State-of-the-Art Techniques on Asymmetrical Perylene Diimide Derivatives: Efficient Electron-Transport Materials for Perovskite Solar Cells

Electron-transport materials (ETMs) play an important role in the injection and transfer of electrons at the interface of perovskite solar cells (PSCs). In this work, four PDI (perylene diimide) derivatives (PDI-3, PDI-4, PDI-5, and PDI-6) were designed on the basis of two asymmetrical PDIs (PDI-1 and PDI-2), which were found to have potential application in organic solar cells. State-of-the art techniques were used to investigate the molecular geometry, crystal packing, and electronic properties of these PDIs. The quantum chemical calculations reveal that designed PDIs have more matching energy level with MAPbI3, which will promote the efficient electron extraction and block undesirable hole transport from perovskite to the ETM layer. The crystal structures of asymmetrical PDIs exhibit a quasi-two-dimensional (2D) π–π stacking motif ascribed to the strong interaction of oxygen atoms in alkoxy benzamide/benzoate groups and hydrogen atoms in the molecule. A largely improved electron mobility of PDI-3 to PDI-6 (0.24, 0.41, 0.40, and 0.15 cm2 V–1 s–1, respectively) was obtained compared to the values for PDI-1 and PDI-2 (6.53 × 10–10 and 1.35 × 10–10 cm2 V–1 s–1, respectively). The calculated electron mobility of the designed PDI-4 and PDI-5 (0.41 and 0.40 cm2 V–1 s–1, respectively) is higher than that of a symmetrical PDI derivative (PDI-0, 0.30 cm2 V–1 s–1). The interface property of PDI-4/MAPbI3 and PDI-5/MAPbI3 is better than that of PDI-0/MAPbI3 due to the reinforced O···Pb interaction between the oxygen atom in the branched chain of PDI-4/PDI-5 and Pb of the perovskite surface. The enhanced defect (methylammonium vacancy, lead vacancy, and iodine substituted at the lead site) passivation ability for perovskite was also found in PDI-4 and PDI-5, which will lead to a high-performance perovskite layer. The current investigation not only offers a few promising ETMs but also provides a useful strategy for introducing asymmetrical functional groups into PDIs to obtain more efficient ETMs of PSCs.