Armando Menéndez

Pulsed radiofrequency glow discharge optical emission spectrometry for the direct characterisation of photovoltaic thin film silicon solar cells

The increasing demand for photovoltaic devices has created a silicon supply shortage, providing a great opportunity for hydrogenated amorphous silicon (a-Si:H) used in thin film technology. The potential of continuous and pulsed radiofrequency glow discharge optical emission spectrometry (rf-GD-OES) for the characterisation of thin film solar cells (TFSC), based on a-Si:H, has been investigated in this work. Qualitative in-depth profiles of TFSC obtained by both GD modes were carefully compared, in terms of signal intensity, penetration rate, emission yield, and depth resolution. The influence of rf-GD parameters operating in continuous mode was studied using three types of samples: B doped, P doped and the complete photovoltaic TFSC device based on a-Si:H. Moreover, the effect of different roughness of the Zn substrate on the depth resolution of the a-Si:H layer was evaluated, along with plasma cleaning conditions optimisation. Additionally, intensity signals and relative depth resolution were investigated for different pulsed GD experimental conditions, such as the pulse frequency (from 500 Hz to 10 kHz), duty cycle (in the range of 0.1875-0.5) and rf forward power (from 25 W to 75 W). 450 Pa and 25 W were selected as the optimum conditions for continuous rf-GD-OES analysis, whereas 450 Pa, 75 W, 1000 Hz and a duty cycle of 0.5 were selected for pulsed rf-GD-OES work. Results show that rf-GD-OES is a powerful tool for direct depth-profiling analysis of a-Si:H TFSC, allowing to discriminate the different parts of the photovoltaic devices: the first contact layer with ZnO2 and Al2O3, the a-Si:H layer (where it can be distinguished between the B doped, the intrinsic a-Si:H and the P doped films), the back contact layer and, finally, the Zn substrate. Moreover, diffusion processes between the coating layers, which could have an important influence on the final efficiency of photovoltaic devices, can be identified and could be studied by rf-GD-OES.

Capabilities and limitations of LA-ICP-MS for depth resolved analysis of CdTe photovoltaic devices

The analytical potential of ArF* excimer Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is investigated for fast qualitative depth profile analysis of multi-layer CdTe photovoltaic (PV) devices. Critical parameters (e.g. laser fluence and laser repetition rate) are evaluated and optimized to reduce the aerosol mixing from consecutive laser shots and to achieve a low penetration rate. As a result, a high depth resolution (10 s of nm) is demonstrated through the analyses of superficial and embedded coatings. For instance, a layer with a thickness of 100 nm at a depth of 3 μm is successfully measured. Moreover, qualitative profiles of major and minor elements obtained by LA-ICP-MS are validated using reference techniques such as Secondary Ion Mass Spectrometry (SIMS) or Glow Discharge Time of Flight Mass Spectrometry (GD-TOFMS). Additionally, the shape and morphology of the laser-induced craters, after different numbers of laser shots, are investigated using mechanical profilometry and Atomic Force Microscopy (AFM), respectively.

Radiofrequency glow discharge-optical emission spectrometry for the analysis of metallurgical-grade silicon

The quantitative analysis of metallurgical-grade silicon by direct current (dc) and radiofrequency (rf) glow discharge–optical emission spectrometry (GD-OES) is investigated. This material is highly in demand today, being the source of raw silicon for industrial applications. Samples of different thickness, between 1.6 and 4 mm, were evaluated. To avoid instabilities, soft electrical conditions (600 V and 10–12 mA) had to be selected for dc-GD, thus achieving poor sensitivity. However, results showed a higher performance of the rf-GD for trace analysis. The effect of the sample thickness was also studied by rf-GD-OES: while for samples thinner than about 2 mm the behaviour was similar to that of conductors, for thicker samples the observed behaviour resembled that of an insulator. Finally, the quantitative analysis of aluminium, calcium, iron, titanium and boron has been successfully achieved by using a multi-matrix calibration approach. All in all, these results demonstrate that rf-GD-OES may be a useful technique for the rapid and convenient analysis of metallurgical-grade silicon in industry.

