# BC-TOPCon's Last Backside Battle: Keep the Bot··· | Ooitech

> How next-gen back-contact TOPCon cells push past 27% by freezing the bottom SiOx, zoning the middle layer between AlOx/SiOx and pure SiOx, and crushing metal contact recombination on the p-finger side.

![BC-TOPCon's Last Backside Battle: Keep the Bottom SiOx Frozen, Zone the Middle Layer with AlOx/SiOx](https://cdn.ooitech.com/static/upload/image/20260804/2026080419779051.webp)

- ** 2026-08-04
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- ** [Blog](/Blog.html)

### BC-TOPCon's Last Backside Battle: Keep the Bottom SiOx Frozen, Zone the Middle Layer with AlOx/SiOx

##### Product Introduction

"Old Zhang, that 26.34% front-n-finger plus full-contact back-p double-layer paper — the next move is to make the back-p finger-shaped too, and that's real BC. But under the p-finger, that middle SiOx layer: do we stick with the pure SiOx from 26.66, or swap in AlOx/SiOx to borrow the field effect?" That question came up this morning, right after I dropped the 26.34% paper into the production-line group chat, and the BC project guys started chasing it.

In true BC-TOPCon, the bottom SiOx — the one that grows the passivating pinholes — cannot be touched. But the "middle SiOx" layer, whose original job was to block Ag and thin down the stop layer, can be played by zone once the p-fingers get localized. Non-contact regions go to AlOx/SiOx to borrow the field effect. Contact regions, right under the paste, keep pure SiOx to stop B diffusion from blowing up. That one swap is the single biggest chunk you can eat on the backside inside the 1.3% abs jump from 26.34% to 27.6%+.

##### Technical Parameters

###### First, the two 27%+ BC benchmarks on the table

Zheng, Z. et al. Maximizing carrier extraction in hybrid back-contact silicon solar cells. Nature (2026). [https://doi.org/10.1038/s41586-026-](https://doi.org/10.1038/s41586-026-) (Beijing University of Technology + Golden Stone Energy, certified 27.62%)

LONGi HIBC, 28.13%, ISFH CalTeC Apr 2026 (LONGi HIBC 2.0, HPBC route, freshly set record). Built on HIBC 1.0 (Wang G. et al., Nature 647, 369–374, 2025, DOI:10.1038/s41586-025-09681-w, 27.81%) iterated with iPET + LIC; paper pending, certification on LONGi's official site.

| Metric | JinKo 26.66% (bifacial n-TOPCon) | BJUT + Golden Stone 27.62% (HBC) | LONGi HIBC 28.13% |
| --- | --- | --- | --- |
| Structure | Front boron + back n-TOPCon full-contact double-layer | Front a-SiOx + back p/n interdigitated (HBC) | Front HJT-P + back TOPCon-N finger (HIBC) |
| Voc (mV) | 744.6 | 740 | — |
| Jsc (mA/cm²) | ~41.2 est. | 42.54 | — |
| FF (%) | 85.57 | 87.73 | — |
| Key move | Double-layer SiOx/poly + Ag blocking | Gradient B-doped a-Si + O-doped a-SiOx front passivation | Bipolar hybrid passivation (P-side a-Si:H + N-side poly) |

Watch that FF of 87.73%. 26.66% only hits 85.57%. That 2% abs FF gap is not something a "backside double-layer" alone can make up. It comes from backside finger geometry optimization, front-surface grid-free shading, and pushing contact recombination down to the 400-500 fA/cm² level (more below).

##### Technical Advantages

###### How the double-layer SiOx/poly changes once p-fingers are localized

Both the 26.66% and 26.34% "double-layers" are full-contact backsides — poly covers the whole back surface, the middle SiOx runs as one continuous sheet. Once real BC makes both back-n and back-p finger-shaped, the stack goes from "blanket" to "interdigitated," and those two logics have to be re-divided.

**Bottom SiOx (~1.x nm, the tunnel site) — never moves.**

- This is the home turf of that Das Solar pinhole-passivation paper. It grows 10¹² cm⁻² class oxygen-bearing pinholes, and pushing J₀ down to 2-4 fA/cm² leans on it.
- In BC, the bottom SiOx under the p-finger faces B-doped poly, not P. B sits with higher Dit at the Si/SiO₂ interface, and B segregation wrecks the SiOx stoichiometry. So the p-finger-side bottom SiOx actually wants to be slightly thicker than the n-finger side (+0.2-0.3 nm), with a 1050°C pre-anneal to activate B and pull crystallinity to 98% (the baseline the 26.34% paper gives).

**Middle SiOx (originally 1.5 nm, the Ag-blocking site) — in BC, the material can be zoned.**

This is the core variable. The middle in 26.66/26.34 is pure thermal SiO₂, single-function (block Ag + thin the stop layer). Under the interdigitated BC structure, the p-finger region hits two new pain points that full-contact never had to face.

