In the IONMIGRATIONPSC project, we are studying how different materials used in solar cells—specifically the hole transport layers (HTLs)—affect the movement of ions inside the device. We are comparing two commonly used materials: MeO-2PACz and PTAA, in a type of solar cell design called “inverted architecture,” and looking at how they behave when a voltage is applied (called forward bias). One focus of our study is the hidden interface between the HTL and the perovskite layer. We found that MeO-2PACz creates a rougher surface, which helps the perovskite layer grow in the intended form, using a stable mix of elements. On the other hand, PTAA has a smooth and water-repelling surface, which causes the perovskite to form in a less stable way, similar to a simpler structure called MAPI, known to break down more easily during use. This uneven layering inside the PTAA-based devices causes ions—especially iodine—to move around more when the device is operating, which leads to faster aging and lower stability. To further understand this, we also studied other similar molecules (called self-assembled monolayers or SAMs), such as Me-4PACz, 2PACz, and Br-2PACz. These tests help us see how the texture and chemistry of the buried interface impact ion movement. Our results show that SAM-based HTLs offer clear advantages over polymer-based ones like PTAA. They help control the structure of the perovskite layer more effectively and reduce unwanted ion movement, which improves the long-term stability of the solar cells. In addition to forward bias testing, we also explored how these solar cells respond to reverse bias (when the voltage is applied in the opposite direction), especially when stored in the dark. This allowed us to study reverse ion migration and better understand the full range of conditions that affect device stability.
