Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells

Journal Pre-proof Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells Cong Chen, Xinmeng Zhuang, Wenbo Bi, Yanjie Wu, Yanbo Gao, Gencai Pan, Dali Liu, Qilin Dai, Hongwei Song PII:

S2211-2855(19)31022-5

DOI:

https://doi.org/10.1016/j.nanoen.2019.104315

Reference:

NANOEN 104315

To appear in:

Nano Energy

Received Date: 10 October 2019 Revised Date:

13 November 2019

Accepted Date: 19 November 2019

Please cite this article as: C. Chen, X. Zhuang, W. Bi, Y. Wu, Y. Gao, G. Pan, D. Liu, Q. Dai, H. Song, Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104315. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Graphical abstract The chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, attractive PCE of 21.5% and superior durability of 5000 h.

Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells Cong Chena,b, Xinmeng Zhuanga, Wenbo Bia, Yanjie Wua, Yanbo Gaoa, Gencai Pana, Dali Liua*, Qilin Daic* and Hongwei Songa* a State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China. b School of Material Science and Engineering, Hebei University of Technology, Dingzigu Road 1, Tianjin 300130, People’ s Republic of China. c Department of Chemistry, Physics, and Atmospheric Sciences, Jackson State University, Jackson, Mississippi 39217, USA.

⁎

Corresponding authors. E-mail: [email protected] and [email protected]

Keywords: stability, iodine bromide, perovskite solar cells, chemical inhibition, decomposition

Abstract Despite the remarkable photovoltaic characteristics and printability of perovskite solar cells, their intrinsic instability has been the most serious drawback toward future commercialization. In this work, we have investigated the stability of perovskite films in terms of morphology, electronic properties and chemical compositions. Specifically, the chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, superior durability and attractive PCE of 21.5%. Femto-second transient absorption spectra and GIWAXS measurements provide deep insight into the reduced carrier recombination and indicate the improved crystallinity of the modified perovskite films. Furthermore, an efficient hole-transporting material, PDPP4T, without using any doping process is applied to achieve PSCs with enhanced open-circuit voltage and better repeatability. As a consequence, the modified PSCs could maintain 82% of their initial efficiency after 5000 h of storage in ambient conditions and 90% of their initial efficiency after 1000 h of light soaking process. An excellent water resistance up to 100 h of the PSCs is also obtained by encapsulation technology. Besides, after coating Ce3+-CsPbI3 nanocrystals as luminescent down-shifting layers on the front side of the PSCs, the PCE of the device was further improved to 22.16 %. 1

1. Introduction Mixed halide perovskites have emerged as attractive candidates due to strong light absorption, continuous tuning of the bandgap and low exciton binding energy coupled to the low recombination rate of photo-generated charge carriers [1-5]. Organometallic halide perovskite solar cells (PSCs) have exhibited exceptionally outstanding power conversion efficiency (PCE) of 25.2% in the lab conditions [3, 6, 7]. PCEs are comparable with traditional commercialized silicon, CIGS and CdTe thin-film solar cells [8, 9]. Halide ions in perovskite films are easily exchanged by the same family elements [10, 11]. The bandgap of methylammonium lead iodide/bromide can be tuned to 1.55-2.43 eV by varying the composition of Br- and I- [12-14]. The complex crystal structure and composition of the unit cell may cause lattice distortion, eventually leading to carrier recombination and trapping [14, 15]. The morphology and crystal quality of perovskite films are the critical factors affecting device efficiency. Numerous methods, including anti-solvent assisted crystallization [16], vacuum evaporation process [17, 18], additive-promoted electrical optimization [19, 20], and compositional engineering [21, 22], have been developed to improve perovskite film quality and PSCs efficiency. The use of additives has been previously suggested as a simple and effective method to improve the perovskite film quality and the corresponding photovoltaic performance of PSCs [20, 2

23-26]. Accordingly, many additives including polymers, fullerenes, inorganic acids, small molecules, metal halide salts and organic halide salts have been reported to improve the quality of perovskite films with respect to compactness and large grain size production by slowing down the crystal growth rate [20, 27, 28]. The crystallinity of perovskite films can be enhanced by the reaction of PbI2, MABr, FACl and I2 to form intermediates [29-31]. Wu et al. used functionalized hydrophobic ammonium to increase crystalline grain size and reduce charge recombination of the double cation perovskite films [27]. Yang et al. introduced MABr treatment in the preparation of MAPbI3 films to obtain high-quality MAPbI3-xBrx films [32]. Tai et al. demonstrated that Pb(SCN)2 as an additive could significantly improve the stability of the devices by introducing SCN− into the perovskite crystal lattice [33]. In previous studies, either cations or anions can act as active substances to improve the quality of the perovskite film. The other anions or cations introduced in the additives at the same time usually act the opposite way. Therefore, both cations and anions as active species in one additive could have more significant potential to improve the quality of perovskite films and thus the device performance. From another point of view, the introduction of additives to improve device performance of PSCs by chemical inhibition mechanism has been extensively studied [34, 35]. Back et al. integrated a new amine-mediated titanium suboxide system as 3

an efficient chemical inhibition layer to protect the corrosion of the metal electrodes by chemically neutralizing mobile ionic defects in the perovskite layers [35]. The Lewis acid and base adduction interaction of the S=O or C=O bond with Pb has been used to passivate the defect and regulate the crystallization process of perovskite crystals [27, 36]. Among all the additive strategies, additive engineering mainly affects the crystallinity of perovskite, and then enhances the device stability and photovoltaic properties [20, 27, 37, 38]. Or as an interlayer barrier to inhibit the direct contact of perovskite materials with water, oxygen and other corrosives, thereby improving stability [39, 40]. There is almost no additive to improve the stability of PSCs from the perspective of controlling the decomposition reaction process of perovskite materials. According

