A research team from Northwestern Polytechnical University, jointly with Xi’an Shiyou University and Xidian University, has developed a novel perovskite film crystallisation regulation strategy for tandem wide-bandgap organic-inorganic hybrid halide perovskite solar cells. It was enabled by using CsPb2Br5 as a two dimensional (2D) seeding agent.
“The novelty of our research lies in using all-inorganic 2D CsPb2Br5 flakes as a heteronucleation agent for 1.80 eV wide-bandgap perovskites, which successfully addresses the critical issues of uncontrollable crystallisation and phase separation,” co-corresponding author of the research,” Chenxin Ran, told pv magazine.
The researchers noted that both perovskite-silicon and all-perovskite tandem cells require a high-performance wide-bandgap (WBG) perovskite top cell to match the narrow-bandgap bottom cell, typically with a bandgap range of 1.68 eV to 1.80 eV, but such absorbers have issues with crystallisation and phase separation that negatively affect performance.
To control crystallisation and phase separation in WBG perovskite film made of Cs0.2FA0.8Pb(I0.6Br0.4)3, the researchers used 2D CsPb2Br5 as a seeding agent. The synthesis of the seeding agent was straightforward, according to first author Changbo Li, “employing a cooling precipitation method” for the constituent CsBr and PbBr2 solutions.
“Due to its low solubility, it precipitates immediately at the top surface upon antisolvent dropping to form solid seeds, effectively guiding the perovskite growth downwards and preventing random nucleation,” co-corresponding author Weiyin Gao told pv magazine, explaining that the agent promotes a top-down vertical crystal growth by “lowering the nucleation energy barrier and increasing the defect formation energy. “This enhances charge transport and suppresses phase separation, ultimately achieving a record-high fill factor of 85.39%,” said Gao.
Furthermore, the optimised 1.80 eV WBG had a champion power conversion efficiency of 20.14 % when used in an inverted single junction solar cell, according to the researchers.
In a further demonstration, the group fabricated 4-terminal tandem devices, one based on heterojunction back contact (HBC) silicon solar cell as top subcell, and the other a narrow bandgap FA0.7MA0.3Pb0.5Sn0.5I3 top subcell. The perovskite-silicon device achieved an efficiency of 31.13%, with a stabilised power output of 30.81%, which was “superior” to tandem device based on the control WBG perovskite solar cells, according to the researchers. Moreover, an all-perovskite tandem cell built with the top cell reached a stabilised 28.01% efficiency.
As for stability, unencapsulated devices under light soaking conditions maintained 80% of the initial performance, while a control device dropped to less than 80% after only 300 h, according to the research team. Furthermore, enhanced thermal stability and air stability tests also showed outperformance compared to the control device.
The measurements of the demonstration devices were conducted on small-area cells 0.05 cm2, while the scalability of the method was demonstrated by fabricating a 1 cm2 device, co-corresponding author Weidong Zhu told pv magazine.
Assessing the results, the researchers said that the work shows that the crystallisation control strategies “can effectively resolve the intrinsic instability issues of wide-bandgap perovskites,” with potential for use in future large area devices.
The details of the study appear in “Heterointeraction-Induced Nucleation Promoting Vertical Growth,” published in Research.
“We have successfully constructed perovskite-silicon tandem devices with an efficiency exceeding 31%, and we are currently dedicated to optimizing their long-term stability to meet future application demands,” concluded co-corresponding author Wei Huang.
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