Extremely thin absorber solar cells

Hasti Majidi

doi:10.17918/etd-7380

A typical extremely thin absorber (ETA) cell comprises an extremely thin absorber layer at the interface between nanostructured, interpenetrating n-type and p-type semiconductors. Compared to planar photovoltaics, the large surface area of the nanostructured interface allows a thinner absorber layer to be used without sacrificing absorber volume. Photogenerated charges must be separated at the absorber interfaces before they recombine. Shorter distances for charge separation in ETA cells allow using materials with lower purity and higher defect densities, enabling use of inexpensive, solution-based processing methods. We report on electrodeposition of CdSe coatings onto ZnO nanowire arrays and determine the effect of processing conditions on material properties such as morphology and microstructure. We show that room-temperature electrodeposition enables growth of CdSe coatings that are uniform, and conformal with precise control over thickness and microstructure. Optimal range of current density was determined to achieve uniform and conformal coatings. Absorbers thinner than the charge collection length reduce bulk recombination and enhance charge separation at the interface. The charge collection length is directly related to the microstructure of the absorber coating. We report on the crystallite growth of electrodeposited CdSe coatings on ZnO nanowires during annealing. Both in situ transmission electron microscopy (TEM) and x-ray diffraction (XRD) reveal that the nanocrystal size increases from ~3 to ~10 nm upon annealing at 350 C for 1 h and then to more than 30 nm during another 1 h annealing at 400 C. Increased crystallite size, comparable to the coating thickness, shows a nearly two-fold improvement in power conversion efficiency upon annealing. In addition to the crystallinity of the coating, the absorber thickness affects the charge separation in ETA cells. Coatings that are too thin show poor light harvesting. However, too thick coatings show inefficient charge separation. We demonstrated optimum thickness of CdSe in the planar ETA cell. In addition, extremely thin coatings suffer from large interfacial recombination. Severe shunting in very thin absorbers is tackled by applying an ultra-thin (5 nm) coating of CdS on ZnO nanowires before CdSe electrodeposition. Interface engineering in ETA cells leads to five times improvement in the power conversion efficiency.

Extremely thin absorber solar cells

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Extremely thin absorber solar cells

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