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Two nonfullerene small molecules, TBTT-BORH and TBTT-ORH, which have the same thiophene–benzothiadiazole–thiophene (TBTT) core flanked with butyloctyl (BO)- and octyl (O)-substituted rhodanines (RHs) at both ends, respectively, are developed as electron acceptors for organic solar cells (OSCs). The difference between the alkyl groups introduced into TBTT-BORH and TBTT-ORH strongly influence the intermolecular aggregation in the film state. Differential scanning calorimetry and UV–vis absorption studies reveal that TBTT-ORH exhibited stronger molecular aggregation behavior than TBTT-BORH. On the contrary, the material solubility is greatly improved by the introduction of a BO group in TBTT-BORH, and the inevitably low molecular interaction and packing ability of the as-cast TBTT-BORH film can be effectively increased by a solvent-vapor annealing (SVA) treatment. OSCs based on the two acceptors and PTB7-Th as a polymer donor are fabricated owing to their complementary absorption and sufficient energy-level offsets. The best power conversion efficiency of 8.33% is obtained with the SVA-treated TBTT-BORH device, where, together with a high open-circuit voltage of 1.02 V, the charge-carrier mobility and the short-circuit current density were greatly improved by the SVA treatment to levels comparable to those of the TBTT-ORH device because of the suppressed charge recombination and improved film morphology. In this work, the simultaneous improvement of both material solubility and device performance is achieved through alkyl side-chain engineering to balance the trade-offs among material solubility/crystallinity/device performance.

A series of semi-crystalline, wide band gap (WBG) photovoltaic polymers were synthesized with varying number and topology of fluorine substituents. To decrease intramolecular charge transfer and to modulate the resulting band gap of D–A type copolymers, electron-releasing alkoxy substituents were attached to electron-deficient benzothiadiazole (A) and electron-withdrawing fluorine atoms (0–4F) were substituted onto a 1,4-bis(thiophen-2-yl)benzene unit (D). Intra- and/or interchain noncovalent Coulombic interactions were also incorporated into the polymer backbone to promote planarity and crystalline intermolecular packing. The resulting optical band gap and the valence level were tuned to 1.93–2.15 eV and −5.37 to −5.67 eV, respectively, and strong interchain organization was observed by differential scanning calorimetry, high-resolution transmission electron microscopy and grazing incidence X-ray scattering measurements. The number of fluorine atoms and their position significantly influenced the photophysical, morphological and optoelectronic properties of bulk heterojunctions (BHJs) with these polymers. BHJ photovoltaic devices showed a high power conversion efficiency (PCE) of up to 9.8% with an open-circuit voltage of 0.94–1.03 V. To our knowledge, this PCE is one of the highest values for fullerene-based single BHJ devices with WBG polymers having a band gap of over 1.90 eV. A tandem solar cell was also demonstrated successfully to show a PCE of 10.3% by combining a diketopyrrolopyrrole-based low band gap polymer.

A series of D–A–D type small molecules containing thiophene and benzo[1,2,5]thiadiazole (BT) as electron-donating and electron-accepting units, respectively, were synthesized using palladium-catalyzed Suzuki coupling reactions. The electron-accepting units of BT were modified with pyridyl nitrogen, a single fluorine atom, or two fluorine atoms to enhance their electron-withdrawing abilities, which resulted in DH5TPys, F-DH5TBs, or 2F-DH5TBs, respectively. The solubilizing hexyl groups were introduced at two different β-positions (3- and 4-positions) on both ends of the thiophene rings. The optical, electrochemical, and photovoltaic properties of the small molecules varied, depending on the introduction of the electron-withdrawing substituents as well as the alkyl variation. The highest occupied molecular orbital (HOMO) energy levels of the small molecules were decreased by the increasing number of fluorine substituents; 4-substitution resulted in deeper HOMO energy levels than 3-substitution. 2F-DH5TB-4 had the lowest HOMO energy level of −5.49 eV. In the organic photovoltaic cells with the configuration ITO/PEDOT:PSS/small molecule:PC71BM/LiF/Al, 2F-DH5TB-4 showed the highest power conversion efficiency (PCE) of 1.11 % owing to the lowest HOMO levels and thus increased the open-circuit voltage. However, two DH5TPys showed low PCEs of 0.20 % despite the deep HOMO energy levels and good UV absorption because of the protonation of pyridyl nitrogen with acidic PEDOT:PSS. In the inverted structure of ITO/ZnO/small molecule:PC71BM/MoO3/Ag, both DH5TPys showed improved PCE values of up to 1.16 %. The device performance of 2F-DH5TB-4 also improved in the inverted structure, showing the PCE of 1.27 %.

