Enhanced light absorption of thin perovskite solar cells using textured substrates

Thin perovskite layers are crucial for fabricating semitransparent perovskite solar cells and tandem devices. However, this will reduce the light absorption of the device, resulting in low efficiency. To overcome this dilemma, we use textured fluorine doped SnO2 (FTO) substrates to trap light and enhance light absorption in thin perovskite solar cells. Both optical simulation and experimental demonstration show the successful application of this new strategy in enhancing the efficiency. In the meantime, the transmittance of the full cell beyond the perovskite absorption range is not sacrificed, which is pivotal for fabricating semitransparent perovskite solar cells and tandem devices.

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

Light Harvesting Schemes for High Efficiency Thin Film Silicon Solar Cells

Christophe Ballif, Corsin Battaglia, Mathieu Gérard Boccard, Maximilien Joseph Marie Bonnet-Eymard, Grégory Bugnon, Mathieu Charrière, Peter Cuony, Matthieu Despeisse, Laura Ding, Jordi Escarre Palou, Sylvain Nicolay, Gaetano Parascandolo, Michael Elias Stückelberger

In Thin Film Silicon (TF-Si) solar cells light harvesting schemes must guarantee an efficient light trapping in the thin absorber layers without decreasing the silicon layers quality and consecutively the p-i-n diodes electrical performance. TF-Si solar cells resilience to the substrate roughness is reported to be possibly improved through optimizations of the cell design and of the silicon deposition processes. By further tailoring the superstrate texture, amorphous silicon / microcrystalline silicon (a-Si:H/mu c-Si:H) tandem solar cells with an initial efficiency up to 13.7 % and a stabilized efficiency up to 11.8 % are demonstrated on single-scale textured superstrates. An alternative approach combining large and smooth features nanoimprinted onto a transparent lacquer with small and sharp textures from as-grown LPCVD ZnO is then shown to have a high potential for further increasing TF-Si devices efficiency. First results demonstrate up to 14.1 % initial efficiency for a TF-Si tandem solar cell.