent density (J_sc ) of 16.3 mA cm^-2 , an open-circuit voltage (V_oc ) of 1.05 V and a fill factor (FF) of 77.1%. Remarkably, DSCs sensitized with ZS4 exhibit an outstanding stability, retaining 95% of their initial PCE under continuous light soaking for 1000 h. It is believed that this is a new record efficiency reported so far for copper-electrolyte-based DSCs using a single sensitizer. The work highlights the importance of developing molecularly tailored photosensitizers for highly efficient DSCs with copper electrolyte.

D-A-D-based Unsymmetrical Thiosquaraine Dye for the Dye-Sensitized Solar Cells†

In dye-sensitized solar cell, modulating the electronic properties of the sensitizer by varying the donor, π-spacer, acceptor and anchoring groups help optimizing the structure of the dye for better device performance. Here, a donor-acceptor-donor-based unsymmetrical thiosquaraine sensitizer (SQ5S) has been designed and synthesized. Photophysical, electrochemical, theoretical and photovoltaic characterizations of SQ5S dye have been compared with its oxygen analog, SQ5. The incorporation of the sulfur atom in the acceptor unit of SQ5S dye showed an intense peak at 688 nm, which was 38 nm of red-shifted and showed the panchromatic light harvesting response with the onset of 850 nm compared with SQ5 dye. The LUMO and HOMO energy levels are well aligned with the conduction band of TiO₂ and the redox potential of electrolyte for the charge injection and the dye-regeneration processes, respectively. Photovoltaic efficiency of 1.51% (V_OC 610 mV, J_SC 3.07 mA cm^-2 , ff 81%) has been achieved for SQ5S dye, whereas SQ5 showed the device performance of 5.43% (V_OC 723 mV, J_SC 9.3 mA cm^-2 , ff 80%). The decreased device performance for the dye SQ5S has been attributed to the favorable intersystem crossing process associated with the photoexcited SQ5S that reduces the driving force for the charge injection process.

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells

Alkyl group wrapped donor-acceptor-donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I^-/I₃^-) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO₂ surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO₂ surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced V_OC values of 642, 675, and 699 mV; J_SC values of 6.38, 11.1, and 11.69 mA/cm²; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic V_OC values of 640, 652, and 650 mV; J_SC values of 5.78, 6.79, and 6.24 mA/cm²; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.

Naphthoquinoneoxime-Sensitized Titanium Dioxide Photoanodes: Photoelectrochemical Properties

Naphthoquinoneoxime derivatives, viz., LwOx, 3-hydroxy-4-(hydroxyimino)naphthalen-1 (4H)-one; PthOx, 3-hydroxy-4-(hydroxyimino)-2-methylnaphthalen-1(4H)-one; and Cl_LwOx, 2-chloro-3-hydroxy-4-(hydroxyimino)naphthalen-1(4H)-one, are used in fabrication of dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the sensitizers were studied. The HOMO-LUMO energy gaps of the sensitizers (LwOx, PthOx, and Cl_LwOx) calculated by using the intersection of UV-visible and fluorescence spectra are 2.85, 2.71, and 2.87 eV, respectively. The energy band alignment energy level of the sensitizer, that is, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), should match with the energy level of the TiO₂ conduction band and the redox potential of iodine/triiodide electrolyte to allow smooth electron transfer. The electrochemical characterization of sensitizers was done to find the LUMO and HOMO level of the sensitizer. It shows that the LUMO level of (LwOx, PthOx, and Cl_LwOx) is above the conduction band position of TiO₂. Electrochemical impedance spectroscopy was used to study the charge transport resistance and electron lifetime of DSSCs. The charge transport resistance at the TiO₂ |electrolyte|counter electrode interface was reduced in the Cl_LwOx device; thus, the electron lifetime of Cl_LwOx was enhanced compared to LwOx and PthOx sensitizers. The fabricated device was characterized using photocurrent density-voltage (J-V) measurement. It is observed that there was an enhancement in the overall power conversion efficiency (η) of the DSSCs fabricated by using Cl_LwOx sensitizers as compared to LwOx and PthOx sensitizer-loaded photoanodes. Enhancement in power conversion efficiency, that is, photovoltage and photocurrent, is achieved due to the chlorine substituent. Thus, the chlorine substituent naphthoquinoneoxime pushes the electron density, enhancing the pushing nature and facilitating the lone pair present in the N-OH moiety to attach to TiO₂ more strongly.

