ZnO-Assisted Growth of CH3NH3PbI3âxClx Film and Efficient Planar

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Energy, Environmental, and Catalysis Applications 3

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ZnO-assisted Growth of CHNHPbI Cl film and Efficient Planar Perovskite Solar Cells with a TiO/ZnO/C Electron Transport Trilayer 2

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Jia Xu, Mingde Fang, Jing Chen, Bing Zhang, Jianxi Yao, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05560 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

ZnO-assisted Growth of CH3NH3PbI3-xClx film and Efficient Planar Perovskite Solar Cells with a TiO2/ZnO/C60 Electron Transport Trilayer Jia Xu,a Mingde Fang,a Jing Chen,a Bing Zhang,a,c Jianxi Yaoa,b,* and Songyuan Daia,c a

State Key Laboratory of Alternate Electrical Power System With Renewable Energy Sources, North China Electric Power University, Beijing 102206, China. b Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, China. c Beijing Key Laboratory of Novel Film Solar Cell, North China Electric Power University, Beijing 102206, China. * Corresponding Authors: Email: [email protected]

ABSTRACT: Appropriate electron transport layers (ETL) are essential in perovskite solar cells (PSCs) with high power conversion efficiency (PCE). Herein, a TiO2/ZnO/C60 trilayer fabricated on a transparent fluorine-doped tin oxide (FTO) glass substrate is used as a compound ETL in planar PSCs. The trilayer shows positive effects on both perovskite synthesis and device performance. The ZnO layer assists growth of CH3NH3PbI3−xClx (x ≈ 0) annealed at a lower temperature and with a shorter time, which is due to a more rapid and easier decomposition of the intermediate CH3NH3PbCl3 phase in the growth of CH3NH3PbI3−xClx. All three materials in the trilayer are important for obtaining PSCs with a high PCE. ZnO is critical for enhancing the open circuit voltage by ensuring proper energy alignment with the TiO2 and C60 layers. C60 enhances carrier extraction from the CH3NH3PbI3−xClx layer. TiO2 eliminates charge recombination at the FTO surface and ensures efficient electron collection. The best-performing PSC based on the TiO2/ZnO/C60 electron transport trilayer features a PCE of 18.63% with a fill factor of 79.12%. These findings help develop an understanding of the effects of ZnO-containing ETLs on perovskite film synthesis and show promise for the future development of high-performance PSCs with compound ETLs.

KEYWORDS: Perovskite solar cells; Electron transport trilayer; Intermediate organometallic mixed halide phase; TiO2/ZnO/C60; CH3NH3PbI3−xClx; Thermal instability.

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INTRODUCTION Organic–inorganic lead trihalide (APbX3, A = methylammonium (MA) or

formamidinium, X = Cl, Br, I, or any mixture thereof) perovskites have emerged as a new class of solution processable photovoltaic material.1−3 Solar cells based on these materials, i.e., perovskite solar cells (PSCs), have received much attention and the highest reported power conversion efficiency (PCE) has increased rapidly in recent years.4−6 In less than 10 years, the PCE of PSCs has increased from 3.8% to more than 20%;7,8 the highest certified PCE of such devices is currently 22.1%.9 Like any other type of solar cell, a high-quality active layer requires effective charge transport layers at its interfaces to achieve high performance devices.10,11 Some PSCs operate without electron transport layers (ETLs) or hole transport layers (HTLs);12,13 however, the majority of high efficiency PSCs reported to date have featured both ETLs and HTLs. These layers sandwich the perovskite active layer to selectively and efficiently extract electrons and holes, respectively. In a regular planar-heterojunction PSC, the ETL is placed on transparent conductive electrode substrate, and the HTL is placed on the perovskite layer. An efficient ETL should be pin-hole free, have good electrically conductivity, conduction band minimum that matches with that of the active layer, and be chemical inert in contact with the perovskite materials.14 To date the most widely used ETL material in PSCs is TiO2. Generally, a compact TiO2 layer is used in combination with a mesoporous TiO2 layer as the ETL in mesoporous PSCs.14 The compact TiO2 layer can be used as an ETL on its own in a planar PSC only when either a modified TiO2 1


