Ionic liquid induced surface trap-state passivation for efficient perovskite hybrid solar cells

Ionic liquid induced surface trap-state passivation for efficient perovskite hybrid solar cells

Organic Electronics 41 (2017) 42e48 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel I...

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Organic Electronics 41 (2017) 42e48

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Ionic liquid induced surface trap-state passivation for efficient perovskite hybrid solar cells Xu Huang a, Heng Guo a, Kai Wang b, Xiaobo Liu a, * a Research Branch of Advanced Functional Materials, Institute of Microelectronic & Solid State Electronic, High-Temperature Resistant Polymers and Composites Key Laboratory of Sichuan Province, University of Electronic Science & Technology of China, Chengdu 610054, PR China b College of Polymer Science and Polymer Engineering, University of Akron, Akron, OH 44325, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 October 2016 Received in revised form 7 November 2016 Accepted 20 November 2016

The highly crystalline methylammonium lead halides (MALHs) perovskite has large carrier diffusion length and thus minimal charge loss within the MALHs layers. But the charge extraction from the perovskite to the charge extraction layers can be significantly influenced by the interfacial contact. Since the fast-crystalized MALHs usually have a rough surface with numerous trap-states and crystal grain boundaries near the top surface, which will deteriorate the electrical coherence and physical contact with the top electron extraction layer (EEL) in the conventional device structure. In this study, we introduced an ionic liquid methyltrioctylammonium trifluoromethanesulfonate (MATS) to passivate the traps and boundaries at the interface of the i-n junction. The photoluminance results demonstrated an improved electron extraction upon the MATS surface treatment. And the impedance measurements also showed a reduced charge transfer resistance within the MATS treated device. Consequently, the conventional planar heterojunction solar cells based on the MALHs perovskite treated by MATS, showed an enhanced device efficiency of 16.10%. Moreover, after heating at elevated temperatures under 2 V forward bias and cooling down to room temperature, we found the MATS modified solar cell devices exhibit a further efficiency improvement to 17.51%. © 2016 Elsevier B.V. All rights reserved.

Keywords: Methylammonium lead halides Perovskite solar cell Methyltrioctylammonium trifluoromethanesulfonate Ionic liquid

1. Introduction The organic-inorganic hybrid perovskite materials have drawn growing attentions in recent years due to their great potentials for high-performance and low-cost solar cell applications [1,2]. The state-of-the-art power conversion efficiency (PCE) has been surged over 22.1% for single-junction solar cells [3]based on the methylammonium lead halides (MALHs) perovskite light harvester thanks to their superior optoelectronic properties and extraordinary long charge carrier diffusion lengths [4]. Different from their organic semiconducting counterparts [5,6], the significantly smaller exciton binding energy of MALHs endows them nonexcitonic behavior that the photo excitation-induced electron-hole pairs can be instantaneously dissociated into free carriers within picoseconds [6]. And the free charge carriers are reported to be capable of travelling over distances of up to micron-scales without significant charge recombination [7,8]. Furthermore, the recent success in growing

* Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.orgel.2016.11.031 1566-1199/© 2016 Elsevier B.V. All rights reserved.

larger crystal sizes [2] and minimizing pinholes [9,10] for acquiring higher quality MALHs thin film have further lowered down the energy barriers for charges to transfer through the MALHs layer itself and consequently give rise to negligible loss within the MALHs layer [11]. Nevertheless, the charge extraction from the MALHs layer to the charge extraction layers are generally limited to the inferior interfacial contacts [12]. The general solution-process MALHs thin films usually shows a polycrystalline morphology with a surface roughness ranging from several to dozens of nanometers, which are expected to hinder the intimate contact with the top charge extraction layers, leaving inevitable trap-states at the interfaces. These trap-states are responsible for retarding and/or even trapping the charge carriers, originating in unwanted trapassisted charge recombination and significantly compromising the device performance. Moreover, the grain-boundaries near the surface may also severely deteriorate the charge extraction and cause significant charge loss [13]. In order to reduce the energy loss at the interface and enhance the electrical coherence between the MALHs and the top electron extraction layers (EELs) in the conventional planar heterojunction (PHJ) perovskite solar cell devices, previous reports have revealed

