Xiaoniu Yang

Improving performance of polymer photovoltaic devices using an annealing-free approach via construction of ordered aggregates in solution

Low crystalline order has been proved to be one of the main hindrances for achieving high performance devices based on thin films composed of crystallizable polymer. In this work, we use a facile method to substantially improve crystallinity of poly(3-hexylthiophene) (P3HT) in its pure or composite film via the construction of ordered precursors in the solution used for thin film deposition. These improvements have been confirmed by bright-field transmission electron micrography, electron diffraction, UV-Vis absorption and wide-angle X-ray diffraction. The electrical conductivity of thus obtained P3HT films is increased by almost two orders of magnitude. Polymer solar cells based on P3HT:PCBM ([6,6]-phenyl C61 butyric acid methyl ester) composite fabricated using this method achieve power conversion efficiencies (PCEs) as high as 3.9%, which is almost four times that of pristine devices and also higher than thermally-annealed devices under the same measurement conditions. This simple method paves the way for the fabrication of high performance devices with an “annealing-free” approach, and enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.

Efficient polymer:polymer bulk heterojunction solar cells

An organic bulk heterojunction photovoltaic device based on a blend of two conjugated polymers, a polyphenylenevinylene as the electron donor and a red emitting polyfluorene as the acceptor, is presented with a maximum external quantum efficiency of 52% at 530nm and a power conversion efficiency, measured under AM1.5G, 100mW∕cm2 conditions, of 1.5% on an active area of 0.36cm2.

Precise construction of PCBM aggregates for polymer solar cells via multi-step controlled solvent vapor annealing

Polymer solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid methyl ester (P3HT/PCBM) composite are one of state-of-the-art polymer photovoltaic devices in terms of performance. In this work, we applied two-step controlled solvent vapor annealing (C-SVA) to achieve an optimized morphology for the photoactive layer with both an appropriate size of PCBM aggregates and an improved crystallinity of P3HT. As revealed by bright-field transmission electron microscopy (TEM), and atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-Vis spectroscopy, PCBM forms aggregates with sizes of ca. 30 nm during the first step C-SVA in tetrahydrofuran vapor. The second step treatment using carbon disulfide vapor on one hand reduces the large size of these PCBM aggregates to ca. 20 nm, and on the other hand substantially increases the crystallinity of P3HT. The polymer solar cells employing a thus-treated composite film gave a power conversion efficiency as high as 3.9%, in contrast to 3.2% for the thermally annealed device under the same characterization conditions. This result shows the importance of a precisely controlled morphology of the photoactive layer in device performance.

Solvent-soaking treatment induced morphology evolution in P3HT/PCBM composite films

Morphology of the active layer has been proven to play an important role in determining the final device performance of photovoltaic devices. Herein, we present a facile mixed solvents soaking approach to tailor the morphology of the active layer, in which not only the crystallinity of poly(3-hexylthiophene) (P3HT) in its composite film with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been substantially improved, but also an interpenetrating network composed of highly crystalline P3HT and PCBM nanoaggregates is constructed as confirmed by transmission electron microscopy. Furthermore, X-ray photoelectron spectroscopy analysis reveals that P3HT chains enrich at the active layer/anode interface while more PCBM are found to present on the active layer/cathode interface along the vertical direction of the composite films, which is beneficial for charge carrier transport and will contribute to better device performance. The power conversion efficiency of the device using this method is improved to 3.23%, in contrast to 1.45% for a pristine device and 2.79% for a thermally annealed device. Therefore, this simple technique can simultaneously optimize lateral and vertical nanoscale phase separation of crystalline P3HT and PCBM, and shows high potential application in the preparation of high performance cost-effective polymer solar cells.

Solvent-Induced Crystallization of Poly(3-dodecylthiophene): Morphology and Kinetics

We report on the self-assembly of poly(3-dodecylthiophene) (P3DDT) into nanowhiskers for the first time via addition of the marginal solvent anisole into its well-dissolved solution. By controlling the solvent composition and aging time, we observed a morphology evolution from nanowhiskers to two-dimensional nanoribbons and foliated aggregates, which was ascribed to diverse driving forces for self-assembly in the process of crystallization. UV-vis absorption spectroscopy and dynamic lighting scattering (DLS) measurements were employed to in situ monitor crystallization kinetics of P3DDT induced by mixed solvents. It has been shown that conformational transition serves as a critical factor for P3AT to perform π-π stacking to form nanowhiskers. From a thermodynamic point of view, P3AT dispersion dissolved in mixed solvents is actually not a thermodynamic equilibrium system, but a multicomponent and multiphase case whose phase composition and properties evolve with time. The understanding in morphology transition mechanisms and crystallization kinetics of P3DDT can provide guidelines for optimization of processing parameters and enhance performance of photovoltaic devices.

