An Efficiency of 16.46% and a T₈₀ Lifetime of Over 4000 h for the PM6:Y6 Inverted Organic Solar Cells Enabled by Surface Acid Treatment of the Zinc Oxide Electron Transporting Layer

Abstract

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface −OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface −OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T 80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

Simultaneously Achieving Highly Efficient and Stable Polymer:Non‐Fullerene Solar Cells Enabled By Molecular Structure Optimization and Surface Passivation

“…[ 24 ] By comparing the stability difference of the IT‐4F cells with different ZnO interlayer, we recently proved that photon generated hydroxyl radicals on the ZnO surface is the chemical reactive species that causes the breaking of the C═C bonds. [ 25 ] With the better understanding on the detailed degradation mechanism of the NFA cells, methods to improve device stability, including blending with fullerene molecules, [ 26 ] surface treatment with Lewis acids, [ 27 ] a thin PEI layer, [ 28 ] or a self‐assembled monolayer [ 29 ] were reported, supporting the proposed degradation mechanism. Despite these excellent research works in improving the stability of the cells, these cells showed lower initial efficiencies less than the optimized cell performance, and most of the cells showed a PCE lower than 15%.…”

“…This result unambiguously confirms that the 𝛽-side chains next to the C═C bond have great influence on the stability of the NFA molecules and the corresponding device performance. Our previous finding showed that treating the ZnO surface with Lewis acid, such as 2-phenylethanethiol (PET), [25,27] can decrease the photon reactivity of ZnO film and consequently improve device stability. With this, a PSC with an inverted structure of ITO/ZnO (PET treated)/PM6:L8-BO/MoO 3 /Al was fabricated, which showed the highest PCE of 17.02% (inset in Figure 5e, see also Figure S7 and Table S3, Supporting Information, for more details).…”

Ultrathin and Efficient Organic Photovoltaics with Enhanced Air Stability by Suppression of Zinc Element Diffusion

“…[ 14 ] In addition to the intrinsic morphological changes in bulk heterojunction blends, [ 24 , 25 ] the changes in morphology, induced by the active layer/transporting layer interfaces, can also affect the cell performance over time. [ 26 , 27 ] This highlights the urgent need to understand the effects of internal changes in OPVs under environmental stresses and to prevent or minimize these changes. Some amine‐containing polymer electron transporting layers (ETLs), such as polyethylenimine (PEI), polyethylenimine ethoxylated (PEIE), and poly[(9,9‐bis(3 ′ ‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)] (PFN) have a tendency to react with the nonfullerene active layer, resulting in poor efficiency.…”

Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells

“…Among the metal oxides, zinc oxide (ZnO) has been widely used as the ETLs in fullerene-based OSCs for its high transparency and good conductivity. [9] ZnO ETLs can be prepared by nanoparticle route [10] or sol-gel route. [11] Most recently, Hou et al developed a new method of printable ZnO as the ETL for flexible OSCs.…”

“…ZnO ETLs prepared from nanoparticle methods or sol-gel methods often suffer from plentiful surface defects, which introduce a high density of recombination centers, leading to insufficient charge extraction and transport. [9] To passivate these defects, extensive efforts have been devoted to surface modification. Introducing an extra layer of organic electrolyte is a facile way to passivate the defects and improve the interfacial contact.…”