X. Huang et al. / Organic Electronics 41 (2017) 42e48

the strategies of using solid materials such as thermal-assisted infiltrated fullerene derivatives [13], perovskite-fullerene hybrid solids [14], amphiphilic ionomers [15], and insulating polymers [16] to either passivate the surface trap-states and grain boundaries near surface, or spatially separate the photogenerated electrons with holes via tunneling effects for reducing the recombination loss. In this study, we introduced an ionic liquid methyltrioctylammonium trifluoromethanesulfonate (MATS) for passivating the surface trap-states and grain boundaries near the top surface of the polycrystalline perovskite layer. The MATS was widely applied in the polymer light-emitting electrochemical cells (PLECs) acting as the reservoir for the mobile ions, which can be redistributed by external bias for lowering the contact resistance between the active layer and electrodes and thereby enhancing the electron injection for PLECs [17]. By spin-casting a dilute MATS solution on top of MALHs polycrystalline thin films, the infiltrated MATS are proposed to fill up the grain boundaries near the top surface and passivate the surface trap-states, and consequently we found an enhanced electron extraction efficiency from photoluminescence (PL) spectra at room temperature. The impedance spectroscopic (IS) analysis also confirmed the reduced internal resistance after the surface treatment by MATS. And the fresh PHJ perovskite solar cell with the interfacial modification by the MATS shows a significantly improved PCE of 16.10% at room temperature. Interestingly, after heating at elevated temperatures under 2V forward bias and cooling down to room temperature, we found the MATS modified devices exhibit a PCE improvement to 17.51%. 2. Experimental 2.1. Materials PEDOT:PSS aqueous solution was purchased from H. C. Starck. (Germany). MATS, PCBM, PbI2 (99.999%), anhydrous N,N-dimethylformamide (DMF), ethanol (99.5%), hydroiodic acid (99.99%), DMSO, and methylamine were purchased from Sigma-Aldrich and used as received. The MAI was synthesized according to previous literature [18]. 2.2. Solar cell fabrication Before solar cell device fabrication, the ITO glass substrates were rinsed by sonication in detergent, deionized water, acetone, and isopropanol, in sequence. The pre-cleaned ITO glasses were dried overnight and further treated by an ultraviolet-ozone treatment to remove residual organics. Then the PEDOT:PSS aqueous solution was filtered by a 0.45 mm syringe filter and spin-casted on the top of the ITO glass with a spin-rate of 3500 r.p.m. for 40 s to form a ~50 nm thick film. Afterwards, the PEDOT:PSS coated ITO glasses were annealed at 150  C for 15 min in ambient atmosphere and then transferred into a glove box for DMSO treatment. The solvent treatment was performed by spin-casting 50 mL DMSO on the surface of the PEDOT:PSS thin film with a spin-rate of 4000 r.p.m. for 30 s, followed by thermal annealing at 100  C for 15 min. The MAPbI3 perovskite layer was prepared according to a two-step method: firstly, the solution of PbI2 in DMF (400 mg/mL) was preheated to 70  C, then the solution was spin-casted on top of the DMSO treated PEDOT:PSS substrates with a spin-rate of 3500 r.p.m. for 20 s, under a constant temperature of 75  C; secondly, the MAI solution in ethanol (45 mg/mL) was spin-casted on top of the PbI2 layer with a spin-rate of 3500 r.p.m. for 30 s, at room temperature, followed by annealing at 100  C for 70 min. The MATS was dissolved in toluene in room temperature, then, the MATS interfacial treatment was carried out by spin-casting a solution of MATS in

