Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions

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Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells Ting Wu,† Liam Collins,‡,§ Jia Zhang,† Pei-Ying Lin,†,∥ Mahshid Ahmadi,† Stephen Jesse,‡,§ and Bin Hu*,† ACS Nano 2017.11:11542-11549. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 06/30/18. For personal use only.

†

Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Department of Photonics, National Cheng Kung University, 70101 Tainan, Taiwan, ROC ‡

S Supporting Information *

ABSTRACT: This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance−voltage (CV) signal is observed as a broad “shoulder” in the depletion region from −0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA+ with difficult-rotational formamidinium (FA+) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells. KEYWORDS: perovskite solar cells, photoinduced bulk polarization, chlorine doping, grain boundary passivation, charge dissociation

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hot carriers for breaking the SQ limit. Although the ferroelectric properties of hybrid perovskites are still under debate, recent studies proposed that the dipolar organic cations (e.g., methylammonium cations, MA+) offer a highly polarizable environment to protect charge carriers from scattering of defects based on the observation of long-lived hot carriers (∼100 ps) in hybrid perovskites.14,17 Meanwhile, the early study reported a giant low-frequency dielectric constant, which is shown to be further enhanced by 1000 times with the aid of

rganic−inorganic hybrid perovskites have demonstrated great advantages in photovoltaic applications1−6 due to their high absorption coefficient,1 ultrafast exciton dissociation,7,8 ambipolar charge transport,8,9 low carrier recombination,8 and low-cost solution processability. To date, the power conversion efficiency (PCE) of perovskite solar cells has been boosted to 22%,6,10 but still lies below the theoretical limit, namely, the Shockley−Queisser (SQ) limit (34% for MAPbI3).11 It has been demonstrated that there are mainly two types of energy loss channels: (i) nonradiative recombination associated with the defects or trap states12,13 and (ii) hot carrier cooling associated with the multiphonon process.14 Theoretical studies predicted that MAPbI3 is potentially ferroelectric,15,16 which enables the utilization of © 2017 American Chemical Society

Received: September 8, 2017 Accepted: October 31, 2017 Published: October 31, 2017 11542

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short) from 0 to 0.30. PCEs are dramatically improved from 12.41% (11.81% on average) to 18.19% (17.58% on average) via Cl incorporation with the optimized Cl/I ratio of 0.25. More specifically, the open-circuit voltage (Voc) is significantly increased from 0.86 ± 0.01 V to 1.05 ± 0.01 V; simultaneously the short-circuit current (Jsc) is improved from 17.97 ± 0.16 mA/cm2 to 21.50 ± 0.11 mA/cm2, while the fill-factor (FF) is slightly enhanced from 0.77 ± 0.01 to 0.78 ± 0.01. However, further increasing the CL/I ratio to 0.30 causes a reduction in both Voc and Jsc, consequently decreasing the photovoltaic efficiency. The histogram of photovoltaic parameters for MAbased perovskite solar cells with and without Cl is shown in the Supporting Information, Figure S1. It should be noted that all MA-based perovskite solar cells do not show detectable J−V hysteresis under different scanning directions and delay times (Figure 1b,c and Figure S2, in the Supporting Information). Meanwhile, the devices exhibit promising stability under continuous light exposure and electrical biasing. Figure 1d indicates that the J−V characteristics show a negligible change under the photoexcitation from a sunlight simulator for 10 min, suggesting a negligible light-soaking effect and degradation during device measurements. More specifically, there is no degradation in either Voc or Jsc under continuous tracking for 10 min in ambient conditions (see inset in Figure 1d). The promising operational stability in ambient conditions is likely due to the self-encapsulation from the polyethylenimine (PEI) layer based on the presence of a large number of amine groups, which not only helps block the moisture from the environment but also prevents the iodine migration to the metal electrode. The removal of those complexities provides the necessary conditions to explore photoinduced bulk polarization in the hybrid perovskites by using CV characteristics. For all MA-based perovskite solar cells, we observe an anomalous photoinduced CV signal occurring within the depletion region ranging from −0.5 to +0.5 V at the intermediate frequency (5 kHz) window associated with the dipolar orientational polarization. As schematically shown in Figure 2, a typical CV curve can be divided into three regions:

