Publications by Paramvir Ahlawat

A universal co-solvent dilution strategy enables facile and cost-effective fabrication of perovskite photovoltaics

Paramvir Ahlawat, Masaud Hassan S Almalki, Viacheslav Bolnykh, Ulf Anders Hagfeldt, Anurag Krishna, Linfeng Pan, Dan Ren, Ursula Röthlisberger, Sandy Sanchez Alonso, Tzu-Sen Su, Narges Yaghoobi Nia, Shaik Mohammed Zakeeruddin, Hong Zhang

Cost management and toxic waste generation are two key issues that must be addressed before the commercialization of perovskite optoelectronic devices. We report a ground-breaking strategy for eco-friendly and cost-effective fabrication of highly efficient perovskitesolar cells. This strategy involves the usage of a high volatility co-solvent, which dilutesperovskite precursors to a lower concentration (1.4 M) solution. More than 70% of toxicwaste and material cost can be reduced. Mechanistic insights reveal ultra-rapid evaporationof the co-solvent together with beneficial alteration of the precursor colloidal chemistryupon dilution with co-solvent, which in-situ studies and theoretical simulations confirm.The co-solvent tuned precursor colloidal properties also contribute to the enhancement ofthe stability of precursor solution, which extends its processing window thus minimizing thewaste. This strategy is universally successful across different perovskite compositions, andscales from small devices to large-scale modules using industrial spin-coating, potentiallyeasing the lab-to-fab translation of perovskite technologies.

2022

Perovskite solar cells have emerged as the most promising cheaper photovoltaic technology. Besides solar cells, halide perovskites have a wide range of applications due to their remarkable optoelectronic properties. Starting from 3.8% in 2009, solar to the power conversion efficiency of perovskite solar cells is now >25%, exceeding market leader polycrystalline silicon. These advancements are mainly due to the improvement in their manufacturing process targeted to make better morphology. To this objective, tens of thousands of experimental studies are conducted where a wide range of additives, compositions, and processing conditions are optimized based on a trial and error methodology. Although this approach increased the efficiency, however, perovskite electronics suffer from the problem of stability and reproducibility. Therefore, to industrialize highly efficient perovskite electronics, one needs to understand better and rationalize their fabrication process, i.e., crystallization of halide perovskites. In this thesis, I perform molecular simulations to understand the formation process of halide perovskites. First, I try to lay out the atomic details of the nucleation process of perovskite from the solution. I find that nucleation proceeds in a two-step manner where an intermediate phase forms before the emergence of perovskite crystals from the solution. I observe that monovalent cations can play an essential role in the nucleation process. On this information, simulation-driven experiments are performed by experimental collaborators to demonstrate that the grain size and crystallinity can be tuned by only varying the concentration of monovalent cations in solution. I also attempted to study the effects of different solvents and pseudo-halide additives used to make high-efficiency solar cells. I calculated that by modulating the solute-solvent interactions, one could minimize raw material losses during the coating process. In comparison, pseudo-halide additives are found to passivate the ionic defects and may slow down the growth process that helps to improve the final morphology. Second, I study the solid-solid phase transition in the popular two-step process. I discover a new path to the low-temperature formation of highly efficient metastable cubic phase of formamidinium lead iodide perovskite. I first validate my simulations with in-situ experiments performed by experimental collaborators. Then simulations are employed to design a practical methodology to successfully make thin films of phase pure cubic formamidinium lead iodide at lower temperatures. Moreover, I study the intrinsic stability of this material. I calculate the energy barrier between the metastable photo-active phase and its photo-inactive thermodynamically stable hexagonal phase. I find that once formed; the photo-active phase might be kinetically stable. Additionally, I identify new polymorphs (perovskitoids) of halide perovskites and also study the structural insights of low-dimensional halide perovskites. Overall, I present a fundamental understanding of the crystallization of perovskites which is helpful to design better experiments to make high efficiency and stable perovskite electronics.

EPFL2022

A combined molecular dynamics and experimental study of two-step process enabling low-temperature formation of phase-pure alpha-FAPbI3

Paramvir Ahlawat, Essa Awadh R Alharbi, Haizhou Lu

It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable α phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure α-formamidinium lead iodide.

