Cerium Oxide Bilayer

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[6,6]-Phenyl-C61-Butyric Acid Methyl Ester/Cerium Oxide Bilayer Structure as Efficient and Stable Electron Transport Layer for Inverted Perovskite Solar Cells Rui Fang, Shaohang Wu, Weitao Chen, Zonghao Liu, Shasha Zhang, Rui Chen, Youfeng Yue, Lin-Long Deng, Yibing Cheng, Liyuan Han, and Wei Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07754 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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[6,6]-Phenyl-C61-Butyric Acid Methyl Ester/Cerium Oxide Bilayer Structure as Efficient and Stable Electron Transport Layer for Inverted Perovskite Solar Cells Rui Fang,†,‡ Shaohang Wu,†,‡ Weitao Chen,†,‡ Zonghao Liu,†,‡ Shasha Zhang,† Rui Chen,† Youfeng Yue,§ Linlong Deng,〒 Yi-Bing Cheng,⊥ Liyuan Han,＃ Wei Chen†,*

†

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, 430074, China §

Electronics and Photonics Research Institute, National Institute of Advanced

Industrial Science and Technology (AIST), 1-1-1 Higashi, 305-8565, Tsukuba, Japan 〒

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China.

⊥

Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia

＃

Research Network and Facility Services Division, National Institute for Materials Science, Japan

*

Corresponding author E-mail: [email protected] (Wei Chen). 1 ACS Paragon Plus Environment

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Abstract: Stability issues and high material cost constitute the biggest obstacles of perovskite solar cell (PVSC), hampering its sustainable development. Herein, we demonstrate that, after suitable surface modification, the low-cost cerium oxide (CeOx) nanocrystals can be well dispersed in both polar and non-polar solvents and easily processed into high quality electron transport layers (ETLs). The inverted PVSC with the configuration of "NiMgLiO/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/CeOx" has achieved a high efficiency up to 18.7%. Especially, the corresponding devices without encapsulation can almost keep their initial PCEs in 30% humidity-controlled air in the dark for 30 days, which also show no sign of degradation after continuous light soaking and maximum power point tracking for 200 hours in a N2 atmosphere. These results have been proved to be associated with the dual functions achieved by the PCBM/CeOx bilayer ETLs in both efficient electron extraction and good chemical shielding. Furthermore, all inorganic interfacial layers based PVSC with the configuration of "NiMgLiO/MAPbI3/CeOx" has also achieved a promissing efficiency of 16.7%, reflecting the potential to fabricate efficient PVSCs with extremely low cost.

Keywords: CeOx nanoink; surface modification; electron transport layer; perovskite solar cells; long-term stability

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Organic-inorganic metal halides perovskite solar cells (PVSCs) have attracted extensive attention in recent years due to high power conversion efficiency (PCE) and low-cost solution processible features.1 These important features make PVSCs the most promising technology towards successful industrialization among kinds of next generation photovoltaics in the near future.2,3 In most reported high efficiency PVSCs, organic compounds, such as spiro-OMeTAD and PTAA, were generally used as hole transport materials in regular n-i-p structure devices,4-7 and PCBM was generally used as electron transport material in inverted p-i-n structure devices.8,9 Despite competitive PCEs having been achieved, these organic charge transport materials (CTMs) always require complicated synthesis/purification procedures, which largely increase the materials cost. Furthermore, long-term stability of organic materials themselves and the extra doping (e.g., Li-, Co-salts in spiro-OMeTAD) induced stability issues become a pressing challenge against commercial sustainability of PVSCs.10-12 It thus encourages more effort of the research community to explore efficient CTMs with higher economic feasibility and enhanced stability.13-16 Recently, several groups have applied inorganic CTMs in PVSCs and some have reported encouraging results. For example, by replacing organic hole transport material with CuSCN17 and CuGaO218, the regular PVSCs have been reported with promising PCEs of 20.8% and 18.5%, respectively. The corresponding devices have also achieved much superior stability to the ones based on spiro-OMeTAD. On the other hand, amorphous Ti(Nb)Ox,9 nanocrystalline SnO2,19 Al-doped ZnO20 and ZnO21 have been used as electron transport materials solely or in combination with 3 ACS Paragon Plus Environment

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[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in the inverted PVSCs, which also achieved competitive PCEs and stability in certain aging conditions. Recently, a highly compact inorganic CTM of AZO/SnOx bilayer deposited on PCBM via a low temperature atomic layer deposition technique, has been used in the inverted PVSC. Its perfect shielding effect could prevent perovskites' decomposition both in ambient air and at elevated temperature, resulting in reasonably high stability of the device. However, possibly due to increased technical complexity to prepare the multiple interfacial layers, the devices gave relatively poor efficiency of 12.6%.22 It is known that in order to realize high efficiency of PVSCs, inorganic CTMs should have well-matched band alignment with perovskites, high conductivity and pin-hole free film morphology. All these factors are of critical importance for efficient interfacial charge transport and recombination suppression. It is not easy to fulfill all of these requirements on CTMs simultaneously, and even more difficulty is the deposition of CTMs on top of perovskites, which should prevent heat or solvent inducing perovskite's decomposition.23-25 These used to constitute the core contents of "interfacial engineering" in PVSCs. In current stage of PVSCs research, beyond the efficiency target, interfacial engineering should take comprehensive consideration to resolve the stability issues in the meantime. Note that so far the reported inorganic CTMs didn't always result in high stability of the devices. Some materials, like CuI,26 have been reported with stability problems in solid-state dye-sensitized solar cells.27 Some materials, such as ZnO, have strong tendency to react with the lewis acids of perovskites.22,28 Therefore, application of efficient and stable inorganic CTMs on top 4 ACS Paragon Plus Environment

