Theoretical and experimental investigations of zinc oxide nanostructures as photoelectrode for solar cell application

Scientists have been interested in solar cells for many years due to its ability to generate electrical energy from sunlight. One of the promising recent developments in this field is the quantum dots solar cell (QDSC) that can potentially replace dye sensitized solar cell (DSSC) due to their relatively simpler device structure and similarity. In QDSCs, the photovoltaic (PV) effect occurs at the interface between a redox electrolyte and the quantum dot (QD) conjugated wide bandgap metal oxide semiconductor (MOS), instead of dye molecules. QDs also have several advantages over organic dyes, including better electrical and optical properties due to several reasons, which are: (i) QD size-dependent tuneable bandgap, (ii) larger absorption cross-section, and (iii) ability to produce multiple excitons generation (MEG) from the absorption of a single-photon. In recent years, researchers have been focusing only on fluorophores and electrolytes in solar cells rather than on the photoelectrode. This research aims to (i) fabricate ZnO nanoparticles (NPs) using the thermal evaporation (TE) method and study their opto-electronic properties, (ii) study the effect of evaporation cycles to the properties of fabricated ZnO NPs, and (iii) build and validate realistic cluster models of ZnO NPs using ab-initio density functional theory (DFT) calculations. The TE pressure of 5 × 10-4 Torr and 5 × 10-5 Torr exhibited increments in: (i) bandgap of 2.58 eV and 2.80 eV, and (ii) PV efficiency of device ZnO/PbS/Carboxymethyl cellulose and polyvinyl alcohol (CMC-PVA) of 0.00181% and 0.99%, respectively. The different evaporation cycles i.e., one-, two-, and three-evaporation cycles caused variations in the size range of distribution of ZnO nanosphere (decreasing upon increment of evaporation cycles), and bandgap of ZnO nanosphere (increasing upon increment of evaporation cycles). The structural geometries of ZnO were identified viz., (ZnO)3, (ZnO)5, (ZnO)6, (ZnO)16, (ZnO)24, and (ZnO)30 with the size of 1.788 nm, 2.459 nm, 2.651 nm, 4.997 nm, 6.543 nm, and 7.529 nm respectively. The energy level alignment which involves the lowest occupied molecular orbitals of CdSe (LUMOCdSe) and (ZnO)30 is hypothesized to be able to satisfy the demand of i.e., (i) electron injection from CdSe to the ZnO (LUMOCdSe > LUMOZnO), and (iii) efficient electron transport (LUMOZnO < LUMOCdSe). The first excitonic peak of (ZnO)30 is in a good agreement with that of the ZnO thin film, thus it is hypothesized that the (ZnO)30 realistic cluster could be fabricated. In conclusion, the (ZnO)30 nano-sphere could be fabricated using TE with the fabrication pressure of 5 × 10-5 Torr and three-evaporation cycle for a better adsorption of thin film

Study of ZnO Nanospheres Fabricated via Thermal Evaporation for Solar Cell Application

A solar cell is a device that absorbs light energy to generate electrical energy. A typical example of a solar cell is the quantum dot solar cell (QDSC), which consists of three main components: (i) fluorophore: the component that absorbs light and generates excited state electrons and holes, (ii) photoelectrode: the component that transports the excited state electron and prevents recombination of excited state electrons and holes, and (iii) electrolyte: the component that re-plenishes the vacancy left by the excited electron in the hole. Despite the increasing number of research in the QDSC field, to date, a device with significant photovoltaic efficiency has not been developed. In this study, the mechanism of electron transport in a zinc oxide (ZnO) photoelectrode was investigated. Two ZnO layers were fabricated using thermal evaporation method at different vacuum pressures (5 × 10-4 and 5 × 10-5 Torr). Two solar cells were fabricated using ZnO as photoelectrode, lead sulphide as fluorophore, and a mixture of carboxymethyl cellulose and polyvinyl alcohol as electrolyte. The cell which utilized the ZnO fabricated under 5 × 10-5 Torr showed the highest efficiency ( = 0.98%), with fill factor = 22.07%, short circuit current = 2.85 mA/m2, and open circuit voltage = 80.719 mV

The effect of number of vacuum thermal evaporation cycles to the optoelectronic and morphological properties of ZnO

Zinc oxide (ZnO) is a wide band gap material (~3.37 eV) which has small exciton Bohr radius ~2.34 nm. In dye-sensitized solar cell, ZnO thin film is used as photoelectrode. Light-sensitive organic/ inorganic fluorophores could be adsorbed on the surface of the ZnO film, which later will be sandwiched with electrolyte and a counter electrode. The aim of this paper is to study the effect of number of evaporation cycle to the yielded morphology and size of ZnO building blocks; deposited using one, two, and three cycles of vacuum thermal evaporation technique. The ZnO thin films have been deposited on ITO glass substrate at vacuum pressure of 5 ´ 10-5 Torr, 116 A, and 2.6 V. The morphology of the thin films has been examined under Field Emission Scanning Electron Microscope (FESEM), which showed nanosphere morphology. The morphological observation is supported by a simulation; which calculated based on the crystallographic properties of the synthesized ZnO – characterized by X-ray diffractometer (XRD). Three sets of the ZnO thin films consists of ZnO particles in the range of 8 – 20 nm, 11 – 37 nm, and 6 – 16 nm respectively. According to the optical properties characterized by absorption spectrometer, it has been observed that the band gap of the thin films increased with increasing number of evaporation cycles. The values of the optical bandgap, Eg evaluated from Tauc’s plot, were found in the range between 2.40 eV to 2.60 eV

Characterization of opto-electronic properties of thermally evaporated ZnO

Photoelectrode plays significant role in excitonic solar cells i.e., (i) as an acceptor and (ii) transport media of excited state electron from the fluorophore upon absorption of energy of photon; which prevents from electron-hole recombination in the fluorophore. The evolution of opto-electronic properties of the ZnO upon change of size, however, receives insufficient attention from researchers. Therefore, the aim of this paper is to establish few realistic clusters of (ZnO)n (n = 3, 6, 12, 13, and 21) to study their opto-electronic properties using quantum chemical calculations at the level of B3LYP functional and lanl2dz basis set. Geometry of the clusters were optimized to the lowest energy structures; evaluated as realistic using a combination of harmonic frequency calculations, and experimental works. A device structure of cadmium selenide-based solar cell was used in the study to analyze the energy level alignment, and compatibility of the ZnO realistic clusters

On the structural-optical correlation of ZnO nanospheres synthesized using thermal evaporation technique

Zinc oxide clusters of different sizes and geometrical shapes have been synthesized and characterized experimentally and theoretically. The ZnO cluster models have been built and presented to study the evolution of physico-chemical properties of the metal oxide semiconductor upon reduction of size. However, most of the theoretical models are lack of support from the experimental details. This article aims to correlate the geometry of the theoretical models with their optical properties. The ZnO thin films were synthesized by thermal evaporator at vacuum pressures of 5 × 10−5 Torr. Cluster models of the synthesized ZnO were built based on its crystal structure. The clusters were evaluated as realistic using geometry optimization, and harmonic frequency calculations at the level of B3LYP functional and lanl2dz basis set. We observed that the fabricated ZnO thin film was constructed by small ZnO spherical clusters, which revealed by the Field Emission Electron Microscopy (FESEM) . This observation is in good agreement with our theoretical simulations, which were established based on its crystal structure – characterized by the X-ray diffractometer