Local Time-Dependent Charging in a Perovskite ... - ACS Publications

Subscriber access provided by University of Sussex Library

Article

Local Time-Dependent Charging in a Perovskite Solar Cell Victor W. Bergmann, Yunlong Guo, Hideyuki Tanaka, Ilka M. Hermes, Dan Li, Alexander Klasen, Simon A. Bretschneider, Eiichi Nakamura, Rüdiger Berger, and Stefan A. L. Weber ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04104 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Local Time-Dependent Charging in a Perovskite Solar Cell Victor W. Bergmann,§ Yunlong Guo,† Hideyuki Tanaka,† Ilka M. Hermes,§ Dan Li,§ Alexander Klasen,§ Simon Bretschneider,§ Eiichi Nakamura,† Rüdiger Berger* § and Stefan A.L. Weber* §,‡ § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany † Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1130033, Japan ‡ Institute of Physics, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany KEYWORDS: Perovskite solar cells, Kelvin probe force microscopy, scanning probe microscopy, space charge layer, ion migration, charge trapping

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

ABSTRACT

Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction or interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on timescales of seconds cause hysteresis in the current-voltage characteristics. Here, we localized and investigated these slow processes with frequency modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers were forming at the interfaces to the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at interfaces of the MAPI to adjacent layers. Our results attest that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states cause the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.

INTRODUCTION Scientific research on photovoltaic cells recently has undergone a boost with the use of methylammonium lead iodide (MAPI) as state-of-the-art light-absorbing active material.1-3 Starting at a power conversion efficiency (PCE) of 3.8%,6 only a few years of research have pushed the PCE of solar cells containing the perovskite MAPI to over 20%.7 Evolving from the


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dye-sensitized solar cell, mesoporous scaffolds of TiO2 were used as the electron collector material.6, 8 Low exciton binding energies in the MAPI bulk material lead to free charge carriers and long diffusion lengths, questioning the necessity of the mesoporous structure.9-10 Thus, planar cells have recently been subject of intensive research with PCEs reaching approximately 17%.11 However, both planar and mesoporous MAPI solar cells often show hysteretic behavior in the current density-voltage curve (JV-hysteresis).12-13 In particular, the measured current at a certain voltage does not have a constant value and depends on the “history” of the device.14-15 This effect is often exploited when the devices are preconditioned during the JV scan to measure higher apparent PCEs.14 The preconditioning of the solar cells is either achieved by applying a forward bias around the open circuit voltage,14-15 or by illuminating the device under open circuit conditions.14 Currently, there is an intensive discussion in the literature about the possible origin of the underlying processes during the preconditioning of the device. Xiao et al. showed that ions can migrate through the MAPI material,16 changing the electric field distribution within the device while preconditioning with a bias. Furthermore, Shao et al. showed that the use of a fullerene layer as the electron selective contact resulted in an almost complete disappearance of hysteresis.17 The latter observation suggests that trapped charges at the MAPI-electron conductor interface are mainly responsible for hysteresis and that ion migration only plays a minor role. Recently, O’Regan et al. suggested that several processes play a role in the preconditioning of a MAPI solar cell.14 The time dependent response of the current upon applying a forward bias showed the existence of slow processes on the order of seconds that the authors assigned to the movement of ions or the formation of dipoles. Transient photovoltage measurements revealed processes related to electronic recombination, which are at time scales < 1µs.14 To unravel the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

origin of the hysteretic effects, van Reenen et al. used a drift-diffusion model to simulate the charge and potential distribution inside a planar perovskite MAPI solar cell.18 Starting from a pi-n junction and applying a preconditioning scenario with a stabilized forward bias and a subsequent JV scan, they showed that the hysteretic effect can neither be explained by ion mobility nor interfacial trap states alone. Only by considering both effects simultaneously the experimentally measured JV-hysteresis could be reproduced. A similar study based on a driftdiffusion model was recently introduced by Richardson et al..19 They could confirm the correlation between ion migration and hysteresis without introducing interfacial trap states. Furthermore they emphasized the existence of ionic charge buildup in a Debye layer at the selective interfaces. However they did not rule out that traps states might contribute to JVhysteresis, as well. In order to experimentally disentangle the charge and potential distribution within the light absorbing MAPI layer, advanced microscopy methods are required. One promising way is to apply scanning force microscopy (SFM) methods. In addition to topographical information, such as grain boundaries20, SFM has been used to gain information about other features of MAPI films, such as local photocurrents21 or ferroic properties22-23. Kelvin probe force microscopy (KPFM).24 KPFM measures the local contact potential difference (CPD), which represents the difference in work function of a nanometer sized SFM probe and the sample material underneath. As the work function Φtip of the metal-coated SFM probe does not change within the measurement, the local variations in the measured CPD represent the change in the local electric potential VE of the surface underneath the reference probe. In a layered structure such as a solar cell, the fermi levels of the layers align, forming a common fermi level. To do so, space charge regions at the interfaces are formed that shift the local vacuum level across these layers (band


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bending). When the device is illuminated, the electrostatic interaction between tip and surface is the sum of the equilibrium CPD plus the additional photo-induced potential generated by charges which accumulate locally. Thereby, the electric field E(x) across the layer structure can be calculated from the CPD distribution by ሺxሻቍ = − V୉ ሺxሻ = Eሺxሻ Φ − ௗ௫ CPDሺxሻ = − ௗ௫ ቌV୉ ሺxሻ − ௘ᇣᇧ ௧௜௣ᇧᇥ ᇧᇤᇧ ௗ௫ ୢ

ୢ

ଵ

ୀ௖௢௡௦௧

ୢ

.

