Investigation of degradation mechanisms of perovskite-based photovoltaic devices using laser beam induced current mapping Zhaoning Song, Suneth C. Watthage, Adam B. Phillips, Geethika K. Liyanage, Rajendra R. Khanal, Brandon L. Tompkins, Randy J. Ellingson, and Michael J. Heben* Wright Center for Photovoltaics Innovation and Commercialization, School for Solar and Advanced Renewable Energy, Department of Physics and Astronomy, University of Toledo, Toledo, OH, USA 43606 ABSTRACT Solution processed thin film photovoltaic devices incorporating organohalide perovskites have progressed rapidly in recent years and achieved energy conversion efficiencies greater than 20%. However, an important issue limiting their commercialization is that device efficiencies often drop within the first few hundred hours of operation. To explore the origin of the device degradation and failure in perovskite solar cells, we investigated the spatial uniformity of current collection at different stages of aging using two-dimensional laser beam induced current (LBIC) mapping. We validated that the local decomposition of the perovskite material is likely due to interactions with moisture in the air by comparing photocurrent collection in perovskite devices that were maintained in different controlled environments. We show that the addition of a poly(methyl methacrylate)/single-wall carbon nanotube (PMMA/SWCNT) encapsulation layer prevents degradation of the device in moist air. This suggests a route toward perovskite solar cells with improved operational stability and moisture resistance. Keywords: Organometal halide, perovskite, solar cells, degradation, laser beam induced current (LBIC), single wall carbon nanotube (SWCNT), moisture resistance, stability
1. INTRODUCTION Recently emerged organometal halide perovskites are promising candidates for photovoltaic applications due to their excellent optoelectronic properties,[1-3] the potential for high power conversion efficiency,[4] and a low-temperature solution-based fabrication method that is industrially feasible.[5] Within the last few years, perovskite solar cells have experienced rapid improvements in device performance and reached a certified efficiency of 20.1%.[6] Despite the rapid development, significant challenges remain. One of the major concerns is that long-term stability of perovskite solar cells has yet to be demonstrated.[7] Substantial effort has been put into developing approaches to improve the long-term stability of perovskite solar cells. Results of devices utilizing advanced material preparation methods and device structures are promising. Over 500 h of high performance using solution-processed perovskite solar cells has been demonstrated.[8, 9] When the mesoporous TiO2 was replaced by Al2O3 scaffold, the perovskite devices were more resilient to UV light with an insignificant drop in photocurrent after 1000 h.[10] Compositional engineering of the perovskite absorber materials improves resistance to moisture and heat. Perovskites chemically tuned with iodide and bromide mixture demonstrated improved tolerance towards high humidity (>55%).[11] Modified perovskite materials with different organic cations, such as (5-AVA)x(MA)1-xPbI3[12] and FAPbI3 [4, 13] showed better thermal stability. Lastly, encapsulation can also protect perovskites against moisture. Using carbon nanotube/polymer composites as a hole collection layer dramatically improved the resistance to water.[14] Glass encapsulation using advanced edge sealing techniques recently reported device stability over 2000 h at 60 ºC without significant degradation.[15] In previous studies,[16-20] the degradation of perovskites was attributed to the water catalyzed decomposition reaction,[16] in which perovskite (CH3NH3PbI3) decomposes into robust lead iodide (PbI2) and volatile hydroiodic acid (HI) and methylamine (CH3NH2).[17] The latter two exist in the gas phase at room temperature due to the low boiling points (HI: -35.4 ºC and CH3NH2: -6 ºC) and release from the device once they formed.[18] The decrease of methylammonium iodide molar fraction results in phase segregation of PbI2 from perovskite.[19] Thus, in an open system, a small amount of water is sufficient to cause the irreversible decomposition of perovskite.[20] This proposed
*
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mechanism provides an explanation of perovskite material degradation but does not fully provide insight into the formation and evolution of microscopic failures. Here, we report a study on the stability and degradation of perovskite devices under different environmental conditions. By using spatially resolved laser beam induced current (LBIC) mapping to investigate degradation pathways of solution-processed perovskite solar cells, we are able to develop insight into the spatial uniformity of the quantum efficiency at different stages of aging.
