Morten Vesterager Madsen

Degradation Patterns in Water and Oxygen of an Inverted Polymer Solar Cell

The spatial distribution of reaction products in multilayer polymer solar cells induced by water and oxygen atmospheres was mapped and used to elucidate the degradation patterns and failure mechanisms in an inverted polymer solar cell. The active material comprised a bulk heterojunction formed by poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between a layer of zinc oxide and a layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) that acted as, respectively, electron and hole transporting layers between the active material and the two electrodes indium-tin-oxide (ITO) and printed silver. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) in conjunction with isotopic labeling using H(2)(18)O and (18)O(2) enabled detailed information on where and to what extent uptake took place. A comparison was made between the use of a humid (oxygen-free) atmosphere and a dry oxygen 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 the reactions taking place at the interface between the active layer and the PEDOT:PSS were the major cause of device failure in the case of these inverted devices, which are compatible with full roll-to-roll (R2R) coating and industrial manufacture. The PEDOT:PSS was found to phase separate, with the PEDOT-rich phase being responsible for most of the interface degradation in oxygen atmospheres. In water atmospheres, little chemically induced degradation was observed, whereas a large partially reversible dependence of the open circuit voltage on the relative humidity was observed. In addition, temporal aspects are discussed in regard to degradation mechanisms. Finally, analytical aspects in regard to storing devices are discussed.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Upscaling from single cells to modules – fabrication of vacuum- and ITO-free polymer solar cells on flexible substrates with long lifetime

Fabrication of polymer solar cell (PSC) modules was done on a previously reported compact coating/printing machine and tested in a readily scalable roll process on flexible substrates without applying vacuum, ITO or spin coating. Our aim was to establish loss upon scaling from cells to small modules. We studied from single cells (1 cm2) to modules comprising four serially connected devices with a total active area of 8 cm2. Four different polymers (P3HT, PV-D4610, PDTSTTz-4 and PBDTTTz-4) were applied in the preparation of the modules and efficiencies of more than 3% were achieved which is comparable to single cell devices prepared using the same process. This proves that it is possible to scale up new materials in an ITO free device context to modules without having an efficiency drop, due to reliable and consistent processing. The main loss observed was due to the packaging using barrier materials. The photochemical stability of the materials was therefore studied using intense light along with the operational stability of the corresponding devices according to the ISOS-D-1 and ISOS-L-1 standards. For devices under constant illumination we found that barrier materials from Mitsubishi and 3M provide better operational stability over time, compared to the barrier foil obtained from Amcor, for all the polymers, which is ascribed to the cut-off at higher wavelengths thereby lowering the degree of UV light that reaches the device. When comparing the operational stability of the four polymers under constant illumination, P3HT generally retains its performance better with higher T80 values, while the polymer PV-D4610 shows the highest PCE (1.6%) after 300 hours of operation.

Photochemical stability of conjugated polymers, electron acceptors and blends for polymer solar cells resolved in terms of film thickness and absorbance

Photochemical degradation at 1 sun under AM1.5G illumination was performed on six conjugated polymers and five different electron acceptors. Additionally, the respective polymer:PC60BM and P3HT:electron acceptor blends were studied, and all degradations were resolved in terms of film thickness and absorbance. A fully automated degradation setup allowed for inclusion of in excess of 1000 degradations in this study to enable a discussion of reliability of the technique. Degradation rates were found to increase exponentially with decreasing film absorbance for all materials. The relative stabilities within each material group were found to vary for both the pure polymers and the blends. The stability ranking between the materials of the pure polymers was found to be similar to the ranking for their respective blends, implying that the photochemical stability of a pure polymer is a good measure of its associated blend stability. Different electron acceptors were found to stabilize P3HT decreasingly with decreasing donor–acceptor LUMO–LUMO gap. Destabilization of P3HT was observed in the case of the electron acceptor ICBA. Additionally, the decreased stabilization of P3HT by high LUMO electron acceptors poses a challenge to solar cell encapsulation if these materials are to be of commercial interest. The presented method is generally applicable to all types of organic materials to assess photochemical stabilities. The presented results of conjugated polymers demonstrate that this is a powerful tool for conjugated polymer stability assessment if the results are interpreted correctly.

Comparative studies of photochemical cross-linking methods for stabilizing the bulk hetero-junction morphology in polymer solar cells

We are here presenting a comparative study between four different types of functionalities for cross-linking. With relatively simple means bromine, azide, vinyl and oxetane could be incorporated into the side chains of the low band-gap polymer TQ1. Cross-linking of the polymers was achieved by UV-light illumination to give solvent resistant films and reduced phase separation and growth of PCBM crystallites in polymer:PCBM films. The stability of solar cells based on the cross-linked polymers was tested under various conditions. This study showed that cross-linking can improve morphological stability but that it has little influence on the photochemical stability which is also decisive for stable device operation under constant illumination conditions.

