PhotoluminescenceâVoltage (PLâV

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The Photoluminescence-Voltage (PL-V) Hysteresis of Perovskite Solar Cells Zhihua Xu, Taryn De Rosia, and Kevin Weeks J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06711 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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The Journal of Physical Chemistry

The Photoluminescence-Voltage (PL-V) Hysteresis of Perovskite Solar Cells

Zhihua Xu,* Taryn De Rosia, Kevin Weeks Department of Chemical Engineering, University of Minnesota-Duluth, Duluth, MN 55812

Abstract This article investigates the effect of an external electric field on the photoluminescence (PL) of methylammonium lead iodide (MAPbI3) film in a working solar cell architecture. Our study reveals hysteretic PL intensity responses when changing the voltage scanning direction, namely PL-V hysteresis. The external electric field is found to have multiple effects on the photo-excited states of PSCs. Firstly, an external electric field instantaneously changes the drift velocity of photo-generated charge carriers. Secondly, it drives ion migration and thus generates an induced electric field which screens the external field. Thirdly, the ion migration driven by the external electric field also changes the distribution and density of charge traps that are responsible for nonradiative recombination. The first effect leads to instant PL change which is not responsible for PL-V hysteresis, while the other two effects are closely related to the slow kinetics of ion migration and lead to the PL-V hysteresis in perovskite solar cells.

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Introduction Over the past few years organometal halide perovskites (OHPs) have emerged as promising light harvesting materials for photovoltaics (PVs), owing to the traits such as tunable energy band gap, high light absorption coefficient and long charge diffusion length. 1-9 The record power conversion efficiency (PCE) of perovskite solar cells (PSCs) with OHPs as the light absorber has exceeded 22%, on par with other commercialized thin film PV technologies such as Cadmium telluride (CdTe) and Copper indium gallium selenide (CIGS) solar cells. Furthermore, the OHPs promise to be a cost-effective alternative to the crystalline silicon as the dominant PV materials because they can be synthesized by earth abundant elements and processed with inexpensive solution-based methods.10 Despite the remarkable achievement in device efficiency, perovskite solar cells still face a few critical challenges towards a marketable PV technology, such as device stability and the toxicity of the lead in OHPs.11 Another issue with PSCs is the presence of current density-voltage (J-V) hysteresis, i.e., the J-V curves and the PCE of a specific perovskite solar cell vary with the voltage sweeping direction, prebias, light soaking, etc.12-15 The existence of hysteresis imposes a difficulty for accurate determination of device efficiency. More importantly, it leads to a significant concern of power output stability for the future large-scale PSC panels. Although the origin of the J-V hysteresis in PSCs have been explained by different mechanisms such as ferroelectric polarization, capacitive effects and charge trapping, more and more researches have suggested ion migration plays a dominant role for the J-V hysteresis in PSCs. 16-22 Under the ion migration mechanism, the electrostatic screening effect of the mobile ions in OHPs is generally considered as the origin of the J-V hysteresis. However, some recent researches have suggested that the mobile ions might also be related to charge trapping and nonradiative recombination 2 ACS Paragon Plus Environment

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processes. 23, 24 Numerical modeling has suggested that the hysteresis effects in perovskite solar cells originate from a combination of ionic transport and trapping of electronic carriers. 25, 26 Steady-state and time-resolved PL spectroscopies are powerful tools to study the photophysics of solar cells. 27 They have been also utilized to study the ion migration processes in OHPs under external electric field. For example, Leijtens et al. studied the PL response as a function of electric field intensity for a perovskite thin film sandwiched between two noninjecting electrodes. 28

