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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.

In this paper, two cyclohexyl-substituted non-fullerene small molecules, T2-Cy6MRH and T2-Cy6PRH, are designed to have the same molecular backbone of a bithiophene (T2) core and rhodanine (RH) end groups. T2-Cy6MRH and T2-Cy6PRH can be synthesized via two and four synthesis steps, respectively, from commercially available starting materials. Thermogravimetric analysis shows that both small molecules are thermally stable at temperatures greater than 380 °C. Compared with T2-Cy6PRH, T2-Cy6MRH exhibits a higher thermal-transition temperature and a more negative enthalpy change, indicating that it exhibits greater crystallinity. Unlike T2-Cy6MRH, T2-Cy6PRH can form a uniform film via solution-processing, because the introduction of longer flexible alkyl chains endows T2-Cy6PRH with better solubility than T2-Cy6MRH. The T2-Cy6PRH film (λmax = 490 nm) exhibits complementary UV-vis absorption with the small-bandgap polymer donor PTB7-Th (λmax = 700 nm). In addition, the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels of T2-Cy6PRH are relatively low-lying compared with those of PTB7-Th. An organic solar cell device based on T2-Cy6PRH and PTB7-Th exhibited a power conversion efficiency (PCE) of 4.60% with a high open-circuit voltage of 1.03 V. The addition of [6,6]-phenyl-C61-butyric acid methyl ester substantially improved the device performance, owing to the improved film morphology and the cascade-energy-level alignment, resulting in a PCE of 6.91% under additive- and annealing-free conditions.

Solution processability is one of the advantages of organic solar cells (OSCs). However, most high-efficiency OSCs are prepared using hazardous chlorinated solvents for the deposition of photoactive layers. The replacement of non-halogenated solvents with eco-friendly green solvents for photoactive materials is urgently required. Herein, we have developed a novel asymmetric T2-OEHRH, which is modified from the symmetric T2-ORH. The introduction of asymmetric alkyl side chains onto rhodanine end groups can effectively suppress excessive self-aggregation/crystallization and substantially improve solubility without sacrificing optoelectrical properties. Therefore, ternary-blend OSCs based on PTB7-Th:EH-IDTBR:T2-OEHRH processed using a non-halogenated solvent system exhibit a uniform and favorable morphology and give a high power conversion efficiency (PCE) of 12.10%. More importantly, we demonstrate an impressive PCE of 9.32% for large-area NFA-OSCs (substrate size = 100 cm2 and aperture size = 55.5 cm2) prepared via D-bar coating in air. To our knowledge, this PCE is the highest reported to date for NFA-based large-area OSC modules processed from a non-halogenated solvent. This asymmetric alkyl-chain engineering strategy can be exploited to develop high-performance large-area NFA-OSCs with eco-friendly solvent processing.

A thiophene flanked by strong electron-withdrawing octylrhodanines, T-ORH, was developed as a non-fullerene acceptor (NFA) for organic solar cells (OSCs), and to the best of our knowledge, this is ...

A simple-structured nonfullerene acceptor (NFA), T2-ORH, consisting of a bithiophene core and octyl-substituted rhodanine ends is utilized as the third component in ternary-blend solar cells with PTB7-Th and EH-IDTBr as host materials. T2-ORH is a wide band gap NFA and specially contributes to light absorption, which provides complimentary absorption with a low band gap acceptor, EH-IDTBR. In the ternary device, a PCE of 11.55%, a short circuit current (JSC) of 17.51 mA cm−2, a fill factor (FF) of 63% and an open circuit voltage (VOC) of 1.05 V were achieved with a 1 : 1 : 1 blend ratio. In this ternary system, it is estimated that the two NFAs work as one alloy-like acceptor and energy transfer model from T2-ORH to EH-IDTBR for efficient ternary-blend organic solar cells (OSCs). On conducting thermal, light, and air-process stability tests, the ternary-blend systems show superior stability performance owing to the formation of a stable and robust morphology. Finally, we fabricated a large-scale device (5 cm × 5 cm substrate, 13.5 cm2 aperture size, and a geometric FF of 54%) with the same ternary system. The sub-module ternary-blend OSCs displayed the highest PCE of 10.08% with JSC = 2.83 mA cm−2, VOC = 6.36 V and FF = 56%. As a result, ternary-blend OSCs show tremendous promise for use in actual applications.

