Boosting Perovskite Light-Emitting Diode Performance via Tailoring

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Surfaces, Interfaces, and Applications

Boosting Perovskite Light-Emitting-Diodes Performance via Tailoring Interfacial Contact Yatao Zou, Muyang Ban, Yingguo Yang, Sai Bai, Chen Wu, Yujie Han, Tian Wu, Yeshu Tan, Qi Huang, Xingyu Gao, Tao Song, Qiao Zhang, and Baoquan Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07438 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Materials & Interfaces

Boosting Perovskite Light-Emitting-Diodes Performance via Tailoring Interfacial Contact Yatao Zou1‡, Muyang Ban1‡, Yingguo Yang2, Sai Bai3, Chen Wu1, Yujie Han1, Tian Wu1, Yeshu Tan1, Qi Huang1, Xingyu Gao2, Tao Song1*, Qiao Zhang1*, and Baoquan Sun1* 1

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, 199 Ren’ai Road, Suzhou 215123, People’s Republic of China 2

Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied

Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, Shanghai 201204, China 3

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden

Keywords: perovskite, light emitting diodes, charge carrier injection, high-efficiency, stability.

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Abstract Solution-processed perovskite light emitting diodes (LEDs) have attracted wide attentions in the past several years. However, the overall efficiency and stability of perovskite-based LEDs remains inferior to those of organic or quantum dot LEDs. Non-radiative charge recombination and the unbalanced charge injection are two critical factors that limit the device efficiency and operational stability of perovskite LEDs. Here, we develop a strategy to modify the interface between hole transport layer and the perovskite emissive layer with an amphiphilic conjugated polymer of poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluore ne)] (PFN). We show evidences that PFN improves the quality of perovskite film, which effectively suppresses non-radiative recombination. With further improving charge injection balance rate, a green perovskite LED with a champion current efficiency of 45.2 cd/A, corresponding to an external quantum efficiency (EQE) of 14.4%, is achieved. In addition, the device based on PFN layer exhibits improved operational lifetime. Our work paves a facile way for the development of efficient and stable perovskite LEDs.

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Introduction High-performance light emitting diodes (LEDs) based on low-cost solution processed perovskite materials are promising for next generation displays and lighting source, due to their tunable emission wavelength, wide gamut, and flexible compatibility1-5. Previous efforts on the composition of perovskites, and device engineering have boosted the external quantum efficiency (EQE) of perovskite LEDs to over 10%6-14. In a typical LED devices, the emissive layers are usually sandwiched between electron transport layers (ETLs) and hole transport layers (HTLs)15, 16. The device efficiency is closely related to the exciton radiative recombination in the emissive layer. The defects in perovskite films, which mainly locate at the grain boundaries and/or at the interfaces between the charge injection interlayers and perovskite emissive layers17-20, result in non-radiative recombination centers and inhibit the device performance of perovskite LEDs. In addition, the energy level alignments between the charge transport layers and the perovskite layers are critical to achieve efficient charge injection in a perovskite LED. In previous reports, conjugated molecules

such

as

poly(9-vinylcarbazole)

2’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-

benzimidazole))

(PVK), (TPBi),

2’,

exhibiting

minimized charge injection barriers at the interfaces have been widely used as HTL and ETL, respectively6, 21, 22. However, the hole mobility of PVK (2.5×10-6 cm2 V-1 S-1) is over one order magnitude lower than electron mobility of TPBi (3.3×10-5 cm2 V-1 S-1)16, 23, which may result in imbalanced charge carrier injection rate and low device efficiency for perovskite LEDs. Organic molecules with high hole mobility,

