Organic solar cells (OSCs), which have recently achieved 20% efficiency, have drawn growing attention from the research community due to their potential in optoelectronic device applications with advantages of flexibility, transparency, low weight, ease of manufacturing, and so on. The advance stems from the development of non-fullerene acceptors, particularly the Y-series acceptors, and high-performance polymer donors like PM6. While efficiency improvements have been substantial, the underlying charge carrier dynamics remain a topic of debate. This uncertainty hinders a deeper understanding of performance gains and limits broader applications of organic photovoltaics, such as in transparent solar technologies.

In this work, to understand the working principle of OSCs, including charge generation, exciton dissociation, charge transfer (CT) state, and charge recombination, a series of devices based on PM6:Y6 were fabricated with a wide range of donor-to-acceptor (D:A) ratios, from 5% to 95%. This variation allows for systematic control of domain sizes, producing either donor- or acceptor-rich regions and enabling precise tuning of the donor–acceptor interface, from sparse, isolated contacts at extreme ratios to well-mixed morphologies near a 1:1 ratio.

As the D:A ratio varies, the absorbance spectra reveal a gradual shift in both peak intensity and position. Correspondingly, the photoluminescence (PL) spectra show varying degrees of quenching for both donor and acceptor components. Notably, a dilution effect is observed for the acceptor, evidenced by a gradual blue shift in its PL peak with increasing donor content. Time-resolved photoluminescence (TrPL) measurements provide exciton lifetimes for each component. The photoluminescence quenching efficiency (PLQE) of the donor remains consistently high—exceeding 90% across all D:A ratios—indicating efficient energy transfer. In contrast, the PLQE of the acceptor follows an asymmetric parabolic trend: it increases from 26% to 92% with rising donor content, then slightly declines to 87% at higher donor concentrations.

Turning to the device performance, Y6 with 5% PM6 can work decently with an efficiency of 4%, whereas PM6 with 5% Y6 can hardly function as a solar cell. Short-circuit current displays an asymmetric parabolic trend corresponding to the exciton splitting efficiency. A similar trend is also observed in the fill factor, which also indicates the change in charge recombination. While open-circuit voltage shows a linear change. Detailed characterization of charge carriers dynamics using ultrafast spectroscopy has been conducted from fs-ps to ns-µs. Preliminary results reveal distinct charge generation and recombination behaviors depending on the donor–acceptor ratio. This work helps to understand the processes from exciton to CT, CT to free carriers, and charge recombination, with a focus on the role of interfacial dimensions.