Thomas Paul Baumeler

Surface Passivation of FAPbI3-Rich Perovskite with Cesium Iodide Outperforms Bulk Incorporation

Essa Awadh R Alharbi, Thomas Paul Baumeler, Brian Irving Carlsen, Felix Thomas Eickemeyer, George Cameron Fish, Jing Gao, Michael Graetzel, Mounir Driss Mensi, Jacques-Edouard Moser, Lukas Pfeifer, Kevin Sivula, Jun Ho Yum, Shaik Mohammed Zakeeruddin

Metal halide perovskites (MHPs) have shown an incredible increase in efficiency, reaching as high as 25.7%, which now competes with traditional photovoltaic technologies. Herein, we excluded CsX and RbX (X = I–, Br–, Cl–), the most commonly used cations to stabilize α-FAPbI3, from the bulk of perovskite thin films and applied them on the surface as passivation agents. Extensive device optimization led to a power conversion efficiency (PCE) of 24.1% with a high fill factor (FF) of 82.2% upon passivation with CsI. We investigated in depth the effect of CsI passivation on structural and optoelectronic properties using X-ray diffraction (XRD), angle-resolved X-ray photoelectron spectroscopy (ARXPS), Kelvin probe force microscopy (KPFM), time-resolved photoluminescence (TRPL), photoluminescence quantum yield (PLQY), and electroabsorption spectroscopy (TREAS). Furthermore, passivated devices exhibit enhanced operational stability, with optimized passivation with CsI leading to a retention of ∼90% of the initial PCE under 1 sun illumination with maximum power point tracking for 600 h.

2023

Bulk and surface engineering to improve performance and stability of perovskite solar cells

Thomas Paul Baumeler

Human society is craving new and renewable energy sources. Indeed, the need for novel sources is an obligation in order to cope with the exponentially increasing global energy demand. Also, environmentally friendly alternatives to fossil fuels are more crucial than ever in order to limit the effects of global warming. Finally, the current gas and oil crisis underlies the importance for each state to have some alternatives in case of shortage, i.e. to gain some energetic independence. In that regard, solar energy represents the best candidate, given its huge potential and its universal character. In particular, perovskite solar cells (PSCs) show the highest promise amongst emerging technologies. From their first report in 2009, the power conversion efficiency rocketed, to cross 25 % recently. Unfortunately, several important issues remain.Chapter 2. Its lower bandgap makes formamidinium lead iodide (FAPbI3) a more suitable candidate for single-junction solar cells than pure methylammonium lead iodide (MAPbI3). However, its structural and thermodynamic stability is improved by introducing a significant amount of MA and bromide, both of which increase the bandgap and amplify trade-off between the photocurrent and photovoltage. Here, we simultaneously stabilized FAPbI3 into a cubic lattice and minimized the formation of photoinactive phases, such as hexagonal FAPbI3 and PbI2, by introducing 5% MAPbBr3, as revealed by synchrotron X-ray scattering. We were able to stabilize the composition (FA0.95MA0.05Cs0.05)Pb(I0.95Br0.05)3, which exhibits a minimal trade-off between the photocurrent and photovoltage. This material shows low energetic disorder and improved charge-carrier dynamics as revealed by photothermal deflection spectroscopy (PDS) and transient absorption spectroscopy (TAS), respectively. This allowed the fabrication of operationally stable perovskite solar cells yielding reproducible efficiencies approaching 22%.Chapter 3. Herein, we successfully engineered a multi-cation halide composition of perovskite solar cells via the two-step sequential deposition method. Strikingly we find that adding mixtures of 1D polymorphs of orthorhombic d-RbPbI3 and d-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA/MA cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic a-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the d-CsPbI3 also is fully integrated into the 3-D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%, with an open-circuit voltage of 1.15V, a short-circuit photocurrent of 24.82 mA·cm-2, and a fill factor of 78% at a bandgap of 1.55 eV. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1-Sun illumination with maximum power point tracking for 400 h.Chapter 4. A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA+) and formamidinium (FA+) as the monovalent cat

EPFL2022

Cyclopentadiene-Based Hole-Transport Material for Cost-Reduced Stabilized Perovskite Solar Cells with Power Conversion Efficiencies Over 23%

Michael Bauer, Thomas Paul Baumeler, Felix Thomas Eickemeyer, Michael Graetzel, Yuhang Liu, Shaik Mohammed Zakeeruddin, Hongwei Zhu

Hole transport materials (HTM) are an important component in perovskite solar cells (PSC). Despite a multitude of HTMs developed in recent years, only few of them lead to solar cells with efficiencies over 20%. Therefore, it is still a challenge to develop high-performing HTMs, which have ideal energy levels of the frontier orbitals, are highly efficient in transporting charges, and stabilize the solar cell at the same time. In this work, the development of a structurally novel molecular HTM, CPDA 1, is described which is based on a common cyclopentadiene core and can be efficiently and inexpensively synthesized from readily available starting materials, which is important for future realization of low-cost photovoltaics on larger scale. Due to excellent optoelectronic, thermal, and transport properties, CPDA 1 not only meets the envisioned properties by reaching high efficiencies of 23.1%, which is among the highest reported to date, but also contributes to a respectable long-term stability of the PSCs.

