DOI: 1.138/NMAT415 Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices Zhengguo Xiao 1,2, Yongbo Yuan 1,2, Yuchuan Shao 1,2, Qi Wang, 1,2 Qingfeng Dong, 1,2 Cheng Bi 1,2, Pankaj Sharma 2,3, Alexei Gruverman 2,3 and Jinsong Huang 1,2 * 1 Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-656, USA. 2 Nebraska Center for Materials, Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-298, USA. 3 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-299, USA *Correspondence to: jhuang2@unl.edu -2-4 -6-8 -1..5.1.15 NATURE MATERIALS www.nature.com/naturematerials 1 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 Figure S1. J-V characteristics of the as-prepared vertical structure devices with the structure of (ITO)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) /Perovskite (3 nm) /Au. Scanning rate was.14 V/s and the scanning direction was labeled in the figure. Fig. S1 shows current density (J)-voltage (V) characteristics of the as-prepared device with 3 nm perovskite layers. The arrow in the figure depicts the scanning direction. Here, the scanning direction for J-V characteristics was from zero to positive voltage to rule out the possibility for a high J SC caused by the un-intentional poling process during device test. It s impressive that the device with gold electrode, which usually serves as a contact for hole-only device, has a high short current density (J SC ) of 8.2 ma/cm 2. The result indicates the junction has been partially formed before poling. The reason for junction formation before poling should be related to different interface structures between perovskite/pedot:pss and perovskite/au contacts, because the J SC from the as-fabricated device using gold as both anode and cathode is much smaller. 2 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION a Current density(ma/cm 2 ) c V OC (V) 2 1-1 -2-3 -2-1 1 2 3.8.6.4 -.4 -.6 -.8.5 V/s.1 V/s.25 V/s Forward scanning Reverse scanning.5.1.15.2.25 Scanning rate (V/s) b J SC (ma/cm 2 ) d 4 3 2 1-1 -2.5 V/s.1 V/s.25 V/s -3-2 -1 1 2 3 25 2 15-15 -2-25 Forward scanning Reverse scanning.5.1.15.2.25 Scanning rate (V/s) Figure S2. Dark current (a) and photocurrents (b) of the vertical structure devices with the structure of ITO/PEDOT:PSS /Perovskite (3 nm)/au tested under different scanning rate and directions.(c-d) V OC (c) and J SC statistics based on five devices with different scanning rates from.5 V/s to.25 V/s. Forward scanning refers scanning from negative to positive bias. In all the measurements, ITO was connected as the cathode and Au was connected as the anode. Fig. S2 shows the dark current and photocurrent hysteresis of the same ITO/PEDOT:PSS /Perovskite (3 nm)/au vertical structure device with different scanning rates and directions. The arrows in the figure depict the bias scanning directions. A large memristive effect from the NATURE MATERIALS www.nature.com/naturematerials 3 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 devices was observed because the dark- and photocurrent curves have a strong dependence on the scanning history. a c 6 4 2 2 4 6 8 1 Time (s).8.4. -.4 -.8 +2.5 V poling b -8 1 2 3 4 5 6 Time (s) Poling bias= -1V Measured device V OC -1.2 8.4 s 11.3 s Accumulated.2 s 1.5 s 4.2 s poling time -1.6 5 1 15 2 25 Time (s) -2-4 -6-2.5 V poling - + - + + - + - Figure S3. a-b, Dark current of the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure devices during positive and negative poling process, c, Dynamic poling process of the ITO/PEDOT:PSS/Perovskite/Au vertical structure device with 1,15 nm thick perovskite layer using -1. V pulse. As shown in Fig. S3 a-b, in the positive poling (+2.5 V) process, the current density is very small at the beginning of the poling process, which corresponds to the reverse-biased dark current of the n-i-p structure device (region I). The current increased quickly with increased 4 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION poling time, and a current peak showed up, which should be due to the ion motion in the perovskite under reverse bias (region II). The current begun to reduce after tens of seconds poling due to the depletion of easily mobile ions, and the current came to a plateau (region III) with high current density, indicating the perovskite film was switched to p-i-n structure and the device worked at forward bias with a large injection current. The negative poling showed a short poling time than positive poling, indicating the non-symmetrical composition/morphology profile along the vertical direction. Fig. S3c shows the dynamic poling process of the device with 1,15 nm thick perovskite layer. Here a train of -1. V pulses with different width