**1. Introduction**

Among the various new energy technologies, solar power is one of the most favorable technologies [1,2]. In this regard, solar cells which can directly convert the sunlight to electricity are growing rapidly in their use [3,4]. Currently, silicon-based solar cells, utilized in industrial applications, have attained a power conversion efficiency (PCE) of more than 20% [5,6]. Nevertheless, such silicon solar cells require a thicker absorber layer due to the low absorption coefficient of Silicon and so they involve higher manufacturing costs [7,8]. In previous decades, different types of photovoltaic devices were established [9–15]. Perovskite solar cells (PSCs) have experienced speedy development during the last decade because of several advantages, including low density, flexibility and low-cost production [16–20]. Despite the fast growth of the reported PCE of lead-based PSCs from an initial

**Citation:** Salem, M.S.; Shaker, A.; Zekry, A.; Abouelatta, M.; Alanazi, A.; Alshammari, M.T.; Gontand, C. Analysis of Hybrid Hetero-Homo Junction Lead-Free Perovskite Solar Cells by SCAPS Simulator. *Energies* **2021**, *14*, 5741. https://doi.org/ 10.3390/en14185741

Academic Editor: Marcin Kami ´nski

Received: 17 August 2021 Accepted: 9 September 2021 Published: 12 September 2021

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value of 3.8% [21] to values higher than 25% [22], their usage is limited as they are not eco-friendly and cause serious environmental concern.

To avoid the instability and toxicity issues of lead, two principal approaches have been presented in the literature. The first technique is accomplished by mixing other metals with lead, where the metals used in the mixture have lower toxicity. One of the most widely used mixtures is tin-lead alloyed perovskite (CH3NH3Sn*x*Pb1−*x*) [23]. The second methodology is performed by entirely substituting lead with similar metals. As a candidate of the toxic lead, the innocuous Tin is regarded as the most appropriate metal because both metals are in the same group in the periodic table [24]. In this regard, CH3NH3SnI3 is considered a promising competitor to replace lead-based PSCs [25]. This material demonstrates a direct band gap of 1.3 eV, which is considered a proper range for the solar cell absorber material [26]. Recently, several groups have effectively fabricated and simulated CH3NH3SnI3-based PSCs, which yielded competitive PCEs [27–29]. Further progress in the solar cell efficiency is constrained by the charge carrier recombination, mainly in the absorber layer when utilizing heterojunction-based structures. Recent works have shown that reducing the charge carrier recombination in the absorber layer and at the interfaces between the absorber layer and adjacent layers allows a PCE to get closer to theoretical values [30,31].

In general, a perovskite material could be either a p-type or an n-type via managing the process condition and the ratio of composition stoichiometry [32–35]. Therefore, it is possible to produce a p–n homojunction PSC because of this self-doping property. Homojunction could decrease the defects/traps that operate as recombination centers [36]. Compared to heterojunction PSC, a homojunction cell has an extra built-in electric field that can boost the transport of photoexcited electrons and holes which, in turn, can reduce the recombination losses. Therefore, the homojunction device is extremely attractive for further enhancement of PSCs [36]. Recent research works are concerned with investigating the lead-based homojunction PSCs [35–40]. However, the lead-free homojunction PSCs have not been investigated yet. To our best knowledge, this is the first study to inspect the device characteristics of hybrid hetero-homojunction lead-free PSCs. The design guidelines provided in this work regarding the lead-free homojunction PSCs are completely different from those encountered in the lead-based homojunction cells. This is mainly due to the difference in absorber material properties, especially the energy band gap, as will be seen hereafter.

Moreover, the most widely used hole transport layer (HTL) material in PSCs is based on organics [41–43], which results in instability issues and an overall expensive cost of PSCs. To conquer these concerns, lots of research has focused on the HTL-free PSCs to simplify the cell architecture and reduce the overall production cost [44,45]. However, the lack of the HTL is accompanied by a poorer hole extraction. This weak extraction behavior might limit the cell performance. Therefore, new strategies have to be carried out to unravel this major issue. As a result, designing efficient p–n homojunctions in the HTL-free PSCs could be a useful and favorable approach to further improve the solar cell performance and to decrease the production cost as well.

Numerical simulation is a basic technique by which the feasibility of a novel device can be tested. In addition, the impact of physical and technological parameters on device performance could be easily investigated. In this work, we propose a hybrid heterohomojunction-based, lead-free PSCs utilizing an n-type CH3NH3SnI3/p-type CH3NH3SnI3 bilayer as an absorber layer by deploying SCAPS for the numerical analysis. This configuration allows for omitting the HTL layers, making an HTL-free device structure. A thorough investigation of the device working principles along with their underlying physics is performed. Firstly, the simulation model is justified by means of comparing the theoretical performance of a conventional pin lead-free PSC with experimental results. Then, the effect of both absorber (n-type and p-type) thickness and doping concentration is inspected in order to get an optimum value for the PCE. Moreover, the HTL-free configuration is investigated, and the work function of the back contact and defect density of the absorber are studied to determine their influence on the device performance. Based on the presented results, we provide some recommendations and guidance to the design of hetero-homojunction lead-free PSCs, either with or without HTL.