They observed that at room temperature an electric field instantaneously quenches the PL

intensity due to field-induced charge separation and drifting, while over a longer time scale an external electric field reduces the nonradiative recombination rate by facilitating ions migration. Jacob et al. characterized the slow transient PL responses (0.5-100 s) of methylammonium lead iodide (MAPbI3) on a lateral structure (Au/MAPbI3/Au), and observed a reversible and an irreversible PL quenching under application of an electrical field.29 The reversible PL response was attributed to charge trapping, whereas the irreversible response, occurring at higher electric field, was attributed to ion migration. Similar reversible and irreversible PL quenching was considered as the result of ion migration and moisture-assisted electric field induced decomposition by Deng et al.30 Qiu and Grey studied electric field-dependent PL from MAPbBr3 perovskite crystals and they attributed field-induced reorientation of the MA+ dipole moment as the dominant mechanism responsible for the strong PL modulation by electric field.31 Most recently, Zhang et al. studied the temperature-dependent electric field poling effect on Photocurrent (PC) and PL of a lateral device (Au/MAPbI3/Au), and they considered the “selfdoping” effect of the mobile ions and the dipole alignment of MA+ as the origins for the PC and PL hysteresis.32 In summary, the studies conducted on the OHP thin films with non-injecting or symmetric contacts have all suggested certain links between the PL intensity and ion migration

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under an external electric field. In order to further clarify the roles of ion migration in perovskite solar cells, this article investigates steady-state and time-resolved PL of the OHP thin films in a working solar cell architecture under external electric field. Our study reveals a PL intensity voltage (PL-V) hysteresis, resulting from the effects of ion migration on charge drifting and recombination processes in PSCs. Experimental Methods Solar Cell Fabrication: Aqueous TiCl4 solution (2M) is spin-casted on a pre-cleaned Fluorinedoped SnO2 (FTO) substrate and annealed at 500oC for 20 min to yield a compact TiO2 layer. A TiO2 paste (NR18T, Dyesol), diluted with terpineol and ethanol, is then spin-casted on the compact-TiO2 layer followed by annealing at 500 oC for 40 min in air to yield a mesoporous TiO2 (mp-TiO2) film. CH3NH3I (MAI, 1-Materials) and PbI2 (Aldrich) are dissolved in a mixture of γ-butyrolactone (GBL) and N,N-dimethylformamide (DMSO) (7:3 v/v) to yield a 0.6M MAPbI3 solution, which is then deposited on the mp-TiO2 substrate by a consecutive twostep spin-coating process at 1,000 and 4,000 r.p.m for 10 and 30 s, respectively. And toluene dripping is applied for 5s at the beginning of second spin-coating step.33 After the MAPbI3 film is dried on a hot plate at 100 oC for 10 min, a hole transporting layer is deposited by spin coating a solution containing 72.3 mg (2,29,7,79-tetrakis(N,N-di-p-methoxyphenylamine)-9,9spirobifluorene) (spiro-MeOTAD), 28.8 ml 4-tert-butylpyridine, 17.5 ml of a stock solution of 520mg/ml lithium bis(trifluoromethylsulphonyl) imide in acetonitrile, and 1 ml of chlorobenzene. Finally, 60nm of gold is thermally evaporated on top of the device to form the back electrode. The active area of this electrode is determined by a shadow mask at 0.12 cm2. Characterizations: The photovoltaic properties of the perovskite solar cells are measured by a Keithley 2400 sourcemeter under 100mW/cm2, AM 1.5 G illumination generated by a solar 4 ACS Paragon Plus Environment

