Water Vapor Treatment of Low-Temperature Deposited SnO2 Electron

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Water Vapor Treatment of Low-temperature Deposited SnO2 Electron Selective Layers for Efficient Flexible Perovskite Solar Cells Changlei Wang, Lei Guan, DEWEI ZHAO, Yue Yu, Corey R. Grice, Zhaoning Song, Rasha Awni, Jing Chen, Jianbo Wang, Xingzhong Zhao, and Yanfa Yan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00644 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Energy Letters

Water Vapor Treatment of Low-temperature Deposited SnO2 Electron Selective Layers for Efficient Flexible Perovskite Solar Cells

Changlei Wang,†,‡,1 Lei Guan,†,¶,1 Dewei Zhao,† Yue Yu,† Corey R. Grice,† Zhaoning Song,† Rasha A. Awni,† Jing Chen,§ Jianbo Wang,*,¶,# Xingzhong Zhao,*,‡ and Yanfa Yan*,†

†

Department of Physics and Astronomy, and Wright Center for Photovoltaics

Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA. ‡

Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, School

of Physics and Technology, Wuhan University, Wuhan, 430072, China ¶

School of Physics and Technology, Center for Electron Microscopy, MOE Key

Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan, 430072, China §

School of Electronic Science and Engineering, Southeast University, Nanjing,

210096, China #

Science and Technology of High Strength Structural Materials Laboratory, Central

South University, Changsha 410083, China 1

These authors contributed equally to this work.

1

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Abstract: Tin oxide (SnO2) electron selective layers (ESLs) processed by low-temperature plasma enhanced atomic layer deposition (PEALD) hold promise for fabricating light-weight and efficient flexible lead halide perovskite solar cells (PVSCs). However, the as-synthesized SnO2 ESLs typically lead to flexible PVSCs with lower open-circuit voltage (VOC) and fill factor (FF) as well as higher degree of current density-voltage (J-V) hysteresis, compared to PVSCs fabricated on rigid substrates. Here, we report that a facile water vapor treatment of PEALD-synthesized SnO2 ESLs can effectively improve VOC and FF while reducing the degree of J-V hysteresis. The improvement in device performance is mainly attributed to the improved conductivity and electrical mobility of SnO2 ESLs enabled by the water vapor treatment. With such a treatment, our best flexible PVSC fabricated on a commercial substrate shows a power conversion efficiency of 18.36 (17.12)% when measured under reverse (forward) voltage scan and a stabilized efficiency of 17.08%, which is the highest reported efficiency for flexible PVSCs with the regular structure.

TOC GRAPHICS

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Organic-inorganic metal halide perovskite solar cells (PVSCs) have gained tremendous progress in the past few years.1-7 In addition to the rapid increase of record power conversion efficiency (PCE), the low-temperature solution processing and high optical absorption coefficients of perovskite absorber layers provide an exceptional opportunity for fabricating light-weight flexible PVSCs,8-12 which is an important key to reduce the cost of the PVSC technology. Though the development of flexible PVSCs has recently made significant progress,13-17 their PCEs still lag much behind that of the best PVSCs grown on rigid substrates, which have exceeded 22%. So far, low-cost and light-weight flexible substrates are mostly polymer-based materials such as poly(ethylene terephthalate) (PET). Such flexible substrates typically cannot survive high processing temperatures (> 150 oC). Therefore, a challenge for fabricating flexible PVSCs is that every component of the cell must be deposited and treated at relatively low temperatures (preferably ≤ 100 oC). This challenge limits the selection of the electron selective layers (ESLs) and hole selective layers (HSLs), which are critical for fabricating efficient PVSCs.17-19 While most of polymer

and

small

molecule

HSLs

and

ESLs

such

as

poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA), phenyl-C61-butyric acid methyl ester (PCBM), and C60 can be deposited at low temperatures (or even room temperature), many inorganic HSLs and ESLs such as NiOx and TiO2 always be deposited or treated at high temperatures. To reduce thermal budget and enable flexible PVSCs, efforts have also been paid to develop low-temperature processed inorganic HSL and ESL oxides.10, 13, 16-17, 20-22 Despite inorganic ESLs and HSLs being

