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Multi-Influences of Ionic Migration on Illumination-Dependent Electrical Performances of Inverted Perovskite Solar Cells Jie Liu, Xingtian Yin,* Xiaobin Liu, Meidan Que, and Wenxiu Que* Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China S Supporting Information *

ABSTRACT: ZnO films are employed as the electron transport layers for perovskite solar cells. Such a device exhibits an ultralong time increase in Voc (∼100 s) and Jsc (∼1000 s) and a weakening hysteresis under continuous illumination. Besides, a slow (∼20 s) Voc decay when illumination is switched off is also observed. The electrical measurements performed under illumination and under voltage bias before being illuminated, suggest the influences of ionic accumulation/redistribution in causing above phenomena. Ionic accumulation happening in dark and ionic redistribution under illumination lead to band bending which affects the excitons separation and carrier extraction. These can account for the ultralong time increase in Voc and Jsc as well as the slow Voc decay. Also, the time-dependent photocurrent response under stepwise scan proves the presence of a capacitive effect in the device which can be dramatically reduced by the ionic redistribution under illumination. The ionic redistribution is also an important reason for the weakening hysteresis. perovskite film has already been demonstrated by the dynamic electrical behavior analysis of perovskite solar cells. For instance, the photoinduced halide redistribution in perovskite films has been demonstrated by using confocal photoluminescence microscopy.27 Kelvin probe force microscopy has also been successfully employed to observe the influence of ionic migration on the surface potential.28,29 However, the detailed mechanism of ionic migration effect on device performance under different conditions is still under debate. In this work, we fabricated inverted planar heterojunction perovskite solar cells with an architecture of ITO/ PEDOT:PSS/MAPbI3/ZnO/Al, which shows a reduced hysteresis behavior and significantly increased open-circuit voltage (Voc) and short-circuit photocurrent (Jsc) under illumination. A slow Voc decay is also observed when the illumination is switched off. The dynamic V oc and J sc characterizations were conducted to investigate the influence of ionic migration based on a band-bending model. Timedependent photocurrent responses and the use of J−V curve tests with bias pretreatment, illustrate the impact of ionic migration on the capacitive effect, which also contributes to the weakening of the hysteresis. Hence, our work provides some new insights on the influences of ionic migration on the complex performances of perovskite solar cells.

1. INTRODUCTION Organic−inorganic hybrid perovskites have shown great potential as candidates for low-cost high-efficiency solar cells with a demonstrated power conversion efficiency (PCE) increasing rapidly over the past few years from 3.8% to 22.1%.1,2 This unprecedented progress is contributed by many factors including the modification of perovskite compositions,3−5 optimization of fabricating processes,6−8 device structures,9,10 as well as interface engineering.11 However, a hysteresis phenomenon for the J−V curve is observed, which is usually affected by device preconditioning,12,13 the scan rate,12,14−16 and voltage range of the J−V curve,17 the thickness and quality of perovskite layer,18 and device architecture,18−20 leading to inaccurate measurement of the device PCE. Therefore, many methods have been explored to weaken or eliminate the hysteresis phenomenon such as by improving the perovskite film quality21 and the use of novel device structures and modified carrier transport materials.22,23 Researchers have also focused their attentions on the origins of hysteresis in perovskite devices. Several mechanisms have been proposed to explain the origin of hysteresis in perovskite devices such as ferroelectric polarization, ionic transport, charge trapping, and capacitive effects.14,24 For example, Chen et al. mainly attributed the hysteresis behavior to the slow transient capacitive current, ionic transport and ferroelectric polarization, rather than due to the trapping process based on its time scale consideration.25 Beilsten-Edmands et al., on the other hand, ruled out any ferroelectric response for perovskite and simply put hysteresis down to ionic migration occurring on a time scale of seconds.26 The existence of ionic migration in the © XXXX American Chemical Society

Received: June 28, 2017 Revised: July 6, 2017

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DOI: 10.1021/acs.jpcc.7b06329 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) TEM images, diffraction rings and HR-TEM image of prepared ZnO nanoparticles. (b) Cross-sectional SEM image of the spin-coated ZnO layer on the perovskite film.

Figure 2. Illumination-dependent electrical performances of the device. (a) J−V curves of the device under illuminated for different durations. (b) Dark J−V curves of the device before illumination and after 30 min illumination. Variations of (c) Voc and (d) Jsc with time under continuous illumination.