Effect of H2/Ar mixtures on the analysis of conducting and insulating materials by radiofrequency glow discharge mass spectrometry

The effect of the addition of hydrogen to a radiofrequency–argon glow discharge coupled to a time of flight mass spectrometer has been studied. Pressure and forward power conditions in the discharge were kept constant. The hydrogen concentrations added to the argon plasma gas were 0.5%, 1% and 10% v/v and two types of homogeneous samples were investigated: a conducting material (nickel-base) and two glasses. A decrease of the dc-bias voltage with increasing H2 concentration was observed at every forward power assayed. For the nickel material, the analytical signals turned out to be higher in 10% H2/Ar and 1% H2/Ar discharges than in pure Ar, while for 0.5% H2/Ar the increases were just obtained at the lowest power assayed. Concerning the glass samples, signal increases were obtained at the powers assayed for most analytical signals using 0.5% H2/Ar instead of pure Ar.

Quantitative depth profile analysis of metallic coatings by pulsed radiofrequency glow discharge optical emission spectrometry.

In recent years particular effort is being devoted towards the development of pulsed GDs because this powering operation mode could offer important analytical advantages. However, the capabilities of radiofrequency (rf) powered glow discharge (GD) in pulsed mode coupled to optical emission spectrometry (OES) for real depth profile quantification has not been demonstrated yet. Therefore, the first part of this work is focussed on assessing the expected advantages of the pulsed GD mode, in comparison with its continuous mode counterpart, in terms of analytical emission intensities and emission yield parameters. Then, the capability of pulsed rf-GD-OES for determination of thickness and compositional depth profiles is demonstrated by resorting to a simple multi-matrix calibration procedure. A rf forward power of 50 W, a pressure of 600 Pa, 1000 Hz pulse frequency and 50% duty cycle were selected. The quantification procedure used was validated by analysing conductive layers of thicknesses ranging from a few tens of nanometer up to about 20 μm and varied compositions (hot-dipped zinc, galvanneal, back contact of thin film photovoltaic solar cells and tinplates).

Depth Profile Analysis of Amorphous Silicon Thin Film Solar Cells by Pulsed Radiofrequency Glow Discharge Time of Flight Mass Spectrometry

AbstractAmong the different solar cell technologies, amorphous silicon (a-Si:H) thin film solar cells (TFSCs) are today very promising and, so, TFSCs analytical characterization for quality control issues is increasingly demanding. In this line, depth profile analysis of a-Si:H TFSCs on steel substrate has been investigated by using pulsed radiofrequency glow discharge-time of flight mass spectrometry (rf-PGD-TOFMS). First, to discriminate potential polyatomic interferences for several analytes (e.g., 28Si+, 31P+, and 16O+) appropriate time positions along the GD pulse profile were selected. A multi-matrix calibration approach, using homogeneous certified reference materials without hydrogen as well as coated laboratory-made standards containing hydrogen, was employed for the methodological calibration. Different calibration strategies (in terms of time interval selection on the pulse profile within the afterglow region) have been compared, searching for optimal calibration graphs correlation. Results showed that reliable and fast quantitative depth profile analysis of a-Si:H TFSCs by rf-PGD-TOFMS can be achieved. Graphical Abstractᅟ

Influence of the hydrogen contained in amorphous silicon thin films on a pulsed radiofrequency argon glow discharge coupled to time of flight mass spectrometry. Comparison with the addition of hydrogen as discharge gas

Thin film solar cells technology based on hydrogenated amorphous silicon (a-Si:H) has undergone a great expansion during recent years. Pulsed radiofrequency glow discharge time-of-flight mass spectrometry (rf-PGD-ToFMS) is able to perform depth profiling analysis of coated materials, providing an excellent tool for rapid and high sensitive chemical characterisation of photovoltaic devices. The hydrogen concentration on a-Si:H thin films is around 10%, which represents a challenge for quantitative depth profile analyses by using GD sources due to the so-called “hydrogen effect”. It is well-known that when hydrogen is present in the Ar discharge, even in small quantities, significant changes can occur in the ion signal intensities and sputtering rates measured. Therefore, a critical comparison has been carried out by rf-PGD-ToFMS in terms of pulse profiles, spectral interferences and depth resolution for two modes of hydrogen introduction in the discharge, exogenous hydrogen in molecular gaseous form (using the mixture 0.2% H2 + Ar as discharge gas) or endogenous hydrogen, sputtered as a sample constituent. For this purpose, non-hydrogenated materials (containing B, P and Si) and three types of a-Si:H thin films were investigated. Exogenous hydrogen was found to produce a noteworthy influence on the pulse profiles of the analytes, whereas the effect of the hydrogen sputtered from the samples could be considered less notorious. Moreover, the proper selection of the after-peak region was found to be critical to obtain optimum mass spectra (i.e. high analyte sensitivities free of interferences).

