engleski

Polymer-infiltrated titania nanotube-based hybrid organic solar cell investigation

Upgrade in efficiency of modern solar cell is known to depend not only on chemical composition of the active material but in significant amount also on their morphology. Titania is established semiconducting material with known application as both charge transfer and absorbing material. Furthermore in the case of specially tailored nanoarhitecture of titania material one can expect very favourable properties of thin films, for example in the case of titania nanotube based thin films. Such thin film can show significant enhancement in solar cell properties when placed between absorbing material and conductive transparent layer i.e. electrode. Preparation of TiO2 nanotube (NT) layer is concerned as a critical one for enabling high efficiency electron transport between absorbing material and electrode. The Ti anodization process was introduced as a convenient way to produce the TiO2 NT with various geometries in terms of nanotube length, diameter and wall thickness (in NH4F electrolyte of various concentrations under different current). The deposition of Ti layer (structure, morphology, thickness) was found to be crucial for ability to have successful anodization. Only several selected samples yield interface properties acceptable for further processing. Also it is worth mentioning that thermal transformation to crystalline anatase is desirable. The successfulness of this production step (in combination with previously mentioned Ti layer properties) is a necessity for homogeneous surface which is needed for solar cell application where in this case a fully controllable and reproducible preparation procedure still presents a challenge. Understanding of the transport mechanisms in titania nanotubes (with respect to their geometry) is still a matter of controversy and acts as a milestone in the future upgrade of the titania NT based solar cell performance. Common titania NT solar cells were od DSSC type, where dye was used as absorbing active material. In this investigation assembly of titania organic hybrid solar was considered. Namely small molecules and polymers could enter the tubes and ensure charge transfer effect. The idea is to quantify how favourable the mentioned effect can be. Several preparation procedures were investigated. ZnO was deposited by magnetron on glass substrate, followed by Ti layer deposition, which was anodized to form TiO2 NT, then polymer/small molecules were infiltrated by spin coating and finally metal was evaporated as an electrode. Non-contacted layers were characterized using structural methods (X-ray diffraction XRD, grazing-incidence GI), spectroscopy methods (absorbance spectroscopy UV-VIS, photoluminescence spectroscopy PL) and microscopy methods (scanning electron microscopy SEM, energy dispersive spectroscopy EDS), offering information on interfacial phenomena, excited state kinetics. Overall contacted solar cell properties were checked for electric properties (current-voltage measurements I-V, impedance spectroscopy IS) in dark and under illumination, offering information on relevant time scales for electronic transport and recombination, as well as overall stability. Effect of the charge transfer was observed, however it definitively can be better upon achieving more large scale homogeneous titania nanotube surfaces. Despite this investigation aims to investigate the TiO2 NT layer and its interaction to adjacent layers (especially absorbing one) it was not easy to confirm is the extent of infiltration of the absorbing material within the nanotubes. This work offer valuable date on the often used layers for the preparation of inorganic-organic hybrid solar cell. Best solar cell samples are worth further upgrading.

TiO2 nanotubes; Hybrid OPV; Infiltration

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