**8. Conclusion**

*Solar Cells - Theory, Materials and Recent Advances*

films by fast laser beam scanning. Under optimum conditions, high-quality perovskite films with good crystallinity, preferred orientation, and low density of defects device offers PCE nearly about 20%. Wang *et al.* [137] showed an efficient strategy to tune the band structure and electron mobility of the ETL by adding NH4Cl to the sol–gel-derived ZnO precursor. Low temperature (160°C) fabricated CsPbIBr2solar cells recorded high efficiency of 10.16%. Li *et al.* [138] fabricated the MAPbI3perovskite solar cells and showed the device performance is strongly influenced by the TiO2 electron transport layer. Oz *et al.* [139] studied the effect of lead(II)propionate additive on the stabilization of CsPbI2Brall-inorganic perovskite, and the use of a novel dopant-freepolymer hole transport material (synthesized by

us) for photovoltaic performance assessment of CsPbI2Br solar cells.

Fu and his co-workers report a dual-protection strategy via incorporating monomer trimethylolpropane triacrylate (TMTA) intoCsPbI2Br perovskite bulk and capping the surface with 2-thiophenemethylammonium iodide (Th − NI) [140]. The fabricated devices show a greatly improved efficiency from 12.17 to 15.58% with an opening circuit voltage (Voc) of 1.286 V. **Figure 9a** presents a schematic illustration of the device and the dual-protection CsPbI2Br film. The UPS measurements are conducted to reveal the electronic structure changes the calculated results are drawn in **Figure 9b**. The photocurrent density-voltage (J-V) curves of the optimal devices under AM 1.5 illumination are presented in **Figure 9c.** The ref. device shows the best efficiency of 12.17% with a Voc of 1.151 V, and TMTA doped film (BT) devices show an improved efficiency of 13.88% after incorporating 1 mg/mL of TMTA. In **Figure 9e**, the Th − NI modified BT film (BTSTh) device exhibits the improved output efficiency with 14.93% for 1000 s, while the ref. shows the attenuated efficiency and remains 10.51% under the same operation, which indicates the better operational stability for

Li *et al.* [141] reported vertically aligned 2D/3D Pb − Sn perovskites with enhanced charge extraction and suppressed phase segregation for efficient printable solar cells. Wang *et al.* [142] successfully fabricated high-quality CsPbBr3 films

It is crucial that the entire energy of any absorbed photon is harvested for next-generation photovoltaics. The unique features of 2d layered materials such as high crystalline quality, transparency, atomic thickness, make them special to use in photovoltaic solar cells. Thus, it is important to synthesized and demonstrate 2d flexible photovoltaic devices on industrial scale. While some different issues and problems will be faced by the experimentalist regarding an efficiency performance. The specific requirements in the fabrication of photovoltaic devices are the large-area synthesis, highly controllable, low cost, atomically thin, and recyclable fabrication of materials and their devices. The above features are specified not only for all potential electronic devices but also for optoelectronic devices. Increased number of layers of 2d materials, the conductivity of the material improves at the cost of reduced transparency. The application of 2dmaterials like TMDs and perovskites in photovoltaic devices has also been investigated over the last few years. The advantages of using such materials for solar cells have been explored based on the high absorption coefficient of these materials in the visible to the near-infrared part of the solar spectrum. So there is a need for more investigation of the heterostructure based on these materials, which can synergize the performance of the

into the precursor solution, a smooth and dense CsPbBr3 film with

+

and pseudo-

via additive engineering with NH4SCN. The incorporation of NH4

**7. Neoteric challenges in development of 2d photovoltaic cells**

good crystallinity and low trap state density can be obtained.

**280**

the BTSTh devices.

halide ion SCN<sup>−</sup>

In this chapter, we have enlightened specific properties and synthesis techniques of graphene, TMDs, and perovskite. We have described recent progress made with graphene, graphene-based 2D materials for solar photovoltaics. In addition, 2d Schottky junction, homojunction, and heterojunction are explained briefly. The unique account of the charge carrier transport layer as ETL and HTL has been done fairly. Moreover, regular n-i-p and inverted p-i-n structure are the key features. Furthermore, 2DRP perovskite, upconversion/downconversion of perovskite cells are also discussed briefly. In the last of the section very recent developments on 2d perovskite photovoltaic cells have been critically explained with the facts. Various challenges in the fabrication and development of today's devices are pointed out. Therefore, the outlook towards 2d materials should be optimistic and needs more attention in the near future.
