Hydrogenated amorphous silicon oxide (a-SiO:H) films have been prepared in a conventional single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide , silane and hydrogen gas mixture. Variation in optical gap from 1.96 to 2.12 eV has been obtained by controlling the deposition pressure and radio frequency (rf) power. The photoconductivities of films in each set are similar to each other for similar optical gaps. High rf power does not have any remarkable deteriorative effect on material but the rate of deposition of film and optical gap are enhanced. At rf power density the and are and 2.11 eV respectively whereas under similar other conditions but with rf power density the corresponding and are and 1.96 eV respectively. By lowering the deposition pressure the optical gap of films has also been enhanced. At 1.5 Torr pressure, the optical gap is 1.96 eV and at 0.13 Torr it becomes 2.12 eV. Here variation of the dilution of hydrogen is adopted in order to look into the role of hydrogen atoms in plasma and characteristics of the deposited films. A model in which incorporation of oxygen is described through plasma kinetics is proposed in this regard. With a higher optical gap the photoconductivity reduces but this reduction is mostly due to there being a lower optical absorption coefficient , partly due to there being a lower value of the product of the mobility , lifetime and quantum efficiency and also partly due to the mid-gap defect density . For the power variation set of samples, with optical gap up to about 1.99 eV, and the photoconductivity remain around and respectively and, with this optical gap, the dark conductivity is . When the material is degraded by soaking in light, a modification of mid-gap states and also of the valence band tail state takes place and falls, together with the photoconductivity. For material of 1.95 eV optical gap the initial and are reduced to and respectively after 3 h of soaking in light.

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This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a-Si:H)-based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a-Si:H-based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E a ) and Urbach parameter (E u ) of n-type hydrogenated amorphous silicon (a-Si:H(n)) on current transport in an a-Si:H-based n/p TRJ has been investigated. The photoluminescence spectra and temperature-dependent current-voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a-Si:H-based n/p-type TRJ. The fabrication of a tandem cell structure consists of an a-Si:H-based top cell and an HIT-type bottom cell with the a-Si:H-based n/p junction developed as a TRJ in between. The development of a-Si:H-based n/p junction as a TRJ leads to an improved a-Si:H/HIT-type tandem cell with a better open circuit voltage (V oc ), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band-tail states in the a-Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V oc = 1430 mV, short circuit current density = 10.51 mA/cm 2 , FF = 0.65, and efficiency = 9.75%.

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