Nam Su Kang

J-aggregation induced low bandgap anthracene-based conjugated molecule for solution-processed solar cells

A new anthracene-based X-shaped conjugated molecule, HBTATHT, was synthesized. Thin film transistors based on unannealed HBTATHT showed a carrier mobility of 0.15 cm(2) V(-1) s(-1) (I(on/off) = 7.9 × 10(6)). Further, a solution processed solar cell made of HBTATHT exhibited promising power conversion efficiencies of 4.84% and 4.70% with PC(61)BM (1:0.8 wt ratio) and PC(71)BM (1:0.6 wt ratio), respectively.

Role of the interfacial layer in the efficiency and lifetime of polymeric light emitting devices

The effects of the interfacial layers on the properties of fluorescent polymeric light emitting devices were investigated. The interfacial layer material, poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-ethoxycarbonylphenyl)-bis-N,N′-phenyl-benzidine (BFEC), has uniform surface roughness and high hole mobility upon thermal treatment up to 250 °C. Both for commercially available fluorescent red and green polymeric light emitters, the insertion of a BFEC interfacial layer resulted in a more than 30% increase of efficiency and a three times longer half-lifetime, whereas the blue fluorescent material showed no enhancement of power efficiency and lifetime. We attribute the improved device performance afforded by the interfacial layer to the bilayered devices having a more charge-balanced structure. The facilitated hole injection and electron/exciton blocking of the interfacial layer induced an infinitesimal shift of the recombination region, which was characterized by the electroluminescent spectra of the devices and...

Enhanced performance of organic photovoltaic devices by photo-crosslinkable buffer layer

AbstractThe performance of organic photovoltaic devices was enhanced by insertion of the photo-crosslinkable buffer layer. This buffer layer was formed by a photo curable precursor with bithiophene and pentadienyl moieties. The fill factor of the device with this buffer layer exceeded 0.7 in the organic photovoltaic cell. The characteristic of the photo-crosslinkable property enabled this buffer layer to be inserted between the hole extraction layer and the active layer, which formed an ohmic contact with both layers. The insertion of the buffer layer induced a 20% enhancement in conversion efficiency by small increases in the short-circuit current and the open-circuit voltage, and a huge increase in the fill factor. This photo-crosslinkable buffer layer acted as a leakage current reducing layer as measured by the reduced dark current. The high crystallinity and smooth surface of this buffer layer resulted in improved surface morphology and internal packing, thus enhanced the fill factor.

Correlation of photoluminescent quantum efficiency and device characteristics for the soluble electrophosphorescent light emitter with interfacial layers

We have investigated the effects of interfacial layers on the properties of soluble phosphorescent organic light emitting devices. Two kinds of polyfluorene-based interfacial layer materials have been studied; both were spin coated on top of PEDOT:PSS to form the insoluble layers by thermal annealing. The molecular-doped, phosphorescent light emitting layer comprising a polymeric host, small molecular host, and guest molecule was fabricated onto the thin interfacial layer. The photoluminescence quantum yield (PLQY) of these layers was measured with an integrating sphere. We have calculated the PLQY values of the single phosphorescent light emitting layer and various organic multilayers incorporating the interfacial layers, showing that a reduction in PLQY due to the interfacial quenching is more significant in the thicker interfacial layer structures. In spite of the decrease in PLQY induced by the triplet energy mismatch, polyfluorene-based interfacial layers improved the charge injection from PEDOT:PSS ...

Methods for balanced charge carriers and enhanced efficiency of organic phosphorescent devices

In this paper, the light emitting efficiency, spectrum, and the lifetime of the phosphorescent devices, whose emission characteristics are strongly dominated not only by the energy transfer but also by the charge carrier trapping induced by the emissive dopant, are explained by differences in the energy levels of the host, dopant, and nearby transport layers. On the basis of our finding on device performance and photocurrent measurement data by time-of-flight (TOF), we suggest a detailed emission mechanism, along with a physical interpretation and practical design scheme for improving the efficiency and lifetime of devices. Moreover, the effects of the modification of charge (both hole and electrons) transporting layers which results in a drop of operating voltage and improved efficiency on the performance of electrophosphorescent device are investigated experimentally. Using the Ir-based phosphorescent emitter, vacuumevaporated, solution-processed, and composition of their hybrid structures are designed for better charge carrier balance and efficient exciton blocking behavior.