either photoexcitation or charge injection in hybrid perovskites.18 Later studies suggested that such a low-frequency dielectric response is likely due to the ions at the electrode interface.19,20 Therefore, exploring photoinduced polarization in hybrid perovskites and its effects on photovoltaic processes becomes a vital issue for the further advancement of perovskite solar cells. Here, we demonstrate the photoinduced bulk polarization in hysteresis-free MA-based perovskite solar cells by gradually scanning a dc bias from −0.5 to +1.5 V at a low alternating bias (50 mV at 5 kHz) based on capacitance−voltage (CV) measurements under different photoexcitation intensities. In CV measurements gradually scanning a dc bias can cause a drifting on mobile ions, generating continuously moving ions. In this situation, a low alternating bias can negligibly influence the drifting ions but largely interacts with local polarization, leading to a CV signal only from local polarization. This forms a convenient approach to explore photoinduced bulk polarization in perovskite solar cells by using CV measurements under photoexcitation.

RESULTS AND DISCUSSION As shown in Figure 1a, all solar cells studied in this work were fabricated with a planar structure of ITO/PEDOT:PSS/

Figure 1. MA-based perovskite solar cells and photovoltaic performance. (a) Device structure; (b) J−V characteristics of perovskite solar cells with the optimized Cl/I ratio of 0.25 under forward (FW) and reverse (RV) scanning modes with zero delay time; (c) J−V characteristics under different delay times (RV scan); (d) J−V characteristics with different light soaking times (RV scan, zero delay time); inset shows the evolution of Voc and Jsc tracked for 10 min in ambient conditions. Figure 2. Schematic diagram of typical photoinduced CV characteristics in a solar cell.

perovskite/PCBM/PEI/Ag. Table 1 presents the photovoltaic parameters of the optimal MA-based perovskite solar cells with different molar ratios of PbCl2 to PbI2 (CL/I ratio is used for

(i) depletion region, (ii) charge-accumulation region, and (iii) charge-recombination region. In the depletion region, the capacitance tends to approach a geometrical value, namely, Aε ε geometrical capacitance, Cge = Lr 0 (A is the effective area, εr is the relative dielectric constant of the medium, ε0 is the permittivity of the vacuum, and L is the thickness of the medium).21,22 When sweeping a forward bias toward the peak (Vpeak), the photogenerated charge carriers start to accumulate at the electrode interface due to the weakening of the built-in potential (Vbi), leading to an increase in the capacitance. Generally, the capacitance in this region can be described by

Table 1. Photovoltaic Parameters of MA-Based Perovskite Solar Cellsa CL/I molar ratio

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

0 0.15 0.25 0.30

18.20 19.51 21.57 20.97

0.87 1.00 1.07 1.01

78 77 79 78

12.41 15.08 18.19 16.47

a

All parameters were averaged from the J−V characteristics of the champion devices under forward and reverse scans. 11543

