Hybrid transparent contacts based on combinations of a transparent conductive oxide and a few graphene monolayers were developed in order to evaluate their optical and electrical performance with the main aim to use them as front contacts in optoelectronic devices. The assessment of the most suitable strategies for their fabrication was performed by testing different protocols addressing such issues as the protection of the device structure underneath, the limitation of sample temperature during the graphene-monolayer transfer process and the determination of the most suitable stacking structure. Suitable metal ohmic electrodes were also evaluated. Among a number of options tested, the metal contact based on Ti + Ag showed the highest reproducibility and the lowest contact resistivity. Finally, with the objective of extracting the current generated from optoelectronic devices to the output pins of an external package, focusing on a near future commercial application, the electrical properties of the connections made with an ultrasonic bonding machine (sonic welding) between the optimized Ti + Ag metal contacts and Al or Au micro-wires were also evaluated. All these results have an enormous potential as hybrid electrodes based on graphene to be used in novel designs of a future generation of optoelectronic devices, such as solar cells.

The present work addresses the exhaustive study of the surfaces of multicrystalline silicon wafers after being subjected to a texturization process for silicon heterojunction solar cell applications. The investigations described include the effect that the time of isotropic etching based on combinations of hydrofluoric and nitric acids has on the reflectance, the morphology of the surfaces and the surface recombination through the evolution of the implicit open-circuit voltage. The influence of previous treatments and the elimination of porous silicon or silicon oxide formed on wafer surfaces as a consequence of these texturization processes are also addressed. Textured multicrystalline silicon wafer surfaces with a good uniformity and low weighted hemispherical reflectances (23–24%) have been achieved with short etching times. These texturization processes have also been tested on upgraded metallurgical silicon wafers, resulting in weighted hemispherical reflectance values around 23%, but at the cost of the appearance of important surface defects.

New architectures of transparent conductive electrodes (TCEs) incorporating graphene monolayers in different configurations have been explored with the aim to improve the performance of silicon-heterojunction (SHJ) cell front transparent contacts. In SHJ technology, front electrodes play an important additional role as anti-reflectance (AR) coatings. In this work, different transparent-conductive-oxide (TCO) thin films have been combined with graphene monolayers in different configurations, yielding advanced transparent electrodes specifically designed to minimize surface reflection over a wide range of wavelengths and angles of incidence and to improve electrical performance. A preliminary analysis reveals a strong dependence of the optoelectronic properties of the TCEs on (i) the order in which the different thin films are deposited or the graphene is transferred and (ii) the specific TCO material used. The results shows a clear electrical improvement when three graphene monolayers are placed on top on 80-nm-thick ITO thin film. This optimum TCE presents sheet resistances as low as 55 Ω/sq and an average conductance as high as 13.12 mS. In addition, the spectral reflectance of this TCE also shows an important reduction in its weighted reflectance value of 2–3%. Hence, the work undergone so far clearly suggests the possibility to noticeably improve transparent electrodes with this approach and therefore to further enhance silicon-heterojunction cell performance. These results achieved so far clearly open the possibility to noticeably improve TCEs and therefore to further enhance SHJ contact-technology performance.

10-um-thick non-hydrogenated amorphous-silicon (a-Si) films were deposited at relatively high rates (_>10 A/s) by radio-frequency magnetron sputtering (RFMS) on different large-area buffer-layer-coated glass substrates at deposition temperatures ranging from room temperature (RT) to 300oC. These amorphous samples were subsequently crystallized by means of a continuous-wave diode laser, looking for conditions to reach liquid-phase crystallization. The influence of deposition conditions on the quality of the final micro-crystalline silicon films has been studied.

We report a hybrid solar cell based on single walled carbon nanotubes (SWNTs) interfaced with amorphous silicon (a-Si). The high quality carbon nanotube network was dry transferred onto intrinsic a-Si forming Schottky junction for metallic SWNT bundles and heterojunctions for semiconducting SWNT bundles. The nanotube chemical doping and a-Si surface treatment minimized the hysteresis effect in current–voltage characteristics allowing an increase in the conversion efficiency to 1.5% under an air mass 1.5 solar spectrum simulator. We demonstrated that the thin SWNT film is able to replace a simultaneously p-doped a-Si layer and transparent conductive electrode in conventional amorphous silicon thin film photovoltaics.

