Enhanced properties of photovoltaic devices tailored with novel supramolecular structures based on reduced graphene oxide nanosheets grafted/functionalized with thiophenic materials

For verifying the influence of donor–acceptor supramolecules on photovoltaic properties, different hybrids were designed and used in organic solar cells. In this respect, reduced graphene oxide (rGO) was functionalization with 2‐thiophene acetic acid (rGO‐f‐TAA) and grafted with poly(3‐dodecylthiophene) (rGO‐g‐PDDT) and poly(3‐thiophene ethanol) (rGO‐g‐PTEt) to manipulate orientation of poly(3‐hexylthiophene) (P3HT) assemblies. Face‐on, edge‐on, and flat‐on orientations were detected for assembled P3HTs on rGO and its functionalized and grafted derivatives, respectively. Alteration of P3HT orientation from face‐on to flat‐on enhanced current density (J _sc), fill factor (FF), and power conversion efficiency (PCE) and thus J _sc = 7.11 mA cm^?2, FF = 47%, and PCE = 2.14% were acquired. By adding phenyl‐C71‐butyric acid methyl ester (PC71BM) to active layers composed of pre‐designed P3HT/rGO, P3HT/rGO‐f‐TAA, P3HT/rGO‐g‐PDDT, and P3HT/rGO‐g‐PTEt hybrids, photovoltaic characteristics further improved, demonstrating that supramolecules appropriately mediated in P3HT:PC71BM solar cells. Phase separation was more intensified in best‐performing photovoltaic systems. Larger P3HT crystals assembled onto grafted rGOs (95–143 nm) may have acted as convenient templates for the larger and more intensified phase separation in P3HT:PCBM films. The best performances were reached for P3HT:P3HT/rGO‐g‐PDDT:PCBM (J _sc = 9.45 mA cm^?2, FF = 54%, and PCE = 3.16%) and P3HT:P3HT/rGO‐g‐PTEt:PCBM (J _sc = 9.32 mA cm^?2, FF = 53%, and PCE = 3.11%) photovoltaic systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1877–1889     

Toward the Realization of A Practical Diketopyrrolopyrrole‐Based Small Molecule for Improved Efficiency in Ternary BHJ Solar Cells

An easily accessible DPP‐based small molecule ( DMPA‐DTDPP ) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM‐based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA‐DTDPP as an additive yields an increase in the short circuit current density (J_sc) because DMPA‐DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble‐free and cost effective synthesis of DMPA‐DTDPP , it may prove very useful in high‐performance solar cells.     

Broadening the absorption of conjugated polymers by "click" functionalization with phthalocyanines

Conjugated copolymer derivatives of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT) containing 10% of alkyne functionalities in the side chains have been prepared using the sulfinyl precursor route and the Rieke method, respectively. With the aim of expanding the absorption range of these conjugated polymers for their use in bulk heterojunction (BHJ) polymer:fullerene solar cells, appropriate phthalocyanine (Pc) molecules have been covalently bound through a post-polymerization "click chemistry" reaction between the alkyne functionalities in the side chains of the copolymers and a Pc functionalized with an azide moiety. The resulting poly(p-phenylenevinylene)-Pc (PPV-Pc) material holds a 9 mol% content of Pcs, while the polythiophene-Pc material (PT-Pc) contains a 8 mol% of Pc-functionalization in the side chains. As expected, the presence of the Pc contributes to the extension of the absorption up to 700 nm. BHJ solar cells have been prepared using PPV-Pc and PT-Pc materials in combination with PCBM. Although the Pc absorption contributes to the generation of photocurrent, the overall power conversion efficiencies (PCE) obtained from these cells are lower than those obtained with BHJ P3HT:PCBM (1:1) and MDMO-PPV:PCBM (1:4) solar cells. A plausible explanation could be the moderate solubility of the PPV-Pc and PT-Pc materials that limits the processing into thin films.     