to

previous

studies,

the

light-induced

or

thermal

decomposition of perovskite materials are usually chemical reversible reactions, in which the device performance can be restored [41, 42]. As we all know, in the reversible reaction process, increasing the concentration of the product can effectively inhibit the positive reaction process. That is to say, increasing the concentration of final product in perovskite crystal will effectively inhibit the decomposition reaction of perovskite [43, 44]. Herein, we report that iodine bromide (IBr, contains I+ and Br− ions) as an additive can significantly improve the quality of perovskite films. Unlike I- ions, I+ ions can effectively neutralize I- ions in 4

perovskite materials, thus producing I2, inhibiting the decomposition of materials and improving stability. At the same time, Br- can play an effective anionic passivation effect. The synergistic effect of I+ and Br- in IBr is favorable for controlling the nucleation process of the crystal growth and inhibiting the decomposition of perovskite materials. Furthermore, the development of dopant-free HTMs toward the high-stability devices has been an essential issue in the field of PSCs. Small molecules have been widely used as organic HTMs due to monodisperse

molecular

weights,

simple

purification

process,

well-defined molecular structures, and batch-to-batch reproducibility. Herein, While using IBr to optimize perovskite layer, we report a strategy for improving the photovoltaic performance PSCs by using an efficient hole-transporting

small

molecules,

poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione -3,6-diyl]-alt-(2,2’;5’,2’’;5’’,2’’’-quaterthiophen-5,5’’’-diyl)]

(PDPP4T),

without using any doping process. The small molecules PDPP4T could facilitate efficient charge transport and provide appropriate energy levels for hole extraction, leading to the optimized PCE of 21.5% without a hysteresis effect for the novel PSCs [45-47]. The results firstly suggest that the synergistic effect of anions and cations in additives is an effective strategy for achieving efficient and stable PSCs [24]. Moreover, small molecules as hole transporting layer paves a new way to prepare efficient 5

PSCs with excellent device stability. 2. Experiment 2.1 Materials: All the reagents and chemicals were purchased from the commercial sources and used without further purification. The spiro-OMeTAD, Bis(trifluoromethane)sulfonimide

lithium

salt

(Li-TFSI)

and

4-tert-butylpyridine (tBP) were obtained from Sigma-Aldrich. Iodine bromide (98%), Lead iodide (PbI2, 99.9985%), anhydrous N, N-dimethylformamide (DMF), and γ -butyrolactone (GBL) were purchased from Alfa Aesar. High mobility p-type polymer PDPP4T (CAS:1267540-03-3, Chemical structure: (C62H90N2O2S4)n) was obtained from OSSILA.

2.2 Device fabrication: FTO on glass was removed for anode contact by etching the FTO with dilute hydrochloric acid (add 10 ml HCl (57%) to 25 ml deionized water) and zinc powder. Cp-SnO2 (compact SnO2) electron transporting layer was deposited on FTO/glass by E-beam evaporated technique. The optimal deposition rate explored was 0.5 nm/s. The film thickness was controlled by deposition time and rate. After the deposition, the FTO/cp-SnO2 substrates were annealed at 100℃ in air for 30 mins. Then the FTO/cp-SnO2 substrates were treated with UV-ozone for 15 min. The 6

perovskite films were deposited through an antisolvent assisted one-step solvent engineering method. Perovskite precursor solution contains FAI (1.0 M), PbI2 (1.1 M), MABr (0.2 M), and PbBr2 (0.2 M) in anhydrous dimethyl formamide (DMF): dimethyl sulfoxide (DMSO) (4:1, volume ratio). Then the perovskite precursor was spin-coated first at 800 rpm for 6 s and then at 4000 rpm for 35 s. During the second step, 500 µL anhydrous diethyl ether was dropped onto the spinning substrates at the first 15 s. The substrates were then annealed at 100 °C for 12 min. The whole process was operated in a nitrogen-filled glovebox. As for IBr and I2 modified perovskite films, a certain amount of IBr and I2 powder were first dissolved in DMF solution, and then added to the precursor solution of perovskite in proportion. For MABr modified perovskite films, MABr with the same molar mass is directly added to the precursor solution of perovskite, and then stirred at 1500 rpm for 20 minutes before use. The hole transporting layer was prepared by spin-coating the spiro-OMeTAD solution at 3000 rpm for 30 s. The spiro-OMeTAD solution consists of 73.2 mg spiro-OMeTAD, 28.8 µL 4-tert-butylpyridine (t-BP), 17.5 µL bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) solution (520 mg in acetonitrile), and 1 mL chlorobenzene. For the pure PDPP4T hole transporting layer, we first dissolve PDPP4T in a chlorobenzene solution with a concentration of 15 mg/ml, then stirred at 1000 rpm for 30 minutes before use. For the spiro-OMeTAD/PDPP4T hole transport layer, we first 7

prepare the doped spiro-OMeTAD precursor solution, and then add the PDPP4T with concentration of 5 mg/ml. Finally, a 100 nm Au or 120nm Ag layer was deposited via thermal evaporation in a vacuum chamber (9 × 10−4 Pa).

