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Halide-substituted electronic properties of organometal halide perovskite films: direct and inverse photoemission studies Chi Li, Jian Wei, Mikio Sato, Harunobu Koike, Zhong-Zhi Xie, YanQing Li, Kaname Kanai, Satoshi Kera, Nobuo Ueno, and Jian-Xin Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02692 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016
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ACS Applied Materials & Interfaces
Halide-substituted electronic properties of organometal halide perovskite films: direct and inverse photoemission studies
Chi Li,1 Jian Wei,1 Mikio Sato,2 Harunobu Koike,2 Zhong-Zhi Xie,1 Yan-Qing Li,1 Kaname Kanai,2,* Satoshi Kera,3,* Nobuo Ueno,4 and Jian-Xin Tang1,*
1
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China. E-mail:
[email protected] 2
Department of Physics, Faculty of Science and Technology, Tokyo University of Science,
2641
Yamazaki,
Noda,
Chiba
278-8510,
Japan.
Email:
[email protected] 3
Division of Photo-Molecular Science III, Department of Photo-Molecular Science, Institute for Molecular Science, National institute of Natural Sciences, 38 Nishigonaka, Myodaiji, Okazaki 444-8585, Japan. Email:
[email protected] 4
Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoicho, Inage-ku,Chiba 263-8522, Japan
* Corresponding authors. E-mail addresses:
[email protected] (J.X. Tang),
[email protected] (K. Kanai),
[email protected] (S. Kera)
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ABSTRACT Solution-processed perovskite solar cells are attracting increasing interest due to the potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions in various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of electronic properties with the qualitatively blue shift along the I Br Cl series, showing the increase in ionization potentials from 5.96 eV to 7.04 eV and the change of transport band gaps in the range from 1.70 eV to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 nm to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes.
KEYWORDS: perovskite, electronic structures, band gap, photoemission spectroscopy, inverse photoemission spectroscopy
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1. INTRODUCTION The extremely fast development of perovskite solar cells based on organometal halide perovskite films has triggered tremendous research interests in photovoltaic applications with respect to the rapidly enhanced power conversion efficiency (PCE) exceeding 20%.1-3 Methylammonium lead trihalides (MAPbX3, X = Cl, Br, or I) are the most commonly used perovskite absorbers, which are capable of both strong light absorption over the visible to near infrared region of the solar spectrum4-6 and efficient transport of photo-generated carriers with micro-scale diffusion length.2,7-12 In addition to excellent photovoltaic properties, these perovskite films exhibit high photoluminescence quantum efficiencies, implying the potential use in light-emitting diodes.13,14 Despite the rapid and continuous improvements in perovskite solar cells with morphological and structural engineering,15,16 a key requirement for further optimizing the efficiency is the precise control over the energetics of these systems, i.e., band gap engineering and energy level alignment with adjacent transport layers for minimal energy loss.17 For example, it has been reported that the mixed halide perovoskite films, e.g., MAPbI3-xClx with the partial substitution of chloride for iodide, exhibit a comparable and even better cell performance, which has been attributed to an enhanced carrier diffusion length.2,18 In addition, perovoskite solar cells using different MAPbX3 absorbers yield various open-circuit voltage (Voc) when matched with an identical hole-transport layer. Yet some of the material aspects are not yet totally understood. Particularly, the effect of halide substitution to the electronic properties, crystal structure and light absorption capability remains unclear, and relative positions of relevant electronic bands have not been investigated for these materials. For the design of high-performance structures, the reliable information and
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deeper understanding about electronic structures of various MAPbX3 perovskite films based on experimental determination is thus indispensable to unraveling the photon harvesting process inside a perovskite-based solar cell. To provide insight into the properties of mixed halide MAPbX3 perovskites in relation to their potential application in phovotoltaics, the role of halide substitution during the preparation processes on their electronic structures is systematically investigated by simultaneously determining their valence and conduction bands with a combination of direct and inverse photoemission spectroscopies. The correlating investigations between crystal structure and optical properties are also performed for different halide-substituted MAPbX3 by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-vis absorption spectroscopy. Fundamental information about the electronic structures of MAPbX3 can be expected to guide the synthesis of new perovskite films and the design of optimal cell structures.
2. EXPERIMENTAL DETAILS 2.1. Perovskite Film Preparation. Methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I) were prepared from different precursor solutions. Precursor materials, methylammonium iodide (CH3NH3I), methylammonium chloride (CH3NH3Cl), methylammonium bromide (CH3NH3Br), were purchased from Polymer Light Technology Corporation with a purity of over 99.5% and four times purification. Lead chloride (PbCl2, purity 98%) and lead iodide (PbI2, purity 99%) were purchased from Sigma-Aldrich Company, while lead bromide (PbBr2, purity 98%) was purchased from Alfa Aesar Company. Precursor solutions with a concentration of 40 wt% were prepared by mixing CH3NH3I, CH3NH3Br, or CH3NH3Cl salts with PbI2, PbBr2, or PbCl2 in various molar ratios into the
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dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) at 60°C for 12 hours. Prior to the spin-coating of MAPbX3 pervoskite films, the indium-tin-oxide (ITO) substrates were ultrasonically cleaned in turn with detergent, ethanol, acetone, ethanol and deionized water, and then dried by nitrogen gas. Precursor solutions were spincoated onto the ITO glass substrates at 6000 rpm for 40 s, and then heated at 110 °C for 40 min under nitrogen atmosphere. The obtained film thickness was about 200 nm as determined by the alpha-SE™ Spectroscopic Ellipsometer. Moreover, extensive precautions in film preparation and transfer were taken to minimize the impact of surface contaminants on the characterization of direct and inverse photoemission characterization. 2.2. Direct Photoemission Spectroscopy Measurements. The valence band levels and core levels of the perovskite films were determined by ultraviolet and Xray photoemission spectroscopies (UPS and XPS), respectively, in a Kratos AXIS UltraDLD ultrahigh vacuum (UHV) surface analysis system, which consists of a fast load lock (base pressure