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accumulation of photogenerated carriers. However, the CV peak shift becomes negligible after the Cl incorporation in Figure 3c, minimizing surface accumulation of photogenerated carriers. Simultaneously, the Cl incorporation can largely increase the photoexcitation dependence of the anomalous CV signal. When the photoexcitation increases from 0.1 sun to 1 sun, the anomalous CV signal is increased by a factor of 3 upon incorporating Cl (Cl/I ratio = 0.25) with respect to devices without Cl incorporation. Clearly, the Cl incorporation can effectively suppress the photoinduced surface polarization but largely increases the anomalous CV signal occurring in the depletion region. Therefore, we propose that the anomalous CV signal in the depletion region is an evidence for photoinduced bulk polarization in perovskite solar cells. Furthermore, this anomalous photoinduced CV signal is shown to exhibit a similar dependence on the Cl/I ratio with the device efficiency (see Figure S4, in the Supporting Information). This suggests that the photoinduced polarization effect plays an important role in determining the photovoltaic performance of perovskite solar cells. We can further confirm that surface accumulation of photogenerated carriers is not responsible for the anomalous CV signal by analyzing the CV characteristics of a Si cell under different photoexcitation intensities at the frequency of 5 kHz (Figure 3a). We can clearly see that increasing photoexcitation does not change the CV signal within the depletion region, but causes a Vpeak shift in the Si cell. The Vpeak shift serves as an indication that increasing photoexcitation leads to the surface accumulation of photogenerated carriers. Clearly, the surface accumulation of photogenerated carriers does not give rise to an anomalous photoinduced CV signal within the depletion region in the Si cell. Therefore, we believe that the anomalous CV signal within the depletion region is related to the photoinduced bulk polarization in organic−inorganic hybrid perovskites. It should be noted that cations/anions in organic−inorganic hybrid perovskites can interfere with the photoinduced bulk polarization, which brings a technical difficulty to detect photoinduced bulk polarization. Here, we should point out that our measurements scan a dc bias to continuously drift the mobile ions during the detection of photoinduced bulk polarization detected by a low alternating bias. In this situation, the continuously drifted ions become inactive in the CV detection under a low alternating bias. Specifically, when the mobile organic ions are continuously drifted by scanning a dc bias, a low alternating bias (50 mV at 5 kHz) can induce a CV signal only from local static polarizations. Therefore, simultaneously scanning a dc bias with applying a low alternating bias can separate the continuously moving ions from local static polarizations during CV detection. Clearly, our measurements present an effective approach to detect photoinduced bulk polarization in organic−inorganic hybrid perovskites. To understand the underlying mechanism of the photoinduced bulk polarization in MA-based perovskite solar cells, we need to discuss how a bulk polarization can be generated within the crystalline structure by photoexcitation. It is known that photoexcitation can induce electronic polarization with a relaxation rate in the high-f regime (∼1014 Hz). However, bulk polarization is normally dominated by dipolar polarization with relaxation rate in the intermediate frequency regime (∼105 Hz). Apparently, the photoinduced electronic polarization and the dipolar polarization dominated bulk polarization have largely different time constants, leading to the difficulty to generate

2(Vbi − V ) A2 eεrε0N

(N represents the doping level).21,23 Further increasing bias beyond Vpeak, the photogenerated charge carriers tend to recombine when Vbi is completely diminished, leading to a decrease in capacitance. Therefore, the typical CV characteristics exhibit a peak, namely, a CV peak, which can be clearly observed in a commercial silicon cell (Figure 3a). Such a CV

Figure 3. Photoinduced CV characteristics measured under simulated sunlight with variable intensities. (a) Commercial silicon cell; (b) MA-based perovskite solar cell without Cl incorporation; (c) MA-based perovskite solar cell with Cl incorporation; (d) control device without perovskite layer. An alternating bias of 50 mV (5 kHz) was applied for all samples. ΔC0V represents the capacitance change under short-circuit conditions upon increasing excitation intensity from 0.1 sun to 1 sun.

peak can also be seen in all of our MA-based perovskite solar cells with and without Cl incorporation to show accumulation and recombination features (Figure 3b and c). Most interestingly, a broad CV signal appears in the depletion region from −0.5 to +0.5 V when increasing the excitation intensity from 0.1 sun to 1 sun, for all MA-based perovskite solar cells. More specifically, when the photoexcitation intensity is increased from 0.1 sun to 1 sun, the amplitude of the anomalous CV signal at zero dc bias (short-circuit condition) and an intermediate frequency (5 kHz) is enhanced by 23% and 73% in the perovskite solar cells with the CL/I ratio of 0 and 0.25, respectively. Such photoinduced CV behavior is reproducible as shown in Figure S3. However, this anomalous CV signal disappears when the perovskite layer is removed from the device, leading to a standard CV curve as shown in Figure 3d. Clearly, the anomalous CV signal is associated with the perovskite layer in our hysteresis-free devices, suggesting a photoinduced bulk polarization appearing after depleting the surface polarization. To confirm whether the anomalous CV signal can serve as a signature of photoinduced bulk polarization, we further analyze the CV peak shift with increasing the photoexcitation intensity, namely, the Vpeak shift. In our early studies we showed that the Vpeak shift is essentially caused by surface accumulation of photogenerated carriers upon changing photoexcitation intensity in solar cells.23−26 It should be noted that, when the surface polarization involves ion migration/accumulation, it can cause an IV hysteresis in perovskite solar cells.27−29 As shown in Figure 3b, without Cl incorporation the device shows a large CV peak shift of 119 mV upon increasing the photoexcitation intensity from 0.1 sun to 1 sun, indicating a surface 11544

