In a brand new paper printed in Nature Communications, the Chinese photo voltaic producer defined that the heterojunction again contact cell it unveiled in late 2023 achieved one of many highest open-circuit voltages ever. reported since this cell expertise, in addition to smaller than typical short-circuit. present losses. The outcomes have been confirmed by the Institute for Solar Energy Research (ISFH) in Germany.
A gaggle of scientists from Chinese photo voltaic module maker Longi has described in a brand new scientific paper the 27.09%-efficient heterojunction again contact (HBC) photo voltaic cell it unveiled in December 2023.
At the time, Longi solely mentioned that the consequence was enabled by a brand new laser graphics course of that’s cheaper than typical high-cost photolithography processes, with out giving additional particulars. In the brand new paper, the corporate explains that the laser sample approach was developed by Longi himself. In addition, it not solely offers a full technical description of the cell structure, but additionally presents a method on enhance the contact resistivity, collection resistance, and the again sample design.
The group specified that HBC photo voltaic cells have provider recombination loss points and mentioned this stems from the hole-selective contact area and polarity boundaries. “We suggest options to those points and set up a transparent relationship between contact resistance, collection resistance, and the bottom sample design,” they added.
The analysis group established the 243.0 cm2 cell with 200 µm-new Longi M6 274.3 cm2 Czochralski wafer based mostly on n-type monocrystalline silicon. The entrance aspect has anti-reflection coatings (ARC), and the again aspect is split into 4 areas, that are hole-selective contact (HSC), hole area, electron-selective contact (ESC), and HSC plus hole.
“We took benefit of amorphous silicon as passivating contact layers and laser ablation as a mass-production expertise for the manufacturing of HBC photo voltaic cells,” defined the lecturers. “This offers benefits together with low leakage within the space close to the np polarity boundary, low price, low deposition problem, and excessive uniformity.”
Through laser patterning, the researchers utilized a pulsed inexperienced picosecond laser to ablate the overlying ia-Si:H/pa-Si:H stack within the designed ESC area. The clear conducting oxide (TCO) layer is deposited utilizing the magnetron sputtering approach.
They additionally used a pulsed ultra-violet picosecond laser to ablate the designed hole area to take away the TCO/pa-Si:H/ia-Si:H stack for isolation, which they are saying avoids the leakage path. Silicon nitride (SiNx) layer is used to forestall the efficiency adjustment of the ia-Si:H/na-Si:H stack from being broken.
Tested beneath customary lighting circumstances, the machine achieved an influence conversion effectivity of 27.07% and an open-circuit voltage of 751 mV, with outcomes confirmed by Germany’s Institute for Solar Energy Research (ISFH). The open-circuit voltage worth was proven by Longi as one of many highest recorded on the analysis degree for this sort of photo voltaic cell.
Their evaluation additionally exhibits that the cell, like different HBC gadgets, suffers from the consequences {of electrical} shading, as a result of the carriers for a similar polarities are collected on the identical aspect, and the resultant discount in short-circuit present density. They additionally discovered, nonetheless, that these losses have been smaller than these seen in earlier gadgets.
“This is especially attributed to the elevated present obtained from the thicker silicon wafer and the designated illumination space used within the measurements, excluding the consequences of recombination and edge leakage on the silicon wafer,” they additional defined.
The group additionally discovered that contact resistance performs an vital position in high-efficiency HBC photo voltaic cells and mentioned that minimizing the contact space within the HSC or ESC area is vital to enhance cell effectivity. “By optimizing the HSC area, we achieved the bottom contact resistance utilizing the pa-Si:H movie,” it specified. “Furthermore, we all know that the contribution of contact resistance to resistivity will be instantly calculated based mostly on the protection space ratio.”
Looking forward, the researchers mentioned they need to harness the consequences of the wafer edge and implement “meticulous” optimization of the entrance anti-reflection coating and rear reflector, which they consider will assist the cell which achieves an effectivity of as much as 27.7%.
The photo voltaic cell structure is described in “27.09%-efficiency silicon heterojunction again contact photo voltaic cell and past,” printed in environmental communication. “The analysis provides many methods and pointers for optimizing the structural design and fixing the main contradictions inside back-contact photo voltaic cells,” the group emphasised.
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