Hysteresis Analysis Based on Ferroelectric Effect in Hybrid Perovskite ...
Hysteresis Analysis Based on Ferroelectric Effect in Hybrid Perovskite Solar Cells Jing Wei,†a Yicheng Zhao,†a Heng Li,a Guobao Li,c Jinlong Pan,c Dongsheng Xu,c Qing Zhao,a,b* and Dapeng Yua,b* a
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China. Email: [email protected]; [email protected] b
c
Collaborative Innovation Center of Quantum Matter, Beijing, China
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
Corresponding Author *E-mail: [email protected], [email protected].
Experimental Methods Solar cell device fabrication. All chemicals were purchased from Sigma-Aldrich or J&K Scientific Ltd. unless expressly stated. Spiro-MeOTAD was purchased from Yingkou OPV Tech New Energy Co. Ltd. CH3NH3I was synthesized according to reported procedure.1 In a typical synthesis, 33.77mL methylamine (33% in methanol) and 30mL of hydroiodic acid (57% in water) were reacting in 250mL round bottomed
flask at 0oC for 2h with stirring. The precipitate was recovered by evaporation at 50oC for 1 h. The product, methylammonium iodide CH3NH3I, was washed with diethyl ether by stirring the solution for 30 min, which was repeated three times, and then finally dried at 60oC in vacuum oven for 24 h. The photovoltaic devices were fabricated on fluorine-doped tin oxide (FTO) coated glass (Pilkington, Nippon Sheet Glass). First, laser-patterned, FTO-coated glass substrates were cleaned by ultrasonication in detergent water, rinsed with deionized water, acetone and ethanol, then subjected to an ultraviolet treatment for 5 min. Compact layers were deposited on the substrates by spin coating titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol) diluted in ethanol (1:30, volume ratio) and annealed at 45oC for 30 min. After cooling to room temperature, the substrates were treated in a 0.06 M aqueous solution of TiCl4 for 30 min at 70oC, rinsed with deionized water and dried at 450oC for 30 min. The mesoporous TiO2 layer composed of 20-nm-sized particles was deposited by spin coating TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol. For different scaffold thickness, the spin speed ranged from 2000 r.p.m. to 5000 r.p.m. And the concentration of TiO2 paste were 40%, 29%, 22%, respectively. After drying at 125oC, the TiO2 films were heated to 450oC, baked at this temperature for 30 min and cooled to room temperature. PbI2 and PbCl2 was dissolved in N,N-dimethylformamide (DMF) at a ratio of 0.8 M:0.4 M under stirring at 70oC. The solution was kept at 70oC during the whole procedure. The mesoporous TiO2 films were then infiltrated with the PbI2 and PbCl2
DMF solution by spin coating at 2500 r.p.m. for 20 s and dried at 70oC for 30 min. For different thickness of the perovskite layer, the spin speed range from 2000 r.p.m. to 8000 r.p.m.
The details to make four types of solar cells are listed on Table S1.
After cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2-propanol (10 mg/ml) for 20 s, rinsed with 2-propanol and anealed at 70oC for 1h. The HTM was then deposited by spin coating at 2,000 r.p.m. for 30 s. The spin-coating
formulation
was
prepared
by
dissolving
72.3
mg
2,2',7,7'-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), 30 µl 4-tert-butylpyridine (tBP) and 20 µl of a stock solution of 520 mg/ml lithium bis(trifluoromethylsulphonyl)imide(Li-TFSI) in acetonitrile in 1 ml chlorobenzene. Finally, 80 nm of gold electrodes were deposied on top of the devices by evaporation at ~10-6 bar. Table S1.
