Facile synthesis of highly branched jacks-like ZnO nanorods and their applications in dye-sensitized solar cells

Highly branched, jacks-like ZnO nanorods architecture were explored as a photoanode in dye-sensitized solar cells, and their photovoltaic performance was compared with that of branch-free ZnO nanorods photoanodes. The highly branched network and large pores of the jacks-like ZnO nanorods electrodes enhances the charge transport, and electrolyte penetration. Thus, the jacks-like ZnO nanorods DSSCs render a higher conversion efficiency of η = 1.82% (V_oc = 0.59 V, J_sc = 5.52 mA cm⁻²) than that of the branch-free ZnO nanorods electrodes (η = 1.08%, V_oc = 0.49 V, J_sc = 4.02 mA cm⁻²). The incident photon-to-current conversion efficiency measurements reveal that the jacks-like ZnO nanorods DSSCs exhibit higher internal quantum efficiency (∼59.1%) than do the branch-free ZnO nanorods DSSC (∼52.5%). The charge transfer resistances at the ZnO/dye/electrolyte interfaces investigated using electrochemical impedance spectroscopy showed that the jacks-like ZnO nanorods DSSC had high charge transfer resistance and a slightly longer electron lifetime, thus improving the solar-cell performance.     

Effects of Na-doping on the efficiency of ZnO nanorods-based dye sensitized solar cells

Na-doped ZnO nanorods (Zn_1?xNa_xO: x = 0.0, 0.02, 0.04) were grown by a chemical bath deposition method on ZnO seeded FTO substrates. The influence of Na-doping on the efficiency of ZnO nanorods-based dye-sensitized solar cells (DSSCs) was investigated. Undoped and Na-doped ZnO nanorods were used as photo-anodes for the fabricated DSSCs. X-ray diffraction measurements exhibited that all the samples had a wurtzite structure of ZnO with a preferred orientation of (002) plane. Scanning electron microscopy images of the samples revealed that all the samples displayed hexagonal shaped nanorods. It was observed from optical measurements that the band gap energy gradually decreased from 3.29 to 3.21 eV for undoped and 4 at.% Na-doped ZnO nanorods, respectively. Photoluminescence spectrum for undoped ZnO showed three peaks located at 379, 422, and 585 nm corresponding to UV emission, zinc vacancy, and deep level emission (DLE) peaks, respectively. When ZnO nanorods were doped with 2 at.% Na, the intensity of UV peak increased whereas the intensity of DLE peak decreased. The maximum conversion efficiency of DSSCs was found to be 0.22 % with a J_sc of 0.80 mA/cm², V_oc of 0.49 V, and fill factor of 0.523 as ZnO nanorods were doped with 2 at.% Na atoms.     

Low-temperature synthesis of ZnO nanoparticles for inverted polymer solar cell application

Zinc oxide (ZnO) nanoparticles were synthesized by a simple wet chemical method at low temperature. Morphologies, crystalline structure, and optical transmission of ZnO nanoparticles were investigated. The results showed that the average diameter of as-synthesized ZnO nanoparticles was about 4.9 nm, the nanoparticles were wurtzite-structured (hexagonal) ZnO and had optical band gap of 3.28 eV. Very high optical transmission (>80 %) in visible light region of ZnO nanoparticulate thin films was achieved. Furthermore, an inverted polymer solar cell consisted of ZnO nanoparticles and polymer were fabricated. The device exhibited an open circuit voltage (V_oc) of 0.50 V, a short circuit current density (J_sc) of 1.76 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.42 %.     

Low-temperature growth of well-aligned ZnO nanowire arrays by chemical bath deposition for hybrid solar cell application

Well-aligned ZnO nanowire arrays were grown on indium tin oxide coated glass substrates by a facile chemical bath deposition technique. Morphologies, crystalline structure and optical transmission were investigated by field-emission scanning electron microscope, X-ray diffraction and UV–visible transmission spectrum, respectively. The results showed that ZnO nanowires were aligned in a dense array approximately perpendicular to substrate surface, they were wurtzite-structured (hexagonal) ZnO. In addition, the nanowire arrays exhibited high optical transmission (>85 %) in the visible region. Furthermore, an inverted inorganic/polymer hybrid solar cell was built using as-grown well-aligned ZnO nanowire arrays as inorganic layer, under the AM 1.5 illumination with a light intensity of 80 mW/cm², the device showed an open circuit voltage (V_oc) of 0.44 V, a short circuit current (J_sc) of 3.23 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.68 %.     