We developed p-n junction organic photodetectors (OPDs) composed of a polymer donor and a nonfullerene acceptor (NFA) to increase both the responsivity (R) and detectivity (D*) while maintaining a narrow wavelength selectivity. The selection of the polymer donor and NFA with similar green (G) absorption is important for achieving G-wavelength selectivity in these OPDs, which differentiates them from current fullerene-based OPDs and NFA-based panchromatic absorption OPDs. In addition, mixing the polymer donor and asymmetric NFA was efficient toward increasing the miscibility and decreasing the interfacial energy difference of the blended films, resulting in the formation of a uniform and well-mixed nanomorphology in the photoconductive layer. Two-dimensional (2D) grazing incidence X-ray diffraction and Fourier-transform infrared spectroscopy revealed that the lamellar ordering of the polymer donor was enhanced in the blend film prepared with an asymmetric NFA, whereas the aggregation of a symmetric NFA in the blend films did not increase the lamellar ordering of the polymer donor. Consequently, we achieved an R value of 0.31 A/W and D* value of 2.0 × 1013 Jones with a full width at half-maximum value of 230 nm at -2 V and fast response time of 27 μs without any external bias in the asymmetric NFA-based OPDs. The enhancement in the lamellar ordering and miscibility of the blended films are crucial toward increasing the static and dynamic properties of OPDs.

Controlling interchain interactions in conjugated polymers is critical to the development of high performance materials. These interchain interactions are dictated by the aggregation and self-assembly of conjugated polymers in solution and from processing steps, such as thermal annealing, in the solid state. Herein, a macrocyclic benzodithiophene building block for conjugated polymers is developed, and the properties of the resulting polymers are compared to analogous acyclic derivatives. The properties of small molecule macrocyclic BDT compounds show the influence of the side-chain substitution on the thermodynamic forces of self-assembly. Comparison of the optical properties of conjugated polymers with macrocyclic and acyclic side-chains in solution and the solid state reveals the ability of the macrocyclic side-chain to modify the structure of aggregates. Grazing incidence wide-angle X-ray scattering shows that the macrocyclic polymers can remain ordered in the solid state while having higher photolumi...

Organic bulk heterojunction (BHJ) solar cells require broad absorption of the incident solar spectrum by the donor and acceptor in the blend. Fullerene derivatives with covalently linked dyes, referred to as dyads, were designed to have strong optical absorption and appropriate electronic levels for electron transfer with common donor polymers in BHJs. Dyads with dyes based on diketopyrrolopyrrole and benzothiadiazole were synthesized using either a methanofullerene or a fulleropyrrolidine linkage. The performance of these dyad acceptors in BHJ solar cells with poly(3-hexylthiophene) (P3HT) and a low optical gap co-polymer of thiophene and diketopyrrolopyrrole (PDPP2FT) were examined. Although the solar power conversion efficiencies were low, charge generation from the molecular dye in the dyad could be observed in BHJs with PDPP2FT. The low power conversion efficiency was attributed to the morphology of the BHJs based on grazing incidence wide angle X-ray scattering patterns. Density functional theory was used to examine the charge transfer states between the donor and the dyad. The lowest energy charge-transfer state was found to be a transition from the donor polymer to the fullerene portion of the dyad.

A new pentacyclic building block, benzodibenzofuranquinone (BDBFQ), is synthesized in a single step from the inexpensive and readily available feedstocks chloranil and 3-bromophenol. This versatile repeat unit is incorporated into a series of conjugated materials as either the redox active quinone form or as an electron-rich dialkylated benzodibenzofuran (BDBF) unit. To illustrate the performance that can be obtained from these systems, optical and electrochemical properties were studied via UV–vis–NIR spectroscopy and cyclic voltammetry. Thin-film morphology was characterized via grazing incidence wide-angle X-ray scattering with thin-film field-effect transistor measurements further demonstrating the utility of this system. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

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.

We report the synthesis of a series of A1–π–A2–π–A1-type small molecules, DPP2T-RH, DPPFT-RH, and DPPT-RH, in which thiophene and furan were introduced to bridge the two strong electron-withdrawing units of DPP and rhodanine (RH). Interestingly, these DPP-RH-based small molecules exhibited ambipolar characteristics, showing photovoltaic performance both as donors and acceptors in combination with PC71BM and P3HT, respectively. The best power conversion efficiencies were obtained from DPPFT-RH (1.77%) and DPP2T-RH (1.28%) as donor and acceptor, respectively. Their physical properties, film packing ability, and photovoltaic performances were fine-tuned by rational design of the π-bridges.

