Exploring low-temperature processed multifunctional HEPES-Au NSs-modified SnO2 for efficient planar perovskite solar cells

In the development of high-efficiency n-i-p planar heterojunction perovskite solar cells (PSCs), the development of dense electron-transporting layers (ETLs) with excellent electrical properties and hetero-interface quality is very important for promoting perovskite growth and carrier transport dynamics. Herein, we propose a simple and effective strategy to simultaneously improve the electronic properties and interface quality of a tin oxide (SnO2) ETL by introducing 2-hydroxyethyl modified gold nanostars (HEPES-Au NSs) into a commercial SnO2 colloidal dispersion in order to synthesize mixed ETLs at low temperatures (<= 150 celcius). Based on the unique plasmonic structure of the HEPES-Au NSs, we demonstrate that the modified ETLs exhibit higher conductivity and greater efficiency in the extraction, transfer, and collection of photogenerated electrons than conventional SnO2 ETLs. Moreover, the chemical bond interaction between HEPES-Au NSs and the perovskite enhances the affinity of the SnO2/perovskite hetero-interface, which effectively improves the nucleation and crystallization kinetics of the perovskite, thus producing a high-quality absorber with high crystallinity and superior absorbance. Meanwhile, the 2-hydroxyethyl (HEPES) adsorbed onto the SnO2 surface effectively passivates perovskiterelated trap states, suppressing the non-radiative recombination and leakage current, and ultimately improving the relative electronic properties and photovoltaic performance output of the modified device. The results show that the power conversion efficiency (PCE) of the modified PSCs is significantly better than that of the original SnO2-based planar devices, and the optimal PCE reached 21.13%, with negligible hysteresis. Additionally, owing to the multifunctional effects of SnO2 modified by HEPES-Au NSs, the functionalized devices without encapsulation also demonstrate reliable reproducibility and good stability, which shows great advantages in the development of high-efficiency flexible PSCs and monolithic tandem devices.