Modulating Hysteresis of Perovskite Solar Cells by a Poling Voltage

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Modulating Hysteresis of Perovskite Solar Cells by a Poling Voltage Xiaobing Cao, Yahui Li, Can Li, Fei Fang, Youwei Yao, Xian Cui, and Jinquan Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05775 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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The Journal of Physical Chemistry

Modulating Hysteresis of Perovskite Solar Cells by a Poling Voltage Xiaobing Cao†, Yahui Li†, Can Li†, Fei Fang‡, Youwei Yao‡, Xian Cui†, Jinquan Wei†* †Key Lab for Advanced Materials Processing Technology of Education Ministry; State Key Lab of New Ceramic and Fine Processing; School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China ‡Institute of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P.R. China *E-mail: [email protected]; Tel: +86-10-62781065

ACS Paragon Plus Environment

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ABSTRACT

Perovskite solar cell has a puzzling phenomenon of current hysteresis. Here, we modulate the hysteresis by applying a poling voltage. Charge accumulations are detected in the perovskite solar cells. Two interacting capacitors are identified through dynamic voltage measurement. We elucidate that the current hysteresis origins mainly from the polarization and depolarization of electric dipoles of CH3NH3+ under an external electric field due to the intrinsic ferroelectric properties of perovskite. The polarization leads to charge accumulation at the surface of perovskite, which establishes a polarization-induced electric field. The polarization-induced electric field affects the charge transport inside the solar cells, resulting in the current hysteresis. The polarization of electric dipoles can be modified by the poling and sweeping voltage, which makes the hysteresis exhibit a history-dependent effect.


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1. Introduction Recently, organic–inorganic halide perovskites of CH3NH3PbX3 (X=Cl, Br, I) have achieved considerable attentions due to their outstanding optoelectronic properties, such as suitable band gap, high absorption coefficient, high carrier mobility, long carrier lifetime and diffusion length.1-6 In 2009, Miyasaka introduced CH3NH3PbX3 into liquid-electrolyte-based dyesensitized solar cells and achieved a power conversion efficiency (PCE) of 3.8%.7 In 2012, Kim reported a stable, all-solid-state, mesoscopic solar cell from CH3NH3PbI3 with a relative high PCE of 9.7%.8 Since then, the perovskite solar cells (PSCs) developed quickly. Many methods, including one-step spin coating,8 two-step spin coating,

9

sequential deposition

10

and vapor

deposition,11 have been developed to fabricate the PSCs. The structure of solar cells also evolved from dye-sensitized, to mesoporous scaffold,

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meso-superstructure

12

and planar pn

junction.11,13 With these progress, the PCEs have been improved significantly from initial 3.8% to above 20%.7, 14, 15 It is very interesting and also puzzling that there are evident current hysteresis phenomena in the PSCs, where there is evident discrepancy between the forward and reverse light current densityvoltage (J-V) curves. It is also very strange that the hysteresis behaviors are significantly reduced and even eliminated in some devices with inverted structure

16, 17

or using the NH2CH=NH2PbI3

perovskite as the active layers.14 It is reported that the hysteresis behaviors are affected by several factors, such as scan rate,

18

scan range,

19, 20

delay time,21 and poling voltage.22 Several

mechanisms have been proposed to explain the causes for the anomalous hysteresis behaviors: ion migration,

23-26

interface charge accumulation,

and de-trapping process,33,

34

27-29

polarization,

19, 22, 30-32

charge trapping

and even interface of compact layer.35 Actually, charge

accumulations are frequently detected in the hysteresis inverstigation.23-29, 36 Some researchers


