Perovskite-perovskite tandem solar cells (PTSCs) offer the possibility to surpass the detailed balance limit of single junction solar cells, without the need of the established but relatively expensive silicon photovoltaic technology [1]. PTSCs employ a wide bandgap (WBG) perovskite top cell (E_g: 1.75– 1.85 eV), together with a narrow bandgap (NBG) perovskite bottom cell (E_g: 1.25 eV) [2]. In the past years, the power conversion efficiencies of two-terminal (2T) PTSCs have been steadily increased with the highest reported values approaching 30% [3]. This progress has been mainly driven by improvements in open circuit voltage (V_oc) and fill factor (FF), while the short circuit current density (J_sc) has rather stagnated [4-7]. To increase the J_sc under current matching conditions in 2T PTSCs, light management strategies need to be employed, but dedicated studies for PTSCs are still lacking up to now.

In this work, sinusoidal nanotextures fabricated by nanoimprint lithography, as reported in earlier works [8], are investigated for PTSCs. The nanotextures are incorporated in superstrate configuration where the PTSC is deposited atop. Potential effects of the altered substrate morphology on the optoelectronic properties of the WBG  are studied and compared to planar references. WBG films fabricated on top of nanotextures only show minor differences with respect to the reference case. The photoluminescence quantum yield of pristine films is slightly enhanced, corresponding to a quasi-Fermi level splitting of around 1.3 eV on nanotextures. This shows that nanotexturing the substrate is not detrimental to perovskite film quality. Additionally, atomic force microscopy measurements show comparable roughness of the nanotextured and planar substrates, indicating that the nanotexture pattern is not transferred to the surface of the top cell. We demonstrate that nanotextures at the glass/TCO/perovskite interfaces reduce reflection losses by 0.5 mA/cm² for the top cell and and 0.3 mA/cm² for the bottom cell. Consequently the external quantum efficiency of both subcells is increased, leading to higher J_sc at current matching conditions. To further understand the experimental findings, optical simulations with the finite element method are performed, which study the impact of nanotextures integrated at different positions in the PTSC. The optical improvements induced by the nanotexture can serve as steppingstone for further measures in the PTSC, e.g. light-in coupling with anti-reflective coating at the air/glass interface, or light trapping strategies for improved near infrared spectral responses. The combined effects could then further increase current matching conditions and subsequently the efficiency of PTSCs.