Passivating pinholes for large-area and high-efficiency silicon solar cells with tunnel oxide passivated contact

Domain: biology
Published: 2026-03-14
Editorial outcome: major_revision

Abstract
The tunnel oxide passivated contact (TOPCon) solar cell is poised to dominate silicon photovoltaics, yet the atomic-scale nature of pinholes—local disruptions in the SiO x layer enabling direct conduction—remains unresolved despite its critical importance for device performance. Here, using spherical aberration-corrected transmission electron microscopy, the TOPCon interface is uncovered at the atomic level, revealing two distinct pinhole types: recombinational pinholes with oxygen-depleted Si–Si contacts, and previously unknown passivating pinholes that retain sufficient oxygen to passivate dangling bonds while enabling carrier tunneling. These passivating pinholes exhibit cross-sectional sizes of approximately 1.6 ± 0.2 nm × 1.4 ± 0.3 nm and an area density of 2 × 10 12 cm -2 . Fischer model analysis demonstrates that pinhole passivation, not geometry, governs device performance. Translating these insights, industrial large-area (333.3 cm 2 ) TOPCon solar cells achieve certified efficiencies of 25.40% and open-circuit voltages of 738.7 mV. Our findings provide atomic-level insights into the TOPCon interface and offer direct guidance for fabricating high-efficiency solar cells.

Introduction
Developing a photovoltaic (PV) technology with high-performance, good reliability, low-cost and being environmentally friendly has been an inexorable trend for future energy and carbon neutrality. With the increase of market share (Supplementary Fig. 1) 1–3 , tunnel oxide passivated contact (TOPCon) solar cells are gradually becoming the mainstream choice of silicon PV technologies because of their high efficiency and being readily into mass production 4 . Recently, the TOPCon cells achieved a rapid progress. In the aspect of power conversion efficiency (PCE), the Fraunhofer Institute for Solar Energy Systems (Fraunhofer-ISE) has achieved a PCE of 26% (4 cm 2 ) for p-type TOPCon solar cells, while Jiangsu Runergy New Energy Technology Co., Ltd. (Runergy) has achieved a PCE of 26.55% (335.2 cm 2 ) for n-type TOPCon solar cells 5,6 . Recent reports shows that JinkoSolar’s perovskite stack cells based on n-type TOPCon have a high PCE of 33.24% 7 . In the aspect of the levelized cost of energy (LCOE), a below 0.30 RMB/kWh cost has happened in China 2 and even 0.07 RMB/kWh in Saudi Arabia 8,9 . Such alarming numbers make the global PV industry to have truly entered the n-type crystalline Si (c-Si) age, and it is expected that the TOPCon technology will dominate this industry for å 5 years in the future 1,2,10,11 .

Although the TOPCon cells have such high performance-price ratio and huge industry scale, the understanding of its nature principle is not completely clear. For instance, Peibst et al. have reported arguments that laterally homogenous tunneling might occur in carrier-selective poly-Si/c-Si junctions with an interfacial oxide layer and also in pinhole formation, where it is commonly considered as poor oxygen passivation 12 . In particular, the role of the pinholes, where the lattice of the poly-Si is in direct contact with the lattice of c-Si, and carrier transport within pinholes, are still not fully understood. This leads to the PCE of TOPCon solar cells has not yet reached their highest level in technology. The theoretical efficiency limit of single-sided n-type TOPCon solar cells is 27.79% 13 . Especially, open circuit voltage (V oc ) and fill factor (FF) to be determined by interface passivation and carrier tunneling, strongly related to the TOPCon interface, including poly-Si, interfacial oxide and pinholes.

The FF of TOPCon cells would depend on two factors: interfacial defect passivation (simultaneously affecting V oc ) and carrier tunneling probability. A thicker oxide layer would contribute to better interface passivation but reduce carrier tunneling probability, resulting in decent V oc but low FF 14 . Too thin oxide layer would form more pinholes, although they are beneficial to carrier tunneling, but results in poor passivation and eventually both low V oc and FF. The ideal situation is that there is an interfacial oxide layer, being thin enough while possessing sufficient oxygen especially even into pinholes (referred to as “passivating pinholes”). In this case, the passivation effect and carrier tunneling probability will achieve a perfect balance, presenting the high V oc and FF simultaneously in the device. However, in the actual TOPCon production process, after the back alkaline polishing, the surface may appear uneven state of ups and downs, resulting in uneven thicknesses of the oxide layer, and presenting three cases: the oxide layer is thick (>~1.7 nm 12,15 ), defect passivation is good but carrier tunneling probability is not the optimal state (Case 1); and the oxide layer is thin (<~1.3 nm 16,17 ), which brings to two cases: one is the lack of oxygen passivation into the lattice contact between poly-Si and c-Si, forming the recombinational pinholes, detrimental to device performance (Case 2); the other is that there are still sufficient oxygen into the lattice contact between poly-Si and c-Si, forming the passivating pinholes, beneficial to device performance (Case 3).

Cases 1 and 2 have been extensively studied and demonstrated by high-resolution transmission electron microscopy (HR-TEM) 12,15–17 . An etching method to determine of pinhole density and electron beam-induced current (EBIC) to visualize current flow through pinholes have also been tried 15 . The rupture of the poly-Si/SiO x /c-Si interface and the oxide layer thicknesses with 1.0–2.2 nm 15,16,18,19 have been well observed by HR-TEM. Choi et al. observed the nanopite with the cross-sectional size of ~0.8 nm × 11.2 nm and ~0.5 nm × 26.7 nm in the tunnel oxide layer when annealed at a high temperature 19 . Liu et al also found that subnanometer pits with the cross-sectional size of ~1.5 nm × 4.6 nm and ~1.5 nm × 7 nm are formed in the oxide layer 16 . However, those observations are only nanopites rather that pinholes. Identification of pinholes the poly-Si/SiO x /c-Si interface is still challenging due to the insufficient resolution of HR-TEM. Therefore, to date, in c-Si PV, Case 3 has yet to be discovered.

The spherical aberration (C s )-corrected high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM) is a more advanced technique, from which many atomic-level structure mechanisms, such as the c-Si/a-Si:H interface in silicon heterojunction solar cells, have been uncovered 20 . In this study, the passivating pinholes, with the cross-sectional size of ~1.6 nm × 1.4 nm, i.e., Case 3 as state above, are identified at the poly-Si/c-Si interface in large-area, high-PCE and industrial TOPCon solar cells using C s -corrected HAADF-STEM. The sufficient oxygen is observed into pinholes, corresponding to the high-PCE TOPCon cells. It presents a new understanding that there are actually two types of pinholes, recombinational pinholes and passivating pinholes. The former is as traditionally understood and reported in the literature, the direct contact of the poly-Si into c-Si leads to large number of dangling bond defects; and the latter is a new discovery that the poly-Si/c-Si interface still has sufficient oxygen passivation on dangling bond defects. This kind of microstructure would not happen in silicon heterojunction (SHJ) solar cells or passivated emitter and rear contact (PERC) solar cells, potentially indicating that the TOPCon cell can possess higher performance, which is consistent with the theoretical result 13,21 . The guidance for improving PCE of TOPCon industry can be concluded: device optimization should focus on optimizing back polishing surface morphologies, oxide layer processing, and polycrystalline layer processing in order to obtain more passivating holes for trade-off between passivation effects and carrier tunneling probability, simultaneously achieving high V oc and FF. Eventually, the TOPCon solar cell with passivating pinholes present a certified power conversion efficiencies of up to 25.40% and open circuit voltage up to 738.7 mV on industry-grade silicon wafers (total area of 333.3 cm 2 ).

Results
Large-area and high-efficiency tunnel oxide passivated contact solar cells
Figure 1a illustrates the structural diagram of the TOPCon solar cell with passivating pinholes. On the front of the solar cell, a state-of-the-art selective emitter (SE) structure used by industry was implemented. This consisted of a SE by boron diffusion and laser doping, Al 2 O 3 passivation and silicon nitride (SiN x ) antireflection layer. The SE contributes the suitable junction doping profile with a very low contact resistivity (ρ c ) of ~1 mΩ cm 2 and a low carrier recombination parameter (J 0,emiter ) of <5 fA/cm 2 simultaneously (Supplementary Fig. 2). On the back of the solar cell, poly-Si junctions are formed by embedding a thin insulating layer of silicon oxide (SiO x ) between the c-Si wafer and a highly doped poly-Si layer. The disruptions in the SiO x , often referred to as pinholes, with less oxygen and sufficient oxygen, respectively, corresponding to recombinational pinholes and passivating pinholes, are illustrated in Fig. 1a. Using optimized oxidation processes in low-pressure chemical vapor deposition (LPCVD) procedure, (Supplementary Fig. 3) combining with suitable back polishing and poly-Si fabrication, on the quasi-square silicon wafer with the industrial size of 182 mm × 183.75 mm (Fig. 1b), a high PCE of 25.40% is certified by ISFH with a total area of 333.3 cm 2 measured under standard test conditions (Fig. 1c). For comparison study, we also fabricated TOPCon cells with a low PCE of 24.07% using the controlled LPCVD oxidation process (Supplementary Fig. 3 and Fig. 1d).

