Understanding How Ambiance Affects the Performance of Hole

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Understanding How Ambiance Affect the Performance of HoleConductor-Free Perovskite Solar Cells from a Chemical Perspective Duo Xu, Xin Hua, Liang Xu, Wenjun Wu, Yi-Tao Long, and He Tian ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Energy Materials

Understanding How Ambiance Affect the Performance of HoleConductor-Free Perovskite Solar Cells from a Chemical Perspective Duo Xu, Xin Hua*, Liang Xu, Wen-Jun Wu, Yi-Tao Long*, and He Tian Key Laboratory for Advanced Materials & School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai, 200237, P. R. China. ABSTRACT: The development of sufficiently stable and efficient perovskite solar cells for practical applications is in great demand. Thus, a deeper understanding of the relationship between synthesis/storing conditions and cell performance is of significance, especially from chemical perspective. Herein, Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to investigate how the synthesis atmosphere relates to the efficiency of hole-conductor-free perovskite solar cells. Time-resolved composition changes of the perovskite solar cells under N2 or air atmosphere are also unraveled, providing detailed molecular evidence of the perovskite solar cells’ performance. KEYWORDS: perovskite solar cells; ToF-SIMS; atmosphere; performance; stability; degradation

rganometal halide perovskites have been noted for their impressive photovoltaic performance and simple production process.1-5 Methylammonium lead halide (MAPbX3, X=Cl, Br or I) is a class of the most representative perovskite materials used in perovskite solar cells (PSCs).6 The maximum power conversion efficiency (PCE) of these PSCs have raised to 23.7%.7 Efficiency and stability are the two most important issues in the development of PSCs.8 Different experimental conditions, including raw materials, oxygen, moisture, light, temperature and synthesis methods, lead to different impacts on the performance of perovskite materials and devices.9-13 For example, Pathak et al. investigated the influence of annealing conditions (time, atmosphere and temperature) on the crystal formation of the perovskite films, and found the correlation between photophysical properties and device performance.14 Comparing with solution deposition, Wang et al. found that the perovskite fabricated through two-step vapor-assisted solution deposition had better properties, which improved the performance of solar cells.13 Song and co-workers presented an effective strategy to prepare Sn-based PSCs under reductive vapor atmosphere, which resulted in improved device reproducibility and PCE.15 Hole-conductor-free PSCs with carbon counter electrodes (CEs) have drawn great interest due to the simple manufacture process and low cost by avoiding the use of gold CEs and hole transporting materials.16 The most frequently used assembly method of these PSCs is to infiltrate perovskite precursor into TiO2/ZrO2/C triple layer structure.16 5-ammonium valeric acid iodide (5AVAI) was known to improve the stability of holeconductor-free PSCs by increasing contact with TiO2 surface and lowering defect concentration after forming perovskite (5-AVA)x(MA)1-xPbI3.17 However, few reports

O

focus on the stability study of these PSCs under different synthesis or storing atmosphere, especially from molecular level. Various analysis techniques like X-ray Diffraction (XRD),18 ultraviolet-visible (UV-vis) spectrum19 and Xray photoelectron spectroscopy (XPS)20 were frequently applied to characterize perovskite materials or PSCs. The above-mentioned techniques provided useful information regarding the crystal structure, light absorbance property and chemical bonding state of perovskite materials. However, few of these methods directly monitored the molecular changes of perovskite materials, which is of significant importance in understanding the detailed chemical changes under different conditions. It is known that subtle compositional differences between samples could lead to different photovoltaic performances.21,22 Thus, highly sensitive analytical techniques were required to capture these small differences. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) could provide mass-resolved characterization of materials at

Figure 1. Schematic illustration of using ToF-SIMS to investigate the relationship between synthesis atmosphere and efficiency of the hole-conductor-free PSC.

