Phenanthroline diimide as an organic electron-injecting material for organic light-emitting devices

We report a diimide-type organic electron-injecting material, bis-[1,10]phenanthrolin-5-yl-pyromellitic diimide (Bphen-PMDI), for organic light-emitting devices (OLEDs), which was synthesized from its monomers, pyromellitic dianhydride (PMDA) and 1,10-phenanthrolin-5-amine (PTA). The vacuum-purified Bphen-PMDI powder showed high glass transition (∼230°C) and thermal decomposition (∼400°C) temperatures, whereas neither melting point nor particular long-range crystal nanostructures were observed from its solid samples. The optical band gap energy and the ionization potential of the Bphen-PMDI film were 3.6 eV and 6.0 eV, respectively, leading to the lowest unoccupied molecular orbital (LUMO) energy of 2.4 eV. Inserting a 1 nm thick Bphen-PMDI layer between the emission layer and the cathode layer improved the device current density by 10-fold and the luminance by 6-fold, compared to the OLED without the Bphen-PMDI layer. The result suggests that an effective electron tunnel injection process occurs through the Bphen-PMDI layer. © The Royal Society of Chemistry 2012.1

Becoming a doctor: using social constructivism and situated learning to understand the clinical clerkship experiences of undergraduate medical students

Abstract Background Despite the emphasis on the uniqueness and educational importance of clinical clerkships in medical education, there is a lack of deep understanding of their educational process and outcomes. Especially due to an inherent trait of clinical clerkships which requires participation in the workplace outside the classroom, it is difficult to fully comprehend their educational potential using traditional learning perspectives such as imbibing outside knowledge. Accordingly, this study aims to explore the experiences of a rotation-based clerkship of medical school students from the perspective of social constructivism of learning, which can empirically examine what and how medical students learn during clinical clerkship in South Korea. By providing an insight into the workings of the clerkship process, this study contributes to a better understanding of how a learning-friendly environment can be cultivated at clinical clerkships. Methods The study utilized a basic qualitative study to understand what and how medical students learn during their clinical clerkships. Semi-structured, in-depth individual interviews were conducted with eight sixth-graders who had experienced a two-year clerkship at Ajou University Medical School. Data were analyzed based on Lave and Wenger’s situated learning theory and Wenger’s social theory in learning. Results We found that the medical students had developed different aspects of their professional identities such as values, functionality, career decisions, sociality, and situating during their clinical clerkships. Further, professional identity was formed through a combination of participation and reification—the processes involved in the negotiation of meaning. This combination was facilitated by the students’ first experience and relationships with professors, classmates, and patients. Finally, non-learning occurred in the context of over-participation (learning anxiety and alienation) or over-reification (evaluation and e-portfolio). Conclusions This study revealed five sub-professional identities and their formation process from the learners’ perspective, thereby uncovering the unique learning characteristics and advantages of rotated-based clerkship and contributing to a further understanding of how gradual improvements can be made to the traditional clerkship education of medical students

Hybrid Solar Cells with In-Situ Prepared Inorganic Nanoparticles/Polymer Bulk Heterojunction Films

Lead sulfide (PbS) nanoparticles were generated by the in-situ reaction of lead chloride (II) and sulfur in the presence of poly (3-hexylthiophene) (P3HT) dissolved in the co-solvents. Three different reaction times were used to examine the effect of the PbS reaction conditions. The in-situ generated PbS nanoparticles in the P3HT matrix were spherical with a diameter of 15~25 nm. The P3HT:PbS composite materials were mixed with a soluble fullerene derivative ([6,6]-phenyl-C61-butyric acid methyl ester-PCBM) for the fabrication of hybrid solar cells. The hybrid (P3HT:PCBM:PbS) solar cells exhibited similar performance to each other, but their performance was better than the control (P3HT:PCBM) solar cells. In particular, the optimized hybrid solar cells showed ca. 14% improved power conversion efficiency due to the larger increase in short circuit current density (~30%). This improvement was attributed to the PbS nanoparticles acting as electron acceptors and a minor contribution from scattering effect. Copyright © 2012 American Scientific Publishers.