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ascribed the charge accumulation to ion migration.23-26 Very recently, Li et al.37 measured the lateral atomic ratio of I/Pb through X-ray photoelectron spectroscopy by applying an external electrical bias through two silver electrodes. They claimed that charge accumulation and current hysteresis are dominated by ion migration of iodide ions. They also claimed that an electrochemical reaction between iodine and silver occurs during the biasing process. Eames et al. 25

calculated the activation energies of I−, Pb2+ and CH3NH3+ (denoted as MA+) for ion migration

in CH3NH3PbI3, and claimed that the ions can migrate inside the perovskite crystal. However, it will lead to increase of defect density, chemical compositional change, and even irreversible structural decomposition,38 which make the perovskite materials degrade quickly. Furthermore, it is difficult to understand how the ions return to the defect sites when the external electric field is cancelled. Some researchers believed that the hysteresis derives from the intrinsic ferroelectric properties of the perovskite. Actually, the ferroelectric domain in CH3NH3PbI3 has been directly observed by piezoelectric force microscopy (PFM).39-42 Very recently, Leguy et al. found that dipolar MA+ ions re-orientate, rather than free rotate and diffuse in the perovskite crystals. 43 So, the hysteresis might derive from the reorientation of the dipolar MA+ ions under electric fields. Chen et al. investigated the effects of poling voltage on perovskite solar cells with various structures (plane, mesoporous TiO2, and mesoporous Al2O3). 22 They found that the ferroelectric polarization is the main cause for the current hysteresis. Here, we investigate the hysteresis behaviors of the perovskite solar cells with different perovskite grain sizes by applying a poling voltage, and measured the dynamic voltage of the PSCs. We found that there are two interacting capacitors within the devices. We discussed the hysteresis behaviors basing on the polarization and depolarization of the electric dipoles.

2. Experimental methods


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Device fabrication: FTO glass (TEC7, Pilkington) was subsequently cleaned with detergent, deionized water, acetone, 2-propanol, ethanol, deionized water, and then dried by nitrogen gas. A compact layer of TiO2 was deposited on the FTO glass by spin coating from titanium isopropoxy solution in ethanol at 2000 rpm for 30 s, and then annealed at 500 °C for 30 min. A mesoporous TiO2 layer was deposited on the compact TiO2 layer by spin coating diluted TiO2 paste (Dyesol18NRT, Dyesol) in ethanol (2:7, weight ratio) at 4500 rpm for 30 s, and then annealed at 500 ℃ for 30 min. The mesoporous TiO2 layer was then infiltrated in PbI2/DMF (1.4 M) solution at 100 °C to get a PbI2 film. In order to obtain perovskite crystals with different grain sizes, the PbI2 film was dipped in solution of CH3NH3I (MAI) in 2-propanol with various concentration (7.5 mg mL−1 or 10 mg mL−1) for 15 min, followed by rinsing with 2-propanol. After drying, a HTM layer was deposited by spin-coating a solution prepared by dissolving 100 mg SpiroOMeTAD, 40 µL 4-tert-butylpyridine (tBP), 36.3 µL of a stock solution of 520 mg mL−1 TFSI in acetonitrile and 60 µL of a stock solution of 300 mg mL−1 FK102 dopant in acetonitrile in 1 mL chlorobenzene. Finally, a thin layer of gold (60 nm) was thermally evaporated on the top of HTM to form a back electrode. The active area of this electrode was fixed at 0.16 cm2. Characterization: The perovskite films were characterized by field-emission scanning electron microscopy (SEM, MERLIN VP Compact) and X-ray diffraction (XRD, D8-Advance). Light current density-voltage (J-V) curves of solar cells were measured by a semiconductor characterization system (4200-SCS, Keithley) under one solar illumination (AM 1.5G, 100 mWcm−2, 91195, Newport). The hysteresis behavior of PSCs was measured by forward and reverse scan in a range of 0 to 1 V subsequently at a scan rate of 5 mV s−1.

