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The colorimetric and photovoltaic efficiency of different structural color coatings, including cellulose nanocrystals, were compared with spectrophotometry, photography, and power loss analysis in view of their application in solar architecture.

The daily Sun supplies the continents of the Earth with four times more energy than humanity consumes in a year. This enormous potential of solar energy to generate clean energy is therefore driving great efforts to replace conventional and unsustainable fossil fuel consumption that damages our climate and our environment. Solar photovoltaic (PV) is emerging as the fastest growing renewable energy technology in the world, yet its share to the electricity production currently is less than 3%. While coal and gas remain key to electricity production, the climate crisis demands a fast transition to a carbon-neutral energy system. In the year 2019, the PV industry produced solar panels with a capacity of about 115 GW. To reach multi-TW levels by 2030, PV production capacity needs to scale at high rates, and continued innovation is necessary. One way to increase production is to reduce silicon wafer thickness, making more cells from the amount of silicon produced. Thinning the cell will not only allow making more of them in less time but will also reduce the cost for a module, and for the electricity it generates. A thinner absorber reduces the absorption of sunlight. To counter this loss, optical concepts are needed. In this chapter, we review the most promising approaches in photonics that have made physically thinner but optically thicker solar cells possible. By enabling thinner solar cells, photonic solutions offer the possibility of efficiency enhancements at lower costs. We will explain their applications, discuss their challenges and estimate their impact on energy yield. Finally, we outline how non-silicon thin-film PV is opening up a plethora of consumer-oriented applications. Since the higher flexibility/bendability of thinner cells broadens their applicability range, thin-film PV will empower our transition to a 100% green society in many aspects of our lives.

Dye-sensitized solar cells (DSSCs) are an efficient photovoltaic technology for powering electronic applications such as wireless sensors with indoor light. Their low cost and abundant materials, as well as their capability to be manufactured as thin and light-weight flexible solar modules highlight their potential for economic indoor photovoltaics. However, their fabrication methods must be scaled to industrial manufacturing with high photovoltaic efficiency and performance stability under typical indoor conditions. This paper reviews the recent progress in DSSC research towards this goal through the development of new device structures, alternative redox shuttles, solid-state hole conductors, TiO2 photoelectrodes, catalyst materials, and sealing techniques. We discuss how each functional component of a DSSC has been improved with these new materials and fabrication techniques. In addition, we propose a scalable cell fabrication process that integrates these developments to a new monolithic cell design based on several features including inkjet and screen printing of the dye, a solid state hole conductor, PEDOT contact, compact TiO2, mesoporous TiO2, carbon nanotubes counter electrode, epoxy encapsulation layers and silver conductors. Finally, we discuss the need to design new stability testing protocols to assess the probable deployment of DSSCs in portable electronics and internet-of-things devices., Dye-sensitized solar cells (DSSCs) are an efficient photovoltaic technology for powering electronic applications such as wireless sensors with indoor light.

Although single-wall carbon nanotubes (SWCNTs) exhibit various colors in suspension, directly synthesized SWCNT films usually appear black. Recently, a unique one-step method for directly fabricating green and brown films has been developed. Such remarkable progress, however, has brought up several new questions. The coloration mechanism, potentially achievable colors, and color controllability of SWCNTs are unknown. Here, a quantitative model is reported that can predict the specific colors of SWCNT films and unambiguously identify the coloration mechanism. Using this model, colors of 466 different SWCNT species are calculated, which reveals a broad spectrum of potentially achievable colors of SWCNTs. The calculated colors are in excellent agreement with existing experimental data. Furthermore, the theory predicts the existence of many brilliantly colored SWCNT films, which are experimentally expected. This study shows that SWCNTs as a form of pure carbon, can display a full spectrum of vivid colors, which is expected to complement the general understanding of carbon materials.

The success of perovskite and dye-sensitized solar cells will depend on their stability over the whole life-time. Aging tests are of utmost importance to identify deficiencies and to suggest cell improvements. Here we analyzed the quality of 261 recent aging tests and found serious shortcomings in current practices. For example, in about 50% of the studies only one sample was considered, meaning that the sample size was too small for statistical significance. We propose a new procedure for aging tests based on careful planning and scientific reporting. This includes estimating the required sample size for an aging test and avoiding so-called nuisance factors, i.e. unintended variations always present in real world testing. The improved procedure can provide more reliable information on stability and lifetime, which could contribute to better understanding of degradation mechanisms important for improving these photovoltaic technologies.

