**3.3 Optoelectronic properties**

In this subsection, we will discuss some of the 2D-perovskite's optoelectronic properties, like how reducing dimensionality can affect the bandgap, exciton's influence on charge transport, and so on. In the 2D-perovskites, the large-sized organic cation interlayers can restrict or limit the charge carriers. These organic interlayers act as dielectric regulators, determining the electrostatic force on the electron–hole pairs. The alternating arrangement of inorganic sheets and

**Figure 6.**

*The D.J. device is more stable and more efficient than the R.P. counterpart. Reprinted with permission from Ref. [64].*

bulky organic interlayers results in a multiple-quantum-well (MQW) electronic structure [68]. In other words, the inorganic slabs serve as the potential "wells" while the organic layers function as the potential "barriers" [10] as shown in **Figure 7**. The high organic and inorganic dielectric contrast leads to a huge electron–hole binding energy (Eb) in 2D perovskites [69, 70]. The confinement effect of 2D-perovskites directly affects the bandgap. For an R.P. hybrid perovskite, the bandgap depends on the good width of layer thickness [71]. The total bandgap energy is determined by the base 3D-structure and extra quantization energies of the electron–hole [72]. The optical bandgap of 2D-perovskite generally decreases as the "n" value increases. For example, the bandgap value for the 2D (CH3(CH2)3NH3)2(CH3NH3)n–1PbnI3n + 1 perovskites decreases with increased layer thickness from 2.24 eV (at n = 1) to 1.52 eV (at n = ∞) due to the quantum-confinement effects associated with the dimensional increase [14]. This flexibility of bandgap tuning can facilitate various optoelectronic applications with targeted optical bandgap materials like tandem solar cells. Those in tandem solar cells, the upper absorber layer needs to have a higher bandgap than the bottom one [73].

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performance standards.

**Figure 7.**

**3.4 Device fabrication of 2D-perovskite films**

*framework (red areas). Reprinted with permission from Ref. [70].*

*Mixed 2D-3D Halide Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.97684*

Quasi-two-dimensional perovskites have been shown to have strong excitonic effects, and their structure generally shows a large exciton binding energy (Eb) of several hundreds of meV. This improves the interaction between electrons and holes compared to the exchange in 3D-perovskites [74]. The sizeable binding energy Eb in low "n" 2D-perovskites may be detrimental for charge separation in solar cells. So, the considerable binding energy of excitons is one of the main reasons for declining

*(A) A schematic of a projection of the 3D-hybrid perovskite, showing an inorganic network of corner-sharing metal halide octahedra (red) with interstitial organic cations (blue, black). (green) highlighting the restriction on cation size. (B) Schematic of a projection down the c-axis of the 2D-hybrid perovskite, showing the alternation of organic (blue, black) and corner-sharing inorganic (red) layers for n = 1 with inorganic layer thickness d and organic layer thickness L. (green), highlighting the restriction solely on the cross-sectional area, but not the length of the organic cation. (C) Energy diagram corresponding to the 2D-structure in (B). Labeling of the valence band (V. B), conduction band (C. B), electronic band gap Eg (gray) and the optical band gap Eexc (blue) of the inorganic framework, and the larger HOMO-LUMO gap of the organic cations (green). The organic framework (gray regions) has a dielectric constant ϵ2, which is smaller than the dielectric constant ϵ1 of the inorganic* 

The finite preparation methods of 2D-perovskite films are different from the multiple preparation methods of 3D-perovskite films. *One-step spin-coating methods* are the most used to prepare 2D-perovskite films [75]. In this method, organics and metal halides are dissolved in solvents, e.g., DMF or (DMSO/GBL(1/1)) on substrates. By adjusting the ratio of the precursors, the dimension of perovskite is changed. 2D-perovskite films in both n-i-p and p-i-n structures are fabricated using one-step spin-coating methods as (PEA)2(MA)n − 1PbnI3n + 1, (BA)2(MA)n − 1PbnI3n + 1 and (PEI)2(MA)n − 1PbnI3n + 1 [65, 76, 77]. The *fast deposition-crystallization procedure* was also introduced to fabricate 2D-perovskite by dropping antisolvent, e.g., chlorobenzene, during the spin-coating process. It is shown, homogeneous nuclei are formed immediately and grow up slowly. Finally, dense and uniform films are obtained without oversize grains that may destroy the morphology [78]. The preferred *hot-cast method* was introduced for (BA)2(MA)3Pb4I13 films on PEDOT:PSS substrate [66]. The FTO/PEDOT:PSS substrate was heated before the precursor solution was spin-coated on it. **Figure 8** shows the photograph of (BA)2(MA)3Pb4I13 films that were prepared on substrates with different hot-cast temperatures from room temperature (R.T.) to 150°C. The films became dark and shiny with lower

#### **Figure 7.**

*Solar Cells - Theory, Materials and Recent Advances*

bulky organic interlayers results in a multiple-quantum-well (MQW) electronic structure [68]. In other words, the inorganic slabs serve as the potential "wells" while the organic layers function as the potential "barriers" [10] as shown in **Figure 7**. The high organic and inorganic dielectric contrast leads to a huge electron–hole binding energy (Eb) in 2D perovskites [69, 70]. The confinement effect of 2D-perovskites directly affects the bandgap. For an R.P. hybrid perovskite, the bandgap depends on the good width of layer thickness [71]. The total bandgap energy is determined by the base 3D-structure and extra quantization energies of the electron–hole [72]. The optical bandgap of 2D-perovskite generally decreases as the "n" value increases. For example, the bandgap value for the 2D (CH3(CH2)3NH3)2(CH3NH3)n–1PbnI3n + 1 perovskites decreases with increased layer thickness from 2.24 eV (at n = 1) to 1.52 eV (at n = ∞) due to the quantum-confinement effects associated with the dimensional increase [14]. This flexibility of bandgap tuning can facilitate various optoelectronic applications with targeted optical bandgap materials like tandem solar cells. Those in tandem solar cells, the upper absorber layer needs to have a higher bandgap than

*The D.J. device is more stable and more efficient than the R.P. counterpart. Reprinted with permission from* 

**170**

**Figure 6.**

*Ref. [64].*

the bottom one [73].

*(A) A schematic of a projection of the 3D-hybrid perovskite, showing an inorganic network of corner-sharing metal halide octahedra (red) with interstitial organic cations (blue, black). (green) highlighting the restriction on cation size. (B) Schematic of a projection down the c-axis of the 2D-hybrid perovskite, showing the alternation of organic (blue, black) and corner-sharing inorganic (red) layers for n = 1 with inorganic layer thickness d and organic layer thickness L. (green), highlighting the restriction solely on the cross-sectional area, but not the length of the organic cation. (C) Energy diagram corresponding to the 2D-structure in (B). Labeling of the valence band (V. B), conduction band (C. B), electronic band gap Eg (gray) and the optical band gap Eexc (blue) of the inorganic framework, and the larger HOMO-LUMO gap of the organic cations (green). The organic framework (gray regions) has a dielectric constant ϵ2, which is smaller than the dielectric constant ϵ1 of the inorganic framework (red areas). Reprinted with permission from Ref. [70].*

Quasi-two-dimensional perovskites have been shown to have strong excitonic effects, and their structure generally shows a large exciton binding energy (Eb) of several hundreds of meV. This improves the interaction between electrons and holes compared to the exchange in 3D-perovskites [74]. The sizeable binding energy Eb in low "n" 2D-perovskites may be detrimental for charge separation in solar cells. So, the considerable binding energy of excitons is one of the main reasons for declining performance standards.
