ZnO compact layers used in third-generation photovoltaic devices: a review

Journal of Materials Science - ZnO is a well-known semitransparent semiconductor with wide applicability in semiconducting devices such as solar cells, LEDs, MOSFETs, gas sensor devices, or...     

Electroluminescence dependence on the organic thickness in ZnO nano rods/Alq3 heterostructure devices

ZnO nanorods are synthesised by a hydrothermal method on ITO glass. Their crystallization and morphology are detected by XRD and SEM, respectively. The results show that the ZnO nanorod array has grown primarily along a direction aligned perpendicular to the ITO substrate. The average height and diameter of the nanorods is about 130 nm and 30 nm, respectively. Then ZnO nano rods/Alq3 heterostructure LEDs are prepared by thermal evaporation of Alq3 molecules. The thicknesses of the Alq3 layers are 130 nm, 150 nm, 170 nm and 190 nm, respectively. The electroluminescence of the devices is detected under different DC bias voltages. The exciton emission of Alq3 is detected in all devices. When the thickness of Alq3 is 130 nm, the UV electroluminescence of ZnO is around 382 nm, and defect emissions around 670 nm and 740 nm are detected. Defect emissions of ZnO nanorods are prominent. When the thickness of Alq3 increases to over 170 nm, it is difficult to observe defect emissions from the ZnO nano rods. In such devices, the exciton emission of Alq3 is more prominent than other emissions under different bias voltage.     

Improved efficiency of organic/inorganic photovoltaic devices by electrospun ZnO nanofibers

The efficiency of the photovoltaic (PV) device based on P3HT and PCBM bulk heterojunction is improved by introducing small-diameter electrospun ZnO diffused nanofibers network. Diameter, diffusion and melting of nanofibers are controlled by calcination temperature. The thickness of the active layer is optimized for efficient PV devices by varying electrospinning (ES) time. Increased nanofiber's mat thickness by an increase in electrospinning time beyond a certain optimum value reduces the device performance due to increased series resistance, increased traps and reduced blend infiltration through the nanofiber pores. ES time suggests optimized active area for energy absorption and exciton dissociation. In this study, we report the improvement in power conversion efficiency (PCE) from 0.9% to 2.23%, for optimum ES time (∼300 s).     

Structural and optical properties of electrospun ZnO nanofibres applied to P3HT:PCBM organic photovoltaic devices

Electrospun ZnO nanofibres with a diameter in the range of 74–125 nm were synthesised by optimising various parameters. Zinc acetate was used as a precursor and optimised to obtain a homogeneous and interconnected porous ZnO nanofibres network. The ZnO/poly(vinylpyrrolidone) composite nanofibres were calcined at 450 °C for 2.5 h to obtain continuous nanofibres network. The morphology of the nanofibres was studied by scanning electron microscopy and imageJ software. The structural and optical properties of the synthesised nanofibres were studied by surface profiler, X-ray diffraction and ultraviolet–visible spectroscopy. The grain size increased while the bandgap was observed to decrease with increased precursor concentration. The effect of precursor concentration was also studied to improve the power conversion efficiency of ZnO nanofibres/poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) photovoltaic device. The efficiency was improved from 0.896 ± 0.007% to 2.29 ± 0.03% by introducing the electrospun ZnO nanofibres.     

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.     

Study of solvent dependence of Methyl Red and C60 based organic photovoltaic devices

In this work, the effect of solvent on the photovoltaic properties of Methyl Red (MR) dye and C₆₀ based device has been reported. It can be assumed for a dye based photovoltaic device that there is an effect of solvent on the film morphology which controls the charge transport mechanism. To observe this effect of solvent on the film morphology two different solvents namely chlorobenzene and toluene are used to prepare the solutions of MR and C₆₀. The devices made with chlorobenzene and toluene solvents are termed as chlorobenzene cell and toluene cell respectively. For each solvent different devices are prepared by varying the weight ratio of MR and C_60. These cells are characterized through different photovoltaic measurements. Experimental data reveal that the photovoltaic response is higher in case of chlorobenzene cell than that of the toluene cell. The power conversion efficiency for toluene cell for a particular concentration of MR:C₆₀ is 0.781 × 10^− 7 whereas for chlorobenzene cell it is 1.026 × 10^− 5. Maximum V_oc and J_sc obtained for toluene cell are 19.5 mV and 33.73 ?nA/cm² whereas for chlorobenzene cell these are 720 mV and 158.0 nA/cm² respectively. Scanning electron microscope images indicate the difference in nano-morphology and cluster sizes between the cells. The cluster size is quite large in case of toluene cell with comparison to chlorobenzene cell. From the transient photocurrent measurement it is observed that the charge transport process is quite faster in chlorobenzene cell which prevents excess recombination resulting in a higher efficiency in chlorobenzene cell.     

