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Publisher Copyright: © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH Degradation of dye solar cell performance based on the early changes in electrolyte color is predicted, allowing to estimate the lifetime of the dye solar cells even before their efficiency declines. Previous predictive models commonly rely on regression analysis of the predicted parameter; thus, they are unable to capture degradation before a significant decrease in performance. Degradation tests, even when accelerated, may take thousands of hours. As such, recognizing degradation trends early can lead to rewarding cuts in the duration of solar cell development pipelines. With accurate lifetime predictions, researchers can steer materials research to reach longer lifetimes in shorter cycles. The predictive power of our model relies on color changes in the electrolyte that directly correlate with the concentration of tri-iodide charge carriers within it, the loss of which is the predominant degradation mechanism for most liquid-electrolyte dye solar cells. By linking the physical mechanisms inside the cell, which eventually start to degrade the performance of dye solar cells, an early prediction of the lifetime can be made even when the device performance still appears stable. It is exemplified with dye solar cells that integrating architecture-specific knowledge on degradation mechanisms has potential to improve lifetime predictions for photovoltaics.

Funding Information: K.A. is grateful for Jane and Aatos Erkko foundation project ASPIRE for funding the study. G.G.S. J.K. and K.M. thank Academy of Finland (projects BioEST and Substainable, and flagship project FinnCERES). X.H. thanks for the scholarship by the Chinese Science Council, No. 201706250038. Solar cell encapsulation literature is reviewed broadly in this paper. Commercial solar cells, such as silicon and thin film solar cells, are typically encapsulated with ethylene vinyl acetate polymer (EVA) layer and rigid layers (usually glass) and edge sealants. In our paper, we cover the encapsulation materials and methods of some emerging solar cell types, that is, those of the organic solar cells, the dye-sensitized solar cells and the perovskite solar cells, and we focus on the latter of the three as the newest contender in the solar cell arena. The PSC encapsulation literature is summarized in a comprehensive table we hope the reader may use as a “handbook” when designing encapsulation and long-term stability experiments. Some additional functionalities included in encapsulants are also discussed in our paper, e.g. improving the encapsulants’ optical properties and manufacturing them sustainably from biobased materials.

Present practice to avoid harmful effects of UV light on dye solar cells (DSC) is to use a UV filter. However, we show here that a standard 400 nm UV cutoff filter offers inadequate protection from UV-induced degradation. DSCs that were exposed to only visible light by LED lamps maintained 100% of their initial efficiency after 3000 hours of exposure, whereas the efficiency of DSCs subjected to full light spectrum (Xenon arc lamp) with an efficient UV filter dropped down to 10% of their initial performance already after 1500 hours. Optical analysis of the UV filter confirmed that the amount of light transmitted below 400 nm was negligible. These observations indicate that (a) DSCs can be very sensitive to even minor amount of UV and (b) eliminating the effects of UV light on DSC stability cannot easily be avoided by a UV filter on top of the cell. A detailed analysis of the degradation mechanisms revealed that the culprit to loss of performance was accelerated loss of charge carriers in the electrolyte of theDSCs-a typical symptom of UV exposure. These results suggest that commonly used stability tests under LED illumination are insufficient in predicting the lifetime of DSCs in outdoor conditions. Instead, for such purpose, we recommend solar cell stability to be tested with a full light spectrum and with a suitable UV filter.

This review addresses the reconstruction of structural plant components (cellulose, lignin, and hemicelluloses) into materials displaying advanced optical properties. The strategies to isolate the main building blocks are discussed, and the effects of fibrillation, fibril alignment, densification, self-assembly, surface-patterning, and compositing are presented considering their role in engineering optical performance. Then, key elements that enable lignocellulosic to be translated into materials that present optical functionality, such as transparency, haze, reflectance, UV-blocking, luminescence, and structural colors, are described. Mapping the optical landscape that is accessible from lignocellulosics is shown as an essential step toward their utilization in smart devices. Advanced materials built from sustainable resources, including those obtained from industrial or agricultural side streams, demonstrate enormous promise in optoelectronics due to their potentially lower cost, while meeting or even exceeding current demands in performance. The requirements are summarized for the production and application of plant-based optically functional materials in different smart material applications and the review is concluded with a perspective about this active field of knowledge.

