Conjugated Polyelectrolytes as Efficient Hole Transport Layers in

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Conjugated Polyelectrolytes as Efficient Hole Transport Layers in Perovskite Light-Emitting Diodes Bo Ram Lee, Jae Choul Yu, Jong Hyun Park, Seungjin Lee, Cheng-Kang Mai, Baodan Zhao, Matthew S. Wong, Eui Dae Jung, Yun Seok Nam, Song Yi Park, Daniele Di Nuzzo, Jin Young Kim, Samuel D Stranks, Guillermo C. Bazan, Hyosung Choi, Myoung Hoon Song, and Richard H. Friend ACS Nano, Just Accepted Manuscript • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Nano

Conjugated Polyelectrolytes as Efficient Hole Transport Layers in Perovskite Light-Emitting Diodes

Bo Ram Lee,¶,†,‡ Jae Choul Yu,§,‡ Jong Hyun Park,§ Seungjin Lee,§ Cheng-Kang Mai,⊥ Baodan Zhao,† Matthew S. Wong,⊥ Eui Dae Jung,§ Yun Seok Nam,§ Song Yi Park,₤ Daniele Di Nuzzo,† Jin Young Kim,₤ Samuel D. Stranks,† Guillermo C. Bazan,⊥ Hyosung Choi,∥,* Myoung Hoon Song§,* and Richard H. Friend†,*

¶

Department of Physics, Pukyong National University, 45 Yongso-ro, Nam-Gu, Busan 48513, Republic of Korea †

Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK

§

School of Materials Science Engineering and KIST-UNIST Ulsan Center for Convergent

Materials/Low Dimensional Carbon Center/Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan, 44919, Republic of Korea ⊥

Center for Polymers and Organic Solids, University of California, Santa Barbara (UCSB) California, 93106, USA

∥

Department of Chemistry and Research Institute for Natural Sciences, Hanyang University Seoul, 04763, Republic of Korea ₤

Department of Energy Engineering, Ulsan National Institute of Science and Technology

ACS Paragon Plus Environment

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(UNIST), Ulsan 44919, Republic of Korea ‡

*

These two authors contributed equally to this work and are co-first authors.

Corresponding

authors,

E-mail:

[email protected],

[email protected]

and

[email protected] Phone numbers to Richard H. Friend Office phone: +44-0-1223-337218 Fax+

Phone numbers to Myoung Hoon Song Office phone: +82-52-217-2316 Fax+82-52-217-2109

Phone numbers to Hyosung Choi Office phone: +82-2-2220-2619 Fax+82-2-2299-0762

Keywords: perovskite light-emitting diodes (PeLEDs); conjugated polyelectrolytes (CPEs); hole transport layer (HTL); PEDOT:PSS; electroluminescence; LEDs; lead halide perovskites;


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Abstract Perovskite-based optoelectronic devices have been rapidly developing in the past 5 years. Since the first report the external quantum efficiency (EQE) of perovskite light-emitting diodes (PeLEDs) has increased rapidly through the control of morphology and structure from 0.1% to more than 11%. Here, we report the use of various conjugated polyelectrolytes (CPEs) as the hole injection layer in PeLEDs. In particular, we find that poly[2,6-(4,4-bis-potassium butanylsulfonate)4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene)] (PCPDT-K) transfers holes effectively, blocks electron transport from the perovskite to the underlying ITO layer and reduces luminescence quenching at the perovskite/PCPDT-K interface. Our optimized PeLEDs with PCPDT-K show enhanced EQE by a factors of approximately 4 compared to control PeLEDs with PEDOT:PSS, reaching EQE values of 5.66%, and exhibit improved device stability.


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Organic–inorganic hybrid perovskite materials are attractive for optoelectronic devices such as solar cells (SCs) and light-emitting diodes (LEDs) because of their key properties such as low material cost, high charge-carrier mobility, solution processing and easy band-gap tunability. Perovskite-based optoelectronic devices have achieved in the last 5 years rapid and remarkable enhancements in the power conversion efficiency (PCE) of perovskite solar cells (PeSCs)1-6 and external quantum efficiency (EQE)7-15 of perovskite light-emitting diodes (PeLEDs). PeSCs have been extensively investigated and reached PCEs of up to 22%, which makes them promising as a next-generation energy source.6 At the same time, perovskite materials have raised interest as lighting materials for displays and lasers,16,17 due to their excellent colour purity, with full width at half maximum (FWHM) as low as 20nm and photoluminescence quantum efficiency (PLQE) of ~70%.18 PeLED EQEs of 11.7%13 have been achieved using self-organized multiple quantum wells (MQWs) for near-infrared emission and 10.4%15 for green emission in PeLEDs introducing a small amount of methylammonium cations into the CsPbBr3 lattice and depositing a hydrophilic and insulating polyvinyl pyrrolidine polymer. Extensive efforts have been dedicated to enhance their EQE by the morphological control of perovskite films8,19-21 to reduce the number defect sites, the use of perovskite-based quantum dots15,22,23 for quantum confinement effects and the modification of the perovskite structure1214,24