were applied on the device, after which the V OC of the device was measured. The accumulated poling time, poling bias and measured device V OC were also marked in the figure. As shown in the figure, the device was switched after 11.3 s accumulated poling. Normalized J SC (a.u.) 1..5. -.5-1. J SC (After negative poling) J SC (After positive poling) 1 1 1 Time (h) 5 NATURE MATERIALS www.nature.com/naturematerials 5 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 Figure S4. The stability test of short circuit current for the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure device after positive and negative poling for 2 s at ±2 V. (The J SC were normalized by the value measured 24 h after the fabrication of the device (18.2 ma/cm 2 and -19. ma/cm 2 ). Fig. S4 shows the stability test result of the short circuit photocurrent for devices after positive and negative poling. The photocurrent direction after poling remained unchanged after two months. Two devices were negatively and positively poled, respectively, under a steady bias of 2.5 V for about ten seconds. After that, the devices were kept under ambient illumination in a glove box and no further poling was applied during the test. In order to eliminate the influence of scanning voltage, we only test the J SC to identify the stability of current direction. As illustrate in the figure, the J SC for both directions changed a small rate of ~1% for the first 1 hours. It s amazing no obvious reduction was identified for device with negative current after 1 hours. 6 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION a c Count 12 9 6 3-3 -6 Negative poling -9-1. -.5..5 1. 12 Negative poling 1 8 6 4 2-1. -.5..5 1. V OC (V) b J SC (ma/cm 2 ) d 2 15 1 5 Negative poling -5-1 -15-2 -1. -.5..5 1. V OC (V) 7 6 Negative poling 5 4 3 2 1 2 1-1 -2 J SC (ma/cm 2 ) Count Figure S5. Performance variation of the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure devices. Scanning rate was.14 V/s. Photovoltaic performance statistics of the vertical structure devices. The vertical structure devices were poled at 6 V/µm for 2 s. The photocurrents were measured under 1 sun illumination at a sweep rate of.14 V/s, b, Distribution of the photovoltaic performance of vertical structure devices in the J SC -V OC coordinate. c, V OC distribution of the poled vertical structure devices; d, J SC distribution of the poled vertical structure device; It is found that there is a large variation for the performances of the ITO/PEDOT:PSS /Perovskite (3 nm)/au vertical structure devices. Some devices showed larger photocurrent NATURE MATERIALS www.nature.com/naturematerials 7 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 while some others showed larger V OC. Fig. S5 shows one device with large switchable photovoltage between.87 V and -.75 V. a 2 Negative poling 15 1 MAPbI x Cl 3-x 5-5 -1-15 -2 -.6 -.4 -.2..2.4.6 c 9 6 3-3 -6 b Negative poling 2 15 1 5-5 -1-15 Negative poling HC(NH2) 2 PbI 3 -.4 -.2..2.4.6 MAPbBr 3 -.4 -.2..2.4 Figure S6. Switchable photovoltaic effect of the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure devices with different perovskite materials, a, CH 3 NH 3 PbI 3-x Cl x, b, HC(NH 2 ) 2 PbI 3 and c, CH 3 NH 3 PbBr 3. The devices were scanned between ±2.5 V at a scanning rate of.14 V/s. 8 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION In order to confirm the ion drift induced photovoltaic switching mechanism, we examined other three organolead trihalide perovksite materials, CH 3 NH 3 PbI 3-x Cl x, HC(NH 2 ) 2 PbI 3 and CH 3 NH 3 PbBr 3 (Fig. S6). It is found all the devices with these materials as active layers showed field switchable photovoltaic behavior. a 2. 15. 1. 5.. Negative poling Pt cathode -5. -2-1. -4-15. Positive Poling -6-2. -1. -.5..5 1. -1. -.5..5 1. Ga cathode c 6. 4. 2.. -2. -4. -6. -8. 6 4 2 Negative poling Negative poling -.4 -.2..2.4.6 b Ni cathode Figure S7. Switchable photovoltaic effect of the ITO/PEDOT:PSS/Perovskite (3 nm)/metal vertical structure devices with different metal contacts of Pt, Ni and Ga. The devices were scanned between ±2.5 V at a scanning rate of.14 V/s. In order to examine whether gold is required for the switchable OTP devices, we tested other metals like aluminum (Al), silver (Ag), platinum (Pt), gallium (Ga) and nickel (Ni) using the 9 NATURE MATERIALS www.nature.com/naturematerials 9 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 vertical device structure. Metal electrodes including Al, Ag, Pt, were deposited by thermal evaporation or sputtering, metal electrode of Ga was formed by directly drop the Ga liquid on perovskite films, and metal electrode of Ni was formed using Ni