The fabrication, thickness, and structure of aerogel films composed of covalently cross-linked cellulose nanocrystals (CNCs) and poly(oligoethylene glycol methacrylate) (POEGMA) were optimized for use as electrolyte absorbers in dye-sensitized solar cells (DSSCs). The aerogel films were cast directly on transparent conducting counter electrode substrates (glass and flexible poly(ethylene terephthalate) plastic) and then used to absorb drop-cast liquid electrolyte, thus providing an alternative method of filling electrolyte in DSSCs. This approach eliminates the use of electrolyte-filling holes, which are a typical pathway of electrolyte leakage, and furthermore enables a homogeneous distribution of electrolyte components within the photoelectrode. Unlike typical in situ electrolyte gelation approaches, the phase inversion method used here results in a highly porous (>99%) electrolyte scaffold with excellent ionic conductivity and interfacial properties. DSSCs prepared with CNC-POEGMA aerogels reached similar power conversion efficiencies as compared to liquid electrolyte devices, indicating that the aerogel does not interfere with the operation of the device. These aerogels retain their structural integrity upon bending, which is critical for their application in flexible devices. Furthermore, the aerogels demonstrate impressive chemical and mechanical stability in typical electrolyte solvents because of their stable covalent cross-linking. Overall, this work demonstrates that the DSSC fabrication process can be simplified and made more easily upscalable by taking advantage of CNCs, being an abundant and sustainable bio-based material.

We explore biocarbon as a low-cost, abundant, and environmentally friendly replacement for Pt in dye solar cells. We introduce a novel biochar based on brewery residues with good performance and stability potential as a counter electrode in complete dye solar cells, and present the first long-term stability test results of a biocarbon in complete dye solar cells. The hydrothermally carbonized and KOH-activated brewer's spent grain (BSG) offers an extremely high surface area for catalytic reactions (2190 m2/g). Counter electrodes based on this material provide a promising initial performance (efficiency of 3.6 ± 0.2% for biocarbon solar cells compared to 5.3 ± 0.2% for reference cells with Pt catalyst) with current production and the total resistance of solar cells very close to that of Pt based solar cells. In an extended accelerated aging test, the best biocarbon dye solar cell maintained over 86% of its initial efficiency for 3000 h. Moreover, the biocarbon reduced the degradation via loss of electrolyte charge carriers during aging. Based on these results, the activated BSG biocarbon provides a promising alternative for Pt catalysts.

We found that the electrical state in which dye solar cells operate affect their ageing. Three states were analysed: open-circuit (OC), short circuit (SC), and under maximum power point (MPP) / load. OC and SC are more or less atypical states, which are relevant while storing cells or in the event of malfunction, whereas the MPP/load corresponds to real life operation of the cells. Our results indicate that keeping the cells at OC or near the MPP lead to practically identical stability, whilst the cells at SC degraded much faster in a 1000 h light soaking test. The underlying cause for the degradation of all the cells was the loss of tri-iodide (i.e. limiting charge carriers) in the electrolyte. While the degradation mechanism appears to be the same, the loss rate of tri-iodide was about five times faster with SC than with OC and MPP cells. In the SC cells, the loss of tri-iodide decreased both the short-circuit current and fill factor resulting in a 36% efficiency loss by the end of the test. In contrast, the efficiency of the OC and load cells remained quite stable throughout the test. Since OC is the most commonly used state in aging tests, it is good news for ageing studies that the real life MPP state and the OC state yield roughly similar results. The identification of the degradation pathway, the loss of charge carriers, and the related degradation rate were used to estimate the remaining lifetime of cells which did not degrade during the 1000 h test. Based on the degradation rate related to the charge carrier loss, full degradation of the OC and MPP cells is expected in approximately 3000 h of operational time.