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of perovskites in PVSCs is still a big challenge. A general methodology from material synthesis to property control of inorganic CTMs is even more desolate in today’s PVSCs. In this work, we exploited a generalized method (see the demonstration in Figure S1) to fabricate high quality nanocrystalline cerium oxide (CeOx) films, and examined the application effects of PCBM/CeOx bilayer and sole CeOx based ETLs in inverted PVSCs. CeOx is one of the most earth-abundant rare metal oxides (the crustal abundance of Ce is close to Cu and Zn), implying its potential for low-cost mass production. This material has been reported with promising electrical properties such as high electron mobility (103 cm2 kV-1 s-1) and suitable band energy level for efficient selective charge extraction in solar cells.29-31 Besides, it is chemically stable.32 Thanks to the versatile solvothermal method to synthesize metal oxides nanocrystals (NCs),33 monodisperse CeOx NCs with narrow size distribution and well-defined surface planes can be obtained massively at a low cost. However, the raw CeOx NCs with long-chain oleic acid (OA) absorbed on their surfaces, made the as-deposited film's electric behavior quite different from the bulk one.34 Therefore, we further developed a facile surface modification method to exchange their surface capping agent from long-chain OA to short-chain acetylacetone. This key step of surface modification enables the CeOx NCs to be highly dispersed in opposite polarity solvents, such as chlorobenzene or methanol. The so-called CeOx nanoinks are very stable, and can be selected to safely deposite on both MAPbI3 and MAPbI3/PCBM underlayers by a simple spin-coating method. The as-prepared CeOx nanocrystalline 5 ACS Paragon Plus Environment

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films show highly compact, pin-hole free morphology and relatively high conductivity of 10-4 S cm-1. These important features allow the corresponding inverted PVSCs, especially the PCBM/CeOx bilayer ETLs based ones, to simultaneously achieve encouraging PCEs and dramatically improved long-term stability. RESULTS AND DISCUSSION The CeOx NCs in this work were synthesized according to Yang’s report.35 This generalized synthesis method can ensure the raw NCs' monodispersity in non-polar solvents due to steric hindrance from the surface-capping agent of long-chain OA molecules. Transmission electron microscopy (TEM) image (Figure 1a) shows that monodisperse, cubic-shaped CeOx NCs with the sizes of 5-10 nm are obtained. Highresolution TEM image (Figure 1b) reflects that the cubic-shaped CeOx NCs are highly crystallized and composed of well-defined surface planes which can be indexed to high energy {100} planes of cubic fluorite CeOx.35 The selected-area electron diffraction pattern shown as the inset in Figure 1b, in consistent with the Xray diffraction (XRD) pattern in Figure S2a, further confirms cubic fluorite crystal structure of the CeOx NCs. Based on the well-known Scherrer equation, the average particle size of CeOx NCs is determined to be 7.6 nm, using the width at half height of (200) peak in the XRD pattern (Figure S2a) for calculation. Surfaces of the raw CeOx NCs are tightly capped by OA molecules due to esterification of carboxyl group with oxide during the solvothermal reaction. The long alkyl chain of OA can prevent aggregation between CeOx NCs, but hamper fluent charge transfer between them. It therefore requires the implementation of a surface 6 ACS Paragon Plus Environment

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modification process by ligand exchange of OA with acetylacetone as depicted in Figure 1c. To verify these processes, Fourier transform infrared spectroscopy (FTIR) was used to characterize the ligand exchange. As shown in Figure 1d, the intensities of three bands at 2925 cm-1, 2855 cm-1 and 1707 cm-1 owing to the stretching vibrations of –CH2–CH3, –CH2–CH2– and C=O of OA molecules, retain strong with positions unshifted after the rinsing step. The bands at 1537 cm-1 and 1432 cm-1 are the characteristics of the asymmetric and symmetric stretches of COO− groups. All of these are consistent with the results in the literatures,36,37 which indicate that the OA molecules are chemically bonded to the CeOx NCs and cannot be washed away by a simple rinsing. After several drops of organic alkali solution (tetrabutylammonium hydroxide) were added to the rinsed sample solution, the hydrolysis reaction was promoted immediately and resulted in quick desorption of OA molecules from the surfaces of CeOx NCs. That is the reason why in the FTIR spectrum of the desorbed sample, the signals of the OA bands are undetectable (see the magnified image in the right inset of Figure 1d). But the surface clean CeOx NCs show poor dispersity in both polar and non-polar solvents (Figure S2b). In order to redisperse the CeOx NCs again, a key step of surface modification was implemented subsequently by adding acetylacetone to the surface clean CeOx precipitate. The surface-capping ligands through this step was changed from OA to acetylacetone. This is approved by the FTIR spectral similarity in the wavenumber range of 2863-3004 cm-1 between the redispersed sample (acetylacetone capped CeOx NCs) and the pure acetylacetone sample, where three characteristic peaks should be assigned to the vibrational bands 7 ACS Paragon Plus Environment