Coating on a Cold Substrate Largely Enhances Power Conversion Efficiency of the Bulk Heterojunction Solar Cell

Spin‐coating a mixture solution of P3HT and PCBM on a cold substrate largely enhanced the power conversion efficiency (PCE) of the bulk heterojunction (BHJ) solar cells. This concept was based on the abrupt decrease in the solubility of P3HT as solution temperature decreased. The selective precipitation of P3HT on the PEDOT:PSS‐coated cold substrate facilitated a desirable rich composition of P3HT at the interface with the PEDOT:PSS layer. The high crystallinity of P3HT suppressed the movement of PCBM during thermal annealing, preventing aggregation of PCBM. The morphological excellence of the pristine film gave a comparable PCE to that made by the conventional fabrication process. After thermal annealing, the device made via coating on a cold substrate showed above 30% increase in PCE from the BHJ solar cells made by the conventional method.

     

Facile single-step preparation of Pt/N-graphene catalysts with improved methanol electrooxidation activity

In this work, we describe a facile single-step approach for the simultaneous reduction of graphene oxide to graphene, functional doping of graphene with nitrogen, and loading of the doped graphene with well-dispersed platinum (Pt) nanoparticles using a solvent mixture of ethylene glycol and N-methyl-2-pyrrolidone. The as-prepared Pt/nitrogen-doped graphene (N-graphene) catalysts are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy while the electrocatalytic methanol oxidation properties of the catalysts are evaluated by cyclic voltammetry and chronoamperometry. Compared with an updoped Pt/graphene control catalyst, the Pt/N-graphene catalyst shows a narrower particle size distribution and improved catalytic performance. Considering the facile, green and effective single-step synthetic process for the Pt/N-graphene catalyst, the results are promising for the potential application of these materials in emerging fuel cell technologies.     

Controlling the surface composition of PCBM in P3HT/PCBM blend films by using mixed solvents with different evaporation rates

The surface composition of poly(3-hexylthiophene-2,5-diyl) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) blend films could be changed by controlling the film formation process via using mixed solvents with different evaporation rates. The second solvent, with a higher boiling point than that of the first solvent and much better solubility for PCBM than P3HT, is chosen to mix with the first solvent with a lower boiling point and good solubility for both PCBM and P3HT. The slow evaporation rate of the second solvent provides enough time for PCBM to diffuse upwards during the solvent evaporation. Thus, the weight ratio of PCBM and P3HT (m _PCBM/m _P3HT) at surface of the blend films was varied from ca. 0.1 to ca. 0.72, i.e., it increases about seven times by changing from single solvent to mixed solvents. Meanwhile, the mixed solvents were in favor to form P3HT naonofiber network and enhance phase separation of P3HT/PCBM blend films. As a result, the power conversion efficiency of the device from mixed solvents with slow evaporation process was about 1.5 times of the one from single solvents.     

Preparation of aryl azides of aryl boronic acids and one-pot synthesis of 1,4-diaryl-1,2,3-triazoles by a magnetic cysteine functionalized GO–CuI/II nanocomposite

A mixed Cu^I/Cu^IIcatalyst based on magnetic cysteine functionalized graphene oxide (Cu^I/II@Cys-MGO) was prepared and used for the azidonation reaction of aryl boronic acids and one-pot synthesis of 1,4-diaryl −1,2,3-triazoles. Aryl azides were obtained in good yields and short reaction times via cross-coupling of aryl boronic acids with sodium azide in the presence of Cu^II catalytic species in this catalytic system. The azide-alkyne cycloaddition reaction was catalyzed by CuI catalytic species in Cu^I/^II@Cys-MGO nanocomposite.     

In situ thermo-optical studies of polymer:fullerene blend films

Optical properties of a blend thin film (1:1 wt) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exposed to a stepwise heating and cooling, have been reported and compared with the properties of pure PCBM and P3HT films. The UV–Vis(T) absorption measurements were performed in situ, during annealing and cooling runs, at the precisely defined temperatures, in a range of 20–210 °C. It was demonstrated that this new method allows to observe the changes of absorption coefficient spectra and absorption edge parameters: the energy gap (E_G) and the Urbach energy (E_U), connected with the length of conjugation and structural disorder of thin film, respectively. Several stages, during annealing/cooling runs, were distinguished for the P3HT:PCBM blend film and related to the following processes, as an increase of P3HT crystallinity in the blend, the orderly stacking of polymer chains, thermally induced structural defects and the phase separation, caused by an aggregation of PCBM in the polymer matrix. These changes were also observed on the P3HT:PCBM film surface, by means to the microscopic studies.     