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toluene with different concentrations (0, 1, 5, 10, 50 mg/mL) on the perovskite surface with a spin-rate of 5000 r.p.m. for 30 s, followed by thermal annealing at 100  C for 30 min. Here we also used the pristine toluene to treat the perovskite surface (the control device, i.e., 0 mg/mL) to eliminate the solvent influence on device performance. Subsequently, the PCBM solution (20 mg/mL in chlorobenzene) was spin-casted at 2000 r.p.m. for 60 s. Finally, a 100 nm thick Al counter electrode was deposited by thermal evaporation. The effective area of the solar cell device is 0.16 cm2. 2.3. Characterization The AFM images were obtained using a Nano-Scope NS3A system (Digital Instrument). The current-voltage (J-V) curves were measured by a Keithley 2400 source-measure unit under one-sun illumination (AM 1.5 G irradiation, 100 mW/cm2) from a 150 W solar simulator (Newport, USA). The light intensity was calibrated by a standardized monosilicon cell (Oriel PN 91150V, Newport, USA) before each test. A shadow mask was also utilized to cover the active area to avoid the adjacent scattering light induced current overestimation. The EQE spectra was obtained by measuring the photon response of the devices in their shortcircuit conditions, under one-sun illumination provided by a 500 W Xe lamp-based solar simulator calibrated by a silicon photodiode (PRL-12, Newport, USA). The IS measurement was performed on an electrochemical workstation, under one-sun illumination with a frequency range from 0.1 Hz to 1 MHz. The PL spectra of various samples were measured by a Picoharp 300 at an excitation incident light wavelength of 530 nm. The postannealing treatment for each device was performed in the glove-box on a hotplate. The devices were connected to an external power supplier to provide a forward bias of 2 V during the annealing at 300 K, 320 K, 350 K for 90 s. Then the devices were cooled down to room temperatures for J-V testing. 3. Results and discussion Fig. 1a displays the chemical structure of the MATS ionic liquid. The extremely low vapor pressure makes this nonvolatile liquid remain in the pinholes and grain boundaries at the surface of MALHs thin film after solvent evaporation during the annealing process [19]. Moreover, the ionic interactions between the CF3SO 3 anion (or [CH3(CH2)6CH2]3CH3Nþ cation) of MATS and MAþ cation (or [PbI6]4-) in the perovskite crystal lattice ensure its excellent affinity to the MALHs perovskite surface [20], resulting in a smooth infiltration into the strap-states and grain boundaries near the perovskite top surface. Fig. 1b shows the PHJ devices structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) doped with dimethyl sulfoxide (DMSO) (50 nm)/MALHs perovskite (~300 nm MAPbI3) (treated by MATS or not)/Phenyl-C61-butyric acid methyl ester (PCBM) (20 nm)/aluminum (Al, 100 nm). The DMSO treated PEDOT:PSS layer possesses an enhanced electrical conductivity of 395 S cm1, confirmed by the four point electrical conductivity measurement, which improves the hole extraction and reduce the sheet resistance for the solar cell device [21]. And the MAPbI3 thin film was prepared from the two-step method followed by solvent annealing for getting a more compact polycrystalline film morphology with larger grain size. The MATS interfacial treatment was further carried out on top of the MAPbI3 layer by spin-casting the MATS solution with different concentrations, followed with thermal annealing for solvent evaporation. The solution processed PCBM acts as the EEL for the solar cell device. Here we applied this low-temperature processed PHJ conventional device structure mainly due to the cost

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(a)

(b)

Al

PCBM MATS

P erovvsk skite Perovskite Methyltrioctylammonium trifluoromethanesulfonate (MATS)

Grain boundary

PEDOT:PSS ITO Fig. 1. (a) Chemical structure of methyltrioctylammonium trifluoromethanesulfonate (MATS) and (b) device architecture of MATS treated planar heterojunction (PHJ) conventional perovskite solar cells.

considerations, knowing that the inverted device structure incorporated with high-temperature sintered TiOx layer and expensive hole extraction layer such as Spiro-MeOTAD is less compatible with future commercialization. Fig. 2 compares the atomic force microscopy (AFM) height images of the MAPbI3 thin films treated by MATS/toluene and pristine toluene, respectively. The untreated MAPbI3 film shows a typical polycrystalline morphology with large grains sizes (~1 mm) but deep gully of grain boundaries [22] near the top surface (Fig. 2a). In contrast, such grain boundaries were sufficiently infiltrated and became more obscure upon MATS treatment (Fig. 2b). Fig. 2c and d shows the corresponding 3-dimensional images, where a much smoother and more homogeneous surface morphology was observed in the MATS treated MAPbI3 thin films, in comparison with the rough surface of the untreated MAPbI3 thin films. The depth profile for MAPbI3 thin films with/without MATS treatment

