**3. Electronic and optical properties of inorganic–organic solar cells materials**

In the present decade organic-inorganic halide perovskite solar cells has been the most significant development in the field of photovoltaics for best bet at satisfying the need for high efficiencies while allowing for low cost manufacturing solutions. Since the first reports of stable solid state solar cells based on CH3NH3PbI3 perovskite in middle of 2012, the power conversion efficiencies of the hybrid solar cells have already exceeded 17%, surpassing every other solar cells produced by solution-processing methods. The wide range of efficient perovskite solar cell device design indicated point towards a considerable semiconducting material with excellent electrical and optical properties. Early pioneering research [31] in organic-inorganic halides field has clearly shown that this hybrid materials are good candidates for low dimen‐ sional electronic systems with tunable properties, permitting for the development of newer perovskite materials for solar cells in addition to CH3NH3PbI3. This section focuses on the recent progresses (*i.e.*, up to Feb 2014) in the area of perovskite solar cells as well as their electronic, optical properties and the dynamics of charge carriers [32]. We first review the electronic properties of this class of hybrid perovskites, followed by its progress as a solar cell material. Due to the rapid pace of research in this area, this section does not aim to be com‐ prehensive but will highlight key work and findings.

Initial studies on the electronic band structures of organic-inorganic (3-D and low-dimension‐ al) perovskites can be traced to the works as below, in 1996 koutselas and his team using band structure calculations by a semi-empirical method based on the extended Huckel theory and an *ab-initio* approach based on the Hartree-Fock theory [33]. Then T. Umebayashi *et. al*. using ultraviolet photoelectron spectroscopy and first principles density functional theory (DFT) band calculations for the room temperature cubic phase [34] and Chang team using first principles pseudopotential calculations in 2004 [35]. As shown in Figure 12 DFT calculations for the three dimensional CH3NH3PbI3 crystal shown that the maxima of valence band consist of the Pb 6p - I 5p σ-anti-bonding orbital, while the minima of conduction band contains Pb 6p –I 5s σ anti-bonding and Pb 6p - I 5p π anti-bonding orbitals [34].

In line with respect to perovskite solar cells, interests in the DFT studies of 3D perovskites began renewed in earnest with the work of E. Mosconi together with F. De Angelis and their collaborators [37]. They calculated the band structure for CH3NH3PbX3 (cubic phase) and the mixed halide CH3NH3PbI2X (tetragonal phase) (X = Cl, Br and I) with the surrounding CH3NH3 + , which were ignored in the earlier studies. Nevertheless, the organic component had little influence to the bandgap energy, of which is mainly determined by the [PbI4] 6- network. In addition, the authors highlight that their calculated bandgaps (by ignoring spin-orbit coupling (SOC)) are in good agreement with the experimental results. These findings are consistent with those in the later works by T. Baikie *et. al.*[37] and Y. Wang *et. al.* [38].

Figure 13 show the absorption spectra of the perovskite quantum well structures. Sharp resonance are due to the exciton state associated with the inorganic layers. So, by replacing different metal cations or halides in organic framework, the positions of the resonance can be manipulated[33, 39]. Room-temperature UV–vis absorption spectra for thin films of

**3. Electronic and optical properties of inorganic–organic solar cells**

prehensive but will highlight key work and findings.

6p –I 5s σ anti-bonding and Pb 6p - I 5p π anti-bonding orbitals [34].

In the present decade organic-inorganic halide perovskite solar cells has been the most significant development in the field of photovoltaics for best bet at satisfying the need for high efficiencies while allowing for low cost manufacturing solutions. Since the first reports of stable solid state solar cells based on CH3NH3PbI3 perovskite in middle of 2012, the power conversion efficiencies of the hybrid solar cells have already exceeded 17%, surpassing every other solar cells produced by solution-processing methods. The wide range of efficient perovskite solar cell device design indicated point towards a considerable semiconducting material with excellent electrical and optical properties. Early pioneering research [31] in organic-inorganic halides field has clearly shown that this hybrid materials are good candidates for low dimen‐ sional electronic systems with tunable properties, permitting for the development of newer perovskite materials for solar cells in addition to CH3NH3PbI3. This section focuses on the recent progresses (*i.e.*, up to Feb 2014) in the area of perovskite solar cells as well as their electronic, optical properties and the dynamics of charge carriers [32]. We first review the electronic properties of this class of hybrid perovskites, followed by its progress as a solar cell material. Due to the rapid pace of research in this area, this section does not aim to be com‐

