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Theoretical modeling of the incidence angle dependence is important for the performance evaluation of solar cells especially when they employ structural photonics that enhance their aesthetics for building-integrated photovoltaic applications. We present an optical model based on transfer matrix method, which is tailored for the analysis of the angular optical characteristics of dye-sensitized solar cells (DSSC) that incorporate a porous one-dimensional photonic-crystal (1DPC) back reflector. We provide complete mathematical description of the angular optics and electromagnetic energetics of DSSC, including solutions that avoid numerical instabilities in the case of thick, strongly light-absorbing layers. The model showed excellent quantitative agreement with angle-dependent spectral transmittance measurements recorded from complete DSSCs, allowing detailed analysis of the light harvesting enhancement by the dye provided by back-reflection of light from the 1DPC. Determining the ratio of the device transmittance measured with and without a 1DPC is proposed as a simple and practical way o determine the internal reflectance of the 1DPC embedded in the device structure. The reported model provides an accurate description of the coherent angular optics of DSSCs, applicable for situations where light scattering in the device structure can be neglected.

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.

Colour can improve the visual aesthetics of solar cells for building and product integration but constitutes an optical loss from the perspective of photovoltaic energy conversion. To quantify this compromise, we report the theoretical efficiency limits of ideal coloured opaque single-band-gap solar cells. The colours were optimized by allowing the solar cells to reflect light within two distinct wavelength bands in the visible region. Colour chromaticity and brightness were controlled by modifying the position and width of the reflection bands, while optimising the band-gap energy for each colour. We found that almost the entire sRGB colour space has an efficiency limit greater than 29%, when relative luminosity is less than 0.25. This corresponds to a relative performance loss of less than 14% compared to an ideal black solar cell. Yellow-green is the most efficient photovoltaic colour, whereas highly saturated blue, red and purple colours produce the lowest efficiencies, when compared at equal brightness. The colour-dependence is explained by the photopic sensitivity of the human eye, which peaks at yellow-green wavelengths and tails towards the blue and red ends of the visible spectrum. For most colours, except the darkest ones, the optimal band-gap energy for a theoretically ideal solar cell is between 1.115 eV and 1.135 eV, matching the value for crystalline silicon. The results clarify the link between the solar cell colour and efficiency and establish a reference point and guidelines for optimizing coloured photovoltaics.

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.

Over the years numerous mixes of chemical compounds have been tried in the electrolytes of dye-sensitized solar cells in efforts to improve their efficiency. How these chemicals interact with each other and the photoelectrode has received surprisingly little attention. Here we report results from a systematic study of two I–/I3– electrolytes and their additives using infrared and Raman spectroscopy together with quantum chemical calculations. In the LiI electrolyte competing interactions between lithium cation and the solvent MPN and the additives TBP, NMBI, and GuSCN were identified. These interactions could inhibit the interaction of lithium ions with the TiO2 surface. It was found that under Raman excitation of PMII solution in contact with the photoelectrode, efficient generation of I3– takes place. For LiI solution, in addition, a Dye–I2 complex is formed. The results could be explained by diffusion-limited buildup of I3– and depletion of I– concentrations in the focal area of the excitation beam and by reduction of I3– via conduction band electrons of TiO2 beyond the focal region. To explain the formation of Dye–I2 complexes in the LiI electrolyte solutions a multistep regeneration mechanism is proposed. It was found that GuSCN reduces the I3– concentration in the electrolyte solutions studied; in the LiI electrolyte in addition it binds to lithium ions and nearly depletes the Dye–I2 complexes. From infrared spectra it became clear that preventing water from entering the DSCs during the preparation stages in ambient air is a demanding task. The identified interactions paint an intriguing new photochemical landscape of the function of the dye-sensitized solar cells giving guidelines for further development of the devices.

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

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

3rd Generation photovoltaic systems such as dye-sensitized solar cells offer almost limitless material base that can be tested in variety of combinations with unique cell designs. One of the key challenges that remains with the dye-sensitized solar cells is to reduce their fabrication cost without compromising the performance. Such a target can only be achieved by fabricating inexpensive and abundantly available materials with simple and rapid preparatory methods that could deliver robust characteristics within the cell. With this motivation, we report here successful implementation of a highly conductive flexible fiber based substrate in dye-sensitized solar cell which is fabricated by coating a low temperature single walled carbon nanotube ink over a laminated fiber sheet. The substrate exhibited extraordinary durability under severe mechanical stability test conditions, reasonable overall efficiency (6.0±0.3%) and charge transfer resistance (3.3±1.6 Ω cm 2 ).

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.

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

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.

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.

The recent surge in investigating electrocatalysts for the H2 evolution reaction is based on finding a cheap alternative to Pt. However platinum’s excellent catalytic activity means very little catalyst needs to be used. The present study combines model experiments with numerical modeling to determine exactly how little catalyst is needed. Specifically we investigate ultra-low Pt loadings for use in photoelectrochemical H2 evolution using TiO2–Ti-pn+Si photocathodes. At a current density of 10 mA cm-2, we photocathodically evolve H2 at +465, +450, +350 and +270 mV vs., RHE at Pt loadings of 1000, 200, 50, and 10 ng cm-2 corresponding to HER overpotentials of η1000ng = 32 mV, η200ng = 46mV, η50ng = 142 mV, and η10ng = 231 mV. To put this in perspective, if 30% of the world’s current annual Pt production was used for H2 evolution catalysis, using a loading of 100 ng cm-2 and a current of 10 mA cm-2 would produce 1 TWaverage of H2. The photoelectrochemical data matched the modeling calculations implying that we were near the fundamental maximum in performance for our system. Furthermore modeling indicated that the overpotentials were dominated by mass transfer effects, rather than catalysis unless catalyst loadings were less than 1000 ng cm-2.

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.

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.

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).

It is demonstrated that the molecular filtering effect of TiO2 has a significant influence on dye solar cell (DSC) performance. As electrolyte is injected to a DSC, some of the electrolyte components adsorb to the surface TiO2 (here 4-tert-butylpyridine and 1-methyl-benzimidazole) and accumulate near the electrolyte filling hole resulting in varying electrolyte composition and performance across the cell. The spatial performance distribution was investigated with a new method, the segment cell method. Not only is the segmented cell method simple and cheap when compared to the only other method for examining spatial variation (photocurrent mapping), it also has the major advantage of allowing the spatial variation in all other operating parameters to be assessed. Here the molecular filtering effect was to influence the cell performance in case of all the five studied electrolytes causing up to 35% losses in efficiency. Raman spectra indicated that the loss in photocurrent in the electrolyte filling was in correlation with the loss of thiocyanate ligands suggesting that dye regeneration may also be a significant factor in addition to electron injection in some of the cells. There were also shifts in the absorption spectra the photoelectrodes which further supported changes in the thiocyanate ligands. Besides absorption changes, there were additional shifts in the IPCE spectra which may relate to deprotonation of the dye. The efficiency losses were reduced to ∼10% with contemporary electrolyte compositions.

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 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.

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.