Haowei Tang

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.

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.

Sol–gel transition of poly(3-hexylthiophene) revealed by capillary measurements: phase behaviors, gelation kinetics and the formation mechanism

π-Conjugated organogels of poly(3-hexylthiophene) (P3HT) are prepared via the addition of a marginal solvent, anisole, into solutions of P3HT. We initiate a novel and facile route to determine the gelation threshold of P3HT nanowire dispersions by capillary measurements. The effects of the P3HT concentration (c), anisole volume fraction (φ) and temperature (T) on the phase behaviors are discussed. A thermodynamic c–φ phase diagram is constructed, in which the P3HT dispersion is divided into four regions: solution, sol, sol–gel blend and gel. The concentration and solvent dependent sol–gel transition kinetics shows that the gelation process could be accelerated via increasing c and φ. The gelation temperature gives an exponential relationship with the concentration of P3HT and the anisole volume fraction. Morphological studies reveal the formation process and the topological structure of the P3HT microgel clusters. Based on the above results, a three-stage physical scenario for the sol–gel transition of P3HT dispersions is proposed.

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.

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.

An aqueous soaking treatment for efficient polymer solar cells

Polymer solar cells, which convert clean renewable solar energy into electricity, have been considered as most promising technology. It is well recognized that the power conversion efficiency of the cell device greatly depends on the morphology of the polymer active layer, and the post-treatments commonly used, such as energy consuming thermal annealing and solvent vapor treatment, are not suitable for commercial applications on large-area polymer solar cells. Herein, we propose a facile aqueous solution post-treatment, which involves only water and a small amount of carbon disulfide (0.13 wt% of CS2), based on regioregular poly (3-hexylthiophene) (P3HT) and [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film to improve the active layer morphology and increase the device efficiency. Upon soaking the blend film in the aqueous solution, P3HT crystallinity is increased and an optimum morphology of nanoscale phase separation with an interpenetrating network is constructed, which offers percolating pathways for charge carrier transport. Furthermore, AFM and XPS analyses reveal that a coarser structure with heave-like PCBM-rich domains is built up at the active layer/cathode (top) interface, which improves the contact with the metal cathode for efficient electron transportation and collection. The feasibility of this method is verified by J–V characteristic of the photovoltaic device, which demonstrates an increased PCE of 3.09% compared with 1.84% of the pristine device, indicating its potential implementation on the application of large-area polymer solar cells.