Interfacial Charge Carrier Recombination At Organic Semiconductor Heterojunctions

Because of their advantageous features,such as flexible,in next generation of flat-panel displays,solid-state lighting and renewable energy,organic semiconductor devices are now under intensive research and development by researchers from both academic institutions and industries worldwide.Although organic semiconductor devices currently have progressed significant in terms of efficiency and have been used in many high-tech electronic products such as smart phones,the physical mechanisms in the operation of these devices remain unclear.Organic semiconductor devices are composed of stacked organic thin films.Recombination of electrons and holes at organic/organic heterojunction interfaces is the key to the efficiency and stability of devices.In this thesis,many organic heterojunction devices are fabricated,and the mechanisms of electron and hole recombination at these organic/organic heterojunction interfaces are systematically studied.The research findings include:(1)Organic rectifying diodes with high rectifying ratio and tunable characteristics are fabricated based on non-radiative charge transfer(CT)exciton recombination at organic/organic heterojunction interfaces.(2)The rate for non-radiative CT exciton recombination is proved both in theory and experiment to follow an exponential relationship with not only the energy difference between the HOMO of donor and the LUMO of acceptor but also the donor-acceptor separation distance.(3)The mechanism for Auger recombination at organic/organic heterojunction interface is studied,and ultralow-voltage organic light-emitting diodes(OLEDs)with working voltages below the HOMO-LUMO energy gap of emitters(i.e.,the output photon energies are higher than the input electrical energies)are fabricated based on interfacial Auger-electron injections.(4)Two competing excitonic processes,i.e.,non-radiative Auger recombination and radiative exciplex emission,are discovered to coexist at an organic/organic heterojunction.These two excitonic processes are also found to be modulated by the relative concentration of electrons and holes accumulated at heterojunction interfaces:the Auger recombination is dominant when a heterojunction is electron rich;the exciplex emission is dominant when a heterojunction is hole rich;both Auger recombination and exciplex emission are present when a heterojunction is charge balanced.The work reported in this thesis not only provides a strong theoretical guidance to design and fabricate efficient OLEDs and organic photovoltaics,but also provides new ideas to develop novel organic devices,such as organic rectifying diodes,ultralow-voltage OLEDs.