Sequential Processing: Control of Nanomorphology in Bulk Heterojunction Solar Cells

Bulk heterojunction organic photovoltaic devices based on poly[N-9 ''-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT)/[6,6]-phenyl C-70 butyric acid methyl ester (PC70BM) can be successfully fabricated by a sequential solution deposition process. When the top layer is deposited from an appropriate cosolvent, the PC70BM penetrates a predeposited bottom layer of PCDTBT during the spin-casting process, resulting in an interdiffused structure with a layer-evolved bulk heterojunction (LE-BHJ) nanomorphology. The PCDTBT:PC70BM LE-BHJ solar cells prepared with an optimized cosolvent ratio have comparable power conversion efficiency to the conventional BHJ solar cells. The nanomorphology of the optimized PCDTBT:PC70BM LE-BHJ mixture was found to have better vertical connectivity than the conventional BHJ material.