Interfacial and bulk properties of hole transporting materials in perovskite solar cells: spiro-MeTAD versus spiro-OMeTAD

Two spiro-MeTAD compounds (1 and 2) were synthesized, characterized by experimental and quantum mechanical methods, and used as hole transporting materials (HTMs) in perovskite solar cells (PSCs). The new compounds differ from spiro-OMeTAD only by the presence of methyl substituents as compared to methoxy groups. This modification results in the absorption band blue shifting by similar to 20 nm as compared to spiro-OMeTAD, increased glass transition temperature for 2, and reduced ionization potentials by 0.02-0.12 eV. Hole mobilities five times larger were obtained for spiro-MeTAD/spiro-MeTAD, which is maintained in the presence of additives. Despite this improvement, J-V measurements in PSCs resulted in a power conversion efficiency (PCE) of 17.2% and 17.05% for 1 and 2 HTMs, respectively, as compared to 19.24% for spiro-OMeTAD. Photoluminescence measurements of perovskite:HTM layers indicate much stronger quenching in the case of spiro-OMeTAD/spiro-MeTAD. These results point to the dominant importance of the perovskite:HTM interfacial properties as compared to the HTM hole-transport properties in the bulk. Given that improved hole-mobility and energy-level alignment are the main targets of the current research efforts in this domain, our results alert to the necessity to prioritize the improvement of perovskite-HTM interaction properties.

Formate as Anti-Oxidation Additives for Pb-Free FASnI(3) Perovskite Solar Cells

Jaeki Jeong, Jinhyun Kim, Jiyoun Seo

Pb-free Sn-based perovskite solar cells (PSCs) have significant potential for application in photovoltaic devices because oftheir suitable bandgap, low exciton binding energy, high carrier mobility, and long diffusion length. However, their performance is hampered by several issues. Sn-based perovskites are highly susceptible to oxidation, which induces a high concentration of defects and degrades the chemical stability of the perovskite crystals. Herein, the anion formate (HCOO-) can effectively suppress the oxidation of Sn in the pure formamidinium tin triiodide (FASnI(3)) perovskite without using A-site cationic additives. Moreover, the presence of formate results in a uniform pinhole-free perovskite film with a low trap density, reduced charge carrier recombination, and improved charge extraction. Density functional theory calculations show that formate-treated FASnI(3) has improved stability against oxidative Sn degradation. The formate-treated PSC achieves a power conversion efficiency of 12.11% at a high open-circuit voltage of 0.71 V. The device exhibits improved stability in ambient air in which it maintained over 80% of its initial power conversion efficiency after 180 min because oxidation is inhibited owing to the strong interaction between Sn and formate.

WILEY-V C H VERLAG GMBH2022

Composition and Interface Engineering of High Performing Perovskite Solar Cells

Mousa Abdulla M E Abuhelaiqa

Perovskite solar cells have been established as a disruptive technology in the domain of photo-voltaics. Their facile, low-temperature processing and high-power conversion efficiency make them stand-out among other mature solar technologies. The performance of perovskite solar cells is pro-foundly tied to an interplay of material properties of singular thin films that are assembled to form the photovoltaic device stack. An understanding of optoelectronic, morphological, and other key charac-teristics of each layer and its interfaces is instrumental in advancing the science of perovskite solar cells. Since their inception after just over a decade, much work on perovskite solar cell has been stag-nant to bring the technology to the market. The long-term stability remains foremostly the main hin-drance to facilitating technology transfer. Therefore, the work presented in this thesis explores multi-ple interface and compositional approaches to promote stability and other photovoltaic parameters of perovskite solar cells.In the second chapter of the thesis, a light-soaking exploration was done on perovskite sam-ples that utilize single (TiO2 and SnO2) and bilayered (TiO2/SnO2) electron transporting materials. The work aims to expand consensus on the optoelectronic and morphological features of light-soaked samples and link them with long-term stability. Upon preliminary testing, maximum power point tracking on each sample reveals efficiency preservation of SnO2 and TiO2/SnO2 samples over the course of 1000 hours while prominent degradation was revealed for the TiO2 sample. On a secondary investigation, light-soaking characterizations were performed where fresh and light-soaked samples were characterized to obtain photoluminescence, microscopic, and crystallographic features. The work reveals the added benefit of employing TiO2/SnO2 transporting bilayers to improve both the stability and power conversion efficiency of a perovskite solar cell. Building on an effort to understand TiO2/SnO2 bilayer behaviors, the third chapter investigates the role of halide passivating elements in SnO2 on the performance of perovskite solar cells. The study utilizes three metalorganic precursors based on acetylacetone complexes with chloride and bromide groups to fabricate SnO2 layers at different annealing temperatures. Upon thermal and elemental in-vestigations, the study clearly links the presence of amorphous halide elements in SnO2 to the power conversion efficiency of perovskite solar cells. More-in-depth, the optimal SnO2 annealing tempera-ture for bromide containing samples 250 ï°C compared with 220 ï°C for chloride containing samples. The higher temperature tolerance tendency was correlated to bromideâs lower sublimation sensitivity. Overall, the study highlights the importance of amorphous passivating elements in SnO2 for high per-forming planar perovskite solar cells. Finally, the fourth chapter involves a novel cation mixing approach for 2D perovskites at the interface of the 3D perovskite and the hole transporting layer. Alkyl ammonium halides have been widely used to form light-stable 2D perovskite films upon interaction with excess PbI2. In this study, different alkyl chain cations (propylammonium iodide and octylammonium iodide) were mixed in one solution to form a novel 2D perovskite crystal lattice. Photoluminescence and x-ray diffraction spec-troscopy investigations reveal a unique crystal lattice and a uniform quasi-2

EPFL2022

Atomic-Level Microstructure of Efficient Formamidinium-Based Perovskite Solar Cells Stabilized by 5-Ammonium Valeric Acid Iodide Revealed by Multinuclear and Two-Dimensional Solid-State NMR

Anwar Qasem M Alanazi, Essa Awadh R Alharbi, Marine Eva Fedora Bouduban, David Lyndon Emsley, Fabrizio Giordano, Michael Graetzel, Farzaneh Jahanbakhshi, Dominik Józef Kubicki, Jovana Milic, Marko Mladenovic, Jacques-Edouard Moser, Daniel Prochowicz, Dan Ren, Ursula Röthlisberger, Shaik Mohammed Zakeeruddin

Chemical doping of inorganic–organic hybrid perovskites is an effective way of improving the performance and operational stability of perovskite solar cells (PSCs). Here we use 5-ammonium valeric acid iodide (AVAI) to chemically stabilize the structure of α-FAPbI3. Using solid-state MAS NMR, we demonstrate the atomic-level interaction between the molecular modulator and the perovskite lattice and propose a structural model of the stabilized three-dimensional structure, further aided by density functional theory (DFT) calculations. We find that one-step deposition of the perovskite in the presence of AVAI produces highly crystalline films with large, micrometer-sized grains and enhanced charge-carrier lifetimes, as probed by transient absorption spectroscopy. As a result, we achieve greatly enhanced solar cell performance for the optimized AVA-based devices with a maximum power conversion efficiency (PCE) of 18.94%. The devices retain 90% of the initial efficiency after 300 h under continuous white light illumination and maximum-power point-tracking measurement.

2019

Composition and Interface Engineering of High Performing Perovskite Solar Cells

Mousa Abdulla M E Abuhelaiqa

EPFL2022