Effect of the TiO2 shell thickness on the dye-sensitized solar cells with ZnO–TiO2 core–shell nanorod electrodes

ZnO nanowires for solar cells: a comprehensive review

As an abundant and non-toxic wide band gap semiconductor with a high electron mobility, ZnO in the form of nanowires (NWs) has emerged as an important electron transporting material in a vast number of nanostructured solar cells. ZnO NWs are grown by low-cost chemical deposition techniques and their integration into solar cells presents, in principle, significant advantages including efficient optical absorption through light trapping phenomena and enhanced charge carrier separation and collection. However, they also raise some significant issues related to the control of the interface properties and to the technological integration. The present review is intended to report a detailed analysis of the state-of-the-art of all types of nanostructured solar cells integrating ZnO NWs, including extremely thin absorber solar cells, quantum dot solar cells, dye-sensitized solar cells, organic and hybrid solar cells, as well as halide perovskite-based solar cells.

Eosin-Y sensitized core-shell TiO2-ZnO nano-structured photoanodes for dye-sensitized solar cell applications

In the current investigation, TiO₂ and TiO₂-ZnO (core-shell) spherical nanoparticles were synthesized by simple combined hydrolysis and refluxing method. A TiO₂ core nanomaterial on the shell material of ZnO was synthesized by utilizing variable ratios of ZnO. The structural characterization of TiO₂-ZnO core/shell nanoparticles were done by XRD analysis. The spherical structured morphology of the TiO₂-ZnO has been confirmed through field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) studies. The UV-visible spectra of TiO₂-ZnO nanostructures were also compared with the pristine TiO₂ to investigate the shift of wavelength. The TiO₂-ZnO core/shell nanoparticles at the interface efficiently collect the photogenarated electrons from ZnO and also ZnO act a barrier for reduced charge recombination of electrolyte and dye-nanoparticles interface. This combination improved the light absorption which induced the charge transfer ability and dye loading capacity of core-shell nanoparticles. An enhancement in the short circuit current (J_sc) from 1.67 mA/cm² to 2.1 mA/cm² has been observed for TiO₂-ZnObased photoanode (with platinum free counter electrode), promises an improvement in the energy conversion efficiency by 57% in comparison with that of the DSSCs based on the pristine TiO₂. Henceforth, TiO₂-ZnO photoelectrode in ZnO will effectively act as barrier at the interface of TiO₂-ZnO and TiO₂, ensuring the potential for DSSC application.

Eco-friendly synthesis of core-shell structured (TiO2/Li2CO3) nanomaterials for low cost dye-sensitized solar cells

Core-shell structured TiO₂/Li₂CO₃ electrode was successfully synthesized by eco-friendly solution growth technique. TiO₂/Li₂CO₃ electrodes were characterized using X-ray Diffractometer (XRD), Scanning electron microscopy (SEM) and photocurrent-voltage measurements. The synthesized core-shell electrode material was sensitized with tetrabutylammonium cis-di(thiocyanato)-N,N'-bis(4-carboxylato-4'-carboxylic acid-2,2'-bipyridine)ruthenate(II) (N-719). The performance of dye-sensitized solar cells (DSCs) based on N719 dye modified TiO₂/Li₂CO₃ electrodes was investigated. The effect of various shell thickness on the photovoltaic performance of the core-shell structured electrode is also investigated. We found that Li₂CO₃ shells of all thicknesses perform as inert barriers which improve open-circuit voltage (V_oc) of the DSCs. The energy conversion efficiency was greatly dependent on the thickness of Li₂CO₃ on TiO₂ film, and the highest efficiency of 3.7% was achieved at the optimum Li₂CO₃ shell layer.

Hole-conductor-free perovskite solar cells with carbon counter electrodes based on ZnO nanorod arrays

ZnO/TiO₂ NR array is a novelty candidate as an electron collector for hole-conductor-free perovskite solar cells with carbon counter electrodes.

