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Band-Edges of Hybrid Halide Perovskites under the Influence of Mixed Cation Approach: A Scanning Tunneling Spectroscopic Insight Hrishikesh Bhunia, Soumyo Chatterjee, and Amlan J. Pal ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018
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ACS Applied Energy Materials
Band-Edges of Hybrid Halide Perovskites under the Influence of Mixed Cation Approach: A Scanning Tunneling Spectroscopic Insight Hrishikesh Bhunia, Soumyo Chatterjee, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
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Abstract
Mixed-cation perovskite thin-films are prepared and characterized by introducing formamidinium (FA+) and cesium (Cs+) ions at the methylammonium (MA+)-site of the traditional methylammonium lead triiodide (MAPbI3) perovskite structure. From scanning tunneling spectroscopy (STS) and density of states (DOS) spectra thereof, the exact band-edges of the perovskites with all possible binary cation combinations have been estimated. From the obtained band locations, we could correlate the change in band-energies with the size of the cation and have discussed the results in terms of orbital overlap. The band-edge(s) responsible for the change in band gap in the multication systems could be identified and correlated with the origin of the bands. The obtained band-energies provided the band-gap of mixedcation perovskites that are commonly derived from optical absorption spectroscopy. From the perspective of device applications, the results are hence useful in drawing band-diagrams and designing band-engineered mixed-cation hybrid perovskites for solar cell applications.
KEYWORDS: Hybrid Halide Perovskites; Double- and Triple-Cation Approach; Scanning Tunneling Spectroscopy; Energy Level Modification; Correlation between Optical and Transport Gaps.
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INTRODUCTION The current decade has witnessed a revolutionary rise of organometallic halide perovskites in the research arena of photovoltaics and other optoelectronic applications.1 These wonder materials have so far yielded a certified power conversion efficiency (PCE) of 22.1%2 and a module PCE of 11.2% (for 10 × 10 cm2 size cells),3 which is comparable to the performance level of commercialized silicon solar modules. To proceed further, elemental substitution in traditional perovskite crystal structures has been considered to enhance the performance of perovskite solar cells4 or to address the stability issues of the material.5 Chiefly, research in perovskite photovoltaics has revolved around the methylammonium lead triiodide (CH3NH3PbI3 alias MAPbI3) perovskite, which has now become the origin of a large family of isostructural perovskites obtained by suitable substitution at all the three sites with appropriate elements or organic ions.6 Substitutions in hybrid halide perovskites has been principally governed by the empirical approach of Goldschmidt’s tolerance factor (t) and octahedral factor (µ).7 The value of these two parameters depends on the effective or operative ionic radii of the constituent ions; structurally stable perovskites are typically formed with 0.75 < t < 1.0 and 0.44 < μ < 0.90.6,8 Following such a concept, a range of elements has been proposed over the years as suitable alternatives to the basic constituents of AMX3, where A and M represent a monovalent and a bivalent cation, respectively, and X symbolizes a halide. For example, several studies established Cl-, Br-, I-, and their combinations as the choice for halogen sites.9 At the metal sites, on the other hand, a range of elements, such as Sn(II),4 Sb(III),10 Co(II),11 etc, as alternatives to Pb(II), have been proposed. Substitution at the organic cation site has resulted in some significant accomplishments, starting from modification of dimensionality, electronic structure, and optical properties to the enhancement of PCE and also the stability of solar cells.12-14 Such a versatile role of the A-site cation on the properties and performance of AMX3 perovskites has been investigated by substituting the traditional CH3NH3+ (MA+) ions by a range of monovalent (1) inorganic cations (e.g., K+, Rb+, Cs+, etc.)12,13,15 and (2) organic cations, such as, formamidinium, [CH(NH2)2]+ (FA+),16 guanidinium [(NH2)3C]+,14 ethylammonium [(CH3CH2)NH3]+,17 or a combination of these.18 A high ACS Paragon Plus Environment
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content of an inorganic cation however leads to formation of a mixed phase inhibiting efficient charge collection and thereby limiting the solar cell performance.12,13,15 In hybrid halide perovskites, the organic cation as such plays an interesting role in tuning the band gap. Though they do not contribute directly to the formation of band edges, but, depending on their size, they influence the degree of metal-halide orbital overlap; the moiety at the A-site hence alters the valence and conduction band energies and also the band gap.19,20 Besides, such a change in band positions would also manipulate other properties of the perovskite, such as stability, charge separation abilities, carrier transport, and so forth. Hence, in mixed-cation perovskite systems, it is extremely important to locate the band-edges from the viewpoint of charge carriers in addition to the band gap, which is commonly derived from optical absorption spectroscopy. In this direction, conventional bandlocating techniques like cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), optical spectroscopy, etc. have their own limitations. The solution-based approach of CV makes the outcome incomplete, whereas spectroscopic approaches do not provide the information of band-energies as seen by the carriers in the system. Scanning tunneling microscopy and spectroscopy (STM and STS), on the other hand, are surface analytical techniques that probe the local density of states (LDOS) of semiconductors with a high spatial and energetic resolution. As a result, it is possible to draw band diagram of devices with energies as actually encountered by electrons and holes in (opto-)electronic devices.21 In this work, we have formed mixed-cation perovskite thin-films by introducing FA+ and Cs+, the most widely used alternatives, at the MA+-site of traditional MAPbI3 perovskite structure. Mixed-cation perovskites with all three possible binary cation combinations and a range of concentrations were prepared through a one-step approach and characterized. To locate the exact band-energies of these perovskites, their ultrathin-films were subjected to STS studies. From the obtained band-locations we could infer the dependence of band-energies on the size of the A-site cation, which can further be generalized for other alternative cations with comparable ionic-radii. Application of STS in photovoltaic materials is a relatively new area of research and band-energy determination of mixed-cation or mixedACS Paragon Plus Environment
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halogen hybrid perovskites in general will allow formation of band-diagrams of heterojunctions. Therefore, from the viewpoint of device applications, the results will be useful in inspiring fabrication of band-engineered hybrid-halide perovskite solar cells.
EXPERIMENTAL SECTION Materials. Methylammonium iodide (CH3NH3I) was purchased from M/s Dyesol Australia Pty. Ltd. Formamidinium iodide, CH(NH2)2I, cesium iodide (CsI), lead(II) iodide (99%) (PbI2), N,Ndimethylformamide (anhydrous 99.8%) (DMF), and polyvinylpyrrolidone (PVP, MW = 55000) were purchased from Sigma-Aldrich, Inc. The materials, which were stored in a nitrogen-filled glovebox to prevent adsorption of humidity and oxygen, were used without further purification or treatment. Formation of Perovskite Thin-Films by Single Precursor Approach. To form thin-films of the perovskites, we have followed a conventional single-precursor approach.22 Though such a one-step approach has been reported to produce a not-so-ideal film for device applications, it allowed to maintain the same degree of structural control on crystallization during the conversion process. Typically, in this method, the precursor solution was prepared by blending a desired amount of the precursors, namely PbI2 and AI (A: MA+, or FA+, or Cs+) in DMF and stirred overnight at 70 °C in a glovebox with moisture and oxygen level below