from 3-dimension (3D) to 2-dimension (2D).

Recently, PeLEDs with a bottom electrode selected for hole-injection, have shown reasonable

performance.

Poly(3,4-ethylenedioxythiophene):poly-styrene

sulfonate

(PEDOT:PSS) has been used as the hole transport layer (HTL) of PeLEDs because it is suitable for solution-processing, thereby eliminating the need for high-temperature fabrication steps, resulting in the realization of flexible devices and roll-to-roll device fabrication. However, the hygroscopic and acidic nature of PEDOT:PSS tends to reduce


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device stability because it can gradually corrode indium tin oxide (ITO) electrodes.25-28 In addition, charge carrier quenching can occur at the interface between PEDOT:PSS and the perovskite layer. 29 There have been two reports of PeLEDs using metal oxides such as nickel oxide (NiOx), 24 and PEDOT:PPS/MoO3 composites.30 Chih et al. demonstrated a highly efficient PeLED using NiOx as an electrode for hole injection. However, NiOx requires thermal annealing at 400 oC. The PEDOT:PSS/MoO3 composite has been shown to enhance the crystallinity of perovskite and reduce the barrier for hole injection. Again, the PEDOT:PSS-based HTL, is problematic for stability as mentioned above, and there is still considerable room for further improvements in performance and stability of PeLEDs. Various strategies including effective hole injection through well-matched energy levels between the perovskite layer and the HTL, superior blocking of injected electrons in the emissive layer, reduction of charge carrier quenching and uniform and well-grown perovskite films on the HTL. Here, we demonstrate how a series of conjugated polyelectrolytes (CPEs) can be used as an alternative to PEDOT:PSS in PeLEDs. The best PeLEDs using the newly-synthesized poly[2,6-(4,4-bis-potassium

butanylsulfonate)-4H-cyclopenta-[2,1-b;3,4-b’]-dithiophene)]

(PCPDT-K) shows a luminance of 18,100 cd m-2, luminous efficiency (LE) of 25.97 cd A-1, and EQE of 5.66%. This EQE value is 4-fold that of control devices with PEDOT:PSS. In addition, pH-neutral and low-temperature solution-processable CPEs provide improved device stability and higher electrical conductivity compared to PEDOT:PSS. Furthermore, the compatibility and wetting property between CPEs and perovskite precursor solutions are excellent, which results in the formation of uniform and dense perovskite films on the CPEs.

Result & Discussion


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Figure 1(a),(b) show illustrations of the full device structure for PeLEDs and the chemical structures for the different CPEs. Our PeLEDs consist of ITO as a transparent anode, CPEs as a HTL, MAPbBr3 as an emissive layer, 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole) (TPBi) as an ETL and lithium fluoride (LiF)/silver (Ag) as a cathode. The detailed fabrication of PeLEDs and CPEs are given in the Experimental section. Figure 1(b) shows various CPEs materials including (i) poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-K),

(ii)

poly[2,6-(4,4-bis-sodium

butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b’]-

dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-Na), (iii) poly[2,6-(4,4-bis-potassium butanylsulfonate)-4H-cyclopenta-[2,1-b;3,4-b’]-dithiophene)] (PCPDT-K) and (iv) poly[9,9(4,4-bis-sodium butanylsulfonate)fluorene-alt-4,7-(2,1,3-benzothiadiazole)] (PFBT-Na). The methods for synthesizing CPEs are reported in ref. 31-32 and Fig. S1. Energy levels and electrical conductivity of CPEs are dependent on the polymer backbone, counter-ions, and lengths of alkyl side chains.26,31,32 CPE-K, CPE-Na and PCPDT-K have a polymer backbone based on cyclopentadithiophene (CPDT), while PFBT-Na has a polyfluorene backbone (PFO).

