Understand Photovoltage in Nanocrystalline Porous TiO2

[Yosty22]

Before I get into my question, it is helpful to note the paper I am referring to.
"Photovoltage in nanocrystalline porous TiO₂" by V. Duzhko, et al. DOI: 10.1103/PhysRevB.64.075204

In Section B. "Spectral photovoltage in well-passivated porous TiO₂ layers", the authors mention that "The [photovoltage] amplitude increases strongly in the region of the forbidden gap." From the data, it is clear that this is true, but I am struggling trying to determine exactly why. I understand that photovoltages can be measured because an incident beam of light causes excess carriers in space (as per the introduction of the paper). From what I understand, the light can move some of the electrons around, leading to an anisotropic charge distribution in the material. This altered charge distribution leads to a (fairly weak) induced electric field which creates a voltage across the sample which can then be measured. (Please correct me if I am thinking about this wrong).

It is stated slightly further along in the paper (same section as above) that the photovoltage signal arises "due to the concentration gradient of excess carriers in the porous layer." From my description of a photovoltage above, it makes sense to me that a carrier concentration gradient would lead to a photovoltage signal, simply because of the anisotropy of the charge density, thus inducing an electric field and a voltage. However, would this describe such a strong increase in measured photovoltage in middle of the band gap? Surely charges cannot reside in the forbidden gap. I know what matters is spatial charge separation, not necessarily energy disparities between carriers, but I still cannot reason why such a sharp increase in photovoltage is measured in the gap.

Any suggestions or ideas would be greatly appreciated. Thanks in advance.

 

[eljose79]

Thank you for bringing up this interesting paper and question. I am a scientist who specializes in the study of materials and their properties, including photovoltaic materials like TiO2. I will do my best to provide a clear and accurate explanation for the observed increase in photovoltage in the forbidden gap region.

Firstly, your understanding of photovoltaic materials and their behavior is correct. When light is incident on a material, it can excite electrons and create an excess of carriers, leading to an anisotropic charge distribution and an induced electric field. This can result in a measurable voltage across the material.

Now, to address your main question about the sharp increase in photovoltage in the forbidden gap region, we must consider the properties of TiO2. This material has a wide band gap, which means that there is a large energy difference between its valence band (where electrons reside) and its conduction band (where electrons can move freely). In the case of TiO2, this energy difference is around 3.2 electron volts (eV).

When light is incident on TiO2, it can excite electrons from the valence band to the conduction band, creating an excess of carriers. However, in the forbidden gap region, there are no available energy states for the electrons to occupy. This means that the electrons cannot simply remain in this region, as you correctly pointed out. Instead, they are quickly pulled back to the valence band through a process known as recombination.

So why do we see such a strong increase in photovoltage in this region? This is because of the unique structure of nanocrystalline porous TiO2. The porous structure creates a large surface area, which allows for a high concentration of excess carriers to be generated. And since these carriers are quickly pulled back to the valence band, they create a large concentration gradient in the material. This gradient in turn leads to a strong induced electric field and a corresponding increase in the measured photovoltage.

In summary, the observed increase in photovoltage in the forbidden gap region is a result of the unique properties of nanocrystalline porous TiO2, including its wide band gap and high surface area. I hope this explanation helps to clarify the concept and provides some insight into this interesting phenomenon. If you have any further questions, please do not hesitate to ask.