Initial studies on the electronic band structures of organic-inorganic (3-D and low-dimension‐ al) perovskites can be traced to the works as below, in 1996 koutselas and his team using band structure calculations by a semi-empirical method based on the extended Huckel theory and an *ab-initio* approach based on the Hartree-Fock theory [33]. Then T. Umebayashi *et. al*. using ultraviolet photoelectron spectroscopy and first principles density functional theory (DFT) band calculations for the room temperature cubic phase [34] and Chang team using first principles pseudopotential calculations in 2004 [35]. As shown in Figure 12 DFT calculations for the three dimensional CH3NH3PbI3 crystal shown that the maxima of valence band consist of the Pb 6p - I 5p σ-anti-bonding orbital, while the minima of conduction band contains Pb

In line with respect to perovskite solar cells, interests in the DFT studies of 3D perovskites began renewed in earnest with the work of E. Mosconi together with F. De Angelis and their collaborators [37]. They calculated the band structure for CH3NH3PbX3 (cubic phase) and the mixed halide CH3NH3PbI2X (tetragonal phase) (X = Cl, Br and I) with the surrounding

In addition, the authors highlight that their calculated bandgaps (by ignoring spin-orbit coupling (SOC)) are in good agreement with the experimental results. These findings are

Figure 13 show the absorption spectra of the perovskite quantum well structures. Sharp resonance are due to the exciton state associated with the inorganic layers. So, by replacing different metal cations or halides in organic framework, the positions of the resonance can be manipulated[33, 39]. Room-temperature UV–vis absorption spectra for thin films of

little influence to the bandgap energy, of which is mainly determined by the [PbI4]

consistent with those in the later works by T. Baikie *et. al.*[37] and Y. Wang *et. al.* [38].

, which were ignored in the earlier studies. Nevertheless, the organic component had

6- network.

**materials**

234 Solar Cells - New Approaches and Reviews

CH3NH3 +

**Figure 12.** Bonding diagram of (a) [PbI6]4- cluster (0-D), (b) CH3NH3PbI3 (3-D) and (b) (C4H9NH3)2PbI4 (2-D) at the top of the valence band and the bottom of the conduction band [34].

(C4H9NH3)2PbX4 with (a) X = Cl, (b) X= Br, (c) X = I are shown by Mitzi *et. al.* as shown in Figure 13. In each spectrum, the arrow demonstrates the position of the exciton absorption peak. The corresponding photoluminescence (PL) spectrum (λex = 370 nm) is shown by the dashed curve in figure 13-c. A stokes shift of about 15nm between peaks of the absorption and emission peaks for the excitonic transition is notable.

**Figure 13.** Room-temperature UV–vis absorption spectra for thin films of (C4H9NH3)2PbX4 with (a) X= Cl, (b) X= Br, (c) X = I [39].

Because of the two-dimensionality of the inorganic structure, coupled with the dielectric modulation between the organic and inorganic layers, the strong binding energy of the excitons arise, which enables the optical features to be observed at room temperature. Also strong photoluminescence, nonlinear optical effects and tunable polariton absorption arise from the large exciton binding energy and oscillator strength [39].

The excitonic absorption and light emission closely relate to the different metal halide in 2D perovskite. For instance, the absorption and photoluminescence of (C5H4CH2NH3)2PbX4 varied with substitution of different halogens. As show in Figure 14, the light emissions change by green to blue and blue to ultraviolet when X= I → Br → Cl [40]. The small FWHM of the peaks

and very small Stokes shift between the UV-vis absorption and PL emission spectra are the signature of exciton.

**Figure 14.** Optical absorption of (a, b, c) and photoluminescence (a´, b´, c´) spectra for (C5H4CH2NH3)2PbI4 (a, a´), (C5H4CH2NH3)2PbBr4 (b, b´) and (C5H4CH2NH3)2PbCl4 (c,c´)

The noticeable feature of the exciton state in this system is the extremely large binding energy. For example, the binding energy in (C6H5–C2H4NH3)2PbI4 are 220 meV. For comparison, the exciton state in bulk PbI2 has a binding energy of only 30 meV. According to the other studies the larger binding energy is due to the unusual alternating organic–inorganic layered structure and the effect of dielectric confinement. The screening of carriers in organic layer is small due to lower dielectric constant of the inert organic molecules. Also lower dielectric constant of organic layer lead to enhancement of the coulomb interaction between electron and hole (higher exciton binding energy) [40].

