Light-Induced Formation of Pb3+ Paramagnetic Species in Lead

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Light-Induced Formation of Pb Paramagnetic Species in Lead Halide Perovskites Silvia Colella, Michela Todaro, Sofia Masi, Andrea Listorti, Davide Altamura, Rocco Caliandro, Cinzia Giannini, Elisa Carignani, Marco Geppi, Daniele Meggiolaro, Gianpiero Buscarino, Filippo De Angelis, and Aurora Rizzo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00944 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Energy Letters

Light-Induced Formation of Pb3+ Paramagnetic Species in Lead Halide Perovskites

Silvia Colella,†,‡ Michela Todaro,# Sofia Masi,†,‡ Andrea Listorti,†,‡ Davide Altamura, ∥ Rocco Caliandro, ∥ ,* Cinzia Giannini, ∥ Elisa Carignani,§ Marco Geppi,§ Daniele Meggiolaro,¶,₸ Gianpiero Buscarino, #,* Filippo De Angelis, ¶,₸* Aurora Rizzo.‡

†

Dipartimento di Matematica e Fisica “E. De Giorgi”, Università del Salento, Campus Ecotekne,

via Arnesano, 73100 Lecce, Italy ‡

CNR NANOTEC – Institute of Nanotechnology, Polo di Nanotecnologia, c/o Campus Ecotekne,

Via Monteroni, 73100 Lecce, Italy #

Dipartimento di Fisica e Chimica, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy

∥ IC §

CNR – Istituto di Cristallografia, via Amendola 122/O, 70126 Bari, Italy

Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi, 13, Pisa, PI,

56124, Italy ¶

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di

Sotto 8, 06123, Perugia, Italy ₸ D3-CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected].

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ABSTRACT

Hybrid halide perovskites are soft materials processed at room temperature, revolutionary players in the photovoltaic field. Nowadays, the investigation on the nature and role of defects is seen as one of the key challenges toward the full comprehension of their behavior and the achievement of high device stability under working conditions. We reveal the reversible generation, under illumination, of paramagnetic Pb3+ defects in CH3NH3PbI3, synthesized in ambient conditions, induced by the presence of Pb-O defects in the perovskite structure that may trap photo-generated holes, possibly mediated by the concomitant oxidation and migration of ions. According to the mechanism we hypothesize, one charge is trapped for each paramagnetic center generated, thus it does not contribute to the photocurrent, potentially limiting the solar cell performance. Our study, based on combined experimental/theoretical approach, reveals the dynamic evolution of the perovskite characteristics under illumination that needs to be considered when investigating the material physical-chemical properties.

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ACS Energy Letters

Hybrid perovskites, extensively used in the field of optoelectronics, are a class of materials extremely promising for their excellent properties combined with the mild synthetic protocol. Mainly used in solar cells1–3 and light emitting diodes,4,5 perovskites are formed from solution by self-assembly of precursors. The resulting soft material is often unable to express its ideal potential, due to unsuitable morphology or elevated density of electronic defects as a consequence of the lowtemperature processing. The investigation on the nature and role of these defects is currently seen as one of the key challenges in the field.6,7 Among many, the exceptional prerogative of hybrid halide perovskites8 which cannot be found in conventional solution-processed semiconductor, is the long carrier diffusion length (~1 µm)9 and carrier lifetimes,10 enabling a transport length larger than the depth of absorbed photons, resulting in high photovoltaic performances. These features indicate a low density of traps, contrary to what expected for a low-temperature solution-processed material. Trap density for perovskite has been found to be as low as ~1011 cm-3 in single crystals,10,11 and ~1016 for polycrystalline materials used in solar cells.10,12,13 However, controversial reports can be found in literature on perovskites with very different optoelectronic quality. Variations in perovskite preparation procedure and ambient conditions, (i.e. the presence of oxidizing agents)14 can indeed lead to formation of a material loaded of defects, mainly vacancies, interstitials or anti-sites which could limit the solar cell output voltage due to trapping of photogenerated carriers.15,16 Recently, the less abundant iodine defects has been identified as the source of photochemically active deep electron and hole traps in MAPbI3, which can however be converted into kinetically inactive electron traps by adjusting the oxidizing conditions.17 In addition, when perovskite film is exposed to an intense illumination, as during photovoltaic device operating condition, the basic characteristics of the material investigated in dark, possibly inducing local reactions, ion migration, charge transfer between different sites, could dynamically change.18–20 Rationalizing the effect of illumination on perovskite materials is essential towards the fabrication of reliable and stable photovoltaic devices.


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Here, we reveal the formation of paramagnetic species in CH3NH3PbI3 (MAPbI3) perovskite polycrystalline powder by Electron Paramagnetic Resonance (EPR) that is found to be a powerful tool to spot defect structures in the material. We use a combined experimental/theoretical approach to unveil the kind of defects involved, namely by Density Functional Theory (DFT), Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS-NMR), and state-the-art synchrotron X-Ray powder diffraction (XRPD) setup, coupled with Pair Distribution Function (PDF) analysis. Importantly, we found that the inclusion of oxygen in the structure during the synthesis of MAPbI3, as revealed by DFT calculations, appears to be the responsible for the formation of Pb-O defects that, under illumination, induce a charge (hole) trapping mechanism and the dynamic formation of paramagnetic Pb3+ species. Evidences of this kind of defects have been found by advanced PDF analysis, as structural deviations from the fitted model at short interatomic distances (R12 Å show the same trend of those in the slice 9-12 Å. ACS Paragon Plus Environment

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In order to determine possible structural variations induced by the above-described defects, leading to the generation of paramagnetic centers under illumination, the perovskite powder was analysed by an in situ experiment with state-the-art synchrotron XRPD setup, coupled with advanced PDF analysis. The whole PDF dataset is represented by the matrix shown in Figure 3a, where variables are the interatomic distance (R), monitored up to 50 Å, and the sequential measurement number. Each MAPbI3 PDF pattern in the data matrix has been described and fitted (Figure 3b) in terms of the tetragonal MAPbI3 phase. As a common feature of all patterns, structural deviations from the fitted model arise at short interatomic distances (R

Light-Induced Formation of Pb3+ Paramagnetic Species in Lead

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