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typically more stable than their organic and small molecule counterparts, the high temperature processes limit their use in flexible PVSCs. Recently, using PTAA as HSL and fullerene as ESL, Huang and co-workers fabricated flexible PVSCs with the inverted structure showing a remarkable PCE of 18.1%.12 Though PTAA and fullerene may not be viable for large scale industrial manufacturing, their work demonstrates the great potential of flexible PVSCs and suggests that more research needs to be done to fabricate efficient flexible PVSCs using low-cost ESL and HSL materials that can be deposited at large scales. Tin oxide (SnO2) is such a low-cost ESL that can be deposited at large scales. Additionally, low-temperature processed SnO2 ESLs can be used to fabricate efficient PVSCs on rigid substrates with PCEs up to 21%.23 Recently, we demonstrated that plasma enhanced atomic layer deposition (PEALD) can reduce the deposition temperature to around 100 oC without sacrificing the device performance.14,

24-27

However, when using as-deposited PEALD-synthesized SnO2 ESLs to fabricate flexible PVSCs with the regular cell structure, the PVSCs generally showed lower open-circuit voltage (VOC) and fill factor (FF) as well as higher degree of current density-voltage (J-V) hysteresis with respect to the control devices fabricated on rigid substrates. Our recent work has revealed that the as-grown SnO2 ESLs are amorphous in nature and exhibit poor electrical conductivity and low electron mobility, which hinder further improvements in device performance.25 Elam et al. reported that SnO2 films deposited using ALD method at temperatures lower than 200 oC contain impurities,28 the residue of impurities will result in relatively high resistivity

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compared to pure SnO2 films. Post-deposition treatments have been shown to improve the conductivity of low-temperature PEALD-processed SnO2 ESLs,14,

24-25

though

these studies did not examine the influence of water vapor, specifically, upon the treatment efficacy. Here, we report a facile approach to effectively improve the charge transport in the PEALD-synthesized SnO2 ESLs by post-annealing in the presence of water vapor. We find that the water vapor treatment at 100 oC can improve the electrical conductivity of SnO2 ESLs more than annealing without water vapor. This is because annealing with water vapor facilitates the complete reaction of organic materials upon annealing process, leading to the formation of purer SnO2.29 The resulting PVSCs exhibit significant improvements in VOC and FF, and reduction in J-V hysteresis, as compared to PVSCs made using the as-deposited SnO2 ESLs. With the water vapor treatment, our best performing flexible PVSC shows a PCE of 18.36 (17.12)% measured under reverse (forward) voltage scan and a stabilized efficiency over 17% using a commercial indium doped SnO2 (ITO)/PET substrate, which is the highest PCE among all reported flexible PVSCs with the regular structure.15, 17, 19

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Figure 1. (a) AFM image of SnO2 as-deposited on ITO/PET substrate and (b) transmittance spectra of ITO/glass, ITO/PET and SnO2/ITO/PET substrates. (c) J-V curves and (d) EQE plots of PVSCs on ITO/glass and ITO/PET substrates.

As shown in Figure 1(a), the flexible SnO2/ITO/PET substrate has a very smooth surface with a root-mean-square roughness of 0.7 nm, which is suitable for fabricating planar PVSCs. The transmittance spectra of ITO/glass, ITO/PET and SnO2/ITO/PET substrates are shown in Figure 1(b). The flexible conductive substrates show lower transmittance in the visible range due to the lower transparency of the polymer substrate. In addition, ITO/PET has a higher sheet resistance of 45 Ω/square than ITO/Glass (15 Ω/square),24 likely due to the low-temperature fabrication process of ITO layer limited by the flexible polymer substrates. Higher ITO sheet resistance will

