ALD-SnOx as a Top Layer and Hole Transport Layer to Improve The Thermal Stability of Lead-Free Tin Perovskite Solar Cells

Lead perovskite solar cells (Pb-PSCs) have attracted significant worldwide attention due to their rapid increase in power conversion efficiency (PCE) from 3.8% to 25.8%.However, lead toxicity is a concern for their commercialization.<br/>Among lead-free alternatives, tin perovskite solar cells (Sn-PSCs) have the highest PCE (&gt;14%) due to their narrow band gap (1.4 eV), low exciton binding energy, and high carrier mobility.<br/>Despite the vigorous efforts to improve the efficiency of Sn-PSCs, thermal stability is still a critical issue that has not been solved.Inverse structure devices using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonic acid) (PEDOT:PSS) as the hole transport layer (HTL) are commonly used for highly efficient Sn-PSCs.However, the PEDOT:PSS layer has hygroscopic and acidic properties that reduce the stability of the solar cell.Therefore, Sn-PSCs without PEDOT:PSS is an important research item in terms of stability.In addition, the diffusion of iodine ions within the tin perovskite layer reduces device stability. Therefore, preventing iodine ion diffusion is also important to improve the stability of Sn-PSCs.<br/>Previous studies have reported that ALD-SnOx (x = 1.7-1.8) can serve as HTLs in tin-lead alloy perovskite solar cells. Furthermore, our previous study demonstrated that SnOx (x = 1.7-1.8), obtained by plasma oxidation of metallic Sn, can serve as HTLs in Sn-PVK-PV with PCEs exceeding 14%. We have also found that tin oxide nanoparticle layers, which are commonly used as electron transport layers (ETLs) in Pb-PSCs, can operate as HTLs in Sn-PSCs.These SnOx layers are partially composed of Sn²⁺ rather than Sn⁴⁺ and exhibit bipolar carrier transport.Sn-PSCs with tin oxide nanoparticle HTLs exhibit higher stability than PEDOT:PSS HTL devices.Therefore, tin oxide HTLs may be effective to enhance the stability of Sn-PSCs.<br/>ALD-SnOx in the top layer has been reported to improve stability in Pb-PSCs by preventing the diffusion of iodine ions.<br/>In this report, we first replaced HTL with ALD-SnOx to improve the thermal stability of Sn-PSCs. In addition, ALD-SnOx was also introduced in the top layer.<br/>In the presentation, the mechanism of thermal stability improvement by ALD-SnOx will be discussed.