Interfacial Effects in Hybrid Perovskite Solar Cells : Insights from First Principles Studies

Development of sustainable energy sources is among the most urgent needs of modern society. Solar energy technologies, and in particular, hybrid perovskite solar cells (HPSCs), have demonstrated hard-to-beat performances with large capacities for improvement. However, maintaining their efficiencies, over 25% as of this date, under operating conditions, remains the first and foremost obstacle to their becoming a competitive commercial technology. In spite of massive endeavors in the field of perovskite solar technologies, their limited tolerance for moisture, extended periods of light exposure and high temperature have yet to be addressed before they can reach their Shockley-Queisser limit. It is however well established that both interlayer proximities (interfaces of the light absorbing layer (LAL) with the electron and the hole transporting layers, ETL and HTL), and intralayer proximities (organic-inorganic interfaces in 2D hybrid perovskites (2DHPs)) have paramount impact on determining the power conversion efficiency (PCE) as well as the stability of solar cells. In addition to well-known interlayer phenomena such as energy-level misalignment and formation of trap states that subsequently interfere with transport properties, intralayer organic-inorganic interfaces remain an interesting point of discussion, given recent blast-off in the investigation and the application of 2DHPs. Studies have revealed that incorporation of longer or bulkier cationic ligands into 3DHPs will provoke dimensionality reduction and formation of either 2D/3D or pure 2D phases comprising successive organic-inorganic layers that offer higher moisture and thermal stabilities. Crystal structures and optoelectronic properties of these systems are evidently governed to a great extent, by the intralayer effects otherwise known as "cross-linking of the conducting framework by the spacing layer" defined mainly by the nature of the cationic ligands, hence offering possibilities of compositional and structural tailoring towards desirable optoelectronic properties. In this thesis, we provide a comprehensive understanding of structural and optoelectronic properties of 2D perovskites based on a series of organic spacers and attempt to address cross-linking effects of organic-inorganic interfaces from first principles calculations also validated by experiments. On the same note, from another point of view, we explore a novel passivation strategy to alleviate deteriorating interfacial effects at the surface of formammidinium lead iodide as one of the most prominent light harvesting materials. Unfavorable interfacial trap states and structural degradations occur predominantly due to presence of surface intrinsic defects in the perovskite film and the aim is to find suitable strate- gies to diminish these deteriorating effects. By employing density functional theory (DFT), we unravel at the atomic level, the underlying mechanism and the role of crownether surface modulation that occurs through complexation of the under-coordinated surface ions. Eventually, we study the LAL/ETL interface as a potential defect accumulation and carrier recombination center. We computationally obtain the energetically most stable interfaces of MAPbI3/TiO2 at DFT level and probe the impact of different surface properties, including terminations, presence of vacancies as well as ion substitution, on the energy-level misalignment and mitigation of trap states that improve the PCE in MAPbI3 PSCs.

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

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

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