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surface for more proper extraction carriers is obtained by some additives or special deposition approach15,16 or a high quality perovskite film is achieved.17 Therefore, many different film deposition techniques have been examined to improve the film quality, including high-vacuum vapor deposition, vapor-assisted deposition, and solvent engineering.18 As another n-type metal oxide semiconductor, ZnO has also been used as a replacement for TiO2 in ETL in planar devices, owing to its high electron mobility and the lower temperature required for annealing ZnO films.19−21 Unfortunately, thermal instability has been observed in PSCs fabricated with ZnO ETLs, which was thought to be due to deprotonation of the methylammonium cation caused by the basic nature of the ZnO surface.22−25 The process can be accelerated by the presence of surface hydroxyl groups and/or residual acetate ligands.22 To address these issues, researchers have used steps such as high-temperature calcination of the ZnO film,22 addition of a poly(ethylenimine) buffer layer between the ZnO and perovskite,23 and application of restricted volume solvent annealing procedure on the formation of perovskite film.26 Perovskite materials deposited on ZnO substrates for PSCs are typically based on MAPbI3 fabricated with PbI2 and MAI through a solution phase approach. The synthesis may involve one-step deposition, two-step sequential deposition, or two-step dipping methods. It has been widely reported that the introduction of chloride (Cl) into the CH3NH3PbI3 system to form MAPbI3−xClx (x ≈ 0) can effectively improve PSC performance.2,27,28 The final Cl content in the annealed perovskite film is below the detection limit of most instruments, including energy 2



dispersive X-ray analysis, electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), even when the precursor solution contains a high Cl content (e.g., MAI:PbCl2 = 3:1).29−31 The loss of chloride has been attributed to the release of gaseous MACl (or other organic chlorides) through an intermediate organometallic mixed halide phase.32,33 Owing to the formation and decomposing of the intermediate phase, the annealing process for MAPbI3−xClx films is generally performed at temperatures above 100 °C and for a duration of no less than 2 hours. These annealing conditions are higher and longer than those used to anneal of MAPbI3 films. Thus, if such an intermediate phase was formed on a ZnO substrate, its decomposition rate might be increased as a result of the thermal instability of perovskites on ZnO surfaces. However, there have been few studies on the formation of MAPbI3−xClx on ZnO substrates. Furthermore, fullerene (C60) and its derivatives are another important family of ETL materials used in PSCs because of the good alignment of their lowest unoccupied energy level with the conduction band of MAPbI3.14 These materials have been frequently used in inverted-structured planar PSCs, in which C60 and its derivatives were deposited on the surface of perovskite film. If you wanted to use C60 as ETL in a regular-structured planar PSC, the problem of partially dissolution of C60 into the dimethylformamide (DMF) which was generally used as the solvent for perovskites synthesis must be taken into consideration. Generally, vacuum thermal evaporation could be applied to obtain a relatively compact C60 layer to withstand the partial dissolution in DMF in the subsequent procedures.34,35 Alternatively, the subsequent 3


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deposition of perovskite film could be fabricated through vapor process to avoid the meeting of DMF and C60.36 As summarized above, as three kinds of the most-widely used ETL materials, TiO2, ZnO and C60 could often be found in the high efficiency PSCs. However, when they were used as single ETLs, much care must be taken in their and their adjacent perovskite films’ qualities. Generally, the need of mesoporous TiO2 layers, the issue of thermal instability of the adjacent perovskite films next to ZnO layers, and the damage of C60 layers in the subsequent DMF process troubled the applications of single TiO2, ZnO and C60 layer prepared by solution method as efficient ETLs in high-performing PSCs with a planar structure. It should be restatement that these difficulties did not mean their incompetent in efficient planar PSCs as single ETLs. As reviewed above, various methods have been designed and carried out to overcome the disadvantages, and high efficiency planar PSCs have been achieved with just single ETL. However, some research groups with a view to combining the advantages of each material to extract electrons and reduce recombination more effectively than possible for a single ETL material attempted to combine two of TiO2, ZnO and C60 materials as compound electron transport bilayers. TiO2/C60 bilayer was fabricated through modifying the TiO2 compact layer surface with a self-assembled C60 monolayer and used to enhance the PCE of the PSCs.37 However, the self-assembly procedure took more than 24 hours. C60/ZnO bilayer was fabricated on the surface of perovskite film in an inverted-structured PSC.38 In that scenario, sol-gel method followed by a sintering process was not applicable for the preparation of ZnO due to 4



the instability of the perovskite materials. Therein, ZnO layer was fabricated through sputtering. The function of a C60 layer deposited before ZnO on the perovskite film was not only enhancing the collection of electrons but also limiting the damage to the MAPbI3 during sputtering. TiO2/ZnO bilayer was fabricated and demonstrated as an effective compound ETL in PSCs based on MAPbI3 perovskite.39 These results all confirm the advantages of the electron transport bilayers. However, some issues still exist and need to be investigated. First, further easier and cheaper fabrication methods for the compound ETLs need to be explored. Second, application of ZnO/C60 compound ETLs in regular-structured planar PSCs needs to be achieved. Third, synthesis of MAPbI3−xClx films on the compound ETL substrates which are consisted of ZnO needs to be studied. In the work reported herein, a TiO2/ZnO/C60 trilayer was fabricated throuth a simple and cheap, three-step spin-coating method on transparent fluorine-doped tin oxide (FTO) substrate. Then regular-structured planar PSCs in which MAPbI3-xClx prepared through the simple one-step spin-coating method were assembled. Both the effects of the TiO2/ZnO/C60 trilayer on perovskite synthesis and device performane were investigated. And the key roles of each layer in the compound ETL on annealling of MAPbI3-xClx and photovoltaic parameters were stuied respectively. It was confirmed finally that all the three materials were indispensable in our experiments to achieve efficient PSCs with a maximun PCE of 18.62%.

