were the main concern, and a variety of fabrication processes were suggested, such as anti-solvent and two-step methods. [1][2][3][4] Researchers shifted focus from synthetic methods to additional treatments on the existing perovskite layers that provide high PCEs and performance since the discovery of multiple processes to obtain flawless and homogeneous perovskite films. [5][6][7][8][9][10][11] 2D perovskite materials, such as phenethylamine halides (PEAX), have been favored by researchers lately for surface treatments. [8,[12][13][14] The surface treatment using the 2D materials significantly improves general photovoltaic properties of 3D perovskites. However, the working mechanism of surface treatments using 2D materials remains unclear. The surface passivation effect best explains the positive effects of surface treatments on 3D perovskites. [13][14][15][16] 2D perovskites existing either in the 3D perovskite grain boundaries or on the top of perovskite surfaces reduce the defects in 3D perovskites because the defect sites mostly exist near the grain boundaries. Multiple research groups reported improved stability against air and moisture because the rigid and robust low-dimensional materials protected perovskite layers. [13,[15][16][17] A recent report by You et al. suggested that unannealed phenethylamine iodide (PEAI) was more effective for surface passivation than annealed PEAI. [8] You et al. claimed that the 2D perovskite materials fabricated using heat were less effective compared to other 2D perovskites. However, many other reports state that the 2D perovskite materials were effective in surface treatments irrespective of their fabrication process. [15,16,[18][19][20] As the clear reasons behind this have not manifested, the right way of using these materials in PSCs has not still been established.The term passivation used in many reports regarding PSCs is also ambiguous and blended with the term interlayers. [21,22] Especially, 2D perovskite materials coated on the perovskite layers such as PEAX are often referred to passivation layers because they are highly reactive with perovskite layers to form protective layers. [8,23,24] Herein, we propose a mechanism for the application and analysis of 2D perovskite materials in surface treatments. We developed new hole transport layers (HTLs) consisting of a mixture of two small molecules having donor-acceptor (D-A) structures instead of using spiro-OMeTAD, a typical HTL for PSCs. This is because the electronic properties of spiro-OMeTAD dynamically change over time due to its self-oxidization making the precise Surface modification of 3D hybrid perovskites using 2D perovskites, such as phenethylamine halides (PEAX), increases the overall power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). The effect is based on a surface passivation phenomenon where PEAX is in direct contact with the perovskite and hole transport layer (HTL). However, it is herein observed that the PCE of PSCs containing PEAX increases significantly when they a...