An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. In the present work, thin-film amorphous- silicon samples were irradiated with a continuous-wave green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. Micro-Raman spectroscopy was used to study the crystallization induced on the irradiated surface. Both laser peak power density and irradiation time are identified as key variables in the crystallization process, but within the parametric window considered, the enhancement of the crystalline factor, is more sensitive to the power density than to the irradiation time. The optimum parameters are then used for crystallizing a large sample area by means of overlapped laser scanned lines. Ellipsometric data experimentally show that the whole volume of a micron-thick sample is crystallized. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).

espanolLa situacion del mercado energetico en Espana es similar a la de otros paises europeos. El modelo energetico esta en fase de transicion. El pais tiene optimos recursos energeticos renovables y un nivel tecnologico excelente en energias renovables y especificamente en energia solar fotovoltaica (ESF). Espana fue el pais con mayor potencia fotovoltaica instalada en el mundo en 2008, debido a la combinacion de esos factores con una legislacion que habria necesitado un ajuste muy preciso para ser mucho mas beneficiosa. Despues de esto, diferentes factores causaron el colapso del sector fotovoltaico espanol, forzando a las empresas del mismo a concentrar su actividad en el mercado internacional. A pesar de esta situacion transitoria, la energia solar fotovoltaica representa a medio y largo plazo una impresionante oportunidad de negocio en Espana EnglishThe situation of the energy market in Spain is similar to that in other European countries. The energy model is in a transition phase. The country has optimum renewable-energy resources and an excellent technological level in renewables and specifically in photovoltaics (PV). Spain was the country with the highest amount of installed photovoltaic power worldwide in 2008, owing to the combination of these factors and a legislation that should have needed an accurate tuning in order to be much more beneficial. After this, a number of factors caused the collapse of the Spanish PV sector, thus forcing the companies to concentrate their activity on the international market. In spite of this interim situation, photovoltaics represent in the midand long term a huge business opportunity in Spain.

This work approaches the problem of increasing the growth rate of device quality a-Si:H by using the simplest case, a standard RF-PECVD system and pure silane as feedstock gas. Starting from plasma conditions which provided a high-quality material at very low deposition rates, the silane flow and the applied power (RFP) were proportionally increased. As a result, the growth rate showed an almost linear increase with the RFP. An exhaustive analysis of the material obtained revealed the existence of a wide window in which the structural/optoelectronic properties remain unchanged. Within this window, a-Si:H p-i-n solar cells were fabricated in order to verify the applicability of the procedure proposed to the development of device structures. The initial results showed an excellent behaviour of the solar cells at higher growth rates, without any relevant detriment in the collection capability and fill factor. Thus, the constant power-to-flow ratio is presented as an easy and reliable method to reduce the deposition time which, additionally, could be applied to any variant of plasma CVD system.

An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. Laser energy is used to heat the a-Si thin film to change the microstructure to poly-Si.Thin film samples of a-Si were irradiated with a CW-green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times.These parameters are identified as key variables in the crystallization process. The power threshold for crystallization is reduced as the irradiation time is increased. When this threshold is reached the crystalline fraction increases lineally with power for each irradiation time.The experimental results are analysed with the aid of a numerical thermal model and the presence of two crystallization mechanisms are observed: one due to melting and the other due to solid phase transformation.

This work studies the capability of using a nanosecond laser source to texture ZnO:Al (AZO) thin films deposited by magnetron sputtering onto a Corning glass 7059. A Q-switched laser working at 355 nm is used to pattern the surface of the AZO thin film by performing an array of parallel and perpendicular laser lines. This method leads to the creation of a micro-texturing similar to a grating on the AZO surface. Subsequently, a-Si p-i-n solar cells were deposited onto substrates with different AZO morphologies. Current-voltage (J-V) characteristics of the resulting devices were measured under 100-mW/cm2 AM1.5G illumination conditions at 25 °C.

In this work we study the contact resistance introduced through the P2 laser patterning of a-Si:H PV. The process is evaluated for different transparent conductive oxides (TCO) of interest and significant irradiation condition. In particular we study the ablation process for the a-Si structure deposited in AZO and SnO2. Backscribing and direct writing configuration for ns and ps pulse duration, using visible wavelengths (532nm) and UV (355nm) were investigated. A comparison of the contact resistance Rc and the open circuit resistance Roc for different scribing procedures is presented. The results obtained from the morphological and electrical studies of the P2 scribes are used to assess the quality of the formed contact. Non- optimized scribes can lead to very low quality contacts with the formation of Schottky barriers or non-ohmic contacts leading to final devices presenting anomalous JV characteristic and low fill factors.

