Polymeric Hole Transport Materials for Red CsPbI3 Perovskite Quantum-Dot Light-Emitting Diodes

polymers Article Polymeric Hole Transport Materials for Red CsPbI 3 Perovskite Quantum-Dot Light-Emitting Diodes Zong-Liang Tseng 1, *, Shih-Hung Lin 2, Jian-Fu Tang 3, Yu-Ching Huang 4, Hsiang-Chih Cheng 5, Wei-Lun Huang 1, Yi-Ting Lee 6 Lung-Chien Chen 5, * 1 Department Electronic Engineering, Ming Chi University Technology, No. 84, Gungjuan Rd., New Taipei City 24301, Taiwan; a88061446@gmail.com 2 Department Electronic Engineering, National Yunlin University Science Technology, Yunlin 64002, Taiwan; isshokenmei@yuntech.edu.tw 3 Bachelor Program in Interdisciplinary Studies, National Yunlin University Science Technology, Yunlin 64002, Taiwan; jftang@yuntech.edu.tw 4 Department Materials Engineering, Ming Chi University Technology, No. 84, Gungjuan Rd., New Taipei City 24301, Taiwan; huangyc@mail.mcut.edu.tw 5 Department Electro-Optical Engineering, National Taipei University Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan; kokusoro860127@gmail.com 6 Center for Organic Photonics Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan; ytlee@opera.kyushu-u.ac.jp * Correspondence: zltseng@mail.mcut.edu.tw (Z.-L.T.); ocean@ntut.edu.tw (L.-C.C.) Citation: Tseng, Z.-L.; Lin, S.-H.; Tang, J.-F.; Huang, Y.-C.; Cheng, H.-C.; Huang, W.-L.; Lee, Y.-T.; Chen, L.-C. Polymeric Hole Transport Materials for Red CsPbI 3 Perovskite Quantum-Dot Light-Emitting Diodes. Polymers 2021, 13, 896. https:// doi.org/10.3390/polym13060896 Abstract: In this study, the performances red CsPbI 3 -based all-inorganic perovskite quantum-dot light-emitting diodes (IPQLEDs) employing polymeric crystalline Poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(9-vinycarbazole) (PVK), Poly(N,N -bis-4-butylphenyl-n,n -bisphenyl)benzidine (Poly- TPD) 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl-N2,N7-diphenyl- 9H-fluorene-2,7-diamine (VB-FNPD) as the hole transporting layers (HTLs) have been demonstrated. The purpose this work is an attempt to promote the development device structures hole transporting materials for the CsPbI 3 -based IPQLEDs via a comparative study different HTLs. A full-coverage quantum dot (QD) film without the aggregation can be obtained by coating it with VB-FNPD, thus, the best external quantum efficiency (EQE) 7.28% was achieved in the VB- FNPD device. We also reported a sting method to further improve the degree VB-FNPD polymerization, resulting in the improved device performance, with the EQE 8.64%. Academic Editor: Agnieszka Tercjak Keywords: perovskite; CsPbI 3 ; QD; light-emitting diodes Received: 14 February 2021 Accepted: 8 March 2021 Published: 15 March 2021 Publisher s Note: MDPI stays neutral with regard to jurisdictional claims in published maps institutional affiliations. Copyright: 2021 by the authors. Licensee MDPI, Basel, Switzerl. This article is an open access article distributed under the terms conditions the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1. Introduction Organic inorganic hybrid perovskite APbX 3 (A is an organic cation, such as CH 3 NH 3 + NH 2 CH=NH 2 +, X is a halide) has opened new avenues for optoelectronic materials in the recent years [1 4]. It is well known that CH 3 NH 3 PbX 3 (MAPbX 3 ) NH 2 CH=NH 2 PbX 3 (FAPbX 3 ) are easily decomposed into PbX 2 volatile MAX FAX in the presence heat moisture. To address the issue, more stable all-inorganic perovskite has been developed, such as CsPbX 3. In 2015, size-controlled compositioncontrolled CsPbX 3 quantum dots (QDs) were first synthesized using a colloidal method [5]. The colloidal CsPbX 3 