# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 1 Introduction  
Nowadays, the continuous consumption of fossil fuels and the sustained environmental crisis have prompted people to seek for new sustainable and clean energy sources. Solar energy is an ideal candidate, not only because of its tremendous reserve, but also it can be directly converted to be electricity through solar cell devices. Recently, organic solar cells (OSC), such as conductive polymers, small molecules, fullerene-based and non-fullerene-based system [1; 2; 3], have attracted much attention due to the advantages of low cost, easy synthesis and structural tuning performance etc. [2; 3]. Higher power conversion efficiency (PCE) and stability of OSC have also been reported [2], which provide a channel to efficient utility of solar energy in a clean and environmentally friendly way. Meanwhile, organic-inorganic halide perovskite solar cells (PVSCs) have been paid extensive attention owing to many their advantages, such as intense broad-band absorption (covering UV and near infrared regions), photo-absorption coefficients (ten times bigger than organic dyes), low-band gap (1.25 eV), long charge diffusion lengths (charge transport is about 1 \(\upmu\)m) and high charge mobility, et al. [4; 5; 6] In the last year, PCE of PVSCs has made impressive progress, from 3.8% to a reported 22% [6; 7], which have revolutionized the field of photovoltaic technologies. The basic structure of perovskite solar cells is usually three layers (electronic transport layer (titanium dioxide)/perovskite absorption layer/hole transport layer) [8], which are placed between substrate materials (see Supplementary Materials Figure S1). Here, the hole transport material (HTM) plays an indispensable role in PVSCs, and an idealHTM can increase the short-circuit current density (J\({}_{\text{SC}}\)), open-circuit voltage (V\({}_{\text{OC}}\)) and have a marked impact on the stability of PVSCs [9; 10]. Good HTM must indeed exhibit the following features [11; 12; 13; 14]: firstly, a good energy match between HTM molecular orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) and contacted interfaces for efficient charge injection; second, high hole transporting properties to reduce charge recombination; stability with characteristics in protection of corrosion and resistance of oxidation. Some HTMs have been reported using spirobiluorens, conjugated polymers, triphenylamines, triazatruxenes and so on [11]. Although the 2,2',7',7'-tetrakis (N,N-p-dimethoxy-phenylamino)-9,9'-spirobi fluorine (Spiro-OMe TAD) as HTM is viewed to be a most successful material, there are some shortcomings in the utility of Spiro-OMeTAD [12; 13], such as cost-performance, long-term stability and low charge-carrier mobility, etc. As an alternative to Spiro-OMeTAD, carbazole-based material (X51) was designed and used as HTM owing to easy substitution with functional groups to control optical and electrical properties [13]. From the viewpoint of molecular design, a new hole transport material Jy5 was experimentally reported on by introduction of benzothiadiazole (BT). Based on HTM X51, and mono-fluorinated benzothiadiazole (BT) and difluorinated BT (FBT) have been used as core structure to design new HTM Jy6 and Jy7 [14] (the core structures of Jy5-Jy7 are represented by the red groups, and the chemical structures of them can be found in Figure 1). Experimentally, the designed Jy6 obtains high open circuit voltage (V\({}_{\text{OC}}\)) for 1.06 V, large short-circuit current density (J\({}_{\text{SC}}\)) for 21.39 mA/cm\({}^{2}\) and outstanding fill factor (FF) up to 81%, which leads to the PCE of 18.54% for the Jy6 system.