Degradation of organic solar cells due to air exposure

We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.     

Stability/degradation of polymer solar cells

Polymer and organic solar cells degrade during illumination and in the dark. This is in contrast to photovoltaics based on inorganic semiconductors such as silicon. Long operational lifetimes of solar cell devices are required in real-life application and the understanding and alleviation of the degradation phenomena are a prerequisite for successful application of this new and promising technology. In this review, the current understanding of stability/degradation in organic and polymer solar cell devices is presented and the methods for studying and elucidating degradation are discussed. Methods for enhancing the stability through the choice of better active materials, encapsulation, application of getter materials and UV-filters are also discussed.     

Degradation mechanism of organic solar cells with aluminum cathode

Polymer-based solar cells with aluminum (Al) cathode often suffer from degradation in air. Here the study focuses on the degradation mechanism at the interface between Al and organic active layer. By performing interface modification combined with electrical and chemical characterization, it is demonstrated that the rapid degradation originates from the formation of a charge blocking layer between evaporated Al cathode and organic active layer. Insertion of a thin interfacial layer of thermally evaporated CrO_x between organic active layer and Al cathode can greatly improve the device stability. It is found that the CrO_x interfacial layer functions as a protective layer by stopping or minimizing penetration of thermally evaporated Al into the active layer to form a diffused organic-Al interface, which will then result in a large oxidized interfacial area upon air exposure.     

Photo- and thermal degradation of MDMO-PPV:PCBM blends

This paper is devoted to the study of the photo- and thermal-ageing of MDMO-poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (PPV) blended with methano-fullerene[6,6]-phenyl C₆₁-butyric acid methyl ester ([60] PCBM), a commonly used active layer of organic solar cells. Thin films of MDMO-PPV, PCBM and MDMO-PPV:PCBM were submitted to photo- and thermal ageing in presence or in absence of oxygen. The modifications of the chemical structure of the materials were analysed by UV-visible and IR spectroscopies. Studying the degradation of pristine materials (MDMO-PPV and PCBM) permit the identification of the markers for characterizing the degradation of each component of the active layer. The degradation of MDMO-PPV was shown to involve a radicalar oxidation implying at first the ethers functions and secondly the double bonds; the photo-oxidation of PCBM was shown to involve the oxidation of its C₆₀ moiety. Photo-oxidation of MDMO-PPV:PCBM blends involved the degradation of both components and the presence of PCBM was observed to largely reduce the rate of degradation of MDMO-PPV in comparison with pristine MDMO-PPV. On a kinetic point of view, the same conclusion was obtained for thermo-oxidation and photolysis (irradiation in absence of oxygen). Our experimental results suggest that the decrease of the rate of degradation of MDMO-PPV in blended films during the three kind of ageing (hυ, T, O₂) is mainly related to the radical scavenging properties of the PCBM.     

Evidence of the improvement of photovoltaic efficiency by polar molecule orientation in a new semiconducting polymer

Storage of an internal field in a polymeric semiconductor device should be of great interest for applications like photovoltaic solar cells to facilitate exciton dissociation and improve charge transport in the structure. Orientation of polar molecules, contained inside a polymer binder, induces a rectifying effect, behaving as a distributed homojunction within a single polymeric film. To investigate this concept, a new poly(p-phenylenevinylene) (PPV) derivative bearing push–pull like molecules was purposefully designed and synthesized. Effect of polar molecules’ orientation on carrier injection and transport properties was studied. In the test systems, we demonstrate an increase of the external quantum efficiency upon orientation.     

Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation

A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO₂ dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO₂ loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO₂ loading, 6 g l⁻¹; circulation flow rate, 4.9 l min⁻¹; and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents.     

Progress in stability of organic solar cells exposed to air

In this paper, the stability of small-molecule organic solar cells based on copper phthalocyanine (CuPc) and fullerene (C₆₀) is investigated. The use of silver instead of aluminum as the metal electrode in these solar cells, with smaller grain size and grain boundaries as well as with more uniform grain size distribution in the film, results in significant improvement in the lifetime of the devices. The substantial role of silver in the protection of the cells against permeation of oxygen and/or water molecules into the organic thin films is confirmed. Substitution of a thin buffer layer (70 Å) of bathophenanthroline (Bphen) for bathocuproine (BCP), sandwiched between C₆₀ and the cathode, makes considerable progress in the lifetime of the device.     

The significance of fullerene electron acceptors in organic solar cell photo-oxidation

Stability of organic solar cells requires development before their commercialisation is possible. This review will give a brief overview of organic solar cells and their stability, before focussing on the photochemical stability of the active layer. The photo-oxidation of the donor polymers will be looked at first which has been studied quite extensively and then fullerene electron acceptors, such as widely used phenyl-C61-butyric acid methyl ester, which has been considerably less studied. It has been shown that oxidation of the fullerene cage on phenyl-C61-butyric acid methyl ester results in oxides with a deeper lowest unoccupied molecular orbital (LUMO) level than the fresh electron acceptor. These oxides act as electron traps, leading to deterioration of the blend photoconductivity. The significance of fullerene photo-oxidation on device stability has been indirectly shown via research on: photoconductivity; organic solar cells made with an oxidised fullerene derivative and organic field effect transistors. Techniques that could be developed to increase photochemical stability of fullerene electron acceptor resistance to photo-oxidation include: reducing its LUMO level; increasing its crystallinity or aggregation and changing its chemical structure. Improving the photochemical stability of organic solar cells would move us one step closer to a more accessible solar power.     

