**1. Introduction**

Perovskite solar cells (PSCs) have received a great deal of attention in the past few decades due to their impressively high power conversion efficiency (PCE) [1]. To date, PCE as high as 25.6% has been successfully recorded. This performance has already been compared with the single-crystalline silicon solar cells system. With the advancement in the perovskite properties control, including the crystallinity properties, grain size, and stability properties, further improvement in the PCE is expected to be achieved soon. The continuous growth in the preparation of the highperformance charge selective layer in the perovskite solar cells further contributes to the rapid progress in the PCE improvement of the PSC [2].

Along with the transparent conducting electrode (TCE) and the top metal contact, a PSC device is composed of an electron-transport layer (ETL), an organometalhalide perovskite active layer, and a hole-transport layer (HTL). In these solar cells, the perovskite and its photoelectrical properties are the keys to the overall

photovoltaic process. Its unique high-optical absorption constant drives massive photon absorption and exciton generation in the device. Despite this key fact, the carrier transport and interfacial charge transfer dynamics play another crucial factor for the generation of the overall PSC performance. These two parameters depend on the nature of the surface and the crystallinity properties of the charge-selective layers [3].

One of the serious problems in perovskite solar cell devices is the loss of charge carriers during the transport process in the carrier layer. This is because, the carrier layer has low crystallization, high grain boundary resistance as well as experiences loss of carrier charge during extraction to the outer electrode. The main factor of carrier charge lost during extraction to the outer electrode is due to the high interface resistance between the electrode and the carrier layer. Therefore, it is expected that when a carrier layer that has high crystallinity, very low thickness, and good coupling conditions with external electrodes is used, then the performance of the device will increase.

The electron transport layer (ETL), for example, TiO2, and other semiconducting oxides, such as SnO2, ZnO, have been widely applied in the perovskite solar cells fabrication. Despite the excellent performance demonstrated by them, this ETL suffers from large-density surface defects related to oxygen vacancy, particularly in the TiO2 system. The defect from such vacancy causes immense trap-limited (Shockley-Read-Hull) transport in the extraction of the photogenerated carrier to the external electrode. This in many cases degrades the photovoltaic performance of the PSC up to a certain degree, reducing the power conversion efficiency of the device. Even though there exist several methods in the passivation of such defects, such as acid passivation, etc., the improvement is minute. In addition, this method may add additional resistance

#### **Figure 1.**

*Mesoporous TiO2 ETL. (A and C) Top and side view of mesoporous TiO2 layer on compact layer TiO2. (B and D) Top and side of mesoporous TiO2 layer. (Reprinted from [4]. © 2017 American Chemical Society).*

*Two-Dimensional Transition Metal Dichalcogenide as Electron Transport Layer of Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.103854*

to the photocarrier transport reducing the power conversion efficiency. Along with these crucial factors, the crystallinity properties of the ETL add an additional issue to the photocarrier transport dynamic in the device. As normal in the high-performance PSC fabrication, mesoporous TiO2 or SnO2 was used as ETL along with a compact layer of TiO2 or SnO2 (See **Figure 1**), [4]. As the figure reveals, the mesoporous layer is composed of a large number of interconnected small grain particles that produce grain boundary resistance due to lattice mismatch among the connected particles. This resistance should be massive due to their large-scale existence on the layer. This certainly complicates the transport of photogenerated electrons to the electrode layer, such as high internal resistance or radiationless recombination [5, 6]. Therefore, the selection of the right material for the carrier layer is important in determining the performance of a device. Such resistance boundary further augments the presence of mesoporouscompact layer interface resistance in the ETL system of the PSC. From this picture, we can estimate the loss would be suffered by the device during the photovoltaic process. This means that if such ETL is replaced with the single-crystalline ETL system, the performance of the perovskite solar cells can be improved.
