Invited Lecture
The diffusion of molecules in organic solar cells

Organic Solar Cells (OSCs) have emerged as a well-performing, solution processed class of thin film solar cells. In such devices, the active layer is composed of organic electron donor and electron acceptor moieties blended on the nano-scale. The complex morphology of the blend, which includes ordered, disordered and mixed domains, determines the overall device performance. However, the blend is prone to kinetically and/or thermodynamically driven changes, due to its “soft” features, which often limit and deteriorate the devices. Here we will show that the dynamic behaviour of the blend morphology can be harnessed to improve performances and stability of OSCs. More specifically, we use the diffusion of small molecules through the organic films to direct and study the morphology and performance. This approach will be demonstrated in two systems: i) the spontaneous migration of selected additives from within the active layer to form interlayers at the organic/metal interface. Using a plethora of techniques, we show that the diffusion, which occurs during the electrode deposition, is driven by the metal-additives interactions and can significantly enhance the device efficiency. And ii) the diffusion of atomic layer deposition (ALD) metal-organic precursors into the pre-formed active layer is used to map the blend morphology. The selective precursor diffusion through the amorphous domains spatially maps the amorphous and crystalline domain of the donor polymer, and the distribution of the acceptor in the film. This allows us to directly correlate the nano-scale morphology with macro-scale device performance.