Plastic (or polymer) semiconductors offer a unique combination of mechanical, electrical and optical properties paired with the possibility to process them on flexible substrates by printing techniques. This makes them attractive as a cheap solution for energy harvesting, lighting or display applications. Their inherent strength are strong absorption and emission of light which are fundamentally important for solar cells or light-emitting diodes (LEDs). However, the most efficient solar cells and LEDs additionally require an exceptionally good and barrier-free transport of charges.  

Unfortunately, the electrical properties of plastic semiconductors are not an inherent strength: The relatively weakly bound and almost chaotic microstructure of plastic semiconductors as well as their comparatively low chemical purity, tend to limit their performance. We show, that a lot of this is caused by water molecules trapped in these soft materials. We have found a way to passivate detrimental water molecules in plastic semiconductors and achieved significant improvements in their performance. The passivation is done by a chemical trick that binds water molecules to an additive in the polymer, preventing it from interacting unfavorably with the semiconductor (Figure below). This way, the electrical properties of polymer diodes (such as used in solar cells or LEDs) could be improved by several orders of magnitude. We were even able to demonstrate that we can now approach a material intrinsic limit, a first for plastic semiconductors. The removal of water using our “additive trick” furthermore yielded a significant improvement in the overall lifetime and stability of diodes made from these materials.

Since the electrical performance is one of the key factors that currently limits the performance of printed solar cells and diodes, our work, therefore, constitutes an important proof-of-concept demonstration how their overall efficiency can be improved in the future. Our findings are likely to have important applications for improving not only the charge transport properties of polymer diodes, LEDs and solar cells, but might also allow the study and improvement of other fundamental characteristics of these devices.

Blog post and illustrations jointly conceptualized by Mark Nikolka and Deepak Venkateshvaran.