Arup Dasgupta

Effects of substrate temperature on structural properties of undoped silicon thin films

Undoped silicon thin films were deposited by a radio frequency plasma enhanced chemical vapor deposition technique over a wide range of substrate temperatures (170–370 °C) using a mixture of silane and hydrogen gas. A low power density (35 mW cm−2) was chosen. The effects of substrate temperature on the structural properties of the films was studied. A distinct transition from amorphous to microcrystalline phase is observed with an increase in the substrate temperature (Ts). Raman spectroscopy shows the variation of amorphous and crystalline volume fractions in the silicon films. The amorphous matrix seems to be composed mainly of monohydrides in contrast to the usual dominance of polyhydrides. At the onset of crystallinity, the films have tiny crystallites and the grain size (δ) increases with Ts (at Ts∼370 °C, δ∼350 A). The deposition rate increases with Ts and attains its maximum (28.2 A min−1) at the amorphous to microcrystalline transition region. All the microcrystalline films with their differing c...

N-side illuminated microcrystalline silicon solar cells

Thin-film microcrystalline silicon solar cells illuminated through the n layer were studied and compared with classical p-layer illuminated cells. To investigate the corresponding charge carrier extraction properties, variation of the intrinsic absorber layer thickness was carried out. It was found that the J–V characteristic and the quantum efficiency of the n- and p-side illuminated cells are almost identical in the thickness range investigated, up to 7 μm. No differences in the collection of photogenerated electrons or holes are observed. Hence, the illumination side of μc-Si:H single junction solar cells of conventional thickness may be randomly chosen without adverse effect on their performance.

Influence of boron doping and hydrogen dilution on p‐type microcrystalline silicon carbide thin films prepared by photochemical vapor deposition

Boron doped μc‐SiC:H films have been prepared by low power (10 mW/cm2) photochemical vapor decomposition of SiH4, C2H2, and B2H6 gases diluted with hydrogen. The effect of boron doping and hydrogen dilution on structural and opto‐electronic properties have been studied. The microstructure consists of Si crystallites while carbon remains at the grain boundaries and amorphous parts. Diborane doping beyond an optimum value has been observed to deteriorate the formation of microcrystallites. By optimizing the process parameters, p‐type μc‐SiC:H having σd∼3.0×10−3 S cm−1 with an E04 of 2.34 eV has been obtained. This film exhibits high optical transmittivity compared to its amorphous counterpart.

Effect of chamber pressure on p-type μc-SiC:H thin films prepared by photo-CVD

Abstract Highly conducting boron-doped microcrystalline silicon carbide ( μ c-SiC:H) thin films have been prepared by mercury sensitised photochemical vapor deposition. The chamber pressure was identified as one of the most crucial parameters governing the microcrystalline growth as well as the dopant incorporation in the microcrystalline thin films. Raman studies show the crystalline size and volume fraction decreases with increasing pressure. Transmission electron microscopy of such films reveal that the crystalline phase contains silicon only, so that carbon is incorporated only in the amorphous phase. The presence of carbon in these films was confirmed by secondary ion mass spectroscopy. There is an optimum pressure (0.5 Torr), depending upon the other deposition parameters, for which the conductivity of p-type μ c-SiC:H film is highest (1.2×10 −4 S cm −1 ).

Accelerated degradation in amorphous silicon solar cells by a combination of current injection and light insolation

Abstract A new degradation technique for amorphous silicon solar cells comprising of a combination of current injection and insolation has been developed. Compared to the conventional light degradation technique, current-induced degradation which involves forward biased current stress, results in a lower stabilized cell parameters including efficiency. This stabilized efficiency is, however, independent of the amount of current injected. The excess degradation is recovered under light illumination. The stabilized value of the conversion efficiency is within experimental error identical to that observed in long term, illumination only degradation. The method is fast, accurate and reliable. Its reliability has also been tested for single junction solar cells having intrinsic layers of different band gaps, as well.

Hole diffusion at the recombination junction of thin film tandem solar cells and its effect on the illuminated current–voltage characteristic

Computer simulation of experimental current density–voltage (J–V) and quantum efficiency characteristics of thin film p1-i1-n1-p2 structures and of double junction solar cells (p1-i1-n1-p2-i2-n2), has been used to understand the hole transport mechanisms near the np “tunnel” junction between two subcells of a multijunction structure. Two different types of p layers at the junction have been studied: (i) hydrogenated microcrystalline silicon (μc-Si:H) and (ii) hydrogenated amorphous silicon carbide (a-SiC:H). There is a striking difference between the experimental J–V characteristics for the p1-i1-n1-p2 structures, with case (i) having a fairly high fill factor (FF) and conversion efficiency (η), as against a very low FF and η in case (ii). Although the difference is much smaller for double junction cells employing these two types of materials as the p layer at the junction, the fill factor of the cell employing μc-Si:H is about 8% higher. Analysis of transport properties as a function of position by compu...

Development of p-type microcrystalline silicon carbon alloy films by the very high frequency plasma-enhanced chemical vapor deposition technique

We developed p -type μc-silicon carbon alloy thin films by the very high frequency plasma-enhanced chemical vapour deposition technique using a SiH 4 , H 2 , CH 4 , and B 2 H 6 gas mixture at low power (55 mW/cm 2 ) and low substrate temperatures (150–250 °C). Effects of substrate temperature and plasma excitation frequency on the optoelectronic and structural properties of the films were studied. A film with conductivity 5.75 Scm −1 and 1.93 eV optical gap ( E 04 ) was obtained at a low substrate temperature of 200 °C using 63.75 MHz plasma frequency. The crystalline volume fractions of the films were estimated from the Raman spectra. We observed that crystallinity in silicon carbon alloy films depends critically on plasma excitation frequency. When higher power (117 mW/cm 2 ) at 180 °C with 66 MHz frequency was applied, the deposition rate of the film increased to 50.7 A/min without any significant change in optoelectronic properties.

Measurement of impurity profiles in microcrystalline silicon solar cells by SIMS

The concentration profiles of B, P, Zn, and O in the active intrinsic (i) μc-Si:H layer and across the interfaces in pin and nip structures have been measured with SIMS. For the ZnO/μc-p/μc-i sequences, an apparent B and O profile extends over several hundred nanometers into the i-layer, and high levels of Zn can be found well above the ZnO substrate layer. These profiles are not affected by annealing at the deposition temperature. Much lower impurity concentrations are measured for n-i-p deposition sequences, or when the p-layer or the i-layer is amorphous. Ruling out diffusion and intermixing processes, evidence for the presence of pinholes in the material is presented, which explain most of the experimental findings.