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simulator (Newport, Oriel Class A, 91195A). Steady-state PL characterizations are carried out with a Horiba Nanolog spectrofluorometer. For PL-V measurements, The FTO side of PSC is covered by a black mask with a small opening, which is used for light excitation and PL collection. The location of the opening is carefully aligned to the gold electrode to ensure the PL signal is collected from the active area of the solar cells. While collecting PL signal, a fixed voltage or sweeping voltages can be applied to the solar cells by the Keithley sourcemeter. The intensity of the 480nm excitation light is controlled in a way to yield the same short-circuit current for the device as it is exposed to standard 1 sun illumination. In order to minimize the light-induced effect on PL change, the PL intensity of the device is monitored and let to be stabilized before applying the external voltage. For time-resolved PL measurements, perovskite solar cells are held on an inverted microscope and illuminated through an objective lens with a 480 nm laser supplied by the doubled output of a mode-locked Ti:sapphire laser. The PL emission is separated from the laser excitation with a long-pass dichroic beam splitter and longpass interference filter, and detected by an avalanche photodiode. PL Lifetimes are then characterized by a time-correlated single-photon counting electronics (PicoHarp 300, PicoQuant, Inc.). Results and Discussion Fig. 1a shows a typical mesoscopic device structure (FTO/Compact TiO2/Meso-TiO2/ MAPbI3/Spiro-OMeTAD/Au) of the perovskite solar cells used for this study, where MAPbI3 perovskite is spin-casted on a mesoporous TiO2 (Meso-TiO2) layer, forming a continuous capping layer (Fig. 1b). And the perovskite layer is sandwiched between a compact TiO2 and spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene), which serve as electron transport layer (ETL) and hole transporting layer (HTL), respectively. Finally, 5 ACS Paragon Plus Environment


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fluorine-doped tin oxide (FTO) and Au form the two electrode contacts. The PL measurements are carried out on the transparent FTO glass side with excitation wavelength at 480 nm. Based on the absorption spectrum of the MAPbI3 film (Fig. S1, Supporting Information), the penetration depth at this wavelength is about 350nm, sufficient to collect the PL signal from the bulk OHP film in perovskite solar cells. When collecting the PL signal from the active area of the solar cells, a constant or sweeping of voltage bias could be applied to the film by a Keithley sourcemeter. The polarity of the external electric field (Eex) is defined as shown in Fig.1a, where a positive Eex refers to a field directed from Au electrode to FTO electrode. Fig. 1c presents typical PL spectra obtained from perovskite solar cells under external biases. We note that the application of the electric field does not lead to visible change in PL peak position. The PSCs show evident J-V hysteresis (Fig. 1d), and a reverse scan (from +1 to 0 V) usually leads to higher efficiency than a forward scan (from 0 to +1 V). Fig. 2 and Fig. 3 represent the typical PL intensity vs voltage (PL-V) characteristics we have observed with the perovskite solar cells. Generally, applying an electric field will facilitate the drift and separation of photo-generated charge carriers, i.e., electrons and holes, and subsequently suppress the radiative charge recombination and PL emission in organic and inorganic semiconductors.28, 34, 35 However, as shown in Fig. 2a, the PL intensity of the device, namely PSC1, increases with increasing voltage between 0 to +1 V. This is clearly different from what has been observed from the OHP films sandwiched by symmetric contacts where an external electric field (Eex) quenches PL intensity. 28-2930 This discrepancy is attributed to the existence of an internal built-in electric field (Ebi), resulted from the p-i-n device structure of the perovskite solar cells.18,19 Furthermore, as shown in Fig. 4a, the Eex generated by a positive voltage acts in the opposite direction with respect to the Ebi. As a result, the net electric field (En)


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applied on the perovskite film decreases with increasing voltage, and consequently leads to PL enhancement. Similar PL enhancement by a positive bias has been observed with GaAs and InGaAs solar cells.36,37 Within this framework, a maximal PL intensity can be achieved by applying a positive voltage, namely VMPL, which completely offsets the Ebi and leads to En =0. Indeed, when the PL-V measurement for this cell is extended to +1.5V, we do observe the PL intensity plateaued at +1.2 V (Fig. S2, Supporting Information). However, the PL measurement at high voltage is complicated by the occurrence of electroluminescence (EL). As shown in Figure S3 (Supplementary Information), the turn-on voltage of EL is between 1.2 to 1.4 V for the perovskite solar cells used for this study. We note that limiting the highest testing voltage at 1V not only avoids the influence of EL, but also improves the device stability for achieving repeatable PL-V results. As shown in Fig. 2a, the PL-V curves reveal evident difference (hysteresis) between forward (0 to +1V) and reverse (+1 to 0 V) scans. Furthermore, the PL-V hysteresis changes with voltage scanning rate. Here we hypothesize the electrostatic screening effect of the mobile ions in MAPbI3 as the common origin for both J-V hysteresis and PL-V hysteresis. According to the ion migration mechanism, an external electric field (Eex) drives the mobile anions and cations in the perovksite film towards the interfaces with ETL and HTL, forming an induced internal electric field (Ein) which partially screens the Eex (Fig. 4a).19 As a result, the magnitude of net electric field, | |, can be calculated by Eq. 1 | | = | | − | | + | |