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RESULTS AND DISCUSSION The compound ETL was composed of TiO2, ZnO and C60 three layers, which

were deposited on FTO sequentially. The surface morphologies of the FTO/TiO2, and FTO/TiO2/ZnO samples were characterized by field emission scanning electron microscope (FESEM) observations, as shown in Fig. 1(a) and (b) respectively. In Fig. 1(a), the TiO2 layer deposited on top of the FTO substrate was so thin that the grain-boundaries of the FTO surface (inset in Fig. 1(a)) were still visible. Nonetheless, the deposition of TiO2 layer reduced the surface roughness a bit (bare FTO: 14.6 nm, FTO/TiO2: 14.1 nm) (Fig. S1(a) and (b)). After the ZnO layer (Fig. 1(b)) was deposited on top of the TiO2 layer, by a sol-gel method with hydrolysis of the precursor solution, the layer appeared thicker and was composed of many aggregated nanoparticles. And the surface roughness re-rose to 14.5nm (Fig. S1(c)). A continuous but not very smooth layer could be found on the top of the FTO layer in the cross-sectional SEM image as shown in Fig. 1(c). This layer is a compound of TiO2 and ZnO, which could be confirmed by the XPS characterizations as shown in the inset of Fig. S2. The Zn 2p1/2 and Zn 2p3/2 electrons featured binding energies of 1044.1 and 1021.2 eV, respectively, which agreed with reference values for ZnO.40 The binding energy of the Ti 2p1/2 and Ti 2p3/2 peaks were at 464.3 and 458.8, respectively, corresponding with literature values for TiO2.41 The thickness of the TiO2/ZnO layer was obviously nonuniform. Considering the surface roughness and its morphology, an average thickness of 33.8 nm was roughly measured. The subsequent deposition of C60 layer (Fig. 1(d)) on FTO/TiO2/ZnO surface 6



could help obtaining a much smoother surface with a surface roughness of 4.35 nm (Fig. S1(d)) than the one without C60. As shown in Fig. 1(e), a continuous and smooth layer was formed on the FTO substrate with an average value of 55.0 nm. Additionally, it has been reported that solution-processed C60 layers may partially dissolve during the subsequent deposition of the PbI2 layer, which is spin-coated from a DMF solution.36 Thus, to simulate these effects on the compound ETL the surface morphology was also be characterized after a blank DMF solution was spin-coated on top of the FTO/TiO2/ZnO/C60 substrate followed by annealing at 80 °C for 30 min. As shown in Fig. 1(d), the morphology of the C60 film changed markedly after the DMF processing. The C60 layer appeared more granular with nanoparticles smaller than 100 nm covering the surface. Transmission spectra of the compound ETL in the wavelength range 300–800 nm were measured for each layer processing step, as shown in Fig. 1(f). The addition of each layer gradually decreased the transmittance particularly in the short wavelength region. The increase in the transmittance of the FTO/TiO2/ZnO/C60 substrate after the DMF processing could also be attributed to partial dissolution of the film and changes of its surface morphology. The enhancement of the PCE in PSCs with the TiO2/ZnO/C60 ETLs compared with that of PSCs with only TiO2 ETLs which will be shown in following section indicated that the compound ETL improved electron extraction and reduced recombination. These effects compensated the reduction of incident light available to the perovskite active layer.

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Figure 1. Characterization of the electron transport trilayer. Top-view FESEM images of (a) TiO2 on FTO substrate, (b) ZnO on FTO/TiO2 substrate. (c) Cross-sectional FESEM images of FTO/TiO2/ZnO sample. (d) Top-view FESEM images of C60 on FTO/TiO2/ZnO substrates before and (inset in (d)) after DMF processing. (e) Cross-sectional FESEM images of FTO/TiO2/ZnO/C60 sample. (f) Transmission spectra of the FTO substrate, TiO2 layer on an FTO substrate, TiO2/ZnO layers on FTO substrate, and TiO2/ZnO/C60 layers on FTO substrates before and after DMF processing.