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such as poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4′-(N-(4-butylphenyl) (TFB) and poly[bis(4-phenyl)(4-butylphenyl)amine] (poly-TPD) would be better choices for efficient charge injection hence that performance of perovskite LED devices. Nevertheless, perovskite precursor solutions composing of ionic ingredients (e.g. lead halide and cesium halide) in polar solvents such as dimethylsulfoxide (DMSO) or dimethyl formamide (DMF), exhibit distinct polarities to most conjugated polymer HTLs24. Therefore, perovskite films suffer from poor coverage on these conjugated HTLs, because their precursor solutions are non-wetting on these HTLs. This phenomenon mainly arises from hydrophobic-branched alkyl groups in their molecular structures. Herein,

an

amphiphilic

conjugated

polymer

of

poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluore ne)] (PFN) is introduced to improve the interfacial contact between the HTLs and perovskite layer. We observe improved film quality and reduced defects of perovskite film on PFN modified substrates. Because amphiphilic PFN can increase surface energy of hydrophobic polymers, HTLs with higher mobility, e.g. poly-TPD, is used to achieve efficient and balanced charge injection. Consequently, we fabricate efficient green LED with a champion current efficiency of 45.2 cd/A, corresponding to an EQE of 14.4%. Moreover, the PFN based perovskite LED devices exhibit improved operational lifetime. This work proposes a facile way to tailor charge carrier injection rate as well as reducing the defects in perovskite-based optoelectronic devices, hence to improve their efficiency and stability.

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Results and Discussions Previous works have demonstrated that surface defects in perovskite films are dramatically suppressed by surface engineering to improve interfacial contact18, 25-27. With inserting a thin PFN layer (unless specially mentioned, the PFN concentration on HTLs is 0.050wt%) between PVK and perovskite film, a slight improvement of perovskite film morphology is achieved, as atomic force microscopy (AFM) and scanning electron microscopy (SEM) images depicted in Figure 1a and b. The root mean square (RMS) roughness values extracted from AFM images for perovskite films on PVK and PVK/PFN are ~1.3, ~1.2 nm, respectively. The identical contact angles of PVK and PVK/PFN in Figure S1 indicate that surface energy of PVK would not change after PFN modification. However, reduced surface defects of perovskite film on PVK/PFN are observed, as depicted by time-resolved PL spectra in Figure 2a. Here, perovskite film on PVK/PFN displays a slower PL lifetime decay in comparison to that on bare PVK one. The curves of PL intensity against time are fitted with a cubic-exponential function28. The fitted average PL lifetime of perovskite films on PVK and PVK/PFN are 37.8 and 45.9 ns, respectively, which reveals reduced defect states in perovskite film incorporating with PFN. We believe that the reduced defects should be attributed to the passivation effects of PFN due to the electron-donating nitrogen in PFN structure. PFN can be regarded as a Lewis base, which has been successfully demonstrated to effectively passivate the surface defects in perovskite films29,

30

. Benefiting from the reduced defects, PL efficiency exhibits obvious

enhancement, as shown in Figure 2b and c. The PLQY of perovskite films deposited

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on PVK and PVK/PFN are 41% and 52%, respectively. To get more insight into the passivation effects of PFN, PL lifetime of perovskite film deposited on quartz from precursor solution with and without extra PFN are conducted. With adding a small amount of extra PFN (0.02 mg/mL) into perovskite precursor solution, perovskite film shows much longer PL lifetime (~97.8 ns) than that of pristine one (~46.5 ns) (Figure 2d), which further confirms the passivation effects of PFN. We noticed that there is a trace amount of PFN that can be re-dissolved in perovskite precursor DMSO solution when perovskite deposited on PFN layer. In Figure S2, we show the UV-vis absorption of deposited PFN after DMSO washing, where optical density of PFN layer slightly reduces, which should be ascribed to the loss of PFN in DMSO solution. In addition, this evidence also confirms that most of PFN layer remains when depositing perovskite in DMSO solution. Although perovskite on PVK/PFN exhibits improved film quality and PLQY, these perovskite LEDs still suffer from imbalanced charge carrier injection if PVK is chosen as HTL to build LED devices. Herein, higher mobility HTL of poly-TPD is selected to explore the influence of interfacial contact on perovskite LEDs performance. To ensure a high quality perovskite film on hydrophobic poly-TPD, an interfacial layer of PFN is required to improve the wettability of perovskite solution on the poly-TPD. Contact angle measurements in Figure S1 reveal that poly-TPD film displays hydrophobic characteristic, whereas turning into hydrophilic upon depositing a thin PFN layer. The contact angle decreases from ~103o (poly-TPD) to ~80o (poly-TPD/PFN). The amphiphilic PFN consists of both hydrophobic backbones and