WILEY-V C H VERLAG GMBH2021

Formation of High-Performance Multi-Cation Halide Perovskites Photovoltaics by delta-CsPbI3/delta-RbPbI3 Seed-Assisted Heterogeneous Nucleation

Essa Awadh R Alharbi, Masaud Hassan S Almalki, Thomas Paul Baumeler, Felix Thomas Eickemeyer, Ulf Anders Hagfeldt, Anurag Krishna, Mounir Driss Mensi, Olivier Ouellette, Linfeng Pan, Zaiwei Wang, Bowen Yang, Shaik Mohammed Zakeeruddin

The performance of perovskite solar cells is highly dependent on the fabrication method; thus, controlling the growth mechanism of perovskite crystals is a promising way towards increasing their efficiency and stability. Herein, a multi-cation halide composition of perovskite solar cells is engineered via the two-step sequential deposition method. Strikingly, it is found that adding mixtures of 1D polymorphs of orthorhombic delta-RbPbI3 and delta-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA (formamidinium)/MA (methylammonium) cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic alpha-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the delta-CsPbI3 also is fully integrated into the 3D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1 Sun illumination with maximum power point tracking for 400 h.

WILEY-V C H VERLAG GMBH2021

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

Quantifying Stabilized Phase Purity in Formamidinium-Based Multiple-Cation Hybrid Perovskites

Neha Arora, Thomas Paul Baumeler, Mohammad Ibrahim Dar, Michael Graetzel, Shaik Mohammed Zakeeruddin

A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed-ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA(+)) and formamidinium (FA(+)) as a monovalent cation. We compare and quantify the methods of stabilizing FA(-) based perovskites involving the additional blending of the smaller inorganic cations cesium (Cs+) and rubidium (Rb+), which can lead to an improvement in phase purity of black cubic perovskite modification. Even under excess lead iodide conditions, the presence of a separate PbI2 phase as well as hexagonal phases, which are very common for formamidinium-containing perovskites, can be drastically reduced or even completely prevented. In this aspect, adding both Cs+ and Rb+ showed greater effectivity than only adding Cs+, enabling an increase in the percentage of the cubic phase within the material from 45% in the double-cation FA:MA mixture to 97.8% in the quadruple composition. The impact of admixing inorganic cations on the perovskite crystal structure resulted in enlarged homogeneous crystallite sizes and a less pronounced orientational order and indicated also minor modifications of unit cell sizes. Finally, we discuss the impact of the phase purity on charge-carrier recombination dynamics and solar cell performance.

2021

Minimizing the Trade-Off between Photocurrent and Photovoltage in Triple-Cation Mixed-Halide Perovskite Solar Cells

Mojtaba Abdi Jalebi, Seckin Akin, Neha Arora, Thomas Paul Baumeler, Mohammad Ibrahim Dar, Michael Graetzel, Yuhang Liu, Ryusuke Uchida, Shaik Mohammed Zakeeruddin

Its lower bandgap makes formamidinium lead iodide (FAPbI(3)) a more suitable candidate for single-junction solar cells than pure methylammonium lead iodide (MAPbI(3)). However, its structural and thermodynamic stability is improved by introducing a significant amount of MA and bromide, both of which increase the bandgap and amplify trade-off between the photocurrent and photovoltage. Here, we simultaneously stabilized FAPbI(3 )into a cubic lattice and minimized the formation of photoinactive phases such as hexagonal FAPbI(3) and PbI2 by introducing 5% MAPbBr(3) , as revealed by synchrotron X-ray scattering. We were able to stabilize the composition (FA(0.95)MA(0.05)Cs(0.05))Pb(I-0.95 Br-0.05)(3), which exhibits a minimal trade-off between the photocurrent and photovoltage. This material shows low energetic disorder and improved charge-carrier dynamics as revealed by photothermal deflection spectroscopy (PDS) and transient absorption spectroscopy (TAS), respectively. This allowed the fabrication of operationally stable perovskite solar cells yielding reproducible efficiencies approaching 22%.

AMER CHEMICAL SOC2020

Low-Cost and Highly Efficient Carbon-Based Perovskite Solar Cells Exhibiting Excellent Long-Term Operational and UV Stability

Seckin Akin, Neha Arora, Thomas Paul Baumeler, Mohammad Ibrahim Dar, Michael Graetzel, Yuhang Liu, Ryusuke Uchida, Shaik Mohammed Zakeeruddin

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large-scale deployment of PSCs, these components need to be replaced with cost-effective and robust ones that maintain high efficiency while ascertaining long-term operational stability. Here, a simple and low-cost PSC architecture employing dopant-free TiO2 and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain approximate to 95% of their initial efficiencies for >2000 h at the maximum power point under full-sun illumination at 60 degrees C. In addition, the CuSCN/carbon-based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro-MeOTAD/Au based devices degrade severely.

WILEY-V C H VERLAG GMBH2019

Bulk and surface engineering to improve performance and stability of perovskite solar cells

Thomas Paul Baumeler

EPFL2022