Laser-doped Selective Emitter Research Articles

Review of Laser Doping and its Applications in Silicon Solar Cells

Laser-doped selective emitter diffusion techniques have become mainstream in solar cell manufacture covering 60% of the market share in 2022 and are expected to continue to grow to above 90% within the next five years (ITRPV). This was a very rapid uptake of technology, coming from only ∼10% penetration in 2018, and has enabled over 20 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> front recombination current reductions on the dominant passivated emitter and rear cell concepts in the same short period. In this article, a broad overview of key concepts in relation to laser doping methods relevant to solar cell manufacturing is given. We first discuss the basic mechanisms behind laser doping along with the benefits over conventional doping methods. The main laser doping approaches reported in the literature are then discussed, along with implications for metallization strategy, particularly in relation to selective emitter and back surface field formation in the dominant passivated emitter and rear cell technology. Different cell concepts that have benefited from the application of laser doping are also discussed. In the last section, we discuss the main defects induced by laser processing of silicon which affect the finished devices, potential and debated causes, as well as some commonly applied treatments for their mitigation.

Improved Laser-Induced Defect Passivation and Simultaneous Elimination of Light-Induced Degradation in p-Type Czochralski Silicon

Laser doping is a typical industrial method to introduce a local highly doped region in silicon solar cells to form a selective emitter. Such a process inherently introduces defects that can be a concern to the overall performance of the solar cell. Here, we investigate the effectiveness of laser-induced defect (LasID) passivation on lifetime test structures through different annealing processes, including high-temperature belt-furnace firing, low-temperature belt-furnace annealing, and an advanced hydrogenation process (AHP) for n+ laser-doped selective emitters. We demonstrate clear advantages of post treatment using a rapid 10 s AHP at 300 °C when the lifetime structures are prefired. For the examined laser speeds of 0.5–6 m/s (sheet resistances of 4--70 Ω/□), AHP is the most effective treatment method. For example, for a typical laser doping speed of 4 m/s, starting from the same effective carrier lifetime of 36.9±2.4 μs after laser-doping step for all the passivation treatments, the AHP not only surpasses the conventional approaches by showing the highest recovery of the effective carrier lifetime (∼79% compared with ∼63% and ∼41% for the firing and belt-furnace annealing treatments, respectively) and dark saturation current density reduction in the regions affected by LasIDs but also simultaneously suppresses light-induced degradation (maximum of 4% effective lifetime degradation with respect to the passivated state, as opposed to 14% and 16% degradation for the firing and belt-furnace annealing treatments, respectively) common in Cz grown boron-doped p-type monocrystalline silicon.

Monitoring of Monocrystalline Silicon PERC Solar Cell with Laser-Doped Selective Emitter Using Infrared and Electroluminescence Imaging

Monitoring of Monocrystalline Silicon PERC Solar Cell with Laser-Doped Selective Emitter Using Infrared and Electroluminescence Imaging

Pathway towards 24% efficiency for fully screen-printed passivated emitter and rear contact solar cells

In this work, the efficiency potential of the fully screen-printed passivated emitter and rear contact (PERC) solar cell structure is investigated via numerical simulations. A series of improvements and optimizations are performed on bulk quality, emitter properties and metallization of screen-printed PERC solar cells based on experimental results obtained in both industry and laboratory environments. With significantly improved bulk and surface passivation quality, we find that carrier recombination losses at the metal/silicon interface will impose a substantial limitation on efficiencies, highlighting the need for developing new screen-printing technologies to overcome the limitation from contact recombination. By improving the effectiveness of the back-surface field, reducing coverage area of laser-doped selective emitters and the front metal/silicon interface contact area, a 15 mV improvement in open-circuit voltage (V OC) was achieved in our modelled cells, due to greatly reduced contact recombination losses. With the further implementation of a multi-busbar and fine-line printing technologies, efficiency above 24% was obtained from simulations. Subsequently, a comprehensive pathway towards 24% efficiency for screen-printed PERC solar cells is proposed, without the need to implement passivated contacts or transition to a plated metallisation scheme. Key target requirements for future developments are also identified.

