Effects of Postsynthesis Thermal Conditions on Methylammonium

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Effects of Post-Synthesis Thermal Conditions on Methylammonium Lead Halide Perovskite: Band Bending at Grain Boundaries and Its Impacts on Solar Cell Performance Daehan Kim, Gee Yeong Kim, Changhyun Ko, Seong Ryul Pae, Yun Seog Lee, Oki Gunawan, D. Frank Ogletree, William Jo, and Byungha Shin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08744 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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The Journal of Physical Chemistry

Effects of Post-synthesis Thermal Conditions on Methylammonium Lead Halide Perovskite: Band Bending at Grain Boundaries and Its Impacts on Solar Cell Performance

Daehan Kim1, Gee Yeong Kim2, Changhyun Ko3, Seong Ryul Pae1, Yun Seog Lee4, Oki Gunawan4, D. Frank Ogletree5,6, William Jo*2, Byungha Shin*1

1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea. 2

Department of Physics, Ewha Womans University, Seoul 120-750, South Korea.

3

Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA.

4

IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA

5

Molecular Foundry, Lawrence Berkeley National Laboratory, California 94720, USA.

6

Materials Sciences Division, Lawrence Berkeley National Laboratory, California 94720, USA.

1

ACS Paragon Plus Environment


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Abstract We studied the effects of post-synthesis thermal treatments on material properties of CH3NH3PbI3 perovskite films and their photovoltaic performance. Kelvin probe force microscopy revealed the existence of a positive potential barrier at grain boundaries, which is known to be beneficial by suppressing carrier recombination. The height of the barrier increased with the annealing duration, which directly correlated with the device performance. The origin of the potential barrier appeared to be chemical inhomogeneity as revealed by Nano-Auger electron spectroscopy. Qualitatively similar temperature-dependence of current-density vs bias characteristics were observed regardless of the annealing duration, where efficiency collapsed at low temperatures due to a diverging series resistance. The freeze-out of free carries and/or a Schottky-type barrier(s) at the interface(s) is likely responsible for the diverging series resistance.

Introduction

Solar cells based on organic-inorganic hybrid perovskite such as methylammonium lead tri-iodide (MAPbI3) have been a very popular topic in photovoltaic research due to the rapid progress in their power conversion efficiency witnessed in the past several years. Initially, MAPbI3 was used as a replacement for a component in dye-sensitized solar cells—first replacing dyes1 and then the hole transporting liquid electrolyte, demonstrating an efficiency close to 10%.2 Further studies2,3 suggested that the working principles of the perovskite solar cells are similar to those of minority carrier type devices as opposed to those of excitonic devices such as dyesensitized solar cells. In line with this, solid state thin-film device architecture was adopted for the perovskite absorbers, which resulted in a record efficiency of over 20% in just a few years.4 A number of variations in the fabrication processes of perovskite thin-films have been reported.5-8 The structural and electrical properties of the resulting perovskite thin-films are substantially influenced by various components of the fabrication steps. For instance, in a two-step sequential deposition method, a cuboid-shaped perovskite layer is often formed when a dilute methyl ammonium iodide (MAI) solution is used, whereas a compact and planar surface morphology is observed when a highly concentrated MAI solution is used.9 One of essential steps in fabricating high-quality perovskite thin-films is an annealing process in which the solvent from the precursor solution is evaporated and 2


