Molecular Orientation and Layer Formation in SoS-Deposited Organic/Perovskite Solar Cells

Organic (and perovskite) photovoltaics (PVs) are poised to deliver lightweight, flexible, and potentially low-cost solar modules – but their performance hinges critically on the molecular orientation and nanostructure of the active layer. In many high-efficiency systems, a face-on orientation of donor and acceptor molecules, combined with optimized phase separation, is essential for efficient charge generation and transport.

Current deposition methods such as spin coating or vacuum processing offer limited control over orientation at scale. Spray-on-Screen (SoS) is an emerging alternative that can produce uniform ultra-thin layers over large areas via droplet-mediated forced wetting through a mesh. The unique droplet-to-film pathway in SoS may be exploited to actively tune molecular orientation. This has however not yet been systematically explored.

In this PhD project, you will connect SoS process physics and engineering to OPV device performance. You will investigate how droplet dynamics, mesh design, and solvent choice influence film formation, crystallinity and molecular orientation, using GIWAXS, NEXAFS and AFM to characterize structure, and complete PV devices to correlate these structures with efficiency and stability. You will work at the interface of thin-film device engineering, organic electronics, and advanced characterization, collaborating with international partners experienced in PV morphology and orientation studies.

In PVs specifically, the orientation and packing of donor and acceptor molecules in the active layer strongly influence exciton dissociation, charge transport, and recombination. Recent studies show that controlled orientation can raise power conversion efficiencies beyond 18% in optimized systems. However, achieving such control in scalable, solution-processed layers remains a major challenge. In our research group, we have proposed SoS as a platform for large-area, ultra-thin OPV layers (<15 nm), combining an ultrasonic spray with a mesh to manipulate droplet impact and forced wetting. This approach has clear advantages in uniformity and scalability, and is compatible with integration with spatial ALD for transport and barrier layers for upscaled devices. Yet, the link between SoS processing conditions and molecular orientation in OPV materials is unknown.

Furthermore, SoS introduces additional degrees of freedom absent in spin coating: droplet size distributions, mesh geometry and material, and the dynamic wetting regime at the mesh and substrate. Together with solvent selection and post-treatments (thermal/solvent annealing), these parameters may provide a powerful toolbox to steer molecular packing and phase separation. The key open question is how to exploit SoS-specific parameters to deliberately induce favorable molecular orientation and microstructure in OPV active layers, thereby enabling high-efficiency devices produced via a scalable, environmentally friendly process.