(1)

Where, | |, | |, and | |are the magnitude of the built-in electric field, the external electric field and the ion-migration-induced electric field, respectively. Comparing to the high drift velocity of the electrons and holes under an electric field, ion migration is usually a much slower 7 ACS Paragon Plus Environment


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process.17, 20 The characteristic time of ion migration could be of the same order of magnitude as the external voltage scanning time. As a result, the ion-migration-induced electric field Ein not only depends on the magnitude of the external bias, but also the voltage scanning parameters, and subsequently leads to the PL-V hysteresis shown in Fig. 2a. To further test this hypothesis, we have characterized the time-dependent PL intensity of PSC1 under a constant external bias. As shown in Fig. 2b, after +1 V bias is turned on, the PL intensity shows an instant enhancement followed by a slow quenching process which lasts over tens of second. The instant PL enhancement suggests an instant reduction of | | due to an increase of | | when the bias is turned on, while the slow PL decay indicates a slow enhancement of | |, which is consistent with an increase of | | resulting from ion migration and subsequent accumulation at interfaces.19 When the polarity of the bias is switched from positive to negative, the direction of Ein will be switched accordingly, while Ebi will remain unchanged (Fig 4b). As a result, | | should be calculated as Eq. 2. | | = | | + | | − | |

(2 )

Since the | | can only partially offset | |, we expect PL intensity decreases with increasing negative voltage. Indeed, when the cell is scanned with increasing negative bias from 0 to -1 V (forward scan), we observe a monotonous PL intensity decay (black curve, Fig 2c). However, when the cell is swept reversibly (from -1 to 0 V), we observe a nonmonotonous change of PL intensity. As shown in Figure 2c (red curve), the PL intensity decreases and reaches a minimum at around -0.5 V and rises afterwards. Fig. 2d shows the time-dependent PL intensity under a constant -1 V bias. The PL intensity of the PSC shows three consecutive changes: an instant


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quenching, a slow decay, and a slow enhancement. Based on Eq. 2, the instant PL quenching can be certainly attributed to instantly increasing of | | when the negative bias is applied, and the slow PL enhancement is consistent with the increase of | | resulting from ion migration. However, the slow PL quenching process in Fig. 2d, as well as the nonmonotonous change of PL intensity observed in Fig. 2c, cannot be satisfactorily explained by the screening effect of the mobile ions. In addition to the typical PL-V characteristics shown in Fig 2, we also observed some devices demonstrate another type of hysteretic PL-V curves as shown in Fig 3. When the cell, namely PSC2, is exposed to forward scan from 0 to +1 V (Fig 3a), the PL intensity increases and reaches a maximum (VMPL = +0.8V) and decreases afterwards. At the reverse scan (+1 to 0 V), the VMPL is even smaller (+0.5V). Comparing with PSC1, which has VMPL greater than +1 V, this marked distinction suggests a significant lower Ebi in PSC2, which might result from ion accumulation at or close to the electrode interfaces.19, 23, 26 We note that ,comparing with PSC1-type devices, PSC2-type devices generally show smaller open circuit voltage (VOC) under 1-sun illumination, which also suggest a lower built-in potential.19 As a result, under a constant +1V bias, the Eex is higher than the Ebi in PSC2, and thus the direction of En is aligned with the Eex, as shown Fig. 4c. In this scenario, the | | should be calculated from Eq. 3 | | = | | − | | − | |

(3)