are further extracted and shown in Fig. 2e and f. A much smaller root-mean-square (RMS) surface roughness (Rq) of 8.16 nm was obtained for the MATS treated MAPbI3 thin films, in comparison with that of 17.52 nm for the untreated MAPbI3 thin films. Such reduced Rq can be ascribed to the MATS infiltration. In microscopic scale, the ionic interaction between MATS and MAPbI3 may induce the microscopic crystal grain refining, making the sharp crystal domain edges merge together for giving a smoother surface morphology [23]; in macroscopic scale, the accumulated MATS at the traps and the boundaries can make up the rough perovskite layer to get a smoother surface with better contact to the EELs on top. Fig. 3a compares the photocurrent density-voltage (J-V) characteristics of the solar cells based on the MAPbI3 perovskite photoactive layer with/without MATS treatment, under one-sun illumination. The MATS treated device shows a higher short-circuit

Fig. 2. AFM height images, 3-dimensional AFM height images, and depth profiles for MAPbI3 perovskites thin films without (a, c, e) and with (b, d, f) MATS treatment.

X. Huang et al. / Organic Electronics 41 (2017) 42e48

(b)100

4 0

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80

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w/ MATS

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Current Density (mA/cm )

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400

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Fig. 3. (a) Photocurrent density-voltage (J-V) characteristics and (b) external quantum efficiency spectra of solar cells based on the MAPbI3 perovskite photoactive layer with/ without MATS treatment.

current density (JSC) of 22.65 mA/cm2, higher open-circuit voltage (VOC) of 0.90 V, higher fill factor (FF) of 0.79, and a decent PCE of 16.10%, in contrast to the JSC of 18.07 mA/cm2, VOC of 0.85 V, FF of 0.73, and PCE of 11.21% for the untreated device. To verify the significantly improved JSC, we further employed the external quantum efficiency (EQE) measurement for tracking the photon response at each wavelength. Fig. 3b compares the EQE spectra of solar cell devices based on the MAPbI3 perovskite photoactive layer with/without MATS treatment. The untreated device shows a mild EQE less than 80% from visible to near infrared region with a typical cut-off around 800 nm [24]. While the MATS treated device shows a dramatically enhanced EQE close to 90% from 375 nm to 800 nm, indicating vast majority of the photo-induced carriers have been extracted and collected by the electrodes. The integrated photocurrent density from the EQE spectra for solar cells with and without MATS treatment are 22.59 mA/cm2 and 18.05 mA/cm2, respectively, which are in good agreement with those (22.65 mA/ cm2 and 18.07 mA/cm2, respectively) extracted from the J-V characteristics, implying there is negligible spectral mismatch between these two measurements and a high convincible accuracy for the JSC values. Fig. 4 and Table 1 compare the device performance of the PHJ perovskite solar cells by interfacial treatment of MATS with different concentrations, where the parameters are the average values from approximately 20 different devices. As the MATS concentration increases from 0 to 10 mg/mL, all the performance

(a)

parameters simultaneously increases and less deviations are observed. It should be noted that as the concentration increases to 10 mg/mL, a highest FF of 0.804 is obtained from the best device. The FF can be more sensitive to the interfacial charge transfer and thus to the interfacial modification. Such a high FF suggest the charge loss during the interfacial extraction between the perovskite and the EEL is significantly reduced, giving a more balanced charge collection at the electrodes and minimized charge recombination thereby [25]. Consequently, simultaneously improved JSC, VOC and FF are acquired. While as further increase the MATS concentration to 50 mg/mL, we observed a dramatic reduction in solar cell performance and a larger deviation in performance parameters. This can be ascribed to the excess amount of the MATS residual at the interface. The insulating nature at low temperatures for the MATS is expected to enlarge the internal series resistance of the solar cell device, inhibiting the charge extraction in turn. To understand the charge extraction and transport behavior at the interface, the PL and IS measurement were further carried out. Fig. 5a shows the PL spectra of glass/MAPbI3, glass/MAPbI3/PCBM, glass/MAPbI3/MATS/PCBM, glass/PEDOT:PSS/MAPbI3, respectively. We used bare glass as substrate during the PL measurement. Since the charge carrier mobility of PEDOT:PSS is better than that of PCBM [26,27], the glass/PEDOT:PSS/MAPbI3 exhibits lower PL intensity (or higher PL quenching) than that of glass/MAPbI3/PCBM. In comparison with the glass/MAPbI3 without any electron or hole extraction layers, the glass/PEDOT:PSS/MAPbI3 and glass/MAPbI3/

(b)

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Fig. 4. Device performances of the PHJ perovskite solar cells based on the MAPbI3 photoactive layer with/without MATS treatment as a function of the concentration of MATS.

