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Yook, Kyoung Soo,Jeon, Soon Ok,Min, Sung‐,Yong,Lee, Jun Yeob,Yang, Ha‐,Jin,Noh, Taeyong,Kang, Sung‐,Kee,Lee, Tae‐,Woo WILEY‐VCH Verlag 2010 Advanced Functional Materials Vol.20 No.11
<P><B>Abstract</B></P><P>Cesium azide (CsN<SUB>3</SUB>) is employed as a novel n‐dopant because of its air stability and low deposition temperature. CsN<SUB>3</SUB> is easily co‐deposited with the electron transporting materials in an organic molecular beam deposition chamber so that it works well as an n‐dopant in the electron transport layer because its evaporation temperature is similar to that of common organic materials. The driving voltage of the p‐i‐n device with the CsN<SUB>3</SUB>‐doped n‐type layer and a MoO<SUB>3</SUB>‐doped p‐type layer is greatly reduced, and this device exhibits a very high power efficiency (57 lm W<SUP>−1</SUP>). Additionally, an n‐doping mechanism study reveals that CsN<SUB>3</SUB> was decomposed into Cs and N<SUB>2</SUB> during the evaporation. The charge injection mechanism was investigated using transient electroluminescence and capacitance–voltage measurements. A very highly efficient tandem organic light‐emitting diodes (OLED; 84 cd A<SUP>−1</SUP>) is also created using an n–p junction that is composed of the CsN<SUB>3</SUB>‐doped n‐type organic layer/MoO<SUB>3</SUB> p‐type inorganic layer as the interconnecting unit. This work demonstrates that an air‐stable and low‐temperature‐evaporable inorganic n‐dopant can very effectively enhance the device performance in p‐i‐n and tandem OLEDs, as well as simplify the material handling for the vacuum deposition process.</P>
Highly Enhanced Light Extraction from Surface Plasmonic Loss Minimized Organic Light‐Emitting Diodes
Kim, Jung‐,Bum,Lee, Jeong‐,Hwan,Moon, Chang‐,Ki,Kim, Sei‐,Yong,Kim, Jang‐,Joo WILEY‐VCH Verlag 2013 ADVANCED MATERIALS Vol.25 No.26
<P><B>Extremely high light out‐coupling efficiency</B> from a transparent organic light‐emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well‐matched analysis of optical simulations.</P>
Park, Sang‐,Hee K.,Hwang, Chi‐,Sun,Ryu, Minki,Yang, Shinhyuk,Byun, Chunwon,Shin, Jaeheon,Lee, Jeong‐,Ik,Lee, Kimoon,Oh, Min Suk,Im, Seongil WILEY‐VCH Verlag 2009 ADVANCED MATERIALS Vol.21 No.6
<P><B>Transparent ZnO thin‐film transistors (TFTs)</B> with a defect‐controlled channel and channel/dielectric interface maintain good photo‐stability during device operation. The figure shows a cross‐sectional view of a top‐gate ZnO‐based transparent TFT/storage capacitor cell structure, connected to front‐panel organic‐light‐emitting‐diode pixels to operate in bottom emission mode. </P>
Jang, Lee‐,Woon,Jeon, Dae‐,Woo,Kim, Myoung,Jeon, Ju‐,Won,Polyakov, Alexander Y.,Ju, Jin‐,Woo,Lee, Seung‐,Jae,Baek, Jong‐,Hyeob,Yang, Jin‐,Kyu,Lee, In‐,H WILEY‐VCH Verlag 2012 Advanced functional materials Vol.22 No.13
<P><B>Abstract</B></P><P>Localized surface plasmon (LSP) effects due to Ag and Ag/SiO<SUB>2</SUB> nanoparticles (NPs) deposited on GaN/InGaN multiquantum well (MQW) light‐emitting diode (LED) structures are studied. The colloidal NPs are synthesized by a sol‐gel method and drop‐cased on the LED structures. The surface density of NPs its controlled by the concentration of the NP solution. Theoretical modeling is performed for the emission spectrum and the electric field distribution of LSP resonance for Ag/SiO<SUB>2</SUB> NPs. Enhanced photoluminescence (PL) efficiency is observed in the LED structures and the amount of PL enhancement increases with increasing the surface density of Ag and Ag/SiO<SUB>2</SUB> NPs. These effects are attributed to resonance coupling between the MQW and LSP in the NPs. It is also shown that the PL enhancement attainable with Ag NPs and Ag/SiO<SUB>2</SUB> NPs is comparable, but the latter displays a much higher stability with respect to long‐term storage and annealing due to a barrier for NP agglomeration, Ag oxidation, and impurity diffusion provided by the SiO<SUB>2</SUB> shell.</P>
Ko, Young‐,Ho,Kim, Je‐,Hyung,Jin, Li‐,Hua,Ko, Suk‐,Min,Kwon, Bong‐,Joon,Kim, Joosung,Kim, Taek,Cho, Yong‐,Hoon WILEY‐VCH Verlag 2011 ADVANCED MATERIALS Vol.23 No.45
