In high performance polymer:fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimised through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to 'High T g PPV':PCBM based solar cells, where the 'High T g PPV' is characterised by its high glass transition temperature (138°C). In-situ monitoring of the photo-current-voltage characteristics at elevated temperatures, in combination with a systematic Transmission Electron Microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation 1 kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms.

Mixtures of conjugated polymers and fullerenes command considerable attention for application in organic solar cells. To increase their efficiency, the design of new materials that absorb at longer wavelengths is of substantial interest. We have prepared such low band gap polymers using the donor-acceptor route, which is based on the concept that the interaction between alternating donors and acceptors results in a compressed band gap. Furthermore, for application in photovoltaic devices, sufficient polymer solubility is required. We have prepared four low band gap conjugated polymers, with a bis(1-cyano-2-thienylvinylene)phenylene base structure, and achieved an excellent solubility by the introduction of long alkoxy and alkyl side chains. The polymers were synthesized via an oxidative polymerization. Their electronic properties were determined from electrochemical and optical measurements, which confirm that they indeed have a low band gap. In the blend of such a low band gap polymer with PCBM, evidence for efficient charge transfer was obtained from PL and EPR measurements. However, bulk heterostructure solar cells made of such blends display only low efficiencies, which is attributed to low charge carrier mobilities.

No abstract

By means of a custom-made fiber bundle for optical excitation and light collection we demonstrate the feasibility of W-band (95 GHz) optically detected magnetic resonance spectroscopy using a commercial high-frequency electron paramagnetic resonance spectrometer with a standard cylindrical cavity.

No abstract