2.3 Device Characterization: The J–V measurements of all PSC devices were performed using a solar cell J–V testing system (SolarIV-150A, Zolix) at AM1.5G and 100 mW cm−2 illumination using a Keithley 2400 as a source-meter. The light was calibrated using a certified reference cell (RERA Solutions RR-1002). The bias voltage for the steady-state measurements was chosen as the average of the maximum power point (MPP) voltage of the J–V measurement. Photo-spectral response was measured by an EQE measurement system (QEX10, PV Measurement), which was equipped with monochromator, a lock-in amplifier, Xe lamp, and current-voltage amplifier. Light soaking (100 mWcm-2) test was conducted on a simulated Xe lamp for continuous illumination of 12 h, and then stored in dark for 12 h. The J-V scan test was carried out at some certain time. XRD measurements for perovskite films were carried out on a Rigaku D/max 2550 X-ray diffractometer using a monochromatized Cu target radiation source at a scanning rate of 18° min−1. The SEM surface morphology of the films was characterized by a SIRION field-emission. 8

The steady state PL measurements were made a fluorescence luminescence

spectrometer. UV/vis/NIR

absorption

spectra

were

collected with a Shimadzu UV-1800 in the range from 300 to 1100 nm. All measurements were undertaken at room temperature in ambient conditions.

Mott-Schottky

(M-S)

and

Impedance

spectroscopy

measurement was characterized on a model Solartron Analytical electrochemical analyzer (AMETEK, Inc. Berwyn, USA) in the frequency range of 0.1–105 Hz with different the applied bias voltage. The local roughness of the thin films was measured by atomic force microscopy (Asylum Research, an Oxford Instruments company, 5500, Agilent, Santa Clara, CA) operated in contact mode. The RMS values were obtained from AFM images in Igor Pro 7. The encapsulation of the device was carried out by a semi-automatic hot press packaging machine (YHMC3050HP, YH electronic equipment business). The hot melt materials for packaging are ethylene-vinyl acetate copolymer.

3. Results and discussion As shown in Fig. 1a, all the devices were fabricated using a low-temperature solution process (<120 °C) with a conventional n-i-p planar

heterojunction

(PHJ)

configuration

of

FTO/cp-SnO2/IBr-FA0.83MA0.17Pb(I0.83Br0.17)3/(Spiro-OMeTAD)/PDPP4T 9

/Au. The electron transport layer SnO2 were prepared by low-temperature E-beam technique on FTO substrates. Importantly, we have studied the effect of IBr as dopant on the crystallinity of perovskite films and device performance. Fig. 1b illustrates the cross-sectional SEM image of the PSC device with FA0.83MA0.17Pb(I0.83Br0.17)3 as photoactive layer. It can be seen that each layer was prepared uniformly. In order to avoid the doping drawbacks of traditional spiro-OMeTAD materials, we introduce novel non-doped hole transport material, PDPP4T, as hole transport layer. The bandgap alignment in the device configuration in Fig. 1c shows that PDPP4T could serve as an efficient hole transporting material, as well as an energy level realignment layer, which allows efficient carrier transport from photoactive layer to top electrode [48, 49]. Fig. 1c illustrates the chemical structure and photograph of PDPP4T dark blue color powders. Fig. 1d shows the photographs of IBr powders (in reagent bottle), IBr solution (DMF solvent), FA0.83MA0.17Pb(I0.83Br0.17)3 solutions (in DMF: DMSO mixed solvent) and IBr-FA0.83MA0.17Pb(I0.83Br0.17)3 mixed solutions are presented in sequence. It is observed that IBr in DMF solution shows black color. At the same time, the yellow perovskite precursor solution turns to black once IBr is added to the perovskite solution. IBr-FA0.83MA0.17Pb(I0.83Br0.17)3 films were prepared with different IBr concentrations (0 mg/ml, 5 mg/ml, 8 mg/ml, 12 mg/ml, 15 mg/ml). The energy level diagram of PSCs in this work is depicted in Fig. 10

1e. The HOMO level of the PDPP4T locates at around −5.07 eV, slightly higher than that of spiro-OMeTAD (~−5.2 eV), indicating a more favorable band alignment with the work function of the counter electrode, which will reduce the energy loss during carrier extraction and transport in PSCs.

Fig. 1. (a) Schematic illustration of the device structure of a PSC, Abbreviation “FAMA” stands for the FA0.83MA0.17Pb(I0.83Br0.17)3 perovskite films. (b) Cross-sectional SEM characteristics of the planar PSC devices. (c) Molecular structural formula of the conjugated polymers. (d) Photographs of the IBr and the corresponding solutions. (e) Band alignment of every layer.

To study the impact of IBr on the perovskite film growth, different amount of IBr was introduced in the perovskite precursor. More details on the device preparation and configuration can be found in the experimental section. The effect of IBr solutions on topography and crystallinity of perovskite film is investigated by scanning electron microscope (SEM) and atomic force microscopy (AFM), as shown in Fig. 2a-d. The grain size of the modified perovskite films is much larger as IBr additives are introduced, compared to that of the control film. When 8 mg/ml IBr is introduced, the grain size of perovskite films slightly increases. The grain 11

size reaches the maximum value as IBr concentration increases to 12 mg/ml. No crystal grains and pinholes are observed in the 12 mg/ml IBr based FA0.83MA0.17Pb(I0.83Br0.17)3 film. Furthermore, the grain size decreases when the IBr concentration is higher than 15 mg/ml. The 12 mg/ml IBr-based film exhibits the maximum grain size (~900 nm) compared to that of other films, which has significant potential to obtain high-efficiency PSCs. For the 15 mg/ml IBr-based film, the excess IBr may retain at the perovskite grain boundaries, leading to some small grains on the surface of the film. Fig. 2e and g show photographs of control and 12 mg/ml IBr modified perovskite films, and the corresponding AFM images are shown in Fig. 2f and h. The SEM and photographs indicate that the introduction of IBr can effectively improve the surface morphology of perovskite films. The AFM images show that IBr modified perovskite film displays markedly lower surface roughness than other films, indicating the smoother and preferable surface structure. The surface roughness values of the control and IBr modified films are 26.9 and 21.2 nm, respectively. These results prove that the perovskite films modified by IBr exhibit larger grain size, indicating that IBr can significantly improve the film quality. As is illustrated from the cross-sectional SEM images in Fig. 2i and j, the IBr modified film also exhibits more flat and uniform morphology than that of the control film. Therefore, as the IBr is introduced to perovskite films, the films exhibit 12

uniform and dense with high surface coverage and low roughness, resulting

in

increased

charge

transport

and

decreased

charge

recombination, which is beneficial to device performance.