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Figure 5. FA-based perovskite solar cells and photoinduced CV characteristics. (a) J−V characteristics of FA-based perovskite solar cells with the device structure of ITO/PEDOT:PSS/FAPbI3(Cs)/ PCBM/PEI/Ag. (b) Photoinduced CV characteristics ( f = 5 kHz) measured under simulated sunlight with variable intensities from 0.1 to 1 sun.

suggesting the presence of large surface polarization due to interface charge accumulation. Most importantly, the CV characteristics of the FA-based perovskite solar cells do not show a broad CV signal associated with the photoinduced bulk polarization in the depletion region, opposite of the MA-based perovskite solar cells. Instead, the CV characteristics of the FAbased perovskite solar cells in Figure 5b are more similar to the typical CV behavior. The lack of photoinduced bulk polarization in the FA-based perovskite solar cells is likely due to (i) the much weaker dipole moment of FA+ as compared to that of MA+ and (ii) the larger rotational barrier of FA+ caused by the larger molecular size and enhanced hydrogen bonding between FA+ and the inorganic framework.34 Clearly, the comparison between MA-based and FA-based perovskites confirms that the photoinduced bulk polarization relies on the orientational/ rotational freedom of dipolar organic cations manipulated by photoexcitation through redistribution of photogenerated carriers. Next, we discuss the effects of Cl incorporation on photoinduced bulk polarization in MA-based perovskite solar cells. In the published works, the roles of Cl− in MA-based perovskite solar cells have been widely studied with the focus on morphological development35,36 and optoelectronic properties,37−40 such as improving film quality, modifying energy level or interface band bending, increasing carrier diffusion length, and reducing nonradiative recombination. However, the present knowledge of the Cl incorporation is difficult to correlate with the bulk polarization of hybrid perovskite. To address how Cl incorporation influences bulk polarization, we performed Kelvin probe force microscopy (KPFM) measurements on the MA-based perovskite solar cells with and without Cl incorporation. KPFM is known to be an effective tool to map the surface potential on local grain boundaries and grain bulk. Essentially, the surface potential arises from the contact potential difference (CPD) between the tip and sample surface, which can reflect local polarization. The topography images of perovskite thin film with and without Cl incorporation do not show a significant change in morphology (Figure 6a and b). However, the perovskite thin film without Cl incorporation shows a blurry mapping of CPD (Figure 6d), whereas the perovskite thin film with Cl incorporation exhibits clear contrast in CPD with a lower potential about ∼20 mV at grain boundaries as compared to the grain bulk (Figure 6e). The potential difference between the grain boundaries and grain bulk is in good agreement with the height difference as shown in Figure S5, which can exclude the effects of charging or band bending at grain boundaries. Meanwhile, the blurry contrast in CPD for a perovskite thin film without Cl incorporation is highly associated with the surface charged

Figure 4. Proposed mechanism for photoinduced bulk polarization. (a) Random dipolar organic cations in perovskite crystalline structure in dark conditions; (b) alignment of dipolar organic cations due to reduced potential barrier of orientating dipolar organic cations upon distributing photoexcited electrons and holes; (c) schematic diagram to show potential barriers between random and aligned dipolar organic cations in dark (dashed) and photoexcitation (solid) conditions. ΔE and ΔE′ represent the rotational barriers in the dark condition and under photoexcitation.

