Effect of Bathocuproine Organic Additive on Optoelectronic Properties

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Effect of BCP Organic Additive on Optoelectronic Properties of Highly Efficient Methylammonium Lead Bromide Perovskite Light Emitting Diodes Xiaojuan Sun, Changfeng Han, Kai Wang, Haomiao Yu, Jinpeng Li, Kai Lu, Jiajun Qin, Hanjun Yang, Liangliang Deng, Fenggui Zhao, Qin Yang, and Bin Hu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01410 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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ACS Applied Energy Materials

Effect of BCP Organic Additive on Optoelectronic Properties of Highly Efficient Methylammonium Lead Bromide Perovskite Light Emitting Diodes Xiaojuan Sun,a Changfeng Han,a Kai Wang,a,* Haomiao Yu,a Jinpeng Li,a Kai Lu,b Jiajun Qin,c Hanjun Yang,a Liangliang Deng,a Fenggui Zhao,a Qin Yang,a Bin Hua,b,c,*

a

Key Laboratory of Luminescence and Optical Information, Ministry of Education,

School of Science, Beijing Jiaotong University, Beijing 100044, China

b

Wu Han National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Wu Han 430074, China

c

Department of Materials Science and Engineering, University of Tennessee,

Knoxville, Tennessee, 37996, United States

KEYWORDS: organic-inorganic hybrid perovskites, CH3NH3PbBr3, BCP, perovskite light emitting diodes, high-color purity

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ABSTRACT

Recent studies have shown that introductions of some organic additives are beneficial for optimizing organic-inorganic hybrid perovskites (OIHPs) optoelectronic properties in order to promote their industrial realizations. In this work, a typical organic molecule bathocuproine (BCP) has been utilized during the preparation of methyl-ammonium lead tri-bromide (CH3NH3PbBr3) precursor, and a perovskite-based light

emitting

diode

(PeLED)

consisting

of

ITO(glass)/PEDOT:PSS/CH3NH3PbBr3:BCP/Bphen/Ag was fabricated. Owing to the participation of BCP, the PeLED exhibits excellent light emissive properties without any crystalline distortions of CH3NH3PbBr3, such as low turn-on voltage 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 =

2.5 V , high luminescent intensity 𝐿𝐿 = 1.4 × 104 cd/m2 and high efficiency 𝜂𝜂 =

3.2 cd/A. Based on electronic transport and optically spectroscopic characterizations,

it has been verified that such remarkable and desirable enhancements are attributed to three contributions, (i) a decrease of CH3NH3PbBr3 crystalline grain size, (ii) a reduction of trap density (𝑛𝑛𝑡𝑡 ), and (iii) an improvement of electronic charge mobility (𝜇𝜇) . We thus believe this work may not only pave the way for the present fast

development of PeLEDs, but it also provides some insightful physics studies for device physicists.


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1.

INTRODUCTION

In the present information era, display technologies not only focus on light emission efficiencies and image resolutions, but also work for high-color purity in order to have vivid displays. Solution-processed novel hybrid materials like organicinorganic hybrid perovskites (OIHPs) exhibit outstanding optoelectronic properties possessing high charge carrier mobility, low non-radiative recombination rates and strong photoluminescence (PL) with tunable optical bandgaps from the visible to infrared regions.1,

2

The low temperature solution fabrication method ensures their

compatibility with flexible substrates.3 Despite these, another unique property is the generation of size-insensitively very narrow full width at half maximum (FWHM) for a pure electroluminescence (EL), usually ≤ 20 nm, in a broad spectral range from 400 nm to 780 nm.4, 5 In contrast, FWHM are as large as more than 40 nm in pure organic

emitters.6 Although, it can be constrained down to 30 nm in inorganic quantum dots (QD) emitters, emissive spectra are inevitably size-sensitive.7,

8

All the inherently

promising optoelectronic properties highlight the possibility for developing perovskite light emitting devices (PeLEDs) for future applications in display panels and solid-state lightings.