Spray-pyrolytically deposited n-CuInSe2/polysulphide photoelectrochemical solar cells

n-CuInSe₂ photoanode has been prepared by spray pyrolysis onto SnO₂-deposited glass substrate. The effect of etching (HCl:HNO₃=5 : 1 by volume) on photoanode properties has been studied. The best cell had the following parameters:V _oc=0·446 V,J _sc=18·32 mA/cm², ff=0·53 andη=6·22%.     

Ag nanoparticle-decorated SiO2@TiO2 hierarchical microspheres improve the efficiency of dye-sensitized solar cells

SiO₂@TiO₂-Ag (STA) microspheres decorated with Ag nanoparticles (Ag NPs) were prepared and assembled into the photoanode. The photoanode composed of STA microspheres and TiO₂ nanoparticles (P25) was prepared by doctor blade method. UV–vis measurement indicates that the introduction of a few STA microspheres observably enhances the light scattering and capturing ability of the photoanode. The photoelectric conversion efficiency of the DSSCs with 2wt% STA photoanode increased to 7.4% from 4.3% comparing with pure P25 TiO₂ nanoparticles. The configuration DSSCs have the maximum short circuit current density (J_sc) of 16.0 mA cm^?2 and open-circuit voltage (V_oc) of 0.780 V, which are significantly higher than the pure TiO₂ DSSCs. The significant improvement of the DSSCs performance can be due to the synergistic effect of the superior light scattering of STA and the localized surface plasma resonance (LSPR) effect of Ag NPs modified on the microspheres surface.

     

Novel D-π-A dye sensitizers of polymeric metal complexes with phenylethyl or carbazole derivatives as donors for dye-sensitized solar cells: synthesis,characterization, and photovoltaic application

Four novel polymeric metal complexes containing D-π-A type structures were synthesized, characterized and applied as dye sensitizers in dye-sensitized solar cells (DSSCs). The alkoxy benzene or carbazole (CZ) derivative acts as an electron donor (D), C=C acts as π-bridge (π) and the 8-hydroxyquinoline derivative complex acts as electron acceptor. Bipyridine derivative was ancillary ligand as well as providing anchoring group. FT-IR, gel permeation chromatography, thermogravimetric analyses, differential scanning calorimetry, UV–Vis absorption spectroscopy, Elemental analysis, cyclic voltammetry, J-V curves and input photon to converted current efficiency plots were introduced to investigate the four dyes. These dyes exhibit good thermal stability with 5 % weight loss at temperatures (T_d) of around 300 °C. For the DSSCs devices using dyes with CZ derivatives as electron D (P2, P4) exhibited higher power conversion efficiency (PCE) than that with alkoxy benzene (P1, P3). The DSSC based on P4 exhibited the highest PCE value of 2.42 % (J _sc  = 4.93 mA/cm², V _oc  = 0.73 V, FF = 67.2 %) under AM 1.5G solar irradiation. This indicates a new way to design dye sensitizers for DSSCs.     

The Impact of Grain Alignment of the Electron Transporting Layer on the Performance of Inverted Bulk Heterojunction Solar Cells

This report presents a new strategy for improving solar cell power conversion efficiencies (PCEs) through grain alignment and morphology control of the ZnO electron transport layer (ETL) prepared by radio frequency (RF) magnetron sputtering. The systematic control over the ETL's grain alignment and thickness is shown, by varying the deposition pressure and operating substrate temperature during the deposition. Notably, a high PCE of 6.9%, short circuit current density (J_sc) of 12.8 mA cm^?2, open circuit voltage (V_oc) of 910 mV, and fill factor of 59% are demonstrated using the poly(benzo[1,2‐b:4,5‐b′]dithiophene–thieno[3,4‐c]pyrrole‐4,6‐dione):[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester polymer blend with ETLs prepared at room temperature exhibiting oriented and aligned rod‐like ZnO grains. Increasing the deposition temperature during the ZnO sputtering induces morphological cleavage of the rod‐like ZnO grains and therefore reduced conductivity from 7.2 × 10^?13 to ≈1.7 × 10^?14 S m^?1 and PCE from 6.9% to 4.28%. An investigation of the charge carrier dynamics by femtosecond (fs) transient absorption spectroscopy with broadband capability reveals clear evidence of faster carrier recombination for a ZnO layer deposited at higher temperature, which is consistent with the conductivity and device performance.     

