High‐Performance Cadmium‐Free Blue Quantum Dot Light‐Emitting Devices with Stepwise Double Hole‐Transport Layers

ZnSe/ZnS core/shell quantum dots (QDs) are environmental‐friendly blue light‐emitting material, which can easily achieve deep blue emission upon external excitation. However, its deep valence band (VB) and numerous defect states remain handicap to realize sufficient performance of quantum dot light‐emitting diodes (QLEDs). In this work, high‐performance cadmium‐free ZnSe/ZnS QLEDs by constructing a double organic hole‐transport layer (HTL) to obtain carrier balance are presented. The double HTLs, which consist of poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)diphenylamine) (TFB) and 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT), can suppress the accumulation of electrons between the HTL and the emissive layer (EML), leading to more hole and electron recombination luminescence in QD layer. In addition, the C8‐BTBT layer is conducive to improve the uniformity of QDs film. Thus, the resulting device achieves an external quantum efficiency of 7.23% with TFB/C8‐BTBT double HTLs, which is almost 150% higher than that of traditional devices based on a single hole‐transport layer (4.84%). The authors anticipate that these results can provide a guidance for the optimization of cadmium‐free blue QLEDs.


Introduction
In the past few decades, quantum dot light-emitting diodes (QLEDs) exhibit great prospect in electroluminescent fullcolor displays field, including high color purity, narrow halfmaximum width, and high quantum efficiency. [1][2][3][4][5][6][7][8][9][10] Although the single transport layer, which cannot effectively reduce the height of the hole injection barrier. [13] Therefore, to construct a stepwise double HTL is of great value to modulate charge injection balance by gentling the energy barrier. [34] The double HTL of TFB/PVK was already reported; [35] however, the film formation of PVK depends heavily on the ambient temperature and conditions, which is difficult to obtain smooth surface. [36,37] In addition, the lower conductivity of PVK would lead to low holetransmission capability. [38] Therefore, it is necessary to explore a new double HTL structure to improve QLED performance.
In this work, we introduced 2,7-dioctyl [1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) as an additional hole-transport layer to cooperate with the original TFB hole-transport layer, which formed TFB/C8-BTBT stepwise double HTLs. We demonstrate that the TFB/C8-BTBT stepwise double HTLs can effectively suppress hole accumulation at the QDs/HTL interface, which enhance the carrier-transport capability and passivate the defects at the interface between emissive layer (EML) and HTL. Benefit from TFB/C8-BTBT stepwise double HTLs, the device exhibits maximum external quantum efficiency (EQE) of 7.23%, which is 150% higher than that of the control device (4.84%).

