Kati Miettunen

Device Physics of Dye Solar Cells

Design of new materials for nanostructured dye solar cells (DSC) requires understanding the link between the material properties and cell efficiency. This paper gives an overview of the fundamental and practical aspects of the modeling and characterization of DSCs, and integrates the knowledge into a user-friendly DSC device model. Starting from basic physical and electrochemical concepts, mathematical expressions for the IV curve and differential resistance of all resistive cell components are derived and their relation to electrochemical impedance spectroscopy (EIS) is explained. The current understanding of the associated physics is discussed in detail and clarified. It is shown how the model parameters can be determined from complete DSCs by current dependent EIS and incident-photon-to-collected-electron (IPCE) measurements, supplemented by optical characterization, and used to quantify performance losses in DSCs. The paper aims to give a necessary theoretical background and practical guidelines for establishing an effective feedback-loop for DSC testing and development.

Interpretation of Optoelectronic Transient and Charge Extraction Measurements in Dye‐Sensitized Solar Cells

Tools that assess the limitations of dye sensitized solar cells (DSSCs) made with new materials are critical for progress. Measuring the transient electrical signals (voltage or current) after optically perturbing a DSSC is an approach which can give information about electron concentration, transport and recombination. Here we describe the theory and practice of this class of optoelectronic measurements, illustrated with numerous examples. The measurements are interpreted with the multiple trapping continuum model which describes electrons in a semiconductor with an exponential distribution of trapping states. We review standard small perturbation photocurrent and photovoltage transients, and introduce the photovoltage time of flight measurement which allows the simultaneous derivation of both effective diffusion and recombination coefficients. We then consider the utility of large perturbation measurements such as charge extraction and the current interrupt technique for finding the internal charge and voltage within a device. Combining these measurements allows differences between DSSCs to be understood in terms such as electron collection efficiency, semiconductor conduction band edge shifts and recombination kinetics.

Review of materials and manufacturing options for large area flexible dye solar cells

This review covers the current state of the art related to up-scaling and commercialization of dye solar cells (DSC). The cost analysis of the different components and manufacturing of DSC gives an estimate on the overall production costs. Moreover, it provides an insight in which areas improvement is needed in order to reach significant cost reductions. As a result of the cost analysis, transferring the technology to flexible substrates and employment of simple roll-to-roll production methods were found the key issues. The focus of this work was set accordingly. In this work, appropriate materials along with their unique fabrication processes and different design methods are investigated highlighting their advantages and limitations. The basic goal is to identify the best materials and preparation techniques suitable for an ideal roll-to-roll process of flexible dye solar module fabrication as well as the areas where further development is still needed.

Dye Solar Cells on ITO-PET Substrate with TiO2 Recombination Blocking Layers

Atomic-layer-deposited TiO 2 recombination blocking layers were prepared on indium tin oxide-poly(ethylene terephthalate) (ITO-PET) photoelectrode substrates for dye solar cells and were examined using several electrochemical methods. The blocking layers increased the open-circuit voltage at low light intensities. At high light intensities, a decrease in the fill factor (FF) due to the additional resistance of the current transport through the layer was more significant than the positive effect by the reduced recombination. The decrease in the FF was reduced by a thermal treatment that made the blocking layer more conductive due to a structural change from an amorphous to a crystalline form. Therefore, thinner blocking layers of this type are required for plastic cells prepared at low temperature than for conventional glass dye solar cells made with temperature processing.

Stability of Dye Solar Cells with Photoelectrode on Metal Substrates

In this study, the stability of dye solar cells DSCs with different kinds of metals as the photoelectrode substrate is studied.Stainless steels StSs, Inconel, and titanium substrates were tested to find stable substrate options. Photovoltaic characterization,electrochemical impedance spectroscopy EIS, scanning electron microscopy, and substrate polarization measurements were usedin the characterization. DSCs based on different grades of StS suffered from rapid degradation of efficiency within few hours inlight soaking. Good stability was demonstrated with DSCs with Inconel and Ti photoelectrode substrates. The Inconel substrateshave a thick passive oxide layer, which is likely related to good stability. However, according to the EIS analysis, the oxide layerof Inconel substrates increased resistive losses, which caused a lower fill factor and photovoltaic efficiency compared to theTi-based cells.© 2010 The Electrochemical Society. DOI: 10.1149/1.3374645 All rights reserved.Manuscript submitted December 17, 2009; revised manuscript received February 19, 2010. Published April 21, 2010.

