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Improved Stability of Organo-metal Halide Perovskite Films and Solar Cells Towards Humidity Via Surface Passivation with Oleic Acid Ghada Abdelmageed, Heather Renee Sully, Sara Bonabi Naghadeh, A. El-Hag Ali, Sue Anne Carter, and Jin Z. Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00069 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Applied Energy Materials

Improved Stability of Organo-metal Halide Perovskite Films and Solar Cells Towards Humidity Via Surface Passivation with Oleic Acid

Ghada Abdelmageed,a, d Heather Renee Sully,b Sara Bonabi Naghadeh,a Amr El-Hag Ali,e Sue A. Carter,c and Jin Z. Zhanga*

a

Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, United States

b

Department of Electrical Engineering, University of California, Santa Cruz, CA 95064, United States c

d

Department of Physics, University of California, Santa Cruz, CA 95064, United States

Department of Radiation Physics, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Nasr City, Cairo 11787, Egypt

e

Department of Polymer Chemistry, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Nasr City, Cairo 11787, Egypt

*Corresponding Author: E-mail: [email protected]

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Abstract Organo-metal halide (OMH) perovskites are highly promising for photovoltaic (PV) and other applications. However, their instability towards environmental factors such as humidity presents a major challenge in their potential commercial use. In this study, we developed a method to modify the surface of CH3NH3PbI3 perovskite films by spin coating oleic acid (OA) to create a water resistant layer that results in enhanced stability and PV performance. The OA-surface passivated perovskites were studied using FT-IR spectroscopy, UV-Vis absorption spectroscopy and X- ray diffraction (XRD). The samples were aged in dark humid air at ∼ 76% relative humidity (RH) for four weeks. The surface passivated films showed minimal sign of decomposition and the PV devices showed better performance than the unpassivated devices. A possible explanation is the carboxyl group (-COO-) of OA binds to surface Pb2+ and/or CH3NH3+ to both passivate these surface defect sites, resulting in the formation of a thin layer of OA with their hydrophobic tail away from the perovskite film surface that effectively prevents water molecules from reaching the perovskite.

Keywords: Perovskite solar cell; Humidity; Surface passivation; Oleic acid; Stability.


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1. Introduction Organometal halide (OMH) perovskites such as methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) has demonstrated great promise for photovoltaic (PV) applications. The rapid progress of power conversion efficiency (PCE) of (OMH) perovskite solar cells from 3.8% at 2009 to 22% at 2016 has been unprecedented in the renewable energy industry. However, to date, the stability of OMH perovskites remains a huge challenge.1-3 Despite their unique properties, their vulnerability to environmental factors such as light, oxygen, and humidity presents a major hurdle to industrial applications.4-6 Each of these factors can induce a specific degradation pathway and often some of these factors act in tandem, leading to complex degradation processes.7,

8

Humidity is a particularly serious issue due to the strongly ionic character of the material.9, 10 Understanding the mechanism of humidity induced degradation and the role of moisture in the improvement of the perovskite film properties have been the subject of several previous studies.11-16 It was proposed that as H2O moves along the film grain boundaries, MAPbI3 would decompose into the hydrate intermediate compound according to equation (1). 4 + 2 → ( ) · 2 + 3 (1) The degradation pathway suggests that the formation of the isolated PbI octahydra hydrated compound initiates the degradation process that results in the separation and crystallization of PbI2. The PbI2 phase separation is observable through the color change and irreversible damage to the film.16 However, a study by Müller et al. showed that upon short-term exposure on the order of one minute, water molecules enter perovskite unit cells without significantly altering the tetragonal crystal structure or the band gap.17 The 3 ACS Paragon Plus Environment


water molecule influences the hydrogen bonding between the MA cations and the Pb-I cage and may increase ion conductance or act as a dopant but likely a combination center. This can explain why small amounts of water have been reported to be beneficial for device performance. Nevertheless, for long-term device stability, it is crucial to control the amount of moisture the perovskite film is exposed to. Guo et al. reported another interesting finding, where radiation (UV, X-ray and electron beam) was found to stimulate recovery for perovskite films after subjecting to humidity (60 ± 5% relative humidity) for 14 days.18, 19 However, the study showed that the recovery via radiation was more successful for perovskite films prepared by drop casting than those prepared by spin coating. An approach to enhance the long-term stability of the perovskite against humidity is to apply a humidity-blocking layer. Han et al. compared two encapsulation methods, one with a glass front cover sealed with epoxy resin and the other adapted from OLED technology that employed desiccant.20 Both failed to prevent the perovskite film from dissolving into its precursors. Hwang et al. spin-coated Teflon on the back of a fully prepared perovskite solar cell as a form of polymer encapsulation.21 The Teflon layer worked as moisture repellent that led to a stable performance for the solar cells when stored in ambient air for 30 days. In another study, Guo et al. mixed PbI2 and MAI in terpineol and found the films to be stable for 70 days at 30% RH.22 However, no tests were done on the effect of terpineol on device performance. Manshor et al. found that adding polyvinylprrolidone extended the lifetime of films, however has a negative effect on charge transfer for devices due to a lower conductivity.23 In addition, Xiong et al. very recently reported that applying a hydrophobic agent (fluoroalkyl silane) on films and in solar cells has improved stability significantly during a storage process in humid air (50%