(1)

The local charge density ρ(x) is then given by ߩሺxሻ = −߳௥ ߳଴

ௗ

ௗ௫

E௫ ሺxሻ = −߳௥ ߳଴ ௗ௫ మ Vா ሺxሻ , ௗమ

(2)

where ϵ0 is the vacuum permittivity and ϵr is the relative permittivity of the material in the respective layer of the solar cell. KPFM has a lateral resolution < 50 nm 25-27 and can be readily applied to study optoelectronic materials,24 in particular different types of solar cells under illumination.28-33 The interface of devices can be accessed by preparing cross section through the devices by cleaving,28-29 focused ion beam milling30, 32-33 or microtome cutting.34 Here, the KPFM analysis of cross sections of solar cells allows studying effects under illumination and different device configuration such as short or open circuit conditions.32-33, 35-38 Recently, we reported that trapped charges in the MAPI layer of a mesoporous perovskite solar cell can be visualized by means of cross sectional (KPFM).33 Furthermore, Jiang et al. studied the potential distribution in different device architectures of perovskite solar cells upon applying an external bias in the dark.38 However, time dependent effects, as expected for preconditioning of the perovskite solar cell devices, remain unexplored by cross sectional KPFM.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Here, we used frequency modulation KPFM (FM-KPFM) to investigate cross sections of planar MAPI solar cells. Compared to the commonly used amplitude modulation KPFM (AMKPFM) technique it is known to exhibit a higher lateral resolution and is less prone to cross talk from adjacent areas with large potential differences.4-5, 27, 39 The latter is a requirement to distinguish ion migration from charge trapping processes within MAPI solar cells. To elucidate both effects on a local scale, we performed time dependent FM-KPFM measurements in dark and under illumination on an operating planar MAPI device. Our measurements revealed that both ion migration across the MAPI layer and trapped charges at different interfaces contribute to the JV- hysteresis and device preconditioning effects. Furthermore, by analyzing the time dependence of the CPD profiles, we found that the formation of space charges at the interface to the hole transport material was slower than the charging at the interface to the electron transport layer. Results and Discussion For this study, two devices with the configuration ITO/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag were used. In order to study the underlying mechanisms for variations in device efficiency, we deliberately chose two devices from the same batch with a high and a low PCE. Both devices exhibited hysteresis in their JV characteristics. For the device with higher efficiency, this resulted in a PCE of 8.1% for the short circuit to open circuit scan (Figure S1; scan rate = 0.02 V/s) and 7.1% for the open circuit to short circuit scan. For the device with lower efficiency, this resulted in a PCE of 3.3 % and 4.8 % respectively (Figure S4; scan rate = 0.02 V/s). For the preparation of the cross sections we used a procedure comprising of cleaving the device and


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


subsequent focused ion beam (FIB) polishing.33 SFM on the as prepared samples showed the different layers in the device and thus their respective interfaces could be identified (Figure 1a). In a first experiment, we performed cross sectional FM-KPFM measurements under shortcircuit conditions of the device in dark and under illumination (Figure 1b). For clarity, we have illustrated the position of the interfaces with grey bars in all diagrams. We measured a potential difference between both electrodes of 100mV, which is consistent with the workfunction values of ITO and Ag from literature.40-41 For short circuit conditions in dark (Figure 1b) we measured a potential drop of ~0.27 V at the TiO2/MAPI interface (blue arrow). At a distance of ~100 nm from the TiO2 interface we found a ~120 nm wide region of almost constant CPD in the MAPI layer. At the HTL/Ag interface an additional drop of 50 mV in CPD (orange arrow) was

Figure 1 (a) SFM height image, which shows the layer structure of the MAPI solar cell. The interfaces are marked with white lines. (b) Respective contact potential difference line profiles for short circuit conditions in dark (black profile) and under white light (red profile). The width of the grey bars indicates the error in the position of the interface, which is given by the interface roughness. ACS Paragon Plus Environment

7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

measured. We attribute the drop of the CPD at the TiO2/MAPI interface to a charge depletion zone, in agreement with previous results.38, 42-43 At the HTL/Ag interface, we measured a depletion width of ~100 nm. Due to the smaller difference between the CPD compared to the TiO2/MAPI interface the slope is reduced and the electric field is much smaller (equation (1)). In particular, the constant CPD in the MAPI bulk corresponds to a vanishing electric field. Thus, the motion of charge carriers in this area of the MAPI film is controlled by diffusion. However, the electric fields in the depletion zones at both interfaces to the selective contacts guide the charge carriers to their respective electrodes. A field-free MAPI layer with depletion zones at both sides was found in simulations performed by van Reenen et al. considering the presence of mobile ions.18 According to these simulations, mobile ions redistribute at the selective interfaces under the influence of the electric field inside the p-i-n junction resulting in a field free MAPI bulk, which was also suggested by Eames et al..44 Furthermore time dependent analysis on charging transients experiments also concluded the accumulation of charges at the electrode vicinity, resulting in confined potential drops at the selective interfaces.45 The measured CPD profile by FM-KPFM with potential drops at the interfaces to the selective contacts thereby confirms the recent studies about the potential profile inside the perovskite solar cell.18, 44-45 Upon illumination with white light, the device did not show any change in CPD within the error of the measurement (~20 mV, see methods section) compared to the measurement under dark conditions (Figure 1b, red profile; measured after ~10 s illumination). This result was confirmed in a second measurement on the same device on a separately prepared cross section (not shown). Since there was no response upon illumination, we conclude that photogenerated charges do not give rise to additional space charges. For comparison, we measured a solar cell


