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Enhancing the performance of mixed-halide perovskite-based light-emitting devices by organic additive inclusion
Synthetic Metals 253 (2019) 88–93
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Enhancing the performance of mixed-halide perovskite-based light-emitting devices by organic additive inclusion Jie Ren, Qi Wang, Xiaxia Ji, Xuefeng Peng, Zewu Xiao, Yanting Wu, Xiaohui Yang
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School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed-halide perovskites Halide phase separation Ligand-assisted crystallization Perovskite light-emitting devices
Mixed-halide perovskites show the phase instability under optical and electrical excitation, which limits their applications in tandem solar cells and color-tunable light-emitting devices. Herein, we study the effects of organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb (Br0.5I0.5)3 thin films. Organic additive incorporation reduces halide demixing, which can be attributed to its effect of defect passivation. As a result, the additive-encompassing light-emitting devices show the luminance efficiency of 0.32 cd/A and external quantum efficiency of 0.42%, which are ca. 5 times those of the counterparts without the additive, as well as improved electroluminescence spectrum stability.
1. Introduction Hybrid organic-inorganic lead halide perovskites are emerging as a new generation of semiconducting materials due to their outstanding properties such as solution processability, high charge carrier mobility, tunable bandgap, and large defect tolerance [1,2]. The R&D of perovskites is mainly focusing on solar cells, which promotes the powerconversion efficiency of the devices from an initially reported 3.8% to over 23% [3,4]. Meanwhile, the R&D is expanded to other optoelectronic devices including light-emitting devices [5–9], lasers [10], and photodetectors [11]. In 2014, Tan et al. [5] developed room-temperature operating perovskite light-emitting devices with the external quantum efficiencies (EQEs) in the range of 0.1–0.8%. Since then, the EQEs of perovskite light-emitting devices increase rapidly [12–15]. Halide alloying allows for facile perovskite bandgap tuning, for example, CH3NH3Pb(BrxI1-x)3 shows halide-composition-dependent bandgap, which can serve as a large bandgap absorber in tandem solar cells or a color-tunable emitter in light-emitting devices [16,17]. However, halide phase separation severely limits the use of CH3NH3Pb (BrxI1-x)3 in such applications. Namely, a low bandgap I-rich phase is formed when CH3NH3Pb(BrxI1-x)3 is subjected to optical and electrical excitation, leading to pinning of the photovoltage and emission wavelength at such I-rich phase [18–20]. There are still controversies about the underlying mechanism for halide phase separation. Bischak et al. [21] proposed that the lattice strain arising from the polaron formation induced I− migration, leading to halide segregation and the initial mixed state was restored upon the strain relaxation in the dark. Draguta
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et al. [20] argued that the composition-dependent free energy difference for MAPb(BrxI1-x)3 (0.2 < x < 0.5) was positive under illumination, resulting in halide demixing. It is commonly recognized that halide ion migration plays a dictating role in the halide phase separation process [17]. Several methods such as B-site alloying [22], A-site mixing [23], and the increase of sample crystallinity [24] have been reported to mitigate halide segregation. Ligand-assisted crystallization is an effective method to control the surface morphology and reduce the defect density of perovskite layers [25–28]. Compared with the commonly practiced post-treatment approach, ligand-assisted crystallization method allows for enhanced additive penetration into perovskite layers, thereby promoting passivation of defect states. The additive can improve charge injection and transport of perovskite layers as well. In many cases, Lewis base is introduced as electron-pair donor to coordinate with the lead precursor, enabling the effective reduction of defect density in perovskite layers [29]. The formation of the Lewis acid-base abduct may compensate halide vacancies and therefore is expected to affect the halide ion migration dynamics. In a pioneering study, Noel et al. [30] reported that the incorporation of pyridine decreased the defect density in MAPbI3 layers from 3.5 × 1016 to 0.2 × 1016 cm−3. Similarly, treatment of FAPbI3 layers with benzylamine increased the radiative recombination due to defect passivation [31]. La-Placa et al. [27] showed that the incorporation of 9,9-spirobifluoren-2-yl-diphenylphosphine oxide (SPPO13) via ligand-assisted crystallization method not only reduced the grain size of the quasi-two-dimensional layered perovskites, but also healed defects, resulting in a ca. 2.3 times improvement in the PLQY.
Corresponding author. E-mail address: [email protected] (X. Yang).
https://doi.org/10.1016/j.synthmet.2019.05.002 Received 15 November 2018; Received in revised form 20 April 2019; Accepted 4 May 2019 Available online 17 May 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic configuration (a) and energy level diagram (b) of light-emitting devices, the preparation of CH3NH3Pb(Br0.5I0.5)3 films using SPPO13-assisted crystallization method and chemical structure of SPPO13 (c).
CH3NH3I (0.37 mmol)/PbI2 (0.37 mmol) in DMF, respectively, which were stirred for 12 h. Afterwards, the CH3NH3Pb(Br0.5I0.5)3 precursor solution was prepared by mixing the CH3NH3PbBr3 and CH3NH3PbI3 precursor solutions (1:1 v/v). SPPO13 and TPBI were dissolved in CB. An S-PSS modified PEDOT:PSS layer [36] was spin-coated onto UVozone treated ITO substrates and subsequently baked at 170 ℃ for 10 min to remove residual water. The CH3NH3Pb(Br0.5I0.5)3 precursor solution was spin-coated at 3000 rpm onto the PEDOT:PSS layers. During the film forming process, 350 μl CB, SPPO13 or TPBI solution was dropped onto the spinning substrate to accelerate the crystallization process [37]. (Fig. 1(c)). The samples were thermally treated for 10 min and then transferred into an evaporation chamber residing inside a glovebox, where 60 nm TmPyPB, 1 nm CsF, and 100 nm Al were sequentially deposited. The thickness was 50 nm for PEDOT:PSS layer and 150 nm for CH3NH3Pb(Br0.5I0.5)3 layer as measured by a surface profiler. Device fabrication was conducted inside a glovebox with water and oxygen concentrations below 1 ppm except for PEDOT:PSS deposition. The CH3NH3Pb(Br0.5I0.5)3 layers were irradiated with constant wave 405 nm light to study their stabilities regarding halide segregation. The voltage − current density − luminance (V − I–L) property of the devices was measured with a programmed Keithley 2400 source − meter and Konica-Minolta CS − 100A chroma meter. The electroluminescence (EL) spectra were recorded with an Ocean Optics USB4000 UV–vis spectrometer. The photoluminescence (PL) spectra were probed with an Edinburgh FLS920 fluorescence spectrometer. The crystallographic structure was analyzed using a Rigaku D/Max-B X-ray diffractometer (XRD) equipped with a Cu Kα radiation source. The morphologies of the perovskite films were studied using a JSM−7100 F scanning electron microscope (SEM). All the measurements were carried out at room temperature under ambient conditions.