A comparative analysis of perovskite structured cathode materials, La0.65Sr0.35MnO3 (LSM), La0.8Sr0.2CoO3 (LSC), La0.6Sr0.4FeO3 (LSF) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), was performed for a ceramic-carbonate nanocomposite fuel cell using composite electrolyte consisting of Gd0.1Ce0.9O1.95 (GDC) and a eutectic mixture of Na2CO3 and Li2CO3. The compatibility of these nanocomposite electrode powder materials was investigated under air, CO2 and air/CO2 atmospheres at 550 °C. Microscopy measurements together with energy dispersive X-ray spectroscopy (EDS) elementary analysis revealed few spots with higher counts of manganese relative to lanthanum and strontium under pure CO2 atmosphere. Furthermore, electrochemical impedance (EIS) analysis showed that LSC had the lowest resistance to oxygen reduction reaction (ORR) (14.12 Ω∙cm2) followed by LSF (15.23 Ω∙cm2), LSCF (19.38 Ω∙cm2) and LSM (>300 Ω∙cm2). In addition, low frequency EIS measurements (down to 50 μHz) revealed two additional semi-circles at frequencies around 1 Hz. These semicircles can yield additional information about electrochemical reactions in the device. Finally, a fuel cell was fabricated using GDC/NLC nanocomposite electrolyte and its composite with NiO and LSCF as anode and cathode, respectively. The cell produced an excellent power density of 1.06 W/cm2 at 550 °C under fuel cell conditions. [Figure not available: see fulltext.]

With electrocatalysts it is important to be able to distinguish between the effects of mass transport and reaction kinetics on the performance of the catalyst. When the hydrogen evolution reaction (HER) is considered, an additional and often neglected detail of mass transport in liquid is the evolution and transport of gaseous H2, since HER leads to the continuous formation of H2 bubbles near the electrode. We present a numerical model that includes the transport of both gaseous and dissolved H2, as well as mass exchange between them, and combine it with a kinetic model of HER at platinum (Pt) nanoparticle electrodes.We study the effect of the diffusion layer thickness and H2 dissolution rate constant on the importance of gaseous transport, and the effect of equilibrium hydrogen coverage and Pt loading on the kinetic and mass transport overpotentials. Gaseous transport becomes significant when the gas volume fraction is sufficiently high to facilitate H2 transfer to bubbles within a distance shorter than the diffusion layer thickness. At current densities below about 40 mA/cm2 the model reduces to an analytical approximation that has characteristics similar to the diffusion of H2. At higher current densities the increase in the gas volume fraction makes the H2 surface concentration nonlinear with respect to the current density. Compared to the typical diffusion layer model, our model is an extension that allows more detailed studies of reaction kinetics and mass transport in the electrolyte and the effects of gas bubbles on them.

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. Copyright © 2015 John Wiley & Sons, Ltd.

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.

A semitransparent, flexible single-walled carbon nanotube (SWCNT) film was efficiently used in place of evaporated silver as the counter electrode of a poly(3,4-ethylenedioxythiophene) polymer-based solid-state dye solar cell (SSDSC): the solar-to-electrical energy conversion efficiency of the SWCNT-SSDSC was 4.8 % when it was 5.2 % for the Ag-SSDSC. The efficiency difference stemmed from a 0.1-V difference in the open-circuit voltage, whose reason was speculated to be related to the different recombination processes in the two types of SSDSCs.

Plasmonic metal nanoparticles have shown great promise in enhancing the light absorption of organic dyes and thus improving the performance of dye-sensitized solar cells (DSSCs). However, as the plasmon resonance of spherical nanoparticles is limited to a single wavelength maximum (e.g., ~ 520 nm for Au nanoparticles), we have here utilized silica-coated gold nanorods (Au@SiO2 NRs) to improve the performance at higher wavelengths as well. By adjusting the aspect ratio of the Au@SiO2 NRs, we can shift their absorption maxima to better match the absorption spectrum of the utilized dye (here we targeted the 600–800 nm range). The main challenge in utilizing anisotropic nanoparticles in DSSCs is their deformation during the heating step required to sinter the mesoporous TiO2 photoanode and we show that the Au@SiO2 NRs start to deform already at temperatures as low as 200 °C. In order to circumvent this problem, we incorporated the Au@SiO2 NRs in a TiO2 nanoparticle suspension that does not need high sintering temperatures to produce a functional photoanode. With various characterization methods, we observed that adding the plasmonic particles also affected the structure of the produced films. Nonetheless, utilizing this low-temperature processing protocol, we were able to minimize the structural deformation of the gold nanorods and preserve their characteristic plasmon peaks. This allowed us to see a clear redshift of the maximum in the incident photon-to-current efficiency spectra of the plasmonic devices (Δλ ~ 14 nm), which further proves the great potential of utilizing Au@SiO2 NRs in DSSCs.

