PV cell charge separation by an external electric field

[askingask]

I have read myself through a lot of information on photovoltaics. Yet I still don't understand how it really works. From what I think I understand, the charge separation in PN junctions happens due to an electric potential build up in the depletion zone. Now according to a wikipage on organic solar cells, single layer organic PVs seperate charges because of the electric potential build up between the two metal electrodes of the cell. Apparently the different work functions of these metals are responsible for this?

Now to my primary concern. The same wikipage on organic PVs also described Bilayer cells, etc. And one of the reasons Bilayer cells are more efficient according to this page, is because of the interface between an electron donor and acceptor, which is also different to a PN junction, what ever that is supposed to mean. Anyway this interface exhibits a stronger electric field. Which makes it more efficient then a single layer organic PV.

Now here comes an idea, what if instead of making the composition of the cell create this electric potential, we put a simple semiconductor cell of some sort (like I said I just have very little understanding on this) between two capacitor disks charged at some high voltage like 1000 volts or something. Could this be useful in separating the charges created by the excitation of an electron entering the conduction band? I‘d be happy if you can help me out and correct me here.

 

[Drakkith]

Now here comes an idea, what if instead of making the composition of the cell create this electric potential, we put a simple semiconductor cell of some sort (like I said I just have very little understanding on this) between two capacitor disks charged at some high voltage like 1000 volts or something. Could this be useful in separating the charges created by the excitation of an electron entering the conduction band? I‘d be happy if you can help me out and correct me here.

What would this accomplish in the context of photovoltaics? Light itself is used to create the charge separation, which is then used to produce electrical current and power. An external field would require the input of energy and wouldn't be useful to a PV system.

 

[askingask]

What would this accomplish in the context of photovoltaics? Light itself is used to create the charge separation, which is then used to produce electrical current and power. An external field would require the input of energy and wouldn't be useful to a PV system.

This question is more about trying to understand how PVs work. Not some practical solution.

Once we have two plates charged up they establish that field. No need to recharge it. The field stays, and the device is inside the field.

 

[Lord Jestocost]

Semiconductor based solar cells contain junctions between (different) materials of different doping and these junctions are crucial for the operation of the solar cells. When, for example, a p-n junction is illuminated, additional electron-hole pairs are generated in the semiconductor. Due to the presence of the “internal” electric field of the p–n junction, the photogenerated holes in the crystal are forced to drift toward the direction of the electric field and accumulate on the p-type region, whereas the photogenerated electrons drift to the opposite direction and accumulate on the n-type region.

This leads to an "internal" separation of electron-hole pairs that are generated in the junction region by illumination and this is essential. Under constant illumination, the cell is thus driven into a non-equilibrium steady state which can be used to "externally extract" part of the energy which was supplied by the illumination to form electron-hole pairs in the semiconductor.

 

[Drakkith]

This question is more about trying to understand how PVs work. Not some practical solution.

Once we have two plates charged up they establish that field. No need to recharge it. The field stays, and the device is inside the field.

I think there's some confusion over 'charge separation' here. The PN junction itself naturally creates charge separation and thus an electric field via the diffusion of charge carriers from each side to the other. This leads to the N side being positively charged and the P side being negatively charged, creating an electric field.

Photovoltaics work by using light to excite electrons from one energy band to another. This doesn't create a charge separation in and of itself, as the electron-hole pair must move apart for that to happen. In fact, if the electron-hole pair never recombined then the electric field in the PN junction would die out. Put simply, the electric field naturally generated by the PN junction pushes electrons one way, holes the other, and if the load on the circuit that the PN junction is part of is small enough then it is easier to for an electron to go through the circuit to recombine with the hole than for each one to diffuse back across the PN junction to recombine*.

A static external field would create a charge separation along the direction of the field lines, but this cannot be used to create current/voltage in the circuit. The field must be inside the device.

*Note that the hole can recombine with any electron it comes across, not just the same one it was paired with at creation. Electrons already present in the conductor typically move in and recombine with holes while electrons generated via the photovoltaic effect move into the conductor. The generation of electron-hole pairs and the recombination of electron-hole pairs happens simultaneously and continuously in an illuminated solar cell.

Have a look here: https://en.wikipedia.org/wiki/Theory_of_solar_cells

 

[askingask]

I think there's some confusion over 'charge separation' here. The PN junction itself naturally creates charge separation and thus an electric field via the diffusion of charge carriers from each side to the other. This leads to the N side being positively charged and the P side being negatively charged, creating an electric field.