Fabrication details to make the four types of solar cells. Structures
TiO2 scaffold concentration spin speed
Perovskite layer PbI2:PbCl2
spin speed
without scaffold(heterojunction) thick capping layer
—
—
0.8M:0.4M
2000
22%
5000
0.8M:0.4M
3000
thin capping layer
29%
3000
0.8M:0.4M
6000
no capping layer(sensitized)
40%
2000
0.6M:0.3M
8000
Characterization and Electrical Measurement. The surface morphology of the electrode was characterized by scanning electron microscopy (SEM) (Nano430, FEI). The spectral optical properties were measured using a QTest Station 1000AD system (Crown Tech Inc.). The current−voltage (J−V) characteristics were obtained using an
Agilent B2900 Series precision source/measure unit, and the cell was illuminated by a solar simulator (Solar IV-150A, Zolix) under AM1.5 irradiation (100 mW cm−2). X-ray diffraction measurement. For XRD measurement, flat PbI2 and TiO2/PbI2 nanocomposites were deposited on glass slides using the above mentioned procedures. X-ray powder diagrams were recorded on an X’PertMPD PRO from PANalytical equipped with a ceramic tube (Cu anode, λ=1.54060A), a secondary graphite (002) monochromator
and
a
RTMS
X’Celerator
detector,
and
operated
in
BRAGG-BRENTANO geometry. The samples were mounted without further modification, and the automatic divergence slit and beam mask were adjusted to the dimensions of the thin films. A step size of 0.008 deg was chosen and an acquisition time of up to 7.5 min/deg. A baseline correction was applied to all X-ray powder diagrams to remove the broad diffraction peak arising from the amorphous glass slide. Raman and FTIR. For FTIR and Raman measurements, the hybrid perovskite material was deposited on glass slides using the above mentioned procedures, and the glass slides were used as the blank control group. For the FTIR, the instrument type we use is Bruker-VERPX80v FT-IR spectrometer at 4 cm-1resolution averaging over 256 scans. All measurements were conducted in vacuum at room temperature, and the signal we collect is the intensity of reflect light. The Raman scattering measurements were performed in a back-scattering geometry using the 514 nm line of a He-Ne laser for excitation. The Raman spectra are analyzed using a Horiba Jobin Yvon LabRAM HR
Evolution
spectrometer,
equipped
with
1800
gr/mm
gratings,
a
liquid-nitrogen-cooled CCD detector, and Brag Grate notch filters that allow for
measurements down to very low wave numbers. The Raman system is based on an optical microscope used to focus the excitation light. The sample is fabricated on the FTO glass as described above. The system has been calibrated against the 520.5 cm-1 line of an internal silicon wafer. The spectra have been registered in the 50-200 cm-1 range containing the Pb-I modes. The data is averaged by four times’ scan, in which the scan time and laser’s power is 60 s and 65 µw, respectively. The measurements were conducted at room temperature in air. All measurements guaranteed the non-degradation of these samples. Ferroelectric measurement. The devices are made of three parts: perovskite material, alumina insulating film, and gold electrode. The architecture is shown in Figure S2 in the Supporting Information. The ferroelectric hysteresis was measured with a Radiant Technologies Inc. Precision Premier II. We tried different measurement frequency and maximum applied voltage, getting the optimized measurement condition: 20 Hz and 15 V.
Figure S1. X-ray diffraction (XRD) data of perovskite material making the solar cells.
Figure
S2.
Device
architecture
(Glass/FTO/Au/Ti/Perovskite/Al2O3/Au)
for
ferroelectric E-P measurement. For the control device, there is no perovskite sandwiched between alumina layer and gold electrode. For the perovskite material device, due to the relatively low resistance of the perovskite material, it is hard to apply a large electric field cross it, so the insulating alumina here is to force the electron or hole accumulate in the boundary, leading to the energy band bending and get detectable signal.
Figure S3. Typical I-V curve of a normal diode, which is an exponential relationship between voltage and current. The current value is very small when the applied voltage is less than the open voltage Vopen, and this area is called high resistance area. If the applied voltage is larger than Vopen, the current value increases exponentially, and this area is low resistance area.
Figure S4. Different architecture with different potential distribution. (a)(b) Typical schematic diagram of architecture of planar type and sensitized type. Because perovskite material prepared by solution method usually has many pin-holes and tetragonal crystal particles, the planar architecture looks a little different from general planar type. (c)(d) their corresponding potential distributions. The arrows a/b/c in (c,d) point to three different typical sites with varied potential values.
We demonstrate the potential distribution between two different types of solar cells. In Fig. (c), x direction is parallel with FTO glass, and y direction is perpendicular to it in planar architecture solar cell. We draw a curve to show the potential distribution cross the p/n type material, along x-axis and y-axis, respectively. Electric field distributes over the perovskite material along y-axis in Fig (c), however, distributes over the hole transport material (HTM) and TiO2&FTO in Fig (d). Along x-axis, in Fig (c), the potential value is constant in either n-type or p-type material. However, it is not the same in sensitized architecture, of which the n-type and p-type as well as absorber intercalate each other, so the potential distribution shows periodical variation with small electric-field strength. This explains why the ferroelectric effect of perovskite has little influence on the material in sensitized architecture, since it is sandwiched between ETM and HTM, the built-in potential in the perovskite is very small, so the ferroelectric-state varies subtly. On the other hand, the collect efficiency is decided mainly by the distribution along y-axis, and when the applied voltage changes, it influences the distribution primarily along y-axis, not x-axis. In conclusion, I-V curves are mainly decided by electron transport material (ETM) and HTM in sensitized architecture, regardless of the ferroelectric effect in perovskite.