The main issue in using low cost metals in dye solar cells is the corrosion caused by the liquid electrolyte. Contrary to typical applications of metals, the adverse effects of corrosion in dye solar cells are related to irreversible depletion of charge carriers from the electrolyte rather than consumption of the metal itself. It is calculated that the penetration rate due to corrosion should not exceed 10−4 mpy (a couple of nanometers per year) to ensure device lifetime longer than 1 year. This is 10 000 times slower rate than what is considered to be a general benchmark value for very low corrosion rate in the field of corrosion science and has a major effect on how corrosion should be investigated in the case of dye solar cells. Different methods, their applicability, and limitations to investigate corrosion in dye solar cells are evaluated here. The issue with most techniques is that they can detect metals that are clearly corroding, but they have significant limitations in proving a metal stable. Our investigation shows that the most reliable information on corrosion is obtained from complete dye solar cells that are exposed to working conditions. A combination of color analysis of the electrolyte to such measurement is proposed as a means to extrapolate future performance of the cells and estimate potential lifetimes of the dye solar cells in regards to corrosion. Copyright © 2014 John Wiley & Sons, Ltd.

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 symmetries

Volatile electrolytes are a stability concern in dye solar cells (DSCs) due to their tendency for leakage. A composite electrolyte consisting of iodide-based ionic liquid and polyaniline-coated carbon black has been previously reported to provide good current transport while being leakage proof due to a quasi-solid structure and the absence of volatile constituents. In this paper we investigate the operating principle of this type of electrolyte and especially its exceptional feature of operating efficiently without added iodine. The absence of additive iodine is significant due to the fact that it is usually required to form the current carrying the I–/I3– redox couple. We modified an electrolyte mass transport model from the literature to estimate the upper limit for the charge transport capability of the composite electrolyte. Comparison of experimental results with the estimated upper limit for the diffusion limiting current density shows clearly that the high current densities observed experimentally...

Yes. Testing 7 different metals as a substrate for a counter electrode in dye sensitized solar cells (DSSC) showed that some metals can be a good option for use with cobalt electrolyte. It was found that Stainless steels 304 and 321 as well as Ni and Ti suit well to the counter electrodes in DSSCs with cobalt electrolyte. In these 4 cases both the efficiency and the lifetime were similar to the reference cells on conducting glass substrates. In contrast, the cells with Al, Cu and Zn substrates suffered from both a low efficiency and a poor stability. These three metals had clear marks of corrosion such as apparent corrosion products in the aged cells. Additionally, we also investigated how the different types of catalyst materials perform in the case of a metal counter electrode (stainless steel 304) with cobalt electrolyte in comparison to reference glass cells. Among the 5 different catalyst layers the best results for stainless steel electrode were achieved with low temperature platinization whereas polymer catalysts poly(3,4-ethylenedioxythiophene)-p-toluenesulfone and poly(3,4-ethylenedioxythiophene)-polystyrenesulfone that worked well on the glass worked very poorly on the metal.

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 the main 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. C � 2012 John Wiley & Sons, Ltd.

The counter electrodes (CEs) for flexible dye solar cells (DSCs) are normally prepared by sputtering platinum on indium tin oxide (ITO) plastic substrate. However both ITO and platinum are expensive materials that need to be replaced with cheaper alternatives in large scale production of low-cost DSCs. We fabricated a flexible and completely carbon-based CE for DSCs based on electropolymerized poly (3,4-ethylenedioxythiophene) (PEDOT) on single-walled carbon nanotube (SWCNT) film on a plain plastic substrate. The DSCs with such a CE had an efficiency of 4.0%, which is similar to the efficiency of the reference DSCs (3.9%) based on conventional sputtered platinum on ITO-plastic CE. The carbon-based electrode was prepared by a simple press-transfer method of SWCNTs from the collection filter used in the gas phase synthesis and by electrochemical deposition of PEDOT on it. Electrochemical impedance spectroscopy confirmed that the PEDOT–SWCNT film had the best catalytic performance among the studied CE materials, and the film was also slightly transparent. The results demonstrate a successful combination of the conductive and catalytic properties of SWCNTs and PEDOT, respectively.