Novel in situ crosslinked polymer electrolyte for solid-state dye-sensitized solar cells

A novel poly(citric acid-ethylene glycol)/LiI/I₂ (PCE/LiI/I₂) solid polymer electrolyte (SPE) based on the biodegradable PCE matrix has been prepared in situ, by penetrating of the PCE prepolymer sol into mesoporous TiO₂ photoanode, followed by curing. The PCE prepolymer can easily penetrate into the mesoporous photoanode, which could induce good interfacial contact between the SPE and photoanode. Assembled with the SPE, highly efficient and stable solid-state dye-sensitized solar cells (DSSCs) have been gained due to the good interfacial contact of SPE/TiO₂ photoanode as well as the favorable ionic conductivity of the SPE. The results show that the contents of CA determine the aggregation structure such as the inter-segmental distance and free volume of the PCE matrix, which consequently affects the ionic diffusion coefficient and conductivity of the PCE/LiI/I₂ electrolyte, and accordingly the photoelectric performance of the DSSCs. With CA content of 32.4 wt%, the SPE reaches the optimal ionic conductivity of 5.43 × 10^?5 S cm^?1 and the solid-state DSSCs obtain the best overall photoelectric conversion efficiency of 1.22 % at 60 mW cm^?2.     

Copper phthalocyanine based Schottky diode solar cells

Copper phthalocyanine (CuPc)/Aluminum (Al) Schottky diode solar cells were studied. The thickness of the CuPc layer was varied from 15 nm to 140 nm. Short circuit current densities (J_sc) increased with thickness from 0.042 mA/cm² at 15 nm to 0.124 mA/cm² at 120 nm reaching saturation at that level. Open circuit voltages (V_oc) increased from 220 mV at 15 nm to 907 mV at 140 nm. Analysis of the current-voltage characteristics indicated that tunneling and interface recombination current mechanisms are important components of the current transport at the CuPc/Al junction.     

Hybrid photovoltaic devices of polymer and ZnO nanofiber composites

Organic semiconductor-based photovoltaic devices offer the promise of a low-cost photovoltaic technology that could be manufactured via large-scale, roll-to-roll printing techniques. Existing organic photovoltaic devices have currently achieved solar power conversion efficiencies greater than 3%. Although encouraging, the reasons higher efficiencies have not been achieved are poor overlap between the absorption spectrum of the organic chromophores and the solar spectrum, non-ideal band alignment between the donor and acceptor species, and low charge carrier mobilities resulting from the disordered nature of organic semiconductors. To address the latter issues, we are investigating the development of nanostructured oxide/conjugated polymer composite photovoltaic (PV) devices. These composites can take advantage of the high electron mobilities attainable in oxide semiconductors and can be fabricated using low-temperature solution-based growth techniques. Additionally, the morphology of the composite can be controlled in a systematic way through control of the nanostructured oxide growth. ZnO nanostructures that are vertically aligned with respect to the substrate have been grown. Here we discuss the fabrication of such nanostructures and present results from ZnO nanofiber/poly(3-hexylthiophene) (P3HT) composite PV devices. The best performance with this cell structure produced an open circuit voltage (V_oc) of 440 mV, a short circuit current density (J_sc) of 2.2 mA/cm², a fill factor (FF) of 0.56, and a conversion efficiency (η) of 0.53%. Incorporation of a blend of P3HT and (6,6)-phenyl C₆₁ butyric acid methyl ester (PCBM) into the ZnO nanofibers produced enhanced performance with a V_oc of 475 mV, J_sc of 10.0 mA/cm², FF of 0.43, and η of 2.03%. The power efficiency is limited in these devices by the large fiber spacing and the reduced V_oc.     

Europium and Acetate Co‐doping Strategy for Developing Stable and Efficient CsPbI2Br Perovskite Solar Cells

All‐inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their future commercial application. The mixed‐halide inorganic CsPbI₂Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI₂Br perovskites limit the open‐circuit voltage (V_oc < 1.2 V), short‐circuit current density (J_sc < 15 mA cm^?2), and fill factor (FF < 75%) of CsPbI₂Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI₂Br perovskite is doped by Eu(Ac)₃ to obtain a high‐quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable V_oc of 1.25 V, a reasonable J_sc of 15.44 mA cm^?2, and a high FF of 79.00% are realized for CsPbI₂Br solar cells. Moreover, the CsPbI₂Br solar cells with Eu(Ac)₃ doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35–40%) for 30 days.     

