Benefit of Grain Boundaries in Organic–Inorganic Halide Planar

Letter pubs.acs.org/JPCL

Benefit of Grain Boundaries in Organic−Inorganic Halide Planar Perovskite Solar Cells Jae S. Yun,*,† Anita Ho-Baillie,† Shujuan Huang,† Sang H. Woo,† Yooun Heo,‡ Jan Seidel,‡ Fuzhi Huang,§ Yi-Bing Cheng,§ and Martin A. Green† †

Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia ‡ School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia § Department of Materials Engineering, Monash University, Victoria 3800, Australia S Supporting Information *

ABSTRACT: The past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic−inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell, and they play an important role that could be benign or detrimental to solar-cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure. We observe the presence of a potential barrier along the grain boundaries under dark conditions and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries, confirming the beneficial roles grain boundaries play in collecting carriers efficiently.

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carrier collections.12−18 It is reported that GBs in these solar cells are electrically charged by impurities or different types of vacancies and create downward band bending toward the GBs in a p-type absorber.12−18 Thus, photogenerated electrons and holes near the GBs will be easily separated by the built-in field at the GBs, and the recombination rate of these carriers will be lower at the GBs. In some cases, it is suggested that in a p-type absorber the photogenerated electrons are flowing along the GB core by a downward band bending created near the GB and subsequently transported to the junction while the main route for photogenerated holes transport is within the grain interior.19 In organic−inorganic perovskite thin-film solar cells, the presence of high densities of grain boundaries does not appear to degrade their electrical performance. Yin et al.20 reported that GBs in CH3NH3PbI3 are intrinsically benign based on firstprinciple calculations. GBs such as Σ5 (310) and Σ3 (111) do not generate deep states in the band gap. Edri et al.21,22 performed contact potential difference (CPD) mapping on GBs in CH3NH3PbI3−xClx. It was shown that ∼40 mV higher surface potential across the GBs was found in the dark and is reduced under illumination. Chen et al.23 reported that grain boundary potential can be controlled via passivation. By releasing the organic species during annealing of the film, higher surface potential at the GBs are changed from 50 mV to lower surface potential around 30 mV. It was suspected that

he past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic−inorganic halide perovskite. Organic−inorganic hybrid materials have been demonstrated to be excellent photovoltaic materials having a large absorption coefficient, high carrier mobility, high carrier diffusion length, and direct band gap.1−3 Conversion efficiencies have increased from 3.8%3 in 2009 to a recently certified 20.1%.4 All early reports utilized an absorber1 in combination with a mesoporous TiO2 or Al2O3 network with TiO2 compact layer as an electron transport material (ETM) and a polymeric hole transport material (HTM). Recently, planar heterojunction structure devices without a mesoporous layer have been demonstrated with efficiencies of 12−18%.5−8 There are different techniques to synthesize planar perovskite thin-film cells with high uniformity and coverage such as thermal coevaporation,5 spin-coating,6,7,9 and vapor-assisted deposition.8 Sizes of the polycrystalline grains of these films range from 100 to 1000 nm. Grain boundaries (GBs) in a polycrystalline thin-film solar cell play an important role that could be benign or detrimental to solar-cell performance. In polycrystalline Si thin-film solar cells, dangling bonds at the GBs create deep states within the band gap and act as a recombination center. Thus, hydrogen passivation is performed to cure grain boundary defects by reducing the density of dangling bonds.10,11 First principle calculations demonstrated that deep levels are not found at the GBs of CuInSe2 films. Therefore, grain boundaries are electrically benign12 in these films. Many studies have shown beneficial effects of GBs in CZTS, CZTSSe, CIGS, and CdTe films enhancing minority © XXXX American Chemical Society

Received: January 27, 2015 Accepted: February 19, 2015

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DOI: 10.1021/acs.jpclett.5b00182 J. Phys. Chem. Lett. 2015, 6, 875−880

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Figure 1. Illustration of the (a) KPFM and (b) C-AFM set up. Relevant vacuum energy levels (in eV) for corresponding materials are indicated.