Diimide nanoclusters play hole trapping and electron injection roles in organic light-emitting devices

We report thermally stable diimide nanoclusters that could potentially replace the conventional thick electron transport layer (ETL) in organic light-emitting devices (OLEDs). Bis-[1,10]phenanthrolin-5-yl-bicyclo[2.2.2]oct- 7-ene-2,3,5,6-tetracarboxylic diimide (Bphen-BCDI) was synthesized from the corresponding dianhydride and amine moieties, and its purified product exhibited a high glass transition temperature (232 °C) and a wide band gap (3.8 eV). The Bphen-BCDI subnanolayers deposited on substrates were found to form organic nanoclusters, not a conventional layer. The OLED made with a subnanolayer of Bphen-BCDI nanoclusters, instead of a conventional ETL, showed greatly improved efficiency (about 2-fold) compared with an OLED without the diimide nanoclusters. The role of the BPhen-BCDI nanoclusters was assigned to hole trapping and electron injection in the present OLED structure. © 2011 The Royal Society of Chemistry.

Influence of Controlled Acidity of Hole-Collecting Buffer Layers on the Performance and Lifetime of Polymer:Fullerene Solar Cells

We report the influence of the controlled acidity of the holecollecting buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), on the performance and lifetime of polymer:fullerene solar cells. The acidity was controlled by adding a strong base (NaOH) to the pristine PEDOT:PSS solutions. The NaOH-modified PEDOT:PSS layers were used for fabricating polymer:fullerene solar cells with active layers made from blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The results showed that a small addition of NaOH (0.2 molar ratio) removed 23% of the sulfonic acid groups but did not change the device performance, even though further NaOH addition degraded the device performance owing to an increased sheet resistance and lowered work function, as well as a changed surface morphology. Storage lifetime tests showed that the device with the modified PEDOT:PSS layer (0.2 molar ratio NaOH) was almost not degraded, whereas the pristine PEDOT:PSS layer might affect the deterioration at the interface with the active layer (P3HT: PCBM). Under 1 sun illumination for 10 h, the modified PEDOT:PSS layer (0.2 molar ratio NaOH) resulted in ∼25% improved lifetime, which is in excellent agreement with the extent of reduction of the sulfonic acid groups (∼23%).© 2011 American Chemical Society.

Solution-processable all-small molecular bulk heterojunction films for stable organic photodetectors: near UV and visible light sensing

We report stable organic photodetectors with all-small molecular bulk heterojunction (BHJ) sensing layers prepared using solutions of electron-donating and electron-accepting small molecules. As an electrondonating molecule, 2,5-bis(2-ethylhexyl)-3,6-bis(4'-methyl-[2,2'-bithiophen]-5-yl) pyrrolo[3,4-c] pyrrole-1,4(2H, 5H)-dione (EHTPPD-MT) was synthesized via a Stille coupling reaction, whereas [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) was used as an electron-accepting component. The devices with the EHTPPD-MT: PC61BM BHJ layer could detect photons at a wavelength of 400-800 nm and exhibited a stable photoresponse under on/off modulation of near UV (405 nm) and visible (532 nm and 650 nm) light even at bias voltage conditions. The corrected responsivity reached similar to 175 mA W-1 for the near UV detection at -1 V. An extremely durable photoresponse was measured for the present devices (including flexible devices) under illumination with high intensity green light (133.4 mW cm(-2) at 532 nm) which is much stronger than standard sun light (100 mW cm(-2), white). The excellent stability has been attributed to the tiny EHTPPD-MT crystals, which are formed in the EHTPPD-MT: PC61BM layers during the coating processesclos

A Pronounced Dispersion Effect of Crystalline Silicon Nanoparticles on the Performance and Stability of Polymer:Fullerene Solar Cells

We investigated the dispersion effect of crystalline silicon nanoparticles (SiNP) on the performance and stability of organic solar cells with the bulk heterojunction (BHJ) films of poly­(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). To improve the dispersion of SiNP in the BHJ films, we attached octanoic acid (OA) to the SiNP surface via esterification reaction and characterized it with Raman spectroscopy and high-resolution transmission electron microscopy. The OA-attached SiNP (SiNP-OA) showed improved dispersion in chlorobenzene without change of optical absorption, ionization potential and crystal nanostructure of SiNP. The device performance was significantly deteriorated upon high loading of SiNP (10 wt %), whereas relatively good performance was maintained without large degradation in the case of SiNP-OA. Compared to the control device (P3HT:PC₆₁BM), the device performance was improved by adding 2 wt % SiNP-OA, but it was degraded by adding 2 wt % SiNP. In particular, the device stability (lifetime under short time exposure to 1 sun condition) was improved by adding 2 wt % SiNP-OA even though it became significantly decreased by adding 2 wt % SiNP. This result suggests that the dispersion of nanoparticles greatly affects the device performance and stability (lifetime)