CPDT-based CPEs have an estimated highest occupied

molecular orbital (HOMO) energy of 4.9 eV, while PFBT-Na has a HOMO energy of 5.4 eV. CPE-K and CPE-Na with (3-benzothiadiazole) BT units have a narrower band gap than PCPDT-K, due to the alternating donor (CPDT)-acceptor (BT) structure that effectively lowers the bandgap. In addition, the anionic side chains of CPE-K and CPE-Na provide stabilization for polarons on the conjugated backbone; thus CPE-K can be doped during the dialysis to provide a bench stable doped polymer. On the other hand, PCPDT-K and PFBTNa are not doped under the same conditions, because of the relatively higher oxidation potential. CPE-K and CPE-Na have the same backbone of CPDT with different counter ions of K and Na, which have the same the lowest unoccupied molecular orbital (LUMO) and


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optical bandgap, as shown in Figure 1(c) and Fig. S2 (absorption spectra), whilst having a significantly different conductivity according to its counter ion. The conductivity of CPE-Na (0.16 S cm-1) is approximately 7-fold higher than that of CPE-K (0.024 S cm-1).26,31 CPE-Na has smaller π-π stacking distance (likely due to the smaller ion size) and larger crystal correlation length, both likely contributing to better conductivity. The conductivity of CPEs films was measured via I-V curve measurements using field-effect transistors (FETs), as shown in Fig. S3. A summary of the conductivity is presented in Table S1. Figure 1(c) shows the energy diagrams of PeLEDs for flat band conditions. To investigate the stability of CPE layers toward solvents used for perovskite deposition, the modification of their thin films was examined using atomic force microscopy (AFM) with and without DMF/DMSO (7:3 ratio) co-solvent treatment, as shown in Fig. S4 (top row: without treatment, bottom row: with treatment). The thickness of all CPEs was approximately 8 nm and there was only a negligible change after deposition of DMF/DMSO (Table S2). CPE-K, CPE-Na, PCPDT-K and PFBT-Na have a root-mean-square (rms) roughness (Rrms) of 0.55nm, 0.58nm, 0.41nm and 0.49nm, respectively, which are very smooth (Rrms < 1nm) and uniform. These AFM results confirm that Rrms and thickness are hardly modified by DMF/DMSO treatment. To demonstrate the wettability and compatibility between the CPEs and perovskite solution, we measured contact angles with DMF/DMSO atop various HTLs such as (a) PEDOT:PSS (2o), (b) CPE-K (2o), (c) CPE-Na (2o), (d) PCPDT-K (3o) and (e) PFBT-Na (3o) (Fig. S5). These low contact angles of indicate excellent wettability and also provide evidence that the physical properties for device fabrication are compatible with perovskite solution and enable the formation of full-coverage perovskite films. However, PFBT-Na shows non-uniform perovskite film (See Fig. S7e) favourable wettability. More details are given in the section on scanning electron microscopy (SEM).


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To investigate the crystallinity of perovskite film on PEDOT:PSS and various CPE layers, XRD measurements were performed as shown in Fig. S6. All perovskite films indicated diffraction peaks at 15.04o, 30.24o and 45.98o, corresponding to (100), (200) and (300) planes of the perovskite phase.33,34 These results confirm the formation of well-grown perovskite atop PEDOT:PSS and all CPEs. Fig. S7 shows the morphology and cross-section of perovskites on PEDOT:PSS and different CPEs layers. The morphology of MAPbBr3 perovskite films on (a) PEDOT:PSS (b) CPE-K, (c) CPE-Na and (d) PCPDT-K is uniform and dense while (e) PFBT-Na has a nonuniform morphology of MAPbBr3 perovskite film and defect sites as pinholes due to the different surface energies between relatively hydrophobic PTBT-Na and the hydrophilic perovskite layer. The PFO backbone of PTBT-Na is more hydrophobic compared to the CPDT backbone of other CPEs. The thickness of perovskite films is similar (~500nm), which is confirmed via cross-section (Fig. S7f). To optimise device performance in the PeLEDs, we also controlled the thickness of the CPEs from 3 to 15 nm. Among the tested devices, the PeLED with 8 nm of PCPDT-K exhibited the highest EQE of 1.47 %, as shown in Fig. S8 and Table S3. On the other hand, the device with thin CPE (< 3nm) shows high current density and no luminance. Device characterizations of optimized PeLEDs with various HTLs such as PEDOT:PSS, CPE-K, CPE-Na, PCPDT-K and PFBT-Na are presented in Fig. S11 and key results are summarized in Table S4. The data in Fig. S11 include (a) current density versus voltage (J-V), (b) luminance versus applied voltage (L-V); inset: photographs of the electroluminescence (EL) of a device with PCPDT-K, (c) luminous efficiency versus voltage (LE-V) and (d) EQE versus voltage (EQE-V). The optimized PeLEDs with PEDOT:PSS (reference device) exhibited peak LE of 3.47 cd A-1 and EQE of 0.75%. PeLEDs with CDBT-based CPEs such as CPE-K, CPE-Na and PCPDT-K show improved device performance compared to the


