Investigation of ZnO nanrod solar cells with layer-by-layer deposited CdTe quantum dot absorbers

Content Provider	Semantic Scholar
Author	Briscoe, Joe
Copyright Year	2011
Abstract	Innovation in solar cell design is required to reduce cost and compete with traditional power generation. Current innovative solar technologies include nanostructured dye-sensitised solar cells and polymer solar cells, which both contain organic materials with limited lifetime. This project aims to combine the advantages of ZnO nanorods and quantum dot (QD) absorbers in an all-inorganic solar cell, using the layer-by-layer (LbL) process to increase light absorption in the cell. The parameters that affect the aqueous chemical growth of ZnO nanorods were investigated on Ag-coated substrates in order to improve the density and alignment of the nanorods. Rods 3–6μm long and 90–500 nm in diameter were grown on fluorinedoped tin oxide (FTO)-coated glass substrates for use in solar cells. ZnO nanorods were doped with antimony (Sb) in-situ during their aqueous synthesis to make them p-type. Direct addition of Sb acetate to the reaction adversely affected the nanorod morphology, which was avoided by first dissolving the Sb acetate in ethylene glycol. Optical and electrical properties of the nanorods were altered with Sb-doping, but p-type behaviour was not proven conclusively. ZnO nanorods were conformally coated with CdTe QD-polymer films using a LbL process. Increasing the number of coated layers increased the level of light absorption at wavelengths of 500-900 nm due to absorption by the QDs. Air annealing of the QD-polymer films above 200 ◦C led to oxidation of the film, which did not occur when annealing in vacuum. Annealing in vacuum up to 350 ◦C led to a slight reduction in quantum confinement effects attributed to increased interaction between the nanoparticles due to reduced separation. At 450 ◦C the polymer was completely removed and no quantum confinement remained in the CdTe. To complete the solar cells CuSCN was deposited between the LbL-coated ZnO nanorods by repeatedly spreading a solution of CuSCN in propyl sulphide on the surface and allowing it to dry. This film filled between the coated nanorods, but the drying and quantity of solution used had to be carefully controlled to avoid cracking in the film. Spin-coating of CuSCN solutions was attempted, but films suitable for solar cells were not produced. Poly(styrenesulfonate)-doped poly(3,4ethylenedioxythiophene) (PEDOT:PSS) was deposited by spin-coating as an alternative to CuSCN, but the film only penetrated ∼200 nm below the nanorod tips. Solar cells were produced with different thicknesses of LbL films, annealed components and other variations. A model was proposed whereby carriers are extracted from the LbL film through exciton transfer between QDs. Annealing of the ZnO nanorods in air and reduction of the cracks in the CuSCN film both improved the efficiency of the solar cells. Annealing of the LbL film in vacuum improved the performance of the cells by increasing the efficiency of charge transfer. In devices with annealed LbL films 50-layer devices had higher efficiency than 30-layer devices and cells using CuSCN had a higher efficiency than those with PEDOT:PSS. The best cells produced used 50 layer CdTe-polymer films annealed at 350 ◦C in vacuum with CuSCN. These produced an energy conversion efficiency of 0.0062 %, which compares to 1–3 % for similar cells in the literature and 10–20 % for commercial devices.
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Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article