Barrier Design to Prevent Metal-Induced Degradation and Improve Thermal Stability in Perovskite Solar Cells † † † † † Caleb C

Letter

Cite This: ACS Energy Lett. 2018, 3, 1772−1778

Barrier Design to Prevent Metal-Induced Degradation and Improve Thermal Stability in Perovskite Solar Cells † † † † † Caleb C. Boyd, Rongrong Cheacharoen, Kevin A. Bush, Rohit Prasanna, Tomas Leijtens, † ‡ and Michael D. McGehee*, , † Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ‡ Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, United States

*S Supporting Information

ABSTRACT: Metal-contact-induced degradation and escape of volatile species from perovskite solar cells necessitate excellent diﬀusion barrier layers. We show that metal-induced degradation limits thermal stability in several perovskite chemistries with Au, Cu, and Ag gridlines even when the metal is separated from the perovskite by a layer of indium tin oxide (ITO). Channels in a sputtered ITO layer that align with perovskite grain boundaries are pathways for metal and halide diﬀusion into or out of the perovskite. Planarizing the perovskite morphology with a spin-cast organic charge- transport layer results in a subsequently deposited ITO layer that is uniform and impermeable. We show that it is critical to seal the edges of the active layers to prevent escape of volatile species. We demonstrate 1000 h thermal stability at 85 °Cin CH3NH3PbI3 solar cells with complete-coverage silver contacts. Our barrier layer design enables long-term thermal stability of perovskite solar cells, a critical step to commercialization.

erovskite solar cells (PSCs) have the potential to often used in single-junction thin-film modules and perovskite combine high efficiencies with cost-efficient manufac- tandem solar cells not only because it is necessary for the turing techniques for terawatt energy production, electrode that is deposited on top of the perovskite to be P 1−9 4,26 especially when used in low-cost tandem solar cells. transparent but also to improve stability. However, ITO has However, to realize the benefits of easy-to-manufacture, 2 orders of magnitude higher resistivity than silver27 and can solution-processable PSCs, their stability must be improved only be used to carry current for approximately 1 cm without to enable at least a 20−30 year lifetime.10 One of the most causing the series resistance to be unacceptably high.28 There important obstacles is metal-contact-induced degradation due is considerable interest in coating 6 in. wide silicon solar cells

Downloaded via UNIV OF COLORADO BOULDER on August 13, 2018 at 17:32:42 (UTC). to the ubiquitous reactivity of metals and halides and the desire with a perovskite cell to make a tandem. Such a cell must have

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. to make the entire electrode out of metal or metal gridlines to metal gridlines on top of the TCO. reduce series resistance. Although some metals may be stable To improve PSC stability while maintaining performance, with respect to the perovskite structure,11 almost all metal conductive barrier layers must be developed that prevent both species, even platinum, are energetically disposed to form metal and halide species from diffusing in or out of the halides.12 Thus, metal contacts act as sinks for halide or perovskite active layer. Potential barrier layers include 29 halogen species that escape from the perovskite under the nanostructured carbon layers, cross-linked charge-transport 30 31 stressors of moisture, oxygen, light, and heat, causing layers, copper phthalocyanine (CuPC), chromium oxide/ 32 33−35 irreversible degradation of both the metal contacts and the metal bilayers, molybdenum oxide/aluminum bilayers, − 36−39 perovskite active layer.13 23 Gold has also been shown to aluminum zinc oxide/tin oxide bilayers, tantalum-doped 40 diffuse through an organic transport layer into the perovskite tungsten oxide/conjugated polymer bilayers, and TCOs such 26 layer, setting up a two-fold degradation mechanism in which as ITO. Designing a barrier that can be broadly applied to halogen/halide species can diffuse up to the metal and the multiple PSC architectures, prevents decomposition of the metal can diffuse into the perovskite (Figure 1A).24 perovskite layer, and allows for metal contacts while Nonmetal contact layers such as carbon electrodes have enabled excellent PSC stability, but they limit solar cell Received: June 3, 2018 performance by increasing series resistance losses.25 Trans- Accepted: July 2, 2018 parent conducting oxide (TCO) electrodes such as ITO are Published: July 2, 2018

