Stability and charge transport analysis of high-performance PM6:Y7 nonfullerene organic solar cells using the metal–insulator–metal model

Materials for Renewable and Sustainable Energy https://doi.org/10.1007/s40243-025-00300-2 (2025) 14:26 ORIGINAL PAPER Stability and charge transport analysis of high-performance PM6:Y7 nonfullerene organic solar cells using the metal–insulator–metal model Liliana Fernanda Hernández-García1 · Luis Reséndiz1 · Magaly Ramírez-Como1 · Angel Sacramento2 · Víctor Cabrera1 · Magali Estrada2 · Josep Pallarès3 · Lluis F. Marsal3 Received: 29 February 2024 / Accepted: 19 February 2025 © The Author(s) 2025 Abstract Non-fullerene acceptors are promising materials for organic solar cells because of their flexibility and low cost; however, their long-term stability remains a critical challenge. In this study, we investigate the degradation mechanisms of conventionally structured solar cells (ITO/PEDOT: PSS/PM6/Y7/PDINO/Ag) under different environmental conditions: nitrogen preservation, encapsulation, and air exposure. Using the metal-insulator-metal (MIM) model, we simulate the current-voltage characteristics and extract key parameters to understand the physical mechanisms governing device degradation. The results show that air exposure primarily affects the anode interface, reducing the interfacial dipole energy and shifting the Fermi-level alignment of PEDOT: PSS, which is crucial for efficient hole extraction. This process leads to a deterioration in the hole transport properties over time, significantly affecting device performance. In contrast, the cathodic interface remains stable, suggesting that degradation is largely driven by changes in the hole transport layer. These findings provide critical insights into the interfacial degradation mechanisms of the NFA-based solar cells. Understanding these effects will aid in the development of strategies to enhance the stability and efficiency of organic photovoltaic devices for long-term operation. Keywords Nonfullerene organic solar cells · Anodic interface degradation · Energy alignment · Numerical simulation Introduction Organic solar cells (OSCs), also known as plastic solar cells, use conductive organic polymers or small organic molecules for light absorption and charge transport, thereby enabling the conversion of sunlight into electricity through the photovoltaic effect. Luis Reséndiz 1 Sección de Estudios de Posgrado e Investigación, UPIITA, Instituto Politécnico Nacional, México City 07340, México 2 Sección de Electrónica del Estado Sólido, Departamento de Ingeniería Eléctrica, CINVESTAV–IPN, México City 07360, México 3 Department of Electric, Electronic and Automatic Engineering, Universitat Rovira i Virgili, Tarragona 43007, Spain The developments in molecular engineering ensure the design of organic molecules optimized for optoelectronic applications. Modifying functional groups attached to the molecule or changing molecule length allows for bandgap adjustments of the material, thereby enabling optical tuning. An advantage of OSCs is their high absorption coefficient. A small amount of organic material, generally having a size in the range of hundreds of nanometers, is sufficient to absorb a large amount of light. However, OSCs present some disadvantages, including lower power conversion efficiency (PCE), stability, and strength, compared to inorganic photovoltaic cells, such as silicon solar cells. Despite these inherent challenges, the allure of OSCs lies in their lightweight construction, disposability, inexpensive fabrication, flexibility, and potential for low environmental impact. These distinctive features make polymer solar cells an attractive research topic. Recently, in the search for an ideal polymer for light absorption and charge conduction, a wide-bandgap polymer, PM6, has been developed that has 13 26 Page 2 of 14 shown excellent photovoltaic performance. PM6 has been successfully used to design OSC with a high PCE of > 15% [1–4], which is a crucial achievement in the field of organic photovoltaics. PM6, in conjunction with a low-band-gap nonfullerene acceptor, such as 2,2’-((2Z,2’Z)-((12,13Bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro- [1, 2, 5] thiadiazolo[3,4-e]thieno-[2”,3’’:4’,5’]-thieno[2’,3’:4,5] pyrrolo[3,2-g]thieno-[2’,3’:4,5]thieno[3,2-b]-indole-2,10diyl)bis(methanylylidene))-bis(5,6-dichloro-3-oxo-2,3dihydro-1 H-indene-2,1-diylidene))dimalononitrile (Y7), forms a bulk heterojunction with high efficiency for photon splitting and charge transfer at the interface; such an acceptor facilitates the rapid development of OSCs with the PCE exceeding 18% [5]. Additionally, OSCs utilizing PM6 in the active layer have demonstrated excellent absorption in the visible and near-infrared ranges [6], maintaining a stable morphology of the active layer over extended periods. However, nonfullerene acceptor OSCs (NFA-OSCs) present challenges that must be addressed before they can be used in commercial devices, particularly regarding their continued performance over time in the ambient atmosphere. To prevent the degradation mechanisms in the photostability of NFA-OSCs, some studies have adopted diverse strategies such as suppressing trap-mediated recombination [7] and incorporating interlayer materials or modulating the electrode work function (WF) to match the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor [8]. A well-matched WF of the anode with the IP of the donors prevents the loss of photovoltaic performance. However, an in-depth exploration of the degradation mechanisms in these devices is imperative. Incorporating a transparent electrode into any OSC design is essential; thus, indium tin oxide (ITO) is an excellent material owing to its transparency, conductivity, and high WF. Similarly, PEDOT: PSS a commercially available and accessible polymer blend, is widely used as a buffer layer in OSCs, achieving a selective electrode for hole collection [9]. Simultaneously, it acts as an electronblocking layer in the solar cells. Furthermore, its popularity stems from the band energy level alignment that reduces the energy barrier between ITO and the highest occupied molecular orbital (HOMO) of the active layer material [10]. To minimize the degradation effects on the OSC and enhance its efficiency, determining the energy level diagram of the materials involved in the heterostructure is crucial. Determination of the degradation effect suffered by the various layers and interfaces of an OSC and its direct consequences on the charge generation and extraction processes is possible by knowing the coupling of the energy levels of the interfaces. Herein, sophisticated experimental techniques, such as ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and cyclic 13 Materials for Renewable and Sustainable Energy (2025) 14:26 voltammetry, are usually used to obtain approximated values for the IP and EA of donors and acceptors, respectively, to determine the solar cell energy configuration experimentally. However, these technologi (...truncated)