Red Halide Perovskite Quantum Dot Light-Emitting Diodes

In recent years,metal halide perovskite quantum dots（PQDs）have attracted much attention in the field of optoelectronics due to their unique properties,such as high charge carrier mobility,high defect tolerance,high photoluminescence quantum yield（PLQY）,narrow full width at half maximum（FWHM）and easily tunable bandgap.Additionally,their solution processability,flexibility,and cost-effectiveness make them highly appealing to the industry.PQD-based light-emitting diodes（LEDs）with PQDs as the active layer have broad prospects in applications such as lighting and displays.Over the past decade,there have been significant advancements in the external quantum efficiency（EQE）of PQD LEDs in various emission colors.However,there are still some challenges to be addressed.At the initial stage of this paper’s research,the field faced challenges faced issues such as low device efficiency,low device luminance,spectral instability due to ion migration,and incomplete understanding of the quantum dot-ligand interface interaction mechanisms.In order to address the aforementioned issues,we have optimized the charge transfer through device structure design;regulated the optoelectronic properties of PQDs by investigating the mechanisms of surface ligand interactions;and prepared PQD thin films with excellent optoelectronic performance by optimizing the surface state of PQDs.As a result,efficient,bright,and spectrally stable red PQD LEDs are successfully fabricated.（1）Establishing an efficient device structure is a prerequisite for realizing high-performance PQD LEDs,which requires efficient carrier injection and balanced carrier transport within the device.For the commonly used inverted device structure（ITO/Zn O/PEI/PQDs/TCTA/Mo O₃/Au）,the low hole mobility of the hole transport layer（HTL）results in imbalanced charge transfer and higher overall operating voltage of the device.This will cause the accumulation of interface carriers,quenching of luminescence,decreased device efficiency,and exacerbate the ion migration issue of PQDs,leading to serious spectral instability.The latter is more severe for mixed halide(Cs Pb Br_xI_1-x)PQD LEDs.Investigation reveals that in this device structure,the voltage at which Cs Pb Br_xI_1-x PQD LEDs exhibit spectral redshift（indicating significant ion migration）is 5.5 V,while the voltage at which the device reaches its maximum luminance is 7.1 V,greatly limiting its applicability.Given these findings,we adjusted the HTL structure by using a bilayer HTL structure of TCTA/TAPC to control the carrier transport.Due to TAPC’s hole mobility being three orders of magnitude higher than that of TCTA,it effectively balances the transport of electrons and holes.Additionally,the device’s turn-on voltage decreases from 2.1 V to 1.7 V,exhibiting a phenomenon of sub-bandgap turn-on（where the turn-on voltage is lower than the EML bandgap energy/e,1.92 V）.This indicates an enhanced hole injection capability.Further investigation reveals that the above phenomenon is attributed to thermally and electrically induced thermionic emission.As a result,after structural optimization,the maximum EQE of the device increased from 6.7%to 11.7%,the maximum luminance rose from 676 cd m^-2 to 4070 cd m^-2,and the voltage required to achieve maximum luminance decreased from 7.1 V to 5.9 V.The lower operating voltage significantly mitigated ion migration under high electric field conditions.（2）Through the optimization of the device structure,we have found that although it can greatly avoid the issue of ion migration under high electric fields,it is still not completely resolved.Therefore,it is necessary to address the fundamental problem causing ion migration,which is the optimization of the PQDs material itself.Here,we used octylphosphonic acid:potassium（OPA:K）for surface modification of Cs Pb Br_xI_1-x PQDs.On one hand,the strong interaction between OPA and Pb enables the removal of surface defects and achieves effective surface passivation of PQDs.On the other hand,the introduction of K⁺enhances its interaction with halogens,improving passivation efficiency and increasing the activation energy for halogen migration,thus enhancing spectral stability.Furthermore,the substitution of long-chain organic ligands（oleic acid and oleamine）on the surface of PQDs with shorter-chain OPA enhances charge transfer capability of the EML.As a result,the LEDs we fabricated achieved a maximum EQE of up to 16.7%and a high luminance of 13730 cd m^-2 at only 4.9 V.At the same time,the spectral stability of the LEDs has also been greatly improved.