Nines Photovoltaics

In this article, we present an optimization of the emitter diffusion for nanotextured p-type monocrystalline silicon solar cells using atmospheric pressure dry etching (ADE) in conjunction with a post-ADE short acidic etch in a passivated emitter and rear cell (PERC) architecture. The optimization of the phosphorus oxychloride diffusion process was realized by first investigating the emitter sheet resistance and emitter recombination current density to achieve improved electrical properties and cell performances at a later stage. The optimization of the diffusion process enables an excellent homogeneity for emitter sheet resistance of 105 Ω/sq with minimized standard deviation of 3%, a decreased emitter saturation current density of ∼120 fA/cm2, a peak doping concentration of 2.2 × 1020 cm−3 and depth of the highly doped surface region of 20 nm, still in the range that is required for good contact formation. By step optimization of the emitter formation of ADE textured PERC solar cells, an efficiency improvement of 0.6% $_\text{abs}$ could be reached leading to best conversion efficiency of 20.9%.

Single-sided etching (SSE) of a-Si/poly-Si is typically considered a challenge for realizing a cost–efficient TOPCon production sequence, as there is a certain degree of unwanted wrap–around for various poly-Si deposition technologies such as LPCVD, PECVD and APCVD. To date, alkaline or acidic wet-chemical solutions in either inline or batch configurations are used for this purpose. In this work, we propose an alternative SSE process using an inline dry etching tool, which applies molecular fluorine (F2) as the etching gas under atmospheric pressure conditions. The developed etching process performs complete etching of both as–deposited amorphous silicon (a–Si) and annealed polycrystalline silicon (poly–Si) layers, either intrinsic or doped, and with measured etch rates of > 3 μm/min at 10% F2 concentration allows etching of typical layer thickness of 200 nm in just a few seconds. The etching process is also configured to perform excellent edge isolation, while maintaining a low wrap–around etching (drear<500 μm) at the opposing-side. The etching process is successfully transferred to the industrial TOPCon solar cell architecture, yielding high parallel resistances (Sshunt,avg. > 1500 kΩ cm²) and low reverse current density (Jrev,avg<0.8 mA/cm²) measured at a bias voltage of -12 V, and independently certified conversion efficiencies of up to 23.3%.

In this paper, we study the plasma-less etching of crystalline silicon (c-Si) by F2/N2 gas mixture at moderately elevated temperatures. The etching is performed in an inline etching tool, which is specifically developed to lower costs for products needing a high volume manufacturing etching platform such as silicon photovoltaics. Specifically, the current study focuses on developing an effective front-side texturing process on Si(100) wafers. Statistical variation of the tool parameters is performed to achieve high etching rates and low surface reflection of the textured silicon surface. It is observed that the rate and anisotropy of the etching process are strongly defined by the interaction effects between process parameters such as substrate temperature, F2 concentration, and process duration. The etching forms features of sub-micron dimensions on c-Si surface. By maintaining the anisotropic nature of etching, weighted surface reflection (Rw) as low as Rw < 2% in Si(100) is achievable. The lowering of Rw is mainly due to the formation of deep, density grade nanostructures, so-called black silicon, with lateral dimensions that are smaller to the major wavelength ranges of interest in silicon photovoltaics.

In this paper, we report significant progress in development and integration of a plasma-less atmospheric pressure dry texturing (ADE) process, performed on multicrystalline silicon (mc-Si) wafers, into high efficiency PERC solar cell architectures using industrially applied process steps. The mechanism of forming sub-micron features on monocrystalline and multicrystalline silicon wafers with commercial grade fluorine gas (F2) is briefly presented. Low weighted surface reflection (Rw,min < 10%) is achieved for mc-Si substrates regardless of the wafer sawing method. Mc-Si PERC solar cells with average conversion efficiencies of 20% are fabricated in the industrial pilot line of Hanwha Q-Cells in Thalheim. A detailed characterization of ADE-textured solar cells suggest that an enhancement in conversion efficiency of up to +0.8% absolute is possible in comparison to the reference-textured solar cells by narrowing the distribution of reflectivity in the textured wafer surface.

