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.     

A compact multi-chamber setup for degradation and lifetime studies of organic solar cells

A controlled atmosphere setup designed for long-term degradation studies of organic solar cells under illumination is presented. The setup was designed with ease-of-use and compactness in mind and allows for multiple solar cells distributed on four glass substrates to be studied in four different chambers with temperature and atmosphere control. The four chambers are situated at close proximity in the setup thereby allowing the solar cells to be subjected to as uniform an illumination distribution as possible for the given solar simulator employed. The cell substrates serve as the front window and present a tight seal. Hence no illumination correction needs to be performed due to transmission and reflection losses as otherwise seen with test chambers employing a window as a seal. The solar cells in each chamber are continuously and individually electrically monitored under biased conditions by means of a computer controlled multiplexer and source meter. The dimensions of the setup allow it to pass through a mid-size load lock in most common glove box systems allowing for mounting of tested samples under inert conditions.As a demonstration of the applicability of the chamber design, a degradation study of standard P3HT:PCBM solar cells was performed under four different environmental conditions.     

Applicability of X-ray reflectometry to studies of polymer solar cell degradation

Although degradation of polymer solar cells is widely acknowledged, the cause, physical or chemical, has not been identified. The purpose of this work is to determine the applicability of X-ray reflectometry for in situ observation of physical degradation mechanisms. We find that the rough interfaces of the polymer solar cell constituent layers seriously obstruct the sensitivity of the technique, rendering it impossible to elucidate changes in the layer/interface structure at the sub-nanometer level.     

Patterns of efficiency and degradation of composite polymer solar cells

Bulk-heterojunction plastic solar cells (PSC) produced from a conjugated polymer, poly(2-methoxy-5-(3′,7′-dimethyloctyl-oxy)-1,4-phenylenevinylene) (MDMO-PPV), and a methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) were investigated using photocurrent imaging techniques to determine characteristic patterns of efficiency and degradation. The solar cells with power efficiencies of up to 2.6% showed significant inhomogeneities and variations depending on the preparation steps (e.g. aluminum deposition), suggesting there is still room for improvements. A characteristic feature of the well-known photoinduced and dark cell degradation is the formation of islands of higher efficiency. Degradation mechanisms appear to have a morphological component. The imaging technique will open opportunities for combinatorial plastic solar cell research.     

Total charge amount as indicator for the degradation of small molecule organic solar cells

We show that the number of extracted charge carriers is a suitable measure to compare lifetime measurements on organic solar cells at different intensities. In detail, we used pin-structures with active layers containing a bulk heterojunction of Zincphthalocyanine (ZnPc) and C₆₀. Extended lifetime measurements under constant monochromatic or white illumination at defined temperatures of 50 °C or 90 °C are done. On the one hand, we show that the number of extracted charge carriers is important to determine the degree of degradation. On the other hand, our results show that the energy of irradiated photons is significant for accelerated measurements. This is an major advantage for the realisation of accelerated lifetime measurements. Additionally, we find that not single charge carriers, but excitons cause the degradation of the observed solar cells.     

Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru^II(LH₂)₂(NCS)₂ with LH₂=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I₂, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO₂-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.     

Stability study of CdS/CdTe solar cells made with Ag and Ni back-contacts

The stability and performance of CdS/CdTe solar cells made using four different back contact structures was studied. Two of these device sets were made with Ag and Ni deposited on a Cu-doped graphite layer. In the remaining sets, the graphite layer was removed before the application of the Ag or Ni. For long-term stability study, all devices were subjected to elevated temperature (100 °C), open-circuit voltage bias, and one-sun (100 mW/cm²) illumination for about 707 h. In general, significantly greater device stability was observed when the graphite was present. These devices exhibited a 10–15% decrease in efficiency primarily due to fill factor loss during stress. In the absence of graphite, the Ag contact degraded faster due to the fast diffusion of Ag and the formation of metastable resistive shunts. Ni metal contacts, in contrast, were plagued by Ni₃Te₂ phase formation that helped minimize Ni diffusion, but also contributed to a loss in fill factor. Devices without the graphite layer showed larger reductions in efficiency ranging from 25% to 45% for Ag and Ni contacts, respectively.     

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.     

Long-term operation of solid oxide fuel cells and preliminary findings on accelerated testing

Stationary applications of Solid Oxide Fuel Cell systems require operating times of 40,000 to 80,000 h for market introduction. Therefore, extended lifetime tests are essential for learning about the long-term behavior and various degradation mechanisms and to foster ideas about accelerated stack testing. The Forschungszentrum Jülich has been gradually extending the testing time, resulting in successful short-stack operating times of between 20,000 and 40,000 h. This work highlights the results of these long-term tests and compares the observations for different material combinations, operating temperatures of 700 and 800 °C, including different fuel utilizations and gas compositions. An increase of temperature from 700 to 800 °C leads to an acceleration of the degradation rate by a factor of 1.5–2. Meanwhile, an increase in fuel utilization from 40 to 80% did not result in increased degradation. The same was found for higher current densities of up to 1 Acm⁻².     

Mechanism for the anomalous degradation of proton- or electron-irradiated silicon solar cells

A mechanism of the anomalous increase of the short-circuit current of n⁺–p–p⁺ silicon space solar cells under high fluence of the high-energy 10 MeV protons or 1 Mev electrons is proposed. In distinction to other models this mechanism takes place as a result of the conversion of conductivity type and increased minority carrier lifetime with respect to that of majority carriers. This mechanism occurs in solar cells with deep centers, whose energy level is close to the middle of the band gap.     

