Structure and power generation principle of perovskite solar cells

1. Perovskite solar cell anatomy

Similar to crystalline silicon cells, perovskite solar cells also have different "layers" stacked together, each layer has its own special function and role.

The battery prototype in the image above consists of an N-type compact layer, a mesoporous oxide layer, a light-trapping perovskite layer, a hole transport layer, and two electrodes. The common structure of the PSC and the different layers are gradually deposited as instructed.

Step 1: Fluorine-doped tin oxide (FTO)/ indium-doped tin oxide (ITO) coated glass is used as a substrate for the photoanode of perovskite devices, and the flexible substrate is usually ITO/PEN.

Step 2: Above it, there is a dense layer of semiconductor material, primarily TiO2, used as a hole blocking layer or dense layer, usually deposited on top of the FTO substrate by spin coating or spray coating. It prevents the holes extracted by the electron selective layer above from coming into contact with the FTO/ITO glass and inhibits recombination losses.

Step 3: Next comes ETL, which promotes the diffusion of electrons from the photoexcited perovskite layer into the FTO/ITO glass and thus to the external circuit.

Step 4: The perovskite layer can be used as a sensitizer or absorber or an electron or hole transporter, although its primary function is a sensitizer, but spin coated on the electron transport layer.

Step 5: Adjacent to the perovskite layer is the hole transport layer, which allows the holes in the excited perovskite to move towards the metal cathode for extraction. The graphene structure makes hole transport more efficient.

Step 6: Finally, there is a metal contact layer, usually deposited on the top of the solar cell by thermal evaporation, used as the counterelectrode, also known as the back contact.

For the practical application of perovskite components, the above perovskite solar cells need to be further packaged. Similar to crystalline silicon components, packaging film and cover glass are required.

2. The general working principle of perovskite solar cells:

The perovskite layer absorbs sunlight, and the energy in the photons is used to excite electrons. This absorption appears as the excitation of electrons from the valence band edge (or highest occupied molecular orbital, HOMO) of the perovskite sensitizing agent to its conduction band edge (or lowest unoccupied molecular orbital, LUMO), leaving the perovskite in an oxidized state, i.e., neutralized by electrons moving from the HOMO of the adjacent hole transport layer.

The electrons excited to the perovskite LUMO are then injected into the LUMO of the ETL and transported to the pre-contact by diffusion. The energy levels are thermodynamically aligned in such a way that when an electron from the edge of the perovskite valence band is excited to the edge of the conduction band, it leaves a hole in the perovskite, and then another electron from the HOMO of HTL can fill in place.

Thus, an electric current is generated through the jumping motion of electrons and holes. HTL allows the holes extracted from the perovskite layer to pass through and extract them into the external circuit. HTL also acts as an electron barrier and prevents any electrons from passing through.

Although perovskite materials have shown encouraging results in terms of improving efficiency, they do have some drawbacks that hinder their commercialization. Because they are formed from organic cations, they are susceptible to moisture, temperature, ultraviolet radiation, and oxygen, which can reduce the performance of solar cells for a short time, with maximum stability values reported to be just over 1,000 hours.