Study On Blue Polymer Light-emitting Diodes And Charge Carriers Balancing

Organic light-emitting diodes?OLED?have been generally recognized as the next-generation in display and lighting applications due to its advantages of self-luminescence,high contrast,wide viewing angle,low power consumption,fast response,flexibility and large area processing.Blue emitters restrict the industrial production because of their low efficiency and short lifetime.Based on a series of polymer PPF-SO,this thesis focuses on optimizing the efficiency and lifetime of blue-OLED by charge carriers balanceing in OLED heterojunctions structure.To solve the problem of energy level matching and carrier balanceing in blue OLED,the device structure has been optimized.As the result,blue OLED with better efficiency and longer lifetime may be fabricated.The research focuses on two aspects:One approach is to construct multi-layer device structure by adding the specific functional layers for control carrier injection and transport.And intermixing issue of layers which is caused by the similar solubility between organic materials,also can be solved.Another way is to change the anode work function by doping in PEDOT:PSS for achieving energy level matching and carrier balance.In the research of construction of multi-layer device structure of printing OLED,one approach is to construct orthogonal solvent systems by selecting appropriate functional layer materials and solvents.1,4-dioxane is a bad solvent for the hole transport material PFO-TF,but a good solvent for the emitter of PPF-SO15.PFO-TF can be introduced in the devices as the hole transport layer?HTL?by the solubility differences between adjacent layers for specific solvents.The devices based on PPF-SO15 emitter with PFO-TF as hole transport layer can achieve a peak current efficiency(LE_max)increasement from 3.51 cd/A to 5.06 cd/A and the half-life increasement from 1 h to 4 h due to good hole transport characteristics,energy levels matching and solution machinability of the HTL PFO-TF.The second way is to employed two hole-transport layers consisting of a crosslinkable poly?indenofluorene-co-triphenylamine?copolymer?X-IFTPA?and an additional poly?9-vinylcarbazole??PVK?layer.The crosslinkable X-IFTPA was used as the bottom HTL due to insolubility after thermal treatment.A thin layer of PVK is deposited on the top of the pre-fabricated X-IFTPA layer and p-xylenethe is a bad solvent for PVK,but a good solvent for the emitter of PPF-SO20.As a result,both X-IFTPA and PVK are robust enough to resist the processing solvent and can be used to fabricate multi-layer OLEDs.The combination of such two HTLs leads to a cascade-like hole-injection from the anode to the emitter and improves the balance of charge carriers,which significantly enhance the maximum luminous efficiency to7.67 cd/A and the maximum external quantum efficiency(EQE_max)to 6.04%.And the devices based on such two-layer HTL exhibitedexhibited remarkable stability,with luminous efficiency remained about 6.6 cd/A at a luminance of 10000 cd/m².In the research of doping of PEDOT:PSS,high work function hole injection layer can be available by modifying the PEDOT:PSS with a perfluorinated ionomer?PFI?.The modified hole-injection layer,which increases the work function of ITO anode to 5.9 eV by optimizing the doping amount of PFI,can well match the deep HOMO of PPF-SO25.With such a HIL,highly efficient hole-transport-layer-free PPF-SO25 devices could be obtained,exhibiting a LEmax over 13 cd/A,EQEmax of 8.51%and the half-life of 16 h.