# Nines Photovoltaics

• Dublin, Dublin, Ireland
Recent publications
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.