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Determining Band-edge Energies and Morphology-dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy R. Clayton Shallcross, Yilong Zheng, S. Scott Saavedra, and Neal R. Armstrong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00516 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017
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Journal of the American Chemical Society
Determining Band-edge Energies and Morphology-dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy R. Clayton Shallcross,* Yilong Zheng,† S. Scott Saavedra and Neal R. Armstrong Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States KEYWORDS. Perovskite, hybrid perovskite, electrochemistry, photoelectron spectroscopy, energetics, stability. ABSTRACT: We show for the first time that the frontier orbital energetics (conduction band minimum (CBM) and valence band maximum (VBM)) of device-relevant, methylammonium bromide (MABr)-doped, formamidinium lead trihalide perovskite (FAPVSK) thin films can be characterized using UV-Vis spectroelectrochemistry, which provides an additional and straightforward experimental technique for determining energy band values relative to more traditional methods based on photoelectron spectroscopy. FA-PVSK films are processed via a two-step deposition process, known to provide high efficiency solar cells, on semitransparent indium tin oxide (ITO) and titanium dioxide (TiO2) electrodes. Spectroelectrochemical characterization is carried out in a non-solvent electrolyte, and the onset potential for bleaching of the FA-PVSK absorbance is used to estimate the CBM, which provides values of ca. -4.0 eV versus vacuum on both ITO and TiO2 electrodes. Since electron injection occurs from the electrode to the perovskite, the CBM is uniquely probed at the buried metal oxide/FA-PVSK interface, which is otherwise difficult to characterize for thick films. UPS characterization of the same FA-PVSK thin films provide complementary near-surface measurements of the VBM and electrode-dependent energetics. In addition to energetics, controlled electrochemical charge injection experiments in the non-solvent electrolyte reveal decomposition pathways that are related to morphology-dependent heterogeneity in the electrochemical and chemical stability of these films. X-ray photoelectron spectroscopy of these electrochemically-treated FA-PVSK films shows changes in the average near-surface stoichiometry, which suggests that lead-rich crystal termination planes are the most likely sites for electron trapping and, thus, nanometer-scale perovskite decomposition.
INTRODUCTION Hybrid inorganic-organic perovskite (PVSK) materials are appealing active layers for emerging optoelectronic devices such as light-emitting diodes (LEDs) and solar cells due to their bandgap tunability, high charge carrier mobility, long electron-hole diffusion lengths, ease of processing, and low material costs.1–4 Photovoltaic (PV) technologies based on PVSK materials are intriguing due to the potential to demonstrate both high energy conversion efficiencies (η) and scalability for low-cost, large area device platforms.5–7 The first high performance PV active layers originated from singlehalide, stoichiometric hybrid perovskites (i.e., AMX3) that rely on Pb- or Sn-based metals (M2+), a particular A-site organic or inorganic cation (A+: methylammonium (MA), formamidinium (FA) or cesium (Cs+)), and halogen anions (X-: chloride, bromide or iodide).8,9 More recent PVSK active layers have included a variety of compositional dopants to boost both energy conversion efficiency and improve device stability.10,11 For PV platforms based on methylammonium bromide (MABr)-doped, formamidinium lead trihalide perovskite (FAPVSK) active layers, research scale PVs have shown η > 20%.12 There are significant knowledge gaps, however, that remain in understanding and controlling the factors that determine efficiency, scalability to large areas, and long term stability of PVSK PV devices.
