FULL PAPER * E-mail: yoonys@yonsei.ac.kr; jack@kist.re.kr Received July 6, 2009; revised September 22, 2009; accepted October 23, 2009. Project supported by the IT R&D program of MKE/KEIT (2009-F-016-01, Development of Eco-Emotional OLED Flat-Panel Lighting) and a grant (code#: 2009K000463) from 'Center for Nanostructured Materials Technology' under '21st Frontier R&D Programs' of the Ministry of Science and Technology, Korea. Chin. J. Chem. 2010, 28, 115—118 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 115 Performance Improvement of Organic Light Emitting Diodes Using Poly(N-vinylcarbazole) (PVK) as a Blocking Layer Lee, Joo-Won a Kim, Jai-Kyeong b Yoon, Young-Soo* ,c a OLED Lighting Team, Convergence and Components & Materials Research Laboratory, New Devices & Materi- als Research Department, Electronics and Telecommunications Research Institute, 138 Gajeongro, yuseong-gu, Daejeon, 305-700, Korea b Opto-electronic Materials Research Laboratory, Korea Institute of Science and Technology, Seoul, 136-791, Korea c Department of Materials Science and Engineering, Yonsei University, Seoul, 120-749, Korea High efficiency organic light-emitting-devices (OLED) have been fabricated by incorporation of a polymeric layer as a controller of the unbalanced charge. In device configuration of ITO/PEDOT:PSS/PVK/Alq 3 /LiF:Al, poly(N-vinylcarbazole) (PVK) was selected as a blocking layer (BL) because it has a hole transporting property and a higher band gap, especially a lower LUMO level than the emitting layer (Alq 3 ) and a higher HOMO level than the hole injection layer (PEDOT: PSS). As a result, the optimal structure with this bl layer showed a peak efficiency of 6.89 cd/A and 2.30 lm/W compared to the device without the PVK layer of 1.08 cd/A, 0.27 lm/W. This result shows that the PVK layer could effectively block the electrons from metal cathode and confine them in the emitting layer and accomplish the charge balance, which leads to enhanced hole-electron balance for achieving high recombina- tion efficiency. Keywords organic light emitting diode, blocking layer, charge balance, poly(N-vinylcarbazole) (PVK), elec- tron-hole balance, hybrid structure Introduction A variety of organic light-emitting diode (OLED) using either small molecules or polymers are being in- vestigated 1 for many applications, including display technology and solid state lighting. Development and improvement of OLED technology rely on the under- standing and control of the elementary mechanisms in- fluencing the emission of light: charge injection, charge transport, exciton formation and recombination. These have been established as lower charge injection barriers at the electrodes, the better efficiency and the longer- lifetime by the fine device. Charge balance is an important issue in achieving high efficiency OLED devices. Unbalanced injection of either electrons or holes will result in building up of one charge carrier species at the heterostructure interface during device operation. In order to obtain high charge injection balance, the injection at the interface between organic layer and electrode should be taken into account. The charge transport characteristics influence the posi- tion of the exciton formation zone and the recombina- tion processes. The excitons formed in the device tend to be localized at the interface between the emitting layer (EML) and the electron or hole transport layer (ETL and HTL), respectively. Therefore, this enhances the imbalance of charges in the emissive layer and in turn decreases the efficiency of the device. A number of attempts have been made to achieve charge balance of holes and electrons. 2-9 Among them, an inserting of thin layer of conducting, semiconducting or insulating materials such as polyaniline, poly(phenyl- enevinylene) and Teflon 10-12 between the electrode and the emitting layer was used as simple methods to in- crease efficiency. In these studies, it seems that the ex- cess holes from an anode were controlled by the in- serted thin layers, which may help improve the balance of hole and electron. In this paper, to increase the recombination effi- ciency, we have associated with poly(N-vinylcarbazole) (PVK) 13,14 as a hole control layer and Alq 3 as an elec- tron control layer by adjusting the solution concentra- tion and the thickness of layer. It was reported 15 that the hole mobility (10 －3 cm 2 /Vs) in the TPD or NPB film was 2 orders of magnitude higher than the electron mo- bility 16 (10 －5 cm 2 /Vs) in Alq 3 , while the hole mobility 15 in PVK (10 －5 cm 2 /Vs) was quite comparable with the