Fullerene Interface Enhances the Inverted

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Surfaces, Interfaces, and Applications

Strengthened Perovskite/Fullerene Interface Enhances the Inverted Planar Perovskite Solar Cells Regarding Efficiency and Stability via Tetrafluoroterephthalic Acid Interlayer Minhua Zou, Xuefeng Xia, Yihua Jiang, Jiayi Peng, Zhenrong Jia, Xiaofeng Wang, and Fan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12961 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Materials & Interfaces

Strengthened Perovskite/Fullerene Interface Enhances the Inverted Planar Perovskite Solar Cells Regarding Efficiency and Stability via Tetrafluoroterephthalic Acid Interlayer Minhua Zou1, Xuefeng Xia1, Yihua Jiang1, Jiayi Peng1, Zhenrong Jia2, Xiaofeng Wang1, Fan Li*1 Department of Materials Science and Engineering, Nanchang University, 999 Xuefu

1

Avenue, Nanchang 330031, China 2

National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Abstract In this work, a novel back contact interface engineering is developed for the inverted planar perovskite solar cells, in which a tetrafluoroterephthalic acid (TFTPA) interlayer is inserted between CH3NH3PbI3 and PC61BM to strengthen the interface contact. Benefiting from the strong Coulombic interactions between positive electron-poor tetrafluoroterephthalate moieties and negative electron-rich fullerene molecules, as well as the coordinate effect between -COOH groups of TFTPA and Pb2+ ions of perovskites

surface, a tightly-jointing and defect-passivated

CH3NH3PbI3/PC61BM interface is formed. The strengthened CH3NH3PbI3/PC61BM back contact can significantly facilitate electron transport and simultaneously diminish the charge accumulation and recombination. Therefore, the power conversion efficiency (PCE) of TFTPA device is up to 19.39% while the hysteresis effect is weak, and the PCE is improved by 20.4% compared with the control device which does not have a TFTPA interlayer. Particularly, the moisture stability of the TFTPA device is greatly improved as compared to the control device. Our findings illustrate that the back contact interface engineering is an important and promising approach for inverted planar PSCs.

Keywords: Perovskite solar cell; Interface; Stability; Efficiency

ACS Paragon Plus Environment

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1. Introduction Although organic-inorganic halide perovskite (OIHP) was discovered in 1978 for the first time,1-2 Miyasaka et al. triggered its great research enthusiasm in 2009 by applying CH3NH3PbBr3 (MAPbBr3) and CH3NH3PbI3 (MAPbI3) as sensitizers in the dye-sensitized solar cells.3 Since then, perovskite solar cells (PSCs) have seen an improved power conversion efficiency (PCE) of 25.2%4 through morphology, composition and device structure optimization, interface engineering, and so on. Such amazing development of PSCs is mainly caused by the unique optoelectronic properties presented by OIHPs, like proper bandgap, higher coefficient of absorption, longer carrier diffusion length and lifetime, smaller exciton binding energy, as well as the ambipolar charge transport.5-8 Moreover, the solution processability of OIHPs makes it possible to achieve low-cost and large-scale devices and PSCs have become the superstar in the third photovoltaic technology. To date, PSCs mainly contain two device architectures, namely mesoporous structure and planar heterojunction structure. The planar structure is composed by conventional n-i-p configuration and inverted p-i-n configuration. While nearly all high-efficiency PSCs have been achieved with mesoporous device structure, more and more people are interested in the development of inverted planar device architectures mainly due to its simplicity, low-temperature processability and low hysteresis.9-13

As we know, the photovoltaic properties of PSCs is determined by numerous factors, including the morphologies and compositions of perovskite active layers, charge transport layers (CTLs) and interfaces. Among them, interfaces assume key roles in the realization of high-performance photovoltaic devices and studies have demonstrated the effect of interface engineering on effectively improving PSCs regarding their stability and efficiency.14-17 In inverted planar PSCs, there are two extremely important interfaces: hole transport

layer (HTL)/perovskite and

perovskite/electron transport layer (ETL). Up to now, many research effort has been devoted to improving the HTL/perovskite interface in order to increase hole extraction/transport efficiency, optimize energy level alignment, improve perovskite


