Preface: the past and present of photovoltaic cells – the end of the P-type era, the beginning of the N-type era
The photovoltaic cell industry belongs to the midstream of the photovoltaic industry chain and is made of silicon wafers through cleaning, texturing, and other steps. Photovoltaic panels generate voltage and current under light, enabling photovoltaic power generation, essentially similar to low-end semiconductor manufacturing.
According to the different raw materials and cell preparation technology, photovoltaic cells can be divided into P-type cells and N-type cells. P-type silicon wafers are made by doping boron in silicon materials; N-type silicon wafers are made by doping silicon materials with phosphorus elements. P-type battery preparation technology has traditional AL-BSF (aluminum backfield) and PERC technology. There are many N-type battery preparation technologies, including PERT/PERL, TOPCon, IBC, and HJT (heterojunction).
2015 is the first year of photovoltaic cell technology transformation. Before 2015, BSF batteries were the mainstream, accounting for 90% of the total market. In 2015, PERC completed commercialization verification, and the battery mass production efficiency exceeded the BSF bottleneck by 20% for the first time, and officially entered the expansion stage. In the following two years, with the advancement of PERC technology, the improvement of efficiency, and the reduction of non-silicon costs, the economic benefits of PERC cells have been reflected. In 2018, the market share of PERC cells reached 33%, and then the production capacity increased explosively. By 2020, the market share has increased to 87%, which has basically surpassed BSF cells, but the efficiency limit of PERC cells is 24.5%. The current conversion efficiency of PERC cells is close to the limit, so in order to reduce costs and increase efficiency, battery companies must seek technological breakthroughs again.
2021 is the turning point of battery technology transformation. What the photovoltaic industry has been pursuing is to reduce costs and increase efficiency. Due to their high conversion efficiency, N-type batteries have gradually entered the stage and have been accepted by people. According to ISFH data, the theoretical limit efficiencies of PERC, HJT, and TOPCon cells are 24.5%, 27.5%, and 28.7%, respectively.
On the whole, aluminum backfield BSF batteries were the mainstream before 2017, and PERC batteries have almost completely replaced aluminum backfield batteries since 2017. However, since the current PERC cell has approached the theoretical limit efficiency of 24.5%, the room for improvement is limited. After 2021, N-type batteries will begin to develop rapidly, dominated by TOPCon and HJT, which are currently in the early stage of large-scale commercialization. The potential future technical route also includes HBC and perovskite tandem cells, which are equivalent to upgrades after combining with HJT so that the conversion efficiency can achieve another leap.
What is HJT
A solar cell is a device that uses the photovoltaic effect to convert solar energy into electrical energy, and its core is a semiconductor PN junction. Using different doping processes, the P-type semiconductor and the N-type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate through diffusion, and a space charge region is formed at their interface called a PN junction. The PN junction has unidirectional conductivity, a property utilized by many devices in electronic technology.
Heterojunction (HIT) is a special PN junction, which is formed by amorphous silicon and crystalline silicon materials. It is a kind of N-type battery by deposits an amorphous silicon film on crystalline silicon. HIT (Heterojunction with Intrinsic Thinfilm) battery was first successfully developed by Sanyo in Japan in 1990. Because HIT has been registered as a trademark by Sanyo, it is also called HJT, HDT, or SHJ. Sanyo’s patent protection period expired in 2015, and HJT technology began to be fully promoted.
What are the advantages of HJT compared to traditional Solar Panels?
As a mature solar cell technology, heterojunction technology has been widely proven to provide higher efficiency, better performance, and more reliable product practical stability. Compared with other solar cell technologies, heterojunctions have higher production efficiency, simpler processes, and fewer production steps than other cell technologies. Heterojunction cells are natural double-sided cells, and the cell color is more stable and controllable.
- No PID phenomenon: Since the upper surface of the battery is TCO, the charge will not generate polarization on the TCO on the battery surface, and there is no PID phenomenon. At the same time, the experimental data also confirmed this. Technical Application and Prospect of Heterojunction Solar Cells
- Low-temperature manufacturing process: The processing temperature of all processes of HJT battery is lower than 250, which avoids the process of high-temperature diffusion junction formation with low production efficiency and high cost, and the low-temperature process makes the optical band gap, deposition rate, The absorption coefficient, and hydrogen content are more precisely controlled, and adverse effects such as thermal stress caused by high temperature can also be avoided.
- High efficiency: HJT batteries have been setting the world record for mass-produced battery conversion efficiency. The efficiency of HJT cells is 1-2% higher than that of P-type monocrystalline cells, and the difference is slowly increasing.
- Technical application and prospect of heterojunction solar cells with high light stability: The Staebler-Wronski effect, which is common in amorphous silicon solar cells, does not appear in HJT solar cells. At the same time, in the N-type silicon wafer used in the HJT cell, the dopant is phosphorus, and there is almost no light-induced attenuation phenomenon.
- It can be developed towards thinning: the process temperature of HJT batteries is low, the upper and lower surface structures are symmetrical, and there is no mechanical stress, so thinning can be achieved smoothly; in addition, research has shown that for N-type batteries with high minority carrier lifetime (SRV<100cm/s) Silicon substrate, the thinner the wafer, the higher the open circuit voltage can be.
How to Make HJT Solar Panel
HJT 4 major process steps:
- Texturing cleaning
- Amorphous silicon film deposition
- TCO preparation
- Electrode preparation
The corresponding equipment is:
- Cleaning and texturing equipment
- PECVD equipment
- PVD/RPD equipment
- Screen printing equipment
The performance of HJT in actual projects