Periodic Reporting for period 1 - IONMIGRATIONPSC (Study of Ion Migration in Perovskite Solar Cells via X-ray Photoemission Spectroscopy Imaging and Photoluminescence Microscopy)
Berichtszeitraum: 2022-11-01 bis 2024-10-31
Overview: Organic-inorganic hybrid perovskite solar cells (PSCs) attract enormous attention in the field of emerging photovoltaics due to their unique opto-electronic properties and unprecedented advances in performance. In just over a decade, the power conversion efficiency (PCE) of PSCs has increased to more than 26%, suggesting this technology might be ready for large-scale exploitation in industrial applications. However, stability, scalability and toxicity are some of the major concerns that impede the commercialization of PSCs. Among these, exploring strategies to enhance the long-term stability of PSCs is a main research topic in the perovskite community. While it is generally considered that ionic defect states in the perovskite layer such as vacancies, interstitial sites, anti-site substitutions as well as surface and grain boundary defects inevitably lead to the degradation of PSCs, much remains unknown about the fundamental nature of ionic defects and in particular their migration in perovskite materials. The IONMIGRATIONPSC project aims to provide fundamental understanding of ion migration in perovskite solar cells (PSCs) and valuable insights for the enhanced device stability by a unique combination of electrical and spectroscopic methods. This project focus on tracking the changes in both compositional and optoelectronic properties of perovskite materials and correlate with the operational stability of the devices.
Objectives:
Quantification of ion migration in PSCs.
Tracking changes in both the compositional and optoelectronic properties of perovskites as a function of applied bias in perovskite devices, with and without illumination.
Correlation between the spatially resolved optoelectronic measurements and the study of perovskite degradation.
Exploring the effects of passivation techniques on the ion migration.
Elucidation of the role of ion migration in PSCs degradation.
Objectives:
Quantification of ion migration in PSCs.
Tracking changes in both the compositional and optoelectronic properties of perovskites as a function of applied bias in perovskite devices, with and without illumination.
Correlation between the spatially resolved optoelectronic measurements and the study of perovskite degradation.
Exploring the effects of passivation techniques on the ion migration.
Elucidation of the role of ion migration in PSCs degradation.
To study how ions move inside perovskite solar cells (PSCs), I first built devices using a common material called methylammonium lead iodide (MAPI) as the light-absorbing layer. I used PTAA and PCBM60 as layers that help move electrical charges (called hole and electron transport layers, respectively). These early devices reached a power conversion efficiency (PCE) of 19%. Since ion movement in MAPI-based PSCs is already well-known—especially in the research group of Prof. Vaynzof—I compared my results with theirs. My findings confirmed that iodine from the MAPI layer can move through the electron transport layer and react with the silver electrode, forming silver iodide (AgI), which damages the device. I used special techniques (X-ray and ultraviolet photoelectron spectroscopy) to study these chemical changes and energy levels inside the device. After confirming that my methods worked on these standard devices, I moved on to more advanced perovskite materials, like the triple-cation composition Cs₀.₀₅[(FA)₅/₆(MA)₁/₆]₀.₉₅Pb(I₀.₉Br₀.₁)₃. With this, I achieved over 21% efficiency without adding any passivating layers to protect the surface. I also found that the type of hole transport layer plays a key role in how much ion migration happens. I compared two materials: PTAA and MeO-2PACz. Both gave similar efficiency, but devices made with MeO-2PACz were much more stable during operation. To better understand this difference, I used imaging techniques to see how ions move when a voltage is applied. The results showed that ion migration was much stronger in PTAA-based devices, which explained their faster degradation. Additional analysis revealed that PTAA leads to the formation of an unwanted MAPI-like layer at the bottom interface, which can contribute to instability. Overall, the goals of the IONMIGRATIONPSC project have been successfully achieved within the planned two years (Nov. 2022–Oct. 2024). Work is still ongoing to explore how surface treatments can reduce ion migration. For this, I’m currently using a chemical called Cl-PEAI to improve both the efficiency and long-term stability of the solar cells.
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.