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from the same batch that showed a reduced PCE of 3.3 % and much stronger hysteresis in the JV behavior (Figure S4). For this device we observed an increase in CPD in the MAPI layer upon illumination under short circuit conditions (Figure S5). We measured a difference in CPD of up to +500 mV under illumination compared to the dark measurement, which indicates the presence of holes. Thus, in this case charge extraction of holes into the adjacent HTM was reduced. Subsequently, we measured the CPD distribution in open circuit conditions and under illumination (Figure 2a). The red and the blue line profiles correspond to a floating Ag and ITO electrode, respectively. The measured CPD differences under dark and under illumination on the Ag and ITO electrodes were found to be ∆CPDITO = -0.83 V and ∆CPDAg = 0.85 V, respectively (arrows in Figure 2a). These values correspond well to the open circuit voltage of 0.9 V. To visualize the light induced potential, i.e. the photo potential (VPhoto), we subtracted the CPD profile measured under dark conditions from the CPD profile measured under illumination. To compare both CPD line profiles, we added ∆CPDITO to the lower curve. This analysis showed that within the error both measurements lead to an identical VPhoto distribution in the device (Figure 2b). Under open circuit conditions, the VPhoto was building up across the selective contacts, respectively. Furthermore, the middle part of the MAPI film stayed on a constant VPhoto. We then calculated the light induced charge density distribution (∆ρ/ε0εr) of the average of both VPhoto profiles using equation (2) (Figure 2c). Inside the MAPI, in front of each selective contact, space charge regions with an opposite sign and a width of ~100 nm formed. The arrangement of charges at the interfaces to the selective contacts shields the electric field inside the MAPI film as suggested earlier by O’Regan et al..14


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

Figure 2 (a) CPD distribution under open circuit conditions. While the black profile shows the initial CPD profile for dark conditions, the red and blue profile is the illuminated case, leaving one electrode floating directly after switching on the illumination. The white arrows mark the built up of the VOC, respectively. (b) Respective photo potential (VPhoto), i.e. potential change due to the illumination. The excellent overlap of the two curves demonstrates the accuracy of the FM-KPFM method and the absence of crosstalk from the Ag electrode. Here it is worth to note that for the floating Ag electrode a potential of 0.85 V builds up and although the KPFMtip scans across this electrode we did not notice any difference to the measurement where the ITO was floating. Thus there is no cross talk caused by the charged electrodes. The latter would be a severe problem for AM-KPFM.4-5 (c) Calculated charge density (∆ρ/ε0εr) profile from averaging over both VPhoto profiles. We smoothed the VPhoto curve before taking the derivative in order to reduce the noise on the data (see experimental for details), slightly reducing the spatial resolution of the charge density data compared to the potential profiles.

In another experiment, we investigated the time dependence of the CPD distribution and the role of preconditioning of devices. In our setup, preconditioning is realized by illumination under open circuit conditions. As a first step, we started CPD measurements at short circuit conditions and subsequently switched to open circuit conditions under illumination. The measured CPD


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 Time dependent open circuit conditions leaving the Ag electrode floating. (a) CPD for dark conditions (black profile) after 2 s and 20 s floating. The same line was continuously scanned at a scan rate of 1 Hz, i.e. one line per second (slow scan disabled). (b) Respective photo-generated charge density calculated from the CPD. After time more positive charges accumulate at the TiO2/MAPI. (c) Time evolution of the integrated area of the CPD profiles (black squares). The red profile shows the double exponential decay fit (equation 3). The time constant for the electronic process is lower than the time resolution of our experiment (1 s) and thereby noted as τprec1 < 1s. The slower process has a time constant of τprec2 = 4.2 s (± 0.5 s). recorded after 2 s (Figure 3a, red line) showed an instantaneous increase in photovoltage, which was beyond the time resolution of a FM-KPFM line scan (~1 s). After 20 s (blue line) the CPD further increased between the ITO/TiO2 interface and the HTL/Ag interface compared to the profile recorded after 2 s. In the MAPI layer, a constant CPD offset of ~100 mV was observed between the measurement after 2 s and 20 s. For both measurements we calculated the change in charge density according to equation (2) (Figure 3b). The most significant observation here was an increase of positive charges at the TiO2/MAPI interface (Figure 3b, green arrow). These additional positive charges in front of the TiO2 could be caused by the formation of a surface dipole or the migration of positive ions towards the TiO2/MAPI interface. These effects have


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

recently been suggested to be the underlying preconditioning process, causing the hysteretic behavior in JV-curves.12, 14, 16, 18, 44-50 An identical increase in CPD was observed in a subsequent experiment where the ITO electrode was floating instead of the Ag electrode (Figure S2). To further analyze the time dependence of the charging process during the preconditioning, we calculated the VPhoto for each profile recorded at time intervals of 1 s and integrated them for each time step over the width of the cross section (Figure 3c, black squares). In order to take the fast initial change in potential and the slower change at a time scale of seconds into account, we fitted the data with a double exponential decay fit y = y଴ + Aଵ ∙ e

షሺ౮ష౮బ ሻ ൰ ಜ౜౗౩౪

൬

+ Aଶ ∙ e

షሺ౮ష౮బ ሻ ൰ ಜ౩ౢ౥౭

൬

.