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the grain growth of polycrystalline perovskite films often occurs. Various annealing methods have been implemented to improve the solar cell efficiency,10-12 and the typical annealing conditions have been relatively short, between 10 and 20 minutes, at a mild temperature of around 100 ˚C.13,14 We studied the effect of annealing duration on the properties of the resulting MAPbI3 films and devices, focusing on fundamental material properties of MAPbI3 films and devices. In-depth characterizations were carried out using Kelvin probe force microscopy (KPFM), Nano-Auger electron spectroscopy (Nano-AES), conductive atomic force microscopy (c-AFM), and temperature-dependent current-density vs. voltage (J-V) measurements under simulated 1-Sun illumination. KPFM is a tool well-suited for studying surface potential across grain boundaries (GBs) and has been successfully used to identify the presence of potential barriers at grain boundaries in polycrystalline chalcogenide thin-films such as Cu(In, Ga)Se2 (CIGS)15,16 and Cu2ZnSn(S,Se)4 (CZTS).17,18 In recent years, there have been some reports in the literature of applying KPFM to perovskites.19-24 Yun et al.21 used KPFM and c-AFM to investigate the charge carrier dynamics at grain boundaries of MAPbI3 perovskite and suggested that GBs are beneficial; however, no clear correlation between the barrier heights and device performance has been reported yet.

Methods

We prepared perovskite thin-films with compact and flat morphology using sequential two-step spincoating deposition followed by annealing in N2-filled glove box at 100 ˚C, with a duration that varied from 10 minutes to 5 hours. Enhanced grain size and microstructure of the perovskite thin-films were observed with an increased annealing duration (Figure S3), while an annealing duration longer than 5 hours degraded the film quality. The structural and electrical properties of the perovskite films were analyzed with advanced AFM techniques such as KPFM, c-AFM and Nano-AES. We fabricated solar cells with a conventional structure consisting of a bottom TiO2 electron transporting layer (ETL), a top 2,2',7,7'-Tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) hole transporting layer (HTL), and perovskite absorbers, which received a different duration of post-synthesis annealing, and we measured their device performance over a temperature range between 240 and 340 K. We found that there was a positive potential barrier at the GB of the MAPbI3 films and that the barrier height could be varied by the annealing time via the 3



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change in stoichiometry of the grain boundaries as revealed by Nano-AES. A correlation between the barrier height at the GB and the device efficiency was noted, suggesting that the beneficial role of a positive potential barrier claimed in CIGS and CZTS solar cells is also operative in the perovskite solar cells.

Results and Discussions

The photovoltaic performance of the perovskite solar cells annealed for 10 minutes and 5 hours is shown in Figure S1a with the photovoltaic parameters listed in Table S1. All photovoltaic parameters except for the shortcircuit current density (JSC)—efficiency, open-circuit voltage (VOC), and fill factor (FF)—were improved for the 5-hour-annealed sample over the 10-minute-annealed sample. The JSC of the 5-hour-annealed sample was consistent with the value integrated from the external quantum efficiency (EQE) spectra shown in Figure S1b. The scanning electron microscopy (SEM) images (Figures S1c and S1d) revealed that the 5-hour-annealed sample had an average grain size of 300 nm, approximately 10 times larger than that of the 10-minute-annealedsample. The x-ray diffraction pattern from the 5-hour-annealed sample exhibited perovskite peaks with a narrower full width at half maximum (FWHM) than the 10-minute-annealed sample, indicating better crystallinity with longer annealing (Figure S2). No hint of residual PbI2 was observed from the XRD of the 5hour-annealed sample. On the other hand, peaks from a residual lead iodide phase were observed from the 10minute-annealed sample, which suggested that there was insufficient reaction time for the complete conversion to the perovskite phase.25 To further understand effects of the change in the annealing duration on the material properties of the MAPbI3 films, we carried out KPFM and c-AFM measurements. Figure 1 shows the surface morphology and surface potential mapping of the perovskite films with annealing durations of 10 minutes and 5 hours. The measurements were performed inside of a glove box filled with high-purity N2 (99.999%) under dark condition. H2O and O2 concentrations in the box were kept lower than 0.1 ppm, which prevented potential degradation of the films. The AFM topography images shown in Figures 1a and 1d are consistent with the SEM images, which illustrates the enhanced grain size growth with the longer annealing duration. The comparison of the surface potential distribution is shown in Figures 1b and 1e. In both cases, the GBs exhibit a higher potential than the grain interior (GI) regions. However, the surface potential difference between the GB and GI is significantly more pronounced in the 5-hour-annealed sample than the 10-minute-annealed sample. 4


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Figure 1. (a) and (d) AFM topography images, (b) and (e) surface potential mapping from KPFM, (c) and (f) line profiles of the perovskite layer using KPFM. The region of the line profiles is marked in topography and surface potential images. The surface potential values at the GBs of the 10-minute-annealed-sample are lower than 30 mV. However, the 5-hour-annealed perovskite sample exhibits a potential as high as 80 mV at GBs.

