Influence of PECVD Poly-Si Layer Thickness on the Wrap-Around and the Quantum Efficiency of Bifacial n-TOPCon Solar Cells

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/pssa.202100156. This article is protected by copyright. All rights reserved Influence of PECVD Poly-Si Layer Thickness on the Wrap-Around and the Quantum Efficiency of Bifacial n-TOPCon Solar Cells Benjamin Grübel 1 *, Henning Nagel 1 , Bernd Steinhauser 1 , Frank Feldmann 2 , Sven Kluska 1 , Martin Hermle 1 1 Fraunhofer Institute for Solar Energy Systems ISE Heidenhofstraße 2, 79110, Freiburg im Breisgau, Germany 2 Solarlab Aiko Europe GmbH Berliner Allee 29, 79110 Freiburg im Breisgau E-Mail: benjamin.gruebel@ise.fraunhofer.de Keywords: TOPCon, PECVD, Metallization, Passivated Contacts Abstract –In typical industrial processing of tunnel oxide passivated contact (TOPCon) solar cells, poly-Si is deposited on the entire back of the cells. During the deposition process, a wrap-around of poly-Si onto the edges and the front side of the cells is virtually unavoidable if chemical vapor deposition processes are used. Plasma enhanced chemical vapor deposition (PECVD) is used to investigate very thin poly-Si films and their effect on wrap-around on bifacial TOPCon solar cells fabricated without wrap-around etching. As a result, reduction of the poly-Si thickness down to 30 nm significantly increases the shunt resistance, reduces the reverse bias current, and thus reduces the risk of hot spots as measured by IR imaging and micro characterization by secondary electron microscopy. Electroplated metallization proves to be a suitable candidate for contacting such thin TOPCon layers being less sensitive than screen-printed metallization. 1. Introduction Conventional silicon solar cell concepts such as passivated emitter and rear cells (PERC) are limited by, among others, excess-carrier recombination at the metal-semiconductor interfaces of the front and rear side metallization to the semiconductor. [1] Introducing a Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved passivating layer in-between that interface such as TOPCon consisting of an ultra-thin SiO x combined with a highly doped poly-Si layer allows to significantly reduce contact recombination losses. [2, 3] This way recent achievements of Richter et al. led to a certified record efficiency of 26.0% for a both side contacted TOPCon solar cell. [4] Industrial upscaling of process steps includes the development of poly-Si film deposition. Currently, the widely used industrial method is low pressure chemical vapor deposition (LPCVD). It enables an in-situ formation of the tunnel oxide and allows a conformal deposition of intrinsic or in-situ doped poly-Si layer. An inherent disadvantage of the process is a severe wrap-around even in the case of back-to-back loading, which has to be removed in an additional process step. [5–7] Plasma enhanced chemical vapor deposition (PECVD) is considered a more single-sided deposition technique, although minimal wrap-around can still occur. Metal contacting of TOPCon layers represents a main challenge in transferring this solar cell concept into an industrially scaled process. In order to guarantee the full functionality of the TOPCon layer, the metallization process shall not damage this layer. [8] It is shown from different research groups, that TOPCon solar cells with poly-Si thicknesses below 100 nm featuring screen-printed screen printing Ag contacts revealed an increased contact recombination. [8–11] However, thinner poly-Si layers are desirable to reduce the deposition time and thereby reducing cost of ownership [12, 13] , the free carrier absorption in the highly doped poly-Si layer [14] as well as improving the bifaciality factor. [15] Additionally, thinner layers could reduce the etch back time. [7] Laser ablation and plating of Ni/Cu/Ag contacts was found to be a candidate to allow for metallizing of TOPCon solar cells with thinner poly-Si layers without inducing increased excess carrier recombination. [9, 16] In this paper, the impact of the reduction of the poly-Si layer thickness on the wrap-around during PECVD and the metallization of these TOPCon layers are investigated. The shunt resistance R sh and the current I rev under reverse bias voltage (-12 V) were investigated as a Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved measure of the influence of the wrap-around. Reverse-biased IR imaging [17–19] for hot spot prediction and micro characterization by scanning electron microscope (SEM) were used to visualize the poly-Si wrap-around. The effect of reducing the poly-Si layer thickness down to 30 nm onto the performance of solar cells were characterized for both electroplated and screen-printed metallization of the TOPCon rear side. 2. Experimental 2.1. Sample Preparation Bifacial TOPCon solar cells were manufactured from 156.75 x 156.75 mm² large n-type Cz-silicon wafers featuring a resistivity of 1 Ω·cm. The exact processing of the solar cells is discussed in detail in the publication of Varun et al [16] The schematic cross-sectional layout is shown in Figure 1. The produced TOPCon solar cells feature a thermal SiO x layer prepared in a tube furnace and a phosphorous-doped a-Si layer deposited in a Centrotherm c.PLASMA tube PECVD with horizontal carrier. The a-Si was transformed to poly-Si by annealing at 900 °C for 10 minutes under nitrogen atmosphere. The samples were subjected to an O3/HF solution to remove the poly-Si wrap-around reducing the poly-Si thickness by about 10 nm. The variation of the poly-Si thickness resulted in thicknesses of 30 nm, 50 nm, 70 nm, and 90 nm, respectively. The poly-Si on the rear side was coated with a SiN x whereas the front side features a p-type boron emitter passivated by a AlO x /SiN x stack. The solar cells were screen-printed with a Ag grid on the rear side and a AgAl grid on the front side and fired at a set peak temperature of 820°C, corresponding to an actual wafer temperature of approximately 720°C. Additionally, TOPCon solar cells featuring screen-printed contacts on the front side and plated contacts on the rear side were also manufactured for all poly-Si thicknesses. Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved Figure 1. Schematic drawing of the cross section of both TOPCon solar cell designs with screen-printed contacts on the front side and screen-printed or plated contacts on the rear side, respectively. 2.2. Characterization Methods The finished solar cells were subjected to IV measurements (cetisPV-Celltest3 / halm), quantum efficiency and reflectance measurements with a step size of 10 nm (pv-tools LOANA) as well as microcharacterization by SEM (Auriga / Zeiss). The IV measurement routine also includes a fast, reliable, yet quantitive inline hot-spot detection using reverse-biased IR imaging allowing to resolve smallest temperature changes. Because of the high local power density of hot spots an approach proposed by Ramspeck et al. is used in this work where an IR image after a very short time (few tens of milliseconds) under reverse voltage of -12 V is taken and subtracted with an IR reference image taken before applying the voltage displaying the temperature change during biasing. [17] The microcharacterization to visualize the poly-Si wrap-around in cross section was performed by use of an SEM. This characterization method remains challenging, as the poly- Si layer features the same n-type doping as the bulk material even though different doping levels are present. The thermal oxide between the poly-Si and the bulk is only 1-2 nm thick and is therefore below the resolution limit of the SEM. The approach selected here is to mechanically break the solar cell prior to introducing into the SEM without further sample Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved preparation. The poly-Si layer can be distinguished from crystalline silicon bulk material due to columnar shape structure of the poly Si. [20] 3. Results and Discussion 3.1. Reverse IV Characteristic and Thermography In Figure 2 the shunt resistance R sh and the reverse bias current I Rev at -12 V of the solar cells are displayed depending on the poly-Si layer thickness. As plated and screen-printed solar cells revealed the same trend the results are merged in Figure 2. For a poly-Si thickness of 90 nm, I rev reaches values over 10 A and R sh of around 10 kΩcm². Both parameters improve for decreasing poly-Si thicknesses to mean values between 2 A and 4 A and above 100 kΩcm² for a poly-Si thickness of 30 nm. Figure 2. Reverse bias current Irev at -12 V and shunt resistance Rsh as a function of poly-Si thicknesses. The screen-printed and plated results are grouped in the same boxes as the trend remained similar independent of the metallization type. The reverse current and the shunt resistance usually refer to the presence of shunts that bear the risk of hot spot generation in PV modules manufactured with these cells. This trend decreases with thinner poly-Si layers. Figure 3 shows thermography images taken under Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved reverse bias voltage of - 12 V of solar cells for all poly-Si thicknesses (30 – 90 nm). For the sample with 90 nm poly-Si almost the whole edge of the solar cell reveals a significant temperature increase. With decreasing poly-Si thickness the edge proportion and brightness of increased temperature is decreasing as well, reducing the risk and severity of hot spots. For the sample with a 30 nm thick poly-Si layer, a smaller share of the edge is thermally visible whereas a larger share remains thermally inactive. Figure 3. Thermography images taken under reverse voltage (-12 V) of TOPCon solar cells featuring poly-Si layer thicknesses of 30 nm, 50 nm, 70 nm, and 90 nm, respectively. 3.2. Microcharacterization Figure 4 shows cross section SEM images of two TOPCon solar cells shown in Figure 3 at several positions for 50 nm (A-B) and 90 nm (a-c). The SEM characterization was performed at thermally active positions according to the thermography images as indicated by the green box in Figure 3. On both samples, images were taken at the transition of the edge to the front side (A, a) and on the edge close to the rear side (B, b). For the sample with a poly-Si Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved thickness of 90 nm an additional image was taken on the front side 20 µm from the edge. All cross-section images reveal the presence of two layers onto the Si bulk material. The outer layer represents the SiN x , whereas the intermediate layer represents the TOPCon layer. The sample with a poly-Si layer thickness of 90 nm reveals a poly-Si thickness of 65 nm at position (b) whereas at position (a) a thickness 45-55 nm is measured. On the front side, at position (c) 20 µm from the edge, a poly-Si layer is still measurable with a thickness above 20 nm. Characterization even further away from the edge to identify the wrap-around extent became challenging as the poly-Si layer reaches the limitation of resolution of the used SEM ( 20 nm).The sample with a poly-Si thickness of 50 nm (left) at position (B) reveals a poly- Si layer with a thickness of 38 nm. Towards the front side the poly-Si thickness decreases down to a thickness of 25 nm. On the front side itself the poly-Si layer could no longer be identified anymore within range of 20 µm from the edge. The comparison of both samples shows a decrease of the thickness of the poly-Si wrap-around for a lower poly-Si thickness. Accepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted ArticleAccepted Article This article is protected by copyright. All rights reserved Figure 4. Cross section images of a TOPCon solar cell with a