Light-emitting ﬁeld-effect transistors with EQE over 20% enabled by a dielectric-quantum dots-dielectric sandwich structure

Emerging quantum dots (QDs) based light-emitting ﬁeld-effect transistors (QLEFETs) could generate light emission with high color purity and provide facile route to tune optoelectronic properties at a low fabrication cost. Considerable efforts have been devoted to designing device structure and to understanding the underlying physics, yet the overall performance of QLEFETs remains low due to the charge/exciton loss at the interface and the large band offset of a QD layer with respect to the adjacent carrier transport layers. Here, we report highly efﬁcient QLEFETs with an external quantum efﬁciency (EQE) of over 20% by employing a dielectric-QDs-dielectric (DQD) sandwich structure. Such DQD structure is used to control the carrier behavior by modulating energy band alignment, thus shifting the exciton recombination zone into the emissive layer. Also, enhanced radiative recombination is achieved by preventing the exciton loss due to presence of surface traps and the luminescence quenching induced by interfacial charge transfer. The DQD sandwiched design presents a new concept to improve the electroluminescence performance of QLEFETs, which can be transferred to other material systems and hence can facilitate exploitation of QDs in a new type of optoelectronic devices. (cid:1) 2021 Science China Press. Published by Elsevier B

Quantum dots (QDs) have been successfully integrated into the LEFETs (QLEFETs) and offer advantages of high color purity and tunability, as well as solution-processable fabrication [8,11,[26][27][28][29][30].However, the progress made with QLEFETs is still limited, especially in the blue and green QLEFETs.To date, the QLEFETs with the emission wavelengths below 550 nm show the external quantum efficiencies (EQEs) of only less than 12% [29].In these devices, the interfacial contact is known to have a critical effect on optoelectronic performance, especially for the in-plane topcontact source-drain electrode structures [12,31].When both source and drain electrodes are deposited on the top of the carrier transport layer (CTL), the in-plane field is generated to facilitate the carrier transport from the top layer preferentially.The efficient carrier transport from the CTL to the QD emissive layer (EML) is prevented by large band offset or poor interfacial contact.Therefore, a high source-drain voltage is imperative to realize efficient carrier injection [32][33][34][35][36][37][38][39][40], however it leads to the reduced device lifetime and possible increase in energy consumption.
Here, we report a solution processable high-performance green QLEFETs based on dielectric-EML-dielectric sandwich structure, namely, methylammonium bromine (MABr)-QDspolyethylenimine ethoxylated (PEIE).In these dielectric-QDsdielectric (DQD) structures, the PEIE thin layer on the QDs is used to create a favourable valence band offset and to improve the interfacial contact between the QDs and the hole transport layer (HTL), which makes the exciton recombination zone shift into the QDs emissive layer.The MABr thin layer below the QD layer is introduced to enhance the electron transport by passivating the surface defects on ZnO nanoparticles electron transport layer (ETL) and to suppress the luminescence quenching of the QDs layer induced by interfacial charge transfer.We perform a comprehensive evaluation of the performance of these QLEFET devices and support our findings by the density functional theory (DFT) studies, and demonstrate that the device concept proposed here enables to produce structures with favourable energy level alignment and an improved interfacial contact.The green QLEFETs with the DQD structure developed here demonstrate a record-high current efficiency (CE) of 93.6 cd A À1 corresponding to an EQE of 21%.

Synthesis of ZnO nanoparticles solution
A low-temperature solution-precipitation method was used to synthesize ZnO nanoparticles [41].Briefly, 3 mmol anhydrous zinc acetate was mixed with 30 mL DMSO.5.5 mmol tetramethylammonium hydroxide dissolved in 10 mL ethanol was added drop-wise into the zinc acetate solution, stirred for 1 h at room temperature.The formed ZnO nanoparticles were collected by centrifugation, washed with ethyl acetate twice and re-dispersed in ethanol.The solutions were filtered before use.