By far, an increasing number of studied have attempted to design and fabricate PeLEDs, meanwhile to continuously optimize their performance using diverse strategies in order to minimize crystalline grain size, greatly enhance radiative recombination rates, enlarge binding energies, make surface morphology smooth,



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eliminate trap density and reduce energy mis-match at different interfaces.9-12 These can be done generally by two ways, to effectively implement appropriate electron and hole transport layers into devices; and, to introduce organic additives or a class of quasi2D OIHPs into their 3D forms.13-16 The later treatment directly aims at modifying the morphology, crystalline grain size and electrical conductivity of OIHPs. Besides, it is generally known that the naturally formed OIHP crystalline grains and grain boundaries contain a large number of traps/defects by the solution mean.17, 18 It is detrimental for PeLED operations since the trap-assist non-radiative recombination may occur. The OIHPs based emissive layers are usually desirable for smaller crystalline grain sizes with film thicknesses ranging from 150 nm to 250 nm. The reduction of grain size inevitably leads to the increase of the number of grain boundaries, but the decrease of charge carrier mobility due to frequent charge scattering at the boundaries. It is thus eagerly important to deal with all the problems simultaneously in order to ultimately elaborate PeLED performance.

In this work, a bathocuproine (BCP) molecule was utilized as an effective organic additive during the preparation of methyl-ammonium lead tri-bromide (CH3NH3PbBr3) precursor. We have found three remarkable features when it was introduced into a PeLED

with

the

device

configuration

ITO(glass)/PEDOT:PSS/CH3NH3Br3:BCP/Bphen/Ag. (i) It leads to a reduction of crystalline grain size without any crystalline distortions. (ii) The affiliated nitrogen (N) atoms of the BCP molecule are beneficial for passivating crystalline defects leading to a decrease of trap density (𝑛𝑛𝑡𝑡 ). (iii) By further optimizing a stoichiometry of BCP in ACS Paragon Plus Environment

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an anti-solvent such as chlorobenzene (CB), the PeLED can reach a turn-on voltage of 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 = 2.5 V, maximum luminance of 𝐿𝐿 = 14000 cd/m2 , and maximum current efficiency of 𝜂𝜂 = 3.2 cd/A respectively. 2.

EXPERIMENTS

Prior to the fabrication, a CH3NH3PbBr3 precursor solution was prepared by dissolving methyl-ammonium bromide (CH3NH3Br) and lead (II) bromide (PbBr2) powders into dimethyl sulfoxide (DMSO) with a molar ratio of 1.1:1. The small molecular material BCP (Figure 1(a)) was dissolved into chlorobenzene (CB) with various concentrations of 1.5 mg/ml, 3 mg/ml, and 4 mg/ml, respectively. 10 mg/ml Bphen solution was made by dissolving into CB. Indium tin oxide (ITO) coated glass substrates were cleaned by detergent, deionized water, acetone and iso-propyl alcohol (IPA) successively in an ultra-sonic bath. They were then treated by plasma oxidation for approximately 1 min. 40 nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layers were spin-coated onto the ITO(glass) substrates and annealed subsequently at 150 oC on a hot plate for about 15 min. After this, all the samples were immediately transferred into a purely nitrogen-filled glove-box equipped with a thermal evaporation system. The CH3NH3PbBr3 precursor solution was spincoated onto the ITO(glass)/PEDOT:PSS substrates at 8000 rpm for 50 s. The thickness of the CH3NH3PbBr3 layer was 250 nm. During the process, a 50 μL as-prepared BCP solution was dripped into the samples at approximately 20 s. Then, the samples were



