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1 DOI: /NCHEM.2324 Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides Xiong Li 1, M. Ibrahim Dar 1, Chenyi Yi 1, Jingshan Luo 1, Manuel Tschumi 1, Shaik M. Zakeeruddin 1, Mohammad Khaja Nazeeruddin 1, Hongwei Han 2 and Michael Grätzel 1 1 Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland. 2 Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan , Hubei, People s Republic of China. NATURE CHEMISTRY 1

2 Index Experimental section Figure S1 Comparison of the morphology for the perovskite films containing 3-APPACl, 4-ABPACl and 5-APPACl deposited on the mp-tio2/fto substrates Figure S2 Characterization for the 4-ABPA-crosslinked perovskite films deposited on the mp-tio2/fto substrates...8 Figure S3 Structural characterization of 4-ABPA-crosslinked CH3NH3PbI3 nanocrystals prepared from the perovskite precursor solution containing 4-ABPACl..9 Figure S4 1 H NMR analysis of hydrogen bond formation between the P-OH protons in butylphosphonic acid 4-ammonium (4-ABPA) and 1,4-hexanebisphosphonic acid (DHP) with halide ions Figure S5 Comparison of the morphology for the 4-ABPA-crosslinked (a) and pristine (b) perovskite films deposited on the mp-tio2/fto substrates...11 Figure S6 Comparison of the morphology for the PbI2 films without (a) and with (b) the addition of 4- ABPACl deposited on the mp-tio2/fto substrates.12 Figure S7 Coordinative interaction between the titania surface and the phosphonic acid groups of 4- ABPACl during one-step spin-coating process.13 Figure S8 Investigation of the role of chloride in morphological control for the 4-ABPA-crosslinked perovskite film deposited on the mp-tio2/fto substrates...14 Figure S9 Hysteresis measurement..15 Figure S10 Temporal evolution of photovoltaic performance metrics of unsealed perovskite solar cells based on pristine CH3NH3PbI3 (green) and 4-ABPA-crosslinked CH3NH3PbI3 (red) films in the dark and ambient air at ~55% humidity..16 Figure S11 Temporal evolution of photovoltaic performance metrics of encapsulated perovskite solar cells based on pristine CH3NH3PbI3 (green) and 4-ABPA-crosslinked CH3NH3PbI3 (red) films aged under continuous 10 mw cm -2 UV-filtered simulated sun light soaking at 45 o C...17 Table S1. Photovoltaic parameters of a batch of ten devices based on pristine perovskite measured under 100 mw cm -2 simulated AM1.5G sunlight..18 Table S2. Photovoltaic parameters of a batch of ten devices based on 4-ABPA-crosslinked perovskite measured under 100 mw cm -2 simulated AM1.5G sun light.19 NATURE CHEMISTRY 2

3 Table S3. The initial and final photovoltaic parameters of a batch of four unsealed perovskite devices during the stability test in dark and ambient conditions at ~55 % humidity...20 Table S4. The initial and final photovoltaic parameters of a batch of four encapsulated perovskite devices during the long term light soaking test under 10 mw cm -2 UV-filtered simulated sun light at 45 o C.21 Table S5. The initial and final photovoltaic parameters of a batch of four encapsulated 4-ABPA-crosslinked perovskite devices measured under 10 mw cm -2 light intensity emitted from a white LED light source during heat stress test at 85 o C in the dark...22 NATURE CHEMISTRY 3