3. Results and discussion


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PbI2 thin films convert to perovskite grains when dipping in a solution of CH3NH3I/2-propanol (MAI/IPA). The grain size of perovskite depends on its nucleation number and growth rate, which is here controlled by the concentration of MAI. Small perovskite grains are expected from the high concentration of MAI due to large nucleation number and fast growth rate; while large grains are expected from low concentration of MAI due to low nucleation number and slow growth rate. Figure 1 shows scanning electron microscope (SEM) images of two perovskite samples prepared from MAI/IPA solution with MAI concentration of 7.5 and 10 mg mL−1, respectively. The two samples have similar morphology, but different grain sizes. The average grain size prepared from low MAI concentration of 7.5 mg mL−1 is about 500 nm (Figure 1a), while it is only ~160 nm when the MAI concentration increase to 10 mg mL−1 (Figure 1b). It is noted that pinholes are occasionally detected in the perovskite layer prepared from a MAI solution of 7.5 mg mL−1 (Figure 1a). The two samples have similar crystalline structure, according to X-ray diffraction patterns (see Figure S1). In our conditions, the perovskite grains increase significantly to 2 µm when the concentration of MAI is reduced to 5 mg mL-1 (Figure S2a). But the grains cannot fully cover the FTO layer, which leads to short-circuit of the devices. The efficiency of the perovskite solar cells fabricated from a low MAI concentration of 5 mg mL-1 are too low to make any comparison. The grain size can be further reduced to ~100 nm when the concentration of MAI increases to 12 mg mL−1 (Figure S2b, and S2c). But, some large cuboid crystals present on the surface of perovskite layer. As a result, the HTL layer cannot contact with the perovskite film very well, which hinders the collection and transportation of charge carriers. The PSCs fabricated from solution with high concentration of MAI also exhibit poor PCE.


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Figure 1 also shows cross-sectional SEM images of the PSCs, which reveal that the two PSCs have similar structure, but different thickness of the perovskite layer. The solar cells with small perovskite grains have higher PCEs than those with large grains (Figure S3a). The solar cells with large perovskite grains have higher short-circuit current density (Jsc), but lower open-circuit voltage (Voc) and fill-factor (FF) than those with smaller grains (Figure S3a). On one hand, large grains lead to less grain boundary, which benefits to the carrier transportation in the perovskite layer. According to Agarwal’s research, the charge mobility depends on the slope of

J dark − V

curve at the high bias regime.44 The charge mobility in the solar cell prepared from 7.5 mg mL−1 MAI solution is about 1.4 times higher than that from 10 mg mL−1 according to the curves (Figure S3b). In addition, low defect density, long carrier lifetime, absorption capability

46

45

J dark − V

and strong light

are expected in the large perovskite grains. Thus, the PSCs with large

grain size have higher Jsc. On the other hand, Spiro-OMeTAD might penetrate the perovskite layer through the pinholes in the devices with large perovskite grains, leading to decrease of Voc and FF. We investigate the hysteresis behaviors by applying an external voltage (poling) on the PSCs in dark for about 200 seconds before the light J-V curve measurement (see inset of Figure 2). The scan sequence is forward (from 0 to 1 V) and then reverse (from 1 to 0 V). Figure 2 presents the light J-V curves obtained from the forward and reverse scan at different poling voltages. It is clearly that the J-V curves obtained from the both reverse and forward scans are all suppressed by the negative poling voltage. The Jsc decreases as the increase of the negative poling voltage. The higher negative poling voltage, the lower current density. At the same negative poling voltage, the current densities obtained from the reverse scans are higher than those from the forward scans, showing typical hysteresis behaviors. The hysteresis depends not only on the


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poling voltage, but also on the grain size of perovskite. The hysteresis of the PSC with small perovskite grains is more sensitive to scan direction and poling voltage than that with large grain size. The Voc also decreases evidently as the negative poling voltage getting higher, especially for the solar cells with small grain size (Figure 2b). In order to describe the hysteresis behaviors, we introduced a J-V hysteresis index (HI)

22, 47

to

quantitate the difference of current densities between reverse and forward scan. Here, HI is defined as: V V J RS ( OC ) - J FS ( OC ) 2 2 HI = V OC ) J RS ( 2

