Boron Emitter Research Articles

Investigation on Effects of the Laser‐Enhanced Contact Optimization Process With Ag Paste in a Boron Emitter for n‐TOPCon Solar Cell

ABSTRACTTOPCon solar cell with boron (B)‐doped emitters plays an important role in photovoltaic cell technology. However, a major challenge to further improving the metallization‐induced recombination and electrical contact of B‐doped emitters. Laser‐enhanced contact optimization (LECO) technology is one of ideal candidates for reducing the metallization recombination and contact resistivity. In this study, we investigate the influence of LECO technology using special Ag paste with a decreased Pb content on the performance of the metallization‐induced recombination (J0,metal), contact resistivity (ρc), microtopography of the contact, the I–V parameters, and possible conductive mechanisms. The results showed that the linear resistivity is reduced from 3.56 to 2.60 μΩ·cm owing to special Ag paste, and after LECO treatment, it also has lower ρc about 0.91 mohm·cm2. Both of them have a large contribution to the FF enhancement. Meanwhile, the J0,metal drops from 500 to 200 fA/cm2, which provides a great contribution to the improvement in open‐circuit voltage. The efficiency improved by 0.26% absolute to 25.94%, mainly because of the increased open‐circuit voltage (Voc) of 4 mV and a fill factor (FF) of 0.26%. Simulated by COMSOL, the electron concentration rises to 4 × 1019 cm−3 after LECO treatment, which can generate a larger reverse current to provide a melting temperature for the glass frit, increasing the interface glass phase conductivity. The possible current transport mechanism of LECO is current tunneling effect, resulting in the decrease in the metallization recombination. After the optimization of the LECO process with low‐corrosion paste, we manufactured industrial‐grade TOPCon cells with Eff, Voc, Jsc, and FF values as high as 26.5%, 736 mV, 42.1 mA/cm2, and 85.5%, respectively.

Development and Characterization of N2O-Plasma Oxide Layers for High-Temperature p-Type Passivating Contacts in Silicon Solar Cells.

Full-area passivating contacts based on SiOx/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiOx) layer. Thin SiOx layers formed via ultraviolet (UV)-O3 exposure are compared with layers obtained through a plasma treatment with nitrous oxide (N2O). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O3 oxide shows a pronounced degradation when using high thermal budget annealing (T > 860 °C), the N2O-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the N2O-plasma oxide layer is comparable to that yielded by UV-O3-grown oxides. To unravel the mechanisms behind the improved performance obtained with the N2O-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both N2O and UV-O3 oxides after RTP. A breakup of the UV-O3 oxide at high thermal budget is observed, whereas the N2O oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the N2O oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O3 oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the N2O oxide under higher annealing temperatures and extended dwell times.

Investigation of the Ag–Si contact characteristics of boron emitters for n‐TOPCon solar cells metallized by laser‐assisted current‐injection treatment

Laser‐assisted current‐injection treatment, also known as Laser Enhanced Contact Optimization (LECO), has great potential to reduce the front contact resistance and metal‐induced recombination of n‐type tunnel oxide passivated contact (n‐TOPCon) solar cells, thereby improving the cell efficiency. Herein, we investigate the interfacial Ag–Si contact characteristics on boron‐doped p+ emitters and the electrical properties of industrial n‐TOPCon solar cells that feature a LECO treatment and a specific Ag paste and compare them with those of n‐TOPCon solar cells with a standard Ag/Al paste process. LECO causes some Current Fired Contacts (CFCs) that, when removed by sequential selective etching, leave bowl‐shaped imprints on the emitter, indicating that isotropic alloying behavior occurs between Ag and Si at these local positions during LECO. Unlike the standard Ag/Al metallization process, the LECO process does not significantly damage the passivation layer or emitter. More interestingly, the n‐TOPCon solar cells prepared with the specific Ag paste did not initially form an effective metal‐semiconductor contact, with an average efficiency of only 0.14%, which increased to 25.65% after LECO treatment, even 0.2%abs higher than that of the reference counterparts with standard Ag/Al electrodes. Ultimately, a physical model of LECO‐induced Ag–Si contact formation on boron emitters is proposed.This article is protected by copyright. All rights reserved.