Fig. 2. (a-d) The top-view SEM images of perovskite film with different amount of IBr additives (0 mg/ml, 8 mg/ml, 12 mg/ml and 15 mg/ml). (e) and (g) are the photographs of control and 12 mg/ml IBr modified perovskite films. (f) and (h) are the corresponding AFM images. (i) and (g) are the corresponding cross-sectional SEM images.

X-ray diffraction (XRD) measurements were conducted to investigate the crystal structure of the perovskite films affected by IBr modification. As shown in Fig. 3a, IBr additive does not cause obvious changes in the crystal structure. The strongest characteristic (110) peak of the perovskite film with 12 mg/ml IBr additive slightly shifts to lower diffraction angle side at 14.18° compared with that of the control perovskite film at 14.42°, which indicates the successful incorporation of 13

IBr into perovskite crystal lattice. It is also suggested that the lattice parameter of IBr modified film decreases compared to control film. Furthermore, IBr modification results in obviously increased (110) diffraction peak intensities, indicating the improved crystallinity of the perovskite films by IBr modification [30]. This is consistent with the grain morphology results in Fig. 2. The UV-Vis absorption spectrum in Fig. 3b shows that the modified perovskite film could exhibit increased absorption intensity in the wavelength range of 350–700 nm. In addition, the introduction of a small amount of Br- has no obvious influence on the perovskite film absorption spectrum profile. We also explored the effects of different types of hole transport materials on devices. As in illustrated in Fig. S1, it can be found that PSCs used pure PDPP4T as a hole transport layer exhibit higher open-circuit voltage (Voc) and better repeatability. PSCs based on pure PDPP4T as a hole transport layer exhibit a maximum PCE of 19.0%, while the PSCs with spiro-OMeTAD exhibit higher short-circuit current density (Jsc) and fill factor (FF) with a maximum PCE of 19.7%. Spiro-OMeTAD with 5 mg/ml PDPP4T could boost the PCE up to 20.02%. Based on the above optimized hybrid hole transport materials, the effect of IBr on the photovoltaic performance of the PSCs is systemically studied. The PSC devices are prepared with different IBr concentrations (0 to 15 mg /mL). The best-performing PCE, Jsc, Voc and 14

FF values of devices are presented in Fig. 3c and Fig. S2. As is shown, the J–V characteristic curves and relevant parameters reveal that the photovoltaic performance of the devices gradually increases as the amount of IBr additive increases, and the best performance is achieved by 12 mg/mL IBr. When the concentration is more than 12 mg/ml, the PCE values slightly decrease. The optimal IBr concentration is 12 mg/ml, which produces an average PCE over 20%. Fig. 3d displays the J–V curves of the optimized devices. For the control device with optimized hybrid hole transport materials (spiro-OMeTAD with 5 mg/ml PDPP4T), Jsc, Voc and FF values are 22.59 mA cm-2, 1141 mV and 77.6%, respectively, achieving a PCE of 20.02 %. For the 12 mg/ mL IBr modified device, these values are 23.45 mA cm-2, 1181 mV and 77.7%, respectively, resulting in a PCE of 21.5%. IBr modification leads Voc to increase from 1141 mV to 1181 mV, which can mainly be attributed to the increased band gap of perovskite films caused by Br- [32, 50]. The increased Jsc (from 22.59 mAcm-2 to 23.45 mAcm-2) can be explained by the enhanced crystallinity. The FF values do not change significantly. Hysteresis effect during J–V measurements is a device stability issue to be solved urgently in perovskite photovoltaics. The hysteresis effect, which is caused by forward bias to short circuit (Reverse) and short circuit to forward bias (Forward) scan modes, is significantly reduced in IBr modified device compared to that of the 15

control device, as illustrated in Fig. 3d. The IBr modified PSCs exhibit high reproducibility and negligible hysteresis. Fig. 3e shows the representative EQE spectra of the PSCs. It can be observed that the IBr modified device presents enhanced response in a broad wavelength range from 350 nm to 800 nm. A larger Jsc of 21.3 mA cm-2 is obtained by integrating the EQE curves compared to that of control devices (20.2 mA cm-2). The enhanced EQE response in whole wavelength range suggests that IBr modification can effectively improve perovskite film crystallinity, and then facilitate charge transport and suppress charge recombination in the devices [4]. Additionally, Fig. 3f shows the steady-state PCEs with an external bias for 100 s. For the control PSC, the rapid dropping rate and degraded power output indicate that the photo-induced halide segregation seriously deteriorates the power output of the devices. In contrast, a highly stable efficiency is achieved for IBr modified PSC due to low density of defect states and the enhanced phase stability under illumination.

