Project

Lead-free halide perovskites for the highest efficient solar energy conversion

Goal: The goal of this project is demonstrating a tin-based perovskite solar cell with power conversion efficiency over 20% and stability for 25 years. The research strategy to enable this disruptive outcome comprises innovative perovskites formulations and unconventional supramolecular interactions.

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Antonio Abate
added an update
Statistical analysis of the stability of tin perovskite solar cells is only possible if we can age hundreds of devices under identical conditions. We have a dedicated setup that will accelerate the progress in making stable tin perovskite solar cells.
 
Antonio Abate
added 4 research items
Lead (Pb) is one of the most toxic elements in existence and has been used by humans for thousands of years. With only a few exceptions, each widespread application of lead has been banned systematically due to dramatic environmental and health consequences. However, we are now at the dawn of the perovskite era, potentially requiring yet again the widespread application of lead.
Current protocols for solution‐processed tin halide perovskite thin films rely on the use of DMSO. However, this solvent was recently found to oxidise tin species. This source of defects in the films may be the reason for the bottlenecks in device performance and reproducibility. DMSO‐free processes may avoid this problem, but lack a controlled crystallization so far. Understanding the chemistry of tin perovskites in new solvent systems would allow fabricating thin films of the highest quality.
Antonio Abate
added 6 research items
Lead halide perovskites revolutionized photovoltaics thanks to their explosive growth in efficiency in just few years, faster than any other photovoltaic technology. Nevertheless, their long-term stability is the main obstacle to their commercialization and the safeness of the lead content of the cell is another insidious issue that is being underestimated by the scientific community. Some studies are currently suggesting that the bioavailability of the lead in the mixed organic-inorganic perovskites is higher than that of the lead commonly present in the soil. For this reason, it is important to explore and develop a safer alternative to lead halide perovskites, relying on the extremely vast knowledge accumulated on these compounds. Tin is the obvious choice for this task due to its similarity to lead but it has a lower bioavailability. Moreover, the bandgap of tin halide perovskites could ensure a better exploitation of the solar spectrum, leading to a higher Shockley–Queisser limit. At the current state of the art, tin-perovskites are still very far from the expected efficiencies. This is related to the same process that makes it so valuable for health. Tin is readily oxidized from Sn²⁺ to Sn⁴⁺ when exposed to air, forming very stable and inert compounds, that are very slowly uptaken by plants. This oxidation is very difficult to contrast and prevents the formation of stoichiometric perovskite. Many different approaches could be adopted to solve this issue and, in this chapter, we will give a comprehensive picture of this new and promising field. The chapter will be divided into three sections. In the first we will introduce ASnX3, illustrating the main features of these compounds in respect to lead counterparts. In the second part we will describe the most recent progress in the performances of ASnX3 devices. Finally, in the third section we will present the most promising approaches for the stabilization of such systems that could lead to stable and efficient ASnX3 devices.
Halide perovskites are crystalline semiconductors gaining increasing attention as low-cost, high-performance materials for optoelectronics. Their processing from solution at low temperatures is compatible with rapid manufacturing of thin-film devices, including solar cells and light-emitting diodes. Therefore, understanding the coordination chemistry in metal halide perovskite precursor solutions would allow controlling the crystallization of thin films, their material properties and device performance. Here, we present a direct nanostructural technique to characterize the colloidal structure of perovskites in precursor solutions. Small-angle scattering is particularly adept for measuring nanoparticles in solution. Applying this technique to perovskite precursor solutions, we can study their colloidal properties. We show that not only do the colloids themselves matter, but also we can reveal their strong interactions in the early stages of crystallization. In particular, we focus on the prearrangement of particles into cluster-like formations. As an example, we present the concentration dependence, which is additionally supported using²⁰⁷Pb NMR.
Tin halide perovskites attract incremental attention to deliver lead‐free perovskite solar cells. Nevertheless, disordered crystal growth and low defect formation energy, related to Sn(II) oxidation to Sn(IV), limit the efficiency and stability of solar cells. Engineering the processing from perovskite precursor solution preparation to film crystallization is crucial to tackle these issues and enable the full photovoltaic potential of tin halide perovskites. Herein, the ionic liquid n‐butylammonium acetate (BAAc) is used to tune the tin coordination with specific O…Sn chelating bonds and NH…X hydrogen bonds. The coordination between BAAc and tin enables modulation of the crystallization of the perovskite in a thin film. The resulting BAAc‐containing perovskite films are more compact and have a preferential crystal orientation. Moreover, a lower amount of Sn(IV) and related chemical defects are found for the BAAc‐containing perovskites. Tin halide perovskite solar cells processed with BAAc show a power conversion efficiency of over 10%. This value is retained after storing the devices for over 1000 h in nitrogen. This work paves the way toward a more controlled tin‐based perovskite crystallization for stable and efficient lead‐free perovskite photovoltaics. The synergistic strategy of tuning the solution coordination and crystallization process by introducing ionic liquid is implemented to successfully fabricate pinhole‐free tin perovskite films with preferential crystal orientation, which possess improved oxidation repellency for Sn(II) and enhanced hydrophobicity. As a result, the stabilization of high‐efficiency lead‐free tin halide perovskite solar cells is achieved.
Zafar Iqbal
added 2 research items