Effect of electrolyte bleaching on the stability and performance of dye solar cells

Degradation of dye solar cells (DSCs) under severe ageing conditions may lead to loss of the tri-iodide in the electrolyte - a phenomenon known as electrolyte bleaching. Monitoring changes in the tri-iodide concentration as a result of degradation mechanisms and understanding their causes and effects are fundamental for improving the long-term stability of DSCs. In this contribution a strongly accelerated ageing test (1 Sun visible light, 1.5 Suns UV light, T = 110 °C for 12 h) was performed on DSCs in a double-sealed masterplate configuration to purposely induce severe electrolyte bleaching, and its effects on the performance and stability of DSCs with different initial tri-iodide concentrations [I3(-)]0 were investigated. The cells with low [I3(-)]0 suffered a severe loss in short circuit current density JSC (up to 85%). Also a significant loss of open circuit voltage VOC was observed and this loss was proportional to [I3(-)]0 with the highest VOC drop observed with the highest [I3(-)]0. Non-destructive analysis techniques based on the limited current density, JSCvs. light intensity, and photographic image analysis, were used to quantify the [I3(-)] loss, which was found to be ca. 50 mM and independent of [I3(-)]0. Quantitative model based VOC analysis in terms of changing [I3(-)] revealed that the degradation responsible for the VOC drop was dominated by an unknown mechanism that is unrelated to [I3(-)]0. The methods and results reported here help separating and identifying different degradation mechanisms related to electrolyte bleaching in DSCs.

Analysis of dye degradation products and assessment of the dye purity in dye-sensitized solar cells.

RATIONALE For commercialization of dye-sensitized solar cells (DSSCs), improvement of their long-term stability and efficiency is important. A key component in solar cells is the dye, its high purity and high stability. Here, methods for dye extraction and purification, and for determination of dye purity and dye degradation in DSSCs, were developed. METHODS A method was developed for extraction of the dye Z907 from intact solar cells using a water/ethanol mixture containing tetrabutylammonium hydroxide. The N719 dye synthesized in our laboratory was purified by gel filtration on Sephadex LH20. These dyes, along with the dyes N3 and RuL2 (NC)2, were analyzed using nuclear magnetic resonance (NMR) spectroscopy and liquid chromatography coupled to an electrospray ionization quadrupole-time-of-flight mass analyzer (LC/MS) operating in negative ionization mode. RESULTS Purification of the synthesized N719 removed several impurities, including its undesired isomer with the thiocyanate ligand attached to ruthenium through sulfur instead of nitrogen. The dyes N719 and Z907 were successfully extracted from solar cells and together with N3 and RuL2 (NC)2 analyzed by LC/MS, although N719 isomerized almost immediately in basic aqueous solution. The [M-H](-1) ions were observed and the measured mass was within a ±6 ppm range from the exact mass. CONCLUSIONS LC/MS in combination with NMR spectroscopy was shown to provide useful information on dye structure, purity, and on the efficiency of the purification methods. These methods allow for further studies of solar cell dyes, which may provide the detailed information needed for the improvement and eventual commercialization of the solar cell technology.

Biobased aerogels with different surface charge as electrolyte carrier membranes in quantum dot-sensitized solar cell

Biobased aerogels were used as environmentally friendly replacement for synthetic polymers as electrolyte carrier membranes in quantum dot-sensitized solar cell (QDSC). Integration of polymeric components in solar cells has received increased attention for sustainable energy generation. In this context, biobased aerogels were fabricated to apply as freestanding, porous and eco-friendly electrolyte holding membranes in QDSC. Bacterial cellulose (BC), cellulose nanofibers (CNF), chitin nanofibers (ChNF) and TEMPO-oxidized CNF (TOCNF) were selected because of their fibrilar structures and water-holding capability to investigate their inherent differences in terms of surface groups and electrostatic charge on the electrolyte redox reaction and the photocell function. BC, CNF, ChNF and TOCNF were selected due to different surface functional groups (hydroxyl, N-acetylglucosamine and carboxyl units) and fibrilar structures that can form highly interconnected and robust network. These aerogels enabled easy handling, effective electrolyte filling and efficient redox reactions, while keeping the solar cell performance on par to that of traditional reference cells without membranes. The aerogel membranes maintained the photocell performance since they took only a very small space of the electrolyte volume, which allowed efficient charge transfer. The results indicated that aerogels did not interfere with the cell operation, as confirmed by quartz crystal microgravimetry with bio-interphases in contact with the polysulfide-based electrolyte. The electrochemical measurements also suggested that the respective functional groups (hydroxyl, N-acetylglucosamine and carboxyl units) did not interfere with the redox reaction of the polysulfide electrolyte.

The performance enhanced by back reflection in nanostructured dye-sensitized solar cells

Dye-sensitized solar cell (DSSC) cannot absorb all the incident light, and some parts of light transits the cell and is unused. The work presented here dealt with transmitted light to enhance the performance by using the back reflection of stainless steel substrate. The influence of surface roughness of stainless steel substrate DSSC was investigated. Three kind of surface roughness were used in the DSSCs. The result showed that the I–V parameters were not sensitive to the surface properties of stainless steel. Some controlled surfaces of substrate were suggested in the further research. The influence of the thickness of TiO2 photoelectrode film was also researched. The optimal thickness of stainless steel DSSC was larger than glass substrate DSSC and over 30 µm.

Application of dye-sensitized and perovskite solar cells on flexible substrates

In this paper, a review of applying dye-sensitized (DSC) and perovskite solar cells (PSC) on flexible substrates is presented. Metallic and polymeric materials are the most common flexible substrates used. Cell integration into a textile substrate is also considered here as a future alternative. Common challenges with these include penetration of humidity, cell stability, and lifetime. Flexible DSC and PSC solar cells are still a niche technology, but have an inherent potential for cheap roll-to-roll massproduction of photovoltaics.