Laser Contact Opening Research Articles

Improvement of Laser Contact Opening in Bifacial PERC Solar Cells by Optimizing the Dashed Pattern

Improvement of Laser Contact Opening in Bifacial PERC Solar Cells by Optimizing the Dashed Pattern

Lithography‐free fabrication of a local contact interlayer for Si‐based tandem solar cells

AbstractCurrently, the Si solar cell market share is dominated by PERC solar cells. Although the efficiency of PERC solar cells has been steadily increasing, it is expected to reach the practical efficiency limit in the near future. The thin film/PERC Si tandem cell technique can be one of the solutions to overcome the single‐cell efficiency limit. In this study, we developed a novel interlayer fabrication technology for the diffused junction Si solar cells of the PERC and Al BSF cell architectures. We combined laser contact opening (LCO) and laser‐induced forward transfer (LIFT) processes to fabricate local contact opening with low contact resistance while maintaining the high passivation performance of the Si bottom cell. The dielectric‐passivated emitter of the Si solar cell was ablated locally by the LCO process, and subsequently, the Ti nanoparticles were transferred selectively by the LIFT process to the opened emitter region followed by transparent conducting oxide deposition. Laser process parameters were carefully optimized to fabricate low‐loss interlayers. We applied the developed interlayer fabrication technology to the Si bottom cells of Al BSF and PERC cells. Finally, we demonstrated successfully the perovskite/PERC Si tandem cell with an interlayer developed in this study. The developed interlayer fabrication technology does not include a photolithography step and vacuum deposition processes for buffer metals; thus, we expect it may be more compatible with the mass production of thin film/diffused junction Si tandem solar cells.

Paste-based silver reduction for iTOPCon rear side metallization

This work presents two approaches to reduce the silver (Ag) amount on the rear side of M2-sized industrial tunnel oxide passivated contact (iTOPCon) solar cells by (partially) substituting conventional Ag pastes with copper (Cu) and aluminum (Al) pastes. The first approach is Cu-based and works by screen printing an Ag layer with ca. 75% Ag paste reduction compared to a conventional Ag layer (functioning as a contact and Cu barrier layer) and screen printing a full-area Cu layer on top of the Ag layer (functioning as a conduction layer compensating the reduced Ag amount). This Cu-based approach yields similar power conversion efficiency (η) compared to the conventional approach, showing the Cu-based approach is promising for iTOPCon solar cells. The second approach is Al-based and works by laser contact opening of the dielectric layers and screen printing a full-area layer with an Al paste – adapted to properly contact an n+-poly-Si layer –, substituting Ag on the rear side entirely. This Al-based approach exhibits a 0.9%absη gap to the conventional approach, mainly stemming from the Al paste damaging the poly-Si layer, while – on a positive note – neither rear p+ layer formation nor tunnel oxide layer penetration by the Al paste is detected. Even though the abovementioned η gap must be eliminated in the future, it shows the principal functionality and potential of such type of contact for iTOPCon solar cells.

Overcoming Throughput Limitations of Laser Systems in Solar Cell Manufacturing via On‐The‐Fly Processing Using Polygon Scanners

In this paper, a high‐throughput on‐the‐fly laser processing system for the manufacture of silicon solar cells is proposed. Industrial laser processing machines are systematically limited in a trade‐off between throughput and focal spot size. Additionally, today's laser power and repetition rates cannot be fully utilized because of limited galvanometer scan speeds. This work aims to overcome this using a polygon scanner in an on‐the‐fly laser processing operation. Unlike conventional approaches, this setup does not use high‐precision positioning systems, but reduces wafer transport to a simple conveyor belt to optimize system cost, space, integrability, and cycle rate. The concept is implemented in a demonstrator setup. Applying a laser contact opening process, a throughput of 10 000 wafers per hour is demonstrated. A potential throughput of 13400 wafers per hour is calculated, considering the utilization of more powerful lasers. Inaccuracies due to the unevenness of the surface as well as transport speed oscillations are analyzed and shown not to have an impact on the efficiency of the cells processed with this setup. The throughput is increased five times compared to the theoretical upper‐throughput limitation for the same structure processed by a galvanometer‐driven laser system.