Figure 2. KPFM measurements performed on a CH3NH3PbI3/TiO2/FTO/glass structure over an area of 3 μm2. (a) Topography map and (b) CPD images taken in the dark. (c−e) CPD maps under various laser illumination intensities at a wavelength of 500 nm. (f) Intensity dependence of CPD of the sample at a wavelength of 500 nm as measured by KPFM.

a tool to measure the CPD and the surface photovoltage (SPV), while c-AFM has been used to measure local photocurrent of thin films. These techniques allow direct measurements of spatial variations of charge-transport behavior of the thin-film device on the nanometer scale. The sample we examined in this study was a CH3NH3PbI3/ TiO2/FTO/glass heterojunction solar cell, as schematically illustrated in Figure 1. No capping layer or hole transport material (HTM) is deposited on top of the CH3NH3PbI3 film such that the surface is directly accessible by the AFM probe. Detailed sample preparation methods are described in the Supporting Information. The thicknesses of CH3NH3PbI3 and TiO2 films are 300 and 50 nm, respectively.

PbI2 phase is presented in perovskite GBs and passivates the GBs. Higher surface potential at the GBs in these studies can be due to band bending in the energy band diagram. However, it is still unclear how photogenerated charges behave at the GBs. In this work, GBs of organic−inorganic halide perovskite films were characterized by Kelvin probe force microscopy (KPFM) and conductive-AFM (c-AFM) to investigate their role in a CH3NH3PbI3/TiO2 heterojunction device. Such a planar structure is chosen due to the demonstrated device performance potential, and its simple design allows the electrical properties at the GBs to be studied without the complication of the mesoporous-TiO2. KPFM has been used as 876

DOI: 10.1021/acs.jpclett.5b00182 J. Phys. Chem. Lett. 2015, 6, 875−880

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The Journal of Physical Chemistry Letters Topography and CPD map of the underlying TiO2 layer were also examined to ascertain the possible effect of the underlying TiO2, as shown in Figure S1 in the Supporting Information. Both the topography and CPD images demonstrate uniform surface and work function across the TiO2 layer, indicating that the topographical variance of the underlying TiO2 layer does not affect the measured topography and CPD of the perovskite layer. The geometry of our experiment is described in Figure 1. For the KPFM setup (Figure 1a), the CPD measured is the CPD between the FTO layer and the gold probe tip. c-AFM measures short-circuit current using a gold probe, and a dc bias voltage was applied when required. Figure 2a shows a spatial topography map of the surface of a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure. As can be seen, the grains varying in size between 50 and 500 nm are found in these solar cell devices. Most of the grains are usually larger than the film thickness. Therefore, GBs are likely to be in direct contact with both the probe tip and the c-TiO2 underlayer, as depicted in ref 6. Light I−V characteristics of the gold-coated solar device are shown in Figure S2 in the Supporting Information. Figure 2b shows the CPD obtained in the dark where no excess charge should be generated. Therefore, the CPD should be constant across the film surface and correspond to the work function difference between the tip and the FTO. Any changes in CPD indicate the presence of an internal field inside the device. In Figure 2b, individual grains are clearly distinguishable and GBs have lower CPD values compared with the CPD within the grains. One possible reason is the presence of builtin potential around the charged GBs. At the GBs, interstitials and vacancies are present, and thus the polarity of GBs can be different compared with that within the grain interiors.24−26 GBs in perovskite oxides are often found to be depleted and form space-charge regions due to the existence of oxygen vacancies at the GBs.24−26 Figure 2c−e shows CPD maps under various illumination conditions of the same area. Figure 2f plots the average CPDs as a function of light intensities. The increase in CPD as a function of light intensity indicates that photogenerated carriers are efficiently separated at the CH3NH3PbI3/TiO2 heterojunction. Under open-circuit conditions, the diffusion current of charges away from the interface is balanced by photogenerated drift current caused by electric field built up in the device and the net current is zero. Therefore, a net CPD between the FTO anode and the perovskite surface can be obtained. The graph depicts a sublinear relationship between the CPD and the light intensity, which follows a similar relationship between the open-circuit voltage and the light intensity of the gold-coated HTM free device in Figure S2b in the Supporting Information. Such similar behavior ensures that our obtained SPV is correctly showing the open-circuit potential under illumination; in other words, the charge-separation process of the device using the AFM probe. Under illumination, individual grains are also distinguishable as under the dark condition. However, it is apparent that the grain boundaries now have a higher CPD than the grain interiors that is opposite to what is observed under the dark condition. Figure 3c−f shows calculated SPV profiles obtained by subtracting CPD in the dark, where Figure 3c is from CPD under illumination at various intensities. The topography map in Figure 3a and line profile in Figure 3b are also shown for reference showing varying grain sizes and heights. In KPFM measurement, surface roughness can lead to CPD differences.

