Investigations Of Carrier Transport And Exciton Dynamics In Doped Disordered Organic Semiconcuctors

Doped disordered organic semiconcuctors have been widely used in organic opto-electronic devices.Therefore,studies on the charge transport and exciton dynamics in these materials,which are vital for understanding the device performance,are of great importance.Based on theoretical and experimental methods,this dissertation investiogates the impact of dopping on charge transport in disordered organic semiconcuctors.Through exciton dynamics studies,this dissertation also investigates the efficiency roll-off and lifetime of the thermally activated delayed fluorescence?TADF?sensitized luminescence.The main results of this dissertation are as follows:1.The impact of traps on charge transport in disordered organic semiconcuctors is studied through time-of-flight experiments and Monte Carlo?MC?simulations.Both the experiments and MC simulations show that the shallow traps greatly decrease the mobility,whereas deep traps mainly decrease the effective carrier density without significantly decreasing the mobility.Studies based on the Gaussian disorder model indicate that shallow traps increase the effective energetic disorder greatly,while deep traps hardly change it.Based on the concept that carrier occupying state closer to the transport energy has larger probability to hopping to the transport energy,a competitive hopping model is carrier out to interpreting the trap effect in disordered organic semiconcuctors.The theoretical results of this model well fit the temperature,carrier density and trap depth dependences of the carrier mobility.2.Carrier mobility of tris-?8-hydroxyquinoline?aluminum?Alq₃?:4,4'-bis?carbazol-9-yl?-biphenyl?CBP?co-vaparated films are investigated.As the CBP dopping ratio increases,the carrier mobility decreases at low CBP dopping ratio owing to the scattering effect,and increases with CBP dopping ratio following a percolation model when CBP dopping ratio is larger than 10 vol.%.The percolation thresholds are 9 vol.% and 8.3 vol.%,respectively for the electron and hole transporting.Possible microcosmic transporting mechanism is established with a model combining charge transfer integral calculation with bond percolation.Through this model,the experimental percolation thresholds in the co-vaparated films can be well predicated.3.Based on exciton dynamics study of the host-guest organic light-emitting diodes,we demonstrate that TADF sensitized luminescence can simultaneously achieve highquantum efficiency,low efficiency roll-off and long device lifetime,given small singlet-triplet splitting(?E_ST)of the host and high host-guest energy transfer rate.Efficiency roll-off of the TADF devices is always serious because small ?E_ST is always followed by small radiative rate owing to the Fermi's golden rule,so that low triplet density is hard to be achived.By exploiting the much faster host-guest energy transfer,devices with TADF sensitized luminescence can realize low triplet density and reduce the efficiency loss under high brightness.The low triplet density can also reduce the density of defects generated by triplet-polaron annilation,hence,suppressing the intrinsic luminescence loss and increasing the lifetime.