In recent decades, renewable energy sources, such as wind, solar, and hydroelectric power, have garnered immense interest in various scientific and technological areas of research, with solar power being one of the most promising fields. Rapid advancements in the power conversion efficiency of organic solar cells, from 2% to 19.19% [1, 2] in just over fifteen years of research, have shown the conceivable possibility of replacing silicon-based photovoltaic (PV) devices with cheaper polymer-based alternatives. Key factors in such a possible transition are the unique properties of organic solar cells. The manufacturing costs of polymer-based devices are significantly lower compared to traditional silicon PVs, they are less dependent on finite natural resources, and they are generally less eco-toxic [3, 4]. Additionally, polymer-based devices can be flexible and transparent, with their color modifiable and adjustable, allowing for a plethora of aesthetic and functional organic PV device options [5].
This work investigates the charge carrier dynamics of organic bulk-heterojunction solar cells based on a D18:Y6 donor/acceptor blend, with different hole-transporting layers (HTLs) composed of PEDOT:PSS, PTAA (hole-transporting materials), 2PACz, and MeO-2PACz (self-assembling monolayers). Charge carrier lifetimes of classical inverted ITO/HTL/D18:Y6/PMMA/Ag and modified inverted ITO/HTL/D18:Y6 organic solar cells were investigated by applying the kinetic photoconductivity method and varying the excitation conditions of the samples. The charge carrier dynamics of both cell structures were compared. The results showed that PEDOT:PSS and PTAA performed best as the hole-transporting layers in classical inverted solar cells, while only PTAA showed a significant effect in modified inverted cells. Additionally, the results demonstrated that the modified inverted structure is a suitable alternative for investigations of charge carrier dynamics in organic solar cells.