(3)

Within the accuracy of the measurement two different processes can be distinguished: The time constant τfast which represents the electronic nature of generating the open circuit potential is faster than the time resolution of the measurement and thereby consequently noted as τfast < 1 s. The second time constant is slower (τslow= 4.2 s ± 0.5 s) and therefore the underlying mechanism can be more diverse. This mechanism can be both an electronic and a chemical process, such as the formation of a surface dipole or the migration of positive ions towards the TiO2/MAPI interface. Almora et al. recently observed a similar characteristic response time for slow ionic charging (τ =10 s). They used it to calculate the ionic diffusion constant and found that ionic charging is restricted to the vicinity of the electrode consistent with our observations.45 Furthermore Chen et al. recently studied the dynamic current transient process when switching between different voltages. They ascribed the hysteresis to a large capacitive effect due to both ionic and electronic charge accumulation.49 By taking a closer look at the charge distribution inside the MAPI after 20 s (Figure 3b), we noticed that the positive space charge peak in front of


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the TiO2 is larger than the negative space charge peak in front of the HTM layer. Thus, more positive charges are present inside the MAPI layer upon illumination and the sum of the total charge density in the MAPI layer is not zero. The fact that the total charge density in the MAPI layer is not zero is a proof that the overall charge accumulation cannot solely result from a rearrangement of ions because they are confined within the MAPI layer. Therefore, positive and negative charges in sum should cancel out. Consequently, additional positive charge carriers must be present in front of the TiO2 interface, to shield the open circuit potential in agreement with the results of Zarazua et al..50 The time constants calculated from the integrated area are representative for the charge rearrangement in the complete device, i.e. for charge accumulations at all interfaces in the solar cell. Since KPFM provides the detailed potential distribution within the device, we are able to extract the time constants at distinct locations inside the device. Therefore, we selected three

Figure 4 Time evolution of the integrated area of the normalized photo potential profiles limited to the charge density peak positions as marked in Figure 3b. The double exponential fits revealed comparable values for the slow component for the peaks in the MAPI (4.8 s & 5.1 s), while the one in the TiO2 was even slower (7.7 s).


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

different regions, namely the TiO2 layer, the MAPI in front of the TiO2 and the MAPI in front of the HTL, which are also marked in the charge density profile (Figure 3b, shaded areas). Then we integrated the VPhoto limited to the corresponding peak positions and normalized the values to the corresponding maximum after 20 s (Figure 4). Then the data was fitted by equation 3 for the calculation of the local slow time constants τ for the three areas (Figure 4). We found that the time constant τslow is lowest for the TiO2 layer (τslow ≈ 7.7 s ± 0.4 s) whereas for the MAPI-TiO2 and MAPI-HTL interfaces we obtained a time constant of τslow ≈ 5 s. In a fourth experiment, we measured the CPD profiles after preconditioning, i.e. while switching from open circuit conditions under illumination to short circuit conditions in dark (Figure 5a). The CPD profile recorded directly (i.e. ~1 s) after returning to dark and short circuit conditions was even lower than the initial CPD profile before illumination (red and black line). Afterwards, the measured CPD returned to the initial state within ~25 s. The corresponding charge density profile shows accumulations of negative charges in the MAPI layer next to both selective contacts (Figure 5b). Accordingly, both selective contacts were charged positively. The presence of almost exclusively negative charges in the MAPI indicates that these left-over charges cannot result from pure ion-rearrangement. In order to calculate the time dependence for this process, we integrated the full VPhoto profiles recorded at each time step. The single exponential fit over the whole width of the solar cell revealed a time constant of τafter = 8.8 s ± 1.1 s (Figure 5c).


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Compared to the slow time constant during the preconditioning process (τslow), τafter is more than a factor two higher (please note that the single exponential fit for the preconditioning process, by excluding t = 0 s, does not change the slow time constant τslow). Such a difference in time constants indicates that the processes behind the effects are different, as well. A possible explanation for the slower time constant τafter is the presence of interfacial trap states. Previously migrated ions start to diffuse, leaving behind the trapped electrons. Such trapped charges are again in agreement with the predicted scenario of the simulation studies by van Reenen et al..18 The same observations were made switching from a floating ITO electrode back to short circuit conditions (Figure S3).

Figure 5 (a) Comparison of the CPD profiles for short circuit conditions before and after preconditioning for 20 s with open circuit conditions Ag floating. (b) Respective photo-generated charge density. (c) Time evolution of the integrated area of the VPhoto profiles (black squares). The red profile shows the single exponential decay fit (equation 3) with a time constant of τafter = 8.8 s (± 1.1 s).


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

Conclusions We demonstrated that cross-sectional FM-KPFM is a suitable method to investigate and identify charge distributions at internal interfaces in solar cells under working conditions. The perovskite MAPI layer is field free under short and open circuit conditions, which indicates a high amount of mobile charges that can screen the outer electric field. With higher mobilities in the perovskite, compared to the charge selective layers (TiO2 and Spiro-OMeTAD) the field drops at the corresponding selective interfaces. In a recent simulation study, van Reenen et al. described the effect of a potential barrier at the interface to the selective contacts, which can lead to a pile up of charges and thereby an increase in the hysteretic effect.18 On a less efficient device (Figure S4 and S5) as well as in our previously reported measurements on a mesoporous device33 we observed a positive charging in the MAPI capping layer upon illumination under short circuit conditions. We attributed the photo-generated potential to an unbalanced charge transport of holes and electrons in the device. On the planar device studied here we did not observe such an effect (Figure 1), indicating a balanced charge extraction and thus the absence of a potential barrier at the interfaces to the selective contacts. Such variations in the behavior of perovskite solar cells can also be caused by slight variations in the amount of mobile ions in the perovskite layer 51-53. The measured time constants for the interfacial charging of several seconds during preconditioning suggest ion migration or the formation of a dipole as main cause.14 However, locally resolving the time constants revealed distinct differences between the charging processes in different regions of the device. This observation suggests that more than one process contributes to the time scale or that the material influences the time scale. Furthermore,