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annealed at 80 oC for 10 min. After annealing, thin Bphen layers of 30 nm thick were spin-coated at 8000 rpm for 30 s. The top electrodes were made by depositing 75 nm thick Ag layers in the thermal evaporation system. The electronic transport areas of the PeLEDs were all made for approximately 4 mm2. The complete PeLED structure that is ITO(glass)/PEDOT:PSS/CH3NH3PbBr3:BCP/Bphen/Ag has been schematically drawn in Figure 1(a). Figure 1(b) is the corresponding cross-sectional view of the scanning electron microscopic (SEM) image. An energy diagram regarding all the materials is given in Figure 1(c). In the experiment, we have also fabricated a control PeLED (i.e. ITO(glass)/PEDOT:PSS/CH3NH3PbBr3/Bphen/Ag) without including BCP during the dripping process. Figure 1(d) provides a photographic image for the bright light emission of the PeLED at a turn-on condition. Current density-luminancevoltage (𝐽𝐽 − 𝐿𝐿 − 𝑉𝑉) characteristic curves were measured by using a source meter unit

(Keithley 2400) and a silicon photodiode. Time-Resolved Photoluminescence (TRPL) and

Stead

State

Photoluminescence

(SSPL)

spectra

were

obtain

by

a

photoluminescence spectrometer (Jobinyvon Horiba Fluorolog-3), and a pulsed laser source with a wavelength of 405 nm was used.

3.

RESULTS AND DISCUSSION

We started with the measurements of 𝐽𝐽 − 𝐿𝐿 − 𝑉𝑉 characteristic curves for the

PeLEDs comprising ITO(glass)/PEDOT:PSS/CH3NH3PbBr3:BCP/Bphen/Ag with different concentrations of BCP, such as 0 mg/ml, 1.5 mg/ml, 3 mg/ml and 4 mg/ml,


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respectively. The results are given in Figure 2(a). In this case, 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 (i.e. V at

which L reaches 1 cd/m2) of the control PeLED (i.e. without BCP) is approximately equal to 2.8 V. With the assists of 1.5 mg/ml and 3 mg/ml BCP additive during the dripping process, 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 can be reduced to approximately 2.5 V. When the

concentration of BCP is further increased to 4 mg/ml, it is close in agreement with the one of the control PeLED. At the 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 condition, the current density 𝐽𝐽 is equal

to 20 mA/cm2 for the control PeLED. In comparison, the one with amounts of 1.5

mg/ml and 4 mg/ml BCP produce higher 𝐽𝐽 up to 50 mA/cm2 and 110 mA/cm2, respectively. When 𝑉𝑉 is increased above 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 , 𝐽𝐽 of all the PeLEDs tend to

increase and eventually become saturated. The maximum L are about 1.1 × 104 cd/m2

and 1.4 × 104 cd/m2 for the BCP concentrations of 3 mg/ml and 1.5 mg/ml,

respectively; on the other hand, the control PeLED produces 5 times smaller luminescence 𝐿𝐿 = 2 × 103 cd/m2 . In Figure 2(b), the maximum 𝜂𝜂 is about 3.2 cd/A for PeLED with 3 mg/ml BCP. By comparison, the control PeLED produces the

maximum 𝜂𝜂 = 0.6 cd/A at higher V. As we can see from both Figure 2(a) and (b), it seems that PeLED with 3.0 mg/ml BCP requires relatively lower 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 , generates

smaller 𝐽𝐽 , and produces the best 𝜂𝜂 when compared with other PeLEDs. In the following studies, we will mainly focus on functionalities for the 3 mg/ml BCP based and the control PeLEDs.

The surface morphologies of CH3NH3PbBr3 films made with and without BCP decoration are provided in Figure 3(a) and (b) respectively. The inset of Figure 3(a) is the BCP decorated top surface of CH3NH3PbBr3 after the sample was rinsed by



chlorobenzene for 4 times. Some residual BCP was still left at grain boundaries and the surface, indicating the fabrication of Bphen may has less influence on the top surface. The one with the absence of BCP displays many large crystalline grains of ≥ 300 nm with very clear grain boundaries (Figure 3(b)). When the CH3NH3PbBr3 film is treated

