Tanesh Bansal

Molybdenum disulphide/titanium dioxide nanocomposite-poly 3-hexylthiophene bulk heterojunction solar cell

Demonstration of hybrid bulk heterojunction (BHJ) solar photovoltaic cell employing molybdenum disulfide (MoS2)/titanium dioxide (TiO2) nanocomposite (∼15 μm thick) and poly 3-hexylthiophene (P3HT) active layers is presented in this letter. The dominant Raman peak at 146 cm−1 confirmed TiO2, while two other peaks observed at 383 cm−1 and 407 cm−1 asserted MoS2 in the nanocomposite film. The demonstrated BHJ solar cell, having a stacked structure of indium tin oxide/TiO2/MoS2/P3HT/gold, exhibits a short circuit current density of 4.7 mA/cm2, open circuit voltage of 560 mV, and photoconversion efficiency of 1.3% under standard AM1.5 illumination condition. We observe that the quality of TiO2/MoS2/P3HT interfaces, as reflected in the dark saturation current in low- and medium-forward-bias region, plays a key role in impacting solar cell performance due to interfacial recombination effect.

Graphene-Based Interconnects on Hexagonal Boron Nitride Substrate

We demonstrated graphene interconnects on layered insulator-hexagonal boron nitride ( h-BN). Performance metrics are compared among three material systems: CVD graphene on h-BN, CVD graphene on SiO₂, and exfoliated graphene on SiO₂. CVD graphene on h-BN shows approximately 19 times and 8 times improved conductivity as compared with CVD graphene on SiO₂ and exfoliated graphene on SiO₂, respectively. For graphene on h-BN, an ultrahigh carrier mobility (~ 15 000 cm²/V·s at a carrier density of 1 × 10¹² cm^-2) is observed. The breakdown power density is much increased, attributed to the higher thermal conductivity of h-BN (than that of SiO₂) that facilitates heat dissipation. Electrical annealing reduces graphene sheet resistance. Unlike the case for SiO₂ substrate, the absence of positive shift of Dirac point could be due to the interface-state-free nature of h-BN substrate. The research suggests that h-BN could be used as an alternative substrate material for graphene-based interconnects, overcoming performance limit and reliability issues caused by the SiO₂ substrate.

Gated spin transport through an individual single wall carbon nanotube

Hysteretic switching in the magnetoresistance of short-channel, ferromagnetically contacted individual single wall carbon nanotubes is observed, providing strong evidence for nanotube spin transport. By varying the voltage on a capacitively coupled gate, the magnetoresistance can be reproducibly modified between +10% and −15%. The results are explained in terms of wave vector matching of the spin polarized electron states at the ferromagnetic ∕ nanotube interfaces.

Schottky-barrier solar cell based on layered semiconductor tungsten disulfide nanofilm

We demonstrated Schottky-barrier solar cell using layer-structured semiconductor tungsten disulfide (WS2) nanofilm (NF) as the photo-active material. WS2 NFs were synthesized by chemical-vapor-deposition initiated on the surface of tungsten. The growth of WS2 NF was confirmed by Raman signature peaks representing active modes of E12g (351.5 cm−1) for in-plane and A1g (420.1 cm−1) for out-of-plane atomic vibrations, respectively. The ITO/WS2/Au Schottky-barrier solar cell was demonstrated by a layer-enabled assembling process, showing a photo-conversion efficiency of 1.7% and effective photon absorption in the wavelength range of 350 nm–950 nm. The Mott-Schottky characteristic suggests low density of bulk and interface defects in WS2 NF attributed to surfaces with negligible amount of dangling bonds which is the essential nature of layered semiconductors.

Graphene nanoribbons exfoliated from graphite surface dislocation bands by electrostatic force

We have developed a novel technique to produce long and narrow graphene ribbons with smooth edges. This technique is free of any chemical treatments and involves a combination of two steps: (i) creation of surface dislocation ribbons by high velocity clusters impacting the graphite surface and (ii) electrostatic transferring of the dislocation ribbons to a desired substrate. The width of the ribbons can be controlled by varying the impact velocity of a cluster jet stream from a gas jet impactor. The electrical transport properties were investigated on the ribbons in field effect transistor (FET) configuration. The p-type behavior observed under ambient conditions was found to be reversed upon annealing at 180 degrees C in a vacuum of 10( - 7) Torr. Charge transfer effects were observed when the degassed graphene was exposed to N(2)O and NH(3).

Multilayer Graphene Oxide/Cadmium Selenide Quantum-Dot-Coated Titanium Dioxide Heterojunction Solar Cell

We demonstrate a heterojunction solar cell employing multilayer graphene oxide (ML-GO) as the hole-conducting layer and cadmium selenide (CdSe) quantum-dot-coated titanium dioxide (TiO2) as the photoactive material. Multilayer graphene, synthesized by a chemical-vapor-deposition technique, is treated with low-power (25-W) oxygen plasma (~100 sccm) to convert into ML-GO, as confirmed by Raman spectroscopy. The ML-GO exhibits an optical bandgap of 3.6 eV extracted from UV-visible absorption spectroscopy. The heterojunction solar cell is implemented with a stacked heterostructure of ITO/TiO2/CdSe/ML-GO/Pt, yielding a short-circuit current density of 11.2 mA/cm2, an open-circuit voltage of 652 mV, and a photoconversion efficiency of 4.1% under a standard AM 1.5 illumination condition.