Figure 3. (a) Topography map, (b) topography line profile, and (c) CPD profile under dark conditions. Panels d−f are SPV profiles obtained by subtracting the CPD values in the dark (c) from CPD results under light illumination at various intensities. The line profiles were analyzed along the white line indicated in the topography map. Numbers denote locations of grain boundaries.

It is clear that the CPD maps in Figure 3 are attributed to the electrical features of the sample’s surface rather than the surface roughness. For instance, GB3, which has the lowest height (Figure 3b), does not have the lowest CPD (Figure 3c). As we’ve seen in Figure 3b, CPDs at the GBs are lower compared with those within the interior of grains under the dark condition. At the lowest intensity of 0.5 kW/cm2, SPVs at the GBs 1, 2, and 3 are only a few millivolts higher compared with the SPVs within the grain interiors. As the light intensity increases, SPVs at the GBs are enhanced and the SPV peaks at the GBs become more prominent with light intensity. This indicates that the photogenerated carriers are more effectively separated at the GBs compared with grain interiors. Such higher SPVs at the GBs could be attributed to broadening of bandgap at the GBs.27 According to first-principle calculations, valence band maximum (VBM) of CH3NH3PbI3 antibonding s−p has the strongest coupling at the perovskite Oh symmetry. If the symmetry is lowered at the GB, the coupling becomes weak and the antibonding state (VBM) drops and enlarges the bandgap.12 Photocurrent mapping was performed using c-AFM in contact mode to confirm that GBs act as efficient charge separator, which is shown in Figure 4. In the dark, there is no noticeable current observed as shown in Figure 4b, indicating a negligible effect of topography on current image. In this case, 877

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Figure 4. C-AFM measurements performed on a CH3NH3PbI3/TiO2/FTO/glass structure over an area of 5 μm2. (a) Topographic image and (b) current image are taken in the dark at 0 V. Current images (c) under illumination at 0 V and (d) under illumination at 0.3 V. Insets in panels b−d are overlap of corresponding c-AFM maps and a topography map of the region with a white outline in panel a. Wavelength and intensity of the illumination were 500 nm and 1.1 kW/cm2, respectively.