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lead to higher series resistance values in solar cells made with this material, which in turn causes poor FFs and PCEs of those PVSCs.15 Figure 1(c) shows the performance of PVSCs using as-deposited PEALD-synthesized SnO2 ESLs on rigid ITO/glass and flexible ITO/PET substrates fabricated in the same batch. Our devices have the regular

configuration

of

glass

(or

PET)/ITO/SnO2/C60-SAM/perovskite/Spiro-OMeTAD/Au, the cross-sectional SEM image of one PVSC fabricated on rigid ITO/glass substrate is shown in Figure S2, and the morphology and crystal structure of perovskite films deposited on rigid and flexible substrates are shown in Figure S3. The PVSC on the rigid substrate has a VOC of 1.121 (1.108) V, a short-circuit current density (JSC) of 22.64 (22.64) mA/cm2 and a FF of 78.95 (71.05) %, yielding a PCE of 20.04 (17.82)% when measured under reverse (forward) voltage scan, while the flexible PVSC shows a VOC of 1.061 (1.030) V, a JSC of 21.53 (21.53) mA/cm2 and a FF of 72.10 (66.80)%, yielding a PCE of 16.47 (14.81) %. The major differences in the J-V curves are reflected in the values of

VOC and FF, which are likely due to the higher sheet resistance of flexible conductive film. The lower JSC of flexible PVSC is possibly caused by the inferior transmission of polymer substrate. Figure 1(d) shows the external quantum efficiency (EQE) plots of PVSCs grown on ITO/glass and ITO/PET substrates. The EQE-integrated current densities are 22.33 and 21.24 mA/cm2 for PVSCs on rigid and flexible substrates, respectively, which are consistent with the JSC values obtained from the J-V curves. The performances of PVSCs fabricated on both ITO/glass rigid substrate and ITO/PET flexible substrate are comparable to the state of the art devices.12, 15, 17, 19

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In our previous work, we found that the low-temperature PEALD deposited SnO2 has an amorphous nature, resulting in poor electrical conductivity that causes imbalanced charge transportation on the ESL/perovskite and perovskite/HSL interfaces of PVSC.25 We identified that post-deposition annealing in air can improve the electrical conductivity of SnO2 ESLs, and subsequently, improve the device performance. We employed post-deposition air-annealing treatment to SnO2 ESLs to fabricate flexible PVSCs, but only obtained moderate performance improvement. However,

in this work,

we find that the post-deposition annealing of

PEALD-synthesized SnO2 ESLs in the presence of excess water vapor can significantly improve the performance of flexible PVSC.

Figure 2. (a) Transmittance spectra of a bare soda lime glass substrate and SnO2 layers having different treatments; the inset shows Tauc-plots of the sample with various SnO2 ESLs. J-V curves of as deposited and air- and water vapor- annealed SnO2 layers embedded in (b) two Au layers and (c) ITO and Ag layers.

Figure 2 shows the optical and electrical properties of PEALD-synthesized SnO2 ESLs as deposited and post-deposition annealed in the presence and absence of water vapor at 100 oC. While the surface morphology and crystal structure of these three types of SnO2 are almost identical due to the amorphous nature (Figure S4),25, 29 the 8


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major differences should be their optical and electrical properties. The inset of Figure 2(a) shows the Tauc plots of these SnO2 ESLs. The as-deposited SnO2 film exhibits a larger absorption tail than the treated ones. The water vapor-treated SnO2 film shows the smallest tail, indicating that post-deposition annealing in the presence of water vapor reduces the density of defect states presenting in the bandgap of SnO2.25 The reduction of defect density can lead to improved carrier mobility and electric conductivity. As shown in Figure 2(b), post-deposition annealing improves the electrical conductivity, but the treatment with water vapor increases the conductivity further. The estimated electrical conductivity is about 2.09 ×10-4, 3.27 ×10-4 and 4.37 ×10-4 S·cm-1 for as-deposited SnO2 film, and post-annealed SnO2 films without and with water vapor, respectively. The improved conductivity can be attributed to the increased carrier mobility. To further investigate carrier mobility of SnO2 films, the space charge limited current (SCLC) method was used by fabricating film stacks with a structure of ITO/SnO2/Ag.25,

