engleski

The role of perovskite type of barium titanate buffer layer on hybrid solar cell performance

The most important task in multilayer solar cell (SC) is to ensure overall compatibility of the active and passive layers. In the 3rd generation solar cells the charge transfer layer (CTO) is concerned as a critical one for enabling high efficiency electron transport between the absorbing material and electrode. The use of various nanostructural arrays may allow additional tuning in addition to chemical composition. Namely, layers roughness ensures active material can be thin enough to overcome the problem of low lifetime of photogenerated carriers, the optical gap and surface chemistry determine transfer of photogenerated carriers towards the external circuit. Altogether transparency, efficiency and stability influence the applicability of the SC. Titania and zincite are a common choice for the CTO layer. Utilisation of these materials in advanced nanostructural forms is known to be especially favourable for the SC performance. Therefore titanium anodization etching process was employed as a convenient way to produce the titania nanotubes (TiO2 NT) (thermally treated to crystallize in anatase) having various geometries (length, diameter and wall thickness). Two step growth from solution was employed to prepare nanorods of zincite (ZnO NR) having various geometries (length, distance). These nanostructured materials were prepared as ordered thin films on indium doped tin oxide (ITO) glass substrate. Commonly CTO layers are in direct contact to the absorbing active material. It was found that all SC problems basically start at the interface of the layers therefore multi-layer concepts often turn out to be favourable and sensitive. This investigation aims to interface the mentioned layers to additional buffer layer of barium titanate in perovskite type of structure. Based on its electric properties this material should present specific electric environment which should enable additional separation of the charge carriers. Assembly of SC where core-shell structure of barium titanate on CTO was infiltrated with organic absorbers was considered. Small molecules (squaraines) should fully infiltrate the nanoarchitecture and ensure charge transfer effect. The idea is not only to evidence this concept is operational but also to quantify how favourable the mentioned solution can be. The preparation procedures included: electrochemical etching, magnetron sputtering, sol-gel processing and spin coating. Only the back hole transmitting material (HTO) and metal electrode were deposited using thermal evaporation. During characterisation process special attention was focused on the thin film interfaces. It is necessary the ordered CTO nanorchitecture remains unaffected after the addition of buffer layer. Thin buffer layer should form core-shell structure with the CTO material. Furthermore, the specific surface should not be diminished by the addition of the buffer layer, i.e. enough space should be available for the infiltration of the active absorbing material. The methods that can ensure the required level of precision in the synthesis are not affordable for the large scale SC production, therefore this investigation aims in control of the layers using only cost effective wet chemistry synthesis methods. Non-contacted layers were characterized using structural methods (X-ray diffraction XRD, grazing-incidence GIXRD), spectroscopy methods (absorbance spectroscopy UV-VIS, photoluminescence spectroscopy PL) and microscopy methods (field emission scanning electron microscopy FESEM, transmission electron microscope TEM, 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, quantum efficiency spectroscopy QE) in dark and under illumination, offering information on relevant time scales for electronic transport and recombination, as well as overall stability. The results of this work show the novel barium titanate interface prepared by affordable methods may significantly attribute to the advantageous behaviour of the cell. Effect of the charge transfer was observed, however some additional effort is necessary to distinguish contributions coming from barium titanate buffer layer to the ones originating from macro and micro scale imperfections of the materials, especially at the boundary. Additionally it was not easy to confirm the extent of infiltration of the absorbing material within the core-shell nanoarchitecture. Understanding of the transport mechanisms between the barium titanate buffer layer and adjecent layers (absorbing and CTO), especially relating achieved properties to layers geometry (core-shell structure) as well as ensuring stability and durability (reproducible preparation process and absence of degradation) is still a matter of further investigation in order to yield the upgrade recipe of the photovoltaic performance.

BaTiO3; Buffer Layer; Hybrid OPV; Solar Cell

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