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crystallization and reduce recombination loss.18-22 On the contrary, very limited investigations are made on the perovskite/ETL interface. In fact, in the inverted planar PSCs, the perovskite/ETL interface, especially the perovskite/fullerence (e.g. [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM)) interface in most cases, plays the same important role as the HTL/perovskite interface and rational manipulation of the perovskite/fullerence interface is indispensable for efficient devices. Based on studies, the perovskite/fullerence interface engineering can improve the device performance, and in the meantime, greatly enhance the device stability along with reduced hysteresis via filling the perforations and passivating traps of perovskite layers, reducing leakage current, enhancing electron-collection efficiency and forming high-quality ETL and hydrophobic protection layers.23-30 However, most of the research work are mainly focused on the modification of the perovskite surface, adjustment of the interface energy level and optimization of electron extraction, less attention is paid on the interface contact between fullerene ETLs and perovskite layers. Actually, the connectivity at perovskite/ETL interface has been demonstrated to be poor owing to the rough and defective surface of the solution-processed perovskite layers,31 which will inevitably deteriorate the device efficiency and stability. Therefore,

strengthening

interface

contact

to

form

tightly-jointing

and

defect-passivated perovskite/fullerene interface will help to enhance the inverted planar PSCs in terms of their stability and efficiency.

Multifluorine substituted benzene possesses a positive electron-depleted carbon ring due to the electron-withdrawing property exhibited by F atoms, which can interact strongly with the negative electron-rich materials, e.g. fullerene derivatives.32 In organic solar cells, multifluorine substituted benzene has been widely applied to control the morphologies of fullerene-containing material systems via the Coulombic interactions between the multifluorophenyl moiety and fullerene.33-37 Inspired by these research work, herein, a tetrafluoroterephthalic acid (TFTPA) interlayer is introduced to strengthen the perovskite/fullerene interface, in which the rigid tetrafluorobenzene ring is connected with 2 carboxyl groups (-COOH) at 1,4-symmetrical position. The



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TFTPA molecule can bond on the perovskite surface as the oxygen-lone pairs of carboxyl groups show the coordinate effect with the Pb2+ ions of the perovskites38-39 and therefore the positive electron-poor tetrafluoroterephthalate moieties lie on the surface of perovskite, which will anchor the top negative electron-rich fullerene molecules through strong Coulombic interactions, thus, forming tightly-jointing and defect-passivated perovskite/fullerene interface. Obviously, such strengthened perovskite/fullerene interface will promote electron extraction and transport, reduce the hysteresis effect, as well as enhance the moisture resistivity, thereby improving the performance of inverted planar PSCs.

2. Experimental section Materials Xi’an Polymer Light Technology Corporation provided the CH3NH3I (MAI) and PbI2 (99%).

Nickel

(Ⅱ)

nitrate

hexahydrate

2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline Sigma-Aldrich.

Super

dehydrated

solvents,

(Ni(NO3)2.6H2O) (BCP) including

were

along provided

chlorobenzene

with by (CB),

N,N-Dimethylformamide (DMF), ethyl acetate (EA) and dimethylsulfoxide (DMSO), were obtained from the Alfar Aesar. J&K Chemicals offered [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and tetrafluoroterephthalic acid (TFTPA). Ag (99.999%) was obtained from Alfa Aesar. All materials used in the study did not receive any purification treatment.

PSC fabrication Detergent water, deionized water, acetone, as well as isopropyl alcohol were used for sequential sonication to clean the patterned indium tin oxide (ITO)-coated glass substrates (15 ohm sq-1) for 15 min each. Following drying treatment under clean N2 flow, the substrate received ten minutes of oxygen plasma treatment (PLASMA CLEANER PDC-002). NiOx nanoparticles aqueous solutions (20 mg mL−1) were spin-coated for 30 s at 4000 rpm, which, followed by 10 minutes of anneal at 140 °C, helped to obtain the NiOx hole-transport layers. As a result, transparent NiOx thin film


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of which the thickness is about 20 nm is finally obtained. A chemical precipitation approach was applied to prepare the NiOx nanoparticles based on previous literature.40 After cooling the NiOx films to the room temperature, we transferred the substrates into a glovebox filled with argon. Dissolving an equal molar ratio of PbI2 (588 mg) and MAI (198 mg) in the DMF and DMSO mixed solvent (DMF/DMSO 4:1 (v/v), 1 mL)

helped

to

prepare

the

perovskite

precursor

solution.