30

From the J-V curves shown in Figure 2(c), the

mobility values were estimated to be 0.244×10-5, 0.373×10-5 and 0.676×10-5 cm2 V-1·s-1 for SnO2 thin films of as deposited, and post-deposition annealed without and with water vapor, respectively. A higher carrier mobility results in a better electron extraction/transportation from the perovskite layer into the SnO2, which reduces the imbalance of charge transportation between ESL/perovskite and perovskite/HSL interfaces, therefore leading to improved VOC and alleviated J-V hysteresis.19, 31

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Figure 3. (a) Photovoltaic performance of PVSCs based on SnO2 ESLs post-deposition annealed with and without water vapor. (b) Impedance spectroscopy Nyquist plots at 0 V bias without illumination, (c) VOC versus light intensity and (d)

JSC versus light intensity of flexible PVSCs based on SnO2 ESLs post-annealed with and without water vapor.

A large number of flexible PVSCs based on air- and water vapor-annealed SnO2 ESLs were fabricated. The performance statistics of 94 devices are shown in Figure S5 and also summarized in Table S1. Figure 3(a) shows the representative photovoltaic performance of two flexible PVSCs based on SnO2 ESLs annealed with and without water vapor. The overall performance of the PVSC made with the SnO2 ESL annealed in water vapor has been improved, compared with that of the cell made using the SnO2 ESL annealed without water vapor, the VOC and FF are especially improved and the degree of J-V hysteresis is reduced. These improvements can be 10


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ascribed to the improved charge transport property of the PEALD processed SnO2 ESL as evidenced by impedance spectroscopy results as shown in Figure 3(b).17 The flexible PVSC with water vapor-annealed SnO2 ESL has lower charge transfer resistance, i.e. a lower semi-circle feature observed at higher probing frequencies, as compared to the device based on SnO2 ESL annealed without water vapor, which means that charge transportation in the former device is more efficient.19,

32

To

investigate the recombination mechanism in flexible PVSCs based on SnO2 ESLs post-deposition annealed in air with and without excess water vapor, we measured the dependence of VOC and JSC on light intensity ranging from 0.794 to 100 mW/cm2, as shown in Figure 3(c) and (d), respectively. The VOC has a linear relationship with natural logarithmic light intensity, showing the fitting slope values of 1.58 and 1.61 kT/q for devices using SnO2 layers treated with and without water vapor, respectively. Both flexible PVSCs show the presence of Shockley-Read-Hall recombination,33 while the slightly smaller slope of the former indicates lower trap-assisted recombination, suggesting that the PVSC fabricated on SnO2 ESL annealed in water vapor has a lower density of defects compared to the one with SnO2 ESL annealed without water vapor.34 The power law dependence of JSC versus light intensity has a linear relation in a double logarithmic scale, and the fitting slope values are 0.961 and 0.946 for PVSCs based on SnO2 ESLs treated with and without water vapor, respectively, as shown in Figure 3(d). The flexible PVSC based on the SnO2 ESL annealed with water vapor shows a slope closer to 1, indicating that this device experiences less space-charge limited photocurrent than the cell made using SnO2

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annealed in air, since a photovoltaic device with no space charge limit will give a slope of α = 1.33 Upon annealing with water vapor, purer SnO2 was obtained due to the increased reaction of residual organic impurities with water,28-29 leading to higher mobility and better charge transport, according to the light intensity dependence of JSC measurement.

Figure 4. Performance data of our best flexible PVSC: (a) J-V curve, (b) EQE plot and integrated JSC curve. (c) J-V curves under reverse scan with different scan speeds. (d) steady-state efficiency with a constant bias of 0.855 V.

