Film Formation Mechanism Of Inkjet-Printing For Electroluminescent Devices

Inkjet printing is one of the most promising technologies to obtain high-resolution,large-scale and low-cost electroluminescent devices,owning to the advantages of high material utilization,mask-free patentability and roll-to-roll compatibility.Thus,inkjet printing technology has attracted extensive attention in academic and industrial communities.However,due to the similar solubility of organic light-emitting and carrier-transporting molecules,the bottom layers will be redissoved in the subsequential printing process,resulting in a non-uniform interface between different functional layers,which will deteriorate the light-emitting,carrier-injecting and transporting abilities in electroluminescent devices.Besides,coffee ring effect will deteriorate the homogeneous morphology of inkjet-printed films,inducing the non-uniform light emission of final electroluminescent displays.In this thesis,a cross-linked hole-transporting layer is realized by inkjet-printing to solve the re-dissolution problem,functional materials and solvents are modulated to adjust the ink rheology,and processing parameters are controlled to investigate the film formation mechanism during inkjet-printing processes.Finally,organic and perovskite based light-emitting diodes（OLEDs or PeLEDs）are obtained by inkjet-printing.The specific research content are as follows:（1）A cross-linkable hole-transporting layer for inkjet-printing OLEDs.In multilayer inkjet-printing process,the re-dissolution problem leads to the destruction of the underlying films and is not suitable for large-area homogeneous films.In this section,a hole-transporting ink comprised of poly（N-vinylcarbazole）（PVK）and oxidative coupling agent phosphomolybdic acid（PMA）is developed,which can be cross-linked at low temperature instantly and obtain an excellent solvent resistance ability.To further suppress the coffee ring phenomenon,high boiling point solvents are introduced into the hole-transporting ink,such as o-dichlorobenzene,butyl benzoate,cyclohexylbenzene and 1-chloronaphthalene.The effect of high boiling point solvents on ink rheology,ink stability,and film formation process is systematically studied,and the corresponding film formation mechanism is proposed.Based on the cross-linked hole-transporting layer,a light-emitting layer with homogeneous morphology is inkjet-printed,and the corresponding OLED obtains a current efficiency of 21.7 cd/A.（2）A polymer-organic small molecular trinary-host system for high-efficient OLEDs.In order to improve the device performance of inkjet-printed OLEDs,a trinary-host system of PVK,hole-transporting material of Tris（4-carbazoyl-9-ylphenyl）amine（TCTA）,and various electron-transporting molecules is proposed to promote the hole-electron carrier balance situation in OLEDs.Whereas,PVK is used as the viscosity modifier,and the electron-transporting materials include 1,3,5-Tris（1-phenyl-1H-benzimidazol-2-yl）benzene（TPBi）,2,7-Bis（diphenylphosphinyl）-9,9spirobifluorene（SPPO13）,2,2′-（1,3-phenyl）bis[5-（4-tert-butylphenyl）-1,3,4-oxadiazole]（OXD-7）,and 2,6-bis（（9H-carbazol-9-yl）-3,1-phenylene）pyridine（26DCz PPy）.We study the effect of different electron-transporting materials on ink rheology,film morphology,carrier transport ability,and device efficiencies of OLEDs systematically.The results show that the PVK:TCTA:TPBi,PVK:TCTA:SPPO13,and PVK:TCTA:OXD-7 trinary-host systems can form exciplexes with lower triplet energy,which decrease the host-guest energy transfer efficiency.The PVK:TCTA:26DCz PPy trinary-host system has uniform film morphology and an efficient energy transfer process,leading to the highest luminous efficiency of 88.1 cd/A and 45.1 lm/W.（3）A stable polymer-perovskite composite ink for inkjet-printing PeLEDs.Differently from the conventional spin-coating process,inkjet-printed perovskite films take longer time for crystallization and film formation,inducing a larger crystalline grain size and a rougher film surface morphology,which is not favorable for the device performance of PeLEDs.Here,fluoropolymers,including polyvinylidene fluoride（PVDF）and polyvinylidene fluoride-co-hexafluoropropylene（PVDF-HFP）,are chosen to modulate the perovskite precursor ink of formamidinium lead bromide（FAPb Br₃）,which can adjust the ink viscosity,control the crystalline grain size and suppress coffee ring formation.Additionally,the strong hydrophobicity of the fluoropolymers improves the stability of perovskite films.Finally,inkjet-printed fluoropolymer-perovskite films are used for information encryption-decryption and PeLEDs,demonstrating a luminance of 3356 cd/m² and a current efficiency of 7.35 cd/A.