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Figure 2. The FE-SEM image, physical photos (A) and photovoltaic parameters Voc, Jsc, FF, PCE (B) of the assembled hole-conductor-free PSCs (N=18).

molecular level.23,24 The detecting limit of ToF-SIMS could go down to sub-ppm range. In combination with principal component analysis (PCA), subtle compositional differences among samples could be visualized in a simplified manner.25 This is vital in distinguishing slight changes in perovskite materials, providing important molecular evidence for efficiency and stability studies of PSCs. In this work, (5-AVA)x(MA)1-xPbI3 PSC is selected as an example to elucidate how the atmosphere relates to the efficiency and stability of hole-conductor-free PSCs by ToFSIMS. Our strategy is shown in Figure 1. In order to investigate the influence of synthesis conditions on PSC performance, we synthesized two (5-AVA)x(MA)1-xPbI3 samples under argon or air atmosphere, respectively (see Table S1, SI). The synthesized perovskite materials were assembled in hole-conductor-free PSCs (defined as PSCArgon or PSC-Air) before current-voltage (J-V) measurement and ToF-SIMS analysis. Thus, the synthesis atmosphere of (5AVA)x(MA)1-xPbI3 and the PECs of the hole-conductor-free PSCs were correlated. Furthermore, time-resolved composition changes of PSCs under different storing atmosphere were also researched. Scanning electron microscopy (SEM), Energy disperse spectroscopy (EDS) and J-V measurement were applied to characterize the assembled hole-conductor-free PSCs. Detailed discussion of morphology characterization is presented in the ESI, indicating a clear triple-layer structure of the solar cell (Figure 2A). J-V measurements were used to compare the photovoltaic properties of PSC-Argon and PSCAir. Standard reporting conditions (AM 1.5 solar light and room temperature) were used in the J-V detection.26 Four key photovoltaic parameters including the open-circuit voltage (Voc), short-circuit photocurrent (Jsc), fill factor (FF), and PCE of both PSCs were compared and shown in the Figure 2B (see details in Table S2, SI). The PSC-Argon showed a Voc of 0.89 V, a Jsc of 20.28 mA cm-2, a FF of 54.36% and a PCE of 9.84%. The results for PSC-Air were: Voc = 0.85 V, Jsc = 18.15 mA cm-2, FF = 42.65%, and PCE = 6.62% (all of these results are average values). As a result, PSC-Argon exhibited higher PCE, FF and Jsc than PSC-Air, showing obvious influence of the synthesis atmosphere on the photovoltaic performances of PSCs. To investigate the synthesis atmosphere-caused chemical differences between two PSCs which caused the photovoltaic performance differences, the crystal structure and composition of both PSCs were characterized by XRD. In Figure S2, the diffraction peaks at 14.16°, 28.54°, 31.92° and 40.74° were observed in PSC-Argon, which corresponded to the (110), (220), (310) and (224) planes of perovskite structure, respectively.27 Similar results were obtained in PSC-Air. Other strong diffraction peaks marked with stars were originated

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Figure 3. The PCA score plots of different PSCs (black: PSCArgon; red: PSC-Air) in negative polarity (A); The PC1 loading plots showing the m/z responsible for the separation at negative polarity (B).

from FTO substrates.28 However, above XRD results showed negligible differences between two PSCs, making it difficult to interpret the effect of different synthesis conditions. To have a better understanding of the differences of the two PSCs, ToF-SIMS analysis was conducted on both samples in negative ion mode. The peak assignment was shown in Table S3 (ESI). All the ToF-SIMS spectra were normalized to total ion intensities. In order to elucidate the differences among samples and simplify ToF-SIMS data, PCA was applied (Figure 3). In PCA, a correlation matrix or covariance matrix is decomposed into principal components (PCs), described by “scores” and “loadings”. Scores describe differences and similarities among samples, and loadings exhibit the contributing variables on the scores.25 As shown in Figure 3, the plots of the PC1 scores versus PC2 scores showed clear distinctions of the two PSCs. A 95% confidence limit of each sample type was defined by an ellipse with the same color to the corresponding sample cluster (black: PSC-Argon; red: PSC-Air). The sum of the variance captured by PC1 (89.6%) and PC2 (7.5%) was over 95%, indicating that the ten spectra can be described by first two principal components (PC1 and PC2). Figure 3B showed the loading plots of PC1 for the corresponding score plots in Figure 3A. The results showed that CNH5Iand CNH6I2- are the main contributors in PSC-Argon, while the rest peaks (IO-, C2H3O2-, I2- and PbxIy-) are for those in PSC-Air. In our previous work, we found that the degradation process caused the intensity increases of Ix-, PbxIy- and the decrease of CNHxIy-.28 Thus, in the synthesis procedure, the degradation of perovskite was also observed. Besides, under the effect of water/oxygen in air, the oxygen-containing ions increased. As a result, the main contributors of PSC-Air are oxygen-containing secondary ions and degradation-related secondary ions. This is in agreement with the fact that the perovskite material in PSC-Argon was synthesized in argon atmosphere while PSC-Air was synthesized in air. Perovskite materials are very sensitive to impurities in the solute and solvent. The water and oxygen in air atmosphere would cause the irreversible decomposition of perovskite.26 For example, it was found that perovskite CH3NH3PbI3 first react with water to decompose into PbI2, CH3NH2 and HI, and then react with O2 to form I2.29 These steps will cause the degradation of perovskite. Therefore, the quality of perovskite precursor will not only directly determine the photovoltaic performance, but also influence the stability and degradation process of the device. Compared with other methods, the combination of ToF-SIMS and PCA could distinguish the two PSCs from molecular level. These results confirmed the potential of ToF-SIMS in