Figure 1. Au, Cu, and Ag contacts degrade PSCs. (A) Diagram of devices in this study showing possible degradation mechanisms of metal ff and iodide di usion through C60/PCBM, SnO2, and ITO layers. (B) FA0.83Cs0.17Pb(I0.83Br0.17)3 cell with Cu contacts before and after ° heating for 1000 h at 85 CinN2 in the dark. The light areas indicate degradation of the perovskite active layer, which originates around the fi JV metal ngers or bus bars. (C) curves of FA0.83Cs0.17Pb(I0.83Br0.17)3 cells with no metal and Ag, Cu, and Au contacts before (blue) and after ° − (red) thermal testing at 85 C for 1000 h in N2. All cells with metal degrade similarly, while cells without metal do not. See also Tables S1 S3 and Figures S1 and S2. demonstrating impressive thermal stability has proven to be a without metal gridlines on top of the ITO, and we could easily challenge for the community. see the perovskite film turn yellow near the gridlines. Au, Cu, and Ag Contacts Degrade PSCs. In the first stage of Because it is not surprising that a reaction between a metal this study, we fabricated PSCs on glass that are identical to and perovskites would be undesirable, we will not attempt to those used in world record perovskite−silicon tandem cells explain in this Letter precisely what reaction took place previously reported.4 They had an “inverted” p−i−n between the metals and the perovskites or why the JV curves changed in the way they did. We speculate that metal diffusing architecture ITO/NiOx/FA0.83Cs0.17Pb(I0.83Br0.17)3/evaporated into the perovskite layer may react with the perovskite at grain C60/SnO2/ITO/Ag (details in the Experimental Methods) section with semitransparent ITO contacts. We chose this boundaries, creating an insulating layer that leads to decreases perovskite composition because the use of formamidinium in JSC and slight increases in VOC via passivation. We leave this to future work and focus here on demonstrating a highly (FA, CH2(NH2)2) and Cs at the A site in ABX3 is known to − ff improve thermal stability.41 43 For this study, we added metal e ective solution for improving the barrier properties of the fingers on top of these cells to reduce the series resistance TCO to prevent the reaction between perovskites and the metal that is needed to reduce series resistance. (Figure 1). We found that the introduction of the metal fingers Ag Diffuses through the ITO Layer into the Perovskite Layer substantially accelerated thermal degradation at 85 °C, a and Volatile Halide Species Penetrate the ITO and React with Ag. common temperature for accelerated testing of solar cells. We We were surprised by the persistence of metal-induced visually saw degradation of the perovskite active layer degradation through both an 8 nm thick atomic-layer- emanating from Au, Cu, or Ag fingers deposited on the ITO deposited (ALD) SnO2 layer and a 150 nm thick sputtered contacts, corresponding to changes in the JV curves of the aged ITO layer. To confirm that degradation shown in the previous cells and overall PCE loss (Figures 1B,C, S1, and S2 and Table section was due to metal−perovskite reactions, we performed a S1). Identical cells without the metal gridlines showed no series of experiments using X-ray photoelectron spectroscopy visible signs of degradation and little to no change in their (XPS) to characterize diffusion of Ag and halide species in and current−voltage characteristics (Figure S1). We found similar out of the perovskite active layer. results when we aged MAPbI3 and FA0.75Cs0.25Sn0.5Pb0.5I3 Cells aged at 85 °C in a dark, nitrogen environment for 1000 devices with metal gridlines (Figure S1 and Tables S2 and h with Ag contacts were depth profiled to determine whether S3). It is very clear that the metal accelerates the degradation Ag diffused into the perovskite layer or not. We used tape to ff because we observed many times that devices made with three peel o the SnO2, ITO, and metal contacts before depth different perovskite compositions were much more stable profiling down into the perovskite layer to avoid diffusion of

