Bi-Functional Inorganic Interlayers via Low-Temperature Atomic Layer Deposition Toward Highly Stable and Efficient Perovskite Solar Cells (>23%)

Power conversion efficiency (PCE) of Perovskite solar cells (PSCs) is over 25.7%. Despite PSCs’ high PCEs, their operational as well as environmental instability remains to be solved and restrict PSCs’ commercialization. Currently, the state-of-the-art PSC is used almost pure α-FAPbI₃ (Formamidinium lead tri-iodide) as an absorber and 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) as hole transporting layer (HTL) in the <i>n-i-p</i> normal structure of PSCs. A chemical-deriven phase transition from photoactive α-FAPbI₃ to non-photoactive δ-FAPbI₃ is pointed as a significant challenge. In addition to phase transition issue, much lower stability of Spiro-OMeTAD is critical for the device performance degradation. As an interlayer in between top metallic electrodes (i.e., Au and/or Ag) and Spiro-OMeTAD/α-FAPbI₃, inorganic HTLs prepared by a low-temperature atomic layer deposition (ALD) technique provide bi-functionality to stabilize the PSCs during operation in ambient. Transition metal oxide (TMO) as an inorganic interlayer can be a strong candidate. Most of TMO layers generally require a high processing temperature and the lack of <i>p</i>-type characteristics inhibits application to PSCs as hole-transporting interlayers. In this study, we adopt ALD to fabricate TMO layers at low temperatures (~ 50 °C) and intentionally induce oxygen deficient traps to form empty <i>d</i>-bands to induce and further enhance hole transporting properties. With their oxygen deficient trap assisted hole transporting properties, we fabricated n-i-p normal structure PSCs of α-FAPbI₃ and Spiro-OMeTAD with ALD grown TMOs (amorphous-TiO₂, MoO_x and V₂O_5-X) on top of Spiro-OMeTAD to enhance device stability. During fabrication, we adopt funnel like 2D perovskite passivation layers for defect passivation and enhance perovskite’s stability coming from hydrophobic characteristics. ^[1] As a result, highly efficient PSCs of PCE of over 23% with TMOs are fabricated with pin-hole free hole transporting and protection bi-functional ALD layers.^[2,3] The environmental stability of PSCs with ALD-V₂O_5-x interlayers is over 90% initial PCE after ~ 600 hrs, while without the interlayer started to be degraded under 80% of initial PCE just after around 200 hrs without any special encapsulation. In conclusion, this study demonstrates the application of TMOs as hole transport/protective bi-functional interlayers and ALD as an optimal deposition method without damaging the underlying perovskite absorber. Furthermore, this study shows the possibility that ALD TMOs can be also applicable to tandem device fabrication with p-i-n type PSCs and stable PSCs’ commercialization.<br/>References<br/>[1] <b>Shin, H. et. al.</b>, “Cyclohexylammonium-Based 2D/3D Perovskite Heterojunction with Funnel-Like Energy Band Alignment for Efficient Solar Cells (23.91 %)”, <i>Adv. Energy Mater.</i> 2102236 (2021) [2] <b>Shin, H. et. al.</b>, “Amorphous TiO₂ Coatings Stabilize Perovskite Solar Cells”, <i>ACS Energy Lett</i> <b>6</b>, 3332 – 3341 (2021) [3] <b>Shin, H. et. al.</b>, “Hole Transporting Vanadium-Containing Oxide (V₂O_5-x) Interlayers Enhance Stability of a-FAPbI₃-Based Perovskite Solar Cells (~ 23%)”, <i>ACS Appl. Mater. & Interfaces, <b>14</b>, </i>37, 42007 – 42017 (2022)