Tandem organic photovoltaics incorporating two solution-processed small molecule donor layers

We develop a partially solution-processed small molecule tandem organic photovoltaic cell using an organic/inorganic interlayer structure that provides efficient charge recombination while protecting underlying layers from degradation due to attack from solvents applied during the deposition of subsequent sub-cells. Each sub-cell consists of a functionalized squaraine (fSQ) blend donor that is cast from solution, followed by evaporation of other functional layers. The first fSQ layer is cast from chloroform, while the second is cast from a tetrahydrofuran, thereby minimizing dissolution of the relatively insoluble, underlying fullerene layer that acts to protect the first donor layer. Solvent vapor annealing increases the sub-cell performance while decreasing the damage caused by spin-coating of the second fSQ layer, both of which result from increased film crystallinity that reduces the rate of solvent penetration. The tandem cell has a power conversion efficiency of 6.2% ± 0.3% and an open circuit volta...