I. Martín

Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiC x :H films

Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, teff, on the excess carrier density, ?n, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30?cm?s-1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.

Pentacene thin-films obtained by thermal evaporation in high vacuum

Pentacene thin-films were obtained by thermal evaporation in high vacuum of a pure (98%) commercially available source. All the samples were grown at room temperature (25 °C) and high deposition rates (>20 A/s) on Corning glass substrates. The microstructure of the pentacene thin-films evidences the coexistence of thin-film and bulk triclinic crystalline phases. The optical absorption edge close to 1.7 eV, together with the high optical absorption in the visible range, make pentacene a promising candidate for low cost solar cells. Doping series of samples were obtained by dipping pieces cut from the same pentacene sample into a solution of iodine in acetonitrile for different times (<1 h). Thereby, room temperature dark conductivity changes from 3×10-7 to 4×10-5 O-1 cm-1. Finally, glass/SnO/pentacene/Al and glass/ZnO:Al/pentacene/Au structures were fabricated showing rectifying characteristics with on/off ratios of approximately 3 orders of magnitude.

Surface passivation of n-type crystalline Si by plasma-enhanced-chemical-vapor-deposited amorphous SiCx:H and amorphous SiCxNy:H films

Excellent passivation of n-type crystalline silicon surface is demonstrated by means of intrinsic amorphous silicon carbide (a-SiCx:H) thin films. An optimum CH4/SiH4 ratio is determined, leading to an effective surface recombination velocity, Seff, lower than 54 cm s−1. By adding a constant flow of N2 to the precursor gases, the surface passivation is improved to Seff⩽16 cm s−1. From infrared spectroscopy measurements of these films, it can be deduced that the N2 flow increases the carbon content of the layers for a constant CH4/SiH4 ratio. The dependence of the effective lifetime, τeff, on the excess charge carrier density, Δn, is measured using the quasisteady-state photoconductance technique, and these curves are simulated through an electrical model based on an insulator/semiconductor structure.

Characterization of a-Si:H∕c-Si interfaces by effective-lifetime measurements

This article studies theoretically and experimentally the recombination at the amorphous/crystalline silicon interface of a heterojunction with intrinsic thin layer (HIT) structure without metallization. We propose a physical model to calculate the interface recombination rate under illumination. This model calculates the effective lifetime τeff as a function of the average excess minority carrier concentration ⟨Δn⟩. In order to test the model, we prepared a set of HIT structures. The dependence of τeff vs ⟨Δn⟩ of the samples is measured using the quasi-steady-state photoconductance technique. By fitting our model to the experimental data, we determine the a-Si:H∕c-Si interface parameters and the doping density of the amorphous layer.

Effect of amorphous silicon carbide layer thickness on the passivation quality of crystalline silicon surface

Surface passivation of p-type crystalline silicon wafers by means of phosphorus-doped hydrogenated amorphous silicon carbide films [a-SiCx(n):H] has been investigated. Particularly, we focused on the effects of layer thickness on the c-Si surface passivation quality resulting in the determination of the fixed charge density, Qf, within the a-SiCx(n):H film and the fundamental recombination of holes, Sp0. The main result is that surface recombination velocity decreases with film thickness up to 40nm and then saturates. The evolution of the interface parameters indicates that Qf could be located in a layer less than 10nm thick. In addition, Sp0 increases with thinner films probably due to different hydrogenation and saturation of interface dangling bonds during forming gas annealing.

Surface passivation and optical characterization of Al2O3/a-SiCx stacks on c-Si substrates

Summary The aim of this work is to study the surface passivation of aluminum oxide/amorphous silicon carbide (Al2O3/a-SiCx) stacks on both p-type and n-type crystalline silicon (c-Si) substrates as well as the optical characterization of these stacks. Al2O3 films of different thicknesses were deposited by thermal atomic layer deposition (ALD) at 200 °C and were complemented with a layer of a-SiCx deposited by plasma-enhanced chemical vapor deposition (PECVD) to form anti-reflection coating (ARC) stacks with a total thickness of 75 nm. A comparative study has been carried out on polished and randomly textured wafers. We have experimentally determined the optimum thickness of the stack for photovoltaic applications by minimizing the reflection losses over a wide wavelength range (300–1200 nm) without compromising the outstanding passivation properties of the Al2O3 films. The upper limit of the surface recombination velocity (S eff,max) was evaluated at a carrier injection level corresponding to 1-sun illumination, which led to values below 10 cm/s. Reflectance values below 2% were measured on textured samples over the wavelength range of 450–1000 nm.