Qualitative Damage Assessment of Laser Contact Opening and Microstructural Evolution at the Rear Contact of a Perc Mono Crystalline Si Solar Cell upon Co-Firing with Si Doped Al Paste

Qualitative Damage Assessment of Laser Contact Opening and Microstructural Evolution at the Rear Contact of a Perc Mono Crystalline Si Solar Cell upon Co-Firing with Si Doped Al Paste

Enabling savings in silver consumption and poly-Si thickness by integration of plated Ni/Cu/Ag contacts for bifacial TOPCon solar cells

Plated Ni/Cu/Ag contacts offer the possibility to significantly reduce silver consumption for tunnel oxide passivated contact (TOPCon) solar cells. This work demonstrates industrial bifacial TOPCon solar cells with plated Ni/Cu/Ag metallization achieving champion solar cell efficiencies of up to 24.0%. The influence of reduced poly-Si thickness down to 60 nm of the TOPCon rear side on the open circuit voltage ( V oc ) is analysed and the impact of laser-induced damage during laser contact opening (LCO) is characterized. Furthermore, mitigation strategies to systematic fill factor losses are developed including laser-induced nano-roughness within the LCO to increase contact adhesion and current-annealing to improve contact resistance. • 24% cell efficiency was realized with Ni/Cu/Ag metallized TOPCon solar cells. • Low laser damage was demonstrated for poly-thickness down to 60 nm. • Current annealing was applied to improve contact resistance of plated contacts. • Systematic fill factor losses are identified and mitigation strategies are proposed.

Investigation of the Defect Distribution of Laser Contact Opening Applied to Poly‐Si/SiNxStacks

Herein, an analysis on the impact of laser contact opening of TOPCon/SiNxstacks is presented. By etching in tetramethylammonium hydroxide (TMAH), the defect distribution in the interfacial tunnel oxide is accessed and analyzed. The defect density is significantly increased in areas where adjacent laser contact openings (LCO) overlap. Using microscopic photoluminescence (μ‐PL) spectroscopy, it is verified that correlates with an increase in the local recombination rate and thus an increase in theJ0,Met. Therefore, overlapping LCO of SiNxin TOPCon/SiNxstacks should be avoided as much as possible. Furthermore, the investigations indicate that defects in the interfacial oxide are dominantly created along exposed structures like tips and edges of (etched‐back) pyramids. A comparison of TOPCon/SiNx stacks with a variation of TOPCon thicknesses indicate that etch pits, and thus the defect density, related to LCO become more significant at lower thicknesses.

Progress of plated metallization for industrial bifacial TOPCon silicon solar cells

AbstractIndustrial tunnel oxide and passivated contact (i‐TOPCon) solar cells were metallized at Fraunhofer ISE using ultrashort pulse laser ablation of the passivation layers for the subsequent Ni/Cu/Ag plating process. The solar cells feature a tunnel SiOx and n‐type doped polysilicon layer covered by a SiNx at the rear side, whereas the front side is made of a boron emitter passivated with a AlOx/SiNx stack. The reference i‐TOPCon solar cells screen‐printed at the supplier reach an efficiency of 23.46% measured by Fraunhofer ISE CalLab. The impact of the laser process on the implied open circuit voltage (iVoc) is characterized showing minor impact on the TOPCon side, while the emitter side reveals an increased iVoc loss due to laser damage. Loss analysis by simulating the plated solar cells points out the benefit of reducing the laser contact opening (LCO) area in terms of shading and contact recombination. Optimization of laser ablation and hydrofluoric acid (HF) pretreatment process result in Voc > 700 mV and FF > 82% leading to a mean efficiency 23.6% measured in‐house and a champion efficiency of 23.84% measured at Fraunhofer ISE CalLab thus outperforming the references by 0.4%abs.

