Electrostatically Enhanced Buried Interface Binding of Self-Assembled Monolayers for Efficient And Stable Inverted Perovskite Solar Cells.

Abstract

Inverted p-i-n structure perovskite solar cells (PSCs) have outperformed traditional n-i-p PSCs in recent years. A key advancement is the use of self-assembled monolayers (SAMs) as hole transport layers. One class of widely used SAMs is carbazole-based phosphonic acids. However, it is found that these SAMs lack strong binding with transparent conducting oxides (TCO) and perovskite. The weak binding strength results in suboptimal interfacial adhesion of the buried interface, which limits the device's stability. Here, interfacial binding is enhanced by increasing the dipole moment that creates a strong interfacial electric field that enhances electrostatic interactions at the TCO/perovskite interface, while incorporating tailored functional groups in SAMs to improve chemical anchoring to TCO and binding to perovskite. Specifically, the donor-acceptor SAM molecule 4-(7-(4-(bis(4-methoxyphenyl)amino)-2,5-difluorophenyl)benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (PAFTB) is employed, which features an enhanced dipole moment along with electron-donating and electron-withdrawing functional groups to optimize interfacial interactions. Compared to extensively used [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz), PAFTB enhances total interfacial adhesion by 2.8 times, thereby improving the thermal stability of the layer. Using this approach, PSCs are demonstrated with a certified quasi-steady-state power conversion efficiency of 24.9% and maintain 80% of the initial efficiency after 900 h of maximum power point tracking at 85 °C.