Vertical phase separation of conjugated polymer and fullerene bulk heterojunction films induced by high pressure carbon dioxide treatment at ambient temperature

The morphology of bulk-heterojunctions (BHJ) is critically important for conjugated polymer and fullerene blend solar cells. To alter the morphology, high pressure (gas phase) carbon dioxide (CO(2)) treatment is applied to poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films under ambient temperature. This process can achieve vertically phase separated morphology such that PCBM distributes toward the film surface, which is suggested by secondary ion mass spectroscopy (SIMS), contact angle, X-ray photoelectron spectroscopy (XPS) and cross-sectional scanning electron microscope (SEM) studies. While pristine P3HT films do not show a significant change upon CO(2) treatment, pristine PCBM films are plasticized in high pressure CO(2). Thus, PCBM is selectively plasticized by CO(2) in the blend film and is drawn towards the surface due to depressed surface energy, although P3HT tends to distribute around the surface without CO(2). This stratification process can enhance solar cell performance. 55% improvement is achieved in the power conversion efficiency of the CO(2) treated device compared to the untreated one, indicating that CO(2) treatment can be a good candidate for optimizing the morphology and enhancing the performance of BHJ polymer solar cells.     

Impact of side‐chain length on the phase structures of P3ATs and P3AT:PCBM films as revealed by SSNMR and FTIR

It is known that poly(3‐alkylthiophene) (P3AT) side‐chain length notably influences the photovoltaic performances of relating devices. However, comprehensively study on its impact on the structures of P3ATs and their blends with [6, 6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) is insufficient. By using solid‐state NMR and FTIR techniques, four P3ATs and their PCBM blends are investigated in this work, focusing on the phase structures as modulated by side‐chain length. Recently, we revealed multiple crystalline main‐chain packings of packing a and b together with a mesophase in poly(3‐butylthiophene) (P3BT) films (DOI: 10.1021/acs.macromol.6b01828). Here, the semicrystalline structures are investigated on poly(3‐hexylthiophene) (P3HT), poly(3‐octylthiophene) (P3OT), and poly(3‐dodecylthiophene) (P3DDT) with traditional form I modification, where packing a and the amorphous phase are probed. Furthermore, crystallized side chain within packing a is detected in both P3OT and P3DDT films, which shows a FTIR absorption at 806 cm⁻¹. Structural studies are also conducted on P3AT:PCBM blends. Compared with the pure P3ATs, the polymer crystallinities of the blends show reduction of about 40% for P3OT and P3DDT, whereas only about 10% for P3HT. Moreover, in P3BT:PCBM and P3HT:PCBM, the crystalline polymers and PCBM are phase separated, while in P3OT:PCBM and P3DDT:PCBM, blend components are mostly miscible. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 751–761     

P3HT/graphene composites synthesized using In situ GRIM methods

Poly(3‐hexylthiophene)/graphene (P3HT/G) materials are synthesized using an in situ Grignard metathesis approach. The structural properties and spectroscopy of the materials are studied using NMR, FTIR, and UV–vis absorption spectroscopies, wide‐angle X‐ray scattering, atomic force microscopy and cyclic voltammetry. P3HT regioregularities ≥90% are observed in composites with approximate graphene compositions as high as 20% by weight. ¹H and ¹³C NMR spectra of liquid phase dispersions reveal signals that are not observed in pristine P3HT. The intensity of these signals correlates with the graphene content, thereby indicating that the graphene is influencing the polymerization process. Anomalous features in the FTIR spectra of P3HT/G powders are also observed. AFM images of P3HT/G films show morphological differences between P3HT on graphene compared with P3HT domains on the silicon substrates. Cyclic voltammetry reveals a monotonic decrease in the energy of the HOMO and LUMO levels of P3HT with increasing graphene loading. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 60–76     

Unsymmetrical zinc phthalocyanines containing thiophene and amine groups as donor for bulk heterojunction solar cells

