Mesoporous TiO2 Beads Offer Improved Mass Transport for Cobalt-Based Redox Couples Leading to High Efficiency Dye-Sensitized Solar Cells

Overcoming ionic diffusion limitations is essential for the development of high-efficiency dye-sensitized solar cells based on cobalt redox mediators. Here, improved mass transport is reported for photoanodes composed of mesoporous TiO2 beads of varying pore sizes and porosities in combination with the high extinction YD2-o-C8 porphyrin dye. Compared to a photoanode made of 20 nm-sized TiO2 particles, electrolyte diffusion through these films is greatly improved due to the large interstitial pores between the TiO2 beads, resulting in up to 70% increase in diffusion-limited current. Simultaneously, transient photocurrent measurements reveal no mass transport limitations for films of up to 10 mu m thickness. In contrast, standard photoanodes made of 20 nm-sized TiO2 particles show non-linear behavior in photocurrent under 1 sun illumination for a film thickness as low as 7 mu m. By including a transparent thin mesoporous TiO2 underlayer in order to reduce optical losses at the fluorine-doped tin oxide (FTO)-TiO2 interface, an efficiency of 11.4% under AM1.5G 1 sun illumination is achieved. The combination of high surface area, strong scattering behavior, and high porosity makes these mesoporous TiO2 beads particularly suitable for dye-sensitized solar cells using bulky redox couples and/or viscous electrolytes.

Solar photoelectrolysis of water with translucent nano-structured hematite photoanodes

Ilkay Cesar

In the context of the diminishing fossil energy resources and global warming, much research is focused on sustainable energy sources. The research that is presented in this thesis falls in this category as it contributes to the development of a device that stores solar energy into a chemical fuel. This device is based on the photoelectrolysis of water in a tandem-cell that produces hydrogen and oxygen under illumination of sunlight. This can be achieved by connecting a larger band gap semiconductor photoanode for oxygen evolution (such as WO3 with Eg = 2.6 eV or Fe2O3 with Eg = 2.0 eV) in series with two dye sensitized solar cells and a hydrogen evolving cathode. Red and green light transmitted by the photoanode is absorbed by the dye, so that a large part of the solar spectrum can be utilized. Fe2O3 absorbs a larger part of the visible solar spectrum as compared to WO3, which results in a 3 times higher theoretical conversion efficiency for the tandem-cell. However, the small hole diffusion length in Fe2O3 of 2 to 20nm is seen as an important reason for the lower practical conversion efficiencies reported in literature. The aim of this thesis project is to improve the photooxidation activity of iron oxide photoanodes in order to improve the practical conversion efficiency of the tandem-cell. For this purpose we have prepared thin films of nano-structured iron oxide with various deposition methods and characterized their performance on the photooxidation of water. The photoanodes are measured in aqueous solution of 1 M NaOH (pH=13.6) under illumination of simulated sunlight (AM 1.5 global of 1000 W/m2). We report the photocurrent at the reversible water oxidation potential of 1.23 V vs. RHE. The first chapter briefly discusses the potential of hydrogen as an energy carrier in a global hydrogen economy and places the tandem-cell in this context. In chapter two the current status of photoelectrolysis of water with iron oxide is briefly reviewed. The experimental set-up for the measurement of the photocurrent under simulated sun light and for the acquisition of photocurrent action spectra are described. This chapter also deals with the spectral mismatch error that is associated with the solar simulator calibration procedure. Chapter three introduces a new solution strategy for efficient photoanodes based on a nanocomposite structure with a hematite film thickness that is commensurate to the hole diffusion length alleviating the problem of poor charge transport. For this purpose nano-porous films of doped and undoped tinoxide and arrays of perpendicularly oriented nanorods of the same material are conformally coated with a thin film of hematite (2-20nm). These electrodes are characterized by SEM, TEM, XEDS elemental analysis, Raman spectroscopy in addition to the photoelectrochemical response. Although, evidence is presented for the successful preparation of the nano-composite structures with a sufficient optical density, the photocurrents obtained from these electrodes are very low. In chapter four we report on thin silicon-doped nanocrystalline α-Fe2O3 films (thickness of 250-500nm) that have been deposited on F-doped SnO2 substrates by ultrasonic spray pyrolysis (USP) and chemical vapor deposition at atmospheric pressure (APCVD). The photoanodes prepared by USP and APCVD gave a photocurrent of 1.17 and 1.45 mA/cm2 respectively. The morphology of the α-Fe2O3 was strongly influenced by the silicon doping, decreasing the feature size of the mesoscopic film. The silicon-doped α-Fe2O3 nano-leaflets show a preferred orientation with the (001) basal plane normal to the substrate. Chapter five treats the preparation and characterization of hematite photoanodes prepared by an improved APCVD setup. Under illumination water is oxidized at the Fe2O3 electrode with higher efficiency (IPCE = 42% at 370 nm and 2.2 mA/cm2) than at the best reported single crystalline Fe2O3 electrodes. This unprecedented efficiency is in part attributed to the dendritic nanostructure which minimizes the distance photo generated holes have to diffuse to reach the Fe2O3/electrolyte interface while still allowing efficient light absorption. Part of the gain in efficiency is obtained by depositing a thin insulating SiO2 interfacial layer between the SnO2 substrate and the Fe2O3 film, and a catalytic cobalt monolayer on the Fe2O3 surface. A mechanistic model for water photooxidation is presented, involving stepwise accumulation of four holes by two vicinal iron or cobalt surface sites. In chapter six we present a detailed investigation of the dependency of the photocurrent response on film thickness and feature size of doped and undoped hematite photoanodes prepared by the APCVD setup discussed in chapter five, in absence of a redox catalyst. Evidence is presented of an unusual high donor density of 1020 cm-3 in the silicon doped photoanodes which would allow a space charge layer to be formed inside the nano-sized features of the polycrystalline film. This suggests that electric field induced charge separation could be an important reason for the photocurrent improvement reported in chapter five. In addition, a strong correlation between feature size and the intensity of a Raman band at 660cm-1 is presented and discussed in the context of disorder in the crystal structure. Chapter seven presents a study of the influence that the deposition parameters have on the photoresponse, morphology, and crystal structure of films prepared by ultrasonic spray pyrolysis. Films are characterized by SEM, XEDS elemental analysis, TEM, XRD, Mössbauer and Raman spectroscopy. A strong correlation is found between the formation of β-Fe2O3 (Bixbyite) and low photocurrents. Silicon doped films enable the electron transport across a film thickness of 1300nm. The absorbed-photon-to-current-conversion-efficiency (APCE) increases with film thickness below 150nm, which indicates an increased recombination near the substrate interface. This could be an important reason for the low photoresponse of nanocomposite electrodes studied in chapter three. In chapter eight we draw some general conclusions with regards to the future development of these photoanodes.

EPFL2007

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

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