are bound with the inorganic framework, particularly with halogens, through hydrogen bonding. In dark conditions and room temperature, the MAPbI3 is in the tetragonal phase where the dipolar organic cations are randomly oriented (Figure 4a) with a large rotational barrier (ΔE) as compared to the cubic phase at higher temperature. Under photoexcitation, the interband excitation can occur through electron transfer from the hybridized Pb 6s−I 5p orbital to the Pb 6p orbital within the inorganic framework [PbI6]− (Figure 4b). The distribution of photoexcited electrons and holes is expected to decrease the rotational barrier for MA+ dipoles, consequently increasing the dipolar polarization (Figure 4c). The binding energy between MA+ and [PbI6]− has been theoretically shown to decrease by interband excitation.30,31 Therefore, a photoexcitation can essentially induce bulk polarization in perovskite solar cells under device-operating conditions by decreasing the potential barrier to realize the orientation of dipolar organic cations. To address the role of organic cations in developing photoinduced bulk polarization shown as an anomalous CV characteristic, we further studied formamidinium (FA)-based perovskites (FAPbI3(Cs)) as compared to MA-based perovskites. Here, the FA+ cations have a much lower dipole moment (μ = 0.2 D) and larger orientation-potential barrier (Erot = 13.9 kJ/mol) than the MA+ cations (μ = 2.3 D, Erot = 1.3 kJ/ mol).15,32 Meanwhile, 15 mol % Cs+ (cesium) was introduced into this type of perovskites to stabilize the photoactive black phase (α-phase) of FAPbI3 due to the stronger interaction between the Cs+ and the inorganic framework.33 The FA-based perovskite solar cells, sharing the same device geometry as MAbased perovskite solar cells, can give a PCE of 14.27% with a slight J−V hysteresis, as shown in Figure 5a. The CV characteristics of the device exhibit a large Vpeak shift of about 360 mV when excitation intensity increases up to 1 sun, 11545

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Figure 6. Kelvin probe force microscopy images of the surface potential of MA-based perovskite thin films. Topography height images of a perovskite thin film without Cl incorporation (a), with Cl incorporation (b), and aged sample with Cl incorporation (c); contact potential difference (CPD) images of a perovskite thin film without Cl incorporation (d), with Cl incorporation (e), and aged sample with Cl incorporation (f). Images are zero-order flattened for clear comparison. The perovskite thin film was spin-cast on top of the PEDOT:PSS/Si wafer. The aged sample was kept in ambient conditions over 24 h without any protection.

defects. This is further confirmed by the blurred contrast in the CPD image of the aged perovskite thin film with Cl incorporation (Figure 6c and f), which was produced by keeping the previous sample with Cl incorporation at ambient conditions over 24 h without any protection. Therefore, the KPFM study demonstrates that the Cl incorporation can effectively reduce the surface charged defects during crystallization. It should be noted that charged defects can screen the bulk polarization in CV measurements. Therefore, the enhanced photoinduced bulk polarization upon Cl incorporation can be attributed to the reduced screening effect from the charged defects. In solar cells, radiative recombination should be suppressed to become a useful component for generating photocurrent, while the nonradiative recombination can be suppressed by reducing grain boundary defects. In this study, we explore the effects of photoinduced bulk polarization on radiative recombination in a series of MA-based perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag] with and without Cl incorporation by using bias-dependent photoluminescence (PL) measurements. Our early study has shown that bias-dependent PL can be conveniently performed on perovskite solar cells by measuring the PL intensity under a forward bias changed from Voc condition, where the built-in field is negligible, to Jsc conditions, where the built-in field exists.26 Essentially, changing the forward bias from Voc to Jsc conditions leads to a decrease in PL intensity, generating a ΔPL. This ΔPL can reflect the effects of the built-in field on the radiative recombination of photogenerated carriers. We should note that, when the bulk polarization is increased by Cl incorporation through the passivation of grain boundary defects, changing the forward bias from Voc to Jsc conditions is expected to enlarge the amplitude of ΔPL. Therefore, comparing the ΔPL measured from Voc to Jsc conditions can indicate how bulk polarization affects the radiative recombination upon chlorine incorporation. Figure 7 presents the results