In this contribution, atmospheric pressure dry etching (ADE) texture is integrated into a passivated emitter and rear cell (PERC) process. In order to make the texture suitable for the solar cell processing, two post-etching processes (inverted pyramid, spherical cap) are implemented and compared. The passivation of the front surface could be improved substantially by the introduction of an Al2O3 layer deposited via atomic layer deposition under the classical antireflection coating SiNX. The impact of the addition of the Al2O3 layer on the contact resistance cannot be neglected and becomes significant for thicknesses higher than 4 nm. Multicrystalline silicon (mc-Si) PERC solar cells are fabricated and measured, leading to a maximum efficiency of about 18.6% for the acidic texture, and for both posts-processed dry chemical etching (ADE) textures. No current gain for the ADE textured wafer compared to the acidic texture could be observed on cell level due to an inhomogeneous etching. However, a gain of 0.4 mA/cm² for the ADE texture was calculated from local EQE measurements. ADE texture presents therefore 2 advantages: compatibility to diamond wire sawing and a potential current gain for PERC solar cell structure.

In this paper, we study the impact of change in emitter diffusion profiles on the electrical characteristics of nanotextured surfaces formed by an inline plasma-less dry-chemical etching process. Our experimental results and process simulations suggest that a deeper highly doped region and a significantly higher inactive P concentration in the emitter plays a determining role in defining recombination, as well as the resistive losses in nanotextured surfaces. Low emitter saturation current densities on phosphorous-diffused surfaces are achievable after passivation with either SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,m in</sub> ≈ 81 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) or AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,m in</sub> ≈ 31 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) if the emitter recombination channels are suppressed. Based upon macroscopic measurement of contact resistivity and microscopic analysis of the contact areas, we propose that the formation of numerous metal-semiconductor direct contact points on the peak and the plateaus of the nanostructures are mainly responsible for a low specific contact resistivity (ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c,m in</sub> ≈ 1.2 mΩ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) achievable in these surfaces.

In this paper, we study the effect of an enlarged surface area of nanotextured crystalline silicon wafers on the formation of n-type emitters using a tube diffusion process applying POCl3 as P dopant source. A fast, single-step and industrially viable F2-based dry texturing process is used to perform nanotexturing of Si wafers. This process is presented as an alternative route of nanotexturing in comparison to the two-step nanotexturing approach of creating black silicon and then modifying it with an alkaline or acidic solution. Predictive simulations of phosphorous in-diffusion aided by microscopical characterization of the selected emitters are used to understand the formation of emitter in nanotextured surfaces. Based on these investigations, we show that the optimized emitter leads to a significant improvement in short circuit current density (Jsc≥0.7 mA/cm2) of nanotextured mc-Si solar cells in comparison to the industrial standard acidic textured solar cells.

In this paper, we study the influence of modifying the geometry of nanotexture on its electrical properties. Nanotexture is formed by an industrially feasible dry-chemical etching process performed entirely in atmospheric pressure conditions. A surface modification process is developed that allows low surface recombination velocities (Seff,min ? 10 cm/s) on nanotextured surfaces. By simultaneously improving the surface passivation and the emitter diffusion processes, we achieve an equivalent passivation level (VOC,impl ? 670 mV) for nanotextured surfaces to that of reference textured surfaces after applying either PECVD or ALD based deposition techniques.

We report recent achievements in adapting industrially used solar cell processes on nanotextured surfaces. Nanostructures were etched into c-Si surfaces by dry exothermic plasma-less reaction of F species with Si in atmospheric pressure conditions and then modified using a short post-etching process. Nanotextured multicrystalline wafers are used to prepare Al-BSF solar cells using industrially feasible solar cell proc- essing steps. In comparison to the reference acidic textured solar cells, the nanostructured cells showed gain in short circuit current (Jsc) of up to 0.8 mA/cm2 and absolute gain in conversion efficiency of up to 0.3%. The best nanotextured solar cell was independently certified to reach the conversion efficiency of 18.0%. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

A novel atmospheric pressure dry texture process is investigated in order to create nanostructures at the c-Si surface. The texture process uses diluted molecular fluorine (F2) as the process gas. F2 is partially dissociated at an elevated temperature before it is delivered to the c-Si wafer. Thermal activation of fluorine occurs on Si wafer surface in a dissociative chemisorption process leading to the removal of Si in the form of volatile SiFx species. The etching process can be controlled to form nanostructures with different aspect ratios and surface reflection values. In this work, we dry textured multicrystalline (mc) Si wafers to reach weighted surface reflection ∼12% in the wavelength range of 250–1200 nm. Nanotextured mc Si wafers were used to prepare p-type Al-BSF solar cells. The fabricated nanostructured cells show a gain in short circuit current (Jsc) of ∼0.5 mA/cm2 and reached a conversion efficiency of 17.3%.