Probing interactions of molecules and pigments on titanium dioxide surfaces for photovoltaic cells, using QCM-D

Photovoltaic cells offer a sustainable way of generating electricity from sunlight, and they will be of growing economical importance for decades to come. Hybrid organic-inorganic solar cells, such as dye sensitized solar cells or perovskite solar cells, are an area of active research, and fundamental scientific advances can lead to impressive improvements of device efficiency. In this thesis, I present experimental studies on some of the fundamental properties of molecules and pigments on titanium dioxide surfaces for solar energy conversion. My main instrument is a quartz crystal microbalance with dissipation technique (QCM-D), and I develop special methods of measurement, which enable the characterization of mass uptake in mesoporous systems. In dye sensitized solar cells (DSC), dye molecules are adsorbed onto a mesoporous titanium dioxide (TiO2) photoanode. The dye molecule absorbs light, subsequently injects an electron into the conducting TiO2, and is regenerated from a hole transport material. Furthermore, the molecular film of dye molecules blocks recombination between electrons in the TiO2 conduction band and the oxidized form of the redox mediator in the hole transport material. In the first chapter, I study the adsorption of the ruthenium-based dye Z907 by QCM-D on flat TiO2 as a model system with sub-monolayer resolution. An adsorption isotherm is measured for Z907, which leads to a formal adsorption equilibrium constant of 5.1e6 M-1 and a number density of 0.76 molecules per nm2. The coadsorbate chenodeoxycholic acid (cheno) enhances the recombination-blocking capability of the adsorbed molecular layer and it prevents aggregation of dye molecules. In chapter two, I develop a method to quantify the ratio between cheno and dye molecules when coadsorbed on the TiO2 using a combination of QCM-D and subsequent fluorescence spectroscopy. I apply this method to study coadsorption with triphenylamine-, porphyrin-, ulllazine- and diketopyrrolopyrrole-based dyes. The role of cheno depends on its specific interaction with the dye molecule. In chapter three, I present a microscopic study on the molecular layer of the amphiphilic dye Z907 adsorbed on TiO2. In collaboration with two groups at EPFL, we present in-situ atomic force microscopy images with sub-nanometer resolution in a functionally relevant liquid environment. Our results reveal changes in the conformation and the lateral arrangement of the dye molecules, depending on their average packing density on the surface, and these results are confirmed by molecular dynamics simulations. Chapter four aims to understand the QCM-D signals on mesoporous TiO2 films attached to the QCM-D sensor. Their high accessible surface area makes mesoporous films interesting for sensing applications. For films of up to 200 nm thickness, I find a regular inertial loading, which allows for an archimedic measure of molecules adsorbed inside the mesoporous matrix. For thicker films, I observe a particular behaviour that is explained by the resonant coupling of the quartz crystal driving frequency to an eigenmode of the TiO2 film. In chapter five, I contribute to the fundamental understanding of the perovskite conversion reaction between PbI2 and CH3NH3PbI3 perovskite. This material is central to the latest, record-breaking perovskite solar cells. I provide evidence for that the back-conversion is reversible by release of methylammonium iodide from the perovskite film into selective solvents.

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

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.

EPFL2022

In-situ mapping of the molecular arrangement of amphiphilic dye molecules at the TiO2 surface of dye sensitized solar cells

Negar Ashari Astani, Hauke Arne Harms, Ursula Röthlisberger, Francesco Stellacci, Ivano Tavernelli, Nicolas Tétreault, Kislon Voitchovsky

Amphiphilic sensitizers are central to the function of dye-sensitized solar cells. It is known that the cells performance depends on the molecular arrangement and the density of the dye on the semiconductor surface, but a molecular-level picture of the cell-electrolyte interface is still lacking. Here, we present subnanometer in situ atomic force microscopy images of the Z907 dye at the surface of TiO2 in a relevant liquid. Our results reveal changes in the conformation and the lateral arrangement of the dye molecules, depending on their average packing density on the surface. Complementary quantitative measurements on the ensemble of the film are obtained by the quartz-crystal microbalance with dissipation technique. An atomistic picture of the dye coverage-dependent packing, the effectiveness of the hydrophobic alkyl chains as blocking layer, and the solvent accessibility is obtained from molecular dynamics simulations.