QDs exhibit high color purity in photoluminescence (PL) spectrum, photoluminescence quantum yields (PLQYs) as high as 100%, which makes them cidates for light-emitting diodes (LEDs) [6 21] solar cells [22 28]. Up until now, the development the colloidal synthesis has been simultaneously promoted in parallel with the performance CsPbX3 QDs-based optoelectronic devices. All-inorganic perovskite QD-based LEDs (IPQLEDs) have drawn much attention for their solution-processed fabrication more flexible application [6 21]. To improve the IPQLED performance, highly efficient exciton recombination in the QD films is a Polymers 2021, 13, 896. https://doi.org/10.3390/polym13060896 https://www.mdpi.com/journal/polymers

Polymers 2021, 13, 896 2 10 significantly critical issue. The hole transporting layers (HTLs) reduce the hole injection barrier block the electron to balance the hole electron enhance efficient exciton recombination in the QD films. The poly(9-vinycarbazole) (PVK) was usually used as the HTL in the preliminary stage the IPQLED development [6,7]. Subsequently, PVK was replaced with Poly(N,N -bis-4-butylphenyl-n,n -bisphenyl)benzidine (Poly- TPD), because improved hole injection efficiency, which could be attributed to the hole mobility Poly-TPD by about two orders magnitude higher than that PVK [8]. To date, Poly-TPD has been selected as the hole transporting material in most IPQLED studies [12 21]. Poly(triaryl)amine (PTAA) also shows a high hole mobility, which makes it another good choice for the hole transporting materials. The defect during the formation process the QD films could be reduced by PTAA [9,10], which is effective in enhancing the radiative recombination. The more crystalline Poly(3-hexylthiophene- 2,5-diyl) (P3HT) has a relatively higher hole mobility than that noncrystalline organic HTLs is ten used as light-harvesting hole transporting materials for CsPbI 3 -based solar cells [25]. Our previous report demonstrated that a thermal crosslinkable HTL, 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl-N2,N7- diphenyl-9h-fluorene-2,7-diamine (VB-FNPD), also provides excellent hole mobility improves the interface between the HTL the CsPbBr 3 QD film [11]. On the other h, the studies the HTLs for deep red CsPbI 3 -based IPQLEDs are still lacking. Table 1 shows only Poly-TPD PTAA have been used as the HTLs in the CsPbI 3 -based IPQLEDs. No literature reported the CsPbI 3 -based IPQLEDs using VB-FNPD P3HT as the HTLs. Therefore, a comparative study different HTLs for the CsPbI 3 -based IPQLEDs is necessary. Table 1. Summary recent reports CsPbI 3 -based all-inorganic perovskite quantum-dot light-emitting diodes (IPQLEDs). Years Emission Layer EL Wavelength (nm) HTLs Peak EQE (%) Maximal LUMINANCE (cd/m 2 ) Reference 2017 CsPbI3 688 Poly-TPD 5.02 748 [15] 2018 CsPbI3 694 Poly-TPD 14.08 1444 [16] 2019 CsPbI3 682 Poly-TPD 1.8 365 [17] 2020 CsPbI3 687 PTAA 14.6 378 [18] 2020 CsPbI3 676 Poly-TPD 6.2 3762 [19] 2020 CsPbI3 675 Poly-TPD 10.21 401 [20] 2020 CsPbI3 685 Poly-TPD 6.02 587 [21] - CsPbI3 680 VB-FNPD 8.64 632 This work Herein, we studied the performance CsPbI 3 -based IPQLEDs employing P3HT, PVK, Poly-TPD VB-FNPD as the HTLs. Meanwhile, a dense smooth CsPbI 3 QDs film can be achieved using VB-FNPD HTLs, which are an important factor for the device performance the IPQLED. We then demonstrated highly bright efficient CsPbI 3 IPQLED based on VB-FNPD HTLs, achieving an external quantum efficiency (EQE) 8.64%. Therefore, we believe that our results may promote the development device structures hole transporting materials to achieve stable low-cost IPQLEDs. 2. Experimental Section 2.1. Materials P3HT was purchased from Solarmer (El Monte, CA, USA). Cesium carbonate (Cs 2 CO 3 ; 99.995%), octadecene (ODE; 90%), oleic acid (OA; 90%), octylamine (OAm; 90%), hexane (95%), octane (98 + %), mehyl acetate (99%), PbI 2 (99.999%), PEDOT:PSS (AI 4083), TPBi PVK were purchased from Sigma Aldrich (Munich, Germany). Poly-TPD,