by BCP, the grain boundaries and the top surface of CH3NH3PbBr3 can be appropriately passivated. In this case, oxygen and moisture can be effectively blocked in order to avoid permeating through the surface and grain boundaries. Indeed, to appropriately apply organic small molecules and polymers for the passivation has been confirmed as an effective tool for the reduction of naturally formed defects, for example, nonfullerene ITIC molecules have been demonstrated for passivating CH3NH3PbIxCl1-x based solar cells and it exhibits a gradient distribution along the vertical crystal growth direction.19 Regarding the change of the average grain size upon adding BCP, Figure 3(c) shows the XRD spectra for the same two samples, and the diffracted peaks have been indexed as (100) and (200) crystalline planes. As we can see from the insets for each pair of the indexed XRD bands around 2𝜃𝜃 = 15.1o and 30.2o respectively, the

spectrum measured due to the presence of BCP possesses two significant features, (i)

the remarkable increase of the full width at half maximum (FWHM) of the diffracted bands, (ii) the relative shift of the X-ray diffraction band towards smaller diffracted angles (2𝜃𝜃).20-22 The former case manifestly indicates that to incorporate BCP into

CH3NH3PbBr3 gives rise to the reduction of the crystalline grain size, and such effect is more favorable in PeLED. The latter case can be ascribed to the residual strain effect at CH3NH3PbBr3 grain boundaries due to the two dissimilar materials.


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In spite of the crystallinity, the electron-hole recombination mechanism is also a major concern by device physicists. The reduction of OIHP grain size may increase electron-hole radiative recombination rates due to a decrease of exciton diffusion length. The phenomenon can be revealed by steady state photoluminescence (SSPL) and timeresolved photoluminescence (TRPL) spectroscopies. Figure 4(a) displays SSPL spectra measured in the ambient temperature for the samples with different concentrations of BCP additive. All the dominating peaks locate at 530 nm exactly without any significant shifts, which means the emission occurs from the CH3NH3PbBr3 energy bands mainly rather than defect states in its energy gap. More importantly, the SSPL intensity of CH3NH3PbBr3 with 3 mg/ml BCP is increased more than 5 times larger than the one of pristine CH3NH3PbBr3. The enhancement of the SSPL intensity can be attributed to two major reasons, (1) the reduction of crystalline grain size leads to the increase of radiative recombination; (2) the molecular passivation can eliminate non-radiative recombination.12, 23 In fact, the photo-excitation intensity (𝐼𝐼𝑒𝑒𝑒𝑒 ) dependence of SSPL intensity

(𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 ) at maxima is a reliable indicator for evaluating the competition between the

radiative and non-radiative recombination processes.24-26 Within these, a few recombination processes are involved for the near-band-edge SSPL of a semiconductor. T. Schmidt et. al have postulated that a change of electron concentration (𝑛𝑛) due to

photoexcited electron-hole recombination in the conduction band can be described by a set of differential equations involving some electronic transitions, 24



𝑑𝑑𝑑𝑑 = 𝑖𝑖𝐼𝐼𝑒𝑒𝑒𝑒 − 𝑎𝑎𝑛𝑛2 − 𝑔𝑔𝑔𝑔(𝑁𝑁𝐷𝐷 − 𝑁𝑁𝐷𝐷0 ) − 𝑒𝑒𝑒𝑒𝑁𝑁𝐴𝐴0 𝑑𝑑𝑑𝑑

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equation (1)

in which, the coefficients such as 𝑖𝑖, 𝑎𝑎, 𝑔𝑔, 𝑒𝑒 denote electronic transition rates. n is

the electron concentration. NA and ND are the concentrations of acceptors and donors

respectively. Depending on material properties, the transitions may involve photoexcitation, free exciton recombination, radiative recombination of bounded excitons, donor-acceptor pair recombination, radiative recombination of a free electron and a neutral acceptor, radiative recombination of a free hole and a neutral donor, and nonradiative recombination. In fact, 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 is decided by the power of photoexcitation 𝑘𝑘 𝐼𝐼𝑒𝑒𝑒𝑒 , such as 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 ∝ 𝐼𝐼𝑒𝑒𝑒𝑒 , and the factor 𝑘𝑘 is called the power index. H. Shibata et. al

