Nearâ Infrared Ternary Tandem Solar Cells

Abstract

The paucity of nearâ infrared (NIR) organic materials with high absorption at long wavelengths, combined with large diffusion lengths and charge mobilities, is an impediment to progress in achieving highâ efficiency organic tandem solar cells. Here a subcell is employed within a series tandem stack that comprises a solutionâ processed ternary blend of two NIRâ absorbing nonfullerene acceptors and a polymer donor combined with a smallâ molecularâ weight, shortâ wavelength fullereneâ based subcell grown by vacuum thermal evaporation. The ternary cell achieves a power conversion efficiency of 12.6 ± 0.3% with a shortâ circuit current of 25.5 ± 0.3 mA cmâ 2, an openâ circuit voltage of 0.69 ± 0.01 V, and a fill factor of 0.71 ± 0.01 under 1 sun, AM 1.5G spectral illumination. The success of this device is a result of the nearly identical offset energies between the lowest unoccupied molecular orbitals (LUMOs) of the donors with the highest occupied molecular orbital (HOMO) of the acceptor, resulting in a high openâ circuit voltage. A tandem structure with an antireflection coating combining these subcells demonstrates a power conversion efficiency of 15.4 ± 0.3%.A tandem organic photovoltaic cell combining a nonfullereneâ acceptorâ based ternary cell with a fullerene smallâ molecule binary subcell is reported. The cell combines vacuumâ and solutionâ deposited layers, achieving a power conversion efficiency of 15.4%.