Despite the fact that the reduction of defect densities derived by organic additive incorporation may significantly affect the halide ion migration dynamics and the correlating photoluminescence and electroluminescence spectrum stabilities, an aspect which is crucial to realize color-tunable light-emitting devices, there is little work investigating this subject. We report the effects of the organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb (Br0.5I0.5)3 films. The addition of SPPO13 reduces halide phase separation in CH3NH3Pb(Br0.5I0.5)3. As a result, the SPPO13-encompassing light-emitting devices show improved electroluminescence spectrum stability and enhanced EQEs compared with those of the counterparts without SPPO13. 2. Experimental section Fig. 1 shows the schematic configuration of light-emitting devices, where poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) acts as the hole injection layer, cesium fluoride (CsF) as the electron injection layer, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) as the electron-transporting layer, indium tin oxide (ITO) and Al serve as transparent anode and cathode. The energy level diagram of the devices is presented in Fig. 1(b), in which the energy levels of the materials and electrode work function have been obtained from the literature [32–34]. There are the large offsets between the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital levels of SPPO13 and the conduction band minimum and valence band maximum of CH3NH3Pb(Br0.5I0.5)3, direct charge transfers between CH3NH3Pb(Br0.5I0.5)3 and SPPO13 are therefore unlikely. Park et al. [35] reported current density of the devices increased upon the addition of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI) additive and pointed out that poorly contacted MAPbBr3 grain boundaries may block electron injection and the presence of TPBI in MAPbBr3 grain boundaries may provide electron injection pathways, thereby improving electron injection into MAPbBr3 layer. Methylammonium bromide (CH3NH3Br), methylammonium iodide (CH3NH3I), lead bromide (PbBr2), lead iodide (PbI2), PEDOT:PSS, TmPyPB, TPBI, and SPPO13 were purchased from Xi’an Polymer Technology Corporation (China) and sodium-poly(styrenesulfonate) (SPSS), dimethylformamide (DMF), and chlorobenzene (CB) were obtained from Sigma-Aldrich. All materials were used as received. The CH3NH3PbBr3 and CH3NH3PbI3 precursor solutions were prepared by dissolving CH3NH3Br (0.37 mmol)/PbBr2 (0.37 mmol) and
3. Results and discussion Fig. 2 shows top-view scanning electron microscopy (SEM) images of the CH3NH3Pb(Br0.5I0.5)3 films. All the samples show a continuous crystalline morphology and high substrate coverage. The sample without SPPO13 incorporation exhibits the grain size of ca. 450 nm as calculated using Image J. With increasing SPPO13 concentration, the grain size continues to decrease to ca. 250 nm and 50 nm for the samples prepared with 0.5 mg/ml and 1 mg/ml SPPO13, respectively. During the film formation process, SPPO13 blocks diffusion of the constituents inside the sample and impedes crystallization, resulting in 89
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Fig. 2. Top-view SEM images of the samples without SPPO13 (a), prepared with 0.5 mg/ml (b), 1 mg/ml (c), and 2 mg/ml (d) SPPO13, insets: high-resolution images and the distribution of grain sizes.
prepared with 2.0 mg/ml SPPO13 is ca. 5 − 8 times that of the CBcrystallized sample. The PLQY of the CB-crystallized samples is 0.2 − 0.3%, in line with the result of a previous study [40], which is ca 20 − 30% that of the sample processed with 1 mg/ml SPPO13 (0.8 − 1%). The large PL intensity enhancement can be attributed to the healing of defects by SPPO13 originating from Lewis acid-base interactions between undercoordinated lead atom and O atom in SPPO13 [27,41]. Similar increase in the PL intensity of the samples upon TPBI addition is shown in Figure S1 in the supplementary content. As shown in Fig. 4(b), the PL spectrum of the sample prepared without SPPO13 shows the red-shifted maxima at 653 and 665 nm, respectively, after 2 and 4 min irradiation, along with a substantial intensity growth at ca. 700 nm, indicating the formation of I-rich phase [20]. By contrast, the post-illuminated PL spectra of the sample with SPPO13 vary in a much less extent. The results clearly reveal that the addition of SPPO13 suppresses halide demixing, in accord with the results of the XRD measurements. The PL intensity–irradiation time plots of the samples are presented in Fig. 4(d). The PL intensity of the sample without SPPO13 is decreased to ca. 80% of its initial intensity after 5 min irradiation, whereas the PL intensity of the sample with SPPO13 remains 98% of its initial intensity after the same treatment. The PL intensities of the samples fully recover after the samples are kept in the dark for 30 min. In addition, the PL maxima of both samples revert back to 645 nm, identical to those of the preilluminated samples. The results confirm that the changes in the PL spectra and intensities upon
decreased grain size. The sample prepared with 2 mg/ml SPPO13 shows a mesh-like surface and exhibits increased grain size probably as a result of the formation of a thin SPPO13 layer on the CH3NH3Pb (Br0.5I0.5)3 surface. As presented in Fig. 3(a), the samples show the diffraction peaks at ca. 14.4 and 29.2°, which can be indexed to the (100) and (200) crystal planes of the pseudocubic phase, respectively, consistent with a previous report [38]. With increasing SPPO13 concentration, the fullwidth at the half maximum (FWHM) of the (100) diffraction peak is enlarged, as shown in Fig. 3(a), indicating that the crystallinity decreases, which is in line with the SEM measurement results. The XRD measurements reveal that SPPO13 is mainly located in grain boundaries or on the top surface of the CH3NH3Pb(Br0.5I0.5)3 samples. After the CBcrystallized sample is illuminated with 405 nm light, the FWHM of the (100) diffraction peak increases from 0.26° to 0.35° (Fig. 3(b)), indicating the occurrence of halide phase segregation [39]. Compared with the CB-crystallized sample, the SPPO13-crystallized sample shows much less broadening of the (100) diffraction peak following the same treatment, revealing that the addition of SPPO13 inhibits halide phase separation. Fig. 4(a) shows the PL spectra of the samples processed with various SPPO13 concentrations. The PL spectra of all samples show a single emission peak at 645 nm, in agreement with a previous report [18]. The PL intensity of the sample monotonically increases with increasing SPPO13 concentration, for example, the PL intensity of the sample
Fig. 3. (a) XRD patterns of the samples; (b) the (100) diffraction peaks of the samples. prepared without SPPO13 (left panel) or with 1 mg/ml SPPO13 (right panel) before and after exposure to 405 nm light. 90
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Fig. 4. PL spectra of the CH3NH3Pb(Br0.5I0.5)3 samples processed with various SPPO13 concentrations (a), the evolution of PL spectra (b, c) and PL intensities (d) of the samples without SPPO13 (b) or processed with 1 mg/ml SPPO13 (c) with the illumination time as well as the PL spectra and intensities of the samples measured after holding the samples in the dark for 30 min.
