Cerium-Oxide-Modified Anodes for Efficient and UV-Stable ZnO

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Cerium Oxide Modified Anode for Efficient and UV-stable ZnO-Based Perovskite Solar Cells Rui-Qian Meng, Xiao-Xia Feng, Yi-Wei Yang, Xudong Lv, Jing Cao, and Yu Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Applied Materials & Interfaces

Cerium Oxide Modified Anode for Efficient and UV-stable ZnO-Based Perovskite Solar Cells Ruiqian Meng, Xiaoxia Feng, Yiwei Yang, Xudong Lv, Jing Cao* and Yu Tang*

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China.

ABSTRACT: CeOx has widely used in optoelectronic devices due to its special electronic and optical structure. Herein, CeOx was directly doped into ZnO to successfully construct a ZnO/CeOx electron transport material (ETM) used in perovskite solar cells (PSCs). The corporation of CeOx can regulate the chemical compatibility between ZnO and perovskite, unmatched energy levels and poor UV stability, further enhancing the cell performance and stability of PSCs. As expected, the best efficiency of fabricated CH3NH3PbI3-PSCs based on ZnO/CeOx as ETM was up to 19.5%. In contrast, the PSC with pure ZnO was 16.0%. Moreover, compared with PSC based on ZnO, the ZnO/CeOx-based PSC exhibited significantly enhanced moisture, thermal and

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UV stability. These results point to the introduction of rare-earth oxides could accelerate the industrialization of PSCs.

KEYWORDS: ZnO, CeOx, Perovskite solar cells, Electron transport material, UV stability


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INTRODUCTION

Solar cells based on perovskite absorbers offer a promising route to convert sunlight to electricity efficiently, which depends on the excellent optoelectronic properties of perovskite.1 Properties such as tunable direct bandgap, long charge-carrier diffusion length, high absorption coefficient, and small exciton binding energy meet the demands as a high-efficiency photovoltaic device. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has skyrocketed to more than 23% within a few years,2 on par with commercial photovoltaic devices. The further enhancement of PCE and stability are critical to the development of PSCs.3-5 As a crucial part of cell structure, electron transporting materials (ETMs) play an important role in charge extraction and transfer.6 A large number of researches have verified that the presence of unmatched energy levels as well as interfacial charge recombination between ETMs and perovskite seriously affects the PCE, which are also related to stability.7-12 Thus, the selection of suitable ETM is of great importance for the fabrication of efficient and stable PSCs. Among alternative ETMs, ZnO has gained widespread attention as a candidate ETM,13-15 since it has higher electron mobility, easy production and modification.16-19 These features can speed up the transport of photogenerated electrons. Nonetheless, it was discovered that the basic surface of ZnO can result in the presence of poor interfacial stability between ZnO and perovskite interface, further causing the severe degradation of perovskite during thermal treatment.20-21 This will restrict the development of PSCs with ZnO as ETM. In addition, the conduction band minimum (CBM) of ZnO is much lower than that of perovskite, further generating a severely interfacial barrier. Moreover, ZnO as an n-type semiconductor exhibits high photocatalysis activity under the irradiation of ultraviolet light, further causing the decomposition of perovskite.20, 22-24 Many researches have been applied to overcome the above



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faults. For example, Zheng et al. demonstrated that the introduction of MgO bridged ethanolamine made ZnO compatible with perovskite and improved the stability of ZnO/perovskite interface.25 Jang et al. used alkali-metal to modify ZnO, thus achieving the surficial defects passivation and balanced energy levels.26 However, it is still challenging to realize the synergistic effect of enhanced chemical compatibility between ZnO and perovskite interface, unmatched energy levels and excellent UV stability for further enhancement of the PCE and stability of PSCs. The rare-earth ions, due to the presence of unique 4f electron structure, were commonly recognized as a type of potential dopants to regulate their structure.27-31 CeOx is one of the most abundant rare metal oxides on earth, which renders it promising candidates of practical application. Besides, CeOx have gained extensive attention due to wide band gap, high ionic conductivity, thermal and chemical stability, which have been used as photoelectrode, catalysts and electrochemical displays.32 Considering the presence of suitable energy level for effective charge extraction and the ability of transferring the UV absorption to visible emission, CeOx has exhibited great potential in solar cells,27, 33 especially for the enhancement of cell UV-stability.34 It makes sense to take full use of advantages of CeOx in PSCs. Herein, CeOx was directly doped into ZnO to successfully construct a ZnO/CeOx ETM in PSC (Figure 1a). The introduction of CeOx triggered an upward shift compared with the CBM of ZnO, and thus facilitating the charge injection and transfer from the perovskite to the ETM, which also inhibit charge recombination at the interface between ETL and perovskite. Moreover, a stable interface contact between ETM/perovskite was achieved. Based on these effects, the as-optimized PSCs gained a PCE up to 19.5% (16.0% for control PSCs), obtaining from 3 mol% CeOx doping. Especially, the enhanced moisture, UV and thermal stability were simultaneously


