Xiaomeng Li

Date of Graduation

2016

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Lane Department of Computer Science and Electrical Engineering

Committee Chair

Xian-An Cao

Committee Co-Chair

Parviz Famouri

Committee Member

Mark Jerabek

Committee Member

Lianshin Lin

Committee Member

Yuxin Liu

Abstract

Organic light-emitting diodes (OLEDs) are generally considered as the next generation display and lighting sources owing to their many attractive properties, including low power consumption, wide viewing angle, vibrant color, high contrast ratios and compatibility with flexible substrates. The research and development of OLEDs has attracted considerable interest and has led to significant progress during the last two decades. The use of OLEDs in small-area displays such as cell phone screens, digital cameras, and wearable devices has become a reality. However, the OLED technology is still far from mature, posing a challenge for their widespread acceptance for applications in large-area displays and solid-state lighting. In particular, the lifetime of OLEDs is too short for many commercial applications, and the degradation mechanisms are still under debate. This work aims to improve the OLED device lifetime by doping of organic hole transport materials with inorganic transition metal oxides (TMOs), and to reduce the cost by simplifying the device layer structure and manufacturing procedure.;First, stress tests under continuous wave and pulsed currents were conducted to gain a better understanding of the key factors governing the degradation process of phosphorescent OLEDs. Through comparative studies of the aging behaviors of OLEDs with different hole transport layers (HTLs) under different stressing conditions, we have found that joule heating plays an important role in device degradation when a large energy level misalignment exists at the indium-tin-oxide (ITO) anode/HTL interface. The heating was effectively suppressed by reducing the interfacial energy barrier, leading to a prolonged lifetime of the OLEDs.;P-type doping of hole transport materials with TMOs was then developed as an effective way to reduce the interfacial energy barrier and the operational voltage of OLED devices. A systematical study was carried out on the effects of doping 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP), a wide bandgap organic hole transport material, with WO3 and MoO3. The optimal doping conditions including the doping level and doping thickness have been determined by fabricating and characterizing a series of hole-only devices. Integrating the doped HTL into green phosphorescent OLEDs has resulted in a simplified structure, better optoelectronic characteristics, and improved device reliability.;Finally, selective doping of organic materials with the TMOs was developed and the concept of delta doping was applied to OLEDs for the first time. Selective doping was achieved by simple sequential deposition of the organic host and TMO dopant. Hole-only devices with a HTL comprising alternative 0.5 nm TMO-doped/3-10 nm undoped CBP layers exhibited greatly enhanced hole transport and had a turn-on voltage as low as 1.1 V. Simple fluorescent tris-(8-hydroxyquinoline) aluminum (Alq3)-based green OLEDs with a selectively doped CBP HTL showed a lower voltage and longer lifetime under constant-current stressing compared to similar OLEDs with an undoped HTL. Furthermore. delta doping was realized in more thermally stable organic materials, resulting in a marked conductivity increase along the plane of the doped layers by several orders of magnitude. The delta doping effects were explained by hole accumulation in potential wells formed in nanometer-thick doped regions, as revealed by high-resolution secondary ion mass spectrometry (SIMS) measurements.

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