When choosing an n-type buffer layer or electron transport layer (ETL), is there a limit to how high its carrier concentration should be, relative to the absorber layer and the window layer?
For example, E15 cm-3 for absorber, E19 cm-3 for buffer, E21 cm-3 for window layer.
Other than generating a favorable conduction band discontinuity, is the buffer layer also preventing the more insulating absorber layer from coming in contact with a highly doped window layer such as ITO?
My possibly flawed understanding is that a buffer layer with a high carrier concentration (near the absorber/buffer junction interface) will increase the recombination of electrons in the buffer layer with holes in the absorber layer.
Thank you in advance for any insights
Dear Helen,
welcome,
The hole transport layer has specific functions and therefore it must have specific electrical and optical characteristics. Optically it must be transparent for the solar radiation. It must have a matched energy level diagram for both the absorber from one side and the cathode layer from the other side. It must have ohmic contact to the cathode metal or metal like layer. This requires the ETL to have as high doping as possible to form a tunnel ohmic contact. The high doping is also required for high electron conductivity to impart a low resistance to the flow the electrons swept from the active layer by the built in electric field. The interface sate density must be low such that the traps has a negligible effect on the overall recombination losses. In this interface the hole concentration is low because the holes will be reflected by potential barrier at the ETL side.
I developed an analytical model for the organic and perovskite solar cells treating these points: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Dear Helen,
welcome,
The hole transport layer has specific functions and therefore it must have specific electrical and optical characteristics. Optically it must be transparent for the solar radiation. It must have a matched energy level diagram for both the absorber from one side and the cathode layer from the other side. It must have ohmic contact to the cathode metal or metal like layer. This requires the ETL to have as high doping as possible to form a tunnel ohmic contact. The high doping is also required for high electron conductivity to impart a low resistance to the flow the electrons swept from the active layer by the built in electric field. The interface sate density must be low such that the traps has a negligible effect on the overall recombination losses. In this interface the hole concentration is low because the holes will be reflected by potential barrier at the ETL side.
I developed an analytical model for the organic and perovskite solar cells treating these points: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Thank you Professor Abdelhalim Zekry for the excellent insight.
If the ETL has a very high electron density (i.e. a highly doped In2O3, such as ITO with a carrier density of E20 cm-3), will an ohmic contact be formed at the p-absorber/n-buffer junction as well in addition to the ohmic contact at the buffer/window junction, causing a loss of rectification at the p-n junction?
Dear Helen,
welcome again,
Tunnel junctions are formed when both sides of the interface are heavily doped. In case one side is heavily doped and the other is moderately doped, the junction formed will be rectifying.
I would like to invite you see my elementary book: Book Electronic Devices
Please if you find it useful recommend it for your colleagues and students. It covers the basics of semiconductors and semicondcutor devices with physical insight
Best wishes
Yes it is recommended that I thin film solar cells n-type buffer layer carrier concentration should be high.
Yes, good information with helpful links are provided by Prof. Abdelhalim Zekry.
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