Tuning The Quasi-2D Perovskite Films And Interface In Perovskite Light Emitting Diodes

Perovskites are regarded as one of the promising luminescent materials owing to their low-cost,tunable emission wavelength,and high defects tolerance.The past several years have witnessed the rapid progress of perovskite light emitting diodes(PeLEDs),however,in contrast to the well-established organic and quantum dots LEDs technologies,the PeLEDs still lag behind.The non-radiative recombination loss,low light outcoupling efficiency,and poor stability of perovskite films are the main limitations that affects the performance of PeLEDs.According to the aforementioned issues,in this thesis,we mainly investigate to enhance the stability of blue perovskite films,suppress the trap states,balance the charge injection,and reduce the Joule heating to improve the efficiency and stability of PeLEDs.Firstly,to overcome the spectrally instable blue PeLED counterparts,we introduced a Cs4PbBr6 phase to stabilize the Ruddlesden-Popper(R-P)perovskite phase and a diammonium salt to form stable Dion-Jacobson(D-J)perovskite films to achieve spectrally stable blue perovskite LEDs.Secondly,additives with different molecular structures were incorporated into perovskite precursor to improve the crystallization process of perovskite films to reduce the trap states,which enables an efficient near-infrared PeLEDs with external quantum efficiency(EQE)over 20%.Thirdly,through an interfacial engineering for hole transport layers,we improve their hydrophilic properties,which ensures us to deposit high-quality perovskite films upon them.A balanced charge injection promotes an efficient green PeLED with EQE of 14.4%.Finally,by substituting the organic compound with an inorganic one,the Joule heating in a working device can be reduced,which improves the operational stability of PeLEDs.The main works are listed as below:1.Tuning quasi-two dimensional components to enhance the stability of blue perovskite films.Through incorporating excess CsBr in the common R-P perovskite films with formula of PEA2Csn-1PbnBr3n+1,non-luminescent phase of Cs4PbBr6 was firstly formed.This Cs4PbBr6 phase is sensitive to humidity,whereas,the Cs4PbBr6 was decomposed and recrystallized with PEABr to form low dimensional perovskite.Due to the coexistence of Cs4PbBr6 and low-dimensional phase in the films,the further growth of low dimensional phase under thermal and electrical stress can be suppressed,which promotes spectrally stable blue perovskite films as well as PeLEDs.2.Incorporating diammonium salt to prepare D-J low-dimensional perovskite films.The nanoplateletes in R-P perovskites were usually linked by a weak van der Waals force,which results in poor stability and electroluminescence spectra shift during operation.In contrast,the nanoplateletes in D-J perovskite based on diammonium salt were linked by a stronger hydrogen binding,which lead to enhanced stability of the resultant films.As a result,PeLED devices exhibit outstanding spectral stability during the operation.3.We revealed the effects of molecular structure of additives on the crystallization process and quality of the perovskite films.Additives have possibility to reduce the defects of perovskite films and enhance the performance of resultant PeLEDs.However,the quality of perovskite films is highly related with the molecular structure of additives.An additive with two functional sites can strongly couple with lead atoms,which retards the formation process of perovskite films and hence that reduces the defects in the resulting perovskite films.In comparison,more defects exist in the perovskite films with additives consisting only one functional site.As a consequence,the PeLED based on additives with two functional sites show much-improved efficiency and operational stability.A near-infrared PeLEDs based on FAPbI3 achieves high EQE of 20.2%.4.Balancing the charge injection in PeLEDs using an interfacial engineering to improve the hydrophilic property of hole transport layer.In a perovskite LED with conversional device structure,more electrons can be injected into device due to the lower hole mobility of poly(N-vinylcarbazole)(PVK)compare to the electron mobility of 2,2?,2?(1,3,5-benzinetriyl)-tris(I?phenyl-1-H-benzimidazole)(TPBi).These over-injected electrons could accumulate at the perovskite interface,quenching the perovskite films and thus deteriorating the device performance.A hydrophilic hole transport layer is essential for depositing a high-quality perovskite film upon it.Otherwise,non-wetting problem of perovskite precursor can occur when depositing on a hydrophobic layer.The wettability of poly[N,N'-bis(4-butylphenyl)-N,N?-bis(phenyl)-benzidine](poly-TPD)was significantly improved with depositing a thin poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene](PFN)layer on it,which balances the charge injection in the LEDs.Meanwhile,due to the unbonded electron of nitrogen atoms in the PFN molecule,the surface defects were also passivated to further reduce the non-radiative recombination loss,leading to an efficient green PeLED with EQE of 14.4%.5.We investigated the effects of Joule heating on the performance of PeLED and engineered the perovskite composition to lower the Joule heating for a working device to improve the stability of PeLEDs.The Joule heating is unavoidable in a working device,which can quench the photoluminescence of perovskite films.Meanwhile,the Joule heating may degrade the film morphology of electron transport layer of TPBi,which accelerates the diffusion of the Al element into the perovskite interface hence that generates more defects in the perovskite film.Through replacing the organic passivation compound with an inorganic one,the charge transport properties of perovskite was improved,less Joule heating effect was observed during operation,leading to a longer operational lifetime.