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of –CH2– of acetylacetone molecules.38 According to the literatures, the characteristic bands at 1552 cm-1 and 1428 cm-1 of the redispersed sample should be assigned to the asymmetric and symmetric stretching vibrations for propionate (C2H5COO−) due to the chelation of CeOx by acetylacetone.39 And the labeled bands of the desorbed sample at 1564 cm-1 and 1347 cm-1 should be assigned to the characteristic bands for nanocrystalline CeOx.40 Surprisingly, the acetylacetone modified CeOx NCs could disperse very well in both chlorobenzene (a non-polar solvent) and methanol (a polar solvent) (Figure S2b). This important feature allows that, the CeOx nanocrystalline film could be deposited safely upon both PCBM and perovskite by selecting solvents with suitable polarity. The acetylacetone modification enhanced dispersity might be attributed to the keto or enol forms of acetylacetone in different solvents. The enol form is freely soluble in non-polar solvents. In contrast, the keto form can easily dissolve in polar solvents.41 Furthermore, as confirmed by TEM characterization (Figure S3), the surface modification process will not change the morphology of CeOx NCs. And as confirmed by dynamic light scattering measurement (Figure S4), the modified CeOx nanoinks with suitable concentration could be monodisperse and keep stable for several hours without any aggregations. We then fabricated CeOx nanocrystalline films with the above nanoinks and investigated the influence of surface modification on their conductivity. From the current-voltage (I−V) curves shown in Figure S5a, the direct current conductivity can be calculated with the equation of I = σ0Ad-1V, where I is the direct current recorded in the applied potential of V, the sample area (A) and thickness (d) are defined to be 0.1 8 ACS Paragon Plus Environment

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cm2 and 150 nm.42 After calculation, the OA-capped CeOx film shows a very low conductivity of 9.6×10-9 S cm-1 (Figure S5b). While the acetylacetone modified CeOx films exhibit much higher conductivity of 1.1×10-4 S cm-1 (methanol nanoink) and 9.7×10-5 S cm-1 (chlorobenzene nanoink). Short-chain acetylacetone molecules with much weaker steric hindrance than long-chain OA molecules, should hold the key for the significantly enhanced conductivity.34 The higher conductivity allows us to deposite a relative thicker ETL with limited internal resistance loss. The surface morphology of CeOx nanocrystalline film deposited on ITO glass substrate was studied by scanning electron microscopy (SEM). A uniform, flat and pin-hole-free surface morphology can be observed from the SEM image (Figure S6), indicating close packing of the CeOx NCs. This good film morphology is original from the high dispersity and size/shape uniformity of the modified CeOx NCs, which will be of great benefit for the corresponding device to achieve high performance. A favorable band-alignment between perovskite and CTM is one of the essential factors governing charge extraction in PVSCs. To evaluate this, ultraviolet-visible (UV-Vis) spectroscopy and ultraviolet photoelectron spectroscopy (UPS) were used to characterize the band structure of the CeOx nanocrystalline film. The optical band gap (Eg) is calculated to be 3.5 eV on the basis of interpretation of the UV-Vis spectrum (Figure S7). Its work function (EF) and EF-EV values are determined to be 4.12 eV (versus vacuum) and 3.44 eV respectively from the UPS spectrum (Figure S8). Therefore, its EV is determined to be -7.56 eV versus vacuum; by adding Eg to EV, we can further figure out its conduction band (EC) -4.06 eV versus vacuum. The 9 ACS Paragon Plus Environment

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fact that EF is much close to EC than EV, confirms the n-type semiconductor nature of the CeOx film. The small difference between EC of CeOx (-4.06 eV) and EC of MAPbI3 (-3.75 eV) indicates their favorable band-alignment.22 To further elucidate the inherent reasons responsible for the electronic properties of the studied CeOx, X-ray photoelectron spectroscopy (XPS) measurement was carried out. The molar ratio between Ce and O of the sample is determined to be 1.00: 1.68 by XPS, which is in-between the ideal stoichiometric ratios of CeO2 and Ce2O3. This non-stoichiometric feature indicates the presence of mixed valance states in the CeOx sample. Figure S9a presents the Ce 3d5/2 core level spectrum of CeOx, which can be assigned to mixed valence state of Ce3+ and Ce4+. The relative ratio between Ce3+ and Ce4+ in the synthesized CeOx NCs is calculated to be 60.28 at.% versus 39.72 at.%, by deconvoluting the XPS spectrum. The abundant Ce3+ is consistent with the broad photoluminescence (PL) peak of the modified CeOx film in Figure S7b, indicating the exist of abundant intra-band doping states.43 According to the reference,44 the abundant Ce3+ may hold the key for the relatively high conductivity of the CeOx nanocrystalline film in this work. Figure S9b shows the O 1s core level spectrum of CeOx. Three types of O can be observed corresponding to the binding energies of 536.85, 534.55 and 532.05 eV, which should be attributed to the lattice O2- bonding to Ce3+, Ce4+ and surface C-O groups, respectively.45 This further confirms the high content of Ce3+ in the CeOx NCs and the presence of surface absorbed acetylacetone molecules after their surface modification.


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Steady-state PL measurement was then conducted to investigate the charge extraction capability of the modified CeOx film in PVSC and the results are shown in Figure 2a. It can be found that CeOx possesses similar quenching effect to traditionally used PCBM when these two CTMs form contacts with perovskites; their PL quenching ratios are calculated to be 84% and 87% according to the relative peak areas, indicating their comparable and efficient interfacial charge transport efficiencies. To further confirm this, time-resolved PL measurement was also conducted. As shown in Figure 2b, fast decays of PL spectra are observed for both perovskite/CeOx and perovskite/PCBM samples. The fitted PL lifetime is reduced from 132.8 ns for the pristine perovskite to 13.1 ns and 10.5 ns for the perovskite/CeOx sample and the perovskite/PCBM sample, respectively. The results are consistent with the steady-state PL data. It is interesting that the PL quenching is even more efficient when a bilayer CTM of PCBM/CeOx forms contact with perovskite; the stastic PL quenching ratio is calculated to be 91% and the PL lifetime is fitted to be 6.5 ns. This indicates the bilayer ETL is more efficient in electron extraction than the sole ones. To demonstrate the CeOx nanocrystalline films can serve as efficient ETLs in PVSCs,

we

firstly

fabricated

solar

FTO/NiMgLiO/MAPbI3/PCBM/CeOx/Ag

cells

with

the

configuration

(Figure

3a).

