Energy Band Alignment in between Perovskites with Different A-Site Molecules and NiO Interlayers for Highly Efficient and Stable Inverted (p-i-n) Perovskite Solar Cells

To date, most highly efficient perovskite solar cells (PSCs) are based on a normal (n-i-p) structure. However, even normal structure PSCs show higher power conversion efficiency (PCE), the advantage of relatively high stability and applicability to tandem devices draw tremendous attention to the inverted (p-i-n) structure of PSCs. Notoriously, most of inverted PSCs are based on polymer hole transporting layers (HTLs), which restrict the further development of stability due to their hygroscopic and acidic properties. Unlike polymer HTLs, PSCs based on oxide HTLs such as NiO, CuO_x and CuCrO₃ demonstrated more stable than polymer HTLs. Among oxide HTLs, NiO is the most attractive and widely accepted as a potential candidate for HTL in the inverted PSCs. However, surface redox reaction at the surface defects, especially at Ni³⁺ what are detrimental reaction sites with cations and/or halides induced voltage loss, series resistance and band alignment miss-match remained to be solved. In addition, surface properties depending on NiO fabrication methods constrict reproducibility of the inverted PSCs based on the composition of formamidine (FA) cations which show better device performance due to broad light absorption compared to methylammonium (MA), which are before mainstream of perovskite absorbers. Furthermore, to the best of our knowledge, direct deposition of pure-FAPbI₃ or more than 95% of FAPbI₃ on NiO HTLs was not reported yet. Therefore, a suggestion of appropriate deposition techniques of NiO and analysis of the interfaces between NiO/Perovskite layers still much to be investigated. Among various deposition methods, we choose atomic layer deposition (ALD), because of their pin-hole free, uniform and conformal characteristics. To investigate NiO/Perovskite interface, we fabricated inverted PSCs with three perovskite composition (1) pure MAPbI₃, (2) (CsPbI₃)_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃, and (3) FAPbI₃ with trace amount of MACl (hereafter MA, CsFAMA and FA). As the amount of FA cation increased from MA to FA composition, PSCs showed a gradual increase of <i>J_sc</i> up to 23 mA/cm² in FA base PSCs. However, open-circuit voltage (<i>V_oc</i>) showed the decreasing trends from average <i>V_oc</i> around of 1.05 V in MA and 0.99 V and 0.97 V in CsFAMA and FA, respectively. We compared X-ray/Ultraviolet photoemission spectroscope (XPS, UPS) and Kelvin Probe work-function measurement focusing on energy band matching between NiO/Perovskite interfaces. As a result, we figure out that voltage loss dependence on FA cation ratio is caused by energy level miss-marching between NiO/perovskite interface. In addition to energy band mismatch, in all cases of perovskite compositions, Ni³⁺defects induced a non-radiative recombination, which could restrict further performance enhancement. We also modified the NiO surfaces with organic substances containing both Lewis bases and pseudo halides. Synergetic effect of Ni³⁺ defects’ passivation and suppressed nonradiative recombination significantly enhances the device performances, in particular, <i>FF</i> and <i>V_OC</i>. Not only device performance, but also reproducibility of PSCs with MA, CsFAMA and FA composition were increased. Furthermore, the fabricated PSCs were additionally passivated with ALD-grown SnO₂ on top of electron transport layer (ETL). Finally, ALD NiO and ETL dual passivated inverted PSCs showed the enhanced photovoltaic performances and increased stability. In summary, this study demonstrates facile surface defect passivation and energy level alignment optimization to boost charge extraction of NiO/Perovskite interfaces by Lewis base/Ionic molecules. In addition to NiO surface modification, we adopt ALD SnO₂ layer on top of ETL to enhance electron transport and device stability.