Optimization of Hole Transport Layer/Perovskite Interface for MAPbI₃ Solar Cells Fabricated on PET Substrates

Because high-quality films can be made from solution deposition and low-temperature processing, perovskite films can be fabricated on flexible substrates, enabling roll-to-roll (R2R) high-throughput manufacturing of perovskite solar cells (PSCs). To achieve high efficiency, transparent conducting electrodes (TCEs) used in flexible PSCs must be highly conductive, transparent, chemically inert, and smooth. However, the low working temperatures of plastic substrates restrict the processing parameters and lead to lower-quality TCE films. In this work, we fabricate flexible hybrid TCEs by combining flexographically printed silver metal bus lines with blade-coated silver nanowires and an overcoat of indium zinc oxide sol-gel film on polyethylene terephthalate (PET) substrates. With this approach, hybrid TCEs exhibit an average transmittance &gt; 80%, sheet resistance &lt; 10 Ω/sq, and surface roughness &lt; 5 nm.[1] We fabricated perovskite solar cells (PSCs) with a <i>p</i>-<i>i</i>-<i>n</i> structure on hybrid TCEs and compared them to commercially available PET/indium tin oxide (ITO) and PET /insulator/metal/insulator substrates. Due to the thermal limitation of the plastic substrate, we utilized low-temperature solution-processable hole transport layers (HTLs) like poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and nickel oxide nanoparticles for PSC fabrication. Using as received PEDOT:PSS as the HTL, PSCs on hybrid TCEs achieved an average PCE of 5.1%, while those on PET/ITO exhibited an average PCE of 2.5%. Lower device performance on both the TCEs was due to the acidic nature of PEDOT:PSS (pH = 1.7), which can etch metal oxide on the PET substrates and increase its surface roughness.[2] To mitigate such effects, we introduced a bilayer HTL by depositing a neutral PEDOT:PSS (pH = 7) layer on the PET/TCEs followed by the as-received PEDOT:PSS layer. This improved the average PCE of PSCs on hybrid TCEs to 10% and on PET/ITO to 3.9%. Further improvements were observed when MeO-2PACz, a self-assembled monolayer, was deposited on top of the PEDOT:PSS bilayer, enhancing the device performance to an average PCE of 11% on hybrid TCEs and 7.4% on PET/ITO. Additionally, to assess the scalability of PSCs made on hybrid TCEs, we fabricated 1 cm² large-area devices. We employed scanning electron microscope, X-ray diffraction, steady-state photoluminescence, time-resolved photoluminescence, surface photovoltage, and electrochemical impedance spectroscopy to study the structural properties of perovskite utilizing different HTLs on flexible TCEs and the impact of MeO-2PACz on charge transfer dynamics at the HTL/perovskite interface. In addition to providing better PSC performance, incorporating MeO-2PACz into the PEDOT:PSS bilayer can enhance their stability. We evaluated the shelf stability of PSCs fabricated on hybrid TCEs and PET/ITO stored in a nitrogen glovebox without encapsulation. PSCs fabricated on hybrid TCEs retained 50% of their initial average PCE over 1500 hours. While for PSCs fabricated on PET/ITO, the average PCE declined to 10% of its initial value in just over 200 hours. These findings aim to advance the development of high-performance, cost-effective, and stable flexible PSC technologies suitable for large-scale production.<br/><br/>This work is funded by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Solar Energy Technologies Office Award Number DE-EE0009518.<br/><br/>[1] Robert T., Piper, Justin C., Bonner, Bishal, Bhandari, Cody R., Allen, Cynthia T., Bowers, Melinda A., Ostendorf, Matthew, Davis, Marisol, Valdez, Mark, Lee, Julia W. P., <i>Adv. Mater. Technol.</i> <b>2024</b>. Under Review<br/>[2] Bishal, Bhandari, Justin C., Bonner, Ropert T., Piper, Julia W.P., Hsu., <i>Flex. Print. Electron</i>. <b>2024</b>. Under Review