The

perovskite

of layer

(MAPbI3) with the thickness of ~450 nm was deposited by spin-coating with antisolvent dropcasting.46 The CeOx layer was deposited safely on top of PCBM (60 nm) via spin coating of its methanol solution. The corresponding energy diagram of the 11 ACS Paragon Plus Environment

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device is shown in Figure 3b. The cross-sectional SEM image with energy dispersive X-ray (EDX) analysis (Figure 3c) confirms the designed layout in the device. The SEM image in Figure 3d depicts surface morphology of the MAPbI3 film. From both of the cross-sectional (Figure 3c) and top-view (Figure 3e) SEM images, it is confirmed that the CeOx nanocrystalline film is smooth, dense and pin-hole free, by which the perovskite underlayer has been completely covered. The 3D AFM image shown in Figure 3f confirms that the perovskite underlayer after coating by PCBM and CeOx layers, the roughness of the film’s top surface is as low as 1.63 nm. The excellent film morphology is important for the PCBM/CeOx bilayer ETL to suppress interfacial charge recombinations and serve as a permeation barrier for ambient moisture and corrosive perovskite species. The optimal thickness of CeOx layer is tuned to be ~40 nm. That is ~4 times thicker than our previously reported amorphous TiNbOx,9 because of the orders of magnitude improved electric conductivity of nanocrystalline CeOx.47 The photocurrent density-voltage (J−V) curves of the devices based on the CeOx layers with different thicknesses are shown in Figure S10 and their performance parameters are listed in Table S1. Too thick CeOx layer (140 nm) will lead to too much increased internal charge transport resistance and result in obviously degraded device performance. Figure 4a shows the J-V curves of the best device with the configuration of FTO/NiMgLiO/MAPbI3/PCBM/CeOx (40 nm)/Ag. The forward scan curve (from JSC to VOC) gives VOC of 1.092 V, short-circuit current density (JSC) of 21.90 mA cm-2, fill factor (FF) of 0.777 and PCE of 18.57 %, and the reverse scan curve (from VOC to 12 ACS Paragon Plus Environment

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JSC) yields VOC of 1.115 V, JSC of 21.82 mA cm-2, FF of 0.768 and PCE of 18.69 %. The hysteresis phenomenon was nearly negligible in this device. In contrast, the reference device with sole PCBM layer as ETL shows a typical “S”-shaped J-V curve (Figure 4a), caused by the Shottky barrier at the PCBM/Ag interface. After the CeOx layer is inserted in-between, Ohmic contact is formed accordingly. This role of the CeOx interlayer has been proved by the capacitance-voltage (C-V) measurement of the devices. As the quantity of electric charge (Q) can be calculated with the equation of Q = CV, the peak in the C−V curve actually indicates the charge accumulation in the device. Namely, C firstly increases and then decreases as the increasing V, corresponding to a charge-discharge process. It can be found in Figure 4b that, for the reference device with sole PCBM layer as ETL, a recognizable peak is present in the bias range of 0.85-1.10 V of the C−V curve, suggesting serious charge accumulation at such a bias region. In contrast, the PCBM/CeOx bilayer ETL based device shows negligible charge accumulation there. IPCE spectrum of the best device is shown in Figure 4c, from which the integrated JSC (21.05 mA cm-2) agrees well with the measured value in the J−V curve. The stabilized photocurrent density at a constant bias of 0.92 V, near the maximum power point, was recorded for 1 hour in the ambient air for the device without encapsulation (Figure 4d). It delivers a stabilized PCE of 18.4%. Figure 4e shows the statistical PCEs of one batch of 56 devices with the configuration of FTO/NiMgLiO/MAPbI3/PCBM/CeOx (40 nm)/Ag. The narrow distribution of PCEs reflects good reproducibility of the devices.


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Good charge extraction capability of the CeOx nanocrystalline film inspired us to employ this material solely as ETLs to fabricate all-inorganic interfacial layers based PVSCs, avoiding to use expensive PCBM. Devices with the configuration of FTO/NiMgLiO/MAPbI3/CeOx/Ag have also been studied (Figure S11). Since the CeOx layer was deposited directly on top of perovskite in this case, its chlorobenzene solution was selected for spin-coating to avoid solvent corrosion of perovskite. First of all, thickness of the modified CeOx layer has also been optimized to be 40 nm. Too thin or too thick CeOx layer will be not good for the device performance (Figure S12 and Table S2). As confirmed by the AFM images shown in Figure S13, too thin CeOx layer (15 nm) is not enough to fully cover the perovskite underlayer with surface roughness of 32.18 nm, which will lead to shunt paths and corrosion reaction between perovskite and Ag electrode. However, similar to the case of PCBM/CeOx bilayer ETL, too thick CeOx layer (140 nm) in sole CeOx ETL based PVSCs also increases too much internal charge transport resistance and results in poor device performance. With the optimal thickness of ~40 nm, the modified CeOx ETL can sufficiently cover the rough perovskite underlayer. See the SEM image in Figure S14. At this thickness (40 nm), the CeOx NCs before and after surface modification have been compared. The J–V curves are shown in Figure 4f and the performance parameters are listed in Table S3. It can be found that after surface modification, the device based on short-chain acetylacetone capped CeOx NCs shows much higher PCE (16.65% versus 3.66%, from reverse scan data) with improved VOC (1.112 V versus 0.987 V) and much improved JSC (20.993 mA cm-2 versus 11.598 mA cm-2) and FF 14 ACS Paragon Plus Environment

