Enhanced Performance in Fluorene-Free Organometal Halide Perovskite Light-Emitting Diodes using Tunable, Low Electron Affinity Oxide Electron Injectors

Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence are obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH3NH3PbBr3 perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier can also be reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.


DOI: 10.1002/adma.201405044
perovskite layer, and acts as a spacer to the metal cathode to prevent emission quenching. [ 4,13 ] However, F8 is known to be unstable under applied bias in air [ 14 ] and its mobility decreases at low temperatures, [ 15 ] making it incompatible with the cryogenic analysis techniques typically used to study crystalline semiconductors. [ 16 ] Critically, visible-light PeLEDs with F8 show parasitic blue emission due to the formation of excitons in the polyfl uorene across the perovskite-free voids, which reduces their color purity. [ 4 ] Replacing this organic spacer is therefore the crucial next step in PeLED design. More generally, most perovskite LEDs and solar cells rely on expensive, unstable, and low-conductivity fl uorenes for charge injection or extraction, such as 2,2′,7,7′-tetrakis( N , N -dip -methoxyphenylamine)-9,9′spiro-bifl uorene (spiro-OMeTAD) [ 17 ] or F8. Finding new electrode materials that overcome these limitations is thus essential for the commercial application of perovskite optoelectronics.
Stable metal oxide electrodes, such as ZnO, are promising replacements for fl uorenes, but are limited by a fi xed electron injection level. [ 3,18 ] In addition, spray pyrolysis, which is the common method for producing metal oxides in hybrid LEDs, requires deposition temperatures >350 °C. This renders it incompatible with fl exible polymer substrates or direct deposition onto perovskites. [ 3,18,19 ] In this work, we overcame these limitations by using spatial atmospheric atomic layer-deposited (SAALD) ZnO fi lms, formed at only 60 °C and deposited directly onto green-emitting methylammonium lead tri-bromide (CH 3 NH 3 PbBr 3 ) perovskite. Through this, we produced the fi rst PeLEDs not reliant on fl uorene-based layers and which were also brighter than the previous report of devices using F8. We found that our new structure presented several advantages. The electron injection barrier with the perovskite could be reduced by incorporating Mg into ZnO to produce Zn 1− x Mg x O, which decreased the electron affi nity from −3.6 to −3.35 eV relative to the vacuum level and reduced the PeLED turn-on voltage. These PeLEDs were stable under bias, and exhibited an electroluminescence peak half as wide as that of conventional InGaN LEDs. The emission was purely from the perovskite (and not contaminated by any emission from the electron injector), which makes PeLEDs with the perovskite/Zn 1− x Mg x O junction very promising for display applications, as well as electrically pumped lasing. To show that our tunable metal oxide is high quality and can be used in effi cient LEDs, we also applied this electron injector to mature polymer LED (PLED) technology, demonstrating luminous effi ciencies comparable to or exceeding previous records with hybrid green F8BT and blue aryl-F8:0.5 wt% TFB devices, Organic light-emitting diodes (LEDs) are a multibillion dollar industry, with applications in displays, lighting, and consumer devices. [ 1 ] The current limitation is the diffi culty in depositing organic emitters by vacuum sublimation over a large area cost effectively. [1][2][3] Solution-processable materials, such as organometal halide perovskites [ 4 ] and conjugated polymers [ 3 ] have the potential to overcome this limitation, due to their compatibility with roll-to-roll solution-processing techniques and inkjet printing. [ 2,3,5,6 ] The development of complementary new electrode materials is essential to increase the performance and commercial potential of LEDs based on these emitters. [ 3,7 ] Organometal halide perovskites used in solar cells have demonstrated an astonishing increase in power conversion efficiency from 4% in 2009 to 19.3% in 2014, [ 8 ] while their high photoluminescence quantum effi ciency has allowed the fi rst demonstrations of optically pumped lasing. [ 9,10 ] More recently, the demonstration of room temperature electroluminescence from this material also promises a bright future for perovskite LED (PeLED) technology. [ 4 ] The appealing properties of organometal halide perovskites enabling these advances are their sharp band edges, [ 11 ] low non-radiative recombination rates, and suppressed defect formation. [ 12 ] In addition, the perovskite band gap can be tuned through its chemical composition, enabling PeLEDs to achieve emission from the infrared to green. [ 4 ] Previously, poly (9,9-dioctylfl uorene) or F8 was used as a large bandgap charge-blocking layer in PeLEDs, which helps to prevent electrical shorts through the discontinuous as well as the ability to signifi cantly reduce the operating voltage by reducing the electron injection barrier.