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The effect of ZnO film depositions using various film deposition methods such as magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) on the photovoltaic properties of ZnO–Cu₂O heterojunction solar cells is described in this report. In addition, the relationship between the resulting photovoltaic properties and the film deposition conditions such as supply power and substrate arrangement was investigated in Al-doped ZnO (AZO)–Cu₂O heterojunction devices fabricated using AZO thin films prepared by d.c. magnetron sputtering (d.c.MSP) or r.f. magnetron sputtering (r.f.MSP). The results showed that the measured photovoltaic properties of devices fabricated with films deposited on substrates oriented perpendicular to the target were better than those of devices fabricated with films deposited on substrates oriented parallel to the target. It was also found that ZnO film depositions under conditions where a relatively weaker oxidizing atmosphere was used yield better properties than films derived from MSP, which utilizes a high-density and high-energy plasma. Using VAPE and PLD, for example, high efficiencies of 1.52 and 1.42%, respectively, were obtained under AM2 solar illumination in devices fabricated at a substrate temperature around 200 °C.     

Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices

We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.     

Fabrication of oriented ZnO nanopillar self-assemblies and their application for photovoltaic devices

Organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerene have been studied by inserting oriented zinc oxide nanopillars which were fabricated by a new method at low temperature (343?K). The dependence of the photovoltaic performance on the zinc oxide morphology was investigated, and it is concluded that the oriented zinc oxide nanopillar array plays an important role in collecting photogenerated electrons and acts as a conducting path to the electrode. Insertion of the oriented zinc oxide nanopillars in the photovoltaic cells produced enhanced performance with a power conversion efficiency of 1.22% under AM1.5 illumination.     

Very large deflection with quadratic voltage dependence of ZnO on Si(3)N(4) bimorph

ZnO on Si(3)N(4) bimorphs have shown large deflections with quadratic dependence on applied voltages. Several effects are suggested that might explain these large deflections. No conclusion on the origin of these large deflections can yet be given.     

Spin coated graphene films as the transparent electrode in organic photovoltaic devices

Many research efforts have been devoted to the replacement of the traditional indium-tin-oxide (ITO) electrode in organic photovoltaics. Solution-based graphene has been identified as a potential replacement, since it has less than two percent absorption per layer, relative high carrier mobility, and it offers the possibility of deposition on large area and flexible substrates, compatible with roll to roll manufacturing methods. In this work, soluble reduced graphene films with high electrical conductivity and transparency were fabricated and incorporated in poly(3-hexylthiophene) [6,6]-phenyl-C₆₁-butyric acid methyl ester photovoltaic devices, as the transparent electrode. The graphene films were spin coated on glass from an aqueous dispersion of functionalized graphene, followed by a reduction process combining hydrazine vapor and annealing under argon, in order to reduce the sheet resistance. The photovoltaic devices obtained from the graphene films showed lower performance than the reference devices with ITO, due to the higher sheet resistance (2 kΩ/sq) and the poor hydrophilicity of the spin coated graphene films.     

Thickness dependence of amplified spontaneous emission in low-absorbing organic waveguides

The effect of varying film thickness (h) on the amplified spontaneous emission (ASE) properties of 0.5 wt.% perylenediimide-doped polystyrene waveguides is reported. The threshold dependence on h, not previously investigated in detail, is analyzed in terms of the film absorption and photoluminescence, the confinement of the fundamental waveguide mode (TE0), and the presence of high-order modes. For h<400 nm and down to 150 nm, the ASE wavelength blueshifts, while the linewidth and threshold increase. The detrimental ASE operation in very thin films is due to the low absorption as well as to the poor confinement of the TE0 mode.     

Performance of polymer/ZnO hybrid photovoltaic devices determined by reaction time for oriented ZnO nanorod growth

The performance of hybrid polymer/metal oxide photovoltaic devices based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) and oriented ZnO nanorods is studied. The ZnO nanorods on indium tin oxide-coated glass were prepared by hydrothermal method, where the length and the defect concentration of ZnO nanorods were controlled by the reaction time (T_r) for nanorod growth. Increasing T_r results in longer ZnO nanorods and higher defect concentration. Results show that both photocurrent and electron lifetime have strong dependence on the nanorod length (i.e., growth time) due to the exponential attenuation of incident light intensity in the device, offering a peak conversion efficiency of 0.337% under 1.5 AM illumination for T_r = 120 min. Combinational analyses of the data in this experiment and the previous data for the electrodeposited ZnO nanorods provide the insights into the dependence of the device performance on the intrinsic property of the ZnO nanorods.     

Properties of Cu-doped ZnO films by RF sputtering method: Thickness dependence

We present results concerning the thickness dependence of structural, morphological and optical properties of the Zn_0.98Cu_0.02O films deposited on glass substrates using radio frequency (RF) sputtering method. The microstructure and the chemical state of oxygen, copper and zinc in ZnO and Zn_0.98Cu_0.02O films were investigated by X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The results indicate that Zn_0.98Cu_0.02O films are the wurtzite structure with strong c-axis orientation. Crystallinity of the films is closely related to the film thickness. With increasing film thickness, there are more surface (mainly nanopores) defects existing in the Zn_0.98Cu_0.02O films and surface roughness increases. XRD and XPS data show that the valence state of copper in the Zn_0.98Cu_0.02O films is Cu²⁺. The transparency of all films is more than 85% in the visible region.     

