Electronic Structure And Thermoelectric Properties Essay

3. Results and Discussions
3.1. Electronic structure and thermoelectric properties
The electronic structure calculations were carried out on the primitive cell of our chalcopyrite crystals containing two Ag, two In, and four Te (Se) atoms.
The obtained band structures of our ternary chalcopyrite are shown in Fig. 2. It can be clearly seen that for both studied compounds the valence band maximum (VBM), and the conduction band minimum (CBM) occurred at the - point, Hence, the materials are direct band gap semiconductors.
The calculated energy band gaps in the present work are shown in Table1. The available experimental data and results from previous calculation are presented for comparison.
Our results within the mBJ_LDA are in good agreement with experimental and theoretical values. However the standard LDA gave invalid results which is well known for the majority of ab initio band calculations.
From these suggestions, it's clear that the mBJ is often an excellent approach to investigate electronic structure.
Further we mention that replacing telluride by selenium leads to an increase in the band gap energy.
The experimental works to date have found that most chalcopyrite structure tends to form p-type semiconductors, so we focus our discussion on the valence band maximum (VBM). As shown in Fig. 2, the valence band presents a mixture of heavy and light bands. The combination of heavy and light bands near the VBM has been reported in the literature to be responsible for good thermoelectric performance of p-type semiconductors [3].
Moving to the thermopower, in figure 3 we depict the conductivity thermopower, of the investigated compounds together with AgGaTe2, for p-type and n-type, at the temperatures of 300, and 800 K.
We included the AgGaTe2 thermopower in the plot because he was found to be an excellent candidate for thermoelectric application with ZT=0.8[12] we think so that its comparison with our sample will presents an important step in our work.
The thermopower can be easily obtained from our first principles calculations from the canonical Boltzmann transport expressions [12], under the assumption that the scattering time is invariant for all carriers and independent of their energy.
As seen from figure3 at 300 our compounds don't present any bipolar conduction, and the thermopower continues increasing to the highest concentration.
In other words, we are face to a Pisarenko behavior, i.e. logarithmic in carrier concentration, for carrier concentrations between 1018 and 1019 cm3.
However for higher temperature (800k) bipolar effect began to appear at hole dopings ranging between 2.1017 and 5.1017(see figure4)
For high thermoelectric performance, It is clear that we need High thermopower , This is seen by noting that ZT=((S^2 )(K)) where "S" is the Seebeck coefficient, "" the electrical conductivity, and 'K' the thermal conductivity.
High-performance thermoelectric materials are usually considered to have ZT = 1 or greater. Hence, the value of thermopower S should be at least 157 VK, along with low lattice thermal conductivity and good carrier mobility otherwise higher thermopowers are needed if the lattice thermal conductivity is significant.