Pinhole microstructures are caused by the thermal oxidation process during LPCVD poly-Si. Thermal oxidation is a critical step to form an ultrathin silicon oxide layer on the wafer surface. The control of the oxygen content of the two different types of pinholes can be achieved by a variety of factors, such as oxidation temperature, time, and gas atmosphere, etc. Here, a two-step oxidation method was used. Firstly, it is oxidized with oxygen at a certain temperature for a period of time to form a thin layer of silicon dioxide. Then, it undergoes a period of oxygen-deficit treatment without the introduction of oxygen. In this case, the formation of passivating pinholes with sufficient oxygen involves a longer oxygen passage time, higher temperature, larger gas flow rate and higher pressure, which is also more conducive to the formation of a uniform and dense oxide layer. During the subsequent diffusion process of POCl 3 , a lower deposition temperature and time are conducive to improving the microstructure and crystallinity of the polycrystalline silicon film, thereby reducing the probability of recombinational pinholes occurring at the interface. A detailed description of the fabrication procedure (Supplementary Fig. 3) and systematic variation series of the relevant parameters (Supplementary Table 1) for two kinds of solar cell can be found in the Supporting Information.

Observation of passivating pinholes and uncovering tunnel oxide passivated contact solar cell interface
Figure 2a, b gives typical cross-sectional HRTEM images of the high-PCE and relatively low-PCE (the controlled one) as stated above, respectively. Only from HRTEM, no obvious difference can be seen between the high-PCE and controlled cell. The poly-Si/SiO x /c-Si structure shows uniform interfaces at both sides of the tunnel layer, and an interfacial oxide passivation layer with the thickness of 1.2–1.4 nm is observed. Clearly, this result cannot explain why two devices have obvious performance differences, ΔV oc of ~16 mV, ΔFF of ~2.8% and ΔPCE of ~1.33%.

To further understand the structural characteristics of the TOPCon interface and clarify the difference between the two devices, C s -corrected HAADF-STEM was performed, as shown in Fig. 2c, d. Through the local contrast variation (Supplementary Fig. 4), we preliminarily determine that the area with brighter contrast corresponds to the location marked with the red box in the C s -corrected HAADF-STEM diagram, which is the position of the pinhole. Next, we confirmed this phenomenon through STEM-EELS and STEM-EDS. It can be seen that pinholes exist in two TOPCon cell interfaces, and the size of the pinholes can be quantified. Cs‑corrected HAADF‑STEM imaging reveals two distinct types of nanoscale features: recombinational pinholes, characterized by oxygen‑depleted Si–Si direct contact (Fig. 2d, f, h), and passivating pinholes, which retain residual oxygen within the contact region (Fig. 2c, e, g). Pinholes in the high-PCE device have the thickness of 1.55 nm and the size of 1.25 nm, and those in the low-PCE one has the thickness of 1.37 nm and the size of 1.35 nm, respectively. Pinholes were observed in both high-PCE and low-PCE cells (see Supplementary Fig. 5 for more images and Supplementary data 1). Even for the relatively low-PCE TOPCon cell, the pinhole size is also very small, while high-PCE TOPCon cell has the smaller pinhole, only ~1.55 nm × 1.25 nm cross-sectional size, which are smaller than the previously reported data 15,16,19 . More importantly, in the electron energy loss spectroscopy (EELS) distributions, we note that the pinhole in the high-PCE device has still sufficient oxygen into the lattice contact between poly-Si and c-Si (Fig. 2e), corresponding to slightly small oxygen-lack valleys (Fig. 2g). This means that defects in the pinhole are passivated by oxygen, leading to a concept being termed the “passivating pinholes”. In contrast, pinhole in the low-PCE device doesn’t have sufficient oxygen (Fig. 2f), with relatively poor passivation effect, which is termed the “recombinational pinholes” as traditionally understood, corresponding to bigger oxygen-lack valleys (Fig. 2h). The similar conclusion can also be proved in the cross-sectional energy-dispersive spectroscopy (EDS) distributions (Supplementary Fig. 6). Clearly, passivating pinholes achieve efficient carrier tunneling while maintaining high-quality interface passivation.

Precise quantification of pinhole size and density remains challenges. The commonly used methods, such as selective etching of the poly-Si layer 22–25 , conductive atomic force microscopy (c-AFM) 23 , and conventional transmission electron microscopy (TEM) 12,15,23 , suffer from inherent limitations. Selective etching and c‑AFM rely on the differential etch rates between Si and SiO x , but over‑etching often leads to coalescence of etch pits, compromising accuracy. More critically, these indirect techniques cannot detect the passivating pinholes identified in this work, as regions with residual oxygen resist etching 24 . HRTEM is a good choice, but only limited to determine the diameter of ~5 nm pinholes 12,22 . Consequently, reported pinhole diameters and densities have historically ranged from ~5 to >200 nm and pinhole density is 10 6 –10 8 /cm -2 , respectively (Supplementary Table 2). In this study, we overcome these constraints by employing Cs corrected HAADF-STEM, which provides atomic resolution Z contrast imaging. This enables the direct identification of pinholes as small as 1.1–1.8 nm in industrial high-efficiency TOPCon cells. Based on quantified pinholes size, we further proposed an X-Y oriented Cs-corrected HAADF-STEM method, as shown in Fig. 3a. In this approach, we employed X‑Y‑oriented imaging not as a 3D tomography technique, but as a high‑resolution projection method optimized for visualizing and statistically analyzing pinhole features within the SiO x layer. Since the probability of re‑imaging the exact same location along both the X and Y axes on a single specimen is exceedingly low, we sectioned the same solar cell along orthogonal X and Y directions, prepared a statistically significant number of species (Fig. 3c), and derived the pinhole parameters from pooled analysis. Given that the pinholes are sub‑2 nm in size—much smaller than the specimen thickness—their projected images in HAADF‑STEM remain distinct due to strong Z‑contrast, allowing reliable identification and counting. This well‑established approach for quantifying nanoscale features in cross-sectional samples is valid provided the features are spatially separated in projection, which is confirmed by our statistical analysis across multiple regions. Using this method, we can statistically sketch pinhole distributions and quantify their areal density. It reveals an unexpectedly high density of 10 12 cm −2 , specifically, 2 × 10 12 cm −2 for passivating pinholes and 3 × 10 12 cm −2 for recombinational pinholes based on examination of 40 high-efficiency and low-efficiency solar cells (Supplementary Table 3). These values are orders of magnitude greater than those previously reported 22–25 .

It should be noted, however, that the derived areal density represents a projected estimate. Given the sub‑2 nm scale of the pinholes and the typical TEM specimen thickness (∼50–150 nm), multiple pinholes may superimpose along the electron‑beam direction in conventional STEM images, potentially influencing quantitative accuracy. To rigorously address this concern, we performed two complementary analyses: (i) 4D‑STEM‑based ptychographic imaging was performed along the thickness direction, enabling atomic‑resolution reconstruction of the c-Si/SiO x /poly‑Si multilayer structure with atomic resolution (Supplementary Fig. 7 and Fig. 8). While conventional annular dark‑field (ADF) imaging enhances Z‑contrast and improves pinhole visibility, ptychographic phase reconstruction yields linear contrast, rendering these fine nanoscale features less distinct in the final phase map 26 ; (ii) complementary EELS‑mapping was conducted across regions of varying specimen thickness (Supplementary data 2). Quantitative comparison confirmed that pinhole density remains statistically consistent regardless of local thickness, demonstrating that projection‑related superposition does not materially distort our spatial distribution analysis, thereby validating the reliability of the quantified ultra‑high pinhole densities.

The orders-of-magnitude discrepancy between our measured pinhole density and prior literature values stems not from an artifact, but from a fundamental conceptual shift coupled with advanced methodology. The nanoscale features observed in this study are not the classical “recombinational pinholes”—oxygen‑free, direct Si–Si contacts that act as strong recombination centers. Instead, they are predominantly “passivating pinholes”: oxygen‑depleted yet oxygen‑containing nanodomains (~1.5 nm) that form under optimized, lower‑temperature, hydrogen‑rich processing conditions. These features act as metastable precursors to classical pinholes, their evolution kinetically arrested to preserve passivation, as supported by recent molecular dynamics simulations 27 . Therefore, the several orders of magnitude difference in density can be attributed to three synergistic factors: (i) conceptual distinction: a previously inaccessible population of passivating nanodefects is detected, being distinct from terminal recombinational pinholes. (ii) sample state: prior studies often examined test structures or moderate‑efficiency precursors, whereas our analysis probes fully optimized, industry‑leading devices (>25% efficiency). (iii) detection method: atomic‑resolution HAADF‑STEM directly images and counts these sub‑2 nm, partially oxygenated regions, which are invisible to indirect, diffraction‑limited, or etching‑based techniques.

