In-situ Identification of Photo- and Moisture-Dependent Phase Evolution of Perovskite Solar Cell

Transcription

1 Supporting Information In-situ Identification of Photo- and Moisture-Dependent Phase Evolution of Perovskite Solar Cell Bo-An Chen, 1 Jin-Tai Lin, 1 Nian-Tzu Suen, 1 Che-Wei Tsao, 1 Tzu-Chi Chu, 1 Ying-Ya Hsu, 2 Ting-Shan Chan, 2 Yi-Tsu Chan, 1 Jye-Shane Yang, 1 Ching-Wen Chiu 1 and Hao Ming Chen 1 * 1 Department of Chemistry, National Taiwan University, Taipei 106, Taiwan 2 National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan *Corresponding author: Hao Ming Chen (haomingchen@ntu.edu.tw) Authors contributed equally to the work Method Materials. The solid chemicals were used as received without further purification while all the solvents were purified through distillation before used. The chemicals used in this work were listed below. Methylamine (CH 3 NH 2, 40% in water, Acros), hydroiodic acid (HI, 57 wt.% in water, Acros ), hydrochloric acid (HCl, 37% in water, Fisher), 2-propanol (IPA, anhydrous, 99.5%, Aldrich), diethyl ether (99.5%, for analysis, Acros), N,N-dimethylformamide (DMF, anhydrous, 99.8%, Aldrich), acetonitrile (ACN, anhydrous, 99.8%, Aldrich), chlorobenzene (CB, anhydrous, 1

2 99.8%, Aldrich), γ-butyrolactone (99%, ECHO), titanium (IV) isopropoxide (TTIP, 97%, Aldrich) lead iodide (PbI 2, 99%, Acros), Spiro-OMeTAD (Lumtec), Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI, 99.5%, Aldrich), 4-tert-butylpyridine (tbp, 96%, Aldrich). Synthesis of CH 3 NH 3 I and CH 3 NH 3 PbI 3. In this synthesis, hydroiodic acid (30 ml, mol) and methylamine (18.9 ml, mol) were reacted in ice bath (0 C) for 2 hrs with stirring. After reacting at 0 C for 2 hrs, the solution was evaporated at 50 C by rotary evaporator and orange color precipitate (crystal or powder) was obtained. Subsequently, the pure CH 3 NH 3 I could be acquired by washing the precipitate three times with diethyl ether and dried at 60 C under vacuum for 24 hrs. The as-prepared CH 3 NH 3 I (0.200 g) and PbI 2 (0.578 g) were mixed inγ-butyrolactone with stirring at 60 C for 6 hrs. The solution became transparent pale yellow and powder-form CH 3 NH 3 PbI 3 could be retrieved at 100 C under vacuum. Synthesis of CH 3 NH 3 PbI 3 H 2 O. In the synthesis, Pb(NO 3 ) 2 aqueous solution (1 ml, 0.2 M) was dropped into CH 3 NH 3 I aqueous solution (5 ml, 1.2 M) at room temperature. Upon the addition of Pb(NO 3 ) 2, bright yellow precipitates formed immediately. This precipitates were immersed in the mother liquor overnight which the yellow precipitates converted to pale yellow crystals by the time. These crystals were then dried by vacuum filtration and then were characterized by powder XRD. Solar cell fabrication and characterization. The FTO substrates (14 Ω/sq; substrate area ~ 2 cm x 2 cm) were cleaned by ultrasonication with soap (5% Hellmanex in water), deionized water, acetone and isopropanol for 20 mins and subsequently treated with UVO plasma for 20 mins. A compact 2