device A and 208 cd/m2 for device C, then starts to decrease with further increase in SPPO13 concentration (43 cd/m2 for device D). Inset of Fig. 5(a) shows a photograph of a lit device C. Uniform light emission can be clearly seen. Fig. 5(b) shows the luminance efficiency–current density characteristics of the devices. The luminance efficiencies and EQEs of devices B and C are 0.20 cd/A/0.32% and 0.26 cd/A/0.42%, respectively, 4 and 6 times those of device A (0.05 cd/A/0.06%). The large performance improvement can be attributed to defect passivation of SPPO13 as discussed above. Further increase in SPPO13 concentration decreases the luminance efficiency and EQE to 0.12 cd/A/0.14% for device D. Fig. 5(c) and (d) show the EL spectra of devices A and C measured at different current densities. The EL spectra of both devices measured at 40 mA/cm2 show a maximum at 645 nm, almost identical to the PL spectra of the CH3NH3Pb(Br0.5I0.5)3 samples (Fig. 4(a)), indicating that charge recombination is confined within CH3NH3Pb(Br0.5I0.5)3 layer and halide demixing is absent under this condition. CIE coordinates of devices A and C at 40 mA/cm2 are (0.630, 0.306) and (0.630, 0.310), respectively. The EL spectrum of device A shows the red-shifted
illumination stem from reversible halide phase separation and the addition of SPPO13 stabilizes the PL spectrum. The addition of TPBI shows similar beneficial effects on PL spectrum stability as presented in Figure S2 in the supplementary content. We have prepared light-emitting devices with the structure of ITO/ PEDOT:PSS (50 nm)/CH3NH3Pb(Br0.5I0.5)3 (150 nm)/TmPyPB (60 nm)/ CsF (1 nm)/Al (100 nm) to study the effects of SPPO13 addition on the EL properties of CH3NH3Pb(Br0.5I0.5)3, in which devices A–D denote the devices prepared with 0, 0.5, 1, and 2 mg/ml SPPO13, respectively. The thermal treatment protocol for CH3NH3Pb(Br0.5I0.5)3 layer is optimized (Figure S3 in the supplementary content). The results indicate that the devices with a CH3NH3Pb(Br0.5I0.5)3 layer treated at 80 ℃ show the optimal luminance efficiency. As presented in Fig. 5(a), the addition of SPPO13 shifts the I–V curves toward a lower voltage direction, manifesting its effect of promotion of charge injection and transport in the CH3NH3Pb(Br0.5I0.5)3 layer [35]. Current density at a given voltage is reduced as SPPO13 concentration is increased to 2 mg/ml. Similarly, the maximum luminance of the devices first increases with increasing SPPO13 concentration, e.g. the maximum luminance is 20 cd/m2 for
Fig. 5. V–I–L (a) and luminance efficiency−current density (b) characteristics of the devices A–D, inset of (a): the photograph of a lit device C; EL spectra of the device A (c) and C (d) at different current densities. 91
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Fig. 6. Possible mechanism for mitigation of halide phase separation in CH3NH3Pb(Br0.5I0.5)3 upon SPPO13 addition.
maxima at 652 and 658 nm under 60 and 80 mA/cm2, respectively, together with the increased spectrum weight at ca. 700 nm, indicating the formation of I-rich phase under high current densities [39]. By contrast, much weaker EL maximum shift and intensity growth at ca. 700 nm are measured for device C. The results illustrate that the addition of SPPO13 stabilizes the EL spectra of CH3NH3Pb(Br0.5I0.5)3based light-emitting devices, which is in line with the results of the XRD and PL measurements. The luminance efficiency and EL spectrum stability of the TPBI-involved devices are improved as well (Figure S4 and S5 in the supplementary content). It has been shown that perovskite materials with a larger number of halide vacancies exhibit stronger halide phase separation [39]. Hoke et al. [18] report a migration activation energy (Ea) value of 0.27 eV for CH3NH3Pb(Br0.4I0.6)3, which is intermediate between the Ea values for VBr− and VI−, illustrating that halide ion migration via halide vacancies plays a dictating role in the halide phase separation process [17] (Fig. 6(a)). In this context, Lewis acid-base interactions between undercoordinated Pb atom and O atom in SPPO13 may effectively compensate halide vacancies [42], resulting in suppression of halide ion migration (Fig. 6(b)). Meanwhile, the reduction of the defect density in CH3NH3Pb(Br0.5I0.5)3 layers accounts for increased EQEs of lightemitting devices.
[2] Y. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6066. [4] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S.C. Ryu, J.W. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [5] Z.K. Tan, R.S. Moghaddam, M.L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L.M. Pazos, D. Credgington, F. Hanusch, T. Bein, H.J. Snaith, R.H. Friend, Bright light-emitting diodes based on organometal halide perovskite, Nat. Nanotechnol. 9 (2014) 687–692. [6] F.X. Yu, Y. Zhang, Z.Y. Xiong, X.J. Ma, P. Chen, Z.H. Xiong, C.H. Gao, Full coverage all-inorganic cesium lead halide perovskite film for high-efficiency light-emitting diodes assisted by 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, Org. Electron. 50 (2017) 480–484. [7] S.Q. Wu, S.L. Zhao, Z. Xu, D.D. Song, B. Qiao, H.X. Yue, J. Yang, X.G. Zheng, P. Wei, Highly bright and stable all-inorganic perovskite light-emitting diodes with methoxypolyethylene glycols modified CsPbBr3 emission layer, Appl. Phys. Lett. 113 (2018) 313501. [8] X. Ji, X. Peng, Q. Wang, J. Ren, Z. Xiong, X. Yang, On the performance of polymer:organometal halide perovskite composite light emitting devices: The effects of polymer additives, Org. Electron. 52 (2018) 350–355. [9] Q. Wang, J. Ren, X.F. Peng, X.X. Ji, X.H. Yang, Efficient sky-blue perovskite lightemitting devices based on ethylammonium bromide induced layered perovskites, ACS Appl. Mater. Interfaces 9 (2017) 29901–29906. [10] H. Zhu, Y. Fu, F. Meng, X. Wu, Z. Gong, Q. Ding, M.V. Gustafsson, M.T. Trinh, S. Jin, X.Y. Zhu, Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors, Nat. Mater. 14 (2015) 636–642. [11] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X.C. Zeng, J. Huang, High-gain and low-driving-voltage photodetectors based on organolead triiodide perovskites, Adv Mater 27 (2015) 1912–1918. [12] X. Yang, X. Zhang, J. Deng, Z. Chu, Q. Jiang, J. Meng, P. Wang, L. Zhang, Z. Yin, J. You, Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation, Nat. Commun. 9 (2018) 570. [13] N. Wang, L. Cheng, R. Ge, S. Zhang, Y. Miao, W. Zou, C. Yi, Y. Sun, Y. Cao, R. Yang, Y. Wei, Q. Guo, Y. Ke, M. Yu, Y. Jin, Y. Liu, Q. Ding, D. Di, L. Yang, G. Xing, H. Tian, C. Jin, F. Gao, R.H. Friend, J. Wang, W. Huang, Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells, Nat. Photonics 10 (2016) 699–704. [14] B. Zhao, S. Bai, V. Kim, R. Lamboll, R. Shivanna, F. Auras, J.M. Richter, L. Yang, L. Dai, M. Alsari, X. She, L. Liang, J. Zhang, S. Lilliu, P. Gao, H.J. Snaith, J. Wang, N.C. Greenham, R.H. Friend, D. Di, High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes, Nat. Photonics 12 (2018) 783–789. [15] K. Lin, J. Xing, L.N. Quan, F.P.Gd. Arquer, X. Gong, J. Lu, L. Xie, W. Zhao, D. Zhang, C. Yan, W. Li, X. Liu, Y. Lu, J. Kirman, E.H. Sargent, Q. Xiong, Z. Wei, Perovskite light-emitting diodes with external quantum efficiency exceeding 20 percent, Nature 562 (2018) 245–248. [16] D.J. Slotcavage, H.I. Karunadasa, M.D. McGehee, Light-induced phase segregation in halide-perovskite absorbers, ACS Energy Lett. 1 (2016) 1199–1205. [17] M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, Light-induced anion phase segregation in mixed halide perovskites, ACS Energy Lett. 3 (2018) 204–213. [18] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613–617. [19] I.L. Braly, R.J. Stoddard, A. Rajagopal, A.R. Uhl, J.K. Katahara, A.K.Y. Jen, H.W. Hillhouse, Current-induced phase segregation in mixed halide hybrid perovskites and its impact on two-terminal tandem solar cell design, ACS Energy Lett. 2 (2017) 1841–1847. [20] S. Draguta, O. Sharia, S.J. Yoon, M.C. Brennan, Y.V. Morozov, J.S. Manser, P.V. Kamat, W.F. Schneider, M. Kuno, Rationalizing the light-induced phase separation of mixed halide organic-inorganic perovskites, Nat. Commun. 8 (2017) 200. [21] C.G. Bischak, C.L. Hetherington, H. Wu, S. Aloni, D.F. Ogletree, D.T. Limmer, N.S. Ginsberg, Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites, Nano Lett. 17 (2017) 1028–1033.
