P. Balaji Bhargav

Sputtered AZO Thin Films for TCO and Back Reflector Applications in Improving the Efficiency of Thin Film a-Si:H Solar Cells

We report the properties of Al doped ZnO (AZO) thin films on glass substrates and its effect on the efficiency of amorphous silicon (a-Si:H) solar cells as the back reflector. Oriented AZO thin films were grown using DC magnetron sputtering by varying Ar gas flow rates. The influence of Ar flow rate on the structural, electrical and optical properties of AZO thin films suitable for transparent conducting oxide (TCO) and back reflector applications was investigated. The (a-Si:H) solar cells, with and without AZO back reflector, were fabricated on FTO coated glass substrates using the PECVD technique. The solar cells were tested using a Sun simulator under AM 1.5 condition. Enhancement in current density from 12.46 to 14.24 mA/cm2 with the AZO back reflector was observed, thereby increasing the efficiency of the solar cell from 6.38 to 7.82 %, respectively.

Optical and Structural Properties of Ammonia-Free Amorphous Silicon Nitride Thin Films for Photovoltaic Applications

ABSTRACT Hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited through the green chemistry route using silane (SiH4) and nitrogen (N2) as process gases with SiH4 flow being variable and N2 flow being constant without the use of pollutant and corrosive ammonia (NH3) by the plasma-enhanced chemical vapor deposition technique at 13.56 MHz. Fourier transform infrared spectroscopy analysis shows various possible vibrational modes of Si-H, Si-N, and N-H bonds present in the film. Raman spectroscopy is performed on these samples to calculate volume fractions corresponding to amorphous phases present in the a-SiNx:H films. The refractive index (η) values are calculated using Swanepoels method, which are in the range of 2.89 to 3.17. The thickness of the deposited films has been evaluated using transmission spectra. Absorption coefficient and band gap (E g) values are obtained from optical absorption studies. An increase in the E g and a decrease in the η value have been observed for the samples grown with decreasing SiH4 flow.

Application of SixNy:Hz (SiN) as index matching layer in a-Si:H thin film solar cells

The difference in refractive indices of glass substrate and transparent conducting oxide (TCO) electrode causes optical reflection in thin film solar cells, which results in lower absorption of light for devices. An anti-reflection layer between glass and TCO is required to reduce the loss of light due to optical reflection. Silicon nitride (SixNy:Hz) films have shown antireflection property. The refractive index of SixNy:Hz films can be engineered by changing the silicon or nitrogen content in the film. Here, we report the optimization of refractive index of SixNy:Hz to achieve a value between refractive index of glass (1.5) and TCO film (2.0). SixNy:Hz films have been deposited in a RF-plasma enhanced chemical vapour deposition system operating at a frequency of 13.56 MHz. The substrate temperature was fixed at 300 °C. Fourier transform infrared analysis has been used to determine the nature of Si-N, N-H, and Si-H bonding in the films. Refractive index of films has been measured using spectroscopic elli...

Effect of ZnO:Al Thickness on the Open Circuit Voltage of Organic/a-Si:H Based Hybrid Solar Cells

Hybrid solar cells are based on the concept of using both organic and inorganic materials for fabrication of devices. Hybrid solar cells, based on a heterojunction between inorganic electron acceptor layer and organic donor layer, has been fabricated. Effect of electron transport layer on open circuit voltage () of hybrid solar cells was investigated. Hybrid solar cells were fabricated using amorphous silicon as main absorbing layer and as electron acceptor layer while using copper phthalocyanine (CuPc) as the donor materials. Al doped ZnO layer was used as buffer layer between ITO and a-Si:H to prevent ITO from reacting with silane gas during plasma enhanced chemical deposition (PECVD) process. ZnO:Al thin film also acts as electron transport layer. The open circuit voltage of hybrid solar cells studied with varying the thickness of ZnO:Al layer. was increased from 0.30 volt to 0.52 volt with increasing the thickness of ZnO:Al layer from 15 nm to 45 nm. The poor interface between inorganic (a-Si:H) and organic layers may be a possible reason for low fill factor and low photocurrent in hybrid solar cells.

