Gema López

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.

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.

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.

High efficiency interdigitated back-contact c-Si(p) solar cells

In this work we describe a baseline fabrication process of interdigitated-back-contact c-Si(p) solar cells, which combines conventional diffusion oven stages to define base p+ and emitter n+ regions at the backside, with front surface passivation using atomic layer deposited Al₂O₃ films on textured surfaces with random pyramids. Very low reflectance with outstanding surface recombination velocity values around 3 cm/s are achieved in our precursors. Fabricated solar cells reach efficiencies up to 20.3% (AM1.5G 1 kW/m², T=25°C), with short circuit density J_sc, open circuit voltage V_oc and fill factor FF of 40.6 mA/cm², 648 mV and 77.2% respectively.

An IBC solar cell for the UPC CubeSat-1 mission

In this work the fabrication and electrical characterization of interdigitated back contact IBC solar cells is shown. These solar cells have been specifically designed for a CubeSat based satellite under developement at the Universitat Politecnica de Catalunya (UPC). Solar cells incorporate a transparent cover-glass as an extraterrestrial radiation shield. Front surface passivation was achieved using an Al2O3 layer exhibiting surface recombination velocities <; 100 cmls at the final device. Measurements confirm photovoltaic efficiencies ηs-12%, with open circuit voltages Vocs ~650 m V and short circuit current densities Jscs ~25 mA/cm2. A module with 11 IBC solar cells interconnected in series will be integrated in one of the faces of the satellite forming part of the power subsystem. Preliminary results confirm the good electrical performance of the module.

New laser-based approaches to improve the passivation and rear contact quality in high efficiency crystalline silicon solar cells

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

Silicon nitride layers for DopLa-IBC solar cells

In this work, we report on the development of silicon nitride (SiNx) layers to be applied to crystalline silicon high-efficiency solar cells. In particular, our research group has developed the concept of Doped by Laser (DopLa) solar cells where the highly doped regions are based on laser processed dielectric films. This concept has been recently applied to Interdigitated Back-Contacted (IBC) solar cells where some limitations associated to silicon carbide films arisen. The objective is to replace these films by SiNx layers on both the rear and front surface. Focusing on the rear surface, we have determined a SiNx layer that could be etched by conventional wet processes avoiding the plasma etching used in our current devices that degrades their surface passivation. On the other hand, for the front surface we found deposition conditions of SiNx films more transparent than their SiCx counterparts and that could stand the subsequent cleaning procedures included in the solar cell fabrication process. A gain in short-circuit current of 0.1 mA/cm2 is expected.

Cost-effective cleaning solutions based on H 2 O/NH 3 /H 2 O 2 mixtures for ALD Al 2 O 3 passivated IBC c-Si solar cells

In this work we study cost-effective cleaning solutions applied to interdigitated back-contacted solar cells (IBC), which are passivated by means of atomic layer deposited Al<inf>2</inf>O<inf>3</inf> films. The cleaning baths must guarantee very clean surfaces as well as relatively low etching Al<inf>2</inf>O<inf>3</inf> rates to avoid excessive undercutting at the edges of strip-like regions. We compare the standard high-cost cleaning procedure used in the microelectronic industry (RCA1/2) with simpler cleaning baths based on H<inf>2</inf>O/NH<inf>3</inf>/H<inf>2</inf>O<inf>2</inf> mixtures considering different temperatures. The best option is the RCA1/2 sequence yielding surface recombination velocities below 4 cm/s but with a total Al<inf>2</inf>O<inf>3</inf> etch around 500 nm after the cleaning stage. Nevertheless very simple and less aggressive cleaning baths performed at only 45 °C obtain a relatively good surface passivation quality, achieving S<inf>eff</inf> values of 20 ± 5 cm/s reducing the under etch to only 80 nm.

Interdigitated back contacted c-Si(p) solar cells with photovoltaic efficiencies beyond 22%

In this work we show a baseline fabrication process of interdigitated back contacted IBC c-Si(p) solar cells, which combines conventional diffusion oven stages to define base p+ and emitter n+/n++ regions at the back side, with outstanding front surface passivation using atomic layer deposited Al2O3 films over random pyramids surfaces. Cells include a selective phosphorous n++ emitter in order to improve contact resistance and simultaneously reduce recombination current density. Fabricated devices reach efficiencies up to 22.2% (AM1.5G 1 kW/m2, T=25°C). This value is so far the highest efficiency reported by any Spanish institution using silicon substrates. 3D simulations envisage efficiencies beyond 24% introducing little changes in the fabrication process.

Boron diffused emitters passivated with Al 2 O 3 films

In this work we study the fabrication and characterization of boron diffused emitters using FZ c-Si(n) substrates. Emitter surface was passivated with Al2O3(25 nm thick) layers deposited by thermal atomic layer deposition ALD technique. This study covers a broad emitter sheet resistance Rsh range from 20 to 250 Ω/sq using both polished and textured wafers. Emitter electrical quality was tested by means of lifetime measurements using quasi-stationary photoconductance QSS-PC method. Dark saturation emitter current densities Joes were extracted from lifetime measurements resulting in Joes values ranging from 10 to 150 fA/cm2 depending on Rsh. These results are in the-state-of-the-art in boron emitter passivation.