Nanosphere Templated Continuous PEDOT:PSS Films with Low Percolation Threshold for Application in Efficient Polymer Solar Cells

Nanometer-sized monodisperse polystyrene nanospheres (PS NS) were designed as an opal template for the formation of three-dimensionally continuous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The resultant films were successfully applied as the anode buffer layer (ABL) to produce highly efficient polymer solar cells (PSCs) with enhanced stability. The conductivity of the PS NS-PEDOT:PSS films was maintained up to ϕ_PS = 0.75–0.80, indicating that the formation of continuous PEDOT:PSS films using PS NS templates was successful. To demonstrate the applicability of the PS NS-PEDOT:PSS film for organic electronics, the PS NS-PEDOT:PSS films were used as ABLs in two different PSCs: P3HT:PCBM and P3HT:OXCBA. The photovoltaic performances of both PSCs were maintained up to ϕ_PS = 0.8. In particular, the power conversion efficiency of the P3HT:OXCBA PSC with a PS NS-PEDOT:PSS ABL (ϕ_PS = 0.8) was greater than 5% and the air stability of the device was significantly enhanced

Comparative Study of Thermal Stability, Morphology, and Performance of All-Polymer, Fullerene–Polymer, and Ternary Blend Solar Cells Based on the Same Polymer Donor

We compared the thermal and morphological stability of all-polymer solar cells (all-PSCs) and fullerene-based PSCs (fullerene-PSCs) having the same polymer donor (PBDTTTPD), which provided comparable peak power conversion efficiencies (PCEs) of >6%. We observed a remarkable contrast in thermal stability dependent upon the acceptor composition in the active layer, with the performance of the fullerene-PSCs completely deteriorating after annealing for 5 h at 150 °C, whereas that of the all-PSCs remained stable even after annealing for 50 h at 150 °C. Pronounced phase separation was observed in the active layer of the fullerene-PSCs at two different length scales. In stark contrast, almost no morphological changes in the all-PSCs were observed, likely due to the low diffusion kinetics of the polymer acceptors. To develop a comprehensive understanding of the role of polymer acceptor on the thermal stability of devices, the morphology of ternary blend active layers composed of PBDTTTPD:polymer acceptor:fullerene acceptor with different fullerene contents was examined while annealing at 150 °C. The ternary blends showed two extreme trends of all-PSC- and fullerene-PSC-like behavior in thermal stability depending on the PCBM content. When included in the active layer as <30 wt % of the acceptor mixture, fullerene was well-dispersed in the amorphous portion of the donor/acceptor polymer blend under thermal stress and led to thermally stable devices with a higher PCE (7.12%) than both all-PSCs without fullerene (6.67%) and polymer–fullerene active layers without a polymeric acceptor (6.12%)

Bicontinuous Block Copolymer Morphologies Produced by Interfacially Active, Thermally Stable Nanoparticles

Polymeric bicontinuous morphologies were created by thermal annealing mixtures of poly(styrene-<i>b</i>-2-vinylpyridine) (PS-<i>b</i>-P2VP) block copolymers and stabilized Au-core/Pt-shell (Au–Pt) nanoparticles. These Au–Pt nanoparticles have a cross-linked polymeric shell to promote thermal stability and are designed to adsorb strongly to the interface of the PS-<i>b</i>-P2VP block copolymer due to the favorable interaction between P2VP block and the exterior of the cross-linked shell of the nanoparticle. The interfacial activity of these Au–Pt nanoparticles under thermal annealing conditions leads to decrease in domain size of the lamellar diblock copolymer. As nanoparticle volume fraction ϕ_p was increased, a transition from a lamellar to a bicontinuous morphology was observed. Significantly, the effect of these shell-cross-linked Au–Pt nanoparticles under thermal annealing conditions was similar to those of traditional polymer grafted Au nanoparticles under solvent annealing conditions reported previously. These results suggest a general strategy for producing bicontinuous block copolymer structures by thermal processing through judicious selection of polymeric ligands, nanoparticle core, and block copolymer

Novel Templating Route Using Pt Infiltrated Block Copolymer Microparticles for Catalytic Pt Functionalized Macroporous WO₃ Nanofibers and Its Application in Breath Pattern Recognition