A simple and non-destructive method is introduced using image processing to investigate changes in the performance of the dye solar cells (DSCs). The main principle is based on the fact that the most important DSC components (dye, electrolyte, catalyst) have a specific color which often changes as result of degradation. Here the imaging technique is demonstrated in the case of exposing DSCs on very harsh conditions (85 °C temperature and UV + Visible light). The aging of the cells was recorded with a color sensitive camera in a well regulated setup and the photographs were processed using image analysis techniques. A key factor in making the imaging method quantitative and suitable for aging studies is color calibration which is explained in detail. The image analysis of different cell configurations revealed that the bleaching reactions of the electrolyte were related to reactions between TiO 2 and the electrolyte. The dye layer on the TiO 2 was shown slow down the degradation. Furthermore the comparison of image analysis and current–voltage curves indicated that the performance degradation of the cells was only partly due to loss of tri-iodide. The loss of photocurrent and photovoltage was apparently largely due to the harmful effect of the by-products of the bleaching and/or the degradation of the dye. In addition, a small recovery effect due to the generation of tri-iodide under reverse bias condition was seen in both image analysis and electrical measurements.

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.

Three characteristically different carbon nanomaterials were compared and analyzed as platinum-free counter electrodes for dye solar cells: 1) single-walled carbon nanotube (SWCNT) random network films on glass, 2) aligned multi-walled carbon nanotube (MWCNT) forest films on Inconel steel and quartz, and 3) pressed carbon nanoparticle composite films on indium tin oxide-polyethylene terephtalate plastic. Results from electrochemical impedance spectroscopy and electron microscopy were discussed in terms of the catalytic activity, conductivity, thickness, transparency and flexibility of the electrode films. The SWCNT films showed reasonable catalytic performance at similar series resistance compared to platinized fluorine doped tin oxide-coated glass. The MWCNTs had similar catalytic activity, but the electrochemical performance of the films was limited by their high porosity. Carbon nanoparticle films had the lowest charge transfer resistance resulting from a combination of high catalytic activity and dense packing of the material.

The purpose of this study was to identify stable metal based counter electrodes (CE) for dye solar cells (DSC). Previous studies have shown that stainless steel (StS 304) suffers from corrosion when used as a counter electrode. Therefore metals which have inherently higher corrosion resistance, such as stainless steel types 321, 316 and 316L, Inconel 600 and titanium, were investigated here. When using thermal platinization for the preparation of the catalyst layer on CE, only the titanium foil based metal based DSC remained consistently stable in the 1000 h light soaking test. The counter electrodes were also prepared with sputtering ∼20 nm thick layer of Pt which provides a highly uniform layer on the CE which acts also as a protective coating on the metal. With sputtered Pt, DSC on all studied metals expect for Inconel remained at 80–95% of the initial efficiency after light soaking test for 1000 h.

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.

Stainless steel based dye solar cells have been upscaled from small, laboratory size test cells of 0.32 cm2 active area to 6 cm x 6 cm "mini-modules" with active areas ca. 15 cm2. Stainless steel works as the photoelectrode substrate whilst the counter electrode is prepared on indium-doped tin oxide coated polyethyleneterephtalate or polyethylenenaphtalate plastic foil (fluorine-doped tin oxide coated glass as a reference). Additional current collector structures were deposited on the counter electrode substrate with inkjet-printing of silver nanoparticle ink in order to reduce the lateral resistance of the plastic foil. Flexible substrates enable roll-to-roll type industrial manufacturing of the cells and the steel's superior conductivity compared to the typical substrate materials such as glass and plastic makes it possible to prepare even substantially larger modules. The best efficiencies obtained this far with the "mini-module" using a stainless steel photoelectrode are 2.5% with a platinum-sputtered indium-doped tin oxide coated polyethyleneterephtalate counter electrode and 3.4% with a thermally platinized fluorine-doped tin oxide coated glass counter electrode. These efficiencies are on the same level than those measured with small cells prepared with similar methods and materials (3.4%-4.7%, depending on configuration, which are amongst the highest reported for this kind of a dye solar cell). Replacing expensive conducting glass with steel and plastic foils as the substrate materials leads also to economical savings in the cell production.