Polymers 2021, 13, 896 3 10 n-octylammonium iodide VB-FNPD were purchased from LUMTEC (Taipei, Taiwan). All the chemicals were used as received. 2.2. Synthesis CsPbI 3 QDs Cs 2 CO 3 (200 mg) was loaded into a 25 ml three-neck flask, along with ODE (9 ml) OA (0.75 ml), then stirred degassed at 120 C for 30 min under nitrogen flow to obtain a transparent Cs oleate precursor. The Pb precursor solution was prepared by dissolving 0.09 M PbI 2 in 30 ml ODE, 3 ml OA 3 ml OAM then stirring degassing at 120 C under nitrogen flow. After PbI 2 was all dissolved, the temperature was increased to 150 C, then a 0.8 ml Cs oleate precursor was quickly injected into the Pb precursor solution. After 5 s, the reaction was cooled on an ice bath, red CsPbI 3 QD crude was obtained. Then n-octylammonium iodide (0.2 mmol) dissolved in toluene (4 ml), as a capping agent was added into the crude. Subsequently, as-prepared crude solution methyl acetate (16 ml) were centrifugated at 12,000 rpm for 15 min. The precipitate was collected loaded in 8 ml hexane methyl acetate (1:3 v/v), the solution was centrifuged at 12,000 rpm for 10 min. The precipitate was collected dispersed in octane (2 ml) centrifuged for 5 min at 12,000 rpm. Finally, the supernatant was collected stored at 4 C. 2.3. Device Fabrication The IPQLEDs were constructed with the architecture indium tin oxide (ITO)/ PEDOT:PSS (40 nm)/ HTLs (~50 nm)/ CsPbI 3 QD (~40 nm)/ TPBi (40 nm)/ LiF (1 nm)/al (100 nm). Here, P3HT, PVK, Poly-TPD VB-FNPD were used as the HTLs. The patterned ITO substrates were wet-cleaned then O 2 Plasma-cleaned. After cleaning, PEDOT:PSS was spin-coated at 8000 rpm for 40 s on the substrate annealed at 130 C for 15 min. Then, the samples were loaded to N 2 -filled glove box to deposit HTLs CsPbI 3 QDs. All HTLs were spin-coated with a concentration 4 mg/ml on PEDOT:PSS then heated at 100 C for 5 min. The thickness each HTL was controlled at ~50 nm by adjusting the spinning speed. Before heating, VB-FNPD was held sting still for 0, 20, 40 60 min then heated at 100 C for 5 min annealed at 170 C for 30 min for thermal crosslinking. The CsPbI 3 QDs were spin-coated with a concentration 40 mg/ml at 2000 rpm for 60 s. TPBi, LiF Al cathode were deposited by a thermal evaporation using a shadow mask to define the device area 2 2 mm 2. 2.4. Characterization Electroluminescence impedance characteristics were measured through computercontrolled LQ-100R spectrometer (Enlitech, Kaohsiung, Taiwan) Material Lab XM (SOLARTRON analytical, Leicester, UK), respectively. The absorbance photoluminescence (PL)/photoluminescence quantum yield (PLQY) were measured using UV-visible spectrophotometer (V-770, JASCO, Tokyo, Japan) in Table 2 fluorescence spectrophotometer (F-7000, Hitachi, Tokyo, Japan), respectively. The surface roughness was measured using an atomic force microscope (AFM, Bruker, Billerica, MA, USA). The electron microscopy images were obtained by HRTEM (JEM-2100, JEOL, Tokyo, Japan) FESEM (JSM-7610F, JEOL, Tokyo Japan), respectively. Table 2. Summary photoluminescence quantum yield (PLQY) values CsPbI 3 quantum dot (QD) films coated on a glass different hole transporting layers (HTLs). Glass VB-FNPD Poly-TPD PVK P3HT PLQY (%) 46.7 42.6 18.0 17.5 15.3 3. Results Discussion Figure 1a shows the planar SEM image the CsPbI 3 QD film spun on the glass substrate. Highly dense surface good crystalline the CsPbI 3 QD film can be obtained