2. EXPERIMENTAL SECTION

dispersed in 2-propanol (IPA, Aladdin) mixed with 2% (v/v) ethanolamine (EA, SCRC) to form a uniform suspension31 with a concentration of 5 mg mL−1. After ultrasonic bath treatment for over 12 h, the suspension became ultratransparent, as shown in Figure S1. The ZnO dispersion was filtered with polytetrafluoroethylene (TPFE) filters (0.45 μm) twice before use. Preparation of Perovskite Precursor. Before fabricating perovskite solar cells, 2.3 g of PbI2 (p-OLED) and 0.8 g of CH3NH3I (p-OLED) were dissolved in 1.5 mL of dimethyl sulfoxide (DMSO, Alfa-Aesar) and 3.5 mL of N,N-dimethylformamide (DMF, Alfa-Aesar) to prepare the perovskite

Synthesis of ZnO Nanoparticles. The method to synthesize ZnO nanoparticles was referred to ref 30 with a little modification. First, 18.05 mmol Zn(Ac)2·2H2O (SCRS) was dissolved in 84 mL of methanol, and then the solution was stirred vigorously and heated to 65 °C. Subsequently, 14.44 mmol KOH (SCRS) mixed into 46 mL of methanol was poured into the above solution within 10−15 min. Finally, the solution obtained was continuously stirred at 65 °C for 2.5 h to obtain ZnO nanoparticles. Next, the suspension was centrifuged and washed with methanol twice. The final solid was then B

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The Journal of Physical Chemistry C precursor.32 The precursor was stirred at 70 °C for over 3 h, and then filtered through TPFE filters (0.45 μm) twice before use. Solar Cell Fabrication. First, a patterned ITO substrate was cleaned by sonication in acetone, ethanol, and deionized water successively. The ITO substrate was then dried by N2 flow and treated by UV−O3 for 15 min before use. Then (PEDOT: PSS):IPA (1:2, v/v) mixture was spin-coated on the ITO at a speed of 3,000 rpm for 60 s, followed by baking at 130 °C for 20 min. The substrate was then transferred into a glovebox to deposit the perovskite film. A Perovskite layer was spin-coated at a low speed of 1000 rpm for 5 s and then at a high speed of 3000 rpm for 30 s, during which 130 μL of chlorobenzene (CB) was dripped onto the spinning film. Then, the film was baked at 70 °C for 2 min followed by 100 °C for 10 min. When the perovskite film is cooled down, ZnO dispersion was spin coated at a speed of 1500 rpm for 45 s, and then baked at 70 °C for 10 min. Finally, the substrate was transferred into an evaporator, and 80-nm-thick Al electrodes were evaporated onto the ZnO film to complete the device fabrication. Characterization. The microstructural and crystalline properties of the ZnO nanoparticles were characterized by transmission electron microscopy (TEM, JEM-2010, JEOL Inc., Japan) and X-ray diffractometry (XRD, SMARTLAB, Rigaku, Japan). Scanning electron microscopy (SEM, JSM-6390, JEOL Inc., Japan) was used to analyze the morphological properties of the perovskite film and ZnO film. A JASCO V-570 UV/vis/ NIR spectrometer was used to acquire the absorption spectra of ZnO films. All the electrical measurements of devices were conducted at room temperature in air with a relative humidity of ∼30%. The light source applied in our experiment was a sunlight simulator with an OF 450 W Xe arc lamp and an AM1.5G filter (Newport Oriel). The J−V forward and backward scans were carried out by an electrochemical analyzer (CHI660), and the scan rate and sample interval were 50 mV s−1 and 5 mV, respectively. The time-dependent photocurrent responses under stepwise forward and backward scans as well as the dynamic Voc and Jsc transient responses with/without bias pretreatments were also measured by using CHI660. During the measurements of the time-dependent photocurrent responses, the voltage step and time step for stepwise scan were 100 mV and 5 s/20 s, respectively, and the voltage range was from −0.1 to 1 V. During the measurements of the dynamic Voc and Jsc transient responses with/without bias pretreatments, the devices were first pretreated with bias voltages in dark, and then the dynamic Voc and Jsc transient responses of the devices were tested under continuous illumination.

Figure 3. Dynamic (a) Voc and (b) Jsc response under intermittent illumination. The recovery time for Voc and Jsc is related to the duration of the dark period.

