Tandem organic solar cells: energy level alignment and morphology of recombination contacts
Speaker: Dr. K. Xerxes Steirer, University of Arizona
Series: Topical Seminars
Location: Engineering Quadrangle B205
Date/Time: Thursday, June 21, 2012, 3:00 p.m. - 4:00 p.m.
Increasing the efficiency in organic solar cells to levels which make them competitive with other solar energy conversion technologies may require the successful implementation of tandem cell configurations, where thin metal interlayers, combinations of n-doped and p-doped molecular semiconductors, or combinations of thin p-type and n-type charge selective oxides will be required as recombination layers between two current-matched OPVs. In most cases the interface chemistry and electronic properties of these interlayers are poorly understood, and require significant optimization prior to their implementation.
This talk will focus on the formation and characterization of two types of potential interlayer films: a) ultra-thin metal films based on vacuum deposited Ag and Au nanoparticles and, b) ultra-thin oxide films based on combinations of ZnO and NiOx. We will discuss the types of compositional and frontier orbital energy changes which occur between the metal nanoparticles and a common organic donor, CuPc and acceptor, C60 as revealed by X-ray and UV-photoemission spectroscopies (XPS/UPS), and by AFM studies to probe local changes in morphology at sub-micron to nanometer length scales correlating differences in metal type, coverage and structure with changes in device performance in prototype tandem OPVs. In the case of ZnO/NiOx interlayers we demonstrate the formation of rectifying heterojunctions, where the electrical properties appear to be dictated and controlled by the band edge offsets between the two oxides (revealed by UPS), and by the processing conditions previously optimized to make these useful charge selective interlayers in single junction OPV platforms.