4. Conclusion We have studied the influences of the organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb(Br0.5I0.5)3 films. The incorporation of SPPO13 reduces the defect density in the CH3NH3Pb(Br0.5I0.5)3 samples, enhancing PLQY and suppressing halide phase separation. This translates into ca. 5 times enhancement in the luminance efficiency and improved EL spectrum stability of CH3NH3Pb(Br0.5I0.5)3-based light-emitting devices. The results indicate that the incorporation of a suitable organic additive is an effective approach to improve the luminance efficiency and in particular the EL spectrum stability of CH3NH3Pb(Br0.5I0.5)3-based lightemitting devices. Acknowledgement Financial support by the National Natural Science Foundation of China (Grant no: 11474232). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019.05. 002. References [1] S.D. Stranks, H.J. Snaith, Metal-halide perovskites for photovoltaic and lightemitting devices, Nat. Nanotechnol. 10 (2015) 391–402.
92
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[22] Z. Yang, A. Rajagopal, S.B. Jo, C.C. Chueh, S. Williams, C.C. Huang, J.K. Katahara, H.W. Hillhouse, A.K. Jen, Stabilized wide bandgap perovskite solar cells by tin substitution, Nano Lett. 16 (2016) 7739–7747. [23] W. Rehman, D.P. McMeekin, J.B. Patel, R.L. Milot, M.B. Johnston, H.J. Snaith, L.M. Herz, Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties, Energy Environ. Sci. 10 (2017) 361–369. [24] M. Hu, C. Bi, Y. Yuan, Y. Bai, J. Huang, Stabilized wide bandgap MAPbBrx I3-x perovskite by enhanced grain size and improved crystallinity, Adv. Sci. (Weinh) 3 (2016) 1500301. [25] X.Y. Jia, Z.Y. Hu, J. Xu, L. Huang, J. Zhang, J.J. Zhang, Y.J. Zhu, Polymer assisted growth of high-quality perovskite films by Lewis acid-base adduct for efficient planar-heterojunction solar cells, Mater. Res. Bull. 95 (2017) 216–222. [26] P.L. Qin, G. Yang, Z.W. Ren, S.H. Cheung, S.K. So, L. Chen, J.H. Hao, J.H. Hou, G. Li, Stable and efficient organo-metal halide hybrid perovskite solar cells via π -conjugated lewis base polymer induced trap passivation and charge extraction, Adv. Mater. 30 (2018) 1706126. [27] M.G. La-Placa, G. Longo, A. Babaei, L. Martinez-Sarti, M. Sessolo, H.J. Bolink, Photoluminescence quantum yield exceeding 80% in low dimensional perovskite thin-films via passivation control, Chem. Commun. (Camb.) 53 (2017) 8707–8710. [28] X. Hou, S.M. Huang, O.Y. Wei, L.K. Pan, Z. Sun, X.H. Chen, Constructing efficient and stable perovskite solar cells via interconnecting perovskite grains, ACS Appl. Mater. Interfaces 9 (2017) 35200–35208. [29] F. Wang, S. Bai, W. Tress, A. Hagfeldt, F. Gao, Defects engineering for high-performance perovskite solar cells, NPJ Flex Elect 2 (2018) 22. [30] N.K. Noel, A. Abate, S.D. Stranks, E.S. Parrott, V.M. Burlakov, A. Goriely, H.J. Snaith, Enhanced photoluminescence and solar cell performance via Lewis base passivation of organicinorganic lead halide perovskites, ACS Nano 8 (2014) 9815–9821. [31] Y. Zhou, F. Wang, Y. Cao, J.P. Wang, H.H. Fang, M.A. Loi, N. Zhao, C.P. Wong, Benzylamine‐treated wide‐bandgap perovskite with high thermal‐photostability and photovoltaic performance, Adv. Energy Mater. 7 (2017) 1701048. [32] S.A. Veldhuis, P.P. Boix, N. Yantara, M. Li, T.C. Sum, N. Mathews, S.G. Mhaisalkar, Perovskite materials for light-emitting diodes and lasers, Adv Mater 28 (2016) 6804–6834. [33] X. Ji, X. Peng, Y. Lei, Z. Liu, X. Yang, Multilayer light emitting devices with organometal halide perovskite: polymer composite emission layer: the relationship of
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
93
device performance with the compositions of emission layer and device configurations, Org. Electron. 43 (2017) 167–174. J. Chen, C. Shi, Q. Fu, F. Zhao, Y. Hu, Y. Feng, D. Ma, Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs, J. Mater. Chem. 22 (2012) 5164–5170. M.H. Parka, S.H. Jeong, H.K. Seo, C. Wolf, Y.H. Kim, H. Kim, J. Byuna, J.S. Kim, H. Cho, T.W. Lee, Unravelling additive-based nanocrystal pinning for high efficiency organicinorganic halide perovskite light-emitting diodes, Nano Energy 42 (2017) 157–165. X.F. Peng, X.Y. Wu, X.X. Ji, J. Ren, Q. Wang, G.Q. Li, X.H. Yang, Modified conducting polymer hole injection layer for high-efficiency perovskite light-emitting devices: enhanced hole injection and reduced luminescence quenching, J. Phys. Chem. Lett. 8 (2017) 4691–4697. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells, Angew. Chem. Int. Ed. Engl. 53 (2014) 9898–9903. T. Duong, H.K. Mulmudi, Y. Wu, X. Fu, H. Shen, J. Peng, N. Wu, H.T. Nguyen, D. Macdonald, M. Lockrey, T.P. White, K. Weber, K. Catchpole, Light and electrically induced phase segregation and its impact on the stability of quadruple cation high bandgap perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 26859–26866. A.J. Barker, A. Sadhanala, F. Deschler, M. Gandini, S.P. Senanayak, P.M. Pearce, E. Mosconi, A.J. Pearson, Y. Wu, A.R. Srimath Kandada, T. Leijtens, F. De Angelis, S.E. Dutton, A. Petrozza, R.H. Friend, Defect-assisted photoinduced halide segregation in mixed-halide perovskite thin films, ACS Energy Lett. 2 (2017) 1416–1424. R.J. Stoddard, F.T. Eickemeyer, J.K. Katahara, H.W. Hillhouse, Correlation between Photoluminescence and carrier transport and a simple in situ passivation method for high-bandgap hybrid perovskites, J. Phys. Chem. Lett. 8 (2017) 3289–3298. D.W. deQuilettes, S. Koch, S. Burke, R.K. Paranji, A.J. Shropshire, M.E. Ziffer, D.S. Ginger, Photoluminescence lifetimes exceeding 8 μs and quantum yields exceeding 30% in hybrid perovskite thin films by ligand passivation, ACS Energy Lett. 1 (2016) 438–444. Y. Lin, L. Shen, J. Dai, Y. Deng, Y. Wu, Y. Bai, X. Zheng, J. Wang, Y. Fang, H. Wei, W. Ma, X.C. Zeng, X. Zhan, J. Huang, Pi-conjugated Lewis base: efficient trappassivation and charge-extraction for hybrid perovskite solar cells, Adv. Mater. 29 (2017) 1604545.