16

Fig. 3. (a) XRD data of the perovskite films with different IBr concentrations. On the right is the enlarged image. (b) Absorption spectra of the perovskite films with different IBr concentrations. The inset shows the absorption spectra in the region of 400-850 nm. (c) Changes of Jsc and PCE values as the variation of IBr concentration. (d) J–V curves of the control and IBr modified devices measured with the forward and reverse scan modes. Inset shows the histograms of PCE values obtained from 20 devices of each set. (e) EQE spectra for the devices. (f) The time dependence of stabilized power output of the PSCs with and without IBr modification under their maximum power-point (MPP) conditions.

Generally, the photovoltaic performance is affected by the carrier transport and recombination. Interfacial carrier density can be obtained by classical Mott-Schottky (M-S) devices via capacitance-voltage (C-V) measurements. Interfacial carrier density is inversely proportional to the slope of the M-S plot. As shown in Fig. 4a, the slope of the IBr modified device is larger than that of control device, suggesting reduced interfacial carrier density and decreased carrier recombination [51, 52]. Fig. 4b shows the light intensity dependent Voc and ideality factor of devices for investigating the carrier recombination in perovskite films. The Voc depends logarithmically on the light intensity, and the ideality factor n is introduced as a prefactor in the following equation: 17

δ

= n (

T/e)ln( ) + constant,

(1)

where I is the light intensity, T is temperature, e is elementary charge, kB is the Boltzmann constant and Voc stands for the open voltage. A plot of Voc as a function of logarithmic light intensity [ln(I)] is linearly fitted to evaluate the slope, n(kBT/e), which is usually associated with the recombination process affected by trap states in optoelectronic devices [53]. It is obvious that IBr modified devices exhibit a higher Voc than the control device under different light intensities. Ideality factors of 1.42 and 1.27 are obtained for control and IBr modified devices, respectively. The decreased slope indicates that IBr modification can effectively suppress the carrier recombination [54, 55]. We further study the recombination kinetics of the PSCs through fitting Nyquist plots (Fig. S3), which provide information about the internal electrical characteristics. The equivalent circuit model displayed in the inset of Fig. 4c is used to perform data fitting. Two distinct regions exhibit in the curves, including a high frequency region representing charge recombination process and a low frequency region correlating with slow dielectric and ionic relaxations in perovskite films. Rs and Rrec stand for the series and recombination resistances, respectively. Rs values for the two devices are around 2.0 Ω. The fitting results of Rrec are summarized in Fig. 4c. Rrec value of IBr modified device is larger than that of the control device, indicating the obviously reduced recombination 18

in the device. This can be attributed to decreased trap density in IBr modified perovskite films. Fig. 4d shows the PL spectra of control and IBr modified films on FTO/cp-SnO2 substrates. A striking PL quenching effect is observed in the IBr modified perovskite film, which could be attributed to the suppressed electron–hole pair recombination in the perovskite photo-active layer. This also indicates that the carrier lifetime is prolonged by IBr modification, which could be caused by the decreased defect density states. On the basis of IBr modified film, we further introduced spiro-OMeTAD and PDPP4T as hole transport layer. It can be seen that PDPP4T can further reduce the PL intensity,

indicating

the

effective

suppression

of

radiative

recombination [56]. The PL process in IBr modified perovskite film is influenced by the trap-assisted recombination at grain boundaries and the radiative recombination inside the grains [56, 57]. Furthermore, we fabricate electron-only devices with the architecture shown in the inset of Fig. 4e and calculate the trap density values of the devices evaluate the carrier mobility by using space charge limited current (SCLC) method. The trap density can be determined by the trap-filled limit voltage (VTFL) using: =



,

(2)

where nt is the trap-state density; e is the elementary charge; L is the thickness of perovskite films (520 nm); ε is the relative dielectric constant 19

of perovskite (ε= ~35) [58, 59]; ε0 is the vacuum permittivity. The VTFL of the control and IBr modified PSCs is 1.72 and 1.03 V, respectively. The trap density nt of control device is calculated to be 2.47×1016 cm−3. For comparison, the calculated electron trap density nt of the IBr modified devices is 1.48×1016 cm−3, confirming the reduced interfacial charge recombination in IBr modified device. The above results indicate the highly improved FA0.83MA0.17Pb(I0.83Br0.17)3 film quality after IBr modification [60]. Accordingly, in the modified perovskite film, it is possible that the photoexcited electrons exhibit faster electron injection than of the control film. It may be postulated that the IBr additive functions as a bridge for carrier injection from the perovskite to the carrier transporting layer [61]. Furthermore, the dark current density curves of PSCs are shown in Fig. 4f. IBr-based device exhibits a small leakage current, further suggesting an improved charge transport and decreased charge recombination loss in IBr modified device.

20

Fig. 4. (a) Mott-Schottky (M-S) plots at 10 kHz. (b) Voc as a function of light intensity in a semi-log plot. (c) Rrec obtained from fitting Nyquist plots through equivalent circuit in the inset for PSCs with and without IBr modification. (d) PL spectra of the FTO/cp-SnO2/perovskite films and FTO/cp-SnO2/IBr-perovskite films. (e) J-V curves from electron-only devices with the structure shown in the inset. (f) Dark J–V curves for the control and IBr modified devices.

To further study the photoexcited carrier dynamics in perovskite films, we have collected femto-second transient absorption spectra (TAS) of the glass/FA0.83MA0.17Pb(I0.83Br0.17)3

and

glass/IBr-

FA0.83MA0.17Pb(I0.83Br0.17)3 samples [62].

