Morphology Of Blend Films Research Articles

Effects of the Molecular Weight and the Side‐Chain Length on the Photovoltaic Performance of Dithienosilole/Thienopyrrolodione Copolymers

AbstractA series of low‐bandgap alternating copolymers of dithienosilole and thienopyrrolodione (PDTSTPDs) are prepared to investigate the effects of the polymer molecular weight and the alkyl chain length of the thienopyrrole‐4,6‐dione (TPD) unit on the photovoltaic performance. High‐molecular‐weight PDTSTPD leads to a higher hole mobility, lower device series resistance, a larger fill factor, and a higher photocurrent in PDTSTPD:[6,6]‐phenyl C71 butyric acid methyl ester (PC71BM) bulk‐heterojunction solar cells. Different side‐chain lengths show a significant impact on the interchain packing between polymers and affect the blend film morphology due to different solubilities. A high power conversion efficiency of 7.5% is achieved for a solar cell with a 1.0 cm2 active area, along with a maximum external quantum efficiency (EQE) of 63% in the red region.

Synthesis and Characterization of New Selenophene-Based Donor–Acceptor Low-Bandgap Polymers for Organic Photovoltaic Cells

A series of novel thiophene- and selenophene-based low-bandgap polymers were synthesized using a Stille cross-coupling reaction; these polymers contained quinoxaline and diketopyrrolopyrrole as acceptors. Various acceptors were introduced into the selenophene backbones, and the solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the resulting polymers were investigated. The weight-averaged molecular weights (Mw) of P3TDTQ, P3SDTQ, P3TDTDPP, and P3SDTDPP were found to be 12 300, 15 500, 13 300, and 17 200, with polydispersity indices of 1.46, 1.85, 1.58, and 1.63, respectively. Photophysical studies revealed low bandgaps of 1.70 eV for P3TDTQ, 1.63 eV for P3SDTQ, 1.27 eV for P3TDTDPP, and 1.25 eV for P3SDTDPP; the films could harvest a broad solar spectrum, covering the range from 300 to 800 nm (P3TDTQ and P3SDTQ) and from 350 to 950 nm (P3TDTDPP and P3SDTDPP). Solution-processed field-effect transistors fabricated from these polymers had p...

Oligomeric Compatibilizers for Control of phase Separation in Conjugated Polymer Blend Films

Control over phase separation and morphology is critical to optimal function in polymer optoelectronic devices. Here, two fully conjugated oligomeric compatibilizers are introduced, and their effect on the phase separation of blends of poly(9,9′-dioctylfluorene-co-benzo-thiadiazole) (F8BT) with poly(9,9′-dioctylfluorene-co-bis-N,N′-(4,butylphenyl)bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) are reported. AFM and STXM analysis demonstrate that the addition of compatibilizer altered the size and relative composition of phase-separated domains formed during spin-casting. Small structural differences between the two compatibilizers brought about significantly different morphological changes to the blends, suggesting that further development of compatibilizer structure could enable enhanced control toward desired blend film morphologies.

Evolution of polymer photovoltaic performances from subtle chemical structure variations

Conjugated polymers are promising replacements for their inorganic counterparts in photovoltaics due to their low cost, ease of processing, and straightforward thin film formation. New materials have been able to improve the power conversion efficiency of photovoltaic cells up to 8%. However, rules for rational material design are still lacking, and subtle chemical structure variations usually result in large performance discrepancies. The present paper reports a detailed study on the crystalline structure, morphology, and in situ optoelectronic properties of blend films of polythiophene derivatives and [6,6]-phenyl C61-butyric acid methyl ester by changing the alkyl side chain length and position of polythiophene. The correlation among the molecular structure, mesoscopic morphology, mesoscopic optoelectronic property and macroscopic device performance (highest efficiency above 4%) was directly established. Both solubility and intermolecular interactions should be considered in rational molecular design. Knowledge obtained from this study can aid the selection of appropriate processing conditions that improve blend film morphology, charge transport property, and overall solar cell efficiency.