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


remaining negative charges all over the MAPI layer directly after illumination under short circuit conditions indicated the presence of trapped charges for a couple of seconds (time constant of τ = 8.8 ± 1.1 s). Our results are in agreement with the scenario of mobile ions and interfacial trap states for our device as reported by van Reenen et al..18 Such a differentiated analysis of potential distribution and charging rates at the interfaces cannot be done by conventional device measurements. In the here presented implementation, the temporal resolution is on the order of the line scan rate of the SFM, i.e. on the order of 1 s. However, by performing experiments with the tip resting on an area of interest, the temporal resolution is only limited by the bandwidth of the detection electronics and the noise of the system. On an optimized setup, the temporal resolution can in principle be on the order of ms-µs 54

or even less.55 Thus, cross sectional FM-KPFM can locally study a wide range of electronic

processes, not only in solar cells, but in all electronic devices where lengths scales of 10 nm to 100 µm play a role. In this respect FM-KPFM is the ideal tool to identify and to understand the electric processes that happen at the devices’ internal interfaces. Our observations underpin the importance of these interfaces for an efficient charge extraction: The preparation of good selective contacts is the key to reach the theoretical limits of MAPI solar cells.

METHODS/MATERIALS The methods used for SFM preparations and device characterization are mostly similar to the methods used in Ref.33 JV Device Characterization For the current density versus voltage analysis we used a setup consisting of 150W Xe lamp (Oriel) with a UVB/UVA dichroic mirror (280–400 nm) as a light source. The


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

active area was fixed at 4 mm2. A digital source meter (Keithley Model 2400) was used to apply the voltage to the cell while the current was recorded. FIB Device Preparation With a dual-beam FIB (FEI Nova600 Nanolab), we deposited a 2-mm-thick Pt protection layer measuring 50 * 5 µm on the top electrode. This Pt bar was placed 5 µm below the broken edge to eliminate contamination and therefore short cuts of the deposited Pt with the cleaved surface. For the following polishing steps, the ion beam was positioned parallel to the cross section perpendicular to the Pt bar. This geometry was used to reduce the contamination with Ga ions of the Gaussian shaped beam and the layer system. Thereby, the Pt bar acted as a shield and protected the layers from the Ga ions beside the focused position. The polishing of the cross section was then done in two steps. First, a coarse polishing step was performed at an acceleration voltage of 30 kV and a current of 3 nA for a volume of 60 * 5 * 10 µm3. Later, a fine polishing step created a smooth surface with an acceleration voltage of 30 kV and a current of 100 pA for a volume of 30 * 1 * 5 µm3. Particularly, the second polishing step directly milled at the Pt-protected area. After the FIB preparation, the sample was transferred in air to the SFM setup with a transfer time of less than 10 min. Kelvin probe force microscopy For KPFM measurements we used a scanning force microscope (Asylum MFP3d) inside a glove box filled with nitrogen. The KPFM signal processing was made with an additional external lock-in amplifier (Zurich Instruments HF2LI-MOD). We used single scan, double sideband frequency modulation with an excitation frequency of ~70 kHz (PPP-EFM cantilever) with ~70 nm excitation amplitude and a modulation frequency of 1


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kHz (VAC= 3 V). By using FM-KPFM, the lateral resolution is limited by the apex of the measuring probe and with a usual tip radius of 30 nm the resolution can be estimated to be 99.5) from Lumitech, 82.5 mg was dissolved in 1 mL chlorobenzene with 15 µl Li-TFSI (520 mg/ml in CH3CN) and 22.5 µl D-TBP. Fabrication of solar cells The devices were fabricated according to the following procedure. An indium tin oxide (ITO) layer on a glass substrate was used with a thickness of 145 nm and a sheet resistance of 8 Ω/square. The surface roughness, Ra, was 0.7 nm and the Rmax was 8.1 nm. Prior to the formation of the buffer layer, the patterned ITO glass was ultrasonically cleaned using a surfactant, rinsed with water, and then finally given UV−ozone treatment. Then, the TiO2 precursor solution was spin-coated on the ITO at 3000 rpm/30s and it was heated to 470 °C for 30 min in air. The MAPI precursor solution was spin-coated on the TiO2 surface at 500 rpm/3s, 5000 rpm/30s and followed by 50 µl chlorobenzene washing


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


its surface after 10 s under a nitrogen atmosphere. The crystalline MAPI active layer thus prepared was ca. 300 nm thick after 10 min annealing at 100 °C.58 The hole-transporting layer was then deposited by spin coating (2200rpm/30s for Spiro-OMeTAD). The top electrode (Ag, 150 nm) was deposited through a metal shadow mask, which defined a 2 mm stripe pattern perpendicular to the ITO stripe. For Spiro-OMeTAD, we tested the device without encapsulation after storing it for 1 day in ambient air.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available showing additional experimental data for the JV-curves and the KPFM measurements on the device with lower efficiency. AUTHOR INFORMATION Corresponding Author * E-Mail: [email protected] or [email protected] Present Addresses Hideyuki Tanaka, Graduate School of Frontier Science, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8561, Japan. Author Contributions V.W.B performed all the experiments. Y.G. and H.T. fabricated the samples. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

Notes The authors declare no competing financial interest

ACKNOWLEDGEMENTS

Victor Bergmann thanks the IRTG 1404 for financial support. Dan Li thanks China Scholarship Council for financial support. Financial support from MEXT, Japan, is gratefully acknowledged (KAKENHI 15H05754 to E.N. and Strategic Promotion of Innovative Research, JST, to Y.G).

