M. Colina

Surface passivation and optical characterization of Al2O3/a-SiCx stacks on c-Si substrates

Summary The aim of this work is to study the surface passivation of aluminum oxide/amorphous silicon carbide (Al2O3/a-SiCx) stacks on both p-type and n-type crystalline silicon (c-Si) substrates as well as the optical characterization of these stacks. Al2O3 films of different thicknesses were deposited by thermal atomic layer deposition (ALD) at 200 °C and were complemented with a layer of a-SiCx deposited by plasma-enhanced chemical vapor deposition (PECVD) to form anti-reflection coating (ARC) stacks with a total thickness of 75 nm. A comparative study has been carried out on polished and randomly textured wafers. We have experimentally determined the optimum thickness of the stack for photovoltaic applications by minimizing the reflection losses over a wide wavelength range (300–1200 nm) without compromising the outstanding passivation properties of the Al2O3 films. The upper limit of the surface recombination velocity (S eff,max) was evaluated at a carrier injection level corresponding to 1-sun illumination, which led to values below 10 cm/s. Reflectance values below 2% were measured on textured samples over the wavelength range of 450–1000 nm.

Sub-Bandgap External Quantum Efficiency in Ti Implanted Si Heterojunction with Intrinsic Thin Layer Cells

In this work we present the manufacturing processes and results obtained from the characterization of heterojunction with intrinsic thin layer solar cells that include a heavily Ti ion implanted Si absorbing layer. The cells exhibit external circuit photocurrent at photon energies well below the Si bandgap. We discuss the origin of this below-bandgap photocurrent and the modifications in the hydrogenated amorphous intrinsic Si layer thickness to increase the open-circuit voltage.

Crystalline silicon solar cells beyond 20% efficiency

This paper describes a fabrication process to obtain high efficiency c-Si cells (> 20%) based on the Laser Fired Contact Passivated Emitter Rear Cell (LFC-PERC) concept. Photovoltaic efficiencies beyond 20% have been achieved using thermal SiO2 as a rear passivation layer on 2 cm × 2 cm solar cells with 0.45 Qcm Fz c-Si substrates. Efficiencies up to 22% are expected for material resistivities in the 0.4–5 Ωcm using an optimized rear contact grid.

Low Surface Recombination in Silicon-Heterojunction Solar Cells With Rear Laser-Fired Contacts From Aluminum Foils

In this study, an approach to create laser-fired contacts from aluminum foils is studied on p-type silicon-heterojunction solar cells. This alternative approach consists of the use of aluminum foils instead of evaporated layers as a metal source and rear electrode for the laser-firing process. A q-switched infrared laser (1064 nm) was employed to create the local point contacts. Quasi-steady-state photoconductance measurements evidenced a limited degradation in the surface passivation quality during the laser-firing process. Heterojunction solar cells fabricated with these rear contacts reached a best conversion efficiency of 18% with a remarkable open-circuit voltage of 690 mV. These values were very close to those of reference devices fabricated with evaporated aluminum layers. This result suggests a similar effect on the rear surface passivation by both contact strategies. However, external quantum efficiency curves revealed a better response from devices with a rear aluminum foil in the near infrared. Optical measurements indicate that this effect can be related to a higher internal reflection at the back surface. Consequently, laser-fired contacts from aluminum foils appear to be a fast and convenient solution for the rear contact of high-efficiency silicon solar cells.

Progress in silicon heterojunction solar cell fabrication with rear laser-fired contacts

Silicon Heterojunction (SHJ) solar cells are one of the most promising alternatives for high efficiency industrially feasible solar cells. The structure of these devices is based on hydrogenated amorphous silicon (a-Si:H) layers deposited at low temperature on crystalline silicon (c-Si) substrates. This fabrication process reduces the thermal stress on the substrate and is compatible with thinner wafers. In this work, we present our recent progress in the fabrication of SHJ solar cells on p-type c-Si wafers. The deposition conditions of hydrogenated amorphous silicon-carbon (a-SiCx:H) layers obtained by Plasma Enhanced Chemical Vapor Deposition (PECVD) are optimized. We have also applied a novel laser-firing process to contact the rear side of the fabricated devices. In this way, solar cells with point contacts through rear passivating layers can be fabricated without any photolithographic step. Recently, our group has obtained a remarkable conversion efficiency of 17.2 % on 1 cm2 SHJ solar cells fabricated in a fully low temperature process.