3 TiO 2 layer was prepared by spin-coating (2000 r.p.m, 60 s) the TTIP precursor solution (containing TTIP: 369 µl, IPA: 2.56 ml, M HCl solution: 2.56 ml) on FTO. This precursor layer was then dried at 180 C for 5 mins following deposition and sintered at 550 C for 30 mins. The perovskite layer was then deposited via fast deposition-crystallization procedure. The as-prepared CH 3 NH 3 I (0.200 g) and PbI 2 (0.578 g) were mixed in anhydrous N,N-dimethylformamide (1 ml) with stirring in 60 C for 2hr to form a transparent pale yellow CH 3 NH 3 PbI 3 (1.25 M) solution. The CH 3 NH 3 PbI 3 (80 µl) was first dropped onto the compact TiO 2 layer and spin-coated at 5000 rpm for 30 s. While the substrate was still under spinning, dropping chlorobenzene (200 µl) immediately as close as possible to the center of the substrate after 7 s since the starting of spin-coating. As the chlorobenzene was dropped, the color of perovskite film transformed into transparent gradually. The film then dried at 80 C for 10 mins and the color of the film became dark brown. The hole-transport layer was deposited by spin-coating (2000 r.p.m, 30 s) the spiro-meotad solution, which was composed of 80.0 mg ml -1 spiro-meotad, 28.5 µl ml -1 4-tert-butylpyridine, 17.5 µl ml -1 of Li-TFSL stock solution (520 mg Li-TFSL dissolved in 1 ml acetonitrile) and 1 ml chlorobenzene. After the hole-transport layer was deposited, the substrate was stored at a damp-proof cabinet overnight. In the final step, gold electrode (thickness: 100 nm) was thermal evaporated through a shadow mask (area: 0.3 cm x 0.3 cm per electrode) as back contact to complete the solar cell. In terms of sample preparation for cross-section SEM images and top-view SEM images, we took the samples with desired conditions to break into pieces. In that 3

4 case, we could take the cross-section SEM images through using these pieces. Furthermore, in the case of top-view image, the Spiro-MeOTAD layer was removed by using chlorobenzene to show the underlaying Pb-perovskite layer. The films shown in Figure 5 ware prepared with coverage of Spiro-MeOTAD, and all samples were identical to the cells that have aged in desired conditions. In-situ X-ray diffraction combined solar cell performance study. The X-ray diffraction of perovskite solar cell was conducted by using Bruker D2 Phaser diffractometer with a special home-made cell designed for the in-situ measurement. The devices were scanned from 5 to 60 with a 0.05 step size and s/deg acquisition time. The current-voltage measurements were operated with Keithley 2400 source meter and the scan rate of the I-V curves was 0.01V/step from 1.1V to 0.1V. The illumination was provided by using a 370 W Xenon lamp (Newport 66921) and equipped with an optical fiber. The output of light intensity from the optical fiber was carefully calibrated to one sun illumination (100 mw/cm 2 ) and the temperature on the surface of the device was roughly 50 o C after 1 hr illumination (measuring by infrared temperature sensor). The relative humidity in the home-made cell was controlled via bubbling the nitrogen gas into a hot water and then pass into the cell through gas in/out channel. Our in-situ X-ray diffraction combined solar cell performance experiment was done in the fashion that cell was kept exposing to one sun illumination (calibrated) while measuring the I-V curve and X-ray diffraction pattern hourly. In-situ X-ray absorption study. The X-ray absorption measurements of perovskite solar cell were performed utilizing synchrotron radiation. A special home-made cell was designed for the in-situ 4

5 measurement of the Pb L III -edge (13,035 ev) in BL01C1 workstation of National Synchrotron Radiation Research Center (NSRRC), Taiwan. The illumination was provided by using a 370 W Xenon lamp (Newport 66921) equipped with an optical fiber. The light intensity output from the optical fiber was calibrated to one sun illumination (100 mw/cm 2 ), and the relative humidity inside the home-made cell was controlled via bubbling the nitrogen gas into a hot water and then pass into the cell through gas in/out channel. 5

6 Figure SI-1. Full-spectrum Rietveld refinements of (a) α-ch 3 NH 3 PbI 3 phase and (b) β-ch 3 NH 3 PbI 3 phase. Note: The major structural difference between α-ch 3 NH 3 PbI 3 form and β-ch 3 NH 3 PbI 3 form is the I-Pb-I bond angle, which is smaller in β-ch 3 NH 3 PbI 3 form. Furthermore, the I-Pb-I bond angles could significantly influence the electronic structure and further led to different bandgaps for α-ch 3 NH 3 PbI 3 and β-ch 3 NH 3 PbI 3 forms (i.e., the smaller the I-Pb-I bond angle, the higher the bandgap of perovskite material). In terms of the X-ray diffraction patterns, the most distinct difference was the additional diffraction peak appeared at c.a. 24 in the sample of β-ch 3 NH 3 PbI 3 (as arrow indicated). 6