This revised understanding resolves an apparent contradiction with prior simulations, such as those by Peibst et al. 12 , which suggested that high pinhole densities would be detrimental to performance. That conclusion logically applies to recombinational pinholes. Our findings reveal that in high‑performance TOPCon cells, the interfacial landscape is dominated by an ultra‑dense array of passivating pinholes. Their exceptional passivation quality (due to residual oxygen) and minimal size together enable efficient carrier tunneling without triggering significant recombination. Thus, the key to high performance is not the absence of pinholes, but the passivation of pinholes—a paradigm shift that aligns our experimental findings with superior device metrics.

The pinholes observed in this work act as localized dot contacts between the crystalline Si wafer and the poly-Si layer. To assess their impact, we quantified the contribution of surface recombination and pinhole area fraction to the saturation current density J 0 using Fischer’s analytical model. Based on our quantification of recombination at the size and density of pinholes, an approximate Fischer’s model geometry is established 12,28 , as shown in Fig. 3b, the pinhole fraction is defined, f = πr 2 /P 2 , with r being the point radius and p being the period length of the pinhole distribution, which is actually related to the pinholes’ geometry distribution. While the direct application of this model to TOPCon structures has certain limitations—Fischer assumes uniformly distributed dots with identical surface passivation, whereas in real TOPCon cells, pinholes are irregular and heterogeneous contacting, with poly-Si—it nevertheless provides valuable insight into the underlying trends.

The modeling reveals that for a given pinhole geometry, the total J₀ is predominantly controlled by the surface recombination velocity (S) at the pinholes, rather than by their areal fraction alone. This underscores that passivation quality, not pinhole geometry, is the critical factor governing device performance (Fig. 3b, d). Specifically, higher pinhole fractions shift this transition toward lower S-values, rendering J 0 more sensitive to the pinhole surface properties. For the experimentally observed case with f≈0.1, J 0 increases sharply once S≳10 3 cm/s, and then saturates at a high level above S≈10 5 cm/s. When the pinhole fraction is reduced by an order of magnitude (from f=0.1 to f=0.01), the onset of this rapid increase shifts to higher recombination velocities (S∼10 4 cm/s), indicating a reduced sensitivity of J 0 to the pinhole recombination (Fig. 3d). It can be summarized: (i) the geometry of pinholes (J 0 vs. f) is not the key of high performance of the TOPCon cells, but the passivation of pinholes is a truly key technology; (ii) larger or more pinholes require higher passivation level; (iii) for the passivating pinhole cells, higher pinhole density would lead to lower contact resistance. Thus, passivating pinholes are very necessary and meaningful for the next breakthrough in the TOPCon solar cell community.

The excellent passivation properties and large-area uniformity
Using the technique of the transient photoconductance decay (PCD), the minority carrier lifetime for silicon wafers coated symmetrically with poly-Si/SiO x layer using corresponding the high-PCE and controlled process was measured and shown in Fig. 4a. Clearly, the sample using the high-PCE process has high effective carrier lifetime (τ eff ) of 8.9 ms and that using the low-PCE has relatively low τ eff of 2.96 ms. Saturation current densities, J 0 , were also deduced from PCD measurements according the following Eq. 1, and these were 2.6 and 10.6 fA/cm 2 for the high-PCE and controlled one, respectively (Fig. 4b).

1/τ eff = 1/τ bulk + 2J 0,back (Δn + N D )/qn i 2 W (1)

Where the effective minority carrier lifetime of the silicon wafer is denoted by τ eff , the bulk lifetime of the silicon material is denoted by τ bulk , the back field saturation current density is denoted by J 0,back , the carrier concentration change is denoted by Δn, the bulk doping concentration is denoted by N D , the charge amount is denoted by q, the intrinsic carrier concentration is denoted by n i , and the silicon wafer thickness is denoted by W.

The excellent passivation quality directly explain the higher V oc achieved in cells with passivating pinholes (Fig. 1). Note that the measured difference in V oc between high- and low-PCE cells is 15.9 mV. Based solely on the difference in their recombination current densities (J 0 = 2.60 vs. 10.64 fA cm −2 ), the expected ΔV oc is only ~11 mV, leaving a residual of ~5 mV unaccounted for. We attribute this additional voltage gain to an improvement in the bulk Shockley–Read–Hall (SRH) lifetime induced by the optimized junction-formation process. As highlighted by Krügener et al. 29 , the formation of n-type poly-Si on oxide junctions under optimal conditions not only minimizes interface recombination (J 0 ) but also promotes effective gettering of metal impurities, thereby enhancing the bulk SRH lifetime. In our high-PCE cells, the same optimized annealing that fosters passivating pinholes also facilitates this beneficial gettering. Thus, the total V oc advantage stems from the synergistic improvement of both interface and bulk properties—a hallmark of advanced TOPCon processing that is critical for surpassing 25% efficiency.

A highly spatial resolution of photoluminescence (PL) mapping was performed on the 333.3 cm 2 industrial size solar cell, as shown in Fig. 4c, d, a homogenous contrast throughout all measurements in both two cells, but higher PL intensity on the passivating pinhole cell are observed, indicating that a lower number of interfacial defects exist in the passivating pinhole cell. Furthermore, a Corescan tool was employed to scan the V oc spatial distribution of two cells (Fig. 4e, f). Corescan is an instrument to diagnose the problems of the devices through different measurement modes 30,31 . The similar large-area uniformity and an obvious promotion of the V oc on the passivating pinhole cell are proved.

The difference in FF is still associated with series resistance and, furthermore, contact resistance. Suns-V measurement of the solar cell is performed to obtain the pseudo fill factor (pFF) and series resistance (R s ), as shown in Fig. 4g. It can be seen that the difference of pFF between high-PCE and low-PCE cell is 0.61%, lower than the difference of FF between high-PCE and low-PCE cell (2.8%). Such big difference is related to the difference of series resistance (R s ) between the two samples (357 and 619 mΩ cm 2 for the high-PCE and low-PCE solar cells, respectively, measured by a series-resistance-free method 32 , see the supplementary Fig. 9), which is mainly determined by the charge carrier transport across the SiO x /poly-Si junction on the rear. We employed a TLM method to evaluate the contact resistivity (ρ c ), as shown in Fig. 4h, the high-PCE cell has a ρ c of 4.6 mΩ cm 2 and the low-PCE cell has a ρ c of 5.4 mΩ cm 2 , respectively. Only according to pinhole density affecting ρ c , this is in consistence (Fig. 3). In order to clarify this point, we have studied the diffusion behavior difference of phosphorus dopants from the poly-Si into the wafer near the pinholes (Supplementary Fig. 10a, b). It can be seen that phosphorus diffusion happens across the recombinational pinholes, but is effectively barred near the passivating pinholes. The phosphorus diffusion behavior difference leads to different doping profile (see supplementary Fig. 10c) and then affects contact resistance.

Methods
Tunnel oxide passivated contact solar cell fabrication
In this study, we first immersed c-Si wafers in a KOH solution to remove the damage produced during sawing and to form surfaces with random pyramidal structures. Subsequently, the wafers were immersed in hydrochloric acid (HCl) and hydrogen peroxide (H 2 O 2 ) solutions to remove metal ions and hydrofluoric acid (HF) to remove the oxide layer. Uniform p+ emitters were formed by successive high-temperature injection and oxidation processes in a gaseous B diffusion tube. In order to form locally redoped selective emitters (SE), a laser was utilized to scan the localized area covered by the metal grid lines, and then the borosilicate glass (BSG) layer was removed, thus avoiding the complicated process of using selective masks. A high-temperature annealing method was used to repair the damage produced during the laser treatment. The backside BSG layer was removed by floating the c-Si wafer on the surface of HF solution, and then the c-Si wafer was immersed in KOH solution for backside polishing. Ultrathin tunneling SiO x layers were prepared by plasma thermal oxidation. Intrinsic polysilicon was deposited using LPCVD and later diffused in a tube furnace to form doped polysilicon. Afterwards, front and side phosphor-silicon glass (PSG) was removed by a chain cleaning process. The front and side polysilicon layers are removed by polishing with KOH solution. The front-side BSG and back-side PSG were removed by wet over HF. Al 2 O 3 and SiN x :H stacked coatings were deposited on both sides of the wafers using atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) techniques, respectively. In the metallization stage, silver/aluminum electrodes were screen-printed on the front side, and silver electrodes were screen-printed on the back side. Finally, the entire structure is sintered in a belt furnace at 830 °C to complete the solar cell fabrication process.