conductive tapes. None of the devices were optimized to have high performance. The devices with inert metal electrodes like Pt and Ni, and low work function metal of Ga also show switchable photovoltaic effect as shown in Fig. S7. It was found that the thermally-evaporated Al and Ag reacted with the perovskite because the deposited electrodes were black without metallic color, and the device performance did not make any sense. The difference in short circuit current density can be explained by the different active materials, electrode materials, and contacts between the electrodes and the perovskite layers, because of the different electrode materials used and different fabrication methods to form these electrodes. J SC (ma/cm 2 ) a.1.5. -.5 -.1 Lateral structure devices Negative poling -1. -.5..5 1. V OC (V) b Count c Count 1 Negative poling 8 6 4 2-1. -.5..5 1. V OC (V) 6 Negative poling 4 2.1.5. -.5 -.1 J SC (ma/cm 2 ) 1 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION Figure S8. Photovoltaic performance statistics of the lateral structure devices. The lateral structure devices were poled under 1.2 V/µm for 1 s, and photocurrent were measured under a quarter sun illumination with a sweep rate of.5 V/s. a, distribution of the photovoltaic performance of lateral structure devices in the J SC -V OC coordinate. b, V OC distribution of the poled lateral structure devices; c, J SC distribution of the poled lateral structure devices. 1 1-1 1-2 1-3 1-4 d=8 m Normalized J (a.u.) 1 d=5 m 1-1 1-2 1 d=1 m 1-1 1-2 -1. -.5..5 1. 1.5 2. Figure S9. J-V characteristics of the Au/perovskite (3 nm)/au lateral structure devices with different electrode spacing. The devices were poled under 1.2 V/µm for 1 s, and photocurrent were measured under a quarter sun illumination with a sweep rate of.5 V/s. Fig. S9 shows the normalized photocurrent curves with respect to the electrode spacing in the Au/perovskite/Au lateral structure devices. In contrast to ferroelectric photovoltaic devices NATURE MATERIALS www.nature.com/naturematerials 11 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 whose V OC is proportional to the electrode spacing, the V OC of single lateral structure device keeps almost constant at around.5 V regardless the electrode spacing variation from 8 µm to 1 µm. This result can exclude the contribution of potential ferroelectric property of the perovskite, if it has, to the switchable photovoltaic effect observed. a 4 3 Remanent Polarization b 4 3 Remanent Polarization Polarization ( C/cm 2 ) 2 1-1 -2 Polarization ( C/cm 2 ) 2 1-1 -2 c -3-4 Scanning rate:.8v/s at room temperature -2-1 1 2 d -3-4 Scanning rate:.8v/s at 77K -2-1 1 2 e f 12 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION Figure S1. Ferroelectric polarization loops measured at room temperature (a) and at 77 K (b) scanned at the same frequency of photovoltaic switch process. c, Piezoresponse force microscopy (PFM) topology (c), amplitude (e) and phase (f) images of the perovskite (3 nm). d, Representative PFM hysteresis loops (phase and amplitude) signal for any location on the film surface. Theoretical calculation predicted ferroelectric property of MAPbI 3 with spontaneous polarization of 38 µc/cm 2 (1). We measured the ferroelectric hysteresis using the Precision Premier Ⅱ from the Radiant technologies, Inc.. However we did not find any ferroelectric polarization within the measurement range of the equipment from these devices both at room temperature and at 77 K and using the same frequency of photovoltaic switch process. This results further exclude the contribution of potential ferroelectric property of the perovskite, if it has, to the switchable photovoltaic effect observed. 13 NATURE MATERIALS www.nature.com/naturematerials 13 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 Normalized J SC (a.u.) 1..8.6.4.2. -.2 -.4 -.6 Negive poling -.8-1. 1 2 3 4 Time (s) Figure S11. J SC output of the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure devices after negative and positive poling with time. The J SC were normalized by the value measured at the starting point. The devices were poled at 6 V/µm for 2 s. Fig. S11 shows the normalized J SC output measured overtime. It is obvious that, after positive or negative poling, the device can output persistent photocurrent under light, which excluded the contribution of charge traps to the switchable photovoltaic effect. 