7 Figure SI-2. The hysteresis and air stability measurements of the prepared lead perovskite solar cell. 7

8 Figure SI-3. The phase transformation experiment between CH 3 NH 3 PbI 3 H 2 O and CH 3 NH 3 PbI 3 in the presence of spiro-meotad, and the X-ray diffraction patterns of β-ch 3 NH 3 PbI 3 was stored at 30 C for 24 hr, CH 3 NH 3 PbI 3 H 2 O powder, as-deposited β-ch 3 NH 3 PbI 3. Note: To verify the effect of moisture in dark condition, a β-ch 3 NH 3 PbI 3 layer was prepared and stored in desired humidity. The phase evolution of the β-ch 3 NH 3 PbI 3 layer was then monitored 8

9 with X-ray diffraction and was shown in Figure SI-3. Once the β-ch 3 NH 3 PbI 3 was deposited on FTO and exposed to moisture for 24 hour at 30 C, the X-ray diffraction pattern clearly indicated that CH 3 NH 3 PbI 3 underwent a structural transformation to CH 3 NH 3 PbI 3 H 2 O (red line). Several studies have revealed the correlation between lead-halide perovskite material and humidity, 1-7 and mentioned that CH 3 NH 3 PbI 3 reacted with water to form hydrate phases including (CH 3 NH 3 ) 4 PbI 6 2H 2 O and CH 3 NH 3 PbI 3 H 2 O. 5-7 In present study, only the mono-hydrate CH 3 NH 3 PbI 3 H 2 O was generated, potentially due to a different setup with a specific β-form of CH 3 NH 3 PbI 3. It was worth noting that this monohydrate phase could readily transform back to CH 3 NH 3 PbI 3 after 10 mins of heating at approximately 50 C. Upon heating, a perovskite phase of CH 3 NH 3 PbI 3 was formed again with accompanying a gradually disappearing of the CH 3 NH 3 PbI 3 H 2 O. This observation was in accordance with the reported result that CH 3 NH 3 PbI 3 H 2 O could transform back to CH 3 NH 3 PbI 3 by removing the water molecule to recover the cell performance. 5 In addition, we noticed that the crystallinity of the reformed CH 3 NH 3 PbI 3 layer was considerably lower than that of as-deposited β-ch 3 NH 3 PbI 3 layer. This implies that despite the hydration/dehydration process is reversible, the formation of structural defects is inevitable. However, we must note here that we did not observed CH 3 NH 3 PbI 3 H 2 O in our in-situ XRD experiment. The major difference between this observation and those in the in-situ experiment, we suspect, is the constant illumination condition. In the in-situ XRD experiment, the cell was illuminating constantly that can be regarded as heating effect. Therefore, it is reasonable to assume that the CH 3 NH 3 PbI 3 H 2 O still formed and caused the deterioration of the cell performance (stage 1). The nature of CH3NH 3 PbI 3 H 2 O, which it can transfer back to CH 3 NH 3 PbI 3 or decompose to PbI 2 could be a relatively fast process and accounts for its missing in the in-situ XRD experiment. Reference: [1] Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Nano Lett. 2013, 13, [2] Niu, G.; Guo, X.; Wang, L. J. Mater. Chem. A 2015, 3, [3] Niu, G.; Li, W.; Meng, F.; Wang, L.; Dong, H.; Qiu, Y. J. Mater. Chem. A 2014, 2, [4] Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Nano Lett. 2014, 14, [5] Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. Chem. Mater. 2015, 27, [6] Yang, J. L.; Siempelkamp, B. D.; Liu, D. Y.; Kelly, T. L. ACS Nano 2015, 9,

10 [7] Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. J. Am. Chem. Soc. 2015, 137, Figure SI-4. Time-dependent UV-vis measurement of prepared lead perovskite solar cell. Note: From the time-dependent UV-vis curve, it is obvious that the degradation of CH 3 NH 3 PbI 3 is directly related to declined absorption of the solar energy of the cell. Since CH 3 NH 3 PbI 3 is the light capture (absorbed) layer, this degradation (transformed to PbI 2, big drop around hrs in Figure SI-4) will decrease the amount of excited electron-hole pair and result in lowering Jsc of the cell. This is definitely one of the main causes of the reduced Jsc. This phase transformation will also inevitably introduce many defects (trap states) especially within the interface and therefore increase the rate of electron-hole recombination accordingly lower the J sc and V oc. 10

