49
• E 1 eV (to account for the metastability and kinetic stabilization)
at pH = 7 and -0.4 V 2.2 V which is the typical operating voltage
of the device.
The materials meeting all the above criteria are selected from the pool of
2400 are selected from the pool of 2400 and shown in the Figure 5.4. In
the figure E denotes the degree of stability of the material, the position
of the direct and indirect band edge positions are shown in red and black
respectively. It is suprising that out of 2400 materials only a handful of
materials follow all the specified criteria. However, in recent experiments 72
a good control over the stability of semiconductors in water has ben achieved
by using protective polymeric layers. Therefore, the size of the materials space
might be expanded by relaxing the criteria of the stability in aqueous medium.
The compounds written in green and underlined in the Figure 5.4 have been
realized previously for different photoelectrochemical applications, 98, 99, 100,
101, 102, 103, 104, 105 and are therefore expected to serve as viable candidates
for photoelectrochemical water splitting.
5.5 Bandgap engineering of functional perovskites
The previous section dealt with a specific application of solar light absorption
i.e photoelectrochemical water splitting. However, the other applications like
photovoltaics, transparent conducting oxides etc. require different size of the
bandgap.75, 76 We also saw that different factors limit the suitability of
a material for a given application e.g stability in water, toxicity, cost etc.
Therefore, a systematic strategy is required to tune the bandgap of an already
existing material which meets the requirement of toxicity, stability etc.
In this section the possibility of tuning the bandgap by the layering of two
perovskites, namely BaSnO3 and BaTaO2N is explored. The bandgap tuning
especially in perovskites has also been explored in previous works in systems
like SrVO3, SrTiO3 etc.106, 107, 108, 109, 110, 111 The choice of BaSnO3 and
BaTaO2N as a model system here stems from the fact that both the compounds
have been explored recently as light absorbers for the photoelectrochemical
water splitting applications and have similar lattice constants thus the layered
system will not be subjected to a high stress. The protypical structure of
the layered compound is shown in the Figure 5.5. The (BaSnO3) and
(BaTaO2N) are stacked along the z-direction while x and y direction have the
periodicity of the cubic perovskite structure and the tuning of the bandgap is
explored by varying the number of layers of (n) and the number of layers
of (n).