Device fabrication
The indium tin oxide (ITO)-coated glass substrates were cleaned in an ultrasonic bath consecutively using detergent, deionized water, acetone and isopropanol, for 30 min each.Then, a layer of insulating Al 2 O 3 (150 nm) was deposited at 200 °C by atomic layer deposition (ALD) using alternating exposure to Al(CH 3 ) 3 and H 2 O vapor at a deposition rate of 0.66 Å per cycle.Subsequently, the coated substrates were transferred into an N 2 -filled glove box for spin-coating of ZnO nanoparticles, MABr, QDs, PEIE and PEDOT: PSS.The ZnO nanoparticles solution (30 mg mL À1 ) was spin coated at 2000 r min À1 for 40 s and annealed at 150 °C for 30 min.The MABr dissolved in DMSO (0.3, 0.5 and 1.0 mg mL À1 ) was spin coated onto ZnO layer and annealed at 100 °C for 10 min.The QD layer was prepared by spin coating the QD solution (18 mg mL À1 ) onto MABr at 2000 r min À1 for 40 s and annealing at 90 °C for 20 min.The PEIE (diluted with 2-ethoxyethanol to 0.8% w/v) was spin coated onto QDs and annealed at 120 °C for 20 min.PEDOT:PSS solution was spin-coated onto PEIE at 4000 r min À1 and annealed at 130 °C for 20 min.Finally, the Aluminum (Al) layer (120 nm) was deposited through a shadow mask by thermal evaporation under a based vacuum (% 10 À4 Pa) to form the source and drain electrodes with the channel width of 1000 lm and channel length of 50 lm.

Characterization
The I-V characteristics of QLEFETs were acquired by a semiconductor parameter analyzer (Keithley 4200) with a probe station (LakeShore, TTP4).A photomultiplier tube, PMT, R928 (Hamamatsu) was placed over the devices and used for simultaneous light-intensity measurements.The photomultiplier is calibrated by an optical integrating sphere [42].The EL spectra were acquired using Ocean Optics spectrometer.The luminance was obtained by combining the photocurrent collected from the drain electrode with that of QD light-emitting diodes (QLEDs) using the following equation: where L is the luminance, S is the emission area, and I P is the photocurrent of the devices, respectively.The current density was calculated as the channel current divided by its area.The EQE was calculated from the luminance, the drainsource current (I DS ), and the EL emission spectrum.The carrier mobilities were calculated using the formula for the linear regime: where l is the field-effect mobility, C i is the gate dielectric capacitance density and W and L are the channel width (1000 lm) and length (50 lm), respectively.The crosssectional transmission electron microscopy (TEM) was performed on a dual-beam focused ion beam with scanning electron microscopy (SEM) and Omniprobe AutoProbe 200.2 robotic hand.The X-ray photoelectron spectroscopy (XPS) data were collected using an ESCALAB 250Xi (Thermo) system.The work functions of the samples were determined by ultraviolet photoelectron spectroscopy (UPS, ESCALAB 250Xi).Absorption spectra were obtained using a PerkinElmer Lambda 950 UV-vis-NIR spectrometer.The photoluminescence (PL) spectra and time-resolved PL (TRPL) were measured using an Edinburgh FLS920 spectrometer.The surface morphology of the films was measured using atomic force microscope (AFM, Seiko SPA 400) and field emission scanning electron microscopy (JSM-6701F).All measurements were performed at room temperature under ambient conditions.