Efficient charge harvesting and charge injection in thin film LED and PV devices is often limited by the energetic offsets between the frontier orbital energy levels (e.g., conduction band minimum/valence band maximum, CBM/VBM) of the active layer and the conduction/valence band and Fermi energy of the electrode materials used to harvest/inject those charges.13 These energetic offsets can be estimated from “preelectrode” energy levels of the individual components (i.e., the bare electrode and a thick film on the electrode), but it has been well established that formation of the electrode/active layer interfaces often shifts the critical band-edge energies by 0.1 – 0.5 eV as a result of interfacial charge redistribution, creation or removal of “mid-gap states” in the active layer material and electrode, and shifts in the local vacuum level.14 Determination of the CBM/VBM for new PVSK active layers, especially at the buried electrical electrode/active layer interface, is essential for integration into and optimization of nextgeneration optoelectronic device platforms. Electrode/PVSK interfacial energetics have been measured via photoelectron spectroscopy for incrementally evaporated methylammonium lead iodide (MAPbI3) active layers, which have shown electrode-dependent interfacial energetics;15–18 however, PVSK films processed via vacuum-based deposition methods can lead to significantly different stoichiometries and morphologies relative to solution deposition methodologies, which may produce differences in the electronic properties of the electrode/PVSK interface and PVSK bulk.19 There is a need for a
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Journal of the American Chemical Society
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measurement strategy that probes the buried electrode/PVSK energetics, especially for thick films, regardless of the deposition methodology. We show here for the first time that the energy of the CBM at a buried oxide/PVSK interface can be estimated for thick (ca. 500 nm) FA-PVSK films using spectroelectrochemical methods, which probe bleaching of the UV-vis PVSK absorbance associated with charge injection from prototypical transparent conducting oxide (TCO) electrical electrodes (e.g., indium tin oxide, ITO, and titanium dioxide, TiO2) into devicerelevant FA-PVSK films. ITO is a ubiquitous TCO bottom electrode or recombination layer for thin film single junction or tandem solar cells, respectively, and can be modified via low-temperature techniques with thin TiO2 interlayers to provide for selective electron-harvesting.20 In these spectroelectrochemical studies, the reference electrode in contact with the non-solvent electrolyte solution acts effectively as the “topcontact” for the oxide/PVSK platform.21 We have recently demonstrated that this spectroelectrochemical approach can be used for determination of the CBM for thin films and tethered monolayers of molecular semiconductors22–25 and semiconductor quantum dots/nanorods (e.g., CdS, CdSe),26,27 and compliments the determination of the semiconductor frontier orbital energetics using photoemission spectroscopies in high vacuum environments.27–29 Monitoring charge injection via optical absorbance changes has distinct advantages over electrochemical methods that only monitor current flow as a function of applied potential:30 i.) spurious background (displacement) currents due to capacitive effects do not affect the spectroelectrochemical response; ii.) currents associated with redox active impurities (e.g., dissolved molecular oxygen) also do not contribute. For direct bandgap materials with small exciton binding energies, such as these hybrid perovskites,31 determination of the CBM from spectroelectrochemical measurements can be combined with the optical bandgap to estimate the VBM, which are complementary with those derived from in-vacuuo UV photoemission spectroscopy (UPS) measurements, assuming the thin film composition is homogeneous from the oxide/PVSK interface to the PVSK/vacuum interface. We also reveal important morphological and stoichiometric changes to the FA-PVSK films that result from charge trapping during these spectroelectrochemical experiments; i.e., FA-PVSK thin films demonstrate nanoscale redox instability on both ITO and TiO2 electrodes. Such heterogeneity in composition, morphology and electro-activity on sub-micron length scales has also been observed in recent conducting tip AFM studies by Weber-Bargioni et al. for MAPbI3 PVSK thin films, where dark and photoelectrical activities are strongly facet dependent.32 Recent studies by Rand and coworkers have suggested that charge injection from a reactive metal top electrode (e.g., Al) leads to morphological and stoichiometric changes to PVSK thin films.33 Understanding the interplay between morphology, chemical composition, and chemical/electrochemical reactivity on the sub-micron scale is critical for realizing efficient and stable optoelectronic devices based on these promising PVSK active layers. EXPERIMENTAL SECTION Preparation of Thin Films. Amorphous and conformal TiO2 thin film (ca. 30 nm) electrodes are prepared by chemical vapor deposition (CVD) on oxygen plasma treated (activated) ITO electrodes using a previously reported procedure.20 Brief-
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ly, ITO is fixed on a heating stage and pumped down in a home-built CVD chamber to a base pressure of ca. 700-800 mTorr. TiO2 thin films are deposited via CVD from a titanium isopropoxide (TIP) precursor in a N2 carrier gas by flowing over a heated (210 °C) ITO electrode for 25 min. at a flow rate 0.66 cm3/min and working pressure of 1.1 Torr. The TiO2 electrodes are activated with oxygen plasma prior to FAPVSK film processing. FA-PVSK films are processed in a N2 glovebox (