We propose a new route for transferring catalysts onto macroporous metal oxide nanofibers (NFs) using metallic nanoparticles (NPs) infiltrated block copolymer microparticles as sacrificial templates. Pt decorated polystyrene-<i>b</i>-poly­(4-vinylpyridine) (PS-<i>b</i>-P4VP) copolymer microparticles (Pt-BCP MPs), produced from oil-in-water emulsions, were uniformly dispersed within electrospun PVP/W precursor composite NFs. The macropore-loaded WO₃ NFs (macroporous Pt-WO₃ NFs), which are additionally functionalized by Pt NPs (10 nm), were achieved by decomposition of polymeric components and oxidization of W precursor after high-temperature calcination. In particular, macropores with the similar size distribution (50–300 nm) with BCP MPs were also formed on interior and exterior of WO₃ NFs. Chemical sensing performance of macroporous Pt-WO₃ NFs was investigated for pattern recognition of simulated breath gas components at highly humid ambient (95% RH). The result revealed that superior hydrogen sulfide sensitivity (<i>R</i>_air/<i>R</i>_gas = 834.2 ± 20.1 at 5 ppm) and noticeable selectivity were achieved. In addition, H₂S pattern recognition against other chemical components (acetone, toluene, and methyl mercaptan) was clearly identified without any overlapping of each pattern. This work demonstrates the potential application of BCP-templated maroporous Pt-WO₃ NFs in exhaled breath analysis for noninvasive monitoring of physical conditions

Architectural Effects on Solution Self-Assembly of Poly(3-hexylthiophene)-Based Graft Copolymers

While solution assembly of conjugated block copolymers has been widely used to produce long 1-D nanowires (NWs), it remains a great challenge to provide a higher level of control over structure and function of the NWs. Herein, for the first time, we report the solution assembly of graft copolymers containing a conjugated polymer backbone in a selective solvent and demonstrate that their self-assembly behaviors can be manipulated by the molecular structures of the graft copolymers. A series of poly­(3-hexylthiophene)-<i>graft</i>-poly­(2-vinylpyridine) (P3HT-<i>g</i>-P2VP) copolymers was designed with two different architectural parameters: grafting fraction (<i>f</i>_g) and molecular weight of P2VP chains (<i>M</i>_n,P2VP) on the P3HT backbone. Interestingly, crystallization of the P3HT-<i>g</i>-P2VP copolymers was systematically modulated by changes in <i>f</i>_g and <i>M</i>_n,P2VP, thus allowing for control of the growth kinetics and curvatures of solution-assembled NWs. When <i>M</i>_n,P2VP (4.4 to 15.1 kg/mol) or <i>f</i>_g (2.8 to 9.2%) of the P3HT-<i>g</i>-P2VP polymers was increased, the crystallinity of the copolymers was reduced significantly. Steric hindrance from the grafted P2VP chains apparently modified the growth of NWs, leading to shorter NWs with a greater degree of curvature for graft copolymers with more hindrance. Therefore, we envision that such conjugated chain-based graft copolymers can be versatile building blocks for producing NWs with controlled length and shape, which can be important for tailoring the optical and electrical properties of NW-based devices

Poly(benzodithiophene) Homopolymer for High-Performance Polymer Solar Cells with Open-Circuit Voltage of Near 1 V: A Superior Candidate To Substitute for Poly(3-hexylthiophene) as Wide Bandgap Polymer

Conjugated homopolymers can be synthesized more simply and reproducibly at lower cost than widely developing donor–acceptor (D–A) alternating copolymers. However, except for well-known poly­(3-hexylthiophene) (P3HT), almost no successful homopolymer-based polymer solar cells (PSCs) have been reported because of their relatively wide band gap and unoptimized energy levels that limit the values of short circuit current (<i>J</i>_SC) and open-circuit voltage (<i>V</i>_OC) in PSCs. Herein, we report the development of poly­(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-b’]­dithiophene) (PBDTT) homopolymer that has high light absorption coefficients and nearly perfect energy alignment with that of [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Therefore, we were able to produce high-performance PSCs with the power conversion efficiency (PCE) of 6.12%, benefiting from both high <i>V</i>_OC (0.93 V) and <i>J</i>_SC (11.95 mA cm^–2) values. To the best of our knowledge, this PCE value is one of the highest values reported for the homopolymer donor-based PSCs. Significantly, the optimized condition of the device was achieved without any solvent additive or thermal treatment. Therefore, PBDTT is a promising candidate to take over the role of P3HT in tandem solar cells and ternary blend solar cells

Efficient Colorimetric pH Sensor Based on Responsive Polymer–Quantum Dot Integrated Graphene Oxide