DFT simulations
DFT calculations were performed using the Vienna Ab initio Simulations Package (VASP) [43,44] and projected augmented wave (PAW) method [45][46][47].The exchange-correlation interaction was treated with generalized gradient approximation (GGA) in the Perdew, Burke and Ernzerhof (PBE) [48] parameterization.For strongly localized d electrons, GGA has systematic and noncancelling errors [49].Therefore, we adopted GGA + U (U = 8 and J = 1) [50,51] to correct for the self-interaction and over delocalized d states.The cut-off energy of the plane-wave basis was set to 450 eV, and integrations over the first Brillouin zone were calculated using a Gamma-centred k-point set of 6 Â 6 Â 4. With these settings, the total energy converged within 1 meV/atom.The wurtzite ZnO (space group P6 3 mc, No. 186) structure was fully relaxed using GGA + U; the energy was converged within 10 À6 eV cell À1 and the force was converged to less than 10 À3 eV Å À1 .A vacuum spacing no less than 15 Å was used in all slab calculations with the same cut-off energy and similar k-point density to that used for bulk calculations.The (1 0 0) surface was modelled by a sixlayer slab consisting of 144 atoms.The bottom three layers were fixed during relaxation.A Schottky defect was created on the top layer.The adsorption energies of MABr on the perfect surface and defective surface were calculated to be 0.9 and 2.1 eV, respectively.

Device structure and performance
Multilayer heterojunction structures were deposited onto ITO-coated glass by layer-by-layer solution processing.Fig. 1a schematically displays our top-contact QLEFETs with stacked trilayer of ETL, EML, and HTL.In this device, the CdSe/ZnS QDs layer, serving as EML, is sandwiched between dielectric layers of PEIE and MABr to form the DQD structure.The PEDOT:PSS and ZnO are used as the HTL and the ETL, respectively.Al source and the drain electrodes are deposited onto the PEDOT:PSS layer.The channel width and length are 1000 and 50 lm, respectively (see a photograph and an optical microscopy image in Fig. S1 online).As revealed by a cross-sectional SEM image, the functional layers are well defined and compact even after multiple solution processing steps (Fig. 1b).The device architecture and composition were further assessed by energy dispersive spectroscopy (EDS) (Fig. 1c) and high-resolution TEM (Fig. 1d), demonstrating that the thicknesses of PEIE, QDs, and MABr layers in the DQD sandwich structure are $5, 25 and $3 nm, respectively.
The EL spectrum of the resulting QLEFETs revealed a narrow peak centered at k EL $ 534 nm with a full-width-at-halfmaximum (FWHM) = 28 nm (Fig. 2a), which indicates a colorsaturated green emission.The EL spectrum is slightly red-shifted and broadened compared with the PL spectrum of the QD solution (k PL $ 530 nm with a FWHM = 21 nm), resulting from the interdot interactions and electric field-induced quantum confinement Stark effect [52].The green emission observed from the operating device with DQD structure is significantly brighter than that from the device without DQD structure (see the inset in Fig. 2a), suggesting an improved carrier injection and enhanced radiative recombination.
The electrical properties of QLEFETs with a QDs-only layer (control device), QDs/PEIE and MABr/QDs/PEIE were investigated in ambient conditions.The gate voltage (V GS ) was swept from À 60 to 60 V with applied drain voltage (V DS ) = 20 V. A characteristic unipolar p-type behavior is observed in the control device (Fig. 2b, purple circles), suggesting a hole-dominated charge transport in this trilayer structure.The field effect transistor (FET) with a PEDOT:PSS-only layer (Fig. S2a online) demonstrates a very similar hole transport characteristics to that of the control device, indicating that the injected holes are predominantly transported across the top p-type CTL (PEDOT:PSS).To further confirm the mechanism of charge transport, the Mott-Schottky curves were measured under dark conditions (Fig. 2c).A negative slope observed in the control device suggests a p-type conductivity characteristic, confirming the results of electrical transfer characteristics.However, there is a significant difference in the QLEFETs with a thin layer of PEIE deposited on QDs, which show a characteristic ambipolar behavior.This suggests the charge transport from HTL to the QD EML, which is also observed in the QDs-only FET (Fig. S2b online).The electrical transfer characteristics under forward/reverse sweep The device performance was assessed by the measurements of luminance, CE and EQE.For the control device at V DS = 20 V, the following values were measured: highest values of luminance of 260 cd m À2 , CE = 0.44 cd A À1 and EQE = 0.1% at the gate voltage of À53 V (Fig. 2d), e and Fig. S5 online).Following deposition of the PEIE layer onto the QDs layer, the device performance was significantly improved, reaching the maximum luminance of 13,320 cd m À2 , CE = 49.0 cd A À1 and EQE = 11%.This enhancement of device performance was attributed to the optimized hole transport in the emissive layer.The gate leakage current of the device modified with PEIE (Fig. S6 online) is smaller than I DS under the same applied V GS , hence it has a negligible impact on the device performance.Therefore, the observed significant improvement of the EL emission is mainly attributed to the enhanced radiative recombination resulting from the efficient injection of holes into the EML.The efficient hole injection could be ascribed to two main reasons: the improved QDs/PEDOT:PSS interfacial contact and the reduced energy barrier between the QDs and PEDOT:PSS in the presence of the PEIE interlayer.The detailed analysis of the interface and the energy band realignment before and after inserting PEIE is discussed below.
In the presence of a thin layer of MABr, the electron transport is further improved due to the enhancement of radiation recombination rate in the QDs layer.The devices with PEIE and MABr layers exhibit both the negative and the positive slopes in the Mott-Schottky plots, characteristic of an ambipolar behaviour with the hole and electron field-effect mobilities of 8.38 Â 10 À4 cm 2 V À1 s À1 at V DS = 20 V, V GS = À25 V and 7.82 Â 10 À4 cm 2 V À1 s À1 at V DS = 20 V, V GS = 25 V, respectively.With the incorporation of MABr layer deposited on ZnO, the best EL performance is observed for the QLEFETs with the DQD structure (see Figs. S7, S8 online), with the maximum luminance of 12,240 cd m À2 , EQE = 21% and CE = 93.6 cd A À1 .The devices also show good reproducibility with an average EQE of 19% (Fig. 2f).