Photovoltaic technology is an alternative resource for renewable and sustainable energy and low costs organic photovoltaic devices such as bulk-heterojunction (BHJ) solar cells, which are selective candidates for the effective conversion of solar energy into electricity. Asymmetric phthalocyanines containing electron acceptor and donor groups create high photovoltaic conversion efficiency in dye sensitized solar cells. In this study, a new unsymmetrical zinc phthalocyanine was designed and synthesized including thiophene and amine groups at peripherally positions for BHJ solar cell. The structure of the targeted compound (4) was characterized comprehensively by FT-IR, UV–Vis, ¹H-NMR, and MALDI-TOF MS spectroscopies. The potential of this compound in bulk heterojunction (BHJ) photovoltaic devices as donor was also researched as function of blend ratio (blend ratio was varied from 0.5 to 4). For this purpose, a series of BHJ devices with the structure of fluorine doped indium tin oxide (FTO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/ ZnPc:[6,6]- phenyl-C61- butyric acid methyl ester (PCBM) blend/Al with identical thickness of ZnPc:PCBM layer were fabricated and characterized. Photo current measurements in 4 revealed that the observed photo current maximum is consistent with UV-vis spectra of the compound of 4. Preliminary studies showed that the blend ratio has a critical effect on the BHJ device performance parameters. Photovoltaic conversion efficiency of 6.14% was achieved with 4 based BHJ device.     

Phthalimide and Naphthalimide end‐Capped Diketopyrrolopyrrole for Organic Photovoltaic Applications

Two small molecules named PI‐DPP and NI‐DPP with a DPP core as the central strong acceptor unit and phthalimide/naphthalimide as the terminal weak acceptor were designed and synthesized. The effects of terminal phthalimide/naphthalimide units on the thermal behavior, optical and electrochemical properties, as well as the photovoltaic performance of these two materials were systematically studied. Cyclic voltammetry revealed that the lowest unoccupied molecular orbitals (LUMO) (~ ‐3.6 eV) of both molecules were intermediate to common electron donor (P3HT) and acceptor (PCBM). This indicated that PI‐DPP and NI‐DPP may uniquely serve as electron donor when blended with PCBM, and as electron acceptor when blended with P3HT, where sufficient driving forces between DPPs and PCBM, as well as between P3HT and DPPs should be created for exciton dissociation. Using as electron donor materials, PI‐DPP and NI‐DPP devices exhibited low power conversion efficiencies (PCEs) of 0.90% and 0.76% by blending with PCBM, respectively. And a preliminary evaluation of the potential of the NI‐DPP as electron acceptor material was carried out using P3HT as a donor material, and P3HT: NI‐DPP device showed a PCE of 0.6%, with an open circuit voltage (V _OC) of 0.7 V, a short circuit current density (J _SC) of 1.91 mA•cm^‐2, and a fill factor (FF) of 45%.     

Copper catalyzed aryl amidation between Nα-Fmoc-protected amino-acid azides and aryl boronic acids

Abstract

A simple and efficient method for the synthesis of aryl amides via oxidative copper-catalyzed coupling of commercially available aryl boronic acids and bench stable N^α-protected amino-acid azides is reported. The potential utility of this protocol is demonstrated through a survey of diversely substituted aryl boronic acids and several side-chain functionalized amino-acid azides, leading to the preparation of the desired amidated products in good to excellent yields. This amide synthesis is suitable for the preparation of amides (such as peptide aryl amides and sterically hindered amino acids) that are not or hardly accessible via classical approaches.     

Photoluminescence Study on Charge Transfer Behavior of Poly(3‐hexylthiophene) and PCBM Blends

Charge transfer behavior of Poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl eser (PCBM) in solutions and in films were examined by photoluminescence (PL) spectroscopy. PL study in solutions indicated that separation distance between P3HT and PCBM affected charge transfer efficiency more seriously than the interface area issue between P3HT and PCBM. P3HT/PCBM film showed very effective photo‐induced charge transfer before post‐thermal annealing on the bi‐layer P3HT/PCBM film. Charge transfer efficiency was gradually diminished by the annealing‐induced phase separation between P3HT and PCBM as revealed by increasing PL emission intensity of P3HT.     

