**4.3 Quantum confinement effect**

Optical absorption and emission characteristics of a semiconductor can be adjusted by changing the size of the semiconductor. If the size of such materials is in the nanocrystal range, changes in band gaps can be observed. The reduction of the crystal size causes the quantum capture effect to be observed and the bandwidth to shift to blue. If the semiconductor size is too small to compare with the Bohr radius of excitons, quantum trapping can be seen in the optical properties of the semiconductors. For example, the quantum capture effect is quite evident in completely inorganic CsPbBr3 perovskite nanocrystals and in organic-inorganic methyl ammonium lead halide perovskite nanocrystals. This can usually be observed when the nanocrystalline size is comparable to the exciton Bohr radius. **Figure 8a** and **b** demonstrate that the emission of CsPbBr3 perovskite nanocrystals can actually be adjusted from 2.7 eV to 2.4 eV, with a size ranging from 4 nm to 12 nm, which is compatible with the theoretical calculation [34].

#### **4.4 Linear absorption and emission**

Many groups have focused on improving the band spacing, excitonic characteristic and optical properties of photoluminescent quantum yields of halide perovskite nanocrystals for optoelectronic applications in recent years [18, 62]. The most interesting of these features is that bandwidth is adjustable. It is possible to adjust the bandwidth by changing the individual components of the metal halide perovskites (MHP). Optical properties of bulk perovskite thin films could be changed across the entire visible spectrum. Thus, it has been shown that the optical properties of MAPbBr3 nanocrystals, which have an emission of approximately 529 nm, can also be altered throughout the entire visible spectrum [63, 64]. For CsPb(X = Cl, Br or I)3 nanocrystals, using halide components, the emission wavelength is from 410 nm (X = Cl), (X = Br) to 512 nm, (X = I) It has been shown that it can be shifted to 685 nm (**Figure 9**).

Adjustable optical features of perovskite nanocrystals are based on the electronic structure of these materials (**Figure 10**). The conduction band consists of external p orbitals of halid and antibonding orbitals of hybridization of Pb 6p orbitals. The valence band consists of antibonding of the hybridization of Pb 6s

**57**

**Figure 9.**

**Figure 8.**

*(reproduced with permission of Ref. [34]).*

*(a) The emission spectra of CsPbBr3 NCs Quantum-size effects in the absorption and (b) experimental versus effective mass approximation size (theoretical technique) with respect to the band gap energy range* 

*UV*−*vis and photoluminescence spectra shows that the band gap could be tuned by controlling of CsPbX3 NCs* 

*as a function of halide (reproduced with permission of Ref. [34]).*

*Perovskite Nanoparticles*

*DOI: http://dx.doi.org/10.5772/intechopen.94588*

*Perovskite and Piezoelectric Materials*

**4.2 High quantum efficiency**

**4.3 Quantum confinement effect**

supports exciton radial recombination efficiency [61].

compatible with the theoretical calculation [34].

**4.4 Linear absorption and emission**

shifted to 685 nm (**Figure 9**).

cations.

can cover the entire visible area, even close to the infrared or ultraviolet region, by substituting halide elements from chloride to iodine [26, 34, 51, 57, 58]. Another way to adjust the emission is to insert other organic molecules into it or replace anions/

Perovskites are considered superior light emitters owing to their large absorption coefficients and high quantum yields [59, 60]. The high quantum yield generally indicates that most of the absorbed photons are transformed by radiative recombination processes. High quantum yields (90%) have been reported in inorganic ABX3 and organic-inorganic methyl-ammonium halide perovskite nanocrystals without further surface treatment [26, 34]. However, the main reason for reduced quantum efficiency in conventional III –V and II –VI groups is that these nanocrystalline structures are often affected by surface defects or donor-receptor levels. In perovskite, where there are very few electrical charge trapping conditions, high quantum efficiency is the result of the formation of a clear band gap that greatly

Optical absorption and emission characteristics of a semiconductor can be adjusted by changing the size of the semiconductor. If the size of such materials is in the nanocrystal range, changes in band gaps can be observed. The reduction of the crystal size causes the quantum capture effect to be observed and the bandwidth to shift to blue. If the semiconductor size is too small to compare with the Bohr radius of excitons, quantum trapping can be seen in the optical properties of the semiconductors. For example, the quantum capture effect is quite evident in completely inorganic CsPbBr3 perovskite nanocrystals and in organic-inorganic methyl ammonium lead halide perovskite nanocrystals. This can usually be observed when the nanocrystalline size is comparable to the exciton Bohr radius. **Figure 8a** and **b** demonstrate that the emission of CsPbBr3 perovskite nanocrystals can actually be adjusted from 2.7 eV to 2.4 eV, with a size ranging from 4 nm to 12 nm, which is