Low-temperature fabrication of boron-doped amorphous silicon passivating contact as a local selective emitter for high-efficiency n-type TOPCon solar cells

Tunnel oxide passivating contact (TOPCon) solar cells (SCs) currently dominate the photovoltaic industry but grapple with efficiency challenges. A primary concern is the direct contact between front-sided metal electrodes and boron emitters, resulting in substantial carrier recombination losses and limiting further efficiency improvements. Here, we introduce a low-temperature approach to deposit a local boron-doped amorphous silicon [a-Si:H(p)] between front-sided metal electrodes and boron emitters. This method, avoiding issues associated with high-temperature processes, demonstrates excellent passivation and contact properties, featuring the lowest contact resistivity (< 1 mΩ·cm2) and a low saturation current density (< 400 fA/cm2). The outstanding passivation and contact properties remain robust even with variations in diborane flow rates during a-Si:H(p) fabrication, annealing temperatures, and sheet resistances of boron emitters. We elucidate the factors contributing to the enhanced passivation observed in boron emitters with a-Si:H(p) through a combination of simulations and experiments. The a-Si:H(p) layer between boron emitters and metal electrodes acts as a protective barrier, preventing the diffusion of metal atoms and suppressing carrier recombination. A heterojunction is formed between a-Si:H(p) and the boron emitter, facilitating electric-field passivation. Consequently, the TOPCon SCs incorporating a-Si:H(p) achieve an efficiency of 24.50%, surpassing their counterparts without a-Si:H(p) (23.11%). This work utilizes low-temperature technology to achieve fully passivated contact, providing insights for the development of high-efficiency TOPCon SCs.

Blue electroluminescence from homoleptic Ir(III) carbene phosphors and peripheral decorated multiple resonance boron emitters

Blue phosphors are indispensable in constructing organic light-emitting diodes (OLEDs). Herein, we reported two Ir(III) complexes, namely f-CN1 and f-CN2, with asymmetrically arranged benzo[d]imidazolylidenes, to which the peri-cyano substituent has concurrently induced both the selective aryl cyclometalation and emission tuning in giving one single blue Ir(III) phosphor. With the aforementioned ‘one stone for two birds’ tactics, these carbene complexes exhibited photoluminescent peak (λmax) at ∼472 nm and excellent quantum yields (83–100 %) in doped thin films. Consequently, the fabricated OLED devices delivered maximum external quantum efficiencies (EQEmax) of 23.6 % and 20.2 %, respectively. Furthermore, hyper-OLED devices with f-CN1 and multi-resonant TADF terminal emitter m-DiNBO displayed an EQEmax of 30.8 % with CIEx,y coordinates of (0.13, 0.10), together with impressive EQE of 26.9 % at practical brightness of 100 cd m−2.

High‐Performance Narrowband OLED with Low Efficiency Roll‐Off Based on Sulfur‐Incorporated Organoboron Emitter

AbstractMulti‐resonance (MR) based organoboron emitters exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) have become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with high color purity and high color gamut. However, most of the MR‐TADF devices suffer from severe efficiency roll‐off at high current density due to the relatively large singlet–triplet splitting energy, long excited state lifetime, and slow reverse intersystem crossing rate (k RISC). Herein, by utilizing a sulfur atom‐fused donor unit, 7H‐benzo[4,5]thieno[2,3‐b]carbazole, an organic boron emitter (BTC‐BNCz) capable of high efficiency and low efficiency roll‐off narrowband TADF emission is developed. Benefitting from the heavy atom effect of the sulfur atom, kRISC up to 1.6 × 105 s−1 is achieved, which could greatly reduce the undesired quenching effect. Consequently, the OLED using BTC‐BNCz as dopant exhibits high external quantum efficiency (EQE) of 27.0% with full width at half maximum (FWHM) of 35 nm. Moreover, by utilizing a TADF material with a high horizontal dipole ratio as assistant host, outstanding device performance is achieved with a maximum EQE of 35.2% and FWHM of 37 nm, featuring slow efficiency roll‐off.

