ZnO-based Solar Cells Research Articles

Defects in local atomic structure of ZnO films sensitized with tinto and mangrove organic dyes

In this work, the dye/ZnO complex was prepared, using natural dyes extracted from the red tree (Haematoxylum campechianum L.) and mangrove tree (Rhizophora Mangle), with a concentration of 5 × 10−3 M, impregnation time of 2 and 6 h, using a mixture of ethanol-water and ethanol as solvents. The ZnO particles were obtained by sol gel method and deposited on Corning glass by the doctor blade technique. X-ray diffraction indicates that these particles belong to the wurtzite-type hexagonal crystalline structure. Anthraquinone and anthocyanin molecules were identified in the natural dyes used. The results of the Fourier Transform Infrared Spectroscopy of the complexes show the band associated with the functional group of the carboxyls, which represents evidence of the presence of the dyes in the semiconductor. X-ray absorption spectroscopy shows changes in the local environment around the Zn atoms. Both dyes introduce a greater quantity of electrons in the unoccupied p states of Zn. The simulation developed and the analysis of the EXAFS signals show the formation of new bonds between Zn and O with dye atoms thus reducing the Zn O and Zn–Zn bond lengths. Also, the distribution of the bonds of the neighbors closest to the Zn, in the ZnO matrix, is modified due to the generation of vacancies of O and Zn. These results imply that the obtained dyes can effectively be used as photosensitizers in ZnO-based solar cells.

Graphene capping layer for improving the photovoltaic stability of ZnO nanorods solar cells under temperature variations

The present study evaluates the role of the graphene capping layer on the optoelectronic performance of ZnO/Si solar cells at different ambient temperatures. The n-ZnO buffer thin film was sputter-deposited on the p-Si substrates to form a p-n heterojunction. Next, ZnO nanostructures were chemically synthesized on top of the ZnO/Si substrates, and few-layer graphene was deposited on ZnO nanostructures using an electrophoretic deposition technique. The planar and cross-sectional electron microscopy analyses exhibited that the graphene capping layer entirely covered the surface of ZnO nanostructures, and their morphology remained intact after graphene transfer. Their photoluminescence (PL) properties before and after graphene deposition were analyzed at various temperatures to evaluate the graphene role in the optical behavior of ZnO nanostructures. The PL intensity was reduced for the graphene-containing sample because of the slight light absorption in each graphene layer. However, the stability of PL spectra at elevated temperatures was improved after graphene transfer. Next, a heterojunction solar cell was developed by depositing top and back contacts to study the photovoltaic performance of nanostructures at various temperatures. The current-voltage results, recorded under a solar light simulator, illustrated that the efficiency of the bare ZnO device was reduced at high temperatures because of the available interstitials and oxygen vacancies. After graphene transferring, this decay was moderated, which enhanced the photovoltaic stability of ZnO-based solar cells. Therefore, the graphene capping layer can act as a stabilizing thin film to enhance the photovoltaic durability of ZnO nanostructures under harsh conditions.

Increase the current density and reduce the defects of ZnO by modification of the band gap edges with Cu ions implantation for efficient, flexible dye-sensitized solar cells (FDSSCs)

Flexible dye-sensitized solar cells (FDSSCs) have good potential in future photovoltaic technology. The spin coating method deposited the ZnO films on indium-tin-oxide-coated polyethylene terephthalate (ITO-PET) flexible plastic substrates. These films are implanted with Cu-ions with 1 × 1013 ions/cm2, 1 × 1014 ions/cm2, and 1 × 1015 ions/cm2. All the films have a hexagonal structure. The film irradiated with 1 × 1014 ions/cm2 showed high crystallinity and crystallite size. Important optical properties like bandgap energy (Eg), band edges, refractive index, extinction coefficient, and dielectric constants are measured by UV–Vis spectroscopy. Bandgap energy decreases, and the refractive index increases at the fluence of Cu ions. The maximum decrease in Eg is observed at the 1 × 1014 ions/cm2 dose. Photoluminescence spectra suggest that defects-related emission peaks are decreased at 1 × 1014 ions/cm2 Cu ions fluency. J-V measurements have significantly improved photovoltaic performance compared to pristine ZnO-based solar cells. The highest efficiency (2.30%) is observed at a 1 × 1014 ions/cm2 dose. The efficiency increase is related to improving the charge transfer ratio and shifting the fermi level toward the conduction band.