Figure 7. Bulk polarization effects on charge recombination in perovskite solar cells. Bias-dependent PL, shown as ΔPL measured between Voc and Jsc conditions, is given for MA-based perovskite solar cells with different CL/I molar ratios. ΔPL = [PL(V) − PL(Voc)]/PL(Voc).

of bias-dependent PL measurement at different biases between Voc and Jsc conditions. When changing the forward bias from Voc to Jsc conditions, the ΔPL is determined to be −37%, −53%, −71%, and −69% for the perovskite solar cells with CL/ I ratios of 0, 0.15, 0.25, and 0.30. Here, ΔPL is calculated by eq 1: ΔPL =

PL(Jsc ) − PL(Voc) PL(Voc)

(1)

where PL(Voc) and PL(Jsc) are the PL intensities measured at Voc and Jsc conditions. Clearly, chlorine incorporation enlarges the bias-dependent PL in MA-based perovskites. Essentially, this indicates that the chlorine incorporation simultaneously suppresses both nonradiative and radiative recombination by passivating the grain boundary defects and increasing bulk polarization. In general, the charge recombination can be determined by the capture radius (rc), given by eq 2:41 rc =

e2 4πεrε0kBT

(2)

where kB is Boltzmann’s constant, T is temperature, and ε represents the dielectric constant. This equation suggests that 11546

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and DMSO (4:1 v/v) at 70 °C. The FAPbI3(Cs) thin film was prepared by spin coating at 4000 rpm for 40 s. The as-cast thin film was immediately dipped into a toluene bath for 30 s and then annealed at 150 °C for 10 min. The PC61BM layer was coated on top of the perovskite layer at 2000 rpm for 60 s from a 20 mg/mL solution of chlorobenzene (from Sigma-Aldrich); the substrates were then kept in a glass Petri dish for 45 min. Afterward, a solution of PEI (Mw ∼2000, 50 wt % in water) in isopropyl alcohol (0.5 mg/mL) was spin-cast on top of the PCBM at 5000 rpm for 60 s. Finally, a 100 nm silver layer was thermally deposited as the cathode with an effective area of 0.06 cm2 under a vacuum of 7 × 10−7 Torr. Measurements and Characterizations. Current−voltage characteristics were measured using a Keithley 2612 source meter and solar simulator (Thermal Oriel 96000 300 W from Newport) at 100 mW/ cm2, AM 1.5G illumination in a nitrogen gas filled glovebox. CV characteristics were measured by the Agilent E4980A LCR meter under an alternating bias of 50 mV with a frequency at 5 kHz. The samples were measured in dark conditions and under photoexcitation with different intensities. Measurements were performed in a nitrogen environment. KPFM measurements were performed in lift mode (lift height of ∼50 nm) using a commercial AFM (Asylum Research, Cypher) and as-received Pt/Ir-coated (Nanosensors, PPP-EFM) AFM probes with a nominal mechanical resonance frequency and spring constant of 75 kHz and 2.8 N/m, respectively. Measurements were performed while an ac bias of 2 V, tuned to the mechanical resonance frequency of the cantilever, was directly applied to the conductive tip. Bias-dependent PL spectra of the solar cell devices were measured in nitrogen gas filled fluorescence spectrometer (SPEX Fluorolog III) under the photoexcitation of a 532 nm CW laser with controllable bias through a Keithley 2400 source meter.

the capture radius rc can be directly decreased upon increasing the bulk polarization. Based on photoinduced capacitance values with and without Cl incorporation at short-circuit conditions under 1 sun photoexcitation, we can estimate that the dielectric constant can be enhanced by approximately 40%, leading to a decrease in rc from 19 Å (about 2−3 unit cells) to 10 Å (about 1 unit cell) in our best perovskite solar cell with a Cl/I ratio of 0.25, which is estimated by using the static dielectric constant in dark conditions (ε0exp ≈ 30.5).42 This estimation provides a clear picture to show why the charge recombination is low in high-efficiency perovskite solar cells under the influence of photoinduced bulk polarization.