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Table 1 The J-V characteristics parameters of the PHJ perovskite solar cells based on the MAPbI3 photoactive layer treated by MATS of different concentrations or not. MATS concentration (mg/mL)

JSC extracted from J-V curve (mA/cm2)

JSC extracted from EQE (mA/cm2)

VOC (V)

FF (100%)

PCE (%)

0 1 5 10 50

18.07 19.08 21.25 22.65 17.53

18.05 19.09 21.27 22.59 17.03

0.85 0.87 0.88 0.90 0.82

0.73 0.74 0.76 0.79 0.59

11.21 12.28 14.21 16.10 8.48

(b) 120 MAPbI 3 MAPbI /PC BM 3 61 MAPbI /MATS/PC BM 3 61 MAPbI /PEDPT:PSS

w/o MATS w/ MATS

3

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PL intensity (a.u.)

(a) 1.5

0.5

0 650

80

40

700

750

800

Wavelength (nm)

850

0

0

50

Z' (Ohm)

100

150

Fig. 5. (a) Photoluminescence spectra of the glass/MAPbI3, glass/MAPbI3/PCBM, glass/MAPbI3/MATS/PCBM, glass/PEDOT:PSS/MAPbI3, respectively; (b) Nyquist plots of the device with/without MATS treatment.

PCBM exhibit a relative PL intensity of 9.5% and 39.8%, respectively. The lower relative PL intensity (or higher PL quenching) at the p-i junction is ascribed to the efficient hole extraction from MAPbI3 to PEDOT:PSS, while the high relative PL intensity at the i-n junction suggests an insufficient electron extraction from MAPbI3 to PCBM, which is due to the inferior interfacial contact. After MATS interfacial treatment, we observed that the glass/MAPbI3/MATS/PCBM exhibit a significantly reduced relative PL intensity of 13.7%, which is more comparable to that of the glass/MAPbI3/PCBM at the anode side. Therefore, a stronger quenching at the i-n junction verifies the effectiveness of the MATS modification on the electron extraction from MAPbI3 to PCBM, and a reasonable higher FF is obtained thereby. The charge transfer behavior at the interface are further investigated by the IS measurement. Fig. 5b shows the Nyquist plots of the solar cells based on the MAPbI3 with/without MATS treatment at their VOC conditions, where the recombination resistance can be negligible compared with the charge transfer resistance. The series resistance RS is directly influenced by the interfacial charge transfer resistance (RCT) at the interfaces as well as the sheet resistance RSHE of the electrodes. Here we notice the single difference comes from the i-n interface caused by the MATS treatment. And according to the impedance values at low frequency region, the untreated device exhibits a RCT of 118.3 U, while the MATS treated device exhibits a reduced RCT of 67.9 U. Such smaller RCT is originated from the optimized interface at the i-n junction by the MATS treatment. The infiltrated MATS is anticipated to passivate the trap-states and grain boundaries near the interface, which results in a better electrical coherence of the i-n junction. The minimized trap-states suggest less trap-assisted recombination [28] at the interface and the better electrical coherence ensures an efficient charge extraction, both of which lead to enhanced JSC, VOC, and FF for the perovskite solar cells. The hysteresis study was further carried out for the solar cells

