Effects on Metallization of n+-Poly-Si Layer for N-Type Tunnel Oxide Passivated Contact Solar Cells

Thin polysilicon (poly-Si)-based passivating contacts can reduce parasitic absorption and the cost of n-TOPCon solar cells. Herein, n+-poly-Si layers with thicknesses of 30~100 nm were fabricated by low-pressure chemical vapor deposition (LPCVD) to create passivating contacts. We investigated the effect of n+-poly-Si layer thickness on the microstructure of the metallization contact formation, passivation, and electronic performance of n-TOPCon solar cells. The thickness of the poly-Si layer significantly affected the passivation of metallization-induced recombination under the metal contact (J0,metal) and the contact resistivity (ρc) of the cells. However, it had a minimal impact on the short-circuit current density (Jsc), which was primarily associated with corroded silver (Ag) at depths of the n+-poly-Si layer exceeding 40 nm. We introduced a thin n+-poly-Si layer with a thickness of 70 nm and a surface concentration of 5 × 1020 atoms/cm3. This layer can meet the requirements for low J0,metal and ρc values, leading to an increase in conversion efficiency of 25.65%. This optimized process of depositing a phosphorus-doped poly-Si layer can be commercially applied in photovoltaics to reduce processing times and lower costs.


Introduction
A tunnel oxide passivated contact (TOPCon) solar cell with an ultrathin silicon oxide (SiO x ) film and a phosphorus-doped polysilicon (n + -poly-Si) layer has the potential for a high theoretical efficiency limit of 28.7% [1].The market share of TOPCon cells is significant according to the ITRPV [2].An n-TOPCon cell (size: 330.15 cm 2 ) at JinkoSolar has achieved the highest conversion efficiency recorded thus far, 26.89% [3].The n + -poly-Si/SiO x layer with high carrier selectivity exhibits superior passivation quality at the metal/silicon contact, distinguishing it from Passivated Emitter and Rear Cell (PERC) solar cells [4,5].The ultrathin SiO x layer has a large tunneling barrier for holes [6].The doped poly-Si layer separates the metal/silicon contact from minority charge carriers, resulting in low recombination and band bending.The doped poly-Si is typically crystallized from an amorphous silicon layer (a-Si) after a high-temperature heat treatment process.Normally, the desired a-Si layer can be prepared using methods such as plasma-enhanced chemical vapor deposition (PECVD) [7,8], low-pressure CVD (LPCVD) [9,10], atmospheric pressure CVD [11], hot wire CVD [12], sputtering [13,14], and electron beam evaporation [15].Published studies focused on poly-Si passivating contacts fabricated using the standard industrial approach based on LPCVD and ex situ phosphorus (P) doping.This process has high throughput and is simple [4,[16][17][18][19][20].Studies in the literature investigated the parameters of passivating contacts, including a tunneling SiO x film [21], the structural properties and thickness of the poly-Si film [22][23][24], and the metallization of the passivating contact [16].These studies have provided valuable insights into the impacts of LPCVD process parameters on the structure and electrical properties of poly-Si films [17,[25][26][27][28][29].
The doped poly-Si layer could be helpful in achieving good passivation performance and low contact resistivity by controlling interfacial defects in the SiO x and the doping concentration of the poly-Si layer [30,31].After maximizing contact passivation using a SiO x /poly-Si layer, the lowest dark saturation current densities (J 0 ) of emitters in cells with passivation layers comprising an n + -poly-Si/SiO x contact and a boron-doped poly-Si/SiO x contact were 0.66 fA/cm 2 and 4.4 fA/cm 2 , respectively [32].At present, researchers and engineers are making further efforts to improve cell efficiency.One important method is minimizing the parasitic absorption losses of the poly-Si layer for solar cells.However, a poly-Si layer with a thickness of 100 nm is necessary under current metallization conditions [4,33].At the same time, various studies have been conducted to prevent metal finger penetration by using a barrier interlayer, such as titanium nitride (TiN) [34,35] or SiO x [36,37], created through a low-temperature method like Laser-Enhanced Contact Optimization (LECO) [38].Although there are many positive outcomes regarding the formation of planar (n) poly-Si passivating contacts, one of the challenging and critical tasks associated with poly-Si layers is determining how to balance parasitic absorption and contact in poly-Si layers [24,[39][40][41].
In this work, n-TOPCon cells containing P-doped poly-Si with passivating contacts were fabricated.The thickness of the poly-Si layer was changed using the LPCVD method, and the influence of the poly-Si layer thickness on J 0 , metallization recombination (J 0,metal ), and contact resistivity (ρ c ) were studied; a microstructure analysis of the contact formation was conducted; and further, the I-V parameters of the n-TOPCon solar cells were investigated, including their efficiency (E ff ), open-circuit voltage (V oc ), fill factor (FF), series resistance (R ser ), and short-circuit current density (J sc ).

