optiZation of the Performance of a bulk Hetero-Junction Solar Cells

Abstract

The dependence of the performance thickness of bulk heterojunction solar cells based on poly(2-methoxy-5-)3,7-dimethyloctyloxy)1,4- phenylenevinylene) MDMO:PPV as an electron donor and (6,6)-phenyl C61 butyric acid methyl ester PCBM as an electron acceptor, has been studied. The thicknesses of the donor and acceptor (blend) are 25 nm, 40 nm, and 60 mn , and the active surface area of the cell is 1.5 mm 2. All the measurements are taken at a temperature of 300 K. A 0.3 W/ml lamp is used as a source of light . It has been commonly accepted that increasing the blend thickness beyond 100 nm does not increase the efficiency . The results of this work show that the optimum thickness of the blend is about 40 nm . The efficiency of a MDMO: PPV/PCBM solar cell is found to be 2.8%, at a blend thickness of 40 nm . By lowering the offset of (LUMO) A- (LUMO) D, to be 0.5 eV, the device efficiency would increase to about 8%, showing the great importance of matching the electronic levels of donor and acceptor. Ohmic contact is found to increase the short circuit current , it -slightly decreases the open circuit voltage .The experimental results show that the voltage losses ,which could be due to band bending, equal to 0.3 V . The open—circuit voltage is found to decrease with increase in temperature . The rate rate of decrease is shown to be about 7 mV/ OC. X