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Enhancing the performance of mixed-halide perovskite-based light-emitting devices by organic additive inclusion Jie Ren, Qi Wang, Xiaxia Ji, Xuefeng Peng, Zewu Xiao, Yanting Wu, Xiaohui Yang
T
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School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed-halide perovskites Halide phase separation Ligand-assisted crystallization Perovskite light-emitting devices
Mixed-halide perovskites show the phase instability under optical and electrical excitation, which limits their applications in tandem solar cells and color-tunable light-emitting devices. Herein, we study the effects of organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb (Br0.5I0.5)3 thin films. Organic additive incorporation reduces halide demixing, which can be attributed to its effect of defect passivation. As a result, the additive-encompassing light-emitting devices show the luminance efficiency of 0.32 cd/A and external quantum efficiency of 0.42%, which are ca. 5 times those of the counterparts without the additive, as well as improved electroluminescence spectrum stability.
1. Introduction Hybrid organic-inorganic lead halide perovskites are emerging as a new generation of semiconducting materials due to their outstanding properties such as solution processability, high charge carrier mobility, tunable bandgap, and large defect tolerance [1,2]. The R&D of perovskites is mainly focusing on solar cells, which promotes the powerconversion efficiency of the devices from an initially reported 3.8% to over 23% [3,4]. Meanwhile, the R&D is expanded to other optoelectronic devices including light-emitting devices [5–9], lasers [10], and photodetectors [11]. In 2014, Tan et al. [5] developed room-temperature operating perovskite light-emitting devices with the external quantum efficiencies (EQEs) in the range of 0.1–0.8%. Since then, the EQEs of perovskite light-emitting devices increase rapidly [12–15]. Halide alloying allows for facile perovskite bandgap tuning, for example, CH3NH3Pb(BrxI1-x)3 shows halide-composition-dependent bandgap, which can serve as a large bandgap absorber in tandem solar cells or a color-tunable emitter in light-emitting devices [16,17]. However, halide phase separation severely limits the use of CH3NH3Pb (BrxI1-x)3 in such applications. Namely, a low bandgap I-rich phase is formed when CH3NH3Pb(BrxI1-x)3 is subjected to optical and electrical excitation, leading to pinning of the photovoltage and emission wavelength at such I-rich phase [18–20]. There are still controversies about the underlying mechanism for halide phase separation. Bischak et al. [21] proposed that the lattice strain arising from the polaron formation induced I− migration, leading to halide segregation and the initial mixed state was restored upon the strain relaxation in the dark. Draguta
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et al. [20] argued that the composition-dependent free energy difference for MAPb(BrxI1-x)3 (0.2 < x < 0.5) was positive under illumination, resulting in halide demixing. It is commonly recognized that halide ion migration plays a dictating role in the halide phase separation process [17]. Several methods such as B-site alloying [22], A-site mixing [23], and the increase of sample crystallinity [24] have been reported to mitigate halide segregation. Ligand-assisted crystallization is an effective method to control the surface morphology and reduce the defect density of perovskite layers [25–28]. Compared with the commonly practiced post-treatment approach, ligand-assisted crystallization method allows for enhanced additive penetration into perovskite layers, thereby promoting passivation of defect states. The additive can improve charge injection and transport of perovskite layers as well. In many cases, Lewis base is introduced as electron-pair donor to coordinate with the lead precursor, enabling the effective reduction of defect density in perovskite layers [29]. The formation of the Lewis acid-base abduct may compensate halide vacancies and therefore is expected to affect the halide ion migration dynamics. In a pioneering study, Noel et al. [30] reported that the incorporation of pyridine decreased the defect density in MAPbI3 layers from 3.5 × 1016 to 0.2 × 1016 cm−3. Similarly, treatment of FAPbI3 layers with benzylamine increased the radiative recombination due to defect passivation [31]. La-Placa et al. [27] showed that the incorporation of 9,9-spirobifluoren-2-yl-diphenylphosphine oxide (SPPO13) via ligand-assisted crystallization method not only reduced the grain size of the quasi-two-dimensional layered perovskites, but also healed defects, resulting in a ca. 2.3 times improvement in the PLQY.
Corresponding author. E-mail address: [email protected] (X. Yang).
https://doi.org/10.1016/j.synthmet.2019.05.002 Received 15 November 2018; Received in revised form 20 April 2019; Accepted 4 May 2019 Available online 17 May 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic configuration (a) and energy level diagram (b) of light-emitting devices, the preparation of CH3NH3Pb(Br0.5I0.5)3 films using SPPO13-assisted crystallization method and chemical structure of SPPO13 (c).
CH3NH3I (0.37 mmol)/PbI2 (0.37 mmol) in DMF, respectively, which were stirred for 12 h. Afterwards, the CH3NH3Pb(Br0.5I0.5)3 precursor solution was prepared by mixing the CH3NH3PbBr3 and CH3NH3PbI3 precursor solutions (1:1 v/v). SPPO13 and TPBI were dissolved in CB. An S-PSS modified PEDOT:PSS layer [36] was spin-coated onto UVozone treated ITO substrates and subsequently baked at 170 ℃ for 10 min to remove residual water. The CH3NH3Pb(Br0.5I0.5)3 precursor solution was spin-coated at 3000 rpm onto the PEDOT:PSS layers. During the film forming process, 350 μl CB, SPPO13 or TPBI solution was dropped onto the spinning substrate to accelerate the crystallization process [37]. (Fig. 1(c)). The samples were thermally treated for 10 min and then transferred into an evaporation chamber residing inside a glovebox, where 60 nm TmPyPB, 1 nm CsF, and 100 nm Al were sequentially deposited. The thickness was 50 nm for PEDOT:PSS layer and 150 nm for CH3NH3Pb(Br0.5I0.5)3 layer as measured by a surface profiler. Device fabrication was conducted inside a glovebox with water and oxygen concentrations below 1 ppm except for PEDOT:PSS deposition. The CH3NH3Pb(Br0.5I0.5)3 layers were irradiated with constant wave 405 nm light to study their stabilities regarding halide segregation. The voltage − current density − luminance (V − I–L) property of the devices was measured with a programmed Keithley 2400 source − meter and Konica-Minolta CS − 100A chroma meter. The electroluminescence (EL) spectra were recorded with an Ocean Optics USB4000 UV–vis spectrometer. The photoluminescence (PL) spectra were probed with an Edinburgh FLS920 fluorescence spectrometer. The crystallographic structure was analyzed using a Rigaku D/Max-B X-ray diffractometer (XRD) equipped with a Cu Kα radiation source. The morphologies of the perovskite films were studied using a JSM−7100 F scanning electron microscope (SEM). All the measurements were carried out at room temperature under ambient conditions.