4 Experimental section Preparation of TiO2 paste. The TiO2 paste were prepared as following: A TiCl4 solution (Ti: %, Wako Pure Chemical Industries, Ltd.) was gradually added to an aqueous solution of ammonium carbonate ((NH4)2CO3, Wako Pure Chemical Industries, Ltd.). Then the mixture was stirred at 288K for 1 h. The resulting precipitate was filtered and washed repeatedly by deionized water. After the ph of the sol was 8, it was hydrothermally treated in a titanium autoclave at 503K for 13 h (heating rate 70 K/h). The dispersion medium of the resultant TiO2 colloidal suspensions was transferred from water to ethanol. The colloid was thoroughly dispersed using a high speed homogenizer by adding ethyl cellulose (Fluka) as a binder and α- terpineol (Wako Pure Chemical Industries, Ltd.) as a solvent for the TiO2 paste. An evaporator was used to concentrate the paste. Materials characterization: The mp-tio2 films were deposited on FTO glass slides and infiltrated with perovskite following the above-mentioned procedure. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 X-ray diffractometer with a graphite monochromator, using Cu-Kα radiation, at a scanning rate of 0.5 deg min -1. The average crystallite size of samples in the form of thin film was derived from the broadening of a peak in a diffraction pattern using Scherrer equation: Շ = (Kλ)/(βθ), where Շ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor with a value close to unity, with a value close to unity but varies with the actual shape of the crystallite; β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians; θ is the Bragg angle. The perovskite structures were examined by Osiris field-emission transmission electron microscope operating at an accelerating voltage of 200 kv, equipped with an Oxford energy-dispersive X-ray (EDS) detector. The TEM specimens of perovskites were prepared by slow evaporation of dilute solutions deposited on a Formvar-coated holey carbon copper grid under dry conditions. The powder obtained after scratching the film deposited by spin-coating method on mp- TiO2, or the nanocrystals after isolated from the mother liquid, was dispersed by sonication in toluene. A fieldemission scanning electron microscope (FESEM, Merlin) was employed to analyze the morphology of the samples. An electron beam accelerated to 3 kv was used with an in-lens detector. The UV-vis spectra were measured with the perovskite infiltrated mesoscopic TiO2 films supported by FTO glass using a PerkinElmer Lambda 950 spectrophotometer. NATURE CHEMISTRY 4

5 Device characterization. Current voltage characteristics were recorded by applying an external potential bias to the cell while recording the generated photocurrent with a digital source meter (Keithley Model 2400). The light source was a 450-W xenon lamp (Oriel) equipped with a SchottK113 Tempax sunlight filter (Praezisions Glas & OptikGmbH) to match the emission spectrum of the lamp to the AM1.5G standard. Before each measurement, the exact light intensity was determined using a calibrated Si reference diode equipped with an infrared cut-off filter (KG-3, Schott). IPCE spectra were recorded as functions of wavelength under a constant white light bias of approximately 5 mw cm -2 supplied by an array of white light emitting diodes. The excitation beam coming from a 300-W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd) and chopped at approximately 2 Hz. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). All measurements were conducted using a non-reflective metal mask with an aperture area of 0.16 cm 2 to cover the active area of the device and avoid light scattering through the sides. Indoor stability measurements in ambient condition at 55% relative humidity. Unsealed PSC devices were maintained in a box containing a white LED light source and filled with ambient air with 55% relatively humidity inside. Current-voltage curves were recorded by using a computer controlled electronic circuit system featuring a delta-sigma analog to digital converter. The electronic system is responsible for the measurements and maintains the cells at a specified electrical condition like open circuit or maximum power point. In this case, the open circuit condition was applied. The current noise of the measurement is 300 na, 50 µv for the potential and the gain error is < 0.2%. The current-voltage curves were recorded in reverse (from VOC to JSC) mode at a scan rate of 25 mv/s using step of 5 mv. The mask of 0.16 cm 2 aperture area was placed on top of the cell. Data were measured automatically on the cells at an interval of 4 hours. Each measurement comprises a set of two current-voltage curves, i.e. in the dark current and 10 mw cm -2 light intensity using white LED (no UV light). Between the measurements, the cells were maintained at the open circuit condition and stored in dark. Indoor stability measurements at 85 C in dark. PSC devices were covered with a thin glass sheet and encapsulated by Surlyn film to avoid penetration of moisture into its interior. The encapsulated device were maintained in a box with a glass cover that was flushed with a flow of argon at a rate of 20 ml/min. The goal was to simulate an anaerobic environment that is water exempt and to increase the reproducibility of the test conditions. This box was placed inside a larger housing which contains a temperature controller and a white LED light source. The temperature inside the box was maintained at 85 C. The measurement conditions were NATURE CHEMISTRY 5

6 applied the same as explained above. The current-voltage curves were recorded in reverse (from VOC to JSC) mode at a scan rate of 25 mv/s using step of 5 mv. The mask of 0.16 cm 2 aperture area was placed on top of the cell. The data were measured automatically on the cells at an interval of 2 hours. Indoor long term light soaking test. The back contact of the complete device was covered with a thin glass sheet. The latter was separated from the TCO front glass by a Surlyn spacer. The complete device was then sealed with epoxy resin to avoid penetration of moisture into its interior. During the this aging test, the encapsulated devices were continuously subjected to a 10 mw cm -2 UV-filtered simulated sun light at 45 o C in ambient air and maintained at the open circuit condition. They were removed at an interval of 24 hours from stability test and cooled over 30 mins to equilibrate at ambient temperature before recording the photovoltaic device performance metrics. Current-voltage curves were measured under simulated AM 1.5 solar light at 100 mw cm 2 intensity in reverse (from VOC to JSC) mode at a scan rate of 125 mv/s using step of 5 mv. The mask of 0.16 cm 2 aperture area was placed on top of the cell. NATURE CHEMISTRY 6