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reference device. Moreover, the operating voltages of PeLEDs with CPEs were lower in comparison to the reference device because of the higher conductivity of CPEs relative to PEDOT:PSS. Among PeLEDs with CPEs, those using PCPDT-K exhibited the best device performance, which we consider results from the well-balanced charge carrier recombination by efficient hole injection and superior electron blocking capability (Fig. S9). Optimized PeLEDs with PCPDT-K show peak LE of 6.83 cd A-1 and EQE of 1.47%. Both LE and EQE are approximately factor of 2 higher than those with PEDOT:PSS. On the other hand, the PLED performance with PFBT-Na was lower than the reference device, even though the energy barrier between the HOMO (5.4 eV) of PFBT-Na and HOMO (~5.9 eV)11 of MAPbBr3 was minimized, compared to other CPEs and PEDOT:PSS (5.1 eV) and the electron transport from an emissive layer to an ITO electrode could be blocked by the energy barrier between LUMO of PFBT-Na (3.0 eV) and LUMO (~3.6 eV)11 of MAPbBr3 due to the several defects of perovskite film sites and corresponding leakage currents. Detailed device characteristics with various CPEs are summarized in Table S4 and EL spectra of PeLEDs with different HTLs are shown in Fig. S10. To make the optimised device, we have tried the nanocrystal spinning process (NCP) and the 50-nm thickness of TPBi by the thermal evaporation. As a result, we obtained EQE of 5.66% in PeLEDs device with PCPDT-K and EQE of 1.38% in PeLEDs with PEDOT:PSS (reference device), as shown in Figure 2 and Table 1. Figure 3(a)-(d) shows the long-term device stability of PeLEDs with PEDOT:PSS and PCPDT-K. These measurements were performend by encapsulating the PeLEDs and measuing at constant current density 20 mA cm-2 for 12 hours. One observes that the voltage and the luminance of PeLEDs with PCPDT-K remain relatively stable, compared to that of PeLEDs with PEDOT:PSS, as seen from only slight changes in the measured voltage with time. The measurement was repeated on the same pixel


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of each device after 15days. The devices were stored in a nitrogen-filled glovebox between each measurement. The transient characteristics of the voltage and the luminance in PeLEDs with PCPDT-K after 15days remain almost similar to that of the first day, whereas PeLEDs with PEDOT:PSS show significantly more unstable transient characteristics compared to that of the first day. This may be due to the degradation from the ion migration and Joule heating.9,35,36 Overall, this demonstrates a superior long-term stability of the devices with PCPDT-K. To check the recovery characteristics of

PeLEDs with PCPDT-K, the voltage and

luminance were measured for 3 hours under a constant current of 20 mA cm-2, as shown in Figure 3(e),(f). Over 3 hours, the luminance decays from ~102 to ~101 cd m-2. The measurement was repeated on the same pixel of device after 5mins, 3hours and 15days. The voltage was independent of time and the luminescence was almost repeatedly recovered despite the slight decrease from the initial measurement. In particular, the luminescence recovery after 5mins, 3hours and 15days is similar, and this indicates that the poling effect of ion migration37 from bias can be reversed within a few minutes. We also investigated the local photoluminescence (PL) variations as function of time via PL image mapping. Fig. S12 and Fig. S13 (a) show the confocal PL images of MAPbBr3 films on top of PCPDT-K layer as a function of time. And Fig. S13(b-d) present the improvement of the PL intensity in (b) the bright, (c) the dark and (d) the average area. PL measurement was repeated on the same position of perovskite film by decreasing the interval time for the remeasurement (#1→#2; 5min, #2→#3; 5min, #3→#4; 3min, #4→#5; 1min). In the initial state (#1), PL intensity was rapidly decreased to the dark within 1.7s after exposure to the 405 nm laser, and then PL intensity was gradually increased from after 50s, and eventually reached an initial PL intensity at approximately 300s (below Fig. S13a). In