Phase equilibria in the Tl2MoO4-Pr2(MoO4)3-Hf(MoO4)2 system and crystal structure of TlPr(MoO4)2

Subsolidus (450–480°C) phase relations in the Tl₂MoO₄-Pr₂(MoO₄)₃-Hf(MoO₄)₂ system have been studied by X-ray diffraction. The system has been shown to contain molybdates with the compositions Tl₅PrHf(MoO₄)₆ (5: 1: 2), TlPrHf_0.5(MoO₄)₃ (1: 1: 1), and Tl₂PrHf₂(MoO₄)_6.5 (2: 1: 4). Single crystals of the double molybdate TlPr(MoO₄)₂ have been grown for the first time from high-temperature solutions through spontaneous nucleation, and their crystal structure has been determined: tetragonal symmetry, sp. gr. P4/nnc, a = 6.3170(1) Å, c = 9.5529(2) Å, V = 381.204(12) ų, Z = 2.     

Thickness dependence of mobility of pentacene planar bottom-contact organic thin-film transistors

We have fabricated planar bottom-contact organic thin-film transistors as a function of the thickness of the pentacene active layer. The highest mobility of the planar bottom-contact transistors is 0.47 cm²/Vs with only a 7 nm pentacene active layer. Our planar bottom-contact transistors show much higher mobility than conventional bottom-contact counterparts and even higher than the reported mobility values of top-contact counterparts for each thickness in the range from 2.5 to 10 nm. We find that spike at the edges of source and drain electrodes seriously deteriorates device performance.     

Efficient white organic electroluminescent devices consisting of blue- and red-emitting layers

We report the fabrication and the characterization of white-organic-light-emitting devices consisting of a blue-emitting layer of 1,4-bis(2,2-diphenyl vinyl)benzene (DPVBi) and a red-emitting layer of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-8-yl)vinyl]-4H-pyran (DCM2) doped into either 4,4′bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (α-NPD) or tris(8-hydroxyquinoline) aluminum (Alq₃). The spectral emission depends on both the doping location of DCM2 and its doping concentration. The electroluminescence (EL) spectra consist of two broad bands around 460 nm (DPVBi) and 560 nm (α-NPD:DCM2) or 590 nm (Alq₃:DCM2). We obtained an efficient white-light emission from the devices with the 0.2% DCM2 doping in α-NPD layer. The device shows the CIE coordinates of (0.33, 0.36), an external quantum efficiency (QE) of about 3.1%, and a luminous efficiency of 3.75 lm/W at luminance 100 cd/m². The maximum luminance of about 41,000 cd/m² was obtained.     

Enhancement of the lifetime in organic light-emitting devices fabricated utilizing wide-bandgap-impurity-doped emitting layers

The degradation behaviors of the electrical and the optical properties of organic light-emitting devices (OLEDs) fabricated with an emitting layer (EML) doped with or without a wide-bandgap-impurity were investigated. The OLEDs with a wide-bandgap-doped Alq₃ EML were more stable than those with an undoped Alq₃ EML. The existence of the doped wide-bandgap-impurity in the EML decreased the trap-charge density in the EML, resulting in an increase in the number of electrons in the Alq₃ EML. That increases in the number of electron in the Alq₃ EML for the OLEDs with a wide-bandgap-impurity decreased the staying time of the holes in the Alq₃ EML, resulting in an enhanced lifetime for the OLEDs. These results indicate that OLEDs with a wide-bandgap-impurity-doped EML hold promise for potential applications in long-lifetime OLED displays.     

Temperature dependence of microstructure and strain evolution in strained ZnO films on Al(2)O(3)(0001)

We have studied the temperature dependence of the growth mode and microstructure evolution in highly mismatched sputter-grown ZnO/Al(2)O(3)(0001) heteroepitaxial films. The growth mode was studied by real-time synchrotron x-ray scattering. We find that the growth mode changes from a two-dimensional (2D) layer to a 3D island in the early growth stage with temperature (300-600?°C), in sharp contrast to the reported transition from three dimensions to two dimensions in metal-organic vapor phase epitaxy. At around 400?°C intermediate 2D platelets nucleate in the early stage, which act as nucleation cores of 3D islands and transform to a misaligned state during further growth. Meanwhile, at high temperature (above 500?°C), the spinel structure of ZnAl(2)O(4) grows in the early stage, and it undergoes a transition to wurtzite-ZnO (w-ZnO) with thickness. The spinel formation is presumably driven by high temperature and large incident energy of impacting atoms during sputtering. The results of the strain evolution as functions of temperature and thickness during growth suggest that the surface diffusion is a major factor determining the microstructural properties in the strained ZnO/Al(2)O(3)(0001) heteroepitaxy.     

Al/MoO3薄膜厚度对含能半导体桥发火特性的影响

使用微细加工技术在半导体桥上分别集成3μm和6μm厚度的Al/MoO_3纳米含能复合薄膜,制备成含能半导体桥,并使用电容放电的激发方式,研究薄膜厚度对含能半导体桥发火特性的影响。研究发现,随着薄膜厚度的增加,含能半导体桥的临界发火时间和临界发火能量无显著性变化,电容放电的作用总时间、作用总能量和能量利用效率降低,能量输出效率显著增加。