Wei-Ru Wu

Competition between Fullerene Aggregation and Poly(3-hexylthiophene) Crystallization upon Annealing of Bulk Heterojunction Solar Cells

Concomitant development of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å(-1)), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented.

Intermixing-seeded growth for high-performance planar heterojunction perovskite solar cells assisted by precursor-capped nanoparticles

This work proposes a novel approach to modulate the nucleation and growth of perovskite crystals in planar perovskite (CH3NH3PbI3−xClx) solar cells by intermixing precursor-capped inorganic nanoparticles of PbS. A small amount of dispersed PbS nanoparticles which were covered with perovskite precursor molecules of methylammonium iodide (CH3NH3I, MAI) through the ligand-exchange treatment functioned as effective seed-like nucleation sites to promote the formation of perovskite lattice structures. Through this intermixing-seeded growth technique, substantial morphological improvements, such as increased crystal domains, enhanced coverage, and uniformity, were realized in the perovskite thin films, and the corresponding solar cell devices exhibited a promising power conversion efficiency of 17.4%, showing an enhancement of approximately 25% compared to that of the controlled pristine solar cell device. The substantially enhanced crystal orientation, particularly along the direction perpendicular to the substrate, was evident from the synchrotron-based grazing incidence wide-angle X-ray scattering data. This observation was consistent with the enhanced carrier diffusion lengths and excellent reproducibility of high fill factors of the planar heterojunction perovskite devices fabricated through the proposed technique.

Organic photovoltaics based on a crosslinkable PCPDTBT analogue; synthesis, morphological studies, solar cell performance and enhanced lifetime†

A thermally crosslinkable poly(cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) analogue copolymer with 2-ethylhexyl group and 5-hexenyl group on 4,4-positions of cyclopentadithiophene has been synthesised by palladium catalyzed Suzuki coupling reaction. Space-Charge Limited Current (SCLC) devices show hole mobility of up to 3 × 10−4 cm2 V−1 s−1. In addition, organic photovoltaics (OPVs) were fabricated using fullerene derivatives as acceptor materials. The best performing device exhibited Power Conversion Efficiency (PCE) at 2.0%, which could be improved to 3.7% after inclusion of dithiol additives. Most significantly, the lifetime of the OPV was enhanced by the crosslinking; when light soaked at 1 sun of irradiance, a half-life, t1/2 = 419 hours was measured, which is 51% improvement over an OPV using standard PCPDTBT. This higher stability is accounted for the crosslinkable structure of the polymer after annealing. The crosslinking reaction was confirmed by solubility tests, Fourier transform infrared spectroscopy and morphological analysis conducted by Atomic Force Microscopy and simultaneous synchrotron grazing-incidence small-/wide-angle X-ray scattering.

Probing the Acid-Induced Packing Structure Changes of the Molten Globule Domains of a Protein near Equilibrium Unfolding

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I1, I2, and E along the unfolding pathway. The I1-I2 transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol-1, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.

Correlated changes in structure and viscosity during gelatinization and gelation of tapioca starch granules.

Melting of the semicrystalline structure of native tapioca starch granules is correlated to solution viscosity for elucidating gelatinization and gelation processes.

Directed Vertical Diffusion of Photovoltaic Active Layer Components into Porous ZnO-Based Cathode Buffer Layers

Cathode buffer layers (CBLs) can effectively further the efficiency of polymer solar cells (PSCs), after optimization of the active layer. Hidden between the active layer and cathode of the inverted PSC device configuration is the critical yet often unattended vertical diffusion of the active layer components across CBL. Here, a novel methodology of contrast variation with neutron and anomalous X-ray reflectivity to map the multicomponent depth compositions of inverted PSCs, covering from the active layer surface down to the bottom of the ZnO-based CBL, is developed. Uniquely revealed for a high-performance model PSC are the often overlooked porosity distributions of the ZnO-based CBL and the differential diffusions of the polymer PTB7-Th and fullerene derivative PC71 BM of the active layer into the CBL. Interface modification of the ZnO-based CBL with fullerene derivative PCBEOH for size-selective nanochannels can selectively improve the diffusion of PC71 BM more than that of the polymer. The deeper penetration of PC71 BM establishes a gradient distribution of fullerene derivatives over the ZnO/PCBE-OH CBL, resulting in markedly improved electron mobility and device efficiency of the inverted PSC. The result suggests a new CBL design concept of progressive matching of the conduction bands.

Self-assembly of pseudorotaxane films with thermally reversible crystal phases and optical properties

Highly ordered thin films of pseudorotaxane consisting of the host cyclic molecule, dibenzo[24]crown-8 (DB24C8), and the guest axle molecule, N-(xylylammonium)-methylferrocene, were prepared on solid substrates via solution casting followed by annealing. Collective evidence obtained from X-ray diffraction, differential scanning calorimetry, polarised optical microscopy (POM) and grazing incident wide-angle X-ray scattering (GIWAXS) experiments indicates that the specially architectured host and guest molecules can first form a highly ordered crystalline phase upon solvent evaporation. Annealing of this phase at temperatures above 130 °C leads to an irreversible transition of the crystal phase to a high-temperature (HT) crystalline phase with triclinic crystalline domain sizes of 50–100 μm. When cooled back to room temperature, the HT phase undergoes a solid–solid phase transition to a low-temperature (LT) crystalline phase of a similar triclinic form but favouring a tighter packing of axle molecules. Thermally reversible LT–HT phase transitions were consistently evidenced via in situ observations by POM and X-ray scattering and are attributed to a subtle ordering competition between the host cyclic molecules and guest axle molecules. This local structural transition, however, only slightly affects the stability of the layering frame structure of the pseudorotaxane film, in which the supermolecular complex adopts an edge-on orientation with the cyclic molecular plane being perpendicular to the substrate and is interlocked via interactions with the axle molecules. The results demonstrate the feasibility of self-assembled pseudorotaxane films with thermally driven phase transitions as a possible molecular switch.