Effects of ZnO nanowire synthesis parameters on the photovoltaic performance of dye-sensitized solar cells

Determination of the effects of ZnO nanowires on the efficiency of ZnO nanowire-based dye-sensitized solar cells (DSSCs) is important. In this study, we determined the effects of different OH^- precursors, concentrations, the ratio of zinc nitrate to hexamethylene tetramine (HMT), and the hydrothermal synthesis temperature on the physical, crystal, and optical properties of ZnO nanowires and investigated the performance of the resulting DSSCs. We observed that ZnO nanowires synthesized using an equimolar ratio of HMT to zinc nitrate yielded a DSSC with high incident photon-to-current efficiency (IPCE), cell efficiency, short circuit current density (J_sc), and fill factor (FF), and low ZnO-dye-electrolyte interface resistance due to an increased amount of dye and a decreased density of defects. Furthermore, ZnO nanowires made using optimal concentrations and ratios of zinc nitrate to HMT had a high surface area and low defect density. All the photovoltaic performance parameters of DSSCs assessed such as IPCE, cell efficiency, J_sc, open circuit potential (V_oc), and FF increased with synthesis temperature, which was related to a decrease in the resistance at the ZnO-dye-electrolyte interface. We attributed these results to an increased amount of dye facilitated by a large nanowire surface area and fast electron transfer because of the improved crystalline structure of the ZnO nanowires and their low defect density. By optimizing the ZnO nanowires, we increased DSSC efficiency to 0.26% using ZnO nanowires synthesized with 25 mM of both zinc nitrate and HMT at 90 °C, while only a 0.02% increase in efficiency was obtained when NH₄OH was used as OH⁻ precursor.     

Rapid thermal melted TiO2 nano-particles into ZnO nano-rod and its application for dye sensitized solar cells

TiO₂ nano-particles with an anchored ZnO nano-rod structure were synthesized using the hydrothermal method to grow ZnO nano-rods and coated TiO₂ nano-particles on ZnO nano-rods using the rapid thermal annealing method on ITO conducting glass pre-coated with nano porous TiO₂ film. The XRD study showed that there was little difference in crystal composition for various types of TiO₂ nano-particles anchored to ZnO nano-rods. The as-prepared architecture was characterized using field-emission scanning electron microscopy (FE-SEM). Films with TiO₂ nano-particles anchored to ZnO nano-rods were used as electrode materials to fabricate dye sensitized solar cells (DSSCs). The best solar energy conversion efficiency of 2.397% was obtained by modified electrode material, under AM 1.5 illumination, achieved up to J_sc = 15.382 mA/cm², V_oc = 0.479 V and fill factor = 32.8%.     

Rapid Thermal Annealing of ZnO Nanocrystalline Films for Dye-Sensitized Solar Cells

Nanocrystalline ZnO thin films were prepared by the sol–gel method and annealed at 600 °C by conventional (CTA) and rapid thermal annealing (RTA) processes on fluorine-doped tin oxide (FTO)-coated glass substrates for application as the work electrode for a dye-sensitized solar cell (DSSC). ZnO films were crystallized using a conventional furnace and the proposed RTA process at annealing rates of 5 °C/min and 600 °C/min, respectively. The ZnO thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses. Based on the results, the ZnO thin films crystallized by the RTA process presented better crystallization than films crystallized in a conventional furnace. The ZnO films crystallized by RTA showed higher porosity and surface area than those prepared by CTA. The results show that the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) values increased from 4.38 mA/cm² and 0.55 V for the DSSC with the CTA-derived ZnO films to 5.88 mA/cm² and 0.61 V, respectively, for the DSSC containing the RTA-derived ZnO films.     

Thin film CdZnS/CuInSe2 solar cells by spray pyrolysis

Cd_1−xZn_xS/CuInSe₂ solar cells having efficiencies in the range of 2·3% were fabricated by spray pyrolysis. The best cell had the following parameters:V _oc = 305 mV,J _sc = 32 mA/cm², FF = 0·32 area = 0·4 cm² and efficiency = 3·149%.V _oc versus temperature measurements showed that the electron affinity difference was 0·22 eV. Forward dark current versus voltage curves were plotted and a possible current mechanism occurring in these cells has been proposed.     