Crystalline silicon surface passivation with amorphous SiCx:H films deposited by plasma-enhanced chemical-vapor deposition

Surface-passivating properties of hydrogenated amorphous silicon carbide films (a-SiCx:H) deposited by plasma-enhanced chemical-vapor deposition on both p- and n-type crystalline silicon (c-Si) have been extensively studied by our research group in previous publications. We characterized surface recombination by measuring the dependence of the effective lifetime (τeff) on excess carrier density (Δn) through quasi-steady-state photoconductance technique. Additionally, we fitted the measured τeff(Δn) curves applying an insulator/semiconductor model which allows us to determine the surface recombination parameters. In this paper, this model is analyzed in detail focusing on the accuracy in the determination of the fitting parameters and revealing uncertainties not detected up to now. Taking advantage of this analysis, the dependence of surface passivation on film deposition conditions is revised including intrinsic a-SiCx:H films on both p- and n-type c-Si and phosphorus-doped a-SiCx:H films on p-type c-Si. ...

Improvement of crystalline silicon surface passivation by hydrogen plasma treatment

A completely dry low-temperature process has been developed to passivate 3.3 Ω cm p-type crystalline silicon surface with excellent results. Particularly, we have investigated the use of a hydrogen plasma treatment, just before hydrogenated amorphous silicon carbide (a-SiCx:H) deposition, without breaking the vacuum. We measured effective lifetime, τeff, through a quasi-steady-state photoconductance technique. Experimental results show that hydrogen plasma treatment improves surface passivation compared to classical HF dip. Seff values lower than 19 cm s−1 were achieved using a hydrogen plasma treatment and an a-SiCx:H film deposited at 300 °C.

Annealing effects on the conduction mechanisms of p+-amorphous- Si0.8C0.2:H/n-crystalline-Si diodes

P+-type hydrogenated amorphous silicon–carbon (a-Si1−xCx:H) on n-type crystalline silicon (c-Si) heterojunction diodes were fabricated and characterized electrically. The effects of thermal annealing on the electrical transport properties of a-Si0.8C0.2:H/c-Si diodes were investigated by measuring their current–voltage characteristics. From the dark current–voltage characteristics measured at different temperatures (298–373 K), transport mechanisms were analyzed in detail. Two carrier transport mechanisms were found to be the origin of forward current. At low bias voltage and temperatures above 320 K as-deposited diodes are dominated by recombination currents on the amorphous side of the space charge region whereas annealed diodes are mainly dominated by diffusion mechanisms. In contrast, at temperatures below 320 K, both types of diodes are mainly dominated by multitunneling capture emission. At higher voltages, the current becomes space charge limited for both diodes throughout the temperature range stu...

n-type emitter surface passivation in c-Si solar cells by means of antireflective amorphous silicon carbide layers

Emitter saturation current densities (JOe) of phosphorus-diffused planar c-Si solar cell emitters passivated by silicon carbide (SiCx) layers have been determined in a wide sheet resistance range (20–500Ω∕sq). Phosphorus diffusions were performed using solid planar diffusion sources without employing any drive-in step. Stacks of two SiCx layers were deposited by plasma enhanced chemical vapor deposition: first a thin silicon rich layer with excellent passivating properties and then an antireflective carbon rich layer. The thickness of the passivating layer was optimized, reaching a trade-off between the better passivation achieved for thicker layers and the increased light absorption within the layer, which reduced the photocurrent. The surface recombination velocity and the optical losses were determined for each configuration and used to calculate photovoltaic conversion efficiency limits for 50 and 90Ω∕sq emitters. In both cases, optimum configuration is for the stacks with passivating layers that are ...