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Abstract The fabrication of organic solar cells using non-halogenated solvents have been recently explored in order to replace conventionally used halogenated solvents that cause environmental, health and safety problems. For achieving highly efficient organic solar cells using non-halogenated solvents, conjugated polymer should be designed and developed to afford high solubility in non-halogenated solvent and desirable photovoltaic properties in finally fabricated devices. Here, we demonstrated that an isoindigo-based copolymer, PiI2fT, composed of isoindigo with 2-decyltetradecane and bithiophene with fluorination is a promising polymer for organic solar cells using eco-friendly non-halogenated solvent. We found that the introduction of fluorine atom in conjugated polymer affords improved photophysical properties including intense light absorption, lowering of the highest occupied molecular orbital energy level and increase of dipole moment, while the introduction of bulk alkyl chain in the polymer may enhance the solubility in organic solvents. When solar cells were fabricated from using o -xylene and diphenyl ether as solvent and additive, PiI2fT polymer exhibited a high PCE of 8.80% with an open-circuit voltage of 1.06 V. Graphical abstract Image 1 Highlights • Two isoindigo-based polymers composed of isoindigo with 2-decyltetradecane and bithiophene with/without fluorination were synthesized. • Introduction of fluorine afforded improved photophysical properties, while bulk alkyl chain enhanced the solubility of polymers. • When solar cells were fabricated from using o-xylene and diphenyl ether as solvent and additive, fluorinated polymer (PiI2fT) exhibited a high efficiency of 8.80%. [ABSTRACT FROM AUTHOR]

Three π-conjugated small molecules (SMs) with D(π-A-Ar) 2 molecular frameworks are designed and synthesized as the donor materials in organic solar cells (OSCs). Containing tetrafluorobenzene (TFB) as central donor (D) core, thiophene as π-bridge, 2-octylthiophene (T) as terminal (Ar) unit, as well as the different arm of acceptor (A) units, pyridine-diketopyrrolopyrrole (PyDPP), thiophene-diketopyrrolopyrrole (TDPP) and isoindigo (IID), namely DTTFB(PyDPP-T) 2 , DTTFB(TDPP-T) 2 and DTTFB(IID-T) 2 , respectively. The impacts of the different acceptor arms combined with the TFB core on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were studied thoroughly. As a result, these SMs presented various light absorption properties, electrochemical performances, and hole mobilities, which are helpful to the optimization of short-circuit current ( J sc ), open-circuit voltage ( V oc ) and fill factor (FF) properties, respectively. Especially, the DTTFB(TDPP-T) 2 showed a relatively lower optical band gap of 1.53 eV, and also showed the best coplanarity among the three SMs, which help it to get a higher J sc of 12.44 mA cm −2 and a higher FF of 59.50%, and resulting in obtaining a best power conversion efficiency (PCE) of 5.15% in the DTTFB(TDPP-T) 2 /PC 71 BM-based cells, which is about 2.8 and 1.6 times higher than that of DTTFB(PyDPP-T) 2 /PC 71 BM and DTTFB(IID-T) 2 /PC 71 BM based cells, respectively. Obviously, introducing different acceptor units as arms onto the D(π-A-Ar) type SMs could be a promising strategy for designing high-performance TFB-based linear molecular donor materials. [ABSTRACT FROM AUTHOR]

Two A 2 -D-A 1 -D-A 2 small molecules ID-IND and ID-RHO have been designed and synthesized, in which isoindigo, thiophene-benzene-thiophene, and 1,3-indenedione or 3-ethylrhodanine are used as the central acceptor unit (A 1 ), the donor units (D) and the end-caped acceptor units (A 2 ), respectively. The effects of the end-caped acceptor units on molecular aggregation, absorption spectra, film morphology, hole mobilities and photovoltaic performance have been fully studied. The results indicate that ID-IND with 1,3-indenedione as the end-caped acceptor units possesses more regular molecular aggregation, red-shifted absorption spectrum, more appropriate microphase separation, higher hole mobility and better photovoltaic performance than ID-RHO with 3-ethylrhodanine as the end-caped acceptor units. Therefore, the ID-IND -based solar cell shows a power conversion efficiency (PCE) of 4.70% with a open-circuit voltage of 1.01 V, which is one of the highest PCE values in organic solar cells based on the small molecules with isoindigo as the central acceptor unit. [ABSTRACT FROM AUTHOR]