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realized. Our studies provide a novel pathway to further efficiently improve the efficiency and stability of PSCs. RESULTS AND DISCUSSION

Figure 1. The schematic view of (a) cell structure and (b) energy band diagram of PSCs. (c) HAADF-STEM image and elemental maps of ZnO/CeOx sample. In this work, the ZnO/CeOx ETMs were prepared by spin-coating of the mixture solution of Zn(CH3COO)2∙2H2O and Ce(CH3COO)3∙xH2O as well as the stabilizing ligand ethanolamine (EA). The added content of Ce(CH3COO)3∙xH2O in precursor solution was range of 1-5 mol% with respect to the Zn(CH3COO)2∙2H2O content. To understand the positive effect of ZnO/CeOx ETM, the energy band diagram was displayed in Figure 1b. We can clearly observe that the



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CBM is -3.85 eV, -4.43 eV and -4.06 eV for CH3NH3PbI3, ZnO and CeOx respectively.10, 25, 33 The CBM of ZnO is much lower than that of CH3NH3PbI3. For increasing the CBM of ETM and matching the energy level of CH3NH3PbI3 better, the modification of CeOx can raise the Fermi energy level and eliminate the interfacial barrier at the interface between perovskite and ETM, probably facilitating the enhancement of cell performance for PSCs with ZnO/CeOx as ETM. Firstly, for determining the composition and nature formation of the ETM after corporation, some detection methods were applied. The added content of Ce(CH3COO)3∙xH2O being 3 % was selected in the following characterizations. The energy dispersive X-ray spectroscopy mapping revealed that Ce and Zn elements were uniformly dispersed within the sample (Figure 1c), indicating the Ce was successfully doped in ZnO particles. Similar result was found by energy dispersive spectroscopy (EDX) (Figure S1). X-ray photoelectron spectroscopy (XPS) were applied to further determine the electronic structure from ZnO/CeOx sample. Full-scan spectrum was shown in Figure S2. The binding energies of 1044 and 1021 eV corresponded to the Zn 2p1/2 and 2p3/2 peaks, respectively (Figure 2a). For investigating electronic structure of Ce 3d, the spin-orbit split doublets (u, v, u′, v′, u′′, v′′, u′′′ and v′′′) related to the different occupation of the Ce 3d were shown in Figure 2b and Table S1. The u, v, u′′, v′′, u′′′ and v′′′ were attributed to Ce4+ while the u′ and v′ were corresponded to Ce3+.35 O 1s spectra of ZnO and ZnO/CeOx were displayed in Figure S3. The results demonstrated that the oxygen deficient defect peak (approximately 531.7eV) was reduced after CeOx modification, which indicates the introduction of CeOx can efficiently passivate the defects.36 The X-ray diffraction (XRD) was obtained to research the crystal structure of ZnO with 3% CeOx doping. XRD patterns further demonstrated that ZnO together with CeO2 was formed independently after treatment (Figure 2c). All these results clearly confirmed the successful modification of ZnO by CeOx.


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Figure 2. XPS spectra of (a) Zn 2p, (b) Ce 3d peaks for a ZnO/CeOx mixed nanocrystalline. (c) XRD patterns and (d) Transmittance spectra of ZnO and ZnO/CeOx (3mol%) films. To evaluate if the introduction of CeOx into ZnO to form ZnO/CeOx sample could be a suitable ETM, the corresponding transmittance spectra of films were measured and shown in Figure 2d. The increased transmission of the ZnO/CeOx in the visible range further makes it more suitable as an ETM, which guarantees the absorption of the device. With the introduction of CeOx, the particles distribution became more uniform, thereby altering the film transmittance.25 Additionally, the contact angles of precursor with and without CeOx were prepared. As shown in Figure S4, compared to the control sample, the precursor solution added with CeOx was easier to spread on the FTO. Better spreading property allows it for better film