Subsequently,

polytetrafluoroethylene filters (0.22 μm) were used to filter the resultant solution. Then the resultant solution underwent 30 s of spin-coating onto NiOx hole-transport layers at 3500 rpm, and we dripped 200 μL of CB quickly onto the rotating substrate after the initial 10 s. Then, the film received 10 minutes of anneal treatment at 100 °C for getting the dark-brown CH3NH3PbI3 films (500 nm). For the preparation of TFTPA interlayer, TFTPA (0.5 mM in ethyl acetate) underwent 30 s of spin-coating on the top of MAPbI3 film at 3000 rpm and then retained at room temperature for 15 s to allow the -COOH groups of TFTPA molecules reacting with the MAPbI3 surface. Subsequently, the film was rinsed twice with ethyl acetate to remove the physically absorbed TFTPA, followed by 10 min of anneal treatment at 40 ℃ for the removal of residual ethyl acetate. Afterwards, 25 s of spin-coating was performed on the PC61BM (23 mg mL−1 in CB) firstly at 1500 rpm, followed by 30 min of drying treatment at 90 °C. BCP (0.5 mg mL−1 in methanol) was deposited on the PC61BM film by 30 s of spin-coating at 5000 rpm, followed by 5 min of drying treatment. The PC61BM/BCP layer presented a total thickness of about 40 nm. At last, Ag electrodes received thermal evaporation with a thickness of 200 nm on the top of devices. ITO/NiOx/MAPbI3/TFTPA/PC61BM/BCP/Ag

constitutes

configuration.

control

For

comparison,

the

the device

completed structure

device was

ITO/NiOx/MAPbI3/PC61BM/BCP/Ag. Other characterization details are provided in the Supporting Information.

3. Results and Discussion The TFTPA interlayer is introduced to the MAPbI3/PC61BM interface and the preparation process is shown in the schematic of Figure 1a. Typically, TFTPA (0.5 ACS Paragon Plus Environment


mM in ethyl acetate) is spin-coated on the MAPbI3 film at 3000 rpm for thirty seconds, followed by retaining on the perovskite film for 15 s to allow the -COOH groups of TFTPA molecules reacting with the MAPbI3 surface. Subsequently, the film is rinsed twice with ethyl acetate to remove the physically absorbed TFTPA, hereafter, annealed at 40 ℃ for 10 min to remove residual ethyl acetate (Procedures are explained in detail in the Experimental Section and the chemical structure of TFTPA is shown in Scheme S1). In the experiment, the solution used for spin-coating TFTPA onto the MAPbI3 film is critical because it should not deteriorate perovskite materials and also should provide good wettability for spin-coating the PC61BM layer. For these purposes, we choose the ethyl acetate as the solution because it can dissolve the TFTPA and at the same time, it commonly used as the anti-solvent for the preparation of MAPbI3 films. According to Figure S1 and Figure S2, the application of ethyl acetate solution will not affect the morphology of perovskite films and can provide good wettability for PC61BM spin-coating. By spin-coating TFTPA solution onto the MAPbI3 films, the carboxyl groups (-COOH) of TFTPA molecules will react with Pb2+ ions on MAPbI3 surface, leading to the plane-on sites of TFTPA on the MAPbI3 surface. At the same time, the strong electron-withdrawing ability of F atoms in TFTPA will cause the center of the benzene ring to be positively charged. Upon coating PC61BM layer on top of it, the negative electron-rich PC61BM will be strongly anchored through the Coulombic interactions. Therefore, the TFTPA interlayer can act as a “bridge” between MAPbI3 and PC61BM to effectively connect the MAPbI3 active layer and PC61BM ETL, resulting in the creation of the tight MAPbI3/TFTPA/PC61BM interface, as shown in Figure 1b. On the contrary, without the incorporation of TFTPA interlayer, a poor-jointing MAPbI3/PC61BM interface appears (Figure S3). It should be pointed out that it is difficult to identify the TFTPA interlayer in the cross-sectional SEM images because such TFTPA interlayer just occurs in molecular level.

The enhanced contact of the MAPbI3/PC61BM interface will contribute to the improvement of the PSCs performance. The current density versus voltage (J-V)