Electrical characterization of the solar cells
Internal 1-sun current–voltage (I–V) solar cell parameters were measured under standard test conditions (AM1.5G, 100 mW cm −2 , 25 °C) using a sun simulator. Based on certified I–V current calibration measurements (ISFH test) and verified light J–V curves of the solar cells, gold-coated brass chuck was used to mount the solar cells, and the resistance of the back-side grid and the front-side busbar were neglected. The contact resistivity of different samples was determined by the transfer length method (TLM). According to the theory of TLM, the contact resistivity can be extracted from the intercept of a linear fit, whereas the sheet resistance of the substrate can be extracted from the slope.

Microstructural analysis of the solar cells by aberration-corrected transmission electron microscopy
Several long rectangular specimens of 2.3–2.7 mm in length and 0.8–1.2 mm in width were cut by a diamond wire cutter and counter-bonded with 4–6 pieces. Silicon carbide (SiC) paper was used to grind the cross sections of the counter bonded samples to ~30 μm, and the cross sections were pasted onto a 3 mm diameter molybdenum mesh. We prepared a large number of TEM samples from different directions along the orthogonal X and Y axes of the same cell and analyzed them en masse to yield statistically robust metrics. The TEM samples for the experiments were obtained by argon ion thinning in a Gatan Precision Ion Polishing System (PIPS) using a single gun.

High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images were recorded using aberration-corrected transmission electron microscopes (JEM ARM200F), a microscope operating at 200 kV with a cold-field emission gun, a corrector, and a dual-silicon drift detector; and the Spectra 300 (Thermo Fisher Scientific), a 300 kV monochromator configured with a high-speed (Super-X EDS detector). Electron energy loss spectroscopy (EELS, Gatan continum 1069) measurements were performed in STEM mode. The HAADF-STEM data and EELS data were analyzed using GMS software. For each case, a large number of observations and corresponding EDS as well as EELS analyses were carried out for individual specimens and for different locations of various specimens to show the reproducibility of the results. Absolute thickness of EELS specimen is calculated through the Log-ratio (absolute) method and the inelastic mean free path 33 , the parameters are as follows, convergence semi-angle (mrad) is 69.22, collection semi-angle (mrad) is 207.66.

We quantified the linear pinhole density along the X and Y directions in single HAADF-STEM image. We then conducted statistical analysis on a certain amount of measurement results and multiplied the linear densities in these two directions to obtain the projected density within the entire cell region.

Details of 4D data acquisition and ptychographic reconstruction
The 4D data was collected using an operating voltage of 200 kV, a convergence semi-angle of 25.3 mrad and an under focus of 20 nm. The 4D data had a scanning step size of 0.4 Å, and each scanning point’s CBED pattern consisted of 104 × 104 pixels, with a pixel size of 0.0418 1/Å. To achieve a pixel size of 0.133 Å in the real space of reconstructed phases, each CBED was expanded to 180 × 180 pixels during the reconstruction process. Six probe states and 16 object slices were used in the reconstruction process, with each slice of the object having a thickness of 0.9 nm. The ptychographic reconstruction was performed based on the APP algorithm.

Characterization of passivation quality
Photoluminescence (PL) measurements using a PL tester (LIS-R, BT Imaging Pty Ltd.) to extract carrier recombination and passivation properties. A V oc scan test system (SunLab SCRA1-9001, the Netherlands) was used to scan the cell frontal V oc and obtain the cell V oc distribution map to detect whether the cell achieves large area uniformity preparation. A Sinton WCT-120 lifetime tester was utilized to measure double-sided SiO/poly-Si(n+) symmetric samples prepared on double-sided polished n-Si substrates with conductivity 1–3 Ω cm. From these measurements, we obtained the implied open-circuit voltage (iV oc ) at one sun, the saturation current density (J 0 ) at an excess carrier density of 5 × 10 15 cm −3 and the effective minority carrier lifetime (τ eff ) at a carrier density of 1 × 10 15 cm −3 .

