Carbon Dioxide Reduction to Methanol with a Molecular Cobalt-Catalyst-Loaded Porous Carbon Electrode Assisted by a CIGS Photovoltaic Cell**

Conversion of CO2 into valuable compounds, including fuels, with renewable energy sources and sustainable compounds is a challenge addressed by artificial photosynthesis research. In particular, the application of solar assisted electrochemical (EC) processes, in which electrons are furnished by a photovoltaic (PV) cell, is a promising approach. A PV-EC system is described, consisting of a CIGS (copper indium gallium selenide) PV unit linked to a carbon electrode loaded with cobalt phthalocyanine as molecular catalyst, able to achieve the CO2 reduction to CO and then to methanol in aqueous media with limited bias voltage. Using CO as starting material, a partial current density of ca. 0.6 mA cm(-2) for methanol is obtained at a bias voltage corresponding to a low 240 mV overpotential. Remarkably, the liquid fuel production can be sustained for at least 7 h. Under ideal conditions, the CO2-to-CH3OH reaction shows a global Faradaic efficiency of 28 %.

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

Interfacial engineering for competitive Dye-Sensitized Solar Cells production

Natalie Flores Diaz

Renewable energy deployment and implementation of a circular economy will play a crucial role in decreasing CO2 emissions to meet the target from the Paris agreement. Dye-sensitized solar cells (DSSCs) can be designed in various colors and transparency for building-integrated photovoltaics (BIPV). This emerging technology has gained significant attention for its remarkably high power conversion efficiency (PCE) under indoor lighting up to 34%. To improve the performance of DSSCs, various approaches to study and reduce the interfacial electron recombination from the conduction band of the semiconductor to the electrolyte were implemented in this thesis. One of the approaches was the design of a tandem electrolyte containing two redox species, the radical AZADO (AZ+/0) and the [Co(bpy)3]2+/3+ complex. DSSCs with tandem electrolytes showed increased open-circuit voltage (VOC) up to 1009 mV and enhanced PCE of 9.15%. The VOC enhancement was determined to result from the higher redox potential of (AZ+/0) and the slower electron recombination rate. These findings demonstrate the advantages of employing tandem electrolytes to improve the VOC and overall performance of DSSCs. To decrease the solvent's volatility and the interfacial electron recombination in cobalt-based electrolytes, hyperbranched networks (HB) were designed through a click thiol-ene reaction between a thiol-siloxane and an acrylate monomer. The formed polymers were denoted HB1 (thiol-siloxane and PEGMA) and HB2 (thiol-siloxane and PEGMA). The addition of 20% of HB polymers to a cobalt-based electrolyte retained up to 98% of their initial weight until 150°C. The devices employing 10%(v/v) of HB1 attained the highest PCE under AM1.5 of 8.52%, comparable to 9.36% for the cobalt reference electrolyte. All the HB-electrolytes presented higher VOC values than the cobalt reference due to an upward shift in the CB of TiO2 and longer electron lifetimes. It was determined that the HB-polymers reduce the electron recombination and enhance the photovoltaic performance under low light intensity leading to a PCE of 23.19% under 1000 lux compared to 20.37% for the Co-reference. We then developed a new dye, R7, with a strong electron-donating PAH core and BPT2 as the central building block and a bulkier HF donor to decrease electron recombination at the interface TiO2/electrolyte. We found that in the presence of a copper electrolyte, the dye R7 outperformed devices with R6. EIS revealed that the bulky HF-donor introduction effectively suppressed the electron recombination between the TiO2 surface and the redox shuttle. Additionally, we employed a co-sensitized system of R7+Y123 to reach the outstanding parameters of JSC of 16.15 mAcm-2, VOC of 1035 mV, FF of 0.76 to achieve a top PCE of 12.7%. To the best of our knowledge, this is the highest for copper-based DSSCs using a blue photosensitizer. Lastly, the interfacial phenomena in DSSCs with carbon counter electrodes (CCEs) and copper-based electrolytes were studied to determine the feasibility of the production of monolithic DSSCs with copper-based hole-transporting materials. It was found that the electron transport in devices with CCEs is in the same order of magnitude as those with PEDOT. However, the devices with CCEs presented a decrease of 2 orders of magnitude in the recombination resistance, resulting in decreased VOC and JSC, leading to a PCE of 17% compared to 24.5% for PEDOT devices under 1000 lux illumination.

EPFL2021

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.

EPFL2022