Improving Durability of Highly Efficient Perovskite/CIGS Tandem Solar Cells using PTAA as Hole Transporting Layer with a Solution Based Buffer Layer

In recent years, perovskite solar cells (PSCs) have attracted considerable attention due to their potential for enabling lightweight and cost-effective photovoltaic devices. A particularly notable advantage of PSCs is their compatibility with existing silicon (Si) or copper indium gallium selenide (CIGS) solar cells to form tandem solar cells, in which power conversion efficiencies (PCEs) exceeding 40% are theoretically attainable. In the fabrication of tandem solar cells, the metal electrodes typically employed in PSCs must be substituted with transparent electrodes like ITO. These electrodes are usually applied via sputtering; however, the ion bombardment associated with this process often leads to a reduction in PCE. To address this issue, thermally evaporated molybdenum oxide (MoO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf>) is commonly utilized as a buffer layer in NIP-type semitransparent PSCs. However, the durability of semitransparent PSCs employing MoO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> has rarely been studied. In this study, the limitations of using MoO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> as a buffer layer are examined, and a more effective alternative—solution-processed metal oxide nanoparticles (MO-NPs)—is introduced. Moreover, our results demonstrate that Spiro-OMeTAD (Spiro) which is the commonly used hole transport layer (HTL) in NIP-type PSCs suffers from insufficient stability, while poly(triarylamine) (PTAA) leads to a substantial improvement in device durability. When MO-NPs were introduced on PTAA, device durability improved significantly, with nearly 100% of the initial PCE retained after continuous exposure to 100 mW/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> LED illumination at 75 °C under open-circuit conditions. Furthermore, as the optical properties of the target device remained virtually unchanged after the test, a four-terminal tandem device incorporating a flexible CIGS bottom cell—with an initial PCE of approximately 17%—achieved a stabilized PCE of 23% following 150 hours of light soaking at elevated temperature.