based on MAPbI3 with/without MATS treatment. Fig. 6a compares the J-V characteristics for above solar cells at reverse (from forward bias to reverse bias) or forward (from reverse bias to forward bias) scan directions with a scan rate of 0.05 V/s. The untreated device shows a severe PCE deviation of 18.5% from 9.58% at forward scan to 11.35% at reverse scan; while the MATS treated device shows a dramatically reduced deviation of 0.5% from 16.19% at forward scan to 16.27% at reverse scan. Previous reports [29,30] have revealed the trap-state passivation by fullerene would minimize the charge trap and retardation, and/or hamper ion migration, which could reduce the photocurrent hysteresis dramatically. Similarly, the strong ionic interaction between the MATS and the MAPbI3 are expected to localize the ions in the crystal lattice [31] at the surface and grain boundaries near top surface, meanwhile the trap-states are filled up by the accumulated MATS. Therefore, much reduced hysteresis is observed in the MATS treated solar cell device. In PLEC, the MATS have been reported [32,33] as the ion source and release mobile ions at elevated temperatures. These ions can be redistributed by an external electrical field at high temperatures and then be frozen at low temperatures to induce an internal electrical field for lowering the interfacial resistance and enhancing the charge transport efficiency at the interface. Here we also studied this effect on the J-V characteristics of the untreated and MATS treated solar cell devices. The solar cell devices based on the MAPbI3 with/without MATS treatment were heated at different temperatures of 300 K, 320 K, and 350 K with a forward bias of 2 V for 90 s and then cooled down to room temperatures. Fig. 6b and Table 2 show the corresponding J-V characteristics parameters. The solar cells without MATS treatment show a slight performance enhancement from room temperature to high temperature postannealing under forward bias. It has been reported that there exists excess ions in generally MAPbI3 perovskite [34e36]. Such ions will be more mobile at elevated temperatures, and with the external electrical field, the ions can be redistributed and

X. Huang et al. / Organic Electronics 41 (2017) 42e48

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-12 -16

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Fig. 6. (a) Photocurrent hysteresis on perovskite solar cells based on MAPbI3 with/without MATS treatment, the scan rate is 0.05 V/s; (b) The J-V characteristics of the untreated and MATS treated solar cell devices, with the post-annealing at different temperatures under a forward bias of 2 V for 90 s.

Table 2 The J-V characteristics parameters of the untreated and MATS treated solar cell devices, with the post-annealing at different temperatures under a forward bias of 2 V for 90 s. Solar cells

Temperature (K)

JSC (mA/cm2)

VOC (V)

FF (100%)

PCE (%)

w/o MATS

300 320 350 300 320 350

18.12 18.37 19.08 22.66 23.41 24.07

0.86 0.86 0.88 0.89 0.91 0.94

0.72 0.74 0.73 0.80 0.79 0.77

11.35 11.66 12.10 16.27 16.93 17.51

w/MATS

accumulated at the interface to reduce the electrical resistance for charge transfer. Therefore, a slight PCE improvement can be obtained. While such improvement is more obvious in the MATS treated devices, as shown in Table 2, a decent PCE of 17.51% was obtained after a 350 K post annealing under 2 V. This observation is quite similar to those found in the PLEC, where either the electrodynamic model (ED) or the electrochemical doping model (ECD) has been proposed to explain the ionic liquid induced energy barrier reduction at the interface [37]. To fully understand the underlying device physics here, more investigations and experimental evidences need to be sought in future. 4. Conclusion In this study, we employed the ionic liquid MATS to address the inferior interfacial contact between the perovskite polycrystals and the PCBM EEL in the PHJ device structure. The rough surface of the perovskite polycrystals generally generates numerous trap-states and the grain boundaries at the interface, deteriorating the electron extraction in the i-n junction. The MATS can sufficiently infiltrate into the grain boundaries near the surface and passivate the trap-states at the interface, leading to an enhanced PL quenching and reduced charge transfer resistance. Consequently, higher device performance has been realized from the solar cells based on the perovskite with MATS treatment. Interestingly, by post-annealing treatment under a forward bias of 2 V, we found a further enhanced PCE to 17.51%. Such improvement is highly speculated by the ion redistribution induced barrier reduction at the interface. Overall, we prove the MATS interfacial modification is an efficient way to address the electron loss in extraction process at the i-n junction and propose the post-annealing under bias treatment can further optimize the interface and improve the device performance.

