**1. Introduction**

From the last few years, the power-conversion efficiency of perovskite solar cell has jumped from 3.8 to 25.5%, showing a remarkable progress in the history of photovoltaic industry [1]. Among these perovskite materials, methylammonium lead iodide (MAPbI3) has shown an excellent performance for photovoltaic applications. Unfortunately, lead is a highly toxic material and its presence in the environment can seriously destroy our soil, water, and other natural resources [2]. Hence, it is highly recommended to replace lead with other lead-free perovskite materials for future photovoltaic applications.

Recently, a novel lead-free perovskite compound such as cesium titanium (IV) bromide (Cs2TiBr6) is being very popular and many researchers believe that this perovskite material has the full potential to replace MAPbI3 for photovoltaic applications [3–5]. The Cs2TiBr6 perovskite offers excellent electrical, optical, and photovoltaic responses and similarly the incorporation of stable titanium (Ti) formulate Cs2TiBr6 a robust perovskite to tolerate higher thermal, electrical, environmental, and irradiation stresses [6].

For an efficient perovskite solar cell, the proper selection of both electron and holetransport material is also very crucial. In this study, PCBM is used as a dopant free electrontransport layer with absorber Cs2TiBr6 for the novel perovskite solar cell. Generally, PCBM based fullerenes and their derivatives demonstrate excellent electron transport parameters which cause the reduction of the hysteresis of the photovoltaic responses [7,8]. For most cases, dopant-free bathocuproine (BCP) as a buffer layer between the cathode and PCBM is highly recommended to further improve the hysteresis, power-conversion efficiency, and stability of the perovskite solar cell [9]. Therefore, both BCP/PCBM combinations were used as an electron-transport layer for the proposed perovskite solar cell.

For the hole-transport layer, NPB (NPB or NPD both these abbreviations are frequently used for the same material) is a highly stable and commonly reported material for the perovskite solar cell [10,11]. The improved stability response of NPB compared to the other organic/polymer hole transport layer is mainly due to the hydrophobic nature of the NPB layer [12,13]. However, the doping of NPB may initiate many complex chemical processes leading to long-term stability issues for photovoltaic applications especially at a higher temperature [14–17]. Therefore, the dopant free NPB as a hole-transport layer was used for the current proposed perovskite solar cell.

Figure 1a,b depicts the schematic structure and the energy-band alignment diagram for the proposed p-i-n type perovskite solar cell, where ohmic Ag and ITO contacts are used as front-end cathode and back-end anode, respectively. The band alignment, as shown in Figure 1b, is nearly consistent with the natural flow of photocarrier transport in both the conduction as well as the valence band, which is the first step towards the design of an efficient solar cell for maximum photovoltaic response.

**Figure 1.** (**a**) The schematic structure, and (**b**) the energy-band alignment diagram of the proposed Ag/BCP/PCBM/Cs2TiBr6/NPB/ITO solar cell.

Simulation and modelling are one of the simplest, low-cost, and highly regarded methods for the comprehensive investigation and optimization for photovoltaic response [18]. In the literature, various simulation and modelling tools are available and here the solar cell capacitance simulator-1 dimension (SCAPS 1D) is selected for this study. The SCAPS 1D is a general-purpose tool and highly recommended by various research groups for the simulation and modelling of perovskite solar cell [19,20]. Therefore, in this study, we proposed, designed, simulated, and optimized the novel lead-free and dopant-free perovskite solar cell as Ag/BCP/PCBM/Cs2TiBr6/NPB/ITO for maximum power-conversion efficiency with the help of the SCAPS 1D software.
