Novel architectures of transparent electrodes incorporating graphene into its design are explored to improve the silicon-heterojunction-cell technology. Two configurations are studied based on the place where the atomic graphene is. The main results obtained reveal the dramatic dependence of atomic graphene position on the transparent-electrode properties. In addition, the electrode performance is also affected by the transparent conductive oxide material chosen in the designed structure. Specifically, the incorporation of graphene into an electrode shows a significant reduction of its sheet resistance (∼55 Ω/sq.) and the weighted reflectance of whole structure down up to ∼10% when combined with a transparent conductive oxide such as indium tin oxide. Such electrode shows an average conductance value as high as 13.12 mS. These data undergone so far clearly suggest the possibility to noticeably improve transparent electrode technology when graphene is incorporated, and therefore to further enhance the contact technology for solar cells.

Many semiconductor technologies require the patterning of films to create features not easily achieved during growth or deposition. In the case of transparent conductive oxides (TCOs), this is typically realized through direct laser-scribing. Although there are models conceived to predict the depth of a scribe, the necessary parameters to obtain a given depth are usually found by trial and error. This is mostly due to the models usually being highly elaborated and dependent on difficult to measure variables. In this paper we introduce a method for predicting the ablation depth in direct laser-scribing processes based on laser-processing parameters and convenient properties like the ablation threshold fluence and the laser penetration depth. In order to apply this method though, the materials must comply with two conditions: a) the material does not develop incubation with successive pulses and b) the ablation depth obtained at any position by a single pulse is determined by the fluence reaching that point. We present experimental data using nanosecond sources and a wavelength of 355 nm for TCOs Indium doped Tin Oxide and Aluminum doped Zinc Oxide that endorse the proposed method as a tool for predicting the ablated depth in laser scribes. (C) 2017 Published by Elsevier B.V.

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.

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.

Back reflectors based on Aluminum and Molybdenum as the reflecting materials and ZnO:Al as the transparent conductive oxide, were deposited at low temperature on polyethylene naphthalate by magnetron sputtering. The optimization of the layer thickness of the constituent materials and the final texture of the structure was performed in order to achieve structures with appropriate properties to be applied to amorphous silicon solar cells. In this sense, average total reflectance values of 71% in the wavelength range of 400-800 nm were obtained for the layer stack of 80 nm-thick ZnO:Al layer on 235 nm-thick Aluminum deposited directly on the flexible substrate. However, this structure showed a poor adherence onto the substrate. This issue was improved using a thin Molybdenum layer between the metal and the substrate, maintaining similar optical properties. On the other hand, textured back reflectors were obtained etching the transparent conductive oxide with NH4Cl aqueous solution. After the etching process, the highest enhancement was achieved for the sample that used an Mo/Al bilayer as reflective metal. In this case, a roughness of around 29 nm and a haze factor of 25% at 600 nm were achieved. (c) 2013 Elsevier Ltd. All rights reserved.

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 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.

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.

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.

Laser scribing with nanosecond (ns) diode pumped solid-state laser sources is the industry standard in the fabrication of silicon-based thin-film photovoltaic (TFPV) modules. Reducing the interconnection area is one of the on-going challenges for the next generation of TFPV modules. In this regard, replacing ns laser sources by picosecond (ps) laser sources is one of the logical steps. Ps-laser pulses reduce the heat-affected zones compared to ns pulses, and thus enable a reduction of the interconnection zone. This work describes the substrate-side ablation of fluorine-doped tin oxide, amorphous silicon (a-Si:H) and a-Si:H with an aluminum layer on top, using a 10-ps laser with a wavelength of 1064 nm. The investigation of single-pulse ablation and trench scribing demonstrates that the complete monolithic interconnection can be achieved at the fundamental wavelength. In addition, the evaluation of the ablation efficiency shows that the best trench quality is achieved at the efficiency maximum.

In this work we present a detailed study of the wavelength influence in pulsed laser annealing of amorphous silicon thin films, comparing the results for material modification at different fluence regimes in the three fundamental harmonics of standard DPSS (diode pumped solid state) nanosecond laser sources, UV (355 nm), visible (532 nm) and IR (1064 nm).The crystalline fraction (% crystalline silicon) profiles resulted from irradiation of amorphous silicon thin film samples are characterized with MicroRaman techniques. A finite element numerical model (FEM) is developed in COMSOL to simulate the process. The crystalline fraction results and the local temperature evolution in the irradiated area are presented and analyzed in order to establish relevant correlation between theoretical and experimental results.For UV (355 nm) and visible (532 nm) wavelengths, the results of the numerical model are presented together with the experimental results, proving that the process can be easily predicted with an essentially physical model based on heat transport at different wavelengths and fluence regimes. The numerical model helps to establish the optimal operation fluence regime for the annealing process. (C) 2013 Elsevier B.V. All rights reserved.

The formation of a pyramidal structure on the surface of < 100 >-oriented monocrystalline-silicon wafers is an effective and well known method to reduce reflection losses from the front surface of both silicon solar cells and silicon-heterojunction solar cells (SHJs). The consequence of this texturisation is an important optical gain, with a subsequent increase of the short-circuit current density (J(sc)) and thus of the conversion efficiency of the devices. On the, other hand, silicon-heterojunction solar cells are critically affected by the surface quality of the c-Si substrates, so the right combination of optimum texturisation- and cleaning steps previous to emitter (a-Si:H) deposition are indispensable in the fabrication process. The main goal of this work has been to perform a systematic and comprehensive analysis aimed at optimising the texturisation process based on the use of alkali solutions of NaOH with de-ionised water (DIW) and isopropyl alcohol (IPA) in different types of monocrystalline-silicon wafers for silicon-heterojunction solar-cell (a-Si:H/c-Si) applications. Three types of < 100 > silicon substrates have been used: polished float-zone (FZ) wafers and rough- (as-cut) and polished Czochralski (CZ) wafers. The texturisation process has been evaluated from images obtained by Scanning Electron Microscopy (SEM) and from hemispherical-reflectance spectra. Different etching times, temperatures and NaOH concentrations of the solutions as well as cleaning treatments of the wafers prior to the texturisation process have been analysed. Results show different conditions of the optimum texturisation process for each type of silicon wafers. An effective texturisation of FZ and CZ substrates has been achieved. Finally, SHJ solar cells have been obtained from FZ and CZ silicon wafers textured by the chemical processes optimised in this work. (C) 2011 Elsevier Ltd. All rights reserved.