A simple small molecule as the acceptor for fullerene-free organic solar cells

A simple small molecule named DICTiF was designed,synthesized and used as the acceptor for solution processed bulk-heterojunction solar cells with polymer PBDB-T as the donor.A power conversion efficiency of 7.11%was obtained.     

Molecular engineering of organic sensitizers containing p-phenylene vinylene unit for dye-sensitized solar cells

Three organic sensitizers containing bis-dimethylfluorenyl amino donor and a cyanoacrylic acid acceptor bridged by p-phenylene vinylene unit were synthesized. The power conversion efficiency was quite sensitive to the length of bridged phenylene vinylene groups. A nanocrystalline TiO2 dye-sensitized solar cell was fabricated using three sensitizers. The maximum power conversion efficiency of JK-59 reached 7.02%.     

Molecular design of organic dyes based on vinylene hexylthiophene bridge for dye-sensitized solar cells

Three donor-(π-spacer)-acceptor (D-π-A) organic dyes, containing different groups (triphenylamine, di(p-tolyl)phenylamine, and 9-octylcarbazole moieties) as electron donors, were designed and synthesized. Nanocrystalline TiO₂ dye-sensitized solar cells were fabricated by using these dyes. It was found that the variation of electron donors in the D-π-A dyes played an important role in modifying and tuning photophysical properties of organic dyes. Under standard global AM 1.5 solar condition, the DSSC based on the dye D2 showed the best photovoltaic performance: a short-circuit photocurrent density (J _sc ) of 13.93 mA/cm², an open-circuit photovoltage (V _oc ) of 0.71 V, and a fill factor (FF) of 0.679, corresponding to solar-to-electric power conversion efficiency (η) of 6.72%. Supported by the Key Project of Hunan Province of China (Grant No. 2008FJ2004), Natural Science Foundation of Hunan Province of China (Grant Nos. 09JJ3020 & 09JJ4005), and Scientific Research Fund of Hunan Provincial Education Department (Grant No. 08C888).     

Synthesis and optical properties of perylene diimide derivatives with triphenylene-based dendrons linked at the bay positions through a conjugated ethynyl linkage

A number of groups including trimethylsilyl, phenyl, triphenylene, and triphenylene-based dendron have been linked to the bay positions of a perylene diimide (PDI) core through an ethynyl bridge. The photophysical properties of the resulting bay-substituted PDI derivatives have been carefully studied in different solvents and as thin films. Without any capping group, the two ethynyl bay-substituted PDI derivates PAT and PRT both aggregate strongly even in dilute solutions but in different perylene-perylene π–π stacking modes; PRT aggregates through slipped (or longitudinal) stacking while PAT self-assembles by rotational (or cross) stacking. With capping groups, the perylene core stacking is completely blocked for PATS in both solution and solid film. For PRTS, the slipped stacking is observed only for its film sample, while for PTB, association only occurs after excitation (excimer formation). When triphenylene or triphenylene-based G1 dendron is attached to the acetylene bridge, the resulting donor–acceptor systems (PTG0 and PTG1) exhibit strong electronic coupling between the dendritic donors and the PDI acceptor, leading to significantly red-shifted absorption bands. The conjugated linkage also facilitates photoinduced electron transfer from the triphenylene or triphenylene dendron to the PDI core, effectively quenching fluorescence emissions of both the donor and the acceptor. The significantly red-shifted absorption bands and the efficient photoinduced electron transfer observed on PTG0 and PTG1 indicate that these new PDI derivatives may find applications in solar cells.     

New perylene diimide electron acceptors for organic electronics: Synthesis,optoelectronic properties and performance in perovskite solar cells

Two new non-fullerene acceptors based on perylene diimide with acetylenic bridges were designed and synthesized employing Stille and Sonogashira coupling reactions as the key steps. Their optical and electronic properties were explored by UV–VIS spectroscopy and cyclic voltammetry, and energies of frontier molecular orbitals were estimated. Their preliminary studies in perovskite solar cells as electron transport materials showed the best power conversion efficiency for photocells of 14.18% value.     

Bridge effect on the charge transfer and optoelectronic properties of triphenylamine-based organic dye sensitized solar cells: theoretical approach

Research on Chemical Intermediates - In this work, a series of six organic dyes-sensitized solar cells (DSSCs) combining various π-bridges with a fixed donor (triphenylamine) and a fixed...     

Tuning the HOMO and LUMO energy levels of organic chromophores for dye sensitized solar cells

A series of organic chromophores have been synthesized in order to approach optimal energy level composition in the TiO2-dye-iodide/triiodide system in the dye-sensitized solar cells. HOMO and LUMO energy level tuning is achieved by varying the conjugation between the triphenylamine donor and the cyanoacetic acid acceptor. This is supported by spectral and electrochemical experiments and TDDFT calculations. These results show that energetic tuning of the chromophores was successful and fulfilled the thermodynamic criteria for dye-sensitized solar cells, electrical losses depending on the size and orientation of the chromophores were observed.     