The formation of pyramidal structures by anisotropic etching of 〈100〉-oriented monocrystalline silicon wafer surfaces is an effective method to reduce reflection losses originating on the front side of conventional silicon solar cells and silicon-heterojunction (SHJ) solar cells. One of the most common methods of texturization used in the solar-cell industry is based on aqueous solutions of NaOH or KOH and isopropyl alcohol (IPA). However, IPA is toxic and relatively expensive, so efforts are being made to replace it. Among the potential alternatives, solutions based on Na2CO3 and Na2CO3/NaHCO3 mixtures have been proposed. In the present study, solutions of Na2CO3 and Na2CO3/NaHCO3 mixtures were prepared in order to form pyramidal structures on silicon wafer surfaces. It was not possible to obtain uniform and completely textured surfaces by using aqueous solutions consisting only of Na2CO3. NaHCO3 must be added in order to achieve uniform textured surfaces with low hemispherical reflectance suitable for SHJ solar-cell applications. Textured surfaces with good uniformity and low average hemispherical reflectance (15.4%) were prepared from 〈100〉 silicon substrates with relatively low etching times (25min). Good surface passivation (lifetime >600μs and implicit open-circuit voltage of 690±10mV) on these p-type textured wafers were achieved.

The true cause of the low fill factor (FF) observed in amorphous silicon (a-Si:H) p–i–n solar cells deposited onto ZnO:Al (AZO) remains unclear. Some authors explain this phenomenon as a consequence of an AZO/p-a-SiC:H blocking contact, while others sustain that the growth of a more defective a-Si:H p–i interface is actually the cause. This work tries to find out evidences that support either of these hypotheses by analyzing a series of a-Si:H devices prepared on Asahi-U™ type glass/SnO2:F (FTO) coated with different AZO thickness. A detailed study of the diode response at forward bias showed the existence of a light-dependent AZO/p barrier contact, which provided an answer to the sudden worsening of the FF in the transition from bare FTO to AZO coated FTO. On the other hand, the AZO thickness dependence of the device collection voltage indicated a progressive increase of the defect density in the a-Si:H structure, which led to a reduction of the open circuit voltage. These results suggested that both mechanisms could simultaneously affect the device FF. In fact, we propose that both approaches could be valid and they do not have to exclude each other. The uncertainty in the control of AZO/p interface properties could be responsible of the apparent controversy.

Texturing of glass substrate is an interesting alternative to enhance light trapping in thin silicon solar cells instead of using conventional naturally textured transparent conducting oxides. In this work, aluminium induced texturing method is used to obtain borofloat glass substrates with σrms values in the range 70 – 90 nm. Al-doped ZnO is deposited by sputtering onto textured glass to provide the front contact for thin film silicon solar cells. Morphology and optical properties of the textured glass substrates, as well as morphology, optical and electrical properties of the ZnO:Al layers deposited on them are described. The application of these textured substrates to a-Si solar cells is also presented. The use of this approach leads to devices with short-circuit currents similar to those obtained with optimized commercial TCO substrates.

Crystallization and grain growth technique of thin film silicon are among the most promising methods for improving efficiency and lowering cost of solar cells. A major advantage of laser crystallization and annealing over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers[l-3].Laser energy is used to heat the amorphous silicon thin film, melting it and changing the microstructure to polycrystalline silicon (poly-Si) as it cools. Depending on the laser density, the vaporization temperature can be reached at the center of the irradiated area. In these cases ablation effects are expected and the annealing process becomes ineffective. The heating process in the a-Si thin film is governed by the general heat transfer equation [4-5]. The two dimensional non-linear heat transfer equation with a moving heat source is solve numerically using the finite element method (FEM), particularly COMSOL Multiphysics [6]. The numerical model help to establish the density and the process speed range needed to assure the melting and crystallization without damage or ablation of the silicon surface. The samples of a-Si obtained by physical vapour deposition were irradiated with a cw-green laser source (Millennia Prime from Newport-Spectra) that delivers up to 15 W of average power. The morphology of the irradiated area was characterized by confocal laser scanning microscopy (Leica DCM3D) and Scanning Electron Microscopy (SEM Hitachi 3000N). The structural properties were studied by micro-Raman spectroscopy (Renishaw, inVia Raman microscope) [7].

In thin film photovoltaic modules, the different solar cells are interconnected monolithically during the production process, which gives a greater control over the size and output characteristics of the finished module. The interconnection is typically achieved through different laser scribing processes made at different production steps. In thin film modules built in the superstrate configuration, the first laser process is the patterning of the transparent front electrode. This paper presents results on the investigation of this first laser scribing process on fluorine-doped tin oxide deposited onto a glass substrate using nanosecond diode-pumped solid-state laser sources. Processes made with two different wavelengths (1064 nm and 355 nm) and executed from the film-side and from the substrate side are compared and evaluated. The quality of the scribes is assessed with confocal and scanning electron microscopy images. In addition, Raman microscopy is used to study the extension of the heat affected zones. While good quality scribes were obtained using both wavelengths and either film-side or substrate-side irradiation, only using 355 nm and substrate-side scribing yielded grooves with no observable heat affected zones. It also needed the lowest values of energy per ablated volume and allowed for the highest processing speeds. As such, substrate side ablation with 355 nm is proposed as the best ablation strategy. (C) 2013 Elsevier B.V. All rights reserved.