Hyperbranched conjugated polymers with donor-π-acceptor architecture as organic sensitizers for dye-sensitized solar cells

Three novel hyperbranched conjugated polymers (H-tpa, H-cya, and H-pca) with the same conjugated core structure and different functional terminal units were synthesized and applied in dye-sensitized solar cells (DSSCs) as photosensitizers. The photophysical, electrochemical and photovoltaic properties of the three hyperbranched conjugated polymers (HBPs) were investigated in detail. The results showed that donor-π-acceptor architecture in hyperbranched molecule benefited intramolecular charge transfer and consequently increased the generation of photocurrent. The three-dimensional (3D) steric configuration of HBPs could effectively suppress the aggregation of dyes on TiO₂ film, which is beneficial for achieving good photovoltaic performances. Among the three hyperbranched dyes, the highest power conversion efficiency (η) of 3.93% (J_sc = 8.78 mA/cm², V_oc = 0.65 V, FF = 0.688) was obtained with a DSSC based on H-pca dye upon the addition of the same mass ratio chenodeoxycholic acid (CDCA) as coadsorbent under AM 1.5 irradiation with 100 mW/cm² simulated sunlight.     

Mechanofluorochromism of heteropolycyclic donor-π-acceptor type fluorescent dyes

This paper reports that mechanofluorochromism is found for a series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorophores (1a-5a) with different p-substituted phenyl groups as acceptor and dibutylamino groups as donor. Grinding of as-recrystallized dyes 1a-3a with strong electron-accepting group induces a fluorescent color change with an enhanced quantum yield and the fluorescent color is recovered by heating or exposure to solvent vapor. On the basis of experimental results and semi-empirical molecular orbital calculations (AM1 and INDO/S), we clarified that the mechanofluorochromism is attributed to a reversible switching between crystalline and amorphous states with changes of dipole-dipole interaction and intermolecular π-π interaction by changes of the densities of the solids before and after grinding.     

Organic dye bearing asymmetric double donor-π-acceptor chains for dye-sensitized solar cells

A novel efficient metal free sensitizer containing asymmetric double donor-π-acceptor chains (DC) was synthesized for dye-sensitized solar cells (DSSCs). Comparing to 3.80%, 4.40% and 4.64% for the DSSCs based on the dyes with single chain (SC1, SC2) and cosensitizers (SC1 + SC2), the overall conversion efficiency reaches 6.06% for DC-sensitized solar cells as a result of its longer electron lifetime and higher incident monochromatic photon-to-current conversion efficiency.     

Synthesis and photovoltaic performances of donor-π-acceptor dyes utilizing 1,3,5-triazine as π spacers

A series of new-type of donor-π-acceptor dyes (TCT-1-6) utilizing 1,3,5-triazine as π spacers were synthesized. These dyes were characterized by ¹H NMR, ESI-MS, EA, and X-ray crystallography. Their photovoltaic performances were also investigated. An overall photon-to-electron conversion efficiency of 1.8% was achieved with the DSSC based on the dye TCT-1(J_sc = 3.33 mA/cm², V_oc = 757 mV, FF = 71.8%) under AM 1.5G illumination (100 mW/cm²).     

Structure-property relationships of organic dyes with D-π-A structure in dye-sensitized solar cells

Organic dyes with a D-π-A structure have drawn increasing attention as sensitizers in dye-sensitized solar cells (DSSCs), due to their rich photophysical properties, easy molecular tailoring, and low-cost production. This review mainly focuses on the relationship between dye structure and photovoltaic properties for organic dyes containing cyanoacrylic acid as both an anchor and an acceptor. This review also introduces different donors and π-conjugation units as building blocks for sensitizer synthesis.     

Novel D-π-A type II organic sensitizers for dye sensitized solar cells

Four organic donor-π-conjugated-acceptor (D-π-A) type II dyes with different thiophene linkers are reported for dye sensitized solar cells (DSSCs). For the first time, a donor (triphenylamine) was introduced in type II sensitizers, and 2-hydroxybenzonitrile as acceptor/anchoring moiety was covalently linked TiO₂ particles. The dye LS203 in this series gives the best solar energy conversion efficiency of 3.4%, with J_sc = 7.4 mA cm⁻², V_oc = 0.67 V, FF = 0.69, the maximum IPCE value reaches 66.9%.     

