Study of buffer layer thickness on bulk heterojunction solar cell

We studied the effect of the buffer layer (molybdenum-oxide (MoO3)) thickness on the performance of organic solar cell based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester fullerene derivative (PCBM). The thickness of MoO3 was varied from 1 nm to 30 nm for optimization of device performance. The photocurrent-voltage and impedance spectroscopy were measured under dark and AM1.5G solar simulated illumination of 100 mW/cm2 for exploring the role of the buffer layer thickness on carrier collection at an anode. The MoO3 thickness of the optimized device (efficiency approximately 3.7%) was found to be in the range of 5 approximately 10 nm. The short-circuit current and the shunt resistance decrease gradually for thicker MoO3 layer over 5 nm. The device can be modeled as the combination of three RC parallel circuits (each one for the active layer, buffer layer and interface between the buffer layer and the active layer) in series with contact resistance (Rs approximately 60 ohm).     

ZnO and conjugated polymer bulk heterojunction solar cells containing ZnO nanorod photoanode

Hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and ZnO nanoparticle bulk heterojunctions (BHJ) combined with ZnO nanorod arrays were fabricated and analyzed. The dispersion of ZnO nanoparticles in P3HT is assisted by dye molecules, which function as a surface modifier for ZnO nanoparticles to improve compatibility between ZnO nanoparticles and P3HT. Compared to the ZnO nanorod/P3HT devices, the optimized cells with the ZnO nanoparticles dispersed in P3HT can significantly increase the short-circuit current and the overall power conversion efficiency from 1.36 mA cm(-2) to 2.51 mA cm(-2) and from 0.18% to 0.45% with 625 nm long ZnO nanorod arrays, respectively. The novel scheme of using the light-absorbing dye molecules both as light absorber and as surfactant for ZnO nanoparticles presents a facile route towards forming bulk heterojunction hybrid solar cells based on semiconducting nanomaterials and conjugated polymers.     

Two-dimensional simulations of bulk heterojunction solar cell characteristics

We present a two-dimensional model of a bulk heterojunction solar cell in which we include the effects of optical interference, exciton diffusion, charge separation via the formation of polaron pairs, and charge transport in two separate interpenetrating phases. Our model shows that the current is increased by an order of magnitude with a full optical model compared to assuming that absorbed photons have a Lambertian profile, and depends much more strongly on applied bias when dissociation via polaron pairs is considered. We find a power efficiency at solar intensities of 1-3% depending on the morphology, and show that the fill factor decreases from 40% at low intensities to 20% at solar intensities because of the increase in the open circuit voltage and decreases much more rapidly at higher intensities due to the decrease in the power efficiency.     

Structural and antireflective properties of ZnO nanorods synthesized using the sputtered ZnO seed layer for solar cell applications

We report the structural and antireflective properties of ZnO nanorod arrays (NRAs) on silicon (Si) substrate by wet chemical growth using the sputtered ZnO seed layer for solar cell applications. The size, height, shape, and number of ZnO nanorods depend strongly on the ZnO seed layer thickness as well as the molar zinc nitrate concentration. Clearly, the ZnO nanorods are of wurzite crystal structure from the X-ray diffraction analysis. To achieve the low reflectance over a wide wavelength range, the ZnO seed layer thickness, molar concentration, and growth time are optimized. It is found that the specular reflection spectrum of ZnO NRAs is closely related to the ZnO seed layer thickness. The solar weighted reflectance, Rw, of ZnO NRAs as antireflection coatings for Si solar cells is estimated under AM1.5 g illumination. For ZnO NRAs with 50 nm ZnO seed layer in 10 mM aqueous solution for 12 hours, the low specular reflectance (i.e., <7%) is obtained at wavelengths of 300-1200 nm, indicating a low Rw of 3.86%.     

Bulk heterojunction formation between indium tin oxide nanorods and CuInS2 nanoparticles for inorganic thin film solar cell applications

In this study, we developed a novel inorganic thin film solar cell configuration in which bulk heterojunction was formed between indium tin oxide (ITO) nanorods and CuInS(2) (CIS). Specifically, ITO nanorods were first synthesized by the radio frequency magnetron sputtering deposition method followed by deposition of a dense TiO(2) layer and CdS buffer layer using atomic layer deposition and chemical bath deposition method, respectively. The spatial region between the nanorods was then filled with CIS nanoparticle ink, which was presynthesized using the colloidal synthetic method. We observed that complete gap filling was achieved to form bulk heterojunction between the inorganic phases. As a proof-of-concept, solar cell devices were fabricated by depositing an Au electrode on top of the CIS layer, which exhibited the best photovoltaic response with a V(oc), J(sc), FF, and efficiency of 0.287 V, 9.63 mA/cm(2), 0.364, and 1.01%, respectively.     