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RH) for three weeks.24 Likewise, Wang et al. studied the potential application of perovskite in water splitting by applying a hydrophobic layer of alkyl ammonium cations on the perovskite layer inside the device with an additional ultrathin outer layer of Ni on to serve as a second shield.25 Results showed a steady performance for 30 minutes, but there was no result reported on possible application in solar cells. In this work, we have demonstrated that surface passivation of MAPbI3 films and solar cells with oleic acid (OA) can effectively improve their stability against humidity. The samples were aged in dark humid setup for four weeks at ∼76% relative humidity (RH) and studied using FTIR, UV-Vis and XRD techniques. Results show significantly enhanced stability of the films towards humidity. The solar cells exhibited both better performance and improved stability, part of which might be attributed to the added protection of the hole transport layer (HTL) by OA. This work demonstrates a simple and potentially low cost approach to stabilizing OMH perovskites, which is important for PV and other applications.

2. Materials and Methods All the chemicals were used as received without further purification, including toluene (Spectroscopic grade, Fisher Scientific), N, N-dimethylformamide (DMF, spectroscopic grade, Fisher Scientific), oleic acid (90%, Alfa Aesar), 2-propanol (IPA, spectroscopic grade, Fisher Scientific), methylammonium iodide (MAI, Dyesol), lead iodide (PbI2, 99.99%, Alfa Aesar), titanium (IV) ethoxide (Ti(OC2H5)4, technical grade, Sigma-Aldrich), hydrochloric acid (HCl, NF grade, Amersco), anhydrous ethanol (EtOH, spectroscopic grade, Alfa Aesar), chlorobenzene (99.8%, Sigma-Aldrich), titanium



dioxide (TiO2) nanopatricles (Solaronix), and poly-3-hexylthiophene (P3HT, SigmaAldrich).

2.1. Synthesis The perovskite films were prepared as follows: PbI2 (0.461 g) was dissolved in 1 ml DMF on a hotplate at 80°C for 30 minutes then 50µL of the dissolved lead iodide was spin-coated on cleaned borosilicate glass slides at 6000 rpm for 5 seconds followed by annealing on hotplate in air at 80°C for 30 minutes. Then, 150µL of MAI (10 mg/ml in IPA) was spin-coated on the films for 1 minute at 0 rpm (loading time) then for 20 seconds at 3000 rpm to remove the excess. The films were dried again at 80°C for 30 minutes in air. For the surface passivation step: equal amounts of oleic acid and toluene are mixed and stirred for 5 minutes. Then, 50 µL of the diluted oleic acid was spin-coated on some of the prepared MAPbI3 films at 6000 rpm for 25 seconds, followed by heating at 70°C for 5 minutes in air. To prepare MAPbI3 solar cells, pre-patterned ITO slides were cleaned by sonication in alconox solution (10% conc.), di-ionized water, 2 propanol, and ethanol respectively for 15 minutes at 60°C, and then dried using compressed N2 gas. Afterwards, a TiO2 sol gel dense layer and mp-TiO2 nanoparticles layer were each spincoated, respectively. Each TiO2 layer was dried at 125°C for 1 hours then sintered at 450°C for 30 minutes in air. The TiO2 sol gel was prepared by mixing Ti(OC2H5)4 with anhydrous ethanol, HCl and deionized water as discussed in a previous study.26 The MAPbI3 layer was prepared as mentioned earlier as well as the surface passivation step. For the hole transport layer (HTL), poly (3-hexylthiophene-2,5-diyl) (P3HT) with concentration of 10 mg/ml in chlorobenzene was spin-coated on the perovskite layer with


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speed of 2000 rpm for 60 seconds. Finally, 100 nm of gold contact was thermally evaporated under high vacuum.