Polymers 2021, 13, x FOR PEER REVIEW 4 10 3. Results Discussion Polymers 2021, 13, 896 4 10 Figure 1a shows the planar SEM image the CsPbI3 QD film spun on the glass substrate. Highly dense surface good crystalline the CsPbI3 QD film can be obtained without obvious aggregations. Such morphology may be attributed to the well-dispersed without high-stability obvious aggregations. suspensions Such in the morphology as-synthesized may be QD attributed dispersions, to theas well-dispersed shown in the insert in Figure high-stability 1. The suspensions PL spectrum in (Figure the as-synthesized 1b) the QD dispersions, as shown in the CsPbI3 QDs film shows a brightly red insert in Figure 1. The PL spectrum (Figure 1b) the CsPbI 3 QDs film shows a brightly luminescence at 682 nm with a narrow Full width at half maximum (FWHM) 35 nm, red luminescence at 682 nm with a narrow Full width at half maximum (FWHM) implying 35 nm, implying a high color a high purity color purity preferred preferred optical optical property. property. The The absorption absorption edge edge in the absorption in the absorption spectrum spectrum is close to is its close emission to its emission peak, which peak, agrees whichwith agrees previous with previous reports [22 25]. reports TEM [22 25]. image shows TEM image as-synthesized shows as-synthesized CsPbI3 QDs CsPbI are 3 QDs cubic areshaped cubic shaped well-dispersed dispersed with in octan, average with size an average 10.8 size nm (Figure 10.8 nm 1c,d). (Figure All 1c,d). abovementioned All abovementioned characteriza- in octan, tion characterization techniques evidently techniques exhibit evidently that exhibit the CsPbI3 that theqd CsPbI dispersion 3 QD dispersion solutions solutions QD solid films QDwith soliduniform films withsize uniform distribution size distribution have been have successfully been successfully obtained. obtained. Figure Figure 1. (a) 1. SEM (a) SEM image image (b) (b) absorbance absorbance photoluminescence (PL) (PL) spectra spectra CsPbI CsPbI3 3 QDs QDs spun spun on glass on glass substrates. substrates. (c) TEM (c) TEM image image (d) (d) size size distribution distribution CsPbI CsPbI3 3 QDs evaluated by by (c). (c). The The inset inset shows shows the the CsPbI CsPbI3 3 QDs QDs dispersed dispersed in in octane octane excited excited under under UV UV light light at at 365 365 nm. nm. Figure 2a shows the energy b diagram the CsPbI 3 QD layer each HTL. Figure 2a shows the energy b diagram the CsPbI3 The highest occupied molecular orbital (HOMO) lowest unoccupied QD layer molecular each orbital HTL. The highest (LUMO) occupied levels molecular all layers can orbital be referred (HOMO) to the results lowest in [12,25,29]. unoccupied Figure molecular 2b d showorbital (LUMO) the device levels performance all layers the can CsPbI be referred 3 IPQLEDto using the results different in HTLs. [12,25,29]. LUMO Figure levels2b d allshow the HTLs device areperformance much higher than the those CsPbI3 the IPQLED QD layer, using resulting different in good HTLs. electron LUMO blocking levels all HTLs ability are inmuch all HTLs higher (Figure than 2a). those HOMO the levels QD layer, all HTLs resulting are higher in good thanelectron those blocking the ability QD layer, in all indicating HTLs (Figure that reducing 2a). HOMO hole levels injection barrier all HTLs is preferred are higher to the than HTL those with the the QD lower HOMO level. Therefore, the tendencies the current in devices different HTLs layer, indicating that reducing hole injection barrier is preferred to the HTL with the lower correspond with the HOMO level their HTL (Figure 2c). The PVK device shows the HOMO level. Therefore, the tendencies the current in devices different HTLs correspond with the HOMO level their HTL (Figure 2c). The PVK device shows the highest highest current, because the lowest HOMO in PVK, which is agreed with the lowest current, because the lowest HOMO in PVK, which is agreed with the lowest impedance

