Dear Bangolla,

If we examine the structures of solar cells we can find them either p-n junction structures such as the commertial silicon solar cells or pin structures. It is so that all the semiconductor material is active and share in absorbing the incident solar radiation. The shorter waves are absorbed in the emitter layer and the long waves are absorbed in the base layer. So, theoretically speaking a semicondcutor material absorbs incident solar radiation irrespective of it doping. The doping is a requirement for the formation of the barrier layer responsible for the opencircuit voltage or the electromotive force of the cell.

The most probable reason may be is that the electron mobility is greater than the hole mobility allowing lower lifetime of electrons in the p-type material for the electrons to achieve the same minimum diffusion length required to absorb the incident longer wavelengths. The other cause may be a technological cause which means easier for fabrication.

Best wishes

Dear Bangolla,

If we examine the structures of solar cells we can find them either p-n junction structures such as the commertial silicon solar cells or pin structures. It is so that all the semiconductor material is active and share in absorbing the incident solar radiation. The shorter waves are absorbed in the emitter layer and the long waves are absorbed in the base layer. So, theoretically speaking a semicondcutor material absorbs incident solar radiation irrespective of it doping. The doping is a requirement for the formation of the barrier layer responsible for the opencircuit voltage or the electromotive force of the cell.

The most probable reason may be is that the electron mobility is greater than the hole mobility allowing lower lifetime of electrons in the p-type material for the electrons to achieve the same minimum diffusion length required to absorb the incident longer wavelengths. The other cause may be a technological cause which means easier for fabrication.

Best wishes

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Thank you sir Abdelhalim Zekry

I agree with Dr.Abdelhalim answer.

It is more efficiently to have in a p-n solar cell the absorbing layer with a higher carrier mobility, p or n. In this case the created carriers are collected and separated by the electrical field of the barrier from a larger volume.

N-type emitter materials with metallic contact due to adjusted energy level and better carrier transport of bulk minority.

Abdelhalim Zekry thanks for explanation

Minority carriers determine solar cell properties and consequently, considering that electrons present higher mobility values (lower effective mass than holes), it is better to use p-type absorber semiconductor to have a higher contribution from photo-generated electron-hole pairs.

Dear Bangolla,

I agree with Dr Zakery and Dr Courel,

and I add to their explanation that P-type nature of the popular solar cell absorbers also returns to their better material properties. As we are aware we need a p-n junction to fabricate a solar cell, thus we can not (commonly) employ intrinsic materials as solar cell absorbers. therefore we need n or p type absorbers. In the case of n-type materials they require lower activation energy and may create unwanted phases in the structure. for example, consider CIGS solar cells. CIGS is an intrinsic material which can be changed to both n-type or p-type by making it Cu-rich or Cu-poor respectively. n-type CIGS can also be used as solar cell absorbers but the high amount of Cu will create CuSe2 phases in the material which act as recombination centers and significantly reduces the solar cell efficiency. we do not witness such trap states in Cu-poor CIGS and thus p-type CIGS is mostly used as the solar cell absorber.

Many times it depends on the available technology. For example, in CdTe it turns out that it is not intentionally doped. CdTe is slightly p-type and it has the appropriate band gap for solar cells (1.45eV). The same goes for GaAs. The GaAs grown by MBE is slightly type p. Therefore the absorbent layer is type p.

Now, from the theoretical point of view if the solar cell is thick as in the case of mature Silicon solar cells. Is convenient that the absorber layer should be p-type. Because the minority carriers will be electrons and the diffusion length is greater for the electrons. So, the carrier collection will be in the depletion region and in the neutral region. Therefore, in this case p-type absorber layer helps as was mentioned by Courel.

By the other hand, in the case of thin film solar cells, it is often not so convenient for the absorbent layer to be p-type because the collection mainly occurs in the depletion region and often the p-type material is presents lower crystalline quality than the n-type or intrinsic one, which causes the recombination in the depletion region to be greater, and as a result reducing the collection.

I hope it helps you.

Here is a theoretical example that explains why the material technology determines the best structure that can be used for solar cells application:

Article Analysis of the performance of InxGa1−xN based solar cells

In a semiconductor mobility of electrons (e) is always substantially greater than holes (h). This is because electrons are free to move around while motion of holes (via bound electrons) is restricted. In p-type semiconductor, the majority carriers are holes while electrons are minority carriers. When light falls on p-type semiconductor of a p-n junction, e-h pairs are created. The h merges with the majority holes but the photo-generated e dominates because minority electrons are very very few. Since electrons have high mobility charge separation can take place quickly before they recombine. If the absorber was n-type photo-generated h would have separated sluggishly and mostly lost by recombination. Even in Si solar cells the absorber is p-type with a shallow n-type layer on top. Thank.