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curves of optimal PSCs with and without TFTPA interlayer (represented as TFTPA device and control device, respectively) are presented in Figure 1c and 1d. Table 1 lists the corresponding photovoltaic parameters. The PCE of the optimal TFTPA device is 19.39% in reverse scan (the short-circuit current (Jsc) is 21.41 mA cm-2, the open-circuit voltage (Voc) is 1.13 V, the fill factor (FF) is 80.2%) and the comparable PCE reaches 18.61% in forward scan (the Jsc, Voc, and FF are 21.11 mA cm-2, 1.12 V, and 78.7%, respectively). For comparison, the PCE of the optimal control device is 16.10% in reverse scan (the Jsc, Voc, and FF are 19.31 mA cm-2, 1.09 V and 76.5%, respectively) but a lower PCE of 15.05% in forward scan (the Jsc, Voc, and FF are 19.20 mA cm-2, 1.08 V and 72.6%, respectively). Obviously, the hysteresis of the optimal control device is more serious, and the PCE is only 15.05% in forward scan, down 6.52% than that in reverse scan. Oppositely, the hysteresis of the optimal TFTPA becomes weaker, and the PCE is at a high level of 18.61% in forward scan, down 4.02% than that in reverse scan. Correspondingly, Figure 1e shows the incident photon-to-current conversion efficiency (IPCE) spectra exhibited by the optimal control and TFTPA devices. Both the control device and TFTPA device share the typical IPCE pattern of PSCs and it is found that the TFTPA PSC possesses an enhanced photoresponse in comparison with the control PSC. The photocurrent integrated from the IPCE spectra is 21.31 mA cm-2 for the TFTPA device and 19.23 mA cm-2 for the control device. These data well support the Jsc values measured based on J-V measurement (Table 1). Meanwhile, the average J-V characteristics of the control and TFTPA devices determined from 30 devices also demonstrate the superior performance of TFTPA device compared with control device (Table S1). Clearly, incorporation of TFTPA interlayer can significantly boost the device efficiency and reduce the device hysteresis effect. Such dramatic performance improvement in the TFTPA device inspires us to reveal the underlying mechanism originated from the introduction of TFTPA interlayer at the MAPbI3/PC61BM interface.



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Figure 1. (a) Schematic process for the introduction of TFTPA interlayer at the MAPbI3/PC61BM

interface.

(b)

Cross-sectional

Glass/ITO/NiOx/MAPbI3/TFTPA/PC61BM

and

SEM

image

schematic

presented

illustration

of

by the

interaction among MAPbI3, TFTPA interlayer and PC61BM. J-V curves of the optimal (c) TFTPA device and (d) control device. The measurement of J-V curves is performed in forward and reverse scans. (e) IPCE spectra and corresponding integrated Jsc presented by the control device and TFTPA device.

Table 1. Photovoltaic performance parameters of the optimal devices with and without TFTPA interlayer measured in reverse and forward scans. Devices

Control device

TFTPA device

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Forward

1.08

19.20

72.6

15.05

Reverse

1.09

19.31

76.5

16.10

Average

1.09

19.26

74.6

15.58

Forward

1.12

21.11

78.7

18.61

Reverse

1.13

21.41

80.2

19.39

Average

1.13

21.26

79.5

19.01


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For exploring how TFTPA interlayer affects the MAPbI3 perovskite, top-view field-emission scanning electron microscopy (SEM) together with atomic force microscopy (AFM) measurements is applied to characterize the surface morphologies of the MAPbI3 and MAPbI3/TFTPA films. According to Figure 2a and 2b, both films appear as dense and smooth polycrystalline layers with high surface coverage and similar grain size, except for the smaller root-mean-square (RMS) roughness exhibited by MAPbI3/TFTPA. Furthermore, regarding the X-ray diffraction (XRD) patterns, the two films also display the same XRD peaks at 14.1°, 20.1°, 28.5° and 32.0°, corresponding to the (110), (112), (220) and (310) diffraction peaks of the MAPbI3 tetragonal phase (Figure S4). The full width at half maximum (FWHM) of the (110) XRD peak for the MAPbI3 film is 0.371° and that for the MAPbI3/TFTPA film is 0.368°. Moreover, the ultraviolet-visible (UV-vis) absorption spectra presented by the two kinds of films are tested and there is no obvious difference (Figure S5). Above observations indicate that the TFTPA interlayer has no substantial influence on the MAPbI3 film in terms of the morphology, optical absorption and crystallinity properties. However, the contact angle measurements show that the MAPbI3/TFTPA film (18.2°) becomes more hydrophilic than the pristine MAPbI3 film (38.4°) (Figure 2c and 2d). The enhanced hydrophilic characteristics of the MAPbI3/TFTPA film illustrate that the TFTPA interlayer can indeed be bonded on the MAPbI3 perovskite surface with the plane-on orientation, which allow the exposure of the two hydrophilic carboxyl groups at the outmost position, and at the same time the electron-depleted positive charge center of the TFTPA molecule tends to absorb the lone pair electron of the oxygen atoms from water. Similar results have been reported in the literature.37