Unique multiphthalocyanine coordination systems: vibrationally hot excited states and charge transfer states that power high energy triplet charge separated states

Controlling the molecular architecture of well-organized organic building blocks and linking their functionalities with the impact of solar-light converting systems constitutes a grand challenge in materials science. Strong absorption cross-sections across the visible range of the solar spectrum as well as a finely balanced energy- and redox-gradient are all important features that pave the way for either funneling excited state energy or transducing charges. In light of this, we used thiopyridyl-phthalocyanines (PcSPy) and ruthenium (tert-butyl)-phthalocyanines (RuPc) as versatile building blocks and demonstrated the realization of a family of multi-functional PcSPy-RuPc 1-4 by means of axial coordination. Sizeable electronic couplings between the electron donors and acceptors in PcSPy-RuPc 1-4 govern ground-state as well as excited-state reactivity. Time-resolved techniques, in general, and fluorescence and transient absorption spectroscopy, in particular, helped to corroborate a rapid charge separation next to a slow charge recombination. Key to these charge transfer characteristics are higher lying, vibrationally hot states of the singlet excited states in parallel with a charge transfer state and the presence of several heavy atom effects that are provided by ruthenium and sulfur. As such, our advanced investigations confirm that rapid charge separation evolves from both higher lying, vibrationally hot states as well as from a charge transfer state, populating charge separated states, whose energies exceed those of the singlet excited states. Charge recombination involves triplet rather than singlet charge separated states, which delays the charge recombination by one order of magnitude.

Efficient Solar Cells Sensitized by Organic Concerted Companion Dyes Suitable for Indoor Lamps

In this work, organic concerted companion (CC) dyes CCOD-1 and CCOD-2 were constructed by covalently linking two organic dye units with complementary absorption spectra. Both CC dyes exhibited intense absorption from 300 to 650 nm with the band edges extended to 700 nm. These CC dyes were used to fabricate dye-sensitized solar cells (DSSCs), and the photovoltaic performance was investigated using different light sources. CCOD-2 possessed bulkier outer shelter than CCOD-1 owing to the longer carbon chains (C₁₂ ) at the donor moiety, and thus it had stronger anti-aggregation and anti-charge-recombination ability. Under simulated sunlight (AM1.5G), CCOD-2 exhibited enhanced photovoltaic behavior with an open-circuit voltage (V_OC ) of 759 mV, short-circuit current density (J_SC ) of 19.23 mA ⋅ cm^-2 , and power conversion efficiency (PCE) of 10.4 %, respectively. Notably, under the illumination of the indoor T5 fluorescent lamp (2500 lux), CCOD-2 afforded an enhanced PCE of 28.0 % with remarkable V_OC and J_SC of 692 mV and 0.424 mA cm^-2 , respectively. Notably, the PCE achieved for CCOD-2 outperformed those of the reference sensitizer N719 and our previously reported CC dyes XW61 and XW70-C8 under the same indoor lamp conditions. In summary, the novel organic CC dyes developed in this work were demonstrated to be promising for fabricating DSSCs to efficiently harvest the energy of indoor lamps.

From Styrenes to Fluorinated Benzyl Bromides: A Photoinduced Difunctionalization via Atom Transfer Radical Addition

An operationally simple and practical method is disclosed to achieve the difunctionalization of styrenes, generating fluorinated benzyl bromides via a photoinduced atom transfer radical addition process. The developed method is mild, atom-economical, cost-effective, employs very low photocatalyst loading (1000 ppm), and is highly compatible with a broad range of functional groups on styrene. The versatility of the fluorinated benzyl bromides is demonstrated through their derivatization to a variety of valuable compounds.

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%).

