Ohmic contacts for organic optoelectronic devices

Abstract

In last three decades, great progress has been made in the field of organic electronics. Researchers have put tremendous efforts to make new materials and device architectures, which has resulted in a great commercial success of organic light emitting diodes in mobile phone and television display screens. Despite that, still today it is challenging to make organic electronic devices that are efficient in performance, stable in operation and are economical in production at the same time. The objective of this thesis is to understand fundamental charge transport properties of small molecules based organic semiconductors and to develop novel organic electronic device architectures. One of the prime requirements for efficient organic optoelectronic devices is to have ohmic charge injection contacts. Therefore, first a charge injection strategy for making ohmic hole contacts is developed. Using this strategy, ohmic hole contacts are achieved on organic semiconductors with an ionization energy up to 6 eV. As a result, the hole transport in a wide range of organic small molecules with ionization energy between 5 to 6 eV could be investigated. Despite the difference in their chemical structures, similar bulk hole mobilities in the range of $1\times10^{-4}$ cm$^{2}$V$ ^{-1} $s$ ^{-1} $ were observed for all molecules. The hole transport was also investigated using molecular multiscale simulations, an excellent agreement was obtained with the experimental results. Despite fullerene derivatives being known as electron conductors, It was found that the fullerene derivative ICBA has a very good hole mobility of $1.4\times10^{-3}$ cm$^{2}$V$ ^{-1} $s$ ^{-1} $ which is the same as bulk electron mobility, demonstrating the intrinsic bipolar charge-trasnport character of organic semiconductors. It is found that charge trapping is causing the frequently observed unipolarity in organic semiconductors, causing preferential conduction of either holes or electrons. This limits the efficiencies and stabilities of the organic optoelectronic devices. By investigating charge trapping in a wide range of organic semiconductors, we have identified that when the electron affinity is lower than 3.6 eV, electron transport becomes trap limited and when ionization energy is higher than 6 eV hole transport becomes trap limited. As a result, within this energy window of about 2.4 eV trap-free charge transport is observed. Combining this energy window for trap-free transport with our developed charge injection strategy, an efficient and stable single layer OLED based on a neat thermally activated delayed fluorescence emitter is demonstrated. The OLED has a maximum external quantum efficiency of 19% at a luminance of $500 $ cdm$^{-2}$and a lifetime to 50% of initial luminance of $1000$ cdm$^{-2}$ of 1,880 h. It has an exceptionally low operating voltage of 2.9 V at a luminance of $10,000$ cdm$^{-2}$, which resulted in a maximum power efficiency of $ 87 $ lmW$^{-1} $.