Golap Kalita

Graphene constructed carbon thin films as transparent electrodes for solar cell applications

Here, we report fabrication of transparent graphene constructed carbon films (TGFs), from the botanical derivative camphor by controlled pyrolysis, for solar cell applications. TGFs show very good transparency in a wide range of wavelengths (0.3–2 μm) in contrast to indium tin oxide, which shows strong absorptions in the near infrared region. Electrical measurement show the high conductivity of the TGFs and their thickness dependent sheet resistance. An organic solar cell was fabricated on a TGF electrode with a transparency of 81% at 550 nm and a conductivity of 357 S cm−1. The dark light characteristic of the device shows very good rectification with minimum current leakage. Under illumination the device showed very good open circuit voltage, while the short circuit current density and fill factor were affected by the high sheet resistance of the TGFs. This finding shows that TGFs could be suitable electrode materials for organic solar cells, as well as for other transparent electronic devices.

Silicon nanowire array/polymer hybrid solar cell incorporating carbon nanotubes

Here we present a simple and novel approach of fabricating three dimensional (3D) n-Si nanowires (NWs) and poly(3-octylthiophene) hybrid solar cells incorporating carbon nanotubes (CNTs). Vertically aligned n-Si NWs arrays were fabricated by electroless chemical etching of a n-Si [1?1?1] wafer. n-Si NWs/poly(3-octylthiophene) hybrid solar cells were fabricated with and without functionalized CNTs incorporation. Fabricated solar cells incorporating CNTs show open circuit voltage (Voc), short circuit current density (Jsc) fill factor (FF) and conversion efficiency as 0.353, 7.85?mA?cm?2, 22% and 0.61%, respectively. In fabricated devices n-Si NWs arrays form multiple heterojunctions with the polymer and provide efficient electron collection and transportation, whereas CNTs provide efficient hole transportation.

Iodine doping in solid precursor-based CVD growth graphene film

Doping of different elements in intrinsic graphene is of great importance to adjust the electrical and chemical properties for realization of different electronic devices. Here, we demonstrate a simple and controllable synthesis process of iodine-doped graphene film using camphor (C10H16O), a solid botanical derivative. In situ doping of iodine in a graphene film has many difficulties in a conventional chemical vapor deposition process using a gas source. In this technique, iodine was mixed with the carbon precursor and simultaneously evaporated to pyrolysis on a metal catalytic substrate. Raman and X-ray photoelectron spectroscopic studies confirm the presence of elemental iodine in the form of triiodide and pentaiodide. Simultaneously, evaporated iodine atoms remains within the few-layers graphene structure and interact with carbon atoms through a charge transfer process. This shows a straightforward technique for iodine doping in graphene and a similar approach can be adopted to deposit iodine-doped graphene on other metal substrates.

Fullerene (C60) decoration in oxygen plasma treated multiwalled carbon nanotubes for photovoltaic application

Multiwalled carbon nanotubes (MWNTs) were functionalized by oxygen plasma treatment. Photoelectron spectroscopy study of oxygen plasma treated MWNTs (O-MWNTs) shows surface modification with hydroxyl and carboxyl groups. C60 decoration of MWNTs were carried out by thermal evaporation and more dense distribution of C60 was achieved on O-MWNTs. C60 decorated MWNTs were combined with poly(3-octylthiophene) for photovoltaic device fabrication. The device with C60 decorated O-MWNTs shows short circuit current density (Jsc), open circuit voltage (Voc), fill factor, and power conversion efficiency (η) as 1.68mA∕cm2, 0.245V, 27%, and 0.11%, respectively. It is expected that C60 provide large surface area for photoexcitons dissociation and efficient electron transportation, whereas MWNTs provide efficient hole transportation.

Low temperature growth of graphene film by microwave assisted surface wave plasma CVD for transparent electrode application

Here, we report the synthesis of a graphene film on Cu foil by microwave assisted surface wave plasma chemical vapor deposition (MW-SWP-CVD) at a low pressure and temperature using a hydrocarbon gas. Raman spectroscopy and transmission electron microscopy (TEM) studies clearly show deposited carbon films that consist of few layer graphene sheets. Deposition of graphene films with a few layers structure was achieved at a temperature as low as 240 °C. In the nucleation and growth process, carbon radicals are formed with hydrogen termination in the surface wave plasma, which absorb continuously on the Cu surface to form sp2 carbon and eventually a graphene structure. The transparency and conductivity characteristics of the transferred graphene films show suitability for flexible electronics applications.