Comparison of the line-scan profiles of the surfaces (Figures 1c and 1f) indicates not only that the surface potential between the GI and GB is distinct for the 5-hour-annealed sample, but also that the magnitude of the potential barrier is higher than the 10-minute-annealed sample—surface potential as high as ~80 mV (5-hourannealed) compared to ~20 mV (10-minute-annealed). Such a positive potential barrier at the GBs suggests a downward band-bending at the GBs suppressing electron-hole recombination, as schematically illustrated in Figure S4. The downward band-bending at the GBs has also been identified in inorganic thin-film solar cell materials such as CIGS15,16 and CZTS,17,18 which is believed to enhance the collection efficiency of carriers generated near the GB, which thereby improves the device performance.18 MAPbI3 films fabricated by a process similar to ours have been reported to be p-type.26 The minority carriers are, therefore, electrons and they should 5



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flow towards the GBs whereas the majority carriers (holes) are repelled due to the barrier. A larger potential barrier between the GI and GB thus leads to a more efficient suppression of the electron-hole recombination. Our KPFM data confirms that the same mechanism is operative in the case of perovskite solar cells. On the other hand, the potential difference in the 10-minute-annealed sample is not as clear as that of the 5-hourannealed samples, which is consistent with the smaller VOC of the solar cell devices. In the case of CIGS and CZTS films, it has been suggested that the band bending at the GBs originates from the composition variations between the GIs and the GBs.15-18 We note that the direction of band bending of our perovskite films is the opposite when compared to the recent KPFM study by Dymshits et al.27 This illustrates that the electronic properties of the grain boundary are very sensitive to the preparation method of the perovskite film. In order to check if there is a similar link between the height of a potential barrier and the chemistry of the GBs in the perovskite films, we employed Nano-AES with the spatial resolution of 10 nm28,29,30 to investigate possible local variation in composition, especially across GBs. Auger electron spectra of chosen elements (Pb, N, and I) were taken at several different regions in the GBs and GIs for both samples and Figure 2 presents representative spectra. The statistics of Auger spectra data from all the regions are listed in Supporting Information (Table S2). Several interesting observations can be made from the Nano-AES data: (i) in both samples, the GBs are richer in N (representing methylammonium) and deficient in Pb compared to the GIs while the iodine intensity is similar, (ii) the enrichment of N (methylammonium) and deficiency of Pb are more pronounced in the10-minuteannealed film. A recent KPFM study by Yun et al. showed that GBs in polycrystalline MAPbI3 films serve as an efficient path for the migration of mobile ions such as MA+, Pb2+.24 The observed inhomogeneity of Pb and N would most likely exist in the form of charged ions, and it is not surprising that ions would easily segregate towards, be repelled from, or even evaporate from the GBs depending on the conditions imposed on the sample, such as thermal annealing. For example, the reduced difference in the N concentration between the GB and the GI in the 5-hour-annealed film is likely due to the evaporation of volatile MAI at the GBs. And the evaporation of MAI may well be followed by the replenishment of iodine ions—which is known to have a lower activation energy for migration compared to other ions that may be present in MAPbI3—from the bulk keeping constant iodine concentration across the GB. The presence of highly mobile charged ions in perovskite has been reported many times in the literature.24,31 Aguiar and co-workers has shown that Pb ions preferentially segregate towards grain boundaries of formamidinium lead iodide (FAPbI3) prepared at 175 oC.32 Although our MAPbI3 films 6