Polymers 2021, 13, x FOR PEER REVIEW 5 10 Polymers 2021, 13, 896 5 10 (Figure S1). In contrast, the lowest current in the P3HT device is caused by the highest HOMO impedance level (Figure hole S1). injection In contrast, barrier, the lowest leading current to the inhighest the P3HTimpedance device is caused (Figure by S1) the turn-on highest voltage HOMO(biased level voltage hole injection at 1 cd/m barrier, 2 ), as leading shown toin thefigure highest2b. impedance Similar (Figure HOMO S1) levels in VB-FNPD turn-on voltage Poly-TPD (biasedlead voltage to their at 1same cd/m 2 turn-on ), as shown voltages, in Figure but 2b. the Similar excellent HOMO radiative levels in VB-FNPD Poly-TPD lead to their same turn-on voltages, but the excellent recombination efficiency in the VB-FNPD device gives it higher EQE. In addition, the PVK radiative recombination efficiency in the VB-FNPD device gives it higher EQE. In addition, device has the highest current, but it simultaneously shows the lowest EQE (Figure 2d), the PVK device has the highest current, but it simultaneously shows the lowest EQE which (Figure may 2d), be which caused may by be inefficient caused by radiative inefficient recombination radiative recombination in the QD in the layers, QD layers, leading to its leading higher to turn-on its higher voltage turn-on than voltage that than VB-FNPD that VB-FNPD Poly-TPD Poly-TPD devices devices (Figure (Figure 2b). It is interesting 2b). It is interesting to know what to know dominates what dominates as the carrier as the recombination carrier recombination efficiency efficiency for each for device. each device. Figure Figure 2. (a) 2. Energy (a) Energy levels levels for for different different HTLs, (b) luminance-voltage (L-V), (L-V), (c) (c) current current density-voltage density-voltage (J-V) (J-V) (d) external (d) external quantum efficiency (EQE) (EQE) for for IPQLED IPQLED devices devices with with different different HTLs. HTLs. In fact, the determination the carrier recombination efficiency can be easily observed In fact, the determination the carrier recombination efficiency can be easily observed by the naked eye. Figure 3a shows the photograph CsPbI3 in which the VB-FNPD film shows brighter than others. The thicknesses QDs all films QDs films spun on by the naked eye. Figure 3a shows the photograph CsPbI 3 QDs films spun on each HTL, each were HTL, around in which 40 nm, the measured VB-FNPD by Alpha Step. film shows Hence, brighter the brightness than others. the The QDthicknesses solids should all QDs be attributed films were toaround the degree 40 nm, themeasured aggregation by on Alpha Step. the differenthence, HTL surfaces, the brightness rather than the QD solids film should thickness. be When attributed the well-organized to the degree array the aggregation the colloidal CsPbI on the 3 QDs different is formed HTL onsurfaces, the rather surface than the film VB-FNPD thickness. filmswhen without the the well-organized QD aggregations, array the light-induced the colloidal exciton CsPbI3 isqds is limited formed inon a QD the nanoparticle surface the to increase VB-FNPD thefilms