International audienceThe efficiency of organic photovoltaic (OPV) solar cells is constantly improving; however, the lifetime of the devices still requires significant improvement if the potential of OPV is to be realised. In this study, several series of inverted OPV were fabricated and thermally aged in the dark in an inert atmosphere. It was demonstrated that all of the devices undergo short circuit current-driven degradation, which is assigned to morphology changes in the active layer. In addition, a previously unreported, open circuit voltage-driven degradation mechanism was observed that is highly material specific and interfacial in origin. This mechanism was specifically observed in devices containing MoO3 and silver as hole transporting layers and electrode materials, respectively. Devices with this combination were among the worst performing devices with respect to thermal ageing. The physical origins of this mechanism were explored by Rutherford backscattering spectrometry and atomic force microscopy and an increase in roughness with thermal ageing was observed that may be partially responsible for the ageing mechanism

Organic photovoltaic solar cells are a promising option for cheap, renewable energy, but must improve in their stability.

The development of nonfullerene acceptors (NFAs) has led to dramatic improvements in the device efficiencies of organic photovoltaic (OPV) cells. To date it is, however, still unclear how those laboratory‐scale efficiencies transfer to commercial modules, and how stable these devices will be when processed via industrially compatible methods. Herein, the degradation behavior of lab‐scale and scalable OPV devices using similar nonfullerene‐based active layers is assessed. It is demonstrated that the scalable NFA OPV exhibits completely reversible degradation when assessed in ISOS‐O‐1 outdoor conditions, which is in contrast to the laboratory‐scale devices assessed via the indoor ISOS‐L‐2 protocol. Results from transient photovoltage (TPV) indicate the presence of charge trap formation, and a number of potential mechanisms are proposed for the selective occurrence of this in laboratory‐scale devices tested in ISOS‐L laboratory conditions—ultimately concluding that it has its origins in the different device architectures used. The study points at the risk of assessing active layer stability from laboratory‐scale devices and degradation studies alone and highlights the importance of using a diverse range of testing conditions and ISOS protocols for such assessment.

No abstract

No abstract

No abstract

No abstract

<p>Metallomesogens (metal-containing liquid crystals) have been of interest to chemists since the early 1980s. Since this period, many of the studies published on metallomesogens have focussed on the synthesis of novel metallomesogens, and studies of their phase behaviour. As a result there is a substantial body of knowledge of their synthesis and phase behaviour, however many of these studies have overlooked the interesting physical properties that transition metals or lanthanides may impart to the mesophase (liquid crystal state). The studies that have been carried out suggest that the optical and photophysical properties resulting from their self assembly are very different to those observed in the crystalline or isotropic liquid phases, and are highly dependent on the structure of the mesophase. A series of salicylaldimine-based ligands and copper(II) complexes with a variety of structural modifications were synthesised and characterised. The structure, phase behaviour and phase relaxation kinetics of these compounds in the crystalline state were studied using differential scanning calorimetry (DSC), single crystal X-ray crystallography and variable temperature powder X-ray diffraction. The mesomorphism of the compounds was studied using small angle X-ray scattering (SAXS) and polarised optical microscopy (POM). The photophysical properties of the complexes and ligands were studied in the solution phase using ultraviolet-visible (UV-vis) spectroscopy. It was found that the smallest complexes (copper(II) N-alkyl,4-alkoxysalicylaldimine complexes) were not metallomesogens, but did exhibit multiple crystalline phases that formed as a result of changes in the conformation of the N-alkyl chains. The transition temperatures of these crystalline phase changes were strongly dependent on the length of the alkyl chains due to kinetic phenomena. The extension of the rigid core of the complex via synthesis of an N-(4-butylphenyl) derivative was successful in inducing mesomorphism in both the complex and the ligand. The ligand formed an enantiotropic nematic mesophase, while the complex formed a monotropic smectic A mesophase. The structural differences between the non-mesomorphic complexes, the mesomorphic ligand and the mesomorphic complex indicate that the determining factor in the formation of mesophases is the magnitude of lateral interactions between the molecules, which is governed by the size and shape of the rigid core. Further attempts at inducing mesomorphism by formation of bimetallic copper complexes were unsuccessful due to chemical instability. The photophysical properties of the compounds showed that the salicylaldimine ligands exist in solution in a tautomeric equilibrium, which can be influenced by the hydrogen-bonding character of the solvent. The ligands also show evidence of photochromism, while the complexes exhibit LMCT bands, both features which could affect and be affected by self assembly. It was also determined from their UV-vis spectra and DFT studies that the ligands bind to the metal centre in a manner which is intermediate to the two tautomeric forms, but close to the higher energy keto-amine tautomer. These results demonstrate that structural modification can be used to control both the phase behaviour and physical properties of salicylaldimine complexes. The compounds studied here also show potential to exhibit a variety of self assembly-dependent photophysical properties in the mesophase and would be good candidates for future research in this area.</p>