Fabrication
n-TOPCon cells were obtained from commercially available n-type Czochralski silicon (Cz-Si) wafers.Texture was generated in an alkaline (KOH)/textured additive solution.The textured wafers were placed into a quartz tube furnace containing BCl 3 gas using a LYDOP™ system.Subsequently, the B-doping side was treated with a Hymson laser with a beam width of 90-100 µm to obtain front B-selective emitters.After the laser treatment, the rear side was cleansed with a KOH/polished additive solution.One 1.6 ± 0.2 nm thick layer of SiO x and an intrinsic a-Si 30-100 nm thick layer on the rear side were prepared by LPCVD.Subsequently, the a-Si layer was crystallized to form poly-Si in a high-temperature tube furnace at 850 • C for 20-30 min using a gas mixture of POCl 3 , O 2 , and N 2 which could be simultaneously doped with P. Phosphorus silicate glass (PSG) was removed using a 5% HF solution for 5 min, and the P-doping concentration profiles of the SiO x /P-doped poly-Si films were determined by electrochemical capacitance-voltage (ECV) profiling.Wafers with different P-doped poly-Si film thicknesses using two-sided passivation of the SiN x layer were used as J 0 samples.Similar samples with polished surfaces were screen-printed with Ag paste lines on one side only and nine different pitches were used as J 0,metal samples; these which were placed with the fingers facing up and then sintered in a sintering furnace at a peak temperature of 730 • C [42][43][44].P-doped poly-Si contacts were fabricated with four different thicknesses (see Table 1).Figure 1 displays the structure of an n-TOPCon solar cell.After cleaning the wafers and removing the BSG and PSG, a clean P-doped poly-Si/SiOx layer was obtained on one side only by etching the poly-Si wraparound side in a mixed solution of KOH and polishing additives.The front and rear of the cells were passivated with 4 nm thick Al2O3 through ALD/78 nm thick SiNx and 75 nm thick SiNx through PECVD, respectively.The fingers and busbars of the wafers were metallized using a commercial Ag paste, using an H-patterned grid which was screen-printed on both sides with a 16-busbar configuration.Finally, the cells with Ag fingers were heat-treated at a peak temperature of 730 °C.

Characterization
The current-voltage (I-V) parameters of the in-house standard cells were measured using a Wavelabs tester.The J0 and J0,metal values of the samples were measured using a WCT-120 Sinton and extracted at an excess carrier density of 3 × 10 15 cm −3 (Boulder, CO, USA) [45].The SiOx/P-doped poly-Si profiles of the monitor wafers were measured by an ECV device (WEP CVP21), using a 0.1 M NH4F solution as an etchant.We used a fourprobe test rig to determine the sheet resistance (Figure 1b).The ρc values were measured using the transfer-length method (TLM; GP-4 TEST) from the H-patterns printed on one side of the J0 samples.The microstructures and chemical composition of the contact interfaces were observed by energy-filtered scanning electron microscopy (FESEM-EDS, Reg-ulus8230, HITCHI, Tokyo, Japan) and transmission electron microscopy (Thermo Scientific Talos™ STEM, 200 kV, FEI Talos F200X, Waltham, MA, USA).The optical reflectance values and losses of the cells were analyzed using a PVE300-IVT (Bentham Instruments, Reading, UK) instrument and the Current Loss Analysis Calculator V1.4 (Series, from the Solar Energy Research Institute of Singapore, based on the Yablonovitch limit of 46.43 mA/cm 2 [46]).After cleaning the wafers and removing the BSG and PSG, a clean P-doped poly-Si/SiO x layer was obtained on one side only by etching the poly-Si wraparound side in a mixed solution of KOH and polishing additives.The front and rear of the cells were passivated with 4 nm thick Al 2 O 3 through ALD/78 nm thick SiN x and 75 nm thick SiN x through PECVD, respectively.The fingers and busbars of the wafers were metallized using a commercial Ag paste, using an H-patterned grid which was screen-printed on both sides with a 16-busbar configuration.Finally, the cells with Ag fingers were heat-treated at a peak temperature of 730 • C.

