Molecular Engineering Towards High Efficiency Perovskite Solar Cells

High energy demands of our society stimulate the development of sustainable tech-nologies, such as those based on solar energy conversion or photovoltaics. Perovskite solar cells (PSCs) are an emerging photovoltaic technology that consist of the perov-skite film positioned between the electron transport and hole transport layer, while the photovoltaic properties are determined by each component of the structure, as well as the corresponding interfaces. Since 2009, power conversion efficiency of solar cells has already achieved values above 22%. This rapid progress in perovskite solar cell research led to the increased performance and operational stability, which nevertheless remains a challenge towards the commercialization of PSC technologies. In the course of this PhD thesis, new strategies in the design of perovskite solar cells were established based on molecular engineering by addressing (1) perovskite, (2) electron transport, as well as (3) hole transport layers, as detailed in the subsequent chapters. The research objectives focused on two key investigation domains, namely charge recombination in the perovskite grain boundaries or interfaces of perovskite / electron transport layers, and the stability of perovskite and hole transport materials. This approach resulted in the development of highly stable and efficient PSCs. First, with the aim of achieving high performance PSCs, the investigation of crystal growth mechanism was performed to discover the methods to control the grain size distribution within perovskite films that affects their performance by employing or-ganic agents. The study revealed the crystal growth mechanisms and resulted in the achievement of high efficiency of 19.5% in a planar perovskite solar cell architecture. Subsequently, cesium-based perovskites were studied as novel perovskite systems with excellent stability. The role of the cesium in the crystal formation of perovskites was primarily explored, facilitating the rapid room temperature crystallization that led to power conversion efficiency of 18.1% under room temperature. These results are very promising for low temperatue applcation demands, such as the roll-to-roll production and the development of flexible PSCs. In addition to the perovskite absorption layer, the roles of electron and hole transport layers are very important for the PSC performance. In this regard, while the mesoscopic titanium oxide (mp-TiO2) is predominantly used for electron transporting materials, studies of mp-TiO2 are scarce despite its common use in todayâs most effi-cient PSCs. Therefore, following the previous investigation of the perovskite materi-al, we further demonstrated for the first time that bimodal porous TiO2 nanoparticles and their surface doping with cesium halides further strengthens the interaction with the perovskite layer, resulting in very high fill factor of 80% and efficiencies exceed-ing 21%. Finally, a new dopant, Zn(TFSI)2, for organic hole transport materials was explored to enhance thermal stability by replacement of one of the dopant materials. This ap-proach revealed further improved stability with efficiency exceeding 22%. This work on the development of highly efficient and stable perovskite solar cells highlights the utility of molecular engineering and provides the basis for facilitating industrial applications in the near future.

Auto-passivation of crystal defects in hybrid imidazolium/methylammonium lead iodide films by fumigation with methylamine affords high efficiency perovskite solar cells

Paul Joseph Dyson, Farzaneh Fadaei Tirani, Zhaofu Fei, Giulia Grancini, Xuehui Liu, Mohammad Khaja Nazeeruddin, Emadeddin Oveisi, Rosario Scopelliti, Erfan Shirzadi, Yi Zhang

Hybrid perovskite solar cells have attracted tremendous interest in the photovoltaic community. Despite their high defect tolerance, reducing the trap density by material engineering and surface modification is still critical to further boost performance. Here, methylammonium lead(II) iodide perovskite has been doped with imidazolium iodide in high concentrations (10-30 mol%) to boost solar cell performance, by passivating defects. Fumigation with methylamine results in the deprotonation of the embedded imidazolium cations, generating imidazole and methylammonium cations. The resulting (neutral) imidazole is extruded from the 3-D perovskite crystal and distributes around the crystal leading to auto-passivation of crystal defects. The structure of the imidazolium-PbI3 salt intermediate (i.e. formed in the absence of the methylammonium cation) has been determined and the resulting perovskite film characterized. Employed in solar cells, a power conversion efficiency (PCE) up to 20.14% is demonstrated.

2019

Investigation on the impact of passivation in perovskite solar cells

Erfan Shirzadi

Conversion of sunlight to electricity is the most plausible approach towards the generation of renewable energies. During recent decade lead halide perovskites (LHPs) have proven to be promising candidates for photovoltaic applications such as solar cells. LHPs can be made easily in laboratories around the world via solution process methods and many universities can afford to participate in their development. How-ever, LHPs suffer from instability issues and their efficiencies can be further improved. In this thesis, both instability and efficiency challenges are addressed. We showed by doping the precursor of methylammonium lead iodide (MAPbI3) perovskite with a relatively large cation of 2-Choloroethylammonium (CLEA), perovskites films with CLEA incorporated into their structures are formed. The novel perovskite is able to withstand high temperatures as high as 80°C while achieving efficiencies > 19% in MAPbI3-based perovskite solar cells (PSCs). Various approaches have been employed for the passivation of the defects in perovskite solar cells Vari-ous approaches have been employed to passivate the defects in perovskite films used in perovskite solar cells (PSCs). However, the passivation mechanism is often unclear at a molecular level and the reason for enhanced PSC performance remains elusive. Here, we describe passivation based on the deposition of two dimensional (2D) perovskites (obtained from para-halophenylethylammonium iodide) on the surface of perovskite films. Moreover, the mechanism of passivation was elucidated and was traced to the high work function of the 2D perovskites, which induce band-bending at the surface of perovskite film and therefore reduces the charge carrier recombination at the interface of perovskite and the hole transporting material (HTM). Efficiencies >22% with average voltages >1.15 V were achieved using a typical mesoporous-TiO2 and Spiro-OMeTAD device configuration. This study paves the way for PSCs efficiency improvement by rational design. The employment of 2D perovskites is a promising approach to tackle the stability and voltage issues inherent in perovskite solar cells. It remains unclear however whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, utilization of AzPbI3 leads to the passivation resulting in a power conversion efficiency (PCE) of 22.4 %. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single-junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a bandgap of 1.55 eV. Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, we describe a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated. We applied statistical analysis to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, we identified two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (

EPFL2021

Molecular dynamics simulations of nucleation and phase transition in halide perovskites

Paramvir Ahlawat

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.