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(0.714 versus 0.319) than the reference based on long-chain OA capped CeOx NCs. This comparison highlights the importance of surface modification of the CeOx NCs and improved conductivity of the resultant nanocrystalline film to the device performance. Otherwise, ‘S’-shaped J–V curves and pronounced hysteresis (Figure 4f) would be obtained, which indicate the presence of serious interfacial charge accumulation.48 To further elucidate the importance of incorporation of this surface modified CeOx NCs ETL, a reference ETL made of the desorbed CeOx NCs without any surface-capping agents has been compared (Figure 4f). It is found that the asdeposited desorbed CeOx film has many aggregates and worm-like pin-holes (Figure S15), which will lead to huge shunt paths and strong interfacial charge recombination. Thus, the best device based on desorbed CeOx ETL with the configuration of “FTO/NiMgLiO/MAPbI3/d-CeOx” just gave a PCE of 11.06% (reverse scan data, Figure S16 and Table S4), with strange shaped J-V curves and very large hysteresis. Indeed, in this work, the optimal performance of the sole CeOx ETL based device still cannot catch up with that of the PCBM/CeOx bilayer ETL based one. FF is the biggest loss (0.714 versus 0.768 from reverse scan data), which may be associated with their different interfacial quality. For example, the ultrasmall size of PCBM (~1 nm) may facilitate its diffusion into the grain boundaries of perovskite film and passivate the interfacial defects.49 However, in comparison to previously reported allinorganic interfacial layer based PVSCs, the performance of our device is among the best, especially for the inverted structured PVSCs (Table S5).


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Long term stability is the key advantage of metal oxides CTMs applied in PVSCs due to their good chemical stability in ambient air, which can provide additional "chemical shielding effect" to prevent perovskites to corrode metal electrodes and also prevent moisture to destroy perovskites underlayers.23 Especially, with the help of hydrophobic PCBM layer, the PCBM/CeOx bilayer ETL in this work provides the devices with excelent ambient storage stability. The unencapsulated devices were stored in 30% humidity controlled air in the dark and tested periodically in ambient air without humidity control during 30 days. Their performance evolution as a function of time are shown in Figure S17. It can be found that the PCEs increase in the early days with the increasing VOC and FF, and then come to a plateau and keep unchanged in the following days. In our previous work, similar ambient storage stability of the regular planar PVSCs based on spiro-OMeTAD has been tested.19 However, the spiro-OMeTAD based devices lost half of the initial PCEs within only 2 days. This striking difference strongly highlights the robust interfacial layer's "encapsulation effect" to the device stability. Continuous light soaking is really a big challenge to the state of the art of PVSCs. Light soaking arouses current flow and build-in potential across the device, which carry a high risk of accelerating perovskite's decomposition. Four different unencapsulated devices with controlled ETLs, such as sole PCBM (60 nm), PCBM (60 nm)/BCP (5 nm), sole CeOx (40 nm) and PCBM (60 nm)/CeOx (40 nm), have been compared under continuous light soaking conditions (100 mWcm-2 white LED light at room temperature) and maximum power points tracking. The normalized PCEs evolution of the devices as a 16 ACS Paragon Plus Environment

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function of aging time are shown in Figure 5. From Figure 5a, it can be found that when kept in a N2 filled glovebox, the CeOx and PCBM/CeOx based devices maintained 84% and 100% of their initial PCEs after 200 hours, while the PCBM and PCBM/BCP based ones kept 33% and 86% of their initial PCEs. Their striking difference reflects the superiority of PCBM/CeOx bilayer ETL in enhancing the device stability over the others. As in a N2 atmosphere, it is nothing about moisture or O2 induced perovskite decomposition; therefore, the enhanced stability should be ascribed to better isolation between corrosive perovskite (most probably are light/electric field induced decomposed species of perovskite) and Ag electrode by the thicker and denser PCBM/CeOx ETL.21,23 Under the same aging conditions, sole CeOx ETL based device also represented much better stability than sole PCBM ETL based one, which should be due to better ohmic contact and removed interfacial charge accumulation at the ETL/Ag interface as reflected in Figure 4b. In previous reports, interfacial charge accumulation induced performance degradation of PVSCs have been well studied.50 The stability of the desorbed CeOx ETL (40 nm) based PVSC has also been compared under the same aging conditions as described in Figure 5a. After aging for 200 hours, the device can maintain only 54% of its initial PCE, much worse than the device based on the acetylacetone modified CeOx ETL (40 nm) (Figure S18). Obviously, the desorbed CeOx ETL with many pin-holes (Figure S15), which isn’t able to block the volatile species of perovskite layer to corrode the Ag electrode, holds the key for the poor stability shown in Figure S18. From Figure 5b, it can be found that when kept in air with controlled humidity (~30%) under continuous light 17 ACS Paragon Plus Environment

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soaking, the sole PCBM ETL based device died very completely and quickly within only 12 hours; the PCBM/BCP based device also died completely after 200 hours. In contrast, the sole CeOx ETL and PCBM/CeOx based devices still maintained 64% and 91% of their initial PCEs. Admittedly, all of these unencapsulated devices degraded faster in humidity-controlled air than in a N2 atmosphere. This reflects the presence of H2O (and perhaps O2) is a critically detrimental factor to the devices under real working conditions. H2O and/or O2 may act as catalyzers for the decomposition of perovskite layer and organic charge transport layers (PCBM and/or BCP in this work).21,51,52 If use the degraded PCE dividing by the aging time to define the device's degradation rate, the PCBM/CeOx bilayer ETL based device degraded about ~11 times slower than the PCBM/BCP bilayer ETL based one in 30% humidity controlled air; the sole CeOx ETL based devices degraded about ~7 times and ~46 times slower than the sole PCBM ETL based ones when they were kept in a N2 atmosphere and in 30% humidity controlled air, respectively. All of these results confirm that the presence of air-stable inorganic CeOx layer in the ETLs can largely slow down the degradation process within the devices. It is believed that the most stable "PCBM/CeOx" bilayer ETL has provided some kind of "chemical shielding" function. In order to confirm this, we prepared four controlled samples and immersed them directly in water, including bare perovskite (sample A), perovskite/PCBM (60 nm) (sample B), perovskite/PCBM (60 nm)/BCP (5 nm) (sample C) and perovskite/PCBM (60 nm)/CeOx (40 nm) (sample D) films on glasses. The film thicknesses were selected for comparison because they are 18 ACS Paragon Plus Environment

