The perovskite solar cells (PSCs) have experienced a significant increase in efficiency, from 3.8 % in 2009 to 26.1 % in the present day.[
1,
2] These solar cells are typically composed of a perovskite light-absorbing layer sandwiched between an n-type electron transport layer (ETL) and a p-type hole transport layer (HTL).[
3,
4,
5,
6] Upon the incidence of light on the solar cell, an exciton is generated within the active layer, which subsequently separates into electrons and holes. These free carriers are then transported through the ETL and HTL, respectively, to the external electrode.[
7] As for HTLs, molecular layers based on organic or conductive polymers have been widely studied to enhance the efficiency of PSCs.[
8] However, these types of HTL layers have notable limitations in terms of reliability due to their environmental sensitivity.[
9,
10,
11] As a result, there is an urgent need to explore alternative HTL materials that are not only efficient and stable, but also offer potential scalability and a reduction in overall manufacturing costs. Inorganic compounds with energy levels suitable for PSCs have been investigated. Nickel oxide (NiO), an inorganic HTL, exhibits semiconductor properties, including excellent stability, high hole mobility, a wide band gap (~3.5 eV), high optical transparency, and a solution process coating method, demonstrating its potential for use in PSCs.[
12,
13,
14] However, a reduction in efficiency due to surface recombination, traps, and defects remains a challenge.[
15] To address these issues, research on passivation[
16,
17], additives[
18,
19,
20], and buffer layers[
21,
22] is being undertaken. Recently, several efforts have been made to reduce defects and enhance the performance of devices using self-assembled monolayers (SAM).[
23] A SAM molecule is made up of three parts: a head group, a linkage tail and a functional group.[
24] The head group attaches directly to the substrate and helps stabilize the surface, using various substances such as silane, carbonyl acid, phosphoric acid, and thiol.[
25] The linkage tail connects the head group and functional group, and it affects the optical properties and performance, depending on its molecular size and tilt angle.[
26] The functional group, which is located on the surface that is exposed, can alter the properties of the surface based on the material used.
Herein, we fabricated inverted p-i-n PSCs using HC-A1 and HC-A4 molecules. These multi-functional molecules, previously employed in dye-sensitized solar cells, exhibited excellent stability and offered the potential to control charge recombination.[
27,
28] The impact of introducing an additional layer of HC-A1 and HC-A4 between the perovskite and the transporting layer was investigated, both structurally and optically, using various techniques. The application of HC-A1 and HC-A4 resulted in an improvement in the grain morphology and uniformity of the perovskite active layer. In particular, HC-A4 led to significant surface modification and efficient carrier extraction with its pronounced hydrophilic properties. As a result, HC-A4-applied solar cells exhibited the highest power conversion efficiency (PCE) of 20%.