My point is not that the creation of electron hole pairs is responsible for charge separation. From what I understand the P and N type semiconductor are both neutral. But the N type semiconductor has more free electrons while the P type semiconductor has more free „holes“. This creates a chemical potential. Electrons diffuse into the P type semiconductor while holes diffuse into he N type semiconductor. This creates an electric potential opposing the chemical potential, the electric potential grows until it is in equilibrium with the chemical
potential.
Please correct me here if im wrong.

Now i have two question:

Firstly, if the chemical and electrical potential are in equilibrium how does any separation occure?

Secondly,

A static external field would create a charge separation along the direction of the field lines, but this cannot be used to create current/voltage in the circuit. The field must be inside the device.

Could you elaborate on this? Why would it not create a current or voltage? Why does the field have to be inside the device.
Thanks in advance.

 

[Drakkith]

Firstly, if the chemical and electrical potential are in equilibrium how does any separation occure?

I believe it is thermal motion that ultimately causes the separation. This causes diffusion against the building electric field until there is an equilibrium between the thermal motion forcing electrons to diffuse into the P-side and the electric force preventing more electrons from diffusing. I could be mistaken here, as I am not an expert in this area.

Could you elaborate on this? Why would it not create a current or voltage? Why does the field have to be inside the device.

Just imagine putting a wire inside a static external electric field. The charges inside the wire will move until the field inside the conductor becomes zero, at which time the entire situation becomes a static situation where nothing else happens. There is no voltage inside (because the e-field is zero) and there is no current. It doesn't matter the direction of the field or the orientation of the conductor, this will always happen. The same is also true for a semiconductor. A static external electric field will only ever cause charge separation across the bulk of the material, which creates only a brief current until the field is neutralized inside the material.

A voltage/current source will produce a voltage that is not seriously diminished by the movement of charges through the circuit under reasonable conditions in a reasonable timeframe for the particular circuit (i.e. not short circuiting a capacitor or battery or something). This voltage is created along the circuit and its components, not across the diameter of a wire for example.

 

[askingask]

Just imagine putting a wire inside a static external electric field. The charges inside the wire will move until the field inside the conductor becomes zero, at which time the entire situation becomes a static situation where nothing else happens. There is no voltage inside (because the e-field is zero) and there is no current. It doesn't matter the direction of the field or the orientation of the conductor, this will always happen. The same is also true for a semiconductor. A static external electric field will only ever cause charge separation across the bulk of the material, which creates only a brief current until the field is neutralized inside the material.

Could one then not make the same argument for conventional solar cells. As there is an equilibrium of potentials.

 

[Drakkith]

Could one then not make the same argument for conventional solar cells. As there is an equilibrium of potentials.

I'm not sure. I'm afraid I've about reached the limit of my understanding of this topic.

 

[askingask]

I'm not sure. I'm afraid I've about reached the limit of my understanding of this topic.

Thank you for your input so far.

 

[Lord Jestocost]

Could one then not make the same argument for conventional solar cells. As there is an equilibrium of potentials.

The expression "equilibrium of potentials" might be misleading.

In case of a “dynamical equilibrium”, the electrochemical potentials of all charge carriers $i$, denoted by $\eta_i$, exhibit no spatial gradient throughout the conducting material:

$$\eta_i=\mu_i+z_iF\phi$$ and
$$\frac {d\eta_i} {dx}=0$$ for all i.

($\mu_i$ is the chemical potential and $z_i$ the effective charge of carrier $i$, $\phi$ is the electrical potential acting on it and $F$ is Faraday’s constant)

 

[askingask]

The expression "equilibrium of potentials" might be misleading.

In case of a “dynamical equilibrium”, the electrochemical potentials of all charge carriers $i$, denoted by $\eta_i$, exhibit no spatial gradient throughout the conducting material:

$$\eta_i=\mu_i+z_iF\phi$$ and
$$\frac {d\eta_i} {dx}=0$$ for all i.

($\mu_i$ is the chemical potential and $z_i$ the effective charge of carrier $i$, $\phi$ is the electrical potential acting on it and $F$ is Faraday’s constant)

Is that not an equilibrium of potentials if both cancel out?

 

[Lord Jestocost]

Is that not an equilibrium of potentials if both cancel out?

What does that physically mean?

 

[askingask]

What does that physically mean?

Maybe I‘d be more fitting to say: the forces acting on the charge carriers are at equilibrium.