2.2. Degradation process and characterization The stability of the perovskite samples was tested against humidity using a procedure mentioned in a previous paper.27 The samples were placed inside an enclosure with sodium chloride salt mixture with deionized water to achieve relative humidity levels of ~76% at room temperature of ~28-30°C. The salt mixture was rehydrated everyday and humidity level was observed during the whole experiments to assure its steadiness. The relative humidity sometimes reached higher levels that exceeded 85%, especially with a freshly made salt mixture, but it gradually decreased to stay at equilibrium state at 76% in few hours. Before and after the aging processes, the samples were examined by taking their UV-Vis absorbance spectra using a Jasco V-670 spectrophotometer. Also, the structure and the weight percent of the contents of the samples were evaluated by XRD analysis (XRD, Rigaku Americas Miniflex Plus powder diffractometer) was performed at a voltage of 40 kV and current of 44 mA, with a scanning angle range of 10-60° (2θ) with a rate of 2°/min. Fourier Transform Infrared (FT-IR) spectra were recorded with a Perkin Elmer Spectrum One FT-IR spectrophotometer using KBr pellets as substrates. The solar cells parameters were evaluated by measuring current density-voltage (J-V) curves under one sun illumination (100 mW/cm2) at calibrated AM1.5G condition. The area reported of each device formed is 0.03 cm2. 3. Results and Discussion



The FT-IR spectra of OA and the MAPbI3 pre- and post-surface passivation are shown in Figure 1. The OA characteristic vibrational bands located at 726, 947, 1716, 2858, and 2934 cm-1 are assigned to CH2 rocking, O-H stretch, C=O stretch, CH3 symmetric stretch, and CH2 asymmetric stretch, respectively.28, 29 Pristine MAPbI3 (presurface passivation) showed bands at 667, 930, 1257, 1487 and 3208 cm-1 that are assigned to NH wagging, CH3 rocking, NH3 rocking, CH scissoring, and NH stretch, respectively.30 Passivated MAPbI3 film (post-surface passivation) shows additional bands at 1716, 2858, and 2934 cm-1 that are characteristic of OA suggesting presence of the oleic acid layer on the film surface. It has been suggested that the R-COO- (R, alkyl chain) functional group can passivate the Pb2+ defects on the surface of PbS quantum dots (QDs) and methylammonium lead bromide (MAPbBr3) perovskite nanocrystals, which may hinder the formation of the isolated PbI octahedra hydrated compound that initiates the degradation process.31, 32 Similar effect may be in operation with the OA passivation of the perovskites here through the -COO- group of OA.


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Figure 1 FT-IR spectra of oleic acid (top), MAPbI3 (middle), and MAPbI3 with surface passivation with oleic acid (bottom). Figure 2 shows changes in the UV-Vis absorption spectra of the aged MAPbI3 films with and without surface passivation with OA over time, with the inset photo images for the fresh and aged films. It is clear that the film treated with OA (Figure 2b) showed no degradation after one week and a sign of minimal degradation indicated by a very slight decrease in the overall absorption curve after four weeks of storing at 76% RH. Moreover, the control sample in Figure 2a exhibited complete degradation after one week of aging, as the onset at 765 nm corresponding to MAPbI3 vanished.33 Also, the image showed conversion of dark brown perovskite to the bright yellow lead iodide film.

Figure 2 UV-Vis absorption spectra of aged MAPbI3 films with and without surface passivation with OA in dark humid environment. (a) The absorption spectra of MAPbI3 before and after one week of aging. (b) The absorption spectra of MAPbI3 film with OA



before and after one and four weeks of aging. Insets show photographic images of the corresponding samples. To determine the change in the structure of the films with humidity, XRD data of the films were collected as shown in Figure 3. The XRD patterns of the fresh samples with and without passivation using OA show the perovskite characteristic peaks at 2θ = 14.15°, 24.47°, 28.50°, 31.90°, and 43.05° assigned to the (110), (202), (220), (310), and (330) lattice planes, respectively. The results indicate orthorhombic crystal structure as reported in literature.34 The control film degraded completely to hexagonal PbI2 (2θ = 12.64°) within the first week of exposure to humidity. Meanwhile, the film with OA passivation did not show any degradation within the same time period (one week), and showed a weak PbI2 peak after four weeks, which is in agreement with the UV-Vis absorption results.


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Figure 3 XRD patterns of aged MAPbI3 films with and without surface passivation with OA in dark humid environment. (a) XRD patterns of MAPbI3 film without OA before and after one and four weeks. (b) XRD patterns of MAPbI3 film with OA passivation. To evaluate the impact of surface passivation with OA on the perovskite solar cells performance and stability towards humidity, devices were fabricated using a layer-bylayer deposition approach, as shown in Figure 4a. The devices were aged in the same setup as the films in dark humid air (76% HR) with identical devices kept in dark dry environment and tested at the same time intervals to evaluate any parasitic degradation effects. Similar to the films, the devices with OA treatment did not show any noticeable degradation after being stored in humid environment for four weeks (Figure 4b), However, the device with untreated film degraded over time. The current-voltage (JV) curves of the devices aged in humid conditions are shown in Figures 4c and 4d and their device parameters are summarized in Table 1.