1773 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778 ACS Energy Letters Letter

Figure 2. Ag diffuses through the ITO layer into the perovskite layer and volatile halide species penetrate the ITO and react with Ag. (A,C) XPS depth profile to detect Ag present in the perovskite. Tape was used to peel away the top layers of the stack to avoid diffusion of metal into the active layer during sputtering and thus false positives. (C) Ag 3d5 counts measured in the XPS depth profile shown in (A) through ff ° two MAPbI3 PSCs after peeling o the top SnO2/ITO/Ag layers, one kept at 85 C and one kept at room temperature for 1000 h in a dark ff N2 environment. (B,D) XPS with in situ heating to look for halide di usion to the metal electrode. (D) XPS spectra on the surface of Ag ° contacts on MAPbI3 solar cells heated in situ at 150 C with evaporated PCBM, an ALD SnO2 layer, and sputtered ITO covering the surface of the perovskite, showing an increase in iodine species at the surface of the solar cell over the course of 4 h. See also Figures S3−S5.

ff Figure 3. Planarizing the PSC prevents Ag di usion. (A,B) Cross-sectional SEM images of ITO barrier layers on MAPbI3 cells with evaporated (A) and spun (B) PCBM, with red arrows showing diffusion channels in evaporated PCBM that propagate along the rough perovskite grain boundaries. These channels are not present in the layer on spun PCBM. (C,D) Top-down SEM of ITO on MAPbI3 with evaporated (C) and spun PCBM (D). The channels in the ITO layer match grain boundaries of the perovskite layer. The spun layer effectively smooths the rough perovskite morphology. See also Figures S6−S10.

1774 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778 ACS Energy Letters Letter