7 Figure S1 Comparison of the morphology for the perovskite films containing 3-APPACl, 4-ABPACl and 5-APPACl deposited on the mp-tio2/fto substrates. The alkyl chain length of the bifunctional ammonium cations exerted a considerable effect on controlling the perovskite film formation. We suggest that the 3-APPACl molecules featuring propyl groups are too rigid resulting in self-aggregation of the additives themselves in the perovskite precursor solution. On the contrary, the 5-APPACl carrying a pentyl chain is so flexible that the two end groups (ammonium and phosphonic acid functions) can bend in one direction and tend to locate at the surface of the same perovskite crystal rather than acting as a crosslinker binding the neighbouring crystals. NATURE CHEMISTRY 7

8 Figure S2 Characterization of the 4-ABPA-crosslinked perovskite films deposited on the mp- TiO2/FTO substrates. a, ATR-FTIR spectra. The pristine perovskite films deposited on the mp-tio2/fto substrates was used as the reference sample and has been subtracted for clarity. The presence of the 4-ABPACl in 4-ABPA-crosslinked perovskite films is confirmed by the appearance of the P-O stretching bands at 1066 cm -1 and 1122 cm -1 as well as the P=O stretching band at 1230 cm -1. b, STEM-EDS spectrum of the 4-ABPAcrosslinked perovskite grains with mp-tio2 scaffold scraped off the FTO substrate in STEM mode. c, XRD patterns of the mp-tio2/fto substrates deposited by pristine (control) and 4-ABPA-crosslinked perovskites as well as a blank mp-tio2/fto film (black trace). Inset is the comparison of the XRD patterns between control and MAPbI3@ABPA samples in most intense peak (110). NATURE CHEMISTRY 8

9 Figure S3 Structural characterization of 4-ABPA-crosslinked CH3NH3PbI3 nanocrystals prepared from the perovskite precursor solution containing 4-ABPACl. a, Powder XRD spectrum. b, STEM-EDX spectrum. c, The high-angle annular dark field (HAADF) of TEM image and elementary mapping of the P, Pb and I components. NATURE CHEMISTRY 9

10 Figure S4 1 H NMR analysis of hydrogen bond formation between the P-OH protons in butylphosphonic acid 4-ammonium (4-ABPA) and 1,4-hexanebisphosphonic acid (DHP) with halide ions. Because of the poor solubility of butylphosphonic acid 4-ammonium (4-ABPA) in organic solvent, such as DMF, the analogous more soluble diphosphonic acid DHP was utilized as the reference molecule to investigate the chemical shift of the P-OH protons in 4-ABPA upon interaction with halide anions. Left: 1 H NMR Spectra of phosphonic acid group in the reference compound 1,6-hexanebisphosphonic acid (DHP). Effect of the addition of halide anions in N,N-dimethylformamide-d7 (DMF-d7) at 295 K on the chemical shift of the P-O-H protons. Right: Representation of the hydrogen-bonding interaction based on halide anions (hydrogen-bonding acceptors) with phosphonic acid group (-PO(OH)2, hydrogen-bonding donor). As shown in Figure S4, the interaction between the phosphonic acid groups and the halide anion is immediately detected by a dramatic change in the 1 H NMR spectra of the DHP upon the addition of methylammonium chloride or methylammonium iodide (CH3NH3Cl or CH3NH3I). A dilute solution of DHP NATURE CHEMISTRY 10