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hydrophilic protonated ammonium groups, which provides selective interfacial contacts with both poly-TPD and perovskite layers (Figure 1c). Based on similarity compatibility principle, the hydrophobic backbone could adhere on the similar polarity groups (Figure S3) toward poly-TPD, which results in hydrophilic group facing air interface. As a result, the hydrophobic surface converts into hydrophilic one. However, it is worth noting that the surface energy of poly-TPD/PFN is closely linked with PFN molecular orientation. We anticipate increased surface energy if the hydrophilic groups of PFN face to air interface. Otherwise, the underlying poly-TPD should remain hydrophobic. Ideally, one dense monolayer of PFN is preferred to convert hydrophobic surface into hydrophilic one. A higher PFN concentration may result in more hydrophobic groups facing solid/air interface, leading to decreased wettability of the poly-TPD layer. This point is consistent with the increased contact angle when PFN concentration is 0.075wt%, as shown in Figure S1. Perovskite film quality is dramatically improved on poly-TPD when a proper thickness PFN layer exists.

However, all perovskite films on different HTLs are

quite smooth, less pin-holes and smoother surface are observed in both AFM and SEM images of perovskite films on poly-TPD/PFN (Figure 1a and b). The extracted RMS value for perovskite on poly-TPD/PFN is ~1.0±0.2 nm, which is lower than that on either PVK (~1.8±0.6 nm) or PVK/PFN (~2.3±1.1 nm) ones. AFM images in Figure S4 show the polymer film morphologies of poly-TPD and poly-TPD/PFN, which indicates unchanged polymer morphologies after PFN deposition. However, the surface energy of poly-TPD is quite dependent on the PFN concentration. Therefore,

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the perovskite film quality is correlated with PFN concentration. Regarding to higher PFN concentration (0.075wt%) based film, pin-holes appear due to the decreased surface energy, as shown AFM and SEM images in Figure S5. Moreover, incomplete and discontinuous film is observed if the PFN concentration is 0.010wt%, as shown in Figure S6. It is worth to notice that the PFN layer can only exist at top surface of poly-TPD because the solvents used to dissolve poly-TPD and PFN are strict orthogonal. Herein, an optimized thickness of PFN layer on the top of poly-TPD is a key to obtain pin-hole free, continuous and compact perovskite film. Being as similar as perovskite on PVK/PFN layer, we also observe reduced defects for perovskite on poly-TPD/PFN layer, which is verified by enhanced PL properties, e.g. PL lifetime, and PLQY, as illustrated in Figure 2a, b and c and Figure S7. The fitted PL lifetime for perovskite on poly-TPD/PFN with different PFN concentration are ~53.7 ns (0.025wt%), ~65.3 ns (0.050wt%), and ~121.6 ns (0.075wt%), while the PLQY are ~48% (0.025wt%), ~68% (0.050wt%), and ~78% (0.075wt%), respectively. These PLQY values are comparable to those of 2D PEA2(FAPbBr3)n-1PbBr4 (n=3) composition perovskite or well-passivated colloidal nanocrystals30, 31. A structure of an indium tin oxide (ITO)/HTLs/PFN/perovskite/TPBi/LiF/Al is used to build LED devices. The thickness of HTLs/PFN, perovskite, TPBi layers are ~50 nm, ~50 nm, and ~20 nm, respectively, as indicated by cross-section SEM image in Figure 3a. LED devices exhibit pure green electroluminescence (EL) emission at ~512 nm with a FWHM of ~22 nm (Figure 3b). It is worth noting that the EL peak under different bias remains constant and symmetric, which indicates its stable crystal

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phase under electrical bias. Figure 3c shows a working device (1 cm×1.5 cm) images under 3 V or 5 V bias, revealing that the LED device displays uniform and bright emission in a relative large bias range. The angular emission intensity of device indicates EL emission follows a Lambertian profile (Figure S8). The current density-voltage-luminance

