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Bredas-Kahn July 2013 highlight

Bredas-Kahn July 2013 highlight

Read more about this collaborative research in our July 2013 Highlight

Publications collage

Publications collage

An important goal for CISSEM is to facilitate highly visible and wide-spread dissemination of the results of our interfacial research.

Kippelen plastic solar cell

Kippelen plastic solar cell

From left to right, Jaewon Shim, Professor Bernard Kippelen, Canek Fuentes-Hernandez, and Yinhua Zhou (first author on the Science article) from Georgia Tech, displaying a completely plastic solar cell.
Photo credit: Georgia Institute of Technology

GT graduate student in clean room

GT graduate student in clean room

photo credit: Yongjin Kim, Georgia Tech

Arizona graduate student working in the lab

Arizona graduate student working in the lab

photo by davidsandersphotos.com

Students postdocs and scientists receive outstanding training

Students postdocs and scientists receive outstanding training

Students, postdocs, & scientists in CISSEM experience outstanding training. photo by Jim Bosch, NREL

News & Updates

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Research funded as part of CISSEM at Georgia Tech (Brédas, Marder), The University of Arizona (Ratcliff) and the National Renewable Energy Laboratory (Berry) combines theory and experiments, in excellent agreement, to achieve a comprehensive understanding of the energetics for a gallium‐doped zinc oxide (GZO) surface modified with five different organic phosphonic acids (PAs) to tune surface and interface properties. Density functional theory (DFT) calculations (with a repeated-slab approach) and ultraviolet photoelectron spectroscopy measurements reveal and describe changes in the density of states features at the GZO valence band edge after PA depositions. Such insight into energy level alignments of the PA molecule frontier molecular orbitals with the valence band edge and Fermi level of the GZO surface, are important for organic optoelectronic applications. The new interfacial states created by PA surface modifiers can impact charge injection or extraction with adjacent active organic layers, which can be critical to optoelectronic device performance. This excellent agreement between measured and DFT-calculated energy level alignments and density of states features is rather unusual – attributed here to the strong bonding between PAs and GZO surface zinc atoms.

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Organic photovoltaic solar cells potentially offer light weight, mechanical flexibility, and low cost roll-to-roll fabrication – especially when manufactured by vacuum-free processes in ambient air with polymeric electrodes. Transfer lamination is a dry process that avoids issues from wetting and film damage caused by solvents used in spin coating or other additive wet-deposition methods. Research funded as part of CISSEM at Georgia Tech (Kippelen) has demonstrated the first semitransparent, all-plastic solar cells prepared in ambient air using sequential dry film-transfer lamination of the photoactive layer and a high-conductivity polymeric top electrode. Current–voltage characteristics of our all-plastic cells were successfully measured as a function of light irradiance over five orders of magnitude; from the dark to the one-sun standard solar spectrum. By using dry film-transfer lamination we realize a very low density of defects for the active layer – much superior to a spin-coated active layer. Consequently, these all-plastic solar cells have a high photovoltaic dynamic range. They produce a photovoltaic response even when the one-sun incident irradiance is attenuated by as much as one million. The extremely low dark current under reverse bias shows the potential of dry film-transfer lamination to produce quality, well-interfaced, low-defect active layers, while easing solvent selection for organic electronics. Our results demonstrate the prospects of organic solar cells for light-weight, portable, stand-by power generation under room light, and even weaker, illumination.

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Self-assembled monolayers (SAMs) provide versatile, well-studied, and straightforward methods of modifying contact/active-layer interfaces in organic solar cells (OPVs). Research funded as part of CISSEM at the University of Washington (Ginger), Georgia Tech (Marder) and the National Renewable Energy Laboratory (Olson) investigated the effects of phosphonic acid SAMs, possessing dissimilar surface dipoles, on open-circuit potential and recombination kinetics (charge carrier lifetimes). For a series of model, polymer/fullerene-derivative blended-heterojunction OPVs in which each hole-extracting, indium tin oxide (ITO) contact is surface modified with a different SAM to vary the contact work function, we have measured charge-carrier lifetimes and densities using transient photovoltage and charge extraction. Our studies show how charge-carrier recombination rates and charge densities change with SAM type under OPV operating conditions. By altering the internal electric field near the ITO contact, such SAM chemical modification at the contact/semiconductor interface can help prevent undesired surface recombination of charges at the hole-extracting contact. Our results highlight the importance of understanding the roles of surface recombination at charge-collecting contacts in OPVs and reveal the potential to improve photovoltaic performance by careful tailoring of properties at contact/semiconductor interfaces – such as contact work function and interfacial doping.

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Message From The Director

Neal Armstrong and CISSEM map

Welcome to the Center for Interface Science: Solar Electric Materials.  We are an EFRC established in 2009, funded by the U.S. Department of Energy, Office of ScienceOffice of Basic Energy Sciences which contribute to our nation’s development of economical, terawatt-level solar energy sources for the 21st century. CISSEM is comprised of a great team of scientists, engineers, and staff located at major universities and research centers in Arizona, Colorado, Georgia, New Jersey, and Washington. An integral part of our mission is to inspire, recruit, and train future energy scientists and leaders in the basic science of solar electric energy conversion.  Our research is focused on the basic science underpinning the development of thin-film photovoltaic energy conversion technologies by understanding and controlling the electronic properties of critical regions called “interfaces” on nanometer length scales.  The chemical composition and energetics of these interfaces significantly affect the overall efficiency and lifetime of solar cells.

Neal R. Armstrong



Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Basic Energy Sciences,
under Award Number DE-SC0001084
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