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Effect of the Spatial Configuration of Donors on the Photovoltaic Performance of Double D-π-A Organic Dyes

Three double D-π-A sensitizers (A1, A3, and A5) containing different donors (triphenylamine, methoxy-modified triphenylamine, and cyclic thiourea-functionalized triphenylamine) are synthesized to investigate the role of different donors in dye-sensitized solar cells (DSSCs). Detailed investigations of the sensitizers reveal that the spatial characteristics of donor units have a considerable impact on the light-harvesting, electrochemistry, and photovoltaic properties. Benefiting from the strong shielding ability of alkyl chains in the donor to its branch chains as observed in density functional theory (DFT), the open-circuit voltage (V_OC = 712.0 mV) of A5-based DSSC is higher than those of A1 and A3 by 90 and 78 mV, respectively. Therefore, the A5-based DSSC delivers a good efficiency of 8.54%, relying on its effective suppression of interfacial recombination. The results indicate that the judiciously tailored donor unit is an effective approach to optimize dye configurations to further improve power conversion efficiencies.

Comparison in optoelectronic properties of triphenylamine-imidazole or imidazole as donor for dye-sensitized solar cell: theoretical approach

In the present work, the structural and electronic properties of the D-D'-π-A organic dye with two donors have been calculated theoretically by DFT/time-dependent DFT method. In order to prove their efficiency as sensitizers, a comparative study was performed with a series of D-π-A architecture with one donor. The results of light-harvesting efficiency (LHE), open circuit voltage (Voc), free energy injection (∆G_inj), free energy dye regeneration∆G_reg,excited-state lifetimes for the two series reveal that the D-D'-π-A dyes are promising for the design of new sensitive dyes in solar cells.

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.

Effect of π-Conjugated Spacer in N-Alkylphenoxazine-Based Sensitizers Containing Double Anchors for Dye-Sensitized Solar Cells

A series of novel double-anchoring dyes for phenoxazine-based organic dyes with two 2-cyanoacetic acid acceptors/anchors, and the inclusion of a 2-ethylhexyl chain at the nitrogen atom of the phenoxazine that is connected with furan, thiophene, and 3-hexylthiophene as a linker, are used as sensitizers for dye-sensitized solar cells. The double-anchoring dye exhibits strong electronic coupling with TiO₂, provided that there is an efficient charge injection rate. The result showed that the power conversion efficiency of DP-2 with thiophene linker-based cell reached 3.80% higher than that of DP-1 with furan linker (η = 1.53%) under standard illumination. The photovoltaic properties are further tuned by co-adsorption strategy, which improved power conversion efficiencies slightly. Further molecular theoretical computation and electrochemical impedance spectroscopy analysis of the dyes provide further insight into the molecular geometry and the impact of the different π-conjugated spacers on the photophysical and photovoltaic performance.

Single- and co-sensitization of triphenylamine-based and asymmetrical squaraine dyes on the anatase (001) surface for DSSC applications: Periodic DFT calculations

Dye aggregation causes poor performance of dye-sensitized solar cell (DSSC) applications through faster charge recombination of the photosensitizer with electrolyte. Triphenylamine (TBA)-based dyes feature a higher molar absorption coefficient and broadened wavelength but cannot absorb sunlight in the near-infrared (NIR) region. In contrast, the squaraine (SQ) photosensitizer, which is also called an NIR photosensitizer, has a maximum wavelength in the NIR region with high intensity. However, SQ dye suffers from dye aggregation due to its planar structure. The use of a co-sensitizer is one well-tested way to increase the power conversion efficiency (η) of solar cells by reducing dye aggregation and charge recombination. Using density functional theory (DFT) and time-dependent DFT (TDDFT), this work explains from a theoretical perspective the higher η values of the TZC1 and TZC2 dyes compared to that of asymmetric the SQ sensitizer (YR6) as free dyes. The electronic properties, reorganization energies, absorption and emission spectra, ICT parameters, and photovoltage parameters of the TZC1, TZC2, and YR6 dyes were computed using the M06/6-31G(d,p) level of theory in the gas phase and CH₂Cl₂ solvent (CPCM method). Additionally, the mono- and co-adsorption processes of TZC-based sensitizers with YR6 on the anatase (001) surface were investigated using periodic DFT calculations with the PBE + U/PAW method and the dispersion correction of the Grimme method D3. The results reveal that the use of the co-sensitized led to significant stabilization of the formed complexes by at least 1.21 eV, the panchromatic effect on the absorption spectra, and an increase in the light-harvesting ability in the NIR region, which improves the performance of DSSCs.

Design and DFT study of nitrogen-rich donor systems for improved photovoltaic performance in dye-sensitized solar cells

Donor modifications, especially through N-annulation, for enhancing the structure–performance relationship of D–π–A systems for DSSC applications.

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.

The influence of π-linkers configuration on properties of 10-hexylphenoxazine donor-based sensitizer for dye-sensitized solar cell application - Theoretical approach

10-Hexylphenoxazine based dyes with A-(π)_n-D-(π)_n-A architecture is designed and investigated systematically for dye-sensitized solar cell (DSSC) application by Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TD-DFT). The designed sensitizers consist of 10-Hexylphenoxazine as electron donor and cyanoacrylic acid as an acceptor, connected by the Thiophene and Cyanovinyl π-spacers configurations with symmetrical and asymmetrical form. The effect of π-spacers configurations on the electronic and optical properties of the dyes is also investigated. The optimized structure, electronic properties and absorption characteristics of A-(π)_n-D-(π)_n-A dyes were investigated. The charge separation and polarization properties are analyzed by co-planarity, natural bond orbital (NBO), dipole moment and linear polarizability studies. The free energy change of electron injection and dye regeneration of the sensitizers are also calculated. The addition of a greater number of π-spacers improves the electronic, spectroscopic, optical, and free energy properties of the designed sensitizer. The DFT studies also reveal that the position of the π-spacers plays an important role in the electronic and spectroscopic properties.

Fine-Tuning by Triple Bond of Carbazole Derivative Dyes to Obtain High Efficiency for Dye-Sensitized Solar Cells with Copper Electrolyte

Three novel dyes consisting of a 5,8,15-tris(2-ethylhexyl)-8,15-dihydro-5H-benzo[1,2-b:3,4-b':6,5-b″]tricarbazole (BTC) electron-donating group and a 4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (BTBT) π-bridge with an anchoring group of phenyl carboxyl acid were synthesized and applied in dye-sensitized solar cells (DSCs).The AJ202 did not contain any triple bonds, the AJ201's ethynyl group was inserted between the BTC and BTBT units, and the AJ206's ethynyl group was introduced between the BTBT moiety and the anchor group. The inclusion and position of the ethynyl linkage in the sensitizer molecules significantly altered the electrochemical properties of these dyes, which can fine-tune the energy levels of the dyes. The best performing devices contained AJ206 as a sensitizer and a Cu(I/II) redox couple, which resulted in a power conversion efficiency (PCE) up to 10.8% under the standard AM 1.5 G illumination, which obtained PCEs higher than those from the devices that contained AJ201 (9.2%) and AJ202 (9.7%) under the same conditions. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the sensitizers were tuned to be well-suited for the Cu(I/II) redox potential and the Fermi level of TiO₂. The innovative synthesis of a tricarbazole-based donor moiety in a sensitizer used in combination with a Cu(I/II) redox couple has resulted in relatively high PCEs.

Conformational and Compositional Tuning of Phenanthrocarbazole-Based Dopant-Free Hole-Transport Polymers Boosting the Performance of Perovskite Solar Cells