Figure 4 (a) Illustration of structure of the MAPbI3 solar cells with and without OA passivation. (1b) Image of MAPbI3 solar cell with OA aged for four weeks. (2b) Image of 11 ACS Paragon Plus Environment


MAPbI3 solar cell without OA aged for 4 weeks. (c) JV characteristics of MAPbI3 solar cell without OA aged for four weeks in humid condition. (d) JV characteristics of MAPbI3 solar cell with OA aged for four weeks in humid condition. Table 1 Solar cell parameters of MAPbI3 solar cells with and without surface passivation with OA aged in dark at 76% RH.

Fresh 1 week 2 weeks 3 weeks 4 weeks

Fresh 1 week 2 weeks 3 weeks 4 weeks

Dark/76% RH /without oleic acid Jsc (mA/cm2) Voc (V) FF (%) PCE (%) 24.4 0.86 36.0 7.62 20.4 0.87 30.4 5.39 18.4 0.81 31.1 4.69 16.9 0.67 26.4 2.98 12.6 0.66 25.2 2.08 Dark/76% RH /with oleic acid Jsc (mA/cm2) Voc (V) FF (%) PCE (%) 23.5 0.93 41.7 9.11 22.9 0.92 44.4 9.39 23.6 0.91 44.8 9.55 21.9 0.91 46.1 9.18 22.4 0.98 45.7 9.85

It is clear that the device with OA passivation has a more stable performance with aging over time. The recorded efficiencies are within the reported range of perovskite devices utilizing P3HT as HTL.35 P3HT was chosen as a dopant free HTL instead of Spiro-OMeTAD to avoid the additional oxidation processes that might interfere with the reliability and the behaviour of the aged solar cells.36 Furthermore, it is noted that the Voc, FF, and PCE values in the device with OA are slightly higher than the control device without OA. This may be attributed to the large alkyl chains of OA resulting in longer distance between interfaces that reduces the charge recombination rate.37 A previous study has found that OA can strongly affect the synthesis and assembly of SrTiO3 nanocuboids, especially the interparticle distance in the assembled two-dimensional 12 ACS Paragon Plus Environment

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arrays.38 Another study concluded that the added layer of OA created longer distance for the photogenerated electrons to reach and recombine at the hole transport layer interface.39 Hence, the electrons may be collected faster at the electron transport layer interface, the TiO2 layers in our case, and consequently increase the values of Voc and PCE. A similar behaviour was reported in a previous study that investigated the impact of doping and surface modification on the device interface engineering.40 The JV curves and performance parameters of the solar cells aged in dry conditions are reported in supporting information (Table S1 and Figure S1). To investigate in more detail the device performance, the normalized values of PCE, Jsc, Voc, and FF of the devices aged in different conditions are plotted against aging time in Figure 5. Only one device that showed degradation, which is the non-treated with OA stored in humid air. The PCE of the device has dropped

75% of its initial value in four weeks. Therefore, the impact of

OA passivation on the solar cell performance is clearly significant.

Figure 5 Normalized solar cell parameters of MAPbI3 solar cell with and without oleic acid (OA) aged in different conditions. Devices aged in dark dry air are labelled “dry”, 13 ACS Paragon Plus Environment


devices aged in dark humid air (76% RH) are labelled “wet”. (a) Normalized Jsc values. (b) Normalized Voc values. (c) Normalized FF values. (d) Normalized PCE values.

4. Conclusions In summary, despite their attractive properties, OMH perovskite materials have yet to be used in practical PV device applications largely due to their instability issues. Several techniques have been used to stabilize these films such as encapsulation and surface passivation using different hydrophobic molecules. However, it is still challenging to achieve stability and performance at the same time. In this work, we applied oleic acid as a surface passivating layer and significantly improved the stability. In addition to improved stability, the device performance was unchanged or even slightly enhanced compared to unpassivated devices when both were exposed to humidity. The improved stability may be attributed to passivation of Pb2+ and or CH3NH3+ defects on the surface of perovskite films with the carboxyl group of the oleic acid. This method can be potentially applied for commercial applications since it is low cost and easy to implement.

Acknowledgements This work was supported and partially funded by Cultural Affairs and Mission Sector in Egypt and by the Bay Area Photovoltaic Consortium, DOE prime award DE-EE0004946 and subaward 60965033-51077. The XRD experiments were performed by J. Hauser at the UC Santa Cruz X-ray Facility, supervised by S. Oliver and funded by the NSF DMR-1126845. JZZ is grateful to support from NASA (NNX15AQ01A) and UCSC Special Research Fund for financial support. 14 ACS Paragon Plus Environment

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We also acknowledge K. Hellier and L. Seymour for help with measurements and instruments.

Check Supporting information for details on control devices performances and extra information about device behaviour.

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Improved Stability of Organometal Halide ... - ACS Publications

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