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exhibited Pb-deficiency at the grain boundaries unlike the case for the FAPbI3 in Ref. 32, presumably due to the difference in the processing temperatures—100 oC for our MAPbI3 vs. 175 oC for the FAPbI3, enhanced migration of Pb ions towards the grain boundary with the longer anneal (5 hours) is evident from the Nano-AES data. It is unclear at the moment how the relative population of MA and Pb ions at the GBs is linked to the barrier height; for instance, the 10-minute-annealed sample exhibits a more pronounced difference in the concentration of N and Pb between the GIs and GBs, yet the barriers are smaller compared to the 5-hourannealed sample. Nonetheless, it is apparent that the height of the potential barrier originates from the local fluctuation of the stoichiometry.

Figure 2. Representative Nano-Auger electron spectroscopy of the MAPbI3 perovskite film. (a), (b), (c) 10minute-annealed, (d), (e), (f) 5-hour-annealed perovskite. (a), (d) Pb NOO Auger peaks of the grain interior (GI) and grain boundary (GB) regions. (b), (e) N KLL Auger peaks, and (c), (f) I MNN Auger peaks.

To demonstrate how the distinct potential difference observed in the KPFM data affects the current flow in the perovskite films, we employed c-AFM to measure the distribution of current flow across the film thickness (i.e., the vertical direction). The surface morphology, corresponding conduction mapping, and line-scan profiles measured by c-AFM across multiple grains are presented in Figure S5. As can be seen from the c-AFM line profiles shown in Figures S5c and S5f, the current flowed dominantly through the GBs, which has a relatively 7



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lower potential value in both samples as discussed previously. This current path shown by the c-AFM is consistent with the KPFM data in that the minority carrier electrons flow through the grain boundary. However, there are two clear distinctions between the c-AFM data of the two samples. In the 5-hour-annealed sample, there is no dark current through the GI and the current path is mostly contained within the GBs, whereas in the 10-minute-annealed sample, a significant amount of dark current exists; this again is consistent with the larger

VOC from the 5-hour-annealed sample. In addition, the magnitude of the current value in the 5-hour-annealed sample is larger than that of the 10-minute-annealed sample.

We found that the structural and electronic properties of the MAPbI3 films (especially around the GBs) were strongly affected by the annealing duration, and those properties should have a great impact on the device performance at room temperature. To gain insight into the general working principles of perovskite solar cells and to understand how different material properties of the perovskite affect the device operation at both elevated and low temperatures, we carried out temperature-dependent light J-V measurements over a temperature range of between 240 and 340 K. The measurements were performed in order of cooling (from 300 K to 240 K), heating (from 240 K to 340 K), and followed by re-cooling (from 340 K to 310 K). Representative J-V curves from both the samples at various temperatures are shown in Figures 3a and 3d. The change in efficiencies with the temperature is shown in Figures 3b and 3e for the 10-minute-annealed sample and the 5-hour-annealed sample, respectively. Two observations can be made from the figures: (i) the efficiency precipitously decreased as the temperature became lower than 300 K, and it almost recovered as the temperature was raised back, and (ii) raising the temperature over 300 K initially resulted in a slight increase in efficiency while further heating induced an irreversible decline of the efficiency. As can be expected from the J-V curves at lower temperature— whose shapes are greatly distorted—and is confirmed by Figures 3c and 3f, it is a diverging series seriesresistance (RS) that is responsible for the reversible degradation when the samples are cooled below 300 K. The diverging RS could be (i) due to the existence of Schottky-type barrier(s) at perovskite/ETL or perovskite/HTL interface or (ii) due to high activation energy of intrinsic dopants in MAPbI3 which would cause freeze-out of free carriers at a temperature low enough to deactivate the dopants, or (iii) due to low diffusivity of mobile ions such as Pb2+, MA+, I- which restricts ion migration at 240 K. The freeze-out mechanism, in particular, is consistent with the low carrier concentration at room temperature—estimated to be lower than 1013 cm-3—of our 8


