Invited Paper EM+AS+EN+TF-ThM11
Interface Science of Emerging Thin Film Photovoltaic Technologies: The Role of Interface Composition on Charge Harvesting Efficiencies

Thursday, October 31, 2013, 11:20 am, Room 101 B

Organic solar cells, while still not at the efficiencies of more mature PV technologies, are improving in efficiency at the fastest rate of all PV technologies. There are nevertheless significant technical and scientific challenges to be overcome before OPVs can be a legitimate energy conversion technology. This talk will review our recent work in understanding the role that electrical contacts play in determining the performance characteristics of OPVs. We discuss both the role of the contact/active layer interface, and the roles of electron- or hole-selective interlayer materials in modifying the properties of the contact, for both small molecule and polymer based OPVs.

We will focus first of all on the characterization small molecule systems (correlating UPS, conducting tip AFM, and device performance), where the contact can dictate the efficiency of charge harvesting through specific interactions between small molecule components and the contact. We find that for several systems orientation, charge redistribution, heterogeneity of charge harvesting, local fields and leakage currents are all influenced by these interactions.

We secondly deal with electron-selective interlayers (e.g. ZnO, TiO₂), and the role of surface/interface composition in controlling their selectivity. Interlayer materials, typically with thicknesses of 10-50 nm, are increasingly required to ensure efficient and selective charge harvesting in bulk-heterojunction polymer and small molecule based organic solar cells (OPVs) and even in solar cells based on nanocrystalline semiconductors. Selectivity is typically achieved by choosing band edge energies in interlayer materials which provide for efficient collection of only one charge carrier and minimization of surface recombination velocities. New approaches to oxide interlayers will be shown, using conventional (sol-gel) and sputtering approaches, and chemical vapor deposition, focusing on the surface characterization of electronically relevant defect states which appear to control charge selectivity, sub-micron heterogeneity in charge collection, and OPV efficiency.