Advances in organic solar cells

The power conversion efficiency of organic solar cells has increased to values approaching 20% in recent years, largely driven by the development of new acceptor molecules. This special issue provides an overview of the recent advances in the field of organic photovoltaics with a focus on such non-fullerene acceptor materials.

A current prominent research topic in the field is the investigation of the electro-optical properties of these non-fullerene acceptors (NFAs) and their interactions with donor counterparts. Hasenburg et al. address charge transport in the NFA IT-4F using different techniques, such as space-charge-limited current measurements and molecular simulations.¹ Raab et al. investigate the film forming properties of one of the field-leading polymer:NFA blends, PM6:Y6, by in situ transmission measurements during the spin-coating process. With this method, they show that using the additive chloronaphthalene (CN) in the spin-coating solution leads to a vertical segregation and helps form an improved film morphology.² Pranav et al. investigate the non-halogenated sibling to Y6, namely Y5. They demonstrate by time-delayed collection field (TDCF) measurements that the reduced fill factor and photocurrent are limited by a field-dependent free charge carrier generation in such blends.³ Jungbluth et al.⁴ present fundamental studies on the role of different intermolecular interactions and their impact on energy levels, supporting or hindering charge separation by investigating ZnPc and its fluorinated derivatives combined with C₆₀ in donor–acceptor solar cells. As Zhang et al. describe in their review, ternary blends based on NFAs constitute a promising avenue for high-efficiency photovoltaics. Here, the design rules, material selection, fabrication methods, and fundamental working principles are described.⁵ 

Device fabrication considerations, including processing methods and appropriate selective contact layers, are also critical for the performance and scalability of NFA-based organic solar cells. Most NFAs are too large to be vacuum processed and will degrade in an evaporation process. Habib et al. demonstrate initial results on vacuum deposited organic solar cells based on the NFA BTIC-H.⁶ In the article by Georgiou et al., the importance of selective contact layers to the organic active layer is demonstrated. By doping a SnO₂ layer with antimony combined with a PEI layer, they demonstrate an improved performance for P3HT:PCBM and also for a NFA solar cell, P3HT:IDTBR.⁷ A perspective by Hong et al. describes the recent advances in the fabrication of organic solar cells based on low-toxicity or non-toxic solvents. The possibilty of large-scale and large-area production is an important consideration for the fabrication of commercial products. Currently, most devices are still prepared with halogen-containing solvents via spin-coating. While this delivers the best organic solar cell performance to date, eco-friendly methods that can be scaled to high-throughput production will be preferable for commercialization.⁸