As one of the core technologies in the field of solar cells, Tunneling Oxide Passivation Contact (TOPCon) technology, with its excellent backside passivation performance, has been widely used in industrial production. However, the inherent narrow bandgap and high absorption coefficient of poly-si thin-film materials lead to significant parasitic absorption losses in the TOPCon structure, while reducing the thickness of poly-si will lead to burn-through of the metal slurry and increase in contact resistance.

In order to accurately quantify the optical properties of poly-si (e.g., extinction coefficient k, refractive index n), this study employs a full-spectrum spectroscopic ellipsometer from Millennial to characterize the thickness of the thin film and the parasitic absorption behavior, and to synergistically optimize this contradiction in order to break through the bottleneck of efficiency. 

Experimental design

Sample preparation

TOPCon Structure Preparation Flow

PECVD poly-si films: P-doped poly-si was deposited on the backside of 183×183 mm² n-type wafers (thickness 110±20 μm, resistivity 0.5-1.5 Ω-cm), and a gradient thickness of 50-150 nm was obtained by adjusting the deposition time (300-950 s) (G1-G4).

(a) Baseline poly-si structure; (b) schematic of poly-si finger structure

TOPCon cell preparation: alkali polishing to form pyramidal weave, boron diffusion to prepare p⁺ layer, embedded single side etching (SSE) to remove peripheral doping, backside PECVD deposition of SiOₓ/P doped amorphous silicon, and annealing and crystallization into two groups:

Shows the proces flow diagram for the production of baseline poly-Si structured versus poly.si finger structured cells on an industrial production line.

Benchmark group: RCA cleaning to remove SiOₓ mask, alkali polishing to remove positive n⁺poly-Si cladding layer.

Finger structure group: laser (532 nm, 50 W, 100 kHz) to form 7-10 nm SiOₓ mask in poly-Si contact zone, alkali polishing (KOH rate 20 nm/min) to thin non-contact zone to ~50 nm, retain ~150 nm thick poly-Si in contact zone. 

Effect of poly-si thickness

(a-b) Extinction coefficient (k) and refractive index (n) measured by ellipsometry; (c-d) external quantum efficiency curves of different samples

Spectroscopic ellipsometry SE tests show that the extinction coefficient of poly-si poly-Si decreases with decreasing thickness, and the parasitic absorption of thin poly-si (G1, 50 nm) is significantly lower than that of thick poly-si (G4, 150 nm). External quantum efficiency tests further confirm that the quantum efficiency of the thin poly-si structure is higher in the long wavelength range (700-1000 nm), verifying the reduction of parasitic absorption losses.

(a) Symmetric structure (I) and TLM test structures (II-III); (b-e) Passivation performance (lifetime, J₀, iVoc) vs. contact resistance (ρc)

Passivation performance tests showed that poly-si in the thickness range of 50-150 nm exhibited excellent passivation, with lifetime averages exceeding 2000 μs and J₀ maintained at the level of 5.9-6.3 fA/cm². Contact resistance tests show that thick poly-si (150 nm) has a contact resistivity of only 0.7 mΩ/cm², while thin poly-si (50 nm) has a contact resistance of 3.9 mΩ/cm² due to metal burn-through, emphasizing the need for a finger-like structure design.

Finger structure validation

(a) Finger structure of poly-si (G5); (b-e) SEM top view and cross section of poly-si in metal contact/non-contact zone

Finger structure synergistic optimization: laser mask combined with alkali polishing to achieve spatially differentiated thicknesses: thick poly-Si (~150 nm) in the contact zone guarantees low resistance (ρc=0.7 mΩ-cm²), thin poly-Si (~50 nm) in the non-contact zone reduces parasitic absorption.SEM cross section confirms structural feasibility.

Shows the electrical performance test data of poly-si finger struchure solar cell and baeline strmucture solar cel, denoted by G5 and Baseline, respectively.

The TOPCon cell (G5) based on a poly-si finger structure achieves an average conversion efficiency of 25.28% on a 183×183 mm² wafer, an increase of 0.33% over the reference structure. The short-circuit current density (Jₛₑ) improves by 0.52 mA/cm² to 41.97 mA/cm², which is attributed to the reduction of parasitic absorption in the thin poly-si in the non-contact region, whereas the open-circuit voltage (Vₒₑ) and the fill factor (FF) are comparable to that of the benchmark structure, indicating that the passivation performance and the fill factor (FF) are not as high as those of the benchmark structure. comparable to that of the benchmark structure, indicating that passivation properties and ohmic contacts are not affected.

By selectively regulating the thickness of poly-si, the poly-si finger structure successfully balances the contradiction between parasitic absorption and metal contact resistance in TOPCon cells, and realizes a significant improvement in conversion efficiency: the mass production efficiency exceeds 25.28%, providing a new path for TOPCon cells to move towards the theoretical efficiency of 28.7%. The structure preparation process is compatible with PECVD and LPCVD methods, and has the potential for industrial-scale production, providing a new technological path for the development of high-efficiency TOPCon solar cells. 

Millennial Automatic Spectroscopic Ellipsometer

Automatic Spectroscopic Ellipsometer with highly sensitive detector unit and spectroscopic ellipsometry analysis software, specially designed for measuring and analyzing layer structure parameters (e.g. thickness) and physical parameters (e.g. refractive index n, extinction coefficient k) of monolayer or multilayer nano-films in photovoltaic fields

✔  Advanced rotary compensator measurement technique: no measurement dead-end problems.

✔  Highly sensitive measurement of rough velvet nanofilms: advanced optical energy enhancement technique, high signal-to-noise ratio detection technique.

✔  Full-spectrum measurement speed in seconds: typical 5-10 seconds for full-spectrum measurement.

✔  Detection sensitivity on the order of atomic layers: measurement accuracy up to 0.05nm.

Millennial Automatic Spectroscopic Ellipsometer realizes the precision optical verification of poly-si finger structure in this study, which confirms the validity of poly-si finger structure in TOPCon batteries, and also provides a general idea for solving the performance contradiction in semiconductor thin-film materials. In the future, the laser processing parameters and poly-si thickness distribution can be further optimized to explore the application of this structure in advanced technologies such as bifacial passivation and stacked cells, so as to push the efficiency of solar cells towards the theoretical limit. 

Original reference:Enhancing passivation and reducing absorption losses in TOPCon solar cells via Poly-Si finger structure