Concurrently Preparing Front Emitter and Rear Passivating Contact via Continuous PECVD Deposition Plus One‐Step Annealing for High‐Efficiency Tunnel Oxide Passivating Contact Solar Cells

Although tunnel oxide passivating contact (TOPCon) solar cells (SCs) have achieved great success in the photovoltaic (PV) industry, the ultrahigh temperature to prepare boron emitters prolongs the preparation processes and increases the thermal budget. Herein, a new method is presented for simultaneously preparing the front‐side boron emitters and the rear‐side TOPCon structures through continuous plasma enhanced chemical vapor deposition of the precursor layers followed by one‐step high‐temperature annealing. The front‐side boron emitters are fabricated using a stack of nano‐SiOx/B‐doped a‐Si:H layers as the boron source, which can avoid the formation of the stacking faults and lower the annealing temperature to match the rear‐side TOPCon fabrication processes. The rear‐side TOPCon structure, consisting of a plasma‐assisted N2O oxidation (PANO) SiOx and a nitrogen‐doped a‐Si:H(n+), is used to match the B diffusion temperature with high‐quality passivation. The precursor TOPCon cell without electrodes features excellent passivation with the highest implied open‐circuit voltage (iVoc) of 726 mV. As a result, the proof‐of‐concept TOPCon SCs exhibit a remarkable efficiency of 24.07%. This work proposes a flexible and elegant process to prepare high‐efficiency TOPCon devices, which shows great potential for application in the PV industry.

Reducing intersystem crossing rates of boron emitters for high-efficiency and long-lifetime deep-blue OLEDs

An external quantum efficiency of 6.7% and a long operational lifetime of 136 h at 1000 cd m−2 were simultaneously realized for organic light-emitting diodes based on the deep-blue boron emitter (B-N-S-3), owing to the reduced intersystem crossing rate of the emitter.

Stack Diffusion Process for Cost- and Energy-Efficient Boron Emitter Formation

The boron emitter formation for tunnel oxide passivated contact (TOPCon) solar cells faces higher costs compared to the POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> diffusion for passivated emitter and rear (PERC) solar cells due to the requirement for higher temperatures and longer process times. This work presents an alternative energy-efficient and low cost of ownership boron diffusion approach for TOPCon solar cells, enabling a highly increased throughput compared to the typically used gas phase diffusion. We use an atmospheric pressure chemical vapor deposition borosilicate glass layer as the boron dopant source and combine it with a subsequent thermal anneal in a quartz tube furnace for dopant drive-in. Here, we either use a conventional single-slot quartz boat configuration, or, for highly increased throughput, a vertical wafer stack configuration with the wafer surfaces in direct contact with each other. We show that this approach yields an emitter doping profile comparable to the state-of-the-art gas phase diffusion with sufficient uniformity across the wafer area. We further investigate the emitter dark saturation current densities <italic 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> as well as the energy conversion efficiency of TOPCon solar cells fabricated for each configuration and compare the results to those of a BBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> reference process. These solar cells achieve energy conversion efficiencies exceeding 23% for the stack diffusion approach. Additionally, we demonstrate a potential reduction in both the cost of ownership and the specific electricity consumption of the presented approach.

Laser‐enhanced contact optimization oniTOPCon solar cells

AbstractThe authors present their work on laser‐enhanced contact optimization (LECO) oniTOPCon solar cells. LECO improves the metal‐semi‐conductor contact resistivityρcon the boron emitter and the n‐TOPCon side from an underfired (thermal budget too low) state of 2.9 and 14.1 mΩcm2to an enhanced state of 1.8 and 2.9 mΩcm2. Therefore, it enables the reduction of the optimal peak firing temperature, which leads to a decrease of metal induced recombination mainly on the boron emitter side and to minor degree on the n‐TOPCon side, as shown in this work. Ultimately, LECO improvesiTOPCon solar cells conversion efficiency by 0.6%abscompared with the optimal fired, untreated reference. Submitting those cells to 140°C and 2 suns equivalent irradiation for more than 4 h shows no impact on the conversion efficiency or any sign of light and elevated temperature‐induced degradation (LeTID). Confirmed by ISE CalLab, LECO improvediTOPCon solar cells reach 23.8% and 24.1% conversion efficiency on industrial typical wafer formats and processes.