have extensively studied the phenomenon and deduced an analytical formula in order

to describe how the magnitude of 𝑘𝑘 − value determines the recombination mechanism.25 Basically, the 𝑘𝑘 −value varies from 1 to 2 via a numerical fitting. The

deviation of the value from 1 to 2 indicates the gradually rise of the chance for the electron-hole radiative recombination. Figure 4(c) shows the experimental results and fitting curves of 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 versus 𝐼𝐼𝑒𝑒𝑒𝑒 for the CH3NH3PbBr3 films with and without BCP

molecules pumped with the 405 nm laser at room temperature. After fittings, the

𝑘𝑘 −values were found to be 1.45 and 1.15 for the CH3NH3PbBr3 films with and without

BCP molecules respectively, which depicts that the non-radiative loss of free excitons are relatively smaller in the former case.

In order to further explore the recombination process, lifetime associated TRPL measurements for the same samples were carried out and the experiment were


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monitored at λ = 530 nm. Figure 4(b) provides the TRPL spectra measured in the

ambient temperature for the same samples. Numerically, they can be well fitted by a tri-exponential decay model and the fitting parameters have been summarized in table

1. Three decay lifetimes, such as 𝜏𝜏1 , 𝜏𝜏2 and 𝜏𝜏3 , can be obtained from the fitting curves indicating fast, intermediate and slow decays. 𝐴𝐴1 , 𝐴𝐴2 and 𝐴𝐴3 denote the

fractions of the three decay components in percentage. In principle, 𝜏𝜏1 and 𝜏𝜏2 are attributed to trap-assisted recombination, while, 𝜏𝜏3

is related to radiative

recombination.27, 28 As we can see from Table 1, all the decay lifetimes (i.e. 𝜏𝜏1 , 𝜏𝜏2 and 𝜏𝜏3 ) increase with the increasing concentration of BCP. By looking at the resultant 𝜏𝜏𝑎𝑎𝑎𝑎𝑎𝑎 ,

those 3 mg/ml and 4 mg/ml BCP yield two longest lifetimes. The measurements elucidate that the dispersion of BCP molecules in CH3NH3PbBr3 can readily enhance

radiative recombination, and at the same time, suppress trap-assisted recombination due to the passivation effect.

Another way to verify the electron-hole radiative recombination for PeLEDs at working conditions is to perform non-destructive, ac-electrical field modulated C-f measurements.29-31 Figure 5(a)-(d) provide the typical C-f spectra for the CH3NH3PbBr3 based PeLEDs with and without BCP decoration under the applications of dc-bias 𝑉𝑉𝑎𝑎𝑎𝑎𝑎𝑎 = 0 V and 3 V respectively. All the spectra can be divided into three distinct regions, which correspond to interfacial (region I), dipolar (region II) and ionic (region III) polarizations, respectively. As we can see from Figure 5(a) and (c), the variations of 𝐶𝐶 with respect to the increase of 𝑓𝑓 from 20 Hz to 10 MHz are similar. The one with BCP exhibits slightly higher capacitive effect. When the two PeLEDs



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were under dc-electrical charge injection above 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡−𝑜𝑜𝑜𝑜 , such as 𝑉𝑉𝑎𝑎𝑎𝑎𝑎𝑎 = 3 V, both

spectra change significantly, particularly at region I. The sign reversal (i.e. negative

capacitance) of 𝐶𝐶 indicates the generation of electron-hole radiative recombination

effect under the dc-bias. By comparing Figure 5(b) and (d), the one with the incorporated BCP produces even more negative capacitive effect reflecting a significant enhancement of the recombination process. Such behavior is indeed beneficial to the PeLED operation. Furthermore, the relative dielectric constant (𝜀𝜀𝑟𝑟 ) of the active-layer can be extracted from the C-f spectra in Figure 5(a) and (c) by considering the flat part