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correlated to the best-performing devices. A video recording these perovskite layers' different decomposition speeds in these four samples is provided in the supporting information (Video S1). Figure S19 shows the screen shots of the video at different time. The bare perovskite film (sample A) turned to yellow (the typical color of PbI2) immediately when immersed into water due to the loss of organic cation. With the hydrophobic PCBM layer covering on top of perovskite, the brown color of sample B remained roughly 3-5 minutes before turned to yellow. An additional BCP layer covering on top of perovskite/PCBM can further enhance the water resistance of sample C. After immersing in water for 10 minutes, except the side regions and some pin-holes tuned to yellow, a large fraction of sample C still maintained the initial brown color of perovskite. However, the optimal BCP thickness in the solar cells was only 5 nm.9 Obviously, in comparison to PCBM/BCP (5 nm) in sample C, PCBM/CeOx (40 nm) in sample D, has even better waterproof capability. Almost the whole sample D kept the brown color unchanged, with few yellow pin-holes after immersing in water for 10 minutes. It is interesting to find that in another two groups of waterproof capability tests (see Figure S20 and Video S2, S3), by simply increasing sole PCBM ETL's thickness from 60 nm to 100 nm, its waterproof capability cannot be enhanced to the level of bilayer ETL made of PCBM (60 nm)/CeOx (40 nm). This is confirmed to be not due to the hydrophobicity of CeOx layer (Figure S21), but should be ascribed to the structural advantage of bilayer ETL in avoiding pinholes. Although, the PCBM (60 nm)/BCP (40 nm) bilayer ETL could


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achieve similar waterproof capability, it would largely sacrifice PCE (Figure S20, S21 and Video S2, S3). Beyond waterproof experiments, we conducted time of flight secondary ion mass spectroscopy (ToF-SIMS) to clarify the superior "chemical shielding" effect of PCBM/CeOx bilayer ETL. Two kind PVSCs based on bilayer ETLs made of PCBM (60 nm)/BCP (5 nm) and PCBM (60 nm)/CeOx (40 nm), before and after aging for 200 hours at the aging conditions described in Figure 5b, have been compared. Their ionic depth profiles are shown in Figure 6, in which the elemental distributions of I ions and Ag ions before and after aging are specially highlighted. It can be found clearly that, the signal owing to I ions after aging is 1-2 orders of magnitude higher in the PCBM/BCP based device than that in the PCBM/CeOx based one, and also the signal owing to Ag ions penetrates much deeper into the MAPbI3 layer with signal intensity much higher in the PCBM/BCP based device than that in the PCBM/CeOx based one. The results coherently indicate that the migrations of I ions to the Ag electrode and the diffusion of Ag ions to the perovskite layer, occurred more seriously in the PCBM/BCP based device than the PCBM/CeOx based one. According to the literatures, the corrosion of Ag electrode and the migration of Ag induced contamination of MAPbI3 film, they both could lead to irreversible degradation on the device performance.53,54,55 The ToF-SIMS results directly prove the strong "chemical shielding" effect of the PCBM/CeOx bilayer ETL. Furthermore, we have compared two Ag electrodes peeled off from the 100 °C-aged samples

of

“FTO/NiMgLiO/MAPbI3/PCBM/BCP 20 ACS Paragon Plus Environment

(5

nm)/Ag”

and

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“FTO/NiMgLiO/MAPbI3/PCBM/CeOx (40 nm)/Ag”. The SEM-EDX and XRD results shown in Figure S22 confirm that the PCBM/CeOx bilayer possesses better "chemical shelding" effect than the PCBM/BCP bilayer, which can more effectively prevent the released species from MAPbI3 to corrode the Ag electrode. All of these results suggest that a charge transport layer could also be a compact encapsulation layer, which is very important for enhancing the PVSC's long-term stability. CONCLUSION In summary, we have successfully fabricated high quality, bipolar solvents (methanol and chlorobenzene) dissoluble CeOx nanoinks and processed them into nanocrystalline films on top of MAPbI3/PCBM and MAPbI3 underlayers, with compact morphology, suitable band edge position and relatively high conductivity. The exchange of long-chain surface capping agent of CeOx NCs by short-chain acetylacetone holds the key for their high dispersity in desired solvents and improved conductivity of the resultant nanocrystalline films. The inverted PVSC based on the PCBM/CeOx bilayer ETL has achieved an impressive PCE up to 18.7%, and good long-term stability under ambient air storge conditions and N2 atmosphere continuous light soaking conditions. The PCBM/CeOx bilayer ETL can provide special chemical shielding effect to prevent ambient moisture to corrode perovskite and also prevent perovskite species to corrode Ag electrode, which holds the key for the dramatically enhanced stability. This work has demonstrated that high efficiency and high stability could be realized simultaneously for PVSCs, if both electric and chemical properties of CTMs, just like the PCBM/CeOx bilayer ETL, have been systematically addressed 21 ACS Paragon Plus Environment

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in interfacial engineering. Furthermore, the PVSC based on the sole CeOx ETL has achieved a promissing PCE of 16.7%; the initial results have also shown the potential to fabricate efficient and low-cost PVSCs based on all inorganic interfacial layers. Especially, the material synthesis and surface modification methods raised in this work would have general applicability in a broad range of metal oxides and sulfides. METHODS