(J-V-L)

and

current

efficiency-luminance-EQE

(CE-L-EQE) characteristics of the devices are plotted in Figure 3d and 3e, and electrical output characteristics are summarized in Table 1. The PVK based device yields a current efficiency of 35.8 cd/A, corresponding to an EQE of 11.3%, which is comparable to the most reported low-dimensional perovskite based LEDs6, 8. However, in PVK/PFN (0.05wt%) device, an improved EQE of 11.8% is achieved. The improvement in PVK/PFN based device should be ascribed to the reduced surface defects in the presence of PFN. Remarkably, a champion device based on poly-TPD/PFN (0.050wt%) achieves a record EQE of 14.4%. In addition, our LED devices exhibit good reproducibility, as shown in the histogram of EQE from ~60 devices (Figure S9). The PVK, PVK/PFN and poly-TPD/PFN based devices yield an average EQE of ~9.4%, 10.1% and ~12.8%, respectively.

We also investigate the effect of PFN concentration on the device performance (Figure S10 and Table S2). LED based on 0.025wt% and 0.075wt% PFN yields an EQE of 12.8% and 13.8%, respectively. Additionally, 20-nm-thick TPBi is an optimized thickness to achieve champion performance, either increasing or decreasing the TPBi thickness would deteriorate device performance, as shown in Figure S11 and Table S3. The impressive efficiency improvement in poly-TPD/PFN based devices 9



can be ascribed to the following reasons. Firstly, the improved film quality reduces the non-radiative recombination centers in the perovskite emissive layer. Secondly, the improvement of hole injection efficiency with poly-TPD/PFN balances charge carrier injection rate at the perovskite interface. Therefore, the poly-TPD/PFN based device exhibits much higher current density than the PVK and PVK/PFN based ones in the space charge limited region (2.5-5V, J~V2) in J-V-L curves, which manifests enhanced radiative recombination rate. As a result, luminance in poly-TPD/PFN based device shows rapid growth with the luminance of 17210 cd/m2 at 5V compared with 2794 cd/m2 for PVK and 6571 cd/m2 for PVK/PFN based devices, respectively. Additionally, it is also found that trace PFN (0.02 mg/mL) in the perovskite precursor solution (defined as PFN modified perovskite) can improve the optical properties of perovskite films, however, the film morphology is obviously degraded, as shown in Figure S12. Obvious aggregation is observed in the PFN modified perovskite film. As a result, the LED devices based on PFN modified perovskite films exhibit high leakage current level in comparison to the pristine ones, leading to reduced device performance, as shown J-V-L and CE-L-EQE curves in Figure S13. The champion EQEs for PFN modified perovskite LED fabricated on PVK and poly-TPD/PFN are 9.6% and 11.8%, respectively. Notably, the poly-TPD/PFN based device shows a lower turn on voltage (Vt, defined as the voltage for luminance at 1 cd/m2) than PVK based one. A higher Vt is expected in poly-TPD based device due to its shallow HOMO level compared with PVK. According to the ultraviolet photoelectron spectroscopy (UPS) measurement (Figure

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4a), the extracted HOMO values for poly-TPD, poly-TPD/PFN and perovskite are ~5.3 eV, ~5.8 eV and ~5.7 eV, respectively. Previous work has revealed that surface dipoles are formed on PFN modified ITO, which influences the work function of ITO substrate32. In order to explore the surface dipole upon coating PFN layer, the surface potential difference is conducted by scanning Kelvin probe microscopy (SKPM). The experimental set-up is illustrated in Figure S14a. The surface potential of PFN on poly-TPD/ITO side is ~150 meV higher than that on ITO side, as shown in Figure S14b and c. An abrupt bump edge of poly-TPD generates due to the accumulated materials during spin coating process, as shown in Figure S14c. However, the surface potential does not follow the similar abrupt change. The difference of the measured surface potential of PFN on the ITO and poly-TPD side should be contributed to the different dipole direction formed at the interface due to the opposite wettability of ITO and poly-TPD. Consequently, the formed surface dipole can affect the HOMO level of the underlying poly-TPD layer. The schematic of reduced hole injection barrier from poly-TPD/PFN to perovskite layer is illustrated in Figure 4b, which can further improve the hole injection efficiency. In order to explore the carrier injection/transporting behaviors of PVK, PVK/PFN, poly-TPD/PFN, TPBi in LED devices, single carrier devices with only hole or electron induced current are characterized. The hole-only devices are fabricated with a multilayer structure of ITO/HTLs (~40 nm)/perovskite (~50 nm)/HTLs (~40 nm)/MoO3 (~20 nm)/Ag (~100 nm), while the electron-only device is made with a structure of ITO/TPBi/perovskite (~50 nm)/TPBi (~20 nm)/LiF (~1.5 nm)/Al (~100