A Copolymer of Benzodithiophene with TIPS Side Chains for Enhanced Photovoltaic Performance

A new conjugated copolymer PBDTTT-TIPS containing thiazolothiazole acceptor unit and triisopropylsilylethynyl (TIPS)-functionalized benzodithiophene donor unit was synthesized by Pd-catalyzed Stille coupling and compared with its alkoxy-substituted analogue PBDTTT-C12. PBDTTT-TIPS exhibits decomposition temperature of 377 °C, absorption maximum of 598 nm, HOMO level of −5.3 eV, and hole mobility as high as 1.2 × 10–3 cm2 V–1 s–1. Without any post-treatments, polymer solar cells based on the blend of PBDTTT-TIPS and PC71BM exhibited power conversion efficiency as high as 4.33%, which is 2 times that for device based on the PBDTTT-C12:PC71BM blend. TIPS and C12 side chains exhibit a little impact on absorption, HOMO level, and hole mobility of the polymers, but they significantly influence blend film morphology and photovoltaic performance. TIPS side chains induces excellent compatibility between PBDTTT-TIPS and PC71BM, and the PBDTTT-TIPS:PC71BM blend exhibits perfect phase separation scales around 10–20 nm, which is beneficial to charge separation and enhanced efficiency of the device, while C12 side chains leads to large phase separation, which is responsible for the lower efficiency.

Surface Morphology and Properties of Poly(γ-benzyl L-glutamate)/Poly(butyl acrylate-co-methyl Methacrylate) Blend Film

Poly(γ-benzyl L-glutamate)/poly(butyl acrylate-co-methyl methacrylate) (PBLG/Poly(BA-co-MMA)) blend films were prepared by casting the polymer blend solution in dichloroethane. Surface morphology of the polymer blend film was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Thermal and mechanical properties of the polymer blend film were studied using differential scanning calorimeter (DSC) and tensile tests. It was revealed that the introduction of Poly(BA-co-MMA) into PBLG could exert marked effects on the surface morphology and the properties of the PBLG film.

Evolved Phase Separation toward Balanced Charge Transport and High Efficiency in Polymer Solar Cells

Understanding effect of morphology on charge carrier transport within polymer/fullerene bulk heterojunction is necessary to develop high-performance polymer solar cells. In this work, we synthesized a new benzodithiophene-based polymer with good self-organization behavior as well as favorable morphology evolution of its blend films with PC(71)BM under improved processing conditions. Charge carrier transport behavior of blend films was characterized by space charge limited current method. Evolved blend film morphology by controlling blend composition and additive content gradually reaches an optimized state, featured with nanoscale fibrilla polymer phase in moderate size and balanced mobility ratio close to 1:1 for hole and electron. This optimized morphology toward more balanced charge carrier transport accounts for the best power conversion efficiency of 3.2%, measured under simulated AM 1.5 solar irradiation 100 mW/cm(2), through enhancing short circuit current and reducing geminate recombination loss.

Controlling Blend Film Morphology by Varying Alkyl Side Chain in Highly Coplanar Donor–Acceptor Copolymers for Photovoltaic Application

A series of varied length alkyl substituted donor–acceptor (D–A) conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and thiophene rings attached to both sides of the benzothiadiazole (TBT) moieties as acceptors were designed and synthesized. The optical and electrochemical properties showed that the absorption spectrum, the band gaps, and the energy levels of the copolymers were not affected by the varied substituted alkyls, and all these copolymers showed low band gaps around 1.75 eV. In addition, the morphologies of the blend film between copolymers and PCBM can be fine-tuned by increasing the length of substituted alkyl of the copolymers, changing from pea-like aggregation to interpenetrating network to grain-like aggregation. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) as acceptors. The optimized photovoltaic performances sh...

The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices

How 1,8-diiodooctane (DIO) enhances performance of polymer solar cells based on polymer HXS-1 and fullerene [6,6]-phenyl C 71-butyric acid methyl ester (PC 71BM) from 3.6% to 5.4% is scrutinized with several techniques by comparing devices or blend films spin-coated from dichlorobenzene (DCB) to those from DCB/DIO (97.5:2.5 v/v). Morphology of blend films is examined with atomic force microscopy (AFM), transmission electron microscopy (TEM) and electron tomography (3-D TEM), respectively. Charge generation and recombination is studied with photoluminescence, and charge transport with field effect transistors. The morphology with domain size in 10–20 nm and vertical elongated clusters formed in DIO system is supposed to facilitate charge transport and minimize charge carrier recombination, which are the main reasons for enhancing power conversion efficiency (PCE) from 3.6% (without DIO) to 5.4% (with DIO). Furthermore, a two year inspection shows no significant impact of DIO on the shelf-stability of the solar cells. No visible degradation in the second year indicates that the morphology of the active layers in the devices is relatively stable after initial relaxation in the first year.