Discussion about hysteresis analysis based on defect states and excess ions migration For the discrepancy observed between forward and reverse scan, Snaith et al.2 hypothesized three possible origins: (1) a very large defect density at surface and interface; (2) excess ions throughout the film; (3) ferroelectric properties of the organometal trihalide perovskites. Here we give some discussions for origin (1) and (2), and point out some debates between these two origins and experimental results. For the origin (1), combined with the transient photoconductivity, 3 the relaxation time is not consistent with the normal measurement time (around seconds), because timescale is about milliseconds for the filling and hollowing these defect states. More importantly, if this timescale were seconds, sensitized solar cells with hybrid perovskite should have a larger discrepancy between forward and reverse scan than planar heterojunction solar cells in the I-V hysteresis, because we usually agree that there exist a larger portion of defects in sensitized architecture. However, all experiments reveal that sensitization architecture has a smaller discrepancy compared to planar type. For the origin (2), the formation energy of the interstitial defect is as large as 100 meV,4 it’s unlikely to have a large portion of excess ions in this material. Granted that these excess ions can migrate and then accumulate at the interface, this effect should have a disadvantageous effect regardless of scan direction. However, the increase in current value in the forward stable-state scan is not consistent with this inference. Additionally, migration can’t explain the high stability demonstrated by other work,5
because as the accumulation of these ions, the efficiency will be damaged after a long time.
References 1. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-319. 2. Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Grätzel, M.; White, T. J. Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3) PbI3 for solid-state sensitised solar cell applications. J. Mater. Chem. A 2013, 1, 5628-5641. 3. Frey, M. H; Payne, D. A. Grain-size effect on structure and phase transformations for barium titanate. Phys. Rev. B 1996, 3158. 4. Tohge, N.; Takahashi, S.; Minami, T. Preparation of PbZrO3–PbTiO3 ferroelectric thin films by the sol–gel process. J. Am.Ceram. Soc. 1991, 74, 67-71. 5. Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Grätzel, M.; Han, H. A hole-conductor–free, fully printable mesoscopic perovskite solar cell with high stability. Science, 2014, 345, 295-298.
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China. Email: [email protected]; [email protected] b
c
Collaborative Innovation Center of Quantum Matter, Beijing, China
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
Corresponding Author *E-mail: [email protected], [email protected].
Experimental Methods Solar cell device fabrication. All chemicals were purchased from Sigma-Aldrich or J&K Scientific Ltd. unless expressly stated. Spiro-MeOTAD was purchased from Yingkou OPV Tech New Energy Co. Ltd. CH3NH3I was synthesized according to reported procedure.1 In a typical synthesis, 33.77mL methylamine (33% in methanol) and 30mL of hydroiodic acid (57% in water) were reacting in 250mL round bottomed
flask at 0oC for 2h with stirring. The precipitate was recovered by evaporation at 50oC for 1 h. The product, methylammonium iodide CH3NH3I, was washed with diethyl ether by stirring the solution for 30 min, which was repeated three times, and then finally dried at 60oC in vacuum oven for 24 h. The photovoltaic devices were fabricated on fluorine-doped tin oxide (FTO) coated glass (Pilkington, Nippon Sheet Glass). First, laser-patterned, FTO-coated glass substrates were cleaned by ultrasonication in detergent water, rinsed with deionized water, acetone and ethanol, then subjected to an ultraviolet treatment for 5 min. Compact layers were deposited on the substrates by spin coating titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol) diluted in ethanol (1:30, volume ratio) and annealed at 45oC for 30 min. After cooling to room temperature, the substrates were treated in a 0.06 M aqueous solution of TiCl4 for 30 min at 70oC, rinsed with deionized water and dried at 450oC for 30 min. The mesoporous TiO2 layer composed of 20-nm-sized particles was deposited by spin coating TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol. For different scaffold thickness, the spin speed ranged from 2000 r.p.m. to 5000 r.p.m. And the concentration of TiO2 paste were 40%, 29%, 22%, respectively. After drying at 125oC, the TiO2 films were heated to 450oC, baked at this temperature for 30 min and cooled to room temperature. PbI2 and PbCl2 was dissolved in N,N-dimethylformamide (DMF) at a ratio of 0.8 M:0.4 M under stirring at 70oC. The solution was kept at 70oC during the whole procedure. The mesoporous TiO2 films were then infiltrated with the PbI2 and PbCl2
DMF solution by spin coating at 2500 r.p.m. for 20 s and dried at 70oC for 30 min. For different thickness of the perovskite layer, the spin speed range from 2000 r.p.m. to 8000 r.p.m.
