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-Vcurve

AFM

and reasonable explanations were given.Morphology studies on the blend films indicated that excellent miscibility between polymer and PC［70］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.