Characterization
The current-voltage (I-V) parameters of the in-house standard cells were measured using a Wavelabs tester.The J 0 and J 0,metal values of the samples were measured using a WCT-120 Sinton and extracted at an excess carrier density of 3 × 10 15 cm −3 (Boulder, CO, USA) [45].The SiO x /P-doped poly-Si profiles of the monitor wafers were measured by an ECV device (WEP CVP21), using a 0.1 M NH 4 F solution as an etchant.We used a four-probe test rig to determine the sheet resistance (Figure 1b).The ρ c values were measured using the transfer-length method (TLM; GP-4 TEST) from the H-patterns printed on one side of the J 0 samples.The microstructures and chemical composition of the contact interfaces were observed by energy-filtered scanning electron microscopy (FESEM-EDS, Regulus8230, HITCHI, Tokyo, Japan) and transmission electron microscopy (Thermo Scientific Talos™ STEM, 200 kV, FEI Talos F200X, Waltham, MA, USA).The optical reflectance values and losses of the cells were analyzed using a PVE300-IVT (Bentham Instruments, Reading, UK) instrument and the Current Loss Analysis Calculator V1.4 (Series, from the Solar Energy Research Institute of Singapore, based on the Yablonovitch limit of 46.43 mA/cm 2 [46]).

Microstructure and Performance
The n + -poly-Si layers of different thicknesses influenced the n + -poly-Si/SiO x profiles, sheet resistance (R □ ), J 0 , ρ c , and J 0,metal of the n-TOPCon cells, and the results are shown in Table 1. Figure 2 displays the ECV profiles and sheet resistances for different thicknesses of n + -poly-Si/SiO x layers.In Figure 2a, n + -poly-Si layers with different thicknesses have almost the same surface concentration of >5 × 10 20 atoms/cm 3 and nearly the same "kneeshaped" tail ECV profile; these were doped at the same high temperature.On an n-type silicon wafer, when the thickness of the n + -poly-Si layer was reduced from 100 to 30 nm, the sheet resistance (R □ ) of the cell increased from 45 to 57 Ω/sq.This increase occurred because the thickness of the poly-Si layer determined the total doping amount (Figure 2b).Although the surface doping concentration is the same, ECV curves of poly-Si layers with different thickness are integrated into the total doping amount.It can be found that as the thickness of the poly-Si layer increases, the total doping amount also increases, leading to a decrease in sheet resistance.