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Figure 4. The PCEs changes (A) and the intensity changes of selected secondary ions (B, C and D) of the assembled PSCs in 6 weeks storing in air and N2, respectively. In all figures, the blue refers to the changes of PSC-Argon storing in air, the red shows the changes of PSC-Argon storing in N2, the black represents the changes of PSC-Air storing in air and the green indicates the changes of PSC-Air storing in N2.

capturing slight composition differences between different PSCs. ToF-SIMS was further applied to analyse the degradation processes of the assembled PSCs stored under different ambiances. For the purpose of researching the relationship between storing atmosphere and solar cell stability, J-V detection and ToF-SIMS measurement were performed on each PSC at the end of each week. The average PCEs of three PSCs storing in N2 or air at room temperature were compared from 0 to 6 weeks, as shown in Figure 4A. It was found that the changing trends of PCEs that storing in different atmospheres were obviously different. When the PSCs were stored in inert environment, the PCEs changed little after 6 weeks (red and green lines in Figure 4A), so as the corresponded secondary ion intensities (red and green columns in Figure 4B-4D). However, relatively obvious decrease of PCEs could be seen after storing in air for several weeks (blue and black lines in Figure 4A). With the increase of storing time, the intensity of CNH6I2-, CNH6PbI3-, CNH6+ and HOOC(CH2)4NH3+ decreased because of the degradation of perovskite (5-AVA)x(MA)1-xPbI3 to form PbI2, MA, AVA, HI or I2.30 Besides, under the influence of oxygen and moisture in air, the oxygen-containing secondary ions PbOH+ and C2H3O2- increased from 0 to 6 weeks. Interestingly, besides the composition differences between two storing conditions, we also found some differences between two PSCs. In Figure 4B-4D and Figure S3, the gaps between red column and blue column of CNH6I2-, CNH6PbI3- and CNH6+ were bigger than those between green column and black column. On the contrary, larger gaps between green column and black column of HOOC(CH2)4NH3+, C2H3O2-, and PbOH+ than those between red column and blue column were observed. These results indicated that the air atmosphere mainly led to the decomposition of CH3NH3PbI3 in PSCArgon, while in PSC-Air, air atmosphere mostly caused the increase of oxygen-containing species and the decomposition of dopant 5-AVAI. These composition changes clearly revealed the different degradation

processes of PSC-Argon and PSC-Air. As mentioned above, in air atmosphere, intact perovskite CH3NH3PbI3 first react with water to form PbI2, CH3NH2 and HI, and followed by reacting with O2 to form I2. Perovskite (5AVA)x(MA)1-xPbI3 follow the similar process. For PSCArgon, the structure for (5-AVA)x(MA)1-xPbI3 are relatively intact at the beginning of the storing process. After further exposed to air, PSC-Argon will preferentially react with water to undergo the degradation process. Thus, obvious decrease of perovskite structurerelated species (CNH6I2-, CNH6PbI3- and CNH6+) were observed. While for PSC-Air, a part of perovskite (5AVA)x(MA)1-xPbI3 had been decomposed by water in the synthesis step, making the subsequent reaction with O2 more advantageous in the air storing process, leading to larger increase of oxygen-containing species and faster degradation of 5-AVAI in PSA-Air than in PSC-Argon. These results demonstrated the sequential effect of water and O2 on PSCs. In addition, the PCE of both PSCs maintained 70% of the initial value after storing in air for 6 weeks, demonstrating high stability of these holeconductor-free PSCs. Furthermore, the appearances of both PSCs storing in air had little changes (Figure S4). In conclusion, ToF-SIMS was used to research how ambiance relates to the performances of hole-conductorfree PSCs. Compared with other analytical techniques, ToF-SIMS provided a detailed visualization of the composition differences among similar samples. In addition, the analysis of chemical changes of PSC-Argon and PSC-Air revealed different degradation processes of the two PSCs. Owing to the sequential effect of water and O2 on PSCs, it was demonstrated that air atmosphere mainly caused the decomposition of perovskite (5AVA)x(MA)1-xPbI3 in PSC-Argon, while in PSC-Air, air atmosphere mostly led to the increase of oxygencontaining species and the decomposition of dopant 5AVAI. Our results could contribute to studying the PSCs with similar chemical composition, providing new insight into the design of novel PSCs with high stability and efficiency.