Femtosecond to millisecond studies of electron transfer processes in a donor-(π-spacer)-acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.     

Dye-sensitized solar cells based on donor-π-acceptor fluorescent dyes with a pyridine ring as an electron-withdrawing-injecting anchoring group

A new‐type of donor–acceptor π‐conjugated (D‐π‐A) fluorescent dyes NI3 – NI8 with a pyridine ring as electron‐withdrawing‐injecting anchoring group have been developed and their photovoltaic performances in dye‐sensitized solar cells (DSSCs) are investigated. The short‐circuit photocurrent densities and solar energy‐to‐electricity conversion yields of DSSCs based on NI3 – NI8 are greater than those for the conventional D‐π‐A dye sensitizers NI1 and NI2 with a carboxyl group as the electron‐withdrawing anchoring group. The IR spectra of NI3 – NI8 adsorbed on TiO₂ indicate the formation of coordinate bonds between the pyridine ring of dyes NI3 – NI8 and the Lewis acid sites (exposed Tiⁿ⁺ cations) of the TiO₂ surface. This work demonstrates that the pyridine rings of D‐π‐A dye sensitizers that form a coordinate bond with the Lewis acid site of a TiO₂ surface are promising candidates as not only electron‐withdrawing anchoring group but also electron‐injecting group, rather than the carboxyl groups of the conventional D‐π‐A dye sensitizers that form an ester linkage with the Brønsted acid sites of the TiO₂ surface.     

Azulene – Thiophene – Cyanoacrylic acid dyes with donor-π-acceptor structures. Synthesis,characterisation and evaluation in dye-sensitized solar cells

We report the synthesis of five new azulene containing dyes, having D-π-A type structures. These dyes are synthesised using a sulfonium salt cross-coupling reaction. The dyes have been evaluated spectroscopically, electrochemically, crystallographically, and as sensitizers in dye-sensitized solar cells. We propose a rationale for the dyes' spectroscopic properties and performance in cells, based on conformational data derived from their crystal structures.     

A-DA′D-A non-fullerene acceptors for high-performance organic solar cells

Since the world-record power conversion efficiency of 15.7% was achieved for organic solar cells(OSCs) in 2019, the newly developed non-fullerene acceptor(NFA) Y6 with an A-DA′D-A structure(A denotes an electron-accepting moiety, D denotes an electron-donating moiety) has attracted increasing attention. Subsequently, many new A-DA′D-A NFAs have been designed and synthesized, and the A-DA′D-A NFAs have played a significant role in the development of high-performance non-fullerene organic solar cells(NF-OSCs). Compared with the classical A-D-A-type acceptors, A-DA′D-A NFAs contain an electrondeficient core(such as benzothiadiazole(BT), benzotriazole(BTA), quinoxaline(Qx), or their derivatives) in the ladder-type fused rings to fine-tune the energy levels, broaden light absorption and achieve higher electron mobility of the NFAs. This review emphasizes the recent progress on these emerging A-DA′D-A(including Y-series) NFAs. The synthetic methods of DA′D-fused rings are introduced. The relationships between the chemical structure of the A-DA′D-A NFAs and the photovoltaic performance of the corresponding OSCs are summarized and discussed. Finally, issues and prospects for further improving photovoltaic performance of the OSCs are also proposed.     

Influence of end-capped group on structural and electronic properties of the At-π-Ac-π-At small molecule donor for high-performance organic solar cells

Structural Chemistry - To achieve high efficient donors for organic solar cells, structural, electronic, and optical properties of the At-π-Ac-π-At small molecules were systematically...     