Altering alkyl-chains branching positions for boosting the performance of small-molecule acceptors for highly efficient nonfullerene organic solar cells

The emergence of the latest generation of small-molecule acceptor(SMA) materials,with Y6 as a typical example,accounts for the surge in device performance for organic solar cells(OSCs).This study proposes two new acceptors named Y6-C2 and Y6-C3,from judicious alteration of alkyl-chains branching positions away from the Y6 backbone.Compared to the Y6,the Y6-C2 exhibits similar optical and electrochemical properties,but better molecular packing and enhanced crystallinity.In contrast,the Y6-C3 shows a significant blue-shift absorption in the solid state relative to the Y6 and Y6-C2.The as-cast PM6:Y6-C2-based OSC yields a higher power conversion efficiency(PCE) of 15.89% than those based on the Y6(15.24%) and Y6-C3(13.76%),representing the highest known value for as-cast nonfullerene OSCs.Prominently,the Y6-C2 displays a good compatibility with the PC_(71)BM.Therefore,a ternary OSC device based on PM6:Y6-C2:PC_(71)BM(1.0:1.0:0.2) was produced,and it exhibits an outstanding PCE of 17.06% and an impressive fill factor(FF) of 0.772.Our results improve understanding of the structureproperty relationship for state-of-the-art SMAs and demonstrate that modulating the structure of SMAs via fine-tuning of alkylchains branching positions is an effective method to enhance their performance.     

Synthesis and properties of perylenetetracarboxylic diimide dimers linked at the bay position with conjugated chain of different length

Three perylenetetracarboxylic diimide (PDI) dimers linked with a conjugated chain of different lengths have been designed and prepared. The UV-Vis absorption and fluorescence spectra of these three dimers revealed different photophysical properties owing to the different length of the linkage. The intermolecular π-π interactions were found to be enhanced significantly with the increase in the length of the linkage and therefore induced different aggregation behaviors of these molecules. The structure of the molecular aggregates was investigated by X-ray diffraction (XRD), and the morphology of the aggregates was examined by atomic force microscopy (AFM). One-dimensional fibers were observed for the aggregates of compounds 2 and 3, and thin solid films were observed for the aggregates of compound 1.     

Modulating morphology via side-chain engineering of fused ring electron acceptors for high performance organic solar cells

In this work, four fused ring electron acceptors(FREAs), 2F-C5, 2F-C6, 2F-C8 and 2F-C10, are developed to investigate the effect of side-chain size on the molecular properties and photovoltaic performance of FREA systematically. The elongation of side-chains in the FREAs not only improves their solubility in the processing solvent, but also enhances their miscibility with the donor PBDB-T. It helps the FREA diffuse into the donor PBDB-T during film-formation, thus leading to the decrease in domain size and domain purity from PBDB-T:2F-C5 to PBDB-T:2F-C10 blend films in sequence. The smaller domain size affords more D/A interfaces to benefit exciton dissociation and inhibit monomolecular recombination. However, severe bimolecular recombination occurs when the domain purity decreases to a critical point. Due to the dual function of the increment of side-chain length, both short-circuit current density(J_(SC)) and fill factor(FF) of devices exhibit an evolution of first increasing then decreasing from 2F-C5, 2F-C6, 2F-C8 to 2F-C10 based OSCs. The PBDB-T:2F-C8 based OSCs get a fine balance in morphology with moderate domain size as well as high domain purity simultaneously for the least charge carrier recombination, thus achieving the highest power conversion efficiency of 12.28% with the best J_(SC)(21.27 mA cm~(-2)) and FF(71.96%).     

Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free organic dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br(-)/Br(3)(-) and I(-)/I(3)(-). The effects of additive Li(+) ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addition of Li(+) ions in electrolytes can broaden the absorption spectra of the dyes on TiO(2) films and shift both the LUMO levels of the dyes and the conduction band of TiO(2), thus leading to the increase of J(sc) and the decrease of V(oc). Upon using Br(-)/Br(3)(-) instead of I(-)/I(3)(-), a large increase of V(oc) is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO(2), as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochemical impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φ(r)) depend on the potential difference (the driving forces) between the oxidized dyes and the Br(-)/Br(3)(-) redox couple. For the dyes for which the HOMO levels are more positive than the redox potential of Br(-)/Br(3)(-) sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br(-)/Br(3)(-), insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.     

Progress of the key materials for organic solar cells

Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials and device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecular donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.     

