Chandan Banerjee

Surface Passivation of Crystalline and Polycrystalline Silicon Using Hydrogenated Amorphous Silicon Oxide Film

Excellent passivation properties of hydrogenated amorphous silicon oxide (a-SiOx:H) prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) at a low substrate temperature (170 °C) on crystalline and polycrystalline silicon (Si) wafers are reported. Films were characterized by ellipsometry, Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) spectrophotometry, and dark-conductivity and photoconductivity measurements. A comparison of the results with those for different passivation layers such as hydrogenated amorphous silicon carbon nitride (a-SiCxNy:H), hydrogenated amorphous silicon nitride (a-SiNx:H), and hydrogenated amorphous silicon (a-Si:H) reveals their superiority as an excellent passivation layer for p-type crystalline Si as well as polycrystalline Si. A maximum effective lifetime of 400 µs was measured for 1–10 Ω cm, 380-µm-thick p-type c-Si using a micro-photocurrent decay (µ-PCD) system. Fixed charge density (Qf) was estimated by high-frequency (1 MHz) capacitance–voltage measurement using a metal–insulator–silicon structure (CV-MIS). The effect of annealing temperature on surface passivation in a nitrogen atmosphere was also studied.

Fabrication of microcrystalline cubic silicon carbide/crystalline silicon heterojunction solar cell by hot wire chemical vapor deposition

The n-type microcrystalline cubic silicon carbide (µc-3C–SiC:H) films were deposited by hot wire chemical vapor deposition (HWCVD) at a low substrate temperature (~300 °C). Heterojunction silicon based photovoltaic devices were fabricated by depositing wide band gap n-type µc-3C–SiC thin films on p-type Si wafers, whose thickness and resistivity were 200 µm and 1–10 Ω cm, respectively. The silicon wafers were textured using alkaline etchant prior to the device fabrication. The photovoltaic parameters of a typical device were found to be Voc=560 mV, Jsc=35.0 mA/cm2, fill factor (F.F.) = 0.724, and η=14.20%. Numerical analysis was performed using automat for simulation of hetero structures (AFORS-HET), a one-dimensional device simulators to determine the probable cause of the changes in device parameters before and after the ageing of the filament.

Silica nanoparticles on front glass for efficiency enhancement in superstrate-type amorphous silicon solar cells

Antireflective coating on front glass of superstrate-type single junction amorphous silicon solar cells (SCs) has been applied using highly monodispersed and stable silica nanoparticles (NPs). The silica NPs having 300 nm diameter were synthesized by Stober technique where the size of the NPs was controlled by varying the alcohol medium. The synthesized silica NPs were analysed by dynamic light scattering technique and Fourier transform infrared spectroscopy. The NPs were spin coated on glass side of fluorinated tin oxide (SnO2: F) coated glass superstrate and optimization of the concentration of the colloidal solution, spin speed and number of coated layers was done to achieve minimum reflection characteristics. An estimation of the distribution of the NPs for different optimization parameters has been done using field-emission scanning electron microscopy. Subsequently, the transparent conducting oxide coated glass with the layer having the minimum reflectance is used for fabrication of amorphous silicon SC. Electrical analysis of the fabricated cell indicates an improvement of 6.5% in short-circuit current density from a reference of 12.40 mA cm−2 while the open circuit voltage and the fill factor remains unaltered. A realistic optical model has also been proposed to gain an insight into the system.

Reduction of Thickness of N-Type Microcrystalline Hydrogenated Silicon Oxide Film Using Different Types of Seed Layer

By using a seeding technique it has been possible to reduce the thickness of n-µc-SiO:H film for use at the tunnel junction of a double-junction a-Si solar cell from ∼ 300 A to ∼ 185 A with acceptable optoelectronic properties. We have used two types of seed layer, i.e., undoped µc-SiO:H and µc-Si:H. The layers were prepared by the radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) method (13.56 MHz) at low rf power density (14 mW/cm2) and low substrate temperature (200°C). The ultrathin seed layer (∼ 40 A) enhances the growth of microcrystallinity of the n-type µc-SiO:H film as confirmed by the results of transmission electron microscopy (TEM) analysis and Raman spectroscopy.

Development of n-type microcrystalline SiOx:H films and its application by innovative way to improve the performance of single junction µc-Si:H solar cell

Light trapping is one of the fundamental necessities of thin film based solar cell for its performance elevation. Back reflection of unused light of first pass is the key way to improve the light trapping phenomena. In this study we have reported the development of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiO:H) layers of different characteristics. The deposition has been done by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The detailed characterization of the films include the following: (1) electrical properties (2) optical properties like E04 (3) structural studies which include crystalline fraction by Raman spectroscopy and grain size by X-ray diffraction measurement, FTIR spectroscopy, AFM and TEM studies. n-µc-SiO:H layer has been introduced as the n-layer of single junction p–i–n structure µc-Si solar cells. By various techniques the optimum use of n-µc-SiO:H layer for enhancing the performance of µc-Si:H solar cells has been done. It has been found that by using suitable bilayer of two different n-µc-SiO:H layers, it is possible to increase the solar cell performances. The maximum efficiency obtained without any back reflector is 8.44% that is about 8.9% higher than that obtained by using n-µc-Si:H layer as n-layer in the solar cells.

