As we know hole transport rates are affected by various factors like band alignment, pinholes in the layer and other defects, also hole injection properties vary from material to material. Also increasing thickness too much can cause an increase in the series resistance of a layer and decrease in the recombination resistance so more charges will get recombine. Keeping in view all of the above factors is there any hard and fast rule to access the impacts of the increasing thickness of these charge transport layers. Need to clarify this concept Help will be grateful. Thanks
Dear Muhammad,
Hope you are well!
The hole transport layer must have high hole concentration even greater or eqaul to the effective state density in the valence band of the material.
This is required to make a low ohmic contact with the metallization layer.
Its conduction band edge must higher than that of the active material or the absorber. This is required to reflect the electrons back to the active layer.
Its valence band must be lower than the valence band of the absorber layer to facilitate the transfer of holes to it.
The mobility of holes must be high enough to make this layer resistance small.
As for the thickness it will have effect on the series resistance of the cell.
As it increases the resistance increases and the photoconversion efficiency will be smaller. This effect will be more pronounced when the mobility is small as in the organic hole transport layers. In such layers the mobility may be as low as 10^-4 cm^2/Vs. In metallic semiconductors the mobility may be high as 10 cm^2/Vs. So, it could be relatively thick without appreciably detracting the performance of the solar cells.
As for the effect on the short circuit current, the series resistance has a voltage drop which do not affects the built in electric field in the active region. It is assumed that the built in voltage is applied on the active layer establishing an electric field that extracts the photogenerated electrons and holes. The voltage drop in the resistance of the HTL will lead to weaken this electric field that may reduce the collection efficiency.
As for the open circuit voltage it will not affect the opencircuit voltage.
I would like to pay your attention to see the paper: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Dear Muhammad,
Hope you are well!
The hole transport layer must have high hole concentration even greater or eqaul to the effective state density in the valence band of the material.
This is required to make a low ohmic contact with the metallization layer.
Its conduction band edge must higher than that of the active material or the absorber. This is required to reflect the electrons back to the active layer.
Its valence band must be lower than the valence band of the absorber layer to facilitate the transfer of holes to it.
The mobility of holes must be high enough to make this layer resistance small.
As for the thickness it will have effect on the series resistance of the cell.
As it increases the resistance increases and the photoconversion efficiency will be smaller. This effect will be more pronounced when the mobility is small as in the organic hole transport layers. In such layers the mobility may be as low as 10^-4 cm^2/Vs. In metallic semiconductors the mobility may be high as 10 cm^2/Vs. So, it could be relatively thick without appreciably detracting the performance of the solar cells.
As for the effect on the short circuit current, the series resistance has a voltage drop which do not affects the built in electric field in the active region. It is assumed that the built in voltage is applied on the active layer establishing an electric field that extracts the photogenerated electrons and holes. The voltage drop in the resistance of the HTL will lead to weaken this electric field that may reduce the collection efficiency.
As for the open circuit voltage it will not affect the opencircuit voltage.
I would like to pay your attention to see the paper: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Thank you, Sir! for the detailed explanation and the reference paper.
Dear Muhammad ,
I highly recommend this theoretical paper to clarify your quest
-Performance analysis of MAPbI3 based perovskite solar cells employing diverse charge selective contacts: Simulation study
https://www.sciencedirect.com/science/article/abs/pii/S0038092X19309934?via%3Dihub
Best,
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