ABBREVIATIONS

MAPI - methylammonium lead iodide PCE – Photon conversion efficiency HTL – hole transporting layer KPFM – Kelvin probe force microscopy FM-KPFM – frequency modulation Kelvin probe force microscopy CPD – contact potential difference SFM – scanning force microscopy FTO – fluorine tin oxide


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ITO – indium tin oxide FIB – focused ion beam REFERENCES 1. Kazim, S.; Nazeeruddin, M. K.; Grätzel, M.; Ahmad, S., Perovskite as Light Harvester: A Game Changer in Photovoltaics. Angew. Chem., Int. Ed. 2014, 53 (11), 2812-2824. 2. Bretschneider, S. A.; Weickert, J.; Dorman, J. A.; Schmidt-Mende, L., Research Update: Physical and Electrical Characteristics of Lead Halide Perovskites for Solar Cell Applications. APL Mater. 2014, 2 (4), 040701. 3. Stranks, S. D.; Snaith, H. J., Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10 (5), 391-402. 4. Gil, A.; Colchero, J.; Gómez-Herrero, J.; Baró, A. M., Electrostatic Force Gradient Signal: Resolution Enhancement in Electrostatic Force Microscopy and Improved Kelvin Probe Microscopy. Nanotechnology 2003, 14 (2), 332-340. 5. Ziegler, D.; Stemmer, A., Force Gradient Sensitive Detection in Lift-Mode Kelvin Probe Force Microscopy. Nanotechnology 2011, 22 (7), 075501. 6. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 6050-6051. 7. National Renewable Energy Labs (NREL) efficiency chart (2016). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg (accessed 03 March 2016). 8. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Gratzel, M.; Park, N.-G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. 9. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342 (6156), 341-344. 10. Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342 (6156), 344-347. 11. Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.-L.; Mohite, A. D., High-Efficiency Solution-Processed Perovskite Solar Cells With Millimeter-Scale Grains. Science 2015, 347 (6221), 522-525. 12. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W., Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5 (9), 1511-1515. 13. Unger, E. L.; Hoke, E. T.; Bailie, C. D.; Nguyen, W. H.; Bowring, A. R.; Heumuller, T.; Christoforo, M. G.; McGehee, M. D., Hysteresis and Transient Behavior in Current-Voltage Measurements of Hybrid-Perovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7 (11), 3690-3698. 14. O’Regan, B. C.; Barnes, P. R. F.; Li, X.; Law, C.; Palomares, E.; Marin-Beloqui, J. M., Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis. J. Am. Chem. Soc. 2015, 137 (15), 5087-5099. 15. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M., Understanding the RateDependent J-V hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: The Role of a Compensated Electric Field. Energy Environ. Sci. 2015, 8 (3), 995-1004. 16. Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J., Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2015, 14 (2), 193-198. 17. Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J., Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. Nat. Commun. 2014, 5, 5784. 18. van Reenen, S.; Kemerink, M.; Snaith, H. J., Modeling Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6 (19), 3808-3814. 19. Richardson, G.; O'Kane, S. E. J.; Niemann, R. G.; Peltola, T. A.; Foster, J. M.; Cameron, P. J.; Walker, A. B., Can SlowMoving Ions Explain Hysteresis in the Current-Voltage Curves of Perovskite Solar Cells? Energy Environ. Sci. 2016, 9 (4), 14761485.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