Direct Contact Electroplating Sequence Without Initial Seed Layer for Bifacial TOPCon Solar Cell Metallization

The metallization of bifacial tunneling oxide and passivating contacts (TOPCon) solar cells without initial metal seed layer by electroplating of Ni/Cu/Ag is demonstrated. The presented approach allows a lead-free metallization with narrow contact geometries and low contact resistivity. A metal plate provides electrical contact to the silicon via micrometer size laser contact openings and allows electroplating of bifacial TOPCon solar cells using industrial type inline plating tools without the need of a previously applied seed layer. Challenges such as direct contacting of silicon and dissolution of contacts are identified, and potential solutions are discussed. An optimized process sequence is developed and with this approach a solar cell efficiency of 22.5% is demonstrated on industrial bifacial TOPCon solar cells reaching the same level as the screen-printed reference solar cells.

Status and prospective of high-efficiency c-Si solar cells based on tunneling oxide passivation contacts

Current photovoltaic market is dominated by crystalline silicon (c-Si) solar modules and this status will last for next decades. Among all high-efficiency c-Si solar cells, the tunnel oxide passivated contact (TOPCon) solar cell has attracted much attention due to its excellent passivation and compatibility with the traditional c-Si solar cells. The so-called tunnel oxide passivated contact (TOPCon) consists of an ultra-thin silicon oxide layer less than 2 nm in thickness and a heavily doped poly-Si layer, which is used for implementing effective passivation and selective collection of carriers. This TOPCon solar cell has some advantages including no laser contact opening, no light-induced degradation and no elevated temperature-induced degradation because of N-type c-Si wafer, compatibility with high temperature sintering and technical scalability. This paper first introduces the basic structure and principles of TOPCon solar cells, then compares the existing methods of preparing ultra-thin silicon oxide layer and heavily doped poly-Si layer, and finally points out the future research direction of this cell based on the analysis of the current research status.

Optimization of Al Fire-Through Contacts for AlOX–SiNX Rear Passivated Bifacial p-PERC

We present optimizations of rear Al fire-through (FT) contacts for bifacial p -type passivated emitter and rear (AlOX–SiNX) cells. This approach allows omitting the laser contact opening step. Prior to this work, high FT lateral resistance due to its glass frit, and high rear surface recombination due to large passivation damage and back surface field layer (BSF) abscence, decreased the cell performance for cells with Al FT contacts. In this work, a proper BSF has been achieved by adapting the firing process and by printing multilayer fingers. Achievement of increased BSF thickness and reduced rear contact area—by means of dashing or finger narrowing by dispensing (down to 70 μm)—seems to close the gap to the non-FT (NFT) reference level in terms of contact recombination. This is indicated by reaching similar open-circuit voltages for rear-side-only fired (front side plated) cells. For cofired cells, open-circuit voltages were 6 mV below NFT level, stemming most likely from an overfired front side contact due to firing process adaptations. Multilayer printing allows for a decrease of the lateral resistance of the less conductive FT grid due to an increase of the finger cross-sectional area. For best rear-side-only fired FT cells, reduced recombination and resistance closed the efficiency ( η ) gap to the NFT reference. For the best cofired FT cells, η a 21.3% and bifacial power output density of $P_{\beta\_{\rm OUT}}$ a 22.8 mW/cm2 is achieved compared to η a 21.5% and $P_{\beta\_{\rm OUT}}$ a 23.1 mW/cm2 for NFT. The remaining $P_{\beta\_{\rm OUT}}$ gap has the potential to be reduced by the means of mitigating front side contact overfiring by matching thermal tolerance of the front side paste, and reducing rear side finger resistivity by paste optimization.