Improved performance of D149 dye-sensitized ZnO-based solar cell under solvents activation effect

Improved performance of D149 dye-sensitized ZnO-based solar cell under solvents activation effect

Nature of the Ultrafast Interligands Electron Transfers in Dye-Sensitized Solar Cells.

Charge-transfer dynamics and interligand electron transfer (ILET) phenomena play a pivotal role in dye-sensitizers, mostly represented by the Ru-based polypyridyl complexes, for TiO2 and ZnO-based solar cells. Starting from metal-to-ligand charge-transfer (MLCT) excited states, charge dynamics and ILET can influence the overall device efficiency. In this letter, we focus on N34- dye ( [Ru(dcbpy)2(NCS)2]4-, dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) to provide a first direct observation with high time resolution (<20 fs) of the ultrafast electron exchange between bpy-like ligands. ILET is observed in water solution after photoexcitation in the ∼400 nm MLCT band, and assessment of its ultrafast time-scale is here given through a real-time electronic dynamics simulation on the basis of state-of-the-art electronic structure methods. Indirect effects of water at finite temperature are also disentangled by investigating the system in a symmetric gas-phase structure. As main result, remarkably, the ILET mechanism appears to be based upon a purely electronic evolution among the dense, experimentally accessible, MLCT excited states manifold at ∼400 nm, which rules out nuclear-electronic couplings and proves further the importance of the dense electronic manifold in improving the efficiency of dye sensitizers in solar cell devices.

Effect of Al doped ZnO on optical and photovoltaic properties of the p-Cu2O/n-AZO solar cells

Metal oxide thin films have fared so well in the semiconductor industry because of their superior physical, electrical, and optical properties. The applications of these materials in solar cells, biosensors, biomedicine, supercapacitors, photocatalysis, luminous materials, and laser systems are becoming increasingly popular. In this study, the influence of Al concentration on Cu2O/AZO heterojunction thin films was examined systematically. First, arrays of n-ZnO and AZO rods were produced on an ITO substrate using a hydrothermal technique at 140 °C. Then, using an alkaline cupric lactate solution, a thin films of p-Cu2O were electrodeposited at 60 °C onto the ZnO arrays. The structure and morphology of the produced materials and the solar cells were studied using X-ray diffraction and scanning electron microscopy. The optical measurements demonstrate a shift in the absorption edge with increasing Al content. Solar cells have been created with a device structure of ITO/ZnO/Cu2O/Al and ITO/Al-doped ZnO/Cu2O/Al configurations. The power conversion efficiency (ɳ) of the inorganic solar cell with 6% Al-doped ZnO is ɳ = 0.282%, which is greater than the ɳ of the ZnO-based solar cell (ɳ = 0.17%).

A Review of the Impact of Zinc Oxide Nanostructure Morphology on Perovskite Solar Cell Performance

Zinc oxide (ZnO) has been widely studied over the last decade for its remarkable properties in optoelectronic and photovoltaic devices because of its high electron mobility and excitonic properties. It has probably the broadest range of nanostructured forms that are also easy and cheap to synthesize using a wide variety of methods. The volume of recent work on ZnO nanostructures and their devices can potentially overshadow significant developments in the field. Therefore, there is a need for a concise description of the most recent advances in the field. In this review, we focus on the effect of ZnO nanostructure morphologies on the performance of ZnO-based solar cells sensitized using methylammonium lead iodide perovskite. We present an exhaustive discussion of the synthesis routes for different morphologies of the ZnO nanostructure, ways of controlling the morphology, and the impact of morphology on the photoconversion efficiency of a given perovskite solar cell (PSC). We find that although the ZnO nanostructures are empirically similar, one-dimensional structures appear to offer the most promise to increasing photoconversion efficiency (PCE) by their proclivity to align and form vertically stacked layers. This is thought to favor electron hopping, charge mobility, and conductivity by allowing multiple charge conduction pathways and increasing the effective junction cross-sectional area. The combined effect is a net increase in PCE due to the reduced surface reflection, and improved light absorption.

