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ABSTRACT: The transparent conductive oxide (TCO) is commonly used as front electrode in p-i-n solar cells. Its surface state has an important impact on the amorphous silicon (a-Si:H) nucleation, and hence on the device efficiency. Specifically, the TCO surface morphology should be controlled to assure the growth of a dense enough a-Si:H structure. Otherwise, ohmic current paths might develop in the solar cell, causing an electrical loss increase. The application of a plasma treatment allows the elimination of possible imperfections on the TCO surface. Thus, the final device shows a higher shunt resistance, and consequently a better performance. This work addresses the problem of the shunting behaviour observed in our a-Si:H p-i-n devices deposited onto sputtered ZnO:Al (AZO). As a solution, a CF4+O2 plasma treatment was used to smoothen the AZO surface. For exposure times around five minutes, the elimination of surface defects led to a more compact a-Si:H structure, and a notable fill factor improvement was observed. Nevertheless, for long enough exposures, this dry-etching process caused an important deterioration of the AZO electrical characteristics. The effect of the CF4+O2 plasma treatment on the AZO bulk and surface properties was also studied.
ScienceJet. 01/2012; 1(21).
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ABSTRACT: P-a-SiC:H is suitable material for the development of optoelectronic devices. However, its chemical structure is complex, and its optoelectronic properties depend strongly on plasma stochiometry. In that sense, optimisation of the alloy characteristics is a fundamental step previous to the preparation of a-Si:H solar cells, since a high optical transmittance and electrical conductivity are required. In this work, a detailed study of the effect of deposition conditions on the properties of p-a-SiC:H was carried out. Diborane molecule presented a better doping efficiency, but the improvement on the p-a-SiC:H electrical properties saturated for concentrations RDB=[B2H6]/[SiH4]≥0.01. On the contrary, trimethylboron allowed obtaining a higher optical transmittance and gap energy. A clear correlation between carbon content and alloy structural disorder was found. This fact explained the deterioration of its electrical characteristics with a high methane concentration, XCH4-GAS. The strong influence of the XCH4-GAS value on the p-a-SiC:H structure quality was used to improve its optoelectronic properties at low deposition temperatures (T<150ºC) without hydrogen dilution.
ScienceJet. 01/2012; 1(6).
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ABSTRACT: The reliability of the analytical model (respectively equivalent circuit) proposed by Merten, Asensi et al. for p-i-n solar cells has been tested. Devices with a low carrier-drift capability have been characterised under different illumination conditions. The validity of the equivalent circuit has been checked in conditions for which the model assumptions are not strictly fulfilled. The voltage dependence of the photocurrent and the irradiance-level dependence of the external quantum efficiency EQE for these devices have been compared to model predictions. In general, a very low fitting error to the JV curves has been found, progressively decreasing at higher photon fluxes. However, the quality of the fit clearly worsens under forward voltages and shorter illumination wavelengths. Theoretical and experimental EQEs have shown a similar qualitative behaviour with the intensity of light. Under red light, model EQE results have differed less than 3% from solar-cell values at sufficiently high irradiance levels. Nevertheless, under infrared light, these differences have been higher than one order of magnitude.
25th European Photovoltaic Solar Energy Conference (EU PVSEC); 09/2010
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ABSTRACT: The origin of this work is the understanding of the correlation observed between efficiency and emitter-deposition temperature in single silicon-heterojunction solar cells prepared by depositing an n-doped hydrogenated-amorphous-silicon thin film onto a p-type crystalline-silicon wafer. In order to interpret these results, surface-recombination velocities have been determined by two methods, i.e. by fitting the current–voltage characteristics to a theoretical model and by means of the Quasi-Steady-State Photoconductance Technique (QSSPC). In addition, effective diffusion lengths have been estimated from internal quantum efficiencies. The analysis of these data has led to conclude that the performance of the cells studied is limited by back-surface recombination rather than by front-heterojunction quality. A 12%-efficient cell has been prepared by combining optimum emitter-deposition conditions with back-surface-field (BSF) formation by vacuum annealing of the back aluminium contact. This result has been achieved without using any transparent conductive oxide.
Solar Energy Materials and Solar Cells 94(2):282-286. · 4.54 Impact Factor
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ABSTRACT: P-type amorphous hydrogenated silicon thin films have been prepared using plasma-enhanced chemical vapour deposition with boron trifluoride instead of diborane as the dopant gas. The influence of the substrate temperature (Ts), the radio frequency (r.f.) power density (RFP) and the dopant gas concentration (C(BF3)) have been investigated. The applicability of boron triflouride to make device-quality p-type amorphous silicon has been demonstrated. The incorporation of fluorine to the lattice has been deduced from the analysis of the infrared transmission spectra. The experimental results show interesting features such as the inhibition of bandgap degradation in a wide doping range, which has been associated with the presence of fluorine. P-type thin films with 1.82 eV optical gap, 4.8 × 10−4 (ω cm)−1 conductivity and 0.43 eV activation energy, have been produced.
Thin Solid Films.
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ABSTRACT: This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH4) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)−1 and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.
Solar Energy Materials and Solar Cells.
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ABSTRACT: The optical properties of ternary chalcogenide amorphous thin films of chemical composition Ge10As15Se75, deposited by vacuum thermal evaporation, have been determined and analysed. Normal-incidence optical transmission spectra have been measured in the range from 400 to 2500 nm. From these transmission spectra, the optical constants and average thickness of this particular amorphous ternary material were accurately calculated using an optical-characterisation method based on creating the upper and lower envelope curves of the spectrum, which also allows to obtain a parameter indicating the degree of film-thickness uniformity. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical-absorption edge is described using the non-direct transition model proposed by Tauc and the optical band gap is calculated from the absorption coefficient values by Tauc's extrapolation procedure. A comparison between these optical properties and those corresponding to the As30Se70 and Ge25Se75 binary thin films (previously reported) is also presented.
Materials Letters.
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ABSTRACT: One set (A) of undoped and three sets (B, C and D) of doped hydrogenated amorphous silicon carbide samples have been made in the framework of a research plan for obtaining high quality p-type window layers by radiofrequency glow discharge of silane-based gas mixtures. The samples of sets A and B were made using different RF-power-density to mass-flow ratios for various methane percentages in the gas mixture. The best carbon incorporation in the amorphous silicon lattice was obtained at the highest RF-power density. The properties of sets C and D, prepared using different RF-power densities and silane and methane propertions have been analysed as functions of the concentration of borom trifluoride with respect to silane. In both cases, the optical gap , after a slight initial decrease, remains at a value of approximately 2.1 eV without quenching in the dopiong ranges covered. The best conductivity obtained is 2 × 10−7 (μ cm)−1. IR spectra allow to associate these features with the structural quality of the films.
Solar Energy Materials and Solar Cells.