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.     

Life prediction of membrane electrode assembly through load and potential cycling accelerated degradation testing in polymer electrolyte membrane fuel cells

Accelerated degradation tests (ADTs) are commonly used to assess the durability of membrane electrode assembly (MEA) components consisting of polymer electrolyte membrane fuel cells (PEMFC) under harsh stress conditions, estimating their lifetime in actual use condition and uncovering their vital degradation mechanisms. ADTs apply mechanically, chemically, or thermally combined stressors to efficiently investigate the durability of MEAs. However, combined stressors for ADTs might cause biased lifetime prediction because major deterioration mechanisms of MEA components are mixed with each other. This work proposes a method to accurately predict the lifetime of MEA through empirical modeling of its performance degradation through ADTs under potential cycle (carbon corrosion) and load cycle tests (electrocatalysts). To simulate operation modes of fuel cell electric vehicles, MEAs are tested under continuous on-off cycle testing (24 h operating and 1 h break) for 5000 h. Degradation patterns of MEAs are first modeled by the empirical model. The relationship between ADTs (potential and load cycle) and continuous on-off condition is then closely examined to accurately predict MEA lifetime under actual operation environments. The proposed idea has a potential to resolve critical durability issues of MEAs by identifying intermingling effects from other constituents.     

Effect of accelerated ageing tests on PBI HTPEM fuel cells performance degradation

The study presented in this paper aims to evaluate the performance degradation of Polybenzimidazole (PBI) based High Temperature PEM (HTPEM) fuel cells subjected to different ageing tests, according to a methodology already used by the authors. Three HTPEM Membrane Electrode Assemblies (MEAs) were characterized before and after different aging tests and performance compared. The three MEAs have been named MEA C, MEA D and MEA E. MEA C was subjected to 100,000 triangular sweep cycles between Open Circuit Voltage (OCV) and 0.5 A/cm² with 2 s of permanence at OCV at each cycle. MEA D and MEA E were subjected to 440 h of operation at constant load of 0.22 A/cm². In order to assess the cell performance, polarization curves, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were recorded during the ageing tests. Degradation rates have been obtained for MEA C (44 μV/h), for MEA D (30 μV/h) and for MEA E (29 μV/h). ECSA (Electrochemical Surface Area) has been calculated for the three MEAs showing a reduction of approximately 50% for MEA C and of approximately 30% for MEA D and MEA E. Polarization curves during aging tests confirm that load cycling is more detrimental. A comparison with data obtained by the authors in a previous research seems to confirm the repeatability of the test protocol used.     

Prediction of proton-induced degradation of GaAs space solar cells

The aim of this paper is to predict the degradation induced by proton and electron irradiations on the parameters (short-circuit current, open-circuit voltage and maximum power) of solar cells versus fluence, by a direct calculation now that the characteristics of the recombination centers induced by the irradiation have been determined. The calculation is performed for any energy of the irradiating particle and for any thicknesses and doping levels in the base and emitter. This approach allows also deducing the degradation of multijunction cells. The validity of this method is illustrated for the case of GaAs cells of different origins.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

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.     

Water and oxygen induced degradation of small molecule organic solar cells

Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C₆₀) followed by 30 nm C₆₀ for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C₆₀F₃₆ (MeO-TPD:C₆₀F₃₆), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H₂¹⁸O and ¹⁸O₂ provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C₆₀ layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.     

Effects of impurities on the degradation and long-term stability for solid oxide fuel cells

The effect of impurities on the degradation of performances was investigated for the flatten tube type SOFC stack. The durability tests of 20-cells stack were conducted at 750 °C (1023 K) with dry H₂ for more than 5000 h under a constant current density of 0.3 A cm⁻². The voltage loss showed a linear relationship between voltage loss and operation time (about 1.5%/1000 h). The ohmic resistance increased with operation time while the polarization resistance showed constant values. After the long-term operation test, the concentration levels of impurities were measured at cathode and interlayer by secondary ion mass spectrometry (SIMS). The concentrations of several elements were successfully determined in ppm levels. The concentrations of several elements increased with operation time (Na, Al, Si, and Cr), which suggested the transports and depositions on the cell component surface. The increase of resistance and impurity concentration at the interlayer were estimated from the literature data and SIMS impurity analysis.     

A review of polymer electrolyte membrane fuel cell durability test protocols

Durability is one of the major barriers to polymer electrolyte membrane fuel cells (PEMFCs) being accepted as a commercially viable product. It is therefore important to understand their degradation phenomena and analyze degradation mechanisms from the component level to the cell and stack level so that novel component materials can be developed and novel designs for cells/stacks can be achieved to mitigate insufficient fuel cell durability. It is generally impractical and costly to operate a fuel cell under its normal conditions for several thousand hours, so accelerated test methods are preferred to facilitate rapid learning about key durability issues. Based on the US Department of Energy (DOE) and US Fuel Cell Council (USFCC) accelerated test protocols, as well as degradation tests performed by researchers and published in the literature, we review degradation test protocols at both component and cell/stack levels (driving cycles), aiming to gather the available information on accelerated test methods and degradation test protocols for PEMFCs, and thereby provide practitioners with a useful toolbox to study durability issues. These protocols help prevent the prolonged test periods and high costs associated with real lifetime tests, assess the performance and durability of PEMFC components, and ensure that the generated data can be compared.