Photostability of Fullerene and Non-Fullerene Polymer Solar Cells: The Role of the Acceptor

Abstract

Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.

Unraveling the Microstructure‐Related Device Stability for Polymer Solar Cells Based on Nonfullerene Small‐Molecular Acceptors

“…[ 4,16 ] However, studies up to date indicated that the light stability of NFA systems is strongly materials‐dependent. [ 4,17 ] As a general trend, degradation of NFA systems is different compared to those based on fullerene derivatives. For example, J SC degradation as a result of materials incompatibility [ 8,9,18–20 ] or fullerene dimerization [ 21,22 ] has been revealed for several polymer:phenyl‐C 61 ‐butyric acid methyl ester (PCBM) combinations.…”

Evaporation‐Free Nonfullerene Flexible Organic Solar Cell Modules Manufactured by An All‐Solution Process

“…This is due to the high absorption properties of the WO 3 layer in the short wavelength region. [57,58] In contrast, in the bilayer HTLs devices, At the point where the light passed position 2 (interface of WO 3 layer and BHJ layer), a markedly decreased response and transmittance in the short wavelength region was observed (shown in Figure S19-4-S19-7, Supporting Information). The electric field distributions and transmittances at positions 1, 2, and 3 were determined by analyzing the light irradiated from the top to the bottom of the fabricated devices.…”

“…During this process, photodegradation occurred in the NFAs-based BHJ layer due to direct UV-exposure. [57,58] In contrast, in the bilayer HTLs devices, At the point where the light passed position 2 (interface of WO 3 layer and BHJ layer), a markedly decreased response and transmittance in the short wavelength region was observed (shown in Figure S19-4-S19-7, Supporting Information). This is because the WO 3 layer absorbed the short-wavelength light (λ abs = ≈335 nm, shown in Figure S20, Supporting Information).…”

“…[13,54] However, only a few studies have investigated solution-processed devices that contain an NFAsbased BHJ layer, and the incorporation of modified-buffer layer and dual buffer layer that allow energy level matching is also necessary. Since the photostability of the NFAs-based BHJ layer is lower than that of the FAs system, [57,58] it is necessary to develop buffer layer technology for protecting the BHJ layers. Therefore, third, the formation of a buffer layer and electrode via slot-die coating and screen printing is necessary to manufacture OSC modules using an all-solution process.…”

Challenges to the Stability of Active Layer Materials in Organic Solar Cells

“…It seems that NFA‐based devices possess robust storage stability. However, Doumon et al found that NFA with complementary absorption is more likely to deteriorate the device photo‐stability because the increased imbalance of hole and electron mobilities in NFA‐based device would form an undesirable space charge that may obstruct the flow of new charge carriers, leading to less charge extraction during the exposure time and influencing the FF . Recently, Brabec's group further systematically investigated the relationship between molecular structure and photo‐stability, in which the end‐group and side‐chain engineering of the NFA was revealed to have great influence on the photo‐stability of the related devices.…”