Shulou(Shulou.com)11/24 Report--

CTOnews.com, February 20, CTOnews.com learned from official account of the University of Science and Technology of China that professor Xu Jixian's team and collaborators proposed a new structure and breakthrough scheme named PIC (porous Insulation contact) to solve the long-standing contradiction of "passivation-transmission" in perovskite solar cells, which achieved the world record for steady-state certification efficiency of p-i-n trans-structure devices. It shows universal applicability in a variety of substrates and perovskite components. The relevant research results were published in the journal Science on the 17th.

The contradiction of "passivation-transmission" is common in optoelectronic devices, such as solar cells, light-emitting diodes, photodetectors, etc. In order to reduce the non-radiative recombination loss on the semiconductor surface, it is necessary to cover the passive layer to reduce the defect density on the semiconductor surface. The electrical conductivity of these passivation materials is generally very low, increasing their thickness will enhance the passivation effect, but at the same time lead to the limitation of current transmission. Because of this contradiction, the thickness of these ultra-thin passivation layers needs to be controlled very accurately within several or even one nanometer (nm, 1/1000000000 meters). Carriers are transmitted through thickness-sensitive ways such as tunneling effect, which is disadvantageous to low-cost large-scale production.

Perovskite solar cell technology has attracted wide attention in recent years, and its main device types include perovskite single junction, crystalline silicon-perovskite laminated cells, full perovskite laminated cells and so on. it is expected to provide a new low-cost and high-efficiency photovoltaic scheme in addition to the traditional crystalline silicon solar cells. In perovskite batteries, the non-radiative recombination loss caused by heterojunction contact has been widely proved to be the main performance limiting factor. Due to the existence of the contradiction of "passivation-transport", the change of the nanometer thickness of the ultra-thin passivation layer will lead to the decrease of filling factor and current density. Therefore, all kinds of perovskite devices urgently need a new contact structure, which can greatly reduce the sensitivity of passivation thickness while improving the performance.

After long-term thinking and a large number of experiments, the team refined this PIC contact structure scheme (figure 1). The main idea is not to rely on the traditional nano-scale passivation layer and pass-through transmission, but to directly use the porous insulating layer with the thickness of 100 nm to force the carrier to transport through the local opening region and reduce the contact area. The semiconductor device modeling calculation of the research team reveals the key design principle that the period of the PIC structure should match the perovskite carrier transmission length. The PIC scheme is similar to the local contact technology in the field of crystalline silicon solar cells, but the difference is that the carrier diffusion length in perovskite is much shorter than that in monocrystalline silicon, greatly reduced from millimeter to micron or even shorter, which requires the size and structure cycle of PIC to be at the level of 100 nanometers. The traditional local contact process of crystalline silicon can not directly meet this precision requirement, but the use of high-precision micro-nano machining technology has deficiencies in preparation area and cost. To meet this challenge, the team cleverly took advantage of the size effect of nanowires and successfully realized the preparation of this nanostructure by low-temperature and low-cost solution method through the transformation of PIC growth mode from conventional "Stranski-Krastanov" mode to "Volmer-Weber" mode (figure 2).

Design principle and device Simulation of figure 1.PIC (porous insulator contact)

Figure 2. Based on the island growth mode regulated by nano-wafer size effect, the PIC structure team carried out the verification of the PIC scheme in the p-i-n trans structure, which is widely used in laminated devices, and achieved the decrease of hole interface recombination velocity from ~ 60cm / s to 10cm / s for the first time (figure 3). And 25.5% of the highest efficiency of single junction (the steady-state authentication efficiency record of p-i-n structure is 24.7%) (figure 4). This significant improvement in performance is common in perovskite with various band gaps and components, which shows the wide application prospect of PIC. In addition, the PIC structure can improve the perovskite film coverage and crystallization quality on a variety of hydrophobic substrates (the carrier phase lifetime is greatly increased), which is also of great significance for large-scale preparation.

Inhibition of non-radiative recombination of perovskite interface and bulk phase by figure 3.PIC

Figure 4. In the PIC structure verification of p-i-n trans devices, it is worth noting that the PIC scheme is universal and can be further extended in different device structures and interfaces; at the same time, the simulation calculation shows that the current experimental PIC coverage area is far from reaching its design potential, and can be further optimized to achieve greater performance improvement.

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