(1)

where, JRS(Voc/2) and JFS(Voc/2) represent the current density at Voc/2 in the reverse and forward J-V curves, respectively. The value of the HI reflects the extent of hysteresis. The larger value of HI, the more prominent hysteresis. Figure 3a shows the plots of HI against poling voltage. It is clear that the HI increases as the negative poling voltage, especially for the devices with grain size of 160 nm. The PSCs with small perovskite grains have more prominent hysteresis effects than those with large grains. This is consistent with the results reported by Kim.48 They also found that charge carriers are easily trapped or recombined at interface of perovskite due to the high defect density.41 Thus, the hysteresis effect in PSCs with small grain size is severer than in those with large grains. Figure 3b presents the reverse and forward J-V curves at a negative poling voltage of -0.8 V. It shows that the J-V curves depend on the scan direction and grain size of perovskite. Figure 4 depicts the dependence of difference of current density (∆Jsc) and power conversion efficiency (∆PCE) between the reverse and forward scan on the poling voltage. The plots of


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∆PCE against the poling voltage have same tendency to those of ∆Jsc, but different from the Voc and FF (see Figure S4), which indicate that the hysteresis behaviors are mainly influenced by the current density. In general, both of the ∆Jsc and ∆PCE increase as the negative poling voltage. Both of the∆Jsc and ∆PCE in the PSC with perovskite grain size of 160 nm are larger than those of 500 nm. It indicates that the hysteresis of the solar cell with small perovskite grains is more sensitive than that with large grains. The minimum of ∆Jsc and ∆PCE locate at 0 V for the PSC with grain size of 160 nm, while they locate at -0.2 V for the devices with grain size of 500 nm. The initial drop of the ∆Jsc and ∆PCE in the devices with grain size of 500 nm might be related to the initial state of the ferroelectric domains in the perovskite films which can be changed during the J-V curve measurement. 19, 22, 39 Charge accumulation is frequently detected at the surface of PSCs. In order to measure the charge accumulation, a poling voltage is applied to a solar cell with initial efficiency of ~12% for 200 s, and then shut it off. Here, we measure the dynamic voltage, rather than current of the solar cells. Figure 5 shows the dynamic voltage before and after stopping poling. It is clear that the voltage drops immediately when the poling voltage shuts off, and then decreases gradually. All the voltage-time (U-t) curves in Figure 5 are well fitted by a two-exponential time decay function:

U = U 0 + A1 exp(−t / t1 ) + A2 exp(−t / t2 )

(2)

where, U0 is the residual voltage, A1 and A2 are the amplitudes, t1 and t2 are time constants, respectively. It indicates that the PSC comprises two capacitors: internal and external. The electric dipole polarization in CH3NH3PbI3 has been directly observed by several independent groups.

39-42

According to Frost’s report, the electronic polarization of MAPbI3 is about 38

µC/cm2, which is comparable to the traditional ferroelectric oxide perovskites.31 So, we believe


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that the charge accumulation is related to dipolar polarization of CH3NH3PbI3. As shown in the inset of Figure 5, electric dipoles are polarized by an external electric field (poling voltage). We believe that the polarization of electric dipoles stems physically from the rotation of MA+ ions.43 The heads and tails of the internal dipoles are canceled as they polarize. But, no cancellation occurs at the surface of the perovskite, resulting in charges accumulation at the surface. Thus, it forms an internal capacitor within the perovskite layer due to the polarization of electric dipoles. It also forms an external capacitor between the two electrodes separated by HTM, perovskite and TiO2 compact layer. These two capacitors have opposite polarities, and interact with each other, which makes charges cannot recombine quickly. The capacitances of these two capacitors depend not only on the thickness of the perovskite layer, but also on the polarization of electric dipoles. According to the fitting results (Figure S5), the time constant of the internal capacitor can be more than 180 times longer than that of the external one, which reaches to 10 to 100 seconds. Although, the dipolar CH3NH3+ ions re-orientate inside the crystal at a timescale of picoseconds, the polarization and de-polarization of the CH3NH3+ depend on the electric field applying on the perovskite. The initial polarization state of the perovskite is changed before J-V curve measurements by applying an external voltage. Thus, the J-V curves are influenced by the poling voltage, scan sequence, and scan rate, showing a history-dependent effect. Because the energy level at the HTM/Au side is higher than that at the FTO side, the effect of negative poling on the polarization is inequivalent to that of the positive poling (Figure 5). The time constant of U-t curves for the positive poling are larger than those of the same negative poling (Figure S5). Figure 6a shows an electric dipole moment placing in an external electric field (Eex). The electric dipole p can be resolved into vertical (p┴) and horizontal (p//) components. The vertical component tends to make the dipole re-orientate in the electric field; while the horizontal


