Zinc Oxide Nanostructured Extremely Thin Absorber Solar Cells

Abstract

Extremely thin absorber (ETA) solar cells are a relatively new and promising technology that could potentially lead to lower cost solar cells by using cheaper, more abundant materials and solution based processing steps. In this thesis, two aspects of a zinc oxide (ZnO) nanostructured solar cell with a cadmium selenide (CdSe) absorber layer were studied. The first was an attempt to control the density of ZnO nanowires using polystyrene (PS) microspheres, and the second incorporated a cadmium sulfide (CdS) buffer layer to improve solar cell characteristics. Using simple techniques of vertical deposition and dipcoating, PS microspheres were deposited onto ZnO seeded substrates. The desired close packed hexagonal monolayer was obtained using vertical deposition, but the monolayer was not uniform over the entire substrate. Dipcoating yields a very uniform deposition of PS microspheres, but a dense pack can only be achieved with a fast withdrawal rate and slow, controlled evaporation after dipcoating. When growing nanowires using the typical chemical bath deposition technique, it was found that the PS microspheres were unable to block the growth of ZnO nanowires. A CdS buffer layer was incorporated into the ETA cell by using the successive ionic layer adsorption and reaction (SILAR) technique. It was confirmed by both visual and UV-vis inspection that increasing the number of SILAR cycles increases CdS thickness. Scanning electron microscopy (SEM) of 33 cycles of CdS showed that aggregates form on top of the nanowires, which are not present after only 16 cycles. The CdS thickness after 16 cycles was measured using transmission electron microscopy (TEM) to be 5 nm, corresponding to a rate of deposition of 3.12 Å/cycle. Incorporating the CdS layer into both the ZnO nanowire/CdSe structure and indium tin oxide (ITO)/CdSe structure showed improved solar cell characteristics and decreased shunting. This improvement could result from an improved interface between the ZnO/CdSe and ITO/CdSe. In the case of the ZnO structure, CdS may block etching of ZnO, which is known to occur in the CdSe electrodeposition bath. In both structures, the improvement could also be a result of improved band alignment in the cell, favoring charge separation and transport.