Large performance improvement in Cu2ZnSnSe4 based solar cells by surface engineering with a nanometric Ge layer

This work presents a radically new approach based on the application of very small Ge quantities on the CZTSe surface (from 1 nm to 25 nm thick Ge layers), allowing for a liquid assisted improved crystallization due to the formation of a Ge-Se (Se-rich) liquid phase. This modification improves the charge transport properties at this interface and consequently the devices voltage and electro-optical properties in general. Using TEM and TOF-SIMS we demonstrate that Ge is barely incorporated into the absorber; nevertheless we observe a drastic increase of the VOC of the solar cells (from 405 mV for the reference to 470 mV for the best Ge modified one). This in turn has a large impact on the performance, increasing it from 7.0% (reference) to 10.1% (Ge modified), which sets a new record efficiency for a Ge containing kesterite and a VOC among the highest obtained for Se-based kesterite solar cells. First characterizations indicate that this is related to an improved grain growth assisted via Ge-Se liquid phases, the minimization of Sn-reduced species and the formation of Ge-O nano-clusters. Our approach not only allows to go towards high efficiency concepts and to contribute to solve the voltage deficit problems in kesterites, but also opens new perspectives for the possible band-gap engineering of kesterite devices with very low Ge concentrations.

Influence of wavelength on laser doping and laser-fired contact processes for c-Si solar cells

This work investigates the influence of the laser wavelength on laser doping (LD) and laser-fired contact (LFC) formation in solar cell structures. We compare the results obtained using the three first harmonics (corresponding to wavelengths of 1064 nm, 532 nm and 355 nm) of fully commercial solid state laser sources with pulse width in the ns range. The discussion is based on the impact on the morphology and electrical characteristics of test structures. In the case of LFC the study includes the influence of different passivation layers and the assessment of the process quality through electrical resistance measurements of an aluminium single LFC point for the different wavelengths. Values for the normalized LFC resistance far below 1.0 mΩcm2 have been obtained, with better results at shorter wavelengths. To assess the influence of the laser wavelength on LD we have created n+ regions into p-type c-Si wafers, using a dry LD approach to define punctual emitters. J-V characteristics show exponential trends at mid-injection for a broad parametric window in all wavelengths, with local ideality factors well below 1.5. In both processes the best results have been obtained using green (532 nm) and, specially, UV (355 nm). This indicates that to minimize the thermal damage in the material is a clear requisite to obtain the best electrical performance, thus indicating that UV laser shows better potential to be used in high efficiency solar cells.

Study of the Surface Recombination Velocity for Ultraviolet and Visible Laser-Fired Contacts Applied to Silicon Heterojunction Solar Cells

In this study, we investigate the effect of the laser-firing process on the back surface passivation of p-type silicon heterojunction solar cells. For that purpose, two different nanosecond laser sources radiating at ultraviolet (UV) (355 nm) and visible (532 nm) wavelengths are employed. First, we optimize the laser-firing process in terms of the electrical resistance of locally diffused point contacts. Specific contact resistance values as low as 0.91 and 0.57 mΩ·cm2 are achieved for the visible and ultraviolet laser sources, respectively. In addition, the impact of the laser-firing process on the rear surface passivation is studied by analyzing the internal-quantum-efficiency curves of complete devices. Low surface recombination velocities in the range of 300 cm/s are obtained for the ultraviolet laser with a 1% fraction of contacted area. This value increases to about 700 cm/s for the visible laser, which indicates a significantly higher recombination at the contacted area. The best heterojunction solar cells with rear laser-fired contacts are obtained for the ultraviolet laser and reached a 17.5% conversion efficiency.

Optical characterization of the heat-affected zone in laser patterning of thin film a-Si:H

In this paper we present an original approach to estimate the heat affected zone in laser scribing processes for photovoltaic applications. We used high resolution IR-VIS Fourier transform spectrometry at micro-scale level for measuring the refractive index variations at different distances from the scribed line, and discussing then the results obtained for a-Si:H layers irradiated in different conditions that reproduce standard interconnection parameters. In order to properly assess the induced damage by the laser process, these results are compared with measurements of the crystalline state of the material using micro-Raman techniques. Additionally, the authors give details about how this technique could be used to feedback the laser process parametrization in monolithic interconnection of thin film photovoltaic devices based on a-Si:H.

Influence of Amorphous Silicon Carbide Intermediate Layer in the Back-Contact Structure of Cu 2 ZnSnSe 4 Solar Cells

Cu2 ZnSn(S1-ySey )4 (CZTS) thin-film solar cells have been qualified as potential competitors of the more established CIGS ones. One of the more important handicaps of CZTS solar cells is the open-circuit voltage deficit. The rear-contact/absorber interface is known to be very sensitive to the formation of secondary phases, which are detrimental for the electrical behavior of photovoltaic devices. The addition of intermediate layers to favor the formation of an adequate interface has been repeatedly tested. In this work, an amorphous silicon carbide (a-SiC) layer is added to explore its influence on the material properties and electrical performance of CZTSe solar cells. According to scanning electron microscopy (SEM) analysis, when the a-SiC layer thickness is increased, bigger grains along the absorber are obtained. Additionally, a lower [VCu + ZnCu ] defect cluster density is also deduced from the analysis of Raman measurements. Both results indicate a favorable impact of a-SiC films on the material quality of the absorber. Fabricated solar cells show an enhancement of 0.9% abs. of efficiency compared to identical solar cells without a-SiC layers used as a reference. This increase is mainly related to an improvement of open-circuit voltage and fill factor (FF) when the proposed intermediate layer is included.