2. EXPERIMENTAL DETAILS 2.1
Precursor materials
Various precursor solutions were prepared for the solution processing of perovskite solar cells. All chemicals were used as received from Sigma Aldrich unless specified. The precursor solution for compact titanium dioxide (c-TiO2) was prepared by dissolving 0.3 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol) in ethanol.[9] The precursor solution for mesoporous titanium dioxide (mp-TiO2) was prepared by diluting transparent TiO2 nanoparticle paste (18NR-T, Dyesol) with ethanol in a volume ratio of 2:7. Lead iodide (99.999%, Alfa Aesar) precursor solution was prepared by dissolving 1 M PbI2 in N,N-dimethlylformamide (DMF). Methylamine iodide was prepared by reacting hydroiodic acid (57 wt% in water) with methylamine solution (33 wt% in ethanol) in nitrogen atmosphere in a stirred ice bath for 2 hours.[9] The precipitate was washed three times using diethyl ether, dried in vacuum, and then dissolved in anhydrous iso-propanol (IPA) at a concentration of 0.25 M. The hole transport layer (HTL), the solution was composed of 15 mg/mL poly(3-hexylthiophene-2,5-diyl) (P3HT, Rieke Metals), 14 mM tert-butylpyridine (tBP), 2 mM lithium bis-(trifluoromethanesulfonyl)imide (Li-TFSI), and 0.13 M acetonitrile in dichlorobenzene.[14] For the single wall carbon nanotubes (SWCNTs) solution, approximately 1 mg SWCNTs (CoMoCAT) were dispersed in a 20 mL of 0.1 wt% sodium dodecylbenzene sulfonate (SDBS) aqueous solution through probe sonication for 1 h.[21] The encapsulating polymer solution was prepared by dissolving 50 mg/ml poly methyl methacrylate (PMMA) in toluene.[14]
2.2
Device fabrication
Perovskite solar cells were fabricated on fluorine doped tin oxide (FTO, Pilkington) glass substrates as reported previously[22] and the schematic of the device structures are depicted in Figure 1. A 60 nm c-TiO2 layer followed by a 400 nm mp-TiO2 layer were deposited sequentially on FTO by spin coating using their precursor solutions and annealed at 500 ℃ for 30 minutes. A 300 nm MAPbI3 perovskite absorber was then deposited on the mp-TiO2 layer via a two-step method,[23] consisting of sequential spin coatings of PbI2 and MAI solution followed by an annealing process at 150 ℃ for 5 min. Two types of device configurations were employed in this study. One was the conventional perovskite/P3HT configuration (Figure 1a) which employed a 50 nm spin-coated P3HT as the HTL. The other was the perovskite/SWCNT configuration (Figure 1b) using SWCNT/PMMA encapsulation. In this structure, SWCNT thin films (100 nm) were formed on mixed cellulose esters membranes (0.22 µm pore size, Zefon International) by vacuum filtration. After removing the membranes by soaking in acetone, the SWCNT films were deposited on perovskites via toluene-assisted floating transfer. A 200 nm PMMA layer was then spin-coated on SWCNTs to form an encapsulation layer. In both configurations, a 60 nm gold electrode was thermally evaporated to complete the device structure. Laser scribing was used to define individual device areas of 0.16 cm2.
Figure 1. Schematic of perovskite device structures using (a) P3HT and (b) SWCNT/PMMA.
2.3
LBIC measurement
LBIC measurement was performed using a laser system developed in-house (Figure 2). A light beam with adjustable power and a fixed size of 40 µm was generated by a 532 nm Nd:YAG laser operating at a repetition rate of 600 kHz. Computer controlled over-head galvanometers were used to steer and focus the beam on a desired location. A LabVIEW program was developed to scan the laser beam across perovskite solar cells at a speed of 1 mm/s and simultaneously collect the light induced current signal amplified by a Keithley 428 current amplifier through a high speed National Instruments data acquisition board (USB-6251) at an acquisition rate of 250 Hz. The light-induced current signals were converted into local quantum efficiencies using the equation:
η=
1240 × I × 100% Pin × λ (nm)
where I is the measured LBIC signal, Pin is the average laser power, and λ is the laser wave length (532 nm). Timeresolved LBIC was observed at different stages of aging to study the evolution of the LBIC quantum efficiency map.
Figure 2. Schematic of the laser system for the LBIC measurement.
2.4
Environmental testing conditions
An environmental testing apparatus (Figure 3a) was built to study the degradation of the perovskite solar cells under various conditions. Perovskite devices were kept in a closed sample chamber (Figure 3b) which was purged constantly by dry air which subsequently flowed through a gas bubbler. The relative humidity (RH) in the chamber was adjusted by heating the bubbler and monitored by a hydrometer. The temperature of the test device was controlled by an infrared radiant heater outside the sample box and monitored by a thermocouple attached to the sample. Four environmental conditions were studied: (1) a standard condition (25 ºC & 20% RH), (2) high temperature condition (50 ºC & 20% RH), (3) high humidity condition (25 ºC & 80% RH), and (4) high temperature and humidity condition (50 ºC & 80% RH). Three devices of each structure were tested under each of the four conditions.
Figure 3. (a) Schematic of an environmental testing apparatus and (b) photo of the sample chamber.