Pathways for efficiency improvements of industrial PERC silicon solar cells

The global manufacturing capacity of Passivated Emitter and Rear Cell (PERC) devices on p-type Czochralski-grown silicon (Cz-Si) wafers is increasing rapidly. This paper analyses various industrial process improvements carried out in our lab to improve the efficiency of large-area Cz-Si PERC solar cells from 20.7% to 21.9%. The key improvements presented in this paper include the transition from line to dot-shaped laser openings in the rear dielectric stack, a dual printing approach for the front fingers and implementation of a laser-doped selective-emitter structure. A nanosecond (ns) pulsed laser source with 532 nm wavelength was used for both the rear dielectric ablation and the front laser doping. By adopting a systematic power loss analysis approach and targeting the largest power loss mechanisms at various stages of development, the best efficiency of the solar cells in this experimental study was improved from 20.7% to 21.9%. Pathways for further improvements in efficiency are analysed, including the application of passivated-contact structures in a PERC-like cell.

A Scanning Spreading Resistance Microscopy Study on a Laser-Doped Selective Phosphorous Emitter

In this work, we present scanning spreading resistance microscopy (SSRM) measurements of a phosphorous emitter formed via tube furnace diffusion and a laser-doped selective emitter (LDSE) processed by additionally doping the tube furnace diffused emitter. The nonlaser-doped emitter is found to be widely homogeneous along the alkaline textured silicon surface. In contrast, the LDSE features large fluctuations in doping depth, almost constant doping concentration along the (laser-modified) surface within a depth of <0.2 μm, and a steep drop of the doping concentration at the p-n-junction. In some parts of the LDSE, the local doping profile is found to decrease stepwise. These findings are new to the photovoltaic community since the named phenomena are not resolved by common characterization methods such as electrochemical capacitance-voltage (ECV) profiling. The limits of ECV profiling are demonstrated by simulating an ECV profile from an SSRM measurement. In this context, we introduce a new method to correct ECV measurements performed on rough surfaces.

BBr3 diffusion with second deposition for laser-doped selective emitters from borosilicate glass

We study a boron tribromide (BBr3) diffusion process with second deposition realized by active nitrogen flow through the BBr3 bubbler at the end of the process with respect to laser doping. Compared to the reference BBr3 diffusion without second deposition, the new BBr3 diffusion provides a two times higher boron dose within the borosilicate glass (BSG)/silicon dioxide (SiO2) stack layer grown on the silicon surface. The second deposition leads to both thinning of the intermediate SiO2 layer and 11 nm-growth of the BSG layer, which results in an 8 nm thicker BSG/SiO2 stack layer with a total thickness of 42 nm. These altered properties of the BSG/SiO2 layer facilitate the formation of laser-doped selective boron emitters, while the second deposition hardly affects the as diffused charge carrier concentration profile. The obtained emitter properties, sheet resistance Rsh ≈ 100 Ω/sq and emitter dark saturation current density j0e ≈ 25 fA/cm2 (textured surface, Al2O3/SiNX passivation) for the photoactive part of the emitter are not affected by the introduced second deposition step. For the laser-doped part of the emitter, the charge carrier concentration after laser doping is higher for the BBr3 diffusion with second deposition resulting in stronger local doping with up to 10 Ω/sq lower Rsh. The application of an analytical model to calculate specific contact resistances ρC in dependence of e.g. dopant concentration of the laser-doped profiles and crystallite penetration depth dcryst reveals that ρC is expected to benefit largely from the altered profile shapes due to laser doping for the BBr3 diffusion with second deposition. A relative decrease in ρC of up to 90% and 50% is found for small dcryst (depth of several tens of nm) and large dcryst (depth of several hundreds of nm), respectively. Another potential advantage of the second deposition holds for emitter dark saturation current densities at the metal-silicon interface j0,met. A 3D simulation model that is based on metal crystallites penetrating into the emitter considering the different doping profiles after laser processing yields a j0,met ≈ 740 fA/cm2 for small dcryst that is 14% relatively lower for the BBr3 diffusion with second deposition.