TAS has been recently

demonstrated to be a useful tool to investigate the charge separation and recombination behavior of perovskite films. It is widely known that the photo-induced bleaching band dynamics will follow the carrier (electron and hole) population dynamics in the perovskite films as it originates from the transparency induced at the onset of the optical absorption after population of the bottom of the conduction band and top of the valence band by photogenerated electrons and holes, respectively [61, 63, 64]. In the spectroscopy model, the photo-bleaching peaks and photo-absorption peaks locate around 720 nm. The photo-bleaching negative peak at 720 nm is related to the band gap or exciton transition of IBr modified perovskite films. Compared with the control film, a slower decrease of the peak intensity at 720 nm is observed in the kinetic model for the IBr modified perovskite film, which is clearly showed in the normalized decay traces. Small spectral shift of ~2.1 nm between the two samples can be attributed to the composition difference, and it originates from differences in the strains or in the tilted PbI6- octahedra, which can be affected by the defect 21

or trap concentrations [65]. This indicates less trap states or defects in the IBr modified perovskite film, which is further confirmed by the steady-state photoluminescence spectra [62]. Furthermore, the TA curves are fitted using a single exponential decay function (y = A1×exp(−x/t1) + y0) to obtain carrier lifetimes in Fig. S4 and Fig. 5c. In general, a longer charge lifetime indicates the reduced non-radiative recombination or effective defect passivation due to higher perovskite film quality. The fitting carrier lifetime of control and IBr modified perovskite films are, 53.2 and 68.7 ps, respectively, indicating that perovskite film quality can be improved by IBr modification [66]. Therefore, TAS results indicate that the non-radiative recombination has been suppressed in the IBr modified perovskite film compared to the control film. Grazing-incidence wide-angle X-ray scattering (GIWAXS) has been widely employed to investigate organic semiconductor thin films and is an important technique for characterizing perovskite thin films [67, 68]. To investigate the crystalline nature of control and IBr modified perovskite

films,

2D

scattering

patterns

are

collected

with

synchrotron-based GIWAXS. This measurement not only provides crystalline lattice parameters but also reveals thin-film crystallinity and crystallite orientation. The schematic diagram of GIWAXS experimental setup is shown in Fig. 5d. Fig. 5e-f exhibit the 2D GIWAXS patterns of 22

control and IBr modified perovskite films. We do not observe any diffraction peaks from IBr additives due to strong crystalline nature of perovskite films and limited amount of additive contents. The GIWAXS patterns exhibit typical scattering features of FA0.83MA0.17Pb(I0.83Br0.17)3 perovskite films, with a stronger peak in the out-of-plane direction at q =10 nm−1 for the two kinds of films, which corresponds to the (110) peak of FA0.83MA0.17Pb(I0.83Br0.17)3 perovskite film. In addition, the (110) diffraction peak of FA0.83MA0.17Pb(I0.83Br0.17)3 perovskite film in all 2D GIWAXS patterns has the similar intensity scale, indicating similar degree of crystallization for all the perovskite films. The GIWAXS results reveal that the crystallinity of perovskite crystals is significantly boosted by introducing low content of IBr in the antisolvent, which further confirms the results from XRD and SEM measurement. Thus, IBr promotes crystallization of perovskite films, which could be one of the main factors for improved device performance.

23

Fig. 5. (a) Femtosecond transient absorption spectra of corresponding films with 5 ps pump−probe delay under irradiation of CW laser (400 nm). Transient absorption of the a) control and (b) IBr-treated perovskite films on quartz λex 400 nm. ∆A/A is the optical density (OD). (c) The TA curves are fitted using a single exponential decay function (y = A1×exp(−x/t1) + y0) to obtain carrier lifetimes. (d) Schematic diagram of GIWAXS experimental setup. 2D GIWAXS images of (e) control and (f) IBr modified FAMA(FA0.83MA0.17Pb(I0.83Br0.17)3) perovskite films.

The perovskite materials are decomposed to HI, CH3NH2, and PbI2 under high temperature annealing. In order to investigate the effects of IBr on thermal stability of perovskite materials, we study the structural and optical properties of the control and IBr modified perovskite films [69-71]. Usually, thermal stability of thin films could be associated with crystallinity [72-74]. Two perovskite films are annealed up to 30 mins at 210 oC. Fig. 6a and b show that IBr modified perovskite film can maintain a black topography after high temperature annealing, while the control film turns yellow color. This directly indicates that the IBr modified perovskite film has better heat tolerance. To further study the perovskite

film thermal

stability, temperature-dependent 24

UV-Vis

absorption spectra are studied for control and IBr modified films after annealing at different temperatures (150 oC-210 oC), as shown in Fig. 6c. The absorption intensity at 600 nm decreases to 30% and 81% of its initial values for control and IBr modified films after 30 min annealing at 210 oC. According to the statistical data shown in Fig. 6d, in high temperature region, the slope of the temperature-dependent absorption is determined to be 8.2×10-3 for the control film, while one slope of 2.8×10-3 is obtained for IBr modified film. The film shows slightly reduced degradation rate after IBr modification. In relative high temperature region (200-210 oC), one large slope of 6.1×10-2 is shown for control film and a small slope of 1.5×10-2 is obtained for IBr modified film. The degradation rate of perovskite films decreases significantly at relative high temperatures, which is about a quarter of the original rate. The fitting results show that the degradation rate can be effectively reduced by IBr modification. Improved thermal and atmospheric stability has previously been observed as the excessed Br- is incorporated [32]. In order to explore whether Br- or I+ contributes to the improved stability, we also try to introduce the same amount of MABr to clarify the influence of Br-. The results in Fig. 6e show that only slightly improved thermal stability was achieved after the introduction of MABr. Furthermore, we introduced I2 additive with same concentration to explore the impact on thermal stability in Fig. 6e. It can be seen that the introduction of I2 can also promote the thermal stability of the 25

device, but it is slightly weaker than that of IBr based device. These results reveal that the I+ ions play a leading role in improving the thermal stability of the device, while Br- plays an auxiliary role. At the same time, it can be found that the introduction of IBr will increase the band gap of perovskite material, leading to the blue shift of absorption spectrum (Fig. S5), which is mainly due to the introduction of anion Br- [75]. The above results further present that the improved stability of the device mainly results from the introduction of I+. According to literature, the thermal decomposition of perovskite materials can be roughly divided into three processes, as follows [76, 77]: CH3NH3PbI3 <—> CH3NH3I + PbI2(s) CH3NH3I <—> CH3NH2(g) + HI 2HI <—> I2(s) + H2(g) I2(s) <—> I2(g)