Microstructure and Performance
The n + -poly-Si layers of different thicknesses influenced the n + -poly-Si/SiOx profiles, sheet resistance (R□), J0, ρc, and J0,metal of the n-TOPCon cells, and the results are shown in Table 1. Figure 2 displays the ECV profiles and sheet resistances for different thicknesses of n + -poly-Si/SiOx layers.In Figure 2a, n + -poly-Si layers with different thicknesses have almost the same surface concentration of >5 × 10 20 atoms/cm 3 and nearly the same "kneeshaped" tail ECV profile; these were doped at the same high temperature.On an n-type silicon wafer, when the thickness of the n + -poly-Si layer was reduced from 100 to 30 nm, the sheet resistance (R□) of the cell increased from 45 to 57 Ω/sq.This increase occurred because the thickness of the poly-Si layer determined the total doping amount (Figure 2b).Although the surface doping concentration is the same, ECV curves of poly-Si layers with different thickness are integrated into the total doping amount.It can be found that as the thickness of the poly-Si layer increases, the total doping amount also increases, leading to a decrease in sheet resistance.The J0 values of n + -poly-Si/SiOx layers with different poly-Si thicknesses exhibit a slight change of approximately 0.5 fA/cm 2 when the thickness increases from 30 to 100 nm.The penetration depth is determined from the inflection point of the SiOx layer to a doping concentration of N = 1 × 10 19 atoms/cm 3 (Figure 3).It can be seen from Figure 3a that as the poly-Si thickness increases, the penetration depth of p-doping increases, and the inflection point concentration decreases.When the thickness of the poly-Si layer is 30 nm, the inflection point concentration is higher, resulting in a large Auger recombination so the J0 value becomes large.When the thickness of the poly-Si layer is 100 nm, the inflection point concentration is low, the passivation effect is poor, and J0 increases.In order to study the passivation properties of different poly-Si thicknesses, PL tests were performed.The results showed that PL intensity decreased slightly with an increase in thickness.The PL values also showed a minor change of approximately 1000 a.u.This indicates that compared to a 100 nm thick layer of n + -poly-Si/SiOx, a 30 nm thick layer can also provide effective field-effect passivation.This is attributed to the good passivation of the SiOx layer and a 15~20 nm penetration depth of P doped into the Si substrate.The J 0 values of n + -poly-Si/SiO x layers with different poly-Si thicknesses exhibit a slight change of approximately 0.5 fA/cm 2 when the thickness increases from 30 to 100 nm.The penetration depth is determined from the inflection point of the SiO x layer to a doping concentration of N = 1 × 10 19 atoms/cm 3 (Figure 3).It can be seen from Figure 3a that as the poly-Si thickness increases, the penetration depth of p-doping increases, and the inflection point concentration decreases.When the thickness of the poly-Si layer is 30 nm, the inflection point concentration is higher, resulting in a large Auger recombination so the J 0 value becomes large.When the thickness of the poly-Si layer is 100 nm, the inflection point concentration is low, the passivation effect is poor, and J 0 increases.In order to study the passivation properties of different poly-Si thicknesses, PL tests were performed.The results showed that PL intensity decreased slightly with an increase in thickness.The PL values also showed a minor change of approximately 1000 a.u.This indicates that compared to a 100 nm thick layer of n + -poly-Si/SiO x , a 30 nm thick layer can also provide effective field-effect passivation.This is attributed to the good passivation of the SiO x layer and a 15~20 nm penetration depth of P doped into the Si substrate.
J 0,measured plots of n + -poly-Si layers of various thicknesses with different metallization fractions are depicted in Figure 4.The values of J 0,metal are determined through a straightforward linear interpolation of the measured data points [42][43][44].The results indicate that the n + -poly-Si layers with a thickness of 100 nm have the lowest J 0,measured values, approximately 26 fA/cm 2 .This could be attributed to the thick layer's ability to withstand the depth of the corrosion of the poly-Si layer by Ag paste.The thinner the poly layer, the more it is destroyed by the paste, which greatly reduces the passivation performance.However, when the thickness of the layer was decreased from 70 to 30 nm, J 0,metal increased from 304 to 545 fA/cm 2 .It is well known that metal contact recombination is related to doping concentration and corrosion depth.When the poly-Si layer thickness is more than 70 nm, the doping concentration increases (Figure 2a), and a thick poly-Si layer provides good resistance against the corrosion of the slurry.When the poly-Si layer is thin, it cannot meet the demand for metallization.The thinner the poly layer, the more the paste destroys the poly layer, which greatly reduces the passivation performance.
Cross-sectional SEM images of the interface between the screen-printed Ag bulk and the n + -poly-Si/SiO x contacts are shown in Figure 5.The polished cross-section of the cell shows a 100 nm thick n + -poly-Si layer uniformly covered by the glass layer beneath the Ag bulk, with some voids.The interface between a Ag finger and the n + -poly Si/SiO x layer is obvious in Figure 5a; it can be observed that the Ag bulks have a block structure and that the interlayer n + -poly Si/SiO x layer seems to be non-uniform (Figure 5b).The thickness of the interlayer is thin in some regions; however, there are some thick interlayers in the dotted-line region (Figure 5c).It is hypothesized that the numerous small white particles on the local area of the poly-Si layer's surface are the result of Ag particle precipitation.Meanwhile, we further show the chemical composition of the thick interlayer in Figure 5c, the EDX elemental mapping images are displayed in Figure 6.J0,measured plots of n + -poly-Si layers of various thicknesses with different metallization fractions are depicted in Figure 4.The values of J0,metal are determined through a straightforward linear interpolation of the measured data points [42][43][44].The results indicate that the n + -poly-Si layers with a thickness of 100 nm have the lowest J0,measured values, approximately 26 fA/cm 2 .This could be attributed to the thick layer's ability to withstand the depth of the corrosion of the poly-Si layer by Ag paste.The thinner the poly layer, the more it is destroyed by the paste, which greatly reduces the passivation performance.However, when the thickness of the layer was decreased from 70 to 30 nm, J0,metal increased from 304 to 545 fA/cm 2 .It is well known that metal contact recombination is related to doping concentration and corrosion depth.When the poly-Si layer thickness is more than 70 nm, the doping concentration increases (Figure 2a), and a thick poly-Si layer provides good resistance against the corrosion of the slurry.When the poly-Si layer is thin, it cannot meet the demand for metallization.The thinner the poly layer, the more the paste destroys the poly layer, which greatly reduces the passivation performance.J0,measured plots of n + -poly-Si layers of various thicknesses with different metallization fractions are depicted in Figure 4.The values of J0,metal are determined through a straightforward linear interpolation of the measured data points [42][43][44].The results indicate that the n + -poly-Si