Core-shell heterostructured metal oxide arrays enable superior light-harvesting and hysteresis-free mesoscopic perovskite solar cells

To achieve highly efficient mesoscopic perovskite solar cells (PSCs), the structure and properties of an electron transport layer (ETL) or material (ETM) have been shown to be of supreme importance. Particularly, the core-shell heterostructured mesoscopic ETM architecture has been recognized as a successful electrode design, because of its large internal surface area, superior light-harvesting efficiency and its ability to achieve fast charge transport. Here we report the successful fabrication of a hysteresis-free, 15.3% efficient PSC using vertically aligned ZnO nanorod/TiO2 shell (ZNR/TS) core-shell heterostructured ETMs for the first time. We have also added a conjugated polyelectrolyte polymer into the growth solution to promote the growth of high aspect ratio (AR) ZNRs and substantially improve the infiltration of the perovskite light absorber into the ETM. The PSCs based on the as-synthesized core-shell ZnO/TiO2 heterostructured ETMs exhibited excellent performance enhancement credited to the superior light harvesting capability, larger surface area, prolonged charge-transport pathways and lower recombination rate. The unique ETM design together with minimal hysteresis introduces core-shell ZnO/TiO2 heterostructures as a promising mesoscopic electrode approach for the fabrication of efficient PSCs.

Optimization of 1D ZnO@TiO2 core-shell nanostructures for enhanced photoelectrochemical water splitting under solar light illumination

A fast and low-cost sol-gel synthesis used to deposit a shell of TiO2 anatase onto an array of vertically aligned ZnO nanowires (NWs) is reported in this paper. The influence of the annealing atmosphere (air or N2) and of the NWs preannealing process, before TiO2 deposition, on both the physicochemical characteristics and photoelectrochemical (PEC) performance of the resulting heterostructure, was studied. The efficient application of the ZnO@TiO2 core-shells for the PEC water-splitting reaction, under simulated solar light illumination (AM 1.5G) solar light illumination in basic media, is here reported for the first time. This application has had a dual function: to enhance the photoactivity of pristine ZnO NWs and to increase the photodegradation stability, because of the protective role of the TiO2 shell. It was found that an air treatment induces a better charge separation and a lower carrier recombination, which in turn are responsible for an improvement in the PEC performance with respect to N2-treated core-shell materials. Finally, a photocurrent of 0.40 mA/cm(2) at 1.23 V versus RHE (2.2 times with respect to the pristine ZnO NWs) was obtained. This achievement can be regarded as a valuable result, considering similar nanostructured electrodes reported in the literature for this application.

Semiconductor quantum dot-sensitized solar cells

Semiconductor quantum dots (QDs) have been drawing great attention recently as a material for solar energy conversion due to their versatile optical and electrical properties. The QD-sensitized solar cell (QDSC) is one of the burgeoning semiconductor QD solar cells that shows promising developments for the next generation of solar cells. This article focuses on recent developments in QDSCs, including 1) the effect of quantum confinement on QDSCs, 2) the multiple exciton generation (MEG) of QDs, 3) fabrication methods of QDs, and 4) nanocrystalline photoelectrodes for solar cells. We also make suggestions for future research on QDSCs. Although the efficiency of QDSCs is still low, we think there will be major breakthroughs in developing QDSCs in the future.

Nanomaterials for energy conversion and storage

Nanostructured materials are advantageous in offering huge surface to volume ratios, favorable transport properties, altered physical properties, and confinement effects resulting from the nanoscale dimensions, and have been extensively studied for energy-related applications such as solar cells, catalysts, thermoelectrics, lithium ion batteries, supercapacitors, and hydrogen storage systems. This review focuses on a few select aspects regarding these topics, demonstrating that nanostructured materials benefit these applications by (1) providing a large surface area to boost the electrochemical reaction or molecular adsorption occurring at the solid-liquid or solid-gas interface, (2) generating optical effects to improve optical absorption in solar cells, and (3) giving rise to high crystallinity and/or porous structure to facilitate the electron or ion transport and electrolyte diffusion, so as to ensure the electrochemical process occurs with high efficiency. It is emphasized that, to further enhance the capability of nanostructured materials for energy conversion and storage, new mechanisms and structures are anticipated. In addition to highlighting the obvious advantages of nanostructured materials, the limitations and challenges of nanostructured materials while being used for solar cells, lithium ion batteries, supercapacitors, and hydrogen storage systems have also been addressed in this review.