Vinyl‐Type Polynorbornenes with Pendant PCBM: A Novel Acceptor for Organic Solar Cells

Vinyl addition homo‐ and copolymerization of norbornene monomer ( M1 ) functionalized with a PCBM moiety using a Pd(II) catalyst in combination with a 1‐octene chain transfer agent efficiently produces polynorbornenes with side‐chain PCBM groups. Characterization by NMR spectroscopy and elemental analysis reveals that the copolymers constitute a well‐defined polymer structure with controlled incorporation of M1 . Although the homopolymer is insoluble in organic solvents, the copolymers containing 62 mol% ( P2 ) and 50 mol% ( P3 ) of the PCBM moiety are soluble in chlorinated solvents such as o‐dichlorobenzene. The bulk‐heterojunction organic photovoltaic devices fabricated based on the P3HT: P3 blends show that P3 can adequately function as an electron acceptor, leading to a cell with a power conversion efficiency of 1.5%, which is outstanding among the polymeric rivals.     

Synthesis of all‐conjugated poly(3‐hexylthiophene)‐block‐ poly(3‐(4′‐(3″,7″‐dimethyloctyloxy)‐3′‐pyridinyl)thiophene) and its blend for photovoltaic applications

New all‐conjugated block copolythiophene, poly(3‐hexylthiophene)‐block‐poly(3‐(4′‐(3″,7″‐dimethyloctyloxy)‐3′‐pyridinyl)thiophene) (P3HT‐b‐P3PyT) was successfully prepared by Grignard metathesis polymerization. The supramolecular interaction between [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and P3PyT was proposed to control the aggregated size of PCBM and long‐term thermal stability of the photovoltaic cell, as evidenced by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and optical microscopy. The effect of different solvents on the electronic and optoelectronic properties was studied, including chloroform (CL), dichlorobenzene (DCB), and mixed solvent of CL/DCB. The optimized bulk heterojunction solar cell devices using the P3HT‐b‐P3PyT/PCBM blend showed a power conversion efficiency of 2.12%, comparable to that of P3HT/PCBM device despite the fact that former had a lower crystallinity or absorption coefficient. Furthermore, P3HT‐b‐P3PyT could be also used as a surfactant to enhance the long‐term thermal stability of P3HT/PCBM‐based solar cells by limiting the aggregated size of PCBM. This study represents a new supramolecular approach to design all‐conjugated block copolymers for high‐performance photovoltaic devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Thermal stability of P3HT and P3HT:PCBM blends in the molten state

The thermal stability of poly(3-hexylthiophene) (P3HT) in its molten state was investigated in air and nitrogen atmospheres under no illumination conditions, with the aim of testing the feasibility of processing it using polymer melt techniques. A large set of different experimental characterization techniques was used including thermogravimetric analysis (TGA), rotational rheometry, infrared spectroscopy (FTIR-ATR), proton nuclear magnetic resonance spectroscopy (¹H-NMR), gel permeation chromatography (GPC), UV-Vis and fluorescence spectroscopy. The results obtained strongly suggest that the processing of P3HT in its molten state is possible, without noticeable degradation, if carried out under nitrogen atmosphere and if the processing (residence) times are relatively short. Conversely, as expected, in a normal air atmosphere P3HT degrades rapidly at temperatures above its melting point. The effect of PCBM on the thermal stability of P3HT:PCBM blends in the molten state was also studied using TGA, and in air atmosphere PCBM is shown to delay oxidation.     

Synthesis of Phosphorus‐Doped Graphene and its Wide Potential Window in Aqueous Supercapacitors

Phosphorus‐doped (P‐doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X‐ray photoelectron spectroscopy, while the morphology of P‐doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P‐doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H₂SO₄. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P‐doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g^?1, providing a high energy density of 11.64 Wh kg^?1 and a high power density of 831 W kg^?1.     

The origin of DC electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) films

We present here the evidence for the origin of dc electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) (P3HT) films. P3HT has been synthesized and purified to obtain pristine P3HT polymer films. P3HT films are chemically doped to make conducting P3HT films with different conductivity level. Temperature (77–350 K) dependent dc conductivity (σ_dc) and dielectric constant (ε′(ω)) measurements on pristine and doped P3HT films have been conducted to evaluate dc and ac electrical conduction parameters. The relaxation frequency (f_R) and static dielectric constant (ε₀) have been estimated from dielectric constant measurements. A correlation between dc electrical conduction and dielectric relaxation data indicates that both dc and ac electrical conductions originate from the same hopping process in this system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1047–1053, 2010