Tandem solar cells are designed as a serial connection of wide-bandgap and low-bandgap sub-cells with complementary absorption spectra, to improve the overlap with the solar spectrum and to minimize thermalization losses.

With the introduction of non-fullerene acceptors in organic photovoltaics, the loss in open-circuit voltage (V_oc) compared to the energy gap of the absorber (E_g/q) can be below 0.5 V and the internal quantum efficiency concomitantly reaches levels of unity.

We will present our recent work on organic cells based on the PM6:Y6 system, which provide an efficiency of > 16% and a high V_oc. While high efficiencies are known for this donor/acceptor combination, stability concerns exist, especially under illumination. We will show the concomitant improvements in efficiency and stability by the addition of small amounts of fullerene to the active layer and, more critically, we will demonstrate that the optimum choice of hole and electron extraction/transport layers is a key that unlocks substantially improved operational stability in NFA solar cells. Furthermore, we found that the stability of the cells is critically affected by the spectrum of the light source. Cells based on MoO₃ instead of PEDOT:PSS on the anode side, and C60/BCP/Ag on the cathode side, sustain continuous operation in the maximum power point for hundreds of hours without notable degradation, if only the acceptor is excited (i.e. for wavelengths > 680 nm;  hn < 1.82 eV), in contrast to the excitation of both donor and acceptor, which results in some substantial decay of efficiency. This finding opens a favorable opportunity to use PM6:Y6 cells in a tandem architecture where short wavelength components of the AM1.5 solar spectrum are absorbed by a suitable wide gap sub-cell. Here, we exemplarily show such a combination with a wide-gap perovskite cell (E_g = 1.8 eV), that as a single junction shows a V_oc of 1.3 V and an efficiency > 15%. The resulting tandem cells show a high V_oc of 2.16 V, which is the perfect addition of the V_oc’s of the two sub-cells, and a high FF of 75%, which combines to a high efficiency > 21%. This is the highest efficiency for a perovskite/organic tandem cell and the highest efficiency of a solar cells, where an organic sub-cell is involved. Further improvements seed the prospect of efficiencies in the range of 26% for these tandem devices.