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.     

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.     

Metal-free indoline dye sensitized zinc oxide nanowires solar cell

Dye sensitized solar cell (DSSC) based on metal-free indoline dye D102 sensitized zinc oxide (ZnO) nanowires (NWs) derived from aqueous solution on seeded substrate was investigated. The morphology, composition and crystalline structure of the highly oriented ZnO NWs were characterized by field-emission scanning electron microscope, energy dispersive X-ray spectrum spectroscopy and X-ray diffraction, respectively. The chemical bond between D102 and ZnO NWs was confirmed by Fourier transfer infrared spectra. The photovoltaic property of DSSC was characterized at full sun intensity of 100 mW/cm² (AM 1.5) with short circuit current J_sc = 14.06 mA/cm² and energy conversion efficiency η = 2.6%.     

Synthesis of hierarchical nanoparticles-based ZnO spheres for their application as the light blocking layers in dye-sensitized solar cells

Three-dimensional nanoparticles-based ZnO hierarchical spheres (ZnO-HS) with strong light harvesting and dye loading abilities have been fabricated by a simple hydrothermal method in this paper. These ZnO-HS were designed as the overlayer for light blocking and applied to the dye-sensitized solar cells (DSSCs) based on bare ZnO nanoparticles (ZnO-NP) or TiO₂ nanoparticles (TiO₂-NP). The results show that the values of the short-circuit current density (J _sc) and the power conversion efficiency (η) have been heightened up to 12.6 mA cm^?2 and 3.40 % for the ZnO-NP/ZnO-HS double-layered DSSC, far higher than the bare ZnO-NP DSSC. However, another DSSC assembled by the TiO₂-NP/ZnO-HS double-layered film displays an adverse result for the decreasing of J _sc and η even though the ZnO-HS light blocking layer has been established on the TiO₂-NP film. According to the electrochemical impedance data compared between the ZnO-NP/ZnO-HS double-layered and TiO₂-NP/ZnO-HS double-layered DSSC, it is found that the former possesses less possibility for the occurrence of charge recombination and electronic loss, which is responsible for its better photovoltaic response.     

Improved power conversion efficiency of dye-sensitized solar cells using side chain liquid crystal polymer embedded in polymer electrolytes

Side chain liquid crystal polymer (SCLCP) embedded in poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-co-HFP)-based polymer electrolytes (PVdF-co-HFP:side chain liquid crystal polymer (SCLCP)) was prepared for dye-sensitized solar cell (DSSC) application. The polymer electrolytes contained tetrabutylammonium iodide (TBAI), iodine (I₂), and 8 wt% PVdF-co-HFP in acetonitrile. DSSCs comprised of PVdF-co-HFP:SCLCP-based polymer electrolytes displayed enhanced redox couple reduction and reduced charge recombination in comparison to those of the conventional PVdF-co-HFP-based polymer electrolyte. The significantly increased short-circuit current density (J_sc, 10.75 mA cm⁻²) of the DSSCs with PVdF-co-HFP:SCLCP-based polymer electrolytes afforded a high power conversion efficiency (PCE) of 5.32% and a fill factor (FF) of 0.64 under standard light intensity of 100 mW cm⁻² irradiation of AM 1.5 sunlight.     

Fabrication and characterization of a composite ZnO semiconductor as electron transporting layer in dye-sensitized solar cells

ZnO nanocomposites involving nanowires and nanoparticles with a thickness of 4 μm were grown by chemical bath deposition and used as electron transporting layer in dye-sensitized solar cells (DSSCs). The growth of ZnO nanowires was initially achieved in a zinc nitrate and hexamethylenetetramine aqueous solution on a fluorine-doped tin oxide thin film seeded with ZnO nanoparticles. Subsequently, layered hydroxide zinc acetate (LHZA) nanoparticles were deposited on the nanowires by dip coating in a zinc acetate methanolic solution. A relatively conformal deposit of nanoparticles all along the nanowires was revealed by scanning and transmission electron microscopy. It is shown by X-ray diffraction measurements that a subsequent annealing convert the LHZA nanoparticles into ZnO nanoparticles. The resulting DSSCs present a short circuit current density almost three times higher when the ZnO nanowire interstices were filled with ZnO nanoparticles, which is due to a higher dye loading for a constant device thickness. This is correlated with a very high specific surface area in ZnO nanocomposites, which is 250 times larger than the geometrical surface area. Although a decrease in both the open circuit voltage and the fill factor was shown by electrochemical impedance spectroscopy owing to an increase in electron radiative and nonradiative recombinations, the efficiency of ZnO nanocomposite-based-DSSCs was on average 1.75%, which is 70% higher than for single ZnO nanowire-based-DSSCs.     

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.     