SAALD ZnO was deposited onto the perovskite using diethylzinc and water vapor precursors to produce a layered structure PeLED: ITO/PEDOT:PSS/CH 3 NH 3 PbBr 3 /ZnO/Ca/Ag ( Figure 1 a), and this successfully led to devices displaying room temperature electroluminescence (Figure 1 b). Lowering the deposition temperature from 150 °C to 60 °C signifi cantly improved the luminance from 0.2 to 550 cd m −2 , whilst maintaining a low turn-on voltage of 2 V (Figure 1 b). This performance improvement was due to a reduction in the pore size in the perovskite as the deposition temperature was reduced ( Figure 1 c-e) and is a notable improvement over the previous report of green PeLEDs using F8, which had a maximum luminance of 364 cd m −2 . [ 4 ] We also note that the PeLEDs with SAALD ZnO deposited on the perovskite had signifi cantly higher performance than PeLEDs with sol-gel TiO x deposited on the perovskite using a previously reported method, [ 20 ] as shown in Figure 1 b and Figure S1 (Supporting Information). This was due to solvents from the TiO x precursor damaging the perovskite layer during deposition ( Figure S2, Supporting Information).
Depositing ZnO onto organometal halide perovskites using an ALD-based technique is typically challenging because conventional ALD requires pumping down to vacuum, which increases the deposition time and reduces scalability. [ 21 ] In particular, this would require heating the perovskite (typically between 70 °C and 150 °C) [ 22,23 ] for 30 min or more in the ALD chamber, which can detrimentally affect the substrate coverage and morphology. [ 24 ] Using SAALD allowed us to overcome these diffi culties, because the metal oxide can be rapidly deposited in open air at low temperatures (150 °C or below). [ 22,25 ] We were therefore able to directly load and unload the samples from the substrate holder, resulting in the samples being heated only for the time required to deposit the fi lms (3 min herein). This minimizes the adverse effects of heating the perovskite during deposition.
The use of water vapor as an SAALD precursor, however, presents a major drawback as organometal halide perovskites are known to react with water. [ 12 ] As a result, the reliability of the PeLEDs with SAALD ZnO deposited using water vapor as the oxidant was low, with only 15% of the devices turning on. We improved this reliability to more than 60% turning on by instead using oxygen gas as the oxidant, since perovskites are known to be stable to oxygen exposure. [ 26 ] As is shown in Figure 1 b, SAALD ZnO deposited with oxygen gas led to PeLEDs with a higher turn-on voltage of 5 V. This was primarily due to the ZnO deposited with oxygen gas having a larger electron affi nity than that deposited with water vapor. This leads to the former having a smaller band gap, as measured by photoluminescence spectroscopy ( Figure S3, Supporting Information), which would lead to a larger electron injection barrier with the CH 3 NH 3 PbBr 3 . In addition, ZnO deposited with oxygen gas has a higher resistivity (Table S1, Supporting Information). However, a signifi cant benefi t of producing the ZnO by SAALD is that the electron affi nity can be fi nely tuned through Mg incorporation. [ 25 ] The comparison of energy levels presented in Figure 2 a,b suggests that incorporating 44 at% Mg into ZnO leads to barrierless electron injection. Figure 2 c shows that replacing SAALD ZnO with SAALD Zn 0.56 Mg 0.44 O reduces the turn-on voltage by approximately 1 V, to a level comparable with PeLEDs using F8.
To investigate the advantages of replacing the polyfl uorene electron injector with our tunable metal oxide, we synthesized PeLEDs with F8 using the previously reported structure (Figure 2 d), [ 4 ] which reproduced the previously reported performance ( Figure S4, Supporting Information). The electroluminescence spectrum of these PeLEDs ( Figure 3 a) exhibited peaks at 428, 452, and 500 nm, in addition to the perovskite emission peak at 525 nm. These are due to parasitic emission from the F8, as can be seen by comparing with the normalized electroluminescence spectrum of F8-only devices in Figure 3 b, and is also indicated by its band diagram in Figure 2 d. In addition, when the F8 PLED was held at 9 V bias, the initially blue electroluminescence spectrum became dominated by new green peaks (at wavelengths of 520, 570, and 620 nm) after only 7 s. This green emission was irreversible, and is consistent with the formation of on-chain keto defects in the F8 due to reaction with oxygen. [ 14 ] Using SAALD Zn 0.56 Mg 0.44 O instead of F8 in these PeLEDs resulted in a single, well-defi ned electroluminescence peak at 525 nm (Figure 3 c), which matched the photoluminescence peak of the bare perovskite (Figure 3 d). Identical behavior was observed using SAALD ZnO deposited using either oxygen gas Adv. Mater. 