Preparation and Characterization of Cu and Al Doped ZnO Thin Films for Solar Cell Applications

The Al- and Cu-doped ZnO nanostructured films in this study were deposited using a sputtering technique. Investigations based on X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, Hall effect measurements, and optical transmission spectroscopy was performed to analyze the structural, electrical, and optical characteristics of the prepared Al–ZnO and Cu–ZnO nanostructured films. The analyses show that doping results in enhanced conductivity as well as improved mobility in Al–ZnO and Cu–ZnO films in comparison to pure ZnO films. The Al- and Cu-doped ZnO films exhibited low resistivity (2.9 × 10−4 Ω cm for Al–ZnO and 1.7 × 10−4 Ω cm for Cu–ZnO) along with an average transmittance of around 80% in the visible spectrum. Moreover, the optical bandgaps of undoped ZnO, Al–ZnO, and Cu–ZnO nanostructures were observed as 3.3, 3.28, and 3.24 eV, respectively. Finally, solar cells were assembled by employing ZnO nanostructured thin films as photoelectrodes, resulting in efficiencies of 0.492% and 0.559% for Al–ZnO- and Cu–ZnO-based solar cells, respectively.

Effect of dye sensitization’s temperature on ZnO-based solar cells

We have investigated the insights of photo-physical surface characteristics for Ru (II) polypyridyl dye (N719) sensitization and its performance in dye-sensitized solar cells (DSSCs) at different temperatures of ZnO-based photoanodes. The maximum power conversion efficiency of 2.25% with a high open-circuit voltage of 0.87 V was recorded for 40 °C sensitized device under 1SUN 1.5 AM. The 60 °C sensitized device shows the maximum current density of 8.8 mA cm−2 and 25 °C sensitized device results. Therefore, the mode of surface interaction of N719-ZnO to promote better electron conduction has highlighted the prospects of DSSCs under diverse solution sensitization temperature environment.

Effect of Mn Doping on Multilayer PbS Quantum Dots Sensitized Solar Cell

The effect of Mn-ion doping on the device performance of multilayer PbS quantum dot (QD) sensitized solar cells is investigated. Undoped as well as the doped PbS QDs, with different doping concentration, are synthesized using simple chemical methods on poly-vinyl alcohol matrix. QDs are characterized using ultraviolet visible spectroscopy, x-ray diffraction analysis, high-resolution transmission electron microscopy, energy dispersive x-ray, and photoluminescence spectroscopy. The QDs were introduced as sensitizer in ZnO-based solar cells in single as well as multiple layers. The current density vs. voltage characteristics are obtained for different Mn doping concentrations as well as for multiple numbers of QD layers, under artificial illumination. Enhanced photo-conversion efficiency was observed in multiple layered doped PbS QDs sensitized solar cell compared to undoped QDs sensitized solar cell.

A back illuminated solar cell using PbS quantum dots as sensitisers

Quantum dot sensitised solar cells (QDSSC) is generally fabricated using transparent conducting (TCO) glass plates and platinum or gold as electrodes. The article reports a modified back side illuminated QDSSC structure utilising aluminium foil as electrodes. PbS quantum dots have been synthesised using simple chemical method and are utilised as sensitisers in a ZnO-based solar cell structure. Two types of QDSSCs are fabricated using the same quantum dots, i.e., one using ITO glass plate and the other using aluminium. The solar cell parameters are obtained for both the structures and their comparative analysis is reported in the paper.

Effect of different photoanode nanostructures on the initial charge separation and electron injection process in dye sensitized solar cells: A photophysical study with indoline dyes

Ultrafast and fast charge separation processes were investigated for complete cells based on several ZnO-based photoanode nanostructures and standard TiO2 nanoparticle layers sensitized with the indoline dye coded D358. Different ZnO morphologies (nanoparticles, nanowires, mesoporous), synthesis methods (hydrothermal, gas-phase, electrodeposition in aqueous media and ionic liquid media) and coatings (ZnO–ZnO core–shell, ZnO–TiO2 core–shell) were measured by transient absorption techniques in the time scale from 100 fs to 100 μs and in the visible and near-infrared spectral range. All of ZnO cells show worse electron injection yields with respect to those with standard TiO2 material. Lower refractive index of ZnO than that of TiO2 is suggested to be an additional factor, not considered so far, that can decrease the performance of ZnO-based solar cells. Evidence of the participation of the excited charge transfer state of the dye in the charge separation process is provided here. The lifetime of this state in fully working devices extends from several ps to several tens of ps, which is much longer than the typically postulated electron injection times in all-organic dye-sensitized solar cells. The results here provided, comprising a wide variety of morphologies and preparation methods, point to the universality of the poor performance of ZnO as photoanode material with respect to standard TiO2.

