Ruixiang Peng

Chemically Binding Carboxylic Acids onto TiO2 Nanoparticles with Adjustable Coverage by Solvothermal Strategy

This paper presents a solvothermal strategy for chemical modification of TiO(2) nanoparticles with carboxylic acids. Solvothermal reaction between the TiO(2) nanoparticles and carboxylic acid molecules in an autoclave at 100 degrees C provides carboxylic acid-modified TiO(2) particles with a modification efficiency much higher than the conventional immersion method. TiO(2) nanoparticles were prepared by hydrolysis of titanium isopropoxide in nitric acid solution; the modified nanoparticles were characterized by powder X-ray diffraction pattern, scanning electron microscopy, absorption and Fourier transform infrared spectra, and thermogravimetric analysis. Results show that the binding form of the modifier molecules on TiO(2) surface is in a bidentate chelating mode, the crystalline phase composition and morphological structure of the preformed TiO(2) nanoparticles are not affected by the solvothermal reaction, and the surface coverage of the modifier molecules can be adjusted by the weight ratio of modifier/TiO(2) during feeding. It is evident that the reaction processes in the solvothermal strategy involve the formation of double hydrogen bondings between carboxylic acid molecule and TiO(2) at the same Ti site and the coordination at solvothermal temperature by dehydration from the hydrogen bondings. The solvothermal strategy for modifying TiO(2) nanoparticles is expected to find potential applications in many fields; for example, our results demonstrate that the photovoltaic performance of the TiO(2) nanoparticles can be improved by the solvothermal modification even with an insulating modifier and controlled by the modifier coverage.

Efficient polymer solar cells based on the synergy effect of a novel non-conjugated small-molecule electrolyte and polar solvent

A novel non-conjugated small-molecule electrolyte was invented as a cathode interlayer in PTB7:PC71BM-based polymer solar cells (PSCs). We discovered a significant synergy effect for improving the device efficiency between methanol treatment and the interlayer. The methanol treatment mainly contributed to the open-circuit voltage, while the interlayer primarily enhanced the short-circuit current and fill factor. Under the effective synergy effect, power conversion efficiencies (PCEs) of PTB7:PC71BM-based PSCs were largely improved from 3.89% to 9.79% for conventional PSCs and from 7.34% to 9.10% for inverted PSCs. Our findings create a new path of interfacial modification for highly efficient PSCs.

Synthesis, crystal structure, and polymerization of butterfly-shaped thieno[3,2-b]thiophene oligomers

A series of new butterfly-shaped thieno[3,2-b]thiophene oligomers with phenyl and thiophene units were synthesized through Suzuki coupling and Stille coupling reactions. The optical and thermal properties of these materials can be tuned by varying both substituents and the conjugation length. The crystal structures have been determined and showed a syn- or anticlinal conformation in the crystal of molecule 4. The electronic properties of the monomers and their electropolymerization ability are discussed and rationalized as a function of their molecular structure. Moreover, stable cross-linked conjugated polymers were formed by electropolymerization.

Thermal Annealing Effects on the Absorption and Structural Properties of Regioregular Poly(3-Hexylthiophene) Films

Regioregular poly(3-hexylthiophene) (P3HT) was synthesized by a Grignard metathesis method. The annealed and fast cooled P3HT films were studied by UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. It is demonstrated that thermal annealing at around the onset temperature of the melting point of the polymer results in the annealed film having the highest interchain and intrachain orders while preserving the original morphological features. However, the film that has the highest or lowest crystallinity does not have the strongest or weakest absorption. The lack of clear correlation between the absorption spectra and GIXRD data is explained by a quasi-ordered phase model, in which crystalline, amorphous, and quasi-ordered chains interconvert into each other depending on annealing temperature.

Highly efficient polymer solar cells using a non-conjugated small-molecule zwitterion with enhancement of electron transfer and collection

A novel non-conjugated small-molecule zwitterion is developed as a cathode interfacial material to enhance the electron transfer and collection properties of high-performance PSCs. The devices show significantly increased performance with power conversion efficiencies up to 9.51%. It is noteworthy that the results here provide significant scientific insights into further improvement of interfacial modification and performance of polymer solar cells.

