What are the characteristics of shallow or defect states appear in the band gap? How can we explain the presence of these defect states in band gap? More specifically, the defect or shallow states mean those states which appear in the band gap of the material due to doping or vacancies in the actual material.
Secondly, how can we overcome such defect states in the band gap and what are the disadvantages of these states?
To identify trap states, photoluminescence (PL) can show a shift due to trap states in the overall band structure. If compared to the bulk of the material by using a PL probe of surface depth penetration and then of surface depth penetration one can compare to see if there is a presence of a red shift. Trap state transitions occur at lower energies which would correspond to a PL spectra shifting to higher wavelengths. The attached publication looks into this in terms of grain boundaries and trap states.
You can also use TSC (thermally stimulated current) but I have little knowledge on how that works.
As for your second question, overcoming these trap states in thin film materials is usually done with a passivization layer. Specifically in solar cells, this layer is generally serving dual purpose as say an electron transport layer AND a passivization layer. These trap states are exactly what they sound like, places that charge carriers can get trapped which in turn will reduce the efficiency of the solar cell by reducing the carrier lifetime, mobility, and diffusion length.
Article Origin and elimination of photocurrent hysteresis by fullere...
I have modified my question, I am interested to distinguish the defect states appear in the band gap of the material due to doping or vacancy, so that real band gap may be defined accurately.
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