Tube‐type plasma‐enhanced atomic layer deposition of aluminum oxide: Enabling record lab performance for the industry with demonstrated cell efficiencies &gt;24%

AbstractIn this work, single‐side aluminum oxide (Al2O3) deposition enabled by a new tube‐type industrial plasma‐assisted atomic layer deposition (PEALD) technique is presented to meet the increasingly stringent requirements for high‐efficiency solar cell mass production. Extremely low emitter saturation current densities, J0e, down to 15 fA/cm2 are achieved on an industrial textured boron emitter with a sheet resistance of 104 Ω/sq, passivated by PEALD Al2O3/PECVD SiNx stack after firing. An implied open‐circuit voltage of up to 721 mV is obtained on symmetrical lifetime samples. The underlying passivation mechanisms of this new tube‐type PEALD Al2O3 are investigated by contactless corona‐voltage measurements. The results indicate that the superior passivation is mainly attributed to a low interface defect density down to 1.1 × 1011 cm−2 eV−1 and a high negative fixed charge density up to 4.5 × 1012 cm−2. Simulations show that the obtained J0e is close to its intrinsic limit. Lastly, the developed tube‐type PEALD Al2O3 is applied to industrial TOPCon solar cells achieving an average cell efficiency above 24% and a maximum Voc of 707 mV. This work shows that the record level of surface passivation available from lab‐scale PEALD reactors is now available in a flexible high‐throughput industrial PEALD platform, which opens a new route for mass production of high‐efficiency industrial TOPCon solar cells with a lean process at low costs.

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 &gt; 700 mV and FF &gt; 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.

High-efficiency black silicon tunnel oxide passivating contact solar cells through modifying the nano-texture on micron-pyramid surface

Optical loss is a significant factor restricting the conversion efficiency of conventional bifacial tunnel oxide passivating contact (TOPCon) solar cells. Black silicon structure is commonly used to enhance the photogenerated current density ( J ph ) of crystalline solar cells due to its excellent light-trapping capability. However, the photogenerated current gain is cancelled by the increased emitter recombination current originated from the black silicon structure with a high enhanced surface area ratio. In this work, we used a buffered oxide etching solution to modify the surface morphology of nanopore/micron-pyramid composite (NPP) structure silicon. Further, we studied the effects of NPP structures with different enhanced surface area ratio on front-side reflection, boron atom doping, emitter passivation, and cell performance. By identifying the appropriate surface modification processing, we fabricated the large-scale (158.75 mm × 158.75 mm) bifacial TOPCon solar cells using industrial equipment and processes with an average short-circuit current density of 41.12 mA/cm 2 and average conversion efficiency of 23.08%. Through adequately widening nanostructure size and depositing high-quality Al 2 O 3 /SiN x stacked passivation films on NPP structure surface, we achieved lower carrier recombination while maintaining high J ph . • Conversion efficiency of 23.2% achieved by b-Si TOPCon solar cells. • Surface modification of b-Si improves emitter passivation using buffer etching. • Medium specific area of b-Si achieve a balance of light-trapping and electrical loss.

Unlocking the potential of boronsilicate glass passivation for industrial tunnel oxide passivated contact solar cells

AbstractIn this work, we present a breakthrough in boronsilicate glass (BSG) passivated industrial tunnel oxide passivated contact (i‐TOPCon) solar cells. We find that a high‐temperature firing process significantly improves the front side BSG passivation quality; however, the use of such high‐temperatures is undesirable for metallization as it could lead to more junction damage by the metal paste spikes. In this study, we present a simple and industrially viable method to resolve this dilemma. With a high‐temperature industrial firing activation step to maximize the potential of BSG passivation, a low emitter saturation current (J0e) of 34 fA/cm2 has been achieved, demonstrating excellent boron emitter passivation that is comparable to state‐of‐the‐art SiO2 and Al2O3‐based passivation methods on similar structures and boron emitters. Applying this solution to cell device, the open‐circuit voltage (Voc) is improved by about 6 mV, corresponding to an absolute cell efficiency improvement of about 0.2%. Furthermore, after activating the BSG passivation, a lower temperature paste could be used at the rear side which further improves the Voc by around 3 mV. Combined together, an overall improvement of Voc close to 10 mV is achieved, propelling the cell Voc into the 690‐mV era. The effectiveness of this solution was also verified in a mass production line, with average cell efficiencies of around 23.2% (0.5% more than the baseline) and a maximum cell efficiency and Voc of 23.4% and 693 mV, respectively. This work opens new routes for further improving conventional solar cell efficiencies, in particular for BSG‐passivated TOPCon solar cells.