In the synthesis of the perovskite film on the compound ETL substrates, we observed that the temperature and time needed for perovskite annealing differed depending on the presence or absence of a ZnO layer. When ZnO was present in the compound ETL, both the annealing temperature and time necessary for synthesis of the perovskite film were reduced. XRD spectra were measured over time for films on FTO/TiO2/ZnO/C60 substrates with annealing at 80 °C, and on FTO/TiO2/C60 substrates with annealing at 80 °C and 100 °C, as shown in Fig. 2(a), (b), and (c), 8



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respectively. To illustrate the appearance of the as-fabricated films over the course of the annealing process, photographs are inset with each of the XRD patterns. When the precursor solution was spin-coated on the FTO/TiO2/ZnO/C60 substrate, as shown in Fig. 2(a), two strong peaks at 14.2° and 28.5°, corresponding to MAPbI3 can be clearly seen in the films annealed for 10-min.29 This results indicates that formation of MAPbI3 on the FTO/TiO2/ZnO/C60 substrate took place within 10 min. The peaks at 15.7° for the samples with annealing times of 10, 30, 50, and 60 min corresponded to the (110) diffraction peaks of MAPbCl3 or MAx+yPbI2+xCly phases, or a mixture of several phases.29 These peaks could be observed initially during the annealing process but disappeared within 60 min, which indicated the MAPbCl3 or MAx+yPbI2+xCly phase was an intermediate step in the MAPbI3 formation. When the annealing time was longer than 70 min, the crystal structure of the as-fabricated film showed no changes and corresponded to MAPbI3. As can be seen from the photographs of each sample, the films darkened after annealed for 10 min. Moreover, decomposition

of

the

as-fabricated

MAPbI3

perovskite

films

on

the

FTO/TiO2/ZnO/C60 substrate was not observed for 120 min in our experiments. When the FTO/TiO2/C60 compound ETLs were used as substrates for depositing perovskite films at 80 °C, the perovskite films underwent different synthesis process from those deposited on FTO/TiO2/ZnO/C60 substrates, as shown in Fig. 2(b). In this annealing process, though color of the films continued to deepen from the yellowish brown, it had not turned into black completely as the one fabricated on FTO/TiO2/ZnO/C60 substrate. The XRD patterns showed that the peaks at 14.2° and 9


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28.5° corresponding to MAPbI3, had relatively low intensity over the whole process and a strong peak at 15.7°, corresponding to the intermediate phase, was visible at all times. This indicates that the annealing temperature was not high enough to eliminate the intermediate phase and complete the transformation of the precursors to MAPbI3. When the annealing temperature was increased to 100 °C for the samples on FTO/TiO2/C60 substrates, the intermediate phase was eliminated and completely transformed to MAPbI3 within 150 min. Photographs indicated at least 120 min was required for the films to visibly change their color.

Figure 2. XRD patterns of films fabricated on FTO/TiO2/ZnO/C60 substrate at 80 °C (a), and on FTO/TiO2/C60 substrates at 80 °C (b) and at 100 °C (c), for different annealing times. 10



Fig. 3 shows typical FESEM images of top views of the annealed perovskite films on two substrate types, i.e., FTO/TiO2/ZnO/C60 and FTO/TiO2/C60. The perovskite film annealed at 80 °C for 80 min on the substrates of FTO/TiO2/ZnO/C60 (Fig. 3(a)) showed more complete coverage of the ETLs and a smoother surface than that annealed on the FTO/TiO2/C60 substrates (Fig. 3(b)). In separate experiments, the annealing time was increased to 120 min at an annealing temperature at 80 °C (inset in Fig. 3(b)) and the annealing temperature was increased to 100 °C while maintaining an annealing duration of 80 min (inset in Fig. 3(c)); however, these conditions did not lead to formation of perovskite films with high quality crystals. Only when the annealing temperature and duration were respectively raised to 100 °C and 150 min (Fig. 3(c)), were densely packed perovskite grains formed on the FTO/TiO2/C60 substrates.

Figure 3. Top-view FESEM images of the films fabricated on (a) FTO/TiO2/ZnO/C60 at 80 °C for 80 min, (b) FTO/TiO2/C60 substrate at 80 °C for 80 min and for 120 min (inset), and (c) FTO/TiO2/C60 substrate at 100 °C for 150 min and for 80 min (inset).The scale bars in each image are 2 µm.

It has been shown that incorporating chloride into the precursor solution of a 1-step deposition approach results in increased crystallographic orientation, crystal 11


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size, and overall crystallinity, which in turn increases the carrier lifetime in the films.2,27-31 However, very little chlorine remains in the system after annealing provided that an iodide is present in sufficient quantities in the precursor solutions. Possible reaction mechanisms, including the intermediates in the perovskite transformation for one-step deposition with chloride, are summarized in Reaction (1) of Scheme 1. Thermally activated MACl sublimation is believed to be the main route for the chloride loss in the final films.32 This mechanism is common to perovskite materials regardless of the substrates they are deposited. However, the above experimental results show that elimination of MAPbCl3 intermediates was promoted on the ZnO-included substrates. The solution-processed C60 layers especially having been partially dissolved in the subsequent DMF solution process used in our compound ETLs did not completely cover the lower ZnO layer, thus promotion of MAPbI3−xClx formation on ZnO-included substrates is attributed to the thermal instability of MAPbCl3 intermediates on the ZnO surfaces. The thermal instability of MAPbI3 on ZnO surfaces is thought to be caused by reactions between MAPbI3 and hydroxyl groups or acetate ligands on the ZnO surface.24 Numerical calculations showed that deprotonation of the methylammonium cation, induced by the basic nature of the ZnO surface, also contributes to the thermal instability observed in PSCs fabricated with ZnO ETLs.22 These mechanisms likely also account for the enhanced elimination of the MAPbCl3 intermediates and rapid formation of MAPbI3−xClx.