Most non-fullerene acceptors (NFAs) are designed in a complex planar molecular conformation containing fused aromatic rings in high-efficiency organic solar cells (OSCs). To obtain the final molecules, however, numerous synthetic steps are necessary. In this work, a novel simple-structured NFA containing alkoxy-substituted benzothiadiazole and a rhodanine end group (BTDT2R) is designed and synthesized. We also investigate the photovoltaic properties of BTDT2R-based OSCs employing representative polymer donors (wide band gap and high-crystalline P3HT, medium band gap and semicrystalline PPDT2FBT, and narrow band gap and low-crystalline PTB7-Th) to compare the performance capabilities of fullerene acceptor-based OSCs, which are well matched with various polymer donors. OSCs based on P3HT:BTDT2R, PPDT2FBT:BTDT2R, and PTB7-Th:BTDT2R achieved efficiency as high as 5.09, 6.90, and 8.19%, respectively. Importantly, photoactive films incorporating different forms of optical and molecular ordering characteristics exhibit favorable morphologies by means of solvent vapor annealing. This work suggests that the new n-type organic semiconductor developed here is highly promising as a universal NFA that can be paired with various polymer donors with different optical and crystalline properties.

This chapter discusses the thermoelectric properties of conjugated polymers with an emphasis on the role of morphology on their behavior. Because of the difficulty of measuring the thermal properties of polymers, it reviews the state of the thermoelectric power factor of polymers and explains the relevant issues related to the thermal conductivity. Thermoelectric materials can generate an electric potential in response to a thermal gradient, which is known as the Seebeck effect, or generate a thermal gradient in response to an applied potential, known as the Peltier effect. Organic thermoelectric materials, including polymers, have the potential to expand the use of thermoelectric devices. Polyaniline can be prepared with different proportions of its backbone nitrogens oxidized, leading to either insulating or conductive behavior. Poly(ethylenedioxythiophene) (PEDOT) polymers have emerged as promising materials for thermoelectric energy conversion. PEDOT-based systems have demonstrated some of the highest thermoelectric performances for polymers with variation in performance depending on the processing methods.

An acceptor-donor-acceptor (A–D–A)-type small molecule, BDT-IN, having a benzo[1,2-b:4,5-b']dithiophene (BDT) unit as its electron-donating core (D) and an 1,3-indanedione (IN) unit as its electron-withdrawing end group (A), was synthesized by Knoevenagel condensation. The BDT-IN film showed broader UV absorption with a greater red shift (λmax = 622 nm) than that of the BDT-IN solution (λmax = 570 nm). The organic photovoltaic cells were fabricated with an ITO/PEDOT:PSS/poly(3-hexylthiophene): BDT-IN/LiF/Al configuration, and showed a power conversion efficiency of 0.23%.

The donor–acceptor–donor (D–A–D) type small molecule, TPD2T, was synthesized by the Stille coupling reaction and consisted of the electron-withdrawing thiene[3,4-c]pyrrole-4,6-dione (TPD) unit as a...

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 %.

In this work, we utilized two fluorene-based polymers (F8TBTT and F8TTZT) as donor materials in organic photovoltaic cells (OPVs) and compared their performance to explain the effect of substituting fused thiazolothiazole for the bithiazole moiety. The OPV device based on F8TTZT showed a power conversion efficiency of 0.24%, which is three times higher than that of F8TBTT (0.07%) under the same conditions. The better performance can be explained by the increased planarity and enhanced intermolecular charge transport.

We synthesized two new thieno[3,4-c]pyrrole-4,6-dione (TPD)- and diketopyrrolopyrrole (DPP)-based small molecules having pyridine end groups, TPDTPy and DPPTPy, using the Stille and Suzuki coupling reactions, respectively. The optical, electrochemical, and photovoltaic properties of these small molecules depended on the introduced electron-accepting units of TPD and DPP. DPPTPy exhibited a relatively low-energy bandgap (Eg) of 1.82 eV compared to TPDTPy (2.22 eV) because of the stronger electron-withdrawing ability of DPP compared to TPD. Organic photovoltaic cells with the inverted ITO/ZnO/small molecule:(6,6)-phenyl-C71-butyric acid methyl ester/MoO3/Ag structure were fabricated. The DPPTPy-based device showed a relatively high power conversion efficiency of 0.62% compared to TPDTPy (0.16%), mainly because of the red-shifted UV absorption, lower Eg, and the higher short-circuit current of DPPTPy.

© 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.