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contrast, there was no change in PL intensity of dark area. In second state (#2), the change of PL intensities was lower than those of the initial state. Moreover, the change of PL intensities in #3, #4 and #5 states was relatively reduced for both bright and dark area, compared to previous state. In general, A bright area (black square) starts at a comparatively high emission level and rise significantly whereas the relative change in a dark area (red circle) is much less. As the interval time decreased, PL intensities gradually increased. These results indicate that PL lifetime and intensity significantly increased as a function of time under laser exposure. The differences of PL intensity were attributed to the reduction of the bulk trap state density in perovskite film.38 Moreover, we observed heterogeneous grains with defferenct PL intensity. Some regions are particularly bright in emission, whereas other grains and grain boundaries were relatively darker, corresponding to higher trap state densities.39

We further investigated the EL behaviour in PeLEDs with PCPDT-K for 3 hours under constant current of 20 mA cm-2 using an optical microscope, as shown in Fig. S14, S15 and Movie S1. The EL imaging behaviour is similar with J-V-L characteristic (Fig. 3e,f). We also observed a blinking phenomenon related with migration of ions and electronic trap sites.40-43 Thus, it is difficult to confirm what kind of crystalline was recovered from EL behaviour due to the blinking phenomenon. To check the the detailed mechanism of the recovery behaviour, further study will be needed to eliminate the blinking. Fig. S15 shows the EL images during 20 seconds at the point passed 5min under constant current of 20 mA cm-2. The field induced migration allows the large amount of mobile bromide to fill vacancies.44-47 We show the continuous EL emission due to high crystallinity at red circle under bias while show the blinking due to the vacancy and trap site at blue circle.


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To investigate the photoluminescence (PL) lifetime of MAPbBr3 deposited on different HTLs, the PL decay in perovskite films was measured using time-correlated single photon counting (TCSPC), as shown in Figure 4. There is an initial fast component in the PL decay in the presence of the ETL (Figure 4(a), and this initial relaxation is dominant for all the HTL/MAPbBr3 samples (Figure 4(b). This is a result of significant quenching of excitations at the interface between the HTL and the perovskite, most likely due to hole transfer from the MAPbBr3 perovskite to the HTL. The insertion of CPE-K and CPE-Na to the perovskite films result in more or less the same quenching rate compared to that of PEDOT:PSS. In contrast, there is a significant reduction in the quenching rate for PCPDT-K and PFBT-Na due to lower electron-transfer resulting from electron-blocking behaviour. The perovskite layer atop PFBT-Na is susceptible to defects related to poor surface coverage, as seen in Figure 4(c) (inset), and therefore the charge carrier lifetime differs depending on the position of the beam spot; the lifetime is much longer in the area with a good morphology (Figure 4(c), position 1), similar to that of the perovskite films with other HTLs (all ~10ns, slow decay). The charge carrier lifetime decreases significantly at a region with many defect sites (Figure 4(c), position 2) to ~ 2.72 ns (slow decay). These TCSPC results agree with the trend seen in the luminance data from Figure S11(b). TCSPC results show that the initial fast decay is due to quenching (including charge transfer) and the long decay is due to recombination in the perovskite films; (present in all films). The fraction of initial fast decay (A1 factor in bi-exponential fit) is decreased in MAPbBr3/PCPDT-K, indicating reduced charge transfer (electron blocking behaviour). To understand the luminescence quenching process detail, we measured the PLQE of MAPbBr3 with and without HTLs using an integrating sphere method.48 The PLQEs show 13.43%, 1.33%, 1.33%, 1.30%, 1.78% and 1.61% for Quartz/MAPbBr3 (without HTLs) and


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Quartz/HTLs/MAPbBr3 with PEDOT:PSS, CPE-K, CPE-Na, PCPDT-K and PFBT-Na, respectively. On the other hand, the PLQE shows 8.34% for ETL (TPBi)/MAPbBr3 interface, which indicates that the charge carrier quenching at the ETL/MAPbBr3 interface is less than at the HTL/MAPbBr3 interfaces. Compared with MAPbBr3 without HTLs, MAPbBr3 with HTLs significantly reduced the PLQE, implying that charge carrier quenching occurs at the interface between HTL and MAPbBr3 layer. The holes separated from photogenerated charge carrier within the MAPbBr3 layer are extracted from MAPbBr3 to HTLs. This result confirms the quenching process from the perovskite layer to the ITO anode, which is in good agreement with TCSPC data. Moreover, we calculate the internal PLQE depending on the HTLs. The internal PLQE is determined by equation (1):49 = ∙ + 1 − ∙ ∙ + 1 − ∙ ∙ + ⋯ ∙

