ИСТИНА

Organic photovoltaic cells (OPC) show high potentia l to convert solar energy into electricity in a cost-effective way. To date, the best polymer OPC a chieved the efficiency of solar energy conversion o f ~10%, and significant efforts are underway to incre ase their efficiency to the level of practical appl ications. One of the key parameters determining the OPC effic iency is the morphology of the active layer on the molecular scale. Specifically, the order of molecul es in semiconducting polymer materials has signific ant effects on their optoelectronic properties. The mos t studied OPC are polymer - fullerene cells based on poly- 3-hexylthiophene (P3HT) as a donor and soluble full erene derivative [6,6]-phenyl C61 butyric acid meth yl ester (PCBM) as an acceptor. Usually a freshly prep ared P3HT:PCBM blends has non-optimal morphology partially assigned to a high content of the amorpho us phase presumably due to PCBM intercalation into the polymer phase. Post deposition treatment of these b lends, e.g., thermal annealing, is frequently used to achieve optimal ordering of P3HT chains and, theref ore, to optimize the bulk heterojunction morphology . The partially crystalline polymer phase is characte rized by both better absorption at longer wavelengt hs and higher charge carrier mobility resulting in a signi ficant increase in the power conversion efficiency. As recenly shown, Raman spectroscopy allows probing of P3HT molecular order in blends with PCBM 1 . In this work, we have developed a technique for in situ probing the degree of molecular order in OPCs by Raman spectroscopy during thermal annealing . Raman spectra are measured during film annealing directly inside a temperature controlled stage and then are fitted by a superposition of two component s – amorphous and quasi crystalline phases. From these data the fraction of quasi-crystalline polymer phas e is evaluated that gives the dynamics of polymer orderi ng upon annealing. Various fullerene derivatives (methanofullerenes, metal complexes of fullerenes, fluorine fullerene derivatives) are studied as an acceptor component for OPC. Different fullerenes similarly t o PCBM were found to decrease the polymer order in spin-cast blends. We show that the dynamics of the P3HT ordering depends strongly on the type of fulle rene. The developed method was used to determine the opti mal parameters of thermal annealing P3HT:fullerene OPC. Figure 1 shows a clear corrella tion between the energy conversion efficiency and t he degree of polymer order measured in P3HT:PCBM devic es at different annealing temperatures. Therefore, the developed Raman technique can be used to optimi ze the performance of P3HT-based OSC. The potential of the proposed technique for other various types o f OSC is discussed.