Jordi Andreu

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.

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.

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.

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 %.

Simulation data of the performance of amorphous hydrogenated 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 solar cell. The study of the temperature dependence of the electrical parameters of solar cell is realized. The simulations results were compared with experimental data measured in T-Solar laboratory obtaining a good coincidence.

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.