Xinhong Yu

Donor/Acceptor Molecular Orientation-Dependent Photovoltaic Performance in All-Polymer Solar Cells

The correlated donor/acceptor (D/A) molecular orientation plays a crucial role in solution-processed all-polymer solar cells in term of photovoltaic performance. For the conjugated polymers PTB7-th and P(NDI2OD-T2), the preferential molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents. However, an increasing content of face-on molecular orientation in the neat P(NDI2OD-T2) films could be found by changing processing solvents from chloronaphthalene (CN) and o-dichlorobenzene (oDCB) to chlorobenzene (CB). Besides, the neat P(NDI2OD-T2) films also exhibited a transformation of preferential molecular orientation from face-on to edge-on when extending film drying time by casting in the same solution. Consequently, a distribution diagram of molecular orientation for P(NDI2OD-T2) films was depicted and the same trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films. By manufacture of photovoltaic devices with blend films, the relationship between the correlated D/A molecular orientation and device performance was established. The short-circuit current (Jsc) of devices processed by CN, oDCB, and CB enhanced gradually from 1.24 to 8.86 mA/cm(2) with the correlated D/A molecular orientation changing from face-on/edge-on to face-on/face-on, which could be attributed to facile exciton dissociation at D/A interface with the same molecular orientation. Therefore, the power conversion efficiency (PCE) of devices processed by CN, oDCB, and CB improved from 0.53% to 3.52% ultimately.

Microphase Separation of Block Copolymer Thin Films

Today, high-ordered micro- and nano-patterned surfaces are widely used in many areas, such as in the preparation of super-thin dielectric films, photonic crystals, antireflective films, super-non-wetting surfaces, bio-compatible surfaces and microelectric devices. Considering the critical fabrication conditions and the irreducible high cost of the photolithography technique in patterning nano-scale structures (<100 nm), the development of other micro- and nano-patterning techniques that can be used to fabricate long-range ordered features - especially nanoscale arrays - is a promising subject in surface science. In contrast to the traditional photolithography patterning technique, block copolymers can spontaneously phase separate into arrays of periodic patterns with length-scales of 10-50 nm, which provides an efficient pathway to pattern nanoscale features. Today, preparing long-range ordered arrays by block copolymer microphase separation is one of the most promising techniques for the fabrication of nanoscale arrays, not only being a simple process but also having a lower cost than traditional methods. In this feature article, we first summarize the many techniques developed to induce ordering in the microphase separation of the block copolymer thin films. Then, evolution, order-order transitions and reversible switching microdomains are considered, since they are very important in the ordered engineering of microphase separation of the block copolymer thin films. Finally, the outlook of this research area will be given.

Metal printing with modified polymer bonding lithography

A simple and versatile metal thin film patterning method is introduced, using the adhesion between polymer and metal thin film as the driving force. Solvent vapor treatment is used to increase the adhesive ability between polymer and metal film in two reverse processes. After selective transfer printing, metal patterns on polymer film or flat elastomeric stamp can be fabricated. Multilayer metal patterns can also be fabricated with this method through multiple printing. Finally, this method was used to fabricate polymer field-effect transistors. The discussions on the effects of solvent on the transistor performance have been given.

Micropatterning of metal films coated on polymer surfaces with epoxy mold and its application to organic field effect transistor fabrication

In this letter, a simple and versatile approach to micropatterning a metal film, which is evaporated on a Si substrate coated with polymer, is demonstrated by the use of a prepatterned epoxy mold. The polymer interlayer between the metal and the Si substrate is found important for the high quality pattern. When the metal–polymer–Si sandwich structure is heated with the temperature below Tm but above Tg of the polymer, the plastic deformation of the polymer film occurs under sufficiently high pressure applied. It causes the metal to crack locally or weaken along the pattern edges. Further heating while applying a lower pressure results in the formation of an intimate junction between the epoxy stamp and the metal film. Under these conditions the epoxy cures further, ensuring adhesion between the stamp and the film. The lift-off process works because the adhesion between the epoxy and the metal film is stronger than that between the metal film and the polymer. A polymer field effect transistor is fabricated...

A laser-diode-pumped acoustic-optic Q-switched Nd:YVO4 slab laser with a hybrid resonator

A 32.1 W laser-diode-stack pumped acoustic-optic Q-switched Nd:YVO4 slab laser with hybrid resonator at 1064 nm was demonstrated with the pumping power of 112 W and repetition rate of 40 kHz, the pulse duration was 32.47 ns. The slope efficiency and optical-to-optical efficiency were 37 and 28.7%, respectively. At the repetition rate of 20 kHz and pumping power of 90 W, the average output power and pulse duration were 20.4 W and 20.43 ns, respectively. With the pumping power of 112 W, the beam quality M2 factors in CW operation were measured to be 1.3 in stable direction and 1.6 in unstable direction.

Highly oriented tunable wrinkling in polymer bilayer films confined with a soft mold induced by water vapor

The authors report the formation of highly oriented wrinkling on the surface of the bilayer [polystyrene (PS)/poly(vinyl pyrrolidone) (PVP)] confined by a polydimethylsiloxane (PDMS) mold in a water vapor environment. When PVP is subjected to water vapor, the polymer loses its mechanical rigidity and changes to a viscous state, which leads to a dramatic change in Youngs modulus. This change generates the amount of strain in the bilayer to induce the wrinkling. With a shape-controlled mold, they can get the ordered wrinkles perfectly perpendicular or leaned 45 degrees to the channel orientation of the mold because the orientation of the resultant force changes with the process of water diffusion which drives the surface to form the wrinkling. Additionally, they can get much smaller wrinkles than the stripe spacing of PDMS mold about one order. The wrinkle period changes with the power index of about 0.5 for various values of the multiplication product of the film thicknesses of the two layers, namely, lambda approximately (h(PS)h(PVP))(1/2).