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 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.

Funding Information: We thank Glen Forde from Aalto Energy Platform for his design work for Figs. 2 and 6. Kati Miettunen thanks The Academy of Finland (project BioEST, 336577). Publisher Copyright: © 2021 Recycling is rarely considered in the field of dye solar cells. However, recycling should be a critical part of holistic eco-design, which considers the efficiency, lifetime, return of energy investment, safety, and availability of materials. The novelty and focus of this work is recycling, and this is the first contribution systematically analyzing how different material choices and their combinations affect the recycling of dye solar cells. By understanding the recycling processes and how the recycling of materials is interlinked in a multicomponent system, it is possible to eco-design systems and guide future research toward selecting materials that support sustainability and enable economically motivated recycling. Economic incentive is the biggest factor determining whether or not recycling will take place. With eco-design, it is possible to avoid future problems, such as trapping rare and expensive critical metals in waste from which they are difficult or even impossible to recover. In fact, the conventional dye solar cells create harmful waste with no economically profitable way of recycling. Interestingly, many of the alternative materials that enable recycling have not been originally designed for that purpose, and it is rarely obvious how the combination of different materials affects recycling. For instance, using thin flexible substrates, which have been developed for roll-to-roll manufacturing, supports the retrieval of Ag, and using high performance Co- or Cu-based electrolytes instead of iodine electrolyte eliminates toxic gas problems in pyrometallurgical recycling processes.

The long-term stability of a dye-sensitized solar cell (DSSC) is a key issue for upscaling and commercialization of this technology. It is well-known that gel electrolytes can improve the long-term stability and allow easy DSSC manufacturing. However, there is limited knowledge on the long-term stability of cobalt-based gel electrolytes and also how this stability is affected when applying different dye sensitizers. Moreover, long-term stability studies have been done with no, or an imperfect, sealing. In this work we investigated the performance and the stability of cobalt-based polymer gel electrolytes using devices properly sealed. Here, two different dyes, an organic and a ruthenium dye, were selected to investigate the device’s performance. The cobalt liquid electrolyte was gelled with a PEO-based terpolymer (PEO-EM-AGE) and compared to its liquid counterpart. After 1000 h, the efficiencies of the liquid- and gel-based solar cells with the ruthenium dye were statistically similar to each other. On th...

Organic-inorganic layered materials, such as organotalcs, are a promising alternative as gelling agent for liquid electrolytes in dye-sensitized solar cells. Talcs could provide an abundant, low cost and environmentally friendly option for solidifying the electrolyte. This work focuses on generation 5 polyamideamino dendron-modified talc with emphasis is on how it affects the performance and stability of the DSSC. The talc was shown to improve the initial photocurrent by up to 39% by acting as a light scatterer and/or a recombination barrier compared to reference solar cells with liquid electrolyte. Non-destructive analysis based on photographic image technique revealed that during the aging the additive absorbed charge carriers, tri-iodide, from the electrolyte reducing the performance of the solar cells. The degradation could, however, be prevented by intercalating polyiodides into interlamellar space of the talc as the resulting dendron chains did not absorb tri-iodide charge carriers from the electrolyte. These quasi-solid solar cells maintained 95% of their initial efficiency under light-soaking at 1 Sun for about 1000 h. The cells with a quasi-solid electrolyte showed up to 5% higher efficiency than those with liquid electrolyte.

Electrospun nanofibers obtained from cellulose acetate before (CA) and after (DCA) deacetylation were used as electrolyte membranes in dye-sensitized solar cells. As holders of the active components of the device and compared to the reference system, the CA and DCA membranes increased the average device efficiency by as much as 14%. The membranes enhanced the charge transfer at the counter electrode (assessed by the Ohmic and charge transfer resistance and corresponding Helmholtz capacitance). Simultaneously, the photoelectrode did not interfere with the performance as measured by the short-circuit current density, open circuit voltage, fill factor and conversion efficiency. Long-term stability tests (light soaking) showed that the CA- and DCA-based solar cells sustain operation for at least 500 h. For long term use and/or to serve as a scaffold for other purposes, DCA performs better than CA. The proposed active electrolyte membranes are expected to open the way toward rapid and continuous assembly of dye sensitize solar cells using cellulose esters. Graphical abstract: [Figure not available: see fulltext.].