SUMMARY This review presents an overview of the current state of research on nanostructured titanium dioxide dye solar cells (DSCs) on alternative substrates to glass. Replacing the traditionally used heavy, rigid, and expensive glass substrate with materials such as plastic foils or metal sheets is crucial to enable large volume cost-efficient roll-to-roll type industrial scale manufacturing of the cells and to make this solar cell technology properly competitive with silicon and thin film photovoltaic devices. One of the biggest problems with plastic substrates is their low-temperature tolerance, which makes sintering of the photoelectrode films impossible, whereas with metals, their corrosion resistance against the iodine-containing electrolyte typically used in DSCs limits the amount of metal materials suitable for substrates. However, significant progress has been made in developing new materials, electrode film deposition and post-treatment methods suitable for low-temperature processing. Also, metals that do not corrode in the presence of iodine electrolyte have been found and successfully employed as DSC substrates. The highest power conversion efficiencies obtained with plastic and metal substrates are already 7–9%, which is not far from the best glass cell efficiencies, 10–11%, and comparable also to, for example, amorphous silicon solar cell efficiencies. One of the most important of the remaining research challenges of DSCs on flexible substrates is to ensure that the long-term stability of the cells is realistic to consumer applications, for example, with providing efficient enough encapsulation to prevent water and other impurities penetration into the cells. Degradation mechanisms specific to metal-based cells are another issue that needs deeper understanding still. More exotic approaches such as depositing the DSC structure on optical fiber or employing carbon nanomaterials to increase the cell efficiency are also discussed in this paper. Copyright r 2009 John Wiley & Sons, Ltd.

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.

This paper investigates how nonuniform generation and inefficient collection of electrons influence the dynamic photocurrent and photovoltage response of nanostructured photoelectrodes. The standard diffusion model theory of small amplitude light intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopy is refined and generalized to an arbitrary electron generation profile, allowing straightforward coupling to any optical model. Expressions are derived for the local electron concentration and IMPS and IMVS transfer functions, for localized, uniform, and exponential generation profiles. Both limited collection and nonuniform generation of electrons modify the photoelectrode thickness (d) dependence of the characteristic IMPS and IMVS time constants and complicate their interpretation. This can lead to significant overestimation of the electron diffusion coefficient, diffusion length, and collection efficiency when using common approximate relations. With near contact electron generation, theIMPS response exhibits two time constants, only the slower one of which corresponds to electron transport across the film and scales with d. In the presence of this effect it is possible that in case of two equally thick samples, the one with smaller electron diffusion coefficient displays apparently faster electron transport. These errors demonstrated by experimental IMPS data of pressed TiO(2) photoelectrodes can be minimized by using modulated light incident from the counter electrode side and avoided when analyzing the ratio of IMPS at opposite directions Of illumination.

The factors that limit photocurrent in dye solar cells (DSC) were studied by incident-photon-to-collected-electron efficiency (eta(IPCE)), optical, and photovoltaic measurements. Nanostructured TiO2 photoelectrodes were prepared by compression technique on glass substrates, and half of them were given an additional heat treatment at 450 degrees C. The spectral absorbed-photon-to-collected-electron efficiency (eta(APCE)) of the cells was determined as a function of the photoelectrode film thickness (d) and direction of illumination and analyzed in terms of electron injection (eta(INJ)) and collection (eta(COL)) efficiency. The cells with pressed-only photoelectrodes gave significantly lower photocurrents yet their eta(APCE), and thus eta(COL), increased significantly with increasing d. To analyze this result quantitatively, methods were formulated based on the standard diffusion model of electron transport in nanostructured photoelectrodes for the factorization of experimental eta(APCF) data into eta(INJ) and eta(COL) parts and subsequent estimation of the effective steady-state electron diffusion length (L). Consistent decoupling of eta(INJ) and eta(COL) was reached in a spectral region where electron generation rate was independent of d. eta(INJ) was low and strongly wavelength-dependent, which was attributed to a poor energetic matching between dye excited states and TiO2 acceptor states due to unfavorable electrolyte composition. L increased systematically with d in both types of cells. Consistent with the increase of eta(IPCE) with light intensity, the result was attributed qualitatively to the electron concentration dependence of L and for a small part to decrease of film porosity with d. The diffusion model and its predictions were reviewed, and its validity in the present case was discussed critically.