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nm), as shown in Figure S15. Regarding to the electron-only device, we notice that the TPBi layer can be partly dissolved during the perovskite film deposition, as shown UV-vis absorption spectra in Figure S16. However, it is still clearly to tell that there remains a thin TPBi layer after DMSO spin coating. In addition, the electron-only device is also compared with the device with ITO/perovskite (~50 nm)/TPBi (~20 nm)/LiF (~1.5 nm)/Al (~100 nm), as shown in Figure S17, their current level are quite different. Moreover, EL is observed for the later device, which indicates that hole can be injected and recombined with the injected electrons. Due to the high lowest unoccupied orbital (LUMO) level of HTLs, injected electrons are blocked by HTLs, and only holes can pass through the device. According to the J-V curves of the single carrier devices (Figure 4c), the hole current density of devices on poly-TPD/PFN is much higher than that of PVK and PVK/PFN based ones, revealing much enhanced hole injection and transport properties in poly-TPD/PFN. The obvious hole current density enhancement can be attributed to the reduced hole injection barrier along with the high mobility of poly-TPD. The enhanced hole injection rate in poly-TPD/PFN matches well with the electron injection rate of TPBi. Therefore, the charge injection in LED device should be balanced, which promotes most injected electrons and holes to form excitons and improves the device efficiency. Finally, we measure the stability of perovskite LEDs with different HTLs under constant bias at 3.5 V (Figure 3f). All devices start to decay after several minutes, which is in line with previous reported perovskite LEDs with the similar device structure14. The decay mechanism is usually explained by ions migration in

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perovskite-based

optoelectronic

devices.

However,

we

observe

that

the

poly-TPD/PFN based device exhibits much slower decay rate compared with the PVK and PVK/PFN ones, indicating much improved operating lifetime. Conclusion In summary, we have achieved efficient perovskite LEDs through tailoring the carrier injection rate and suppressing non-radiative recombination rate in perovskite layer. Amphiphilic interface layer of PFN is used to increase the surface energy of hydrophobic poly-TPD, and a high quality perovskite film with excellent PL efficiency are obtained. In addition, the high hole mobility of poly-TPD matches well with the electron mobility of TPBi, which results in balanced charge injection rate for the obtained perovskite LEDs. Efficient green LEDs with a champion current efficiency of 45.2 cd/A, corresponding to an EQE of 14.4% are achieved. Moreover, the operating lifetime of our devices is dramatically enhanced in the presence of PFN layer. This work provides a way to develop efficient perovskite LEDs via tuning charge injection efficiency along with suppressing non-radiative recombination channels. Experiment Preparation of perovskite precursor solution: PEABr were synthesized with a previously reported method, where 6.71 g hydrobromic acid (Alfa, 48% in water) was added into 39.8 mmol phenethylamine (Acros, 99%) contained ethanol solution with vigorously stirring at 0 oC for 2 h. Then PEABr was dried at 50 oC for further purification. The prepared PEABr was purified by ethanol for three times and stored

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in glove box. 0.2 mmol CsBr (Sigma Aldrich, 99.999%), 0.2 mmol PbBr2 (Alfa, 99.999%), 0.08 mmol as-synthesized PEABr, and 4 mg 18-Crown-6 (Sigma Aldrich, 99.99%) were dissolved in 1 mL DMSO (Acros, 99.999%) to form perovskite precursor solution. Device fabrication: The LEDs were fabricated on pre-patterned ITO glass substrates. Firstly, the ITO substrates were cleaned in acetone, ethanol and deionized water for 15 min in sequence, followed ozone plasma treatment for 20 min before transferred into a glove-box (H2O

Boosting Perovskite Light-Emitting Diode Performance via Tailoring

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