Biobased cryogel membranes were applied as electrolyte holders in dye solar cells (DSC) while facilitating carrier transport during operation. They also improved device performance and stability. For this purpose, cellulose nanofibers (CNF), TEMPO-oxidized CNF (TOCNF), bacterial cellulose (BC), and chitin nanofibers (ChNF) were investigated. The proposed materials and protocols for incorporating the electrolyte, via simple casting, avoided the typical problems associated with injection of the electrolyte through filling holes, a major difficulty especially in manufacturing large area cells. Owing to the fact that cryogel membranes did not require any orifice for injection, they were effective in minimizing leakage and in retaining liquid electrolyte. The results indicated the reduction of performance losses compared to conventional electrolyte filling, likely due to the better spatial distribution of electrolyte. DSCs based on BC cryogels had an initially higher performance and similar stability compared to those of the reference cells. When compared to reference cells, CNF and ChNF cryogels produced higher initial performance, but they underwent a faster degradation. The difference in stability was attributed to the effect of residual components, including lignin in CNF and proteins in ChNF, as demonstrated in bleaching experiments. TOCNF indicated a relatively poor performance, most likely because of residual aldehydes. Overall, we offer a comprehensive evaluation based on current-voltage (IV) profiles under simulated sunlight, incident photon-to-charge carrier efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and color image processing, together with accelerated DSC stability tests, to unveil the effects of new membrane-based assembly. Our results give guidelines for future developments related in particular to the effects of the tested biomaterials on device stability.

The iridescence displayed by films made from cellulose nanocrystals (CNCs) has long been the subject of fundamental research. This has expanded our understanding of colloidal self-assembly towards the development of advanced materials. However, the application of such findings is less reported for visual designs that exploit structural color. Aesthetic outputs are already in reach, but requires input from trend setters in the design and art industries. In this realm, the CNC-based iridescence uniquely offers broadband, multi-colored reflections through the “coffee ring” effect, which arises upon evaporation-induced self-assembly (EISA). Although this effect has been thoroughly studied in the context of axisymmetric patterns, complex geometries remain to be evaluated for large-scale implementation. This is central to the present efforts, where EISA of CNC suspensions occurred onto non-circular surfaces. We used orientation-dependent contact angle measurements, profilometry and fixed-light source photography tounveil the effect of asymmetric drying fluxes at sharp angles, between 30° and 90°, on CNC particle deposition and resulting color patterns. We also demonstrate the causality between increased capillary fluxes and deposition with the help of modelling via energy minimization of the suspension volume onto a given surface and using the diffusion equation to obtain the local concentration of water vapor during EISA. Lastly, we study the effect of background reflections as well as light and temperature resistance of CNC-based reflectors, both important for any deployment. The results from this multidisciplinary effort, involving applied design, art and colloid chemistry, point to the excellent prospects of CNC films for the development of structured and chromatic patterns. Graphical abstract: [Figure not available: see fulltext.].

A sequence of generations of polyamidoamine dendron modified-talc, PAMAM-talc-Gn (n = 1, 3, 5 and 7), is proposed as additive in a composite gel electrolyte for dye-sensitized solar cells. Polyiodides are intercalated into the organotalc interlamellar space by adsorption of iodine vapor, producing triiodide and polyiodides. We investigate the effect of organotalc content on the charge transport in the electrolyte and solar cell performance and optimize the organotalc content. Without the previous adsorption of iodine molecules, the organotalcs appear to remove iodine from the electrolyte solution decreasing device’s performance significantly. Instead, the samples with additional iodide had higher J sc and efficiency approaching the values of the reference cells containing liquid, which suggests that this kind of gelling method would be suitable for dye solar cells. Charge transport in the gel electrolyte is investigated with electrochemical impedance spectroscopy and cyclic voltammetry analyses using symmetrical CE-CE electrochemical cells.