（3）As mentioned above,ligand modification plays a crucial role in enhancing the optoelectronic properties of PQDs.However,the understanding of surface anchoring sites on PQDs is currently not fully clear,which hinders the ideal design of high-performance PQDs materials and devices.It is commonly believed that the surface anchoring sites on PQDs follow a"one-to-one"fashion,where each functional group corresponds to one anchoring site.However,in this study,we discovered new anchoring sites on the surface of Cs Pb I₃ PQDs using the classic ligand triphenylphosphine（TPP）.Specifically,the P atom of TPP can interact with both Pb and I on the surface of Cs Pb I₃,breaking the conventional understanding of ligand-PQDs surface interactions in a"one-to-one"manner.This simultaneous interaction with Pb and I can significantly suppress the formation of iodine vacancy defects on the PQDs surface and stabilize the structural symmetry of PQDs by reducing surface distortions,thereby enabling superior luminescent performance and material stability.Furthermore,we increased the delocalization characteristics of the PQDs surface using the derivative ligand diphenylphosphinobiphenyl（DPB）,thereby enhancing the carrier transport capability.The change in surface ligands also resulted in a more optimized energy level structure,which facilitated better injection and transport of carriers in the device.Finally,PQD LEDs passivated with DPB achieved a maximum EQE of 22.8%and a peak luminance of 15204 cd m^-2,along with a nearly 35-fold improvement in device lifetime.Exploring ligands with multi-site anchoring functionality also provides new insights into deciphering the molecular image of the dynamic organic-inorganic interface on the surface of PQDs.（4）Although we can increase the PLQY of PQD solutions to nearly 100%through strategies like ligand passivation,the PLQY tends to decrease when the PQDs are assembled into thin films,which is detrimental to the efficiency of devices.Here,we have developed an efficient and convenient surface reconstruction method for PQDs by introducing various small molecule ions（GA⁺,Na⁺and SCN^-）,aiming to suppress the loss of PLQY of PQDs from solution to thin films.Experimental investigations and theoretical calculations demonstrate that these ions exhibit stronger interaction with the surface of PQDs compared to oleylamine and oleic acid.This allows them to passivate defect sites generated during the centrifugation and spin-coating processes.Moreover,the resulting surface protection from this reconstruction approach prevents PQDs from aggregating during centrifugation purification and film formation,thereby maintaining the integrity of their original crystal morphology and effectively suppressing severe FRET in thin film states.Additionally,the uniform particle size distribution allows PQDs to have better orientation during film formation,improving the film’s flatness and charge carrier mobilities.Finally,we successfully prepared PQDs films with a high PLQY of up to 95.1%,leading to a further enhancement of the maximum EQE of PQD LEDs to 24.5%.The innovations of this thesis are as follows:（1）To address the issues of carrier transport imbalance and severe ion migration in inverse-structured LEDs,a dual-hole transport layer structure was utilized.This structure aimed to increase hole injection,balance carrier transport,and reduce device operating voltage.As a result,the device EQE was significantly improved（from 6.7%to 11.7%）,while effectively preventing ion migration under high electric fields.（2）To tackle the problems of low PLQY and low ion migration activation energy in PQDs,surface modification of OPA:K was introduced to passivate surface defects,enhance ion migration activation energy,and increase the maximum EQE of the prepared LEDs to 16.7%,achieving high luminance of 13730 cd m^-2 at only 4.9 V.（3）Addressing the incomplete understanding of the structural relationship between surface ligands and PQDs core,TPP and its derivatives were introduced to explore their interaction with PQD surfaces,revealing new P-I supramolecular interactions and significantly enhancing the optoelectronic performance of PQDs.The prepared PQDs LEDs achieved a maximum EQE of up to 22.8%and a maximum luminance of 15204cd m^-2.（4）To combat the severe loss of PLQY from PQDs solution to film,various small molecule ions were introduced to restructure the surface of PQDs,passivating surface defects,enhancing surface structural stability,suppressing FRET in PQDs films,and improving the orientation and smoothness of the films.The PLQY of the prepared PQDs film reached 95.1%,with the maximum EQE of LEDs further increased to 24.5%.