Acknowledgements The paper was financially supported by the National Natural Science Foundation (No. 51173021 and No. 51403029), and South Wisdom Valley Innovative Research Team Program. References [1] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395e398. [2] W. Nie, H. Tsai, R. Asadpour, J.C. Blancon, A.J. Neukirch, G. Gupta, J.J. Crochet, M. Chhowalla, S. Tretiak, M.A. Alam, Solar cells. High-efficiency solutionprocessed perovskite solar cells with millimeter-scale grains, Science 347 (2015) 522e525. coppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Gr€ [3] X. Li, D. Bi, C. Yi, J.D. De atzel, A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells, Science 353 (2016) 58e62. [4] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, High charge carrier mobilities and lifetimes in organolead trihalide perovskites, Adv. Mater. 26 (2014) 1584e1589. [5] N.C. Giebink, G.P. Wiederrecht, M.R. Wasielewski, S.R. Forrest, Thermodynamic efficiency limit of excitonic solar cells, Phys. Rev. B Condens. Matter 83 (2011) 173e184. [6] M. Hu, B. Cheng, Y. Yuan, Z. Xiao, Q. Dong, Y. Shao, J. Huang, Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system, Small 11 (2015) 2164e2169. [7] S. Stranks, G. Eperon, G. Grancini, C. Menelaou, M. Alcocer, T. Leijtens, L. Herz, A. Petrozza, H. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber, Science 342 (2013) 341e344. [8] S.R. Cowan, L.L. Wei, N. Banerji, G. Dennler, A.J. Heeger, Identifying a threshold impurity level for organic solar cells: enhanced first-order recombination via well-defined PC 84 BM traps in organic bulk heterojunction solar cells, Adv. Funct. Mater. 21 (2011) 3083e3092. [9] G.E. Eperon, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci. 7 (2014) 982e988. [10] L. Bo, J. Tian, L. Guo, C. Fei, T. Shen, X. Qu, G. Cao, Dynamic growth of pinholefree conformal CH3NH3PbI3 film for perovskite solar cells, Acs Appl. Mater. Interfaces 8 (2016) 4684e4690. [11] Y. Zhang, Z. Wang, X. Li, L. Wang, M. Yin, L. Wang, N. Chen, C. Fan, H. Song, Dietary iron oxide nanoparticles delay aging and ameliorate neurodegeneration in drosophila, Adv. Mater. 28 (2016) 1387e1393. [12] K. Wang, C. Liu, C. Yi, L. Chen, J. Zhu, R. Weiss, X. Gong, Efficient perovskite hybrid solar cells via ionomer interfacial engineering, Adv. Funct. Mater. 25 (2015) 6875e6884. [13] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells, Nat. Commun. 5 (2014) 5784e5791. [14] C. Adami, A. Hintze, Evolutionary instability of zero-determinant strategies demonstrates that winning is not everything, Nat. Commun. 5 (2014) 2193e2200. [15] T. Tang, D.J. Coady, A.J. Boydston, O.L. Dykhno, C.W. Bielawski, Pro-ionomers: an anion metathesis approach to amphiphilic block ionomers from neutral precursors, Adv. Mater. 20 (2008) 3096e3099. [16] D.J. Lee, H. Lee, Y.J. Kim, J.K. Park, H.T. Kim, Lithium-oxygen batteries: sustainable redox mediation for lithium-oxygen batteries by a composite

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[17] [18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

X. Huang et al. / Organic Electronics 41 (2017) 42e48 protective layer on the lithium-metal anode (Adv. Mater. 5/2016), Adv. Mater. 28 (2016) 857e863. Y. Shao, G. Bazan, A. Heeger, Long-lifetime polymer light-emitting electrochemical cells, Adv. Mater. 19 (3) (2007) 365e370. K. Wang, C. Liu, P. Du, H.L. Zhang, X. Gong, Efficient perovskite hybrid solar cells through a homogeneous high-quality organolead iodide layer, Small 11 (2015) 3369e3376. A.M.A. Leguy, Y. Hu, M. CampoyQuiles, M.I. Alonso, O.J. Weber, P. Azarhoosh, M.v. Schilfgaarde, M.T. Weller, T. Bein, J. Nelson, Reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells, Chem. Mater. 27 (2015) 3397e3407. dua, Molecular force field for ionic liquids composed of J.N.C. Lopes, A.A.H. Pa triflate or bistriflylimide anions, J. Phys. Chem. B 108 (2004) 16893e16898. H.K. Yong, C. Sachse, M.L. Machala, C. May, L. Müller-Meskamp, K. Leo, Photovoltaic devices: highly conductive PEDOT: PSS electrode with optimized solvent and thermal post-treatment for ITO-free organic solar cells (Adv. Funct. Mater. 6/2011), Adv. Funct. Mater. 21 (2011) 1076e1081. Y. Shao, Y. Fang, T. Li, Q. Wang, Q. Dong, Y. Deng, Y. Yuan, H. Wei, M. Wang, A. Gruverman, Grain boundary dominated ion migration in polycrystalline organiceinorganic halide perovskite films, Energy Environ. Sci. 9 (2016) 1752e1759. C. Wang, X. Xu, W. Zhang, J. Bergqvist, Y. Xia, X. Meng, K. Bini, W. Ma, A. Yartsev, K. Vandewal, Organic photovoltaics: low band gap polymer solar cells with minimal voltage losses, Adv. Energy Mater. : http://dx.doi.org/10.1002/ aenm.201670109. D.X. Yuan, A. Gorka, M.F. Xu, Z.K. Wang, L.S. Liao, Inverted planar NH2CH¼NH2PbI3 perovskite solar cells with 13.56% efficiency via low temperature processing, Phys. Chem. Chem. Phys. 17 (2015) 19745e19750. D.I. Schuster, P. Cheng, P.D. Jarowski, D.M. Guldi, C. Luo, L. Echegoyen, S. Pyo, A.R. Holzwarth, S.E. Braslavsky, R.M. Williams, Design, synthesis, and photophysical studies of a porphyrin-fullerene dyad with parachute topology; charge recombination in the marcus inverted region, J. Am. Chem. Soc. 126 (2004) 7257e7270. H.H. Jin, M.S. You, M.H. Chang, W. Yin, T.K. Ahn, S.J. Lee, S.J. Sung, D.H. Kim,

