Organic photovoltaic (OPV) technology has experienced substantial growth over the past decades [1,2], primarily driven by the design of innovative donor polymer and non-fullerene acceptor materials [3], enabling certified power conversion efficiencies (PCEs) exceeding 20% in single-junction devices [4]. Nevertheless, the performance of single-junction OPVs remains fundamentally limited by thermalization, spectral mismatch, and recombination losses, as defined by the Shockley–Queisser constraint. Tandem solar cell configurations provide an effective approach to mitigate these intrinsic losses by integrating multiple sub-cells with complementary light absorption, thereby enhancing overall utilization of the solar spectrum [5]. In this context, we present our recent work carried out as a joint collaboration between CHOSE labs at the University of Rome Tor Vergata and CAPE center at Southern Denmark University, which focuses on the study and optimization of Interconnecting Layer (ICL) for two-terminal organic-organic tandem devices (2T-TSCs), in inverted architecture. Compared with four-terminal configurations, 2T‑TSC devices offer lower fabrication costs and are compatible with scalable, large-area, roll-to-roll processing, making them industrially relevant [6,7]. However, they require careful current matching and solvent compatibility to prevent interlayer mixing and damage to the active layers. The front and rear sub-cells were independently optimized through the careful selection of donor and non-fullerene acceptor materials exhibiting complementary absorption in the visible and near-infrared light spectral regions. Wide-bandgap and narrow-bandgap bulk heterojunctions were designed to maximize photocurrent generation while minimizing series resistance and recombination losses. The tandem architecture was subsequently realized, through spin coating technique, using a fully solution-processable ICL consisting of n-type and p-type materials sequentially deposited. A systematic investigation of ZnO nanoparticles-based electron transport layer, processed from different solvents provided by different suppliers, was conducted to address challenges associated with interlayer intermixing, film morphology, and voltage losses. This approach enabled the identification of an efficient ohmic n‑type/p‑type interface, resulting in tandem devices with a maximum PCE of 14.9%, that consistently overcome the performance of their corresponding single‑junction sub‑cells. Based on these results, the optimized ICL architecture was successfully transferred to scalable fabrication methods via slot‑die coating, yielding to a maximum PCE of 13.6%. Large‑area tandem slot-die coated modules with an active area of 13.8 cm² were fabricated, exhibiting an open-circuit voltage of 10.8 V, a fill factor of 67.4%, and a PCE of 12.9%. These results highlight the critical role of fully solution‑processable interconnecting layers and demonstrate a viable pathway toward scalable, high-efficiency organic tandem photovoltaic technologies.