(3.1) (3.2) (3.3) (3.4)

In the following, we use typical MAPbI3(CH3NH3PbI3) to stand for some commonly used perovskite materials. It can be seen from the formulas that perovskite materials are mainly decomposed to PbI2, I2 and other materials when heated at high temperatures. The solid I2 produced by decomposition is easily volatilized, which makes the whole process toward positive reaction. After the introduction of IBr, I+ reacts with I- in perovskite materials to form I2, which exists in the interstitial positions of the lattice (not exposed to the perovskite film surface). This makes I2 excessive in the above three reaction processes, and plays a role in inhibiting the reaction toward positive reaction. Although the limited 26

amount of IBr additive is an oxidizing material with high chemical activity and presents moderate thermal stability (boiling point ~116 oC), it could partially react with the I- contained agents including MAI(I-), FAI(I-) and PbI2(I-) in potential, leading to the reaction product of I2 (I+ + I-—> I2), which will further decrease the concentration of deep-level defects, inhibit the whole decomposition process of perovskite and improve the photovoltaic performance and device stability [30]. Furthermore, some content of IBr additives will dope into the lattice of the perovskite materials but not the surface of the films. As for the excessive Br-, it will further improve the stability of perovskite by the anion self-passivation effect [75]. Therefore, the introduction of IBr additive can effectively inhibit the decomposition reaction and improve the thermal stability of perovskite films and devices [78, 79].

Fig. 6. (a) and (b) represent the morphology evolution of two kinds of films annealing at 210 oC for 30 min (a: control film, b: IBr modified film). (c) temperature-dependent UV-Vis absorption 27

decay process was studied for control and IBr modified films after annealing at a high temperature from 150 oC to 210 oC. (d) the temperature-dependent absorption values at 600 nm obtained from Fig. 6c. (e) absorption spectra of control, MABr, I2 and IBr modified FAMA perovskite films before and after annealing at 210 oC for 30 mins.

Next, we address the stability performance of corresponding solar cells. Both of the two PSCs are stored in ambient atmosphere at room temperature without encapsulation, and the corresponding PCEs versus time are plotted inside of Fig. 7a. Light soaking tests show that the control PSC shows a rapid 10% loss in PCE after 50 h continuous illumination, while the IBr modified PSC needs 1000 h to lose the same percentage of the initial efficiency. The rapidly decreased PCE in control device can be mainly explained by the decomposition of perovskite films, leading to the formation of PbI2 over time (Fig. 7b). Fig. 7c shows the stability of the control and IBr modified PSCs versus the exposure time to air (RH=30%) in the dark. It can be seen that the control device degrades rapidly, with a 100% loss of the PCE within 2000 h. In contrast, the best PCE of the IBr modified PSC could maintain at over 82% for more than 5000 h. Based on the typical device structure, the device stability of 5000h far exceeds most existing results [35, 80-83]. Further improvement of the light stability and air stability of PSCs could be attributed to improved crystallinity and effective inhibition of I2 production to boost the stability of the perovskite films. In order to promote the practical applications of the PSC devices, we conduct the external encapsulation of the devices. Encapsulation 28

technology is based on hot melting and solidifying of an ethylene-vinyl acetate (EVA) copolymer. After encapsulation, the device exhibits excellent environmental stability and good water resistance (inset of Fig. 7d). Fig. 7d shows that the device performance of PSC does not change that much after immersing the device into water for more than 100 h. The video of the entire device soaked in water is shown in the supporting information, indicating excellent water resistance and providing a reference for future applications in special environments. According to our previous research, UV light converters can further improve the utilization of the solar spectrum by PSCs devices. The air stable Ce3+-CsPbI3 nanocrystals are coated on the front side (transparent side) of the device as shown in the schematic diagram (inset of Fig. 7e). Fig. 7e shows the J–V curves of the Ce3+-CsPbI3 nanocrystals coated and uncoated PSCs. Voc and FF values of the two devices are almost the same (1.19 V and 79.7%), indicating that the effect of Ce3+-CsPbI3 coating on Voc and FF of the devices is negligible. The enhanced Jsc from 23.45 to 24.13 mAcm-2 can be attributed to the enhanced light harvesting in UV light by Ce3+-CsPbI3 nanocrystals via photon downshifting effect. The PCE exhibits the same tendency as Jsc, and increases from 21.5% to 22.16%. The EQE spectra in Fig. 7f from 360-850 nm further indicate the improved light utilization in UV region [84].

29

Fig. 7. (a) Light stability of the control and IBr modified perovskite films-based PSCs. PCE values with error bar are shown for illustrating the PCE changing of the devices. (b) XRD patterns of the control and IBr modified perovskite films after light soaking for 1000 h. (c) Normalized PCE values versus time of unsealed devices for testing the air stability of PSCs. The PCEs are obtained from at least 15 devices under the same conditions. (d) Normalized PCE versus time of the devices by macromolecule material encapsulation for testing the water stability of PSCs. (e) J–V characteristics curves of the bare device and Ce3+-CsPbI3 nanocrystals coated PSC. Abbreviation of “LDS” stands for the luminescent down-shifting layers of Ce3+-CsPbI3 nanocrystals. (f) The EQE results of the device with and without Ce3+-CsPbI3 coating.