Conjugated polymers are regarded as promising candidates for dopant-free hole-transport materials (HTMs) in efficient and stable perovskite solar cells (PSCs). Thus far, the vast majority of polymeric HTMs feature structurally complicated benzo[1,2-b:4,5-b’]dithiophene (BDT) analogs and electron-withdrawing heterocycles, forming a strong donor–acceptor (D–A) structure. Herein, a new class of phenanthrocarbazole (PC)-based polymeric HTMs (PC1, PC2, and PC3) has been synthesized by inserting a PC unit into a polymeric thiophene or selenophene chain with the aim of enhancing the π–π stacking of adjacent polymer chains and also to efficiently interact with the perovskite surface through the broad and planar conjugated backbone of the PC. Suitable energy levels, excellent thermostability, and humidity resistivity together with remarkable photoelectric properties are obtained via meticulously tuning the conformation and elemental composition of the polymers. As a result, PSCs containing PC3 as dopant-free HTM show a stabilized power conversion efficiency (PCE) of 20.8% and significantly enhanced longevity, rendering one of the best types of PSCs based on dopant-free HTMs. Subsequent experimental and theoretical studies reveal that the planar conformation of the polymers contributes to an ordered and face-on stacking of the polymer chains. Furthermore, introduction of the “Lewis soft” selenium atom can passivate surface trap sites of perovskite films by Pb–Se interaction and facilitate the interfacial charge separation significantly. This work reveals the guiding principles for rational design of dopant-free polymeric HTMs and also inspires rational exploration of small molecular HTMs.

Effect of Multidonor and Insertion Position of a Chromophore on the Photovoltaic Properties of Phenoxazine Dyes

Research and development of new organic semiconductor materials can never be terminated because any structural fine-tuning may result in an important impact on its application performance, although the effect may be negative in many cases. Herein, we designed and synthesized a series of phenoxazine-based dyes, YH1, YH2, YH3, and YH4, whose absorption spectrum, electrochemical cyclic voltammetry, theoretical calculation, dye-sensitized solar cell photovoltaic characteristics, and electrochemical AC impedance are used to analyze the photophysical, electrochemical, and photovoltaic performance of the materials, aiming to study the effect of multidonor and adjustment of the chromophore insertion position on their photovoltaic performance. When donor triphenylamine is added at the end of YH1 and YH3, the absorption spectrum and photovoltaic performance of dyes YH2 and YH4 improved a little. The improvement is much greater when the chromophore (ethylenedioxy)thiophene in YH1 and YH2 is adjusted and inserted on the other side of phenoxazine and the energy conversion efficiencies (photon-to-current conversion efficiency) of the resulting dyes YH3 and YH4 reach 8.02 and 8.97%, respectively, which are 23 and 25% higher than those of YH1 and YH2, respectively. Although the improvement may be because of factors such as the dihedral angle, the result will undoubtedly provide some reference for the future study of the relationship between the structure and performance of organic dyes.

Double Linker Triphenylamine Dyes for Dye-Sensitized Solar Cells

Most organic dyes synthesized for dye-sensitized solar cells (DSC) use a single linker group to bind to the metal oxide photo-anode. Here we describe the synthesis and testing of two new triphenylamine dyes containing either two carboxylic acids 5-[2-(4-diphenylamino-phenyl)-vinyl]-isophthalic acid (10) or two cyanoacrylic acids (2Z, 2′Z)-3, 3′-(5-((E)-4-(diphenylamino) styryl)-1, 3-phenylene) bis (2-cyanoacrylic acid) (8) as linker groups. Full characterization data are reported for these dyes and their synthetic intermediates. DSC devices have been prepared from these new dyes either by passive or fast dyeing and the dyes have also been tested in co-sensitized DSC devices leading to a PCE (η = 5.4%) for the double cyanoacrylate linker dye (8) co-sensitized with D149. The dye:TiO2 surface interactions and dye excitations are interpreted using three modelling methods: density functional theory (at 0 K); molecular dynamics (at 298 K); time dependent density functional theory. The modelling results show the preferred orientation of both dyes on an anatase (1 0 1) TiO2 surface to be horizontal, and both the simulated and experimental absorption spectra of the dye molecules indicate a red shifted band for (8) compared to (10). This is in line with broader light harvesting and Jsc for (8) compared to (10).