Perovskite solar cells are the rising star of third-generation photovoltaic technology. With a power conversion efficiency of 25.5%, the record efficiency is close to the theoretical maximum efficiency of a single-junction solar cell. However, lead toxicity threatens commercialization efforts and market accessibility. In this context, Sn-based perovskites are a safe alternative. Nevertheless, the efficiency of Sn-based devices falls far behind the efficiency of Pb-based counterparts. This concise review sheds light on the challenges that the field faces toward making Sn-based perovskites the perovskite photovoltaic benchmark. We identified four key challenges: materials and solvents, film formation, Sn(II) oxidation, and energy band alignment. We illustrate every single challenge and highlight the most successful attempts to overcome them. Finally, we provide our opinion on the most promising trends of this field in the future.
The power conversion efficiency of the formamidinium tin iodide (FASI) solar cells constantly increases, with the current record power conversion efficiency approaching 15%. The literature reports a broad anomaly distribution of the photoluminescence (PL) peak position. The PL anomaly is particularly relevant to photovoltaic applications since it directly links the material’s bandgap and subgap defects energy, which are crucial to extracting its full photovoltaic potential. Here, we study the PL of FASI polycrystalline thin film and powder. We found a distribution of PL peak positions in line with the distribution available in the literature systematically. We link the distribution in PL to the octahedral tilting and Sn off-centring within the perovskite lattice, influenced by the procedure used to prepare the material. Our finding paves the way towards controlling the energy distribution of tin perovskite and thus preparing high efficient tin halide perovskite solar cells. This article is protected by copyright. All rights reserved.
Antonio Abate
added 2 research items
Die Fluoridchemie in Zinn-Halogenid-Perowskiten optimiert deren Kristallisationsprozess. Fluoridanionen koordinieren und entfernen somit selektiv SnIV und beeinflussen die kolloidalen Eigenschaften in Lösung. Der in dieser Studie vorgestellte Funktionsmechanismus von SnF2 unterstreicht die Bedeutung der Lösungschemie für die Kontrolle der Kristallisation und SnII-Oxidation in Zinn-Halogenid-Perowskiten. Abstract Zinn ist der Top-Favorit für den Ersatz von giftigem Blei in Perowskit-Solarzellen. Allerdings kommt es dabei verstärkt zu der unerwünschten Oxidation von SnII zu SnIV. Die herkömmlichen Verfahren verwenden SnF2 in der Perowskit-Vorläuferlösung, um die Bildung von SnIV zu verhindern. Dennoch bleibt der Wirkmechanismus des Additivs unklar. Um diesen eingehender zu erläutern, untersuchen wir die Fluoridchemie in Zinn-Halogenid-Perowskiten mit einander ergänzenden Analyseverfahren. NMR-Spektroskopie der Vorläuferlösung offenbart eine stark bevorzugte Affinität der Fluoridanionen für SnIV gegenüber SnII, wodurch dieses selektiv als SnF4 komplexiert wird. Harte Röntgenphotoelektronenspektroskopie an Dünnschichten zeigt die geringere Bereitschaft von SnF4 gegenüber SnI4, in die Perowskit-Struktur eingebaut zu werden und verhindert somit den Einschluss von SnIV in der Dünnschicht. Abschließend offenbart Röntgen-Kleinwinkelstreuung den starken Einfluss vom Fluorid auf die kolloidale Chemie der Vorläuferlösungen, der sich direkt auf die darauffolgende Kristallisation auswirkt.
Tin is the frontrunner for substituting toxic lead in perovskite solar cells. However, tin suffers the detrimental oxidation of Sn(II) to Sn(IV). Most of reported strategies employ SnF 2 in the perovskite precursor solution to prevent Sn(IV) formation. Nevertheless, the working mechanism of this additive remains debated. To further elucidate it, we investigate the fluoride chemistry in tin halide perovskites by complementary analytical tools. NMR of the precursor solution discloses a strong preferential affinity of fluoride anions for Sn(IV) over Sn(II), selectively complexing it as SnF 4 . Hard X-ray photoelectron spectroscopy on films shows the lower tendency of SnF 4 than SnI 4 to get included in the perovskite structure, hence preventing the inclusion of Sn(IV) in the film. Finally, small-angle X-ray scattering reveals the strong influence of fluoride on the colloidal chemistry of precursor dispersions, directly affecting perovskite crystallization. Combining these analytical tools, we provide a complete picture of fluoride's working mechanism.
Antonio Abate
added a research item
Tin is one of the most promising alternatives to lead to make lead-free halide perovskites for optoelectronics. However, the stability of tin-based perovskites is hindered by the oxidation of Sn(II) to Sn(IV). Recent works established that dimethyl sulfoxide, which is one of the best-performing solvents for processing perovskite, is the primary source of tin oxidation. The quest for a stable solvent could be a game-changer in the stability of tin-based perovskites. Starting from a database of over 2000 solvents, we identified a series of 12 new solvents suitable for the processing of formamidinium tin iodide perovskite (FASnI3) by investigating (1) the solubility of the precursor chemicals FAI and SnI2, (2) the thermal stability of the precursor solution, and (3) the possibility of forming perovskite. Finally, we demonstrate a new solvent system to produce solar cells outperforming those based on DMSO. Our work provides guidelines for further identification of new solvents or solvent mixtures for preparing stable tin-based perovskites.
Antonio Abate
added a research item
The ability to control the energy levels in semiconductors is compelling for optoelectronic applications. In this study, we managed to tune the work function (WF) of halide perovskite semiconductors using self-assembling monolayers of small molecules to induce stable dipoles at the surface. The direction and intensity of the surface dipoles rely on specific molecule-to-surface interaction. Electron acceptor or donor molecules result in the positive or negative WF shifts up to several hundreds of meV. Our approach provides a versatile tool to control the WF of halide perovskite and adjust the energy level alignment at the interface with charge transport materials in perovskite-based optoelectronics. The impact on perovskite solar cells is reported and discussed in detail with the support of modelling.
Qiong Wang
added a research item
Low toxicity and an ideal energy bandgap make 2D Ruddlesden-Popper tin-based halide perovskites a promising photovoltaic material. However, the disordered crystal orientation and the oxidation of Sn2+ to Sn4+ at the surface still need to be addressed. Here, we demonstrate that the annealing of FASnI3 assisted by phenyl ethyl ammonium chloride enables the formation of more ordered 2D tin-based perovskite crystals oriented vertically. We use in-situ synchrotron-based grazing incident X-ray diffraction (GIXRD) to correlate the higher crystal orientation to the better device performance. We measured a maximum power conversion efficiency of >9%. Furthermore, we demonstrate that the phenyl ethyl ammonium chloride acts as a barrier layer at the surface of the crystals protecting the tin from the oxidation. This work paves the way for more efficient and stable lead-free perovskite solar cells.
Antonio Abate
added an update
We can probe the oxidation of Sn with NMR
 
