FTO is a fluorine doped SnO2 transparent conductive glass, which has the advantages of high visible light transmittance, low electrical resistivity, and strong acid alkali resistance. It is used as a conductive substrate for perovskite solar cells. As the base material of perovskite solar cells, the surface cleanliness is crucial for the quality of the subsequent film layer.

Plasma cleaning

Plasma cleaning can modify the surface and endow it with new surface functional states while maintaining the inherent properties of the material. It is an efficient, economical, environmentally friendly, and reproducible cleaning method.

Plasma cleaning refers to the decomposition of gas molecules into high-speed moving atoms and molecules under high pressure, vacuum, and other conditions. These atoms and molecules dissociate and collide to generate plasma, which acts on the surface of FTO glass to recombine the chemical bonds of surface molecules. Therefore, not only can residual organic impurities be removed, but hydroxyl functional groups can also be injected into the glass surface to make it hydrophilic, enabling better nucleation and crystallization of perovskite on the FTO substrate surface, obtaining high-performance perovskite films, and improving the work function of FTO.

Contact angle size of FTO substrate treated with different cleaning methods

In order to investigate the effect of different cleaning methods on perovskite film formation, we conducted wetting angle tests on FTO substrates treated with different methods. The instrument used is a dynamic contact angle measuring instrument with a measurement accuracy of ± 0.1 °. The contact angle is the angle formed by the two tangent lines of the gas-liquid interface and the solid-liquid interface at the intersection of the solid liquid gas three-phase system. Drop the perovskite precursor solution onto the surface of FTO substrates using different cleaning methods. The test results are shown in Figure 1.1. The contact angle for ordinary chemical cleaning is 36 °, the contact angle for ultraviolet ozone cleaning is 17 °, and the contact angle for plasma cleaning is 3 °. Conventional chemical cleaning cannot completely remove surface impurities and oil stains. The plasma cleaning effect is significantly better, which can effectively remove organic small molecules and oil stains adsorbed on the substrate surface. Better hydrophilicity can enable better spreading of perovskite solution on FTO surface, which is beneficial for perovskite film formation. Perovskite thin films that come into closer contact with FTO can reduce the presence of defects, avoid electron accumulation at the interface between FTO and perovskite, and correspondingly reduce electron hole recombination.

Figure 1.1 Contact angle size of FTO substrate treated with different cleaning methods

The use of plasma cleaning can effectively remove oil stains and organic small molecules on the surface of FTO substrate, improve the interface contact between perovskite layer and FTO substrate, reduce defects at the interface, enhance the electron extraction ability of FTO, avoid the accumulation of electrons in FTO, and reduce the recombination of electrons and holes. After plasma cleaning, FTO conductive glass has improved surface wettability, enhanced film adhesion, and strengthened device adhesion and stability.