Väriaineherkistetyt nanorakenteiset ja orgaaniset aurinkosähkökennot: tekninen kirjallisuuskatsaus ja alustavia kokeita

The solar electricity is presently a rapidly growing but often relatively expensive renewable energyform. Recently however, new molecular photovoltaic (PV) materials have been developed, whichcould enable a production of low-cost solar cells in the future. The thesis begins with a discussion of the current status of the PV technology and a shortintroduction to the different PV technologies and to the basics of photovoltaics. The dye-sensitized solar cell (DSSC) is an electrochemical solar cell where light absorption occursby dye molecules attached to a nanostructured TiO2 electrode. An introduction to the DSSC is givenincluding a short description of the operating principle of the cell and a discussion of the physicaland chemical processes behind it. A systematic literature review is done on the materials and mostessential preparation methods of the standard DSSC. The performance of the DSSC is reviewed in terms of the energy conversion efficiency and the longtermstability. The important directions of development are the transition from glass substrates toplastic foils and from batch processing to continuous processing as well as the use of solid stateelectrolytes. The glass-based DSSC technology is on the verge of commercialization and themanufacturing cost estimates for the technology are close to the projected costs of other PVtechnologies. The purely organic solar cells are discussed individually beginning with the discussion of thefundamentals of organic photovoltaics and an introduction of different types of organic photovoltaicmaterials including semiconducting polymers, dyes, pigments and liquid crystalline materials. Areview is done on the performance results of organic solar cells categorizing the cells by their devicearchitecture. The development of the organic PV materials is still at an early stage and no clearlyoutperforming materials or cell structures have yet emerged. Experimental results are reported including a demonstration of the dye-sensitization with a naturaldye as well as a preparation and testing of a series of ruthenium-dye based DSSCs. An efficiency of0.6% at about 600 W/m2 solar illumination was obtained for the DSSCs in outdoor measurements.Aurinkosähkö on tällä hetkellä nopeasti kasvava mutta usein verrattain kallis uusiutuvaenergiamuoto. Viime aikoina on kuitenkin kehitetty uusia molekulaarisia aurinkosähkömateriaaleja,jotka voivat mahdollistaa tulevaisuudessa halpojen aurinkosähkökennojen tuotannon. Työn alussa käsitellään aurinkosähkön nykytilaa ja luodaan lyhyt katsaus eriaurinkosähköteknologioihin ja aurinkosähkön perusteisiin. Väriaineherkistetty aurinkokenno (väriainekenno) on valosähkökemiallinen aurinkokenno, jossavalon absorptio tapahtuu nanorakenteisen TiO2 -elektrodin pintaan kiinnittyneidenväriainemolekyylien avulla. Työssä esitellään väriainekennon toimintaperiaate ja tarkastellaan sentaustalla olevia fysikaalisia ja kemiallisia prosesseja, sekä tehdään järjestelmällinenkirjallisuuskatsaus perusväriainekennon materiaaleihin ja tärkeimpiin valmistusmenetelmiin. Väriainekennon suorituskykyä tarkastellaan energian konversion hyötysuhteen japitkäaikaisstabiilisuuden osalta. Tärkeitä kehityssuuntia ovat siirtyminen lasisubstraateistamuovikalvoihin ja vaiheittaisesta valmistusprosessista jatkuvaan prosessiin sekä kiinteidenelektrolyyttien käyttö. Lasisubstraattiin perustuva väriainekennoteknologia on kaupallistumisenkynnyksellä ja sen valmistuskustannusarviot ovat lähellä muiden aurinkosähköteknologioidenkustannusennusteita. Puhtaasti orgaanisia aurinkosähkökennoja tarkastellaan erikseen alkaen orgaanistenaurinkosähkömateriaalien fysikaalisista perusteista. Tämän jälkeen esitellään erilaiset orgaanisetaurinkosähkömateriaalit, joihin kuuluu puolijohtavia polymeerejä, väriaineita, pigmenttejä janestekiteisiä materiaaleja, sekä tehdään katsaus orgaanisten kennojen tuloksiin luokitellen kennotniiden rakenteen mukaan. Orgaanisten aurinkosähkömateriaalien kehitys on vielä alkuvaiheessa eikäselkeästi suorituskyvyltään muita parempia materiaaleja ja kennorakenteita ole vielä ilmennyt. Lopuksi esitetään kokeelliset tulokset väriaineherkistyksen havainnollistamisesta luonnonväriaineella sekä ruteeni-väriaineeseen perustuvien väriainekennojen valmistuksesta ja testauksesta.Ulkomittauksissa 600 W/m2 auringonvalossa saavutettiin väriainekennojen hyötysuhteeksi 0.6%

