Page 269 - A Comprehensive Guide to Solar Energy Systems
P. 269

272  A COmPreHenSIVe GuIde TO SOlAr enerGy SySTemS



             oxide which serves as the transparent electrode. Conversely, in a silicon PV the light en-
             ters the device through the top-surface, which is printed with a grid of closely spaced
             silver wires. The silver grid lines are opaque, so light enters the device through the gaps
             in the metal grid. The conducting oxide electrodes used in OPVs comprise a 100–200 nm
             thick layer of a heavily doped oxide such as F:SnO 2  or Sn:In 2 O 3  (ITO) supported on glass,
             and offer a sheet resistance ∼10 Ω per square combined with an average transparency of
             ∼80% over the wavelength range 400–900 nm. (note: The electrical resistance of a square
             sample of thin film conductor is independent of substrate area because the resistance is
             proportional to the length and inversely proportional to the width, so the resistance of
             thin film conductors is most often given per unit square without defining the area). Con-
             ducting oxide electrodes are inherently brittle and are produced using energy intensive
             batch-to-batch manufacturing processes, which renders them poorly matched to OPVs.
             Consequently it is widely recognized that a low-cost alternative is needed that is compat-
             ible with flexible substrates [9,71,72]. A number of solutions to this complex problem have
             been proposed, including electrodes based on conducting polymers, carbon nanotubes,
             graphene, and metal nanowires [71]. However, all have significant disadvantages either
             in terms of environmental concerns relating to the toxicity of the materials used (as for
             nano-Ag and Cd), the complexity of the fabrication process, unacceptably high surface
             roughness or impractically high sheet resistance, and so no clear leader has emerged. For
             large area (practical) applications the electrode sheet resistance cannot realistically exceed
             15 Ω per square without incurring excessive resistive losses, and so it is a false economy
             to increase the electrode transparency at the expense of sheet resistance beyond 15 Ω per
             square [73]. A reduction in the sheet resistance of the transparent electrode to ∼1 Ω per
             square would open the door to new electrode geometries beyond the narrow-strip designs
             currently used, which would reduce module manufacturing complexity and costs [73]. To
             date random arrays of Ag wires with a diameter of the order of 20–100 nm and printed
             silver grid electrodes offer the best performance in terms of sheet resistance and transpar-
             ency [71]. However, for application in OPVs these electrodes must be used in conjunction
             with a heavily doped conducting polymer or doped small molecule layer to span the gaps
             between wires/grid lines and smooth the surface, which inevitably complicates the fabri-
             cation process. Further serious drawbacks are instability resulting from localized heating
             at the contacts between silver nanowires [74], combined with the susceptibility of nano-
             silver toward oxidation particularly by airborne sulfur containing compounds.
                In recent years there has been growing interest in using unpatterned silver films with
             a thickness of 6–12 nm fabricated by vacuum evaporation as the transparent electrode in
             OPVs [9,64,71,72,76,77]. large area roll-to-roll deposition of metals by vacuum evapora-
             tion is an established industrial process for the manufacture of low cost food packaging
             and insulation foils, and offers excellent control over metal thickness and uniformity: For
                                                                                         −2
             example, the 20 nm thick aluminum film used for crisp packaging is deposited at ∼10  Pa
                                    −1
             at a rate of 400–800 m min . In the most rigorous economic model for the large-scale man-
             ufacture of OPVs published to date, vacuum deposition of the metal electrode is shown to
             represent a tiny percentage of the total cost of a solution processed OPV [75]. This method
   264   265   266   267   268   269   270   271   272   273   274