The suitability of stainless steel for dye solar cell substrate was investigated with respect to performance and stability using photovoltaic characterization, electrochemical impedance spectroscopy (EIS), open circuit voltage decay (OCVD), and substrate polarization measurements. Stainless steel was employed both as photoelectrode and as counter electrode substrate gaining initial cell efficiencies of 4.7% and 3.5%, respectively. The leakage current from the stainless steel substrate was found to be very low. The effect of the stainless steel substrate on the performance of the other cell components was also examined. The traditional data analysis based on external cell voltage was shown to be inadequate and even misleading. Here, the voltage over a single cell component was determined computationally on the basis of EIS measurements as a function of cell current; through this approach, we found that the stainless steel counter electrode did not have any impact on the photoelectrode whereas the stainless...

The effect of repeated temperature variations on the performance of both fresh and aged dye-sensitized solar cells with liquid and semisolid electrolytes has been studied. The cell performance was characterized with IV-curves obtained at different cell operating temperatures and electrochemical impedance spectroscopy measurements before and after the temperature treatments. Consecutive temperature rampings of the aged cells did regenerate the cell function, so that the total efficiency drop over the observation period was on average 18%/unit less for the temperature-treated cells than for reference cells aged at constant temperature. r 2007 Elsevier B.V. All rights reserved.

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.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.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.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.

Spray deposition of powder suspensions followed by room temperature compression was studied as a method for preparing nanostructured TiO 2 films for dye-sensitized solar cells. The structure of the films was analyzed with optical and scanning electron microscopy and the films were applied to dye-sensitized solar cells. Continuous and fast deposition of crack-free 7–14 μm thick films was achieved by heating the substrates during the deposition. Scanning electron microscopy revealed small amount of structural imperfections in the compressed films due to the nature of the deposition method. An energy conversion efficiency of 2.8% was achieved at 100 mW/cm 2 light intensity.

Electrochemical impedance spectroscopy was used to determine the effective charge transfer resistances of porous dye-sensitized solar cell counter electrodes prepared by low-temperature spray deposition and compression of conductive carbon and platinized Sb-doped SnO 2 powders on indium tin oxide-coated plastic substrates. The charge transfer resistances were 0.5–2 and 8–13 Ω cm 2 , respectively, when using 3-methoxypropionitrile as the electrolyte solvent. The manufacturing method used lends itself to produce mechanically stable and even-quality electrodes in an easy and fast manner.

A sensor developed for measurement of water concentration inside glass/polymer encapsulation structures with a particular application area in accelerated aging of photovoltaic module encapsulants is described. An approximately 5 μm thick porous TiO 2 film applied to a glass substrate with a conductive coating acts as the moisture-sensitive component. The response is calibrated with weather chamber experiments for sensors open to the environment and with diffusion experiments for sensors laminated under an encapsulant. For the interpretation of diffusion experiment results, a transport model describing the diffusion of water across the polymer/TiO 2 interface is developed. The logarithm of AC resistance shows a linear dependence on water concentration in both open and encapsulated calibration. The first measurable response from an encapsulated 3.5 mm × 8 mm size sensor is obtained when approximately 10 μg of water has entered the film. Implications of the calibration results for sensor usage in accelerated aging tests are discussed.

In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize the conversion efficiency of electrodes of different thicknesses containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thicknesses can be understood as the result of the randomization of the light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparable to the thickness of the film. This simple relationship could also be used as a design rule to attain the optimum optical design in other photovoltaic materials. This journal is © The Royal Society of Chemistry.

This paper discusses the numerical exponential instability of the transfer matrix method (TMM) in the framework of the symmetry formalism. This numerical weakness is attributed to a series of increasingly extreme exponentials that appear in the TMM when it is applied to geometries involving total internal reflection (TIR) or very high absorption. We design a TMM formalism that identifies the internal symmetries of the multilayer geometry. These symmetries suggest particular transformations of a reference system in the TMM that improve its ill-conditioned exponentials. To illustrate the numerical improvements, we present examples with calculations of electric fields. © 2013 IOP Publishing Ltd., This work was partially funded by the Nordic Innovation Center project NORDIC DSC (no. 09053) and the CNB-E project of the TKK/Aalto University Multidisciplinary Institute of Digitalisation and Energy MIDE. H.M. thanks the Spanish Ministry of Economy and Competitiveness for funding under grants MAT2011-23593 and Consolider CSD2007-00007 and Junta de Andalucia for funding under grants FQM- 3579 and FQM-5247. H.M. also thanks the European Research Council for a Starting Grant (POLIGHT)., Nordic Innovation Center, TKK/Aalto University Multidisciplinary Institute of Digitalisation and Energy, Spanish Ministry of Economy and Competitiveness, Junta de Andalucia, European Research Council