Environmental stability of PTB7:PCBM bulk heterojunction solar cell

The short life span of organic photovoltaic (OPV) cell in an ambient laboratory condition is one of the challenges hindering the realization of organic-based devices. The presence of moisture and oxygen in conjugated polymer matrix is the major factors responsible for the degradation of organic molecules. The chemical degradation of OPV cell generally depends on the nature of the semiconductor polymer used in the preparation of the devices. However, the lifespan of unprotected OPV cells often ranges in the order of few hours in simple laboratory environment. We are reporting here the lifetime of organic photovoltaic cell in ambient laboratory condition whose active layer is composed of PTB7:PCBM blend.     

Preparation of ZnO nanoparticles using electrodeposition and co-precipitation techniques for dye-sensitized solar cells applications

Zinc oxide thin films have been successfully prepared by co-precipitation and electrodeposition methods onto Fluorinated tin oxide substrate using zinc nitrate aqueous solutions at various bath temperatures (25–75 °C). The deposition of electrodeposition method was conducted using both using linear sweep voltammetry and Chronoamperometric techniques. The effects of solution composition, agitation and bath temperature on the electrochemical measurements and ZnO film characteristics were fully analyzed. The findings reveal that temperature and nitrate ion concentration have a strong promoting effect on ZnO film formation. Moreover, the obtained powders were investigated by X-ray diffraction, Field emission scanning electron microscopy and UV–Vis Spectroscopy. Structural characterization by X-ray diffraction indicates the formation of ZnO phase and the deposited film exhibits the Zincite structure with crystallite size around 51 nm. The photovoltaic performance of dye-sensitized solar cells based on both ZnO prepared by co-precipitation and electrodeposition methods was investigated. A power conversion efficiency (η) of 3.5 % was achieved for the DSSC with co-precipitation ZnO, which is higher than that of the cell with electrodeposition ZnO (2.5 %). Explanations are substantiated by incident photon to electron conversion efficiency curves.     

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 %.     

Surfactant-assisted hydrothermal synthesis of titania nanoparticles for solar cell applications

Nanostructured anatase TiO₂ powders (~7 nm) with different microstructures have been successfully synthesized using surfactant-assisted hydrothermal route. It can be seen that different morphologies of sphere, flower petal and cauliflower were appeared for the anatase powders formed at hydrothermal temperature 100 °C for 24 h without and with sodium dodecyl sulfate (SDS) and cetyl trimethyl-ammonium bromide (CTAB) as anionic and cationic surfactants, respectively. The specific surface area S_BET was increased from 77.14 m²/g without surfactant to 177.19 m²/g in the presence of SDS as anionic surfactant. The optical properties were measured and the band gap energy of the obtained powders was ~3.3 eV. The UV-absorption band of the anatase phase was at near 295 nm without other observable bands, which proved to exhibit high optical property and might have potential application in solar cells devices.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Polydimethylsiloxane as a macromolecular additive for enhanced performance of molecular bulk heterojunction organic solar cells

The effect of the macromolecular additive, polydimethylsiloxane (PDMS), on the performance of solution processed molecular bulk heterojunction solar cells is investigated, and the addition of PDMS is shown to improve device power conversion efficiency by ～70% and significantly reduce cell-to-cell variation, from a power conversion efficiency of 1.25 ± 0.37% with no PDMS to 2.16 ± 0.09% upon the addition of 0.1 mg/mL PDMS to the casting solution. The cells are based on a thiophene and isoindigo containing oligomer as the electron donor and [6,6]-phenyl-C61 butyric acid methyl ester (PC(61)BM) as the electron acceptor. PDMS is shown to have a strong influence on film morphology, with a significant decrease in film roughness and feature size observed. The morphology change leads to improved performance parameters, most notably an increase in the short circuit current density from 4.3 to 6.8 mA/cm(2) upon addition of 0.1 mg/mL PDMS. The use of PDMS is of particular interest, as this additive appears frequently as a lubricant in plastic syringes commonly used in device fabrication; therefore, PDMS may unintentionally be incorporated into device active layers.     