Design and fabrication of InxGa1 − xN/GaN solar cells with a multiple-quantum-well structure on SiCN/Si(111) substrates

The fabrication and characterization of In_xGa_1 − xN/GaN-based solar cells that use In_xGa_1 − xN multiple quantum wells (MQWs) and a SiCN/Si(111) substrate are reported. Solar cell operation with a low dark current density (J_d), a high open-circuit voltage (V_oc), a high short-circuit current density (J_sc), and a high fill factor (FF) is demonstrated. It was found that the proposed device and fabrication technology are applicable to the realization of solar cells with a low J_d of 2.14 to 8.88 μA/cm², a high V_oc of 2.72 to 2.92 V, a high J_sc of 2.72 to 2.97 mA/cm², and a high FF of 61.51 to 74.89%. The device performance with various quantum-well configurations was investigated under an air mass 1.5 global solar spectrum. A high photovoltaic efficiency of 5.95% in the MQW sample over the p-i-n sample was observed.     

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.     

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%.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

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.     

The role of buffer layer between TCO and p-layer in improving series resistance and carrier recombination of a-Si:H solar cells

The properties of the window layer and transparent conducting oxide (TCO)/p interface in silicon based thin-film solar cells are important factors in determining the cell efficiency. As the potential barrier got larger at the interface, the transmission of photo-generated holes were impeded and the recombination of photo-generated electrons diffusing back toward the TCO interface were enhanced leading to a deterioration of the fill factor. In this paper different p-layers were studied. It was found that using p-type hydrogenated amorphous silicon oxide (a-SiO_x:H) layer as the window layer along with a 5 nm buffer layer which reduced the barrier at the fluorine doped tin oxide (SnO₂:F) TCO/p-layer interface, improved the cell efficiency. a-SiO_x:H was used as the buffer layer. With the buffer layer between TCO and p-type a-SiO_x:H, the potential barrier dropped from 0.506 eV to 0.472 eV. This lowered barrier results in increased short circuit current density (J_sc) and fill factor (FF). With the buffer layer, J_sc increased from 11.9 mA/cm² to 13.35 mA/cm² and FF increased from 73.22% to 74.91%.     

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.

     

High efficiency amorphous silicon solar cells with high absorption coefficient intrinsic amorphous silicon layers

This paper considers the intrinsic layer of hydrogenated amorphous silicon (a-Si:H) solar cells. The deposition temperatures (T_d) and electrode distances (between cathode and anode, E/S) are important factors for a-Si:H solar cells. Thus, this study examines the effects of deposition temperatures and electrode distances in the intrinsic layer of a-Si:H solar cells with regard to enhanced the short-circuit current density (J_sc) and thereby conversion efficiency. It is shown that the J_sc of a-Si:H solar cells can be increased by proper choice of T_d and E/S of the i-a-Si:H layers. The J_sc of the a-Si:H solar cells is largely dependent on light absorption of the i-a-Si:H layer. It is demonstrated that the absorption coefficient in an i-a-Si:H layer can be increased to provide higher J_sc under fixed thickness. Results show that the optimized parameters improve the J_sc of a-Si:H solar cells to 16.52 mA/cm², yielding an initial conversion efficiency of 10.86%.     

Improvement of the efficiency of triple junction n-i-p solar cells with hot-wire CVD proto- and microcrystalline silicon absorber layers

Hot-wire chemical vapour deposition (HWCVD) was applied for the deposition of intrinsic protocrystalline (proto-Si:H) and microcrystalline silicon (μc-Si:H) absorber layers in thin film solar cells. For a single junction μc-Si:H n-i-p cell on a Ag/ZnO textured back reflector (TBR) with a 2.0 μm i-layer, an 8.5% efficiency was obtained, which showed to be stable after 750 h of light-soaking. The short-circuit current density (J_sc) of this cell was 23.4 mA/cm², with a high open-circuit voltage (V_oc) and fill factor (FF) of 0.545 V and 0.67.Triple junction n-i-p cells were deposited using proto-Si:H, plasma-deposited proto-SiGe:H and μc-Si:H as top, middle and bottom cell absorber layers. With Ag/ZnO TBR's from our lab and United Solar Ovonic LLC, respective initial efficiencies of 10.45% (2.030 V, 7.8 mA/cm², 0.66) and 10.50% (2.113 V, 7.4 mA/cm², 0.67) were achieved.     

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 %.     

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.     

Characteristics of dye-sensitized solar cells with surface-modified multi-walled carbon nanotubes as counter electrodes

A polystyrene-based functional block copolymer is employed as a surface modifier for multi-walled carbon nanotube (MWCNT) paste utilized in the fabrication of a MWCNT counter electrode (CE) in dye-sensitized solar cells (DSSCs). The surface modification of MWCNTs paste improves the dispersibility of MWCNTs, resulting in a facilitated fabrication of electrodes through the screen printing procedure, as evidenced by a lower viscosity and more homogeneous paste, as well as a more uniform MWCNT coating. Upon removing organic compounds from the paste through a thermal treatment procedure, the DSSC with the modified CE exhibits enhanced solar energy conversion efficiency (η) compared with that of the neat MWCNT CE. The behavior stems from an improvement in the overall redox reaction kinetics and the short-circuit current (J _sc) of the DSSC. The DSSC also exhibits an improved η value over an extended storage period, which demonstrates a successful combination of processability, performance, and stability of the DSSC achieved by using an optimum amount of surface modifier for MWCNTs.