~23% rear side poly-Si/SiO2 passivated silicon solar cell with optimized ion-implanted boron emitter and screen-printed contacts

We report on the understanding and optimization of ion-implanted boron emitters in combination with screen-printed contacts to produce very low recombination current density and high-efficiency cells with rear poly-Si/SiO2 passivated contact. Due to high bulk lifetime in n-base Si and very low recombination in the rear-side n+ passivated contact, recombination in the emitter limits the efficiency potential of manufacturable single-side passivated contact cells. Emitter recombination is governed by recombination under metal contacts (J0e,metal) and in the passivated regions in between (J0e,pass). Emitter profile engineering and dielectric passivation can lower the J0e,pass while paste chemistry and firing conditions can reduce J0e,metal by altering the extent of screen-printing induced emitter surface etching and the percentage of unetched dielectric islands under the metal contacts. We optimized implanted boron profiles, surface concentration and passivation, and junction depth in the sheet resistance range of 48-200 Ω/□ and achieved very low J0e,pass (<15 fA/cm2) for textured boron emitters with Rsheet≥140 Ω/□. Additionally, screen-printing paste and firing optimization reduced J0e,metal from 1172 to 707 fA/cm2 on 170 Ω/□ boron emitter by virtually eliminating emitter surface etching and creating distributed local contacts with unetched dielectric islands (~40% area coverage) underneath the grid lines. This resulted in very low total metallized J0e of 30 fA/cm2 with 3% metal coverage. Our device modeling showed the optimum boron sheet resistance is in the range of 120-170 Ω/□. The simulated results were validated by fabricating fully screen-printed 100 cm2 22.9% efficient bifacial cell with n+ poly-Si/SiO2 passivated rear contact.

Formation of emitter by boron spin-on doping from SiO2 nanosphere and properties of the related n-PERT solar cells

In this paper, spin-on doping (SOD) process using commercial boron dopant source was applied to form front p-type emitter on n-type mono-crystalline CZ substrate. Results showed that the thickness of boron rich layer (BRL) decreased from 180 nm to 20 nm with SiO2 nanosphere assisted SOD and the BRL can be removed entirely by further in-situ oxidation. Furthermore, the hole concentration of the emitter reduced from 5.5 × 1019 cm3 to 2.8 × 1019 cm3 by integrating these two processes. In a symmetric sample without BRL, a very low emitter saturation current density of 32 fA/cm2 with implied-Voc over 700 mV was obtained. The minority carrier lifetime got improved more than 30% after diffusion. Finally, n-PERT solar cell with a conversion efficiency of 20.26% was achieved using optimized boron emitter, showing an enhanced conversion efficiency of 0.54%.

Concentration quenching–resistant multiresonance thermally activated delayed fluorescence emitters

A deep blue thermally activated delayed fluorescence (TADF) organic light-emitting diode resistant to the concentration quenching effect was developed using a multiresonance-type boron emitter decorated with a di(tert-butylphenyl)amine unit. The di(tert-butylphenyl)amine group surrounding the rigid core structure enabled a high photoluminescence quantum yield and fast reverse intersystem crossing of the multiresonance TADF emitter even at high doping concentrations. The multiresonance B–N type blue TADF emitter showed narrow blue emission spectrum with a full width at half maximum of 25 nm without bathochromic shift by the additional donor. It showed a high external quantum efficiency of more than 26%, a deep blue emission color of (0.13, 0.08), and a very small efficiency decrease of 2% even at high doping concentrations.