Many groups have focused on improving the band spacing, excitonic characteristic and optical properties of photoluminescent quantum yields of halide perovskite nanocrystals for optoelectronic applications in recent years [18, 62]. The most interesting of these features is that bandwidth is adjustable. It is possible to adjust the bandwidth by changing the individual components of the metal halide perovskites (MHP). Optical properties of bulk perovskite thin films could be changed across the entire visible spectrum. Thus, it has been shown that the optical properties of MAPbBr3 nanocrystals, which have an emission of approximately 529 nm, can also be altered throughout the entire visible spectrum [63, 64]. For CsPb(X = Cl, Br or I)3 nanocrystals, using halide components, the emission wavelength is from 410 nm (X = Cl), (X = Br) to 512 nm, (X = I) It has been shown that it can be

Adjustable optical features of perovskite nanocrystals are based on the electronic structure of these materials (**Figure 10**). The conduction band consists of external p orbitals of halid and antibonding orbitals of hybridization of Pb 6p orbitals. The valence band consists of antibonding of the hybridization of Pb 6s

**56**

#### **Figure 8.**

*(a) The emission spectra of CsPbBr3 NCs Quantum-size effects in the absorption and (b) experimental versus effective mass approximation size (theoretical technique) with respect to the band gap energy range (reproduced with permission of Ref. [34]).*

#### **Figure 9.**

*UV*−*vis and photoluminescence spectra shows that the band gap could be tuned by controlling of CsPbX3 NCs as a function of halide (reproduced with permission of Ref. [34]).*

#### **Figure 10.**

*The energy bands forms in a lead iodide perovskite by the crossing of lead and iodide orbitals (reproduced with permission of Ref. [23]).*

and the same halide p-orbitals. The conduction band is generally p-like owing to the high density energy bands lead density contribution [65]. However, as the opposite of this situation, the band gap of gallium arsenide occurs between bonding and antibonding orbitals. As the halide component changes, the valence energy band shifts at the limit value, while only small changes occur in the energy limit value of the transmission band [66]. The cation A does not contribute considerably to the conduction and valence orbitals, but has an important effect on the band gap of perovskite [67]. As a result, emission energies of MA-based perovskite nanocrystals have been demonstrated to range from halide to 407 to 734 nm. It is understood that the FA emissions of nanocrystalline FAs shifted to 408 nm with Cl, to 535 nm with Br and to I and 784 nm, that is, to red [68–70]. In addition, the B cation has a significant mission in changing the optical properties of metal halide perovskites nanocrystals. As is known, the lead is a harmful element for the nature, instead of using the band [71], the band gap and PL emissions of the metal halide perovskites nanocrystals shifted from Cl to 443 nm and from I to 953 nm. The reason is probably a result of the higher electronegativity of Sn2 than Pb2 [72]. But, the stability of Sn2+ and similarly Ge2+ based on perovskite compounds is too weak owing to the reduction of non-interacting electron pair effects corresponding to a decrease in the stability of the divalent oxidation state [73]. As a result, PL emissions or energy band gaps of nanocrystalline structures obtained by various methods depend only on stoichiometry [74]. Stokes shift is an important parameter at the absorption and emission spectra that LHP nanocrystals show typically small, ranging from 20 to 85 meV [75–77]. Stokes shift increases as nanocrystals decrease in size. This is clarified by the creation of a compatible hole state that can be delocalized across the whole nanocrystal [78]. PL line widths of metal halide perovskites are another important point, particularly for LED applications. In fact, the line widths are commonly in the range of 70-110 meV and have been found to vary significantly with respect to the halide content. In many articles, the halide component greatly varies the PL spectra in terms of wavelength, for instance for Cl-perovskites reaches to 10–12 nm and for I-perovskites 40 nm. In terms of photoluminescent quantum yields, LHP nanocrystals display high values with the more epitaxial shell range the more chalcogenide QDs without electronic passivation [69]. Some article abot MA based halide perovskites have been reached to 80 % to 95 % for photoluminescent

**59**

(**Figure 11b**) [87].

**Figure 11.**

**5.1 Optical lasing**

**5. Optoelectronics applications of perovskite**

*Perovskite Nanoparticles*

*DOI: http://dx.doi.org/10.5772/intechopen.94588*

quantum yields, bromide and iodides, respectively [79–82]. The defect tolerance of halide perovskite gives new properties due to orbital structure and the bandgap that forms between two antibonding orbitals. FA-based nanocrystals also reach to 70–90% photoluminescent quantum yield. There is alternative method that doping with additional ions to handle the optical emission of LHP nanocrystals. The addition of Mn2+ ions leads to a strong Stokes shift emission by the band gap given by the perovskite matrix and the emission from the atomic states of the Mn2+ ions (**Figure 11a**) [83]. Mn2+ doping can result in a pair of controllable emissions from both localized Mn2+ states and band gap recombination [84, 85]. In contrary, CsPbBr3 nanocrystals have been shown to cause blue shift of doping, band boundary and PL emission with other divalent cations such as Sn2+, Cd2+ and Zn2+. In these cases, an important portion (0.2–0.7%) of the original Pb ions have been exchanged by new metal cations that produce alloy nanocrystals. CsPbBr3 alloy nanocrystals with 0.2% content of AI3+ ions have a blue shear PL emission with a centre of 456 nm and a relatively high 42% photoluminescence quantum yield [86]. In all these cases, perovskite nanocrystals act as an absorbent host that stimulates dopants through energy transfer. In addition, by selecting specific dopant atoms, the emission wavelengths of the nanocrystals obtained can be easily adjusted. When lanthanide ions are doped, the emission of CsPbCl3 nanocubes ranges from 400 nm to 1000 nm and their quantum yields are 15% and 35%, respectively