20. Li, D.; Bretschneider, S. A.; Bergmann, V. W.; Hermes, I. M.; Mars, J.; Klasen, A.; Lu, H.; Tremel, W.; Mezger, M.; Butt, H.-J.; Weber, S. A. L.; Berger, R., Humidity-Induced Grain Boundaries in MAPbI3 Perovskite Films. J. Phys. Chem. C 2016, 120 (12), 6363-6368. 21. Kutes, Y.; Zhou, Y.; Bosse, J. L.; Steffes, J.; Padture, N. P.; Huey, B. D., Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale. Nano Lett. 2016, 16 (6), 3434-3441. 22. Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P., Direct Observation of Ferroelectric Domains in SolutionProcessed CH3NH3PbI3 Perovskite Thin Films. J. Phys. Chem. Lett. 2014, 5 (19), 3335-3339. 23. Hermes, I. M.; Bretschneider, S. A.; Bergmann, V. W.; Li, D.; Klasen, A.; Mars, J.; Tremel, W.; Laquai, F.; Butt, H.-J.; Mezger, M.; Berger, R.; Rodriguez, B. J.; Weber, S. A. L., Ferroelastic Fingerprints in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. C 2016, 120 (10), 5724-5731. 24. Berger, R.; Domanski, A. L.; Weber, S. A. L., Electrical Characterization of Organic Solar Cell Materials Based on Scanning Force Microscopy. Eur. Polym. J. 2013, 49 (8), 1907-1915. 25. Belaidi, S.; Lebon, F.; Girard, P.; Leveque, G.; Pagano, S., Finite Element Simulations of the Resolution in Electrostatic Force Microscopy. Appl. Phys. A 1998, 66 (1), S239-S243. 26. Colchero, J.; Gil, A.; Baró, A. M., Resolution Enhancement and Improved Data Interpretation in Electrostatic Force Microscopy. Phys. Rev. B 2001, 64 (24), 245403. 27. Zerweck, U.; Loppacher, C.; Otto, T.; Grafström, S.; Eng, L. M., Accuracy and Resolution Limits of Kelvin Probe Force Microscopy. Phys. Rev. B 2005, 71 (12), 125424. 28. Takashi, M.; Takao, U.; Shigeru, K.; Koichi, M., Measurement of Contact Potential of GaAs pn Junctions by Kelvin Probe Force Microscopy. Jpn. J. Appl. Phys. 1999, 38 (8R), 4893. 29. Jiang, C.-S.; Hasoon, F. S.; Moutinho, H. R.; Al-Thani, H. A.; Romero, M. J.; Al-Jassim, M. M., Direct Evidence of a Buried Homojunction in Cu(In,Ga)Se2 Solar Cells. Appl. Phys. Lett. 2003, 82 (1), 127-129. 30. Glatzel, T.; Marrón, D. F.; Schedel-Niedrig, T.; Sadewasser, S.; Lux-Steiner, M. C., CuGaSe2 Solar Cell Cross Section Studied by Kelvin Probe Force Microscopy in Ultrahigh Vacuum. Appl. Phys. Lett. 2002, 81 (11), 2017-2019. 31. Zhang, Z.; Hetterich, M.; Lemmer, U.; Powalla, M.; Hölscher, H., Cross Sections of Operating Cu(In,Ga)Se2 Thin-Film Solar Cells under Defined White Light Illumination Analyzed by Kelvin Probe Force Microscopy. Appl. Phys. Lett. 2013, 102 (2), 023903. 32. Saive, R.; Scherer, M.; Mueller, C.; Daume, D.; Schinke, J.; Kroeger, M.; Kowalsky, W., Imaging the Electric Potential within Organic Solar Cells. Adv. Funct. Mater. 2013, 23 (47), 5854-5860. 33. Bergmann, V. W.; Weber, S. A. L.; Javier Ramos, F.; Nazeeruddin, M. K.; Grätzel, M.; Li, D.; Domanski, A. L.; Lieberwirth, I.; Ahmad, S.; Berger, R., Real-Space Observation of Unbalanced Charge Distribution Inside a Perovskite-Sensitized Solar Cell. Nat. Commun. 2014, 5, 5001. 34. Scherer, M.; Saive, R.; Daume, D.; Kröger, M.; Kowalsky, W., Sample Preparation for Scanning Kelvin Probe Microscopy Studies on Cross Sections of Organic Solar Cells. AIP Adv. 2013, 3 (9), 092134. 35. Lee, J.; Kong, J.; Kim, H.; Kang, S.-O.; Lee, K., Direct Observation of Internal Potential Distributions in a Bulk Heterojunction Solar Cell. Appl. Phys. Lett. 2011, 99 (24), 243301. 36. Kong, J.; Lee, J.; Jeong, Y.; Kim, M.; Kang, S.-O.; Lee, K., Biased Internal Potential Distributions in a Bulk-Heterojunction Organic Solar Cell Incorporated with a TiOx Interlayer. Appl. Phys. Lett. 2012, 100 (21), 213305. 37. Chen, Q.; Mao, L.; Li, Y.; Kong, T.; Wu, N.; Ma, C.; Bai, S.; Jin, Y.; Wu, D.; Lu, W.; Wang, B.; Chen, L., Quantitative Operando Visualization of the Energy Band Depth Profile in Solar Cells. Nat. Commun. 2015, 6, 7745. 38. Jiang, C.-S.; Yang, M.; Zhou, Y.; To, B.; Nanayakkara, S. U.; Luther, J. M.; Zhou, W.; Berry, J. J.; van de Lagemaat, J.; Padture, N. P.; Zhu, K.; Al-Jassim, M. M., Carrier Separation and Transport in Perovskite Solar Cells Studied by Nanometre-Scale Profiling of Electrical Potential. Nat. Commun. 2015, 6, 8397. 39. Glatzel, T.; Sadewasser, S.; Lux-Steiner, M. C., Amplitude or Frequency Modulation-Detection in Kelvin Probe Force Microscopy. Appl. Surf. Sci. 2003, 210 (1–2), 84-89. 40. Dweydari, A. W.; Mee, C. H. B., Work Function Measurements on (100) and (110) Surfaces of Silver. Phys. Status Solidi A 1975, 27 (1), 223-230. 41. Schlaf, R.; Murata, H.; Kafafi, Z. H., Work Function Measurements on Indium Tin Oxide Films. J. Electron Spectrosc. Relat. Phenom. 2001, 120 (1–3), 149-154. 42. Guerrero, A.; Juarez-Perez, E. J.; Bisquert, J.; Mora-Sero, I.; Garcia-Belmonte, G., Electrical Field Profile and Doping in Planar Lead Halide Perovskite Solar Cells. Appl. Phys. Lett. 2014, 105 (13), 133902. 43. Dymshits, A.; Henning, A.; Segev, G.; Rosenwaks, Y.; Etgar, L., The Electronic Structure of Metal Oxide/Organo Metal Halide Perovskite Junctions in Perovskite Based Solar Cells. Sci. Rep. 2015, 5, 8704. 44. 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. Nat. Commun. 2015, 6, 7497. 45. Almora, O.; Guerrero, A.; Garcia-Belmonte, G., Ionic Charging by Local Imbalance at Interfaces in Hybrid Lead Halide Perovskites. Appl. Phys. Lett. 2016, 108 (4), 043903.