layers with a thickness of 100 nm have the lowest J0,measured values, approximately 26 fA/cm 2 .This could be attributed to the thick layer's ability to withstand the depth of the corrosion of the poly-Si layer by Ag paste.The thinner the poly layer, the more it is destroyed by the paste, which greatly reduces the passivation performance.However, when the thickness of the layer was decreased from 70 to 30 nm, J0,metal increased from 304 to 545 fA/cm 2 .It is well known that metal contact recombination is related to doping concentration and corrosion depth.When the poly-Si layer thickness is more than 70 nm, the doping concentration increases (Figure 2a), and a thick poly-Si layer provides good resistance against the corrosion of the slurry.When the poly-Si layer is thin, it cannot meet the demand for metallization.The thinner the poly layer, the more the paste destroys the poly layer, which greatly reduces the passivation performance.Cross-sectional SEM images of the interface between the screen-printed Ag bulk and the n + -poly-Si/SiOx contacts are shown in Figure 5.The polished cross-section of the cell shows a 100 nm thick n + -poly-Si layer uniformly covered by the glass layer beneath the Ag bulk, with some voids.The interface between a Ag finger and the n + -poly Si/SiOx layer is obvious in Figure 5a; it can be observed that the Ag bulks have a block structure and that the interlayer n + -poly Si/SiOx layer seems to be non-uniform (Figure 5b).The thickness of the interlayer is thin in some regions; however, there are some thick interlayers in the dotted-line region (Figure 5c).It is hypothesized that the numerous small white particles on the local area of the poly-Si layer's surface are the result of Ag particle precipitation.Meanwhile, we further show the chemical composition of the thick interlayer in Figure 5c, the EDX elemental mapping images are displayed in Figure 6.A large number of white schistose Ag particles embedded in the SiOx-based glass phase are visible in a HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy, HITCHI, Japan) image of the contact interface between the Ag grid finger and the polished surface of the n + -poly-Si layer (Figure 6a).The corresponding elemental distribution was measured using EDX spectroscopy and is shown in Figure 6b as an overlay of Ag, Si, N, and O; the matrix elements are shown separately in Figure 6b-f.The partial SiNx layer can still be observed at the interface between the Ag finger and the n + -poly-Si/SiOx layer, which could influence the contact performance (Figure 6e).Some Ag particles have corroded the poly-Si layer, which is crucial for metallization recombi- A large number of white schistose Ag particles embedded in the SiOx-based glass phase are visible in a HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy, HITCHI, Japan) image of the contact interface between the Ag grid finger and the polished surface of the n + -poly-Si layer (Figure 6a).The corresponding elemental distribution was measured using EDX spectroscopy and is shown in Figure 6b as an overlay of Ag, Si, N, and O; the matrix elements are shown separately in Figure 6b-f.The partial SiN x layer can still be observed at the interface between the Ag finger and the n + -poly-Si/SiO x layer, which could influence the contact performance (Figure 6e).Some Ag particles have corroded the poly-Si layer, which is crucial for metallization recombination (Figure 6f).However, the SiO x tunneling layer is clearly present in Figure 6b,d.
As shown in the EDX line scan (Figure 7), the corrosion depth of the Ag particles is approximately 0.06 µm, which is consistent with the J 0.metal analysis mentioned above.This imposes a constraint on the minimum thickness of the n + -poly-Si layer.Therefore, when the thickness of the poly-Si layer is thin, it cannot resist the corrosion caused by the Ag paste.This leads to the formation of metal-semiconductor contact to some extent, resulting in a higher J 0,metal .
mission Electron Microscopy, HITCHI, Japan) image of the contact interface between the Ag grid finger and the polished surface of the n + -poly-Si layer (Figure 6a).The correspond ing elemental distribution was measured using EDX spectroscopy and is shown in Figure 6b as an overlay of Ag, Si, N, and O; the matrix elements are shown separately in Figure 6b-f.The partial SiNx layer can still be observed at the interface between the Ag finger and the n + -poly-Si/SiOx layer, which could influence the contact performance (Figure 6e).Some Ag particles have corroded the poly-Si layer, which is crucial for metallization recombi nation (Figure 6f).However, the SiOx tunneling layer is clearly present in Figure 6b,d.
As shown in the EDX line scan (Figure 7), the corrosion depth of the Ag particles is approximately 0.06 μm, which is consistent with the J0.metal analysis mentioned above.This imposes a constraint on the minimum thickness of the n + -poly-Si layer.Therefore, when the thickness of the poly-Si layer is thin, it cannot resist the corrosion caused by the Ag paste.This leads to the formation of metal-semiconductor contact to some extent, result ing in a higher J0,metal.TEM was conducted and revealed the composition of the contact cross-section formed between the screen-printed Ag bulk and the n + -poly-Si/SiO x layers.As shown in Figure 8, the thickness of the n + -poly-Si/SiO x layer is 50 nm.The Ag-Si alloy can be observed in specific regions of the n + -poly-Si layer which are only 10 nm away from the SiO x layer.This means that the depth of the Ag-Si alloy can reach up to 40 nm, which is consistent with Figure 7.There are also some Ag embryos in certain areas of the n + -poly-Si layer (Figure 8b).This can explain why the contact resistance is low but the metallization recombination increased.However, the Ag embryos did not penetrate the Si substrate; instead, a clear SiO x layer can be observed (Figure 8c).The above analysis indicates that under the current process conditions, the thickness of the n + -poly-Si layer should be more than 40 nm.
The contact resistivity of the solar cells with varying n + -poly-Si layer thicknesses is depicted in Figure 9.There is a remarkable change in ρ c , which decreases from 40.4 to 7.8 mΩ•cm 2 with an increase in the layer thickness from 30 nm to 100 nm.Due to an increase in the total doping concentration, the width of the internal depletion region in a silicon wafer can be narrowed, enabling the quantum mechanical tunneling of charge carriers through Schottky barriers [47].
layer.This means that the depth of the Ag-Si alloy can reach up to 40 nm, which is con-sistent with Figure 7.There are also some Ag embryos in certain areas of the n + -poly-Si layer (Figure 8b).This can explain why the contact resistance is low but the metallization recombination increased.However, the Ag embryos did not penetrate the Si substrate; instead, a clear SiOx layer can be observed (Figure 8c).The above analysis indicates that under the current process conditions, the thickness of the n + -poly-Si layer should be more than 40 nm.The contact resistivity of the solar cells with varying n + -poly-Si layer thicknesses is depicted in Figure 9.There is a remarkable change in ρc, which decreases from 40.4 to 7.8 mΩ•cm 2 with an increase in the layer thickness from 30 nm to 100 nm.Due to an increase in the total doping concentration, the width of the internal depletion region in a silicon wafer can be narrowed, enabling the quantum mechanical tunneling of charge carriers through Schottky barriers [47].