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perovskite films.33,34 Incidentally, the freeze-out mechanism has been identified as the main reason behind the collapsing efficiency of CZTS solar cells at low temperatures.35,36 Comparison between two successive measurements at 300 and 310 K showed an increase in the efficiency due to the improved FF (from decreasing

RS). This suggests that the height of the barrier (either a Schottky-type at the interface or dopant activation) that causes the diverging RS at lower temperature is comparable to kT of 300 K, where k is the Boltzmann constant and T is the device temperature. In ion migration mechanism, activation energies for Pb2+, MA+, and I- have been reported to be 2.31 eV, 0.84 eV, and 0.58 eV, respectively.37 Using these values, we could calculate the diffusivity, D difference between 240 K and 300 K for each ions using diffusion equation, D = D0*exp(-EA/kT), where D0 is the diffusivity prefactor and EA is the activation energy. At 240 K, Pb2+, MA+, I- ions would have a diffusivity 3.7 x 108, 1.3 x 103, and 135 times lower compare to the ion diffusivity at 300 K. Therefore, ion migration at 240 K is greatly suppressed, which would increase the series resistance if ion migration contributes the electrical conductivity of the films. With regard to the irreversible degradation of the samples at a temperature higher than 320 K, we speculate that phase transition of MAPbI3 from tetragonal to cubic,38 which is known to occur at ~333 K could be responsible; decomposition of the perovskite can be ruled out because the sample remained dark without any noticeable change in the appearance after the measurement at 340 K (see Figure S6). An interesting point is that both devices (10-minute-annealed and 5-hours-annealed) exhibited qualitatively similar temperature-dependence despite the remarkable difference in the properties of grain boundaries of the perovskite absorbers as well as in the efficiency at room temperature. From this, we speculate that temperature-dependence of J-V characteristics mainly originates from the bulk (interior of grains) properties of the perovskite. Note that the degradation of the device performance with cooling appears less severe in the 10-minute-annealed device; however, more statistics are needed to draw a firm conclusion.

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Figure 3. (a) and (d) light J-V curves taken at various temperatures, (b) and (e) efficiency vs. temperature plots, (c) and (f) series resistance (RS) vs. temperature for 10-minute-annealed sample (left) and 5-hour-annealed sample (right)

Conclusion

In summary, we studied the effect of duration of the post-synthesis annealing at 100 ˚C for 10 minutes and 5 hours. The 5-hour-annealed sample showed enhanced photovoltaic performance over that of the 10-minuteannealed sample. Extensive characterizations with advanced AFM techniques suggested that the presence of a positive potential barrier at GBs, the height of which is larger for the 5-hour-annealed sample and which appears 10


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to be correlated with chemical inhomogeneity as revealed by Nano-AES. Measurements using c-AFM revealed that current dominantly flows through the GBs, which was consistent with the potential distribution measured by KPFM. In temperature-dependent light J-V measurements, both solar cells (with 10-minute-annealed and 5hour-annealed MAPbI3 absorbers) exhibited similar temperature-dependence, where there were reversible and irreversible changes in PV parameters and series resistance during cooling below 300 K and heating above 300 K, respectively. These measurements suggest either the existence of Schottky-type barrier(s) at the interface(s) or high activation energy of intrinsic dopants in MAPbI3.

Supporting Information The Supporting Information contains a description of XRD analysis, SEM images with annealing time, c-AFM data, band bending schematics, table for nano auger spectroscopy, perovskite device image after temperaturedependent light J-V measurement, and plots showing photovoltaic parameter changes within temperature range.

Author information Corresponding author *E-mail: [email protected], [email protected] 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,

Daejeon 34141, South Korea. Tel: +82-42-350-3315 2

Department of Physics, Ewha Womans University, Seoul 120-750, South Korea. Tel: +82-2-3277-4066

Acknowledgments This research was supported by the National Research Foundation of Korea under Grant No. 2014R1A1A1004282. Nano-auger electron spectroscopy experiments at the Molecular Foundry were supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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Effects of Postsynthesis Thermal Conditions on Methylammonium

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