Revealing Hole Transporting Properties of Transition Metal Oxides Deposited via Atomic Layer Deposition as Bi-Functional Interlayers to Highly Efficient Perovskite Solar Cells (>23%)

Power Conversion Efficiency (PCE) of Perovskite Solar Cells (PSCs) is over 25.7%. Despite solar cell’s high PCE, low device stability coming from the inherent stability of perovskite absorber remains to be solved and restrict PSCs’ commercialization. Currently, the state-of-the-art PSCs used FAPbI₃ (Formamidinium lead tri-iodide) as the main composition. Although pure FAPbI₃-based PSCs show higher PCE due to a wider absorption spectrum owing to a lower band gap than MAPbI₃(Methylammonium lead tri-iodide), a spontaneous phase transition from photoactive α-FAPbI₃ to non-photoactive δ-FAPbI₃ is pointed as a significant challenge. In addition to phase transition issue, low device’s stability because of 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) which is mainly used as hole transporting layer(HTL) in n-i-p normal structure PSCs is also raised as a challenge. Oxide hole transporting layers (HTL) can be strong candidates. However, the oxide transporting layers generally require high processing temperature and the lack of p-type characteristics inhibits application to PSCs as HTLs. Transition metal oxide (TMO) such as TiO₂, WO_X, MoO_X and V₂O_5-X are also recognized as a solution, but the control of hole transporting properties related to oxidation state of metals in TMO and a proper ohmic junction with organic layer and/or suitable deposition methods without damaging the perovskite absorber layer should be solved in advance. Motivated from here, we adopt atomic layer deposition (ALD) to fabricate TMO as HTLs with low temperature process 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 as an HTL, we fabricated n-i-p normal structure PSCs of FAPbI₃ and Spiro-OMeTAD with ALD grown TMOs (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 stability of PSCs with ALD TMO layer is over 90% initial PCE after ~ 570 hrs and maintained 94.9% of initial PCE after 130 hrs operating condition without UV cut-off filter. In conclusion, this study demonstrates the application of TMOs as hole transport/protective bi-functional HTLs and ALD as an optimal deposition method without damaging the underlying perovskite absorber. Furthermore, this study shows the possibility that ALD TMO can be also applicable to tandem device fabrication and stable PSCs’ commercialization.