We propose here a performance mode) where the spectral mismatching under overcast conditions is evaluated from an empirical clearness index, which can be easily determined from irradiance data of the site. The model allows to determine the site dependend yields and losses related to the technology of the modules. As an example, a single and a double junction amorphous silicon module is examined. The series connection in the double junction modules leads to a higher loss under low illumination conditions, which results in related yearly losses of 7% instead of 4% for the single junction module. The better spectral matching under real operating conditions than under standard conditions increases the amorphous silicon module performance in Barcelona by about 5%.

As optoelectronic devices continue to improve, control over film thickness has become crucial, especially in applications that require ultra-thin films. A variety of undesired effects may arise depending on the specific growth mechanism of each material, for instance a percolation threshold thickness is present in Volmer-Webber growth of materials such as silver. In this paper, we explore the introduction of aluminum in silver films as a mechanism to grow ultrathin metallic films of high transparency and low sheet resistance, suitable for many optoelectronic applications. Furthermore, we implemented such ultra-thin metallic films in Dielectric/Metal/Dielectric (DMD) structures based on Aluminum-doped Zinc Oxide (AZO) as the dielectric with an ultra-thin silver aluminum (Ag:Al) metallic interlayer. The multilayer structures were deposited by magnetron sputtering, which offers an industrial advantage and superior reliability over thermally evaporated DMDs. Finally, we tested the optimized DMD structures as a front contact for n-type silicon solar cells by introducing a hole-selective vanadium pentoxide (V2O5) dielectric layer.

Amorphous Silicon p-i-nphotodiodes were obtained by PECVD in a reactor with a rotating substrate holder. Reverse currents as low as 5 × 10−11 A/cm2 at a bias of − 2V were measured using a guard ring electrode to minimize lateral edge currents. The devices were degraded by a Xenon flash lamp in open circuit conditions. The kinetics of the degradation process was evaluated by studying the long time dark current transient under reverse bias.

Dielectric/metal/dielectric structures based on vanadium pentoxide with a thin silver interlayer have been optimized to replace traditional transparent electrodes. As would be expected, there is a tradeoff in the metal thickness to achieve high transparency and low sheet resistance simultaneously. It has been demonstrated that an ultra-thin gold seed prevents the tendency of silver to form clusters. This wetting effect reduces the metal thickness needed to form a continuous film, which leads to a higher averaged transmittance and very low sheet resistance. On the other hand, vanadium pentoxide on silicon forms a high-quality hole-selective contact. Thus, these structures can be used as an all-in-one transparent electrode and selective contact for a new kind of heterojunction solar cells. This concept has been proved in a 13.3% efficient solar cell fabricated on n-type silicon wafers. Besides being dopant-free, the complete fabrication route did not require any sputtered transparent electrode.

In a previous paper, we reported that thin films of ZnO:Al [aluminum-zinc oxide (AZO)] deposited after achieving a very low base pressure [from 4.0×10–7 Torr (5.6×10–5 Pa) to 5.7×10–7 Torr (7.6×10–5 Pa)] result dark yellow in color and are resistive. These are undesirable characteristics for the application of AZO thin films as front electrodes in solar cells. However, given the increasingly tendency in the acquisition of equipment that allow us to reach excellent vacuum levels, it is necessary to find the deposition conditions that lead to an improving of transmittance without greatly impacting the electrical properties of materials deposited after achieving these levels of vacuum. In this way, the present work is focused on AZO thin films deposited after achieving a very low base pressure value: 4.2×10–7 Torr (5.6×10–5 Pa). For this, we studied the effect of the variation of the oxygen volume percent in the argon/oxygen mixture (by maintaining the deposition pressure constant) and the effect of deposition pressure with only argon gas on the main properties of AZO thin films. The depositions were done at room temperature on glass substrates by direct-current magnetron sputtering with a power of 120 W (corresponding to a power density of 2.63 W/cm2). As results, we found that the variation of deposition pressure with only argon gas is a good option for the control of optical and electrical properties, since the addition of oxygen, although improves transmittance, greatly impacts on the electrical properties. Furthermore, an interesting correlation was found between the optical and electrical properties and the chemical composition of the AZO films, the latter depending on the argon pressure (for this, a careful X-ray photoelectron spectroscopy analysis was performed). Also, the inverse relationship between crystallinity and deposition rate was confirmed, in which deposition rate inversely depends on argon pressure.