(PDF#36-1451). The absorption spectrum and corresponding Tauc plot (Figure S2b) suggest a band gap of ∼3.43 eV for the ZnO nanoparticles. Compact ZnO nanoparticles films were deposited on the top of perovskite films by spin-coating the ZnO colloid nanoparticles. The SEM image in Figure S3a shows that the ZnO film capping layer is very flat and no void can be observed. It is further ascertained by cross-sectional SEM image shown in Figure 1b that the thicknesses of the ZnO layer, perovskite layer and PEDOT:PSS layer to be 50, 400, and 40 nm, respectively. The perovskite solar cell device was completed by evaporating ∼100 nm Al cathode, and the cross-sectional SEM image of a completed device is provided in Figure S3b. Figure 2a shows the J−V curves for the device measured after being illuminated by a simulated sun light (AM 1.5) for different durations. It shows a weakening of the current hysteresis between the forward and backward voltage scans with the increasing duration of illumination. Figure 2b presents the dark J−V performances before and after 30 min illumination. Although hysteresis behavior is commonly observed to depend on scan rate and voltage range in normal planar perovskite solar cells,33−35 hysteresis behavior as a function of illumination is rarely reported. After the device is stored in dark for over 12 h, the Voc and Jsc increase gradually to their final values within hundreds of seconds (Figure 2, parts c and d) under illumination, respectively. It should be mentioned here that this phenomenon is not the same as the frequently reported light soaking processes which happen within only several seconds.22,36,37 To figure out the reasons for the ultralong time for Voc and Jsc to reach their stable values of ∼0.9 V and ∼15 mA cm−2 when continuously illuminated, the devices were subjected to light which is turned on and off, instead (Figure 3). The off period is varied from 5 to 25 s in increasing steps of 5 s. The corresponding responses are shown in Figure 3, parts a and b. It

3. RESULTS AND DISCUSSION SECTION 3.1. Characterizations and Illumination-Dependent Electrical Performances of the Device. The TEM images of prepared ZnO nanoparticles are presented in Figure 1a, in which uniform nanoparticles with a diameter of ∼5 nm can be clearly observed. The diffraction rings shown in the corresponding selected area confirm good crystalline property of these particles. HR-TEM image of a ZnO nanoparticle presents clear lattice fringes with a spacing of 0.28 nm, which should be indexed to the (1000) planes of wurtzite ZnO crystal. The crystalline property of the ZnO nanoparticles can also be demonstrated by the XRD pattern (Figure S2a), where all the observed diffraction peaks can be indexed to wurtzite ZnO C

DOI: 10.1021/acs.jpcc.7b06329 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Energy bands change induced by ionic migration process in our device. (a) In the dark, charged mobile ions are drifted to contacts so that built-in field is screened inside the perovskite layer and the corresponding bands diagram is flat. (b) When the device is exposed to light after being kept in the dark, the ions have no time to redistribute and the photogenerated free carriers in region II go diffuse slowly to ZnO. Meanwhile, part of photogenerated carriers associated with ions modifies the charge concentration near the contacts and then the accumulated ions will redistribute as illumination continues. (c) After a certain duration of illumination, ionic accumulation nearly disappears and the built-in field recovers due to the disappearance of compensating field. The photogenerated carriers can thus be extracted quickly by both drift and diffusion; (d) When the illumination is switched off, free carriers are no longer generated and the accumulated ions associated with carriers redistribute. After a certain period, the energy bands goes back to that seen in part a.

and Jsc to recover to their stable values. The longer the device is kept in dark, the longer is the recovery time for Voc and Jsc. 3.2. The Influence of Ionic Migration. It has been reported that organic−inorganic perovskite is a mixed ionicelectronic conductor.38 The migration activation energies has been demonstrated to be as low as 0.1 eV for iodine vacancies and interstitials, 0.5 and 0.8 eV for MA and Pb vacancies, respectively.39 To explain the above phenomena better, we illustrate the band diagrams for the device in Figure 4 under different conditions. The illustrations are presented as follows: i In dark (Figure 4a), built-in field (Vbi) drifts mobile cations and anions to accumulate near p-type PEDOT:PSS and n-type ZnO, respectively. Meanwhile, ionic accumulation forms a compensating field (Vcom) screening Vbi. So that the corresponding bands in region II is flat and the potential drop of Vbi only occurs in region I and III. ii It is reported that both free carriers and weakly bound excitons coexist in perovskite at room temperature.40 When the device is exposed to light after being kept in the dark (Figure 4b), the ionic distribution has no time to change. Most bound excitons recombine quickly because flat bands in region II cannot separate them efficiently. Moreover, the extraction efficiency of free carriers in region II is low compared with the condition where a field exists. Because photocurrent is the amount of carriers extracted per second, both the above reasons result in a poor initial Jsc. The carriers which are not extracted recombine inside the device. The above factors also contribute to a low carrier density at relative contacts under open-circuit condition as well, resulting in a low initial Voc. The detailed discussion can be found in the Supporting Information (Figure S4). Simultaneously, the accumulated ions can associate with free carriers due to Coulomb interaction and form neutral interfacial doping near the contacts.19 Then the associated ions start to redistribute due to the high concentration near the contacts, leading to the recovery of space electrical field inside perovskite layer, which promotes the separation of bound excitons and free carriers extraction. As a result,

Figure 5. Dynamic Voc transient of the device under illumination after the device was biased with positive and negative voltages for 60 s in dark.