the measured current is in the range of −1 to +1 pA, which is at the same level as the background noise from the instrument. When light with a wavelength of 500 nm is shone onto the sample with an intensity of 1.1 kW/cm2, a sizable short-circuit current with highest magnitude of −53 pA was measured, as shown in Figure 4c. It is clear that the current is not evenly distributed over the surface of the film. Rather, the current is most prominent around the GBs, as shown in the overlapped c-AFM and topography maps in insets of Figure 4c,d. When a forward bias of 0.3 V is applied, the magnitude of the current is decreased to −33 pA, as shown in Figure 4d, but is still confined in the vicinity of the GBs, as depicted in insets of Figure 4c,d. One may expect that the higher current around GBs could lead to reduction of open-circuit voltage by creating shunting path. In fact, numerical simulations on GBs of CIGS solar cells demonstrated that open-circuit voltage could be higher or lower depending on the potential barrier height at GBs.28 Our KPFM results showed higher SPV at the GBs, and thus we suspect that the potential barrier formed in our perovskite sample does not lead to reduction of open-circuit voltage. Figure 5 shows a line profile of a topography that crosses three GBs. The result shows that there is no direct correlation between the magnitude of the measured current and the height. For example, no short-circuit current is measured in the vicinity of GB2 where there is a dip in the film thickness. Not all GBs behave the same and are affected by crystallography and defect chemistry.12 The higher current collection near GBs is consistent with our previous KPFM results, which indicate that photogenerated carriers are more efficiently separated and transported along the GBs. On the basis of our KPFM results, lower CPD at the GBs under the dark condition implies that downward band bending is present at GBs. Although there are studies on the polarity of the CH3NH3PbI3 films, which is determined by different types of vacancies,29,30 it is not yet known what type of vacancies could possibly be present at the GBs of polycrystalline

Figure 5. C-AFM current (black) and height (blue) line profiles at 0 V under illumination.

CH3NH3PbI3 films that charge the GBs and promote efficient charge separation. We have demonstrated that the GBs in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure play a beneficial role. KPFM measurement showed that a potential barrier is formed along the GBs and higher SPV is found along the GBs. The c-AFM measurement showed the higher shortcircuit current collection near GBs compared with that within grain interior. These two measurements confirm that photogenerated carriers are effectively separated and collected at GBs. Further studies on the composition or different types of vacancies at GBs are required to fully understand how the GBs play these beneficial roles.

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EXPERIMENTAL SECTION KPFM and c-AFM measurements were carried out using an AFM (AIST-NT SmartSPM 1000) in air. Nitrogen gun was 878

