Interdigitated back contact (IBC) solar cells have the potential for high efficiency due to their front-electrode-free design and double-sided passivated contact characteristics. However, the traditional IBC cell manufacturing process is complex, involving multiple doping and electrode patterning steps, which increases the cost and manufacturing difficulty. The SABC technology proposed in this article significantly simplifies the process flow by depositing an n-type polysilicon layer through PVD and combining it with self-aligned separation.

SABC solar cells are an advanced back contact (BC) solar cell technology. Its core feature is the separation of the positive and negative electrodes on the back of the cell through self-alignment technology, thereby improving the efficiency and performance of the cell. 

Manufacturing method of self-aligned back contact (SABC) solar cells

SABC (Self-Aligned Back Contact) Solar Cell Structure

Front surface:

●AlOx/SiNx passivation layer: used to reduce surface recombination and improve light capture efficiency.

●n-type silicon substrate: the main material of the solar cell.

Back surface:

●The first tunneling oxide layer: located between p-type polysilicon (p-poly Si) and the silicon Substrate, used for passivation and promoting selective carrier transport.

●p-type polysilicon layer (p-poly Si): as an emitter, forming a tunneling junction with the n-type polysilicon layer.

●n-type polysilicon layer (n-poly Si): covers the p-type polysilicon emitter and the exposed silicon substrate (base contact). Through PVD directional deposition, it is automatically disconnected at the trench due to the shadow effect to achieve self-aligned separation.

●The second tunneling oxide layer: located between p-type and n-type polysilicon, optimizing carrier transport.

●Metal electrode: a single-step printed metallization layer that covers all n-type polysilicon areas (emitter and base contacts).

SABC solar cell manufacturing process

Structural advantages: Through self-alignment technology, SABC solar cells achieve precise separation of positive and negative electrodes on the back, reducing manufacturing steps and improving cell efficiency.

Process simplification: Compared with traditional TOPCon solar cells, SABC solar cells only add two additional tools (laser ablation tool and PVD deposition tool), which greatly simplifies the manufacturing process.

Fully passivated design: By using passivation technology on the front and back, surface recombination is significantly reduced, and the open circuit voltage (Voc) and short circuit current (Isc) of the cell are improved.

Single metallization: Since the n-type polysilicon layer covers the entire back, the same slurry can be used for metallization of both polarities, further simplifying the manufacturing steps.

Performance of n-type polysilicon passivated contacts

iVoc and Sheet Resistance Performance of PVD n-type Polysilicon Passivated Contacts

Relationship between iVoc and annealing temperature:

The highest iVoc (738 mV) is achieved at 880°C annealing, but the sheet resistance is high (208 Ω/sq).

Low temperature (<860°C) leads to insufficient doping activation (increased Rsh), and high temperature (>880°C) causes degradation of the tunneling oxide layer (decreased iVoc).

SIMS analysis: Phosphorus concentration (Cp=1×1021 cm−3) is similar at all temperatures, but phosphorus diffusion into the silicon substrate is aggravated at high temperatures, increasing Auger recombination.

It is necessary to balance the passivation quality (iVoc) and conductivity (Rsh), which may be achieved by adjusting the PVD deposition rate or doping concentration.

Passivation and Conductive Properties of p-type/n-type Polysilicon Stacks

Performance of p-type poly-Si/n-type poly-Si stacked structure

iVoc comparison:

p-type single layer iVoc = 727 mV (870°C), while p/n stack iVoc = 715 mV, indicating that the n-type layer introduces slight passivation loss.

Sheet resistance:

Rsh = 97 Ω/sq for the stacked structure, significantly lower than that of a single n-type layer (208 Ω/sq), attributed to the parallel conduction of the n-type and p-type layers. The calculated value is consistent with the measured value, verifying the low resistance characteristics of the tunnel junction.

The passivation loss may be caused by the interface defects between the n-type polysilicon and the p-type layer, which needs to be improved by optimizing the interface oxide layer.

Trench structure and n-type polysilicon self-aligned separation

SEM image of trench cross section

p-type polysilicon overhang:

Thickness is about 300 nm, remaining at the top of the trench due to etching rate differences (p-poly Si < c-Si). Protrudes about 1–1.5 μm horizontally, forming a "roof" structure, providing shadow shielding for self-aligned separation.

n-type polysilicon layer (n-poly Si):

Continuous area: Uniform thickness at the bottom of the trench and the plane area (120–130 nm).

Separation area: Gradually thinning to disappear below the overhang, confirming the directional deposition characteristics of PVD.

Auxiliary SiNx layer:

Used only for SEM imaging comparison, thickness is about 100 nm, clearly distinguishing polysilicon from c-Si substrate.

Etching depth:

The trench depth is about 1.6 μm, indicating that the removal rate of isotropic etching for c-Si is significantly higher than that of p-type polysilicon.

Self-aligned separation is achieved by PVD directional deposition and etching overhangs. This structure successfully avoids the complex patterning steps of traditional IBC and provides the possibility for ultra-narrow electrode spacing.

Self-aligned separation of n-type polysilicon is achieved through physical vapor deposition (PVD) technology, successfully simplifying the manufacturing process of traditional interdigitated back contact (IBC) solar cells. Experimental results show that this technology can not only achieve excellent passivation performance (iVoc = 738 mV) and low sheet resistance (Rsh = 97 Ω/sq), but also avoid complex photolithography processes through self-aligned separation of trench structures, providing a feasible technical path for efficient and low-cost IBC cells.