© 2018 American Chemical Society. Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F4TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F4TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the electrical conductivity is significantly higher, improving the thermoelectric power factor.

We synthesized three diketopyrrolopyrrole-thiophene-based small molecules (p-, m-, and o-DPP-PhCN) substituted with electron-withdrawing cyanide groups on both end phenyl rings in different positions. The physical properties of the oligomers varied based on the position of the CN groups. Compared to m- and o-DPP-PhCN, the p-DPP-PhCN film had a more red-shifted, strong UV absorption (λmax = 670 nm). p-DPP-PhCN also exhibited a relatively well-aligned arrangement in the X-ray diffraction pattern, owing to a high degree of molecular packing in p-DPP-PhCN. Such an exceptionally strong aggregation of p-DPP-PhCN is expected to give rise to strong molecular orbital interactions and a subsequent decrease in the energy band gap (Eg). p-DPP-PhCN has a lower optical Eg (1.75 eV) than m- and o-DPP-PhCN (∼1.80 eV). Organic photovoltaic cells with the structure ITO/PEDOT:PSS/poly(3-hexylthiophene) (P3HT):DPP-PhCN/LiF/Al were fabricated. Two D/A-type binary cells using p- or o-DPP-PhCN showed similar power conversion efficiencies (PCEs) of 0.5% although the device parameters were different. A high open circuit voltage of 1.09 V in P3HT:o-DPP-PhCN comes from a high-lying lowest unoccupied molecular orbital energy level of o-DPP-PhCN. In contrast, the relatively high short circuit current density of P3HT:p-DPP-PhCN can be explained by the red-shifted UV absorption and superior molecular packing in p-DPP-PhCN. Furthermore, the maximum photocurrent response (13%) of P3HT:p-DPP-PhCN was observed at the λmax of p-DPP-PhCN. In other words, the light absorption of p-DPP-PhCN contributes to the photocurrent along with the absorption of P3HT (e.g., “channel II” charge generation). Finally, a PCE of 1.00%, more than twice that of binary cells, was achieved in the D/A/A-type ternary cells composed of P3HT, p-, and o-DPP-PhCN. The contribution of the electron acceptor to the photocurrent of the devices was enhanced by adding a second acceptor. Improved film morphology and better charge separation were observed in the ternary cells.

Six small molecules with three different dye units, ethyl cyanoacetate (ECA), 1,3-indandione (IN), and 3-ethylrhodanine (RH), attached to both ends of thiophene-flanked carbazole (Cz) or fluorene (Flu) cores were synthesized and used as acceptors in organic photovoltaic cells (OPVs). Their optical and electrochemical, and consequent photovoltaic parameters varied mainly according to the dye units. The OPV cells were fabricated with the configuration ITO/PEDOT:PSS/poly(3-hexylthiophene):small molecule/LiF/Al and all devices except for Cz-IN showed photovoltaic performances with power conversion efficiencies (PCEs) in the range of 1.03–3.08% and open-circuit voltages as high as 1.03 V. Among them, the ECA and RH series had relatively high open-circuit voltages values because of their high-lying lowest unoccupied molecular orbital energy levels. Devices based on Flu-IN and the RH series showed high short-circuit currents (JSC) because of the strong molecular aggregation, as evidenced by red-shifted UV absorption from solution to film. The adequate molecular aggregation in Flu-IN and RH films enhanced electron transport between the molecules, resulting in devices with high JSC and PCE values. This work demonstrates that the introduction of dye end units onto carbazole- and fluorene-based small molecules provides good candidates for non-fullerene acceptors.

The orientational correlation length of domains in a semiconducting polymer controls its thermoelectric performance., The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m−1 K−2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m−1 K−2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.