Among all flexible PVSCs that we have fabricated using water vapor-treated SnO2 ESLs, Figure 4(a) shows the J-V curve of our champion flexible PVSC. This cell has a VOC of 1.101 (1.095) V, a JSC of 22.11 (22.11) mA/cm2 and a FF of 75.42 (70.71)%,

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yielding a PCE of 18.36 (17.12)% when measured under reverse (forward) voltage scan. To the best of our knowledge, this is the highest performance of flexible PVSCs with regular structure. Figure 4(b) shows the EQE plot of the champion flexible PVSC. The integrated JSC over the AM 1.5G spectrum of 22.09 mA/cm2, which is in good agreement with the JSC value obtained from the J-V curve, indicating no JSC overestimation in our J-V measurements. Due to the relatively small degree of hysteresis, our PVSCs show almost the same performance irrespective of sweep speed, as shown in Figure 4(c), maintaining a consistent PCE with negligible variation when measured under reverse voltage scan with different scan rates. In order to avoid overestimation of the real performance, steady-state efficiency (SSE) of flexible PVSC was measured under AM 1.5G 100 mW/cm2 illumination. The steady-state current density was 19.98 mA/cm2, at a constant bias of 0.855 V, yielding a stable maximum power output of 17.08% for 300 seconds (Figure 4(d)). The stable maximum power output of our flexible PVSCs benefits from the balanced charge transfer at both carrier selective interfaces and by having a high-quality perovskite film (Figure S3). It is also worth mentioning that our SnO2 ESL is a strong UV-light filter, which may partially contributes to the stable SSE of our flexible PVSC.

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Figure 5. Bend fatigue testing of a flexible PVSC with a bending radius of 1 cm. (a) J-V curves measured after various bending cycles. (b) Performance parameters as a function of bending cycles.

To test the bending fatigue properties of flexible PVSCs, a device using SnO2 ESL annealed with water vapor was repeatedly bent to a radius of 1 cm, with the performance effects shown in Figure 5. The VOC and JSC did not significantly decrease with increased bending cycles, while the FF decreased dramatically from 72.43% to 58.07% after 1000 bending cycles, leading to a reduction in PCE from of 17.43% to of 13.28%, with about 76% of the relative initial PCE being retained. The increased sheet resistance of the flexible ITO/PET substrate with increased bending cycles is likely to be proposed as the predominant reason for the decreased FF and PCE.14-15, 20, 35-36

In summary, we have found that post-deposition annealing at 100 oC in the presence of water vapor can effectively improve the charge transport ability and electrical conductivity of PEALD processed SnO2 ESLs. As a result, the PVSCs made using these water-vapor treated SnO2 ESLs have shown improved VOC and FF and reduced J-V hysteresis. Our best performing flexible PVSC showed a PCE of 18.36 (17.12)% measured under reverse (forward) voltage scan and a stabilized efficiency over 17% using a commercial ITO/PET substrate, which is the highest PCE among all reported flexible PVSCs with regular structure. Our results reveal a facile approach to

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effectively improve the performance of flexible PVSCs using low-temperature processed SnO2 ESLs. ASSOCIATED CONTENT

Supporting Information.

Experimental details and supplementary characterizations of materials and devices. (PDF)

AUTHOR INFORMATION

Corresponding Authors *Email: [email protected] (J. Wang)

*Email: [email protected] (X. Zhao)

*Email: [email protected] (Y. Yan)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the U.S. Department of Energy (DOE) SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA-0000990), National Science Foundation under contract no. CHE−1230246 and DMR-1534686, Office of Naval Research (N00014-17-I-2223), and the Ohio Research Scholar Program. This work also received financial support from National Basic Research 15


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Program of China (2011CB933300), National Science Fund for Distinguished Young Scholars (50125309), National Natural Science Foundation of China (Grants 51272184 and 91433203).