Materials: All reagents and solvents were purchased from Sigma-Aldrich and used as received without further purification unless otherwise statement. PbI2 (98%) was purchased from Tokyo Chemical Industry Co., Japan. Methyl ammonium iodide was synthesized according to previously reported method.4 PC61BM (99.5%) was purchased from Lumtec Co., Taiwan. Synthesis and surface modification of CeOx NCs: The CeOx NCs were synthesized according to a previous report.35 Typically, Ce(NO3)3∙6H2O (0.16 g), tertbutylamine (0.15 mL) and OA (1.5 mL) were dissolved in H2O/methylbenzene (70 ml) mixed solvent (1: 1 by volume). The solvothermal reaction was carried out at 180 °C for 24 hours in a 100 ml Teflon-lined autoclave. After solvothermal reaction, the brown color precipitate was gathered by centrifugal separation from the mother solution. Then, the precipitate was rinsed thoroughly with excess ethanol and gathered again by centrifugal separation. The wet sample after the rinsing step could be easily redispersed in methylbenzene to form a transparent brown solution, denoted as “the rinsed sample”. Several drops of tetrabutylammonium hydroxide solution were added into the above methylbenzene solution. The hydrolysis reaction was promoted 22 ACS Paragon Plus Environment

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immediately, leading to quick desorption of OA capping molecules from the CeOx NCs. By repeating centrifugal separation and washing with ethanol for three times, light-yellow powder consisting of surface clean CeOx NCs was obtained. This is denoted as “the desorbed sample”. The wet sample was then redispersed in acetylacetone (20 mL) by vigorous stirring for 30 minutes followed by ultrasonication for another 30 minutes. After that, the excess acetylacetone was removed by rotary evaporation. The surface modified NCs could be easily redispersed in selected solvents (methanol and chlorobenzol) for further use. The concentrations of standard nanoinks were controlled at ca. 50 mg ml-1, which were diluted by the same solvents by 2.5 to 15 times. Hereafter, the corresponding nanoinks were denoted as “CeOx2.5”, “CeOx-15”, etc. Fabrication of solar cells: FTO glasses (TEC-15, Nippon Sheet Glass Co., Japan) were ultrasonically washed with detergent solution, distilled water, alcohol, and acetone in sequence. A p-type NiMgLiO compact layer (~20 nm thick) was then deposited on the FTO glass by spray pyrolysis according to our previous work.9 The anti-solvent method was used for the perovskite layer deposition: DMF/DMSO (4: 1 by volume) solution (80 µl of 1.5 M) of PbI2/MAI (1.05: 1 by molar ratio) was spincoated at a rotation speed of 5000 rpm for 30 seconds, followed by quickly dropcasting diethyl ether (200 µl) as anti-solvent within 10 seconds. A chlorobenzol solution of PCBM (20 mg mL−1) was spin–coated on top of the perovskite film at the rotation speed of 1500 rpm for 30 seconds. Subsequently, the methanol solutions of surface modified CeOx NCs (with different concentrations by dilution) were spin– 23 ACS Paragon Plus Environment

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coated on top of the PCBM layer at 1000 rpm for 30 s. At last, one batch of films were transferred to the evaporator chamber, 100 nm thick Ag electrodes were deposited under high vacuum (< 3×10–4 Pa). For the sole CeOx ETL based device, CeOx NCs layer was directly deposited onto perovskite layer via spin-coating its chlorobenzene solution. Characterizations: SEM and TEM images were obtained using a Nova Nano 450 scanning electron microscope (FEI Co., Netherlands) and a Tecnai G2 F30 transmission electron microscope (FEI Co., Netherlands), respectively. XRD characterization was performed on an Empyrean X-ray diffractometer with Cu Kα radiation (PANalytical B.V. Co., Netherlands). The infrared spectra were obtained on a VERTEX 70 Infrared Fourier transform microscope (Bruker Co., Germany). The film roughness was measured on a SPM 9700 atomic force microscope (Shimadzu Co., Japan). The XPS and UPS measurements were performed on an AXIS–ULTRA DLD–600W Ultra Spectrometer (Kratos Co. Japan). The PL spectra were recorded on an Edinburgh FLS920 fluorescence spectrometer (Edinburgh Co., UK). The UV–Vis spectra were measured on a Lambda 950 spectrophotometer (PerkinElmer Co., USA). ToF-SIMS depth profiles were carried out using an IonToF ToF-SIMS 5 instrument, where the pulsed primary ions from a 30 keV Bi+ liquid-metal ion gun were used to bombard the sample surface to produce secondary ions, and a 1 keV O2 source was used to sputter out the layers from the sample surface to analysis the depth. A solar simulator (Oriel, model 9119) with AM 1.5G filter (Oriel, model 91192) was used to give an irradiance of 100 mW cm−2. The light intensity was precisely calibrated with a 24 ACS Paragon Plus Environment

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standard Si photodiode detector. The active area of solar cell was determined to be 0.09 cm2 by a black metal mask. IPCE was measured on a Newport IPCE system (Newport, USA). The long-term stability was measured with a CHI1000c multichannel electrochemistry workstation (Chenhua, Co., China).


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ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (51672094, 51661135023), the National Key R&D Program of China (2016YFC0205002), the Self-determined and Innovative Research Funds of HUST (2016JCTD111), Wuhan Youth Science and Technology Plan (2017050304010297), the open research funds of Engineering Research Center of Nano-Geo Materials of Ministry of Education, China University of Geosciences (NGM2017KF013), Guangdong Natural Science Foundation (2017A030313342), the Basic Research Project of Shenzhen Science and Technology Plan (JCYJ201005280434A). The authors thank Analytical and Testing Center of Huazhong University Science and Technology for the sample measurements.