In order to identify the existence of TFTPA interlayer, as well as the interaction between the TFTPA interlayer and the MAPbI3 film surface, X-ray photoelectron spectroscopy (XPS) is performed on the pristine MAPbI3 and MAPbI3/TFTPA films. It is specially pointed out that the MAPbI3/TFTPA film is fabricated through spin-coating the TFTPA solution on the surface of MAPbI3 followed by rinsing twice



with ethyl acetate to remove the physically absorbed TFTPA. According to Figure S6 and Figure 2e, the signals of Pb, I, C and N elements from the MAPbI3 film can be clearly observed in the XPS spectra of the MAPbI3 and MAPbI3/TFTPA film, while the signals of F and O elements from TFTPA molecules can only be identified in the XPS spectrum of the MAPbI3/TFTPA film. Meanwhile, the high-resolution C 1s peak also exhibits some changes and the peaks of C-C/C-H and C-F/O-C=O bonds becomes dominant and distinct for the MAPbI3/TFTPA film (Figure S7).41-43 Following these results, the TFTPA interlayer is formed on the perovskite film surface. The high-resolution Pb 4f peaks of the MAPbI3 and MAPbI3/TFTPA film are presented in Figure 2f. For the pristine MAPbI3 film, the binding energy of 143.3 eV and 138.4 eV are observed for the Pb 4f5/2 and Pb 4f7/2, respectively. For the MAPbI3/TFTPA film, the binding energy of 143.1 eV and 138.2 eV are observed for the Pb 4f5/2 and Pb 4f7/2, respectively. Compared to the pristine MAPbI3 film, insertion of TFTPA interlayer slightly shifts the Pb 4f5/2 and Pb 4f7/2 peaks to lower binding energy. We infer that the shift strongly suggests the formation of chemical bonds between MAPbI3 and TFTPA through the coordination between Pb2+ and -COOH groups in TFTPA.44 In addition, the C=O vibration band of TFTPA in MAPbI3/TFTPA is observed to move to 1710 from 1700 cm-1 in the Fourier transform infrared (FTIR) spectra, further verifying the interaction between TFTPA molecules and MAPbI3 surface (Figure S8). Furthermore, XPS depth profiling is carried out to detect the location of TFTAP (Figure S9, S10 and S11). Also, after etching for 18 s, the MAPbI3/TFTPA film shows only 9% atomic content of F. With etching time increasing, the F signal slowly weakens and Pb signal increases. After etching 72 s, the F signal disappears completely. Apparently, the TFTPA mainly exists on the MAPbI3/TFTPA film surface.


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Figure 2. SEM images regarding (a) the MAPbI3 film and (b) the MAPbI3/TFTPA film (Inset: AFM images of the MAPbI3 film and MAPbI3/TFTPA film, respectively). Contact angles with the deionized water drops on (c) the MAPbI3 film and (d) the MAPbI3/TFTPA film. XPS spectra of the MAPbI3 and MAPbI3/TFTPA films for (e) F 1s and (f) Pb 4f.

The effect of TFTPA interlayer-treatment of perovskite surface on the formation of PC61BM layer is examined by means of SEM and AFM (Figure 3a and 3b). Both the MAPbI3/PC61BM film and the MAPbI3/TFTPA/PC61BM film show amorphous and smooth morphologies of PC61BM with similar RMS roughness values reaching 3.0 and 2.8 nm, respectively. Despite the comparable morphologies of PC61BM films, there are obvious differences in the tightness of interface contact between the MAPbI3/PC61BM film and the MAPbI3/TFTPA/PC61BM film. In order to test the



interface contact tightness, the MAPbI3/PC61BM and MAPbI3/TFTPA/PC61BM films are rinsed with chlorobenzene, followed by the measurement of the contact angle. According to Figure 3c, the MAPbI3/PC61BM film presents obvious contact angle change (67.6° → 38.9°) before and after chlorobenzene washing. Contrarily, the MAPbI3/TFTPA/PC61BM gives only slight change (67.3° → 67.1°). Further, XPS and FTIR analysis are carried out to reveal the origins for the contact angle change. Based on Figure 3d, for MAPbI3/PC61BM film washed by chlorobenzene, the C-C/C-H bond locating at 283.3 eV displays a remarkably decreasing intensity, almost similar to that of the MAPbI3 film, which indicates that it is easy to wash away PC61BM from the MAPbI3/PC61BM film by chlorobenzene solution.45-46 In contrast, for the MAPbI3/TFTPA/PC61BM film, after chlorobenzene washing, the intensity of C-C/C-H bond presents no distinct change, confirming the presence of PC61BM. Likewise, based on Figure 3e, the fingerprint peak at 524 cm-1 for fullerene47-48 can still be observed in the FTIR spectrum of the MAPbI3/TFTPA/PC61BM film rinsed by chlorobenzene while it is hard to be distinguished in the MAPbI3/PC61BM film rinsed by chlorobenzene. Above results reveal that the insertion of TFTPA interlayer can tighten the MAPbI3/PC61BM interface mainly due to the strong interaction between TFTPA and PC61BM.