Photovoltaic and photophysical properties of a novel bis-3-hexylthiophene substituted quinoxaline derivative

We report on the photophysical properties and photovoltaic performance of a polythiophene derivative, poly-2,3-bis(4-tert-butylphenyl)-5,8-bis(4-hexylthiophen-2-yl)quinoxaline (PHTQ) as an electron donor in bulk heterojunction solar cells blended with the acceptor 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM). Devices were composed of PHTQ and varying amounts of PCBM (1:1, 1:2, 1:3, 1:4 w-w ratio). The components were spin cast from ortho-dichlorobenzene (ODCB) and characterized by measuring current–voltage characteristics under simulated AM 1.5 conditions. Efficiencies up to 0.3% have been reached. Incident photon to current efficiency (IPCE) is reported and the nanoscale morphology was investigated with atomic force microscopy (AFM). Photoinduced absorption spectroscopy confirms the photoinduced charge transfer in such donor acceptor blends.     

Abnormally high photocurrent of a degraded organic solar cell under chopped light

Photocurrent of an organic solar cell under chopped monochromatic light was studied at different stages, i.e. just after fabrication (without degradation) and after serious degradation. A large photocurrent under chopped light was observed for a seriously degraded device, unexpected from intuitionistic reckoning. This work demonstrates that both light bias and chopper frequency affect the photocurrent measurement of an organic solar cell, especially for a degraded device. The influence of light bias and chopper frequency is explained as the result of traps produced during the degradation of an organic solar cell. Thus measuring photocurrent under one sun light bias at low chopper frequency is suggested to more accurately determine the external quantum efficiency.     

Bulk heterojunction organic photovoltaic based on polythiophene–polyelectrolyte carbon nanotube composites

It is shown that carbon nanotubes can be used to enhance carrier mobility for efficient removal of the charges in thin film polymer-conjugated/fullerene photovoltaic devices. The fabricated photovoltaic devices consist of poly(3-octylthiophene) (P3OT) polymer blended with undoped multiwalled carbon nanotubes (MWNTs) and carbon nanotubes doped with nitrogen (CNx-MWNTs). Nanophase formation and dispersion problems associated with the use of carbon nanotubes in polymer devices were addressed through the generation of functional groups and electrostatic attaching of the polyelectrolyte poly(dimethyldiallylamine) chloride (PDDA) in both MWNTs and CNx-MWNT systems. The resultant nanophase was highly dispersed allowing for excellent bulk heterojunction formation. Our results indicate that CNx-MWNTs enhance the efficiency of P3OT solar cells in comparison with MWNTs.     

Efficiency enhancement of solar cells by luminescent up-conversion of sunlight

Significant improvements in the efficiency of solar cells by combination with luminescent up- or down-converters have recently been predicted theoretically. Here, we extend the theoretical analysis of the limiting efficiency of the up-conversion (UC)-system to realistic Airmass spectra and analyse the spectral robustness of the UC-system. We also present initial experimental results from prototypes involving bifacial silicon solar cells with UC-phosphors attached to the rear surface, and discuss the possibility of realizing efficient UC with low-band-gap solar cells in combination with a light emitting diode.     

Photo-degradation and stabilization effects in operating organic photovoltaic devices by joint photo-current and morphological monitoring

We report on a joint morphological/photoelectrical study of polymer-based photovoltaic (PV) cells in working conditions. The bulk heterojunction devices investigated are based on an active layer of poly(3-hexyl thiophene) blended with methano-fullerene, combining good PV performances with promising stability. The set-up adopted allowed the electrical properties of the device to be directly correlated to the modification of the electrode morphological parameters (thickness and roughness), which were obtained by in situ energy dispersive X-ray reflectometry (EDXR). The results of this joint time-dependent characterization demonstrated how the observed photo-induced oxidation process, limited to the buried electrode interface, is responsible for a fast decrease in the photo-current. The time-resolved measurements allowed to rule out the dynamics of the morphological changes and showed that the interface morphology may be stabilized by annealing treatments, with a significant improvement of the cell efficiency.     