Control of the domain size and morphology of ferroelectric-semiconductor polymer blend thin films is essential for producing working organic ferroelectric resistive switches that can be used for low-cost, flexible memory applications. However, improvements in characterization techniques that can selectively probe these polymers are still needed. The unique core-level absorption profiles of these polymers make synchrotron based soft X-ray techniques ideal to achieve contrast and chemical sensitivity between polymers and characterize thin film morphology. Transmission soft X-ray microscopy and scattering reveal that a phase separated structure exists through the bulk for a blend of a semicrystalline semiconducting polythiophene with a functionalized side chain and a well-studied ferroelectric polymer. Surface sensitive soft X-ray spectroscopy and wide-angle X-ray scattering suggest a potential enhancement of polythiophene at the film surface, and that the surface layer is more amorphous in character. This w...

The introduction of oligothiophenes (b-5T and b-5TB) improved the performance of F85TB:PCBM bulk heterojunction OPV cells due to improved UV-vis absorption and the well-matched cascade energy levels between components.

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.

Side chain modification of a semiconducting polythiophene changes the resulting phase separation length scales when blended with a ferroelectric polymer for use in organic ferroelectric resistive switches. The domain size of the semiconducting portion of blends of poly[3-(ethyl- 5-pentanoate)thiophene-2,5-diyl] (P3EPT) and poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) in thin film blends are smaller than previously reported and easily controllable in size through simple tuning of the weight fraction of the semiconducting polymer. Furthermore, P3EPT has a relatively high degree of crystallinity and bimodal crystallite orientations, as probed by wide-angle X-ray scattering. Resistive switches fabricated from blends of P3EPT and PVDF-TrFE show memristive switching behavior over a wide range of polythiophene content and good ON/OFF ratios.

We synthesized a linear π-conjugated small molecule (β-DH5T) based on thiophene, using the palladium-catalyzed Suzuki coupling. The oligothiophene was designed to contain β-substituted solubilizing alkyl groups to facilitate solution processing. The organic photovoltaic cell based on β-DH5T:PC71BM gave a power conversion efficiency of 0.46% with a short-circuit current of 4.69 mA/cm2, an open-circuit voltage of 0.60 V, and a fill factor of 0.35, giving under AM 1.5G illumination (100 mW/cm2). The optical and electrochemical properties and molecular alignment of oligothiophene were investigated using UV-visible absorption spectroscopy, cyclic voltammetry, and thin-film X-ray diffraction studies.

Polymer-capped silicon nanoparticles (Si NPs) were prepared using poly(vinylpyrrolidone) (PVP) as a protection layer to minimize the surface oxidation of the surface. The wet synthetic procedure were performed at room temperature via a one-pot synthesis. The PVP capped Si-NPs were characterized by thermo gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), high resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy (FE-SEM). HR-TEM micrograms confirm the presence of crystalline Si-NPs. The pattern by Selected Area Electron Diffraction (SAED) reveals (111), (220) lattice planes which are consistent with the cubic-structured crystalline silicon. The mean size of Si-NPs is estimated to be ca. 10 nm, which is larger than those from conventional wet synthetic methods for silicon quantum dots.

Two benzo[1,2-b:4,5-b’]dithiophene and thiophene-based materials, a small molecule (SM-BDT) and a polymer (P-BDT), were synthesized and their photovoltaic properties were investigated by fabricating their binary and ternary cells. The introduction of the ternary blend system effectively enhanced device performance. The power conversion efficiency (PCE) of the ternary SM-BDT:P-BDT:[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) film (0.19%) was nearly double that of the pristine binary films of SM-BDT:PCBM and P-BDT:PCBM. The optical and electrochemical properties were investigated to explain the improved photovoltaic properties of the ternary blended cells.

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.

Two π-conjugated small molecules based on diketopyrrolopyrrole (DPP), DPP4T and DPP2F2T, were synthesized using the Suzuki coupling reaction. DPP4T and DPP2F2T contained furan and thiophene, respectively, next to a DPP core. Organic photovoltaic cells (OPVs) were fabricated using two DPP-based oligothiophenes as donors. DPP4T showed higher power conversion efficiency (PCE) (1.44%) than DPP2F2T (0.85%). The short-circuit current (JSC) of DPP4T (4.38 mA cm−2) was nearly twice that of DPP2F2T (2.49 mA cm−2). The improved photovoltaic properties of DPP4T could be explained by the optical properties and the film morphology.

T.L. and Y.E. contributed equally to this work. This work was financially supported by the Korea Institute of Energy Technology Evaluation & Planning (KETEP) and the Ministry of Trade, Industry & Energy (No. 20173010012960) and by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science & ICT (NRF-2015R1A1A3A04001498 & NRF-2019R1A2C1003679) of the Republic of Korea.