A newly formulated four-component modified Butler-Volmer model has been developed to evaluate global oxidation kinetic parameters for the various types of carbon monoxide adsorbates (COads) on a nanoparticle Pt surface determined by the type of bonding as well as the local structure of the adsorption site. Partial coverages of COads were prepared by potentiostatic adsorption of methanol followed by potentiostatic partial oxidation at various elevated potentials and for various durations. Anodic linear sweep voltammetry was then performed, and the COads oxidation peaks were fitted with the model to analyze the kinetics. According to the model, preferential oxidation with respect to COads bonding and Pt substrate structure can be achieved dependent upon the potential and extent of oxidation. Partial oxidation at 450 mV vs. RHE for 60 min. resulted in a majority population of linearly bonded COads on cubic-packed Pt sites; whereas partial oxidation at 650 mV vs. RHE for 220 sec. resulted in a majority population of bridged-bonded COads on close-packed Pt sites. © 2014 Elsevier Ltd.

This review provides a current status report of the various n-type polymer acceptors for use as active materials in organic photovoltaic cells (OPVs). The polymer acceptors are divided into four categories. The first section of this review focuses on rylene diimide-based polymers, including perylene diimide, naphthalene diimide, and dithienocoronene diimide-based polymers. The high electron mobility and good stability of rylene diimides make them suitable for use as polymer acceptors in OPVs. The second section deals with fluorene and benzothiadiazole-based polymers such as poly(9,9’-dioctylfluorene-co-benzothiadiazole), and the ensuing section focuses on the cyano-substituted polymer acceptors. Cyano-poly(phenylenevinylene) and poly(3-cyano-4-hexylthiophene) have been used as acceptors in OPVs and exhibit high electron affinity arising from the electron-withdrawing cyano groups in the vinylene group of poly(phenylenevinylene) or the thiophene ring of polythiophene. Lastly, a number of other electron-deficient groups such as thiazole, diketopyrrolopyrrole, and oxadiazole have also been introduced onto polymer backbones to induce n-type characteristics in the polymer. Since the first report on all-polymer solar cells in 1995, the best power conversion efficiency obtained with these devices to date has been 3.45%. The overall trend in the development of n-type polymer acceptors is presented in this review.

Solution-processed organic light-emitting diodes (OLEDs) are attractive due to their low-cost, large area displays, and lighting features. Small molecules as well as polymers can be used as host materials within the solution-processed emitting layer. Herein, we report two 3,3′-bicarbazole-based host small molecules, which possess a structural isomer relationship. 9,9′-Di-4-n-butylphenyl-9H,9′H-3,3′-bicarbazole (BCz-nBuPh) and 9,9′-di-4-t-butylphenyl-9H,9′H-3,3′-bicarbazole (BCz-tBuPh) exhibited similar optical properties within solutions but different photoluminescence within films. A solution-processed green phosphorescent OLED with the BCz-tBuPh host exhibited a high maximum current efficiency and power efficiency of 43.1 cd/A and 40.0 lm/W, respectively, compared to the device with the BCz-nBuPh host.

Graphene sheets were produced by chemical reduction of graphite oxides in the solution of ionic conductive polymer, Nafion. The obtained graphene, coated with Nafion, can be re-dispersed in water, and readily forms stable dispersed state. The polymer-coated graphene had been characterized by FT-IR spectroscopy, UV–vis and X-ray photoelectron spectroscopy (XPS). The PEDOT film with Nafion-coated graphene increased significantly from 0.25 S/cm for pure PEDOT film and reached 12 S/cm. Further, the films of PEDOT doped Nafion-coated graphene had also higher conductivities compared to films doped PSS-coated graphene.

Two fluorene-based conjugated copolymers containing hexyl-thiophene derivatives, PF-1T and PF-4T, were synthesized via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weights (M n ) of PF-1T and PF-4T were found to be 19,100 and 13,200, respectively. These polymers were soluble in common organic solvents such as chloroform, chlorobenzene, toluene, etc. The UV-vis absorption maximum peaks of PF-1T and PF-4T in the film state were found to be 410 nm and 431 nm, respectively. Electrochemical characterization revealed that these polymers have low highest occupied molecular orbital (HOMO) levels, indicating good resistance against oxidative doping. Thin film transistor devices were fabricated using the top contact geometry. PF-1T showed much better thin-film transistor performance than PF-4T. A thin film of PF-1T gave a saturation mobility of 0.001-0.003 cm 2 V -1 s -1 , an on/off ratio of 1.0 x 10 5 , and a small threshold voltage of -8.3 V. To support TFT performance, we carried out DSC, AFM, and XRD measurements.