3. RESULTS AND DISCUSSION 3.1
Perovskite/P3HT devices
To investigate the impacts of temperature and humidity on the perovskite device stability, we examined the conventional perovskite/P3HT devices under four conditions (1 – 4 above). The devices had an initial average device efficiency of 10% with a short circuit current of 17 mA/cm2 and an external quantum efficiency of 70% at 532 nm.[22] Figure 4 shows how the average LBIC efficiency evolves with elapsed time for devices under each condition. Although the full experiments were completed in relatively short time periods (less than 340 h), the changes in the device stability are evident. The devices exposed to the low temperature and humidity (1) were the most stable but still showed a decrease in LBIC efficiency. Visual inspection showed no significant difference compared with a new device. The devices at elevated temperature and low humidity (2) experienced LBIC efficiency drops from 78% to 36% after 300 h. Despite the significant degradation, the devices were still functioning after the experiment was completed and demonstrated ~5% energy conversion efficiency under a simulated AM1.5 spectrum. In contrast, the devices in high humidity conditions (3 & 4) showed a rapid photocurrent loss in a short period of time. With the first 24 h, devices were significantly degraded and became yellow due to the conversion to PbI2. To quantify the rate of decomposition, we define the lifetime of the device, te as the point at which the LBIC efficiency was e-1 (0.37) of its initial value. In the high humidity conditions, the lifetimes of the devices were te = 24 h and te = 52 h at the high and low temperatures, respectively. In the low humidity conditions, the degradation processes were slow and the lifetimes of the devices were estimated to be te = 500 h at the high temperature and te > 1000 h at room temperature, as determined by a linear extrapolation. Although high temperature accelerated the degradation, the results indicate that device stability is more sensitive to moisture.
Figure 4. Stability tests of perovskite/P3HT devices under different environmental conditions.
To provide insights to the origin of the perovskite degradation and failure of the devices, the evolution of current collection efficiency was mapped using LBIC (Figure 5). Inhomogeneous current collection was observed in most of the devices. Some low-current collection spots are likely due to physical pin contact damages during the prior J-V measurement, while others may be related to non-uniform surface coverage of perovskite thin films. In the low humidity conditions (1 & 2), the device degradation in the first 48 h was not significant, and the perovskite materials gradually degraded after 100 h. The most vulnerable regions to degradation were the outside edges and randomly distributed low-current spots. Those regions with defects and irregular morphology have more surface exposure to ambience and, thus, are more prone to degrade. In comparison, high humidity led to much faster degradation with the dark brown perovskite rapidly decomposing into yellow PbI2 in 1 h. The degradation rapidly spread out over the whole device area and the LBIC efficiency dramatically decreased. It is likely that high temperature introduces the localized decomposition while high humidity leads to large-area decomposition.
Figure 5. Evolution of the LBIC mapping of the perovskite/P3HT devices in different conditions.
3.2
Perovskite/SWCNT devices
To protect perovskite thin films against moisture, we added a SWCNT/PMMA complex as the encapsulation layer.[14] In this device structure, the SWCNT network serves as a hole conducting material that connects the perovskite and gold electrodes, while the PMMA infiltrates into the SWCNT network and fills the pores to form an encapsulation layer that improve the resistance to water ingress. The preliminary devices showed an average efficiency around 7%, which is lower than the standard perovskite/P3HT devices. Due to the addition of the insulating PMMA, the open circuit voltage decreased and the series resistance significantly increased. However, those devices demonstrate better current collection uniformity and improved resistance to device degradation (Figures 6 & 7). The addition of SWCNT/PMMA encapsulation enhanced te for the devices from 24 to 300 h for the high temperature and 52 to 700 h for the low temperature condition. With the optimization of fabrication process and application of glass encapsulation, the lifetime is likely to improve further. The SWCNT/PMMA electrode offers high resilience against heat and moisture and shows a great potential toward perovskite solar cells with a long-term stability.
Figure 6. Evolution of the LBIC mapping of the perovskite/SWCNT devices under high humidity conditions.
Figure 7. Stability tests of perovskite/SWCNT devices under high humidity conditions.
4. CONCLUSION The evolution of spatial quantum efficiency mapping for perovskite solar cells under different environmental conditions was quantitatively evaluated using the LBIC mapping. The result shows that the presence of moisture is most responsible for the rapid device degradation due to the water catalyzed decomposition of perovskite materials. Improved stability in the high humidity conditions was demonstrated by adopting the SWCNT/PMMA encapsulation layer.
ACKNOWLEDGEMENTS The work was supported by the Air Force Research Laboratory, Space Vehicles Directorate (contract No. FA945311-C-0253) and the National Science Foundation (contract No. CHE-1230246).
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