Results and Discussion
First, we chose ZnSe/ZnS quantum dots with suitable emission wavelength as light-emitting material. The absorption spectra and normalized photoluminescence (PL) spectra of ZnSe/ ZnS QDs films are shown in Figure 1a. The inset of Figure 1a shows the photoluminescence of the QDs ink, with the PL peak at 456 nm. For the hole-transport material, C8-BTBT is often used as a p-type active layer material in thin-film transistors (TFTs), [39] due to its high mobility (the highest mobility of TFT devices fabricated using C8-BTBT can reach 43 cm 2 V −1 s −1 , the average mobility is up to 25 cm 2 V −1 s −1 ). [40,41] Moreover, it can be processed by using the solution method. [39] The absorption spectrum of C8-BTBT is shown in Figure 1b, and the inset shows the energy band value of the C8-BTBT calculated based on absorption spectrum. Ultraviolet photoelectron spectroscopy (UPS) spectrum is shown in Figure 1c, from which the lowest unoccupied molecular orbital (LUMO) energy levels of C8-BTBT is calculated to be −2.51 eV and the highest unoccupied molecular orbital (HOMO) energy level to be −5.85 eV. The result shows that C8-BTBT possesses deep valence band energy level and large bandgap. Therefore, it can block electrons from permeating into the hole-transport layer and promote the hole injection. Combining its excellent hole-transport ability and energy level matching with the deep HOMO of ZnSe QDs, C8-BTBT is selected as an additional HTL material cooperating with TFB HTL. We adopted the strategy of layering the two HTLs. The challenge of appending an extra layer in the device structure is to prevent the under layer being damaged and dissolved. Therefore, choosing a suitable solvent to fabricate the C8-BTBT film is crucial. C8-BTBT is a kind of highly crystalline small molecule and has good solubility in benzene solvents. [39] However, the solvent of the TFB layer is chlorobenzene. Hence, selecting benzene or its homologues as the solvent of C8-BTBT may damage the TFB film after the spin-coating process. On the basis of the principle of orthogonal solvent selection, N,Ndimethylformamide (DMF), 1,4-dioxane, and n-octane were selected as the solvent of C8-BTBT, respectively. The concentration of each solution was kept the same to guarantee the consistency. Figure 2a-c shows the root-mean-square (RMS) of spin-coated films of C8-BTBT with different solvents. It can be seen that C8-BTBT film with 1,4-dioxane had higher surface roughness than others, showing inhomogeneous morphology. Many particles appeared on C8-BTBT film with DMF, which may be attributed to aggregation of C8-BTBT molecules. In contrast, the C8-BTBT film was uniformly deposited on glass by using n-octane solvent, which had smooth surface roughness of 0.08 nm, as shown in Figure 2c.
To investigate the effect of C8-BTBT with different solvents on the TFB layer, the PL spectra of TFB/C8-BTBT film were measured, as shown in Figure 2d. Obviously, the PL intensity of TFB was significantly reduced under the solvent washing of 1,4-dioxane. However, DMF and n-octane had almost no influence to the PL intensity of TFB. Based on the above tests, n-octane is found to be the most favorable solvent of C8-BTBT.
To further examine the effects of the C8-BTBT layer to HTL and EML interface, we analyze the interface exciton dynamics. The PL spectrum of TFB/C8-BTBT/QDs indicates that the fluorescence intensity of QDs layer was strengthened by the stepwise double HTLs, which can be attributed to the reduction of nonradiative energy, as shown in Figure 2e. The PL spectrum of TFB/QDs film had a supernumerary peak at around 425 nm, which belongs to TFB. While the C8-BTBT layer effectively inhibited the PL of TFB to make more holes and electrons combined in the QDs layer and enhanced its PL intensity. The time-resolved PL (TRPL) spectrum was further measured, as demonstrated in Figure 2f. The average fluorescence lifetime was calculated to be 10.80 ns for TFB/ QDs films and 12.93 ns for TFB/C8-BTBT/QDs films. The increased fluorescence lifetime can be attributed to the effective suppressing of exciton quenching and reduction of the nonradiative energy transfer from QDs to TFB with the help of C8-BTBT layer.
Meanwhile, the roughness of C8-BTBT layers with different thicknesses was also measured to further investigate its influence to devices' performance. Figure 3a shows the root-meansquare (RMS) of C8-BTBT films with different thicknesses deposited on TFB layer, which decreased initially and followed by an increase. The high roughness of C8-BTBT film with thin thickness can be ascribed to the sparse small molecular of C8-BTBT. Then, the minimum RMS with 0.65 nm was obtained at thickness of 15 nm, which indicates the uniform deposition of C8-BTBT. The C8-BTBT deposited on TFB film is even more uniform than TFB-only film, which reduces the roughness of HTL and is conducive to the subsequent deposition of QDs layer. The roughness increased again when the thickness exceeded 15 nm, which may be due to the stacking of small molecules. The morphology investigation illustrates that proper thickness of C8-BTBT layer can optimize the interfacial contact. Moreover, the additional smooth surface can effectively diminish the leakage current in the device.  Figure 3b, which roughly divided into two stages. In the first stage, the current density is enhanced as the thickness of C8-BTBT layer increases, indicating that more holes were introduced into EML by reducing the hole injection barrier from HTL to EML. When the thickness of C8-BTBT layer reached 15 nm, the current density of the hole-only device was close to the electron-only device, indicating the balance of the transportation of electron and hole. In the second stage with the thickness over 15 nm, the current density gradually decreased that is probably due to the increase of the device series resistance.
In order to confirm the interfacial passivation of C8-BTBT, X-ray photoelectron spectroscopy (XPS) tests were performed on the films of TFB and TFB/C8-BTBT, as shown in Figure 3c. It can be seen that the characteristic peaks of TFB do not shift significantly after adding C8-BTBT film. In addition, the space charge limited current (SCLC) measurement was also performed on the hole-only devices to identify the interface passivation. The trap density of QD films could be calculated by [21] ε ε = 2 trap 0 T FL 2 n V eL (1) where ε 0 is the vacuum permittivity, ε is the relative dielectric constants of ZnSe (about 2.0), [42] L is the thickness of QD films, and e is the elementary charge. And the trap-filling limit voltage (V TFL ) was calculated in Figure 3d. The hole trap density was reduced from 5.77 × 10 20 to 4.71 × 10 20 cm -3 after adding the C8-BTBT film. The additional C8-BTBT layer plays the role of interface passivation that decreases the nonradiative recombination and reduces the defects in the carrier transfer process. In view of the above analysis, the stepwise TFB/C8-BTBT double HTLs are designed to enhance the hole injection with balanced charge injection. The schematic of this process is shown in Figure 3e,f. Because of the low hole mobility and the high energy level of HOMO of TFB layer, electrons are accumulated at the EML/HTL interface in the pristine structure as shown in Figure 3e. Electrons partially pass through the EML into the TFB layer, which forms excitons with holes and leads to recombination in nonradiative form. For the device with double HTLs, C8-BTBT possessed high hole mobility and decreased HTL/EML interface injection barrier, resulting in more holes entering into EML for radiative recombination and the reduction of leakage current.
In order to elucidate the effect of different thicknesses of TFB/C8-BTBT stepwise double HTLs on the device performance, QLEDs consisting of multistory layers were fabricated. Figure 4a shows the schematic of QLEDs device structure with the stepwise double HTLs, consisting of ITO/ PEDOT:PSS/TFB/C8-BTBT/QDs/ZnO:poly(vinylpyrrolidone) (PVP)/Ag. High-resolution transmission electron microscopy (HRTEM) was carried out to obtain the cross-section view of the device, as shown in Figure 4b. It was clearly seen that each layer is distinct and can still be uniformly deposited after the addition of C8-BTBT layer. Figure 4c presents the energy level alignment for the various layers, in which the ZnSe/ZnS QDs and C8-BTBT energy level were determined by ultraviolet photoelectronic spectroscopy (UPS) measurement and the others were obtained from the literatures. [34] It is revealed that the C8-BTBT layer commendably coordinated with the TFB and QDs layer to reduce the hole injection barrier. Figure 4d shows the electroluminescence (EL) of the device in which the emission peak is located at 456 nm, exhibiting pure blue emission. And the inset of Figure 4d shows the photograph of QLEDs under working condition. The addition of the C8-BTBT layer did not affect the pure blue emission of the device, because the exciton recombination region of the QLEDs is unchanged.
The current density (J)-voltage (V)-luminescence (L) and current efficiency (CE)-J-EQE characteristics of these devices are shown in Figure 4e,f. After adding C8-BTBT layer, the device performances are enhanced, manifesting the effectiveness of this strategy. The result is also consistent with the previous analysis, which shows that the device performance increases first and then decreases as the thicknesses of C8-BTBT layer increase. When the thickness of C8-BTBT layer reaches 15 nm, the device achieved the maximum EQE of 7.23% and the Based on the above results, the effect of C8-BTBT includes interface passivation, energy band adjustment, and the hole mobility promotion. First of all, the flat surface of the C8-BTBT layer with the optimal thickness is conducive to passivate the defects at interface. Nonradiative recombination is effectively reduced by avoiding the direct contact between TFB and QDs layers. Second, the C8-BTBT layer decreases the hole injection barrier and improves the hole injection, which results in more radiative recombination in EML. Finally, the high hole mobility of C8-BTBT improves the hole current density for reducing the excessive electron accumulation at the EML/HTL interface.

