# Research Progress of Zero-Busbar Technology Based on Heterojunction Photovoltaic Modules
## 1 Introduction  
The development of photovoltaic (PV) modules has been directed toward achieving higher efficiency [1, 2], lower consumption [3, 4], and lower costs [5, 6, 7]. Compared with p-type crystalline silicon (c-Si) substrates [8, 9], n-type c-Si substrates have some advantages, such as a higher minority carrier lifetime [10] and lower light-induced degradation (LID) [11], resulting in a higher power conversion efficiency [12, 13, 14, 15]. Among n-type solar cells, HJT solar cells use hydrogenated amorphous silicon (a-Si:H) as passivation layers to achieve open-circuit voltages of up to 750 mV [16, 17]. Additionally, the high flugicality, favorable temperature coefficient, and low LID loss all make HJT solar cells strong performers [18, 19]. Silver, currently the primary material used in solar cell metal pastes, accounts for 30-40% of cell production costs. To meet the climate targets set by the COP-21 Paris Agreement, a drastic increase is required in the production capacity of the PV industry to the terawatt (TW) level [3]. However, this is a challenge due to the current high level of silver consumption. To address this, two strategies are employed for the fabrication of HJT modules: (1) eliminating busbar lines while retaining fingers, shifting from super-multi-busbar (SMBB) to zero-busbar (0 BB) designs, and (2) using a non-noble metal and silver mixture paste such as Ag@Cu for fingers. However, although these approaches reducesilver consumption, they pose a greater challenge for HJT-0 BB interconnections. Thus, new interconnection techniques for HJT-0 BB PV modules have emerged, and they can be divided into four types: Smart Wire Connection Technology (SWCT) [20], Integrated Film Covering (IFC), adhesive dispensing, and soldering with adhesive dispensing. The 0 BB technology was first proposed by the Day4 Energy company (later acquired by the Meyer Burger company) in 2006, and it is also known as SWCT [21]. This technology uses a pre-fabricated compound foil and up to 38 Cu wires, forming ohmic contact with cells during lamination [22]. However, due to Meyer Burger's patent protection on 0 BB technology, there are relatively few research papers on the topic. [PERSON] and co-workers simulated the output power of modules with 0 BB cells interconnected using wave-shaped wires, and it exceeded that of 5 BB by 1.0% rel. and that of 12 BB by 0.7% rel. [23]. [PERSON]'s team investigated different soldering methods using three types of 0 BB cells and found that infrared soldering could form reproducible and stable solder joints [24]. They further found that a lower series resistance could be achieved for 0 BB-cell interconnections, and that 20-cell modules passed the TC-200 test without considerable degradation. [PERSON]'s group designed 0 BB cells for shingled PV modules, which exhibited almost the same efficiency as conventional ones but with a 60% decrease in Ag usage [25]. Even now, few companies have managed to overcome Meyer Burger's patent restrictions to produce other photovoltaic modules based on 0 BB technology. In 2022, the PV equipment manufacturer Ningxia XN Automation Equipment Co., Ltd. (Yinchuan, China) proposed an IFC-based 0 BB technology in which a carrier film (a type of pre-curing film) is used to pressure solder ribbons onto cell surfaces for pre-interconnections, and ohmic contact is formed with cells during lamination, like in SWCT. This technology provides an all-new fabrication process for the interconnection of HJT-0 BB PV modules. \"Adhesive Dispensing\" technology uses insulating or conductive adhesives to create contact between solder ribbons and cells. Ohmic contact between solder ribbons and fingers on cells is also formed during lamination.