− Figure 4. 1000 h thermal stability of MAPbI3 solar cells with an impermeable TCO. (A D) Schematics of barrier layers evaluated in this study showing degradation mechanisms that occur with each design. (C,D) Barrier layers evaluated in long-term thermal stability testing, − ffi with results shown in (E G). (E) Normalized plot of the power conversion e ciency during thermal stability testing of MAPbI3 solar cells ° with an ITO barrier layer and Ag electrodes at 85 C in a dark N2 environment. Points are averages for each data set, and error bars bound the range of all cells tested. Cells with spun PCBM have an impermeable ITO layer that effectively blocks Ag diffusion and iodide diffusion and enables 1000 h thermal stability, while cells with evaporated PCBM have a permeable ITO layer that allows for Ag and iodide diffusion, − causing a drop in PCE. (F,G) Current voltage curves of MAPbI3 cells with evaporated (F) and spun (G) PCBM before and after aging. See also Figures S11−S15. metal into the active layer during sputtering (Figure 2A). We to characterize the worst-case scenario. Iodine (or iodide) confirmed that the peel method delaminates the solar cell in presence on the surface of the Ag contacts increased drastically the C60 or PCBM layer through XPS, as previously reported.44 during heating, indicating that iodine or iodide species were We positively identified Ag peaks in the perovskite active layers diffusing out of the active layer to the silver surface. Thus, not ff of FA0.83Cs0.17Pb(I0.83Br0.17) 3 ,MAPbI3 ,and only does Ag di use into the active layer, but the Ag contact FA0.75Cs0.25Sn0.5Pb0.5I3 cells that exhibited degradation after also acts as a sink for iodine from the perovskite, aiding in aging at 85 °C(Figures 2B and S3−S5). We used secondary decomposition of the active layer. Both of these diffusion ion mass spectroscopy (SIMS) with depth profiling to further processes are clearly harmful for device operation and must be confirm the presence of Ag in the perovskite layer after heating prevented for stable PSCs. and validate the use of XPS depth profiling to assess the Planarizing the PSC for an Impermeable ITO Layer. ITO has presence of silver (Figure S3). We note that it is difficult to say been shown to effectively prevent metal, oxygen, and water − with certainty where in the perovskite layer the Ag lies due to vapor diffusion,45 48 contrary to our evidence of degradation convoluting effects of roughness during ion milling, ion knock due to metal and iodine/iodide diffusion. Because it is not ff on, and the fact that the grains in the MAPbI3 do not all likely that di usion is occurring through the ITO bulk, traverse through the entire film. However, we did not detect improving the morphology of the ITO to prevent diffusion at Ag with XPS in the perovskite in fresh cells of any architecture grain boundaries or through pin holes is the key to improving studied in this Letter, in cells that were kept at room thermal stability. We tried replacing ITO with the amorphous temperature for 1000 h, or in cells that were heated for 1000 h TCO indium zinc oxide (IZO), which has no grain boundaries without Ag contacts (Figures 2B, S4, and S5). We can and has been shown to be more mechanically stable and therefore strongly conclude that Ag did penetrate the thermally stable than ITO, but found that it did not improve perovskite film aged for 1000 h at 85 °C and that the Ag the barrier morphology or resistance to metal-contact-induced signal observed by XPS is not an experimental artifact. degradation (Figures S6 and S7).27,49,50 Using scanning While metal diffusion into the active layer is one source of electron microscopy (SEM) and atomic force microscopy potential degradation, iodide/iodine species escaping the (AFM), we saw that the morphology of both TCOs drastically active layer is potentially a more ubiquitous and challenging changed with planarization of the perovskite layer through degradation mechanism under both heat and light.15,22,23 We spin-coating of the electron-transport layer (ETL) versus ° − performed in situ heating at 150 C on fresh MAPbI3 solar evaporating the ETL (Figures 3A D, S6, and S8). Evaporation cells with evaporated PCBM, ALD SnO2, and sputtered ITO conformally coats the perovskite surface, and the sputtered layers with opaque silver contacts (complete active area TCO process cannot fill in the deep perovskite grain coverage) in an XPS tool, taking high-sensitivity scans on the boundaries. Instead, the perovskite grain boundaries propagate metal contact surfaces over the course of 4 h to look for through the TCO layer as channels, as can clearly be seen in diffusion of elements out of the perovskite active layer (Figure the cross-sectional and top-down SEM images in Figure 3A− 2B,D). We chose MA because it is the most volatile A-site D. We observed this grain boundary propagation even with a cation used in ABX3 metal halide perovskites, and we wanted 500 nm thick TCO (Figure S9). A spun PCBM layer, in