Based on Eq. 3, when +1 V bias is switched on, the PL intensity of PSC2 should show an instant change due to the increase of | | followed by a slow enhancement due to the increase of | | resulted from ion migration. However, as shown in Fig. 3b, the PL intensity response to +1V bias reveals an instant quenching followed by a slow decay. When the device is exposed to a constant -1V bias, as shown in Fig. 4d, the Ebi and Eex are in the same direction, and | | should 9 ACS Paragon Plus Environment


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be calculated with Eq. 2, which suggests a PL recovery with increasing| |. However, it can be seen in Fig 3d that after the initial drop, the PL intensity of PSC2 continues to grow and eventually exceeds the original intensity. This surprisingly large PL enhancement can’t be solely attributed to the change of the magnitude of Ein resulting from ion migration. In summary, some of the PL-V characteristics of the perovskite solar cells cannot be satisfactorily explained within the framework of field-assisted charge drifting and the screening effect of ion migration. Our observations strongly suggest a more complex role of the mobile ions in regulating the PL intensity of PSCs. Under an external electric field, the radiative recombination, which leads to PL emission, is competing with charge collection and nonradiative recombination processes in the PSCs. As a result, the PL intensity decreases when charge drift velocity and nonradiative recombination rate are enhanced. The charge drifting is assisted by electric field, while the nonradiative radiative recombination rate is mainly determined by the density of charge traps.38, 39 Applying an external electric field (Eex) can instantaneously change the charge drift velocity and lead to an instant PL intensity change. It will also drive ion migration which leads to a slow change of Ein, and hence a slow PL intensity change, which contributes to the PL-V hysteresis. Our time-resolved PL decay result suggest that ion migration also changes the nonradiative recombination rate in the OHP films, as has been suggested by other publications. 24, 28, 32 Fig. 5 shows that the PL lifetimes of PSC2 is shortened by applying a constant +1V bias, but it is clearly elongated by -1V bias. Here we consider the PL lifetimes of the PSC under external electric field is regulated by both charge drifting and nonradiative recombination. Lower drift velocity and nonradiative recombination rate will lead to longer PL lifetimes. We also note that the collection of PL signal from the biased device for lifetime analysis lasts one minute. In this time frame, the external bias is largely compensated by the ion-


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migration-induced internal field, leading to minimized effect on charge drift velocity. Therefore, the PL decays shown in Fig. 5a mainly reflect the ion migration effect on nonradiative recombination dynamics under voltage biases. As a result, the elongation of lifetime under -1V bias tends to suggest a reduced nonradiative recombination rate, while the shortening of lifetime by +1V bias indicates an enhancement of nonradiative recombination. The external electric field effect on nonradiative recombination rate is further examined by the excitation density dependence of the PL intensity of a PSC2-type device. Generally, the PL intensity (IPL) of MAPbI3 increases super-linearly with the excitation density (IEX) and can be fitted with a power law: ∝ .The super-linear increase of PL intensity with excitation

density stems from the competition between the trap-mediated nonradiative recombination and the radiative bimolecular recombination.40, 41 At the low excitation, the nonradiative recombination is the dominant mechanism due to abundant traps available to the charge carriers, leading to low PL quantum yield. With increasing excitation density, the radiative recombination becomes the dominant process due to saturation of traps, leading to higher PL quantum yield. The value of power exponent α roughly represents the saturation degree, and the larger α corresponds to the larger density of traps.41 As shown in Fig. 5b, the power exponent α is extracted to be 1.57 when the device is short-circuited (0 V bias). The value of α increases to 1.78 under +1 V bias, and it slightly decreases to 1.53 under -1 V bias. This result further suggests that +1 V bias enhances trap density and nonradiative recombination rate, while -1 V bias plays an opposite role. However, direct evidence about the trap density change is still needed to fully support this conclusion. Based on the PL-V results, here we propose a preliminary mechanism for the effect of ion migration on nonradiative charge recombination in perovskite solar cells. During ion migration 11 ACS Paragon Plus Environment


