Wei Peng Goh

Imbalanced charge mobility in oxygen treated polythiophene/fullerene based bulk heterojunction solar cells

The effect of oxygen induced traps on charge mobility in bulk heterojunction solar cells using poly(3-hexylthiophene) (P3HT):l-(3-methoxycarbonyl)-propyl-l-phenyl-(6, 6) methanofullerene (PCBM) blend have been studied using photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) technique. The solar cells exposed to oxygen exhibit dual PhotoCELIV peaks, whereas the solar cell without oxygen treatment show single PhotoCELIV peak with the charge mobility of the order of 10−4 cm2/V s. It is demonstrated that the oxygen treatment imbalance the charge mobility in the P3HT/PCBM photoactive layer, which affects the power conversion efficiency and lifetime of the solar cell. The single PhotoCELIV peak for the device without oxygen treatment indicates that the charge mobility is balanced, that causes the overlapping of electron and hole transients.

Improvement of CH3NH3PbI3 Formation for Efficient and Better Reproducible Mesoscopic Perovskite Solar Cells

High-performance perovskite solar cells (PSCs) are obtained through optimization of the formation of CH3NH3PbI3 nanocrystals on mesoporous TiO2 film, using a two-step sequential deposition process by first spin-coating a PbI2 film and then submerging it into CH3NH3I solution for perovskite conversion (PbI2 + CH3NH3I → CH3NH3PbI3). It is found that the PbI2 morphology from different film formation process (thermal drying, solvent extraction, and as-deposited) has a profound effect on the CH3NH3PbI3 active layer formation and its nanocrystalline composition. The residual PbI2 in the active layer contributes to substantial photocurrent losses, thus resulting in low and inconsistent PSC performances. The PbI2 film dried by solvent extraction shows enhanced CH3NH3PbI3 conversion as the loosely packed disk-like PbI2 crystals allow better CH3NH3I penetration and reaction in comparison to the multicrystal aggregates that are commonly obtained in the thermally dried PbI2 film. The as-deposited PbI2 wet film, without any further drying, exhibits complete conversion to CH3NH3PbI3 in MAI solution. The resulting PSCs reveal high power conversion efficiency of 15.60% with a batch-to-batch consistency of 14.60 ± 0.55%, whereas a lower efficiency of 13.80% with a poorer consistency of 11.20 ± 3.10% are obtained from the PSCs using thermally dried PbI2 films.

Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions

Strong visible electroluminescence (EL) has been observed from a 30 nm silicon nitride thin film multiply implanted with Si ions and annealed at 1100 °C. The EL intensity shows a linear relationship with the current transport in the thin film at lower voltages, but a departure from the linear relationship with a quenching in the EL intensity is observed at higher voltages. The EL spectra show two primary EL bands including the predominant violet band at ∼3.0 eV (415 nm) and the strong green-yellow band at ∼2.2 eV (560 nm). Two weak bands including the ultraviolet band at ∼3.8 eV and the near infrared band at ∼1.45 eV emerge at high voltages. The evolution of each EL band with the voltage has been examined. The phenomena observed are explained, and the EL mechanisms are discussed.

Three-Dimensional Optoelectronic Model for Organic Bulk Heterojunction Solar Cells

This paper describes a 3-D optoelectronic device model for organic bulk heterojunction solar cells. Three-dimensional full-wave optical simulation enables us to incorporate different modern light trapping techniques, such as subwavelength nanostructures, in a typical organic bulk heterojunction solar cell, while 3-D electrical simulation allows us to handle localized enhancement or reduction of polaron/charge generation, recombination, and transport induced by modern light trapping techniques in the device. We calibrate our model with an experimental poly (3-hexylthiophene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) organic bulk heterojunction solar cell by tuning only one free parameter as compared with other device models, which have multiple fitting parameters. A 3-D example of a silver nanoparticle array in a typical P3HT:PCBM organic bulk heterojunction cell is also demonstrated, and the current density-voltage relation is predicted with our model.