Influence of Structural Properties on the Performance of Dye-Sensitized Solar Cells Based on Electrodeposited ZnO

ZnO is an interesting material for application in the dye-sensitized solar cell (DCS) related to the large variety of low-temperature synthesis methods that allow for easy manipulation of the morphology and texture of the material. In particular, electrodeposition of crystalline ZnO can be achieved at room temperature, making it an excellent method for the preparation of solar cells based on flexible plastic substrates. We will present results on the performance of dye-sensitized solar cells based on electrodeposited ZnO, describing the electrodeposition method in detail, and compare the results to ZnO-based solar cells prepared using sol-gel synthesis and CVD. The morphology and texture are expected to influence the performance of the dye solar cell via a variety of effects. For instance, the exposed crystal facets may affect the dye bonding, dye coverage, and injection efficiency, the electron transfer kinetics to the solution, and the trap state distribution [1]. Using these materials, we have compared the electron transport and recombination properties using small-signal perturbation techniques [2]. We have also addressed the influence of solution additives on the performance of ZnO-based dye-sensitized solar cells and rationalized the results. The influence of surface facet exposure on dye bonding is highlighted.

Enhanced power conversion efficiency of CdS quantum dot sensitized solar cells with ZnO nanowire arrays as the photoanodes

We report the fabrication of CdS quantum dot sensitized solar cells with ZnO nanowire arrays as the photoanodes. The influences of precursor solution temperature and sensitizing cycles on the performance of CdS quantum dots sensitized ZnO nanowires solar cells were studied. Successive ionic layer adsorption and reaction (SILAR) method was applied to deposit CdS quantum dots on the surface of ZnO nanowire arrays for assembling ZnO/CdS electrodes. The results of scanning electron microscopic (SEM), X-ray diffraction (XRD) patterns and UV–vis absorption spectroscopy indicated that the ZnO nanowires electrodes were well-covered with CdS quantum dots. The temperature of the ethanol sensitizing solutions significantly influenced the performance of ZnO/CdS electrodes by affecting the rate of deposition reaction and the penetration ability of ethanol solution. The CdS quantum dots sensitized ZnO-based solar cells exhibited a short-circuit current density (Jsc) of 3.1mA/cm2, an open-circuit voltage (Voc) of 0.55V and a photovoltaic conversion efficiency of 0.72%, which is much higher than that reported in literatures, under the illumination of one sun (AM 1.5, 100mW/cm2) when the temperature of the ethanol solutions was 60°C and ZnO arrays were sensitized for seven times.

Higher Open Circuit Voltage and Reduced UV-Induced Reverse Current in ZnO-Based Solar Cells by a Chemically Modified Blocking Layer

Solid-state semiconductor-sensitized solar cells require a thin, dense hole-blocking layer at the conducting glass substrate (F-doped tin oxide (FTO)) to prevent shorting beween the FTO and hole conductor. We found that by adding a small amount of Sb ions to a ZnO chemical deposition bath a thin (few tens of nanometers thick) dense and uniform layer of Sb-incorporated ZnO forms. Here we investigate the electronic properties of this layer in comparison to the continuous ZnO layer at the base of the ZnO rods formed in the standard preparation. Devices incorporating the Sb-incorporated dense layer followed by a standard ZnO nanorod growth, onto which CdS or CdSe was grown followed by a CuSCN hole conductor, showed 100–200 mV higher photovoltage together with occasional improvement in the short-circuit current. Electrochemical and electrical measurements indicated complete coverage of the FTO substrate by both preparations; however, the shunt resistance (resistance to a reverse leakage current) in the cells (and films) made using the Sb-incorporated ZnO layer is dramatically increased. Using bias-dependent incident photon-to-electron conversion efficiency studies, we found that an increased dark or leakage current develops in the cell on illumination with UV light together with application of a forward bias. This can be explained by the presence of a “Schottky junction” at the FTO\ZnO interface. This increased leakage current is significantly larger in cells without the Sb-incorporated ZnO compact layer.

Impedance spectroscopy study of N719-sensitized ZnO-based solar cells

ZnO porous films prepared by electrodeposition and by sol–gel techniques have been sensitized by the N719 dye and used as photoelectrode in dye-sensitized solar cells. Similar stationary current–voltage characteristics have been measured for films having two drastically different morphologies and nanostructures. The solar cells have been studied by impedance spectroscopy over large frequency and applied voltage ranges in order to elucidate the inherent correlation between the photoelectrode structure and properties. The electrical response has been analyzed using an ad-hoc electrical model to extrapolate the electronic structure and charge carrier kinetic properties of the photoelectrodes. The two films exhibit very different density distributions of their trap states below the conduction band edge. Moreover, their electron lifetimes and transport times vary differently with the DOS due to different surface and conduction properties. The charge collection efficiency has been calculated by two different approaches, one being derived from first principles. Very high values are reported in all cases. Globally, the limitation of N719/ZnO solar cell performances is related to their open circuit voltage and their short circuit current. The latter point is discussed in the light of our impedance results and photoelectrode light absorption data.

A suitable chemical conversion route to synthesize ZnO/CdS core/shell heterostructures for photovoltaic applications

In order to improve the performance of ZnO-based solar cells, ZnO/CdS core/shell heterostructures were synthesized via a facile chemical conversion route. Large area and quasi-aligned ZnO nanowire arrays were firstly fabricated on conductive glass substrates, and then the synthesis of ZnO/ZnS and ZnO/CdS heterostructures were realized by a chemical conversion method. The morphology and structure of the obtained products has been confirmed by field-emission scanning electron microscopy and X-ray diffraction measurements. The synthesized ZnO/CdS core/shell heterostructures were successfully applied in solar cells. The photovoltaic performance investigation demonstrates that the synthesized ZnO/CdS heterostructure-based solar cell shows a higher short-circuit current density compared with the N719 dye-sensitized ZnO nanowire array counterpart. The enhancement in current density may be due to the fact that the surface coverage of the light absorption layer in the ZnO/CdS core/shell heterostructure is very high. These results indicate that the synthesized ZnO/CdS heterostructures by the present method are very suitable for the application as photoelectrode in solar cells.

Accurate determination of interface trap state parameters by admittance spectroscopy in the presence of a Schottky barrier contact: Application to ZnO-based solar cells

This work shows that when a Schottky barrier is present in a photovoltaic device, such as in a device with an ITO/ZnO contact, equivalent circuit analysis must be performed with admittance spectroscopy to accurately determine the pn junction interface recombination parameters (i.e., capture cross section and density of trap states). Without equivalent circuit analysis, a Schottky barrier can produce an error of ∼4-orders of magnitude in the capture cross section and ∼50% error in the measured density of trap states. Using a solution processed ZnO/Cu2O photovoltaic test system, we apply our analysis to clearly separate the contributions of interface states at the pn junction from the Schottky barrier at the ITO/ZnO contact so that the interface state recombination parameters can be accurately characterized. This work is widely applicable to the multitude of photovoltaic devices, which use ZnO adjacent to ITO.

Recent Progress in ZnO-Based Nanostructured Ceramics in Solar Cell Applications

ZnO is widely used as an n-type semiconductor in various solar cell structures; including dye-sensitized, organic, hybrid and solid-state solar cells. Here, we review advances in ZnO-based solar cell applications, looking at the influence of morphology, as well as the effect of different materials and sensitizers. ZnO morphologies play an important role in changing the surface area and charge transport properties, affecting the performance of the solar cells. External quantum efficiencies using purely ZnO as the active material has generally been below 3% with some dye-sensitized solar cells using liquid electrolytes above 5%. Sensitizers such as organic and inorganic dyes, quantum dots and hole conductors have been shown to influence cell efficiency by improving the absorption or providing improved charge transport. The combination of ZnO with other nanomaterials such as, TiO2, SiO2 and ZrO2 in core-shell structures or buffer layers creates improved electron transport by controlling recombination at interfaces and increasing stability of the device. The highest reported efficiencies to date were from combinational structures at 7.07% for ZnO nanosheets with TiO2 nanoparticulate coating and 7% for ZnO core-TiO2 shell structures.

ZnO-Based Dye-Sensitized Solar Cells

ZnO was one of the first metal oxides used in dye-sensitized solar cells (DSCs). It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amount of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is suggested.