Organic semiconducting polymers and small-molecules belong to the emerging materials for photovoltaics (PVs). Organic semiconductors have shown their potential by advancing the technology for light-emitting diodes,¹ offering thinner and flexible displays with higher image quality and response time. Meanwhile, organic semiconductors for PVs remain to lack commercial impact. On the other hand, there is continuous and rapid development that is expected to bring organic-based PVs closer to meeting the marketplace standards. Relative to other emerging PV materials, organic semiconductors offer excellent environmental and biocompatibility. As the primary purpose of scientific innovations is to push forward green technologies for environmental conservation, the potential risks and irreversible damage that modern technologies could cause should be given serious consideration. We must have already learned from ancient history that highly advanced civilizations may collapse and be erased from the historical timeline when technology abuses the environment. Hence, the search for renewable energy technologies and materials should remain at the forefront of modern research.

Given the significant advantages of organic semiconductors, allowing versatile PV applications (i.e., beyond traditional solar farms and rooftop installations) without posing a significant threat to the environment and the living organisms, we strive to continue developing up-to-date strategies to keep pushing forward the performance of organic semiconductors for PVs. Currently, we primarily focus on photovoltaic efficiency and operational stability. Recently, we have found that the energetic disorder at the donor-acceptor heterojunctions is not completely undesired,² as when properly tuned, it can offer an energetic barrier to suppress the bimolecular recombination losses and foster device fill factors > 80 %. Likewise, by introducing guest components to act as nanofillers in these heterojunctions,³ the trade-offs arising from the use of non-halogenated processing solvents can be minimized. These non-halogenated solvents are critical for transitioning towards large-scale operations,⁴ as we have further elaborated in our latest review article. By acknowledging the role of heterojunctions in refining the electronic processes and FFs, our ongoing work has focused on updating the design principles for the selection of guest components, promoting synergistic improvements toward short-circuit current density (Jsc) and open-circuit voltage (Voc). In particular, we have observed that the molecular polarizability of these guest components will influence the inherent short-range mobility of free carriers at the heterojunctions, which impacts the recombination probability. Furthermore, we also explored the use of solid additives to regulate nanomorphology beyond the limits of photo-active molecules and common post-processing treatments. We have found that the choice of processing solvent boiling point should be dependent on the volatility of the solid additive, ensuring maximized operational stability.

Overall, these findings demonstrate that there are a lot of design principles for organic-based PVs that remain to be discovered, thereby requiring the combined efforts of the academic community.