An electron-transporting triazole (Tz) unit was introduced into the fluorene-thiophene-based copolymer backbone via a Suzuki coupling reaction. The resulting copolymer, poly[9,9'-dioctyl-fluorene-co-5, 5-(4', 7'-di-2-thienyl-2', 1',3'-benzothiadiazole)-co-(4-(4-butyl-phenyl)-3, 5-diphenyl-4H-1,2,4]triazole)] (PF3TBTz), was soluble in common organic solvents and can be easily used as the active layer in organic photovoltaic cells (OPVs). By the introduction of the triazole unit, the OPV performance was nearly doubled from 0.62% (PF3TB) to 1.25% (PF3TBTz) under the same conditions. The higher performance can be explained by the improved surface morphology, resulting in better charge photogeneration and higher short circuit current (J(sc)) value in PF3TBTz in comparison with PF3TB. The possibility of the use of triazole units in OPV applications are described herein.

We report a low-temperature aqueous solution growth of uniformly aligned ZnO nanorod arrays on flexible substrates. The substrate is Indium Tin Oxide (ITO) film coated on polyethylene terephthalate (PET). Solutions with five different concentrations of the precursors with equimolar Zinc Nitrate and Hexamethylenetetramine (HMT) in distilled water were prepared to systematically study the effect of precursor solution concentration on the structural and optical properties of ZnO nanorods. It was concluded that the precursor concentration have great influence on the morphology, crystal quality, and optical property of ZnO nanorods. The diameter, density, and orientation of the nanorods are dependent on the precursor solution concentration. X-ray diffraction and micro-Raman spectroscopy showed that the ZnO nanorods with the highest concentration of 50 mM were highly aligned and have the highest level of surface coverage. It was also found that the diameter and length of the nanorods increases upon increasing precursor solution concentration. This is the first systematic investigation of studying the effect of precursor solution concentration on the quality of ZnO nanorods grown on ITO/PET substrates by low-temperature solution method. We believe that our work will contribute to the realization of flexible organic-inorganic hybrid solar cell based on ZnO nanorods and conjugated polymer.

This article describes the first surface-initiated polymerization of isocyanide monomers onto various surfaces in a controlled manner. Brushes up to 200 nm could be easily obtained within 3 h of polymerization, and the polymer growth was studied as a function of reaction time, monomer concentration, and growth conditions, using atomic force microscopy (AFM) and ellipsometry. The monomers used in this study contain dipeptide (L,L-Ala-Ala) side groups; circular dichroism (CD) measurements, Fourier transform infrared spectroscopy (FTIR), and AFM confirmed that the well-defined helical conformation was retained in the polyisocyanide brushes with hydrogen bonds along the polymer chains.

Correction for ‘Effect of dye end groups in non-fullerene fluorene- and carbazole-based small molecule acceptors on photovoltaic performance’ by Yujeong Kim et al., RSC Adv., 2015, 5, 62739–62746.

New amorphous semiconducting materials consisting of fluorene-based thiophene copolymers, poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-3-hexylthiophen-2-yl)-5-(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene) P1 and poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-4-hexylthiophen-2-yl)-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene) P2, have been successfully synthesized via a palladium-catalyzed Suzuki coupling reaction. The number-average molecular weights (Mn) of P1 and P2 were found to be 18 300, and 15 800, respectively. These polymers dissolve in common organic solvents such as chloroform, chlorobenzene, and toluene. The UV-vis absorption maxima of P1 and P2 appeared at 436 and 427 nm in solution and 441 and 431 nm in the film state, respectively. X-Ray diffraction (XRD) analysis showed no reflection peaks indicating amorphous collections of randomly oriented polymer chains. Atomic force microscopy (AFM) images of P1 and P2 showed amorphous film morphologies. Field-effect transistor mobilities of stable amorphous OTFTs of P1 and P2 under ambient conditions have been achieved up to 5.4 × 10−4 cm2V−1 s−1 and 1.6 × 10−4 cm2V−1 s−1, respectively. The high stability and mobility of fluorene-based thiophene copolymers in the amorphous state make them a new family of promising candidates for organic thin-film transistors.

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