11 (0.05 mm DHP in 1 ml DMF-d7) was titrated with up to 4 equivalents of CH3NH3Cl or CH3NH3I with concomitant downfield shifts in P-OH proton signals. Resonance signals for the P-OH protons appear at δ= 5.89 ppm in the presence of CH3NH3I and δ = 9.46 ppm in the presence of CH3NH3Cl, respectively. The chemical shift of the P-OH protons of the DHP (in the absence of halide anions) are found at δ = 5.08 ppm, indicating that the presence of iodide and chloride anions induced a downfield shift of Δδ = 0.8 ppm and 4.38 ppm, respectively. Compared to iodide ions, the chloride anions with higher electronegativity inductively withdraw much more electron density out of the P-OH protons, which in turn induce a more substantial downfield shift in resonance signals. Furthermore, after the addition of an excess amount of CsPbI3 precursor solution (0.5 mm) into the mixture of DHP in the presence of 4 equivalents of CH3NH3Cl, the iodide anions from iodoplumbate complex ion induce a upfield shift by Δδ = 3.45 ppm for the P-OH hydrogen signals. This suggests that the hydrogen-bonding interaction between P-OH and halide ions has change from O-H Cl - to O-H I - due to strong binding with iodide ions from CsPbI3. Figure S5 Comparison of the morphology for the (a) 4-ABPA-crosslinked and pristine (b) perovskite films deposited on the mp-tio2/fto substrates. The dense and fully covered film on mp-tio2 is only achieved by spin-coating the ABPA-CH3NH3PbI3 precursor solution. NATURE CHEMISTRY 11

12 Figure S6 Comparison of the morphology for the PbI2 films without (a) and with (b) the addition of 4-ABPACl deposited on the mp-tio2/fto substrates. The pristine PbI2 film is composed of plateletshaped PbI2 crystals that are between nm in diameter and fully cover the TiO2/FTO surface, while the grains in the film obtained with the addition of 4-ABPACl cohere with each together much more tightly and forms immediately continuous hills uniformly covering the entire substrate surface. The commercial PbI2 for 0.9 M pristine PbI2 solution was stirred in a DMF solvent at 60 C for 12 h. The 0.9 M PbI2 solution containing 4-ABPACl (denoted as ABPA-PbI2) was prepared as follows: 0.03 mm 4-ABPACl and 0.9 mm PbI2 were dissolved in 1ml DMF solvent and then stirred at 60 C overnight. The pristine PbI2 and ABPA- PbI2 were coated onto the mp-tio2/bl-tio2/fto substrate by spin-coating process at 6500 r.p.m for 5 s. After spin-coating, the pristine PbI2 and ABPA-PbI2 films were left to dry at room temperature in the dry box for 10 minutes, then annealed on a hotplate at 100 C for 5 mins. NATURE CHEMISTRY 12

13 Figure S7 Coordinative interaction between the titania surface and the phosphonic acid groups of 4- ABPACl during one-step spin-coating process. ATR-FTIR spectra for mp-tio2/fto substrate coated with 4-ABPACl. The mp-tio2/fto reference substrate without pre-treatment has been subtracted for clarity of presentation. The adsorption of 4-ABPACl onto TiO2 surface based on their strong coordinative interaction with surface titanium ions is confirmed by the appearance of the P-O stretching bands at 1036 cm -1 and 1078 cm -1 as well as the P=O stretching band at 1236 cm -1. Samples for ATR-FTIR were prepared as follows: 0.05 M 4-ABPACl solution in mixed GBL/DMF (4:6 V/V) solvent was loaded onto the mp-tio2/fto substrate by a consecutive two-step spin-coating process at 1,000 and 3,000 r.p.m for 10 and 15 s, respectively. After spin-coating, the film was washed by mixed GBL/DMF (4:6 V/V) solvent, then dried on a hotplate at 100 C for 30 mins in dry air atmosphere. NATURE CHEMISTRY 13

14 Figure S8 Investigation of the role of chloride in morphological control for the 4-ABPA-crosslinked perovskite film deposited on the mp-tio2/fto substrates. a, Surface SEM image of the perovskite film prepared by spin-coating a precursor solution containing CH3NH3Cl on the mp-tio2/fto substrate, b, Surface SEM images of the pristine perovskite films deposited on the mp-tio2/fto substrate. The perovskite precursor solution containing CH3NH3Cl was prepared as follows: 1.2 mm CH3NH3I, 0.05 mm CH3NH3Cl and 1.25 mm PbI2 were dissolved in 1ml GBL/DMF (4:6 v/v) mixed solvent and then stirred at 60 C overnight. As shown in Figure S8, compared to film morphology of the pristine perovskite, the perovskire films prepared by the precursor solution containing CH3NH3Cl did not show obvious improvement in the morphological control and exhibits incomplete coverage, indicating that the chloride in 4-ABPACl does not affect the crystal growth of plumbohalide perovskites in mesoscopic TiO2 scaffold, accordingly make little contribution to the improvement of the perovskite film quality. NATURE CHEMISTRY 14