Theoretical analysis of the absorption spectrum, electronic structure, excitation, and intramolecular electron transfer of D-A'-π-A porphyrin dyes for dye-sensitized solar cells

A series of porphyrin dyes with D-A'-π-A structure were designed and theoretically investigated by DFT and TD-DFT methods. Different electron-withdrawing auxiliary acceptor units were introduced into the dye molecule skeleton to shed light on how the type and position of auxiliary acceptors influence the photoelectric performance of the dyes. It was found that the introduction of electron-withdrawing units, BTD, TPZ, BTZ, PP and DPPZ, between the Zn-porphyrin core and the anchoring group had a significantly positive influence on the performance of the dye molecules. Also, more appropriate electron distribution in the molecular orbital and the lower HOMO-LUMO energy gap, more extensive absorption coverage, higher light-harvesting efficiency, lower orbital overlap, and more effective long-range intramolecular electron transfer (IET) process can be achieved as compared to the reference dye. Among these five electron-withdrawing units, BTD and TPZ had the effect leading to the greatest improvement and therefore, are the best candidates for auxiliary acceptors. The calculated results indicated that the longitudinal π-conjugation of the electron-withdrawing unit also had an obvious effect on the performance of the dye molecule, and NTD is expected to be a more effective auxiliary acceptor than BTD. The effects of the relative positions of the auxiliary acceptors in the dye molecular skeleton were also investigated. A comparative study of AX1-3 and AA1-3 showed that the introduction of BTD, TPZ and BTZ units between the donor part and the Zn-porphyrin core may have a negative impact on the performance of the dyes. Our studies are expected to provide new insight for the design and screening of high-efficiency porphyrin dyes for DSSCs applications.

The Hagfeldt Donor and Use of Next-Generation Bulky Donor Designs in Dye-Sensitized Solar Cells

"The Hagfeldt donor" is a bulky triarylamine building block with four alkyl chains in a 3-dimensional arrangement that is used with organic dyes in dye-sensitized solar cells (DSCs) in over 140 publications. Many of the highest performing DSC devices in literature make use of this group due to exceptional TiO₂ surface protection properties, which slows recombination of electrons in TiO₂ with the electrolyte. Importantly, record-setting cobalt and copper redox shuttle-based DSCs require exceptional surface protection to slow a facile recombination of electrons to these positively charged redox shuttles. Several syntheses have emerged for the Hagfeldt donor due to the need for iterative aryl-halide cross- coupling reactions complicating a straightforward route. Six synthetic strategies found in literature are described along with the challenges of each route. A recent method that has been put forward in the literature as a scalable, regioisomerically pure route is highlighted.

Investigations of New Phenothiazine-Based Compounds for Dye-Sensitized Solar Cells with Theoretical Insight

New D-π-D-π-A low-molecular-weight compounds, based on a phenothiazine scaffold linked via an acetylene unit with various donor moiety and cyanoacrylic acid anchoring groups, respectively, were successfully synthesized. The prepared phenothiazine dyes were entirely characterized using nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The compounds were designed to study the relationship between end-capping donor groups’ structure on their optoelectronic and thermal properties as well as the dye-sensitized solar cells’ performance. The effect of π-conjugation enlargement by incorporation of different heterocyclic substituents possessing various electron–donor affinities was systematically experimentally and theoretically examined. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were implemented to determine the electronic properties of the novel molecules.

New Oxindole-Bridged Acceptors for Organic Sensitizers: Substitution and Performance Studies in Dye-Sensitized Solar Cells

New D-π-A configured organic sensitizers featuring halogen-substituted oxindole-bridged acceptor units have been synthesized for dye-sensitized solar cells applications. Among fluorine, bromine, and iodine substitution, the cell based on bromine incorporated dye exhibited the highest efficiency. The oxindoles in these sensitizers were found to assist the electron injection through the chelation of their amide carbonyl groups to the TiO2 surface. This study provides an alternate approach for future rational dye design to gain excellent DSSC performance.