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under an external electric field, the anions and cations will inevitably interact with other mobile ions or immobile defects. The interaction might change the density and distribution of the charge-trapping defects which are responsible for nonradiative recombination. For example, when the iodine vacancy, which are widely considered as the mobile ions in OHPs,17-18, 24 migrate into a defect area (such as grain boundaries) with high density of iodine interstitials may lead to ion recombination and defect annihilation which reduces the nonradiative recombination rate. Under another scenario, when mobile ions move into a perfectly crystallized film area, the so-called “self-doping” effect may generate new charge-trapping defects which facilitate nonradiative recombination.32, 42 Therefore, the net effect of ion migration on nonradiative recombination should vary with the landscape of mobile ions and charge traps in the perovskite film, which largely depends on film deposition and interface engineering procedures.14, 33, 43 Leijtens et al. have suggested that ion migration to the interfaces will reduce the density of charge traps in the bulk MAPbI3 film and enhance PL intensity.28 While Zhang et al. have observed PL quenching due to the “self-doping” effect of ion migration in MAPbI3 film.32 For the MAPbI3 films incorporated in a solar cell architecture, we found the effect of ion migration on nonraradiative recombination varies significantly with the polarity of external electric field. For example, for PSC2, the PL decay shown in Fig. 3b can be attributed to an enhancement of nonradiative recombination under a positive bias, while the large PL enhancement shown in Fig. 3d strongly suggests a reduced nonradiative recombination rate by a negative bias. The built-in electric field in a solar cell architecture, which inevitably leads to certain degree of polarization and inhomogeneous distribution of the mobile ions and charge traps in the OHP film, might be responsible for the polarity-dependent ion migration effect on nonradiative recombination. For example, in our devices, the traps formed during device fabrication may be preferably located


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near one of the charge extraction interfaces, i.e., ETL or HTL. And the mobile ions moving into or out of this interface area under different voltage biases, e.g. +1V and -1V, will either increase or decrease the trap density, and thus lead to the bias-dependent PL intensity change. Therefore, the slow PL rise under -1V bias (Fig. 3d) could be attributed to the double effects of ion migration: screening of external bias and reduction of trap density. Nevertheless, a clear picture about the nature of the mobile ions and the landscape of the charge traps in the OHPs films is certainly needed to fully understand the variation of the ion migration effect on nonradiative recombination in perovskite solar cells. We note that there are other factors could potentially influence PL-V hysteresis. For example, the power conversion efficiency of the device and the thermal effect induced by the illumination. Our champion device with 15.5% efficiency clearly exhibits PSC1-type PL-V hysteresis as shown in Fig. S4 and Fig. S5. However, devices with lower efficiency could show either PSC1 or PSC2 type of PL-V hysteresis. Therefore, more systematic study is certainly needed to unveil the effect of photovoltaic performance on PL-V hysteresis in perovskite solar cells.” Continuous light illumination will increase the temperature of the devices, and thus could potentially change PL intensity. We have compared the PL intensity of our device biased with +1V under illumination of two different power density (100 mW/cm2 and 20 mW/cm2) as shown in Fig. S5. The trivial difference between the two PL intensity curves suggest the thermal effect induced by light illumination does not play a significant role in determining the PL-V characteristics. Conclusions In conclusion, we have investigated the external electric field effect on the PL of MAPbI3 films in a working solar cell architecture. Applying an external electric field will instantaneously 13 ACS Paragon Plus Environment


