Sequential processing: Control of nanomorphology in bulk heterojunction solar cells

Bulk heterojunction organic photovoltaic devices based on poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)/[6,6]-phenyl C₇₀ butyric acid methyl ester (PC ₇₀BM) 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.