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Xiaolu Zheng1 Yulong Wang2 Hongwei Lei1 Guang Yang1 Zongxiang Xu2 Guojia Fang1

1, School of Physics and Technology, Wuhan University, Wuhan, Hubei, China
2, Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong, China

Organo-lead halide perovskite solar cells (PSCs) have attracted tremendous attention owing to their superior photovoltaic properties. However, despite the excellent power conversion efficiencies (PCEs) that have recently been achieved, the device stabilities are still a challenge for the commercialization of PSCs. Spiro-OMeTAD is a widely used hole transport layer (HTL) in conventional n-i-p PSCs, which has been reported to suffer degradation from the permeate of moisture due to the hygroscopic additive and the presence of pinholes. To fix the relatively low device stability of PSCs based on spiro-OMeTAD, numerous strategies have been developed and applied in PSCs.
One approach to diminishing these adverse effects introduced by the moisture permeate and ion migration is to insert a buffer layer. To avoid decreasing the device performance while improving the stability, this p-type semiconductor needs possess high conductivity and superior hole mobility besides hydrophobicity. Lead sulphide (PbS) is a traditional direct bandgap semiconductor with high hole mobility. We found that when inserting a thin layer of PbS between the metal electrode and spiro-OMeTAD, the PSCs with PbS buffer layer exhibited a better photovoltaic performance and significantly enhanced stability with respect to the reference cells. The superior hole mobility of spiro-OMeTAD/PbS bilayer was considered to be the dominated origin of the device performance improvement. And the hydrophobic nature and dense morphology of PbS enable it to provide an efficient permeation barrier against moisture and metal migration. The champion cell with PbS buffer layer displayed a PCE of 19.58% and maintained almost 100% of its initial PCE after 1000 h stored in ambient air. While in this structure the spiro-OMeTAD is still requisite.
Metallophthalocyanine (MPc) compounds that consist of an 18-π electron conjugated macrocycle skeletal structure are potential candidates for stable HTLs in PSCs. We reported a novel heavy atom Pc derivative, octamethyl-substituted palladium(II) phthalocyanine (PdMe2Pc), which shows promise as an HTL in PSCs. The introduction of the heavy Pd atom endows the material with a long carrier lifetime and without dramatically reducing its mobility. This PdMe2Pc exhibited a long carrier diffusion length (LD) which is benefit to reducing the charge recombination. As a result, the devices based on PdMe2Pc displayed a relatively high PCE of 16.28% and good long-term stability.

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