(i.e. intermediate frequency part) of region II. The corresponding capacitance is known as the geometrical capacitance and 𝜀𝜀𝑟𝑟 =

𝐶𝐶𝑔𝑔𝑔𝑔𝑔𝑔 ∙𝐿𝐿 𝜀𝜀0 ∙𝐴𝐴

, where, 𝐿𝐿 is the total layer thickness,

𝜀𝜀0 is the permittivity of the free space, and 𝐴𝐴 is the electronic transport area. In this

case, 𝜀𝜀𝑟𝑟 were calculated to be approximately 40.6 F/m and 37.8 F/m for the PeLEDs

with and without BCP respectively. This indicates that BCP is functional for the

reduction of 𝜀𝜀𝑟𝑟 , and consequently, electrically and optically induced polarizations can

be effectively suppressed.

Apart from this, a similar characterization method that involves temperature dependence of C-f measurements can be utilized to quantitatively determine the trap density 𝑛𝑛𝑡𝑡 in the OIHPs. The experimental results are given in Figure 6(a) for the

PeLEDs with and without BCP respectively. The relationship of 𝑛𝑛𝑡𝑡 versus electronic

energies can be written as, 𝑛𝑛𝑡𝑡 = −

𝑉𝑉𝑏𝑏𝑏𝑏 𝑑𝑑𝑑𝑑 𝜔𝜔 𝑞𝑞𝑤𝑤0 𝑑𝑑𝑑𝑑 𝑘𝑘𝐵𝐵 𝑇𝑇 ACS Paragon Plus Environment

equation (2)

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where Vbi is the built-in potential, ω is the angular frequency, q is the elementary charge, 𝑤𝑤0 is the depletion width, kB is the Boltzmann’s constant and T is the

temperature.32-34 Values of Vbi and W can be extracted from C-V measurements in dark

condition. In Figure 6(b), the PeLED with BCP produces a significantly lower 𝑛𝑛𝑡𝑡 in-

between 0.3 eV and 0.45 eV, and the largest gap reaches 38% at 0.36 eV. It is evidently to confirm that the introduction of BCP into CH3NH3PbBr3 can effectively eliminate 𝑛𝑛𝑡𝑡 for the PeLED at working conditions. 35, 36 In spite of this, we have also examined the electron transport abilities of CH3NH3PbBr3 with/without BCP molecules by the space-charge-limited current (SCLC) method.37-39 Electron-only devices were therefore fabricated consisting of ITO(glass)/SnO2/CH3NH3PbBr3:BCP(with

or without)/Bphen/Ag. A schematic

drawing of the device configuration is also provided in the inset of Figure 6(b). The JV characteristics of the devices measured at dark condition are presented in Figure 6(b). Clearly, to disperse BCP molecules into CH3NH3PbBr3 films causes an increase of the electron mobility (𝜇𝜇𝑒𝑒 ) for CH3NH3PbBr3 from 1.77×10-4 cm2V-1s-1 to 2.44×10-4 cm2V-1s-1. This is of particular importance since CH3NH3PbBr3 has been known to have

the p-type semiconducting characteristic.23, 40, 41 Such enhancement is inevitable to make bi-polar charge transport efficient.

A surface/interface depletion region is naturally formed when a semiconductor is in the contact with a metal. It creates the so-called surface depletion capacitance (𝐶𝐶𝑑𝑑 ).