**Pain point A**: The p-TOPCon side is naturally weak on field effect. The fixed charge from B at the Si/SiO₂ interface is positive (opposite of the n+ side). Pure SiOx gives no p-side field-effect passivation, so you have to borrow negative charge from an outer AlOx stack.

**Pain point B**: Metal contact recombination J₀,metal under the p-finger is the BC efficiency ceiling. Silver-free BC today sits at Al-contact J₀,metal ~2400-2500 fA/cm², corresponding to ~25.9% efficiency. To touch the 26.8% Ag-system level, you have to press J₀,metal below 400-500 fA/cm².

A pure-SiOx middle can't handle A. Swapping the whole thing to AlOx/SiOx steps on another landmine.

###### AlOx/SiOx as a tunnel layer is a trap, but as a middle layer it might be candy

Two scenarios get mixed up in the literature and must be kept apart.

**Scenario 1: AlOx directly replaces the bottom SiOx as the tunnel layer.** Kaur's work (Solar Energy Materials 2021) is the cautionary tale here:

- AlOx and AlOx/SiOx double-layers as tunnel layers passivate worse than pure SiOx.
- Mechanism: AlOx/SiOx markedly enhances B diffusion and suppresses P diffusion. B piles into a "reservoir" in the AlOx region then pushes into c-Si — a double blast of Auger recombination plus B-O pair defects.
- On top of that, Al diffuses from the AlOx into c-Si forming deep levels, so SRH recombination climbs too.

So the bottom SiOx absolutely cannot be swapped for AlOx — that's the root reason the 26.66/26.34 baselines don't move, and it's exactly why LONGi HIBC runs "P-side with a-Si:H (HJT logic), N-side with poly (TOPCon logic)." The P-side simply sidesteps the SiOx/poly setup and walks the HJT amorphous + hydrogen route to dodge the B-SiOx trap.

**Scenario 2: AlOx/SiOx sits in the "middle" position (no tunneling, only field effect + Ag blocking).** This is the BC-only play:

- Bottom SiOx (1.x nm) stays pure thermal SiO₂, grows the passivating pinholes.
- Above the inner poly (p+, high crystallinity), below the outer poly (p++, heavily doped for metallization) sits the pure SiOx middle (1-1.5 nm).
- In non-contact regions (between fingers, and inside fingers away from paste), the middle swaps to an ultra-thin AlOx (2-4 nm) / SiOx (1 nm) stack. AlOx negative fixed charge ~10¹²-10¹³ cm⁻² lends the p-finger a field effect and presses the p-TOPCon-side Dit down.
- In contact regions (under the paste opening), the middle keeps pure SiOx — to stop AlOx from letting B diffusion explode during firing (Ag paste fires at 700-800°C, and AlOx goes unstable above 550°C as Si-H dissociates).

There's no public BC cross-section data on this "zoned middle" yet, but the logic chain holds. a-SiOx:H/AlOx:H at 6nm/12nm on n-type front-surface passivation already runs τeff 5.1 ms with no anneal, which says AlOx not directly touching c-Si (with a-SiOx or SiOx in between) lets field effect and chemical passivation cooperate. The BC p-finger non-contact region is exactly that case: bottom SiOx separates c-Si, the middle AlOx/SiOx separates the inner poly, AlOx never touches c-Si, and the B diffusion path is blocked by two barriers, bottom SiOx plus inner poly — far safer than AlOx as a direct tunnel layer.

##### Product Application

###### Three supporting moves for p-finger localization (the 26.34 to 27.6 increment)

| Move | 26.34% baseline | BC p-finger localization increment | Bottleneck addressed |
| --- | --- | --- | --- |
| Bottom SiOx | Full-contact 1.8 nm (p-TOPCon side) | 1.8→2.0 nm under p-finger (anti-B pinning), 1.3-1.5 nm under n-finger | B segregation wrecks SiOx |
| Middle SiOx | Pure SiOx 1 nm (in-situ) | Zoned: non-contact AlOx(3nm)/SiOx(1nm), contact pure SiOx 1 nm | Weak p-side field effect |
| Outer poly | 90 nm p++, alkali-metal paste | Laser LCO opening under p-finger, opens only AlOx/SiNx without hurting poly/SiOx (iVoc verified stable) | J₀,metal 2400→400 |
| Anneal | 1050°C pre-anneal, 98% crystallinity | Same, but p/n finger co-anneal window must compromise — P-side 880-920, B-side 1050, split at ~950-980°C long anneal | Thermal budget conflict |
| Metallization | Alkali-metal Ag paste (4wt% Na₂O/K₂O) | Silver-free Al contact or Ag-Al mixed paste, p-finger 700°C firing J₀,metal ~2400, n-finger ~2500 same condition | Silver-free + press J₀,metal to 400 |

That silver-free BC data matters: after LCO (laser contact opening) iVoc barely drops and Raman shows no amorphous signal, proving "open the window without breaking poly/SiOx passivation" is doable — that's the process foundation for "keep pure SiOx middle in contact regions + LCO paste window" under the BC p-finger. But J₀,metal 2400 fA/cm² only maps to 25.9% efficiency. That 0.9% abs gap to the Ag-system 26.8% is entirely eaten by contact recombination. The next-jump bottleneck is not the passivation layer, it's metallization.

