# Solar cells (1 Viewer)

#### Drsoccerball

##### Well-Known Member
Does this sound right ?
When light hits an n type semi-conductor, if the frequency is higher than the work function electrons will be emitted towards the p-type semi-conductor from the n-type. Light must hit the n-type semiconductor as the electrons are in the n-type thus it requires significantly less energy to circulate around the circuit. The energy provided also has to be high enough to prevent the recombination of electrons.

#### iforgotmyname

##### Metallic Oxide
you need to mention how the contact between the n type and p type creates an electric field as the free electrons of n-type move towards the holes of the p types. Which means that electrons can only go from p to n and not n to p

#### PhysicsMaths

##### Active Member
Does this sound right ?
When light hits an n type semi-conductor, if the frequency is higher than the work function electrons will be emitted towards the p-type semi-conductor from the n-type. Light must hit the n-type semiconductor as the electrons are in the n-type thus it requires significantly less energy to circulate around the circuit. The energy provided also has to be high enough to prevent the recombination of electrons.
I don't understand the last two sentences.

If that's your whole description of how a solar cell works, then that is not enough

the reason why electrons move from the n to p type is not entirely due to the photoelectric effect, but rather due to the process of "diffusion" where excess electrons in the n-type move to the p-type until an equilibrium is established.

#### anomalousdecay

Basically, the pn junction consists of an electric field due to the depletion region. The incident photons will excite the electrons. The frequency of the photons will decide whether the electrons will obtain enough energy to leave the depletion area and hence providing a free carrier (hole or electron) in the circuit. The electric field is important as it determines where free carriers go after the photons have separated the electrons and holes in the depletion area (as iforgotmyname pointed out).

Here's a nice video to illustrate the whole process:

#### el_manu

##### Member
Can someone answer this question to get ALL six marks? Please? I'll be forever in your debt.

#### Attachments

• 42.8 KB Views: 730

#### sharoooooo

##### Active Member
havent really revised physics completely but here it goes:

The Photoelectric effect is defined as when electrons are emitted when an incident light ray hits the surface of a metal at threshold frequency. In the solar cell, it uses the p-n semiconducting junction; solid state device; where incident light energy is converted into electrical energy.
The n-type semiconductor is exposed to the light (as shown in the diagram above), whereas the transparent p-type conduct is not.
This allows the diffusion of electrons at the p-n junction from n-type to p-type, and creates a current to flow due to the movement of the electrons to switch the light bulb on.
Hence, the n-type becomes positively charged in the region closest to the depletion zone while p-type becomes negatively charged nearest to the depletion zone. Therefore, this creates a potential gradient across the depletion zone which prevents further flow of the electrons.
Then, by the Photoelectric effect, when an electron is struck in the depletion zone, it accelerates to the n-type semiconductor by E-field.
Ultimately, excess electrons in the n-type flow through the external circuit to replenish the deficiency in the p-type and thus, the current continues to flow and keep the light bulb glowing.

#### Mathsisfun15

##### Active Member
Someone correct me if i'm wrong this is what I learnt
The Photoelectric effect is defined as when electrons are emitted when an incident light ray hits the surface of a metal at threshold frequency. In the solar cell, it uses the p-n semiconducting junction; solid state device; where incident light energy is converted into electrical energy.
The n-type semiconductor is exposed to the light (as shown in the diagram above), whereas the transparent p-type conduct is not.
This allows the diffusion of electrons at the p-n junction from n-type to p-type, and creates a current to flow due to the movement of the electrons to switch the light bulb on.
Hence, the n-type becomes positively charged in the region closest to the depletion zone while p-type becomes negatively charged nearest to the depletion zone. Therefore, this creates a potential gradient across the depletion zone which prevents further flow of the electrons.
Then, by the Photoelectric effect, when an electron is struck in the depletion zone, it accelerates to the n-type semiconductor by E-field.
Ultimately, excess electrons in the n-type flow through the external circuit to replenish the deficiency in the p-type and thus, the current continues to flow and keep the light bulb glowing.
It doesn't need sunlight to be shone on it for the p-n junction form. Simply connecting the P and N semiconductor will cause a p-n junction and hence a potential difference to occur
The PV cell isn't the photoelectric effect since the photoelectric effect is for metals only. It is simply providing electrons with enough energy to move from the valence to the conduction band in silicon and due to the potential difference they can move across the PN junction

#### leehuan

##### Well-Known Member
Someone correct me if i'm wrong this is what I learnt

It doesn't need sunlight to be shone on it for the p-n junction form. Simply connecting the P and N semiconductor will cause a p-n junction and hence a potential difference to occur
The PV cell isn't the photoelectric effect since the photoelectric effect is for metals only. It is simply providing electrons with enough energy to move from the valence to the conduction band in silicon and due to the potential difference they can move across the PN junction
Physics in Focus talks about photoelectric effect

Jacaranda does not
__________________
I would argue you're right, but at the same time semiconductors could also have their E=hf-W (W = work function of semiconductor)?

Though, yes, the depletion zone creates the potential difference because in effect the depletion zone establishes the electron gradient.

#### Mathsisfun15

##### Active Member
Physics in Focus talks about photoelectric effect

Jacaranda does not
__________________
I would argue you're right, but at the same time semiconductors could also have their E=hf-W (W = work function of semiconductor)?
I reckon the formula should still apply but it is beyond the scope of the syllabus to apply it
Both these textbooks define the photoelectric effect for metals only so I would be reluctant to say that solar cells operate using the photelectric effect

#### leehuan

##### Well-Known Member
HSC Physics in Focus - Pg 225 it has photoelectric effect in bold.

[Reliability of information in a second hand investigation can be achieved through cross referencing with multiple sources, especially reputable sources] except I'm lazy

#### leehuan

##### Well-Known Member
2009 HSC Q27(c) forces you to relate between the two, so now I'm going to say yes most likely PE effect does matter.

#### Mathsisfun15

##### Active Member
2009 HSC Q27(c) forces you to relate between the two, so now I'm going to say yes most likely PE effect does matter.

I don't see it

#### Mr_Kap

##### Well-Known Member

I don't see it
Yeh. THis isn't a solar cell. This is just normal PE.

#### Mr_Kap

##### Well-Known Member

I don't see it
For B) what would I say? Something along the lines of this?

The significance of this observation is that in all metals above the relationship between the Kinetic Energy of the emitted electron, and the incoming frequency is the same, which is the 'h' (plank's constant). This led to a formula to find a value for the Kinetic energy of the emitted electron, Ek = hf - w, where the kinetic energy is ALWAYS equal to the energy of the incoming photon, minus the work function of the metal (the minimum energy to remove a valnce electron )

What else do i need to mention.

Last edited:

#### Mathsisfun15

##### Active Member
For B) what would I say? Something along the lines of this?

The significance of this observation is that in all metals above the relationship between the Kinetic Energy of the emitted electron, and the incoming frequency is the same, which is the 'h' (plank's constant). This led to a formula to find a value for the Kinetic energy of the emitted electron according to the formula Ek = hf - w, where the kinetic energy is ALWAYS equal to the energy of the incoming photon, minus the work function of the metal (the minimum energy to remove a valnce electron )

What else do i need to mention.
its only one mark so I think it is just asking to state the significance of Planck's constant

#### Mr_Kap

##### Well-Known Member
its only one mark so I think it is just asking to state the significance of Planck's constant
Oh. woops.

#### leehuan

##### Well-Known Member
Pretty sure that is not 2009. I did that question before in I'm pretty sure was 2007...........