Making concentrated solar power competitive with coal: The costs of a European feed-in tariff

The European Union has yet to determine how exactly to reach its greenhouse gas emissions targets for the future. One potential answer involves large-scale development of concentrated solar power (CSP) in the North African region, transmitting the power to Europe. CSP is a relatively young and little utilized technology and is expensive when compared to other methods of generation. Feasibility studies have shown it is possible to generate enough power from CSP plants in Africa to spearhead the EUs climate goals. However, the costs of such a project are less well known. Currently, CSP must compete with low cost coal-fired electricity plants, severely hindering development. We examine the possible investment costs required for North African CSP levelized electricity cost to equal those of coal-fired plants and the potential subsidy costs needed to encourage growth until the technologies reach price parity. We also examine the sensitivity of investment and subsidies to changes in key factors. We find that estimates of subsidy amounts are reasonable for the EU and that sensitivity to such factors as perceived risk and learning rates would enable policy-makers to positively influence the cost of subsidies and time required for CSP to be competitive with coal.     

Long-term stability of tandem solar cells containing small organic molecules

In this paper, we discuss long-term stability measurements of tandem solar cells with mixed phthalocyanine: fullerene photoactive layers that exhibit an initial power conversion efficiency of about 4%. These devices are remarkably stable against exposure to halogen light as their power conversion efficiency decreases by less than 3% within more than 1400 h of permanent illumination at an intensity of approximately at . In addition, long-term stability measurements at an elevated temperature of are performed. In comparison to the illumination experiment, the cells show a much faster degradation which is attributed to the low glass transition temperature of the hole transport layer.     

NiAl coatings on carbon steel by self-propagating high-temperature synthesis assisted with concentrated solar energy: mass influence on adherence and porosity

NiAl have been produced by a self-propagating high-temperature synthesis (SHS). The power source that ignite the SHS reaction is concentrated solar energy. NiAl coatings are obtained in few seconds and the processes are economic and environment friendly. Three different NiAl mass are tested: 0.3; 0.6 y 1.7 g. Coating porosity and adherence to substrate depends on the NiAl mass. Pores are large in samples with 1.7 g while the other specimens have small pores. Coating adherence is better when the amount of reactive powder is larger.     

Study of cracking of methane for hydrogen production using concentrated solar energy

In this study, the cracking phenomenon of methane taking place in a cylindrical cavity of 16 cm in diameter and 40 cm in length under the heat of concentrated solar radiation without any catalyst is analysed. Three cases have been chosen; in all cases the primary phase contains methane and hydrogen gases. In the first case, we consider two phases; the secondary phase is a homogeneous carbon black powder with 50 nm of diameter; in the second case we have three phases where the two secondary phases are a particles powder with two diameters 20 and 80 nm and finally, a third case of five phases with a powder of four different diameters 20, 40, 60 and 80 nm. The low Reynolds K-ε turbulence model was applied. A calculation code "ANSYS FLUENT" is used to simulate the cracking phenomena where an Eulerian – Eulerian model is applied. The choice of several diameters greatly increases the calculation time but it approaches more of the physical reality of the radiation by these particles during the cracking. Results have shown that increasing the number of diameters gives higher cracking rates; the case of the powder of 4 different diameters gives the highest cracking rate. A parametric study as a function of the inlet velocity, carbon particle diameters and the intensity of solar radiation is realized. For the cracking heat, provided by the choice of the two concentrators of 5 and 16 MW/m² used in this simulation, the CH₄ inlet velocity is a decisive parameter for the cracking rate. Any increase in the inlet velocity requires more heat and this leads to a decrease in the cracking rate. For a velocity not exceeding 0.177 m/s (i.e. 0.3 L/min), both solar concentrations give the same amount of hydrogen produced. These quantities of hydrogen obtained reach maximum values for an inlet flow rate of CH₄ between 0.58 L/min (i.e. 0.34 m/s) and 0.62 L/min (i.e. 0.3655 m/s) for both reactors. The results are interpreted and compared with experimental work.     

Integration of direct carbon fuel cells with concentrated solar power

In this paper, we will report on a study on the thermodynamic feasibility of a concept that realizes the cracking of methane with a concentrated solar power (CSP) reactor and electricity production with a direct carbon fuel cell (DCFC) and its possible contribution to a clean energy supply for Europe in the long-term future. The natural gas (methane) is decomposed in an endothermic reaction into hydrogen and carbon. The separated carbon is fed to a direct carbon fuel cell (DCFC) and converted with high efficiency to electric power. A model of the proposed concept is carried out in the flow sheet program Cycle-Tempo and the results of the simulations and the corresponding analysis are presented in this paper. Finally the location factors influencing the implementation of this concept in the north of Africa are evaluated.     

Mechanism for the anomalous degradation of Si solar cells induced by high-energy proton irradiation

High-energy and high-fluence proton irradiation of Si space solar cells has provoked an anomalous increase in short-circuit current, followed by its abrupt decrease and cell failure. A model is proposed which explains the phenomena by expressing a reduction in the carrier concentration of the base region, in addition to a decrease of minority-carrier diffusion length. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has the effect of (1) broadening the depletion region width and (2) increasing the resistivity of the base layer. The anomalous change in the quantum efficiency of the cells under high-fluence ( 10¹⁴cm⁻²) irradiation is also explained by considering the generation of a donor-type defect level with the irradiation.