A group of researchers led by the Jinan University in China has developed a two-terminal (2T) perovskite-silicon tandem solar cell based on special hybrid interconnecting layers (ICLs) that prevent direct contact between the perovskite absorber and transparent conductive oxide (TCO).
“Typical self–assembled monolayers (SAMs) directly on transparent-conductive-oxide (TCO) recombination layer exhibit poor uniformity and compactness, thus resulting in significant current leakage losses and poor reproducibility of tandem solar cells,” the research's corresponding author, Yousheng Wang, told pv magazine. “To address this issue, we proposed a sputtered nickel oxide (NiOx) as the seed layer of SAMs to build the hybrid interconnecting layers. The sputtered treatment technique provides an easy coating on a complex substrate and high reproducibility.”
According to Wang, NiOx materials can further increase the coupling of SAM molecules with the substrate. Thus, the hybrid ICLs could improve the uniformity of the interface between the TCO and the SAM based on MeO-2PACz, which is also known as [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid, and reduce the leakage current. “Then, a good energy level alignment between perovskite and hybrid ICLs was built, which is beneficial to carrier extraction and transportation,” he added. “The hybrid ICLs further decrease the interface defects and bulk defects.”
The scientists built the tandem cell with a bottom silicon heterojunction device and a 19.73%-efficient top inverted perovskite solar cell with an energy bandgap of 1.71 eV integrating the ICLs. The latter was designed with a substrate made of indium tin oxide (ITO), a hole transport layer (HTL) made of nickel(II) oxide (NiOx), the MeO-2PACz SAM, a perovskite absorber doped with 2-phenylethylammonium iodide (PEAI), an electron transport layer (ETL) based on a buckminsterfullerene (C60), a transparent back contact made of indium zinc oxide (IZO), and a silver (Ag) metal contact.
The performance of the tandem device was analyzed and compared to that of a refence cell based on a NiOx HTL but without the MeO-2PACz SAM. The latter achieved a power conversion efficiency of 22.27%, an open-circuit voltage of 1.75 V, a short-circuit current density of 17.85 mA cm−2, and a fill factor of 71.15%. The former obtained an efficiency of 28.47%, an open-circuit voltage of 1.88 V, a short-circuit current density of 18.25 mA cm−2, and a fill factor of 81.8%.
The group emphasized that the tandem cell's fill factor is currently one of the highest in the scientific literature for perovskite-silicon devices. “It is found that the hybrid ICLs of NiOx/MeO-2PACz significantly reduce current leakage and non-radiative recombination losses by avoiding direct contact between perovskites and TCO,” it further explained. “Our results provide an effective method to improve the uniformity and reduce the leakage current of MeO-2PACz coating on the substrate.”
The solar cell was presented in the paper “Hybrid interconnecting layers reduce current leakage losses in perovskite/silicon tandems with 81.8% fill factor,” published in Cell Reports Physical Science. The research group comprised academics from Germany's Forschungszentrum Jülich GmbH, as well as from the Wuyi University and the University of Macau in China.
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