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ASSOCIATED CONTENT

Organometal

Supporting Information

Rationalized: Ultrafast Charge Generation, High and

This Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, the morphology characterization, synthesis conditions, performances of PSCs, peak assignment and EDS mapping. The photos of solar cells storing in air from 0 to 6 weeks.

Microsecond-Long

AUTHOR INFORMATION

Halide

Perovskite Balanced

Solar

Cell

Mobilities,

Materials

and

Slow

Recombination. J. Am. Chem. Soc. 2014, 136, 5189-5192. (5) Casaluci, S.; Cinà, L.; Pockett, A.; Kubiak, P. S.; Niemann, R. G.; Reale, A.; Di Carlo, A.; Cameron, P. J. A Simple Approach for the Fabrication of Perovskite Solar Cells in Air. J. Power Sources 2015, 297, 504-510.

Corresponding Author

(6) Berhe, T. A.; Su, W. N.; Chen, C. H.; Pan, C. J.;

*E-mail: [email protected].

Cheng, J. H.; Chen, H. M.; Tsai, M. C.; Chen, L. Y.; Dubale,

*E-mail: [email protected]

A. A.; Hwang, B. J. Organometal Halide Perovskite Solar

ORCID

Cells: Degradation and Stability. Energ. Environ. Sci. 2016,

9, 323-356.

Duo Xu: 0000-0003-2554-3602

(7)

Xin Hua: 0000-0003-1064-083X

NREL.

Best

Research-Cell

Efficiencies.

http://www.nrel.gov/pv/assets/pdfs/pv-efficiency-

Liang Xu: 0000-0002-1883-0577

chart.20190103. pdf (accessed Jan 9, 2019).

Wenjun Wu: 0000-0003-0044-1917

(8) Zhang, H. Y.; Shi, J. J.; Zhu, L. F.; Luo, Y. H.; Li, D. M.;

Yi-Tao Long: 0000-0003-2571-7457

Wu, H. J.; Meng, Q. B. Polystyrene Stabilized Perovskite

He Tian: 0000-0003-3547-7485

Component,

Notes

Grain

and

Microstructure

for

Improved

Efficiency and Stability of Planar Solar Cells. Nano Energy

The authors declare no competing financial interests.

2018, 43, 383-392.

ACKNOWLEDGMENT

(9) Matteocci, F.; Busby, Y.; Pireaux, J. J.; Divitini, G.;

This work was supported by National Natural Science Foundation of China (21421004, 21834001, 21705046), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-0002-E00023), the Fundamental Research Funds for the Central University (222201718001, 222201717003, 222201814015), Shanghai Sailing Program (17YF1403000) and Shanghai Natural Science Foundation (17ZR1407700).

Mater. Interfaces 2015, 7, 26176-26183.

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Figure 1. Schematic illustration of using ToF-SIMS to investi-gate the relationship between synthesis atmosphere and effi-ciency of the hole-conductor-free PSC. 82x36mm (300 x 300 DPI)



Figure 2. The FE-SEM image, physical photos (A) and photo-voltaic parameters Voc, Jsc, FF, PCE (B) of the assembled hole-conductor-free PSCs (N=18). 82x32mm (300 x 300 DPI)


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Figure 3. The PCA score plots of different PSCs (black: PSC-Argon; red: PSC-Air) in negative polarity (A); The PC1 loading plots showing the m/z responsible for the separation at negative polarity (B). 82x30mm (300 x 300 DPI)



Figure 4. The PCEs changes (A) and the intensity changes of selected secondary ions (B, C and D) of the assembled PSCs in 6 weeks storing in air and N2, respectively. In all figures, the blue refers to the changes of PSC-Argon storing in air, the red shows the changes of PSC-Argon storing in N2, the black represents the changes of PSC-Air storing in air and the green indicates the changes of PSC-Air storing in N2. 171x71mm (300 x 300 DPI)


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Understanding How Ambiance Affects the Performance of Hole

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