The effect that the base pressure achieved prior to deposition has upon the electrical, optical, structural, and chemical properties of ZnO:Al [aluminum-zinc oxide (AZO)] thin films was studied. The thin films were obtained at room temperature on glass substrates by direct-current magnetron sputtering with a power of 120 W (corresponding to a power density of 2.63 W/cm2) and a total deposition pressure of 4.0 mTorr (0.53 Pa) with only argon gas. It was observed that all AZO characteristics and properties varied with base pressure without following a simple tendency, although some correlation was found between the crystallinity and deposition rate, with the latter depending directly upon the achieved base pressure. However, three distinct vacuum zones were identified, each of which produced AZO thin films that exhibited similar characteristics and properties. Among the base pressures studied, the lower base pressure zone [best vacuum level: under ∼5.7 × 10−7 Torr (∼7.6 × 10−5 Pa); dark yellow zone] produced dark yellow AZO thin films with moderate transmittance, but the highest resistivity and the worst stability to the environment. The middle base pressure zone [∼5.7 × 10−7 Torr (∼7.6 × 10−5 Pa) to ∼7.3 × 10−7 Torr (∼9.7 × 10−5 Pa); opaque zone] produced dark gray or brown AZO thin films with good conductivity, but that exhibited the worst transmittance. The higher base pressure zone [bad vacuum level: higher than ∼7.3 × 10−7 Torr (∼9.7 × 10−5 Pa); transparent zone] produced highly transparent AZO thin films with moderate resistivity. The best AZO thin film was deposited after achieving this transparent base pressure zone, but specifically in the transparent–opaque border [∼7.35 × 10−7 Torr (∼9.80 × 10−5 Pa)]. It was found that the films deposited near this transparent–opaque border within a wide margin in the direction of the transparent zone (to higher base pressure) exhibited the best transmittance, conductivity, and stability to the environment. The use of this moderate vacuum level is therefore recommended for the deposition of AZO thin films.

In this work, 50-nm thick Al2O3 thin films were deposited at room temperature by magnetron sputtering from an Al2O3 ceramic target at different RF power and argon pressure values. The sputtering technique could be preferred to conventional atomic layer deposition for an industrial application, owing to its simplicity, availability, and higher deposition rate. The resulting thin films were characterized by UV/Vis/NIR spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The deposited Al2O3 material was always highly transparent and amorphous in nature. It was found that the O/Al ratio is higher when the Al2O3 layer is deposited at lower RF power or higher argon pressure. Also, some argon incorporation into the films was observed at low deposition pressure. On the other hand, the performance of the previously characterized Al2O3 thin films in the passivation of 2.25-Ωcm p-type float zone c-Si wafer surfaces was evaluated by the quasi-steady-state photoconductance technique. The best effective carrier lifetime value at one-sun illumination, 0.34ms (corresponding to a surface recombination velocity of 41cm/s), was obtained with the 50-nm Al2O3 deposited at the higher argon pressure studied, 0.67Pa (5.0mTorr), with the lowest RF power studied, 150W (corresponding to a power density of 3.3W/cm2), and after an annealing process, in this case at 350°C for 20min with forming gas. It was assumed that the reduction of the surface passivation quality at higher RF power or lower argon pressure is a consequence of an increased surface damage, and, probably, to a decrease of the O/Al ratio of the Al2O3 passivation material. These assumptions were confirmed with the obtainment of a lifetime of 0.73ms (a surface recombination velocity equal to 19cm/s) with a simple experiment with Al2O3 deposited with progressively varied sputtering conditions started from minimal silicon surface damage conditions: 50W (corresponding to a power density of 1.1W/cm2) and 6.67Pa (50mTorr). Finally, comments about further improvement of the effective lifetime (up to 1.25ms, corresponding to a surface recombination velocity of 11cm/s) with preliminary experiments about the incorporation of an intrinsic hydrogenated amorphous silicon interlayer are included.

Aluminium induced texturing (AIT) method has been used to texture glass substrates to enhance photon absorption in microcrystalline thin film Si solar cells. In this process, a thin Al film is deposited on a glass substrate and a non-uniform redox reaction between the glass and the Al film occurs when they are annealed at high temperature. After etching the reaction products, the resultant glass surface presents a uniform and rough morphology. In this work, three different textures (σrms~85, ~95, ~125nm) have been achieved by tuning the dc sputtering power and over them and over smooth glass, pin microcrystalline silicon solar cells have been fabricated. The cells deposited over the textured substrates showed an efficiency improvement in comparison to the cells deposited over the smooth glass. The best result was given for the glass texture σrms~125nm that led to an average efficiency 2.1% higher than that given by the cell deposited on smooth glass.

Analysis of continuously acquired electrical data of amorphous silicon (a-Si) modules working under ambient conditions enable us to separate the effects of various factors, as e.g, module temperature, sun spectrum, ambient temperature, influencing a-Si module performance. This results in an improved understanding of the seasonal fluctuations of electric characteristics that can be expected in different ambient conditions. For the test site of Orense, seasonal oscillations lead to electrical power variation of 19% (± 9.5%) around an annual average value with a minimum around January and a maximum around mid of July. Variation of the sun spectrum has the highest impact on the outdoor performance of a-Si modules with amplitude corresponding to about 5.5%. The Staebler Wronski effect has a slightly lower influence with amplitude of about 4 %.