Recent years has seen enormous and impressive progress in the search for non-precious hydrogen evolution reaction (HER) catalysts for both electrolysis and photoelectrochemical water splitting (PEC) with the molybdenum sulfide– and the cobalt phosphide families of HER catalysts showing great potential [1]. Despite the progress, no non-precious catalyst which matches platinum in HER performance has yet been found. To a large degree, the hunt for non-precious HER catalysts is motived by the notion that the physical scarcity and high price of platinum precludes it from usage on the terawatt scale. Using numerical modelling supported by benchmarking experiments on very well-defined platinum-based PEC electrodes we demonstrate that platinum in principle offers sufficiently fast kinetics to be useful on the terawatt level. As may be seen in the insert in figure 1 just 54 tons of platinum (which corresponds to ca. 30% of the global annual production [2]) would in principle be enough to support 1 TW of HER at a PEC current density of 10 mA/cm2 at a modest overpotential of 50 mV.[3] We discuss the platinum requirement under different assumptions and also the potential problems of using platinum in real-world PEC systems. Refs [1] P.C.K. Vesborg, et al., J. Phys. Chem. Lett., 6, (6) p. 951-957, (2015) [2] P.C.K. Vesborg & T.F. Jaramillo, RSC Advances, 2, (21), p. 7933-7947, (2012) [3] E. Kemppainen et al., Energy & Environmental Science, 8, p. 2991-2999 (2015) Figure: Overpotential vs. of platinum loading for PEC-relevant current densities and (insert) total platinum requirement to implement 1 TW (average power, at a 15% capacity factor) of PEC hydrogen production. Figure 1

Herein, we present an integral optical and electrical theoretical analysis of the effect of different diffuse light scattering designs on the performance of dye solar cells. Light harvesting efficiencies and electron generation functions extracted from optical numerical calculations based on a Monte Carlo approach are introduced in a standard electron diffusion model to obtain the steady-state characteristics of the different configurations considered. We demonstrate that there is a strong dependence of the incident photon to current conversion efficiency, and thus of the overall conversion efficiency, on the interplay between the value of the electron diffusion length considered and the type of light scattering design employed, which determines the spatial dependence of the electron generation function. Other effects, like the influence of increased photoelectron generation on the photovoltage, are also discussed. Optimized scattering designs for different combinations of electrode thickness and electron diffusion length are proposed. © 2012 American Chemical Society., H.M. thanks the Spanish Ministry of Science and Innovation for funding provided under grants MAT2008-02166, MAT2011-23593, and CONSOLIDER HOPE CSD2007-00007 and to Junta de Andalucía for grants FQM3579 and FQM5247. H.M. and J.H. are thankful for funding under a grant from Nordic Innovation Centre (NICe).

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.

A Single-Walled Carbon Nanotube Coated Flexible PVC Counter Electrode for Dye-Sensitized Solar Cells

This paper unifies the analytical models used widely but thus far mostly separately for electrical and optical small amplitude perturbation measurements of nanostructured electrochemical dye solar cells (DSC): electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). The models are linked by expressing the kinetic boundary condition used for solving the time-dependent continuity equation of electrons in IMPS and IMVS analysis in terms of the series and parallel impedance components found in the complete equivalent circuit impedance model of DSC. As a result, analytical expressions are derived for potentiostatic IMPS and galvanostatic IMVS transfer functions of complete DSCs that are applicable at any operating point along the solar cell current-voltage (IV) curve. In agreement with the theory, impedance spectrum calculated as a ratio of IMVS and IMPS transfer functions measured near the maximum power point matches exactly with the impedance spectrum measured directly with EIS. Consequently, both IMPS-IMVS and EIS yield equal estimates for the electron diffusion length. The role of the chemical capacitance of the nanostructured semiconductor photoelectrode in the interpretation of the so-called RC attenuation of the IMPS response is clarified, as well as the capacitive frequency dispersion in IMPS and IMVS.

The current status of the long-term stability of dye solar cells (DSCs) and factors affecting it is reviewed. The purpose is to clarify present knowledge of degradation phenomena and factors in these cells by critically separating the assumptions from the solid experimental evidence reported in the literature. Important degradation processes such as dye desorption, decrease in the tri-iodide concentration, degradation at the photoelectrode and counter electrode, affect of ultraviolet light and moisture, and issues related to the sealing, are covered. It is concluded that techniques giving chemical information are needed for the stability investigations of DSCs to reveal possible ways to improve their lifetime. In this regard, experimental methods suitable for separating degradation mechanisms in complete cells during long-term testing are proposed employing specifically designed sealed cell structures, called segmented cells, that provide windows to measure specific cell components without being obscured by the others.