Evaluating a Procedure to Align Local Laser Doping and Metallization

We evaluate a procedure that is meant to ensure a highly accurate alignment between laser-doped lines and either screen-printed metal contacts or laser contact openings (LCO) for subsequent galvanic plating of the metal contacts. For both process routes, we prove that the application of the procedure significantly increases the alignment accuracy, discusses potential error sources, and identifies the major challenges for avoiding misalignment. As shown by the presented experiments, the use of screen-printed contacts is challenging for the alignment, since the laser doping process “needs to foresee” the screen print. For plated contacts, however, a much more accurate alignment can be achieved, since the LCO process “can react” on the previously applied laser-doped pattern. As we demonstrate, we are able to combine 10-μm-wide LCO lines with 36-μm-wide laser-doped lines (while ensuring that the passivation layer is only opened within the laser-doped area).

Investigation on Industrial Screen-Printed Aluminum Point Contact and Its Application in n-PERT Rear Junction Solar Cells

As compared with the traditional back surface field (BSF) solar cells, the n -type passivated emitter rear and totally diffused rear-junction ( n -PERT-RJ) solar cells have much lower carrier recombination losses on their rear sides. In order to obtain more benefits (higher open-circuit voltages) from the currently passivated rear sides, the implementation of the point contact concept is an intuitive solution. In this article, we present industrial screen-printed point contacts that are metallized with aluminum (Al) pastes containing silicon additives (Si-add). To understand the impacts of the Si-add on the performance our n -PERT-RJ solar cells, the detailed loss analysis on the open-circuit voltage ( $V_{OC}$ ), short-circuit current density ( $J_{SC}$ ), and fill factor are performed separately. The effect of Si-add and pitch of laser contact openings (LCO) on the point contact characteristics are also investigated. In the last part, we introduce an innovative “point-line” contact concept on the rear side, which is composed of dot-shaped LCO and Al metal finger design, resulting in an n -PERT-RJ solar cell to reach $V_{OC}$ of 692 mV and peak efficiency of 22%. The 60% bifaciality of the device opens the way for an additional power output on module level, which further reduces the levelized cost of electricity. These features make the rear-side point contact design more flexible to be applied in industrial mass production.

Optimization of Ni/Cu Plating Process for Silicon Solar Cells through Understanding of Underlying Electrochemical Reaction Mechanisms

To meet future energy demand and limit global warming, researchers and industrials aim to make solar technologies more efficient and cost effective. Si-wafer based photovoltaic technology accounted for about 95 % of the total production in 2017 [1] and one of the key processing step allowing to reduce cost and increase efficiency of these solar cells is the metallization step [2]. Nowadays, screen printing is the leading metallization technology as it is simple, fast and suitable for mass production. However, this method relies on silver containing pastes, an expensive metal with a fluctuating cost [3]. Moreover, screen-printing has almost reached its limitations in term of contact quality and different ways to apply metallic contacts are required for high efficiency cell architectures and advanced cells concepts. In this context, nickel copper (Ni/Cu) electrochemical metallization, also called “plating”, is under investigation as it not only gives a solution to metal cost reduction by using cheaper metals but also has the potential to overcome screen-printing limitations [4]. Indeed, Ni/Cu plating is suitable for advanced silicon solar cell architectures and, among others interests, form very thin and non-porous metallic contacts with low contact resistivity and better aspect ratio. Despite this, Ni/Cu plating requires more improvements, especially in terms of reliability and adhesion before being implemented in mass production. Plating process optimization is only achievable through a complete understanding of electrochemical reactions mechanisms involved which is the purpose of this work. We have investigated the Ni/Cu electrochemical metallization for n-PERT (Passivated Emitter Rear Totally diffused) bifacial silicon solar cells having a front textured side and rear polished side. The different steps of the Ni/Cu plating process are the following: 1) localized laser contact opening of dielectric layers using a UV picosecond laser, 2) silicon surface deoxydation to remove silicon oxide, 3) palladium activation of the silicon surface through galvanic displacement, 4) electroless nickel deposition, 5) electrolytic copper deposition, 6) final annealing to form NiSi metallic contact. Firstly, we have demonstrated that the silicon surface morphology and composition were non-homogenous after localized laser ablation over all the laser ablation conditions. Indeed, after laser ablation, silicon oxide is still present on the silicon surface with different thicknesses over the laser opened area geometry. Consequently, palladium activation appears to be non-homogeneous with a higher number of palladium particles on the border of laser opened area. Since nickel deposition rate depends on the number of palladium particles deposited, border effects of the nickel layer have been observed. Adherence and quality of the metallic contacts are thus adversely affected. Accordingly, we have investigated in details the roles of the different parameters involved in surface deoxydation and palladium galvanic displacement reactions in order to obtain a more homogeneous activated silicon surface before nickel deposition. This allowed us to control palladium deposition process by promoting palladium nucleation over growth, enabling to have more homogeneous silicon activated surface. A changer: “”is currently being done for nickel electroless deposition and copper electrolytic deposition to increase the quality of metallic contact and silicon metal interface through a better control of deposition conditions. Moreover, a direct plating route without palladium activation, which is a challenge for metallization of bifacial solar cells, will be presented.