Efficiency enhancement of bifacial PERC solar cells with laser‐doped selective emitter and double‐screen‐printed Al grid

AbstractWe report that the application of a laser‐doped selective emitter (SE) can improve the trade‐off between the recombination in the emitter and the Ag‐Si specific contact resistance and that a double‐screen‐printed rear aluminum grid can decrease the series resistance in the industrial bifacial passivated emitter and rear cell (PERC). Our results reveal that a front‐side efficiency of 21.9% can be achieved in the PERC (SE)+ solar cell by using low surface‐recombination Al2O3/SiNx films as passivation layers, low‐recombination emitter, and low resistive Ag‐Si contact with an optimized sheet resistance of the lightly doped area of SE with 125Ω/sq and sheet resistance of the highly doped area of SE with 70 Ω/sq. The highest front‐side efficiency of 22.0%, with open‐circuit voltage of 680 mV, short‐circuit current density of 40.1 mA/cm2, and fill factors of 80.8%, is obtained by using double‐screen‐printed rear Al grid process, which is 0.1% higher than the best PERC (SE)+ solar cell. The rear‐side double‐layer antireflection coating consists of 50 nm SiNx:H (n = 2.1) and 10 nm SiNx:H (n = 2.37), which enables a large bifacial gain with the output power density PMPP of 25.5 mW/cm2 in the PERC (SE)+ solar cell by using double‐screen‐printed rear Al grid process when rear‐side illumination intensity Erear = 0.25 suns.

Selective emitter solar cell through simultaneous laser doping and grooving of silicon followed by self-aligned metal plating

Both buried contact solar cells (BCSC) and laser doped selective emitter (LDSE) solar cells have achieved considerable success in large-scale manufacturing. Both technologies are based on plated contacts. High metal aspect ratios achieved by BCSC allow low shading loss while the buried metal contacts in the grooves provide good contact adhesion strength. In comparison, although the LDSE cell achieves significantly higher efficiencies and is a much simpler approach for forming the selective emitter region and self-aligned metal plating, the metal adhesion strength falls well short of that achieved by the BCSC. Recent studies show that plated contacts based on the latter can be more durable than screen-printed contacts. This work introduces a new concept of laser doping with grooving to form narrow grooves with heavily doped walls in a simultaneous step, with the self-aligned metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The laser-doped grooves are only 3–5µm wide and 10–15µm deep; the very steep walls of these grooves remain exposed even after the subsequent deposition of the antireflection coating (ARC). This unique feature significantly reduces the formation of laser-induced defects since the stress due to the thermal expansion mismatch between the ARC and silicon is avoided. Furthermore, the exposed walls allow nucleation of the subsequent metal plating. This novel structure also benefits from greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon surface in the same way as the BCSC. Cell efficiencies over 19% are achieved by using this technology on p-type Czochralski (Cz) wafers with a full area aluminum (Al) back surface field (BSF) rear contact. It is expected that much higher voltages and consequently higher efficiencies could be achieved if this technology is combined with a passivated rear approach.

Plated contacts for solar cells with superior adhesion strength to screen printed solar cells

The improvement of adhesion strength and durability of plated contacts is required for cell manufacturers to gain confidence for large-scale manufacturing. To overcome weak adhesion at the metal/Si interface, new approaches were developed. These involve the formation of laser-ablated anchor points, or grooves in the extreme case of overlapping anchor points, in the heavily doped silicon surface. When plated, these features greatly strengthen the mechanical adhesion strength of the metal. A stylus-based adhesion tester was developed specifically for evaluating the effectiveness of plated contacts to smooth silicon surfaces. The use of such a tester was also extended in this work to textured and roughened surfaces to allow evaluation of different metal contacting approaches. The adhesion strengths for various metal contacting schemes were evaluated, including screenprinted silver contacts, nickel/copper (Ni/Cu) lightinduced plated (LIP) contacts for laser-doped selective emitter (LDSE) cells, buried-contact solar cells (BCSCs), and Ni/Cu LIP contacts formed with laser-ablated anchoring points in selective emitter (LAASE) cells. The latter has superior adhesion strength. The standard “peel test” of the industry was compared to the stylus-based adhesion testing, with the latter shown value for testing metal contacts on smooth surfaces but with caution needed for use with textured or roughened surfaces.