15 Figure S9 Hysteresis measurement. a, Influence of scan rates on the current-voltage characteristics for a typical device based on ABPA-anchored perovskite. b, Current-voltage curves with reverse and forward scans for a typical cell constructed from pristine CH3NH3PbI3 films. Measurement in reverse (from VOC to JSC) and forward (from JSC to VOC) modes were taken under simulated AM1.5 solar light at 100 mw cm -2 intensity. NATURE CHEMISTRY 15

16 Figure S10 Temporal evolution of photovoltaic performance metrics of unsealed perovskite solar cells based on pristine CH3NH3PbI3 (green) and 4-ABPA-crosslinked CH3NH3PbI3 (red) films in the dark and ambient air at ~55% humidity. The cell performance was measured under 10 mw cm -2 light intensity emitted from a white LED light source. NATURE CHEMISTRY 16

17 Figure S11 Temporal evolution of photovoltaic performance metrics of encapsulated perovskite solar cells based on pristine CH3NH3PbI3 (green) and 4-ABPA-crosslinked CH3NH3PbI3 (red) films aged under continuous 10 mw cm -2 UV-filtered simulated sun light soaking at 45 o C. The cell performance was measured under simulated AM1.5 solar light at 100 mw/cm 2 intensity. 17 NATURE CHEMISTRY 17

18 Table S1. Photovoltaic parameters of a batch of ten devices based on pristine perovskite measured under 100 mw cm -2 simulated AM1.5G sunlight. The current-voltage curves were recorded at a scan rate of 125 mv s -1 using step of 5 mv in reverse (from VOC to JSC) mode. Cell no VOC [v] JSC [ma cm -2 ] FF PCE % Average 0.833± ± ± ±0.70 NATURE CHEMISTRY 18

19 Table S2. Photovoltaic parameters of a batch of ten devices based on 4-ABPA-crosslinked perovskite measured under 100 mw cm -2 simulated AM1.5G sun light. The current-voltage curves were recorded at a scan rate of 125 mv s -1 using step of 5 mv in reverse (from VOC to JSC) mode. Cell VOC [V] JSC [ma cm -2 ] FF PCE % Average 0.998± ± ± ±0.81 NATURE CHEMISTRY 19

20 Table S3. The initial and final photovoltaic parameters of a batch of four unsealed perovskite devices during the stability test in dark and ambient conditions at ~55 % humidity. The current-voltage curves were measured under 10 mw cm -2 light intensity emitted from a white LED light source Cell VOC [V] JSC [ma cm -2 ] FF PCE % 1 I I I I Pristine devices perovskite Average I 0.735± ± ± ± F F F F Average F 0.642± ± ± ± I I I ABPA-crosslinked perovskite devices 4 I Average I 0.800± ± ± ± F F F F Average F 0.764± ± ± ±0.56 I The initial photovoltaic parameters during the stability test over 1000 h. F The final photovoltaic parameters during the stability test over 1000 h. NATURE CHEMISTRY 20

21 Table S4. The initial and final photovoltaic parameters of a batch of four encapsulated perovskite devices during the long term light soaking test under 10 mw cm -2 UV-filtered simulated sun light at 45 o C. The currentvoltage curves were measured under 100 mw cm -2 simulated AM1.5G sunlight. Cell VOC [V] JSC [ma cm -2 ] FF PCE % 1 I I I I Pristine devices perovskite Average I 0.842± ± ± ± F F F F Average F 0.792± ± ± ± I I I ABPA-crosslinked perovskite devices 4 I Average I 1.025± ± ± ± F F F F Average F 1.008± ± ± ±0.54 I The initial photovoltaic parameters during the stability test over one week. F The final photovoltaic parameters during the stability test over one week. NATURE CHEMISTRY 21

22 Table S5. The initial and final photovoltaic parameters of a batch of four encapsulated 4-ABPA-crosslinked perovskite devices measured under 10 mw cm -2 light intensity emitted from a white LED light source during heat stress test at 85 o C in the dark. Cell VOC [V] JSC [ma cm -2 ] FF PCE % 1 I I I I Average I 0.762± ± ± ± F F F F Average F 0.664± ± ± ±0.33 I The initial photovoltaic parameters during the stability test over 350 h. F The final photovoltaic parameters during the stability test over 350 h. NATURE CHEMISTRY 22