Investigation of dielectric layers laser ablation mechanism on n-PERT silicon solar cells for (Ni) plating process: Laser impact on surface morphology, composition, electrical properties and metallization quality

Laser contact opening is a critical step for solar cells manufacturing and needs to be optimized to achieve high efficiencies. In this paper, laser contact opening using a picosecond laser (wavelength 355 nm, pulse duration 10 ps) has been carried out on n-PERT precursors composed of a SiOx/SiOxNy stack on the rear polished side and a SiOxNy layer on the front textured side. By varying peak fluence from 0.130 J/cm2 to 2.159 J/cm2 and spot overlapping, ninety parameters combinations have been tested to open these dielectric layers. Surface morphology characterization, before and after laser ablation, has been realized using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. Bulk and surface compositions have also been investigated by Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy analysis, respectively. Results have shown the existence of four separated laser impacted areas on the polished side and a related ablation mechanism is suggested. Also, electrical characterization using four probe measurements and calibrated lifetime photoluminescence revealed that electrical properties of the silicon underlying increased when post laser annealing was performed associated with no spot overlapping. Then, nickel electroless deposition has been performed and first characterizations indicate adherence issues and inhomogeneous metallization. Characterization of metallized samples revealed that these observations were closely linked to the non-homogenous surface morphology and composition after laser ablation.

Laser contact openings for local poly-Si-metal contacts enabling 26.1%-efficient POLO-IBC solar cells

We demonstrate damage-free laser contact openings in silicon oxide layers on polycrystalline silicon on oxide (POLO) passivating contacts. A pulsed UV-laser evaporates the upper part of the polycrystalline silicon layer, lifting off the silicon oxide layer on top. On n-type POLO (and p-type POLO, respectively) samples a saturation current density of 2 fA cm−2 (6 fA cm−2) and an implied open-circuit voltage of 733 mV (727 mV) are achieved with a laser contact opening area fraction of 12.3% (8.7%). The application of this ablation process in an interdigitated back contact solar cell leads to an independently confirmed power conversion efficiency of 26.1%. The excellent contact quality of the laser contact openings is proven by the low series resistance of 0.1 Ω cm2 on the solar cell with a contact area of only 3%.

Understanding the rear-side layout of p-doped bifacial PERC solar cells with simulation driven experiments

To investigate the rear side of bifacial p-type Czochraslki-grown silicon PERC solar cells, the present work combines Sentaurus Device simulation – calibrated with extensively characterized samples – and the subsequent fabrication of solar cells according to the simulation findings. The authors investigate the physical alteration of rear-side characteristics in the context of an additional rear-side illumination. The additional injection represents an further factor for the balance of carrier generation, recombination and series resistances which in turn influences the design rules for the rear side layout. Our detailed bifacial simulations include these physical aspects and we derive design solutions for different bifacial illumination scenarios for a bifacial p-doped PERC solar cell. Using an industrial PERC process, solar cells with laser contact openings (LCO) and a rear aluminum grid were produced according to the simulation results with a wide variation in rear side layout parameters. The PERC batches showed a rather constant medium (front side) efficiency of η = 20.8±0.2% and a bifaciality of 66 to 77% depending on the rear layout, allowing us to investigate the rear-side characteristics in detail and to compare them with the effects predicted by the simulations. We processed an aluminum rear contact grid with finger widths as small as 100 µm and successfully aligned it onto the LCO with 30 µm contact openings on full-area 156x156 mm2 wafers. We reached good accordance between the monofacial measurements from front and rear side and our simulation model and could thus predict bifacial illumination results by modeling for two issues: 1. Planar rear sides have an advantage over pyramid textured rear sides for 1000 W/m² front illumination unless additional rear illumination exceeds 250 W/m². 2. As soon as any rear illumination is added to the front-illuminated PERC solar cell, 100 µm thin fingers at the rear side have an output power advantage compared to 150 µm and 200 µm wide fingers.

