Performance of perovskite solar cells under simulated temperature-illumination real-world operating conditions

Since reaching 20% efficiency, research in perovskite photovoltaics has shifted from a race for efficiency to a race for stability. For efficiency, the standard test conditions set the rules for the race. However, the term 'stability' is used very broadly and assessed in various ways, meaning different groups are running different races. For the application, only energy yields that can be achieved under real-world, long-term operation matter. Here, we characterize and analyse the performance of an efficient perovskite solar cell (PSC) under simulated ambient conditions based on real temperature and irradiance data from selected days over one year at a location in central Europe. We find that the PSC shows only a low decrease of efficiency with elevated temperature and low light intensity, maintaining almost optimum values for ambient conditions, under which most of the solar energy is incident on the solar cell. The overall energy yield differs from what is expected from standard test condition measurements and is influenced by reversible degradation (delivering the highest performance in the morning) and by a slight permanent degradation that is observable during the year. With reference to tandem cells, we compare the PSC with a silicon device.

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. 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. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009

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

Investigation of Lead-free perovskites for Optoelectronic Application

Alexander Fedorovskiy

The search for new energy sources and materials is at the foundations of scientific and technological progress. These two, often interconnected, fundamental elements open a way to create novel designs and improve existing ones for all types of machines and devices.Perovskite solar cells (PSCs) are a great example of this kind of system. Achievements in halide perovskite materials research made a remarkably rapid growth of energy conversion efficiency in PSC technology possible. In turn, a manufacturing process for the technology is potentially cheaper and more straightforward (relative to Si-based). As a result, the PSCs can be a solid background for producing the next generation of solar cells.However, unfortunately, PSCs are still not ready for a market, primarily because of three problems: large-scale production, stability, and toxicity. The first problem is mainly a result of the rawness of the technology, requires mostly resolving engineering tasks, and is not a fundamental barrier. The second problem â stability has a deeper connection to the properties of the material itself. The nature of degradation processes in PSCs is not completely clear. Still, different studies have found some promising ways to stabilize PSCs for a range of thousands of hours. Thus, a combination of engineering and material research can solve this problem, despite its deeper basis.The third problem, toxicity, is a fundamental property of the high efficient PSCs because of its lead-based compound. Lead is one of the critical components of halide perovskites used in the technology, and at the same time, this metal has confirmed toxicity. The water solubility of lead in halide perovskites (primarily as PbI2 and PbBr2) complicates the situation for the whole lifecycle of the devices - from production to recycling. The toxicity of the materials used in PSCs can affect one of the main goals of solar energy: to be produced safe, clean, and sustainable.Taking into account these facts, I focused this thesis on searching for lead-free alternatives for PSCs.Initially, I focused on searching for a model to predict promising vacancy-ordered double perovskites' formability as part of initial screening. I demonstrated the applicability of the geometrical approach based on Goldschmidt's Tolerance Factor for a simple, easy-to-use formability prediction technique for this class of materials. During this part of the study, I also demonstrated a difference between using the method for classic and vacancy-ordered perovskites.In the next part, I used a novel approach based on deep learning to design a new model of perovskite formability. As a result, I successfully forecast perovskite formability with an accuracy of over 98% for the best case. Moreover, I found that the resulting models were able to find some inaccuracies in the original data.Following, I synthesized and researched the structure and properties of tellurium-based vacancy-ordered perovskites. During the research, I found an unexpected complex situation of the optical gap determination for the materials. To solve this challenge, I demonstrated different approaches of absorption spectra fitting and cross-referenced it with other experimental data.

EPFL2021