Highly Efficient Organic Solar Cells with Improved Vertical Donor-Acceptor Compositional Gradient Via an Inverted Off-Center Spinning Method

A highly efficient single junction organic solar cells (OSCs) were demonstrated with an averaged power conversion effi ciencies (PCE) of 10.71% that can be realized from a novel, yet simple inverted off-center solution fabrication method. For the inverted device fabrication, a 20 nm sol-gel ZnO_x layer was spin coated onto indium tin oxide (ITO)-coated glass substrates with a sheet resistance from ZnO _x precursor solution at 4000 rpm for 70 s and then annealed at 200°C for 1 h. The ZnO_x precursor solution was prepared by dissolving zinc acetate dehydrate and monoethanolamine in 2-methoxyethanol under stirring for 8 h for hydrolysis reaction and aging. For conventional central spin-coating method, the spin speed varied from 700 rpm to 1200 rpm for 150 s and 2000 rpm for 10 s. The centrifugal force and gravity during the inverted off-center spinning alters the BHJ film morphology and creates a more favorable vertical D:A composition gradient. More importantly, this simple method allows us to achieve near 100% internal quantum efficiency (IQE) in film thicknesses ranging from 100 to 215 nm contributing to high short circuit current density and steady fill factor, which results in high performance OSCs.