Evolution of electroluminescence from multiple Si-implanted silicon nitride films with thermal annealing

Influence of thermal annealing on electroluminescence (EL) from multiple-Si-implanted silicon nitride films has been investigated. A reduced injection current and an enhanced EL intensity have been obtained simultaneously by increasing the annealing temperature, which results in a higher EL quantum efficiency. In addition, four emission bands are identified, and their peak energies, intensities, and full widths at half maxima are found to change with annealing temperature. A model is proposed to illustrate the carrier transport, the mechanisms of the four emission bands, and the evolution of the EL bands with annealing as well. The two major bands and the minor ultraviolet band are explained in terms of the recombination of the injected electrons in either the silicon dangling-bond (≡Si0) states or the nitride conduction band with the injected holes in either the band tail states above the nitride valence band or the valence band itself, while the minor near infrared band is attributed to the Si nanocryst...

Quenching and Reactivation of Electroluminescence by Charge Trapping and Detrapping in Si-Implanted Silicon Nitride Thin Film

In this brief, quenching of electroluminescence (EL) from Si-implanted silicon nitride (SNR) thin film under a forward bias has been observed. The quenching phenomenon is shown to be due to charge trapping in the defect states involved in the radiative recombination. The composite EL bands have different quenching rates, causing a change in the EL spectrum shape by the EL quenching. Release of the trapped charges by a low-temperature annealing at 120degC or an application of a reverse gate bias can partially recover the quenched EL both in the intensity and spectrum shape. The quenching phenomenon poses a serious challenge to the application of Si-implanted SNR thin films in light-emitting devices.

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al2O3 layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT(+) polaron population on the Al2O3 thickness. The optimal thickness was found to be 3-5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells.

AQUEOUS-BASED FORMATION OF GOLD NANOPARTICLES ON SURFACE-MODIFIED COTTON TEXTILES

A facile and environmental friendly route has been developed to synthesize gold nanoparticles (AuNPs) by using amine stabilizers and N-vinyl pyrrolidone (NVP) as the reductant in an aqueous medium. This synthetic concept can be applied to the formation of AuNPs on normal cotton fibers via in situ surface modification of the cotton fibers by (3-aminopropyl)triethoxysilane (ATS) followed by nanoparticle formation. This scheme produces red AuNP-dyed cotton textiles. Surface treatment of the cotton textiles by ATS was found to be crucial for the formation of nano-Au. X-ray photoelectron spectroscopy (XPS), diffused reflectance UV-visible spectroscopy (UV-vis) and energy dispersive X-ray (EDX) were used to investigate the fiber surfaces. The distribution of AuNPs as well as surface treatment agent on the fiber surface was located by time of flight secondary ion mass spectroscopy (TOF-SIMS).

Translucent Polymer Solar Cells

The recent developments on the construction of translucent cathodes for the application of tandem or multi-junction organic solar cells are reviewed in this report. The translucent cathode plays a major role in determining the efficiency of translucent solar cells by controlling the intensity of incident light absorption at the photoactive layer of each cell. As a result, there are varieties of translucent cathodes based on conductive polymer, thin metallic layer, and transparent conducting oxides (TCO) have been developed by various research groups. A translucent cathode based on a combination of thin metallic (i.e. calcium and silver) and transparent conductive oxide layers (indium tin oxide (ITO)) is developed in our group and the advantages of this translucent cathode for organic solar cell application are presented. In addition, the experimental results obtained from polymeric solar cells, based on poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) blends, and ITO-based translucent cathode, are also presented.

Improved cathode for semitransparent organic solar cells

High quality transparent cathode is one of the major impediments to achieve high performance semitransparent organic solar cells (OSCs). This work reports a study of the effect of Al/indium tin oxide (ITO) cathode for application in poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C60 (PCBM)-based semitransparent OSCs. The power conversion efficiency (PCE) and lifetime of OSCs made with Al/ITO-based cathode are studied and compared with structure identical OSCs made with Ca/Ag/ITO-based cathode. The interfacial properties at polymer/cathode interface were investigated using X-ray Photoelectron Spectroscopy (XPS). It is found that an ultrathin Al:AlOx interlayer was formed at polymer/cathode interface, induced during the ITO deposition. Semitransparent OSCs with Al/ITO cathode having a PCE of up to 2.37% with an average transmittance >40% at light wavelength of 650nm were demonstrated. The results reveal that the presence of an ultrathin AlOx layer at the polymer/cathode interface plays a role in determining the stability of the semitransparent OSCs. The possible designs to improve the performance of semitransparent organic solar cells over the two competing indexes of power conversion efficiency and transmittance are discussed.