Compositional and interfacial optimization for enhancement of perovskite solar cells performance

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.

Solar Energy Stored as Hydrogen

Jérémie Minh-Châu Brillet

The tandem photoelectochemical (PEC) cell based on oxide semiconductors for water splitting offers a potentially inexpensive route for solar hydrogen generation. At the heart of the device, a nanostructured photoanode for water oxidation is connected in series with one or two dye-sensitized solar cells (DSCs) that provide an extra bias to photogenerated electrons in order to perform water reduction at the platinum cathode. In Part A of this thesis, after reviewing the different technologies available for solar hydrogen production, I focus on different possible architectures for photoanode / DSC tandem cells enabled by the most recent advances in the field. First, the development of all organic squaraine dyes having a narrow absorption bandwidth extending into the near infrared region and a broad transparent window in the visible region of the spectrum have allowed the design of a new photoanode / DSC / DSC tandem architecture. The "Back DSC" tandem cell, where two DSCs placed side-byside exploit the photons transmitted by a single photoanode is the conventional architecture. In this thesis, I demonstrate the inverted "Front DSC" architecture, which allows the use of non-transparent lower cost metallic substrate for the photoanode, as well as the "tri-level" tandem cell where a panchromatic "black" dyebased DSC exploits the energy transmitted by the photoanode and the squaraine dyebased DSC. Opto-electronic studies on each of these three architecture are performed and the respective solar-to-hydrogen conversion (STH) efficiencies of 1.16 %STH, 0.76 %STH and 1.36 %STH are assessed. Finally, I leverage recent findings in the field of high voltage DSCs to demonstrate for the first time a tandem cell using only one photovoltaic cell to perform complete water splitting at efficiencies as high as 3.10 % STH. Such a result represents a ten-fold improvement over previous demonstrations with this class of device. This work describes a breakthrough in the inexpensive solar-to-chemical conversion using improved photon management in a dual-absorber tandem cell and undoubtedly constitutes a benchmark for solar fuel production by solution processable oxide-based devices. In Part B, I focus on the photoanode for the oxygen evolution reaction. Hematite (α-Fe2O3) is a great candidate material for this application due to its availability, low cost, non-toxicity and appropriate band gap allowing extensive absorption in the visible part of the spectrum. However, it suffers severe drawbacks, among which are poor majority carrier conductivity and a short diffusion length of minority charge carrier with regard to photon penetration depth. This circumstance causes most photogenerated charges to have a low probability of reaching the semiconductor / liquid junction and thus to participate in the water oxidation reaction. This feature implies the use of hematite morphologies having feature size in the range of 10-20 nm. A solution-based colloidal approach offers a simple and easily scalable method to obtain nanostructured hematite. However, this type of nanostructure needs to be exposed to an annealing step at 800 °C to become photoactive. This has been attributed to the diffusion of dopants from the substrate. In addition, this annealing step sinters the 10 nm particles colloid into a feature size approaching 100 nm. Here, I first show the effect of intentionally doping this material on the sintering and photoactivity. I then demonstrate a method that allows for the first time to apply high temperature annealing steps while controlling the feature size of the nanoparticles. Such an approach can be applied to any kind of nanostructure. This latter strategy coupled to further passivation of surface states allowed an improvement of the net photoactivity of this type of photoanode by a factor of two. A reproducible net photocurrent exceeding 4 mA.cm-2 was obtained. This result represents the highest performance reported for hematite under one sun illumination. In Part C of this thesis, I present several synthesis methods for titania nanostructures acting as photoanodes in the DSC. In this photovoltaic cell, the electronic loss that is typically discussed is the slow transport induced recombination. Charges recombining before reaching the electrode have a detrimental influence on the photocurrent and photovoltage. Several approaches have been proposed in order to reduce interfacial recombination and improve charge collection in liquid electrolyte and solid state-DSCs (ss-DSCs) including the use of radial collection nanostructures, one-dimensional ZnO and TiO2 nanorods, and nanowires as photoanodes. Even though these approaches show great promise, they have yet to achieve power conversion efficiencies above 5% in liquid electrolyte DSCs and 1.7% in ss-DSCs. In this part of my thesis, I expose results on the strategies I have pursued during the course of my PhD to improve the dynamics in the liquid and ss-DSC. From one dimensional titania nanotube arrays, I have moved to tri-dimensional fibrous network of crystalline TiO2 and more complex host-passivation-guest photoelectrodes.

EPFL2012

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008