Linnaeus Physics Colloquium: Processes at molecular interfaces: opportunities and challenges in polymer solar cells

Lecturer: Professor Ellen Moons, Karlstad University, Sweden
Title: Processes at molecular interfaces: opportunities and challenges in polymer solar cells

Place: Kalmar – room N304, Norrgård. Växjö – through link, room D0073, building D. Live – through Adobe Connect, https://connect.sunet.se/cmp-kalmar.

Coffee and buns at 13.45 at Norrgård, room N304.

Abstract

Light absorption, charge generation and transport processes in a polymer solar cell are strongly affected by the distribution of the electron-donating and the electron-accepting molecules in the thin molecular films that make up the photoactive core of the solar cell. This nanostructured network of electron-donating polymers and electron-accepting molecules (usually fullerenes) forms a bulk heterojunction, which is a result of the demixing that occurs when a thin film is coated from a blend solution. Morphology tuning of this bulk heterojunction has indeed been a major key to the recent improvements in energy conversion efficiency of polymer-based solar cells, with record values of 13%. The nanostructure is determined by the donor-acceptor-solvent interactions, the molecules' tendency to self-organise, and the kinetics of the film formation.

To achieve the ability to predict and control the morphology, a better understanding of the molecular interactions and the drying process is needed. We use scanning probe methods and synchrotron-based X-ray spectroscopy and microscopy to study the structure and composition of the thin films of polymer/fullerene blends. For instance, with near-edge absorption fine structure (NEXAFS) spectroscopy we probe unoccupied electronic states, which results in spectra that are fingerprints of the molecules in the surface region of the film. With Scanning Transmission X-ray Microscopy (STXM) we can then probe compositional variations in the plane of these blend films.

Also the interfaces between the photoactive layer and the electrodes are crucial to an adequate charge collection. We develop therefore stable, transparent and conducting materials that are used as interfacial layers in order to improve the efficiency and stability of polymer solar cells.

To achieve commercially viable solution-processed solar modules, materials with a good thermal and photochemical stability are required. Understanding the degradation processes of the molecular semiconducting components is therefore also a critical question to enhance the stability of the solar cells, which are continuously exposed to external factors such as light, heat, in-diffusing oxygen and humidity. Fullerene derivatives, which are among the most used electron-acceptor materials, have a tendency to photo-oxidize in the presence of oxygen. Therefore, finding non-fullerene acceptors with a good photochemical stability is one of the new challenges for the field.