Effect of surfactant on the physical properties of ZnO nanorods and the performance of ZnO photoelectrochemical cell

The performance of photoelectrochemical cell (PEC) strongly depends on the physical properties of photovoltaic material. Polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB) and hexamethylenetetramine (HMT) surfactants were used to modify the morphology nanostructure of ZnO films by a simple technique, namely, ammonia-assisted controlled hydrolysis technique during their growth process. The film treated with PVP, CTAB and HMT produce the nanostructure shape of nanoflower, nanowire and nanorod (NR), respectively. These ZnO samples were utilised as photovoltaic materials in a PEC of FTO (fluorine tin oxide)/ZnO/electrolyte/platinum. It was found that the photovoltaic parameters such as short-circuit current density (J_sc), open-circuit voltage (V_oc) and fill factor (FF) are influenced by the morphology in term of shape and particle size and optical property of the ZnO NR. The PEC utilising the ZnO sample treated with HMT surfactant demonstrated the highest J_sc of 0.47 mAcm^?2, V_oc of 0.46 V, FF of 29.2% and η of 0.06%, respectively, since it possesses the lowest energy gap.     

n-CuInS2/polysulfide photoelectrochemical solar cells prepared by spray pyrolysis

n-CuInS₂ photoanode has been prepared by spray pyrolysis onto SnO₂ deposited glass substrate at 350°C. The conductivity type of the photoanode was tested by hot-probe method and was ofn type. The conductivity of the photoanode was of the order of 2–4Θ⁻¹cm⁻¹ and was measured by using four-probe method. The effect of etching (HCl:HNO₃ = 5:1 by volume) on photoanode properties has been studied. The best cell had the following parameters:V _oc = 0·29V,I _sc = 5·33 mA/cm²,ff = 0·571 and η = 1·275%.     

The effect of alcohol solvent treatment on the performance of inverted polymer solar cells

Significant performance change has been observed for inverted polymer solar cells (PSCs) with the structure of ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag when the photoactive layer was rinsed by spin-coating alcohol solvent before the deposition of MoO₃ and Ag anode. As a result, isopropanol (IPA) treatment can dramatically enhance the device performance of inverted PSCs while methanol, ethanol, and 1-butanol treatment led to worse photovoltaic performance. The enhancing device performance should be attributed to the remove of PC₆₁BM near the top of the P3HT:PC₆₁BM active layer by spin-coating IPA due to better wetting with the photoactive layer, resulting in the power conversion efficiency (PCE) and short-circuit current (J_sc) increased from 3.96% and 8.97 mA/cm² to 4.49% and 9.92 mA/cm² for IPA treatment, respectively. This facile alcohol-treated route provides a promising method to enhance the device performance of inverted PSCs.     

Optimization of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin film absorber layer growth without sulphurization using triethanolamine as complexing agent for thin film solar cells applications

Quaternary kesterite Cu₂ZnSnS₄ (CZTS) thin films have been prepared via a simple spin-coating technique based on a sol–gel precursor of 2-methoxyethanol solution with metal salts and thiourea. Solution processed CZTS thin film growth parameters using complexing agent triethanolamine (TEA) have been investigated. Effects of complexing agent TEA on structural, morphological, optical, electrical and photovoltaic properties of CZTS thin films were systematically investigated. X-ray diffraction and Raman spectroscopy studies reveal that amorphous nature of CZTS thin film changes into polycrystalline with kesterite crystal structure with optimized TEA concentartion. Surface morphology of CZTS films were analyzed by field emission scanning electron microscope and atomic force microscope, which revealed the smooth, uniform, homogeneous and densely packed grains and systematic grain growth formation with varying TEA concentrations. UV–Vis spectra revealed a direct energy band gap ranging from 1.78 to 1.50 eV, which was found to depend upon the TEA concentration. X-ray photoelectron spectroscopy demonstrated stoichiometric atomic ratios of multicationic quaternary CZTS thin film grown without sulphurization. p-type conductivity was confirmed using Hall measurements and the effect of varying concentartion of TEA on electrical and photovoltaic properties are studied. The SLG/FTO/ZnO/CZTS/Al thin film solar cell is fabricated with the CZTS absorber layer grown at optimized TAE concentration of 0.06 M. It shows a power conversion efficiency of 0.87% for a 0.16 cm² area with V_oc = 0.257 mV, J_sc = 8.95 mA/cm² and FF?=?38%.