To investigate the length of the alkyl side-chains on the photovoltaic performance, three isoindigo-based polymers with different-length side chains on the thiophene-benzene-thiophene (TBT) (electron-donating) moiety and isoindigo (electron-withdrawing) moiety have been designed and synthesized, in which the total carbon atom numbers of the side chains in a constitutional repeating unit are completely identical. The results indicate that the polymer with longer branched side chains on the TBT units and shorter linear side chains on the isoindigo units shows stronger interchain interaction and light-harvesting capacity in 1,2-dichlorobenzene solution and thin film, lower band gaps, stronger π−π stacking interaction, more appropriate microphase separation with PC 71 BM, higher hole mobility and better photovoltaic performance. As a result, the bulk heterojunction (BHJ) device based on PTBTOD-IDB with 2-octyldodecyl on the TBT moiety and n -butyl on the isoindigo moiety exhibits a power conversion efficiency (PCE) of 5.29%, which is almost eight times as high as that of PTBTEH-IDHD with 2-ethylhexyl side chain on TBT moiety and 2-hexyldecyl side chain on isoindigo moiety. [ABSTRACT FROM AUTHOR]

Herein, we report design and synthesis of DPP-Ii-DPP, a conjugated small molecule (SM) composed of central isoindigo flanked with diketopyrrolopyrrole (DPP), for organic solar cell (OSC) application. The DPP-Ii-DPP exhibited broad and intense absorption in 400–800 nm in wavelength with a low optical band gap of 1.46 eV. However, in contrast to common DPP-based SMs with strong electron-donating central moiety, DPP-Ii-DPP exhibited much lowered energy levels (HOMO = −5.56 eV), which is beneficial to achieve high open circuit voltage (V OC ) of the devices. For the purpose to optimize power conversion efficiency (PCE) of OSCs based on DPP-Ii-DPP, the photoactive blend film was treated with thermal/solvent annealing or processing additive. Among which, the device with solvent annealing exhibited an optimized PCE of 5.86% with a V OC of 0.86 V, a J SC of 11.75 mA/cm 2 and FF of 0.58. This work shows the potential of DPP-Ii-DPP as a promising SM-based electron donor materials for efficient OSCs. [ABSTRACT FROM AUTHOR]

Two random electron donor-acceptor (D-A) polymers comprised of two acceptor moieties, difluorobenzothiadiazole and isoindigo, and one donor moiety ( P1 and P2 ) have been synthesized in good yields to study the effects of donor structures on organic thin-film transistor (OTFT) and bulk-heterojunction organic solar cell (BHJ-OSC) performance. Both polymers possess reasonably good solubility in common processing solvents for organic electronics. P1 , which contains a benzodithiophene donor structure, shows poor charge transport ability in OTFTs and low solar power conversion efficiency (PCE) in BHJ-OSC devices with PC 71 BM acceptor. On the other hand, P2 , with a dithienylthieno[3,2- b ]thiophene donor moiety, exhibits distinctly higher hole field-effect mobility and significantly much better PCE under similar conditions. These results have demonstrated the decisive influence of donor moiety over the optoelectronic properties of D-A polymers. [ABSTRACT FROM AUTHOR]

Two isoindigo-based conjugated polymers, based on fluorinated elctron donor units (dithienyldifluorobenzothiadiazole(DT2FBT) and dithienyltetrafluorobenzene (DT4FBZ)), have been synthesized and their optical, electrochemical, morphological, and crystallinity features were evaluated. The combination of highly conjugated isoindigo moiety with fluorinated electron donor units induces high crystallinity of the derived polymers, resulting in p -type charge transport characteristics with high hole mobility exceeding 7.0 × 10 −2 cm 2 V −1 s −1 . Furthermore, the low-lying energy levels due to the electron-deficient fluorinated electron donors allow not only effective electrical contact with the source/drain electrodes but also excellent ambient stability of the derived transistor devices, enabling the polymers to be promising candidates for the environmentally stable optoelectronics. [ABSTRACT FROM AUTHOR]

Two D–π-A–π-A organic dyes ( YC-1 and YC-2 ) with 5-phenyl-5 H -dibenzo[ b,f ]azepine derivatives as donor, thiophene as π bridge, and isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a panchromatic absorption between 300 nm and 800 nm both in solution and neat film. The photovoltaic performances of both dyes were evaluated in dye-sensitized solar cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based device displays better device performance with open-circuit photocurrent density of 12.12 mA cm −2 , open-circuit voltage of 0.53 V, and fill factor of 68.9%, corresponding to overall conversion efficiency ( η ) of 4.38%. The inferior performance of device based on YC-2 ( η = 1.46%) is ascribed to short electron lifetime as evidenced from electrochemical impedance spectroscopy measurement. This research provided a potential promising donor unit for organic dyes and revealed the influence of donor size in organic dyes for photovoltaic performances. [ABSTRACT FROM AUTHOR]