Promotion of boron diffusion and gettering by employing thin Al2O3 layer as a reaction barrier

The formation of a boron-rich layer (BRL) is a common phenomenon during the fabrication of boron (B) emitter due to different solubility of B in the silicon oxide layer and the silicon substrate. In our work, we employ alumina oxide (Al2O3) layers as reaction barriers to avoid the production of the silicon oxide layer and eliminate the BRL. The B diffusion in silicon becomes much more accessible that the diffusion concentration and depth are increased dramatically with the absence of BRL. The Al2O3 layer can also improve the uniformity of surface sheet resistance due to its hydrophilic, bringing superior quality of liquid boron source spin coating. We investigate the influence of Al2O3 with different thicknesses and refractive indexes on the B diffusion. The diffusion profiles of B in the silicon capped with Al2O3 become broader as the thicknesses and refractive indexes of Al2O3 increase. Besides, iron is employed as a model impurity to study the gettering effects during the B diffusion. The increase of the maximum B concentration and the total amount of B doping contribute to a superior gettering effect with the interstitial iron concentration decrease from 3.91e12 cm−3 to a minimal value of 1.65e11 cm−3, and the iron concentration even dropped to 7.24e10 cm−3 after attaching a second deposition. We analyze the mechanisms of gettering systematically during the B diffusion. The present work will help deepen the understanding of B doping and the fabrication of B emitter.

Imaging and quantifying carrier collection in silicon solar cells: A submicron study using electron beam induced current

In this work electron-beam-induced current (EBIC) is used to study the collection efficiency of emitters in industrial silicon solar cells. Laser-doped local emitters have been deployed industrially, yet in mas production they are designed wider than the screen-printed silver fingers to allow alignment tolerances. EBIC has allowed to image and quantify the laser-induced damage that occurs in these local emitter regions. A model is developed to account for such damage, so that losses in EQE could be quantified from the observed EBIC collection characteristics. The damaged regions present ~12% lower collection efficiency at short wavelength (300–500 nm) than the homogenous emitter. Sentaurus TCAD simulations reveal that eliminating such damage would improve cell efficiency by ~0.12%. Additional degradation is found in a region 1–2 µm wide adjacent to the silver fingers, which has not been detected before. It is also found that pulsed laser doping leads to ~15 µm long un-doped gaps, along the direction of laser movement. As laser doping becomes a key part of industrial cell fabrication, it is crucial to develop a better understanding of the potential pitfalls, and future improvements to the process. The versatility of EBIC imaging is also demonstrated using FIB milling to improve lateral resolution and study the depth profile of boron emitters in newly developed industrial i-TOPCon cells. EBIC imaging, in combination with advanced device simulations, have proven powerful tools to elucidate carrier collection characteristics and drawbacks, thus helping to understand and improve fabrication processes at industrial level.

Analysis of the negative charges injected into a SiO2/SiNx stack using plasma charging technology for field‐effect passivation on a boron‐doped silicon surface

AbstractWe investigated field‐effect passivation by injecting negative charges into SiO2/SiNx stack using a plasma charge injection technique. The Si/SiO2/SiNx samples exhibited a very high flat‐band shift with a high injected negative charge density (&gt;3.0 × 1013 cm2) after plasma negative charge injection; this density was higher than that for the well‐known Al2O3 layer. Most injected negative charges were present within approximately 90 nm of the surface of the SiNx layer deposited by plasma‐enhanced chemical vapor deposition (PECVD) when comparing the capacitance–voltage analysis results obtained while etching the SiNx film considering four assumptions of the injected negative charge distribution. The saturation current density in a 90‐ohm/sq boron emitter decreased from ~90 to 50 fA/cm2 after negative charge injection, which is equivalent to the J0e of the structure passivated with an Al2O3/SiNx stack. Six‐inch n‐type bifacial cells with an approximately 100‐ohm/sq boron emitter passivated with SiO2/SiNx displayed an approximately 0.2% increase in absolute cell efficiency after negative charge injection. In addition, n‐PERT bifacial cells with a high boron sheet resistance of ~150 ohm/sq exhibited a 1.0% or higher absolute efficiency enhancement from a relatively low precharging efficiency of approximately 19.0%. We also demonstrated that the final efficiency after charging was comparable with n‐PERT bifacial cells with Al2O3 passivation, suggesting that the proposed process is a potential low‐cost alternative method that could replace expensive Al2O3 processes.