The details to make four types of solar cells are listed on Table S1.
After cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2-propanol (10 mg/ml) for 20 s, rinsed with 2-propanol and anealed at 70oC for 1h. The HTM was then deposited by spin coating at 2,000 r.p.m. for 30 s. The spin-coating
formulation
was
prepared
by
dissolving
72.3
mg
2,2',7,7'-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), 30 µl 4-tert-butylpyridine (tBP) and 20 µl of a stock solution of 520 mg/ml lithium bis(trifluoromethylsulphonyl)imide(Li-TFSI) in acetonitrile in 1 ml chlorobenzene. Finally, 80 nm of gold electrodes were deposied on top of the devices by evaporation at ~10-6 bar. Table S1.
Fabrication details to make the four types of solar cells. Structures
TiO2 scaffold concentration spin speed
Perovskite layer PbI2:PbCl2
spin speed
without scaffold(heterojunction) thick capping layer
—
—
0.8M:0.4M
2000
22%
5000
0.8M:0.4M
3000
thin capping layer
29%
3000
0.8M:0.4M
6000
no capping layer(sensitized)
40%
2000
0.6M:0.3M
8000
Characterization and Electrical Measurement. The surface morphology of the electrode was characterized by scanning electron microscopy (SEM) (Nano430, FEI). The spectral optical properties were measured using a QTest Station 1000AD system (Crown Tech Inc.). The current−voltage (J−V) characteristics were obtained using an
Agilent B2900 Series precision source/measure unit, and the cell was illuminated by a solar simulator (Solar IV-150A, Zolix) under AM1.5 irradiation (100 mW cm−2). X-ray diffraction measurement. For XRD measurement, flat PbI2 and TiO2/PbI2 nanocomposites were deposited on glass slides using the above mentioned procedures. X-ray powder diagrams were recorded on an X’PertMPD PRO from PANalytical equipped with a ceramic tube (Cu anode, λ=1.54060A), a secondary graphite (002) monochromator
and
a
RTMS
X’Celerator
detector,
and
operated
in
BRAGG-BRENTANO geometry. The samples were mounted without further modification, and the automatic divergence slit and beam mask were adjusted to the dimensions of the thin films. A step size of 0.008 deg was chosen and an acquisition time of up to 7.5 min/deg. A baseline correction was applied to all X-ray powder diagrams to remove the broad diffraction peak arising from the amorphous glass slide. Raman and FTIR. For FTIR and Raman measurements, the hybrid perovskite material was deposited on glass slides using the above mentioned procedures, and the glass slides were used as the blank control group. For the FTIR, the instrument type we use is Bruker-VERPX80v FT-IR spectrometer at 4 cm-1resolution averaging over 256 scans. All measurements were conducted in vacuum at room temperature, and the signal we collect is the intensity of reflect light. The Raman scattering measurements were performed in a back-scattering geometry using the 514 nm line of a He-Ne laser for excitation. The Raman spectra are analyzed using a Horiba Jobin Yvon LabRAM HR
Evolution
spectrometer,
equipped
with
1800
gr/mm
gratings,
a
liquid-nitrogen-cooled CCD detector, and Brag Grate notch filters that allow for
measurements down to very low wave numbers. The Raman system is based on an optical microscope used to focus the excitation light. The sample is fabricated on the FTO glass as described above. The system has been calibrated against the 520.5 cm-1 line of an internal silicon wafer. The spectra have been registered in the 50-200 cm-1 range containing the Pb-I modes. The data is averaged by four times’ scan, in which the scan time and laser’s power is 60 s and 65 µw, respectively. The measurements were conducted at room temperature in air. All measurements guaranteed the non-degradation of these samples. Ferroelectric measurement. The devices are made of three parts: perovskite material, alumina insulating film, and gold electrode. The architecture is shown in Figure S2 in the Supporting Information. The ferroelectric hysteresis was measured with a Radiant Technologies Inc. Precision Premier II. We tried different measurement frequency and maximum applied voltage, getting the optimized measurement condition: 20 Hz and 15 V.