Tuning the moisture stability of metal-organic frameworks by incorporating hydrophobic functional groups at different positions of ligands

The introduction of hydrophobic groups (e.g. methyl) at the most adjacent sites of each and every coordinating nitrogen atom of the bipyridine pillar linker in a carboxylate-based bridging MOF could shield the metal ions from attack by water molecules, and thus enhance the water resistance of the MOF structure significantly.     

Alkoxy substitution on simple non-fused electron acceptors for tuning the photoelectric properties of organic solar cells

In order to identify high-performance non-fused ring electron acceptors for bulk heterojunction (BHJ) solar cells, six structurally diverse molecules are designed and categorized into two series. The first series is anchored by R1 as the reference molecule, featured fixed BDT, IC-2F end groups and modified π bridges. The second series is anchored by R2 as the reference molecule, incorporated a terminal IC-2F and a central core modified with EDOT bridge. The electronic structure and photoelectric properties of all acceptor molecules were investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Charge transfer matrix (CTM), Density of states (DOS) and Reorganization energy (RE) etc. were analyzed to provide fundamental knowledge on charge transport and electronic excitation. Among the studied molecules, W2 exhibited a smaller energy gap (1.74 eV) compared to the other molecules, effectively transferring its electron from HOMO to LUMO. At the same time, the W2 molecule has excellent V_oc and FF. Furthermore, W2 displayed the largest λ_max redshift compared to R1. Although W3 had a smaller value of λ_h, the comprehensive photovoltaic parameters of W2 were more excellent. The research results not only demonstrated the feasibility of introducing different alkoxy groups to alter the structure of the π bridge and central core is a feasible method for constructing high-performance NFREAs, but also highlighted that BDT cores combined with EDOT bridges are among the most promising small molecule acceptors (SMAs) that could be considered as reasonable candidates for synthesis and incorporation into organic solar cells. The results of this study are expected to provide seminal ideas for the design of high-performance non-fullerene acceptors.     

Tuning the adsorption and fluorescence properties of aminal‐linked porous organic polymers through N‐heterocyclic group decoration

Aiming at tuning the adsorption and fluorescence properties of targeted porous organic polymer, four new aminal‐linked porous organic polymers (NAPOPs) were synthesized through the reaction of 1,4‐Bis(4,6‐diamino‐s‐triazin‐2‐yl) benzene (BATB) with four kinds of aldehydes substituted with different N‐heterocyclic groups. Among the polymers, NAPOP‐3 decorated with 5‐phenyl‐tetrazole group shows the largest CO₂ adsorption capacity (2.52 mmol g^?1 at 273 K and 100 kPa) because of its relative large surface area, while NAPOP‐1 decorated with piperazine groups shows relative large CO₂/N₂ adsorption selectivity (77 at 273 K and 100 kPa), attributable to its large CO₂ adsorption heats and cabined pore (<4 Å). Meanwhile, NAPOP‐1 and ?3 exhibit high adsorption rate toward iodine with a high capacity (>240 wt %). In addition, different luminescence emissions were also observed for NAPOPs, indicating different intramolecular charger transfer occurred inside polymer networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1724–1730     

Tuning the Bergman cyclization by introduction of metal fragments at various positions of the enediyne. Metalla-Bergman cyclizations

We expand the scope of the Bergman cyclization by exploring computationally the rearrangement of two osmaenediynes and one rhodaenediyne. The three hypothetical metallaenediynes are constructed by substituting the 14-electron Os(PH3)3 fragment for the C fragment, or the 15-electron Os(PH3)3H or Rh(PH3)3 fragments for the sp2 CH fragment, of 3-ene-1,5-diyne. This replacement is guided by the isolobal analogy and previous metallabenzene chemistry. The rearrangement of osmaenediyne with an Os(PH3)3 fragment in place of C is exothermic by 3 kcal/mol (the parent Bergman reaction is computed to be endothermic by 5 kcal/mol) and associated with a significant decrease in the barrier to rearrangement to 13 kcal/mol (the Ea of the parent reaction computed at the same level of theory is 33 kcal/mol). The replacement of a CH by the isolobal analogue Os(PH3)3H reduces the energy of activation for the rearrangement to 23 kcal/mol and produces a corresponding metalladiradical that is 8 kcal/mol less stable that the corresponding osmaenediyne. The activation energy corresponding to the rearrangement of the rhodaenediyne is the same as that of the organic parent enediyne. Interesting polytopal rearrangements of metallaenediynes and the diradical nature of the resulting intermediates are also explored.     