*(a) Photoluminescence emission & absorption of Mn-doped CsPbCl3 NCs, (b) photoluminescence curves of CsPbCl3 NCs doped with different lanthanide ions (reproduced with permission of Refs. [23, 87]).*

High absorption coefficient and strong photoluminescence is the most powerful side of metal halide perovskite. It is possible to obtain a laser with a high quantum efficiency material and a suitable optical band spacing. With the understanding

#### **Figure 11.**

*Perovskite and Piezoelectric Materials*

**Figure 10.**

*permission of Ref. [23]).*

and the same halide p-orbitals. The conduction band is generally p-like owing to the high density energy bands lead density contribution [65]. However, as the opposite of this situation, the band gap of gallium arsenide occurs between bonding and antibonding orbitals. As the halide component changes, the valence energy band shifts at the limit value, while only small changes occur in the energy limit value of the transmission band [66]. The cation A does not contribute considerably to the conduction and valence orbitals, but has an important effect on the band gap of perovskite [67]. As a result, emission energies of MA-based perovskite nanocrystals have been demonstrated to range from halide to 407 to 734 nm. It is understood that the FA emissions of nanocrystalline FAs shifted to 408 nm with Cl, to 535 nm with Br and to I and 784 nm, that is, to red [68–70]. In addition, the B cation has a significant mission in changing the optical properties of metal halide perovskites nanocrystals. As is known, the lead is a harmful element for the nature, instead of using the band [71], the band gap and PL emissions of the metal halide perovskites nanocrystals shifted from Cl to 443 nm and from I to 953 nm. The reason is probably a result of the higher electronegativity of Sn2 than Pb2 [72]. But, the stability of Sn2+ and similarly Ge2+ based on perovskite compounds is too weak owing to the reduction of non-interacting electron pair effects corresponding to a decrease in the stability of the divalent oxidation state [73]. As a result, PL emissions or energy band gaps of nanocrystalline structures obtained by various methods depend only on stoichiometry [74]. Stokes shift is an important parameter at the absorption and emission spectra that LHP nanocrystals show typically small, ranging from 20 to 85 meV [75–77]. Stokes shift increases as nanocrystals decrease in size. This is clarified by the creation of a compatible hole state that can be delocalized across the whole nanocrystal [78]. PL line widths of metal halide perovskites are another important point, particularly for LED applications. In fact, the line widths are commonly in the range of 70-110 meV and have been found to vary significantly with respect to the halide content. In many articles, the halide component greatly varies the PL spectra in terms of wavelength, for instance for Cl-perovskites reaches to 10–12 nm and for I-perovskites 40 nm. In terms of photoluminescent quantum yields, LHP nanocrystals display high values with the more epitaxial shell range the more chalcogenide QDs without electronic passivation [69]. Some article abot MA based halide perovskites have been reached to 80 % to 95 % for photoluminescent

*The energy bands forms in a lead iodide perovskite by the crossing of lead and iodide orbitals (reproduced with* 

**58**

*(a) Photoluminescence emission & absorption of Mn-doped CsPbCl3 NCs, (b) photoluminescence curves of CsPbCl3 NCs doped with different lanthanide ions (reproduced with permission of Refs. [23, 87]).*

quantum yields, bromide and iodides, respectively [79–82]. The defect tolerance of halide perovskite gives new properties due to orbital structure and the bandgap that forms between two antibonding orbitals. FA-based nanocrystals also reach to 70–90% photoluminescent quantum yield. There is alternative method that doping with additional ions to handle the optical emission of LHP nanocrystals. The addition of Mn2+ ions leads to a strong Stokes shift emission by the band gap given by the perovskite matrix and the emission from the atomic states of the Mn2+ ions (**Figure 11a**) [83]. Mn2+ doping can result in a pair of controllable emissions from both localized Mn2+ states and band gap recombination [84, 85]. In contrary, CsPbBr3 nanocrystals have been shown to cause blue shift of doping, band boundary and PL emission with other divalent cations such as Sn2+, Cd2+ and Zn2+. In these cases, an important portion (0.2–0.7%) of the original Pb ions have been exchanged by new metal cations that produce alloy nanocrystals. CsPbBr3 alloy nanocrystals with 0.2% content of AI3+ ions have a blue shear PL emission with a centre of 456 nm and a relatively high 42% photoluminescence quantum yield [86]. In all these cases, perovskite nanocrystals act as an absorbent host that stimulates dopants through energy transfer. In addition, by selecting specific dopant atoms, the emission wavelengths of the nanocrystals obtained can be easily adjusted. When lanthanide ions are doped, the emission of CsPbCl3 nanocubes ranges from 400 nm to 1000 nm and their quantum yields are 15% and 35%, respectively (**Figure 11b**) [87].