I-V Parameters
Table 2 lists the I-V parameters of the cells as a function of the thickness of the n +poly-Si layer.The cells with 70 nm and 100 nm thick n + -poly-Si layers have the same efficiency, approximately 25.45%, which is mainly due to the current gain compensating for the loss in Voc.When the thickness of n + -poly-Si layer is equal to 70 nm, the cells have a high J0,metal value, but the cells only exhibit a slight decrease in the Voc value of 1mV and have the best FF.This result is consistent with the aforementioned analysis of the ρc value.However, as shown in Figure 10, when the thickness of the n + -poly-Si layer ranged from 30 to 50 nm, there was a sharp decline in the efficiency of the cells by over 0.2%.There is a slight decrease in the Voc of 1mV for the cells with 50 nm thick n + -poly-Si layers, while there is a significant decrease in the Voc of 8 mV for the cells with 30 nm thick n + -poly-Si layers.This occurred because the silver embryos penetrated the passivation layer of SiOx.Meanwhile, the Jsc value of the cell does not exhibit any advantages with the decreased thickness of the poly-Si layer.The decrease in the passivation performance of the n + -poly-Si layer is primarily attributed to Ag corrosion, which impacts the generation of photogenerated electrons.Based on the above data, it can be inferred that the optimal thickness

I-V Parameters
Table 2 lists the I-V parameters of the cells as a function of the thickness of the n +poly-Si layer.The cells with 70 nm and 100 nm thick n + -poly-Si layers have the same efficiency, approximately 25.45%, which is mainly due to the current gain compensating for the loss in V oc .When the thickness of n + -poly-Si layer is equal to 70 nm, the cells have a high J 0,metal value, but the cells only exhibit a slight decrease in the V oc value of 1 mV and have the best FF.This result is consistent with the aforementioned analysis of the ρ c value.However, as shown in Figure 10, when the thickness of the n + -poly-Si layer ranged from 30 to 50 nm, there was a sharp decline in the efficiency of the cells by over 0.2%.There is a slight decrease in the V oc of 1 mV for the cells with 50 nm thick n + -poly-Si layers, while there is a significant decrease in the V oc of 8 mV for the cells with 30 nm thick n + -poly-Si layers.This occurred because the silver embryos penetrated the passivation layer of SiO x .Meanwhile, the J sc value of the cell does not exhibit any advantages with the decreased thickness of the poly-Si layer.The decrease in the passivation performance of the n + -poly-Si layer is primarily attributed to Ag corrosion, which impacts the generation of photogenerated electrons.Based on the above data, it can be inferred that the optimal thickness of the n + -poly-Si layer of cell for industrial production is 70 nm.