ZnO nanoparticles and porous coatings for dye-sensitized solar cell application: Photoelectrochemical characterization

Nanoscale zinc oxide (ZnO) powder with Brunauer-Emmelt-Teller surface area of 43 m² g^− 1 has been synthesized by soft chemistry at low temperature via reaction of zinc acetate dehydrate (Zn(CH₃COO)₂.2H₂O) and sodium hydroxide (NaOH). The influence of the pH value of the sol on the structure and morphology of ZnO powder have been investigated by X-Ray Diffraction, Scanning Electron Microscopy and High Resolution Transmission Electron Microcopy. Their thermal properties have been determined by simultaneous Differential Thermal Analysis and Thermogravimetry coupled to Mass Spectroscopy analysis. The nanoparticles are single crystals with (101) preferred orientation but agglomerated. Their crystallite size can be adjusted from 15 nm to 35 nm by controlling the pH value between 7 and 13. Thick porous crystalline coatings have been obtained by doctor blade coating on conducting SnO₂:F glass substrates using pastes prepared by wetting the crystalline powders with polyethylene glycol and water. After sintering at 400 °C and Ruthenium 535 dye sensitization, the coatings have been tested in a three electrode electrochemical cell containing an appropriate electrolyte in the dark and under 450 W Xenon lamp illumination. The influence of the electrolyte iodine concentration, the film thickness and the light intensity on the current density are presented and discussed. Such coatings appeared promising for the realization of dye sensitized solar cells.     

Modification of ZnO nanorods through Au nanoparticles surface coating for dye-sensitized solar cells applications

Dye-sensitized solar cells (DSSC) based on ZnO nanorods were fabricated and modified through the addition of Au nanoparticles. The as-synthesized ZnO nanorods were well-dispersed and of high crystallinity quality leading to a high cell efficiency of 5.2%. On the other hand, thick layer of Au nanoparticles aggregation may have led to distortion of plasmonic effect. Also, the addition of Au nanoparticles have effectively decreased the surface area of ZnO nanorods with direct contact to the dye molecules, resulting in a lower amount of adhered dye molecules to convert sunlight. The electrons generated by the photo-absorption through thick aggregated Au nanoparticles layer may have a lower injection rate to ZnO nanorods as compared to those absorbed by the dye.     

Organic photovoltaic cell employing organic heterojunction as buffer layer

Hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C₆₀. The F₁₆CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells.     

Chemical bath deposition of ZnO nanorods for dye sensitized solar cell applications

ZnO nanorods using various molar concentrations have been synthesized through the chemical bath deposition method. X-ray diffraction result shows that the ZnO nanorods are of hexagonal structure. The morphology of the ZnO nanorods has been examined by scanning electron microscopy. The ZnO nanorods have diameters ranging from 100 to 200 nm and length of 1–3 μm. Dye-sensitized solar cells have been assembled by using ZnO nanorod film photoelectrode sensitized using natural dye extracted from lantana camara as sensitizer. The ZnO nanorods have been used as electrode material to fabricate dye sensitized solar cells which exhibited an efficiency of 0.71 %, the maximum efficiency was obtained for films deposited for 0.07 M concentration.     

Influence of organic salt concentration on the performance of bulk heterojunction organic solar cell

The effect of organic salt on the performance of bulk heterojunction organic solar cell was investigated by varying the concentration of tetrabutylammonium hexafluorophosphate (TBAPF₆). Organic solar cells based on TBAPF₆-blended poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV): (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) thin films with aluminium (Al) as cathode have been fabricated on ITO substrates. The MEHPPV:PCBM films with different concentrations of TBAPF₆ (10, 20, 30 and 40 wt% with respect to MEHPPV) were deposited onto the ITO by spin coating technique, followed by deposition of Al using electron gun evaporation technique to build the devices. Experimental results showed that the short circuit current density and open circuit voltage improved with increasing of TBAPF₆ concentration up to 20 wt% since more dissociated ions accumulated at the photoactive layer-electrode interfaces resulted in higher built in electric field. However, the short circuit current density and open circuit voltage started to decrease at TBAPF₆ concentration of 30 wt%, indicating higher charge recombination as a result of agglomeration of TBAPF₆.     

Formation and characterization of nanoparticles based CuInS2 absorbing layer for solar cell

CuInS2 powders have been successfully prepared by simple heating a mixture of copper nitrate, indium nitrate and thiourea in ethylene glycol. The prepared powders were fully characterized by SEM, XRD and UV-Vis spectroscopy. The particle shape of the powders obtained with an optimal condition showed spherical shape with about 50 nm in size. The XRD results showed more enhanced crystallized CuInS2 with chalcopyrite structure with increasing reaction time. The values in band-gap energy of nano-sized CuInS2 powders would be estimated 1.63 eV, blue-shifted from that of micron-sized powders due to size quantization effect.