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The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by the morphology of the active layer on the nanoscale level. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements show that ordering of P3HT plays a key role in optimizing the photovoltaic performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. The crystallinity of the photo-active blend is typically restored by an annealing procedure resulting in improved device performance, characterized by a spectral broadening of the optical absorption.
The morphological changes upon annealing of the P3HT:PCBM blends are accompanied by electrical changes as shown in charge carrier mobility measurements. Space-charge limited current measurements have been performed in hole-only devices with various P3HT:PCBM blend ratios. The mobility before and after annealing is compared and from temperature dependent measurements the width of the density of states distribution (DOS) is determined. The hole mobility in pristine P3HT remains practically unaffected by the annealing treatment. The as-produced P3HT:PCBM blends on the other hand, with a more disordered P3HT phase, have a much lower hole mobility. Annealing is capable of increasing the P3HT ordering with as a result an orders of magnitude larger hole mobility, approaching the value found in pristine P3HT. The DOS bandwidths are affected similarly. In the as-produced blend films a value of 100 meV is found, larger than in the annealed films, there reaching a value around 70 meV similar as in pristine P3HT. Variation of the processing solvent demonstrated however that an optimized morphology and charge transport situation can also be obtained without an additional annealing step. It is shown that in that case the as-produced active layer has already a favorable crystalline morphology. We argue that the high boiling point of the solvent plays an important role in this by influencing the evaporation speed during deposition of the photo-active blend. Further proof is delivered that indeed slowing down the evaporation speed can beneficially influence the solar cell performance. Power conversion efficiency over 4% has been achieved in this way.
Screen-printing is studied as deposition technique for conjugated material based layers. Photovoltaics based on the principle of bulk donor-acceptor heterojunction are tested using a blend of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) mixed with the C60-derivative (6, 6)-phenyl C61-butyric acid methyl ester (PCBM). First, different solution concentrations of the donor MEH-PPV material and of the blend are subjected to rheology measurements. Addition of the acceptor (PCBM) to a donor material based solution induces a decrease of the solution viscosity. However, the overall flow behaviour of the blend remains similar to that of the MEH-PPV based solution. Secondly, it is shown that specific printer settings have to be used to obtain active layers that are suitable for opto-electronic applications. Finally, devices with an overall energy conversion efficiency of 1.25% under standardized simulated solar illumination (AM1.5G; 100mW/cm2) have been obtained showing that screen-printing can be a suitable technique for the deposition of the active layer of polymer solar cells.
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