Failure Analysis
Current loss was investigated to understand the effects of the n + -poly-Si layer thickness.As shown in Figure 11a, there is no significant difference in optical reflection between the 70 nm and 100 nm samples, as represented by a dashed curve.The solid line curve represents the internal quantum efficiency (IQE).A notable disparity in the IQE between 70 nm and 100 nm is observed at short wavelengths of <550 nm.The blue responses of these sizes were notably enhanced by the low total phosphorus doping concentration.The IQE for the 70 nm cell exhibited a good response at wavelengths < 900 nm.The curve shows that the values of the Jsc are increased by decreasing the thickness of the poly-Si layer owing to the low degree of parasitic absorption on the rear surface.

Failure Analysis
Current loss was investigated to understand the effects of the n + -poly-Si layer thickness.As shown in Figure 11a, there is no significant difference in optical reflection between the 70 nm and 100 nm samples, as represented by a dashed curve.The solid line curve represents the internal quantum efficiency (IQE).A notable disparity in the IQE between 70 nm and 100 nm is observed at short wavelengths of <550 nm.The blue responses of these sizes were notably enhanced by the low total phosphorus doping concentration.The IQE for the 70 nm cell exhibited a good response at wavelengths < 900 nm.The curve shows that the values of the J sc are increased by decreasing the thickness of the poly-Si layer owing to the low degree of parasitic absorption on the rear surface.
A current loss analysis of n-TOPCon cells with 70 nm and 100 nm thick n + -poly-Si layers is shown in Figure 11b.The cells with an n + -poly-Si layer thickness ranging from 70 nm to 100 nm have three advantages.The first is the "NIR parasitic absorption loss", which results in an increase of 0.14 mA/cm 2 in the J sc value due to the thin n + -poly-Si layer.The other advantages are "blue loss" and "base collection loss", which benefit from the low total dopant concentration of P.However, there is one disadvantage of "ARC reflectance" for the cells with a 70 nm thick n + -poly-Si layer.This issue is related to the wraparound thin poly-Si layer, which is may damage the texture of the front surface and increase the reflectance [48].
70 nm and 100 nm is observed at short wavelengths of <550 nm.The blue responses of these sizes were notably enhanced by the low total phosphorus doping concentration.The IQE for the 70 nm cell exhibited a good response at wavelengths < 900 nm.The curve shows that the values of the Jsc are increased by decreasing the thickness of the poly-Si layer owing to the low degree of parasitic absorption on the rear surface.A current loss analysis of n-TOPCon cells with 70 nm and 100 nm thick n + -poly-Si layers is shown in Figure 11b.The cells with an n + -poly-Si layer thickness ranging from 70 nm to 100 nm have three advantages.The first is the "NIR parasitic absorption loss", which results in an increase of 0.14 mA/cm 2 in the Jsc value due to the thin n + -poly-Si layer.The other advantages are "blue loss" and "base collection loss", which benefit from the After process optimization, the thin poly-Si layer can lead to an increase in J sc value.However, it is essential to strike a balance between maximizing the J sc value and minimizing the recombination effect of metallization on the etched depth of the poly-Si layer.Therefore, the development in metallization of industrialized poly-Si selective emitter technology will be the next focus of research.

Conclusions
J 0 , J 0,metal , ρ c , and microstructure analyses of contact formation were conducted, and the I-V parameters of solar cells were investigated as functions of the n + -poly-Si layer thickness.We introduced an n + -poly-Si layer with a thickness of 70 nm and a surface concentration of 5 × 10 20 atoms/cm 3 to enhance the conversion efficiency to 25.65%.The results showed that the J 0 values of n + -poly-Si layers exhibit a minor change of approximately 0.5 fA/cm 2 when the thickness ranges from 30 to 100 nm.However, the thickness of the n + -poly-Si layer had a significant impact on J 0,metal values, which increased from 304 to 545 fA/cm 2 , and ρ c values, which decreased from 9 to 40.4 mΩ•cm 2 , when the thickness decreased from 70 to 30 nm.The analysis above indicates that the thickness of the n + -poly-Si layer cannot be less than 40 nm under current process conditions.This limitation is primarily attributed to the presence of corroded silver (Ag) particles at a certain depth within the n + -poly-Si layer.A reduction in the thickness of the poly-Si layer by 30-50 nm did not result in an increase in the short-circuit current density, resulting in an inability to effectively collect current.The ρ c and J 0,metal results, along with the I-V characteristics, indicate that the thickness of the n + -poly-Si layer needs to exceed the depth of Ag particle corrosion to achieve a low ρ c .Simultaneously, it is crucial to ensure that the J 0,metal value is low.This optimized n + -poly-Si layer process can be commercially applied in photovoltaics to reduce the processing time and thus lower costs.

