Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
We investigate the correlation between molecular scale morphology and charge generation across hybrid photovoltaic interfaces made of metal oxides (ZnO and TiO2) and a prototypical electron donor polymer, P3HT. We use device characterization and UV-NIR transient absorption spectroscopy to demonstrate that the local disorder of the polymer chains on the surface of the metal-oxide film provides better electron injection efficiencies than the crystalline phases, though the latter are essential for energy and charge transport.
One of the major challenges of the ultra-fast spectroscopic studies is to disentangle interface specific injection dynamics from the bulk polymer contribution. In this work, we directly monitor the charge injection into the metal-oxide by tracking the Burstein shift of its band-edge using 150 fs transient absorption spectroscopy. This technique is sensitive to the conduction band occupation in the oxide semiconductor. Thus it provides an unambiguous fingerprint of the charge transfer at the interface to a site where they can effectively be transported. Combining the spectroscopic data with those on device parameters, we assess the role of disorder in charge generation across the interface.
a) Ultrafast Burstein shift dynamics in ZnO at the probe wavelength of 365 nm, after electron injection from the photo-excited P3HT (regio-regular and regio-random morphologies). b) Illustration of efficient injection from the disordered polymer morphology into the metal-oxide