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C: Energy Conversion and Storage; Energy and Charge Transport
Self-Doping in Hybrid Halide Perovskites via Precursor Stoichiometry: To Probe Type of Conductivity through Scanning Tunneling Spectroscopy Goutam Paul, Soumyo Chatterjee, Hrishikesh Bhunia, and Amlan J. Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06968 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018
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The Journal of Physical Chemistry
Self-Doping in Hybrid Halide Perovskites via Precursor Stoichiometry: To Probe Type of Conductivity through Scanning Tunneling Spectroscopy Goutam Paul, Soumyo Chatterjee, Hrishikesh Bhunia, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
Corresponding Author *Tel.: +91-33-24734971. Fax: +91-33-24732805. E-mail:
[email protected] ORCID Amlan J. Pal: 0000-0002-7651-9779
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ABSTRACT
In this work, direct observation of ambipolar self-doping in hybrid lead iodide perovskites has been reported through scanning tunneling spectroscopy (STS) and thereby density of states (DOS). Selfdoping phenomenon in CH3NH3PbI3 (MAPbI3) and CH(NH2)2PbI3 (FAPbI3) through precursor stoichiometry has led to an alteration in the Fermi energy and hence a change in the type of electronic conductivity without affecting the inherent band gap of the materials. From STS and the respective DOS spectra, the band-energies of the perovskites with respect to the Fermi energy for a range of precursor ratios have been estimated. The ‘direct’ measurement of band-edges with respect to Fermi energy inferred a gradual change in electronic conductivity from p-type to n-type as both the perovskites were reacted from PbI2-deficient to PbI2-rich precursors. The results have been correlated to point defects generated due to growth environment (stoichiometry of precursors) of perovskites providing a new dimension to probe Fermi energy of hybrid perovskites.
KEYWORDS Self-doping, hybrid halide perovskites, scanning tunneling spectroscopy, precursor stoichiometry, type of conductivity. ACS Paragon Plus Environment
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The Journal of Physical Chemistry
INTRODUCTION The remarkable progress in the development of solar cells based on hybrid halide perovskites has impelled a considerable interest in engineering of these materials. In the perovskite structure (AMX3), elemental substitution or alloying of the parent compound, namely CH3NH3PbI3 has been reported to be effective routes to tune their optical and electronic properties and also the band gap.1-6 In AMX3 perovskites, A and M represent a monovalent organic cation and a bivalent metal ion, respectively, and X symbolizes a halide. Accordingly, the power conversion efficiency (PCE) of optimized perovskite solar cells (PSCs) has improved significantly to a staggering value of 22.1%. 7 Amongst multiple features behind such an impressive PCE, a high level of defect tolerance in the perovskite crystals has been considered to be an important aspect. Intrinsic point defects, which are a mode of unintentional doping, have been predicted to leave the band gap of perovskites unaffected. They are not expected to generate mid-gap states since no foreign element is involved here; as a result, ambipolar or unipolar self-doping is possible through defect engineering in these perovskites sans any change in the band gap.8,9 However, since the (concentration of) defects can substantially tune the position of Fermi energy, such a nature of doping can effectively lead to an alteration in the type of electronic conductivity, for example, from p-type to n-type or viceversa.8,9 The defects can therefore engineer band energies and accordingly energy-diagram of heterojunctions with carrier-selective contacts. Needless to state, an optimized energy-diagram is a prerequisite to excel performance of any devices including perovskite solar cells. Interestingly, point defects in perovskite crystals can be controlled by stoichiometry of the precursors, since formation energy of the defects depends on the growth environment.8,10 If we recall, the hybrid perovskite unit is composed of coordinately-bonded inorganic regular octahedron (MX6-) and an organic entity (A+) at the centre. Therefore, with a change in the stoichiometry of respective precursors, namely MX2 and AX, it is possible to vary the point defects in AMX3. The point-defects with a least formation energy would form and dictate the type of electronic conductivity in a perovskite.
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The analysis on the point defects and hence the type of conductivity in hybrid halide perovskites has so far been based on computational point of view.8,9 Experimentally, n/p-type behavior of self-doped CH3NH3PbI3 perovskites has been evidenced through Hall voltage measurements.11 Band gap, in conjunction with Fermi energy, measured through ultraviolet photoelectron spectroscopy (UPS) confirmed the effect of doping. However, to excel device performance with such defect-engineered perovskites, both band-energies need to be estimated from the view-point of charge carriers, so that an energy level diagram of regular (n–i–p) or inverted (p–i–n) heterojunctions can be formed to engineer efficient devices. In this direction, surface analytical techniques like scanning tunneling microscopy and spectroscopy (STM and STS, respectively) are extremely useful tools over the conventional methods like optical spectroscopy, cyclic voltammetry (CV), UPS, and so forth. STM/S probes the local density of states (LDOS) of semiconductors with a high spatial and energetic resolution; it is hence possible to draw band diagram of heterojunction devices with energies as actually encountered by charge carriers.12,13 The results would hence offer a direction while fabricating solar cells based on self-doped CH3NH3PbI3 perovskites. In this report, we present a direct route to investigate the impact of precursor composition and hence the self-doping effect on the band energies of APbI3 with A being methylammonium (MA) or formamidinium (FA) from a scanning tunneling spectroscopic (STS) insight. The stoichiometry of the precursors in forming the perovskite films were varied from in a PbI2-rich condition to an AI-rich extremity in steps. For each such growth condition of hybrid lead iodide perovskites, their band-edge energies were estimated from STS studies. From the position of Fermi energy with respect to the bandedges, the type of electronic conductivity of the perovskites was determined as well. Reports on such a ‘direct’ measurement method of band-edges and Fermi energy in a series of hybrid lead iodide perovskites can be useful from the standpoint of device applications and research in perovskite photovoltaics.
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The Journal of Physical Chemistry
EXPERIMENTAL METHODS Materials. Methylammonium iodide, CH3NH3I (MAI) was purchased from Dyesol. Formamidinium iodide, CH(NH2)2I (FAI), lead(II) iodide (99%), and N,N-dimethylformamide (anhydrous 99.8%) (DMF) were purchased from Sigma-Aldrich Chemical Co. The materials were stored in a nitrogen-filled glovebox and used without further purification. Fabrication of Perovskite Thin-Films. To form thin-films of the perovskites, we have followed a usual one-step method,14 which was based on co-deposition of both organic and inorganic components through solution processing. The one-step method allowed to maintain a uniformity of all the perovskite films. In this method, a desired amount of MAI (or FAI) and PbI2 in DMF was stirred overnight at 70 °C to dissolve in a glovebox with moisture and oxygen level below
