Computational studies for the characterization and design of dye sensitized solar cells (DSSCs) with improved efficiencies

During millions of years of evolution, all creatures have found their ways of harvesting, storing and utilizing the energy coming from the sun. Plants use photosynthesis to convert carbon dioxide to glucose. With a low efficiency of only 3%, plants still capture enough energy to grow their roots, fruits and leaves. Animals regulate their body temperature by basking in the sun or seeking a shadow when needed. Human beings are the only species that is not satisfied with this natural share. With the exponential population growth and a gross domestic product per capita that doubles every fifty years, finding a clean, secure, and sustainable source of energy is inevitable. Photovoltaic solar cells together with a proper energy storage technology, like water splitting and a fuel cell to burn the stored hydrogen, can form the promising green cycle that solves the energy problem. Among different photovoltaic technologies, hybrid organic-inorganic solar cells, such as dye sensitized solar cells (DSSCs) or perovskite solar cells, are subject of active research due to their low-fabrication costs and easy production. In order to become commercially viable, these modules need to reach their theoretical Shockley-Queisser efficiency limit while acquiring enough durability and stability. In this context ab initio first-principles studies are of great help in order to give a fundamental insight of the chemistry and physics going on inside the cell. The main purpose of this dissertation is to make use of computational methods for a characterization of the fundamental processes in dye-sensitized and perovskite solar cells and the use of this knowledge for a rational design of solar cells with higher efficiencies. Starting from a systematic investigation of bio-inspired porphyrin-based dyes focused on their light harvesting properties, we were able to outline some key guidelines for designing efficient dye sensitizers. The outcome of this study was an efficient light harvester, which when integrated in an optimized device achieved the record-breaking 13% efficiency porphyrin- based dye sensitized solar cell. While having good sensitizers is a necessary condition for a high performance DSSC, it is often not enough. It is essential to know the interactions of different components of a DSSC and how they work together. For this reason, we performed atomistic molecular dynamics simulations of a comprehensive model of a DSSC consisting of dye molecules, TiO2 substrate, solvent and electrolyte. In particular, in these simulations, we studied the accessibility of oxidized electrolyte to the TiO2 substrate for different dye coverages and show that at medium to high coverage, peripheral alkyl groups of the dyes are highly efficient in shielding the TiO2 substrate from direct contact with the oxidized electrolyte preventing recombination. In a next chapter, and as a proof of principles study, we investigated the importance of solvation effects on the stability of a carbocation (as a prototype for the cationic dye species after electron injection). The last two parts of this thesis focus on perovskite solar cells where we show the origins of the highly tunable band gap and propose a simple model in terms of orbital overlap that can be used to rationalize and predict variations of the band gap as a function of crystal structure and chemical composition of the perovskite. Finally, we study the efficient carrier transport properties of these materials and show that there is a direct correlation between the efficiency of carrier transport and the band gap.

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.

EPFL2019

Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

Frederico Alves Lima, Paul Beaud, Majed Chergui, Amal El Nahhas, Thomas James Penfold, Van Thai Pham, Ivano Tavernelli, Renske Marjan van der Veen

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L-1,L-3 edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I-/I-0(H2O)(n=1-4) clusters. The L-1 edge sub-picosecond spectra point to the existence of a transient species that is not present at SO ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I-0(OH2) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3-4 Ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.

2011

Nitranilic acid hexahydrate, a novel benchmark system of the Zundel cation in an intrinsically asymmetric environment: spectroscopic features and hydrogen bond dynamics characterised by experimental and theoretical methods

Nitranilic acid (2,5-dihydroxy-3,6-dinitro-2,5-cyclohexadiene-1,4-dione) as a strong dibasic acid in acidic aqueous media creates the Zundel cation, H5O2+. The structural unit in a crystal comprises (H5O2)(2)(+)(2,5-dihydroxy-3,6-dinitro-1,4-benzoquinonate)(2-) dihydrate where the Zundel cation reveals no symmetry, being an ideal case for studying proton dynamics and its stability. The Zundel cation and proton transfer dynamics are studied by variable-temperature X-ray diffraction, IR and solid-state NMR spectroscopy, and various quantum chemical methods, including periodic DFT calculations, ab initio molecular dynamics simulation, and quantization of nuclear motion along three fully coupled internal coordinates. The Zundel cation features a short H-bond with the O center dot center dot center dot O distance of 2.433(2) angstrom with an asymmetric placement of hydrogen. The proton potential is of a single well type and, due to the non-symmetric surroundings, of asymmetric shape. The formation of the Zundel cation is facilitated by the electronegative NO2 groups. The employed spectroscopic techniques supported by calculations confirm the presence of a short H-bond with a complex proton dynamics.

Royal Soc Chemistry2014

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

EPFL2019