Kion Norrman

Stability of Polymer Solar Cells

Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

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.

Using light-induced thermocleavage in a roll-to-roll process for polymer solar cells.

We report on the use of intense visible light with a narrow spectral distribution matched to the region where the conjugated polymer material absorbs to selectively heat the active material and induce thermocleavage. We show a full roll-to-roll process, leading to complete large-area polymer solar cell modules using light-induced thermocleavage. The process employs full solution processing in air for all five layers in the device and does not employ indium-tin oxide or vacuum processing. The process steps were carefully analyzed using X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, attenuated total reflectance infrared, and transmission/reflection UV-vis techniques.

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.

H 2 ­ H 2 O ­ Ni ­ YSZ Electrode Performance Effect of Segregation to the Interface

The electrochemical properties of the interface between Ni and yttria-stabilized zirconia (YSZ) in the H 2 -H 2 O-Ni-YSZ electrode have been investigated in order to obtain knowledge on the effect of segregation of impurities to the electrode-electrolyte interface on performance. Even for a relatively pure nickel electrode, a film of impurities was found at the contact between Ni and YSZ and on the surface of YSZ. Impurity ridges were also found at the three-phase boundary. Impurities in these locations all have the potential of impeding processes and reactions at the electrode-electrolyte interface. The pure electrodes showed a lower resistance compared to most electrodes discussed in the literature.

All printed transparent electrodes through an electrical switching mechanism: A convincing alternative to indium-tin-oxide, silver and vacuum

Here we show polymer solar cells manufactured using only printing and coating of abundant materials directly on flexible plastic substrates or barrier foil using only roll-to-roll methods. Central to the development is a particular roll-to-roll compatible post-processing step that converts the pristine and non-functional multilayer-coated stack into a functional solar cell through formation of a charge selective interface, in situ, following a short electrical pulse with a high current density. After the fast post-processing step the device stack becomes active and all devices are functional with a technical yield and consistency that is compelling.

Detrimental Effect of Inert Atmospheres on Hybrid Solar Cells Based on Semiconductor Oxides

We report the improvement observed in J sc , V oc , and current-voltage (1-V) curves when hybrid solar cells (HSCs) are transferred from inert conditions to ambient atmosphere. The effect is observed regardless of the semiconductor oxide applied and has been attributed to the reversible incorporation of oxygen from the atmosphere into the semiconductor oxide surface during illumination. The HSCs were prepared as bilayers of thin-film semiconductor oxides (TiO 2 , Nb 2 O 5 , and ZnO) made by the sol-gel technique and the polymer poly[(2-methoxy-5-ethylhexyloxy)-l,4-phenylenevinylene] (MEH-PPV), applying a final device configuration of the type indium tin oxide/oxide thin film /MEH-PPV/Ag. The photovoltaic response was studied in terms of inert atmosphere by recording the initial values of open-circuit voltage (V oc ) and current density (J sc ). Solar decay curves, I-V curves, the effect of filter and resting time, as well as photophysical analyses were also carried out for each type of device.

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.

Rapid flash annealing of thermally reactive copolymers in a roll-to-roll process for polymer solar cells

Light induced thermocleaving of a thermally reactive copolymer based on dithienylthiazolo[5,4-d]thiazole (DTZ) and silolodithiophene (SDT) in contact with the heat sensitive substrate the heat sensitive substrate polyethyleneterphthalate (PET) was effectively demonstrated with the use of high intensity pulsed light, delivered by a commercial photonic sintering system. Thermally labile ester groups are positioned on the DTZ unit of the copolymer that can be eliminated thermally for enhanced photochemical stability and advantages in terms of processing (solubility/insolubility switching). The photonic sintering system was successfully implemented in a full roll-to-roll process on flexible PET substrates and large-area polymer solar cell modules were prepared by solution processing of five layers under ambient conditions using the photonic sintering system for thermocleaving of the active layer. The PET foil did not show any deformation after exposure to the high intensity light only leaving the insoluble thermocleaved active layer. The active layer remained planar after light exposure thereby allowing the coating of supplementary material on top.

Nucleophilic and electrophilic displacements on covalently modified carbon: introducing 4,4′-bipyridinium on grafted glassy carbon electrodes

4,4′-Bipyridinium (i.e., viologen) was immobilized on 4-(chloromethyl)phenyl grafted glassy carbon electrodes by a nucleophilic substitution reaction involving 1-ethyl- or 1-benzyl-4-(4′-pyridyl)pyridinium. Reaction times of about 5 days were required for these surface-constrained processes to go to completion in aqueous solution at room temperature. The applicability of the described procedure was demonstrated by performing the equivalent modification in 2 steps by reacting first with 4,4′-bipyridine, followed by quaternization of the available nitrogen to obtain the viologen functionality, that is, the surface acts as a nucleophile in a substitution reaction. However, the quaternization step was found to be possible for introducing the benzyl group but not the ethyl group. The covalently modified electrodes were reasonably stable to repeated electrochemical sweeping in acetonitrile with a 25% decrease in the observed electroactivity after 100 sweeps at a sweep rate of 2 V s−1. The coverage was determined from the electrochemical response of the viologen moiety to be approximately 3 × 10−10 mol cm−2. In addition to cyclic voltammetry, the presence of viologen was demonstrated by means of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Scanning images (500 × 500 µm2) obtained by the latter technique indicated that the molecules were distributed uniformly over the entire surface. Scanning tunnelling microscopy was used to follow the individual steps of the modification procedure on highly ordered pyrolytic graphite.