Rubidium as an Alternative Cation for Efficient Perovskite Light

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Surfaces, Interfaces, and Applications

Rubidium as an Alternative Cation for Efficient Perovskite Light Emitting Diodes Anil Kanwat, Eric Moyen, Sinyoung Cho, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01292 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Rubidium as an Alternative Cation for Efficient Perovskite Light Emitting Diodes Anil Kanwat, Eric Moyen, Sinyoung Cho, and Jin Jang *

Advanced Display Research Center (ADRC), Department of Information Display, Kyung Hee University, Dongdaemoon-ku, Seoul, 130-701, Korea *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Incorporation of Rubidium (Rb) into mixed lead halide perovskites has recently achieved record power conversion efficiency and excellent stability in perovskite solar cells. Inspired by these tremendous advances in photovoltaics, this study demonstrates the impact of Rb incorporation into MAPbBr3-based light emitters. Rb partially substitutes MA (methyl ammonium), resulting in a mixed cation perovskite with the formula MA(1-x)RbxPbBr3. Pure MAPbBr3 crystallizes into a polycrystalline layer with highly defective sub-micrometer grains. However, the addition of a small amount of Rb forms MA(1-x)RbxPbBr3 nanocrystals (10 nm) embedded in an amorphous matrix of MA/Rb Br. These nanocrystals grow into defect free sub-micrometer sized crystallites with further addition of Rb, resulting in a threefold increase in exciton lifetime when the molar ratio of MABr:RbBr is 1:1. A thin film fabricated with a 1:1 molar ratio of MABr:RbBr showed the best electroluminescent properties with a current efficiency (CE) of 9.45 cd/A and a luminance of 7694cd/m2. These values of CE efficiency and luminance are respectively nineteen and ten times larger than those achieved by pure MAPbBr3 devices (0.5 cd/A and 790 cd/m2). We believe this work provides important information on the future compositional optimization of Rb+-based mixed cation perovskites for obtaining highperformance light emitting diodes.

KEYWORDS: Light emitting diode, Mixed halide cations, Nanocrystals, Perovskite, Polycrystalline


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INTRODUCTION Organic-inorganic metal halide perovskites (OMHPs) or all-inorganic metal halide perovskites (MHPs) with the structure ABX3 (where A and B are cations, and X is an anion (a halide or oxide)), represent a new and disruptive technology in the field of optoelectronics.1–6 Perovskites exhibit excellent optoelectronic properties such as high optical absorption coefficients, long diffusion lengths for charge carriers, high photoluminescence quantum yields (PLQYs) and easy bandgap tunability.7–15 These properties make them ideal candidates for solar cells and light emitting diode (LED) applications. OMHPs have revolutionized the technology from dye-sensitized solar cells in 2009 with a power conversion efficiency (PCE) of only 3.8% to solid state solar cells in 2016 with a certified PCE of 22.1%.16 Perovskites for electroluminance (EL) have recently attracted the attention of many researchers worldwide. EL from perovskite LEDs (PeLEDs) was first reported in 2015 by Z. K. Tan et al.2 The authors demonstrated green and red LEDs with external quantum efficiency (EQE) of 0.1 and 0.76% by utilizing MAPbBr3 and MAPbI3-xClx OMHP, respectively. This opened the doors to the extensive study of green emission PeLEDs.17–27 More recently, EQE of 8.53% has been achieved by a nanocrystal pinning (NCP) method.6 Pinning of nanocrystallite (NCs) results in the formation of 100 nm-sized crystallites. A nonstoichiometric ratio of precursors (MABr:PbBr2 =1.05:1) reduces the concentration of Pb+ ions at grain boundaries, thus reducing the density of non-radiative recombination centers. Perovskite thin films obtained by NCP method exhibit a poor surface coverage resulting in a high leakage current when integrated into PeLED devices. An excess of MABr (MABr:PbBr2, 3:1) can improve the surface coverage. The excess of MABr forms a matrix which passivates 12 nm sized MAPbBr3 NCs and reduces leakage current.23 Such thin films enabled to obtain current efficiency as high as 34.46 CdA-1 when integrated into a