quantum without confinement the QD effect, aggregations, resulting in the thelight- induced improved exciton radiative is limited recombination, a QD as nanoparticle illustrated into Figure increase 3b. Inthe contrast, quantum the light-induced confinement effect, resulting in the improved radiative recombination, as illustrated in Figure 3b. In con- exciton can transport between nanoparticles, due to the QD aggregations, leading to the increased dissociation possibility the exciton prior to its radiative decay [30,31]. It is trast, the light-induced exciton can transport between nanoparticles, due to the QD aggregations, the reason why the brightness VB-FNPD film is much stronger than that other films, which leading is in good to agreement the increased with dissociation the device results possibility (Figure 2d). the exciton The summary prior to its PLQYs radiative decay [30,31]. It is the reason why the brightness VB-FNPD film is much stronger than that other films, which is in good agreement with the device results (Figure 2d). The summary PLQYs for CsPbI3 QDs layers spun on different HTLs are listed in Table 1. The PLQY CsPbI3 QDs layer on the glass is higher than the PLQY those spun on each

Polymers 2021, 13, 896 6 10 Polymers 2021, 13, x FOR PEER REVIEW 6 10 for CsPbI 3 QDs layers spun on different HTLs are listed in Table 1. The PLQY CsPbI 3 QDs layer on the glass is higher than the PLQY those spun on each HTL, which may HTL, which may be because the exciton dissociation is suppressed at the insulated glass be because the exciton dissociation is suppressed at the insulated glass [32]. On the other [32]. On the other h, the full-coverage QD films confirm the carrier combination. The h, the full-coverage QD films confirm the carrier combination. The films with the QD films aggregations with the provide QD aggregations a leakage path, provide whicha is leakage the reason path, that which the current is the inreason the Poly-TPD that the current device in the is higher Poly-TPD than that device in theis VB-FNPD higher than device that (Figure in the 2c). VB-FNPD The best performance device (Figure 7.28% 2c). The best was performance achieved in the VB-FNPD 7.28% was device. achieved in the VB-FNPD device. Figure Figure 3. (a) 3. CsPbI3 (a) CsPbI3 QDs QDs films films deposited on 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl- N2,N7-diphenyl-9H-fluorene-2,7-diamine (VB-FNPD), Poly(N,N Poly(N,N -bis-4-butylphenyl-n,n -bisphenyl)benzidine -bisphenyl)benzidine (Poly-TPD),(Poly- TPD), poly(9-vinycarbazole) (PVK) (PVK) crystalline crystalline Poly(3-hexylthiophene-2,5-diyl) Poly(3-hexylthiophene-2,5-diyl) (P3HT) (P3HT) excited under excited UV light under at 365 UV nm. light at 365 nm. (b) The (b) illustration The illustration the QDthe aggregations QD aggregations different on different HTLs. HTLs. To To further improve the device performance, the the different sting sting times times were were introducedinto intothe the VB-FNPD film preparation. Figure Figure 4 shows 4 shows current voltage luminance characteristics, introduced characteristics, EQE EQE the normalized electroluminescence electroluminescence (EL) (EL) spectrum spectrum the devices the devices prepared by different sting times. The performances all devices with the sting prepared by different sting times. The performances all devices with the sting treatment show better than that the device without the treatment. EL spectrum shows treatment an emission show peak better at 680 than nm that with a narrow the device FWHM without 32 nm, the treatment. high EL color spectrum purity. shows an The emission EL peak