11 Figure SI-5. Contour plots of in-situ X-ray diffraction measurement and the corresponding cell performance along time evolution at R.H. 40 % condition. 11

12 Figure SI-6. (a) cell performance as function of time at R.H. 65 % condition without consecutively illuminating. All measured conditions were identical to those of Figure 2d except for consecutively illuminating, indicating that degradation of cell performance caused by X-ray damage could be ruled out. This degradation might be attributed to the formation of CH 3 NH 3 PbI 3 H 2 O and PbI 2 under moist condition. (b) XRD patterns of as-prepared CH 3 NH 3 PbI 3 layer and that was stored in the condition of R.H. 65% without covering spiro-meotad in the dark, showing that no PbIOH phase was present except for PbI 2 and CH 3 NH 3 PbI 3 H 2 O even though the perovskite cell was stored for more than 40 hours. 12

13 Figure SI-7. UV-vis spectrum of PbIOH. 13

14 Figure SI-8. The phase stability experiment of β-ch 3 NH 3 PbI 3 with and without spiro-meotad layer. The X-ray diffraction patterns suggested that β-ch 3 NH 3 PbI 3 would decompose to PbI 2 and PbIOH without and with spiro-meotad, respectively. Note: In order to discover the PbIOH observed in phase III and realize the key factor of cell life, CH 3 NH 3 PbI 3 and spiro-meotad/ch 3 NH 3 PbI 3 films were subject to exposure to the humid environment of R.H. 90%. Before exposing, two perovskite samples featured identical diffraction patterns (as-prepared sample w/ spiro-meotad), indicating that the deposition of spiro-meotad would not alter the phase of the underlaying CH 3 NH 3 PbI 3 layer. After exposing to the humidity condition of R.H. 90% for 20 hrs, the X-ray diffraction pattern obtained from pure perovskite film had shown a mixture phase of PbI 2 (major) and CH 3 NH 3 PbI 3 (minor) (red line). Indeed, once the 14

15 spiro-meotad/ CH 3 NH 3 PbI 3 film was exposed to a humid environment of R.H. 90%, a distinctly different pattern (blue line; R.H. 90% for 20 hrs w/ spiro-meotad) comparing to that of pure CH 3 NH 3 PbI 3 (red line) was observed. A reasonable assumption should be that the spiro-meotad might play an important role in the formation of PbIOH. The diffraction peaks clearly indicated PbI 2 and the PbIOH, which was consistent with the finding recognized in the in-situ X-ray diffraction experiment. It is noticed that the peak at around 22.5 (in blue line) seems to present a new phase. After carefully examining all possible phases, it is argued that this peak can be attributed to PbI 2. The reason that it does not appear in the pattern o as-synthesized PbI 2 (orange line) is probably due to the effect of prefer orientation. A reference XRD pattern for PbI 2 showing the diffraction peak at 22.5 could be found in the work of Leguy et al. (Chem. Mater. 2015, 27, 3397). Note that neither (CH 3 NH 3 ) 4 PbI 6 2H 2 O nor CH 3 NH 3 PbI 3 H 2 O could be found, suggesting that the transformation from hydrate to PbI 2 and/or PbIOH was irreversible in the presence of spiro-meotad. As a consequence, these findings demonstrated that deposition of spiro-meotad upon the CH 3 NH 3 PbI 3 layer could clearly dominate the structural transformation of CH 3 NH 3 PbI 3 under humid condition and facilitate the irreversible generation of PbI 2 or PbIOH. 15