Side Chain Engineering of Copolymers Based on Bithiazole and Benzodithiophene for Enhanced Photovoltaic Performance

Four new copolymers P1–P4 containing the same backbone of bithiazole acceptor unit and benzodithiophene donor unit but different side chain pattern were synthesized by Pd-catalyzed Stille coupling. The effect of the side chains on backbone conformation, solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the polymers were experimentally and theoretically investigated. The planarity of the main chain increases in the order P3 < P4 < P1 ≈ P2. Upon increasing the planarity of the main chain, the polymer exhibits red-shifting of absorption maximum in film from 448 to 544 nm, upshifting of HOMO from −6.0 to −5.4 eV, downshifting of LUMO from −2.6 to −2.9 eV, and increasing of hole mobility from 4.7 × 10–4 to 0.06 cm2 V–1 s–1. Upon increasing the planarity of the polymer main chain, the phase separation size in polymer:PC71BM blend increases. The polymer solar cells based on P4:PC71BM (1:1, w/w) exhibit highest power conversion efficiency of 2.54% under AM 1.5, 100 mW cm–2, which is attributed to combination of broad absorption, high mobility, and suitable phase separation benefited from moderate planarity of the main chain.

Influence of Film Thickness on the Phase Separation Mechanism in Ultrathin Conducting Polymer Blend Films

The film morphology of thin polymer blend films based on poly[(1-methoxy)-4-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(N-vinylcarbazole) (PVK) is probed as a function of film thickness. Blend films are prepared with spin-coating of polymer solutions with different concentrations on top of solid supports. The blending ratio of both conducting polymers is kept constant. The film and surface morphology is probed with grazing incidence ultrasmall-angle X-ray scattering (GIUSAXS) and atomic force microscopy (AFM). A linear dependence between the film thickness and the averaged phase separation is found. In addition, X-ray reflectivity measurements show an enrichment of PVK at the substrate interface. UV/vis spectroscopy measurements indicate a linearly increasing amount of both homopolymers in the blend films for increasing film thicknesses. The generalized knowledge about the influence of the film thickness on the phase separation behavior in conducting polymer blend films is finally used to describe the phase separation formation during the spin-coating process, and the results are discussed in the framework of an adapted Flory-Huggins theory for rodlike polymers.

Molecular Engineering of Copolymers with Donor−Acceptor Structure for Bulk Heterojunction Photovoltaic Cells Toward High Photovoltaic Performance

A series of donor−acceptor (D−A) conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and two different electron-accepting groups bithiophenevinyl-2-pyran-4-ylidenemalononitrile (TVM) and benzothiadiazole (BT) moieties as acceptors is designed and synthesized. The optical and electrochemical properties show that the band gaps of the copolymers are in the range of 1.70−1.84 eV, and the HOMO and LUMO energy levels can be tuned effectively by controlling the varied ratios between TVM and BT because of the change of the ICT interaction between donor and acceptor, the electron delocalization degree, and the electron cloud density distribution of the copolymers. Bulk heterojunction photovoltaic devices are fabricated by using the copolymers as donors and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptors. The optimized photovoltaic performances show that the open-circuit voltage (Voc) was gradually increased from 0.7 to 0.94 V when decreasing the HOMO energy levels of copolymers, and the short-circuit current density (Jsc) is greatly improved by increasing the absorption spectrum in the visible region, increasing the hole mobility and optimizing the morphologies of blend films between copolymers and PCBM. The optimized photovoltaic performance with a Voc of 0.78 V, Jsc of 5.47 mA/cm2, fill factor (FF) of 0.40, and power conversion efficiency (PCE) of 1.67% under simulated AM 1.5 solar irradiation of 100 mW/cm2 is obtained by the copolymer PM50 (PM50:PC61BM, 1:3 w/w, in CB solution). This is due to its high hole mobility and interpenetrating network morphology of PM50:PC61BM blend film. The photovoltaic device based on PM50:PC71BM shows a Jsc of 8.32 mA/cm2 and a PCE of 2.89%.

Preparation and properties of chitosan derivative/poly(vinyl alcohol) blend film crosslinked with glutaraldehyde

Novel crosslinked blend films of N-(2-hydroxy)propyl-3-trimethylammonium chitosan chloride (HTCC) and poly(vinyl alcohol) (PVA) were prepared by mixing their aqueous solutions followed by crosslinking with glutaraldehyde (GA). The effects of the GA content and ratio of HTCC to PVA on the appearance, swelling, gel content, morphology and antibacterial activities of blend films were studied and a possible mechanism of crosslinking was proposed based on the results. It was found that the equilibrium degree of swelling (ESD) increased with the increasing content of HTCC, but decreased with the content of GA. The ESD was a maximum (212%) when the ratio of HTCC/PVA/GA was 60/40/2 (wt). Antibacterial activities against Staphylococcus aureus and Escherichia coli of the blend films were weakened slightly by crosslinking, but still showed substantial antibacterial activity. These results demonstrate that, not only the PVA, but also the HTCC reacted with GA. It was the amino groups on HTCC that was un-substituted by quaternary ammonium salt, which reacted with the aldehyde groups on GA.