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component enhances the charge separation. Physically, the polarization is relevant to the rotation of MA+ ions in the MAPbI3 crystal.43 Thus, the initial state of electric dipole is modified from random distribution to orientation by the poling voltage (Figure 6b to 6d). The polarization density depends on the strength and direction of the electric field. By applying an equal poling voltage, the electric field on the perovskite with small grain is stronger than that with large grain due to different thickness of dielectric perovskite layer. There are three kinds of electric fields applying on the perovskite layer during the J-V curve measurements: photo-induced built-in electric field (Eph), external electric field (Eex) due to sweeping voltage, and polarization-induced electric field (Epi). The Epi can be separate into the poling-induced (denoted as Epi1) and sweeping-induced electric field (Epi2). The Epi2 is antiparallel to Eex. Both of the Eex and Epi2 are getting stronger during the forward scan, and getting weaker during the reverse scan. Thus, the effective electric filed applying on the perovskite layer is given by Eeff=Eph+Eex+Epi1+Epi2, which drives the photo-induced carriers transfer in the solar cells, resulting in photocurrent. We consider the hysteresis effects in PSCs with and without poling. In the case without poling, the initial electric dipoles distribute randomly in the perovskite layer as shown in Figure 6b. The effective electric field applied on the perovskite is equal to Eeff=Eph+Eex+Epi2. For a given bias, the Epi2 induced by the forward scan is weaker than that by the reverse scan, thus the current density obtained from the forward scan is lower than that from the reverse scan, showing a typical hysteresis behavior. When a negative poling voltage is applied to a PSC before the light J-V curve measurement, the electric dipoles is pre-polarized. It generates an Epi1 that is antiparallel to the Eph. The Epi1 reduces the strength of Eeff (Figure 6c), resulting in the suppression of the light J-V curves and


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decrease of the photovoltaic properties. The Eeff is getting weaker as the increase of the negative poling voltage. Thus, the hysteresis depends on the poling voltage evidently. The electric field applied on the small grain size perovskite is stronger than that on the large grain at a same poling voltage. Thus, the hysteresis also depends on the grain size of perovskite. It is noted that the coercive electric filed of the MAPbI3 is low, so the state of ferroelectric domain can be changed during J-V curve measurement.39 The initial state of the electric dipoles will affect the transportation of charge carriers, which might be the cause for initial drop in ∆Jsc and ∆PCE in Figure 4a. Also, the polarization state of the electric dipoles will be eliminated by the forward bias during the voltage sweeping, which is the reason for why the greater reduction during forward scan. Figure 7a shows light J-V curves of a PSC under -0.8 V poling voltage measured in a scan sequence of reverse---forward---reverse in a range of 0 to 1 V. The photovoltaic performance obtained from the second reverse scan is improved significantly after the first round of reverse--forward scan. The Epi1 is weakened by the Epi2 gradually. The photovoltaic properties are getting better as the reverse scan going on. If the solar cells are applied by a positive poling voltage, the Epi1 will parallel to Eph, which enhances the Eeff. Thus, the photovoltaic properties will be improved evidently. Figure 7b shows the J-V curves of a solar cell with and without positive poling voltage. By adding a positive poling voltage of +0.6 V, the photocurrents obtained from both reverse and forward scan are improved evidently. Thus, the hysteresis behaviors and photovoltaic properties are modulated by the poling voltage. Currently, the causes for charge accumulation and current hysteresis in PSCs are still under debate. Generally, there are two main possible mechanisms: ion migration18,

23, 25, 49, 50

and


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polarization. 22, 32, 39-41 For the ion migration mechanism, some researchers suggested that I− and MA+ can all migrate in the perovskite due to their relative low activation energy (~0.55 eV for I− and ~0.8 eV for MA+).25 The activation energy I− will decrease evidently, if there are defects and vacancies.