Despite the fact that the reduction of defect densities derived by organic additive incorporation may significantly affect the halide ion migration dynamics and the correlating photoluminescence and electroluminescence spectrum stabilities, an aspect which is crucial to realize color-tunable light-emitting devices, there is little work investigating this subject. We report the effects of the organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb (Br0.5I0.5)3 films. The addition of SPPO13 reduces halide phase separation in CH3NH3Pb(Br0.5I0.5)3. As a result, the SPPO13-encompassing light-emitting devices show improved electroluminescence spectrum stability and enhanced EQEs compared with those of the counterparts without SPPO13. 2. Experimental section Fig. 1 shows the schematic configuration of light-emitting devices, where poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) acts as the hole injection layer, cesium fluoride (CsF) as the electron injection layer, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) as the electron-transporting layer, indium tin oxide (ITO) and Al serve as transparent anode and cathode. The energy level diagram of the devices is presented in Fig. 1(b), in which the energy levels of the materials and electrode work function have been obtained from the literature [32–34]. There are the large offsets between the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital levels of SPPO13 and the conduction band minimum and valence band maximum of CH3NH3Pb(Br0.5I0.5)3, direct charge transfers between CH3NH3Pb(Br0.5I0.5)3 and SPPO13 are therefore unlikely. Park et al. [35] reported current density of the devices increased upon the addition of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI) additive and pointed out that poorly contacted MAPbBr3 grain boundaries may block electron injection and the presence of TPBI in MAPbBr3 grain boundaries may provide electron injection pathways, thereby improving electron injection into MAPbBr3 layer. Methylammonium bromide (CH3NH3Br), methylammonium iodide (CH3NH3I), lead bromide (PbBr2), lead iodide (PbI2), PEDOT:PSS, TmPyPB, TPBI, and SPPO13 were purchased from Xi’an Polymer Technology Corporation (China) and sodium-poly(styrenesulfonate) (SPSS), dimethylformamide (DMF), and chlorobenzene (CB) were obtained from Sigma-Aldrich. All materials were used as received. The CH3NH3PbBr3 and CH3NH3PbI3 precursor solutions were prepared by dissolving CH3NH3Br (0.37 mmol)/PbBr2 (0.37 mmol) and
3. Results and discussion Fig. 2 shows top-view scanning electron microscopy (SEM) images of the CH3NH3Pb(Br0.5I0.5)3 films. All the samples show a continuous crystalline morphology and high substrate coverage. The sample without SPPO13 incorporation exhibits the grain size of ca. 450 nm as calculated using Image J. With increasing SPPO13 concentration, the grain size continues to decrease to ca. 250 nm and 50 nm for the samples prepared with 0.5 mg/ml and 1 mg/ml SPPO13, respectively. During the film formation process, SPPO13 blocks diffusion of the constituents inside the sample and impedes crystallization, resulting in 89
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Fig. 2. Top-view SEM images of the samples without SPPO13 (a), prepared with 0.5 mg/ml (b), 1 mg/ml (c), and 2 mg/ml (d) SPPO13, insets: high-resolution images and the distribution of grain sizes.
prepared with 2.0 mg/ml SPPO13 is ca. 5 − 8 times that of the CBcrystallized sample. The PLQY of the CB-crystallized samples is 0.2 − 0.3%, in line with the result of a previous study [40], which is ca 20 − 30% that of the sample processed with 1 mg/ml SPPO13 (0.8 − 1%). The large PL intensity enhancement can be attributed to the healing of defects by SPPO13 originating from Lewis acid-base interactions between undercoordinated lead atom and O atom in SPPO13 [27,41]. Similar increase in the PL intensity of the samples upon TPBI addition is shown in Figure S1 in the supplementary content. As shown in Fig. 4(b), the PL spectrum of the sample prepared without SPPO13 shows the red-shifted maxima at 653 and 665 nm, respectively, after 2 and 4 min irradiation, along with a substantial intensity growth at ca. 700 nm, indicating the formation of I-rich phase [20]. By contrast, the post-illuminated PL spectra of the sample with SPPO13 vary in a much less extent. The results clearly reveal that the addition of SPPO13 suppresses halide demixing, in accord with the results of the XRD measurements. The PL intensity–irradiation time plots of the samples are presented in Fig. 4(d). The PL intensity of the sample without SPPO13 is decreased to ca. 80% of its initial intensity after 5 min irradiation, whereas the PL intensity of the sample with SPPO13 remains 98% of its initial intensity after the same treatment. The PL intensities of the samples fully recover after the samples are kept in the dark for 30 min. In addition, the PL maxima of both samples revert back to 645 nm, identical to those of the preilluminated samples. The results confirm that the changes in the PL spectra and intensities upon
decreased grain size. The sample prepared with 2 mg/ml SPPO13 shows a mesh-like surface and exhibits increased grain size probably as a result of the formation of a thin SPPO13 layer on the CH3NH3Pb (Br0.5I0.5)3 surface. As presented in Fig. 3(a), the samples show the diffraction peaks at ca. 14.4 and 29.2°, which can be indexed to the (100) and (200) crystal planes of the pseudocubic phase, respectively, consistent with a previous report [38]. With increasing SPPO13 concentration, the fullwidth at the half maximum (FWHM) of the (100) diffraction peak is enlarged, as shown in Fig. 3(a), indicating that the crystallinity decreases, which is in line with the SEM measurement results. The XRD measurements reveal that SPPO13 is mainly located in grain boundaries or on the top surface of the CH3NH3Pb(Br0.5I0.5)3 samples. After the CBcrystallized sample is illuminated with 405 nm light, the FWHM of the (100) diffraction peak increases from 0.26° to 0.35° (Fig. 3(b)), indicating the occurrence of halide phase segregation [39]. Compared with the CB-crystallized sample, the SPPO13-crystallized sample shows much less broadening of the (100) diffraction peak following the same treatment, revealing that the addition of SPPO13 inhibits halide phase separation. Fig. 4(a) shows the PL spectra of the samples processed with various SPPO13 concentrations. The PL spectra of all samples show a single emission peak at 645 nm, in agreement with a previous report [18]. The PL intensity of the sample monotonically increases with increasing SPPO13 concentration, for example, the PL intensity of the sample
Fig. 3. (a) XRD patterns of the samples; (b) the (100) diffraction peaks of the samples. prepared without SPPO13 (left panel) or with 1 mg/ml SPPO13 (right panel) before and after exposure to 405 nm light. 90
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Fig. 4. PL spectra of the CH3NH3Pb(Br0.5I0.5)3 samples processed with various SPPO13 concentrations (a), the evolution of PL spectra (b, c) and PL intensities (d) of the samples without SPPO13 (b) or processed with 1 mg/ml SPPO13 (c) with the illumination time as well as the PL spectra and intensities of the samples measured after holding the samples in the dark for 30 min.