2015, 27, 1414-1419 www.advmat.de www.MaterialsViews.com The full width half maximum (FWHM) of the electroluminescence peak was 25 nm, which is half of the FWHM from industry standard green InGaN LEDs [ 27 ] and less than half the FWHM of the peak from a "fruit-fl y" F8BT PLED (Figure 3 d). The PeLED spectral emission is also sharper than that reported for some green colloidal quantum dot LEDs (>30 nm FWHM), [ 28 ] for which high color purity is considered one of their greatest strengths. [ 29 ] Zn 1− x Mg x O also exhibits a deep valence band at −7 eV, [ 25 ] presenting a barrier of 1.3 eV to injected holes and thereby providing strong leakage suppression. Using SAALD Zn 1− x Mg x O therefore allows PeLEDs to fulfi ll their potential as stable, color-pure devices with sharp emission spectra, a possibility suggested by the sharp band edges of perovskite semiconductors. [ 11 ] This is crucial for ultrahighdefi nition display applications, in which a narrower spectral width of the three primary colors results in better color rendering and range. [ 29 ] Our metal oxide electron injector could also be used to achieve electrically pumped lasing from perovskites, [ 10 ] as the oxide enables electrically induced emission to occur from the perovskite only. In addition, cryogenic measurements can be performed on PeLEDs with Zn 1− x Mg x O, since the metal oxides possess high carrier mobilities even at low temperatures. [ 30,31 ] The use of organometal halide perovskites in LEDs is currently in its infancy but, if their development in the solar cells fi eld can be taken as an example, these materials promise a wealth of discoveries that will lead to rapid effi ciency developments. To demonstrate that our tunable metal oxide is high quality and could be used in effi cient devices, we applied our SAALD Zn 1− x Mg x O to mature PLED technology. We produced green PLEDs using emissive layers of poly(9,9-dioctyl fl uorenealtbenzothiadiazole) or F8BT, as well as blue PLEDs using aryl polyfl uorene mixed with poly(9,9-dioctylfl uoreneco -N -(4-butylphenyl)diphenylamine) or aryl-F8:0.5 wt%TFB. Using a 60 nm SAALD Zn 0.85 Mg 0.15 O electron injector, a luminous effi ciency of 21 cd A −1 was obtained from the green F8BT PLEDs ( Figure 4 a), which is comparable with the highest reported in the literature, [ 3 ] while a luminous effi ciency of 6.8 cd A −1 was obtained from blue aryl-F8:0.5 wt%TFB PLEDs (Figure 4 b), which is higher than the previous report of 5.9 cd A −1 . [ 18 ] Carefully increasing the Mg content in Zn 1− x Mg x O also reduced the bias required to produce 200 cd m −2 luminance from both types of PLEDs, while maintaining luminous effi ciency (Figure 4 c and Figure S6 and S7, Supporting Information). We also note that the bias producing 10 mA cm −2 from the blue PLEDs was reduced from 16 (using SAALD ZnO) to 9.3 V (using SAALD Zn 0.56 Mg 0.44 O), as shown by Figure 4 d, which is signifi cantly lower than the previously reported 16.9 V for aryl-F8:0.5 wt%TFB PLEDs using spraypyrolyzed ZnO. [ 18 ] This emphasizes that our tunable metal oxide is just as effective an electron injector as spray-pyrolyzed ZnO (previously the favored cathode material for effi cient hybrid LEDs), [ 3,18,32 ] but with the additional benefi ts of exhibiting a tunable electron injection level and the capability to be deposited at low temperature, directly onto perovskite emitters to produce PeLEDs that do not rely on fl uorene-based organic transport layers.
In conclusion, we produced fl uorene-free PeLEDs by adopting a very careful synthesis approach that enabled the deposition of SAALD ZnO directly onto CH 3 NH 3 PbBr 3 perovskite. This involved using a short deposition time and low processing temperature of 60 °C, producing green PeLEDs with an improved turn-on voltage and higher luminance than Adv. Mater. 2015, 27, 1414-1419 www.advmat.de www.MaterialsViews.com  [ 4,25,33,34 ] those previously reported using F8. The advantages of using SAALD ZnO as the electron injector instead of F8 are: i) The ability to tune the electron injection level through Mg incorporation, reducing the turn-on voltage, ii) sharp, color-pure emission from the PeLEDs due to a larger hole injection barrier, and iii) enhanced stability in air and compatibility with cryogenic measurements. The improvements obtained with SAALD Zn 1− x Mg x O make these PeLEDs highly appealing for ultrahigh defi nition display applications, and paves the way toward electrically pumped lasers. Being able to deposit a metal oxide with a tunable electron affi nity onto perovskites could also be highly desirable for electron transport layers (ETLs) in inverted perovskite solar cells that are not reliant on fl uorene-based layers, [ 20,24 ] since this enables the ETL electron energy level alignment with the absorber LUMO to be optimized to minimize energy losses. [ 25 ] SAALD Zn 1− x Mg x O can play an important role in fl uorene-free PeLEDs as this new fi eld develops and their effi ciencies increase to a commercially appealing level, as seen from the highly effi cient green F8BT and blue aryl-F8:0.5 wt% TFB PLEDs that were produced using our metal oxide electron injector. This work therefore demonstrates a successful tunable oxide electron injector that can enable high performance in perovskite LEDs that are not dependent on expensive and less stable fl uorene layers.