Multifunctional emitters for efficient simplified non-doped blueish green organic light emitting devices with extremely low efficiency roll-off

Highly efficient organic light-emitting diodes (OLEDs) with simplified device structures are widely desired for both scientific research and industrial applications. However, a very limited number of simplified OLEDs have been reported to date. In this work, two multifunctional blueish green emitters, BPTPETPAI and 2TPETPAI, are designed and synthesized. Owing to the presence of a tetraphenylethene (TPE) moiety, their aggregation induced emission (AIE) properties are also investigated. High photoluminescence efficiencies of the two compounds in non-doped films render them good emitters for non-doped devices. Multilayer non-doped devices based on these emitters achieve maximum external quantum efficiencies (EQEs) and current efficiencies (CEs) of 3.13% and 6.14 cd A−1 as well as 3.25% and 6.70 cd A−1 for BPTPETPAI and 2TPETPAI, respectively. Given their shallow highest occupied molecular orbital (HOMO) energy levels, both emitters can also be used as hole injection and hole transporting materials. Based on this, single layer devices show even higher efficiencies with extremely low efficiency roll-off, achieving maximum CEs as high as 7.12 cd A−1 and 7.80 cd A−1 using BPTPETPAI and 2TPETPAI, respectively. These results demonstrate a bright prospect for the development of highly desired multifunctional emitters as well as simplified OLEDs with significant reduction in the fabrication cost of the device.

Non-Doped Sky-Blue OLEDs Based on Simple Structured AIE Emitters with High Efficiencies at Low Driven Voltages

Blue organic light-emitting diodes (OLEDs) are necessary for flat-panel display technologies and lighting applications. To make more energy-saving, low-cost and long-lasting OLEDs, efficient materials as well as simple structured devices are in high demand. However, a very limited number of blue OLEDs achieving high stability and color purity have been reported. Herein, three new sky-blue emitters, 1,4,5-triphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEI), 1-(4-methoxyphenyl)-4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEMeOPhI) and 1-phenyl-2,4,5-tris(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (3TPEI), with a combination of imidazole and tetraphenylethene groups, have been developed. High photoluminescence quantum yields are obtained for these materials. All derivatives have demonstrated aggregation-induced emission (AIE) behavior, excellent thermal stability with high decomposition and glass transition temperatures. Non-doped sky-blue OLEDs with simple structure have been fabricated employing these materials as emitters and realized high efficiencies of 2.41 % (4.92 cd A-1 , 2.70 lm W-1 ), 2.16 (4.33 cd A-1 , 2.59 lm W-1 ) and 3.13 % (6.97 cd A-1 , 4.74 lm W-1 ) for TPEI, TPEMeOPhI and 3TPEI, with small efficiency roll-off. These are among excellent results for molecules constructed from the combination of imidazole and TPE reported so far. The high performance of a 3TPEI-based device shows the promising potential of the combination of imidazole and AIEgen for synthesizing efficient electroluminescent materials for OLED devices.

Benzophenone-based small molecular cathode interlayers with various polar groups for efficient polymer solar cells

A series of novel benzophenone-based small molecular cathode interfacial materials with different polar groups including hydroxyl groups, neutral amino groups, amino N-oxide, and sulfobetaine ions were synthesized for PTB7:PC71BM-based polymer solar cells between the active layer and Al electrode. The photovoltaic properties of the devices with these interfacial materials were studied. The differences in interface modification performance of hydroxyl and amino interfacial materials were investigated for the first time. The devices with a solution-processed amino N-oxide-based interlayer showed a PCE of 9.34% with the highest short-circuit current density and fill factor by reducing the series resistance and charge recombination compared to the devices with the other interlayers in this work. The study of structure–property relationships proposes the significant guidance for the design of efficient cathode interface materials in organic solar cells.

All‐Solution‐Processed Metal‐Oxide‐Free Flexible Organic Solar Cells with Over 10% Efficiency

All-solution-processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers the possibility of a safe and cost-effective fabrication environment for the devices. Herein, an all-solution-processed flexible organic solar cell (OSC) using poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) electrodes is reported. The all-solution-processed flexible devices yield the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal-oxide-free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high-performance PEDOT:PSS/plastic underlying substrate with a matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibit an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high-performance all-solution-processed flexible OSCs, which is important for the development of printing, blading, and roll-to-roll technologies.

High-Performance Polymer Solar Cells Employing Rhodamines as Cathode Interfacial Layers

The development of simple and water-/alcohol-soluble interfacial materials is crucial for the cost-effective fabrication process of polymer solar cells (PSCs). Herein, highly efficient PSCs are reported employing water-/alcohol-soluble and low-cost rhodamines as cathode interfacial layers (CILs). The results reveal that rhodamine-based CILs can reduce the work function of the Al cathode and simultaneously increase the open-circuit voltage, current density, fill factor, and power conversion efficiency (PCE) of PSCs. The solution-processed rhodamine-based PSCs demonstrated a remarkable PCE of 10.39%, which is one of the best efficiencies reported for thieno[3,4-b]thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester-based PSCs so far. The efficiency is also 42.3% higher than that of the vacuum-deposited Ca-based device (PCE of 7.30%) and 21.5% higher than that of the complicated solution-processable polymeric electrolyte poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-based device (PCE of 8.55%). Notably, rhodamines are very economical and have been extensively used as dyes in industries. Our work indicates that rhodamines have shown a strong potential as CILs compared to their counterparts in the large-area fabrication process of PSCs.