Fusing Thienyl with N-Annulated Perylene Dyes and Photovoltaic Parameters for Dye-Sensitized Solar Cells

Due to the role of dyes in dye-sensitized solar cells (DSSCs), designing novel dye sensitizers is an effective strategy to improve the power conversion efficiency. To this end, the fundamental issue is understanding the sensitizer's trilateral relationship among its molecular structure, optoelectronic properties, and photovoltaic performance. Considering the good performance of N-annulated perlyene dye sensitizers, the geometries, electronic structures, and excitations of the selected representative organic dye sensitizers C276, C277, and C278 as well as dyes adsorbed on TiO₂ clusters were calculated in order to investigate the relationship between molecular structures and properties. It was found that fusing thienyl to N-annulated perlyene can elevate the highest occupied molecular orbital (HOMO) energy, reduce the orbital energy gap, increase the density of states, expand the HOMO to the benzothiadiazole moiety, enhance the charge transfer excitation, elongate the fluorescence lifetime, amplify the light harvesting efficiency, and induce a red-shift of the absorption spectra. The transition configurations and molecular orbitals of the dye-adsorbed systems support that the electron injection in DSSCs based on these dyes is a fast mode. Based on extensive analysis of the electronic structures and excitation properties of these dye sensitizers and the dye-adsorbed systems, we present new quantities as open-circuit voltage and short-circuit current density descriptors that celebrate the quantitative bridge between the photovoltaic parameters and the electronic structure-related properties in order to expose the relationship between properties and performance. The results of this work are critical for the design of novel dye sensitizers for solar cells.

Thioalkyl-Functionalized Bithiophene (SBT)-Based Organic Sensitizers for High-Performance Dye-Sensitized Solar Cells

A series of 3,3'-dithioalkyl-2,2'-bithiophene (SBT)-based organic chromophores were designed and developed for the use in dye-sensitized solar cells (DSSCs). By appropriate structural modification of the SBT π-linkers with different alkyl chains and conjugated thiophene units, chromophore aggregation and interfacial charge recombination could be suppressed to a remarkable degree. Single-crystal and optical/electrochemical data clearly show that the SBT core is nearly planar with the torsional angle <1°, likely via S(alkyl)···S(thiophene) intramolecular locks. Therefore, this highly π-conjugated unit should enhance panchromatic light-harvesting and prove to be an excellent core for organic dye. For comparison, the 3,3'-dialkyl-2,2'-bithiophene (BT)-based dye was also prepared. Under 1 sun (100 mW cm^-2) illumination, an optimized SBT-6 dye-sensitized cell indicates a short-circuit current density (J_SC) of 17.21 mA cm^-2, an open-circuit voltage (V_OC) of 0.78 V, and a fill factor (FF) of 0.71, corresponding to a power conversion efficiency (η) of 9.47%, which is nearly two times higher than that of alkylated bithiophene (BT)-based chromophores. Finally, the proposed sensitizer SBT-6 exhibited an excellent η of 23.57% under the T5 fluorescent illumination of 6000 lux. To the best of our knowledge, this is the highest power conversion efficiencies (PCE) value reported to date among the studied thiophene or bithiophene-based chromophores.

A Blue Photosensitizer Realizing Efficient and Stable Green Solar Cells via Color Tuning by the Electrolyte

Semitransparent dye-sensitized solar cells (DSCs) are appealing as aesthetically pleasing and colorful see-through photovoltaics. Green semitransparent DSCs have been presented, but the best ones rely on green zinc porphyrin photosensitizers and high volatile electrolytes. For potential outdoor applications, the zinc porphyrin DSCs employing ionic liquid electrolytes merely reached a power conversion efficiency (PCE) of 6.3% even with opaque mesoporous TiO₂ films. Herein, the new green DSC is realized by using a blue organic photosensitizer in conjunction with an orange ionic-liquid-based electrolyte, presenting a simple and an effective path for color tuning of photovoltaics. The new approach allows for broadly modulating the color from spring green to cyan by tuning the contributions of the light absorption by the dye-sensitized TiO₂ film and the electrolyte layer. The new semitransparent DSCs with spring green to cyan colors have PCEs ranging from 6.7% to 8.1% and show stability for 1000 h under accelerated ageing test at 80 °C, superior to the zinc porphyrin DSCs. The findings pave a new way to achieve efficient and stable colorful solar cells.