DQD structure characteristics
To gain further insights into the contribution of the DQD structure in improving the QLEFETs performance, the role of PEIE and MABr layers was studied.AFM, SEM, and UPS measurements were carried out to investigate the surface morphology and the energy levels of the QDs and QDs/PEIE films.The interface morphology and alignment of energy levels are the critical parameters defining the carrier transport.It is also noteworthy that surface defects on the nanoparticles can act as charge traps leading to non-radiative recombination.The QD layer in these devices is relatively thin ($25 nm), and hence pinholes can be formed easily during solution-based processing and the deposition onto the ZnO film with a relatively high surface roughness [53].The pinholes in the QDs layer may lead to the direct contact between the HTL and the ETL, having a detrimental effect on the device performance.The AFM images (Fig. 3a-c) revealed the maximum height amplitude in the ZnO underlayer and the QDs layer of $ 10 and $ 13 nm, respectively.When the PEIE thin layer is introduced between QDs and PEDOT:PSS, a decrease of the height amplitude of QD film to 3 nm.This corresponds to a decrease of the root mean square (r.m.s.) value of the surface roughness from 1.25 to 0.42 nm, comfirming the formation of a smooth film surface without pinholes.Note that the thickness of PEIE is reduced when the PEDOT:PSS is deposited on top of it due to the part dissolution of PEIE surface caused by the solvent used.The improved interfacial quality is consistent with the SEM results (Fig. S9 online).These results confirm that the coating with PEIE could improve the interface morphology, hence preventing the carrier loss at the contact of the CTLs.In addition, the PEIE helps to modify the electronic structure of the QDs (Fig. 3f) resulting from the formation of interface dipoles by taking advantage of its aliphatic amine groups with large dipole moment [54,55].The UPS data for the QD films (Fig. 3d, e) and Fig. S10 online) revealed a decreased work function (from 4.1 to 3.9 eV) and a shift of the valance band maximum (VBM) from 6.0 to 5.8 eV, which reduces the energy barrier at the QDs/PEDOT:PSS interface and thus enhances hole injection capability.This optimized band energy alignment is of great The optical properties of MABr/QDs films were investigated to understand the role of MABr in QLEFETs.As shown in Fig. 4a, the PL emission intensity of the QD films is significantly decreased when being in contact with the conductive ZnO layer.The observed decrease of PL intensity is attributed to charge transfer at the interface of ZnO/QDs; while the PL quenching is effectively suppressed when a thin layer of MABr is inserted.It is known that ZnO nanoparticles with high surface-to-volume ratios introduces numerous surface defects, which can act as electron trapping and charge recombination sites [56,57].The ZnO/MABr/QDs film displays a longer PL lifetime than that of ZnO/QDs film (Fig. 4b and Table S1 online), indicating the reduced interfacial non-radiative  recombination.DFT calculations reveal that Schottky defects are easily formed on the surface of ZnO.The adsorption energies for MABr on the perfect surface and defect-rich surface are calculated to be 0.9 and 2.1 eV, respectively (see Experimental).As shown in Fig. 4c (iii and iv), the positions of atoms and bond angles on the defect-rich surface with adsorbed MABr are comparable to those of the perfect surface without MABr.This chemical bonding between the MABr and a Shottky defect significantly reduces the charge-carrier trapping induced by oxygen vacancies [57].The coordination between Zn 2+ and Br À has also been experimentally confirmed by XPS results (Fig. 4d), where the photoemission of Zn 2p 1/2 and Zn 2p 3/2 in the ZnO/MABr film are both shifted to higher binding energies by around 0.3 eV in comparison with that of ZnO film, demonstrating that Br À is bound to Zn 2+ .
To demonstrate the trap states intuitively, the subthreshold swing (SS) without and with MABr modification were obtained to be 5.2 and 1.1 V dec À1 , respectively, through the output characteristics at the V DS voltage of 20 V (Note S1 in the Supplementary materials).We calculated the interface trap density (D IT ) with the equation, where C i , K B , T, and q are the dielectric capacitance, Boltzmann constant, temperature, electron charge, respectively.The D IT for QLEFET without and with MABr layer is calculated to be 3.2 Â 10 13 and 6.3 Â 10 12 cm À2 eV À1 , respectively.We further performed the trap-filled space-charge-limited current studies to quantify the trap density in ZnO layer without and with the MABr interlayer (Fig. 4e).The current density increases linearly in the ohmic regime.The nonlinear regimes are correlated with a trap-filled limited and space charge limited current according to k (J / V k ) [58][59][60].The trap-state density (n trap ) can be determined by the equation: , where e is the relative dielectric constant of ZnO, e 0 is the permittivity of free-space, V TFL is trap-filled limiting voltage, e is the elementary charge, and L is the thickness of the ZnO film.It is found that the MABr interlayer reduces the trap density from 2.1 Â 10 17 cm À3 for ZnO film to 8.5 Â 10 16 cm À3 for ZnO/MABr film, which thus results in a higher electron current in the electron-only device with ZnO/MABr structure.

