ORGANIC/METAL INTERFACES FOR HIGH PERFORMANCE ORGANIC DIODES

Abstract

Organic electronics have received a great attention for next generation electronics due to lots of advantages such as easy patterning, flexibility, light-weight, and potential of large-area application. In spite of such a lot of advantages, however, there are still many issues that need to be improved such as charge injection, mobility, lifetime, operational stability, reliability, and uniformity. One of the important issues to make high performance diode is to improve charge injection efficiency. In this thesis, we investigate organic/metal interface of the diode to enhance charge injection efficiency and demonstrate high performance organic diodes. Two major methods are used to improve device performance: improved charge injection by electrical annealing and reduced hole injection barrier by using permanent dipole moment of self-assembled monolayer (SAM). First, we investigate the effect of electrical annealing on pentacene diode to which electrical annealing has not been applied because it cannot have ionic species. By using molybdenum trioxide (MoO3) instead of ionic species, electrical annealing can be applied to thermally deposited device which is advantageous for fabricating high performance devices. After electrical annealing, The turn-on voltage is reduced from approximately 1.3 V to 0.2 V and current at 3 V is increased from approximately 0.2 mA to 1 mA without increase of the reverse-bias current. In addition to MoO3 as a hole injection layer, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and copper hexadecafluorophthalocyanine (F16CuPc), which have deep highest occupied molecular orbital (HOMO) levels, show electrical annealing effect but poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), 4,4,4-tris(N-3-methylphenylamino)triphenylamin (m-MTDATA), and copper phthalocyanine (CuPc) do not affect electrical annealing. the cutoff frequency was increased from 10.5 MHz to 85.7 MHz. There is no improvement of current or reduction of turn-on voltage by using thermal annealing only, indicating that electric field plays an important role for electrical annealing. From the time of flight secondary ion mass spectrometry (ToF-SIMS) and impedance spectra, we conclude that the device performance of the pentacene diode is improved by electrical annealing due to the creation of the pentacene:MoO3 mixed layer. The mixed layer effectively increases charge injection by reducing small potential barrier which causes the turn-on voltage of current–voltage (I–V) characteristics and the RC-component at Au/MoO3/pentacene interface of impedance spectra. Note that because this uniform and thin pentacene:MoO3 mixed layer cannot be formed by thermal evaporation, electrical annealing is the best technique to form the uniform, thin, and gradual pentacene:MoO3 mixed layer for improving device performance. After electrical annealing, Al penetration into the pentacene layer was also observed. Because Al was deposited on the polycrystalline pentacene, Al spikes are formed at the pentacene grain boundary. These Al spikes can induce a higher electric field, facilitating the penetration of Al. Therefore, the penetrated Al may create rod-like structures that can be modeled as constant phase element (CPE). Second, we investigated the structure–property relationship of pentacene on gold and SAM-treated gold along the vertical direction. From the photoelectron spectrometer, the work function of gold, thiophenol (TP)-modified gold and pentafluorobenzenethiol (PFBT)-modified gold is measured to be 4.78, 4.67, and 5.02 eV, respectively. PFBT-treated gold effectively lower the injection barrier between the anode and the active layer, the forward-bias current density of the diode with PFBT-treated gold is much higher than that with pristine gold and finally current density of 100 A/cm2 is obtained at 3 V. In addition, the rectification ratio of the diode is founded to be 7.47 × 105 at 1 V, and 1.05 × 107 at 2.8 V. The 3-dB frequency, in terms of voltage, of the rectifier which is composed of the diode and a capacitor is obtained to be 1.24 GHz. Finally, Vout of 3.8 V at 1 GHz is obtained when input voltage of 10 V is applied. From the X-ray diffraction (XRD), atomic force microscope (AFM), and Raman analysis, pentacene molecules on gold exhibit lying-down orientation and those on PFBT-treated gold exhibit standing-up orientation. These structure differences change the electrical property. The mobility, calculated by space charge limited current (SCLC), of the pentacene film on gold and PFBT-treated gold is measured to be 6.82 × 10-4 and 0.114 cm2V-1s-1, respectively. The XRD patterns and vertical scanning electron microscope (SEM) images show that pentacene on gold exhibits the entangled and disordered structure whereas pentacene on PFBT-treated gold exhibits dense and ordered structure. This poor molecular ordering for pentacene on gold can limit charge transport property, resulting that the mobility of the pentacene film on gold is smaller than that of on PFBT-treated gold.