Two isoindigos were symmetrically connected by 2,1,3-benzothiadiazole core to form an acceptor2–acceptor1–acceptor2 monomer. The new monomer was used as electron acceptor unit in construction of a unique donor/acceptor polymer with donor–acceptor2–acceptor1–acceptor2 repeating units. The optical and electrochemical properties of the monomer and polymer were investigated. The optical band gap of monomer and polymer were about 1.86 and 1.65 eV, respectively. Both monomer and polymer showed relatively low-lying highest occupied molecular orbital energy levels and the corresponding values were −5.90 and −5.56 eV for monomer and polymer, respectively. The semiconducting properties of new materials were also studied. Organic field effect transistors based on solution processed thin films of the polymer displayed typical p-channel characteristics with a hole mobility of 0.02 cm 2 (V s) −1 in ambient conditions. [ABSTRACT FROM AUTHOR]

Eight novel isoindigo (iI) based small molecules have been successfully synthesized. Their molecular structure consists of an electron acceptor iI core symmetrically linked to two furan (F-series) or thiophene (T-series) rings and end-functionalized with four auxiliary electron withdrawing groups (EWGs) of different strength. The optical properties of the dyes in chloroform solution are uniformly modulated by the terminal EWGs so that absorption maxima wavelengths move to higher values as the EWG's strength increases. A computational (DFT level) analysis provides useful information on the electronic structure of the dyes: upon photoexcitation, the electron density moves away from iI core or towards it according to the different EWG considered. Optical analysis is performed on dyes' thin films as well and a general broadening and red shift of the absorption is observed as compared to the behaviour in solution; all the dye's thin films are characterized by narrow bandgaps (<1.60 eV) and diffused absorption of most of the visible light. From XRD diffraction analysis performed on drop casted films of the dyes, it is possible to observe a lamellar organization in the solid phase with lamellae width clearly linked to the nature of the terminal EWG. HOMO and LUMO energies of the dyes, determined by cyclic voltammetry analysis performed on dyes' thin films, show very stable LUMO and HOMO energy levels, suggesting, respectively, a tendency to act as n-type semiconductors and a very good thermo-oxidative stability. The dyes are finally employed as active layers in organic field-effect transistors to study their charge transport properties: all of them display unipolar n-type charge transport with the presence of the electron accumulation phenomenon under the application of positive gate voltages. For one of the dye, mobility (μ) up to 10−2 cm2/V∙s was measured, whereas values around 10−3 cm2/V∙s were found for the others. [Display omitted] • Novel isoindigo derivatives end capped with auxiliary electron acceptor groups have been synthesized and characterized. • As thin films, the dyes absorb broadly in all the visible spectrum with a narrow bandgap, as low as 1.60 eV. • The dyes present an unipolar n-type charge transport behaviour with electron mobility up to 10−2 cm2/V∙s. [ABSTRACT FROM AUTHOR]

Herein we introduce the π-conjugated organic building block, 8-bromo-indolo[2,3- b ]quinoxaline (indoloquinoxaline, IQ ), that combines both electron withdrawing and donating nitrogen atoms. It is accessible from low-cost starting materials through a straightforward and high yielding condensation reaction. In terms of materials design, indoloquinoxaline is suitable for use as a terminal unit; it can act as a planar extension of the π-conjugated system and is easily functionalized with aliphatic side-chains, which can alter material solubility and self-assembly processes. The indoloquinoxaline framework is highly versatile and can be readily derivatized considering the hundreds of commercially available indole and phenylenediamine starting materials. In this case, the indoloquinoxaline building block is neither strongly electron rich or deficient which allows for material optoelectronic properties to be largely dictated by the character of the molecular core, as shown through the incorporation of three different core units, a weak thiophene donor, a strong benzodithiophene donor and an isoindigo acceptor. [ABSTRACT FROM AUTHOR]