On the large capacitance of nitrogen doped graphene derived by a facile route

Recent research activities on graphene have identified doping of foreign atoms into the honeycomb lattice as a facile route to tailor its bandgap. Moreover, the presence of foreign atoms can act as defective centres in the basal plane, and these centres can enhance the electrochemical activities of the surface of graphene. Here, we report a facile synthetic approach towards the bulk synthesis of nitrogen doped graphene (N-Graphene) from graphene oxide using a hydrothermal process, with significant control over the extent of N-doping. The electrochemical activeness of N-Graphene (with 4.5 atomic% of nitrogen) is studied by conducting supercapacitor measurements. N-Graphene exhibits a remarkably high specific capacitance of 459 Fg−1 at a current density of 1 mA cm−2 in an electrolyte of 1 M H2SO4 with a high cycle stability compared to that of pristine graphene, which has a specific capacitance of 190 Fg−1. The structural destabilisation of graphene in higher pH/high amount alkaline treatment is demonstrated, and hence optimization of the amount of reagents is necessary in developing a graphene based high performance electronic or electrochemical devices.

Cutting carbon nanotubes for solar cell application

This paper presents the application of cutting multiwalled carbon nanotubes (cut-MWNTs) in solar cell. Cutting of MWNTs is performed by plasma fluorination and followed by defluorination. Cut-MWNTs with lengths of 50–200nm are incorporated in a poly(3-octylthiophene)∕n-Si heterojunction solar cell. We found that a device fabricated with cut-MWNTs shows much better performance than that of a device with pristine MWNTs. The device with cut-MWNTs shows short circuit current density, open circuit voltage, fill factor, and power conversion efficiency as 7.65mA∕cm2, 0.23V, 31%, and 0.54%, respectively. Here, we proposed that cut-MWNTs provide efficient hole transportation having a few nanometer transportation path, hence suppressing recombination. Cut-MWNTs can be the solution to the shorting and shunting effects generally observed in the MWNT solar cell.

Direct growth of nanographene films by surface wave plasma chemical vapor deposition and their application in photovoltaic devices

Here, we report direct synthesis of nanographene films on silicon (n-Si) and glass (SiO2) substrates by microwave assisted surface wave plasma (MW-SWP) chemical vapor deposition (CVD) and their application in photovoltaic devices. The technique is a metal catalyst free, rapid growth process and the film can be deposited on different substrates; thus simplifying the synthesis process for various device applications. The directly grown graphene film consists of triangular shaped nanographene domains with sizes of 80–100 nm in length. The nanographene domains interconnect to form a continuous film which shows metallic characteristics. A Schottky junction based photovoltaic device is fabricated with directly grown nanographene film on n-Si and a conversion efficiency of 2.1% is achieved. This finding shows that a transparent nanographene film can be deposited on different substrates and can be integrated for various devices.

Opening of triangular hole in triangular-shaped chemical vapor deposited hexagonal boron nitride crystal

In-plane heterostructure of monolayer hexagonal boron nitride (h-BN) and graphene is of great interest for its tunable bandgap and other unique properties. Here, we reveal a H2-induced etching process to introduce triangular hole in triangular-shaped chemical vapor deposited individual h-BN crystal. In this study, we synthesized regular triangular-shaped h-BN crystals with the sizes around 2-10 μm on Cu foil by chemical vapor deposition (CVD). The etching behavior of individual h-BN crystal was investigated by annealing at different temperature in an H2:Ar atmosphere. Annealing at 900 °C, etching of h-BN was observed from crystal edges with no visible etching at the center of individual crystals. While, annealing at a temperature ≥950 °C, highly anisotropic etching was observed, where the etched areas were equilateral triangle-shaped with same orientation as that of original h-BN crystal. The etching process and well-defined triangular hole formation can be significant platform to fabricate planar heterostructure with graphene or other two-dimensional (2D) materials.

Highly transparent and conducting C:ZnO thin film for field emission displays

Incorporation of carbon into a zinc oxide (ZnO) thin film has led to a potential new application as a transparent and conductive oxide thin film. Here, we fabricated carbon doped zinc oxide (C:ZnO) films by RF sputtering at a low temperature, aiming to produce a transparent and conductive screen for transparent field emission displays. The incorporation of highly inert carbon atoms into ZnO thin films can tune the intrinsic defect sites. A co-sputtering technique was used for carbon doping into ZnO, where the dopant amount was controlled by varying the number, size and position of the graphite plates used as the carbon source. A sheet resistance of 37 Ω □−1 and a transmittance of 84% at a wavelength of 550 nm were achieved for the C:ZnO thin film with a nominal carbon concentration of 2.7%. The bonding of carbon with ZnO was analyzed using de-convolution of XPS peak C1s. We also demonstrated the light emission properties of the C:ZnO thin films by fabricating a field emission device. A high current density of 1 mA cm−2 was obtained with a lower applied electric field of 5 V μm−1. Our findings showed that the C:ZnO thin films can be a promising transparent and conducting phosphor screen for a field emission display.