The corresponding depletion width (𝑤𝑤𝑜𝑜 ) is decided by 𝐶𝐶𝑑𝑑 in the relationship, 𝑤𝑤0 =



𝐴𝐴𝜀𝜀𝑟𝑟 𝜀𝜀0 𝐶𝐶𝑑𝑑

. Indeed, magnitudes of 𝐶𝐶𝑑𝑑 can be extracted from the C-V spectra for both

PeLEDs with and without BCP in Figure 7. As we can see from the plots, the entire spectra can be divided into three zones under application of dc-bias. The most flat parts (zone I) that locate below 1.5 V is due to the presence of the depletion region in the PeLEDs. The application of bias voltage that is within this range cannot suppress the build-in potential. At moderate bias voltages, we have observed continuous increase of C for both PeLEDs indicating the suppression of the depletion regions due to charge injection and accumulation. At even higher bias voltages, C starts to decrease down to negative values due to the electron-hole recombination process. In this case, 𝑤𝑤0 were

found to be approximately 5.5 nm and 6.3 nm respectively for the PeLEDs with and without BCP molecules. Thus, the utilization of BCP helps to reduce 𝑤𝑤0 , and consequently it facilitates electron-hole recombination.

4.

CONCLUSION It has been experimentally confirmed that the performance of CH3NH3PbBr3 based

PeLED can be optimized remarkably by introducing the BCP organic molecules into it during the dripping process. The optimal PeLED exhibits maximum 𝐿𝐿 = 14000 cd/

cm2 and 𝜂𝜂 = 3.2 cd/A. With optical and electrical characterizations, the origin of the enhancement can be mainly attributed to three modifications: (i) The crystalline grain

size of CH3NH3PbBr3 can be reduced, which is beneficial to the electron-hole recombination. (ii) Surface and grain boundary traps can be effectively eliminated causing a decrease of non-radiative recombination. (iii) The bi-polar transport behavior


Page 14 of 28

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of the PeLED can be made possible due to the increase of 𝜇𝜇𝑒𝑒 for CH3NH3PbBr3. We

believe this is valuable work for a comprehensive study of the molecular passivation effect in the application of PeLEDs.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (K. Wang) * E-mail: [email protected] (B. Hu)

ACKNOWLEDGMENT We acknowledge financial supports from the National Natural Science Foundation of China (Grant No. 61604010, 61634001, U1601651), and the research funding from Beijing Jiaotong University Research Program (Grant No. 2018JBM066, 2017JBZ105, 2016RC027).



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Page 20 of 28

Page 21 of 28

a

b Ag Bphen CH3NH3PbBr3:BCP PEDOT:PSS ITO

Ag Bphen CH3NH3PbBr3:BCP PEDOT:PSS

CH3NH3PbBr3

ITO Glass

Glass

-3.0 -3.57 -3.5

ITO

-5.2

PEDOT :PSS -5.70

Bphen

-4.8

BCP

E (eV)

c

CH3NH3PbBr3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BCP

d

-4.3

Ag

-6.4

-7.0

Figure 1. (a) Schematic drawing of a PeLED comprising ITO(glass)/PEDOT:PSS/CH3NH3PbBr3:BCP/Bphen/Ag, (b) Cross sectional view of the SEM image for the PeLED, (c) A schematic drawing of energy diagram for the PeLED, and (d) the photographic image of a working PeLED.



105

a

101

103

100 102 10-1

0 mg/ml 1.5 mg/ml 101 3 mg/ml 4 mg/ml 0

10-2 10-3

0

1

2

3

4

5

6

10

b

100

104

η (cd/A)

102

L (cd/m2)

103

J (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

10-1

0 mg/ml 1.5 mg/ml 3 mg/ml 4 mg/ml

10-2

10-3 2

V (V)

3

4

5

6

V (V)

Figure 2. (a) Current density and luminance versus voltage (𝐽𝐽 − 𝐿𝐿 − 𝑉𝑉) , and (b) efficiencies (η) of PeLEDs with different concentrations of BCP molecules.


b

c

(100)

14.6

14.8

15.0

15.2

15.4

XRD Intensity (a.u.)

a XRD Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XRD Intensity (a.u.)

Page 23 of 28

29.5

(200)

30.0

2θ (deg.)

w/o BCP w BCP

(100)

15

30.5

20

31.0

2θ (deg.)

2θ (deg.)