Black silicon laser-doped selective emitter solar cell with 18.1% efficiency

We report fabrication of nanostructured, laser-doped selective emitter (LDSE) silicon solar cells with power conversion efficiency of 18.1% and a fill factor (FF) of 80.1%. The nanostructured solar cells were realized through a single step, mask-less, scalable reactive ion etch (RIE) texturing of the surface. The selective emitter was formed by means of laser doping using a continuous wave (CW) laser and subsequent contact formation using light-induced plating of Ni and Cu. The combination of RIE-texturing and a LDSE cell design has to our knowledge not been demonstrated previously. The resulting efficiency indicates a promising potential, especially considering that the cell reported in this work is the first proof-of-concept and that the fabricated cell is not fully optimized in terms of plating, emitter sheet resistance and surface passivation. Due to the scalable nature and simplicity of RIE-texturing as well as the LDSE process, we consider this specific combination a promising candidate for a cost-efficient process for future Si solar cells.

Passivation-Induced Cavity Defects in Laser-Doped Selective Emitter Si Solar Cells—Formation Model and Recombination Analysis

Laser-induced selective Si doping and simultaneous ablation of a dielectric passivation layer is a promising technology for the creation of efficient and cost-effective solar cells. In this paper, the electrical quality of emitters produced with a 532-nm continuous-wave laser will be discussed using elaborate analysis of quasi-steady-state photoconductance (QSSPC) measurements. It will be shown that these emitters cause good charge carrier shielding, which leads to emitter saturation current densities as low as 240 fA/cm $^2$ for unpassivated surfaces. If an SiN $_{\it x}$ layer is present during laser doping, the emitter recombination increases by a factor of three. This detrimental effect is put down to the formation of microcavities within the recrystallized Si. A model of the ablation mechanism and cavity formation for long laser pulses is proposed, with the experimental data in this study serving as a limiting case for long irradiation lengths.

Failure Modes Identified during Adhesion Testing of Metal Fingers on Silicon Solar Cells

Abstract A stylus-based adhesion tester was used to characterise the adhesion of metal fingers on silicon solar cells. The measured lateral force together with the failure modes of finger cut-off and dislodgment, were identified for screen-printed silver contacts and nickel/copper plated laser-doped selective emitter cells. In-situ images were recorded to visualise the dynamics of the adhesion testing process. The contours of the measured lateral force were mapped to demonstrate the variability of finger adhesion characteristics across two representative solar cells.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Copper penetration in laser-doped selective-emitter silicon solar cells with plated nickel barrier layers

Copper penetration through light-induced plated and electrolessly-plated nickel barrier layers into the underlying silicon of laser-doped selective-emitter (LDSE) silicon solar cells was investigated. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were employed for chemical identification of copper in silicon after thermal treatments at 200°C and 400°C. Copper precipitates were detected by TEM/EDX analysis in solar cells that were cooled rapidly by quenching in ethylene glycol, indicating that the nickel barrier layer may not completely prevent copper from diffusing into silicon. However, they were not detected in solar cells that were allowed to cool slowly under ambient laboratory conditions. The potential impact of copper ingression on the performance of LDSE solar cells was investigated by capturing open circuit photoluminescence images of the cells before and after heat treatments. Solar cells plated with an electroless nickel barrier layer showed more severe photoluminescence degradation after heat treatment followed by fast cooling, compared to cells plated with a nickel barrier formed by light-induced plating. The photoluminescence intensity of a solar cell with an electroless nickel barrier was partially recovered by a second heat treatment at 250°C followed by slow cooling under ambient laboratory conditions.

Comprehensive study of light induced plating of nickel and its effect on large area laser doped crystalline solar cells

Light-induced plating (LIP) as one of alternative metallization technologies has been widely investigated for solar cell metallization, especially for laser-doped selective emitter (LDSE) silicon solar cells. Plating of nickel layer is very crucial in the whole metallization process because nickel is the initial layer on the laser doped area, acting as both contact and barrier layer. The negative effect of poor nickel layer on performance degradation was initially evaluated on LDSE solar cells. Several parameters of light induced nickel plating were then investigated via a laboratory scale plating bath to understand the mechanism of light induced nickel plating directly on the laser doped silicon. Finally, the effect of nickel plating improvement on the performance of large area laser-doped multi-crystalline solar cells was studied via an industrial plating system. By optimization of the applied voltage in the plating nickel, the operation window of laser power was enlarged. At last, an independent confirmed cell efficiency of 17.8% was achieved on large area (243.4cm2) LDSE multi-crystalline silicon.

