Mingtao Zhang

A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

Controlling the Effective Surface Area and Pore Size Distribution of sp2 Carbon Materials and Their Impact on the Capacitance Performance of These Materials

A series of sp(2) carbon materials with different specific surface area (SSA) and controlled pore size distribution (PSD) were synthesized at large scale through a facile and low-cost method. The SSA and PSD of these carbon materials were controlled by using different carbon sources and preparation methods. With different total and effective SSA (E-SSA) and PSD, the impacts on their capacitance performance were investigated thoroughly, which demonstrated that both E-SSA and PSD played the most important roles in their capacitance performance. Furthermore, theoretical modeling was performed, and the results are in agreement with the experimental results for the influence of E-SSA and PSD on their capacitance performance. Based on these, a general model using the slit/cylindrical NL-DFT approach is proposed for the estimation of the specific capacitance of sp(2) carbon materials, which offers a simple but reliable method to predict the capacitance performance of these materials, thus speeding up the design and screening of the materials for high-performance supercapacitor and other surface area related devices.

Efficient small molecule bulk heterojunction solar cells with high fill factors via introduction of π-stacking moieties as end group

Three new oligothiophene derivatives with an acceptor–donor–acceptor structure incorporating 1,3-indanedione or the derivative of 1,3-indanedione units as the terminal acceptor groups—DIN7T, DINCN7T and DDIN7T—have been designed and synthesized for solution-processable small molecule BHJ solar cells. The impacts of these different end dye moieties on the optical absorption, solubility, electrochemical properties, morphology, mobility and solar cell performance were studied. All three compounds exhibit broad and highly efficient solar absorption with a low bandgap. The DIN7T-based BHJ solar cell device achieved a PCE of 4.93% and a high fill factor of 0.72, under illumination of AM 1.5, 100 mW cm−2.

Impact of dye end groups on acceptor-donor-acceptor type molecules for solution-processed photovoltaic cells†

Two new oligothiophene derivatives of the acceptor–donor–acceptor type, incorporating double rhodanine or 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione units as the terminal acceptor groups, D2R(8 + 2)7T and DTDMP7T, have been designed and synthesized for solution-processable small molecule BHJ solar cells. The impacts of these different end dye moieties on their optical, electrochemical properties, morphology, mobility and solar cell performance were studied. Both compounds exhibit broad and highly efficient solar absorption with low bandgaps of 1.70 and 1.62 eV for D2R(8 + 2)7T and DTDMP7T, respectively. The D2R(8 + 2)7T and DTDMP7T based BHJ solar cell devices achieved a PCE of 2.46% and 4.05%, respectively, under the illumination of AM.1.5, 100 mW cm−2.

Impact of the Electron‐Transport Layer on the Performance of Solution‐Processed Small‐Molecule Organic Solar Cells

Although the performance of polymer solar cells has been improved significantly recently through careful optimization with different interlayers for the same materials, more improvement is needed in this respect for small-molecule-based solar cells, particularly for the electron-transport layers (ETLs). In this work, three different solution-processed ETLs, PFN, ZnO nanoparticles, and LiF, were investigated and compared in the performance of small-molecule-based devices, and power conversion efficiencies (PCEs) of 8.32, 7.30, and 7.38% were achieved, respectively. The mechanism for the ETL-induced enhancement has been studied, and different ETLs have a significantly different impact on the device performance. The clearly improved performance of PFN is attributed to the combination of reduced bimolecular recombination and increased effective photon absorption in the active layer.

Impact of fluorinated end groups on the properties of acceptor–donor–acceptor type oligothiophenes for solution-processed photovoltaic cells

Two new oligothiophene derivatives with the acceptor–donor–acceptor structure incorporating fluorinated alkyl cyanoacetate units as the terminal acceptor groups, DCAE7T-F1 and DCAO7T-F7, have been designed and synthesized for solution-processable BHJ solar cells. The impacts of these fluorinated end groups on the optical absorption, solubility, electrochemical properties, morphology, surface energy, film forming ability, mobility and solar cell performance were studied. We found that as the fluorinated alkyl length increased, the surface energy decreased and the lipophobicity increased. Due to its high lipophobic property and a problem with its wettability, DCAO7T-F7 was not able to produce a uniform film by spin coating. A power conversion efficiency (PCE) of 2.26% was achieved with an open-circuit voltage (Voc) of 0.83 V, short circuit current (Jsc) of 5.55 mA cm−2 and fill factor (FF) of 0.50 for DCAE7T-F1 based solar cells. The low Jsc suggests that controlling the film morphology and molecular assembly is essential for the performance of these fluorinated small molecules.

Expression of γ-tocopherol methyltransferase gene from Brassica napus increased α-tocopherol content in soybean seed

A cDNA encoding γ-tocopherol methyltransferase from Brassica napus (BnTMT) was overexpressed in soybean [Glycine max (L.) Merr.] under the control of seed-specific promoter of Arabidopsis fatty acid elongase 1 (FAE1) or soybean glycinin G1. Two and three transgenic plants were selected, respectively, after Agrobacterium-mediated transformation. Polymerase chain reaction (PCR) and Southern blots confirmed that BnTMT was single-copy integrated into the genome of transgenic plants. RT-PCR analysis showed that the expression of BnTMT was higher in the immature cotyledons than in the mature cotyledons, while no expression was detected in the leaves. Moreover, the expression level under the control of FAE1 was higher than that of G1. HPLC analysis indicated that the seed-specific expression of BnTMT resulted in 11.1-fold and 18.9-fold increase in α- and β-tocopherol content, respectively, in T2 seed. These results suggested that introducing BnTMT into soybean can be used to increase the vitamin E composition in seeds.

What are the practical limits for the specific surface area and capacitance of bulk sp 2 carbon materials

The possible practical limits for the specific surface area and capacitance performance of bulk sp2 carbon materials were investigated experimentally and theoretically using a variety of carbon materials. We find the limit for the specific surface area to be 3500–3700 m2 g−1, and based on this, the corresponding best capacitance was predicted for various electrolyte systems. A model using an effective ionic diameter for the electrolyte ions was proposed and used to calculate the theoretical capacitance. A linear dependence of experimental capacitance versus effective specific surface area of various sp2 carbon materials was obtained for all studied ionic liquid, organic and aqueous electrolyte systems. Furthermore, excellent agreement between the theoretical and experimental capacitance was observed for all the tested sp2 carbon materials in these electrolyte systems, indicating that this model can be applied widely in the evaluation of various carbon materials for supercapacitors.

Pyrene‐Containing Twistarene: Twelve Benzene Rings Fused in a Row

Success in obtaining higher-order twistarenes with precise structures is very important for fundamentally understanding the relationship between the structures and physical properties/optoelectronic applications. In this research, by using the advantages from a retro-Diels-Alder process (clean reaction) and the cross-conjugated nature of the pyrene unit, a novel dodeca-twistarene was prepared for the first time. Its structure, confirmed by single-crystal XRD analysis, indicates that it possesses a twisted angle (≈30°), and two neighboring molecules in the crystal lattice are perpendicular to each other because of the twisted character and the strong intermolecular CH-π interactions. However, its basic physicochemical properties suggest its instability in air derives from its elevated HOMO energy level, although NICS calculations confirm that the pyrene units contribution poorly to the π conjugation of the overall molecule.