This paper reports on work performed in the FP7 European project HELATHIS. It presents the development of a TCO-glass combining a low iron-content substrate with a SnO2-layer with enhanced carrier mobility and therefore higher optical transmission, achieving in average 7.4% single junction module efficiency in TSolar standard production. First results of the further optimization of the fabrication process with this TCO glass indicate an increase of the single junction module efficiency in the future to nearly 8% in the stabilized state. Lightsoaking experiments of modules present very similar stabilized efficiency in the investigated i-layer thickness range between 140 to 260 nm, therefore, decreasing the i-layer thickness permits to increase the factory production capacity from about 55 up to about 75 MW/year. Investigation of the deposition of Al-doped ZnO layers by rf-sputtering from rotatable targets for the back reflector indicates improvement in the Isc and Voc. Implementation of such targets allows to expand the target life time and to reduce the maintenance time in comparison to standard planar targets. Ga-doped ZnO layers, when implemented in the back reflector, present similar cell performance compared with Al-doped ZnO.

Simulation data of the performance of amorphous silicon (a-Si:H) thin film solar cells using the software package Sentaurus TCAD (Synopsis Inc.) are presented. The Sentaurus software is configured with standard theoretical models describing e.g. the density of states in the mobility gap of a-Si:H, generation/recombination statistics, optical data of a-Si:H thin films etc. to calculate illuminated current voltage curves and the respective spectral response for the initial and degraded state of the solar cell. For the selected physical properties of the solar cell the simulation data predicts a maximum of the efficiency for an intrinsic a-Si:H layer thickness between 200-250 nm. Furthermore, a guideline for the optimization of the p-doped layer thickness and the doping concentration is given.

The European project HELATHIS, executed by the five project partners signing this article, is dedicated to the improvement of the efficiency of very large area (5.7m 2 ) silicon thin film photovoltaic (PV) modules. Optical confinement has been identified by the project partners as a major source for efficiency improvement of thin film silicon PV modules. One reason of the performance gap between highly efficient laboratory solar cells and industrial modules is the poorer electrical and optical performance of the industrial TCO-covered front glass substrate (TCO glass) which is investigated in this work. The glass substrate in industrial PV modules is about 3 times thicker than in laboratory solar cells where frequently Asahi U-type TCO glass (about 1 mm thick) is used. Therefore, the glass quality has an important impact on the transmission of industrial TCO glass. Reducing the iron-content in the float glass substrate increases the integrated transmission in the wavelength range from 400-800nm by nearly 2%. The electrical properties, namely the electrical carrier mobility, of the industrial TCO layer of AGC has been increased by about 15% by improving the industrial deposition process, resulting in a thinner TCO layer with higher transmission by maintaining the sheet resistance of about 9-10 /sq. Combining both developments the integrated transmission of industrial TCO glass has been increased by more than 2%. The TCO layer properties of Asahi U-type and standard industrial TCO glass (AGC AN10) have been investigated by SEM, AFM, XRD and ARS, showing that the Asahi U TCO scatters red light more effectively into larger angles.

a-Si:H p-layers doped by trimethylboron (TMB) were obtained by PECVD in a monochamber reactor with a rotating substrate holder. The influence of the substrate temperature (Ts) on the film properties was systematically studied for two different doping gas concentrations. The incorporation of boron, hydrogen and carbon was studied by Secondary Ion Mass Spectrometry (SIMS). Optical properties were determined by means of Photothermal Deplection Spectroscopy (PDS) and optical transmission. Dark conductivity (σd) and activation energy (Eact) were measured electrically. Our results show that Σd has a marked dependence on substrate temperature, although boron atom concentration depends only slightly on Ts. The optical gap for samples obtained at the higher concentration also depends on Ts and its dependence is related to the hydrogen content, as boron content does not change. P-i-n diodes were obtained with the p-layer deposited from TMB.

Polysilicon (poly-Si) thin films have been obtained using hot-wire chemical vapor deposition (HWCVD) from silane-hydrogen mixtures. The films were prepared at low substrate temperatures (down to 200°C) and at very high deposition rates (up to 40 Å/s). They showed good crystalline properties and no amorphous phases were detected. The films can also be efficiently doped by adding diborane or phosphine to gas phase. In this paper, an overview of the properties of the poly-Si films, intrinsic and p and n-doped, deposited at our laboratory by HWCVD is presented and discussed. The properties of the material and the features of the deposition technique which are interesting for their application in photovoltaics are emphasized.