Low-cost high-efficiency system for solar-driven conversion of CO2 to hydrocarbons

Conversion of carbon dioxide into hydrocarbons using solar energy is an attractive strategy for storing such a renewable source of energy into the form of chemical energy (a fuel). This can be achieved in a system coupling a photovoltaic (PV) cell to an electrochemical cell (EC) for CO2 reduction. To be beneficial and applicable, such a system should use low-cost and easily processable photovoltaic cells and display minimal energy losses associated with the catalysts at the anode and cathode and with the electrolyzer device. In this work, we have considered all of these parameters altogether to set up a reference PV-EC system for CO2 reduction to hydrocarbons. By using the same original and efficient Cu-based catalysts at both electrodes of the electrolyzer, and by minimizing all possible energy losses associated with the electrolyzer device, we have achieved CO2 reduction to ethylene and ethane with a 21% energy efficiency. Coupled with a state-of-the-art, low-cost perovskite photovoltaic minimodule, this system reaches a 2.3% solar-to-hydrocarbon efficiency, setting a benchmark for an inexpensive all-earth-abundant PV-EC system.

Cyclometalated Ruthenium Complexes for Dye-Sensitized Solar Cells

Sadig Aghazada

The transition towards carbon-free sources of energy is vital for humankind. This process is feasible through the extensive use of photovoltaic technologies, which may convert solar energy into elec-trical and chemical energy. In the last half-century, many competitive technologies were developed. One promising type of solar cell is Dye-Sensitized Solar Cell (DSC), which differ from other solar cells in many ways. DSCs can be produced in different colors and used as constructing components of urban buildings. Additionally, DSCs have inner beauty related to their working principle. The pro-cesses of light absorption and charge transport are spatially separated in many ways reminiscent of water oxidation in Photosystem II in nature. Historically, DSCs were developed with an electrolyte based on an iodine-iodide redox shuttle. However, the overall performance of these solar cells is restricted due to the high loss of potential imparted by multistep dye regeneration by iodide. Shift to the outer-sphere one-electron redox couple, such as Co3+/2+ imine complexes, boosted the field, and new record efficiencies were achieved. Still, new redox mediators failed to provide reasonable power conversion efficiencies using the classic ruthenium sensitizers with isothiocyanate ligands. Poor performance is related to high charge recombination rates. To tackle this problem, I present my research on new cyclometalated ruthenium (II) complexes as sensitizers for DSCs that employ cobalt-based electrolytes. I introduce tris-heteroleptic cyclometalated Ru (II) complexes, which have various organic substituents. Optical and electrochemi-cal analyses are conducted before applying the new sensitizers in state-of-the-art solar cells. In this work, a record-high power conversion efficiency of 9.4 % was obtained with a ruthenium sensitizer and a cobalt-based redox shuttle. Transient absorbance spectroscopy, electrochemical impedance spec-troscopy, and transient photovoltage and photocurrent decay measurements were conducted to reveal the main factors that limit the performance of the solar cells. The role of dye-loading, which is generally ignored in the field, was shown to be crucial. Additionally, it was shown that organic substituents that are usually attached to improve the photophysical properties of sensitizers cause irreversible sensi-tizer electrochemical oxidation. Further, in this thesis, I discuss the role of sulfur atoms in the sensitizer structure on the performance of the solar cell. Then I introduce new bidentate ligands, which coordinate to the metal with both cyclometalation and N-heterocyclic carbenes. I investigate the potential of new complexes with presented ligands as sensitizers. In the last section, I discuss the extended anchoring ligands and their complexes with ruthenium, which have six-membered chelating rings. In this section I also introduce new binding mode for the pyridine-type of ligand. Finally, I briefly summarize the obtained results.

