Molecules with Multifunctional Groups for Versatile Passivation in Tin-Lead (SnPb) Perovskite Solar Cells (PSCs) to Achieve High Efficiency and Stability

Various passivation has been adopted in the tin-lead (SnPb) perovskite solar cells (PSCs) in order to relieve trap states, defects reparation and energy level alignment for efficiency enhancement. This relies heavily on the molecule used in the passivation layer due to the molecular moieties available in the particular molecule. For instant, amine-based molecules such as ethylene diamine and their family have been used as passivation layer in order to suppress the oxidation of Sn²⁺ by lowering the surface trap density in the SnPb films and also bridging the undercoordinated Sn on the surface yielding a higher quality SnPb films. However, these molecules are usually applied to passivate the surface of the perovskite films near the electron transport layer in the inverted structure of the PSCs. A more versatile molecule with various functional moieties was introduced in this research and was found to be able to passivate the interfacial of the perovskite films from either surface and promote to both efficiency and stability enhancement. An organic molecule with fused-aromatic cores and various functional side chains was used in this study. The incorporation of this molecule aligned the energy level between the SnPb film and the respective carrier selection layers. This further facilitates the carriers injection and yield in higher Voc leading to an efficiency enhancement. We observed the Voc of our target devices improved significantly (from 0.80 V to 0.89 V) suggesting that the carrier injection was significantly enhanced compared to the control devices without adopting the molecule with multifunctional groups as passivation. We are able to minimize Voc loss via this technique and yield an efficiency of more than 22% in our SnPb PSCs. Analysis near the electron transport layer indicates a richer electron concentration near the ETL side, reduced background hole concentration and the oxidation of the Sn²⁺ species into the Sn⁴⁺ species were mitigated thus improving the stability of our SnPb PSCs. The long term stability at 85 degree Celsius in the nitrogen demonstrated no significant change in the efficiency after 500 hours of keeping. The target devices also showed improved light stability under light illumination test. Thus, in this presentation, we will reveal the mechanism of the degradation and the prevention techniques which could be applied especially in the Sn based PSCs to accommodate for higher efficiency and long term stability expectation.