25

(200)

30

Figure 3. (a) and (b) are scanning electron micrograph (SEM) images of CH3NH3PbBr3 films with and without BCP, the inset of (a) is the SEM image for the BCP decorated CH3NH3PbBr3 film after rinsing 4 times with chlorobenzene. (c) XRD spectra for CH3NH3PbBr3 with (black line) and without (red line) BCP treatment. Insets show magnifications of diffracted bands at (100) and (200) crystalline orientations.


475

500

525

550

λ (nm)

575

600

100


b

-1

10

10-2 10-3

c

ISSPL ∝ Iexk

k=1.15 (W/O BCP) k=1.45 (W BCP)

-4

10

0

625

Page 24 of 28

SSPL Intensity (a.u.)


a

Normalized PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SSPL Intensity (a.u.)


50

100

150

200

1E-3

0.01

0.1

1

Iex (mW)

T (ns)

Figure 4. (a) SSPL spectra for different concentrations of BCP molecules, and (b) TRPL spectra obtained for the same samples. (c) Photoexcitation power (𝐼𝐼𝑒𝑒𝑒𝑒 ) dependence of SSPL intensity (𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 ) for samples with and without 3 mg/ml BCP. Table 1. Fitting parameters obtained from Figure 4(b) for several different lifetimes of CH3NH3PbBr3 with different concentrations of BCP molecules. BCP (mg/ml) 0 1.5 3 4

τ1

A1

τ2

A2

τ3

A3

τavg

(ns) 0.15 0.30 0.34 0.36

(%) 31.99 29.01 13.30 12.44

(ns) 2.51 3.22 3.98 3.98

(%) 45.03 46.24 62.11 62.95

(ns) 14.48 18.48 29.00 32.97

(%) 22.98 24.74 24.59 24.60

(ns) 4.51 6.15 9.65 10.66


Page 25 of 28

0.4

CH3NH3PbBr3 (w/o BCP) at 0 V

a

0.2 0.0 0

C (μF/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20

b

CH3NH3PbBr3 (w/o BCP) at 3 V

-40 0.4

CH3NH3PbBr3 (w BCP) at 0 V

c

0.2 0.0 0 -40

d

CH3NH3PbBr3 (w BCP) at 3 V

-80

region Ⅰ

-120 101

102

region Ⅱ 103

104

region Ⅲ 105

106

107

f (Hz) Figure 5. (a) and (b) are C-f spectra for PeLEDs without BCP under applications of 𝑉𝑉𝑎𝑎 = 0 V and 3 V respectively, (c) and (d) are C-f spectra for PeLEDs with BCP under applications of 𝑉𝑉𝑎𝑎 = 0 V and 3 V respectively.



1020

10

19

J (mA/cm2)

10

1018 CH3NH3PbBr3 (w/o BCP) CH3NH3PbBr3 (w BCP)

17

10

b SnO2

nt (eV-1cm-3)

3

0.3

0.4

E(ε) (eV)

0.5

n=2

Bphen BCP

a

CH3NH3PbBr3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

102

J ∝ Vn

n=1

101

0.1

CH3NH3PbBr3 (w/o BCP) CH3NH3PbBr3 (w BCP)

V (V)

1

Figure 6. (a) Trap density (𝑛𝑛𝑡𝑡 ) versus energies 𝐸𝐸 for PeLEDs with and without 3 mg/ml BCP molecules. (b) Space-charge-limited current measurements for electrononly devices with and without BCP molecules.


Page 27 of 28

25.0 12.5

C (µF cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


zone I

0.0

zone II

-12.5 -25.0 -37.5 -50.0 0.0

zone III

PeLED without BCP PeLED with BCP 0.5

1.0

1.5

2.0

2.5

3.0

V (V) Figure 7. C-V spectra are for PeLEDs with (red) and without (black) BCP molecules, zone I, II and III correspond to the depletion zone, the suppression of depletion region at finite bias, and recombination zone, respectively.




Page 28 of 28

Effect of Bathocuproine Organic Additive on Optoelectronic Properties

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