16 Figure SI-9. The influence of Li-salt in the spiro-meotad toward phase degradation. 16

17 Note: The experimental results suggested that spiro-meotad was a critical factor for the formation of PbIOH. In terms of spiro-meotad, the Li-salt was commonly incorporated in spiro-meotad to increase its conductivity, suggesting that Li-salt might play a vital rule in spiro-meotad for this phase degradation behavior. In order to exclusively understand the influence of Li-salt, we also carried out the experiments of Li-salt effect onto spiro-meotad layer. It evidently elucidated that the phase transformation (to PbIOH) occurred regardless the present of Li-slat, yet we did observe that the Li-salt in spiro-meotad would facilitate the phase transformation. It was most likely due to the nature of Li-salt that could absorb more moisture and further induced this phase transformation (higher moisture level in the cell). In other words, the spiro-meotad layer enhances the degradation of the light absorber (CH 3 NH 3 PbI 3 ) in the presence of moisture, suggesting that an alternative hole-transport layer may be required for achieving a stable cell. This was consistent with recent studies, in which spiro-meotad-free architecture exhibited remarkable long-term reliability of lead-halide perovskite solar cell. Although the spiro-meotad could facilitate the formation of PbIOH phase at moist situation, the light illumination definitely was another factor to greatly promote the transformation rate from CH 3 NH 3 PbI 3 (with presence of spiro-meotad) to PbIOH while CH 3 NH 3 PbI 3 (with presence of spiro-meotad) was fairly stable toward photoexcitation. 17

18 Figure SI-10. XRD patterns of as-prepared CH 3 NH 3 PbI 3 layer with PMMA, P3HT and PTAA covering and those were stored in the condition of R.H. 65% and consecutively illuminating for 40 hours, showing that no PbIOH phase was present except for PbI 2. 18

19 Figure SI-11. Crystal structural evolution from CH 3 NH 3 PbI 3, CH 3 NH 3 PbI 3 H 2 O and PbI 2 to PbIOH. Note: The CH 3 NH 3 PbI 3 layer has proven to be very sensitive to moisture and can transform into CH 3 NH 3 PbI 3 H 2 O when exposing to moist conditions. This phase transformation is reversible, and the CH 3 NH 3 PbI 3 H 2 O can transfer back to CH 3 NH 3 PbI 3 through heat treatment. The reformed CH 3 NH 3 PbI 3 is characteristic of poor crystalline nature, which may explicate the decay of perovskite solar performance in stage I. We have to point out here that although both (CH 3 NH 3 ) 4 PbI 6 2H 2 O and/or CH 3 NH 3 PbI 3 H 2 O were found in other reports 23,33, however, only CH 3 NH 3 PbI 3 H 2 O intermediate was observed in present study. Reversible transition from CH 3 NH 3 PbI 3 H 2 O to CH 3 NH 3 PbI 3 is a relatively fast reaction, while CH 3 NH 3 PbI 3 H 2 O could further decompose to PbI 2 in the presence of moisture and/or illuminating (Figure SI-10b and SI-10c) and caused the decay of Jsc in stage II. Recently, Shirayama et al. (J. Appl. Phys. 2016, 119, ) suggested that the decomposition from CH 3 NH 3 PbI 3 to either CH 3 NH 3 PbI 3 H 2 O or PbI 2 was a competing reaction, and thence the direct transformation into PbI 2 was possible as well (Figure SI-10). In the presence of spiro-meotad, the transformation from PbI 2 to PbIOH seems irreversible and leads to instantly decreasing efficiency of perovskite solar cell to nearly zero (stage III). The moisture may further react with axial I of [PbI 6 ] 4- and release HI. This effect results in the formation of [PbI 4 O 2 ] 6- (Figure SI-10d, colored in green). Moreover, the ionic radius of oxygen anion (O 2- : 1.26 Å, Pauling scale) is significantly smaller than that of I anion (I - : 2.06 Å, Pauling scale), 44 and thence the deformation of [PbI 4 O 2 ] 6- octahedrons can be expected. These deformed [PbI 4 O 2 ] 6- octahedrons further connects with each other to become PbIOH. Comparing with other phases 19

20 mentioned here, the most prominent difference between PbIOH and other phases is a fact that the coordination number of Pb has increased from 6 to 8 (with one additional O and I attach to Pb and form [PbI 5 O 3 ] 9-, Figure SI-12). This has been corroborated by in-situ X-ray absorption measurement which reveal that the chemical environment of Pb is considerably different from that of other phases. Figure SI-12. Coordination Environment of PbI 2 ([PbI 6 ] 4- ) and PbIOH ([PbI 5 O 3 ] 9- ). 20

21 Table SI-1. Atomic coordinates and equivalent isotropic displacement parameters (B eq ) for PbIOH. PbIOH Atom Site x y z B eq Pb1 1b I1 2c O1 1b