Figure S1. X-ray diffraction (XRD) data of perovskite material making the solar cells.
Figure
S2.
Device
architecture
(Glass/FTO/Au/Ti/Perovskite/Al2O3/Au)
for
ferroelectric E-P measurement. For the control device, there is no perovskite sandwiched between alumina layer and gold electrode. For the perovskite material device, due to the relatively low resistance of the perovskite material, it is hard to apply a large electric field cross it, so the insulating alumina here is to force the electron or hole accumulate in the boundary, leading to the energy band bending and get detectable signal.
Figure S3. Typical I-V curve of a normal diode, which is an exponential relationship between voltage and current. The current value is very small when the applied voltage is less than the open voltage Vopen, and this area is called high resistance area. If the applied voltage is larger than Vopen, the current value increases exponentially, and this area is low resistance area.
Figure S4. Different architecture with different potential distribution. (a)(b) Typical schematic diagram of architecture of planar type and sensitized type. Because perovskite material prepared by solution method usually has many pin-holes and tetragonal crystal particles, the planar architecture looks a little different from general planar type. (c)(d) their corresponding potential distributions. The arrows a/b/c in (c,d) point to three different typical sites with varied potential values.
We demonstrate the potential distribution between two different types of solar cells. In Fig. (c), x direction is parallel with FTO glass, and y direction is perpendicular to it in planar architecture solar cell. We draw a curve to show the potential distribution cross the p/n type material, along x-axis and y-axis, respectively. Electric field distributes over the perovskite material along y-axis in Fig (c), however, distributes over the hole transport material (HTM) and TiO2&FTO in Fig (d). Along x-axis, in Fig (c), the potential value is constant in either n-type or p-type material. However, it is not the same in sensitized architecture, of which the n-type and p-type as well as absorber intercalate each other, so the potential distribution shows periodical variation with small electric-field strength. This explains why the ferroelectric effect of perovskite has little influence on the material in sensitized architecture, since it is sandwiched between ETM and HTM, the built-in potential in the perovskite is very small, so the ferroelectric-state varies subtly. On the other hand, the collect efficiency is decided mainly by the distribution along y-axis, and when the applied voltage changes, it influences the distribution primarily along y-axis, not x-axis. In conclusion, I-V curves are mainly decided by electron transport material (ETM) and HTM in sensitized architecture, regardless of the ferroelectric effect in perovskite.
Discussion about hysteresis analysis based on defect states and excess ions migration For the discrepancy observed between forward and reverse scan, Snaith et al.2 hypothesized three possible origins: (1) a very large defect density at surface and interface; (2) excess ions throughout the film; (3) ferroelectric properties of the organometal trihalide perovskites. Here we give some discussions for origin (1) and (2), and point out some debates between these two origins and experimental results. For the origin (1), combined with the transient photoconductivity, 3 the relaxation time is not consistent with the normal measurement time (around seconds), because timescale is about milliseconds for the filling and hollowing these defect states. More importantly, if this timescale were seconds, sensitized solar cells with hybrid perovskite should have a larger discrepancy between forward and reverse scan than planar heterojunction solar cells in the I-V hysteresis, because we usually agree that there exist a larger portion of defects in sensitized architecture. However, all experiments reveal that sensitization architecture has a smaller discrepancy compared to planar type. For the origin (2), the formation energy of the interstitial defect is as large as 100 meV,4 it’s unlikely to have a large portion of excess ions in this material. Granted that these excess ions can migrate and then accumulate at the interface, this effect should have a disadvantageous effect regardless of scan direction. However, the increase in current value in the forward stable-state scan is not consistent with this inference. Additionally, migration can’t explain the high stability demonstrated by other work,5
because as the accumulation of these ions, the efficiency will be damaged after a long time.
References 1. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-319. 2. Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Grätzel, M.; White, T. J. Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3) PbI3 for solid-state sensitised solar cell applications. J. Mater. Chem. A 2013, 1, 5628-5641. 3. Frey, M. H; Payne, D. A. Grain-size effect on structure and phase transformations for barium titanate. Phys. Rev. B 1996, 3158. 4. Tohge, N.; Takahashi, S.; Minami, T. Preparation of PbZrO3–PbTiO3 ferroelectric thin films by the sol–gel process. J. Am.Ceram. Soc. 1991, 74, 67-71. 5. Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Grätzel, M.; Han, H. A hole-conductor–free, fully printable mesoscopic perovskite solar cell with high stability. Science, 2014, 345, 295-298.
