In 2024, improving the conversion efficiency of N-type TOPCon solar cells will still be a hot topic. As a leading company in photovoltaic testing equipment in China, Millennial Solar continues to pay attention to the technological development of the industry. In order to deal with the problems in improving conversion efficiency, Millennial proposed high-efficiency TOPCon cell research and development solutions and provided corresponding research and development equipment. Among them, POLY Built-in Thin Film Thickness Tester adopts leading micro-nano thin film optical measurement technology to achieve an ultra-wide measurement range of 20nm-2000nm and ultra-high repeatability accuracy of 0.5nm, and can perform fast and automatic 5-point synchronous scanning of samples. Next, we will show you the impact of different Poly film thicknesses on the performance efficiency of solar cells.

To solve the problem of "improving efficiency", thinning the Poly layer is the key

For TOPCon cell, based on the cell conversion efficiency of 24.8%, the main factors affecting the efficiency are from large to small: 1. Front recombination loss, 2. Optical loss, 3. Front transmission loss, 4. Body recombination loss, 5. Back side Transmission loss, 6. Backside composite loss. It can be seen that the current main efficiency gap of TOPCon cells comes from the front surface. the reason is:

The back surface of N-type TOPCon cell has a passivation contact structure composed of SiO2 and poly layers, while the front surface is only passivated by the Al2O3 layer. Using burn-through slurry, there is still direct metal-silicon substrate contact;

Due to the low doping concentration of boron expansion, in order to achieve better contact, the front fine gate is changed from silver paste to silver aluminum paste. In order to achieve the same conductive effect, the width of the grid line is larger than that of the silver paste.

In order to solve the efficiency loss on the front surface of TOPCon cells, the ultimate solution is to also form a passivation contact structure of SiO2+poly layer on the front surface. However, the passivation ability of the P-type TOPCon layer is inherently weaker than that of the N-type TOPCon layer, and the front surface polysilicon will cause strong optical absorption. Therefore, at present, local poly layers are mostly considered, that is, a small part of SiO2 and poly layers are made under the front surface electrode, but the application level is more difficult.

Based on the current efficiency of 25%, TOPCon cell efficiency can be increased to 26% through optimization processes such as lossless SE technology and thin poly.

For TOPCon backside plating and screen printing,Effects of different Poly film thicknesses on performance

 The following will focus on the electroplating and screen printing metallization on the back of TOPCon to study the impact of reducing the Poly film thickness to 30nm on solar cell performance.

Thermal imaging images of TOPCon solar cells with Poly layer thicknesses of 30, 50, 70 and 90nm taken under reverse voltage (-12 V).

The above figure shows the thermal imaging images of all solar cells with Poly film thickness (30-90nm) under -12V reverse bias voltage. For the sample with a 90nm thick Poly layer, almost the entire solar cell edge shows a significant temperature increase. As polysilicon thickness decreases, edge ratio and brightness at elevated temperatures also decrease, reducing the danger and severity of hot spots. For samples with a 30nm thick Poly layer, a small portion of the edge is thermally visible, while the larger portion remains inactive.

IQE and reflectivity curves of different Poly film thicknesses

(All thicknesses of electroplating (30, 50, 70, 90nm) are shown; screen printing only shows the results of 30 and 70nm thickness)

The graph above shows internal quantum efficiency (IQE) and reflectance measurements as a function of Poly film thickness for electroplated and screen-printed solar cells in the 800-1200nm wavelength range. At a wavelength of 800nm, the IQE curves begin to diverge from each other and do not merge again until a wavelength of around 1100nm. For both metallization methods, the same trend can be seen, that is, as the poly film thickness decreases, the IQE decreases in the above wavelength range.

The sample with a poly film thickness of 90nm using electroplating metallization process showed the highest IQE curve.

The IQE curve of the screen-printed sample with a Poly film thickness of 70nm is almost the same as the IQE curve of the electroplated sample with a Poly film thickness of 30nm.

The screen-printed sample with a Poly film thickness of 30nm has the lowest IQE curve.

The table below shows the average Jsc loss compared to a cell using a 90nm thick Poly layer and plated contacts as a reference. Contact recombination increases, and poly layers below 100nm are still less sensitive to laser ablation than screen-printed metallization.

Conclusion

Single-sided deposition by PECVD causes the poly layer to wrap around to the front side. This parasitic current path may shunt the solar cell, resulting in lower shunt resistance. By combining quantum efficiency results with thermal imaging and scanning electron microscopy micro-features, the Poly surround was identified as the cause of the hot spots. Reducing the deposited Poly thickness from 90nm to 30nm reduces the influence of surround, thereby improving Rsh and Irev. In addition to shortening the deposition time, this method can also reduce the FCA and improve the bifacial coefficient of the solar cell.

POLY Built-in Thin Film Thickness Tester

E-mail: market@millennialsolar.com

Millennial POLY 5000 Built-in Thin Film Thickness Tester is specially designed for photovoltaic process monitoring. It can quickly and automatically scan the sample at 5 points simultaneously to obtain film thickness distribution information at different locations of the sample. The measurement size can be customized according to the size of the customer's sample.

●Effective spectral range 320nm~2400nm

●Fast, automatic 5-point simultaneous scanning

●Repeatability accuracy <0.5nm

●Ultra-wide measurement range 20nm~2000nm

●Online monitoring and detection achieves zero fragmentation rate

●Realize automatic inspection of the entire production line, greatly saving inspection time

Currently, the upgrade and switching of photovoltaic technology from P-type to N-type has brought about changes in the entire industry chain. In the industrialization of N-type high-efficiency battery technology, N-TOPCon solar cells are more cost competitive and have mass production advantages at this stage, and the production capacity has been implemented faster than expected. The continuous improvement of conversion efficiency will become an important factor in the industrialization of N-TOPCon solar cells. Item "problem". In this process, Millennial Solar is committed to helping customers continuously innovate and optimize cell film thickness, front-side metallization, back-side metallization and other technologies to achieve >25% N-TOPCon cell mass production conversion efficiency.