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ASSOCIATED CONTENT The authors declare no competing financial interest.

Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXX/acsnano.XXXXX.

Optical images and XRD patterns of several metal oxide and sulfide nanoinks; XRD spectrum and optical images of CeOx NCs; TEM image of CeOx NCs; particle size distributions of nanoinks; conductivity measurement of CeOx films; SEM image of CeOx on ITO; UV-Vis and PL spectra of CeOx films; UPS measurement results; XPS measurement results; J-V curves of PCBM/CeOx based PVSCs; device structure; J-V curves of sole CeOx based PVSCs; AFM images and SEM image of CeOx on perovskite; SEM image of desorbed CeOx on perovskite; J-V curves of desorbed CeOx based PVSCs; performance evolution of the unencapsulated devices in the ambient; stability of desorbed CeOx ETL based PVSCs; waterproof capability and water contact angles of different ETLs; SEM images and EDX mapping results of the Ag electrodes (PDF)

Video S1-S3: These videos recorded the color change of perovskite films without and with different ETLs covering on top by immersing them in water (MP4).


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AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (Wei Chen).

Author Contributions ‡

Rui Fang, Shaohang Wu, Weitao Chen and Zonghao Liu contributed equally to this

work.


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Figure 1. (a) TEM image of the monodisperse CeOx NCs. (b) HRTEM image and selected-area electron diffraction pattern (the inset) of a CeOx NC. (c) Schematic illustrating the ligand exchange procedures to convert oleic acid (OA) capped CeOx NCs into acetylacetone modified ones. (d) FTIR spectra of CeOx NCs after different treatments, using molecular OA (black), acetylacetone (red) as the references. The 39 ACS Paragon Plus Environment

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rinsed sample (blue) was CeOx NCs after thoroughly washing by ethanol, the desorbed sample (green) was CeOx NCs treated by alkaline solution; the redispersed sample was CeOx NCs after accomplishment of surface modification by acetylacetone (pink). The right inset depicts the local enlarged characteristic bands in the wavenumber range of 2800-3100 cm-1.


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Figure 2. (a) Steady-state and (b) time-resolved PL spectra of MAPbI3 deposited on glass (black square), covered with sole PCBM (red circle), sole CeOx (blue triangle), and bilayer of PCBM/CeOx (green diamond), respectively.


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Figure 3. (a) Device structure of the FTO/NiMgLiO/MAPbI3/PCBM/CeOx/Ag studied in this work, (b) energy levels (relative to vacuum) of the various device components, (c) cross-sectional SEM image and EDX analysis of the multi-layers in the device, the PCBM/CeOx bilayer's interface denoted by the arrow could be clearly identified, top-view SEM images of (d) the perovskite film and (e) the perovskite film covered with PCBM/CeOx bilayer ETL, (f) 3D AFM image of the perovskite film covered with PCBM/CeOx bilayer ETL. 42 ACS Paragon Plus Environment

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Figure 4. (a) J-V curves of the best devices based on PCBM/CeOx (40 nm) bilayer ETL and sole PCBM ETL, obtained at different scan directions (FS: from VOC to JSC; RS: from JSC to VOC) with step width of 10 mV and delaying time of 100 ms. (b) The dependences of capacitance on applied potential for the two devices in (a). (c) IPCE spectrum of the best device based on PCBM/CeOx (40 nm) bilayer ETL and the corresponding integrated photocurrent density with AM1.5 G solar spectrum. (d) The stabilized photocurrent density (black) and PCE (blue), measured under a constant bias of 0.92 V near the maximum power point, of the best device based on 43 ACS Paragon Plus Environment

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PCBM/CeOx (40 nm) bilayer ETL. (e) Device performance distribution for one batch of 56 cells obtained at different scan directions, based on the PCBM/CeOx (40 nm) bilayer

ETL.

(f)

J-V

curves

of

the

devices

with

structure

of

"FTO/NiMgLiO/MAPbI3/CeOx/Ag" based on the sole CeOx ETLs (40 nm) before and after surface modification. A reference ETL made of desorbed CeOx NCs with its optimized thickness of 40 nm has been compared. Solid symbols: forward scan data. Open symbols: reverse scan data.


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Figure 5. Normalized PCEs of four kind PVSCs based on sole PCBM (blue triangle), PCBM/BCP (5 nm) (red circle), sole CeOx (40 nm) (green diamond) and PCBM/CeOx (40 nm) (black square) ETLs. The unencapsulated devices were aging for 200 hours in (a) N2 filled glovebox and (b) air filled environment testing chamber with controlled humidity of ~30%, held at room temperature under continuous light soaking and maximum power point tracking. Light with the intensity of 100 mW cm-2 was provided by white LED array equipped with a UV blocking filter.


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Figure 6. ToF-SIMS elemental depth profiles of PVSCs based on (a) PCBM (60 nm)/BCP (5 nm) and (b) PCBM (60 nm)/CeOx (40 nm) bilayer ETLs. The devices were aged in the 30% humidity and light soaking conditions for 200 hours as described in Figure 5b. Fresh samples: solid lines, aged samples: dash lines.


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BRIEFS Bipolar solvents dissolvable CeOx nanoinks have been prepared via a solvothermal synthesis and subsequent surface modification technique. Based on the resultant highquality CeOx nanocrystalline films, the PCBM/CeOx bilayer electron transport layer has addressed both efficient electron extraction and good chemical shielding functions, which allows the inverted perovskite solar cells to achieve high efficiency and high stability simultaneously.

ToC figure


Cerium Oxide Bilayer

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