Low-bandgap polymers with quinoid unit as π bridge for high-performance solar cells

To construct efficient low band gap polymers, increasing the Quinone structure of the polymer backbone could be one desirable strategy. In this work, two D–Q–A–Q polymers P1 and P2 were designed and synthesized with thiophenopyrrole diketone(TPD) and benzothiadiazole(BT) unit as the core and ester linked thieno[3,4-b]thiophene(TT) segment as π-bridging, and the main focus is to make a comparative analysis of different cores in the influence of the optical, electrochemical, photochemical and morphological properties. Compared with the reported PBDTT_(EH–)TBTT_(HD-i), P1 exhibited the decreased HOMO energy level of-5.38 e V and lower bandgap of 1.48 e V. Furthermore, when replaced with BT core, P2 showed a red-shifted absorption profile of polymer but with up-shifted HOMO energy level. When fabricated the photovoltaic devices in conventional structure, just as expected, the introduction of ester substituent made an obvious increase of V_(OC) from 0.63 to 0.74 V for P1. Besides, due to the deep HOMO energy level,higher hole mobility and excellent phase separation with PC_(71) BM, a superior photovoltaic performance(PCE = 7.13%) was obtained with a short-circuit current density(J_(SC)) of 14.9 m A/cm~2, significantly higher than that of P2(PCE = 2.23%). Generally, this study highlights that the strategy of inserting quinoid moieties into D–A polymers could be optional in LBG-polymers design and presents the importance and comparison of potentially competent core groups.     

The design,synthesis, and characterization of D-π-A-π-A type organic dyes as sensitizers for dye-sensitized solar cells (DSSCs)

New D-π-A-π-A type organic dyes were synthesized and characterized as sensitizers for dye-sensitized solar cells (DSSCs). These dyes showed wide absorption spectra (300–625 nm) and high molar extinction coefficients (ε_467 nm = 60,911 M⁻¹ cm⁻¹). As dye sensitizers in DSSC, the D-π-A-π-A dye having a cyanoacrylic acid as an acceptor gave the best cell performance with a short-circuit photocurrent density (J_sc) of 7.14 mA/cm², an open-circuit voltage (V_oc) of 0.62 V, and a fill factor (FF) of 0.72, corresponding to an overall conversion efficiency η of 3.19%.     

Latest progress on fully non-fused electron acceptors for high-performance organic solar cells

Benefitting from the development of non-fullerene acceptors(NFAs), remarkable advances have been achieved with the power conversion efficiency(PCE) exceeding 19% over the last few years. However, the major achievement comes from fused ring electron acceptors(FREAs) with complex structures, leading to high cost. Hence, it is urgent to design new materials to resolve the cost issues concerning basic commercial requirements of organic solar cells. Recently, great progress has been made in fully non...     

Effects of structure and electronic properties of D-π-A organic dyes on photovoltaic performance of dye-sensitized solar cells

Herein,we examine the performance of dye-sensitized solar cells containing five D-π-A organic dyes designed by systematic modification of π-bridge size and geometric structure.Each dye has a simple push-pull structure with a triarylamino group as an electron donor,bithiophene-4,4-dimethyl-4 H-cyclopenta 1,2-b:5,4-b’]dithiophene(M11),4,4-dimethyl-4 H-cyclopenta[1,2-b:5,4-b’]dithiophenethiophene(M12),thiophene-4,4-dimethyl-4 H-cyclopenta[1,2-b:5,4-b’]dithiophene(M13),4,4-dimethyl-4 H-cyclopenta[1,2-b:5,4-b’]dithiophene-benzene(M14),and 4,4-dimethyl-4 H-cyclopenta[1,2-b:5,4-b’]dithiophene(M15)units asπ-bridges,and cyanoacrylic acid as an electron acceptor/anchor.The extension of theπ-bridge linkage favors wide-range absorption but,because of the concomitant molecular volume increase,hinders the efficient adsorption of dyes on the TiO₂ film surface.Hence,higher loadings are achieved for smaller dye molecules,resulting in(i)a shift of the TiO₂ conduction band edge to more negative values,(ii)a greater photocurrent,and(iii)suppressed charge recombination between the photoanode and the redox couple in the electrolyte.Consequently,under one-sun equivalent illumination(AM 1.5 G,100 mW/cm²),the highest photovoltage,photocurrent,and conversion efficiency(η=7.19%)are observed for M15,which has the smallest molecular volume among M series dyes.     