Figure 6. Dynamic Jsc transient process of the device under illumination after the device is pretreated with positive and negative bias voltages for 60 s in dark.

is interesting to note that Voc and Jsc decay to zero values in different ways when the light is switched off: Voc decreases rapidly from ∼0.9 V to ∼0.55 V and then experiences a longer (∼20 s) decay, while Jsc drops almost immediately to zero. When the light is switched on, it takes some time for the Voc D

DOI: 10.1021/acs.jpcc.7b06329 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 7. Ionic accumulation-induced capacitive effect on J−V performances of the device. (a) J−V performances at a scan rate of 50 mV s−1 and corresponding time dependent forward and backward stepwise photocurrent responses with a voltage step of 100 mV and a time step of 5 s. (b) J−V performances for a device at different scan rates. (c) Cycling-time dependent forward stepwise photocurrent responses under illumination. (d) Variation of capacitive current density at each step extracted from plot c under illumination.

nation of these carriers leads to the slow Voc decay. In short-circuit condition, the generated carriers are extracted immediately, so that Jsc does not show any evidence of decay when illumination is switched off. Then Vbi drifts the released ions to contacts, and finally the ionic distribution and the energy bands recover to that as shown in Figure 4a. To further verify this mechanism, we applied a bias pretreatment (BPT) on the device to change the ionic distribution. A positive bias reduces the ionic accumulation, while a negative bias works in an opposite way. The effect of ionic distribution will be reflected in the performances of Voc. As shown in Figure 5, Voc approaches its stable value slowly after it is exposed to light. When a positive BPT is applied on the device, the Voc increases to its stable value rapidly. On the contrary, when a negative BPT is applied, it takes much longer time to reach the stable value than the untreated sample. So it can be understood that a positive BPT causes the ionic redistribution, while a negative BPT aggravates the ionic accumulation. In other words, a positive BPT can shorten the ionic redistribution process under illumination. As for the difference between forward and backward J−V curves, the ionic migration should make some great difference. A beginning high positive bias during the backward scan rapidly redistributes the accumulated ions, which recovers the Vbi partially and improves the current extraction. However, during the forward scan, the ions redistribution process is much slower as the applied voltage increases gradually. Therefore, the forward scan presents a lower current density as compared with the backward scan at the same voltage.

Figure 8. Effect of bias pretreatment on J−V performance of the device. J−V performances under illumination after the device is pretreated at a bias of 1.0 V for different durations in dark.

the recombination is minimized, and the Voc and Jsc gradually increase under continuous illumination. iii After a certain period of persistent illumination (Figure 4c), ions associated with carriers redistribute evenly throughout the perovskite film.27,38 Ultimately, a uniform potential drop of Vbi appears inside the perovskite layer, which maximizes the separation efficiency of bound excitons and the extraction efficiency of free carriers. Therefore, Voc and Jsc finally reach their stable values. iv When the illumination is switched off as shown in Figure 4d, free photoinduced carriers are no longer generated and the carriers associated with ions are released. Hence, a new potential difference is produced at contacts due to the injection of the released carriers, resulting in a dark Voc in open circuit condition. Then the slow recombiE