A series of isoindigo-based metal-free organic sensitizers have been synthesized and used in dye-sensitized solar cells (DSSCs). Measurement data indicated that the fine-tuning of the donor or linker moiety could conveniently adjust the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. All DSSCs sensitized by three new dyes displayed an efficient panchromatic spectra response with the improved light-harvesting capability in near-infrared region, in which a highest photovoltaic conversion efficiency of 5.56% was obtained by the DSSCs of benzene-bridge isoindigo-based sensitizer, with a short-circuit photocurrent density ( J sc ) of 13.16 mA/cm 2 , an open-circuit photovoltage ( V oc ) of 582 mV, and a fill factor ( ff ) of 0.73. [ABSTRACT FROM AUTHOR]

Abstract: Donor/acceptor polymers with N-acylated isoindigo as acceptor units have been synthesized. The thermal, optical, electrochemical, and charge-transport properties of the polymers have been investigated. The new polymers show broad absorption from 450 to 900 nm. Due to the strong electron accepting characteristic of N-acylated isoindigo, the new polymers exhibit narrow optical band gaps and deep LUMO energy levels compared with the polymers based on N-alkylated isoindigo. The new polymers show n-type transport behavior and balanced ambipolar transport behavior in organic thin film transistors (OTFT) device. [Copyright &y& Elsevier]

Abstract: Two new donor/acceptor π-conjugated polymers with functional dyes, either diketopyrrolopyrrole or isoindigo as the electron accepting units and 5,10-bis(2,3-didecylthiophen-5-yl)-naphtho[1,2-b:5,6-b′]difuran as an electron donating units have been synthesized. The photo-physical, electrochemical and photovoltaic properties of the new polymers have been studied and compared. With the same donor unit, the diketopyrrolopyrrole based polymer showed low bandgap and broad absorption extended into near IR region. The isoindigo based polymer showed little absorption in near IR region. However, the isoindigo based polymer showed more efficient absorption than diketopyrrolopyrrole based polymer in the most of visible region (from 300 to 690 nm) in solution. The isoindigo based polymer showed lower HOMO and LUMO energy levels than diketopyrrolopyrrole based polymer. The isoindigo polymer based solar cell devices consistently showed higher open circuit voltages than the diketopyrrolopyrrole polymer based devices. [Copyright &y& Elsevier]

Abstract: A new easily-accessible solution-processed oligomer, with an isoindigo group as an electron acceptor and a thieno[3,2-b]thiophene flanked by thiophenes as electron donors, has been synthesized by a Stille coupling reaction. Through introducing the extended π-conjugated groups into isoindigo, the electro-optical properties of the material can be fine-tuned. The isoindigo oligomer has a broad absorption in the region from 300 to 800 nm with a narrow bandgap (1.54 eV), which is believed to be an ideal bandgap as donor materials. The oligomer possesses low HOMO energy level (−5.39 eV). The potential of optical and electronic properties encouraged us to explore the photovoltaic performance using the oligomer as the donor material in bulk heterojunction organic solar cell along with 6,6-phenyl-C61-butyric acid methyl ester as the acceptor. The solar cell based on the oligomer with an inverted device configuration provided a power conversion efficiency of 1.41% under the AM 1.5G illumination with an intensity of 100 mW cm−2 from a solar simulator. [Copyright &y& Elsevier]

Abstract: A series of arylmethylidenoxindoles was synthesized by acid or base catalysed condensation of substituted aldehydes with oxindole. The products were formed as mixtures of Z- and E-isomers that were separated using column chromatography and identified by 1H and 13C NMR. The effect of substituents on absorption maxima was investigated both experimentally and theoretically, based on time dependent density functional theory. Low temperature absorption spectroscopy in a solvent glass enabled identification of the position of the 0-0 vibronic bands. The spectral features of the parent Z-benzylidenoxindole were compared to those of trans-isoindigo and trans-stilbene. Fluorescence of arylmethylidenoxindoles in solution was not observed, as E–Z isomerization represents a dominant deactivation channel after irradiation; when this geometrical isomerization was sterically hindered at low temperature solvent glass or in solid, fluorescence of planar Z-isomers was observed. [Copyright &y& Elsevier]