Antonio Abate
added 10 research items
Tin-halide perovskites have great potential as photovoltaic materials, but their performance is hampered by undesirable oxidation of Sn(II) to Sn(IV). In this work, we use nuclear magnetic resonance spectroscopy (NMR) to identify and describe the origins of Sn(IV) in Sn-based perovskites, mainly focusing on direct measurements of Sn oxidation states with ¹¹⁹Sn-NMR in solid-state and solution. We find that dimethylsulfoxide (DMSO), a typical solvent for Sn-based perovskites, oxidizes Sn(II) in acidic conditions under temperatures used for film annealing. We propose a redox reaction between DMSO and Sn(II), catalyzed by hydroiodic acid, with iododimethylsulfonium iodide intermediate. We find that lower temperatures and less acidic conditions abate this reaction, and we assess a range of compositions and solution components for this instability. These results suggest the need for strategies to prevent this reaction and shed light on other solution instabilities beyond Sn(IV) that must be mitigated to achieve high-performance lead-free perovskites.
Already exhibiting solar to electrical power conversion efficiencies of over 16 %, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here we report the first completely lead-free, CH3NH3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.
ASnX3 perovskite solar cells (Sn‐PSC) have the potential to deliver the most efficient solar cell technology with safe materials. In this review, a comprehensive introduction of the field is given, that is suitable for nonexperts, gradually leading the reader to a narrower and detailed analysis of the most recent and significant advances. A brief description is given of the leading alternatives for lead‐free PSC and the reasons for ASnX3 compounds' status as one of the most promising candidates are presented. The last part of the review focuses on the stabilization of ASnX3, which is the most compelling challenge to achieve the highest efficiency PSCs. The most promising approaches toward stable and efficient ASnX3 PSCs are identified and discussed. ASnX3 perovskite solar cells (Sn‐PSC) have the potential to deliver the most efficient solar cell technology using Sn, a less hazardous element for health and environment than Pb. This review gives a comprehensive introduction to the field, in a suitable fashion for nonexpert readers, gradually narrowing and detailing the subject with the most recent and significant advances.
Antonio Abate
added a project goal
The goal of this project is demonstrating a tin-based perovskite solar cell with power conversion efficiency over 20% and stability for 25 years. The research strategy to enable this disruptive outcome comprises innovative perovskites formulations and unconventional supramolecular interactions.