Ziyi Ge

Novel Bipolar Bathophenanthroline Containing Hosts for Highly Efficient Phosphorescent OLEDs

The electronic structures of eight bathophenanthroline derivatives were elucidated by DFT calculations, and four representatives of which CZBP, m-CZBP, m-TPAP, and BPABP were synthesized and employed as the hosts to afford highly efficient phosphorescent OLEDs. The calculated molecular orbital energies agree well with the experimental results, which further demonstrates that the localization of HOMO and LUMO at the respective hole- and electron-transporting moieties is desirable in bipolar molecular designs.

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.

Highly efficient single- and multi-emission-layer fluorescent/phosphorescent hybrid white organic light-emitting diodes with ∼20% external quantum efficiency

A blue fluorophore of N,N-diphenyl-4′′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′:4′,1′′-terphenyl]-4-amine (BBPI) was utilized as the blue fluorescent emitter and the host of a phosphorescent emitter to fabricate highly efficient fluorescent/phosphorescent (F/P) hybrid white organic light-emitting diodes (WOLEDs) in a single- or multi-emission-layer architecture. For the single-emission-layer WOLEDs consisting of only the blue emitter BBPI and yellow complementary phosphorescent emitter PO-01 in the emission layer, a maximal current efficiency (CE) of 49.1 cd A−1, power efficiency (PE) of 52.0 lm W−1, and external quantum efficiency (EQE) of 16.3% were achieved by modulating the dopant concentration, indicating efficient singlet and triplet exciton separation and utilization ability in the emission layer. An improved EQE of 17.1%, broader spectral coverage, and higher color rendering index (CRI) were achieved by inserting a non-doped blue emission layer of BBPI and a green emission layer of CBP:Ir(PPy)3. A further improved EQE of up to 19.7% could be achieved when an orange-red phosphorescent emitter PQ2Ir was used instead of PO-01, and it is one of the best reported values for the F/P hybrid WOLEDs.

Highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio deep-blue emitter with improved concentration of phosphorescent dopant

An improved concentration of phosphorescent dopant for highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio (80%) deep-blue emitter has been developed. The high radiative exciton ratio for the deep-blue emitter was found to be the transfer from the higher triplet (T5) to the lowest singlet state (S1) by a “hot-exciton” process. Notably, when the concentration of Ir(2-phq)3 is up to 0.9 wt%, the OLED still exhibited white emission with a maximum total EQE, CE and PE of 22.3%, 53.7 cd A−1 and 60.2 lm W−1, respectively. The exciton transfer mechanism in a high concentration of phosphorescent dopant was also discussed. The studies provide a way to obtain high performance F/P hybrid WOLEDs with a simple architecture and improved doping concentration.

Precise Control of Photoinduced Birefringence in Azobenzene-Containing Liquid-Crystalline Polymers by Post Functionalization

A series of functionalized liquid-crystalline polymer materials with different degrees of functionality was synthesized by a post Sonogashira cross-coupling reaction of a polymer precursor. The post-functionalization was carried out under mild conditions and showed a high yield. Although a highly birefringent azotolane group was introduced into the polymer precursor, the photoresponse of the functionalized liquid-crystalline materials was not obviously changed. By adjusting the content of azotolane groups, precise control of the photoinduced birefringence was successfully obtained after thermal enhancement upon annealing. The present method to gain precise control of photoinduced birefringence might enable one to finely photocontrol the optical performances of materials, and may have a potential application as an advanced process for photonic materials.

Nonvolatile floating gate organic memory device based on pentacene/CdSe quantum dot heterojuction

An organic floating-gate memory device using CdSe quantum dots (QDs) as a charge-trapping element was fabricated. CdSe QDs were localized beneath a pentacene without any tunneling insulator, and the QD layer played a role as hole-trapping sites. The band bending formed at the junction between pentacene and QD layers inhibited back-injection of holes trapped in CdSe into pentacene, which appeared as a hysteretic capacitance-voltage response during the operation of the device. Nearly, 60% of trapped charge was sustained even after 104 s in programmed state, and this long retention time can be potentially useful in practical applications of non-volatile memory.

Tuning the properties of poly(2,6-dimethyl-1,4-phenylene oxide) anion exchange membranes and their performance in H2/O2 fuel cells

Here, we present a complete investigation of poly(2,6-dimethyl-1,4-phenylene) (PPO) AEMs with quaternary ammonium (QA) groups placed at different locations within the chemical structure of the polymer, e.g. a standard benzyltrimethyl ammonium control (BTMA), side-chain-type (SCQA), long side-chain-type (LSCQA), comb-shaped (CQA), or side-chain type/comb-shaped (SCCQA) AEMs have been designed and studied. These copolymers possess a similar composition but considerably different molecular architectures, the nature of which significantly alters their properties and device performance. Hydroxide conductivity was improved for the SCQA, LSCQA and comb-shaped membranes possessing a C-18 alkyl terminal pendant compared to that of BTMA and comb-shaped samples with a short alkyl chain. In chemical stability experiments under 10 M NaOH and 80 °C for 200 hours, LSCQA and SCCQA samples with a C-18 alkyl terminal pendant architecture showed less decrease in conductivity (∼10%) than the BTMA, SCQA and SCCQA polymers having short alkyl chains, which lost more than 50% conductivity after alkaline stability testing. Significant degradation was observed for the unstable PPO AEM samples by either SN2 substitution or Hoffmann elimination according to 1H NMR analysis. Interestingly, the fuel cell device performance provided counterintuitive data that showed that longer side chains with excellent alkaline stability were not superior in device function assessment. Specifically, the highly alkaline stable long side-chain-type LSCQA-30 membrane showed significant degradation in a fuel cell device with an operating lifetime of 3.3 h at 100 mA cm−2. Analysis of the aged membrane showed SN2 substitution and elimination of trimethylamine to be the dominant degradation mechanisms. In contrast, the BTMA-30 membranes with poor alkaline stability showed good durability in a working device without obvious degradation after 8.3 h operation as confirmed by 1H NMR spectra. This new finding that contrasts membrane alkaline stability and device operation stability is extremely important and gives us directions for new polymer designs for high performance devices. Additionally, this study paves the way for coupled ex situ chemical stability and in-device degradation studies, which are sorely needed in this field.

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.

Anthradithiophene-benzothiadiazole-based small molecule donors for organic solar cells

A new A–D–A small molecule involving anthradithiophene as a donor and benzothiadiazole as an acceptor unit has been synthesized by Stille coupling reaction. Its thermal, optical and electronic properties, hole mobility and photovoltaic properties have been fully characterized. The resulting material shows a broad absorption range (300–750 nm), a low band gap (1.59 eV) and a moderate hole mobility (8.81 × 10−4 cm2 V−1 s−1). We used the new small molecule blended with PC71BM as the active layer to fabricate solution-processed organic solar cells (OSCs), and employed a variety of post-treatment methods to optimize the device performance. With the help of polar solvent exposure, the highest power conversion efficiency (PCE) of 0.55% was obtained. These results would supply useful information to understand the relationship between molecular structure and photovoltaic properties of anthradithiophene/benzothiadiazole-based OSCs.

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.