EPFL2018

Molecular engineering of functional materials for optoelectronic applications

Nikita Drigo

Organic functional materials are indispensable components for the last generation of light-energy conversion devices. Photo- and electroactive small organic molecules constitute thin active (emitting, transporting, blocking, injecting etc.) layers of organic optoelectronics and often play a decisive role for efficiency of an operating device. Since desired optoelectronic and morphological properties of organic materials can be finely tuned at the molecular level, the relationships between molecular structure, physicochemical properties and, ultimately, performance of the corresponding bulk material in an operating device are subject of extensive research nowadays. Moreover, developing understanding of device operation and physics imposes newer stricter and more challenging requirements for organic functional materials urging the search for new molecular architectures with advanced properties. Thus, the work presented in this thesis is focused on synthesis, profound characterization and use of new organic functional materials for light-energy conversion devices. We aim eventually to find out the synthetic guidelines for judicious molecular design of organic low-molecular-weight materials with advanced properties for use in optoelectronic devices. In this work, we introduce rigid bulky symmetric 10H,10'H-9,9'-spirobi[acridine] (SBA) as a universal molecular platform to construct emitting dopants and HTMs for the last generation optoelectronics: thermally-activated delayed fluorescence light-emitting diodes (TADF OLEDS) and perovskite solar cells (PSCs) respectively. The compact SBABz4 TADF emitter exhibited pure blue emission via TADF channel and a significant hypsochromic shift as opposed to simpler DMABz4 with commonly used âmonomericâ donor without the spiro-node. Moreover, stick-like elongated SBABz4 demonstrated increased light-outcoupling efficiency. As the next part of our work, we utilized SBA molecular platform to construct 4 novel HTMs that mimic simultaneously two benchmark materials: 2,2â²,7,7â²,-tetrakis-(N,N-dimethoxyphenyl-amine)-9,9â²-spirobifluorine (spiro-MeOTAD) and poly(triaryl amine) (PTAA) monomer fragment. The new HTMs performed on par with spiro-MeOTAD reference, but the corresponding devices demonstrated no current density-voltage (J-V) hysteresis and excellent durability, strongly outperforming the etalon molecule. Finally, driven by our curiosity to new molecular scaffolds, we explored chemistry and optoelectronic properties of electron excessive polyaromatic heterocycle â ullazine. We performed synthetic optimization and elaborated a straightforward high-yielding protocol towards 3,9-disubstituted ullazine scaffold and a reactive monobromoullazine. Moreover, we developed synthetic approaches towards all possible ullazine monocarbaldehydes and studied the relationships between regioisomerism and photo-/electronic properties of the corresponding conjugates with malonate acceptor. One of the donor-acceptor (D-A) conjugates displayed a suitable energy level and was tested as a small-molecule dopant-free HTM for PSCs without conventional arylamine substituents.

EPFL2020

Compositional and interfacial optimization for enhancement of perovskite solar cells performance

Essa Awadh R Alharbi

Non-renewable sources are responsible for most of the overall greenhouse gas emissions, the development of sustainable energy sources is of prime importance to mitigate the negative effects of climate change. Among renewable sources, solar energy is one of the most abundant forms of energy available. Perovskite photovoltaics has emerged as one of the promising candidates for harvesting solar energy. However, these perovskite solar cells suffer from instability which is primarily due to the bulk and interface defects. In this thesis, we focus on the passivation of the bulk and interface defects to enable high-performance and stable devices. At the beginning of the thesis, we engineered the composition of mixed-cation and mixed-halide perovskite films by judicious incorporation of guanidinium iodide to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. Thereafter, in our next project, we develop a facile strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium, and guanidinium iodide. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium, and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We employed 2D-solid-state NMR (1H-1H correlation) spectroscopy to unravel the atomic-level mechanism of this passivation effect. Following that, we applied a facile and very effective method that employs methylammonium triiodide (MAI3) for bulk passivation together with interface passivation developed in the second project for holistic defect mitigation in perovskite solar cells. As a result, the champion device shows a power conversion efficiency (PCE) of 23.46% with a high fill factor (FF) of over 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. Finally, in our last project, we reconfigured the design of the alkylammonium halide salts to create a holistic bulk and interface engineering. We show that dimethylammonium head along with octyl chain and iodide counter anion works best in the bulk whereas dimethylammonium head along with octyl chain and fluoride counter anion works best at the interface. With a synergistic combination of both bulk and surface passivation, we achieved a high PCE of 24.91% along with exceptional long-term operational stability. In summary, in this thesis we employed different strategies based on organic ammonium salts to engineer the bulk and interface of the PSCs. We did an in-depth investigation of the optoelectronic properties, local structure, and molecular interactions with a combination of characterization at different length and time scales. Our results show that it is imperative to passivate both bulk and interface defects for obtaining high efficiency and stable solar cells. From our work, one can take inspiration from the design and analysis of new materials for the passivation of the PSCs.

EPFL2022