Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

Low-dimensional hybrid perovskites have emerged as promising materials for optoelectronic applications. Although these materials have already demonstrated enhanced stability as compared to their three-dimensional perovskite analogues, their functionality has been limited by the insulating character of the organic moieties that primarily play a structure-directing role. This is particularly the case for the layered (2D) perovskite materials based on formamidinium lead iodide (FAPbI3) that remain scarce. We demonstrate a low-dimensional hybrid perovskite material based on a SPbI4 composition incorporating an electroactive naphthalenediimide (NDI) moiety as an organic spacer (S) between the perovskite slabs and evidence the propensity of the spacer to stabilize the α-FAPbI3 perovskite phase in hybrid low-dimensional SFAn–1PbnI3n+1 perovskite compositions. This has been investigated by means of solid-state nuclear magnetic resonance spectroscopy in conjunction with molecular dynamics simulations and density functional theory calculations. Theoretical calculations suggest an electronic contribution of the organic spacer to the resulting optoelectronic properties, which was confirmed by transient absorption spectroscopy. We have further analyzed these materials by time-resolved microwave conductivity measurements, revealing challenges for their application in photovoltaics.

Fluorination of Organic Spacer Impacts on the Structural and Optical Response of 2D Perovskites

Pietro Caprioglio, Inés Garcia Benito, Giulia Grancini, Mohammad Khaja Nazeeruddin, Valentin Ianis Emmanuel Queloz

Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.

2020

Methylammonium Triiodide for Defect Engineering of High-Efficiency Perovskite Solar Cells

Paramvir Ahlawat, Essa Awadh R Alharbi, Claudia Esther Avalos, Thomas Paul Baumeler, Mathias Dankl, Felix Thomas Eickemeyer, George Cameron Fish, Michael Graetzel, Ulf Anders Hagfeldt, Elsa John, Anurag Krishna, Mounir Driss Mensi, Jacques-Edouard Moser, Olivier Ouellette, Linfeng Pan, Lukas Pfeifer, Ursula Röthlisberger, Pascal Alexander Schouwink, Viktor Skorjanc, Bowen Yang, Shaik Mohammed Zakeeruddin

The defects present in metal halide perovskite are deleterious to both the performance and stability of photovoltaic devices. Consequently, there is an intense focus on developing defect mitigation strategies. Herein we report a facile strategy that employs methylammonium triiodide (MAI3) as an additive to the perovskite precursor solution. We examine the effect of MAI3 on the structural and optoelectronic properties by X-ray diffraction, density functional theory calculations, molecular dynamics simulations, solid-state nuclear magnetic resonance, steady-state, time-resolved photoluminescence (TRPL), and time-resolved terahertz spectroscopy (TRTS). Specifically, TRPL and TRTS show that MAI3 suppresses nonradiative recombination and increases the charge carrier mobility. As a result, the champion device shows a power conversion efficiency (PCE) of 23.46% with a high fill factor of >80%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining ∼90% of its initial PCE under 1 sun illumination with maximum power point tracking for 350 h.

2021

Defect chemistry of methylammonium lead iodide

Alessandro Senocrate

The present work deals with the defect chemistry and charge transport properties in halide perovskites, and in particular in the archetypal methylammonium lead iodide. These materials are extensively researched due to their very promising application as light-harvesters in solar cells and in other optoelectronic devices. Notwithstanding the numerous studies dealing with these materials (especially with their optical and electronic properties, and with device application), a significant portion of the underlying physics and chemistry is still poorly understood. Indeed, the physico-chemical features behind their exceptional photo-electrochemical properties are still largely unknown. Moreover, these materials suffer from severe degradation processes presently impeding their practical application. In addition, the charge transport in these materials is not purely electronic, but rather shows a significant ionic portion due to mobile ionic defects. The nature of such ion conduction, alongside its effect on the photo-electrochemical properties and on the materials stability, has never been systematically investigated. The study of these aspects is the aim of this thesis, where: At first, we study the charge transport properties of methylammonium lead iodide, with particular attention to the ionic contribution, and we perform a defect chemical study of the compound. We show how the ionic conductivity, in equilibrium conditions, can be unambiguously assigned to mobile iodine vacancies, with an electronic contribution due to electron holes or conduction electron depending on the iodine activity. Secondly, we investigate the light effect on the charge transport previously characterized. Alongside an expected increase of the electronic contribution, we observe a striking enhancement of ionic conductivity in methylammonium lead iodide upon illumination. This remarkable observation is of fundamental relevance for both photovoltaics and solid state ionics fields. Here we also discuss a mechanism for such photo-enhanced ion conduction that relies on electron-ion interaction. Subsequently, we analyze methylammonium lead iodide and other hybrid halide perovskites under oxygen exposure, both in the dark and under illumination. We show that light strongly affects the kinetics of oxygen interaction, so much so that under illumination methylammonium lead iodide completely degrades, while it is metastable in the dark. The oxygen, when incorporated in a sufficient amount in the lattice, also acts as acceptor dopant, greatly varying the ionic and electronic conductivities. We then investigate stability of several halide perovskites with respect to temperature, oxygen, and illumination through thermodynamic considerations. Here we show how many of these processes are expected to be extremely severe for some of the compounds, underlining an important -and intrinsic- bottleneck for the application of these materials. At last, we investigate the short-range ion dynamics in methylammonium lead iodide, since these are linked to stability and electronic transport properties. Here, we show that methylammonium dynamics conforms well to a fast bi-axial rotation becoming isotropic in the cubic phase. In the inorganic lattice, a strong nuclear coupling between Pb and I is present, alongside highly active iodine dynamics.