Tuning the energy gap of conjugated polymer zwitterions for efficient interlayers and solar cells

Narrow band gap conjugated polymer zwitterions (CPZs) were synthesized by Suzuki polymerization and characterized to understand their electronic properties and utility as cathode modification layers in solar cells. The polymers were prepared from diketopyrrolopyrrole (DPP) and iso-indigo monomers containing sulfobetaine (SB) pendant groups, benefiting from an ion-rich aqueous phase in the polymerizations. UV–vis absorption spectroscopy revealed the optical energy gap value for the CPZs, ranging from 1.7 to 1.2 eV. Ultraviolet photoelectron spectroscopy of the CPZs as thin layers on Ag metal showed that the pendent zwitterions impart an interfacial dipole (Δ) to the metal and a work function reduction of ∼0.9 eV. OPVs fabricated using a conventional bulk heterojunction (BHJ) device architecture of ITO/PEDOT:PSS/(PTB7:PC₇₁BM)/CPZ/Ag led to dramatic improvements in power conversion efficiency (PCE) values relative to devices having bare Ag cathodes (PCE < 2% for bare Ag vs. 6.7–7.7% for CPZ/Ag). The benzothiadiazole (BT)/DPP polymer denoted as PT₂BTDPPSB gave an optimal PCE of 7.7% in a conventional BHJ OPV device architecture fabricated on a Ag cathode. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 327–336     

Effects of alkyl side chains positioning and presence of fused aromatic units in the backbone of low‐bandgap diketopyrrolopyrrole copolymers on the optoelectronic properties of organic solar cells

The systematic optimization of the chemical structure of low‐bandgap (LBG) donor‐acceptor polymeric semiconductors is a challenging task for which accurate guidelines are yet to be determined. Several different structural and molecular parameters are crucial ingredients for obtaining LBG polymers that simultaneously possess high power conversion efficiencies, good processability in common organic solvents, and enhanced stability in organic photovoltaic devices. In this work, we present an extensive structure–optoelectronic properties–solar cell performance study on the emerging class of diketopyrrolopyrrole‐based LBG polymers. In particular, we investigate alkyl side chain positioning by introducing linear alkyl side chains into two different positions (α‐ and β‐), and the distance of the electron rich and electron deficient monomers within the repeat units of the polymer chain. We demonstrate that anchoring linear alkyl side chains to the α‐positions and introducing fused moieties into the polymer backbone, can be beneficial toward maintaining photocurrents similar to the unsubstituted derivative, and concurrently exhibit better processabiliy in common organic solvents. These results can provide a design rationale towards further optimization of semiconducting polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 138–146     

Synthesis and characterization of three organic dyes with various donors and rhodanine ring acceptor for use in dye-sensitized solar cells

A series of new organic dyes comprising carbazole, iminodibenzyl, and phenothiazine moieties as the electron donors and rhodanine ring as the electron acceptor/anchoring groups were designed and developed for use in dye-sensitized solar cells. HOMO and LUMO energy level tuning was achieved by varying the carbazole, iminodibenzyls and phenothiazine donors. This was evidenced by spectral and electrochemical experiments and density functional theory calculations. Electrochemical studies indicated that the phenothiazine unit was much more effective in lowering the ionization potential than were the iminodibenzyl and carbazole units. The phenothiazine dye shows a solar-energy-to-electricity conversion efficiency (η) of 4.87%; the carbazole and iminodibenzyl dyes show η of 2.54% and 3.52%, respectively. These findings reveal that using carbazole, iminodibenzyl and phenothiazine donors as light-harvesting sensitizers are promising candidates for dye-sensitized solar cells.     

Effect of additives on the photovoltaic properties of organic solar cells based on triphenylamine-containing amorphous molecules

Photovoltaic performance of the organic solar cells(OSCs)based on 2-((5′-(4-((4-((E)-2-(5′-(2,2-dicyanovinyl)-3′,4-dihexyl-2,2′-bithiophen-5-yl)vinyl)phenyl)(phenyl)amino)styryl)-4,4′-dihexyl-2,2′-bithiophen-5-yl)methylene)malononitrile(L(TPAbTV-DCN))as donor and PC70BM as acceptor was optimized using 0.25 vol%high boiling point solvent additive of1-chloronaphthalene(CN),1,6-hexanedithiol(HDT),or 1,8-diodooctane(DIO).The optimized OSC based on L(TPA-bTVDCN)–PC70BM(1:2,w/w)with 0.25 vol%CN exhibits an enhanced power conversion efficiency(PCE)of 2.61%,with Voc of0.87 V,Jsc of 6.95 mA/cm2,and FF of 43.2%,under the illumination of 100 mW/cm2 AM 1.5 G simulated solar light,whereas the PCE of the OSC based on the same active layer without additive is only 1.79%.The effect of the additive on absorption spectra and the atomic force microscopy images of L(TPA-bTV-DCN)–PC70BM blend films were further investigated.The improved efficiency of the device could be ascribed to the enhanced absorption and optimized domain size in the L(TPA-bTV-DCN)–PC70BM blend film.