Endogenous and exogenous hydrogen influence on amorphous silicon thin films analysis by pulsed radiofrequency glow discharge optical emission spectrometry

During the last decade the photovoltaic industry has been growing rapidly. One major strategy to reduce the production costs is the use of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The potential of pulsed radiofrequency glow discharge coupled to optical emission spectrometry (rf-PGD-OES) for the analysis of such type of materials has been investigated in this work. It is known that when hydrogen is present in the argon discharge, even in small quantities, significant changes can occur in the emission intensities and sputtering rates measured. Therefore, a critical comparison has been carried out by rf-PGD-OES, in terms of emission intensities, penetration rates and depth resolution for two modes of hydrogen introduction in the discharge, manually external hydrogen in gaseous form (0.2% H(2)-Ar) or internal hydrogen, sputtered as a sample constituent. First, a comparative optimisation study (at 600 Pa and 50 W) was performed on conducting materials and on a silicon wafer varying the pulse parameters: pulse frequency (500 Hz-20 kHz) and duty cycle (12.5-50%). Finally, 600 Pa, 50 W, 10 kHz and 25% duty cycle were selected as the optimum conditions to analyse three types of hydrogenated samples: an intrinsic, a B-doped and a P-doped layer based on a-Si:H. Enhanced emission intensities have been measured for most elements in the presence of hydrogen (especially for silicon) despite the observed reduced sputtering rate. The influence of externally added hydrogen and that of hydrogen sputtered as sample constituent from the analysed samples has been evaluated.

Characterization of Doped Amorphous Silicon Thin Films through the Investigation of Dopant Elements by Glow Discharge Spectrometry: A Correlation of Conductivity and Bandgap Energy Measurements

The determination of optical parameters, such as absorption and extinction coefficients, refractive index and the bandgap energy, is crucial to understand the behavior and final efficiency of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The influence of small variations of the gas flow rates used for the preparation of the p-a-SiC:H layer on the bandgap energy, as well as on the dopant elements concentration, thickness and conductivity of the p-layer, is investigated in this work using several complementary techniques. UV-NIR spectrophotometry and ellipsometry were used for the determination of bandgap energies of four p-a-SiC:H thin films, prepared by using different B2H6 and SiH4 fluxes (B2H6 from 12 sccm to 20 sccm and SiH4 from 6 sccm to 10 sccm). Moreover, radiofrequency glow discharge optical emission spectrometry technique was used for depth profiling characterization of p-a-SiC:H thin films and valuable information about dopant elements concentration and distribution throughout the coating was found. Finally, a direct relationship between the conductivity of p-a-SiC:H thin films and the dopant elements concentration, particularly boron and carbon, was observed for the four selected samples.

Rf glow discharge optical emission spectrometry for cleaning process control of oil residues in low alloy steel

Radiofrequency glow discharge optical emission spectrometry (rf-GD-OES) has been investigated as an assessment tool in the control of cleaning processes developed to remove oil residues covering the surface of the steel. The control of the cleaning process has a special importance on the production of zinc and zinc alloy coatings on steel as a corrosion protection. One of the most important methods used in producing zinc coatings is the immersion of steel in a liquid bath of zinc or a zinc alloy. Therefore, prior to this immersion, the cleaning of oil residues remaining on the steel sheet from previous processing steps is of great importance. Two cleaning processes have been investigated. Qualitative and quantitative rf-GD-OES results have been used to control the carbon-based impurities present on the surface of the steel sheet.