Photochemical stability of electrochromic polymers and devices

The stability of fully printed flexible organic electrochromics based on 11 different conjugated polymers is explored from the fundamental chemical degradation level to the operational device level. The photochemical stability of the electrochromic polymers (ECPs) is studied enabling an analysis of the influence that the chemical constitution of the conjugated polymer backbone has on the photochemical stability. Based on changes in the UV-visible absorption and IR spectra, the polymers were categorized into two distinct groups, each with a separate degradation mechanism. During irradiation (1000 W m−2, AM 1.5G) under ambient conditions the majority of the polymers degraded within 4–5 hours. Three polymers showed increased stability with degradation rates from 0.44 to 1.58% per hour measured as loss of absorption. Application of oxygen and UV barrier foils was found to drastically slow the photochemical decomposition of the polymer films, such that after 2200 hours of continuous irradiation the less stable polymer films were degraded 27% on average, while the degradation of the most stable polymer films was immeasurable thus indicating that such materials can be sufficiently stable for device operations for many years under indoor conditions and for a few years under outside conditions. Finally, functioning electrochromic devices (ECDs) were made and the effect of illumination on the response time and optical contrast was established. This report shows that encapsulated electrochromic devices based on flexible barrier substrates exhibit increased stability and are indeed viable in devices such as shading elements, light management systems, displays with low switching speed requirements and signage.

Thermally reactive Thiazolo[5,4-d]thiazole based copolymers for high photochemical stability in polymer solar cells

New thermally reactive copolymers based on dithienylthiazolo[5,4-d]thiazole (DTZ) and silolodithiophene (SDT) have been synthesized and explored in bulk heterojunction solar cells as mixtures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In thin films the polymers had optical band gaps in the range of 1.64–1.80 eV. For solubility the polymers have incorporated alkyl groups on the SDT unit and thermally removable ester groups on the DTZ unit that can be eliminated around 200 °C for improved photochemical stability in thin films. The bulkiness of the alkyl chains on the SDT unit proved to be very significant in terms of photovoltaic performance of the polymer:PCBM solar cells. Polymers based on 4,4-dihexyl-4H-silolo[3,2-b:4,5-b′]dithiophene reached power conversion efficiencies (PCEs) up to 1.45% but changing the alkyl groups to more bulky ethylhexyl chains reduced the PCE to 1.17%. More noteworthy is that the photovoltaic performance improves for the polymers based on 4,4-dihexyl-4H-silolo[3,2-b:4,5-b′]dithiophene after the ester groups has been eliminated from the DTZ unit by a thermal treatment around 200 °C. To confirm that elimination of the solubilizing groups improve the long term durability of the materials the photochemical stability was estimated by a novel accelerated degradation method by which the photobleaching of the polymer was followed during degradation at 100 solar intensities. This clearly shows that the pristine polymer films are by far the most unstable under the given conditions emphasizing the unfavorable effect of solubilizing groups on the photochemical stability of conjugated polymers.

Oxygen- and water-induced degradation of an inverted polymer solar cell: the barrier effect

The work focuses on the degradation of performance induced by both water and oxygen in an inverted geometry organic photovoltaic device with emphasis on the accumulated barrier effect of the layers comprising the layer stack. By studying the exchange of oxygen in the zinc oxide (ZnO) layer, the barrier effect is reported in both a dry oxygen atmosphere and an oxygen-free humid atmosphere. The devices under study are comprised of a bulk het- erojunction formed by poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester sandwiched between a layer of zinc oxide (electron transporting layer) and a layer of poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (hole transport layer) and the two electrodes indium tin oxide and silver. Time-of-flight secondary ion mass spectrometry is employed to characterize the accumulated barrier effect. A pronounced barrier effect is observed in the hu- mid atmosphere, correlating well with a long observed lifetime in the same atmosphere. C 2011 Society of Photo-Optical Instrumentation Engineers. C 2011 Society of Photo-Optical Instrumentation

Ultra high open circuit voltage (>1 V) of poly-3-hexylthiophene based organic solar cells with concentrated light

One approach to increasing polymer solar cell efficiency is to blend poly-(3-hexyl-thiophene) with poorly electron accepting fullerene derivatives to obtain higher open circuit voltage (Voc). In this letter concentrated light is used to study the electrical properties of cell operation at up to 2000 solar intensities of these photoactive blends. Comparison of solar cells based on five different fullerene derivatives shows that at both short circuit and open circuit conditions, recombination remains unchanged up to 50 suns. Determination of Voc at 2000 suns demonstrated that the same logarithmic Voc evolution is observed from 0.4 to 2000 suns, where a maximum Voc of 1019 mV was obtained.

The Influence of Conjugated Polymer Side Chain Manipulation on the Efficiency and Stability of Polymer Solar Cells

The stability of polymer solar cells (PSCs) can be influenced by the introduction of particular moieties on the conjugated polymer side chains. In this study, two series of donor-acceptor copolymers, based on bis(thienyl)dialkoxybenzene donor and benzo[c][1,2,5]thiadiazole (BT) or thiazolo[5,4-d]thiazole (TzTz) acceptor units, were selected toward effective device scalability by roll-coating. The influence of the partial exchange (5% or 10%) of the solubilizing 2-hexyldecyloxy by alternative 2-phenylethoxy groups on efficiency and stability was investigated. With an increasing 2-phenylethoxy ratio, a decrease in solar cell efficiency was observed for the BT-based series, whereas the efficiencies for the devices based on the TzTz polymers remained approximately the same. The photochemical degradation rate for PSCs based on the TzTz polymers decreased with an increasing 2-phenylethoxy ratio. Lifetime studies under constant sun irradiance showed a diminishing initial degradation rate for the BT-based devices upon including the alternative side chains, whereas the (more stable) TzTz-based devices degraded at a faster rate from the start of the experiment upon partly exchanging the side chains. No clear trends in the degradation behavior, linked to the copolymer structural changes, could be established at this point, evidencing the complex interplay of events determining PSCs’ lifetime.