The use of a thin carbon nanotube (CNT) counter electrode (CE) on plastic in a dye solar cell (DSC) is demonstrated as an alternative to expensive indium tin oxide and platinum materials. Optically transparent, single-walled CNT films synthesized by the aerosol CVD method and dry-printed on PET plastic substrates functioned as both the catalyst and conducting layer of the DSC CE. The best charge-transfer resistances and sheet resistances for the random network-type film were around 89Ωcm2 and 60Ω∕□ , respectively, making them suitable for low-intensity DSC applications. A solar cell efficiency of 2.5% was reached at an illumination of 8mW/cm2 . The photocurrent generation of the cells was found to decrease when a non-purified CNT-CE was used. The electrochemical removal of iron catalyst particles from the CNT films reduced the detrimental effect and stabilized the performance of the DSC.

A flexible, slightly transparent and metal-free random network of single-walled C nanotubes (SWCNTs) on plain polyethylene terephthalate (PET) plastic substrate outperformed Pt on conductive glass and on plastic as the counter electrode (CE) of a dye solar cell employing a Co(II/III)tris(2,2'-bipyridyl) complex redox mediator in 3-methoxypropionitrile solvent. The CE charge-transfer resistance of the SWCNT film was 0.60 Ω cm2, 4.0 Ω cm2 for sputtered Pt on In Sn oxide-PET substrate and 1.7 Ω cm2 for thermally deposited Pt on F-doped Sn oxide glass, resp. The solar cell efficiencies were in the same range, thus proving that an entirely C-based SWCNT film on plastic is as good CE candidate for the Co electrolyte.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2922-2924, In this contribution we demonstrate a significant enhancement in the performance of dye sensitized solar cell (DSC) by introducing an alternative for the electrolyte filling process. The overall efficiency of an 8 cm long segmented cell made with traditional one way electrolyte filling was increased upto 42% when changed to the new two ways electrolyte filling method. This optimization of the electrolyte was shown to reduce the spatial variations generated during the filling process of the large area DSCs. This electrolyte filling process was then applied to completely rigid, semi flexible and completely flexible stripe type cells. The reverse illuminated fully flexible stripe cell based on titanium and ITO-PEN sheets achieved an efficiency of 3.4%. This new technique decreases the spatial variation losses in the manufacturing of large area DSC modules.

An ITO free, highly conductive PET foil is fabricated by depositing aqueous single-walled carbon nanotube (SWCNT) ink that exhibits remarkable durability when exposed to severe mechanical stability tests. Excellent adhesion of the SWCNT film on PET was obtained by aging the ink overnight at 50 degrees C before deposition. A counter electrode for a dye-sensitized solar cell was fabricated by electro-polymerizing the PEDOT polymer over the SWCNT film which gave 7% solar cell efficiency and low (0.4 U cm(2)) charge transfer resistance.

The slow process in which the light absorbing dye molecules are adsorbed from solution on the nano-crystalline TiO2 photoelectrode film has been a handicap to the fast and cost-effective fabrication of dye-sensitized solar cells (DSSCs) using printing techniques. Here, we report a versatile dye sensitization process, achieved by inkjet printing a concentrated dye solution over the TiO2 film, which produces solar cells with equal performance and stability as obtained using the popular dye drop casting method. In addition to allowing precise control of dye loading required for dispensing just the right amount of dye to achieve uniform and full coloration of the TiO2 films without any need for washing off the excess dye, inkjet printing also makes it possible to freely adjust the amount and position of the dye to create DSSCs with tailored transparency, color density gradients, and patterns of one or more dyes on the same electrode. The method was confirmed to be applicable also for non-transparent, high-efficiency DSSC designs that employ a light scattering layer. The inkjet-dyed DSSCs exhibited high stability, retaining almost 100% of their conversion efficiency (eta = 6.4 +/- 0.2%) and short circuit current density (J(SC) = 14.2 +/- 0.6 mA cm(-2)) when subjected to a 1000 h accelerated aging test under 1 Sun illumination at 35 degrees C, followed by additional 1154 hours under 0.5 Sun at 60 degrees C. These results overcome one of the main hurdles in realizing fully printed DSSCs and open opportunities for entirely new DSSC designs.