<P>Electrically driven hybrid light‐emitting diodes (LEDs) consisting of quantum dots, wires, and wells based on the nanometer‐sized pyramid GaN structure are reported by Taek Kim, Yong‐Hoon Cho, and co‐workers on page 5364. The LEDs exhibit mixed emissions from InGaN quantum dots, wires, and wells formed at the tops, edges, and sidewalls of the pyramids, respectively. The hybrid LEDs containing low‐dimensional quantum structures provide a broad‐band, highly efficient visible lighting source. </P>
Kim, Sei‐,Yong,Jeong, Won‐,Ik,Mayr, Christian,Park, Young‐,Seo,Kim, Kwon‐,Hyeon,Lee, Jeong‐,Hwan,Moon, Chang‐,Ki,Brü,tting, Wolfgang,Kim, Jang‐,Joo WILEY‐VCH Verlag 2013 Advanced Functional Materials Vol.23 No.31
<P><B>Abstract</B></P><P>High‐efficiency phosphorescent organic light‐emitting diodes (OLEDs) doped with Ir(ppy)<SUB>2</SUB>(acac) [bis(2‐phenylpyridine)iridium(III)‐acetylacetonate] in an exciplex forming co‐host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (<I>Θ</I>, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield (<I>q</I><SUB>PL</SUB>) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30%, which matches very well with the experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling. Based on the results, the maximum EQE achievable for a certain emitting dye in a host can be predicted by just measuring <I>q</I><SUB>PL</SUB> and <I>Θ</I> in a neat film on glass without the need to fabricate devices, which offers a universal plot of the maximum EQE as a function of <I>q</I><SUB>PL</SUB> and <I>Θ</I>.</P>
Han, Tae‐,Hee,Choi, Mi‐,Ri,Woo, Seong‐,Hoon,Min, Sung‐,Yong,Lee, Chang‐,Lyoul,Lee, Tae‐,Woo WILEY‐VCH Verlag 2012 ADVANCED MATERIALS Vol.24 No.11
<P><B>A highly efficient simplified organic light‐emitting diode (OLED)</B> with a molecularly controlled strategy to form near‐perfect interfacial layer on top of the anode is demonstrated. A self‐organized polymeric hole injection layer (HIL) is exploited increasing hole injection, electron blocking, and reducing exciton quenching near the electrode or conducting polymers; this HIL allows simplified OLED comprised a single small‐molecule fluorescent layer to exhibits a high current efficiency (∼20 cd/A).</P>
Visible‐Color‐Tunable Light‐Emitting Diodes
Hong, Young Joon,Lee, Chul‐,Ho,Yoon, Aram,Kim, Miyoung,Seong, Han‐,Kyu,Chung, Hun Jae,Sone, Cheolsoo,Park, Yong Jo,Yi, Gyu‐,Chul WILEY‐VCH Verlag 2011 ADVANCED MATERIALS Vol.23 No.29
<P><B>Visible‐color‐tunable light‐emitting diodes (LEDs)</B> with electroluminescent color that changes continuously from red to blue by adjusting the external electric bias are fabricated using multifacetted GaN nanorods with anisotropically formed 3D InGaN multiple‐quantum wells. Monolithically integrated red, green, and blue LEDs on a single substrate, operating at a fixed drive current, are also demonstrated for inorganic full‐color LED display applications. </P>
Huh, Chul,Kim, Kyung‐,Hyun,Kim, Bong Kyu,Kim, Wanjoong,Ko, Hyunsung,Choi, Chel‐,Jong,Sung, Gun Yong WILEY‐VCH Verlag 2010 Advanced Materials Vol.22 No.44
<P><B>The light output power and wall‐plug efficiency</B> of the Si nanocrystal (nc‐Si) light‐emitting diode (LED) were significantly enhanced by employing the multiple‐luminescent structures. This improvement was attributed to a strong confinement of carriers in the SiNx luminescent layers containing the nc‐Si due to the band offset between the luminescent layer and barrier layer. </P>
Lee, Sunghun,Kim, Kwon‐,Hyeon,Limbach, Daniel,Park, Young‐,Seo,Kim, Jang‐,Joo WILEY‐VCH Verlag 2013 Advanced functional materials Vol.23 No.33
<P><B>Abstract</B></P><P>An exciplex forming co‐host is introduced in order to fabricate orange organic light‐emitting diodes (OLEDs) with high efficiency, low driving voltage and an extremely low efficiency roll‐off, by the co‐doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieves a low turn‐on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent‐green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs show low efficiency roll‐off with an EQE of over 21% at 10 000 cdm<SUP>−2</SUP>. The device displays a very good orange color (CIE of (0.501, 0.478) at 1000 cdm<SUP>−2</SUP>) with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicate that both energy transfer and direct charge trapping takes place in the devices.</P>