Comparison of structural and optoelectronic properties of N-type microcrystalline silicon and silicon oxide films with lowering of thickness

We have compared the structural and optoelectronic properties of n-type microcrystalline hydrogenated silicon oxide (n-µc-SiO:H) and n-type microcrystalline hydrogenated silicon (n-µc-Si:H) films with lowering of thickness, prepared by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD, 13.56 MHz) method. At thickness ≤ 300 A, the n-µc-SiO:H film has higher optical gap (E05) and lower optical absorption while retaining the photoconductivity (σph) and activation energy (Ea) similar to those for n-µc-Si:H film. Due to these advantages of n-µc-SiO:H film over that of n-µc-Si:H at low thickness this material has potential for use in improving the performance of single and double junction amorphous silicon solar cells.

Development of large area nanostructured silicon-hydrogen alloy material with improved stability for solar cell application by argon dilution method

Here we have presented the results of large area (30 × 30 cm2) silicon-hydrogen alloy material and solar cell by argon dilution method. As an alternative to hydrogen dilution, argon dilution method has been applied to develop single junction solar cell with appreciable stability. Optimization of deposition conditions revealed that 95% argon dilution gives a nanostructured material with improved transport property and less light induced degradation. The minority carrier diffusion length (Ld) and mobility-lifetime (μτ) product of the material with 95% argon dilution degrades least after light soaking. Also the density of states (DOS) below conduction level reveals that this material is less defective. Solar cell with this argon diluted material has been fabricated with all the layers deposited by argon dilution method. Finally we have compared the argon diluted solar cell results with the optimized hydrogen diluted solar cell. Light soaking study proves that it is possible to develop stable solar cell on large area by argon dilution method and that the degradation of argon diluted solar cell is less than that of hydrogen diluted one.

Innovative Utilization of Improved n-doped μc-SiOx:H Films to Amplify the Performance of Micromorph Solar Cells

Due to in-situ deposition process doped SiOx material attracts the PV community as intermediate reflecting layer (IRL) for the less hazardous deposition process. Previously we have been shown that how to formulate a better quality SiOx layer using seeding technique. In this paper we have shown that how this superior quality material will be beneficial as IRL and improve the performance of micromorph solar cells. Enhanced performance is observed when SiOx material is deposited with the help of highly conducting seed layer. For improving the performance of the micromorph solar cell further we have eliminated the n-layer of the top constituent cell. In the context of substituting the top n-layer, SiOx layer grown on seed layer is more significant as of its suitable optoelectronics properties. Initial efficiency of ∼12% is perceived in micromorph solar cells with the improved SiOx based IRL used innovatively which is 7.9% higher than that of the cell with conventionally developed and typical use of IRL.

Modeling and Analysis of Lifetime Curve of Amorphous Silicon/Crystalline Silicon Heterostructure Solar Cell

The modeling and analysis of recombination properties is one of the critical aspects in the design and development of high efficiency amorphous silicon (a-Si)/crystalline silicon (c-Si) heterostructure solar cell (SH-SC). In this paper, we have developed a recombination model pertinent for a-Si/c-Si SH-SC. Based on the model, an analytical expression for the relationship between effective carrier lifetime (τeff) and excess carrier concentration (An), commonly termed as lifetime curve, is derived and the results are validated with the experimental data. The results show that the inverse of τeff, after correcting for the contributions of Auger and radiative recombination in silicon, when plotted as a function of An, shows a linear characteristic, with each of the slope and the intercept, providing definitive details about the recombination processes occurring in the device.

Silicon heterojunction solar cells with novel fluorinated n-type nanocrystalline silicon oxide emitters on p-type crystalline silicon

A novel fluorinated phosphorus doped silicon oxide based nanocrystalline material have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) Czochralski (CZ) wafers. The n-type nc-SiO:F:H material were deposited by radio frequency plasma enhanced chemical vapor deposition. Deposited films were characterized in detail by using atomic force microscopy (AFM), high resolution transmission electron microscopy (HRTEM), Raman, fourier transform infrared spectroscopy (FTIR) and optoelectronics properties have been studied using temperature dependent conductivity measurement, Ellipsometry, UV–vis spectrum analysis etc. It is observed that the cell fabricated with fluorinated silicon oxide emitter showing higher initial efficiency (η = 15.64%, Jsc = 32.10 mA/cm2, Voc = 0.630 V, FF = 0.77) for 1 cm2 cell area compare to conventional n-a-Si:H emitter (14.73%) on flat c-Si wafer. These results indicate that n type nc-SiO:F:H material is a promising candidate for heterojunction solar cell on p-type crystalline wafers. The high Jsc value is associated with excellent quantum efficiencies at short wavelengths (<500 nm).