For revealing the origin of the strong interaction between TFTPA and PC61BM, we calculate the electrostatic potential (ESP) map of TFTPA molecule using the density functional theory (DFT) to obtain more intuitive electrostatic distribution, where the positive potential is expressed as blue and the negative potential is expressed as red (Figure 3f). As we know, the fluorine (F) atom shows the strongest electronegativity in the periodic table. As a consequence, the strong electron withdrawing nature makes the electrons easily move to the periphery of F atoms, causing the center of the benzene ring to be positively charged. Obviously, we can observe that the color changes gradually from blue to red from the center of benzene ring to the periphery of TFTPA molecule. Such electrostatic characteristic enables the TFTPA molecule to strongly interact with electron-rich fullerene, which is further verified by quantum


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chemical calculations (Figure 3g). The geometry optimizations are performed by using the Gaussian09 suite at the wB97XD/6-311G (d, p) level of theory. The TFTPA molecule and PC61BM shows an optimized average distance of about 3.39 Å and the intermolecular interaction strength is 12.21 kcal mol-1. Based on these results, TFTPA is capable of forming the electrostatic Coulombic attraction with PC61BM, beneficial for the PC61BM film to be firmly absorbed on the surface of perovskite.

Figure 3. SEM images presented by (a) the MAPbI3/PC61BM film and (b) the MAPbI3/TFTPA/PC61BM film (Inset: AFM images of the MAPbI3/PC61BM film and MAPbI3/TFTPA/PC61BM film, respectively). (c) Contact angles with the deionized water drops on the MAPbI3/PC61BM film and the MAPbI3/TFTPA/PC61BM film before and after chlorobenzene washing, respectively. (d) High-resolution C 1s XPS spectra

of

MAPbI3,

MAPbI3/TFTPA,

MAPbI3/PC61BM

(before

and

after

chlorobenzene wash) and MAPbI3/TFTPA/PC61BM (before and after chlorobenzene wash) films. (e) FTIR spectra of TFTPA, MAPbI3, PC61BM, MAPbI3/PC61BM (before and after chlorobenzene wash) and MAPbI3/TFTPA/PC61BM (before and after chlorobenzene wash) films. (f) ESP map of TFTPA molecule. (g) Lateral view of optimized structure of PC61BM together with the TFTPA molecule.



Evidently, the tight MAPbI3/TFTPA/PC61BM film enabled by TFTPA interlayer will facilitate the charge transfer between the two kinds of films. Photoluminescent (PL) studies were conducted to understand the charge transfer at abovementioned interface. Figure 4a and 4b show the steady-state and time-resolved PL (TRPL) spectra of the MAPbI3, MAPbI3/TFTPA, MAPbI3/PC61BM and MAPbI3/TFTPA/PC61BM films, respectively. Table S2 lists the fitting parameters of TRPL curves in detail, obtained from a bi-exponential fitting.49 In this study, all films are spin-coated on quartz substrates. To our knowledge, the PL intensity of perovskite film without CTLs (e.g. HTLs or ETLs) is primarily from the exciton radiative recombination. Compared to the pristine MAPbI3 film, significant enhancement of PL intensity can be observed for the MAPbI3/TFTPA film. Meantime, the average carrier lifetime, estimated to be ≈ 3.14 ns for the pristine MAPbI3 film, becomes longer up to 5.83 ns after introduction of TFTPA interlayer. This demonstrates the effective inhibition on the nonradiative recombination due to the reduction in surface traps after interface passivated by TFTPA interlayer, which has been supported by XPS and FTIR analysis. On the other hand, the introduction of CTLs will lead to the PL quenching resulting from the charge transfer. As we can see, upon coating the PC61BM ETL on the MAPbI3 or MAPbI3/TFTPA film, an apparent PL-quenching effect can be observed. More strikingly, the MAPbI3/TFTPA/PC61BM film displays stronger PL quenching and shorter average carrier lifetime (91.2%, 1.55 ns) compared to the MAPbI3/PC61BM film (85.5%, 2.38 ns), suggesting that the TFTPA interlayer can enhance the contact between perovskite and PC61BM and promote the electron transport at interface. Moreover, ultraviolet photoelectron spectroscopy (UPS) helps to study the variation of perovskite energy levels caused by the insertion of TFTPA interlayer (Figure 4c) and corresponding data can be found in Table S3. Clearly, the application of TFTPA interlayer can slightly adjust the energy level of the MAPbI3 film to optimize the interfacial energy level alignment (Figure 4d).


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Figure 4. (a) Steady-state PL and (b) time-resolved PL spectra regarding MAPbI3, MAPbI3/TFTPA, MAPbI3/PC61BM and MAPbI3/TFTPA/PC61BM films, (c) UPS spectra of MAPbI3 and MAPbI3/TFTPA films, (d) Energy levels diagram of materials in PSCs.

