Visible Light–Induced Degradation of Inverted Polymer:Nonfullerene Acceptor Solar Cells: Initiated by the Light Absorption of ZnO Layer

Abstract

Power conversion efficiencies (PCEs) of polymer solar cells (PSCs) have exceeded 18% in the last few years. Stability has therefore become the next most important issue before commercialization. Herein, the degradation behaviors of the inverted PM6:IT‐4F (PBDB‐T‐2F:3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene) solar cells with different ZnO layers are systematically investigated. The PCE decay rates of the cells and the photobleaching process of the IT‐4F containing organic films on ZnO surface are directly correlated with the light‐absorption ability of the ZnO layer in the visible light range, indicating that photochemical decomposition of IT‐4F is initiated by the light absorption of ZnO layer. By analyzing the products of the aged ZnO/IT‐4F films with matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS), it is confirmed that photochemical reactions at the IT‐4F/ZnO interface include de‐electron‐withdrawing units and dealkylation on the side‐phenyl ring. Hydroxyl radicals generated by the photo‐oxidation of dangling hydroxide by ZnO are confirmed by electron spin resonance (ESR) spectroscopy measurements, which is attributed as the main reason causing the decomposition of IT‐4F. Surface treatment of ZnO with hydroxide and/or hydroxyl radical scavenger is found to be able to improve the stability of the PSCs, which further supports the proposed degradation mechanism.

Mechanism of the Photodegradation of A‐D‐A Acceptors for Organic Photovoltaics**

“…Formed by reversible aldol condensation, the vinylene bond bridging the donor and acceptor moieties was often speculated to be the origin of the photoinduced decomposition of IT-series and many other A-D-A type NFAs. [26,27,29] However, our results unequivocally show that the photodegradation involves the dicyanomethylene moiety, while the vinylene bridge is not the immediate reaction center. The 1 H NMR spectrum of both degradation products features an sp 3 singlet around 4.7 ppm (Figure S3), which was observed in a recent photostability study and attributed to the formation of epoxides upon photooxidation of the vinylene bridge.…”

“…Most of these assume photooxidation as the key mechanism, [25,26] although photocatalytic degradations of the vinylene moiety of A-D-A molecules at the interface with ZnO (electron extraction layer) have also been proposed. [27][28][29] However, no unambiguous identifications of degradation products were reported in the past, and this limited understanding of the molecular origin of degradation remains a key obstacle for improving the stability of OPVs. [30,31] Here, we uncover a new mechanism of the photodegradation of A-D-A NFAs involving 6-e electrocyclic reactions of the 1,1-dicyanomethylene-3indanone (INCN) moiety.…”

Emerging Chemistry in Enhancing the Chemical and Photochemical Stabilities of Fused‐Ring Electron Acceptors in Organic Solar Cells

“…mechanism between IT-4F and ZnO (Figure 5d), which contains four steps: 1) the defects enhance the absorption of ZnO in visible light; 2) the electrons of ZnO are excited to its CB; 3) the left holes on VB of ZnO oxidize the dangling OH − to afford reactive •OH; 4) the decoupling reactions occur between •OH and IT-4F. [41]…”

“…d) Proposed white-light-induced interfacial degradation mechanism between ZnO and IT-4F. Reproduced with permission [41]. Copyright 2020, Wiley-VCH.…”

Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

“…Benefiting from the excellent tunability of the electronic structure and blend morphology for donor (D)-acceptor (A)-type electron acceptors, [1][2][3][4] organic photovoltaics (OPVs) have achieved high power conversion efficiencies (PCEs) above 17% within a short period of time, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] indicating a bright future for commercial applications, provided that the three key issues, efficiency, cost, and stability, can be synergistically addressed. [20][21][22][23][24][25][26][27][28] Currently, most high-performance D-A acceptors contain an electron-rich core and strong electron-deficient 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) terminals (Figure 1a), 29 recognized as one of the main factors that limit the device lifetime because the exocyclic double bonds formed by the kinetically reversible Knoevenagel condensation reaction (KCR) are highly labile upon photooxidation, [30][31][32][33][34] ZnO-catalyzed photodegradation, 35,36 and base-induced [37][38][39] decomposition (Figure 1a). To increase the device stability, stabilizers, …”