Joint ChBE/UMERC Seminar: Teresa Barnes
Monday, April 26, 2010
Room 2110, Chemical and Nuclear Engineering Bldg.
Professor Sheryl Ehrman
Flexible and Solution-Processible Transparent Contacts For Organic Solar Cells
Presented by Teresa Barnes
National Renewable Energy Lab, Golden, CO
Most potentially low-cost photovoltaic (PV) device architectures, including CdTe, CuInxGa1-xSe2 (CIGS), thin-film Si, and organic PV rely on a transparent conducting oxide (TCO) contact. Figure 1 shows the location of the TCO in each structure. The CdTe (a) and OPV (b) superstrate structures require a TCO with very high conductivity and one that is chemically and thermally robust against the subsequent deposition steps. The CIGS (c) and a:Si (d) substrate structures can use a less conductive TCO with a grid, but the deposition process must be compatible with the existing device. These examples show that different device structures require different TCO properties. Here, the focus is on OPV with some discussion of conventional thin-film PV.
Organic solar cells are appealing as a cost-effective PV technology because they are highly amenable to rapid, scalable, low-temperature, and low-cost printing processes. This is certainly true of the polymer active layers in the devices, but high-quality solution processed transparent contacts have not yet been demonstrated. Organic devices are most often fabricated using a sputtered or evaporated In2O3:Sn (ITO) transparent contact because of its excellent opto-electronic properties, commercial availability, and patternability. However, ITO is brittle and subject to corrosion in OPV device structures, particularly those containing PEDOT:PSS.
Here we discuss spray deposited single-wall carbon nanotube (SWCNT) networks and other sprayed nanostructured networks in comparison with ITO. We have demonstrated many OPV devices using SWCNT contacts, including devices made without PEDOT:PSS or any hole transport layer. These devices often out-perform expectations based on contact sheet resistance and transparency by generating high short circuit currents (11.7 mA/cm2, NREL certified measurement) and efficiencies greater than 2.5%. We believe that interfacial interactions between the SWCNT networks and bulk heterojunction polymers contribute strongly to device performance. We will also discuss how to choose and compare transparent contacts for use in OPV devices, and we will use these comparisons to tune the properties of the nanostructured networks for optimal device performance.