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formation and higher film coverage. To verify these results, the surficial morphologies of ZnO films without and with 3% CeOx doping were investigated by scanning electron microscopy (SEM) as shown in Figures 3a and 3b. With the introduction of CeOx, the particles distribution became more uniform, thereby reducing the defects formation in the ETMs and advantageous to the charge transfer. Furthermore, the morphologies of related perovskite films based on ZnO and ZnO/CeOx substrates were obtained (Figures 3c-d). There are no obvious changes in the images before and after modification. It is reasonable that the introduction of CeOx mainly improved the optoelectronic properties.25

Figure 3. Top-view SEM images of the ZnO without (a) and with (b) 3% CeOx doping. Top-view SEM images of the CH3NH3PbI3 films deposited on the ZnO without (c) and with (d) 3% CeOx doping films.


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While the presence of positive effects by the modification of CeOx was certified, it is essential to assess the effect of qualified ETM on the cell performances of PSCs. We used the ZnO

and

ZnO/CeOx

films

as

ETMs

to

fabricate

PSCs

with

FTO/ETM/MAPbI3/Spiro-OMeTAD/Au configuration (Figures 4a and S5) and tested the performance of these devices under AM 1.5 G conditions. Due to the small amount of CeOx added, the film thickness of the two samples was almost identical, which was demonstrated by the measurement of SEM images (Figures 4a and S5). As shown in the Figures 4b-c, S6, S7 and Table S2-S3, the significantly increased cell performance was observed for the doped device. As shown in Figure S6, the photovoltaic results determined 3% doped content of CeOx being the optimum doping amount. The relevant photoelectric performance data were presented in the Table S2. The current density-voltage (J-V) curves of referred devices in Figure 4b showed the excellent properties after the introduction of 3% CeOx. Solar cell based on ZnO/CeOx (3%) has a higher incident photo conversion efficiency (IPCE) response than the ZnO device in the wavelength from 350nm to 700nm (Figure 4c). Conspicuously, the best efficiency of the device with 3% CeOx doping reached 19.52% with Jsc of 23.64 mA cm-2, Voc of 1.09 V and FF of 75.67% (Table S3). Histograms of device efficiencies among 30 cells were summarized in Figure 4d. The average PCE of optimized devices was about 20% higher than pristine devices and displayed valid reproducibility. Then, stabilized power output of PCE with 18.6% further validated the champion cell performance (Figure S8). Compared with the device based on ZnO ETM, the augmented PCE could be mainly owned to the superior Voc and FF. The reduced hysteresis behavior was also observed. Additionally, negligible difference in electron mobility was found for the introduction of CeOx (Figure 4e).



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Figure 4. (a) Cross-sectional SEM image of a PSC device with ZnO/CeOx. (b) Best J-V curves and (c) IPCE of PSCs based on ZnO without and with 3% CeOx doping. (d) Histogram of PCEs among 30 cells. (e) Electron mobilities of ZnO without and with 3% CeOx doping. (f) Nyquist plots of the MAPbI3 perovskite solar cells using ZnO without and with 3% CeOx doping measured at 0 V bias in the dark.


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To further understand the interfacial interaction between the perovskite and the modified ETM, the electrochemical impedance spectroscopy (EIS) measurements were performed on the CH3NH3PbI3 solar cells. The Nyquist plot of the related cells with or without CeOx addition at 0 V bias in the dark was shown in Figure 4f. The semicircle in the low frequency can be mainly ascribed to the recombination resistance (Rrec).9 For the modified sample, the Rrec was obviously increased, indicating the presence of reduced electron recombination.37 To sum up, the modified ETM efficiently represses the charge recombination and enhances the charge transfer, finally improving the cell performance. Besides the high PCE, device stability is also one of the top concerns for the application of PSCs. As a rare earth oxide, CeOx has the ability to resist UV light to a certain extent, further enhancing the UV-stability of the PSC device with ZnO/CeOx. In addition, the basic behavior of ZnO has been weakened by doping with CeOx (Table S4), indicating the presence of improved thermal stability for the PSC device with ZnO/CeOx.20 To verify these supposes, the stability of the devices based on ZnO and ZnO/CeOx (3 mol%) were measured. All the related cells were tested without encapsulation. Firstly, the moisture stabilities of involved devices without any encapsulation were tested under external environment with approximately 45% humidity at room temperature (Figure 5a). The PCE of modified PSC retained 90% of its initial PCE after 600 h, while ZnO-based cell only sustained 1,000 Hour Operational Stability. Nat.

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Table of Contents


Cerium-Oxide-Modified Anodes for Efficient and UV-Stable ZnO

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