QLEDs with a DQD structure
To understand the gating effect on the luminescence emission, the comparison of the device performance parameters of the QLE-FETs and QLEDs was conducted.The QLEDs with the same structure and composition of the active layers were fabricated (Fig. 5a), in which the layer sequence, thickness and growth parameters are the same as those in the QLEFETs.The energy levels of the ZnO films are calculated from the UPS and optical studies (Fig. S12 online).Since the work functions of Al and ITO are similar, the transport behaviors of carriers in the QLEDs can be compared to those in the QLEFETs.The EL spectrum of QLEDs revealed a peak at k EL $ 534 nm with FWHM = 30 nm (Fig. 5b), similar to that of the QLEFETs (Fig. 2).The QLEDs with DQD structure demonstrate a characteristic I-V LED behavior, showing a maximum brightness of 12,400 cd m À2 , CE = 8.5 cd A À1 and EQE = 1.9% (Fig. 5c, d).Note that the relatively low performance for the QLED is mainly due to a poor charge balance in this device in the absence of a HTL (i.e., poly [bis (4-phenyl) (4-butylphenyl) amine] (poly-TPD), poly (9-vinyl carbazole) (PVK) and poly(9,9-dioctylfluorene-alt-N-(4-secbutyl phenyl)-diphenylamine) (TFB), etc.).We envisage that the superior performance in QLEFETs, including luminance and EQE, results from the effective electron injection from ZnO into the QD layer induced by the gating effect.As the V GS increases, the electrons transport from ZnO into QDs will be promoted, which favors the radiative recombination of excitons and the subsequent light emission.The luminance and EQE under different negative gate voltages were also investigated and support this explanation (Fig. S13 online).Both the luminance and EQE consistently increase with the increase of the negative gate voltage, which demonstrates that the gating effect greatly improves the electron transport from ZnO to QDs.As a result, an EL generating device with one-order of magnitude higher efficiency is realized in a DQD structure-based QLE-FETs, owing to the improved carrier transport balance.

