# Effects of Doped Hole-Transporting Layers on Perovskite Solar Cell Performances
## 1 Introduction  
Several emerging solar cells (i.e., organic, dye-sensitized, quantum dot and perovskite) have received growing attention because of the necessity for sustainable and clean energy. Among these emerging solar cells, perovskite solar cells (PSCs) show the sharpest increase in efficiency. PSCs started with an efficiency of 3.8% in 2009 [1] and achieved an efficiency of 26.1% in 2023 [2]. Nowadays, the efficiency of PSCs has developed to the point where it matches to that of the crystalline silicon solar cells. However, PSCs are vulnerable to heat and moisture, making them difficult to commercialize due to its low stability and short lifetime. Silicon solar cells are known to have a lifetime of more than 30 years but typical perovskite cells have a lifetime of less than a year. Therefore, it is necessary to improve the stability of perovskite solar cells for commercialization.  
Another problem for the commercialization of PSCs is that it is difficult to prepare a uniform large-area perovskite film through a single solution process because the uniformity of the formed film varies greatly depending on various conditions. The key point for the fabrication of efficient PSCs is how to make the minimal defect films without pinholes and with a large grain. The inverted structure (i.e., n-i-p type) is widely used because of its mass production capability using low-temperature solution processing and negligible hysteresis behavior [3; 4; 5]. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) has widely been used in inverted PSCs because of its low temperature processability, good electrical conductivity, and mass production capability such as roll-to-roll processing. Theconductivity of PEDOT: PSS films from a stock solution is usually below 1 S/cm, which is much lower than those of metal or inorganic cathode materials. Therefore, the conductivity of PEDOT: PSS films should be increased.  
The uniformity of PEDOT: PSS film morphology is also important to the PSCs' performance. Recently, many studies [6; 7; 8; 9; 10; 11; 12; 13; 14; 15] have reported on the doping of the PEDOT: PSS film that improves the uniformity and roughness of the hole-transporting layers. The addition of dopants increases the uniformity of the PEDOT: PSS film and consequently improves the efficiency and stability of the PSC devices. In this study, we studied a novel processing technology to improve performances of PSCs through adding solvent additives to PEDOT:PSS. Both 1,8-diiodooctane (DIO) and 1,8-dichlorooctane (DCO), high-boiling liquids which are miscible in methanol, are widely used as surfactants for controlling the phase separation of active materials in the organic solar cell industry. DIO is seldom used with PEDOT: PSS and a reduction of PEDOT: PSS film when mixed with DIO was reported [16]. It is expected that the addition of these solvent additives would produce a more homogeneous PEDOT: PSS film with higher conductivity and that the partially electro-negative halogen (i.e., I\({}^{8-}\) and Cl\({}^{8-}\)) in DIO and DCO could interact with the perovskite crystal's Pb\({}^{2+}\), while the hydrophobic aliphatic moiety (i.e., partially positive octyl groups) can passivate the surface of perovskite thin film from moisture.