Effect of boron doping on optical and electrical properties of p-type a-Si∶H films for thin film solar cells application

We report the effect of boron doping on optical and electrical properties of p-type a-Si:H films. The p-type a-Si:H thin films have been deposited by RF-PECVD system varying the diborane flow rate from 5 sccm to 9 sccm. Enhancement in deposition rate from 30.3 nm/min to 33.6 nm reported with increasing diborane gas flow rate. Change in band gap, refractive index and Urbach energy with varying doping level is studied. An increase in dark conductivity is reported with increasing boron concentration. Solar cells fabricated with 4.63% efficiency.

Raman and FTIR Studies on PECVD Grown Ammonia-Free Amorphous Silicon Nitride Thin Films for Solar Cell Applications

Ammonia- (NH3-) free, hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited using silane (SiH4) and nitrogen (N2) as source gases by plasma-enhanced chemical vapour deposition (PECVD). During the experiment, SiH4 flow rate has been kept constant at 5 sccm, whereas N2 flow rate has been varied from 2000 to 1600 sccm. The effect of nitrogen flow on SiNx:H films has been verified using Raman analysis studies. Fourier transform Infrared spectroscopy analysis has been carried out to identify all the possible modes of vibrations such as Si–N, Si–H, and N–H present in the films, and the effect of nitrogen flow on these parameters is correlated. The refractive index of the above-mentioned films has been calculated using UV-VIS spectroscopy measurements by Swanepoel’s method.

Structural and optical investigations on seed layer assisted hydrothermally grown ZnO nanorods on flat and textured substrates

In this article we report the synthesis of vertically aligned ZnO nanorods on plain as well as textured fluorine doped tin oxide (FTO) coated glass substrate using hydrothermal method. Prior to hydrothermal method, AZO seed layer of thickness 5, 10 and 15 nm were deposited on the chosen substrates by DC magnetron sputtering. The as-grown nanorods were annealed at 450 °C for 3 h to improve the crystallinity. Morphology and structure of the nanorods was observed by field emission scanning electron microscopy. The formation of wurtzite structure was confirmed through x-ray diffraction studies. The optical mode of ZnO, E2 (high) at 434 cm−1 present in the samples was confirmed by Raman spectroscopy. The seed layer assisted growth of ZnO nanorods were defect free, which is confirmed from the photoluminescence spectra, and the intensity of band to band emission is much greater than the emission from the defects at the deep level.

Confocal Raman studies in determining crystalline nature of PECVD grown Si nanowires

Silicon nanowires of diameter ∼200 nm and length of 2-4 µm are grown in the plasma enhanced chemical vapour deposition technique using nanoclustered Au catalyst assisted vapour-liquid-solid process. The crystallinity in the as-grown and annealed samples is studied using confocal Raman spectroscopic studies. Amorphous phase is formed in the as-grown samples. Structural studies using high resolution transmission electron microscopy confirm the polycrystalline nature in the annealed sample.

Studies on Al:ZnO thin films for TCO applications in flexible amorphous silicon solar cells

Al doped ZnO thin films are deposited by DC magnetron sputtering on corning glass substrates at different process parameters. The effects of Ar flow rate and power density on the structural, optical and electrical properties are investigated by using XRD, UV-Vis spectroscopy, Four-point probe method and surface roughness of the deposited films were examined by AFM analysis. All the films deposited at different process conditions have a strong c-axis preferred orientation and the transmittance of ∼85% in the visible range. Thickness and Refractive Index (η) values are measured using ellipsometry.

Vibrational Spectroscopic and Optical Absorption Studies on PVA Based Polymer Electrolytes

The electrochemical methods of energy storage and conversion are of great interest for many practical applications. In the present investigations, PVA: MgSO4 based solid polymer electrolytes were prepared at different weight percent ratios using solution cast technique. FTIR spectroscopic studies were carried out to verify the complexation of the dopant with polymer. Force constant measurement was also carried out to ensure the interactions of polymer with salt. Optical absorption studies were carried out in the wave length range 200 to 600 nm. Absorption edge as well as bandgap values were evaluated. In order to ensure the ionic conduction of these electrolyte systems, transference number measurements were also carried out. The dominant conducting species were ions rather than electrons. These studies will help in verification or in investigating the feasibility of these electrolyte systems in polymer batteries, fuel cells, and other electrochemical systems.