Conclusion
The injection of holes in cadmium-free blue QLEDs is effectively improved by introducing the TFB/C8-BTBT stepwise double HTLs, which reduce the charge accumulation between HTL and the ZnSe QD emissive layer. As a result, ZnSe-based blue QLEDs with the maximum EQE of 7.23% and the maximum CE of 7.72 cd A −1 are successfully achieved. The introduction of C8-BTBT layer is regarded with three functions, including enhancing the hole current density, reducing the hole injection barrier, and passivating the interface defects. Our results suggest that the introduction of stepwise double HTLs structure is an efficient approach to develop high-efficiency cadmium-free blue QLEDs.
Device Fabrication: The QLEDs were fabricated on patterned ITOcoated glass substrates with a sheet resistance of 15 per square. PEDOT:PSS was spin-coated at 3k rpm for 40 s and baked at 120 °C for 20 min. TFB was spin-coated at 3k rpm for 40 s and annealed at 120 °C for 20 min. C8-BTBT was spin-coated at 3k rpm for 40 s and baked at 120 °C for 10 min. After that, the sample was sent to a nitrogenfilled glove box. The QD and ZnO:PVP solutions were spin-coated at 3k rpm for 40 s and annealed at 80 °C for 10 min, respectively. Finally, thermal evaporation was utilized at a vacuum of 6 × 10 −4 Pa to deposit Ag cathodes with a thickness of 100 nm.
Characterization: The surface morphology of the films and the thickness of the films was examined with an atomic force microscope (AFM, Bruker, Multimode 8). Electroluminescence (EL) spectra of the QLEDs were measured by using a computer-controlled fiber optic spectrometer (Ocean Optics USB2000+). Time-resolved photoluminescence (TRPL) measurements were performed by using a fluorescence lifetime measurement system (HORIBA scientific Tempro-01). The UV-vis absorption spectrum was measured with A UV/vis/ NIR spectrophotometer (Shimadzu, UV-3600). Photoluminescence (PL) spectra measurement was collected with a fluorescence spectrophotometer (Hitachi F-4600). The current-voltage (I-V)

Supporting Information
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