device A and 208 cd/m2 for device C, then starts to decrease with further increase in SPPO13 concentration (43 cd/m2 for device D). Inset of Fig. 5(a) shows a photograph of a lit device C. Uniform light emission can be clearly seen. Fig. 5(b) shows the luminance efficiency–current density characteristics of the devices. The luminance efficiencies and EQEs of devices B and C are 0.20 cd/A/0.32% and 0.26 cd/A/0.42%, respectively, 4 and 6 times those of device A (0.05 cd/A/0.06%). The large performance improvement can be attributed to defect passivation of SPPO13 as discussed above. Further increase in SPPO13 concentration decreases the luminance efficiency and EQE to 0.12 cd/A/0.14% for device D. Fig. 5(c) and (d) show the EL spectra of devices A and C measured at different current densities. The EL spectra of both devices measured at 40 mA/cm2 show a maximum at 645 nm, almost identical to the PL spectra of the CH3NH3Pb(Br0.5I0.5)3 samples (Fig. 4(a)), indicating that charge recombination is confined within CH3NH3Pb(Br0.5I0.5)3 layer and halide demixing is absent under this condition. CIE coordinates of devices A and C at 40 mA/cm2 are (0.630, 0.306) and (0.630, 0.310), respectively. The EL spectrum of device A shows the red-shifted
illumination stem from reversible halide phase separation and the addition of SPPO13 stabilizes the PL spectrum. The addition of TPBI shows similar beneficial effects on PL spectrum stability as presented in Figure S2 in the supplementary content. We have prepared light-emitting devices with the structure of ITO/ PEDOT:PSS (50 nm)/CH3NH3Pb(Br0.5I0.5)3 (150 nm)/TmPyPB (60 nm)/ CsF (1 nm)/Al (100 nm) to study the effects of SPPO13 addition on the EL properties of CH3NH3Pb(Br0.5I0.5)3, in which devices A–D denote the devices prepared with 0, 0.5, 1, and 2 mg/ml SPPO13, respectively. The thermal treatment protocol for CH3NH3Pb(Br0.5I0.5)3 layer is optimized (Figure S3 in the supplementary content). The results indicate that the devices with a CH3NH3Pb(Br0.5I0.5)3 layer treated at 80 ℃ show the optimal luminance efficiency. As presented in Fig. 5(a), the addition of SPPO13 shifts the I–V curves toward a lower voltage direction, manifesting its effect of promotion of charge injection and transport in the CH3NH3Pb(Br0.5I0.5)3 layer [35]. Current density at a given voltage is reduced as SPPO13 concentration is increased to 2 mg/ml. Similarly, the maximum luminance of the devices first increases with increasing SPPO13 concentration, e.g. the maximum luminance is 20 cd/m2 for
Fig. 5. V–I–L (a) and luminance efficiency−current density (b) characteristics of the devices A–D, inset of (a): the photograph of a lit device C; EL spectra of the device A (c) and C (d) at different current densities. 91
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Fig. 6. Possible mechanism for mitigation of halide phase separation in CH3NH3Pb(Br0.5I0.5)3 upon SPPO13 addition.
maxima at 652 and 658 nm under 60 and 80 mA/cm2, respectively, together with the increased spectrum weight at ca. 700 nm, indicating the formation of I-rich phase under high current densities [39]. By contrast, much weaker EL maximum shift and intensity growth at ca. 700 nm are measured for device C. The results illustrate that the addition of SPPO13 stabilizes the EL spectra of CH3NH3Pb(Br0.5I0.5)3based light-emitting devices, which is in line with the results of the XRD and PL measurements. The luminance efficiency and EL spectrum stability of the TPBI-involved devices are improved as well (Figure S4 and S5 in the supplementary content). It has been shown that perovskite materials with a larger number of halide vacancies exhibit stronger halide phase separation [39]. Hoke et al. [18] report a migration activation energy (Ea) value of 0.27 eV for CH3NH3Pb(Br0.4I0.6)3, which is intermediate between the Ea values for VBr− and VI−, illustrating that halide ion migration via halide vacancies plays a dictating role in the halide phase separation process [17] (Fig. 6(a)). In this context, Lewis acid-base interactions between undercoordinated Pb atom and O atom in SPPO13 may effectively compensate halide vacancies [42], resulting in suppression of halide ion migration (Fig. 6(b)). Meanwhile, the reduction of the defect density in CH3NH3Pb(Br0.5I0.5)3 layers accounts for increased EQEs of lightemitting devices.
[2] Y. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6066. [4] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S.C. Ryu, J.W. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [5] Z.K. Tan, R.S. Moghaddam, M.L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L.M. Pazos, D. Credgington, F. Hanusch, T. Bein, H.J. Snaith, R.H. Friend, Bright light-emitting diodes based on organometal halide perovskite, Nat. Nanotechnol. 9 (2014) 687–692. [6] F.X. Yu, Y. Zhang, Z.Y. Xiong, X.J. Ma, P. Chen, Z.H. Xiong, C.H. Gao, Full coverage all-inorganic cesium lead halide perovskite film for high-efficiency light-emitting diodes assisted by 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, Org. Electron. 50 (2017) 480–484. [7] S.Q. Wu, S.L. Zhao, Z. Xu, D.D. Song, B. Qiao, H.X. Yue, J. Yang, X.G. Zheng, P. Wei, Highly bright and stable all-inorganic perovskite light-emitting diodes with methoxypolyethylene glycols modified CsPbBr3 emission layer, Appl. Phys. Lett. 113 (2018) 313501. [8] X. Ji, X. Peng, Q. Wang, J. Ren, Z. Xiong, X. Yang, On the performance of polymer:organometal halide perovskite composite light emitting devices: The effects of polymer additives, Org. Electron. 52 (2018) 350–355. [9] Q. Wang, J. Ren, X.F. Peng, X.X. Ji, X.H. Yang, Efficient sky-blue perovskite lightemitting devices based on ethylammonium bromide induced layered perovskites, ACS Appl. Mater. Interfaces 9 (2017) 29901–29906. [10] H. Zhu, Y. Fu, F. Meng, X. Wu, Z. Gong, Q. Ding, M.V. Gustafsson, M.T. Trinh, S. Jin, X.Y. Zhu, Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors, Nat. Mater. 14 (2015) 636–642. [11] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X.C. Zeng, J. Huang, High-gain and low-driving-voltage photodetectors based on organolead triiodide perovskites, Adv Mater 27 (2015) 1912–1918. [12] X. Yang, X. Zhang, J. Deng, Z. Chu, Q. Jiang, J. Meng, P. Wang, L. Zhang, Z. Yin, J. You, Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation, Nat. Commun. 9 (2018) 570. [13] N. Wang, L. Cheng, R. Ge, S. Zhang, Y. Miao, W. Zou, C. Yi, Y. Sun, Y. Cao, R. Yang, Y. Wei, Q. Guo, Y. Ke, M. Yu, Y. Jin, Y. Liu, Q. Ding, D. Di, L. Yang, G. Xing, H. Tian, C. Jin, F. Gao, R.H. Friend, J. Wang, W. Huang, Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells, Nat. Photonics 10 (2016) 699–704. [14] B. Zhao, S. Bai, V. Kim, R. Lamboll, R. Shivanna, F. Auras, J.M. Richter, L. Yang, L. Dai, M. Alsari, X. She, L. Liang, J. Zhang, S. Lilliu, P. Gao, H.J. Snaith, J. Wang, N.C. Greenham, R.H. Friend, D. Di, High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes, Nat. Photonics 12 (2018) 783–789. [15] K. Lin, J. Xing, L.N. Quan, F.P.Gd. Arquer, X. Gong, J. Lu, L. Xie, W. Zhao, D. Zhang, C. Yan, W. Li, X. Liu, Y. Lu, J. Kirman, E.H. Sargent, Q. Xiong, Z. Wei, Perovskite light-emitting diodes with external quantum efficiency exceeding 20 percent, Nature 562 (2018) 245–248. [16] D.J. Slotcavage, H.I. Karunadasa, M.D. McGehee, Light-induced phase segregation in halide-perovskite absorbers, ACS Energy Lett. 1 (2016) 1199–1205. [17] M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, Light-induced anion phase segregation in mixed halide perovskites, ACS Energy Lett. 3 (2018) 204–213. [18] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613–617. [19] I.L. Braly, R.J. Stoddard, A. Rajagopal, A.R. Uhl, J.K. Katahara, A.K.Y. Jen, H.W. Hillhouse, Current-induced phase segregation in mixed halide hybrid perovskites and its impact on two-terminal tandem solar cell design, ACS Energy Lett. 2 (2017) 1841–1847. [20] S. Draguta, O. Sharia, S.J. Yoon, M.C. Brennan, Y.V. Morozov, J.S. Manser, P.V. Kamat, W.F. Schneider, M. Kuno, Rationalizing the light-induced phase separation of mixed halide organic-inorganic perovskites, Nat. Commun. 8 (2017) 200. [21] C.G. Bischak, C.L. Hetherington, H. Wu, S. Aloni, D.F. Ogletree, D.T. Limmer, N.S. Ginsberg, Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites, Nano Lett. 17 (2017) 1028–1033.