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript investigates two different types of pinholes in TOPCon solar cells using high-resolution imaging techniques, revealing the critical role of oxygen content in the passivation effect of SiOx pinholes, thereby advancing the understanding of pinhole phenomena in this field. Furthermore, the manuscript integrates PL and Voc spatial distribution analyses to elucidate the impact of these two types of pinholes on device performance. Undoubtedly, understanding and controlling the uneven oxide passivated layer is a critical step in TOPCon solar cell production process, making the subject of this manuscript meaningful. However, I regard that the current manuscript lacks sufficient experimental evidence and theoretical support to the so-called pinholes. HRTEM or STEM-HAADF is a two-dimensional projection in a three-dimensional space. Depending on this technique, the so-called pinhole size is difficult to accurately measure because these pinholes overlap along the direction of the electron beam. How to prove that Case 3 also lacks sufficient evidence. It is very inaccurate to determine the oxygen content in a local area through EDS mapping.\n\nIn a short, there is no direct evidence to support the conclusion obtained from authors: the recombinational pinholes, where the lack of oxygen passivation into the lattice leads to direct contact between the poly-Si surface and the lattice of c-Si and formation of more dangling bond defects; and passivating pinholes, where the sufficient oxygen into the lattice contact between poly-Si and c-Si reduces dangling bond defects. I do not recommend this manuscript publishing in Nature Communications.\n\nIn addition, I have several specific suggestions that could further enhance the quality of this work.\n1. In my opinion, using EDS to analyze light elements like oxygen is not appropriate. The signal intensity it collects is largely related to the thickness of the sample you prepared and the electron beam intensity. I recommend using EELS to implement the characterization of oxygen elements in Fig.2.\n2. In Fig.2 b, the thickness of SiOx is evidently uneven (compared to Fig.2 a). Since the authors have emphasized the critical impact of SiOx thickness on performance, I believe the experimental results should be based on samples with similar SiOx thicknesses, which is not difficult to achieve in a factory.\n3. The size and density of the pinholes should be quantified, as this would help readers gain a deeper understanding of the points proposed in the manuscript. AFM should be used to detect the morphology of SiOx layer from the top, rather than just observing the cross-sections through TEM or STEM.\n4. It is puzzled that the method for controlling the oxygen content of the two different types of pinholes is not specifically discussed but merely briefly mentioned in the supplementary information. This is a highly important aspect, and I believe it should be included in the main text with a more detailed discussion.\n5. Can atomic-scale imaging be directly correlated with the PL intensity/Voc distribution of the device? I believe this might raise doubts among some readers as well. I would like to see some microscopic performance metrics to demonstrate that the decline in device performance is indeed caused by pinholes. This would help establish a cross-scale structure-performance relationship. If appropriate characterization methods are not available, simulations could be incorporated to support the findings.\n6. The beneficial role of oxygen in interface passivation is not a novel perspective. I would like to see more discussion on the nature of the pinholes—whether they originate from the thermal oxidation process or the poly-Si deposition process—and how they can be improved. This is a crucial aspect to address.", "reviewer_2": "Thanks a lot for the possibility to review the manuscript entitled „Passivating pinholes for large-area and high-efficiency TOPCon solar cells”.\nIt is an interesting and relevant paper. I agree with the authors' point of view on the importance of photovoltaics as the main energy conversion technology in a future with renewable energy and record low LCOE. I also agree that TOPCon solar cells have emerged as the new workhorse in photovoltaics and could retain this position for a long time. Especially in combination with other innovations such as LECO and laser transfer printing, an almost perfect (optically transparent and contact recombination free) front side can be combined with a “perfect” poly-Si/SiOx passivated back side to create the “ultimate” solar cell. So the importance of progress in this area is obvious. I also agree that pinholes in the interfacial oxide are a crucial aspect for understanding the device physics of TOPCon solar cells and that one should expect a gradual transition from “initial oxide thickness in the pinhole” to “no oxide in the pinhole” when pinholes are formed. It is interesting to see experimental evidence for this transition.\nThe manuscript is well written. Advanced characterization methods – in particular the HAADF-STEM & EDS measurements - are applied. The efficiency level up to 25.4 % of the solar cells investigated is high, although meanwhile state-of-the-art for the leading TOPCon companies rather than outstanding (see e.g. Trina’s 26.6%, Hanwha Q.Cells 25.7%).\nThe main novelty of the manuscript is the observation of a different amount of remaining oxygen in pinholes, as resolved by HAADF-STEM & EDS. I think this finding is definitely worth to be published. Nevertheless, I’m questioning whether it should be published in Nature Communications. An alternative would be for example Solar RRL. Furthermore, I’I think that prior to any publication, the manuscript should be strongly revised.\nSo far, I think that the story of the manuscript is incomplete due to the following main issues:\n1. Analysis of IV-parameters and their correlation to the properties of the different poly-Si/SiOx junctions: I am afraid that the observed difference in oxygen content in the pinholes is not as crucial for the device performance as the authors conclude from the phenomenological comparison of IV parameters of different devices. The reported difference in passivation quality - 1.5 fA/cm^2- seems far from sufficient to explain the observed difference of ~30 mV in Voc. I calculated the total J_01 for the entire “good” device from Voc and Jsc, assuming a one-diode model and an ideality factor of 1 for simplicity. The resulting value of J_01,total of 13.4 fA/cm^2 seems reasonable, considering the ~5.5 fA/cm^2 for the front side mentioned by the authors, ~5 fA/cm^2 for the recombination in an excellent wafer and the 2.6 fA/cm^2 for the back side. Adding the 1.5 fA/cm^2 from above, the total J_01,total for the “bad” device is 14.9 fA/cm^2. This in turn should still allow a Voc of 736 mV! However, the “bad” device only has 709 mV. This is clearly contradictory! It seems that the “bad” device has other problems besides a slightly worse passivation of the poly-Si/SiOx junction.\nI request the authors to resolve this inconsistency. Is there any further difference between the “good” and “bad” sample besides the additional 1.5 fA/cm^2 on the rear? Have the symmetric lifetime test samples experienced all process steps relevant to be comparable to the final solar cells (e.g. the fast firing)? One hint might be seen in the QSSPC curves in Fig. 3b: not only the slope of the curve 1-tau_eff(Delta_n)-curve is different, but also the intercept with the y-axis. This intercept corresponds to the (inverse) bulk lifetime. The latter is roughly ~12ms for the “good sample” and ~6ms for the “bad” sample.\nFurthermore, the difference in FF needs to be analyzed in more detail. As stated by the authors in the introduction, pinholes without oxide should facilitate efficient charge carrier transport, resulting in a low contact resistance between the poly-Si and the wafer (lower than for the case of remaining oxide in the pinholes). However, the FF of the “bad” device w./o. oxide in the pinholes is by 1,6% abs. lower that of the “good” device w. oxide in the pinholes. This appears puzzling. Parts of the explanation could be the recombination behavior the wafer, since Auger recombination (with a low ideality factor of ~2/3 and thus a high pseudo fill factor) should be more pronounced in the “good” device with high Voc and thus a high injection level.\nThe authors should distinguish between bulk-recombination-induced fill factor limitations and series resistance-induced fill factor limitations to learn anything about the charge carrier transport across the SiOx/poly-Si junction on the rear. For this purpose, series resistance and pseudo fill factor should be analyzed.Optionally, the poly-Si/SiOx junction could be probed directly by e.g. Cox & Strack method.\n2. Quantification of recombination at the different types of pinholes: It is difficult to conclude the difference in recombination behavior caused by the pinholes with and without residual oxide. One complication could be a locally enhanced diffusion of dopants from the poly-Si into the wafer near the pinholes, as demonstrated by Firat et al. using SSRM (ACS Applied Materials & Interfaces, 14(15), pp. 17975-17986, 2022). This diffusion of dopants - which should be enhanced in the pinholes without residual oxide - reduces the local minority carrier concentration and thus the recombination at the pinhole. This effect counteracts the advantage of “residual surface passivation by residual oxide”. Sophisticated modeling would be required to really take into account all potentially relevant aspects (see e.g. Folchert et al., Progress in Photovoltaics: Research and Applications 28 (12), pp. 1289-1307, 2020).\nMost striking, however, is the fact that in previous modelling works – also in Ref. [12] cited by the authors – the highest possible recombination velocity in the pinholes (1e7cm/s as limited by the thermal velocity) was already assumed. Nevertheless, lowest “macroscopic, area-averaged” J0-Values below 1 fA/cm^2 for poly-Si/SiOx are still consisted with this assumption as long as the pinhole area density is low enough (<~1e7 1/cm^2) and the regions in which the interface is still intact is well passivated. Thus, for a low-enough pinhole area density, there seems to be no need to “passivate” the pinholes!\nI request the authors to quantify the recombination behavior of their pinholes (beyond QSSPC measurements) by modelling, and to distinguish the recombination in the pinholes from the recombination in the regions in which the oxide is still intact. For this, an experimental determination of the pinhole area density would be required. If there is no advanced simulation software available, the Fischer model (Plagwitz et al., Progress in Photovoltaics: Research and Applications 14, pp. 1–12 2006.) should be a suitable approach.\n3. Statistical significance: There is always a concern about the statistical significance of the HAADF-STEM & EDS results, in particular when concluding on differences for different samples. I understand that these investigations are very time-consuming, work-intensive and expensive, and I therefore hesitate to request further measurements. Nevertheless, the authors should state on the statistics of the HAADF-STEM & EDS investigations. How many samples have been investigated? What is the thickness of the sample in the analyzed region? How reproducible are the observed differences in the amount of remaining oxide in the pinholes? Was it just observed in one respective sample / at one microscopic location? Do samples from different positions from each wafer yield the same results? Was a wafer-to-wafer reproducibility checked?