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


46. Zhang, Y.; Liu, M.; Eperon, G. E.; Leijtens, T. C.; McMeekin, D.; Saliba, M.; Zhang, W.; de Bastiani, M.; Petrozza, A.; Herz, L. M.; Johnston, M. B.; Lin, H.; Snaith, H. J., Charge Selective Contacts, Mobile Ions and Anomalous Hysteresis in OrganicInorganic Perovskite Solar Cells. Mater. Horiz. 2015, 2 (3), 315-322. 47. Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kochelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O/'Regan, B. C.; Nelson, J.; Cabral, J. T.; Barnes, P. R. F., The Dynamics of Methylammonium Ions in Hybrid Organic-Inorganic Perovskite Solar Cells. Nat. Commun. 2015, 6, 7124. 48. Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F., Defect Migration in Methylammonium Lead Iodide and its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8 (7), 2118-2127. 49. Chen, B.; Yang, M.; Zheng, X.; Wu, C.; Li, W.; Yan, Y.; Bisquert, J.; Garcia-Belmonte, G.; Zhu, K.; Priya, S., Impact of Capacitive Effect and Ion Migration on the Hysteretic Behavior of Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6 (23), 46934700. 50. Zarazua, I.; Bisquert, J.; Garcia-Belmonte, G., Light-Induced Space-Charge Accumulation Zone as Photovoltaic Mechanism in Perovskite Solar Cells. J. Phys. Chem. Lett. 2016, 7 (3), 525-528. 51. Yin, W.-J.; Shi, T.; Yan, Y., Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104 (6), 063903. 52. Kim, J.; Lee, S.-H.; Lee, J. H.; Hong, K.-H., The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2014, 5 (8), 1312-1317. 53. Leijtens, T.; Srimath Kandada, A. R.; 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. J. Am. Chem. Soc. 2015, 137 (49), 15451-15459. 54. Coffey, D. C.; Ginger, D. S., Time-Resolved Electrostatic Force Microscopy of Polymer Solar Cells. Nat. Mater. 2006, 5 (9), 735-740. 55. Giridharagopal, R.; Rayermann, G. E.; Shao, G.; Moore, D. T.; Reid, O. G.; Tillack, A. F.; Masiello, D. J.; Ginger, D. S., Submicrosecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics. Nano Lett. 2012, 12 (2), 893-898. 56. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on MesoSuperstructured Organometal Halide Perovskites. Science 2012, 338 (6107), 643-647. 57. Guo, Y.; Liu, C.; Inoue, K.; Harano, K.; Tanaka, H.; Nakamura, E., Enhancement in the Efficiency of an Organic-Inorganic Hybrid Solar Cell with a Doped P3HT Hole-Transporting Layer on a Void-Free Perovskite Active Layer. J. Mater. Chem. A 2014, 2 (34), 13827-13830. 58. Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y.-B.; Spiccia, L., A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem., Int. Ed. 2014, 126 (37), 10056-10061.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

TABLE OF CONTENTS GRAPHIC


26

Page 27 of 31

a)

11nm

0

0.40

TiO2

0.30

0.20 ITO

Spiro-OMeTAD (HTL)

illumination off illumination on

b)

CPD (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Ag

0.4

0.5

MAPI 0.1

0.2 0.3 position (mm)



MAPI

-0.4 0.0

0.1

0.2

0.3

0.4

position (mm)

MAPI

ITO TiO2

0.2 Ag 0.5

Ag

0.0 0.0

Ag/ITO floating average 100 ITO TiO2 0.0

-

+

0.2

0.3

0.4

position (mm)


0.5

0.0

- + MAPI

- 100 0.1

Spiro-OMeTAD (HTL)

0.0

0.4

Spiro-OMeTAD (HTL)

0.4

c)

Ag floating ITO floating 0.8

VPhoto (V)

ITO TiO2

Spiro-OMeTAD (HTL)

0.8

b)

Dr/ee0 (V/µm2)

Ag floating both electrodes grounded ITO floating

a)

CPD (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 28 of 31

0.1

0.2

0.3

0.4

position (mm)

Ag 0.5

ITO TiO2 MAPI

0.6

0.2 0.0

0.1

Spiro-OMeTAD (HTL)

2

1.0

b)

0.2 0.3 0.4 position (mm)

Ag

0.5

c)

Ag floating 2 s Ag floating 20 s 150

50

ITO TiO2

0.0 - 50

+

-

MAPI

0.0

0.1

0.2 0.3 0.4 position (mm)


calculated data points double exponential decay fit 0.30

area (mm*V)

both electrodes grounded Ag floating 2 s Ag floating 20 s

D Charge density (V/µm )

a)

CPD (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Spiro-OMeTAD (HTL)

Page 29 of 31

0.20

tfast < 1 s

0.10

tslow = 4.2 ± 0.5 s

Ag 0.5

0

5

10 15 time (s)

20


local area TiO2/MAPI-TiO2/Spiro double exponential decay fit 1.0 area (mm*V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

0.9

tslow = 4.8 ± 0.4 s tslow = 5.1 ± 0.2 s tslow = 7.7 ± 0.4 s

0.8

0

5

10 15 time (s)


20

Page 30 of 31

Page 31 of 31

a)

c)

b)

before illumination after illumination 1s

0.2 0.0

0.1

0.2

0.3

0.4

position (mm)

0.5

10

ITO TiO2

MAPI

0 -10 0.0

-

0.1

0.2

0.3

0.4

position (mm)


0.0

area (mm*nV)

0.3

Ag

Spiro-OMeTAD (HTL)

MAPI

ITO TiO2

Dr/ee0 (V/µm2)

Spiro-OMeTAD (HTL)

0.4

calculated data points exponential decay fit

difference after-before 20

CPD (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Ag 0.5

-10.0 tafter = 8.8 ± 1.1 s -20.0 0

5

10

15

time (s)

20

25

Local Time-Dependent Charging in a Perovskite ... - ACS Publications

Recommend Documents