49

Azpiroz et al.

50

estimated that I− ion migrate through the MAPbI3 film very fast

(less than 1 µs). It seems that the ion migration take place easy in the perovskite crystal. However, with the physical migration of ions (mainly I− and MA+), the defect density of perovskite will increase significantly, and the ions enrich at the surface and dilute inside the crystal, which might lead to structural change and even decomposition of the perovskite. Furthermore, it is also unknown how the ions migrate back to the defect sites after releasing the voltage. Actually, ion migration does occur in the perovskite crystal, especially in the case of environments with hydrate phase,38 which leads to decomposition of the perovskite and degradation of the photovoltaic performance. It is noted that the degradation is an irreversible process, which is quite different from the hysteresis. For the hysteresis effect, the I-V curves can recover easily by releasing the poling voltage (see Figure S6). Thus, we believe that charge accumulation and hysteresis derive mainly from the polarization, rather than the ionic migration. In the case of polarization, the electric dipolar MA+ just rotate and re-orientate inside the perovskite crystals.43 Actually, ferroelectric domains of perovskite film have been observed by piezoelectric force microscopy (PFM) at room temperature directly.39-42 The ferroelectric domains can be switched by applying a 0.2 V voltage due to a coercive field.39 The polarization mechanism derives from the intrinsic ferroelectric properties of the perovskite. The I-V curves can recover to its initial state by depolarization (Figure S6). It is noted that both the ion migration and rotation of dipoles are very fast, which can complete in less than 1 microsecond.

43, 50

But the polarized dipoles to release depends on the electric field


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applying on the perovskite, which is determined by the time constant of the inner capacitor. It is reported that the re-oriental diploes can maintain more than 30 min in the CH3NH3PbI3 sized larger than 100 nm,41 and the ferroelectric domain can be switched at low poling voltage.39 It has also been reported that the re-orientation of the diploes in MAPbI3 is a slow dynamic process in a time scale of seconds.47 We also noticed that it has been reported the hysteresis-less phenomena in the PSCs with inverted structure (i.e. PEDOT:PSS/perovskite/PCBM), 17 and hysteresis-free in the PSCs using NH2CHNH3PbI3 (FAPbI3) as the active layers.14 It is hard to understand the hysteresis-less or hysteresis-free phenomena by using the ion migration mechanism, since the ion migration also take place in these perovskite active layers. Here, we also try to explain these phenomena by the polarization mechanism. In the perovskite solar cells with inverted structure,17 charge accumulation effect is much lower than that in the normal structure, since the PEDOT:PSS and PCBM layers have relative high carrier mobility. Thus, the photo-induced charge carriers can be transported effectively through the PEDOT:PSS and PCBM layer, which reduces the hysteresis effect. In the perovskite of FAPbI3, there are two symmetric ammonium radicals in FA+, which implies that the dipole moment of the FA+ is weaker than that of the MA+. Furthermore, the rotation of the FA+ under external electric field is not as easy as the MA+ due to the larger FA+ cations. The polarization effect of FA+ is reduced significantly, and even eliminated in the perovskite. Thus, the solar cells using FAPbI3 as active layer exhibit hysteresis-free behaviors.

4. Conclusions In summary, the effects of poling voltage on the photovoltaic properties and hysteresis behaviors are investigated. The hysteresis is influenced by the poling voltage, direction, and grain size of perovskite, showing an evident history-dependent effect. Charge accumulation are detected at the


14

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surface of the devices. Two interacting capacitors are identified through the dynamic voltage measurement. We elucidate that the current hysteresis origins mainly from the polarization and depolarization of electric dipoles of CH3NH3+ under an external electric field due to the intrinsic ferroelectric properties of perovskite. The polarization makes charge accumulation at the surface of perovskite, which forms a polarization-induced electric field. The polarization-induced electric field affects the charge transfer inside the solar cell, leading to photocurrent hysteresis. The polarization state is modified by the poling and sweeping voltage, which make it possible to modulate the photovoltaic properties by applying a poling voltage.


