Current Matching in Multifold DBP/C70 Organic Solar Cells With Open-Circuit Voltages of up to 6.44 V

In this paper, we demonstrate a novel method for achieving high open-circuit voltages (V_oc) in organic solar cells based on tetraphenyldibenzoperiflanthen (DBP) as donor and fullerene (C₇₀) as acceptor molecules, by fabrication of multifold bilayer single cells stacked on top of each other. As devices based on the material combination of DBP and C₇₀ show relatively high open-circuit voltages of 0.87 V for single junction cells, and as both materials show broad absorption in the visible region with pronounced peaks, they become ideal candidates as active layer materials in tandem stacked solar cells. By using a ten-fold bilayer structure of DBP and C₇₀, sandwiched between two electrodes, we reach, in this paper, an open-circuit voltage of up to 6.44 V for a single device, and thus crossing the 5 V limit that is required to power up several low-power consuming devices. Furthermore, by conducting drift-diffusion-based device modeling of the multifold devices, considering also the optical absorption profile in the stacked tandem cell and the effective exciton diffusion lengths, we demonstrate that effective current matching can be obtained in the devices through thickness optimization for each single cell. As a result, we demonstrate that the efficiency of these novel devices can be improved from 3.1% to 4.4% (best performing devices) in the case of a fivefold device structure, mainly due to the strong increase in the short-circuit current density, and thus lead to efficient small molecule-based solar cells with high open-circuit voltages.