All-perovskite tandem solar cells (APTSCs) have the potential to exceed the detailed balance limit of single‑junction solar cells, and, being a thin-film technology, promise a cost-effective large-scale production. APTSCs combine a wide bandgap (WBG) top cell (E_g ≈ 1.80 eV) with a narrow bandgap (NBG) perovskite bottom cell (E_g ≈ 1.25 eV). In the past years, the power conversion efficiency (PCE) of 2-terminal (2T) APTSCs increased steadily, now surpassing 30% [1]. In this contribution, we will present our recent results on APTSCs, focusing on both, the reduction of electrical and optical losses.

To increase the current density of APTSCs, we apply different strategies to minimize optical losses. The optimization of TCOs allows for the tuning of layer thicknesses to spectrally shift the interferences. Moreover, we implement nanotextures into our APTSCs (Figure 1a) and make use of the reduced parasitic absorption in the NBG’s hole-selective layer. These measures allow to increase the photogenerated current density by more than 1 mA/cm² under current-matching conditions.

For the improvement of the optoelectronic properties of APTSC, we studied the suppression of non-radiative recombination losses through surface treatments at the interface between perovskite and electron transport layer (ETL) in both, WBG and NBG subcell. We found that a treatment with piperazinium iodide (PI) virtually eliminates non-radiative recombination losses at this interface (Figure 1b), enabling V_OCs up to 1.36 V [2]. Further efforts address the replacement of the standard hole-transport layer (HTL) PEDOT:PSS, by self-assembling monolayer (SAM) materials in the NBG perovskite cell [3]. We find that the pseudo-halide additive lead-thiocyanate (Pb(SCN)₂) has a significant influence on the performance for both, PEDOT:PSS- and SAM-based NBG perovskite solar cells. While additive engineering with SCN‑compounds is highly important for PEDOT:PSS-based NBG perovskite solar cells, our study demonstrates that it strongly limits the performance of SAM-based devices. In a next step, to further improve the performance of NBG subcells, we improve the binding of SAMs to the substrate, which yields V_OCs >0.85 V and thereby surpasses the photovoltaic performance of PEDOT:PSS-based devices (Figure 1c).

Finally, the combined efforts described above enable APTSCs with a champion PCE of 27.5% (Figure 1d). Our contribution will also address pathways to improve the optical and electronic quality of APTSC and to enhance their PCE beyond 30%.