The effect of synergistic structural engineering (asymmetric side chains and backbone fluorination) for polymer donors in photovoltaic applications is investigated in this study. Six isoindigo (IID)-based polymer donors with slightly different side-chain and backbone designs are composed of a bithiophene (BT) or fluorinated bithiophene moiety to form two series of polymers [PII2T (P1–P3) and PII2TF (P4–P6)] while symmetric/asymmetric side-chain combinations are composed of decyltetradecane branched alkyl side chain (DT) and siloxane-terminated side chain (SiO–C8). The results indicate the different side chain design and backbone fluorination modulate the energy levels, crystalline properties, and molecular packing behaviors of the polymers. P2 and P5 with asymmetric side chains are showed to deliver the highest power conversion efficiency (PCE) in the respective series of PII2T and PII2TF. The morphological analyses reveals that the mismatched side-chain geometry results in a face-on dominated packing to yield superior device performance. Meanwhile, the fluorinated polymers (P4–P6) show superior performance than the corresponding non-fluorinated polymers (P1–P3). Moreover, backbone fluorination largely reduces the performance difference between the polymers with symmetric side chains and asymmetric sides chains. This work demonstrates that the synergistic effect of asymmetric side chains and backbone fluorination can be a rational design towards high-performance conjugated polymers. [Display omitted] • The synergistic effect of asymmetric side chains and backbone fluorination for polymer donors in photovoltaic applications is investigated. • The polymers with the asymmetric side chains deliver the highest PCE in their respective series with other polymers with symmetric side chains. • Backbone fluorination largely reduces the performance difference between the polymers with different side-chain designs. [ABSTRACT FROM AUTHOR]

The potential of a new series of H-bonded isoindigo (ID) based small molecules as charge transporting materials was examined. The soluble precursor units contained two terminal isoindigo units bearing tert -butoxycarbonyl (Boc) solubilizing substituents. Thiophene, thienothiophene and/or diketo pyrrolopyrrole are used as π-conjugated spacers between the isoindigo units. The protected Boc-ID units are converted to the hydrogen-substituted counterparts by annealing the spin-coated precursor films up to 220 °C. The solid state molecular ordering after Boc thermal decarboxylation was investigated using thermogravimetric analysis, UV–vis spectroscopy and X-ray diffraction analysis. The thermogravimetric analysis confirmed the full removal of Boc groups. It was revealed that the formation of terminal intermolecular hydrogen bonds gave the ability to the deprotected materials to change their molecular ordering to edge-on or face-on orientation based on the spacer size and the π-conjugated length. The [-NH----O C] H-bonding dramatically improved the strength of the intermolecular interactions and induced a significant shift on the wavelength of annealed films. The hole mobilities of isoindigo based organic materials measured by the space charge-limited current (SCLC) method and showed a tenfold improvement after thermal decarboxylation. Image 1 • Synthesis of a new series of H-bonded isoindigo (ID) based small molecules. • Edge-on and/or face-on molecular orientation was studied. • The materials' charge mobilities were examined. • The hole mobility is dramatically a tenfold increased after thermal deprotection. [ABSTRACT FROM AUTHOR]

In this study, a panchromatic absorptive conjugated terpolymer, BDTID-BDT3MT , is synthesized, which consists of electron-donating benzodithiophene (BDT), isoindigo (ID) as a strong electron-accepting unit, and methyl-3-thiophenecarboxylate (3 MT) as a weak electron-accepting unit. By combining these three monomers into the structure of a conjugated terpolymer, the absorption spectrum of BDTID-BDT3MT is induced to exhibit an unusually broad, strong, and uniform band in the wavelength interval from 300 to 750 nm, which helps achieving highly efficient light harvesting under solar illumination. The intriguing panchromatic absorption behavior of BDTID-BDT3MT was explained on the basis of theoretical calculations using simplified repeating units. Polymer solar cells (PSCs) based on BDTID-BDT3MT as a donating polymer and non-fullerene acceptors (e.g., ITIC-4F) exhibited a high power conversion efficiency (PCE) of 5.38%, high open circuit voltage (V oc) of 0.88 V, and short circuit current density (J sc) of 13.74 mA/cm2, while PSCs based on ternary blend systems consisting of BDTID , BDT3MT , and ITIC-4F exhibited lower PCE and J sc of 3.74% and 11.15 mA/cm2, respectively. The superior performance of PSCs based on BDTID-BDT3MT can be attributed to their high light harvesting efficiencies and relatively more favorable nano-phase film morphologies. Our results establish that BDT, ID, and 3 MT units serve as useful building blocks in the structure of conjugated terpolymers due to their remarkably broad panchromatic absorption band. Image 1 • A panchromatic absorptive conjugated terpolymer, BDTID-BDT3MT , was successfully synthesized. • Isoindigo as a strong electron-accepting unit and thiophenecarboxylate as a weak electron-accepting unit were employed. • The absorption spectrum of BDTID-BDT3MT exhibit a broad, strong, and uniform band from 300 to 750 nm. • Polymer solar cells with ITIC-4F exhibited a high power conversion efficiency of 5.38%. [ABSTRACT FROM AUTHOR]