Here a 2000-h light-soaking stability test is reported for cobalt complex based dye solar cells using Y123 organic dye with either platinum or poly(3,4-ethylene dioxythiophene) as counter electrode catalyst, and two different Co(III) electrolyte concentrations. Both the Co(III) concentration and the counter electrode catalyst affected the device stability. Higher Co(III) concentrations were found to slow down the degradation process with both catalyst materials. Different degradation behavior was also identified for devices with different catalyst materials.

Here, we report on the behavior of dye solar cells in real-life weather conditions from a northern outdoor test covering for the first time cell performance measurements in harsh conditions with varying weather from mildly warm conditions to freezing and snowy. The effect of different weather conditions on the cell performance is quantitatively investigated by using correlations coefficients of weather parameters to cell performance. No degradation was observed during the frosty period, but instead during the warmer, rainy periods with high moisture levels. Nevertheless, after 6 weeks of outdoor testing in varying harsh conditions, the cells maintained on average 88% of their initial efficiency. Tracking the cell performance during the aging showed that the test cells generated roughly as much current at subzero temperatures as at warmer temperatures. Investigations of the degradation reactions revealed that while photoelectrode degradation was the main cause of degradation during this test, the loss of charge carriers, which had only a minor effect on performance during the test, would likely become a major degradation factor during the next 1000 h of testing. Furthermore, the test showed that the cells even doubled their efficiency in low light intensity conditions compared with the standard reporting conditions. Thus, the overall conversion efficiency during the whole experiment reached up to 50% higher values compared with the results in standard testing conditions.

In this paper, a review of applying dye-sensitized (DSCs) and perovskite solar cells (PSCs) 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 DSCs and PSCs are still a niche technology, but have an inherent potential for cheap roll-to-roll mass production of photovoltaics.

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.

Organic–inorganic hybrid layered materials are proposed as additives in a quasi-solid gel electrolyte for dye-sensitized solar cells. Talcs could provide a low-cost and environmentally friendly, as well as abundant, option as gelators. Here, talcs were prepared by functionalizing an organotalc with three polyamidopyridine dendron generations, PAMPy-talc-Gn (n = 1, 2 and 3). PAMPy dendrons grow parallel to the lamellae plane and form an organized structure by intermolecular interactions. In addition, polyiodide–dendron charge-transfer complexes were prepared onto the organotalc by adsorption of iodine. In this work, the effect of the dendron generation of PAMPy-talc and the influence of polyiodide intercalation on solar cell performance and stability were investigated. The best results were reached with the use of lowest-generation PAMPy-talc (η = 4.5 ± 0.3%, VOC = 710 ± 19 mV, Jsc = 10.4 ± 0.9 mA cm–2, and FF = 61 ± 2%): 15% higher efficiency compared to similar liquid devices. While some previously studi...

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.

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 effect of electrolyte filling method on the performance of the dye-sensitized solar cells is investigated with the segmented cell method, a recent technique which is very simple but effective as it can be used to examine all the photovoltaic characteristics. The electrolyte filling techniques compared were single injection, which is typically used in small laboratory cells, and pumping the electrolyte through the cell several times, which is often used for larger cells and modules. Significant photovoltage and photocurrent variations occur with the repeated pumping of the electrolyte in the cell preparation. Transient and charge extraction measurements confirmed that the differences in open circuit voltage were due to the shifts of the TiO 2 conduction band and time correlated single photon counting confirmed that the reduction of short circuit current was largely due to reduced electron injection correlated with the increasing conduction band edge in the studied cases. This was interpreted as an effect of molecular filtering by the TiO 2 causing an accumulation of electrolyte additives (4- tert -butylpyridine and benzimidazole) near the electrolyte filling hole, the concentration of which increased with repeated pumping of the electrolyte. Interestingly, spatial variations were seen not only in the relative TiO 2 conduction band energy but also in the density of trap states. In this contribution it is demonstrated how the changes in the conduction band can be separated from the changes in the density of trap states which is an essential for the correct interpretation of the data.