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Associated Content Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM, XRD, J-V curves (light and dark), and fitting results of the dynamic voltage of perovskite solar cells.

Author Information Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by National Natural Science Foundation of China (51172122), Shenzhen Jiawei Photovoltaic Lighting Co., Ltd, and Tsinghua University Initiative Scientific Research Program (20161080165).


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Figures

Figure 1. Low (left) and high (middle) magnification SEM images of the CH3NH3PbI3 crystals, and cross-sectional (right) SEM images of the PSCs with different perovskite grain sizes. (a) 500 nm, (b) 160 nm. The blue dotted lines illustrate the thickness of perovskite layer.

Figure 2. J-V curves obtained from forward (left) and reverse scan (right) under various poling voltages of the PSCs with different perovskite grain sizes. (a) 500nm, (b) 160 nm. Inset represents corresponding structure of the solar cell under negative poling.


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Figure 3. (a) Dependence of the hysteresis index (HI) of PSCs on the poling voltage. (b) J-V curves of the PSCs with different grain sizes under reverse and forward scan at a negative poling voltage of-0.8 V.

Figure 4. The difference (∆) of Jsc and PCE obtained from the reverse and forward scan for the PSCs with different perovskite grain sizes. (a) 500 nm, (b) 160 nm.


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Figure 5. The dependence Voltage on time after stop poling at various voltage. (a) Negative poling, (b) Positive poling. Insets show schematics of polarization of dipoles under negative and positive poling.

Figure 6. Schematic of band structure and orientation of dipoles under different polarization conditions (b) without poling voltage (c) negative poling voltage (d) positive poling voltage.


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Figure 7. (a) J-V curves of a PSC under -0.8 V poling voltage in the scan sequence of reverse→forward→reverse (b). J-V curves of a PSC under positive poling. The photovoltaic parameters are enhanced by applying a positive poling voltage of +0.6 V.


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TOC graphic


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Figure 1. Low (left) and high (middle) magnification SEM images of the CH3NH3PbI3 crystals, and crosssectional (right) SEM images of the PSCs with different perovskite grain sizes. (a) 500 nm, (b) 160 nm. The blue dotted lines illustrate the thickness of perovskite layer. 345x175mm (300 x 300 DPI)


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Figure 2. J-V curves obtained from forward (left) and reverse scan (right) under various poling voltages of the PSCs with different perovskite grain sizes. (a) 500 nm, (b) 160 nm. Inset represents corresponding structure of the solar cell under negative poling. 252x190mm (300 x 300 DPI)



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Figure 3. (a) Dependence of the hysteresis index (HI) of PSCs on the poling voltage. (b) J-V curves of the PSCs with different grain sizes under reverse and forward scan at a negative poling voltage of-0.8 V. 333x165mm (300 x 300 DPI)


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Figure 4. The difference (∆) of Jsc and PCE obtained from the reverse and forward scan for the PSCs with different perovskite grain sizes. (a) 500 nm, (b) 160 nm. 323x161mm (300 x 300 DPI)



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Figure 5. The dependence Voltage on time after stop poling at various voltage. (a) Negative poling, (b) Positive poling. Insets show schematics of polarization of dipoles under negative and positive poling. 327x163mm (300 x 300 DPI)


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Figure 6. Schematic of band structure and orientation of dipoles under different polarization conditions (b) without poling voltage (c) negative poling voltage (d) positive poling voltage. 261x170mm (300 x 300 DPI)



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Figure 7. (a) J-V curves of a PSC under positive poling. The photovoltaic parameters are enhanced by applying a positive poling voltage of 0.6 V. (b) J-V curves of a PSC under -0.8 V poling voltage in the scan sequence of reverse→forward→reverse. 308x154mm (300 x 300 DPI)


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Modulating Hysteresis of Perovskite Solar Cells by a Poling Voltage

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