New benzotrithiophene derivative with a broad band gap for high performance polymer solar cells

A new planar conjugated polymer, poly{benzo(1,2-b:3,4-b′:5,6-d′′)trithiophene-alt-4,4′-dihexyl-2,2′-bithiazole} (BTT-BTz), with a broad band gap and higher Voc based on benzotrithiophene and bithiazole units, was designed and synthesized. This copolymer possesses a deeper HOMO (−5.65 eV), and after 1,8-diiodoctane treatment, the power conversion efficiency of the photovoltaic device based on the BTT-BTz/PC71BM photoactive layer reaches 5.06% with 0.81 V of open-circuit voltage, 10.9 mA cm−2 of short-circuit current and 0.57 of fill factor, which is the highest one among the conjugated polymers based on BTT or BTz derivatives. This new conjugated copolymer seems a promising candidate for the application in tandem solar cells.

Synthesis of aluminium-doped ZnO nanocrystals with controllable morphology and enhanced electrical conductivity

In this work, aluminium-doped zinc oxide nanocrystals (AZONs) with controllable morphology and enhanced electrical conductivity are successfully prepared via a solvothermal method and a subsequent calcination process. Thus obtained near-spherical AZONs with an average particle size of 40 nm show a minimum volume resistivity of ∼22.38 Ω cm, which is eleven orders of magnitude lower than that of pure ZnO. The remarkable enhancement of electrical conductivity is mainly ascribed to the homogenous incorporation of Al3+ into wurtzite ZnO lattice. In addition, the calcination under hydrogen atmosphere further improves electrical conductivity, which is deemed to be caused by the bonding of hydrogen to the lattice oxygen and oxygen vacancy. All these defects result in the generation of free electrons and therefore increase the charge carrier concentration.

Nonisothermal crystallization kinetics: poly(ethylene terephthalate)-poly(ethylene oxide) segmented copolymer and poly(ethylene oxide) homopolymer

Abstract The nonisothermal crystallization behavior of polyethylene oxide (PEO) in poly(ethylene terephthalate)–poly(ethylene oxide) (PET–PEO) segmented copolymer and PEO homopolymer has been studied by means of differential scanning calorimetry, as well as transmission electron microscope. The kinetics of PEO in copolymer and PEO homopolymer under nonisothermal crystallization condition has been analyzed by Ozawa equation. The results show that Ozawa equation only describes the crystallization behavior of PEO-6000 homopolymer successfully, but fails to describe the whole crystallization process of PEO in copolymer because the secondary crystallization in the later stage could not be neglected. Due to the constraint of PET segments imposed on the PEO segments, a distinct two stage of crystallization of PEO in copolymer has been investigated by using Avrami equation modified by Jeziorny to deal with the nonisothermal crystallization data. In the case of PEO-6000 homopolymer, good linear relation for the whole crystallization process is obtained owing to the secondary crystallization does not occur under our experimental condition.

Side‐Chain Engineering for Enhancing the Thermal Stability of Polymer Solar Cells

An effective strategy of engineering side chains is proposed for enhancing solar-cell-device thermal stability. As the conjugated length of the side chains increases, the morphological stability of the blend film is enhanced. The thermal stability of corresponding devices is consequently improved.

Bulk Interpenetration Network of Thermoelectric Polymer in Insulating Supporting Matrix

Thermoelectric properties of conjugated polymers are found to improve upon homogeneously distributing conjugated polymer into an insulating supporting matrix. The local one-dimensional charge transport along the interpenetration conductive network simultaneously leads to lower thermal conductivity, higher electrical conductivity without sacrifice of Seebeck coefficient, and thus a higher figure of merit ZT, as compared with neat conjugated polymer.