Thermal and Dielectric Properties of Cyanate Ester Cured Main Chain Rigid-Rod Epoxy Resin.

Thermal and dielectric properties of rigid-rod bifunctional epoxy resin 4,4-bis(2,3-epoxypropoxy) biphenyl epoxy (BP) and commercial epoxy resin diglycidyl ether of bisphenol A (DGEBA) were studied using differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA), dynamic mechanical analyzer (DMA), thermal mechanical analyzer (TMA) and dielectric analyzer (DEA). These two epoxies were cured with cyanate ester hardener 2,2'-bis(4-cyanatophenyl) propane (AroCy B10). The BP/B10 system consisting of a rigid-rod structure exhibited better thermal properties than the DGEBA/B10 system with a flexible structure. Anisotropic BP/B10 (2:1) had the highest 5% weight loss temperature, the highest amount of residue and a smaller thermal expansion coefficient than the commercial DGEBA/B10 system. The BP/B10 system, which cured at the LC phase temperature, had higher Tg than the commercial DGEBA/B10 system, as found from dynamic mechanical analysis. The BP/B10 system also demonstrated better dielectric properties than the commercial DGEBA/B10 system when enough curing agent was provided.

Correlation between Hierarchical Structure and Processing Control of Large-area Spray-coated Polymer Solar Cells toward High Performance.

The formation mechanism of a spray-coated film is different from that of a spin-coated film. This study employs grazing incidence small- and wide-angle X-ray Scattering (GISAXS and GIWAXS, respectively) quantitatively and systematically to investigate the hierarchical structure and phase-separated behavior of a spray-deposited blend film. The formation of PCBM clusters involves mutual interactions with both the P3HT crystal domains and droplet boundary. The processing control and the formed hierarchical structure of the active layer in the spray-coated polymer/fullerene blend film are compared to those in the spin-coated film. How the different post-treatments, such as thermal and solvent vapor annealing, tailor the hierarchical structure of the spray-coated films is quantitatively studied. Finally, the relationship between the processing control and tailored BHJ structures and the performance of polymer solar cell devices is established here, taking into account the evolution of the device area from 1 × 0.3 and 1 × 1 cm(2). The formation and control of the special networks formed by the PCBM cluster and P3HT crystallites, respectively, are related to the droplet boundary. These structures are favorable for the transverse transport of electrons and holes.

Mechanism and control of the structural evolution of a polymer solar cell from a bulk heterojunction to a thermally unstable hierarchical structure.

We simultaneously employed grazing incidence small-angle and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques to quantitatively study the structural evolution and kinetic behavior of poly(3-hexylthiophene) (P3HT) crystallization, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and amorphous P3HT/PCBM domains from a bulk heterojunction (BHJ) to a thermally unstable structure. The independent phase separation regimes on the nanoscale (∼10 nm), mesoscale (∼100 nm) and macroscale (∼µm) are revealed for the first time. Bis-PCBM molecules as inhibitors incorporated into the P3HT/PCBM blend films were adopted as a case study of a control strategy for improving the thermal stability of P3HT/PCBM solar cell. The detailed information on the formation, growth, transformation and mutual interaction between different phases during the hierarchical structural evolution of P3HT/PCBM:xbis-PCBM (x = 8-100%) blend films are presented herein. This systematic study proposes the mechanisms of thermal instability for a polymer/fullerene-based solar cell. We demonstrate a new fundamental concept that the structural evolution and thermal stability of mesoscale amorphous P3HT/PCBM domains during heating are the origin of controlling thermal instability rather than those of nanoscale thermally-stable BHJ structures. It leads to a low-cost and easy-fabrication control strategy for effectively tailoring the hierarchical morphology against thermal instability from molecular to macro scales. The optimum treatment achieving high thermal stability, control of mesoscale domains, can be effectively designed. It is independent of the original BHJ nanostructure design of a polymer/fullerene-based solar cell with high performance. It advances the general knowledge on the thermal instability directly arising from the nanoscale structure.

Nanoparticle-tuned self-organization of a bulk heterojunction hybrid solar cell with enhanced performance.

We demonstrate here that the nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction (BHJ) can be tuned by inorganic nanoparticles (INPs) for enhanced solar cell performance. The self-organized nanostructural evolution of P3HT/PCBM/INPs thin films was investigated by using simultaneous grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Including INPs into P3HT/PCBM leads to (1) diffusion of PCBM molecules into aggregated PCBM clusters and (2) formation of interpenetrating networks that contain INPs which interact with amorphous P3HT polymer chains that are intercalated with PCBM molecules. Both of the nanostructures provide efficient pathways for free electron transport. The distinctive INP-tuned nanostructures are thermally stable and exhibit significantly enhanced electron mobility, external quantum efficiency, and photovoltaic device performance. These gains over conventional P3HT/PCBM directly result from newly demonstrated nanostructure. This work provides an attractive strategy for manipulating the phase-separated BHJ layers and also increases insight into nanostructural evolution when INPs are incorporated into BHJs.

Quantitative nanoorganized structural evolution for a high efficiency bulk heterojunction polymer solar cell.

We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance.

Manipulation of nanoscale phase separation and optical properties of P3HT/PMMA polymer blends for photoluminescent electron beam resist.

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The lambda(max) of the film absorption is blue-shifted from 520 to 470 nm, and its lambda(max) of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration.

High-sensitivity Raman scattering substrate based on Au/La(0.7)Sr(0.3)MnO(3) periodic arrays.

We have developed Au/La(0.7)Sr(0.3)MnO(3) (Au/LSMO) periodic arrays with tunable surface plasmon properties that can be used as novel surface-enhanced Raman scattering (SERS) substrates. The periodic arrays are created by electron beam lithography of LSMO resist and metal film deposition. The LSMO electron beam resist is unique in that it exhibits either positive or negative resist behaviors depending on the electron beam dosage. Interestingly, surface plasmon behavior of the arrays can be controlled by just changing the electron beam dosage when presented with a fixed design pattern. Scanning confocal microscopy and spectral microreflectometry have been adapted to directly demonstrate this unique behavior. Furthermore, we show that our novel Au/LSMO array can be used as a high-sensitivity Raman scattering substrate. To illustrate this working principle, the Au/LSMO periodic array is applied to enhance the Raman scattering of a thin film containing 0.1 wt % poly-3-hexylthiophene (P3HT) in poly(methyl methacrylate) (PMMA). By controlling the geometry of the patterned substrate that exhibits gold surface plasmon near the excitation wavelength, we can enhance the intensity of Raman scattering of P3HT at 1350 cm(-1) up to 4 orders of magnitude as compared with previously generated planar Au substrates.