= ∙ ∙ ∑ 1 − ∙ = ∙

(1)

where ext is the external PLQE, esc is the escape probability from considering the perovskite-glass and perovskite-air interfaces and is the internal PLQE. For bromide perovskite, the refractive index is 2.4 at the bandedge50 giving transmission probabilities of 9.2% for the perovskite-glass interface and 3.6% for the perovskite-air interface. With an optical density of 0.6, this gives an escape probability of esc=15.9%. Thus, the internal PLQE of MAPbBr3 without HTLs is 49.38%. We assume that the maximized EQE will be 1.46% in PeLEDs even though the internal PLQE of MAPbBr3 is approximately 50%; this decrease is attributed to charge carrier quenching at the HTLs/MAPbBr3 interface and incomplete light extraction. The detailed external and internal PLQE dependence on HTL structures are summarized in Figure 4(c), (d) and Table 2.

Conclusion


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In this work, we successfully demonstrate improvement in PeLED by employing a series of CPEs as an HTL alterative to PEDOT:PSS. Selection of CPEs makes it possible to tune energy levels and electrical transport/doping levels, depending on the polymer backbone, counter-ions, and lengths of alkyl side chains, leading to effective hole injection and electronblocking behaviour. Moreover, solution-processed perovskite films form well defined structures and layers on the CPEs due to excellent wettability and compatibility. Thus, the device performances and the long-term stability of PeLEDs with CPEs are enhanced. In particular, optimized PeLEDs with PCPDT-K exhibit a maximum LE of 25.97 cd A-1 and EQE of 5.66%, which are ~4-fold enhanced, respectively, compared to the reference device using PEDOT:PSS. Furthermore, our approach can be applied to flexible devices via pHneutral and low-temperature solution-processable methods.


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ACS Nano

Experimental Section Preparation of optimized PeLEDs. ITO-coated glass substrates were cleaned using sequential ultra-sonication in acetone and isopropanol for 10 min each. And then, PEDOT:PSS was spin-coated at 5,000 rpm for 40 s and then annealed at 140 °C for 10 mins and CPEs were spin-coated at 4,000 rpm for 45 s and then annealed at 60 °C for 10 mins for HTL. The MAPbBr3 precursor (29.4 wt. %) solution were prepared by dissolving MABr and PbBr2 with molar ratio 1.1:1 in DMF:DMSO (7:3) co-solvent at 60 oC with stirring. Anti-solvent solution for dropping were prepared by dissolving TPBi in chloroform (0.05 wt. %). To form MAPbBr3 layers, MAPbBr3 solution was spin-coated onto HTLs at 3,000 rpm for 50s, after 10s of spin-coating at 500 rpm, ~ 70 µl TPBi dissolved chloroform was quickly dropped onto the centre of the spinning substrate in glovebox. Finally, TPBi (50 nm) , LiF (1 nm) and silver (100 nm) were deposited successively by the thermal evaporation method.

Characteristic of PeLEDs. For PeLEDs, J-V-L characteristics and device performances were measured using a Konica Minolta spectroradiometer (CS-2000) with Keithley 2400 source meter. The emissive area of the device was 13.5 mm2. The devices were encapsulated before testing. The PL and EL from encapsulated PeLEDs with PEDOT:PSS and CPE layers as HTLs were measured under ambient air conditions.

SEM measurement. SEM (LEO GEMINI 1530VP FEG-SEM system) was used to observe the morphology and the cross-section of the MAPbBr3 films.


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Confocal fluorescence imaging. Confocal PL images of the perovskite films were obtained using an LSM 780 NLO laser scanning confocal microscope (Carl Zeiss) with a 100x oil immersion objective (α Plan-APO, NA = 1.46). The samples were excited using a 405 nm diode laser.51

EL imaging. EL microscope images were obtained from samples of the PeLEDs under a constant current density of 20 mA cm-2 using an inverted optical microscope (OE, IX81, Olympus).35

Time-correlated single photon counting (TCSPC). The perovskite films for TCSPC studies were spin-coated onto different hole transporters deposited on pre-cleaned quartz substrates with the same method described in the device fabrication section. The samples were photoexcited using a 407 nm pulsed laser with pulse width

Conjugated Polyelectrolytes as Efficient Hole Transport Layers in

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