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change the PL intensity by modifying the charge drift velocity, but this instant PL change is not considered to be responsible for PL-V hysteresis. The origin of the hysteretic PL-V characteristics is attributed to a combination of the electrostatic screening effect and nonradiative recombination effect of ion migration under external electric field. The screening effect of ion migration leads to a change of the net electric field applied to the MAPbI3 layer and hence changes the drift velocity of the photo-generated charge carriers. Ion migration also modifies the landscape of charge traps which determines the nonradiative recombination rate. Both effects are closely related to the slow kinetics of ion migration and contribute to PL intensity changes which depend on the voltage scanning parameters, such as direction and rate, and consequently lead to the PL-V hysteresis. Although the screening effect of ion migration is generally considered as the dominant mechanism leading to the J-V hysteresis in PSCs, our study suggests the effect of ion migration on charge recombination also contribute to the hysteresis. Further investigation on the interactions between the mobile ions and trap defects in perovskite solar cells is certainly needed to fully understand the hysteresis behaviors in perovskite solar cells. Furthermore, charge recombination in a working solar cell reduces power conversion efficiency, and therefore, an in-depth understanding of the ion-migration-induced charge recombination may lead to a new pathway to enhance the device performance of perovskite solar cells.

ACKNOWLEDGMENTS This work is supported by the Grant-in-Aid program of the University of Minnesota. Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. Use of the Center for Nanoscale Materials, an Office of Science 14 ACS Paragon Plus Environment

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user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This project was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Visiting Faculty Program (VFP). The authors would like to thank Dr. Mircea Cotlet, Dr. Chang-Yong Nam, Dr. Seth Darling, and Dr. David Gosztola for their technical support for solar cell fabrication and characterizations.

Supporting Information. UV-vis absorption spectrum of MAPbI3 films on Mesoporous TiO2 substrate. The Photoluminescenece (PL) and electroluminscence (EL) intensity vs voltage for the perovskite solar cells. The J-V, PL-V, and PL vs time of a perovskite solar cells. This information is available free of charge via the Internet at http://pubs.acs.org



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17. Meloni, S.; Moehl, T.; Tress, W.; Franckevicius, M.; Saliba, M.; Lee, Y. H.; Gao, P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Rothlisberger, U.; Graetzel, M., Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells. Nature Communications 2016, 7, 9. 18. Li, C.; Tscheuschner, S.; Paulus, F.; Hopkinson, P. E.; Kiessling, J.; Kohler, A.; Vaynzof, Y.; Huettner, S., Iodine migration and its effect on hysteresis in perovskite solar cells. Advanced Materials 2016, 28 (12), 2446-2454. 19. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M., Understanding the rate-dependent J-V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy & Environmental Science 2015, 8 (3), 995-1004. 20. Eames, C.; Frost, J. M.; Barnes, P. R. F.; O'Regan, B. C.; Walsh, A.; Islam, M. S., Ionic transport in hybrid lead iodide perovskite solar cells. Nature Communications 2015, 6, 8. 21. Shao, Y. C.; Fang, Y. J.; Li, T.; Wang, Q.; Dong, Q. F.; Deng, Y. H.; Yuan, Y. B.; Wei, H. T.; Wang, M. Y.; Gruverman, A.; Shielda, J.; Huang, J. S., Grain boundary dominated ion migration in polycrystalline organic-inorganic halide perovskite films. Energy & Environmental Science 2016, 9 (5), 1752-1759. 22. Rajagopal, A.; Williams, S. T.; Chueh, C. C.; Jen, A. K. Y., Abnormal current-voltage hysteresis induced by reverse bias in organic-inorganic hybrid perovskite photovoltaics. Journal of Physical Chemistry Letters 2016, 7 (6), 995-1003. 23. Wu, Y. L.; Shen, H. P.; Walter, D.; Jacobs, D.; Duong, T.; Peng, J.; Jiang, L. C.; Cheng, Y. B.; Weber, K., On the origin of hysteresis in perovskite solar cells. Advanced Functional Materials 2016, 26 (37), 6807-6813. 24. Dequilettes, D. W.; Zhang, W.; Burlakov, V. M.; Graham, D. J.; Leijtens, T.; Osherov, A.; Bulovic, V.; Snaith, H. J.; Ginger, D. S.; Stranks, S. D., Photo-induced halide redistribution in organic-inorganic perovskite films. Nature Communications 2016, 7, 9. 25. van Reenen, S.; Kemerink, M.; Snaith, H. J., Modeling anomalous hysteresis in perovskite solar cells. Journal of Physical Chemistry Letters 2015, 6 (19), 3808-3814. 26. Calado, P.; Telford, A. M.; Bryant, D.; Li, X. E.; Nelson, J.; O'Regan, B. C.; Barnes, P. R. F., Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis. Nature Communications 2016, 7, 10. 27. Leijtens, T.; Hoke, E. T.; Grancini, G.; Slotcavage, D. J.; Eperon, G. E.; Ball, J. M.; De Bastiani, M.; Bowring, A. R.; Martino, N.; Wojciechowski, K. et al. Mapping Electric field-induced switchable poling and structural degradation in hybrid lead halide perovskite thin films. Advanced Energy Materials 2015, 5 (20), 11. 28. Leijtens, T.; Kandada, A. R. S.; Eperon, G. E.; Grancini, G.; D'Innocenzo, V.; Ball, J. M.; Stranks, S. D.; Snaith, H. J.; Petrozza, A., Modulating the electron-hole interaction in a hybrid lead halide perovskite with an electric field. Journal of the American Chemical Society 2015, 137 (49), 15451-15459. 29. Jacobs, D. L.; Scarpulla, M. A.; Wang, C.; Bunes, B. R.; Zang, L., Voltage-induced transients in methylammonium lead triiodide probed by dynamic photoluminescence spectroscopy. Journal of Physical Chemistry C 2016, 120 (15), 7893-7902. 30. Deng, X. F.; Wen, X. M.; Lau, C. F. J.; Young, T.; Yun, J.; Green, M. A.; Huang, S. J.; Ho-Baillie, A. W. Y., Electric field induced reversible and irreversible photoluminescence responses in methylammonium lead iodide perovskite. Journal of Materials Chemistry C 2016, 4 (38), 9060-9068. 31. Qiu, C.; Grey, J. K., Modulating charge recombination and structural dynamics in isolated organometal halide perovskite crystals by external electric fields. Journal of Physical Chemistry Letters 2015, 6 (22), 4560-4565. 32. Zhang, C.; Sun, D. L.; Liu, X. J.; Sheng, C. X.; Vardeny, Z. V., Temperature-dependent electric field poling effects in ch3nh3pbi3 optoelectronic devices. Journal of Physical Chemistry Letters 2017, 8 (7), 1429-1435. 17 ACS Paragon Plus Environment