Influence of Controlled Acidity of Hole-Collecting Buffer Layers on the Performance and Lifetime of Polymer:Fullerene Solar Cells

We report the influence of the controlled acidity of the hole-collecting buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), on the performance and lifetime of polymer:fullerene solar cells. The acidity was controlled by adding a strong base (NaOH) to the pristine PEDOT:PSS solutions. The NaOH-modified PEDOT:PSS layers were used for fabricating polymer:fullerene solar cells with active layers made from blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM). The results showed that a small addition of NaOH (0.2 molar ratio) removed 23% of the sulfonic acid groups but did not change the device performance, even though further NaOH addition degraded the, device performance owing to an increased sheet resistance and lowered work function, as well as a changed surface morphology. Storage lifetime tests showed that the device with the modified PEDOT:PSS layer (0.2 molar ratio NaOH) was almost not degraded, whereas the pristine PEDOT:PSS layer might affect the deterioration at the interface with the active layer (P3HT: PCBM). Under 1 sun illumination for 10 h, the modified PEDOT:PSS layer (0.2 molar ratio NaOH) resulted in similar to 25% improved lifetime, which is in excellent agreement with the extent of reduction of the sulfonic acid groups (similar to 23%).X114542sciescopu

Doping Effect of Organosulfonic Acid in Poly(3-hexylthiophene) Films for Organic Field-Effect Transistors

We attempted to dope poly­(3-hexylthiophene) (P3HT) with 2-ethylbenzenesulfonic acid (EBSA), which has good solubility in organic solvents, in order to improve the performance of organic field effect transistors (OFET). The EBSA doping ratio was varied up to 1.0 wt % because the semiconducting property of P3HT could be lost by higher level doping. The doping reaction was confirmed by the emerged absorption peak at the wavelength of ∼970 nm and the shifted S2p peak (X-ray photoelectron spectroscopy), while the ionization potential and nanostructure of P3HT films was slightly affected by the EBSA doping. Interestingly, the EBSA doping delivered significantly improved hole mobility because of the greatly enhanced drain current of OFETs by the presence of the permanently charged parts in the P3HT chains. The hole mobility after the EBSA doping was increased by the factor of 55–86 times depending on the regioregularity at the expense of low on/off ratio in the case of unoptimized devices, while the optimized devices showed ∼10 times increased hole mobility by the 1.0 wt % EBSA doping with the greatly improved on/off ratio even though the source and drain electrodes were made using relatively cheaper silver instead of gold

Interaction effect between NAFLD severity and high carbohydrate diet on gut microbiome alteration and hepatic de novo lipogenesis

Nonalcoholic fatty liver disease (NAFLD) is associated with high carbohydrate (HC) intake. We investigated whether the relationship between carbohydrate intake and NAFLD is mediated by interactions between gut microbial modulation, impaired insulin response, and hepatic de novo lipogenesis (DNL). Stool samples were collected from 204 Korean subjects with biopsy-proven NAFLD (n = 129) and without NAFLD (n = 75). The gut microbiome profiles were analyzed using 16S rRNA amplicon sequencing. Study subjects were grouped by the NAFLD activity score (NAS) and percentage energy intake from dietary carbohydrate. Hepatic DNL-related transcripts were also analyzed (n = 90). Data from the Korean healthy twin cohort (n = 682), a large sample of individuals without NAFLD, were used for comparison and validation. A HC diet rather than a low carbohydrate diet was associated with the altered gut microbiome diversity according to the NAS. Unlike individuals from the twin cohort without NAFLD, the abundances of Enterobacteriaceae and Ruminococcaceae were significantly different among the NAS subgroups in NAFLD subjects who consumed an HC diet. The addition of these two microbial families, along with Veillonellaceae, significantly improved the diagnostic performance of the predictive model, which was based on the body mass index, age, and sex to predict nonalcoholic steatohepatitis in the HC group. In the HC group, two crucial regulators of DNL (SIRT1 and SREBF2) were differentially expressed among the NAS subgroups. In particular, kernel causality analysis revealed a causal effect of the abundance of Enterobacteriaceae on SREBF2 upregulation and of the surrogate markers of insulin resistance on NAFLD activity in the HC group. Consuming an HC diet is associated with alteration in the gut microbiome, impaired glucose homeostasis, and upregulation of hepatic DNL genes, altogether contributing to NAFLD pathogenesis.N