Sudip Adhikari

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.

Improving photovoltaic properties by incorporating both single walled carbon nanotubes and functionalized multiwalled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) and functionalized multiwalled carbon nanotubes (f-MWCNTs) are introduced together for photovoltaic application in a poly(3-octylthiophene)/n-Si heterojunction solar cell. The performance of the device was improved by manyfold by the incorporation of both SWCNTs and f-MWCNTs. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) were 0.44 V, 6.16 mA/cm2, 36%, and 0.98%, respectively. Here, we expect that SWCNTs help in exciton dissociation and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole transportation. CNT incorporation yields better carrier mobility, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action.

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.

Photovoltaic characteristics of postdeposition iodine-doped amorphous carbon films by microwave surface wave plasma chemical vapor deposition

The amorphous carbon thin films have been deposited on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100°C) in Ar∕CH4 phase gas. Doping of iodine has been done in the postdeposited films by exposing them in iodine vapor. The photovoltaic measurements of the films were carried out before and after iodine doping. The results show dramatic decrease of optical gap from 3.4 to 0.9 eV corresponding to nondoping to iodine doping conditions, respectively. The preliminary photovoltaic characteristics of the film deposited on n-type silicon substrate under light illumination (AM1.5, 100mW∕cm2) reveal a short-circuit current density of 1.15μA∕cm2, open-circuit voltage of 177 mV and fill factor of 21.7%.

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.

THE OPTICAL PROPERTIES OF NITROGENATED AMORPHOUS CARBON FILMS GROWN BY A NOVEL SURFACE WAVE MICROWAVE PLASMA CVD METHOD

The effects of annealing temperature on the optical properties of nitrogenated amorphous carbon (a-C:N) films grown on quartz substrates by a novel surface wave microwave plasma chemical vapor deposition (SWMP-CVD) method are reported. The thickness, optical, structural and bonding properties of the as-grown and anneal-treated a-C:N films were measured and compared. The film thickness decreased rapidly with increasing annealing temperature above 350°C. A wide range of optical absorption characteristics is observed, depending on the annealing temperature. The optical band gap of as-grown a-C:N films is approximately 2.8 eV, gradually decreasing to 2.5 eV for the films anneal-treated at 300°C, and beyond that decreasing rapidly down to 0.9 eV at 500°C. The Raman and FTIR spectroscopy measurements have shown that the structural and composition of the films can be tuned by optimizing the annealing temperature. The change of optical, structural and bonding properties of SWMP-CVD-grown a-C:N films with higher annealing temperature was attributed to the fundamental changes in the bonding and band structures of the films.

Structural, Optical and Electrical Characterization of Amorphous Carbon Thin Films Grown on PTFE Substrates for Photovoltaic Application

The n-type conductivity of nitrogen doped amorphous carbon films have been grown on p-type silicon, quartz and heat tolerant (up to 260 degC) flexible polytetrafluoroethene plastic substrates by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) at low temperature (<100 degC). For film deposition at gas composition pressure of 50 Pa in the CVD chamber, we used argon as carrier gas, nitrogen as dopant and methane as carbon plasma source. Photovoltaic effects of the films as well as their chemical composition, bonding and structural properties have been studied. The X-rays photoelectron spectroscopy measurement shows that nitrogen content in the films grown on plastic substrates is higher compared with the films grown on quartz substrates. The optical measurements show that the optical band gap of the films grown on plastic substrate is lower compared with the films grown on quartz at the same parameters. The temperature dependence conductivity and photoresponse measurements show that the electrical conductivity of the films grown on plastic substrates is much higher compared with the films grown on quartz substrates

EFFECTS OF METHANE GAS FLOW RATE ON THE OPTOELECTRICAL PROPERTIES OF NITROGENATED CARBON THIN FILMS GROWN BY SURFACE WAVE MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

We have studied the influence of the methane gas (CH4) flow rate on the composition and structural and electrical properties of nitrogenated amorphous carbon (a-C:N) films grown by surface wave microwave plasma chemical vapor deposition (SWMP-CVD) using Auger electron spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectroscopy, four-point probe and two-probe method resistance measurement. The photoelectrical properties of a-C:N films were also studied. We have succeeded to grow a-C:N films using a novel method of SWMP-CVD at room temperature and found that the deposition rate, bonding and optical and electrical properties of a-C:N films are strongly dependent on the CH4 gas sources, and the a-C:N films grown at higher CH4 gas flow rate have relatively high electrical conductivity for both cases of in dark and under illumination condition.

Amorphous carbon thin film deposition by microwave surface-wave plasma CVD for photovoltaic solar cell

Amorphous carbon (a-C) thin films were successfully deposited on various substrates, such as, silicon, quartz, glass and flexible plastic at low temperature (<100/spl deg/C) by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD), a newly developed deposition method for solar cell application. The camphor dissolved with alcohol mixture and methane were used as carbon precursors, while argon was used as carrier gas. Nitrogen and iodine were used as dopants. The deposited a-C films were characterized by UV/VIS/NIR spectrophotometer, SEM, Raman scattering and XPS measurements. In this paper, we report experimental results of the optical, physical and structural properties of the films; the results suggested that it is possible to control film growth rate and optical band gap, and consequently improve photoconductivity by proper selection of deposition parameters, optimizing dopants and appropriate annealing temperature. Our research work is in a progressive stage to realize cheap, reasonably highly efficient and environmentally friendly a-C-based photovoltaic solar cell in the future.

Few layers isolated graphene domains grown on copper foils by microwave surface wave plasma CVD using camphor as a precursor

Few layers isolated graphene domains were grown by microwave surface wave plasma CVD technique using camphor at low temperature. Graphene nucleation centers were suppressed on pre-annealed copper foils by supplying low dissociation energy. Scanning electron microscopy study of time dependent growth reveals that graphene nucleation centers were preciously suppressed, which indicates the possibility of controlled growth of large area single crystal graphene domains by plasma processing. Raman spectroscopy revealed that the graphene domains are few layered which consist of relatively low defects.