[27]

[28]

[29]

[30]

[31] [32] [33]

[34]

[35] [36]

[37]

H.I. Sang, Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode, Nano Energy 15 (2015) 530e539. H.I. Sang, J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, 18.1% hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells, Energy Environ. Sci. 8 (2015) 1602e1608. € M. Nyman, O.J. Sandberg, R. Osterbacka, 2D and trap-assisted 2D langevin recombination in polymer:fullerene blends, Adv. Energy Mater., http://dx.doi. org/10.1002/aenm.201400890. S. Olthof, S. Singh, S.K. Mohapatra, S. Barlow, S.R. Marder, B. Kippelen, A. Kahn, Passivation of trap states in unpurified and purified C60 and the influence on organic field-effect transistor performance, Appl. Phys. Lett. 101 (2012) 253303e253303-4. Y. Xing, C. Sun, H.L. Yip, G.C. Bazan, F. Huang, Y. Cao, New fullerene design enables efficient passivation of surface traps in high performance p-i-n heterojunction perovskite solar cells, Nano Energy 26 (2016) 7e15. R.S. Nelson, M.W. Thompson, The penetration of energetic ions through the open channels in a crystal lattice, Philos. Mag. 8 (1963) 1677e1690. Y. Shao, G.C. Bazan, A.J. Heeger, Hybrid polymer light-emitting devices, US2010. Y. Shao, G.C. Bazan, A.J. Heeger, Light-emitting polymer solution of a soluble ionic liquid such as methyltrioctylammonium trifluoromethanesulfonate; the polymer may be a phenyl-substituted polyphenylenevinyleneused for light emitting diodes and electrochemical cells, WO2010. B. Joseph, B. Tonio, D.A. Egger, H. Gary, K. Leeor, L. Yueh-Lin, L. Igor, S.R. Marder, M. Yitzhak, J.S. Miller, Hybrid organic-inorganic perovskites (HOIPs): opportunities and challenges, Adv. Mater. 27 (2015) 5102e5112. H.Y. Hsu, L. Ji, M. Du, J. Zhao, E.T. Yu, A.J. Bard, Optimization of PbI2/MAPbI3 Perovskite Composites by Scanning Electrochemical Microscopy, 2016. G.A. Nemnes, C. Goehry, T.L. Mitran, A. Nicolaev, L. Ion, S. Antohe, N. Plugaru, A. Manolescu, Band alignment and charge transfer in rutile-TiO2/CH3NH3PbI3xClx interfaces, Phys. Chem. Chem. Phys. 17 (2015) 30417e30423. s, C. Rold S.B. Meier, D. Tordera, A. Pertega an-Carmona, E. Ortí, H.J. Bolink, Light-emitting electrochemical cells: recent progress and future prospects, Mater. Today 17 (2014) 217e223.