peak position at 680 isnm close with to the a narrow PL spectrum, FWHM which 32 can nm, beindicating attributed high to carrier color recombination EL peak position in the QDis films. close Figure to the 5 shows PL spectrum, the AFMwhich imagescan the be VB-FNPD attributed films to with carrier re- purity. The combination the differentin sting the QD times. films. Figure The AFM5 shows phase image the AFM exhibits images that light the VB-FNPD dark colors films with the are different alternately sting uniformly times. The distributed AFM phase on theimage VB-FNPD exhibits film surface that light without dark the sting treatment, uniformly indicating two-phase distributed coexistence on the VB-FNPD [33] low film degree surface without polymerization. the sting colors are alternately treatment, With the increase indicating the two-phase sting coexistence times, the deepened [33] colors low degree the larger polymerization. domain sizes With on the phase images can be found, which could be attributed to the increased degree the increase in the sting times, the deepened colors the larger domain sizes on the the polymerization. Therefore, the reduced surface roughness can be seen in the AFM phase topography images images, can be leading found, to the which improved could hole be transporting attributed to characteristic the increased the degree device the polymerization. performance. Thus, Therefore, the highest the reduced EQE 8.64% surface in the roughness VB-FNPDcan devices be seen treated in the for AFM 60 mintopog- raphy wereimages, achieved. leading to the improved hole transporting characteristic the device performance. Thus, the highest EQE 8.64% in the VB-FNPD devices treated for 60 min were achieved.

Polymers 2021, 13, 896 7 10 Polymers 2021, 13, x FOR PEER REVIEW 7 10 Figure Figure 4. (a) 4. (a) Luminance-voltage (L-V), (b) current density-voltage (J-V), (c) EQE for IPQLED devices based on on VB-FNPD HTLs, HTLs, prepared by by different sting times (inset is the enlarged high-efficiency region). (d) (d) Electroluminescence (EL) (EL) spectrum the the champion device biased at 3 V. Inset is a photograph the the working device with with red red emission.

Polymers 2021, 13, 896 Polymers 2021, 13, x FOR PEER REVIEW 8 10 8 10 Figure 5. AFM phase (left) topography (right) images for VB-FNPD films prepared by differfigure 5. AFM phase (left) topography (right) images for VB-FNPD films prepared by different ent sting times (top to bottom: W/O sting, 20, 40 60 min). sting times (top to bottom: W/O sting, 20, 40 60 min). 4. Conclusions In conclusion, polymeric hole transport materials employed for red CsPbI3 IPQLEDs have been demonstrated. The b-aligned aggregation characteristics the CsPbI3

Polymers 2021, 13, 896 9 10 References 4. Conclusions In conclusion, polymeric hole transport materials employed for red CsPbI 3 IPQLEDs have been demonstrated. The b-aligned aggregation characteristics the CsPbI 3 layers deposited on P3HT, PVK, Poly-TPD VB-FNPD HTLs were discussed. A fullcoverage QD film without the aggregation can be obtained on the VB-FNPD films, thus, the best performance was 7.28% in the VB-FNPD device. One the key issues associated with the utilization thermal-crosslinking polymer thin films is the control their alignment orientation. A sting method increasing the degree VB-FNPD polymerization was also presented, resulting in the improved device performance with the EQE up to 8.64%. Supplementary Materials: The following are available online at https://www.mdpi.com/2073-436 0/13/6/896/s1, Figure S1: Impedance characteristics IPQLED devices with different HTLs. Author Contributions: Conceptualization, Z.-L.T. L.-C.C.; Data curation, H.-C.C., W.-L.H. J.-F.T.; Formal analysis, Z.-L.T. L.-C.C.; Investigation, H.-C.C., W.-L.H. J.-F.T.; Methodology, S.-H.L., J.-F.T. Y.-C.H.; Resources, Z.-L.T. L.-C.C.; Writing original draft, Z.-L.T. L.-C.C.; Writing review & editing, Z.-L.T., Y.-T.L. L.-C.C. All authors have read agreed to the published version the manuscript. Funding: This work was supported by the Ministry Science Technology, Taiwan, under Grant No. MOST 108-2221-E-131-009-MY2 Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Statement excluded. Conflicts Interest: The authors declare no conflict interest. 1. Lee, M.M.; Teuscher, J.; Miyasaka, T.; Murakami, T.N.; Snaith, H.J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338, 643 647. [CrossRef] 2. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M.K.; Grätzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316 319. [CrossRef] 3. Yang, W.S.; Noh, J.H.; Jeon, N.J.; Kim, Y.C.; Ryu, S.; Seo, J.; Seok, S.I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234 1237. [CrossRef] 4. Wei, H.; Fang, Y.; Mulligan, P.; Chuirazzi, W.; Fang, H.-H.; Wang, C.; Ecker, B.R.; Gao, Y.; Loi, M.A.; Cao, L. Sensitive X-ray detectors made methylammonium lead tribromide perovskite single crystals. Nat. Photonics 2016, 10, 333 339. [CrossRef] 5. Protesescu, L.; Yakunin, S.; Bodnarchuk, M.I.; Krieg, F.; Caputo, R.; Hendon, C.H.; Yang, R.X.; Walsh, A.; Kovalenko, M.V. Nanocrystals cesium lead halide perovskites (CsPbX3, X = Cl, Br, I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15, 3692 3696. [CrossRef] [PubMed] 6. Song, J.; Li, J.; Li, X.; Xu, L.; Dong, Y.; Zeng, H. Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3). Adv. Mater. 2015, 27, 7162 7167. [CrossRef] 7. Pan, J.; Quan, L.N.; Zhao, Y.; Peng, W.; Murali, B.; Sarmah, S.P.; Yuan, M.; Sinatra, L.; Alyami, N.M.; Liu, J.; et al. Highly efficient perovskite-quantum-dot light-emitting diodes by surface engineering. Adv. Mater. 2016, 28, 8718 8725. [CrossRef] [PubMed] 8. Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, B.; Shan, Q.; Han, B.; et al. 50-Fold EQE improvement up to 6.27% solution-processed all-inorganic perovskite CsPbBr3 QLEDs via surface lig density control. Adv. Mater. 2017, 29, 1603885. 9. Dong, Y.; Wang, Y.-K.; Yuan, F.; Johnston, A.; Liu, Y.; Ma, D.; Choi, M.-J.; Chen, B.; Chekini, M.; Baek, S.-W. Bipolar-shell resurfacing for blue LEDs based on strongly confined perovskite quantum dots. Nat. Nanotechnol. 2020, 15, 668 674. [CrossRef] 10. Song, J.; Fang, T.; Li, J.; Xu, L.; Zhang, F.; Han, B.; Shan, Q.; Zeng, H. Organic inorganic hybrid passivation enables perovskite QLEDs with an EQE 16.48%. Adv. Mater. 2018, 30, 1805409. [CrossRef] 11. Lin, C.-C.; Yeh, S.-Y.; Huang, W.-L.; Xu, Y.-X.; Huang, Y.-S.; Yeh, T.-H.; Tien, C.-H.; Chen, L.-C.; Tseng, Z.-L. Using Thermally Crosslinkable Hole Transporting Layer to Improve Interface Characteristics for Perovskite CsPbBr3 Quantum-Dot Light-Emitting Diodes. Polymers 2020, 12, 2243. [CrossRef] 12. Chiba, T.; Hayashi, Y.; Ebe, H.; Hoshi, K.; Sato, J.; Sato, S.; Pu, Y.-J.; Ohisa, S.; Kido, J. Anion-exchange red perovskite quantum dots with ammonium iodine salts for highly efficient light-emitting devices. Nat. Photonics 2018, 12, 681 687. [CrossRef] 13. Chiba, T.; Hoshi, K.; Pu, Y.-J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Kawata, S.; Kido, J. High-efficiency perovskite quantum-dot light-emitting devices by effective washing process interfacial energy level alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054 18060. [CrossRef]

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