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Scheme 1. Mechanisms of perovskite growth and the possible chemical reactions involved in the rapid elimination of the intermediates on the ZnO-containing substrates.

Another reaction cascade may also take place during perovskite deposition. Together with sublimation of MACl, chloride could also be eliminated by decomposition into CN3NH2 and HCl (Reaction (2) in Scheme 1). The reaction of HCl with ZnO (Reaction (3) in Scheme 1) on the ZnO-containing ETL substrates could result in the formation of water, which is known to degrade MAPbX3 (X = Cl, I, Br). Compared with TiO2, ZnO is more active in its reactions with HCl. Thus, such reactions in Reactions (2) and (3) also corresponded to the experimental results that the elimination of MAPbCl3 intermediates is faster on the ZnO-containing substrates than on ZnO-free substrates. In order to further confirm the rapid elimination of MAPbCl3 on the ZnO-containing substrates experimentally, the change of pure MAPbCl3 films facing to abundant MAI on the three concerned kinds of substrates, i.e. FTO/TiO2, FTO/TiO2/ZnO, and FTO/TiO2/ZnO/C60, and the thermal stability of the pure MAPbCl3 films on such three substrates were studied and compared. The MAPbCl3 film was fabricated by spin-coating a DMF solution containing PbCl2 and MACl (mole ratio of 1:1) and a follow-up thermal annealing. In the former experiment, MAPbCl3 film was dipped in an isopropanol solution with MAI (10 mg/mL). The XRD patterns of the films after dipping for 1 and 5 min were shown in Fig. S3. 13


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Judging from the comparison between the (110) peak of the newly formed MAPbI3 and the (100) peak of MAPbCl3 as the reactant on the films with dipping time of 5 min, it is evident that the formation of MAPbI3 was promoted on the ZnO-containing substrates (i.e. Fig. S3(b) and (c)). In other words, the decomposition and conversion of MAPbCl3 were accelerated on the latter two substrates. And, by comparing Fig. S3(b) and (c), it is found that the presence of C60 did not affect the effect of ZnO. In the stability checking experiments, pure MAPbCl3 films the three substrates were annealed at 80 °C with varied times (Fig. S4). MAPbCl3 films on FTO/TiO2/ZnO (Fig. S4(b)) and FTO/TiO2/ZnO/C60 (Fig. S4(c)) substrate decomposed after 3- and 10-hour-long annealing process, respectively. In contrast, the peak corresponding to PbCl2 was not found for the films on the TiO2 surface throughout the experiment (Fig. S4(a)). The results of the above two experiments further confirmed that ZnO in the substrates promoted the decomposition or conversion of MAPbCl3. And, the coverage of C60 in the presence of abundant MAI in solution barely suppressed the promotion effect of ZnO. Since C60 cannot completely suppress the decomposition promoting effect of ZnO on intermediate products (MAPbCl3), whether it can play some positive effect on the final products (MAPbI3) need to be studied further. To obtain the same MAPbI3 film on the three kinds of substrates, non-chloride precursor (PbI2:MAI=1:1) in DMF solutions were used to fabricate the perovskite films. Their stabilities at two annealing temperatures (80 and 100 °C) with varied times were studied by judging from the appearance of characteristic peaks according to PbI2 in the XRD patterns (Fig. S5 and 14