Performance limiting factors in flexible dye solar cells

Photovoltaic cells convert the electromagnetic energy of sunlight to electrical energy. An example of this is the electrochemical dye solar cell (DSC). In this work, preparation of DSCs on flexible plastic and metal substrates was studied concentrating on the factors that limit their energy conversion efficiency. Flexible substrates enable manufacturing of solar cells with cost-effective roll-to-roll techniques, but set restrictions to their materials and fabrication processes. An improved method for the preparation of nanoparticle films on plastic was developed using spray deposition and mechanical pressing at room-temperature. With the method, deposition of nanostructured TiO2 photoelectrodes was fast but their photocurrent output was low. To investigate this, an improved experimental method was introduced that allows determining the quantum efficiencies of photocurrent generation in DSC by conventional optical spectroscopy and spectral response measurements. The reason was low electron collection efficiency due to too short electron diffusion length in the pressed TiO2 films. Stainless steel 304 was found to possess excellent electrochemical properties for its use as the photoelectrode substrate. 4.4 % cell efficiency was reached with a steel based DSC. When the effects of different cell components on the current-voltage curve of the cell were studied using electrochemical impedance spectroscopy (EIS), it was found that the stainless steel decreased the electron recombination resistance of the TiO2 photoelectrode film. The performance of the plastic substrate based cells at low light intensities was significantly limited by electron recombination via the substrate. Preparing a 4 nm thick compact TiO2 layer on the substrate by atomic layer deposition (ALD) suppressed the recombination, but introduced an additional contact resistance that decreased the fill factor and cell efficiency. The dynamic photocurrent and photovoltage response of DSC is significantly affected by non-uniform generation profile and inefficient collection of electrons. This complicates the interpretation of the dynamic data and can lead to erroneous conclusions when a common approximate way of analysis is used. As a solution to this, a new dynamic performance characteristic was introduced, that allows also determining the effective electron diffusion coefficient and lifetime at the short circuit condition consistently with the steady state cell performance

Spatial distribution and decrease of dye solar cell performance induced by electrolyte filling

The spatial performance variation of dye solar cell with standard liquid electrolyte was examined by dividing the cell into segments. Surprisingly large and permanent performance differences were found in different parts of the cell leading to significant losses in the overall cell efficiency. The decrease of open circuit voltage along the electrolyte filling direction suggests that 4-tert-butylpyridine is adsorbed non-uniformly as the electrolyte passes through the dyed TiO2 layer during the filling process. The result indicates that non-uniform electrolyte adsorption may limit the up-scaling of dye solar cells, which calls for the examination of electrolyte filling techniques and electrolyte compositions less prone to this effect.Peer reviewe

Segmented Cell Design for Improved Factoring of Aging Effects in Dye Solar Cells

A new segmented cell design was applied to study the aging of dye solar cell with stainless steel (StS) photoelectrode substrate, in particular the role of electrolyte in the degradation. Photovoltaic characterization indicated that StS photoelectrode cells are subjected to rapid (within hours or days) performance degradation that did not occur in the StS counter electrode cells. Other complementary techniques, open circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS), showed changes in the recombination at the photoelectrode/electrolyte interface. With the segmented cell method, we confirmed that the electrolyte was not contaminated by the StS nor was it subject to other significant changes related to the rapid degradation.Peer reviewe

Physical Modeling of Photoelectrochemical Hydrogen Production Devices

Solar-powered water splitting with photoelectrochemical (PEC) devices is a promising method to simultaneously harvest and store solar energy at a large scale. Highly efficient small prototype PEC devices reported recently demonstrate a move from basic material research toward design and engineering of complete devices and systems. The increased interest in engineering calls for a better understanding about the operational details of PEC devices at different length scales. The relevant physical phenomena and the properties of typical materials are well-known for separate device components, but their interaction in a complete PEC cell has received less attention. Coupled physical models are useful for studying these interactions and understanding the device operation as a whole and for optimizing the devices. We review the central physical processes in solar-powered water splitting cells and the physical models used in their theoretical simulations. Our focus is in particular on how different physical processes have been coupled together to construct device models and how different electrode and device geometries have been taken into account in them. Reflecting on the literature we discuss future opportunities and challenges in the modeling of PEC cells.Peer reviewe

