Perovskite Materials for Energy and Environmental Applications. Группа авторов

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Название Perovskite Materials for Energy and Environmental Applications
Автор произведения Группа авторов
Жанр Техническая литература
Серия
Издательство Техническая литература
Год выпуска 0
isbn 9781119763352



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with the other materials for making solar cells. Voltage at maximum power point (Vmpp) is to be considered as the most relevant voltage for considering maximum efficiency [36]. Because of the manufacturing defects, there is high series resistance, which greatly affects the Vmpp. On optimization of solar cells the Vmpp will improve significantly [1].

      2.5.3 Low Recombinations

      Recombinations are of two types: radiative and nonradiative recombination. When an electron deexcites from conduction band to valence band it releases a photon to release energy called radiative recombination. Whereas in nonradiative recombination, when an electron deexcites the energy is released in the form of heat. It causes harm to the performance of the cell. The device gets heated up during nonradiative recombinations due to which efficiency gets decreased.

      Diffusion length is defined as the average length a carrier moves between its generation/formation and its recombination. On the basis of diffusion length parameters, the semiconductor material can be assessed for solar cell applications. Semiconductor materials have a shorter diffusion length and higher recombinations because they are heavily doped. If the diffusion length is higher, then the longer will be the lifetime of recombinations, the better the collection of carriers at the electrode. CH3NH3PBI3−xClx has a diffusion length of more than 1 micron. This diffusion length is almost three times the thickness of the film in solar cells [37].

      For constructing planar heterojunction solar devices this characteristic is very important. As the diffusion length of perovskite(CH3NH3PBI3) is only a hundred nanometer, so for transportation of charges to terminals, a nanoparticle system of a mesoporous TiO2 is required [38].

      2.5.4 Tunable Bandgap

      For designing the solar cell, it is necessary that the light absorber is absorbing the maximum amount of the sunlight. To achieve this, we have to tune the band gap of the absorber. So here perovskite has been greatly advantageous as a light absorber because its bandgap is tunable/controllable. The perovskite has a structure of ABX3. So the bandgap of the perovskite material (absorber) can easily be regulated by altering the organic cation (A) or the metal atom (B) or the halide (X).

       2.5.4.1 Organic Cation (A)

A substitution M substitution X substitution
Material Band gap (eV) Material Band gap (eV) Material Band gap (eV)
EAPbI3 2.2 MaPbI3 1.5 MAPbI3 1.5
MAPbI3 1.5 MASn0.3Pb0.7I3 1.31 MAPbI2Br 1.8
FAPbI3 1.4 MASn0.5Pb0.5I3 1.28 MAPbBr3 2.20
CsPbI3 1.67 MASn0.9Pb0.1I3 1.18 MAPbCl3 3.11
MASnI3 1.10

       2.5.4.2 Metal Cation (M)

      Lead (Pb) was majorly used as a metal cation. European Union has restricted the use of Pb as it is toxic to the environment (Restriction of the use of certain hazardous substances (RoHS), Directive 2011/65/EU.). These reasons have led to synthesize leadless perovskite for light absorbing. As an alternative of Pb, we can use tin because tin and Pb are in the same group, so obviously tin would be the first choice. After Sn and Pb ratio was optimized in CH3NH3Sn1-xPbxI3, it was observed that bandgap was tunable in the range of 1.17 to 1.55 eV. Because of this, it was found that the absorption capacity of light could be increased from visible region to the near-infrared region (1060 nm) [41].

       2.5.4.3 Halide Anion (X)

      X position in perovskite can be replaced with elements chlorine, bromine, and iodine. There is a considerable proportion of change in bandgap when there is alternation in the element. Altering the position of X from chlorine to iodine the band gap value ranges from 3.11eV to 1.51eV. The smallest band gap of 1.51eV is obtained by tuning the position of X in CH3NH3PbX3 [38].

      M and X elements should be changed simultaneously so