Understanding the External Quantum Efficiency of Organic Homo-Tandem Solar Cells Utilizing a Three-Terminal Device Architecture

A fundamental analysis of the external quantum efficiency (EQE) of organic tandem solar cells with equal absorbers in both subcells (homo-tandem solar cells) is presented. Providing direct access to both subcells by introducing a conductive intermediate polymer electrode into the recombination zone, without changing the optical and electric device properties, the three-terminal device becomes a proxy to the two-terminal tandem solar cell properties. From the spectrally resolved EQE of the subcells in three-terminal configuration wavelength and intensity of suitable bias light as well as bias voltage are determined that in turn allow for accurate EQE measurements of the common two-terminal tandem solar cells. Theoretic considerations allow the prediction of the tandem solar cell's EQE from its subcells' EQEs as well as the prediction of the tandem cell EQE under monochromatic bias light illumination being in excellent agreement with experimental results. All findings discussed herein can be applied to more common hetero-tandem solar cell architectures likewise.