*1.5.2 First-principles electronic band structure of Cs2SnI6*

The self-consistent full-potential linearized augmented plane wave method (LAPW) within density functional theory (DFT) and the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof for the exchange and correlation potential and WIEN2k program is used for electronic structure calculations [18–23]. The modified Becke-Johnson exchange potential is also employed for bandgap correction [24]. From these calculations, Cs2SnI6 estimate a direct bandgap of �1.3 eV at the Γ point comprising filled I-5p orbitals and empty hybrid I-6p/Sn-5 s orbitals for the valence band maximum (VBM) and conduction band minimum (CBM), respectively (**Figure 1(c)**). The valence and conduction bands are surprisingly well dispersed in energy, for a molecular {SnI6} <sup>2</sup>� salt compound, with �1 eV and � 0.5 eV bandwidth, respectively. Such a band configuration appears to justify the remarkably high electron and hole mobility of pristine Cs2SnI6.

#### *1.5.3 Electrical properties*

From Hall effect measurements, electrical resistivity of a pressed polycrystalline pellet of Cs2SnI6 by annealing at 200°C shows a reasonably low value of �100 Ω�cm. The electron carrier concentration is measured to be on the order of �<sup>1</sup> � <sup>10</sup><sup>14</sup> cm�<sup>3</sup> by RT and <sup>≈</sup> � 2.6 � <sup>10</sup><sup>3</sup> <sup>μ</sup>V/K of the Seebeck coefficient. Plus, electron mobility of the pristine bulk material that behaves as an n-type semiconductor shows about 310 cm2 /V�s. Interestingly, with doping Sn2+ (as SnI2) in Cs2SnI6, p-typed semiconductor behavior can be observed (�<sup>1</sup> � <sup>10</sup><sup>14</sup> cm�<sup>3</sup> of a nearly identical carrier concentration and *<sup>S</sup>* <sup>≈</sup> 1.9 � <sup>10</sup><sup>3</sup> <sup>μ</sup>V/K of the Seebeck data). The hole mobility and resistivity of the p-type Cs2SnI6 show �42 cm2 /V�s and 780 Ω�cm, respectively. Although electron mobility level of p-type Cs2SnI6 is lower than that of the CH3NH3PbI3 perovskite, it still preserves a considerable hole mobility [5, 16]. And, we *Lead-Free Perovskite and Improved Processes and Techniques for Creating Future… DOI: http://dx.doi.org/10.5772/intechopen.106256*

can clearly understand the ambipolar nature of Cs2SnI6 from these characteristics of both n- and p-type behavior.

### *1.5.4 Thermal properties of Cs2SnI6*

The thermal stability of Cs2SnI6 is assessed by differential thermal analysis (DTA). The temperature maximum in each scan was varied progressively by a 100°C increment between 400°C and 600°C (**Figure 1(d)**). The melting point Tm was determined to be at 515°C, but X-ray diffraction suggests that the melting proceeds through partial decomposition to CsI (mp = 621°C, bp = 1277°C) and SnI4 (mp = 143°C, bp = 348.5°C). It is not clear at which temperature the decomposition occurs, but it can be tentatively assigned to the reversible process occurring at Td 410°C. The further thermal properties of Cs2SnI6 powder are analyzed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. For the TGA data, the drastic weight loss onset occurs at 269°C, which is attributed to the beginning of the decomposition of Cs2SnI6 to SnI4 and CsI from the perovskite frameworks. Nearly 44% weight loss between 270°C and 350°C of Cs2SnI6 is observed. In general, DSC measurement can be helped to understand a sublimation and a decomposition process for Cs2SnI6. The DSC curve exhibits two small endothermic peaks at 200°C and 308°C. Both TGA and DSC results also confirm that the Cs2SnI6 crystal is relatively good stability and nonexplosive character. In order to verify this behavior, three independent samples of Cs2SnI6 are prepared starting from pristine, solution precipitated material. The first sample consists of the 'as made' fresh material, whereas the other two have been annealed at 350°C and 550°C, respectively, in evacuated ampoules. In agreement with the DTA and TGA data, the material obtained from 350°C annealing remains unchanged, whereas the treatment at 550°C results in a molten solid ingot which is, however, contaminated with orange crystals of SnI4. High-resolution powder X-ray diffraction using synchrotron radiation confirms the decomposition of Cs2SnI6 to SnI4 and CsI above 410°C. This is accompanied by a relative loss of crystallinity, as judged by the loss of diffraction intensity and the relative broadening of the reflections.