CONCLUSIONS In summary, by studying the CV characteristics of a series of MA-based perovskite solar cells, we observed an anomalous CV signal caused by photoexcitation within the depletion region from −0.5 to +0.5 V by gradually scanning a dc bias at a low ac bias (50 mV at 5 kHz) based on CV measurements. This anomalous CV feature can serve as the signature of photoinduced bulk polarization in MA-based perovskite solar cells by simultaneously drifting mobile ions with gradually scanning a dc bias and detecting bulk polarization with a low ac bias in CV measurements. The photoinduced bulk polarization can be attributed to the reduced rotational barrier of the dipolar MA+ cations upon photoexcitation. This is confirmed by comparing the CV characteristics of FA-based and MA-based perovskites. Meanwhile, we found that the photoinduced bulk polarization can be enhanced by a factor of 3 when the Cl incorporation is used to decrease the charge defects. The KPFM study clearly shows that Cl incorporation can reduce the surface charged defects, providing the necessary condition to enhance photoinduced bulk polarization by suppressing the screening effect. Most importantly, the photoinduced bulk polarization is shown to provide an important mechanism to suppress the charge recombination by decreasing the Coulomb capture radius. Clearly, our studies indicate that a photoexcitation can induce a bulk polarization through orientation/rational polarization toward decreasing charge recombination in perovskite solar cells.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06413. Histogram for photovoltaic parameters, evaluation of current−voltage hysteresis, reproducibility of the photoinduced capacitance−voltage characteristics, photoinduced capacitance−voltage characteristics for a series of perovskite solar cells, and local profile of contact potential difference and height across two neighboring grains (PDF)

EXPERIMENTAL SECTION

AUTHOR INFORMATION

Device Fabrication. All solar cells studied in this work were fabricated with a planar structure of ITO/PEDOT:PSS/perovskite/ PCBM/PEI/Ag. The PEDOT:PSS layer was spin-cast on top of precleaned indium tin oxide (ITO) substrates at 4000 rpm for 60 s and then annealed at 150 °C in ambient conditions for 30 min. The MAbased perovskites were prepared from the precursor solutions of MAPbI3 (1.35 M, with or without incorporating Cl) by dissolving methylammonium iodide (MAI, from 1-Material), lead(II) chloride (PbCl2, from Alfa Aesar), and lead(II) iodide (PbI2, from Alfa Aesar) with the molar ratio of PbCl2 to PbI2 varied from 0 to 0.15, 0.25, and 0.30. The mixed solvent of γ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) (7:3 v/v, from Sigma-Aldrich) was used for the precursor solutions, which were stirred at 70 °C for 4 h and then cooled to room temperature before using. The MA-based perovskite thin films were spin-cast on top of PEDOT:PSS with a two-step spincoating process (1000 rpm for 10 s and 4000 rpm for 60 s). The substrates were subjected to a fast antisolvent extraction process with toluene (400 μL) at the 20th second of the second step, followed by thermal annealing at 100 °C for 10 min. Alternatively, the FA-based perovskites were made from the precursor solution of FAPbI3(Cs) by mixing formamidinium iodide (FAI, from 1-Material, 0.8 M), cesium iodide (CsI, ≥99.9% metals basis, from Sigma-Aldrich, 0.2 M), and PbI2 (1 M) into a solvent mixture of N,N-dimethylformamide (DMF)

Corresponding Author

*E-mail: [email protected]. ORCID

Bin Hu: 0000-0002-1573-7625 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the financial support from Air Force Office of Scientific Research (AFOSR) under grant number FA 9550-15-1-0064, AOARD (FA2386-15-1-4104), and National Science Foundation (CBET-1438181). This research was partially conducted at the Center for Nanophase Materials Sciences based on user projects (CNMS2016-279, CNMS2016-R45), which is sponsored by Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. The authors also acknowledge the project support from National Science Foundation of China (Grant Nos. 61475051, 2014CB643506, and 2013CB922104). 11547

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DOI: 10.1021/acsnano.7b06413 ACS Nano 2017, 11, 11542−11549

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DOI: 10.1021/acsnano.7b06413 ACS Nano 2017, 11, 11542−11549