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Chinese researchers have proposed a novel lithium-free doping strategy to fabricate spiro-OMeTAD-based hole transport layers (HTLs) for applications in perovskite solar cells. A photovoltaic device constructed with a salt-doped HTL achieved a record efficiency of 25.45%.
Scientists at Northwestern Polytechnic University in China have used a lithium-free doping strategy to fabricate a perovskite solar cell based on a metal-free hole transport layer (HTL) made of spiro-OMeTAd that reportedly offers remarkable levels of efficiency and stability.
The research team explained that spiro-OMeTAD for perovskite cell applications is typically doped with a compound known as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to improve hole extraction and conductivity. However, this type of doping requires oxidation in air for 24 hours, which is a barrier to commercial production of perovskite photovoltaic devices.
“Inadequate doping efficiency of this post-treatment process can lead to retention of unreacted reagents and/or harmful byproducts in the doped spiro-OMeTAD layer, negatively affecting the device efficiency,” the academics stress. “Moreover, a major challenge restricting its practical application is the trade-off between high efficiency and stability.”
To solve this problem, they used a salt known as Eu(TFSI) for doping spiro-OMeTAD, which can generate oxygen (O2) and facilitate the pre-oxidation of spiro-OMeTAd. “Unlike conventional LiTFSI doping with air exposure, O2-derived spiro-OMeTAD HTLs can instantly obtain high conductivity and ideal work function without the need for post-treatments,” they added.
Adopting this doping strategy, the researchers constructed the solar cell with a fluorine-doped tin oxide (FTO) glass substrate, a titanium oxide (TIO2)-based electron transport layer (ETL), a perovskite absorber, the proposed Eu(TFSI)-doped HTL, and a gold (Au) metal contact.
Tested under standard illumination conditions under reverse scanning, the device achieved a power conversion efficiency of 25.45%, an open-circuit voltage of 1.210 V, a short-circuit current density of 25.41 mA cm2 and a fill factor of 82.50%. A LiTFSI-doped reference cell with the same architecture achieved an efficiency of only 23.19%.
“In addition, the O2-derived devices offer a stabilized power output (SPO) of 25.22% for 100 s, while the SPO of the LiTFSI-doped devices barely stabilizes at 22.48%,” the scientists explained, noting that the cell was also able to retain 90% of its initial efficiency after 1000 h of operation.
The device was then used to build a 6 cm x 6 cm perovskite solar module that achieved an efficiency of 20.35%. According to the team, this result demonstrates the effectiveness of the new doping strategy regardless of the size of the devices.
“In perovskite solar cells with spiro-OMeTAD HTLs without oxidation or without Li, the present O2 doping work ranks among the highest in terms of both efficiency and stability,” the academics concluded.
The novel doping strategy and cell architecture are described in the paper, “ Superoxide radical derived metal- free spiro-OMeTAD for highly stable perovskite solar cells,” published in Nature Communications . |
