# Homework Help: Transmittance & Bandgap

1. Mar 8, 2016

### says

1. The problem statement, all variables and given/known data
I recently completed an experiment where I measured the transmittance of a set of samples (different semiconductors). I'm trying to understand why the transmittance was different for different samples. Only just started learning some solid-state physics.

The spectrum (I think it was Voltage [y-axis] vs. Wavelength [x-axis])

The Transmittance of the surface of a material/semiconductor is its effectiveness in transmitting radiant energy.

The bandgap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can participate in conduction.

So the transmittance of different semiconductors is different because they have different bandgaps. Is that correct to infer that?

If so, I'm wondering how different materials have different bandgaps? Is it purely because of their crystalline structure?

2. Mar 8, 2016

### ehild

Yes, different semiconductors have different bandgaps. They transmit light at longer wavelengths and absorb light with wavelenghts shorter than the one corresponding to the bandgap. The bandgap of different materials depend both on the atoms/molecules they consist of and their crystalline structure.

3. Mar 9, 2016

### says

What about the reflectance of a semiconductor? Is that also due to bandgap? Or just atoms they are made of, crystalline structure and anti-reflective coating?

4. Mar 9, 2016

### ehild

Everything has some reflectance. Highly absorbing materials also have high reflectance. The reflectance can be calculated from the real and imaginary parts of the refractive index, and it also depends on the composition and structure of the material.
Reflectance can be reduced by anti-reflection coatings.

5. Mar 9, 2016

### says

Different semiconductors have different reflectance based on their composition and crystalline structure. So it is beneficial for a solar cell to have anti-reflective coating (ARC) because it reduces the number of reflections and increases the transmittance and absorption, which increases the efficiency of a solar cell?

If a semiconductor is known to reflect light of a certain wavelength range can an ARC be then placed over the semiconductor to reduce the reflectance at that specific wavelength range? i.e. If a semiconductor has high reflectance from say 600 - 800 nm, an ARC can be placed on the semiconductor that reduces the reflections at this wavelength range.

6. Mar 9, 2016

### ehild

Yes, reflection means loss of the intensity of the light entering into the solar cell.
Yes, an ARC coating works in a certain wavelength range. That range must be the same as the working range of the solar cell.