S6). When the annealing temperature was set at 80 °C, the PbI2’s peak appeared in the film on the ZnO surface after annealing for 40 min. But, the films on the FTO/TiO2 and FTO/TiO2/ZnO/C60 substrates even after annealing for 2.5 hours did not show the peaks of PbI2. It is proved that at such temperature C60 covering layer can suppress the negative effect of the ZnO on the stability of the MAPbI3 perovskite layer. But when the annealing temperature was raised to 100 °C, the MAPbI3 became more unstable than them at 80 °C. Even for the films on TiO2 surface, they were found decomposed after 2.5 hours of annealing. Those films on FTO/TiO2/ZnO, and FTO/TiO2/ZnO/C60 substrates began to show the PbI2 peaks after 20 and 30 min of annealing respectively. A very little bit protection effect of the C60 layer can be seen. In can be concluded that C60 covering layer on ZnO is has a positive effect on the stability of MAPbI3 under certain temperature. Considering that the temperature at which of the solar cells tested and worked did not exceed 80 °C, it is reasonable to say that in our experiments C60 can play a protective role on the final PSCs. To study the effects of ETL composition on device performance, compound ETLs with different structures were used to form planar-heterojunction PSCs. Fig. 4(a) shows a schematic diagram of the device architecture, which includes the TiO2/ZnO/C60 electron transport trilayer. A representative cross-sectional SEM image of a PSC fabricated with the electron transport trilayer is shown in Fig. 4(b). ETL (TiO2/ZnO/C60 trilayer), perovskite, spiro-OMeTAD, and Au were deposited successively on top of the FTO glass substrate. Their average thickness were FTO (~360 nm), ETL (~48 nm), perovskite (~290 nm), spiro-OMeTAD (~210 nm), and Au 15


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(~60 nm), respectively. Herein, ETL was slightly thinner than the one shown in Fig. 1(e). The decrease in thickness could be attributed to the partially dissolved C60 in the subsequent deposition of perovskite layer, which is spin-coated from a DMF solution, which had been confirmed in our previous research.42

Figure 4. (a) Schematic device architecture, (b) cross-sectional FESEM image of the planar-heterojunction PSCs with TiO2/ZnO/C60 electron transport trilayer.

In addition to the TiO2/ZnO/C60 compound ETL, three other compound ETLs structures were also used in PSC devices with similar structures, namely, TiO2/C60, TiO2/ZnO, and ZnO/C60. To ensure a well crystallized CH3NH3PbI3−xClx (x ≈ 0) film in the PSCs, the perovskite films were annealed at 100 °C for 150 min on the TiO2/C60 substrates, or at 80 °C for 150 min on the TiO2/ZnO/C60, TiO2/ZnO, and ZnO/C60 substrates. About 20 devices were fabricated in each case. Current density–voltage (J–V) characteristic measurements under simulated AM 1.5G (100 mW cm−2) solar irradiation in a glovebox were used to evaluate the PSC performance. Fig. 5(a) depicts typical J−V curves of four PSCs based on four different compound ETLs. The PCEs shown in Fig. 5(a) for each type of PSC were very close to their corresponding mean 16



values of the 20 devices, as summarized in Table 1. The performance of the devices based on TiO2/ZnO/C60 ETLs was clearly superior to that of PCEs based on other ETLs. The performance of PSCs based on ETLs containing a single electron transport material, i.e. single TiO2, ZnO, and C60 layers, were also recorded. The best-performing PSCs for each kinds of single ETL based PCSs and the corresponding statistical results have been plotted in Fig. S7. Even the best PCE of each kind of PCSs were far lower than that of the reported efficient single-ETL based planar PSCs. Obvious low fill factors (FF) resulted in the low PCE, which meant the existence of large series resistances and small shunt resistances in such PSCs. It suggested the incompetence of our single ETLs in achieving efficient PSCs. It can be seen that when multilayer were used as ETLs, the optimum efficiency for each kinds of PSCs (seen in Table 2) have all been increased significantly compared with the best-performing devices with single ETL. Considering the superiority of the bilayer ETL over the single ETL on the PSCs’ performance, the follow-up analysis on the enhancement of the PCE by the using of TiO2/ZnO/C60 ETLs were basing the comparison with the best-performing samples based on TiO2/ZnO, ZnO/C60 and TiO2/C60, ETLs. Statistical results for characteristic device parameters, including short circuit current densities (Jsc), open circuit voltages (Voc), PCE and FF of PSCs based on the four kinds of compound ETLs are shown in Fig. 5(b)-(e). The corresponding mean values of Jsc, Voc, FF and PCE with their associated the standard deviations (SD) of 17


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devices based on different compound ETLs are summarized in Table 1.

Figure 5. (a) Typical J−V curves of PSCs based on four compound ETLs. Statistical results for (b) Jsc, (c) Voc, (d) FF, and (e) PCE values of PSCs with different compound ETLs, including TiO2/ZnO/C60, TiO2/C60, ZnO/C60, and TiO2/ZnO.

Devices based on TiO2/ZnO/C60 ETLs produced the highest mean PCE of 17.12% among these four compound ETL cases. The most efficient devices produced a Jsc between 20.39 and 22.05 mA·cm−2, Voc between 1.05 and 1.09 V, with a FF between 73.35% and 79.12%. The resulting PCEs ranged from 16.00% to 18.62%. For the TiO2/C60 ETL cases, the Jsc ranged between 19.32 and 22.68 mA·cm−2, the Voc between 0.89 and 0.94 V, with a FF between 61.98% and 78.92%. The resulting PCEs ranged from 11.32% to 16.75%. The devices based on TiO2/ZnO and ZnO/C60 ETLs showed much poorer performance than the former two cases, owing to 18



decreases in both Jsc and FF. And the attributions of each photovoltaic parameter for the TiO2/ZnO/C60 ETLs based samples process the smallest SD among the four kinds of PSCs.