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High-Performance Planar Perovskite Solar Cells: A New Scheme of Pyridine-Promoted Perovskite Formation. Adv. Mater. 2017, 29, 10.1002/adma.201770091. (11) Kaltenbrunner, M.; Adam, G.; Glowacki, E. D.; Drack, M.; Schwodiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; et al. Flexible High Power-Per-Weight Perovskite Solar Cells with Chromium Oxide-Metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14, 1032-1039. (12) Bi, C.; Chen, B.; Wei, H.; DeLuca, S.; Huang, J. Efficient Flexible Solar Cell Based on Composition-Tailored Hybrid Perovskite. Adv. Mater. 2017, 10.1002/adma.201605900. (13) Shin, S. S.; Yang, W. S.; Noh, J. H.; Suk, J. H.; Jeon, N. J.; Park, J. H.; Kim, J. S.; Seong, W. M.; Seok, S. I. High-Performance Flexible Perovskite Solar Cells Exploiting Zn2SnO4 Prepared in Solution Below 100 oC. Nat. Commun. 2015, 6, 7140. (14) Wang, C.; Zhao, D.; Yu, Y.; Shrestha, N.; Grice, C. R.; Liao, W.; Cimaroli, A. J.; Chen, J.; Ellingson, R. J.; Zhao, X.; et al. Compositional and Morphological Engineering of Mixed Cation Perovskite Films for Highly Efficient Planar and Flexible Solar Cells with Reduced Hysteresis. Nano Energy 2017, 35, 223-232. (15) Yoon, J.; Sung, H.; Lee, G.; Cho, W.; Ahn, N.; Jung, H. S.; Choi, M. Super Flexible, High-Efficiency Perovskite Solar Cells Employing Graphene Electrodes: Toward Future Foldable Power Sources. Energy Environ. Sci. 2016, 10, 337-345. (16) Zhang, H.; Cheng, J.; Lin, F.; He, H.; Mao, J.; Wong, K. S.; Jen, A. K.-Y.; Choy, W. C. Pinhole-Free and Surface-Nanostructured NiOx Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility. ACS Nano 2015, 10, 1503-1511. (17) Yang, D.; Yang, R.; Zhang, J.; Yang, Z.; Liu, S. F.; Li, C. High Efficiency Flexible Perovskite Solar Cells Using Superior Low Temperature TiO2. Energy Environ. Sci. 2015, 8, 3208-3214. (18) Kim, B. J.; Kim, D. H.; Lee, Y.-Y.; Shin, H.-W.; Han, G. S.; Hong, J. S.; Mahmood, K.; Ahn, T. K.; Joo, Y.-C.; Hong, K. S.; et al. Highly Efficient and Bending Durable Perovskite Solar Cells: Toward a Wearable Power Source. Energy Environ. Sci. 2015, 8, 916-921. (19) Yang, D.; Yang, R.; Ren, X.; Zhu, X.; Yang, Z.; Li, C.; Liu, S. Hysteresis-Suppressed High-Efficiency Flexible Perovskite Solar Cells Using Solid-State Ionic-Liquids for Effective Electron Transport. Adv. Mater. 2016, 28, 5206-5213. (20) Heo, J. H.; Lee, M. H.; Han, H. J.; Patil, B. R.; Yu, J. S.; Im, S. H. Highly Efficient Low Temperature Solution Processable Planar Type CH3NH3PbI3 Perovskite Flexible Solar Cells. J. Mater. Chem. A 2016, 4, 1572-1578. (21) Zhu, Z.; Xu, J.-Q.; Chueh, C.-C.; Liu, H.; Li, Z. a.; Li, X.; Chen, H.; Jen, A. K. Y. A Low-Temperature, Solution-Processable Organic Electron-Transporting Layer Based on Planar Coronene for High-Performance Conventional Perovskite Solar Cells. Adv. Mater. 2016, 28, 10786-10793. (22) Feng, J.; Yang, Z.; Yang, D.; Ren, X.; Zhu, X.; Jin, Z.; Zi, W.; Wei, Q.; Liu, S. 17


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ACS Energy Letters

Ultrathin, Smooth, and Low-Loss Al-Doped Ag Film and Its Application as a Transparent Electrode in Organic Photovoltaics. Adv. Mater. 2014, 26, 5696-5701. (36) Zhao, D.; Zhang, C.; Kim, H.; Guo, L. J. High-Performance Ta2O5/Al-Doped Ag Electrode for Resonant Light Harvesting in Efficient Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1500768.

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Water Vapor Treatment of Low-Temperature Deposited SnO2 Electron

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