###### So what's the backside really fighting over in that last jump to 27%

Line up all four generations — Das 25.4 → JinKo 26.66 → JinKo 26.34 → Golden Stone 27.62 / LONGi 28.13:

- 25.4% generation: the backside fought over the bottom SiOx's "pinhole personality" (passivation vs recombination), LPCVD two-step oxidation growing 10¹²-class passivating pinholes.
- 26.66% generation: the backside fought over the double-layer SiOx/poly + middle SiOx Ag block + laser thinning, solving metallization degradation and parasitic absorption.
- 26.34% generation: back p-TOPCon full-contact double-layer + alkali-metal paste + 1050°C pre-anneal, gnawing at p-side B segregation and metallization.
- 27.6%+ generation (BC): back p/n interdigitated, bottom SiOx tuned by polarity (p-side 2.0 / n-side 1.3) + zoned middle (non-contact AlOx/SiOx for field effect, contact pure SiOx) + LCO opening without hurting passivation + silver-free/low-Ag metallization pressing J₀,metal to 400.

One counter-intuitive call: above 27%, in the backside "field effect + double-layer combo," AlOx cannot go in the bottom (the tunnel site blows up B diffusion), only in the middle non-contact region. That's the biggest difference from the PERC era's "AlOx directly on c-Si to borrow negative charge." The BC structure happens to hand you the space for a "zoned middle" (interdigitated geometry brings its own zoning). Full-contact TOPCon simply can't play this. That also explains why 26.66/26.34 never put AlOx in the middle — they're full-contact, the middle has to double as stop layer + Ag block, and pure SiOx is safer.

###### What a BC pilot line can try first

1. Move p-finger bottom SiOx from 1.5 to 1.8-2.0 nm — add 30-50% LPCVD 620°C O₂ time, first look at the B segregation ECV profile.
2. Run "two-choice" middle samples: Group A pure SiOx 1.5 nm (26.66 baseline), Group B non-contact ALD AlOx 3nm / SiOx 1nm plus contact pure SiOx 1.5 nm (get the LCO window alignment right).
3. Tune LCO laser to accommodate AlOx — AlOx is more sensitive to 355 nm than SiOx, so the deep-UV 4.8 eV photon "open only SiNx/AlOx without hurting poly/SiOx" recipe needs re-calibrating for B-side and N-side doses separately.
4. Switch metallization to Ag-Al mixed paste or full Al, lock firing at 700°C — the baseline n/p J₀,metal 2400-2500 needs glass frit + firing atmosphere + contact pattern density all tuned together to reach 400-500.

###### Open questions for the BC metallization crew

Manufacturing feasibility of the "zoned middle" under the p-finger — does ALD AlOx only on the non-contact region need a mask, or do you deposit full-face and let LCO sweep away the contact-region AlOx? The former adds a mask step (BC is complex enough already), the latter risks the LCO laser sweeping the AlOx/SiOx stack and hurting the bottom SiOx passivating pinholes (only sweeping AlOx/SiNx was verified as non-damaging to poly/SiOx; the AlOx/SiOx stack was not).

Also, Golden Stone's 27.62% used "gradient B-doped a-Si + O-doped a-SiOx front passivation," with the P-side on HJT logic rather than TOPCon. Will HIBC (P-side HJT + N-side TOPCon) hit 28% earlier than full-TOPCon BC (both p-finger and n-finger poly/SiOx)? LONGi's 28.13% line looks like HIBC won, but the device simplicity of full-TOPCon BC (poly on both N and P sides, shared anneal) might win back on long-term cost. These two BC sub-routes will collide next over the "passivation ceiling vs process complexity" trade-off.

Four papers back to back (Das 25.4 → JinKo 26.66 → JinKo 26.34 → Golden Stone 27.62 / LONGi 28.13), and TOPCon's three-generation backside logic from 25%→26%→27%+ is complete: passivating pinholes → double-layer Ag block + thinning → BC interdigitation + zoned middle + field-effect combo.

##### Ooitech's View

The zoned-middle idea is clever, but honestly the harder fight lives on the line, not on the drawing board. Pushing J₀,metal from 2400 down toward 400 means your LCO laser dosing, paste frit, and firing profile have to hold tolerance across every finger, wafer after wafer — and that's a module-line repeatability problem as much as a cell physics one. Whether the industry lands on full-TOPCon BC or HIBC first, the process window shrinks either way, so metrology and thermal control on the backside stack become the real gatekeepers. Folks building or upgrading module production lines can pick up a lot of these firing and lamination trade-offs on the Ooitech YouTube channel at [www.youtube.com/ooitech](http://www.youtube.com/ooitech).

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