Uniform Plating of Thin Nickel Layers for Silicon Solar Cells

This paper describes a galvanic deposition method for the formation of uniformly thin nickel seed layers for silicon solar cells. Unlike the previously-reported electroless and light-induced plating methods, where the thickness of nickel seed layers can be vary significantly due to non-uniform nucleation of metal which can be exacerbated by variations in surface dopant concentrations and roughness, this method saturates resulting in Ni seed layers of uniform thicknesses in the range of 250-400nm depending on the plating time and patterning process. The method was successfully used in the fabrication of nickel/copper plated homogeneous 100Ω/□ emitter Si solar cells and laser-doped selective emitter (LDSE) cells, both with screen-printed, Al-alloyed back surface fields. Average cell efficiencies of 18.4% and 18.9% were achieved for the homogeneous emitter cells and LDSE cells, respectively, with the best LDSE cell having an efficiency of 19.2%.

Over 700 mV Implied Voc on p-Type CZ Silicon Solar Cells with Double-Sided Laser Doping

Efficiencies above 19% have been achieved by many PV manufacturers by applying different selective emitter technologies on p-type CZ silicon wafers with screen printed aluminium back-surface field which limits the voltage of the cell to below 640mV. In order to overcome this limit, in this paper, laser doping technology was applied into both surfaces of commercial grade p-type silicon wafer passivated with dielectric layer to form a pseudo solar cell structure with standard laser-doped selective emitter on the front and local back-surface field. This paper studies the passivation and thermal property of the dielectric layer. Post deposition annealing is investigated by measuring the minority carrier effective lifetime and implied Voc of the samples by photoluminescence imaging and photoconductance effective lifetime measurements. The influence of laser doping parameters on the implied Voc is also discussed in this work. As a result, implied Voc over 700mV was achieved on commercial grade p-type CZ silicon wafer after double sided laser doping process. This implied Voc is much higher, compared to the ones obtained by single-sided selective emitter structure.

A Method for Imaging the Emitter Saturation Current With Lateral Resolution

A method is developed to measure the emitter saturation current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> with lateral resolution. The method uses photoluminescence lifetime imaging at several different injection densities to evaluate the injection-dependent effective excess carrier lifetime, from which <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> is derived at each pixel using the method of Kane and Swanson. The applicability of the method is successfully demonstrated on laser-doped selective emitters for crystalline Si solar cells. The derived <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> images of selective emitter test structures are discussed, including measurement uncertainties. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> images may, for example, be used to optimize the laser parameters of selective emitters and to quantify laser damage, and may also provide input parameters for device simulation.

Experimental study on the elimination of over-plating problems in industrial manufacturing of large-area acidic-textured laser-doped multi-crystalline solar cells

Light-induced plating (LIP) techniques, which are self-aligned and are used in the fabrication of well-shaped metal grids, have been widely studied and used in solar cells, particularly in laser-doped selective emitter (LDSE) ones. However, the application of LIP techniques in standard acidic-textured multi-crystalline silicon wafers with SiNx-coated surfaces can be challenging because of the presence of some unwanted metals on the SiNx surface during the plating process. This phenomenon is called over-plating, a problem in industrial LDSE multi-crystalline Si solar cell manufacturing that this paper aims to eliminate without the addition of any new process steps. The feasibility of changing the texturing process as well as the pretreatment before plating and the deposition of SiNx to reduce over-plating was investigated. The effects of over-plating on solar cell performance were also investigated to determine the mechanism of performance degradation. The cells reached a maximum efficiency of 17.2% and an average efficiency of 17.1% on large-area, commercial-grade, p-type multi-crystalline Si substrates when the over-plating problem was eliminated.