Table 1. Summary of mean photovoltaic parameters with the standard deviations (SD) of devices based on different compound ETLs. JJsc (mA/cm2) FF (%) PCE (%) Voc (V) ETL Mean (SD) Mean (SD) Mean (SD) Mean (SD) TiO2/C60 21.81 (1.09) 0.92 (0.02) 69.51 (6.74) 14.00 (1.73) ZnO/C60 13.62 (2.52) 0.96 (0.02) 45.35 (7.26) 6.05 (1.87) TiO2/ZnO 9.74 (2.33) 0.98 (0.06) 40.80 (8.77) 4.02 (1.42) TiO2/ZnO/C60 21.06 (0.50) 1.07 (0.01) 76.26 (1.77) 17.12 (0.77)

Two points can be noted from the statistical distribution of the photovoltaic parameters. First, ZnO was critical to promoting the Voc values of the devices. As shown in Fig. 5(c), although the Jsc and FF of the devices based on TiO2/ZnO and ZnO/C60 ETLs were clearly lower than those of the devices based on TiO2/C60 ETLs, their Voc values were higher than that of the TiO2/C60 based devices. The maximum mean Voc was 1.07 V in the devices with the TiO2/ZnO/C60 ETL. The Voc values of PSCs are closely related to the band structure and interfacial recombination across the whole device.43,44 Decreasing interfacial recombination can improve the Voc, which can be achieved by deposition of a defect passivation layer (e.g. ZnO) (see later). Second the combination of TiO2 and C60 layers appears to be important for obtaining high Jsc. As shown in Fig. 5(b), when the compound ETLs contained both TiO2 and C60, the Jsc and FF of such devices had mean values higher than 21 mA·cm−2 and 69%, respectively. If an equivalent circuit was used to examine the 19


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working principle of the PSCs, a high FF means a small series resistance and large shunt resistance.12 In other words, high FF and Jsc indicate that photoinduced carriers within the solar cells can be separated and collected efficiently. This is attributed to enhanced electron extraction from the perovskite layers by the adjacent C60 layers. The placement of C60 layers between the TiO2 and CH3NH3PbI3−xClx layers has been shown

to

reduce

the

non-radiative

recombination

channels

at

the

C60/CH3NH3PbI3−xClx interfaces and passivate or inhibit the formation of trap states at the TiO2/C60 interfaces.35,37 The mean value of Jsc for the devices based on TiO2/C60 ETLs was higher than that for devices based on TiO2/ZnO/C60 ETLs. This result can be attributed to a loss of incident light on the active perovskite layer through the FTO/TiO2/ZnO/C60 layer compared with FTO/TiO2/C60, as shown in Fig. 1(f). The maximum PCE was measured in the device group with TiO2/ZnO/C60 ETLs. This device exhibited outstanding performance with a Jsc of 22.06 mA·cm−2, a Voc of 1.07 mV, a FF of 79.12%, and a PCE of 18.63%, when the bias voltage scanned from forward bias to short circuit (FB-SC) with a rate of 0.082V/s. The incident photon-to-electron conversion efficiency (IPCE) spectrum for the device is shown in Fig. S8. Because hysteresis behavior, i.e. the discrepancy of the performance between two voltage-scanning directions when performing a J-V measurement, also exists in our devices, maximum power point tracking measurement was carried out by setting the bias at 0.892V for 10 min to obtain the stabilized PCE which is thought to be close to the actual output ability of the device. It can be seen that a stabilized PCE of 18.12% was obtained which is a little lower than the one obtained using FB-SC scan 20



model.

Figure 6. Performance of best performing PSC with TiO2/ZnO/C60 electron transport trilayer. The inset shows the maximum power point tracking for 10 min resulting in a stabilized PCE of 18.12% at 0.892 V.

The hysteresis behavior of the best performing PSC based on TiO2/ZnO/C60 ETL was compared with the other three kinds of best-performed PSCs based on TiO2/ZnO, ZnO/C60, and TiO2/C60 respectively. Using the identical scan rate of 0.082V/s, the J-V characteristics for each PSC under two scan directions, i.e. FB-SC and form short circuit to forward bias (SC-FB) were all recorded and shown in Fig. S9. The summary of the photovoltaic parameters were summarized in Table 2. As shown, compared with the parameters obtained under FB-SC conditions, all Jsc, Voc, FF and PCE decrease when SC-FB scan direction was used. Hysteresis index (HI) is used to quantify the hysteresis behavior.45 It can be observed clearly that the HI values decrease following the series: ZnO/C60>TiO2/ZnO>TiO2/C60>TiO2/ZnO/C60. Hysteresis behavior, which could be often seen in many PSCs, has been an common concern due to its complex complex origins and negative impact on evalluating the solar cells’ performance. Recently, many research works reported that the dominant mechanism 21