Advances in PassDop technology: recombination and optics

The PassDop technology is a promising approach to realize passivated emitter and rear locally diffused (PERL) silicon solar cells within a reasonable process sequence. In previous studies, the feasibility of this concept was evaluated, but two topics were only briefly discussed: the recombination at the locally diffused and contacted surface and the optical layer. In this study we present an analysis of the recombination at laser processed spots of local laser diffusion from a-SiNx:P. We show that initially the local recombination prefactor J0b,met for the metallized area is very high (104 fA/cm2) but a dedicated anneal can be used to reduce the recombination to a competitive level (600 fA/cm2). The improvement was allocated to the inner area of the laser-processed spot by measuring μ-PL before and after annealing. To improve the optics we show that SiOx can be used on the rear if an etch-back is applied after the laser diffusion. Applying SiOx, results similar to the reference MgF2 layer for Jsc and FF were achieved, while providing much improved adhesion. Finally, we show large area n-type silicon solar cells featuring a PassDop rear side as well as laser contact opening in combination with nickel- and copper-plating on the front. Combining these technologies, we were able to achieve an energy conversion efficiency of 22.2 %.

Investigation of laser contact opening pitch affecting the void formation on n-Type rear emitter silicon solar cells

The pitch of the rear side laser contact opening (LCO) strongly influences the performance of n-type Passivated Emitter and Rear Totally diffused (n-PERT) solar cells. The rear emitter of a back junction (BJ) cell was formed by boron doping [1,2]. The considered n-PERT, BJ cell structure is based on a p-type passivated emitter and rear cell (p-PERC) process sequence [3] upgraded by a boron diffusion on the rear side. Due to the application on n-type substrates, the cells have no light-induced degradation (LID) [4]. In this work we focus on fine tuning the LCO pitch of the device. We found that the LCO pitch directly affected the quality of the contact formation, leading to a variation on the solar cell performance investigated. By combined electroluminescence (EL) and photoluminescence (PL) measurements. Voids were identified (It’s about 60 μm width, 25 μm depth.) by scanning electron microscopy (SEM) and confirmed the different nature of the defects contributing to EL and PL contrast. Void formation is attributed to the diffusion of Si and Al at the Si/Al metallization interface. We found that void formation can be controlled by different LCO pitches. Based on this result we achieved a 0.35% absolute efficiency gain in average.

Impact of Ag Pads on the Series Resistance of PERC Solar Cells

Screen-printed passivated emitter and rear cells (PERC) require Ag pads on the rear side to enable solderable connections for module integration. These Ag pads are separated from the silicon by a dielectric layer to avoid recombination of minority charge carriers. The drawback of this configuration is an elongated transport path for the majority charge carriers generated above the pads. This results in an increase in series resistance. The strength of this effect depends on charge carrier generation above the Ag pads that critically depends on shading of the cell's front side. Ag pads are usually wider than the busbars or the interconnector ribbons and thus are only partially shaded. We build PERC test structures with various rear side configurations of Ag and Al screen printing as well as with and without laser contact openings (LCO). Using experiments and finite element simulations we investigate the impact of shading the Ag pads by the busbars and other means. While fully shaded regions do not increase the lumped solar cell's series resistance, unshaded Ag pads lead to an increase of about 37%.