2021

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

Multimodal host–guest complexation for efficient and stable perovskite photovoltaics

Paramvir Ahlawat, Zhaowen Dong, Felix Thomas Eickemeyer, David Lyndon Emsley, Michael Graetzel, Jinming Guo, Ulf Anders Hagfeldt, Michael Allan Hope, Farzaneh Jahanbakhshi, Anurag Krishna, Jovana Milic, Aditya Mishra, Marko Mladenovic, Olivier Ouellette, Dan Ren, Ursula Röthlisberger, Tzu-Sen Su, Zaiwei Wang, Shaik Mohammed Zakeeruddin, Hong Zhang, Zhiwen Zhou

Formamidinium lead iodide perovskites are promising light-harvesting materials, yet stabilizing them under operating conditions without compromising optimal optoelectronic properties remains challenging. We report a multimodal host–guest complexation strategy to overcome this challenge using a crown ether, dibenzo-21-crown-7, which acts as a vehicle that assembles at the interface and delivers Cs+ ions into the interior while modulating the material. This provides a local gradient of doping at the nanoscale that assists in photoinduced charge separation while passivating surface and bulk defects, stabilizing the perovskite phase through a synergistic effect of the host, guest, and host–guest complex. The resulting solar cells show power conversion efficiencies exceeding 24% and enhanced operational stability, maintaining over 95% of their performance without encapsulation for 500 h under continuous operation. Moreover, the host contributes to binding lead ions, reducing their environmental impact. This supramolecular strategy illustrates the broad implications of host–guest chemistry in photovoltaics.

2021

Nanoscale Phase Segregation in Supramolecular π-Templating for Hybrid Perovskite Photovoltaics from NMR Crystallography

Paramvir Ahlawat, Loï Charles Carbone, Brian Irving Carlsen, Manuel Cordova, David Lyndon Emsley, Ulf Anders Hagfeldt, Michael Allan Hope, Farzaneh Jahanbakhshi, Dominik Józef Kubicki, Yuhang Liu, Jovana Milic, Aditya Mishra, Marko Mladenovic, Toru Nakamura, Ursula Röthlisberger, Rashmi Runjhun, Thomas Schneeberger, Shaik Mohammed Zakeeruddin

The use of layered perovskites is an important strategy to improve the stability of hybrid perovskite materials and their optoelectronic devices. However, tailoring their properties requires accurate structure determination at the atomic scale, which is a challenge for conventional diffraction-based techniques. We demonstrate the use of nuclear magnetic resonance (NMR) crystallography in determining the structure of layered hybrid perovskites for a mixed-spacer model composed of 2-phenylethylammonium (PEA+) and 2-(perfluorophenyl)ethylammonium (FEA+) moieties, revealing nanoscale phase segregation. Moreover, we illustrate the application of this structure in perovskite solar cells with power conversion efficiencies that exceed 21%, accompanied by enhanced operational stability.

2021

Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

Paramvir Ahlawat, Masaud Hassan S Almalki, Vincent Henri Guillaume Dufoulon, David Lyndon Emsley, George Cameron Fish, Michael Graetzel, Farzaneh Jahanbakhshi, Dominik Józef Kubicki, Jovana Milic, Aditya Mishra, Marko Mladenovic, Jacques-Edouard Moser, Ursula Röthlisberger, Marco Alejandro Ruiz Preciado, Thomas Schneeberger, Pascal Alexander Schouwink, Shaik Mohammed Zakeeruddin

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.

2021

Formamidinium-Based Dion-Jacobson Layered Hybrid Perovskites: Structural Complexity and Optoelectronic Properties

Paramvir Ahlawat, Brian Irving Carlsen, Mohammad Ibrahim Dar, Algirdas Ducinskas, Michael Graetzel, Ulf Anders Hagfeldt, Farzaneh Jahanbakhshi, Yang Li, Jovana Milic, Marko Mladenovic, Ursula Röthlisberger, Wolfgang Richard Tress, Amita Ummadisingu, Shaik Mohammed Zakeeruddin

Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic-inorganic perovskite materials, which are predominantly based on Ruddlesden-Popper structures. Formamidinium (FA)-based Dion-Jacobson perovskite analogs are developed that feature bifunctional organic spacers separating the hybrid perovskite slabs by introducing 1,4-phenylenedimethanammonium (PDMA) organic moieties. While these materials demonstrate competitive performances as compared to other FA-based low-dimensional perovskite solar cells, the underlying mechanisms for this behavior remain elusive. Here, the structural complexity and optoelectronic properties of materials featuring (PDMA)FA(n)(-1)Pb(n)I(3)(n)(+1)(n = 1-3) formulations are unraveled using a combination of techniques, including X-ray scattering measurements in conjunction with molecular dynamics simulations and density functional theory calculations. While theoretical calculations suggest that layered Dion-Jacobson perovskite structures are more prominent with the increasing number of inorganic layers (n), this is accompanied with an increase in formation energies that rendern > 2 compositions difficult to obtain, in accordance with the experimental evidence. Moreover, the underlying intermolecular interactions and their templating effects on the Dion-Jacobson structure are elucidated, defining the optoelectronic properties. Consequently, despite the challenge to obtain phase-puren > 1 compositions, time-resolved microwave conductivity measurements reveal high photoconductivities and long charge carrier lifetimes. This comprehensive analysis thereby reveals critical features for advancing layered hybrid perovskite optoelectronics.

WILEY-V C H VERLAG GMBH2020

Guanine‐Stabilized Formamidinium Lead Iodide Perovskites

Paramvir Ahlawat, David Lyndon Emsley, Ulf Anders Hagfeldt, Li Hong, Dominik Józef Kubicki, Yuhang Liu, Jovana Milic, Marko Mladenovic, Linfeng Pan, Dan Ren, Ursula Röthlisberger, Marco Alejandro Ruiz Preciado, Amita Ummadisingu, Shaik Mohammed Zakeeruddin

Formamidinium (FA) lead iodide perovskite materials feature promising photovoltaic performances and superior thermal stabilities. However, conversion of the perovskite α‐FAPbI3 phase to the thermodynamically stable yet photovoltaically inactive δ‐FAPbI3 phase compromises the photovoltaic performance. A strategy is presented to address this challenge by using low‐dimensional hybrid perovskite materials comprising guaninium (G) organic spacer layers that act as stabilizers of the three‐dimensional α‐FAPbI3 phase. The underlying mode of interaction at the atomic level is unraveled by means of solid‐state nuclear magnetic resonance spectroscopy, X‐ray crystallography, transmission electron microscopy, molecular dynamics simulations, and DFT calculations. Low‐dimensional‐phase‐containing hybrid FAPbI3 perovskite solar cells are obtained with improved performance and enhanced long‐term stability.

2020

Unravelling the structural complexity and photophysical properties of adamantyl-based layered hybrid perovskites

Paramvir Ahlawat, Brian Irving Carlsen, Mohammad Ibrahim Dar, Algirdas Ducinskas, Michael Graetzel, Ulf Anders Hagfeldt, Farzaneh Jahanbakhshi, Yang Li, Jovana Milic, Marko Mladenovic, Ursula Röthlisberger, Wolfgang Richard Tress, Amita Ummadisingu, Shaik Mohammed Zakeeruddin

Layered hybrid perovskites comprising adamantyl spacer (A) cations based on the A(2)FA(n-1)Pb(n)I(3n+1)(n= 1-3, FA = formamidinium) compositions have recently been shown to act as promising materials for photovoltaic applications. While the corresponding perovskite solar cells show performances and stabilities that are superior in comparison to other layered two-dimensional formamidinium-based perovskite solar cells, the underlying reasons for their behaviour are not well understood. We provide a comprehensive investigation of the structural and photophysical properties of this unique class of materials, which is complemented by theoretical analysisviamolecular dynamics simulations and density functional theory calculations. We demonstrate the formation of well-defined structures of lower compositional representatives based onn= 1-2 formulations with (1-adamantyl)methanammonium spacer moieties, whereas higher compositional representatives (n> 2) are shown to consist of mixtures of low-dimensional phases evidenced by grazing incidence X-ray scattering. Furthermore, we reveal high photoconductivities of the corresponding hybrid perovskite materials, which is accompanied by long charge carrier lifetimes. This study thereby unravels features that are relevant for the performance of FA-based low-dimensional hybrid perovskites.

ROYAL SOC CHEMISTRY2020