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(2) Mitzi, D. B.; Feild, C.; Schlesinger, Z.; Laibowitz, R. Transport, Optical, And Magnetic Properties of the Conducting Halide Perovskite CH3NH3SnI3. J. Solid State Chem. 1995, 114 (1), 159−163. (3) 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. (4) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 45). Prog. Photovoltaics 2015, 23 (1), 1−9. (5) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501 (7467), 395−398. (6) Huang, F.; Dkhissi, Y.; Huang, W.; Xiao, M.; Benesperi, I.; Rubanov, S.; Zhu, Y.; Lin, X.; Jiang, L.; Zhou, Y. Gas-Assisted Preparation of Lead Iodide Perovskite Films Consisting of a Monolayer of Single Crystalline Grains for High Efficiency Planar Solar Cells. Nano Energy 2014, 10, 10−18. (7) 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. 2014, 126 (37), 10056−10061. (8) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.H.; Liu, Y.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2013, 136 (2), 622−625. (9) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic−organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13 (9), 897−903. (10) Nickel, N.; Johnson, N.; Jackson, W. Hydrogen Passivation of Grain Boundary Defects in Polycrystalline Silicon Thin Films. Appl. Phys. Lett. 1993, 62 (25), 3285−3287. (11) Terry, M. L.; Straub, A.; Inns, D.; Song, D.; Aberle, A. G. Large Open-Circuit Voltage Improvement by Rapid Thermal Annealing of Evaporated Solid-Phase-Crystallized Thin-Film Silicon Solar Cells on Glass. Appl. Phys. Lett. 2005, 86 (17), 172108. (12) Yan, Y.; Jiang, C.-S.; Noufi, R.; Wei, S.-H.; Moutinho, H.; AlJassim, M. Electrically Benign Behavior of Grain Boundaries in Polycrystalline CuInSe 2 Films. Phys. Rev. Lett. 2007, 99 (23), 235504. (13) Jiang, C.-S.; Noufi, R.; Ramanathan, K.; AbuShama, J.; Moutinho, H.; Al-Jassim, M. Does the Local Built-in Potential on Grain Boundaries of Cu (In, Ga) Se2 Thin Films Benefit Photovoltaic Performance of the Device? Appl. Phys. Lett. 2004, 85 (13), 2625− 2627. (14) Visoly-Fisher, I.; Cohen, S. R.; Gartsman, K.; Ruzin, A.; Cahen, D. Understanding the Beneficial Role of Grain Boundaries in Polycrystalline Solar Cells from Single-Grain-Boundary Scanning Probe Microscopy. Adv. Funct. Mater. 2006, 16 (5), 649−660. (15) Li, J. B.; Chawla, V.; Clemens, B. M. Investigating the Role of Grain Boundaries in CZTS and CZTSSe Thin Film Solar Cells with Scanning Probe Microscopy. Adv. Mater. 2012, 24 (6), 720−723. (16) Kim, G. Y.; Jeong, A. R.; Kim, J. R.; Jo, W.; Son, D.-H.; Kim, D.H.; Kang, J.-K. Surface Potential on Grain Boundaries and Intragrains of Highly Efficient Cu 2 ZnSn (S, Se) 4 Thin-Films Grown by TwoStep Sputtering Process. Sol. Energy Mater. Sol. Cells 2014, 127, 129− 135. (17) Jiang, C. S.; Repins, I. L.; Beall, C.; Moutinho, H. R.; Ramanathan, K.; Al-Jassim, M. M. Investigation of Micro-Electrical Properties of Cu2ZnSnSe4 Thin Films Using Scanning Probe Microscopy. Sol. Energy Mater. Sol. Cells 2015, 132 (0), 342−347. (18) Hafemeister, M.; Siebentritt, S.; Albert, J.; Lux-Steiner, M. C.; Sadewasser, S. Large Neutral Barrier at Grain Boundaries in Chalcopyrite Thin Films. Phys. Rev. Lett. 2010, 104 (19), 196602. (19) Visoly-Fisher, I.; Cohen, S. R.; Ruzin, A.; Cahen, D. How Polycrystalline Devices Can Outperform Single-Crystal Ones: Thin Film CdTe/CdS Solar Cells. Adv. Mater. 2004, 16 (11), 879−883. (20) Yin, W.-J.; Shi, T.; Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26 (27), 4653−4658.

used to remove particles on sample surfaces prior to the measurements. Samples were measured immediately after they were removed from nitrogen storage. Both KPFM and c-AFM measurements have shown consistent results for repetitive measurements. Both c-AFM and KPFM allow for high lateral resolution measurements of spatial variations of the electrical properties of the devices on the nanometre scale. KPFM is a surface potential detection method that determines the CPD during scanning by compensating the electrostatic forces between the probe and the sample.26 KPFM measurements were performed using a gold-coated Si tip with 6 nm a radius of curvature (HYDRA6R100NG-10, APPNANO) as the probe in a noncontact mode with an AC voltage of −1 to +1 V. For c-AFM and KPFM under light illumination, an external tunable laser source with spot size of 9.54 um2 (FemtoPower 1060) was used to irradiated the sample at an angle of ∼30° to the surface so the probe does not block the incident light; see Figure 1 for the illustration of the setup. Both wavelength and intensity of the illumination source were controllable. The wavelength of laser light used for the AFM beam deflection was 1300 nm, which is outside the absorption range of the sample under test.

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ASSOCIATED CONTENT

S Supporting Information *

KPFM measurements performed on a TiO2/FTO/glass structure over an area of 3 μm2. Light I−V curves of the Au/ CH3NH3PbI3/TiO2/FTO/glass solar cell. Open-circuit voltage (Voc) as a function of light intensity of Au/CH3NH3PbI3/ TiO2/FTO/glass solar cell. I−V curves obtained by applying a voltage bias at the AFM tip and Voc as a function of light intensity. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. J.S. acknowledges support by the Australian Research Council under grant numbers FT110100523, DP140100463, and DP140102849. This work was also supported by the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (contract no. NRF-2013S1A2A2035418).

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REFERENCES

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