The optimized morphology of polymer:fullerene bulk heterojunction thin film via reducing pressure

The influence of pressure on the morphology of blend film consisted of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied in the solvent evaporating process. It is found that the crystalline aggregation of the P3HT component was markedly enhanced in the blend film grown under reduced pressure, compared to that under normal atmospheric pressure. Moreover, the blend film grown under reduced pressure offered greater short-circuit current and power conversion efficiency of a device than that grown under normal atmospheric pressure, mainly ascribing to it that the optimized morphology of the blend film via decreasing pressure improved transport of photo-charges. The pressure effect of PCBM vertical diffusion in the blend film is also discussed.

Phase separation of polyphosphazene/poly(lactide-co -glycolide) blends prepared under different conditions

Using a mutual solvent technique, blend films of poly[(alaine ethyl ester)0.62(glycine ethyl ester)0.38]phosphazene/poly(lactide-co-glycolide) (PAGP/PLGA blend) were prepared at different conditions including weight ratios, solvents, environmental humidity, film thickness, and substrates. The morphology and properties of blend films were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDX), X-ray photoelectron spectrometry (XPS), and solvent selective etching. Compared with dichloromethane and tetrahydrofuran (THF), chloroform was the better solvent to form miscible PAGP/PLGA blend films at relatively anhydrous atmosphere. However, in the humid atmosphere, the hexagonal arrangement of holes appeared on the surface of PAGP/PLGA blend films due to the ordered array of water droplets. A sandwich-liked structure was formed with the hydrophilic PAGP component at the top and bottom, while the PLGA component in the middle. In addition, the surface morphology of PAGP/PLGA blend films was also influenced by the film thickness and the property of the substrate. Copyright © 2010 John Wiley & Sons, Ltd.

Changes on surface morphology of corn starch blend films

This study aims at evaluating the influence of enzymatic degradation solution on the surface morphology and thermal properties of a poly(ethylene-vinyl alcohol) copolymer-corn starch thermoplastic blend (SEVA-C), as a function of immersion time. To perform this study, three different batches were assessed using SEVA-C samples of different thicknesses and a fixed weight of 1.6 g, immersed in alpha-amylase (50 u/L) up to 90 days at 37 degrees C. TGA, contact angle measurements, scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques were used. Three degradation mechanisms are considered in these systems: namely, mass loss due to plasticizer leaching (glycerol), starch enzymatic cleavage, and synthetic polymer fractions degradation. Enzymatic hydrolysis of the starch fraction and subsequent leaching from the internal bulk structure led to an increase in surface porosity, pore size, roughness, and to the development of small pits throughout the surface, as observed by SEM and AFM.

Organometallic Photovoltaics: A New and Versatile Approach for Harvesting Solar Energy Using Conjugated Polymetallaynes

Energy remains one of the world's great challenges. Growing concerns about limited fossil fuel resources and the accumulation of CO(2) in the atmosphere from burning those fuels have stimulated tremendous academic and industrial interest. Researchers are focusing both on developing inexpensive renewable energy resources and on improving the technologies for energy conversion. Solar energy has the capacity to meet increasing global energy needs. Harvesting energy directly from sunlight using photovoltaic technology significantly reduces atmospheric emissions, avoiding the detrimental effects of these gases on the environment. Currently inorganic semiconductors dominate the solar cell production market, but these materials require high technology production and expensive materials, making electricity produced in this manner too costly to compete with conventional sources of electricity. Researchers have successfully fabricated efficient organic-based polymer solar cells (PSCs) as a lower cost alternative. Recently, metalated conjugated polymers have shown exceptional promise as donor materials in bulk-heterojunction solar cells and are emerging as viable alternatives to the all-organic congeners currently in use. Among these metalated conjugated polymers, soluble platinum(II)-containing poly(arylene ethynylene)s of variable bandgaps (∼1.4-3.0 eV) represent attractive candidates for a cost-effective, lightweight solar-energy conversion platform. This Account highlights and discusses the recent advances of this research frontier in organometallic photovoltaics. The emerging use of low-bandgap soluble platinum-acetylide polymers in PSCs offers a new and versatile strategy to capture sunlight for efficient solar power generation. Properties of these polyplatinynes--including their chemical structures, absorption coefficients, bandgaps, charge mobilities, accessibility of triplet excitons, molecular weights, and blend film morphologies--critically influence the device performance. Our group has developed a novel strategy that allows for tuning of the optical absorption and charge transport properties as well as the PSC efficiency of these metallopolyynes. The absorbance of these materials can also be tuned to traverse the near-visible and near-infrared spectral regions. Because of the diversity of transition metals available and chemical versatility of the central spacer unit, we anticipate that this class of materials could soon lead to exciting applications in next-generation PSCs and other electronic or photonic devices. Further research in this emerging field could spur new developments in the production of renewable energy.

