Guanghao Lu

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.

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.

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.

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.

Epitaxy-Assisted Creation of PCBM Nanocrystals and Its Application in Constructing Optimized Morphology for Bulk-Heterojunction Polymer Solar Cells

PCBM (a C60 derivative) is so far the most successful electron acceptor for bulk-heterojunction polymer photovoltaic (PV) cells. Here we present a novel method epitaxy-assisted creation of PCBM nanocrystals and their homogeneous distribution in the matrix using freshly cleaved mica sheet as the substrate. The highly matched epitaxy relationship between the unit cell of PCBM crystal and crystallographic (001) surface of mica induces abundant PCBM nuclei, which subsequently develop into nanoscale crystals with homogeneous dispersion in the composite film. Both the shape and size of these nanocrystals could be tuned via choosing the type of matrix polymer, film thickness, ratio of PCBM in the composite film, and annealing temperature. Thus, the obtained thin composite film is removed from the original mica substrate via the flotation technique and transferred to a real substrate for device completion. The success of this method has been verified by the substantially improved device performance, in particular the increased short-circuit current, which is heavily dependent on the morphology of the photoactive layer. Therefore, we have actually demonstrated a novel approach to construct preferred morphology for high-performance optoelectronic devices via resorting to other specific substrates which could induce the formation of this type morphology.

Influence of fluorination on the properties and performance of isoindigo–quaterthiophene-based polymers

Here a series of isoindigo (ID) and quaterthiophene (T4)-based donor–acceptor copolymers are synthesized and compared. The polymer with fluorination on the donor unit exhibits the strongest extent of temperature-dependent aggregation, which leads to a higher hole mobility of the polymer and PSCs with efficiencies up to 7.0% without using any processing additives. Our results provide important insights into how fluorination affects the aggregation properties and performance of isoindigo-based polymers.

Constructing thin polythiophene film composed of aligned lamellae via controlled solvent vapor treatment.

Thin poly(3-butylthiophene) (P3BT) film composed of aligned lamellae attached to the edge of the original film has been achieved via a controlled solvent vapor treatment (C-SVT) method. The polarized optical microscopy operated at both single-polarization and cross-polarization modes has been used to investigate the alignment of the fiber-like lamellae. A numerical simulation method is used to quantitatively calculate angle distributions of the lamellae deviated from the film growth direction. Prepatterned P3BT film edge acts as nuclei which densely initialize subsequent crystal growth by exhausting the materials transported from the partially dissolved film. The growth of new film upon crystallization is actually a self-healing process where the two-dimensional geometric confinement is mainly responsible for this parallel alignment of P3BT crystals. The solvent vapor pressure should be carefully chosen so as to induce crystal growth but avoid liquid instability which will destroy the continuity of the film. The combination of microfabrication technique and C-SVT method provides a novel method to fabricate hierarchical structure within thin polymer film with multiscale morphology via utilizing both up-bottom and bottom-up approaches.

In-situ tuning threshold voltage of field-effect transistors based on blends of poly(3-hexylthiophene) with an insulator electret

Blending the conjugated polymer poly(3-hexylthiophene) (P3HT) with the insulating electret polystyrene (PS), we show that the threshold voltage Vt of organic field-effect transistors (OFETs) can be easily and reversely tuned by applying a gate bias stress at 130 °C. It is proposed that this phenomenon is caused by thermally activated charge injection from P3HT into PS matrix, and that this charge is immobilized within the PS matrix after cooling down to room temperature. Therefore, room-temperature hysteresis-free FETs with desired Vt can be easily achieved. The approach is applied to reversely tune the OFET mode of operation from accumulation to depletion, and to build inverters.

A novel melting behavior of poly(3-alkylthiophene) cocrystals: premelting and recrystallization of component polymers

Poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT) cocrystals were prepared by solution casting and the cocrystals show two endothermic peaks in a differential scanning calorimetry (DSC) thermogram. The former endothermic peak is nearly fixed at the melting temperature of pure P3HT, while the latter one shifts gradually towards the P3BT melting point with increased P3BT content. This melting behavior of the polymer cocrystal is novel and different from the studies commonly reported in which the polymer cocrystal generally has one endothermic peak. After extensive analyses, including powder X-ray diffraction (XRD), synchrotron-based two dimensional grazing-incidence XRD, temperature-dependent UV-Visible spectroscopy and temperature modulated DSC, we propose the melting mechanism of P3HT/P3BT cocrystals involving three steps. In the first step, the P3HT component melts at the former endothermic peak around 241 °C. After that, the P3BT component recrystallizes when the coil-like P3HT chains escape from the cocrystal lattices. Finally, the P3BT component completely melts at the latter endothermic peak. These findings are beneficial to the understanding of the criteria for the formation and the melting behavior of polymer cocrystals.

The Role of Morphology Control in Determining the Performance of P3HT/C-70 Bulk Heterojunction Polymer Solar Cells

Morphology is one of the critical elements in determining the performance of polymer solar cells. As an important member in fullerene family, unsubstituted C-70 is scarcely employed to prepare polymer solar cells as compared with its derivatives, which is ascribed to its poor solution processability from a single organic solvent. Hereby, we show a method by using mixed solvent to prepare poly(3-hexylthiophene) (P3HT)/C-70 photoactive layer for high-performance polymer solar cells. Composite films spin coated from the solution with different mixed solvents are characterized by optical microscopy, transmission electron microscopy, UV-vis spectroscopy, and photovoltaic (PV) device characterization. As a result, heptane/o-dichlorobenzene (ODCB) mixture is found to be a good combination to prepare homogenous P3HT/C-70 film, and a high-performance device is thus obtained. By further optimizing the ratio between the donor P3HT and acceptor C-70 in the composite film, the PV cells with a power conversion efficiency of 2.24% have been achieved, which is much higher than that of the device prepared from ODCB-only solution. Dynamic light scattering measurement implies that C-70 molecules form aggregates with size of ~250 nm in ODCB solution, while in heptane/ODCB mixture, the aggregates are much smaller, which is responsible for the homogenous P3HT/C-70 film and also high device performance. This paper demonstrates a vivid approach to control the morphology of polymer solar cells, and thus significantly improved performance via deliberately designed solvent system for thin-film deposition.