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LED. These thin films are as thick as 800 nm which could be the reason PeLEDs has lower brightness. Alternatively, MA can be substituted by organo-ammonium halides such as butyl ammonium bromide (BABr) and phenethylammonium bromide (PEBr) to constrain the growth of NCs during the film formation.24-26 Such films are 70 nm (BABr) and 50 nm (PEABr) thick with high surface coverage and very low surface roughness. NCs can be as small as 10 nm (BABr) or 6.4 nm (PEABr) and EQEs are as high as 9.3 (BaBr) or 7.0% (PEABr). However, nanocrystalline thin films tend to exhibit a high density of surface defects by increasing the surface/volume ratio, which limits the PLQY and therefore the performance of the PeLEDs. Owing to the sensitivity of perovskite, defects impede fast in humidity which limits the devices stability. Thus, larger crystallites have recently been studied, as they pose ultralow trap densities that are in the order of 1010 cm-3.28,29 For example, an anti-solvent vapor post-treatment of a CsPbBr3 thin film increases its grain size from 250 nm to 5 µm.28 The average exciton lifetime increases from 46.4 µs to 180.6 µs due to the reduction of surface trap density therefore, performances and stability of the PeLEDs are improved.30 However, this method adds additional steps to the fabrication process. Alternatively, it has recently been shown that Rb incorporation into MAPbI3 thin films prepared by the conventional one step approach can induce a significant reduction of trap density, resulting in high photocurrents when integrated in perovskite solar cells. 31 As such reduction of defects could also significantly improve the performances of PeLEDs, here we study the incorporation of Rb into a MAPbBr3 thin films and its consequences on PeLEDs performances. Photophysical properties of the thin films are analyzed by PL, optical


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absorbance, and time resolved photoluminance (TRPL) with different Rb concentrations into MAPbBr3. Furthermore, structural studies are carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), amd X-ray spectroscopy (XPS). RESULTS AND DISCUSSION Two perovskite layers, MAPbBr3 and MAxRb1-xPbBr3, were deposited from solution by first mixing their respective precursors in two solvents; dimethylformaide (DMF) and dimethylsulphoxide (DMSO). The precursors used for MAPbBr3 are MABr and PbBr2. For MAxRb1-xPbBr3, a third precursor, RbBr, is added. The MAPbBr3 solution was prepared with a molar ratio MABr:PbBr2 of 2:1 to achieve a nonstoichiometric layer with excess MA. Three MAxRb1-xPbBr3 solutions were prepared by mixing MABr, RbBr, and PbBr2, with molar ratios of MABr:RbBr of 1:0.33, 1:0.66, and 1:1, respectively. As RbBr has limited solubility in DMF, DMSO was added to DMF (6:4 v/v ratio) to increase its solubility (Fig. S1). Samples with a molar ratio MABr:RbBr of 1:0, 1:0.33, 1:0.66, and 1:1 will be denoted S1, S2, S3 and S4 in the manuscript. Thin films prepared in this manuscript are coated on indium tin oxide (ITO) / poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and annealed at 80°C for 20 min unless otherwise specified. Figure 1a shows XRD patterns of S1, S2, S3 and S4. The diffraction pattern of S1 depicts the typical cubic phase of MAPbBr3. 2,11 After Rb+ incorporation, the diffraction peaks shifted towards higher diffraction angles. Similar shifts are reported in the literature with smaller cation doping. 21, 32 The (100) peak of MAPbBr3 at 15.01o shifted to 15.36o as the concentration of RbBr increased, indicating lattice constant reduction upon Rb incorporation. The reduction in the lattice constant can be attributed to the smaller size of Rb with respect to MA. Shrinkage of the perovskite lattice upon smaller cation incorporation was previously reported when MA or Cs was


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added into formamidinium (FAPbBr3) or Cs was added to MAPbBr3.21, 32, 33 Thus, Rb+ seems to partially replace MA to form a mixed halide perovskite with an MA1-xRbxPbBr3 composition. Moreover, as shown in the details of Figure 1b, XRD peaks exhibited similar full width halfmaximum (FWHM) values for S1, S3 and S4, except for S2, where the FWHM value was higher than the rest. This indicates that the minimal amount of Rb in MAPbBr3 induces the formation of crystallites that are small enough to induce a widening of the XRD peaks; i.e. well below 50 nm. It is not possible to distinguish whether the formation of NCs is a direct consequence of the addition of Rb or the result of a non-stoichiometric ratio of precursors as reported by J. W. Lee et al.23 Thus, TEM images are obtained to thoroughly examine the impact of Rb doping on morphologies. Pure MAPbBr3 (S1) is composed of the crystal grains of

Rubidium as an Alternative Cation for Efficient Perovskite Light

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