Figure 1 .
Figure 1.(a) Structure of n-TOPCon solar cell and (b) schematic diagram of four-probe sheet resistance tester.

Figure 1 .
Figure 1.(a) Structure of n-TOPCon solar cell and (b) schematic diagram of four-probe sheet resistance tester.

Figure 2 .
Figure 2. ECV profiles (a) and sheet resistances (b) of cells with different n + -poly-Si/SiOx layer thicknesses.(The solid pattern represents the test data, and the hollow pattern represents the average value).

Figure 2 .
Figure 2. ECV profiles (a) and sheet resistances (b) of cells with different n + -poly-Si/SiO x layer thicknesses.(The solid pattern represents the test data, and the hollow pattern represents the average value).

Figure 3 .
Figure 3. J0 (a) and PL (b) plots of cells.(The solid pattern represents the test data, and the hollow pattern represents the average value).

Figure 3 . 14 Figure 3 .
Figure 3. J 0 (a) and PL (b) plots of cells.(The solid pattern represents the test data, and the hollow pattern represents the average value).

Figure 4 .
Figure 4. J0,measured plots for n + -poly-Si layers (a) 30 nm, (b) 50 nm, (c) 70 nm, and (d) 100 nm thick with different metallization fractions ranging from 5.9% to 17.3% and (e) J0,metal of different thickness of n + -poly-Si layers.(The solid pattern represents the test data, and the hollow pattern represents the average value).

Figure 5 .
Figure 5. Cross-sectional SEM images of the metal contact on (a) an n + -poly-Si layer, the interface of Ag-Si fingers (b); and numerous little white particles on the local area of the surface of the poly-Si layer (c) shown in (b).

Figure 6 .
Figure 6.(a) HAADF-STEM image of the formed contact of an n+ poly-Si layer from Figure 5c; (b) corresponding EDX elemental mapping and EDX elemental mapping of Si (c), O (d), N (e), and Ag (f).

Figure 6 .
Figure 6.(a) HAADF-STEM image of the formed contact of an n+ poly-Si layer from Figure 5c; (b) corresponding EDX elemental mapping and EDX elemental mapping of Si (c), O (d), N (e), and Ag (f).

Figure 7 .Figure 7 .
Figure 7. STEM/EDX energy element line scan images of the Ag-Si interface.

Figure 8 .
Figure 8. STEM bright-field cross-sectional images of Ag bulk/glass layer/50nm n + poly-Si/SiOx layer contact; high-density of Ag embryos on SiNx layer was observed in (a).Ag-Si alloy in the n + poly-Si layer (b), (c) Ag-embryo on n + poly-Si layer, and (d) selected area of tunneling layer SiOx.

Figure 8 .
Figure 8. STEM bright-field cross-sectional images of Ag bulk/glass layer/50nm n + poly-Si/SiO x layer contact; high-density of Ag embryos on SiN x layer was observed in (a).Ag-Si alloy in the n + poly-Si layer (b), (c) Ag-embryo on n + poly-Si layer, and (d) selected area of tunneling layer SiO x .Materials 2024, 17, x FOR PEER REVIEW 10 of 14

Figure 9 .
Figure 9. Plot of contact resistivity of solar cells.

Figure 9 .
Figure 9. Plot of contact resistivity of solar cells.

Figure 10 .
Figure 10.I-V parameters of n-TOPCon solar cells.

Figure 10 .
Figure 10.I-V parameters of n-TOPCon solar cells.

Figure 11 .
Figure 11.Solid line curves show internal quantum efficiency (IQE), and dashed line curves show optical refection (a) and current loss mechanisms of cells with 70 nm and 100 nm thick n + poly-Si layers (b).

Figure 11 .
Figure 11.Solid line curves show internal quantum efficiency (IQE), and dashed line curves show optical refection (a) and current loss mechanisms of cells with 70 nm and 100 nm thick n + poly-Si layers (b).

Table 1 .
Process conditions and performance of wafers.

Table 1 .
Process conditions and performance of wafers.

Table 2 .
I-V parameters of n-TOPCon solar cells.E ff : efficiency; V oc : open circuit voltage; J sc : short-circuit current density; FF: fill factor.