Thin Film Morphology of Block Copolymer Blends with Tunable Supramolecular Interactions for Lithographic Applications

A modular and hierarchical self-assembly strategy using block copolymer blends (AB/B'C) with tunable supramolecular interactions is reported. By combining supramolecular assembly of hydrogen- bonding units with controlled phase separation of diblock copolymers, highly ordered square arrays or hexagonalarraysofcylindricaldomainswereobtainedformixturesofpoly(ethyleneoxide)-b-poly(styrene-r- 4-hydroxystyrene) (PEO-b-P(S-r-4HS)) and poly(styrene-r-4-vinylpyridine)-b-poly(methyl methacrylate) (P(S-r-4VP)-b-PMMA) diblock copolymers under solvent annealing with controlled high humidity. The fraction of the H-bonded phenolic and pyridyl units was shown to be critical for both the generation of long- range order and controlling the spatial arrangement of the cylindrical domains. Both low absolute numbers and a near-stoichiometric ratio of pyridyl-to-phenolic groups are needed to produce ordered square arrays with separated PEO and PMMA domains, whereas a low ratio of pyridyl-to-phenolic groups facilitated the formation of ordered hexagonal arrays with mixed PEO and PMMA domains. Self-consistent field theory simulations suggest that the effective Flory-Huggins parameters between the various blocks control the stability of the different packing structures in this system. The modularity and tunability of this supramo- lecular block copolymer blending approach is a unique and powerful strategy to fabricate diverse nanostructures for a variety of applications such as block copolymer lithography.

High‐Resolution Spectroscopic Mapping of the Chemical Contrast from Nanometer Domains in P3HT:PCBM Organic Blend Films for Solar‐Cell Applications

AbstractA high‐resolution near‐field spectroscopic mapping technique is successfully applied to investigate the influence of thermal annealing on the morphology of a poly(3‐hexylthiophene) and [6,6]‐penyl‐C61 butyric acid methyl ester (P3HT:PCBM) blend film. Based on the simultaneously recorded morphological and spectroscopic information, the interplay among the blend film morphology, the local P3HT:PCBM molecular distribution, and the P3HT photoluminescence (PL) quenching efficiency are systematically discussed. The PL and Raman signals of the electron donor (P3HT) and acceptor (PCBM) are probed at an optical resolution of approximately 10 nm, which allows the chemical nature of the different domains to be identified directly. In addition, the local PL quenching efficiency, which is related to the electron transfer from P3HT to PCBM, is quantitatively revealed. From these experimental results, it is proposed that high‐resolution near‐field spectroscopic imaging is capable of mapping the local chemical composition and photophysics of the P3HT:PCBM blends on a scale of a few nanometers.

Novel Low Band Gap Small Molecule and Phenylenevinylene Copolymer with Cyanovinylene 4-Nitrophenyl Segments: Synthesis and Application for Efficient Bulk Heterojunction Solar Cells

A novel star-shaped small monomer SM containing a 1,3,5-triazine core and arms with terminal cyanovinylene 4-nitrophenyls was synthesized. Moreover, an alternating p-phenylenevinylene copolymer P containing thiophene with cyanovinylene 4-nitrophenyl side segments was synthesized by Heck coupling. Both SM and P showed broad absorption spectra with long-wavelength maximum at 630−648 nm, which for P is attributable to an intramolecular charge transfer. The optical band gap was 1.57 eV for SM and 1.70 eV for P. Both SM and P were blended with PCBM to study the donor−acceptor interactions on the blend film morphology and device characteristics of organic bulk heterojunction solar cells. A combination of characterization techniques including X-ray diffraction and optical topographical images were used to investigate the film morphology. The HOMO and LUMO levels of both SM and P are well-aligned with those of the PCBM acceptor, allowing efficient electron transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of 2.53 and 1.43% for SM:PCBM and P:PCBM-based devices, respectively. The thermal annealing leads to suitable phase separation due to the increase in crystallinity of donor material and material distribution so that highly effective bulk heterojunction morphologies are obtained which further increases the PCE up to 3.82% and 2.37% for SM:PCBM and P:PCBM-based devices, respectively. These results are preliminary based on the illumination without using a solar simulator.