4. Conclusions In summary, we have demonstrated a chemical inhibition mechanism by iodine bromide for solving the intrinsic instability issue of perovskite films, which significantly improves the long-term stability of the perovskite films at high temperatures, light soaking and humid air conditions, while simultaneously enhances their PCE, yielding 21.5% efficiency. It is suggested that IBr modification facilitates carrier transport with minimized resistance loss, and noticeably suppress the charge recombination,

leading

to

significant 30

reduced

hysteresis.

The

simultaneous enhancement of stability and efficiency is achieved through: i) passivation effects of anion Br-, ii) chemical inhibition for decomposition reaction induced by cation I+, iii) and effective hole transport of PDPP4T. The encapsulated PSCs can even withstand direct contact with water for up to 100 h without obvious decomposition. These stability results are compared to most reported literature. It is expected that the chemical inhibition strategy provides valuable design guidelines for efficient and super-stable perovskite solar cells.

Acknowledgements This work was supported by the Key Program of NSFC-Guangdong Joint Funds of China (U1801253), National Key Research and Development Program (2016YFC0207101), the National Natural Science Foundation of China (Grant Nos. 61674067, 11674126, 11674127, 11874181, 61874049, 61775080，61822506), the Special Project of the Province-University

Co-constructing

Program

of

Jilin

Province

(SXGJXX2017-3).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the 31

online version at http://dx.doi.org/10.1016/j.nanoen.##########.

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35

Personal portrait photos and biosketchs

Cong Chen received his Ph.D. degree from Jilin University in June 2019. Previously, he studied and did scientific research at Jilin University focuses on perovskite and quantum dots sensitized solar cell under the supervision of Prof. Hongwei Song. In July 2019, he joined Hebei University of technology as an associate professor. He is the youngest special-term “Yuanguang scholar” in Hebei University of Technology (HEBUT). His main topic of interest is the design of solar cells having high e℃ciency and long-term stability.

Xinmeng Zhuang received her B.S. degree from Heilongjiang University in 2018. She is currently a master student, majoring in integrated circuit at College of Electronic Science and Engineering, Jilin University. Her current research interests mainly focus on nanomaterial， photovoltaics，energy material and perovskite-based solar cells with good stability.

Wenbo Bi received her M. S. degree from Harbin Normal University of condensed matter physics in 2016. Currently she is pursuing her Ph.D. under the supervision of Prof. Hongwei Song at 36

College of Electronic Science and Engineering, Jilin University. Her research is focused on the design of perovskite solar cells and spectral physics of rare earth ions.

Yanjie Wu received her M.S. degree from Jilin Normal University in 2018. Currently she is pursuing her Ph.D. under the supervision of Prof. Hongwei Song at College of Electronic Science and Engineering, Jilin University. Her current research is focused on the design of perovskite and Cu2ZnSn(S,Se)4 solar cells. Her main topic of interest is the design of efficient and stable solar cells.

Yanbo Gao received her M.S. degree from Jilin Normal University in 2018. Currently he is pursuing his Ph.D. under the supervision of Prof. Yu Zhang at College of Electronic Science and Engineering, Jilin University. His main topic of interest is the design of solar cells having high efficiency and long-term stability.

Gencai Pan received his Ph.D. degree from the College of Electronic Science and Engineering, Jilin University in 2019. Currently, he works at School of Physics and Electronics, Henan University. His research is mainly focused on the synthesis and spectral physics of rare earth ions doped quantum dots and their application in optoelectronic devices. 37

Dali Liu received her Ph.D. degree in Condensed Material Physics from Changchun Institute of Physics, Chinese Academy of Science (CAS) in 1998. Now she works as a professor at College of Electronic Science and Engineering, Jilin University (JLU). Her research focuses on integrated optical waveguide devices and functional devices for energy conversion.

Qilin Dai is an assistant professor of physics in Jackson State University, USA. He earned his Ph.D. degree in Condensed Matter Physics at Chinese Academy of Sciences in 2009, then served as postdoctoral research associate in USA. Dr. Dai is an editorial board member of Scientific Reports-Nature. He received 2016 Distinguished Young Scholar in Silk Road International Symposium (Xi'an), 2009 “Da-Heng” Optics Scholarship, Chinese Academy of Sciences. His research focuses on synthesis of nanoparticles, and applications in solar cells, magnets, and light-emitting diodes.

Hongwei Song received his Ph.D. degree in Condensed Material Physics from Changchun Institute of Physics, Chinese Academy of Science (CAS) in 1996. From 1996 to 2000, he worked as a postdoctoral researcher in Institute of Physics, CAS, Nagoya Institute of Technology, and 38

University of California at Berkeley in turn. From 2007 he works in Jilin University as a full professor. He is currently the editorial advisory board member of Scientific Reports. He has published over 250 scientific papers and two book chapters. His research interests have been focused on spectral physics of rare earth ions, optoelectronics and its application.

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Highlights

1. The chemical decomposition inhibition strategy by IBr could induce superior durability of 5000 h. 2. The IBr modified PSCs could achieve a remarkable PCE of 21.5 %. 3. Novel hole-transporting material PDPP4T without doping help to enhance the Voc and better repeatability of devices. 4. The encapsulated PSC exhibits an excellent resistance effect up to 100 h in water. 5. Femto-second TAS spectra and GIWAXS measurements prove the reduced carrier recombination processes.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: