Efficiency Enhancement of Dye-Sensitized Solar Cells’ Performance with ZnO Nanorods Grown by Low-Temperature Hydrothermal Reaction

In this study, aligned zinc oxide (ZnO) nanorods (NRs) with various lengths (1.5–5 µm) were deposited on ZnO:Al (AZO)-coated glass substrates by using a solution phase deposition method; these NRs were prepared for application as working electrodes to increase the photovoltaic conversion efficiency of solar cells. The results were observed in detail by using X-ray diffraction, field-emission scanning electron microscopy, UV-visible spectrophotometry, electrochemical impedance spectroscopy, incident photo-to-current conversion efficiency, and solar simulation. The results indicated that when the lengths of the ZnO NRs increased, the adsorption of D-719 dyes through the ZnO NRs increased along with enhancing the short-circuit photocurrent and open-circuit voltage of the cell. An optimal power conversion efficiency of 0.64% was obtained in a dye-sensitized solar cell (DSSC) containing the ZnO NR with a length of 5 µm. The objective of this study was to facilitate the development of a ZnO-based DSSC.     

Efficiency Boost in Dye-Sensitized Solar Cells by Post- Annealing UV-Ozone Treatment of TiO2 Mesoporous Layer

The organic residues on titanium(IV) oxide may be a significant factor that decreases the efficiency of dye-sensitized solar cells (DSSC). Here, we suggest the UV-ozone cleaning process to remove impurities from the surface of TiO₂ nanoparticles before dye-sensitizing. Data obtained from scanning electron microscopy, Kelvin probe, Fourier-transform infrared spectroscopy, and Raman spectroscopy showed that the amounts of organic contamination were successfully reduced. Additionally, the UV-VIS spectrophotometry, spectrofluorometry, and secondary ion mass spectrometry proved that after ozonization, the dyeing process was relevantly enhanced. Due to the removal of organics, the power conversion efficiency (PCE) of the prepared DSSC devices was boosted from 4.59% to 5.89%, which was mostly caused by the increment of short circuit current (J_sc) and slight improvement of the open circuit voltage (V_oc).     

Eco-Friendly Dye-Sensitized Solar Cells Based on Water-Electrolytes and Chlorophyll

Organic solvents used for electrolytes of dye-sensitized solar cells (DSSCs) are generally not only toxic and explosive but also prone to leakage due to volatility and low surface tension. The representative dyes of DSSCs are ruthenium-complex molecules, which are expensive and require a complicated synthesis process. In this paper, the eco-friendly DSSCs were presented based on water-based electrolytes and a commercially available organic dye. The effect of aging time after the device fabrication and the electrolyte composition on the photovoltaic performance of the eco-friendly DSSCs were investigated. Plasma treatment of TiO₂ was adopted to improve the dye adsorption as well as the wettability of the water-based electrolytes on TiO₂. It turned out that the plasma treatment was an effective way of improving the photovoltaic performance of the eco-friendly DSSCs by increasing the efficiency by 3.4 times. For more eco-friendly DSSCs, the organic-synthetic dye was replaced by chlorophyll extracted from spinach. With the plasma treatment, the efficiency of the eco-friendly DSSCs based on water-electrolytes and chlorophyll was comparable to those of the previously reported chlorophyll-based DSSCs with non-aqueous electrolytes.     

Structural and Electrical Characterization of Pure and Al-Doped ZnO Nanorods

Pure and Al-doped (3 at.%) ZnO nanorods were prepared by two-step synthesis. In the first step, ZnO thin films were deposited on silicon wafers by spin coating; then, ZnO nanorods (NR) and Al-doped ZnO NR were grown using a chemical bath method. The structural properties of zincite nanorods were determined by X-ray diffraction (XRD) and corroborated well with the morphologic properties obtained by field-emission gun scanning electron microscopy (FEG SEM) with energy-dispersive X-ray spectroscopy (EDS). Morphology results revealed a minute change in the nanorod geometry upon doping, which was also visible by Kelvin probe force microscopy (KPFM). KPFM also showed preliminary electrical properties. Detailed electrical characterization of pure and Al-doped ZnO NR was conducted by temperature-dependent current–voltage (I–V) measurements on Au/(Al)ZnO NR/n-Si junctions. It was shown that Al doping increases the conductivity of ZnO NR by an order of magnitude. The I–V characteristics of pure and Al-doped ZnO NR followed the ohmic regime for lower voltages, whereas, for the higher voltages, significant changes in electric conduction mechanisms were detected and ascribed to Al-doping. In conclusion, for future applications, one should consider the possible influence of the geometry change of (Al)ZnO NRs on their overall electric transport properties.     

Limits of ZnO Electrodeposition in Mesoporous Tin Doped Indium Oxide Films in View of Application in Dye-Sensitized Solar Cells

Well-ordered 3D mesoporous indium tin oxide (ITO) films obtained by a templated sol-gel route are discussed as conductive porous current collectors. This paper explores the use of such films modified by electrochemical deposition of zinc oxide (ZnO) on the pore walls to improve the electron transport in dye-sensitized solar cells (DSSCs). Mesoporous ITO film were dip-coated with pore sizes of 20–25 nm and 40–45 nm employing novel poly(isobutylene)-b-poly(ethylene oxide) block copolymers as structure-directors. After electrochemical deposition of ZnO and sensitization with the indoline dye D149 the films were tested as photoanodes in DSSCs. Short ZnO deposition times led to strong back reaction of photogenerated electrons from non-covered ITO to the electrolyte. ITO films with larger pores enabled longer ZnO deposition times before pore blocking occurred, resulting in higher efficiencies, which could be further increased by using thicker ITO films consisting of five layers, but were still lower compared to nanoporous ZnO films electrodeposited on flat ITO. The major factors that currently limit the application are the still low thickness of the mesoporous ITO films, too small pore sizes and non-ideal geometries that do not allow obtaining full coverage of the ITO surface with ZnO before pore blocking occurs.     

Investigation of Coral-Like Cu2O Nano/Microstructures as Counter Electrodes for Dye-Sensitized Solar Cells

In this study, a chemical oxidation method was employed to fabricate coral-like Cu₂O nano/microstructures on Cu foils as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The Cu₂O nano/microstructures were prepared at various sintering temperatures (400, 500, 600 and 700 °C) to investigate the influences of the sintering temperature on the DSSC characteristics. First, the Cu foil substrates were immersed in an aqueous solution containing (NH₄)₂S₂O₈ and NaOH. After reacting at 25 °C for 30 min, the Cu substrates were converted to Cu(OH)₂ nanostructures. Subsequently, the nanostructures were subjected to nitrogen sintering, leading to Cu(OH)₂ being dehydrated into CuO, which was then deoxidized to form coral-like Cu₂O nano/microstructures. The material properties of the Cu₂O CEs were comprehensively determined using a scanning electron microscope, energy dispersive X-ray spectrometer, X-ray diffractometer, Raman spectrometer, X-ray photoelectron spectroscope, and cyclic voltameter. The Cu₂O CEs sintered at various temperatures were used in DSSC devices and analyzed according to the current density–voltage characteristics, incident photon-to-current conversion efficiency, and electrochemical impedance characteristics. The Cu₂O CEs sintered at 600 °C exhibited the optimal electrode properties and DSSC performance, yielding a power conversion efficiency of 3.62%. The Cu₂O CEs fabricated on Cu foil were generally mechanically flexible and could therefore be applied to flexible DSSCs.     

Density Functional Theory (DFT) Study of Coumarin-based Dyes Adsorbed on TiO2 Nanoclusters—Applications to Dye-Sensitized Solar Cells

Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO₂ nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO₂ cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.     

Fabrication of Well-Aligned ZnO Nanorods Using a Composite Seed Layer of ZnO Nanoparticles and Chitosan Polymer

In this study, by taking the advantage of both inorganic ZnO nanoparticles and the organic material chitosan as a composite seed layer, we have fabricated well-aligned ZnO nanorods on a gold-coated glass substrate using the hydrothermal growth method. The ZnO nanoparticles were characterized by the Raman spectroscopic techniques, which showed the nanocrystalline phase of the ZnO nanoparticles. Different composites of ZnO nanoparticles and chitosan were prepared and used as a seed layer for the fabrication of well-aligned ZnO nanorods. Field emission scanning electron microscopy, energy dispersive X-ray, high-resolution transmission electron microscopy, X-ray diffraction, and infrared reflection absorption spectroscopic techniques were utilized for the structural characterization of the ZnO nanoparticles/chitosan seed layer-coated ZnO nanorods on a gold-coated glass substrate. This study has shown that the ZnO nanorods are well-aligned, uniform, and dense, exhibit the wurtzite hexagonal structure, and are perpendicularly oriented to the substrate. Moreover, the ZnO nanorods are only composed of Zn and O atoms. An optical study was also carried out for the ZnO nanoparticles/chitosan seed layer-coated ZnO nanorods, and the obtained results have shown that the fabricated ZnO nanorods exhibit good crystal quality. This study has provided a cheap fabrication method for the controlled morphology and good alignment of ZnO nanorods, which is of high demand for enhancing the working performance of optoelectronic devices.     

Influence of Tin Doped TiO2 Nanorods on Dye Sensitized Solar Cells

The one-step hydrothermal method was used to synthesize Sn-doped TiO₂ (Sn-TiO₂) thin films, in which the variation in Sn content ranged from 0 to 7-wt % and, further, its influence on the performance of a dye-sensitized solar cell (DSSC) photoanode was studied. The deposited samples were analyzed by X-ray diffraction (XRD) and Raman spectroscopy, which confirmed the existence of the rutile phase of the synthesized samples with crystallite size ranges in between 20.1 to 22.3 nm. In addition, the bare and Sn-TiO₂ thin films showed nanorod morphology. A reduction in the optical band gap from 2.78 to 2.62 eV was observed with increasing Sn content. The X-ray photoelectron spectroscopy (XPS) analysis confirmed Sn⁴⁺ was successfully replaced at the Ti⁴⁺ site. The 3-wt % Sn-TiO₂ based DSSC showed the optimum efficiency of 4.01%, which was superior to 0.87% of bare and other doping concentrations of Sn-TiO₂ based DSSCs. The present work reflects Sn-TiO₂ as an advancing material with excellent capabilities, which can be used in photovoltaic energy conversion devices.     

Co-Sensitization Effects of Indoline and Carbazole Dyes in Solar Cells and Their Neutral–Anion Equilibrium in Solution

Co-sensitization of two or more light-absorbing compounds on a TiO₂ surface has recently become one of the most successful strategies in the development of dye-sensitized solar cells (DSSCs). The specific structure of the dyes for DSSCs implies that they can partly exist in anionic forms in popular solvents used for sensitization. Our study concerns the above two issues being analyzed in detail using the example of the popular carbazole (MK2) and indoline (D205) dyes, studied by stationary absorption and emission, femtosecond transient absorption (in complete cells and in the solutions), current-voltage measurements, DFT and TD-DFT theoretical calculations. After the addition of D205 to DSSC with MK2, the fill factor of the cells was improved, and the electron recombination between TiO₂ and the dyes was blocked (observed on sub-nanosecond time scales). Thus, the active co-adsorbent can take the role of the typically used passive additive, like chenodeoxycholic acid. Evidence of the concentration-dependent equilibrium between neutral and anionic forms of dyes with different lifetimes was found in acetonitrile solutions (the best for sensitization), while in ethanol solution the dominant form was the anion (worse for sensitization). Our findings should help in better understanding the operation and optimization of DSSC.     

Electrolyte Tuning in Iron(II)-Based Dye-Sensitized Solar Cells: Different Ionic Liquids and I2 Concentrations

The effects of different I₂ concentrations and different ionic liquids (ILs) in the electrolyte on the performances of dye-sensitized solar cells (DSCs) containing an iron(II) N-heterocyclic carbene dye and containing the I^–/I₃^– redox shuttle have been investigated. Either no I₂ was added to the electrolyte, or the initial I₂ concentrations were 0.02, 0.05, 0.10, and 0.20 M. The short-circuit current density (J_SC), open-circuit voltage (V_OC), and the fill factor (ff) were influenced by changes in the I₂ concentration for all the ILs. For 1-hexyl-3-methylimidazole iodide (HMII), low V_OC and low ff values led to poor DSC performances. Electrochemical impedance spectroscopy (EIS) showed the causes to be increased electrolyte diffusion resistance and charge transfer resistance at the counter electrode. DSCs containing 1,3-dimethylimidazole iodide (DMII) and 1-ethyl-3-methylimidazole iodide (EMII) showed the highest J_SC values when 0.10 M I₂ was present initially. Short alkyl substituents (Me and Et) were more beneficial than longer chains. The lowest values of the transport resistance in the photoanode semiconductor were found for DMII, EMII, and 1-propyl-2,3-dimethylimidazole iodide (PDMII) when no I₂ was added to the initial electrolyte, or when [I₂] was less than 0.05 M. Higher [I₂] led to decreases in the diffusion resistance in the electrolyte and the counter electrode resistance. The electron lifetime and diffusion length depended upon the [I₂]. Overall, DMII was the most beneficial IL. A combination of DMII and 0.1 M I₂ in the electrolyte produced the best performing DSCs with an average maximum photoconversion efficiency of 0.65% for a series of fully-masked cells.     

Decoration of Zinc Oxide Nanorods into the Surface of Activated Carbon Obtained from Agricultural Waste for Effective Removal of Methylene Blue Dye

Zinc oxide (ZnO) nanorods incorporated activated carbon (AC) composite photocatalyst was synthesized using a hydrothermal process. The AC was prepared from lapsi (Choerospondias axillaris) seed stone, an agricultural waste product, found in Nepal by the chemical activation method. An aqueous suspension of AC with ZnO precursor was subjected to the hydrothermal treatment at 140 °C for 2 h to decorate ZnO rods into the surface of AC. As-obtained ZnO nanorods decorated activated carbon (ZnO/AC) photocatalyst was characterized by various techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. Results showed that highly crystalline hexagonal ZnO nanorods were effectively grown on the surface of porous AC. The photocatalytic property of the as-prepared ZnO/AC composite was studied by degrading methylene blue (MB) dye under UV-light irradiation. The ZnO/AC composite showed better photocatalytic property than that of the pristine ZnO nanorods. The enhanced photocatalytic performance in the case of the ZnO/AC composite is attributed to the combined effects of ZnO nanorods and AC.     

Characteristics of the Dye-Sensitized Solar Cells Using TiO2 Nanotubes Treated with TiCl4

The replacement of oxide semiconducting TiO₂ nano particles with one dimensional TiO₂ nanotubes (TNTs) has been used for improving the electron transport in the dye-sensitized solar cells (DSSCs). Although use of one dimensional structure provides the enhanced photoelectrical performance, it tends to reduce the adsorption of dye on the TiO₂ surface due to decrease of surface area. To overcome this problem, we investigate the effects of TiCl₄ treatment on DSSCs which were constructed with composite films made of TiO₂ nanoparticles and TNTs. To find optimum condition of TNTs concentration in TiO₂ composites film, series of DSSCs with different TNTs concentration were made. In this optimum condition (DSSCs with 10 wt% of TNT), the effects of post treatment are compared for different TiCl₄ concentrations. The results show that the DSSCs using a TiCl₄ (90 mM) post treatment shows a maximum conversion efficiency of 7.83% due to effective electron transport and enhanced adsorption of dye on TiO₂ surface.     

Formamidinium Lead Iodide Perovskite Films with Polyvinylpyrrolidone Additive for Active Layer in Perovskite Solar Cells,Enhanced Stability and Electrical Conductivity

In this paper, we studied the influence of polyvinylpyrrolidone (PVP) as a stabilization additive on optical and electrical properties of perovskite formamidinium lead iodide (FAPI) polycrystalline thin films on ZnO nanorods (ZNR). FAPI (as an active layer) was deposited from a single solution on ZNR (low temperature processed electron transport layer) using a one-step method with the inclusion of an anti-solvent. The role of PVP in the formation of the active layer was investigated by scanning electron microscopy and contact angle measurements to observe the effect on morphology, while X-ray diffraction was used as a method to study the stability of the film in an ambient environment. The effect of the PVP additive on the optical and electrical properties of the perovskite thin films was studied via photoluminescence, UV-Vis measurements, and electrical impedance spectroscopy. We have demonstrated that PVP inclusion in solution-processed perovskite FAPI thin films prevents the degradation of the film in an ambient atmosphere after aging for 2 months. The inclusion of the PVP also improves the infiltration of FAPI perovskite into ZnO nanostructures, increases electrical conductivity and radiative recombination of the photo-generated charge carriers. These results show promising information for promoting PVP stabilized FAPI perovskites for the new generation of photovoltaic devices.     

Controlling Morphological Parameters of Anodized Titania Nanotubes for Optimized Solar Energy Applications

Anodized TiO₂ nanotubes have received much attention for their use in solar energy applications including water oxidation cells and hybrid solar cells [dye-sensitized solar cells (DSSCs) and bulk heterojuntion solar cells (BHJs)]. High surface area allows for increased dye-adsorption and photon absorption. Titania nanotubes grown by anodization of titanium in fluoride-containing electrolytes are aligned perpendicular to the substrate surface, reducing the electron diffusion path to the external circuit in solar cells. The nanotube morphology can be optimized for the various applications by adjusting the anodization parameters but the optimum crystallinity of the nanotube arrays remains to be realized. In addition to morphology and crystallinity, the method of device fabrication significantly affects photon and electron dynamics and its energy conversion efficiency. This paper provides the state-of-the-art knowledge to achieve experimental tailoring of morphological parameters including nanotube diameter, length, wall thickness, array surface smoothness, and annealing of nanotube arrays.     

Electrochemical Impedance Investigation of Dye-Sensitized Solar Cells Based on Electrospun TiO2 Nanofibers Photoanodes

This work investigates an electrochemical impedance analysis based on synthesized TiO₂ nanofibers (NFs) photoanodes, which were fabricated via electrospinning and calcination. The investigated photoanode substrate NFs were studied in terms of physicochemical tools to investigate their morphological character, crystallinity, and chemical contents via scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analyses. As a result, the studied photoanode substrate NFs were applied to fabricate dye-sensitized solar cells (DSCs), and the electrochemical impedance analysis (EIS) was studied in terms of equivalent circuit fitting and impacts of N-doping, the latter of which was approved via XPS analysis. N-doping has a considerable role in the enhancement of charge transfers, which could be due to the strong interactions between active-site N atoms and the used photosensitizer.     

Bi and Sn Doping Improved the Structural,Optical and Photovoltaic Properties of MAPbI3-Based Perovskite Solar Cells

One of the most amazing photovoltaic technologies for the future is the organic–inorganic lead halide perovskite solar cell, which exhibits excellent power conversion efficiency (PCE) and can be produced using a straightforward solution technique. Toxic lead in perovskite can be replaced by non-toxic alkaline earth metal cations because they keep the charge balance in the material and some of them match the Goldschmidt rule’s tolerance factor. Therefore, thin films of MAPbI₃, 1% Bi and 0%, 0.5%, 1% and 1.5% Sn co-doped MAPbI₃ were deposited on FTO-glass substrates by sol-gel spin-coating technique. XRD confirmed the co-doping of Bi–Sn in MAPbI₃. The 1% Bi and 1% Sn co-doped film had a large grain size. The optical properties were calculated by UV-Vis spectroscopy. The 1% Bi and 1% Sn co-doped film had small E_g, which make it a good material for perovskite solar cells. These films were made into perovskite solar cells. The pure MAPbI₃ film-based solar cell had a current density (J_sc) of 9.71 MA-cm⁻², its open-circuit voltage (V_oc) was 1.18 V, its fill factor (FF) was 0.609 and its efficiency (η) was 6.98%. All of these parameters were improved by the co-doping of Bi–Sn. The cell made from a co-doped MAPbI₃ film with 1% Bi and 1% Sn had a high efficiency (10.03%).     

The Role of the Graphene Oxide (GO) and Reduced Graphene Oxide (RGO) Intermediate Layer in CZTSSe Thin-Film Solar Cells

Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells with low cost and eco-friendly characteristics are attractive as future sources of electricity generation, but low conversion efficiency remains an issue. To improve conversion efficiency, a method of inserting intermediate layers between the CZTSSe absorber film and the Mo back contact is used to suppress the formation of MoSe₂ and decomposition of CZTSSe. Among the candidates for the intermediate layer, graphene oxide (GO) and reduced GO have excellent properties, including high-charge mobility and low processing cost. Depending on the type of GO, the solar cell parameters, such as fill factor (FF), were enhanced. Thus, the conversion efficiency of 6.3% was achieved using the chemically reduced GO intermediate layer with significantly improved FF.     

Electrical and Optoelectronic Properties Enhancement of n-ZnO/p-GaAs Heterojunction Solar Cells via an Optimized Design for Higher Efficiency

In this study, we report the fabrication of high quality AZO/NRs-ZnO/n-ZnO/p-GaAs heterojunction via a novel optimized design. First of all, the electrical proprieties of gallium arsenide (GaAs) substrates were enhanced via an optimized gettering treatment that was based on a variable temperature process (VTP) resulting in an obvious increase of the effective minority carrier lifetime (τ_eff) from 8.3 ns to 27.6 ns, measured using time-resolved photoluminescence (TRPL). Afterward, the deposition of a zinc oxide (ZnO) emitter was optimized and examined in view of its use both as a light trapping layer (antireflection) and as the n-type partner for the p-type (GaAs) substrate. Nanorod-shaped ZnO was grown successfully on top of the emitter, as an antireflective coating (ARC), to further boost the absorption of light for a large broadband energy harvesting. The interface state of the prepared heterojunction is a key parameter to improve the prepared heterojunction performance, thus, we used laser ablation to create parallel line microgroove patterns in the GaAs front surface. We studied the effect of each step on the performance of the n-ZnO/GaAs heterojunction. The results demonstrate a significant improvement in V_oc, J_sc, fill factor (FF), and an obvious enhancement in the I–V characteristics, exhibiting good diode properties, giving rise to the photovoltaic conversion efficiency (η) from 8.31% to 19.7%, more than two times higher than the reference.     

Effect of the Source-to-Substrate Distance on Structural,Optoelectronic, and Thermoelectric Properties of Zinc Sulfide Thin Films

Zinc sulfide (ZnS) thin films with variable structural, optical, electrical, and thermoelectric properties were obtained by changing the source-to-substrate (SSD) distance in the physical-vapor-thermal-coating (PVTC) system. The films crystallized into a zinc-blende cubic structure with (111) preferred orientation. The films had a wide 3.54 eV optical band gap. High-quality homogenous thin films were obtained at 60 mm SSD. The sheet resistance and resistivity of the films decreased from 10¹¹ to 10¹⁰ Ω/Sq. and from 10⁶ to 10⁵ Ω-cm, when SSD was increased from 20 mm to 60 mm, respectively. The phase and band gap were also verified by first principles that were in agreement with the experimental results. Thermoelectric characteristics were studied by using the semi-classical Boltzmann transport theory. The high quality, wide band gap, and reduced electrical resistance make ZnS a suitable candidate for the window layer in solar cells.