Patterning organic luminescent materials by solvent-assisted dewetting and polymer-bonding lithography

A combined approach of solvent-assisted dewetting and polymer-bonding lithography was employed to pattern the luminescent materials on the polymer-coated substrate. In saturated solvent vapor atmosphere, light-emitting material films (e.g., Alq3∕PVK,PhE-PdI4∕PS) on the polydimethylsiloxane mold could dewet and form isolated micropatterns directed by the mold. Then, the microstructures were transferred to the polymer-coated substrates based on the surface adhesion to generate the useful isolated and well-defined light-emitting patterns. The optical property of Alq3∕PVK remains almost unaffected in the patterning process. The combined approach is applicable to other organic functional materials and polymer-coated surfaces with different physical and chemical properties and has potential applications in organic microelectronics fabrication.

Formation of parallel aligned nano-fibrils of a donor-acceptor conjugated copolymer via controlling J-aggregates and post treatment

We increased the content of J-aggregates in poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2′ : 5′,2′′ : 5′′,2′′′-quaterthiophene](PDQT) solution by choosing appropriate solvents. It was found that the higher content of J-aggregates in solution in good solvents (i.e. CF) led to the formation of parallel aligned nano-fibrils. This morphological transition from the disordered domains to ordered ones could also occur after thermal treatment or solvent vapor annealing. Parallel aligned fibrils of PDQT formed in films via annealing at 200 °C for 20 min or solvent vapor annealing in a CF atmosphere with a relatively low vapor pressure for 24 hours. In the films filled with parallel aligned fibrils, the PDQT molecules adopted an edge-on orientation and the π–π stacking direction of molecules was perpendicular to the growth direction of fibrils. PDQT chains preferred to adopt the extended conformations without chain-folding due to the extremely rigid backbones. The intact conjugation planes of rigid PDQT chains with extended conformations allow the further growth of parallel aligned fibrils, which enhanced the connectivity between crystalline domains. The results in this work are thought to be helpful to improve the charge-carrier transport in PDQT films.

Restricting the liquid–liquid phase separation of PTB7-Th:PF12TBT:PC71BM by enhanced PTB7-Th solution aggregation to optimize the interpenetrating network

The current understanding of the active layer morphology in ternary organic solar cells (OSCs) is superficial owing to more variables and complexity compared to that of binary OSCs. The PTB7-Th:PF12TBT:PC71BM ternary system with complementary polymer absorption spectra and efficient energy transfer from PF12TBT to PTB7-Th was anticipated to have an outstanding performance. However, only a limited improvement in the power conversion efficiency (PCE) was achieved when the ternary devices were processed from CB/DIO. This is because large PC71BM domains formed with the addition of amorphous PF12TBT and the PF12TBT molecules embedded in the large PC71BM domains served as trap sites. Constraining the formation of large PC71BM domains and avoiding PF12TBT molecules being fully embedded in PC71BM domains are needed for a better performance. Polymer pre-aggregation before liquid–liquid phase separation is beneficial to construct bicontinuous interpenetrating networks with proper phase-separated domains. Therefore, para-xylene was introduced into CB/DIO to cause a weak polymer–solvent interaction for enhanced PTB7-Th solution-phase aggregation. The enhanced PTB7-Th aggregation in the CB/PX/DIO solution restricted the extent of the liquid–liquid phase separation, and a well-developed polymer network formed with improved PTB7-Th crystallinity, which prevented large PC71BM domains from forming via fluid-phase Ostwald ripening in the liquid–solid phase separation stage. The appearance of PF12TBT emission and increased PTB7-Th emission in the ternary film processed from CB/PX/DIO suggested that fewer PF12TBT molecules were embedded in the PC71BM domains, and the energy transfer from PF12TBT to PTB7-Th was more efficient. Thus, PCE increased from 8.09% for binary blends processed from CB/DIO to 9.28% for ternary blends processed from CB/PX/DIO.

Inkjet printed polystyrene sulfuric acid-doped poly(3,4-ethylenedioxythiophene) (PEDOT) uniform thickness films in confined grooves through decreasing the surface tension of PEDOT inks

A uniform thickness of inkjet printed polystyrene sulfuric acid-doped poly(3,4-ethylenedioxythiophene) (PEDOT) films in confined grooves is very important for their application in inkjet printed polymer light-emitting displays (PLEDs). PEDOT films with a concave or convex cross section are often obtained during inkjet printing. We obtained uniform thickness printed PEDOT films through the low surface tension modification of PEDOT inks using isopropanol (IPA). The thickness fluctuation along the cross section of the PEDOT films was decreased from ∼30 nm for the convex condition and ∼20 nm for the concave condition to less than 10 nm through replacing the original PEDOT ink with 10% IPA modified PEDOT ink. The meniscus profile at gelation of the PEDOT inks was used to explain the morphology change of the PEDOT films. IPA modification of PEDOT inks induced lower surface tension and increased the Bond number, which decreased the curvature of the meniscus and improved film uniformity both under concave and convex conditions. IPA modification has no obvious effect on the PEDOT morphology, work function and performance of organic light-emitting diodes.