Conclusion
In summary, we have demonstrated a high-performance QLE-FET with an EQE up to 21% by employing a concept of DQD structure, PEIE-QDs-MABr.The PEIE with a high dipole moment modifies the interface of the QDs and PEDOT:PSS effectively, resulting in a smooth interfacial morphology and a better energy alignment between the QDs and PEDOT:PSS.These enable the exciton recombination zone to move to the interior of QDs emissive layer, consequently leading to a significant improvement of the device performance.In these devices, the thin layer of MABr acts to reduce the electron trapping on the ZnO surface, which increases the electron density in QD layer and suppresses the PL quenching of emissive layer.Thus, we are able to achieve the green QLEFETs with highest efficiency reported to date.Our work paves the way for the development of high-performance QLEFETs and their future applications.

Fig. 1 .
Fig. 1. (Color online) Device structure of QLEFETs with an emission layer of CdSe/ZnS QDs.(a) Schematic diagram of the device structure, (b) cross-sectional SEM image and (c) EDS compositional mapping images of a QLEFET with a DQD sandwiched structure.The scale bars are 100 nm.The thicknesses of Al 2 O 3 , ZnO, and PEDOT:PSS layer are $150, 80 and 60 nm, respectively.(d) Representative high-resolution TEM images of cross-sections of (left) QDs/PEIE/PEDOT:PSS and (right) ZnO/MABr/QDs heterostructures.

Fig. 2 .
Fig. 2. (Color online) Performance characterization of QLEFETs.(a) Normalized PL and EL spectra of QLEFET with a DQD sandwich structure.Inset: the photograph of the operating devices with (top) and without (bottom) the DQD structure.The emitting area is 1 mm Â 0.05 mm at V GS = À50 V.The scale bar is 400 lm.(b) Electrical transfer characteristics of the QLEFETs.(c) Mott-Schottky plots of the control and QLEFETs with DQD structure.(d) Luminance (L)-V GS and (e) EQE-V GS of the QLEFETs.(f) Histogram of the maximum EQEs measured on 20 devices.

Fig. 3 .
Fig. 3. (Color online) Assessment of morphology and energy level alignment of the QD films.AFM images and a representative z-profile of (a) ZnO, (b) QDs and (c) QDs/PEIE films.UPS spectra in (d) secondary electron region and (e) frontier electronic structure region of the QDs and the QDs/PEIE films.(f) Schematic diagram of energy level alignment in the PEIE modified QD films, where E vac is the vacuum energy.

Fig. 5 .
Fig. 5. (Color online) Device structure and performance characterization of QLEDs.(a) Energy level diagram of the QLEDs.(b) Normalized EL and PL spectra, (c) J-V-L characteristics and (d) CE-J-EQE characteristics of QLEDs.Inset in (b): the photograph of the operating device with an emitting area of 2 mm Â 2 mm at applied voltage of 7.5 V.