Furthermore, for obtaining the origin of the enhanced Voc and FF as well as the reduced hysteresis, we apply the space-charge-limited-current (SCLC) technique to the quantitative evaluation of the trap-state density and electron mobility by virtue of the

electron-only

device

with

FTO/TiO2/MAPbI3/with

or

without

TFTPA

interlayer/PC61BM/Ag configuration. Figure 5a and 5b illustrate the representative dark J-V curves of the control device and TFTPA device, respectively, showing linear Ohmic region at low bias, the trap-filled limited region at intermediate bias and SCLC region at high bias. The trap-filled limit voltage (VTFL) helps to determine the trap-state density based on the equation (1):50

N t  20rVTFL  / eL2


(1)


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In the equation, ε0 denotes the vacuum dielectric constant, εr denotes the perovskite dielectric constant (εr = 32),51-52 e denotes the elemental charge, L is the perovskite film thickness (L=500 nm) and VTFL is the starting voltage of the trap filling limit region. We notice that trap density is reduced upon introducing TFTPA interlayer, in which the trap density of the control device is determined to 1.15 × 1015 cm-3 and that of TFTPA device decreases to 0.83 × 1015 cm-3. Besides, SCLC method also helps to estimate the electron mobility using the equation (2):53 J  9 0 r V 2 / 8 L3

(2)

Where J denotes the dark current, εr denotes the relative dielectric constant, ε0 denotes the vacuum permittivity, μ denotes the electron mobility, V denotes the applied voltage, and L denotes the MAPbI3 thickness. For the device without TFTPA interlayer, the electron mobility is 1.9 × 10-2 cm2V-1s-1 while it is increased to be 4.5 × 10-2 cm2V-1s-1 for the device with TFTPA interlayer. These findings indicate that the employment of TFTPA interlayer can effectively passivate the surface densities of pervoskite films. It is expected that reducing the trap density for the TFTPA device can decrease the interfacial charge recombination, as a result, the FF and Voc are improved, and the hysteresis becomes weaker.

Electrical impedance spectroscopy (EIS) and Mott-Schottky analysis provide further understanding concerning the interfacial charge transfer as well as the recombination process. Figure 5c shows the Nyquist plots of the control device and TFTPA device. Insert of Figure 5c displays the corresponding equivalent circuit model of solar cells and fitting parameters are provided in Table S4, which include the series resistance (Rs), chemical capacitance (C), transfer resistance (Rtr) and recombination resistance (Rrec).54 In general, the high-frequency component can be ascribed to the Rtr and the low-frequency component can be ascribed to the Rrec. For each device, the low frequency region can observe the existence of a typical single semicircle, correlated with the Rrec. Drawn from the Nyquist plots, the Rrec for the control device and TFTPA device is 824.6 Ω and 1180.4 Ω, respectively. Obviously, the recombination


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resistance of the TFTPA device is higher than that of the control device, which will improve the extraction/transport of charge, as well as weaken the accumulation of charge at interfaces.

In addition, C-2-V characteristics obtained from the Mott-Schottky model can also help to find how the interfacial charge accumulation affects the potential barrier following the equation (3):55-56 (3) In the equation, Vbi refers to the build-in potential, V refers to the applied bias, A refers to the active area, and q refers to the elementary charge. ε is the static permittivity. ε0 is the permittivity of free space. NA denotes the density of excited states and C denotes the measured capacitance. Figure 5d illustrates the Mott-Schottky plots of the two kinds of devices. Generally, the trap-assisted charge recombination at device interface and perovskite layer has the function of reducing the energetic offset between EFn and Ep, where EFn refers to the quasi Fermi level of CTL/electrode contact and Ep refers to that of the perovskite film, as a result, the flat-band potential will be decreased. The Vbi value of the TFTPA device is 0.91 V, which is higher compared with the control device (0.80 V). The calculated NA value of the TFTPA device is 3.62 × 1016 cm-3, while that of the control device is 1.45 × 1016 cm−3. Obviously, incorporation of TFTPA interlayer can significantly reduce the accumulation of charge at the interface, so as to boost the performance of the device.



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Figure 5. Dark J-V curves of devices with FTO/TiO2/MAPbI3/Ag structure (a) without TFTPA and (b) with TFTPA, (c) Nyquist plots based on the electrochemical impedance spectra (EIS) regarding the control device and TFTPA device (Inset: the equivalent circuit model of the solar cell), (d) capacitance-voltage measurement Mott-Schottky plot of the two kinds of devices.