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Figure Captions Figure 1. (a). A schematic device architecture of pervoskite solar cells. (b) Cross section SEM image of a pervoskite solar cell. (c) The PL spectra of a perovskite solar cell collected under -1, 0, and +1 V bias; (d). The current density-voltage (J-V) curves and device performance parameters with forward and reverse voltage scan at 0.04 V/s. Figure 2. The PL-V characteristics of the PSC1 sample. (a, c). The PL-V curves for forward and reverse scans between 0 and +1 V (a), and between 0 and -1v (c). (b, d).The PL intensity vs time under a constant +1 V (b) and -1 V (d) bias. The scan rate is at 0.04 V/s and 0.13V/s, respectively. Figure 3. The PL-V characteristics of the PSC2 sample. (a, c). The PL-V curves for forward and reverse scans between 0 and +1 V (a), and between 0 and -1v (c). (b, d).The PL intensity vs time under a constant +1 V (b) and -1 V (d) bias. Figure 4. Schematics about the external and internal electric fields for PSC1 under positive (a) and negative (b) voltage scanning, and for PSC2 under a constant +1V (c) and -1 V (d) bias. The arrows represent the electric field directions. The large“-”and “+” symbols in the figures represent the anions and cations, while the small symbols represent the electrons and holes, respectively. Eex: external electric field, Ein: ion-migration-induced electric field, Ebi: built-in electric field, En: net electric field. Figure 5. The time-resolved PL decay curves (a) and excitation density-dependent PL intensity (b) for PSC2 under different electrical biases.



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FIG. 1


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FIG. 2



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FIG. 3


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FIG. 4



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FIG. 5


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