70]FULLERENE-BASED EFFICIENT BULK HETEROJUNCTION SOLAR CELLS

Abstract

A bulk heterojunction photovoltaic cell in which an isomeric mixture of fullerene derivatives,PC[70]BM is used as an electron acceptor in combination with poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene)(MEH-PPV) as an electron donor has been reported.PC[70]BM is the higher fullerene analogue of PC[60]BM,and displays improved light absorption in the visible region.It is found out when this material is used instead of PC[60]BM in a photovoltaic cell with MEH-PPV as donor,a value of 6.07 mA/cm2 current density for the PC[70]BM∶MEH-PPV devices means an increase of over 40% with respect to an optimized PC[60]BM∶MEH-PPV cell.The open-circuit voltage Voc of 0.85 V and a fill factor(FF) of 0.53 for the PC[70]BM∶MDMO-PPV devices resulted in an overall power-conversion efficiency of 3.42%,measured under AM1.5G,solar simulater(80 mW/cm2).The influence of different solvents used for spin-casting of active layer on device performance was studied.The results showed that 1,2-dichlorobenzene was a best solvent for PC[70]BM/MEH-PPV system.The mechanism of the solvent influence on the device performance was analyzed by light and dark I-V curve,AFM,and reasonable explanations were given.Morphology studies on the blend films indicated that excellent miscibility between polymer and PC[60]BM favored exciton separation.Because the exciton diffusion length in organic materials is typically small,the intimate mixture favors exciton separation and therefore higher photocurrent.