\n4. Root cause of different pinhole microstructures: It does not become clear what is exactly causing the difference in the oxygen content in the pinholes between the different samples. The process sequence depicted in Supplementary figures 3 and 4 appear identical, and also the process parameters for the poly-Si/SiOx junction formation appear similar. Is it a difference in surface polishing / cleaning? Is it just the 20 K difference in the oxidation temperature for the interfacial oxide growth? The 3 K difference for the poly-Si deposition? The 3 K or the 6 minutes difference in the POCl3 diffusion? It would be desirable to show a systematic variation series of the relevant parameters rather than only two data points. In any case, the authors should deduce a clear recommendation how to optimize the junctions with respect to the aspect “pinhole structure” as promoted in the manuscript. Otherwise, the practical relevance of the findings is limited.\nMinor issues:\n• Abstract, line 4: typo “ploy”\n• Introduction, end of first paragraph: Why different units for LCOE in different regions?\n• Introduction, end of third paragraph: missing space between “and” and “interface”\n• Introduction, beginning of last paragraph: meaning of “HAADF-STEM” not explained\n• Section “Large-area and high-PCE TOPCon solar cells: SiO2/Al2O3 passivation is mentioned for the front-side passivation but only ALD deposition of Al2O3 is mentioned in the “methods” section\n• Section “The excellent …”, Eq. (1): N_D instead of N_A for n-type material, n_i in denominator\n• Methods section, TOPCon solar cell fabrication: What is “plasma thermal oxidation”?"}, "author_response": "Firstly, we sincerely appreciate the reviewer’s time and effort in evaluating our manuscript. We also agree with the reviewer’s suggestion that the addition of EELS is better for the characterization of oxygen elements. Fortunately, despite indeed a significant amount of additional experimental work and clarification, we have still obtained the EELS data, as shown in the new Fig. 2, with the related discussions in the manuscript. It can be seen that EELS data give the similar results with EDS, i.e., direct evidence, that the pinhole in the high-PCE device has still sufficient oxygen into the lattice contact between poly-Si and c-Si, (Fig. 2e) corresponding to small oxygen-lack valleys (Fig. 2g).\n\nWe fully agree with the reviewer's comments and have re-selected the cells to ensure that the SiO thicknesses were similar, x and TEM and spherical aberration electron microscopy tests were again performed on both the high-efficiency and low-efficiency cells, and the relevant data are updated in Fig. 2.\n\nThis is a very good suggestion. We have achieved the quantization of the size of the pinholes: The cross-sectional size (thickness×diameter) of pinholes is disclosed: passivating pinholes are ~1.6±0.2 nm×1.4±0.3 nm and recombinational pinholes are ~1.5±0.2 nm×1.4±0.3 nm, respectively (the statistical results of 40 samples). Regarding the density of the pinholes, we introduce a X-Y orientated Cs-corrected HAADF-STEM method, i.e., sampling Cs-corrected HAADF-STEM along X-axis and Y-axis respectively and then sketching pinholes distribution (please see Fig. 3a and 3b), to achieve the quantification. We have also obtained the density of the pinholes: passivating pinhole density of 5×1012 cm-2, and recombinational pinhole density of 1×1013 cm-2, being be counted from total 40 high-efficiency and low-efficiency solar cells.\n\nMore relative discussions can be seen in the response to the issue 2 from reviewer 2#.\n\nHowever, regarding the AFM measurement, we understand that the reviewer’ s suggestion is for the area density of the pinholes, it is indeed very difficult. We think about it, it seems like that AFM should be used to detect the morphology of SiOx layer from the top, but the pinholes exist in a very thin interface. If we want to see them from the top of the ultrathin interface, the requirement for sample preparation is extremely high. We are sorry we don’t succeed although we tried a lot. Here, a X-Y orientated Cs-corrected HAADF-STEM method, is firstly used, but it can give sensible results, which also support simulations (please see Fig. 3c).\n\nThe related content has been added in the main text, Page 4, Line 15-26. The formation of the oxygen content of the two different types of pinholes depends mainly on the differences in the process of the thermal oxidation process. This is manifested in the control of parameters such as temperature, time, gas atmosphere and pressure during the thermal oxidation process, as shown in Supplementary Fig. 3. The oxidation process uses a two-step oxidation method, first at a certain temperature for a period of time through the oxygen for oxidation, the formation of a thin layer of silicon oxide, and then smothered oxygen treatment for a period of time, no longer through the oxygen. The formation process of passivation pinhole with sufficient oxygen content passes oxygen for a longer period of time, higher temperature, greater gas flow, lower pressure, more conducive to the line into a uniform dense oxide layer, the formation of the pinhole also contains a sufficient amount of oxygen. The thickness of the oxide layer is highly sensitive and crucial. Therefore, under the superimposed changes of these parameters, it is of vital importance for the quality of the oxide layer and the formation of pinholes.\n\nPL intensity is directly correlated with microscopic defects and can reflect a passivation level in device1. V distribution of oc the device measured by a Corescan tool is not correlated with atomic-scale imaging. Here, we employed the PL to correlate the interface passivation and the Voc distribution to reveal the large-area uniformity. We use the same samples between the C -corrected HAADF-STEM and PL/V measurements, and therefore in fact a cross-scale structure-performance S oc relationship has also been established.\n\nRegarding simulations’ suggestion, we deeply appreciate the reviewers to guide us and refine and elevate the quality of our research significantly. With the help of our cooperator, we have achieved fruitful simulations (see Fig. 3c and the response to the issue 2 from reviewer 2#).\n\nThe more discussion has been added in the main text. In TOPCon cell preparation, thermal oxidation is a critical step to form an ultrathin silicon oxide layer on the wafer surface. However, this process is affected by a variety of factors, such as oxidation temperature, time, and gas atmosphere, etc. If these parameters are not properly controlled, it may lead to uneven growth of the silicon oxide layer, which may result in the formation of pinholes. However, by accurately controlling the parameters such as temperature, time and gas atmosphere during the thermal oxidation process, it can be ensured that the silicon oxide layer can grow uniformly and the oxygen content in the pinholes is sufficient to improve the cell performance. A two-step oxidation method was used, in which oxygen with a flow rate of 20,000 sccm was firstly passed at a temperature of 600°C for oxidation for 400s at a pressure of 85,000 Pa to form a thin layer of silicon oxide, and then smothered oxygen treatment was carried out without further passage of oxygen to continue the smothering process for 400 s, which helped to improve the quality of the oxide layer and obtain passivating pinholes with sufficient oxygen content.\n\nWe thank the reviewer’s positive evaluation of our work. To be honest, the high impact journal is still our top priority when we considered the submission of this short communication because this work is indeed a significant breakthrough for understanding the interface nature of the TOPCon solar cell and guiding silicon photovoltaic industry upgrading. In this fully and thoroughly revised version, a much stronger case of the mechanism of the formation of pinholes and of the nature of these pinholes are presented, again explaining the reasons why this work could be not only a clear understanding of the work mechanism of the TOPCon solar cells but also very valuable for the future TOPCon-based PV (such as TOPCon-BC and perovskite/TOPCon tandem solar cells) industry.\n\nWe appreciate the reviewers' comments regarding the inconsistency between the V and J data for high-efficiency and oc 0 low-efficiency devices. In order to improve this aspect, we have selected new experimental samples and updated the experimental data to resolve this inconsistency. The updated data are shown in Fig. 1d and 4a,b of the main text. In the updated experimental data, the actual J difference between the “good” and “bad” samples was found to be 8.04 fA/cm². 0 According to the one diode model, the open-circuit voltage of the “bad” device is theoretically maintained at approximately 727 mV, with the actual measured open-circuit voltage being 722.8 mV, being under a reasonable range. The symmetric lifetime test samples underwent all processing steps related to the final solar cell. In the updated experimental data, the QSSPC curve in Fig. 4b shows that the two samples have nearly identical y-axis intercepts. Actually, because both samples in this work are from the production line, we can ensure the same bulk lifetime of wafers.\n\nCorrespondingly, a Suns-Voc measurement of the solar cell is performed to obtain the pseudo fill factor (pFF) and series resistance (Rs), as shown in Fig. 4g. It can be seen that the difference of pFF between high-PCE and low-PCE cell is 0.61%, higher than the difference of FF between high-PCE and low-PCE cell (2.8%). Such big difference is related to the difference of Rs between two samples, which is determined by the charge carrier transport across the SiOx/poly-Si junction on the rear. A TLM method is employed to evaluate the contact resistance, as shown in Fig. 4h. The difference of the contact resistance can be explained by the diffusion behavior difference of phosphorus dopants from the poly-Si into the wafer near the pinholes (please see the supplementary Fig. 8a,b). It can be seen that phosphorus diffusion happens across the recombinational pinholes, but is barred near the passivating pinholes. The phosphorus diffusion behavior difference leads to different doping profile (see the supplementary Fig. 8c) and then affects contact resistance.\n\nIt is correct that here a locally enhanced diffusion of dopants from the poly-Si into the wafer near the pinholes happens. However, the new finding is that phosphorus elements can diffuse across the recombination pinholes, but is barred near the passivating pinholes (the supplementary Fig. 8a,b). That is, the passivating pinholes with sufficient oxygen is a diffusion barrier of dopants from the poly-Si into the wafer due to sufficient oxygen into the lattice contact between poly-Si and c-Si. This can not counteract the advantage of “residual surface passivation by residual oxide”. It would affect the doping profile (the supplementary Fig.8c) and favor contact properties (Fig. 4h).