Apart from the device efficiency, moisture stability is another important issue in PSCs due to the ionic character, which has severely limited their real application.57 To assess the long-term moisture stability of devices, the J-V curves of unencapsulated devices with a regular interval in 30 days are measured. The solar cells are kept at room temperature in a 30% relative humidity and tested at ambient environment. Figure 6a illustrates the time evolution regarding PCE for the control and TFTPA devices based on the J-V curves. The control device can keep 78% of the original PCE after 30 days, whereas TFTPA device still retains 92% of the original PCE. The XRD patterns of the MAPbI3/PC61BM film and MAPbI3/TFTPA/PC61BM film for 15, 30 and

50

days,

respectively,

are

recorded

in


Figure

6b.

For

the

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MAPbI3/TFTPA/PC61BM film, only slight PbI2 peak can be observed after 15 days and 30 days, and after 50 days, a weak PbI2 peak signal is found. However, for the MAPbI3/PC61BM film, an obvious PbI2 peak appears after 15 days and 30 days, and

a

the PbI2 peak dominates the XRD patterns after 50 days. Accordingly, the MAPbI3/PC61BM film exhibits a color change from black to yellow after 50 days while the MAPbI3/TFTPA/PC61BM film maintains the black appearance (Figure 6c). The TFTPA devices present an improved long-term stability possibly attributed to the tightly-jointing MAPbI3/PC61BM interface via Coulombic interaction between TFTPA and PC61BM.

Figure 6. (a) Normalized PCE of the control device and TFTPA device, (b) XRD patterns regarding the MAPbI3/PC61BM and MAPbI3/TFTPA/PC61BM films that are stored at room temperature for 30 days with a 30% relative humidity, (c) Photographic images of the MAPbI3/PC61BM films and MAPbI3/TFTPA/PC61BM films stored at room temperature for 30 days with a 30% relative humidity.

c

4. Conclusions In conclusion, a TFTPA interlayer is introduced to strengthen the back contact interface in the inverted planar PSCs and the tightly-jointing and defect-passivated perovskite/fullerene interface is formed by the coordinate effect and physical Coulomb force. Compared to control device without TFTPA interlayer, TFTPA device



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with enhanced perovskite/fullerene back contact can significantly facilitate electron extraction/transport, and simultaneously diminish recombination and accumulation of charge at perovskite/PC61BM interface. The PSC with TFTPA interlayer shows a high PCE of 19.39% and the hysteresis effect is weaker, outperforming the control device. Meanwhile, 92% of the initial PCE of TFTPA device can be retained after 30 days of storage without encapsulation under the condition that the relative humidity is 30 %. In the work, a simple and effective back interlayer strategy is demonstrated to construct inverted planar PSCs with high efficiency and stability.

ASSOCIATED CONTENT Supporting Information Characterization details; Chemical structure of TFTPA molecule; Top-view SEM images of the MAPbI3 films rinsed with ethyl acetate at distinct times; Contact angles with PC61BM drops on the MAPbI3 film, the MAPbI3 film rinsed with ethyl acetate and

the MAPbI3/TFTPA film; Cross-sectional

SEM image

presented

by

Glass/ITO/NiOx/MAPbI3/PC61BM; Performance of PSCs fabricated with TFTPA and without TFTPA under standard AM 1.5 illumination (100 mW cm-2) at active layer area (0.10 cm2); XRD patterns, UV–vis absorption spectra and XPS spectra regarding the MAPbI3 and MAPbI3/TFTPA films; High-resolution C 1s XPS spectra regarding the MAPbI3 film and MAPbI3/TFTPA film; FTIR spectra regarding TFTPA, MAPbI3 and MAPbI3/TFTPA films; XPS depth profiling for the MAPbI3/TFTPA film; TRPL decay fitting parameters of the MAPbI3, MAPbI3/TFTPA, MAPbI3/PC61BM and MAPbI3/TFTPA/PC61BM films; Energy levels of the pristine MAPbI3 and MAPbI3/TFTPA films; EIS parameters for the PSCs fabricated with TFTPA and without TFTPA.

Author Information Corresponding Author Fan Li, E-mail: [email protected]


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ORCID Fan Li: 0000-0001-8972-4715

NOTES These authors declare no competing financial interest.

Acknowledgments The work was completed with the support from the National Natural Science Foundation of China (61664006 and 61464006) and the Natural Science Foundation of Jiangxi Province, China (20171ACB21010). F. L. acknowledges the

help

from

the

Young

Scientist

Project

of

Jiangxi

Province

(20142BCB23002).

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Fullerene Interface Enhances the Inverted

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