Experimental Section
Perovskite Light-Emitting Diode Fabrication : ITO/ glass (Colorado Concepts LLC) substrates were ultrasonically cleaned in acetone and isopropyl alcohol for 15 min, followed by oxygen plasma cleaning for 10 min with a Diener Low Pressure Plasma System. PEDOT:PSS (Clevios) was spin cast at 6000 rpm for 30 s and annealed at 140 °C for 30 min in a nitrogen-fi lled glovebox. CH 3 NH 3 PbBr 3 (prepared according to previous reports) [ 4 ] was spin cast on the PEDOT:PSS at 3000 rpm for 30 s and annealed at 80 °C for 15 min, followed by 100 °C for 2 min. For conventional devices, F8 (Cambridge Display Technology) was spin cast from solution (10 mg mL −1 in chlorobenzene) on the perovskite at 3000 rpm for 30 s. For PeLEDs with SAALD Zn 1− x Mg x O, the metal oxides were deposited in open air onto the perovskite at 60 °C for 3 min using either: i) nitrogen gas bubbled through deionized water at 100 mL min −1 and diluted with 200 mL min −1 nitrogen gas or ii) oxygen gas fl owing at 100 mL min −1 to the gas manifold as the oxidant. For SAALD ZnO, nitrogen gas was bubbled through diethylzinc at 25 mL min −1 and diluted with 100 mL min −1 carrier nitrogen gas before being fed to the gas manifold. Nitrogen gas fl owing at 500 mL min −1 was also fed to the gas manifold to form the inert gas channels. For Zn 1− x Mg x O, the only difference was that the nitrogen gas was bubbled through the diethylzinc at 4 and 200 mL min −1 through bis(ethylcyclopentadienyl)magnesium heated to 55 °C. Sol-gel TiO x was prepared and deposited using a previously reported method. [ 20 ] To complete the devices, 20 nm Ca and 100 nm Ag were thermally evaporated through a shadow mask to give devices with an area of 5.25 mm 2 .
Polymer Light-Emitting Diode Fabrication : ITO/ glass (Colorado Concepts LLC) substrates were ultrasonically cleaned in toluene and isopropyl alcohol for 15 min. SAALD Zn 1− x Mg x O was deposited at 150 °C onto the ITO according to previous reports. [ 25 ] These fi lms were annealed at 400 °C for 15 min. Cs 2 CO 3 (5 mg mL −1 in 2-methoxymethanol) was spin cast on the annealed SAALD Zn 1− x Mg x O at 6000 rpm for 45 s. For effi cient F8BT PLEDs, 1200 nm F8BT (45 mg mL −1 in p-xylene) was spin cast on top at 2000 rpm for 45 s before being annealed at 160 °C for 1 h in a nitrogen-fi lled glovebox. TFB (20 mg mL −1 in p-xylene) was spin cast on top of both types of fi lms at 700 rpm for 45 s. For blue PLEDs, aryl-F8 (30 mg mL −2 in p-xylene) had 0.5 wt% TFB mixed into it, and this mixture was spin cast at 2000 rpm for 45 s to give a 450 nm thick fi lm, which was annealed at 120 °C for 1 h in a nitrogen-fi lled glovebox. For the F8BT and aryl-F8:0.5 wt% TFB PLEDs, MoO 3 (10 nm), and Au (50 nm) were thermally evaporated on top through a shadow mask to produce 5.25 mm 2 pixels.
Device Characterization : The luminous effi ciencies, external quantum effi ciencies, and current densities were measured using a silicon photodiode and Keithley 2400 source measure unit according to previous reports. [ 4 ] The electroluminescence spectra were also measured according to previous reports using a Labsphere CDS-610 spectrometer. [ 4 ] Film Characterization : The photoluminescence of the perovskite fi lm on ITO/PEDOT:PSS was measured with an InGaAs detector (Andor iDus DU490A). These measurements were performed in an integrating sphere under a constant stream of nitrogen and the excitation laser had a wavelength of 405 nm (Omnicron LDM405.100.CWA.L) with a power of 100 mW. Scanning electron microscopy (SEM) images of the perovskite fi lms were obtained using a LEO VP-1530 fi eld emission scanning electron microscope.