\n\nQuantification of recombination behavior of pinholes and introduction of modeling are very constructive suggestions. In response to the reviewers' insightful suggestions, extensive supplementary experiments and simulations have been added, and therefore the overall rigor and academic value of our paper have been remarkably enhanced. Firstly, we introduce a X-Y orientated Cs-corrected HAADF-STEM method, i.e., sampling Cs-corrected HAADF-STEM along X-axis and Y-axis respectively and then sketching pinholes distribution (please see Fig. 3a), to achieve the quantification of pinhole area density. We obtain more detailed pinhole data: 1) pinholes size (thickness × diameter): passivating pinholes are ~1.6±0.2 nm×1.4±0.3 nm and recombinational pinholes are ~1.5±0.2 nm×1.4±0.3 nm, respectively (the statistical results of 40 samples); 2) pinhole area density: passivating pinhole density of 5×1012 cm-2, and recombinational pinhole density of 1×1013 cm-2. So far, pinhole quantification in experimental is unclosed by us using advanced characterization methods –the HAADF-STEM & EDS measurements. These experimental data have just been used for simulations.\n\nAccording to the reviewer’s guidance, we employ Fischer’s approach to calculate “macroscopic, area-averaged” J -Values 0 as function of pinhole passivation, as shown in Fig. 3c. It can be seen that passivating pinholes are necessary and meaningful. Related discussions have been added in Page 6, Line 1-24, “The pinholes observed in this work act as localized dot contacts between the crystalline Si wafer and the poly-Si layer. To assess their impact, we quantified the contribution of surface recombination and pinhole area fraction to the saturation current density J using Fischer’s analytical 0 model. Based on our quantification of recombination at the size and density of pinholes, an approximate Fischer’s model geometry is established, as shown in Fig. 3b, the pinhole fraction is defined, f=πr2/P2, with r being the point radius and p being the period length of the pinhole distribution, which is actually related to the pinholes’ geometry distribution. While the direct application of this model to TOPCon structures has certain limitations—Fischer assumes uniformly distributed dots with identical surface passivation, whereas in real TOPCon cells pinholes are irregular and heterogeneous contacting with poly-Si—it nevertheless provides valuable insight into the underlying trends.\n\nThe modeling reveals that, for a given pinhole geometry (fixed radius and pitch, hence fixed area fraction f), the total J of 0 the device strongly depends on the surface recombination velocity S at the pinholes once a critical threshold is reached. Specifically, higher pinhole fractions shift this transition toward lower S-values, rendering J more sensitive to the pinhole 0 surface properties. For the experimentally observed case with f≈0.1, J increases sharply once S≳103 cm/s, and then 0 saturates at a high level above S≈105 cm/s. When the pinhole fraction is reduced by an order of magnitude (from f=0.1 to f=0.01), the onset of this rapid increase shifts to higher recombination velocities (S∼104 cm/s), indicating a reduced sensitivity of J to the pinhole recombination (Fig. 3c). It can be summarized: (i) the geometry of pinholes (J vs f) is not the 0 0 key of high performance of the TOPCon cells, but the passivation of pinholes is a truly key technology; (ii) bigger or more pinholes require higher passivation level; (iii) for the passivating pinhole cells, higher pinhole density would lead to lower contact resistance. Thus, passivating pinholes are very necessary and meaningful for the next breakthrough in TOPCon solar cell community.”\n\nOur experimental data were obtained through multiple batches and multiple samples. The statistical results are shown in Table S3. We tested 40 samples. By precisely controlling and measuring the thickness of the samples, we ensured that the thickness of the analysis area was within 100 nm. All samples in the manuscript were prepared on an industrial production line, demonstrating excellent cell repeatability and uniformity. These variations were not observed in a single sample or at a single location but were found in multiple samples at different locations, as shown in supplementary Fig. 5.\n\nTable S3. Statistical results of sample preparation information for HAADF-STEM samples.\nGroup Date Sample quantity Sample thickness (nm) Pinhole density (nm-1)\nG1 2024.06.18 4 100 0.15\nG2 2024.08.10 11 100 0.17\nG3 2024.08.12 5 100 0.15\nG4 2025.02.26 5 100 0.15\nG5 2025.04.09 4 100 0.18\nG6 2025.05.17 3 100 0.18\nG7 2025.07.08 4 100 0.15\nG8 2025.07.15 4 100 0.15\n\nWe are very grateful for the reviewers' questions and suggestions. We have provided a detailed description and supplement of the sample preparation process and procedure in the manuscript, Page 4, Line 15-26. We have also explained the reasons for the differences in oxygen content in the pinholes between different samples. It can be seen in the response to the issue 4 and 6 from reviewer 1#).\n\nWe are very grateful to the reviewers for their questions and suggestions. We have corrected all minor issues in the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "A significant reservation persists regarding the methodological approach, which I hope can be adequately addressed. The authors state that they employed an X-Y-oriented Cs-corrected HAADF-STEM method to visualize the distribution of pinholes and perform quantitative analysis.\n\nThe primary objective of the study is to achieve high-resolution three-dimensional imaging to precisely locate and quantify pinholes within the SiOx layer. However, the specific imaging and analytical methodology employed raises substantial questions: Was this accomplished via 4D-STEM or a three-dimensional reconstruction technique? Current microscopy capabilities cast serious doubt on achieving the resolution claimed by the authors.\n\nFurthermore, the preparation of TEM cross-sectional specimens from photovoltaic devices is inherently a destructive process. Consequently, it is highly improbable that the authors could perform re-imaging of the identical location along both the X- and Y-axes on the same specimen. The prospect of achieving genuine 3D imaging and modeling thus appears fundamentally unfeasible under these conditions.\n\nAccording to the authors' descriptions, the majority of the pinholes are sub-2 nm in size. In contrast, the thickness of conventional TEM specimens typically ranges from 50 to 100 nm. Along the electron beam direction (Z-axis), this thickness is sufficient to cause significant superimposition of numerous pinholes within the projected image. This superimposition would severely compromise any analysis of the true spatial distribution of pinholes and introduce critical ambiguities in elemental analysis. The entire framework of the authors' microstructural analysis is predicated on this inherently flawed two-dimensional projection.\n\nTherefore, to validate their core claims, it is imperative that the authors demonstrate Z-direction resolution (tomography) on individual specimens. The implementation of high-resolution 4D-STEM methodology is strongly recommended as a potential solution, as the validity of the manuscript's conclusions fundamentally rests upon this capability.", "reviewer_2": "I would like to thank the authors for their thorough revision of the manuscript. They have seriously addressed my comments (and those of the other reviewer). They have performed additional experiments and calculations. They have mitigated inconsistencies and added further details on sample preparation. The latter facilitates the reproducibility of their results by others and thus increases the significance of the work. I think the manuscript is now almost ready for publication.\nNevertheless, I have two comments that correspond to a recommendation for ‘minor revisions’:\nThe recent findings are quite different from previous works that have coined the term “pinhole” in the community. I assume that most researches consider a “pinhole” to be a nanometer sized hole in the interfacial oxide, with no remaining oxide in this region and with an areal density in the range of ~1e6-1e8 cm^(-2) as determined by various methods such as selective etching, EBIC, …. By contrast, “pinholes” in the recent manuscript do feature remaining, very thin silicon oxide, and their areal destiny – as determined by HAADF-STEM- is ~6! orders of magnitude higher than in previous reports. That’s also why their passivation quality now matters. The difference to the previous picture is that striking that I suggest to further discuss their relation to the structures investigated in previous works, and to even think about specific naming for these structures. Would it be too speculative to assume that the now observed structures are ‘precursors’ to ‘classical pinholes’ that would form from some of these precursors under higher thermal conditions? In other words, that they are not yet ‘fully formed’ pinholes, but rather ‘local thinning’ in the oxide? ‘Emerging pinholes’, so to speak? Some insights on the evolution of pinholes could be obtained from the comparison of the recent experimental results to theoretical predictions from Molecular Dynamic simulations (see Diggs et al., ACS Appl. Energy Mater. 2024, 7, 3414−3423, https://doi.org/10.1021/acsaem.4c00171)\nMy second comment is that I still believe that the remaining discrepancy between the voltage difference observed in the IV measurement between ‘good’ and ‘bad’ solar cells (15.9 mV) and the expectations for the different J0 values (11 mV) is at least partly due to a difference in bulk lifetime in the wafer. The intersection values of the 1/tau_eff (Delta_n) curve with the y-axis in Fig. 4 (b) and the effective lifetimes at low injection levels in Fig. 4 (a) differ significantly. Even if the wafers were identical before processing, different process conditions can lead to different bulk lifetimes after the manufacturing process. In particular, the effectiveness of impurity gettering in poly-Si (which generally improves the bulk lifetime) depends on the properties of the poly-Si/SiOx/c-Si interface. For n-type doped poly-Si and different annealing conditions, the highest tau_SRH values seem to coincide with the lowest J0 values (see Krügener et al., http://dx.doi.org/10.1016/j.solmat.2017.05.055, Fig. 5). I consider this to be worthy of discussion/mentioning because (i) the discrepancy in the voltage difference is still significant and (ii) it is even an advantageous aspect of the optimised process conditions that not only J0 is minimised, but also the bulk lifetime is maximised."}, "author_response": "We sincerely thank the reviewer for their insightful comments and fully appreciate their concerns regarding the methodological approach. We have carefully considered each point and provide the following clarifications and additional evidence to address the raised issues.\n\n1. Feasibility and Rationale of the X‑Y‑Oriented Cs‑corrected HAADF‑STEM Method. The reviewer correctly notes the challenge of specimen thickness causing superposition along the electron‑beam direction. In our study, the X‑Y‑oriented Cs‑corrected HAADF‑STEM imaging was employed not as a 3D tomography technique, but as a high‑resolution projection method optimized for visualizing and statistically analyzing pinhole features within the SiO layer. As the reviewer noted, the probability of x re-imaging the same location along both the X and Y axes on a single specimen is exceedingly low. We sectioned the same cell along orthogonal X and Y directions, extracted a statistically significant number of foils, and derived the pinhole parameters from pooled analysis. Given that the pinholes are sub‑2 nm in size—much smaller than the specimen thickness—their projected images in HAADF‑STEM remain distinct due to strong Z‑contrast, allowing reliable identification and counting. This approach is well‑established for quantifying nanoscale features in cross‑sectional samples, provided the features are spatially separated in projection, which is confirmed by our statistical analysis across multiple regions.