π-Extension and chlorination of non-fullerene acceptors enable more readily processable and sustainable high-performance organic solar cells

Organic solar cells(OSCs) processed without halogenated solvents and complex treatments are essential for future commercialization. Herein, we report three novel small molecule acceptors(NFAs) consisting of a Y6-like core but with π-extended naphthalene with progressively more chlorinated end-capping groups and a longer branched chain on the Nitrogen atom. These NFAs exhibit good solubilities in nonchlorinated organic solvents, broad optical absorptions, close π-π stacking distances(3.63–3.84 ?)...     

Theoretical investigation of novel carbazole-fluorene based D-π-A conjugated organic dyes as dye-sensitizer in dye-sensitized solar cells (DSCs)

The ground state structure and frontier molecular orbital of newly synthesized carbazole-fluorene based D-π-A organic dyes, CFP1A, CFP2A, CFP1CA, and CFP2CA, were theoretically investigated using density functional theory (DFT) at B3LYP/6-31G(d,p) level. These dye molecules have been constructed based on carbazole-fluorene as the electron-donating moiety while introducing benzene units as π-spacer connected to different anchor groups, such as acrylic acid and cyanoacrylic acid, as acceptors. The electronic vertical excitation energies and absorption wavelength were carried out using time-dependent DFT (TD-DFT). Furthermore, the adsorptions of phenylacrylic acid and phenylcyanoacrylic acid on the TiO(2) anatase (101) surface were carried out by means of quantum-chemical periodic calculations employing periodic PBE functional with DNP basis set. The results promise that anchor dyes with strong withdrawing CN group have easier injected electron to the conduction band of semiconductor implying that CFP1CA and CFP2CA show better performance among four dyes. Additionally, the intramolecular charge transfers (ICT) from electron donor group to anchoring group of CFP1CA and CFP2CA have shown better performance. The calculated results provide the efficiency trend of our new dyes as CFP1CA ≈ CFP2CA > CFP1A ≈ CFP2A which are excellently agree with experimental observation.     

The role of π electrons in the formation of benzene-containing lithium-bonded complexes

The intermolecular interactions in C₆H₆???LiX (X=OH, NH₂, F, Cl, Br, NC, CN) complexes are investigated by using second‐order Møller–Plesset perturbation theory (MP2) calculations and quantum theory of “atoms in molecules” (QTAIM) studies, and the role of π electrons is studied in the formation of these benzene‐containing lithium‐bonded complexes. The molecular electrostatic potentials of benzene and LiX determine the geometries of the lithium‐bonded complexes. The electron densities at the lithium bond critical points in the πC₆H₆???LiX complexes are obviously stronger than those in the σC₆H₆???LiX complexes, which indicates that the intermolecular interactions in the C₆H₆???LiX complexes are mainly attributable to π‐type interaction. The topological and energy properties at the lithium bond critical points in both the C₆H₆???LiX and πC₆H₆???LiX complexes are linear with the interaction energies, thereby showing the crucial role of the π electrons in the formation of these complexes. Electron localization function (ELF) analysis indicates that the formation of the lithium bonds leads to the reduction of the ELF π‐electron density and volume, and the reduction of the π‐electron volume is linear with the interaction energies with the correction coefficient 0.9949.     

Molecular engineering and cosensitization for developing efficient solar cells based on porphyrin dyes with an extended π framework

<正>Dye-sensitized solar cells(DSSCs)have a promising future because of their low production cost,easy fabrication and relatively high solar energy conversion efficiency.In the past two decades,increasing interests have been focused on the improvement of cell efficiencies.Thus,many efficient sensitizers with donor-π-acceptor frameworks have been successfully developed.In this respect,porphyrins have