1775 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778 ACS Energy Letters Letter contrast, fills the valleys of the perovskite grains, allowing for a In this Letter, we have shown that if metal is used to reduce dense, uniform TCO layer. Not coincidentally, many of the the series resistance of the electrodes in PSCs then it is devices in other studies that report good stability under light necessary to have a dense, channel-free barrier layer to protect soaking or heating with metal contacts contain barrier layers the metal from the perovskite and contain volatile components deposited on solution-processed transport layers such as of the perovskite. If there are channels between grains of the nanoparticle layers or hole-transport layers.26,29,31,32,35,36,40 perovskite, then it is necessary to solution-cast a contact Using the same XPS depth profiling experiment as that in the material to fill in the channels and disrupt the perovskite previous section with silver contacts that completely cover morphology before sputtering a TCO electrode that serves the − ff MAPbI3 devices to examine a device in which metal dual purposes of carrying current and acting as a di usion perovskite reactions would be most likely to occur, we found barrier. It is also critically important to seal the edges of the that the improved TCO layer morphology successfully blocked device. If modules are made with laser scribes between cells, it silver diffusion into the perovskite layer (Figure S10). will be necessary to make sure that metal in the vias is While a spun PCBM layer successfully blocked Ag diffusion protected from the perovskite. We have shown that solar cells into the perovskite layer, in situ XPS heating showed that the made with even one of the most thermally unstable perovskites ° iodide/iodine signal increased at the surface for cells with both can be stable for 1000 h at 85 C even when Ag, which reacts spun and evaporated PCBM that had ITO only on top of the easily with perovskites, is on top of the TCO. We still solar cell (Figures 4A,B and S11). To test the hypothesis that recommend, however, using the more thermally stable the volatile halide species were able to escape from the perovskites, a dense TCO barrier layer, and high-quality uncovered edges of the active layer, we sealed the edges of the packaging to maximize the chances of having nearly 100% of active layer with ITO (Figure 4C,D). We observed that the the cells operated outside survive for >25 years. amount of iodide/iodine signal that accumulated during in situ In this study, the only stressor that we subjected the solar heating decreased drastically for the cell with evaporated cells to was heat. We suspect, however, that the barrier layers described in this Letter will also improve stability under light PCBM and almost completely disappeared for the cell with because a recent paper shows that light accelerates reactions spun PCBM (Figures S11 and S12). Thus, in addition to between perovskites and metal contacts by creating mobile having an impermeable TCO layer on top of the perovskite halogen vacancies through a process in which photogenerated active layer, it is also necessary to seal the edges of the holes oxidize halogens, making them smaller and allowing perovskite layer, especially for perovskite compounds that them to become interstitials.23 In addition to increasing contain volatile organic species such as MA. halogen mobility, the photogenerated vacancies allow any 1000 h Thermal Stability of MAPbI3 Solar Cells with an oxygen that might be present to penetrate the film and Impermeable TCO. To test the stability of our improved barrier ultimately react with it.14 We therefore predict that properly layer in actual devices with a notoriously volatile perovskite, we protecting perovskites with barrier layers will allow them to fabricated MAPbI3 solar cells with either spun or evaporated return to their original state unharmed after photoexcitation PCBM ETLs. We completely covered the active area of the cell and are currently testing this hypothesis. with metal to put the barrier properties of the TCO through the toughest test. We deposited an 8 nm thick ALD SnO2 layer ■ EXPERIMENTAL METHODS and a 150 nm thick sputtered ITO layer, making sure to extend the ITO layer over the edges of the perovskite active layer to See the Supporting Information. prevent release of the volatile MA species. We used an ■ ASSOCIATED CONTENT evaporated MgF2 insulating strip between the top and bottom ITO layers on one edge of the solar cell in order to prevent *S Supporting Information shunting and allow for complete ITO coverage of the active The Supporting Information is available free of charge on the layer (Figure S13). We kept three unencapsulated cells of each ACS Publications website at DOI: 10.1021/acsenergy- type with initial efficiencies of 10.1−13.1% on a hot plate in a lett.8b00926. N glovebox in the dark for 1000 h at 85 °C.51 We note that 2 Detailed methods, tables showing JV characteristics of the ALD process had been optimized for a different perovskite fi compound that produces record efficiency perovskite/silicon the species studied, and gures showing pictures of the 4 ff tandem solar cells, and we used MAPbI3 as a model case to perovskite cells with di erent metals and active layers, truly challenge the effectiveness of the designed barrier. It is for external quantum efficiencies, depth profiles, XPS ffi this reason that the e ciency of our cells is lower than is spectra, AFM and SEM images, optical microscope frequently observed for MAPbI3. We saw that all of the cells with evaporated PCBM experienced degradation due to metal images, and the acrhitectures of the cells (PDF) diffusion, but remarkably all of the cells with spun PCBM suffered no loss in efficiency (Figures 4 and S14 and Table S2). ■ AUTHOR INFORMATION We also heated the four MAPbI3 architectures shown in Corresponding Author ° ° ∼ Figure 4 at 150 C in ambient air (25 C, 40% RH) for 4 h. *E-mail: [email protected]. These test parameters are relevant for packaging commercial ORCID solar modules, which often use a 10−20 min, 150 °C encapsulation step. All architectures showed considerable Caleb C. Boyd: 0000-0001-7408-7901 visual degradation except the one with a spun PCBM layer Kevin A. Bush: 0000-0003-1813-1300 and ITO on the edges as well as the top of the perovskite Rohit Prasanna: 0000-0002-9741-2348 active layer, which effectively sealed in volatile components Tomas Leijtens: 0000-0001-9313-7281 and kept moisture and oxygen out of the cell (Figure S15). Michael D. McGehee: 0000-0001-9609-9030

