Gautam Ganguly

Material structure and metastability of hydrogenated nanocrystalline silicon solar cells

We find that the volume fraction of amorphous component in hydrogenated nanocrystalline silicon intrinsic layers is not necessarily the determining factor for the light-induced metastability ofn-i-p solar cells. Small grains and/or intermediate range order may play an important role in improving the stability. The distribution of nanocrystallites along the growth direction is also important. Based on the findings, we have optimized the hydrogen dilution profiling for controlling the structural evolution and have reduced the light-induced degradation of solar cells. As a result, we have achieved initial and stable active-area efficiencies of 14.1% and 13.2%, respectively, using ana-Si:H/nc-Si:H/nc-Si:H triple-junction structure.

Importance of surface processes in defect formation in a-Si:H

Abstract The deposition temperature and growth rate dependence of the defect density in a-Si:H is shown to depend on the balance between rates of microscopic dangling bond creation and saturation reactions on the steady state growing surface. The surface dangling bonds are incorporated into the bulk as the film grows. On the basis of this model we show that the defect density can be reduced through precursor assisted defect suppression (PADS) at high substrate temperatures (>350°C), and defect reduction by energized precursors (DREP) at low substrate temperatures (

Optical characterization of hydrogenated silicon thin films using interference technique

This work introduces an application of an “interference spectroscopy technique” (IST) for determination of absorption coefficient and refractive index spectra of amorphous silicon (a-Si:H) and related thin film materials. The technique is based on computer analysis of measurements of optical transmission and specular reflection (T & R) of thin films (including the films on substrates) over a wide range of the incident photon energies (0.5–2.8 eV) using carefully controlled spectrometer conditions. IST is used to investigate the absorption spectrum in the sub-gap energy range (0.8–1.6 eV) of intrinsic and phosphorous-doped a-Si:H, “polymorphous-Si:H,” and microcrystalline silicon films. The enhanced sensitivity of the technique over conventional analysis of T & R data results from utilization of interference to obtain absorption coefficient values at the maxima of transmission. The factors limiting the accuracy of the calculated absorption coefficient are discussed in detail. Measurement on films of thickn...

Reduced light‐induced changes of photoconductivity in duterated amorphous silicon

The light‐induced decrease of the photoconductivity in deuterated amorphous silicon is a factor of 3 less even though the defect density increase is greater than in hydrogenated material having equivalent as‐deposited properties. Consequent changes in the average recombination cross section of the defects is illustrated. Since the differences in the light soaking behavior upon isotopic substitution has been found to disappear in films deposited at low temperatures, the changes are thought to arise from differences in the silicon network occurring during growth.

Electron and Ion Energy Controls in a Radio Frequency Discharge Plasma with Silane

Electron and ion energy distribution functions are controlled in a radio-frequency (rf) discharge plasma with silane for production of hydrogenated amorphous silicon films. We apply the grid-bias method to an rf silane plasma in order to obtain a low electron-temperature (T e ≃ 0.2 eV) and low ion-temperature (T i ≃ 0.1 eV) plasma. The ion beam energy is controlled by biasing the substrate. We find that the room temperature hole drift mobility is increased by two orders of magnitude compared to the conventional value at an ion beam energy between 23 eV and 24eV.

Modeling the dielectric functions of silicon-based films in the amorphous, nanocrystalline and microcrystalline regimes

Abstract We describe simple expressions that use a minimum number of free parameters to fit the dielectric function spectra of a variety of Si-based film materials ranging from amorphous silicon (a-Si:H) and its alloys with Ge and C to nanocrystalline silicon (nc-Si:H) and microcrystalline silicon (μc-Si:H). Three applications of these formulas are presented. First, we demonstrate how the expressions can be used in optical modeling of multijunction solar cells. Second, we analyze a-Si:H materials prepared versus the H 2 -dilution flow ratio, R =[H 2 ]/[SiH 4 ], and observe that improved ordering is obtained at larger R . Finally, we analyze Si films as a function of thickness across the a→μ c phase boundary and quantify effects of electronic confinement in the nc-Si:H regime and grain development in the μc-Si:H regime.

Polycrystalline silicon carbide films deposited by low‐power radio‐frequency plasma decomposition of SiF4‐CF4‐H2 gas mixtures

Polycrystalline silicon carbide thin films have been deposited on amorphous substrates by radio‐frequency plasma‐assisted decomposition of tetrafluoro silane, tetrafluoro methane, and hydrogen gas mixtures using low‐power density and deposition temperatures. The material is shown to possess the α‐SiC structure using transmission electron microscopy. It has highly visible transmittance and exhibits bands due to silicon carbide as well as fluorine bonded to carbon and silicon in the infrared transmission spectra. It is easily doped, both types showing high conductivity (∼10 S/cm) and Hall mobility [∼10 cm2/(V s)] for either carrier type. The conductivity is seen to be independent of thickness down to ∼10 nm when deposited on glass. This behavior and the dependence of both structural and electronic properties on deposition parameters is discussed in terms of the chemical reactions in gas phase and on the growth surface.

Control of photodegradation in amorphous silicon: The effect of deuterium

Abstract Light-induced changes in the photoconductivity and defect density have been compared in deuterated and hydrogenated amorphous silicon. There is no significant difference between the as-deposited properties of the two materials; however, the photoconductivity degrades more slowly but anneals more rapidly in material prepared using the heavier isotope. In hydrogenated material, the average photocarrier recombination cross-section of the defects increases with light soaking but the opposite occurs in deuterated material. These results are explained in terms of two kinds of recombination centre which are photocreated at different rates. The variation in the degradation and annealing behaviour of photoconductivity with deposition temperature implies subtle structural differences between the two materials which, for the first time, gives rise to the possibility of controlling photodegradation in amorphous silicon.

Saturation of the defect density in hydrogenated amorphous silicon by pulsed light soaking

We report the first observation of saturation of the defect density and of the photoconductivity in hydrogenated amorphous silicon after light soaking with laser pulses. The saturated defect density reaches approximately the same value as the density established by soaking with continuous light. Comparison of pulsed with cw light‐soaking experiments, via an effective light‐soaking time, shows the use of pulsed light to be equivalent to that of continuous wave light, in defect saturation.

Photo-induced changes in the properties of undoped and boron-doped a-Si:H films

Abstract Light-induced effects on the dark conductivity, photoconductivity, spectral responses and band gap of hydrogenated amorphous silicon thin films have been studied. The a-Si:H films were prepared by the r.f. glow-discharge decomposition of silane gas in a capacitatively coupled plasma-reactor system. Undoped a-Si:H films were prepared from a mixture of silane and hydrogen with the proportion of hydrogen varying from 0 to 80%. It was found that the magnitude of light-induced effects decreased with an increase in hydrogen concentration, which may be due to the removal of weak bonds contributing to the Staebler-Wronski effect. The effect of boron doping on light-induced effects has also been studied. For nearly compensated films at low boron concentrations the light-induced effects are small. As the boron doping is increased, the dark conductivity increases on light exposure and reaches a maximum which is the reverse of the normal Staebler–Wronski effect. It has been suggested that this effect is due ...