Photovoltage bleaching in bulk heterojunction solar cells through population of the singlet exciton

The effect of illumination by a tunable light source on the open circuit voltage of a poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cell is measured. Illumination at the energy of the PCBM ground state singlet exciton causes a sharp decrease in the photovoltage, while illumination at the MDMO-PPV exciton produces no change. Capacitive photocurrent spectroscopy reveals that the charge dissociated from the PCBM exciton is large despite the small photovoltage signal. This implies that polarons generated by the below gap excitation promote recombination of the above gap photoexcited charge.

Experimental Determination of the Electronic Density of States for Graphene Oxide

CPS is unique in that it specifically measures absorption due to the formation of charge carriers, while being uninfluenced by absorption due to vibrational states of the lattice, or non-mobile impurity states. The set-up is shown in Figure 1. The GO sample lies on top of an ITO coated quartz slide, which is anchored to a copper plate within an evacuated optical cryostat. Electrical contact is made between the copper plate and a gold pad deposited near the edge of the ITO layer. The sample is illuminated with light from an optical parametric amplifier which emits a series of 120 fs light pulses at a 1 kHz repetition rate. Light absorption occurs in the GO film, producing electron-hole pairs. The ITO acts as an acceptor for negative photoexcited charge, causing the electron-hole pairs to separate across the GO / ITO interface. This produces an ac voltage, whose frequency is equal to the laser repetition rate. An applied DC potential causes the negative charge to be attracted to the ITO/quartz interface, amplifying the capacitive photocurrent signal. Figure 2 shows a sample capacitive photocurrent spectrum, while Fig. 3 shows the DOS extracted from the data. Three peaks are observed at approximately 0.7, 1.6 and 3.2 eV (marked as ▲, ●, and ■). Strong minima occur at 2.1 eV, and beyond 4 eV, indicating gaps in the density of states. Similar results are observed for all measured GO samples. The DOS extracted from the data consists of the π / π* peaks, plus three additional mid gap states at -0.5 eV, 0.4 eV and 1.5 eV. For comparison, the density of states for graphene is also shown. The π and π* peaks are asymmetric with respect to the Fermi energy due to interaction with the substrate / environment. 3 The ■ transition in the CPS correlates with the 320 nm shoulder observed in the absorbance spectra of GO, and attributed to n-π* transitions of C=O. This suggests that the -0.5 eV state is the non-bonding orbital of the oxygen atoms. 5

Substrate effect on graphene-based interconnects

We investigated major performance metrics of graphene interconnects on 2D layered insulator - hexagonal boron nitride (h-BN). Comparative study was made on three different material systems, including CVD graphene on h-BN, CVD graphene on SiO₂, and exfoliated graphene on SiO₂, with respect to electrical conduction and breakdown power density. Remarkable improvement in conductivity is observed in CVD graphene on h-BN substrate as compared with CVD graphene on SiO₂ and exfoliated graphene on SiO₂. The carrier mobility in CVD graphene on h-BN exhibits a value of ~15,000 cm²/V-s at carrier density of 1×10¹² cm^-2. Higher thermal conductivity of h-BN (as compared with that of SiO₂) facilitates heat dissipation, leading to improvement in breakdown power density. It is demonstrated that using h-BN as substrate material could help breaking the performance and reliability limits imposed by SiO₂ substrate.

MoS 2 / TiO 2 nanoparticle composite bulk heterojunction solar cell

We investigate hybrid bulk heterojunction (BHJ) solar cell employing layered transition-metal-dichalcogenide molybdenum disulfide (MoS₂) as photon harvesting layer with titanium dioxide (TiO₂) as electron acceptor and poly 3-hexylthiophene (P3HT) as hole conductor. Micro-Raman spectroscopic measurement confirms that MoS₂ coated on porous TiO₂ film consist nanoflakes of monolayer/few layers in the stack. The semiconducting anatase phase of TiO₂ is confirmed by a signature peak at 146 cm^-1, with two other peaks at 383 cm^-1 and 407 cm^-1 corresponding to MoS₂ in the film. The BHJ solar cell having a stacked structure of ITO/TiO₂/MoS₂/P3HT/Au showed a short circuit current density of 4.7 mA/cm², open circuit voltage of 560 mV, and photoconversion efficiency of 1.3% under an illumination of 100 mW/cm². The external quantum efficiency of the BHJ solar cell confirms that MoS₂ nanoflakes absorb solar photons in a wavelength range of 350 nm ~ 800 nm. Dark current-voltage characteristics of the BHJ solar cell under low and medium forward bias regions suggest that electron recombination at TiO₂/MoS₂/P3HT interfaces limits photovoltaic performance of the device.