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underlying hysteresis in PSCs are attributed to the modulation of interfacial barriers at the perovskite/contact interface.46−48 Such modulation can be caused by the migration of iodide ions/interstitials driven by the electrical bias.47,48 Our experimental results show the TiO2/ZnO/C60 ETLs can not only enhance the PCE, but also suppress the hysteresis behaviors of the PSCs to some extent. The following analysis (see later) will show that the TiO2/ZnO/C60 ETLs offer the more proper interfacial conditions for the adjacent perovskite than the other for ETLs concerned in our experiment.

Table 2. Summary of photovoltaic parameters devices based on the four kinds of ETLs. Jsc Voc ETL 2 (mA/cm ) (V) *1 18.700 0.96 FB-SC ZnO/C60 *1 SC-FB 15.39 0.84 FB-SC 11.39 1.03 TiO2/ZnO SC-FB 10.81 0.95 FB-SC 22.50 0.94 TiO2/C60 SC-FB 22.45 0.90 FB-SC 22.06 1.07 TiO2/ZnO/C60 SC-FB 21.78 1.05

of the best performing

FF 0.53 0.36 0.60 0.35 0.79 0.68 0.79 0.73

PCE (%) 9.50 4.63 7.02 3.61 16.75 13.67 18.63 16.74

HI*2 0.392 0.329 0.095 0.057

*1 FB-SC corresponds to the situation that the device was scanned form forward bias to short circuit. SC-FB corresponds to the opposite scan direction to FB-SC. *2

, where OC and SC are abbreviations of open circuit and

short circuit, respectively, and JRS and JFS stand for the current under the scan directions of FB-SC and SC-FB, respectively.

Moreover, the performances of the best-performing PSC under different scan rate were recorded. Besides the rate of 0.082 V/s reported above, two other faster scan rates (0.15 and 0.3 V/s) and a much slower rate (0.001 V/s) were used to sweep the device. As shown in Fig. S10, the hysteresis gets more and more severe as the scan 22



rate increased from 0.082 to 0.3 V/s. And the HI at the rate of 0.082 V/s is the same as the one obtained at a relatively very slow scan rate (0.001 V/s). It means that the hysteresis reduced when the scan rate decreased. Thus, the best PCE reported herein is the value with the smallest HI in our experiments. Stability testing was performed on two kinds of PSCs, i.e. PSCs based on TiO2/ZnO/C60 ETLs and TiO2/C60 ETLs, due to their better performance than the other two kinds. Testing devices were stored in a nitrogen-filled glovebox. And the J–V characteristics of the solar cells under working condition were recorded every two days. As shown in Fig. S11, normalized PCE for two devices showed similar degradation behaviors. The PCE of both cells kept on more than 80 percent of the initial values within two weeks. A correlation between the decrease in the recombination rate and the enhancement of the Voc has been confirmed experimentally and simulated numerically.49,50 Dark current–voltage (I–V) curves of photovoltaic devices are often used to evaluate the charge recombination properties of the devices.51,52 The dark I–V curves, shown in Fig. 7(a), of PSCs based on the four different compound ETLs in our experiments exhibited obvious rectification characteristics but with different turn-on voltages. The turn-on voltage of the TiO2/C60 ETL device was much lower than those of the other PSCs, which contained ZnO layers. Moreover, as shown in the inset in Fig. 7(a) with a logarithmic current coordinate, the dark current of the TiO2/ZnO/C60 ETL device was much smaller than those of the TiO2/ZnO and ZnO/C60 ETL devices by one and two orders of magnitude, respectively. Like many other types of solar cells, 23


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nonradiative recombination dominated the recombination mechanism in PSCs.53,54 Some experiments showed that the trap-assisted recombination at material interfaces was the dominant nonradiative recombination channel limiting the PSCs’ performance.55,56 Thus, recombination current should be the dominant dark current mechanism in PSCs. Physics-based mathematical model for the external voltage-dependent forward dark current of photovoltaic devices considering nonradiative recombination showed the dark current was inversely proportional to the carrier lifetimes and exponentially increased with increasing the injection barrier heights.57 When the band structure was postulated alike for the four cases, the small dark current meant long carrier lifetimes, which also implied suppressed nonradiative recombination. As shown in Fig. 7(a), device based on TiO2/ZnO/C60 ETL showed the minimum recombination among these four cases. Moreover, equivalent circuit analysis was performed to extract more information about the devices from the dark I–V curves. Photovoltaic devices can be considered equivalent to a circuit including a diode, a series resistance (Rs) and a shunt resistance (Rsh). When Rs

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