In this paper, we report the development of a versatile platform for a highly efficient and stable graphene oxide (GO)-based optical sensor that exhibits distinctive ratiometric color responses. To demonstrate the applicability of the platform, we fabricated a colorimetric, GO-based pH sensor that responds to a wide range of pH changes. Our sensing system is based on responsive polymer and quantum dot (QD) hybrids integrated on a single GO sheet (MQD-GO), with the GO providing an excellent signal-to-noise ratio and high dispersion stability in water. The photoluminescence emissions of the blue and orange color-emitting QDs (BQDs and OQDs) in MQD-GO can be controlled independently by different pH-responsive linkers of poly(acrylic acid) (PAA) (p<i>K</i>_a = 4.5) and poly(2-vinylpyridine) (P2VP) (p<i>K</i>_a = 3.0) that can tune the efficiencies of Förster resonance energy transfer from the BQDs to the GO and from the OQDs to the GO, respectively. As a result, the color of MQD-GO changes from orange to near-white to blue over a wide range of pH values. The detailed mechanism of the pH-dependent response of the MQD-GO sensor was elucidated by measurements of time-resolved fluorescence and dynamic light scattering. Furthermore, the MQD-GO sensor showed excellent reversibility and high dispersion stability in pure water, indicating that our system is an ideal platform for biological and environmental applications. Our colorimetric GO-based optical sensor can be expanded easily to various other multifunctional, GO-based sensors by using alternate stimuli-responsive polymers

High-Performance All-Polymer Solar Cells Based on Face-On Stacked Polymer Blends with Low Interfacial Tension

We report highly efficient all-polymer solar cells with power conversion efficiencies of over 4.5% by highly intermixed blends of PTB7-Th donor and P­(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on π–π stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from the active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P­(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency, and <i>J</i>_SC values

Morphological Evolution of Block Copolymer Particles: Effect of Solvent Evaporation Rate on Particle Shape and Morphology

Shape and morphology of polymeric particles are of great importance in controlling their optical properties or self-assembly into unusual superstructures. Confinement of block copolymers (BCPs) in evaporative emulsions affords particles with diverse structures, including prolate ellipsoids, onion-like spheres, oblate ellipsoids, and others. Herein, we report that the evaporation rate of solvent from emulsions encapsulating symmetric polystyrene-<i>b</i>-polybutadiene (PS-<i>b</i>-PB) determines the shape and internal nanostructure of micron-sized BCP particles. A distinct morphological transition from the ellipsoids with striped lamellae to the onion-like spheres was observed with decreasing evaporation rate. Experiments and dissipative particle dynamics (DPD) simulations showed that the evaporation rate affected the organization of BCPs at the particle surface, which determined the final shape and internal nanostructure of the particles. Differences in the solvent diffusion rates in PS and PB at rapid evaporation rates induced alignment of both domains perpendicular to the particle surface, resulting in ellipsoids with axial lamellar stripes. Slower evaporation rates provided sufficient time for BCP organization into onion-like structures with PB as the outermost layer, owing to the preferential interaction of PB with the surroundings. BCP molecular weight was found to influence the critical evaporation rate corresponding to the morphological transition from ellipsoid to onion-like particles, as well as the ellipsoid aspect ratio. DPD simulations produced morphologies similar to those obtained from experiments and thus elucidated the mechanism and driving forces responsible for the evaporation-induced assembly of BCPs into particles with well-defined shapes and morphologies

Effects of Solubilizing Group Modification in Fullerene Bis-Adducts on Normal and Inverted Type Polymer Solar Cells

Structural control of solubilizing side groups in fullerene-based electron acceptors is critically important to optimize their performance in bulk heterojunction (BHJ)-type polymer solar cell (PSC) devices. The structural changes of fullerene derivatives affect not only their optical and electrochemical properties but also their solubility and miscibility with electron donor polymers. Herein, we synthesized a series of <i>o</i>-xylenyl C₆₀ bis-adduct (OXCBA) derivatives with different solubilizing side groups to systematically investigate the effects of fullerene derivative structures on the photovoltaic properties of PSCs. The xylenyl side groups on the OXCBA were modified to produce several different OXCBA derivatives in which the xylenyl groups were functionalized with fluorine (FXCBA), nitro (NXCBA), methoxy and bromine (BMXCBA), and phenyl groups (ACBA). End group modifications of OXCBA dramatically affect photovoltaic performance in blend films with poly­(3-hexylthiophene) (P3HT), resulting in power conversion efficiencies (PCEs) ranging from 1.7 to 5.3%. We found that this large range in PCE values is mainly due to differences in the blend morphology and interfacial area of the P3HT:OXCBA derivative films caused by changes in the hydrophobicity of the OXCBA derivatives and their interaction with P3HT. The trend in photovoltaic performance of the different OXCBA derivatives agrees well with those of the interfacial tension, PL quenching, and exciton dissociation probability, which suggests that changes in the interaction with P3HT are largely responsible for their photovoltaic performances. Finally, the OXCBA derivatives were applied in inverted type PSC devices. We note that P3HT:OXCBA blend devices exhibited more than 5% PCE with excellent air stability, which is one of the best inverted type devices based on the P3HT polymer in a simple device architecture without any extra interlayers