\n\nWe acknowledge that projection-based imaging has inherent limitations for general three-dimensional analysis. However, for the specific objective of this study—to quantify an ultra-high density (~10 ²¹cm ⁻)² of sub‑2 nm interfacial features within a complete, as-fabricated device stack, the methodology employed is not only appropriate but rigorously validated.\n\nThe combination of atomic-resolution Z‑contrast imaging (HAADF‑STEM), orthogonal sampling across a statistically significant number of specimens, and targeted complementary experiments (4D‑STEM ptychography, thickness‑dependent EELS mapping) forms a robust analytical framework. This approach is established for interrogating nanoscale features in functional layered materials where true atomic‑resolution 3D tomography over large areas remains challenging. Our results, confirmed by consistency with theoretical simulations, demonstrate the validity and reliability of this strategy for the present system.\n\n2. High-resolution 4D-STEM Assessment of Superposition Effects on Spatial Distribution Analysis. We acknowledge that superposition in projection could, in principle, affect distribution analysis. Following the reviewer's recommendation, we conducted high‑resolution 4D‑STEM ptychography to obtain phase‑contrast information along the beam direction. We employed 4D-STEM to perform ptychographic imaging along the thickness direction, achieving atomic-resolution imaging of the Si-SiO -poly-Si structure x in Fig. S7 and Fig. S8. However, we note that while ADF imaging provides Z-contrast that makes pinhole structures appear more prominent, the reconstructed ptychographic results yield linear contrast, making these features less distinct in the final reconstruction [1].\n\n3. Large-Area HAADF images and EELS Mapping Confirm Thickness-Independent Response. To directly verify the influence of the superposition effect in the projection on the statistical results, we performed complementary EELS‑mapping across regions of varying specimen thickness (from ~0 nm to ~165 nm). Quantitative comparison of pinhole density and distribution in these areas showed no significant thickness‑dependent variation (Supplementary Excel Data 2). This consistency indicates that the superposition effect does not materially distort our conclusions regarding pinhole spatial distribution. Moreover, our reported pinhole densities align closely with theoretical simulations, further supporting the reliability of our 2D‑projection‑based statistical approach.\n\n4. Corresponding Revisions in the Manuscript & Significance of the Study. In response to the reviewer‘s comments, we have revised the manuscript to (Page 5, Line26 ~ Page 7, Line 10):\nClarify in the Methods section that the X‑Y‑oriented HAADF‑STEM analysis provides a high‑resolution projected 2D statistical assessment, and we have explicitly discussed the rationale for its use along with the steps taken to evaluate potential superposition effects.\nInclude the new 4D‑STEM ptychography data as Supplementary Figures S7 and S8, with a detailed description of the acquisition and reconstruction parameters in the Supplementary Methods.\nPresent the EELS thickness‑variation analysis (Supplementary Excel Data 2) and a corresponding discussion in the Results section to demonstrate the robustness of our distribution analysis against superposition effects.\nStrengthen the discussion by emphasizing that our sample—a world-leading TOPCon solar cell with an intact layer stack—provides a realistic, non‑etched, and non‑AFM‑perturbed representation of the SiOx layer, and that the measured pinhole density matches theoretical simulations, underscoring the physical relevance of our findings.\n\nMost importantly, this study focuses on a critically relevant technological problem: understanding the pinhole distribution in the SiOx layer of TOPCon solar cells, which currently command >70 % of the photovoltaic market and continue to gain share. The mechanism governing the quality of this interfacial layer is urgently needed by the field, and our work—performed on a world-leading, intact device without etching or AFM‑induced artifacts—provides direct experimental insight that aligns closely with theoretical simulations. The consistency between the HAADF‑STEM and 4D‑STEM results further strengthens the reliability of our findings, underscoring that the methodological framework adopted is not only state‑of‑the‑art but also physically conclusive for addressing this pressing industrial and scientific question.\n\nWe thank the reviewer again for raising these important points, which have helped us improve the clarity and robustness of the study. We hope our detailed responses and additional data satisfactorily address the concerns raised.\n\n[1] H. Sha, J. Cui, and R. Yu, Deep sub-angstrom resolution imaging by electron ptychography with misorientation correction, Science Advances 8, eabn2275 (2022)\n\nWe sincerely appreciate the reviewer's insightful comments, which raise crucial conceptual and technical points that help clarify the context and significance of our findings. We have carefully considered both issues and provide the following detailed responses, which have also been incorporated into the revised manuscript and supporting information.\n\nOn the Definition, Density, and Nature of the Observed Pinholes, we fully agree with the reviewer that the term “pinhole” in the classical literature (e.g., in early POLO and TOPCon studies) typically refers to a nanometer‑sized, oxide‑free, through‑going perforation with an areal density in the range of ≈10⁶–10⁸ cm⁻,² often detected by methods such as selective etching or EBIC. In stark contrast, the nanoscale features we observe in the SiOx layer via HAADF‑STEM are fundamentally different, and we thank the reviewer for the constructive suggestion to introduce distinct nomenclature. Accordingly, in the revised manuscript, we explicitly define two cases: “Recombinational pinholes”: Referring to the classical, oxide-free direct Si-Si contacts that act as strong recombination centers. “Passivating pinholes”: Referring to the newly identified, localized oxygen-depleted (yet oxygen-containing) nanodomains (~1.5 nm) within the SiO layer, which retain passivation quality. x\n\nWe attribute the striking difference in reported density (~10 ²¹cm ⁻ ²in our work vs. ≈10⁶–10⁸ cm⁻)² to three synergistic factors, which we now discuss in a dedicated subsection:\n1) Conceptual distinction: “Passivating” vs. “Recombinational” pinholes.\nThe classical “recombinationalpinholes” are formed at higher temperatures (often >1050 °C) and are essentially direct Si‑Si contacts that act as strong recombination centers. In our work, the devices were processed under optimized, lower‑temperature conditions that promote the formation of what we term “passivating pinholes.” These are localized, oxygen‑depleted (yet not oxygen‑free) regions within the SiOx layer that arise from nucleation‑driven phase separation under hydrogen‑rich conditions, as recently modeled by Diggs et al. (ACS Appl. Energy Mater. 2024, 7, 3414‑3423). In their simulations, such “proto‑pinholes” (or “emerging pinholes”) appear as hour‑glass‑shaped, Si‑rich nuclei that do not fully penetrate the oxide unless further elevated temperature or prolonged hydrogen exposure is applied. Our experimental conditions stabilize these features without allowing them to evolve into classical through‑going holes. Hence, they are fundamentally different in structure, formation mechanism, and electronic function.\n2) Sample state: Full high‑efficiency devices vs. precursors.\nMost previous reports have studied pinhole densities on laboratory precursors, test structures, or cells with moderate efficiencies. In contrast, our analysis was performed directly on world-leading TOPCon solar cells (with independent certification of efficiency >25%). The extremely high‑performance of these devices implies that the interfacial oxide layer is in an optimized state, which inherently favors a high density of nanoscale oxygen‑depleted regions that nevertheless maintain excellent passivation. To illustrate this point, we have added a table in the Supplementary Material (Table S2) comparing the pinhole densities reported in the literature with those observed in our work, explicitly linking the values to the device efficiency and measurement method.\n3) Detection method: HAADF‑STEM vs. indirect techniques.\nThe atomic-resolution HAADF‑STEM technique allows direct imaging of these sub‑2 nm, partially oxygenated regions, which are invisible to conventional indirect methods like selective etching or EBIC, explaining part of the historical discrepancy.\n\nIn the revised manuscript, we have explicitly introduced the term “passivating pinholes” and clarified their structural and functional distinction from classical “recombinational pinholes.” We also cite the work of Diggs et al. to provide a theoretical foundation for their nucleation‑mediated formation under hydrogen‑rich, moderate‑temperature conditions.\n\nWe are particularly grateful for the reviewer's conceptual suggestions regarding nomenclature and the link to theoretical precursors. These insights have allowed us to significantly refine the narrative of our work. By adopting the distinct terminology of “recombinational” versus “passivating” pinholes and integrating the concept of “metastable precursors” from the simulations of Diggs et al., we have been able to frame our experimental discovery not merely as an observation, but within a broader and more predictive theoretical context.\n\nThe reviewer's comments have been instrumental in elevating the conceptual clarity and impact of our study, thereby strengthening the connection between our experimental findings and the evolving understanding of interface physics in passivating contacts.\n\nWe thank the reviewer for pointing out the ~5 mV discrepancy and the insightful connection to bulk gettering. We fully agree with this analysis. The additional voltage gain is indeed a signature of enhanced bulk SRH lifetime due to impurity gettering during our optimized junction-formation process, consistent with the mechanism reported by Krügener et al.\n\nWe have revised the manuscript to explicitly discuss this point: a paragraph has been added in the “The excellent passivation quality…” section (Page 8, Line 12) that quantifies this contribution, cites Krügener et al., and frames the synergistic improvement of both interface (J₀) and bulk (SRH lifetime) properties as a key advantage of our optimized TOPCon process.\n\nWe believe these revisions directly address the reviewer‘s concerns and significantly strengthen the manuscript‘s clarity and impact. We thank the reviewer again for the constructive suggestions."}, {"round": 3, "reviewer_comments": {"reviewer_1": "Thanks for the reply. The authors have addressed my concerns."}, "author_response": ""}]

Decision letter