Injection And Transport Mechanism In Organic Light-Emitting Diodes

Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. The primary reason is that a large number of organic materials are known to have extremely high fluorescence quantum efficiencies in the visible spectrum, including the blue region. In this regard, they are ideally suited for multicolor display applications. Charge carrier injection and transport have been the key issues in organic light-emitting diodes (OLEDs). The work performed in this dissertation is based on the fabrication and characterization of various types of organic light-emitting diodes, focusing on the improvement and the explanation of injection and transport mechanisms.White organic light emitting devices (WOLEDs) with an RBG stacked multilayer structure were first demonstrated. In RGB stacked OLEDs, blue emitting, 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN) doped with p-bis(p-N, N-diphenyl-amono-styryl)benzene (DSA-Ph), green emitting, tris-[8-hydroxyquinoline]aluminum (Alq3) doped with C545, and red emitting, tris-[8-hydroxyquinoline]aluminum (Alq3) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), were used. By adjusting the order and thickness of emitting layer in RBG structure, we got a white OLED with current efficiency of 5.60 cd/A and Commission Internationale De L'Eclairage (CIE) coordinates of (0.34, 0.34) at 200 mA/cm2. Its maximum luminance was 20,700 cd/m2 at current density of 400 mA/cm2. The results have been explained on the basis of the theory of excitons generation and diffusion. Based on this theory, an equation has been set up which nicely explains our EL spectra. It has been found that the experimental results are in good agreement with the theoretical values.Highly efficient organic electroluminescent devices (OLEDs) were developed based on 4,7- diphenyl-1, 10-phenanthroline (BPhen) as the electron transport layer (ETL), tris (8- hydroxyquinoline) aluminum (Alq3) as the emission layer (EML) and N,?-bis-[1-naphthy(-N,?diphenyl-1,1′-biphenyl-4,4′-diamine)] (NPB) as the hole transport layer (HTL). The typical device structure was glass substrate/ ITO/ NPB/ Alq3/ BPhen/ LiF/ Al. Since BPhen possesses a considerable high electron mobility of 5×10-4 cm2 V-1 s-1, devices with BPhen as ETL can realize an extremely high luminous efficiency. By optimizing the thickness of both HTL and ETL, we obtained a highly efficient OLED with a current efficiency of 6.80 cd/A and luminance of 1361 cd/m2 at a current density of 20 mA/cm2. To explain the high current efficiency in our devices, we fabricated hole-only (NPB based) and electron-only (Alq3-BPhen) devices. By comparing the J-V characteristics of hole-only and electron-only devices, it has been found that there exits a highly efficient balance for both holes and electrons for particular thicknesses of NPB and BPhen in our devices. Thus, this improvement in the current efficiency has been attributed to the efficient charge balance in the EML.The power efficiency of organic light-emitting diodes was improved significantly with p-i-n Structure by introducing a novel n-doping (4'7- diphyenyl-1, 10-phenanthroline: 33 wt % 8-hydroxy-quinolinato lithium) layer as an electron transport layer (ETL) and a p-doping layer composed of 4,4′,4″-tris (3-methylphenylphenylamono) triphenylamine (m-MTDATA) and tetrafluro-tetracyano-quinodimethane (F4- TCNQ) as a hole transport layer (HTL). Hole-only and electron-only devices were demonstrated to observe the improvement in the conductivity of the transport layers. In this strategy, we demonstrated that the power efficiency was enhanced by ~ 100 %, luminous efficiency was enhanced by ~ 54 % while driving voltage was reduced by 32 % as compared to the control device. It has been observed that all the important parameters such as current efficiency, power efficiency and driving voltage of organic light-emitting devices (OLEDs) are improved significantly. We obtained a power efficiency of 4.44 lm/W which is the best value so far reported for tris (8- hydroxyquinolinato) aluminum (Alq3)- based emitter. This improvement has been ascribed to the improved conductivity of the transport layers and to the better charge balance in the emission zone.The electron mobility of 4,7- diphyenyl-1, 10-phenanthroline (BPhen) at various thicknesses (50–300 nm) has been estimated by using space-charge-limited current (SCLC) measurements. The measured bulk mobility is in excellent agreement with the results from time-of-flight (TOF) method. We have found that at higher thickness of BPhen (i. e., x > 150 nm), the mobility becomes insensitive to the thickness change and approaches the value of intrinsic electron mobility of BPhen. The estimated electron mobility of BPhen at 300 nm is found to be 3.4×10?4 cm2/V s (at 0.3 MV/cm) with weak dependence on electric field, which is in good agreement with the TOF measurements (4.2×10?4 cm2/V s at 0.3 MV/cm). However, TOF technique can not measure the practical mobility in the typical thickness of about 50 nm because it requires quite thick films of several microns. Therefore, we also investigated the electron mobility of BPhen for the thickness (~50nm) typical of organic light-emitting devices by SCLC.Investigations of charge injection and transport in organic electron transport (ET) materials were reported. ITO/ET materials (BPhen, Bpy-OXD, BCP)/ cathode devices have been studied as a function of metal cathodes, showing that the electron conduction is injection limited with Au cathode. Using calcium and Aluminum cathodes, electrons are preponderant and are bulk limited. The injection and transport mechanisms have been investigated using injection limited and space- charge limited models. The devices with Au cathode follow the Fowler-Nordheim mechanism for electron injection at the organic / Au interface and the predicted values are qualitatively consistent with experimental data.For the Ca and Al cathodes, electron conduction is preponderant and is bulk limited. A power law dependence J∝Vm with m > 2 is consistent with experimental data and the trap charge-limited current (TCLC) mechanism has been employed to explain the J-V characteristics of the devices. The total electron trap density is estimated to be 5.0×1018 cm-3. These results may be very meaningful to understand the injection and transport mechanisms in organic semiconductors in general and in electron transport (ET) materials, in particular.