1776 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778 ACS Energy Letters Letter

Author Contributions Uncertainty Analysis and Its Application to Perovskite on Glass C.C.B., R.C., and M.D.M. designed the experiments and Photovoltaic Modules. Prog. Photovoltaics 2017, 25 (5), 390−405. analyzed the data. C.C.B. and R.C. performed the experiments. (11) Zhao, J.; Zheng, X.; Deng, Y.; Li, T.; Shao, Y.; Gruverman, A.; K.A.B., R.P., and T.L. provided expertise and feedback. C.C.B. Shield, J.; Huang, J. Is Cu a Stable Electrode Material in Hybrid and M.D.M. wrote the paper. All authors discussed the results Perovskite Solar Cells for a 30-Year Lifetime? Energy Environ. Sci. 2016, 9 (12), 3650−3656. and commented on the manuscript. (12) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd Notes ed.; Butterworth-Heinemann: Oxford, U.K., 1997. The authors declare no competing ﬁnancial interest. (13) Leijtens, T.; Bush, K.; Cheacharoen, R.; Beal, R.; Bowring, A.; McGehee, M. D. Towards Enabling Stable Lead Halide Perovskite ■ ACKNOWLEDGMENTS Solar Cells; Interplay between Structural, Environmental, and Thermal Stability. J. Mater. Chem. A 2017, 5 (23), 11483−11500. The information, data, and work presented herein were funded (14) Aristidou, N.; Eames, C.; Sanchez-Molina, I.; Bu, X.; Kosco, J.; in part by the U.S. Department of Energy (DOE) PVRD2 Islam, M. S.; Haque, S. A. Fast Oxygen Diffusion and Iodide Defects program under Award Number DE-EE0008154 and by the Mediate Oxygen-Induced Degradation of Perovskite Solar Cells. Nat. Oﬃce of Naval Research under Award Number N00014-17-1- Commun. 2017, 8, 15218. 2525. C.C.B. acknowledges support by the National Science (15) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; Foundation Graduate Research Fellowship under Grant No. D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; DGE-1656518. R.C. acknowledges support from the Ministry Mosconi, E.; et al. Intrinsic Thermal Instability of Methylammonium of Science and Technology of Thailand Fellowship. Part of this Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5 (15), 1500477. work was performed at the Stanford Nano Shared Facilities (16) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized (SNSF), supported by the National Science Foundation under TiO2 with Meso-Superstructured Organometal Tri-Halide Perovskite Award ECCS-1542152. The authors would like to thank Solar Cells. Nat. Commun. 2013, 4, 2885. Chuck Hitzman and Juliet Jamtgaard for insightful discussions (17) Han, Y.; Meyer, S.; Dkhissi, Y.; Weber, K.; Pringle, J. M.; Bach, and expertise on spectroscopy techniques and analysis. We U.; Spiccia, L.; Cheng, Y.-B. Degradation Observations of thank Farhad Moghadam and Wen Ma of Sunpreme for Encapsulated Planar CH3NH3PbI3 Perovskite Solar Cells at High depositing the ITO electrodes. 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1777 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778 ACS Energy Letters Letter

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1778 DOI: 10.1021/acsenergylett.8b00926 ACS Energy Lett. 2018, 3, 1772−1778