Three-Dimensional Nanoarchitectured Transparent Conducting Oxides: Synthesis, Characterization and Photovoltaic Applicat
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Three-Dimensional Nanoarchitectured Transparent Conducting Oxides: Synthesis, Characterization and Photovoltaic Applications Zhenzhen Yang, Tao Xu* Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA * Corresponding author, Email: [email protected] Abstract The photovoltaic materials in solar cells take multiple tasks including absorbing lights, separating the light-induced electron-hole pairs, and consequently transport charges to the corresponding metallic electrodes. These tasks, however, are often mutually conflicting. In particular, a thick PV layer is desired to absorb enough light for creating sufficient light-induced charges, while a thin PV layer is also desired to shorten the charge transport path length insider the PV layer in order to suppress recombination. Using dye-sensitized solar cells as an exploratory platform, this dilemma is mitigated using a non-traditional 3-dimensional (3-D) highly doped fluorinated SnO2 (FTO, core)-TiO2(shell) nanostructured photoanodes. The FTO core serves as conductive core for low-resistance and drift-assisted electron extraction. The thin, conformal and low-doped TiO2 shell layer is coated by atomic layer deposition, which provides a large area for anchoring dyes and maintains a large resistance against recombination. Introduction The search for highly efficient photovoltaic (PV) systems often encounters conflicting demands on materials and structures. This can be particularly exemplified by dye-sensitized solar cells using the traditional configuration of a thick porous semiconducting layer sandwiched between a planar transparent conducting oxide (TCO) electrode and a counter electrode. The semiconducting layer typically consists of a dye-sensitized TiO2 nanoparticle (NP) layer soaked in redox electrolyte. The sensitized TiO2 NP layer must simultaneously capture light (through the anchored dye molecules), separate the light-induced charges, and convey the electrons to the distant TCO. The use of low doping (103 S/cm, (107 times greater than TiO214) due to its high carrier concentration (>1020/cm3) and carrier mobility(65 cm2V-1s-1)15. With all these regards, it is worth scrutinizing the functions of TCO in DSSCs for the purpose of enhancing charge extraction. In conventional DSSCs using a planar TCO, the primary role of the TCO electrode is to extract charges from the base of the TiO2 NP layer to the external circuit (metal wires), while still allowing incident light to be transmitted.14,16 In this sense, a rational electron extraction scheme would be to minimize the transport distance in the slow (diffusive transport controlled) TiO2 layer by relocating the TCO from the base of the TiO2 NP layer into the close proximity of the TiO2-dye interface where charge separation occurs, while still retaining sufficient surface area of the TiO2 layer. To achieve this goal, we aim to aggressively alter the 2-D flat TCO film to become an integral 3-D TCO NP network with all TCO surfaces shelled by a thin conformal TiO2 layer.17 As depicted in Fig
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