Author: Michael Wallace Rowell

Publisher:

Published: 2011

Total Pages:

ISBN-13:

Virtually all solar cells require a transparent and conductive (TC) layer on the top surface that allows sunlight to enter the cell and photoexcited charge to be conducted laterally across the top surface. The impact on power conversion efficiency due to reflection, absorption, resistive losses and lost active area is a reduction by 10 -- 25 %, relative. Historically, doped metal oxides such as tin doped indium oxide (ITO) have been used. In the last several years, however, there has been renewed interest in this area with the development of several new nanostructured materials, many of which have the potential for performance, processing, and cost advantages. In the first portion of this work, we describe TC related efficiency losses in detail for the two major categories of solar cells. For thin film monolithically integrated modules, TC related losses can be as high as 25 % and for standard modules of cell strung together losses are typically 10 -- 15 %. For the purpose of developing new TC materials, we specify the material performance requirements for a competitive TC material and show the expected TC related efficiency losses in photovoltaic (PV) modules for any material of known electrical and optical properties. We then provide an overview of the leading nanostructured materials and show that carbon nanotubes (CNT) have basic optoelectronic material properties that are superior to traditional metal oxides. We exhibit the first demonstration of a highly flexible organic solar cell using carbon nanotube films as the TC. The achieved power conversion efficiency of 2.5 % is comparable to the 3.0 % efficiency of the control device using ITO and the flexibility of the CNT device is far greater. Finally, we detail an in-depth investigation of the electrical properties of carbon nanotube networks in order to determine and understand the performance limitations. For this, we develop a novel method of atomic force microscope (AFM) scratch lithography in order to isolate individual CNT bundles and a novel method of electric force microscopy (EFM) in order to quantitatively measure local contact resistances on the nanoscale between CNT bundles. We measure bundle-to-bundle junction resistance for a range of bundle diameters which reveals a previously unobserved inverse scaling of contact resistance with diameter. Values range from 1 kOhm to 200 kOhm for bundle diameters typical of CNT TC networks. We also measure the resistivity of ropes of bundles and we find that this resistance is very high and limits the conductivity of the CNT TC films studied in this work. The structure property relationships revealed from these measurements clearly show what type of morphology control is necessary for high performance CNT TCs and that increasing tube length and/or decreasing junction resistance are the primary routes for further increases in performance.