14 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION a + poling perovskite b 12.1 nm - poling perovskite 134.6 nm Au Au -95.4 nm -111.6 nm c + poling perovskite d + poling perovskite Figure S12. AFM topography image (a), surface potential image (c) and adhesion image (d) of the perovskite film after positive poling. AFM topography image of another film after negative poling (b). The Au electrode in topography image shown in Fig. S12 is not clear because the thickness of the Au electrode is only about 5 nm, much smaller than the roughness (~15 nm) of the perovskite thin films formed on glass. Nevertheless, we observed a clear difference of Au region and perovskite region as well as the separation line between these two regions by examining the adhesion mapping recorded during AFM scanning. The adhesion mapping of the exact same area of Fig. 3e is shown in Fig. S12d. Adhesion is defined as the minimum tension ( pull-off ) forces 15 NATURE MATERIALS www.nature.com/naturematerials 15 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 required for retracting the AFM tip from the sample surface. Many different types of interaction forces, mainly van der Waals and/or electrostatic force, attribute to adhesion. Because the interaction between tips with different materials have different adhesion, which constitutes the contrast, this technology has been applied to a wide variety of materials to investigate the surface heterogeneity or material distributions. As shown in Fig. 3e and Fig. S12d, the adhesion mapping agrees well with the KPFM mapping. Figure S13. A SEM image of a Au/perovskite/Au lateral structure device after poling It shows the morphology change of perovskite after poling near the anode area. Compared to the area far away from the anode, the area close to the Au electrode showed a lot of pin-holes formed. 16 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION d f a -1-15 2-1.5V pulse Thickness 3 nm at RT 1-1 -2 2-2.5V pulse 1-1 -2 2-4. V pulse 1-1 -2 5 1 15 2 Time (S) 5 Solvent annealing -2.5 V pulse -5-1 -15-2 5 Thermal annealing -5 e 1µm 1µm -2 5 1 15 2 Time (s) b c J SC (ma/cm 2 ) 2 Thickness: 3 nm -2.5 V pulse At RT 1 25 Temperature of measurement: 2 RT 6 o C 15 1 5-5 -1 Film treatment: Thermal annealing -15 Solvent annealing -2 Thermal annealing -25 3 6 9 12 15 Accumulated poling time (s) Figure S14. Dynamic poling process of the ITO/PEDOT:PSS/Perovskite (3 nm)/au vertical structure devices at (a) varied electrical field, (b-c) temperature and (d-f) with different film morphology. The thickness of the films in d-f is 1,15 nm. (g) J SC versus poling time of the devices with different film annealing processes and measurement temperatures. The devices were measured under.1 sun when measured at 6 C. The error bar showed the device performance -1-2 2 1-1 -2 At 6 o C 5 1 15 2 Time (s) 1. Negative poling.5 -.5 g 1.5. 275 K -.4 -.2..2.4.5 Negative poling. -.5 25 K -.4 -.2..2.4 2 1-1 -2 J SC (ma/cm 2 ) NATURE MATERIALS www.nature.com/naturematerials 17 214 Macmillan Publishers Limited. All rights reserved.
DOI: 1.138/NMAT415 variation based on the statistics of five devices of each category. And the scanning rate in Fig. S14c was.14 V/s. The drift of ions is expected to depend on temperature, electric field and the film morphology. We studied the influence of these factors on the poling process by applying a train of short voltage pulse (.95 S) on the device and measured the device at J SC after each pulse to avoid additional poling during the measurement. As shown in Fig. S14a-c, a larger electric field or a higher poling temperature results in faster switching of the device. The switching of photovoltaic was frozen under low temperature below C, where the photocurrent direction cannot be switched by the same bias of ±2.5 V. It should be noted the data shown in Fig. 14a were measured under 1 sun illumination, and the photocurrent in Fig. S14b were measured under around.1 sun illumination due to the probe station setup limitation. Photocurrents in Fig. S14c were measured with incident light penetrating the thin opaque Au electrodes, therefore the short circuit current densities were much smaller than other cases. Our recently developed solvent-annealing process resulted in larger grain size in the perovskite films than the thermal-annealing, as shown in the cross-section scanning electron microscopy images (Fig. S14d-e). We also tested the dynamic poling process of the devices with perovskite films fabricated by solvent-annealing and thermal-annealing. The perovksite film with larger grain size had fewer grain boundaries and thus less ion vacancies, which also resulted in smaller ion drift velocity. As shown in Fig. S14e, the device with the solvent-annealed perovskite films took much longer time to be switched than the thermal-annealed devices. 18 NATURE MATERIALS www.nature.com/naturematerials 214 Macmillan Publishers Limited. All rights reserved.