Metallic and plastic dye solar cells

Dye solar cells (DSCs) are quite a new technology in photovoltaics. The traditional DSCs are prepared on conductively coated glass substrates in high temperature using a batch process. Manufacturing the cells on low-cost metal and plastic substrates would enable significant cost reductions as well as roll-to-roll mass production. There is a selection of metals and possible conducting coatings for plastics with varying electrical, optical, and chemical properties and price. The substrate has a dominant impact on the methods and materials that can be applied to make the cell and consequently on the resulting performance of the device. Furthermore, the substrates influence significantly the stability of the device. The main issue with plastics is their permeability whereas with metals, chemical stability in the electrolyte is themain concern. The leakage of electrolyte and the impact of water intake through the plastics can be affected by the material choices in particular with the electrolyte and dye composition. In the case of the metallic electrodes, the chemical stability can be improved by choosing a corrosion-resistant metal, applying a blocking layer or changing to a less aggressive electrolyte. One major focus of the current research of the flexible DSCs is increasing the efficiency by improved low-temperature preparation methods and materials especially for the photoelectrode. Another significant challenge is the development of noncorrosive electrolyte and dye combinations that work well even in the presence of significant amounts of water.Peer reviewe

An analytical model of hydrogen evolution and oxidation reactions on electrodes partially covered with a catalyst

Our previous theoretical study on the performance limits of the platinum (Pt) nanoparticle catalyst for the hydrogen evolution reaction (HER) had shown that the mass transport losses at a partially catalyst-covered planar electrode are independent of the catalyst loading. This suggests that the two-dimensional (2D) numerical model used could be simplified to a one-dimensional (1D) model to provide an easier but equally accurate description of the operation of these HER electrodes. In this article, we derive an analytical 1D model and show that it indeed gives results that are practically identical to the 2D numerical simulations. We discuss the general principles of the model and how it can be used to extend the applicability of existing electrochemical models of planar electrodes to low catalyst loadings suitable for operating photoelectrochemical devices under unconcentrated sunlight. Since the mass transport losses of the HER are often very sensitive to the H2 concentration, we also discuss the limiting current density of the hydrogen oxidation reaction (HOR) and how it is not necessarily independent of the reaction kinetics. The results give insight into the interplay of kinetic and mass-transport limitations at HER/HOR electrodes with implications for the design of kinetic experiments and the optimization of catalyst loadings in the photoelectrochemical cells.Peer reviewe

Fully stable numerical calculations for finite onedimensional structures: mapping the Transfer Matrix method

We design a fully stable numerical solution of the Maxwell´s equations with the Transfer Matrix Method (TMM) to understand the interaction between an electromagnetic field and a finite, one-dimensional, nonperiodic structure. Such an exact solution can be tailored from a conventional solution by choosing an adequate transformation between its reference systems, which induces a mapping between its associated TMMs. The paper demonstrates theoretically the numerical stability of the TMM for the exact solution within the framework of Maxwell´s equations, but the same formalism can efficiently be applied to resolve other classical or quantum linear wave-propagation interaction in one, two, and three dimensions. This is because the formalism is exclusively built up for an in depth analysis of the TMM´s symmetriesPeer reviewe

Two-Dimensional Time-Dependent Numerical Modeling of Edge Effects in Dye Solar Cells

A two-dimensional transient model of dye solar cells (DSC) describing the electrochemical reactions in the cell has been prepared. The model includes the relevant components of DSCs: the photoelectrode, the electrolyte, and the counter electrode. The solved variables are potential and the concentrations of the different ion species, which can be used to determine, e.g., the current−voltage characteristics of the cell. The largest benefit of this model is its 2D features which enable the study of lateral inhomogeneity. Using the model, a new phenomenon was described: lateral current density distribution caused by a small difference in the size between photoelectrode and counter electrode, typical of laboratory test cells, causes tri-iodide to move from the edge region to the active area of the cell. This process takes a relatively long time (8 min) and can be important for performance characterization and design of DSCs.Peer reviewe

A Novel Machine Learning-Based Approach for Induction Machine Fault Classifier Development—A Broken Rotor Bar Case Study

Rotor bars are one of the most failure-critical components in induction machines. We present an approach for developing a rotor bar fault identification classifier for induction machines. The developed machine learning-based models are based on simulated electrical current and vibration velocity data and measured vibration acceleration data. We introduce an approach that combines sequential model-based optimization and the nested cross-validation procedure to provide a reliable estimation of the classifiers’ generalization performance. These methods have not been combined earlier in this context. Automation of selected parts of the modeling procedure is studied with the measured data. We compare the performance of logistic regression and CatBoost models using the fast Fourier-transformed signals or their extracted statistical features as the input data. We develop a technique to use domain knowledge to extract features from specific frequency ranges of the fast Fourier-transformed signals. While both approaches resulted in similar accuracy with simulated current and measured vibration acceleration data, the feature-based models were faster to develop and run. With measured vibration acceleration data, better accuracy was obtained with the raw fast Fourier-transformed signals. The results demonstrate that an accurate and fast broken rotor bar detection model can be developed with the presented approach