DOI: 10.1021/acs.jpcc.7b06329 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 3.3. Ionic Migration-Induced Capacitive Effect on Hysteresis Behavior. We further investigated the dynamic Jsc transient processes with BPTs in dark. As shown in Figure 6, Jsc exhibits trends different from that of Voc. A positive BPT leads to a decrease of Jsc to the stable value, while a negative BPT leads to an opposite trend. This can be explained by the charging and discharging processes of the device during the jump of voltage due to the existence of ionic migration-induced capacitive effect. As shown in Figure 7a and 7b, time-dependent photocurrent responses (TDPRs) and J−V performances at different scan rates indicate the direct relation between the hysteresis and the capacitive effect, which has been amply discussed by Chen in ref 41. The TDPRs are the non-steady-state photocurrents in response to stepwise variation of the applied voltage from −0.1 to +1 V (forward stepwise scan) or from +1 to −0.1 V (backward stepwise scan) with ΔV = 100 mV and Δt = 5 s. The decay of photocurrent in one voltage step, which is defined as the capacitive current density (Jcap), represents the relative magnitude of capacitive effect. In order to figure out the effects of the ionic migration on the weakening hysteresis, repeated TDPRs (r-TDPRs) in response to the backward stepwise scan with ΔV = 100 mV and Δt = 20 s under continuous illumination were conducted, and the results are shown in Figure 7c. The trends of Jcap in the corresponding voltage steps versus illumination time extracted from Figure 7c are shown in Figure 7d. It is noticed that Jcap decreases with the increase of illumination time, indicating that ions redistribution under illumination leads to a smaller capacitive effect, which shows the same trend as the weakening hysteresis. We further investigated the influence of ionic migration on hysteresis behavior of the device which has been treated by BPT in the dark to the change the ionic distribution. Before tracing the J−V curves, a bias of 1 V is applied on the device for different duration in dark. As shown in Figure 8, a proper duration (less than 150 s) for a BPT can effectively suppress the hysteresis behavior. While a duration longer than 160 s aggravates the hysteresis behavior because of the barriers formed by the ionic accumulation of the reverse polarity, as shown in Figure S5. Therefore, both the proper BPT and the long time illumination are able to reduce the ionic accumulation, which adjust the band bending and decrease the capacitive effect of the device, leading to the weakening hysteresis.

field and recovers the efficiencies of excitons separation and free carriers extraction, so that Voc and Jsc gradually reaches their stable values under continuous illumination. When illumination is switched off, the break of the association between ions and carriers causes long Voc decay. Besides this, ionic redistribution under continuous illumination can reduce the capacitive effect, which contributes to the weaker hysteresis behavior.

4. CONCLUSIONS We fabricated inverted perovskite solar cells by using ZnO nanoparticle films as the electron transport layers. The device exhibits a weakening hysteresis behavior in the J−V curve between the forward and backward scans when the device is illuminated. The Voc and Jsc of the device increase gradually to equilibrium values after a long illumination period. This phenomenon is different from the reported light soaking processes where the increases in the Voc and Jsc take place within only tens of seconds. Besides, when the illumination on the device is switched off, the Voc decays slowly, which is not commonly observed. It is revealed that ionic accumulation due to Vbi in the dark produces a compensating field screening Vbi, and leading to the low excitons separation efficiency and the poor free carriers extraction efficiency, which result in the inferior Jsc and Voc at the start of illumination. Ionic redistribution happens under illumination due to the association with free carriers, which reduces the compensating

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) Research Cell Record Efficiency Chart. National Renewable Energy Laboratory:https://www.nrel.gov/pv/assets/images/ efficiency-chart.png (accessed July 4, 2017). (3) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (4) Chiang, C.-H.; Wu, C.-G. Bulk Heterojunction Perovskite-PCBM Solar Cells with High Fill Factor. Nat. Photonics 2016, 10, 196−200. (5) Rong, Y. G.; Venkatesan, S.; Guo, R.; Wang, Y. N.; Bao, J. M.; Li, W. Z.; Fan, Z. Y.; Yao, Y. Critical Kinetic Control of NonStoichiometric Intermediate Phase Transformation for Efficient Perovskite Solar Cells. Nanoscale 2016, 8, 12892−12899. (6) Jung, H. S.; Park, N.-G. Perovskite Solar Cells: From Materials to Devices. Small 2015, 11, 10−25. (7) Im, J.-H.; Kim, H.-S.; Park, N.-G. Morphology-Photovoltaic Property Correlation in Perovskite Solar Cells: One-Step versus TwoStep Deposition of CH3NH3PbI3. APL Mater. 2014, 2 (8), 081510.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06329. Image of ZnO dispersion in IPA, XRD, UV−vis absorption spectrum and SEM of ZnO nanoparticles, the origin of the open-circuit voltage of the p-i-n junction, and the band diagrams for the device after different poling conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.Y.) E-mail: [email protected]. Telephone: 86-2983395679. *(W.Q.) E-mail: [email protected]. Telephone: 86-2983395679. ORCID

Xingtian Yin: 0000-0001-9077-5982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Doctoral Program of Higher Education of China under Grant 20120201130004, the Science and Technology Developing Project of Shaanxi Province under Grant No. 2015KW-001, the National Natural Science Foundation of China under Grant No. 51502239, China Postdoctoral Science Foundation under Grant 2015M582659, Natural Science Basic Research Plan in Shaanxi Province of China under Grant No. 2016JQ6058, and the 111 Project of China (B14040). The SEM work was done at International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, P. R. China.

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DOI: 10.1021/acs.jpcc.7b06329 J. Phys. Chem. C XXXX, XXX, XXX−XXX