Hybrid Photoelectrochemical Systems Based on Self-Organized TiO2 Nanotubes ALD Coated with Chalcogenides
Authors: Krbal Milos , Bures Filip , Zazpe Raul , Prikryl Jan , Dvorak Filip , Sopha Hanna , Macak Jan M. 
Published: February 2020

Development of advanced types of solar cells has tremendously accelerated in recent years all activities in the photovoltaics (PVs) driven by the needs to produce solar panels with as high efficiency as possible at lowest cost possible [1]. Realizing that classical silicon solar cells have their limits, such as poor function at low light intensities, lots of research has been carried out past two decades towards alternative technologies based on thin film solar cells (amorphous Si-H, CIGS, CdTe) [2], perovskite cells [3], dye-sensitized cells [4], and organic cells [5]. Even though, the overall efficiencies of advanced photovoltaic devices have grown up significantly (and this goes hand in hand with the development of production technologies), there is so far no solar cell that would have reliable stability and performance over many years of the cell service, that would be cheap, environmentally reasonable and potentially flexible. One of most competing technologies to silicon solar cells, when considering the efficiency, low-cost production and stability is based on thin films of semiconducting chalcogenides, such as Cu(In,Ga)Se2 (CIGS) [6,7] and Cu2ZnSn(Se,S)4 (CZTS) [8]. Both became recently materials of the choice as they represent in thin film solar cells chromophores of adjustable band gaps, good radiation stability and high optical absorption coefficient.
For solution processed CIGS and CZTS thin film PVs cells, however, the limiting factors for further enhancement of the conversion efficiency involve the shape, size and grain boundaries of the chromophore films. The film morphology, defects and character of the grain boundaries predetermine the mobility (the loss) of free carriers in the chromophore film resulting in conversion efficiency maximum beyond ~11 % for CZTS materials and multilayer solar cell design [8].
One of the possible solutions how to improve the carrier mobility of semiconducting chalcogenides to the highest possible level is to use hybrid photocells employing a highly ordered TiO2 nanotube film /chromophore interface. However, the major issue to extend the functional range of nanotubes is to coat homogenously tube interiors by semiconducting chalcogenides in order to achieve the best possible contact of both components on their interface. This is especially crucial when high aspect ratio semiconducting TiO2 nanotube arrays are utilized [9, 10] and thus the Atomic Layer Deposition technique becomes beneficial.
The presentation will show initial photo-electrochemical results for anodic TiO2 nanotubes employed as highly ordered electron-conductive supports for host materials coated using ALD with secondary materials to enhance light absorbing capabilities of such hybrid systems. We will focus on all ALD photo-electrochemical devices based on inorganic chalcogenides [11].

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