As already pointed out in a lot of published works it is interesting to compare the lumines‐ cence and absorption properties of the organic–inorganic compounds. As revealed, extensive studies of the excitonic properties of lead halide based organic–inorganic materials (R– NH3)2PbX4 have been performed. The measured absorption and photoluminescence wave‐ lengths of (R–NH3)2PbI4 and (R–NH3)2PbBr4 reported in the literature are summarized in Table 1. Using different organic chains (e.g. simple saturated organic chains and unsaturated chains including aromatic rings and delocalized p electrons) demonstrate enhancement of the photoluminescence and the binding energy of excitons. For the saturated alkylammonium chains organic layers, the length of organic chain and the width of the PbI4 wells does not affect the excitonic properties. This is due to the small difference between the dielectric constants of the inorganic and organic layers which leads to a rather weak impact of the dielectric confine‐ ment (see, for instance, the work of Ishihara et al. on (CnH2n+1NH3)PbI4 with n = 4, 6, 8,..., 12). In contrast, when the organic chains consist of aromatic rings and delocalized p electrons, the binding energy of exciton is low because of the difference between the organic and inorganic dielectric constants (dielectric confinement effect) and the luminescence peak shows red shift [41-44]. This dependence of the saturated/unsaturated nature of the organic chains is summar‐ ized in Table 1.

Because of the two-dimensionality of the inorganic structure, coupled with the dielectric modulation between the organic and inorganic layers, the strong binding energy of the excitons arise, which enables the optical features to be observed at room temperature. Also strong photoluminescence, nonlinear optical effects and tunable polariton absorption arise from the

The excitonic absorption and light emission closely relate to the different metal halide in 2D perovskite. For instance, the absorption and photoluminescence of (C5H4CH2NH3)2PbX4 varied with substitution of different halogens. As show in Figure 14, the light emissions change by green to blue and blue to ultraviolet when X= I → Br → Cl [40]. The small FWHM of the peaks and very small Stokes shift between the UV-vis absorption and PL emission spectra are the

**Figure 14.** Optical absorption of (a, b, c) and photoluminescence (a´, b´, c´) spectra for (C5H4CH2NH3)2PbI4 (a, a´),

The noticeable feature of the exciton state in this system is the extremely large binding energy. For example, the binding energy in (C6H5–C2H4NH3)2PbI4 are 220 meV. For comparison, the exciton state in bulk PbI2 has a binding energy of only 30 meV. According to the other studies the larger binding energy is due to the unusual alternating organic–inorganic layered structure and the effect of dielectric confinement. The screening of carriers in organic layer is small due to lower dielectric constant of the inert organic molecules. Also lower dielectric constant of organic layer lead to enhancement of the coulomb interaction between electron and hole

As already pointed out in a lot of published works it is interesting to compare the lumines‐ cence and absorption properties of the organic–inorganic compounds. As revealed, extensive studies of the excitonic properties of lead halide based organic–inorganic materials (R– NH3)2PbX4 have been performed. The measured absorption and photoluminescence wave‐ lengths of (R–NH3)2PbI4 and (R–NH3)2PbBr4 reported in the literature are summarized in Table 1. Using different organic chains (e.g. simple saturated organic chains and unsaturated chains

large exciton binding energy and oscillator strength [39].

(C5H4CH2NH3)2PbBr4 (b, b´) and (C5H4CH2NH3)2PbCl4 (c,c´)

(higher exciton binding energy) [40].

signature of exciton.

236 Solar Cells - New Approaches and Reviews

Comparison of the absorption and photoluminescence peak wavelengths and the exciton binding energy of (NH3(CH2)6NH3)PbBr4 with those of the homologous bromide and iodide compounds as shown in Table 1. It is clear that the exciton binding energy of compounds (I) and (II) containing saturated organic chains are almost the same (about 180 meV). On the other hand, compounds (III) and (IV) containing unsaturated organic chains, exhibit much lower exciton bending energy. The homologous iodide compound (V) with the same (saturated) organic chain as (NH3(CH2)6NH3)PbBr4 shows strong photoluminescence at room tempera‐ ture. The efficient emitted photoluminescence is observable by naked eyes


**Table 1.** Absorption, photoluminescence wavelengths and Stokes shifts of some reported compounds.