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.

The characteristics of different types of catalyst layers based on Pt, polymer and carbon for flexible dye sensitized solar cells (DSSC) are investigated. These counter electrodes prepared at low temperature on indium doped tin oxide polyethyleneterephtalate (ITO-PET) plastic are compared with high temperature treated platinum and carbon catalyst layers on fluorine doped tin oxide (FTO) coated glass substrates. Here the electrical characteristics of the counter electrode and their optical performance are evaluated in order to compare their suitability for direct and reverse illumination. The morphology of the catalyst layers to the substrate is also investigated. Among the studied materials, the chemically platinized and carbon gel based counter electrodes demonstrated the

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.

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.

Carbon gel catalyst layers were used in dye solar cells. These layers were prepared on flexible plastic substrates at low temperatures (130 °C). The carbon gel, demonstrated excellent flexibility which is an important feature for roll-to-roll production and special applications of dye solar cells. The use of these low cost and highly flexible catalyst layers resulted in good photovoltaic performance; only 10% lower than dye solar cells with rigid glass-based counter electrodes prepared with thermal platinization at ∼400 °C temperature.

By deliberately introducing the thermally degraded form of the dye solar cell sensitizer N719 in dye-sensitized solar cells (DSCs) using synthetically prepared N719-TBP ([Ru(L-H)(2)(NCS)(4-tert-butylpyridine)](-+)N-(Bu)(4)), we have investigated the devastating influence of this ligand substitution product (N719-TBP) on the performance parameters of the cells. Two types of dyed solar cells, based on either N719 or N719-TBP, have been characterized employing standard current-voltage (I-V) performance test, UV-vis optical spectroscopy, incident photon to current efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) methods. The performance tests show a drastic efficiency reduction of similar to 50% in the N719-TBP containing cells as compared to N719-dyed cells. The lower performance of N719-TBP was caused by lower overall light harvesting efficiency due to ca. 30 nm blue shift in the absorption spectrum of the dye, ca. 50% shorter electron diffusion length due to lower electron recombination resistance, and ca. 14% lower charge separation efficiency, which most likely can be ascribed to decreased dye regeneration efficiency caused by the replacement of one NCS ligand with TBP in the substitution product. The observations made in this study of DSC cells dyed with the substitution product, representing a worst case scenario of cells with 100% degraded dye, are in agreement with the characteristics of N719-dyed solar cells degraded at 85 degrees C, where the effect of ligand substitution is somewhat less pronounced.

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.

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 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...

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.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1513-1518, The effects of dye purification on the initial performance and lifetime of dye-sensitized solar cells are studied. The use of UV filters to prevent electrolyte bleaching (the loss of tri-iodide in the electrolyte) during light soaking tests is also investigated. The initial performance and stability were characterized with IV tests, electrical impedance spectroscopy and cell photographing. Statistical methods were applied in the result analysis to increase the reliability of the results. Dye purification did not have statistically significant effect neither on the cell performance nor lifetime in this study. UV filtering did reduce electrolyte bleaching but did not completely prevent it.

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.

Cheap ferritic stainless steel is applied here as the counter electrode substrate in dye sensitized solar cells with cobalt complex electrolyte. A 5.0% efficiency was reached with these type of cells which is more than 2.5 times higher compared to previously reported devices with metal counter electrode and cobalt complex electrolyte. The electrochemical impedance spectra analysis showed that the best cells with the ferritic steel counter electrode had as low charge transfer resistance (3.6 Ωcm2) as the reference glass cells with the same electrolyte. While in previous studies many metals have corroded in the cobalt complex electrolyte, the stability analysis including scanning electron microscope imaging of the aged electrodes suggested that the ferritic stainless steel substrates did not corrode in the electrolyte. Hence ferritic stainless steel appears as a possible alternative counter electrode in dye solar cells with cobalt electrolyte in terms of cost, performance and stability.

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.