Comparing C-60 and C-70 as acceptor in organic solar cells : Influence of the electronic structure and aggregation size on the photovoltaic characteristics

The difference in aggregation size of the C-60 and C-70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly [2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C-70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 degrees C is approximately twice bigger when the C-60 is the acceptor. We attribute this effect to the tendency of C-60 in form smaller aggregate domains relatively to the C-70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C-60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C-60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C-60.Peer reviewe

Inflation and stabilisation in Albania

This article analyses the determinants of inflation in Albania during the transition period. We attempt to explain why Albania was successful in reducing inflation, by the end of 1995, to a level comparable to many countries in the European Union. We discuss the relative importance of monetary, fiscal and external variables, and we highlight the role of remittances by emigrants. Using vector autoregression analysis, we demonstrate how shocks to money growth and remittances affect the subsequent paths of inflation and the exchange rate. Our main conclusion is that, while there are important lessons from the Albanian experience for other ex-communist countries, the presence of large-scale emigration and huge I-emittance flows makes Albania unique among European countries in transition, in the extent to which these factors aided the anti-inflation programme. However, recent events have demonstrated that political stability and control of the informal financial sector ai-e also crucial for low inflation