4. Conclusion We have studied the influences of the organic additives on the morphology, crystallographic structure, and photophysics property of CH3NH3Pb(Br0.5I0.5)3 films. The incorporation of SPPO13 reduces the defect density in the CH3NH3Pb(Br0.5I0.5)3 samples, enhancing PLQY and suppressing halide phase separation. This translates into ca. 5 times enhancement in the luminance efficiency and improved EL spectrum stability of CH3NH3Pb(Br0.5I0.5)3-based light-emitting devices. The results indicate that the incorporation of a suitable organic additive is an effective approach to improve the luminance efficiency and in particular the EL spectrum stability of CH3NH3Pb(Br0.5I0.5)3-based lightemitting devices. Acknowledgement Financial support by the National Natural Science Foundation of China (Grant no: 11474232). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019.05. 002. References [1] S.D. Stranks, H.J. Snaith, Metal-halide perovskites for photovoltaic and lightemitting devices, Nat. Nanotechnol. 10 (2015) 391–402.
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Synthetic Metals 253 (2019) 88–93
J. Ren, et al.
[22] Z. Yang, A. Rajagopal, S.B. Jo, C.C. Chueh, S. Williams, C.C. Huang, J.K. Katahara, H.W. Hillhouse, A.K. Jen, Stabilized wide bandgap perovskite solar cells by tin substitution, Nano Lett. 16 (2016) 7739–7747. [23] W. Rehman, D.P. McMeekin, J.B. Patel, R.L. Milot, M.B. Johnston, H.J. Snaith, L.M. Herz, Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties, Energy Environ. Sci. 10 (2017) 361–369. [24] M. Hu, C. Bi, Y. Yuan, Y. Bai, J. Huang, Stabilized wide bandgap MAPbBrx I3-x perovskite by enhanced grain size and improved crystallinity, Adv. Sci. (Weinh) 3 (2016) 1500301. [25] X.Y. Jia, Z.Y. Hu, J. Xu, L. Huang, J. Zhang, J.J. Zhang, Y.J. Zhu, Polymer assisted growth of high-quality perovskite films by Lewis acid-base adduct for efficient planar-heterojunction solar cells, Mater. Res. Bull. 95 (2017) 216–222. [26] P.L. Qin, G. Yang, Z.W. Ren, S.H. Cheung, S.K. So, L. Chen, J.H. Hao, J.H. Hou, G. Li, Stable and efficient organo-metal halide hybrid perovskite solar cells via π -conjugated lewis base polymer induced trap passivation and charge extraction, Adv. Mater. 30 (2018) 1706126. [27] M.G. La-Placa, G. Longo, A. Babaei, L. Martinez-Sarti, M. Sessolo, H.J. Bolink, Photoluminescence quantum yield exceeding 80% in low dimensional perovskite thin-films via passivation control, Chem. Commun. (Camb.) 53 (2017) 8707–8710. [28] X. Hou, S.M. Huang, O.Y. Wei, L.K. Pan, Z. Sun, X.H. Chen, Constructing efficient and stable perovskite solar cells via interconnecting perovskite grains, ACS Appl. Mater. Interfaces 9 (2017) 35200–35208. [29] F. Wang, S. Bai, W. Tress, A. Hagfeldt, F. Gao, Defects engineering for high-performance perovskite solar cells, NPJ Flex Elect 2 (2018) 22. [30] N.K. Noel, A. Abate, S.D. Stranks, E.S. Parrott, V.M. Burlakov, A. Goriely, H.J. Snaith, Enhanced photoluminescence and solar cell performance via Lewis base passivation of organicinorganic lead halide perovskites, ACS Nano 8 (2014) 9815–9821. [31] Y. Zhou, F. Wang, Y. Cao, J.P. Wang, H.H. Fang, M.A. Loi, N. Zhao, C.P. Wong, Benzylamine‐treated wide‐bandgap perovskite with high thermal‐photostability and photovoltaic performance, Adv. Energy Mater. 7 (2017) 1701048. [32] S.A. Veldhuis, P.P. Boix, N. Yantara, M. Li, T.C. Sum, N. Mathews, S.G. Mhaisalkar, Perovskite materials for light-emitting diodes and lasers, Adv Mater 28 (2016) 6804–6834. [33] X. Ji, X. Peng, Y. Lei, Z. Liu, X. Yang, Multilayer light emitting devices with organometal halide perovskite: polymer composite emission layer: the relationship of
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
93
device performance with the compositions of emission layer and device configurations, Org. Electron. 43 (2017) 167–174. J. Chen, C. Shi, Q. Fu, F. Zhao, Y. Hu, Y. Feng, D. Ma, Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs, J. Mater. Chem. 22 (2012) 5164–5170. M.H. Parka, S.H. Jeong, H.K. Seo, C. Wolf, Y.H. Kim, H. Kim, J. Byuna, J.S. Kim, H. Cho, T.W. Lee, Unravelling additive-based nanocrystal pinning for high efficiency organicinorganic halide perovskite light-emitting diodes, Nano Energy 42 (2017) 157–165. X.F. Peng, X.Y. Wu, X.X. Ji, J. Ren, Q. Wang, G.Q. Li, X.H. Yang, Modified conducting polymer hole injection layer for high-efficiency perovskite light-emitting devices: enhanced hole injection and reduced luminescence quenching, J. Phys. Chem. Lett. 8 (2017) 4691–4697. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells, Angew. Chem. Int. Ed. Engl. 53 (2014) 9898–9903. T. Duong, H.K. Mulmudi, Y. Wu, X. Fu, H. Shen, J. Peng, N. Wu, H.T. Nguyen, D. Macdonald, M. Lockrey, T.P. White, K. Weber, K. Catchpole, Light and electrically induced phase segregation and its impact on the stability of quadruple cation high bandgap perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 26859–26866. A.J. Barker, A. Sadhanala, F. Deschler, M. Gandini, S.P. Senanayak, P.M. Pearce, E. Mosconi, A.J. Pearson, Y. Wu, A.R. Srimath Kandada, T. Leijtens, F. De Angelis, S.E. Dutton, A. Petrozza, R.H. Friend, Defect-assisted photoinduced halide segregation in mixed-halide perovskite thin films, ACS Energy Lett. 2 (2017) 1416–1424. R.J. Stoddard, F.T. Eickemeyer, J.K. Katahara, H.W. Hillhouse, Correlation between Photoluminescence and carrier transport and a simple in situ passivation method for high-bandgap hybrid perovskites, J. Phys. Chem. Lett. 8 (2017) 3289–3298. D.W. deQuilettes, S. Koch, S. Burke, R.K. Paranji, A.J. Shropshire, M.E. Ziffer, D.S. Ginger, Photoluminescence lifetimes exceeding 8 μs and quantum yields exceeding 30% in hybrid perovskite thin films by ligand passivation, ACS Energy Lett. 1 (2016) 438–444. Y. Lin, L. Shen, J. Dai, Y. Deng, Y. Wu, Y. Bai, X. Zheng, J. Wang, Y. Fang, H. Wei, W. Ma, X.C. Zeng, X. Zhan, J. Huang, Pi-conjugated Lewis base: efficient trappassivation and charge-extraction for hybrid perovskite solar cells, Adv. Mater. 29 (2017) 1604545.