The Intrinsic Role of Molecular Mass and Polydispersity Index in High‐Performance Non‐Fullerene Polymer Solar Cells

Abstract

efficiencies (PCEs) rapidly improving from 11% to extend to 17% over the past five years. [9,10,11,12] However, a number of works have shown that non-fullerene PSCs also suffered from rapid performance degradation over time, particularly upon exposure to oxygen, light or elevated temperatures. [13,14,15,16] It can be seen that the development of highly efficient PSCs that show stable photovoltaic performance without adverse degradation remains a significant challenge in this field. [14] Extensive efforts are directed at understanding BHJ morphology degradation resulting from internal factors, [16,17,18,19] synergistic reaction mechanisms, [20,21,22,23] and external stresses. [14] For instance, photooxidation and photostability of active layers are strongly dependent on the sidechain and end-group engineering of the NFAs. [16,24] Increased molecular order and crystallinity have proved to be a benefit for the photostability and thermal stability. [25,26] The high glass transition temperature and proper miscibility of D/A materials can be positively related to the enhanced thermal stability of relevant active layers. [20,27] However, the material structure-device performance correlation is still far from being well understood. Given the greater synthetic complexity of polymer donors compared to the NFAs with relatively well-defined molecular structures, it is interesting to consider whether such PSCs employing polymer donors by fine-tuning their degree of polymerization (including weight-average molecular weight (M w ) and polydispersity (PDI)) can exhibit reduced degradation losses under various external stresses. Recently, a few studies also shed light on the influence of the molecular mass of polymer donors on the underlying degradation mechanisms measured under multiple external stresses. For instance, it has been shown that the composition of the polymer, in terms of M w values, significantly influences the burn-in phase. [28,29,30] In addition, increasing molecular weight but narrowing its distribution can effectively improve the photostability and thermal stability of the fullerene-based PSCs. [29,31] These stability investigations have not only gained a better understanding of the degradation mechanisms associated with the molecular nature but also raised a fundamental question as to whether the degradation behaviors of PSCs is inherent to the conjugated polymers and cannot be overcome. To our knowledge, a subject linking the degradation features to the degree of polymerization has not yet been fully covered in The degree of polymerization can cause significant changes in the blend microstructure and physical mechanism of the active layer of non-fullerene polymer solar cells, resulting in a huge difference in device performance. However, the diversity of stability issues, including photobleaching stability, storage stability, photostability, thermal stability, and mechanical stability, and more, poses a challenge for the degree of polymerization to comprehensively address the trade-off between device efficie...

High-performance polymer solar cells with efficiency over 18% enabled by asymmetric side chain engineering of non-fullerene acceptors

“…The operational stability is one of the most challenging issues for "Y-series" PSCs. Min and co-workers [47] investigated the stability of device with PM6:Y6 active layers by tuning the molecular weight (M w ) and polydispersity (PDI) of PM6. It was found that devices based on different PM6 batches retained about 40%-70% of their initial PCEs after 100 h sun illumination.…”

Viable Mixing Protocol Based on Formulated Equations for Achieving Desired Molecular Weight and Maximal Charge Separation of Photovoltaic Polymer

“…[5][6][7] Despite the success of the field of PSCs, batch-to-batch differences in molecular weight of a conjugated polymer used as one of the active layer components within a BHJ active layer, led to inconsistency in macromolecular ordering, optoelectronic and charge transport characteristics, and ultimately device performance. [8][9][10][11] There have been several attempts to overcome the molecular weight limitations caused by synthetic polymers. For example, You's and Marks's groups independently demonstrated the possibility of controlling polymer's molecular weight via a synthetic strategy based on the classic Carothers equation, considering the comonomer ratios and degree of polymerization.…”

Developing Halogen‐Free Polymer Donors for Efficient Nonfullerene Organic Solar Cells by Addition of Highly Electron‐Deficient Diketopyrrolopyrrole Unit

“…In addition, the number‐averaged molecular weight ( M n ) and polydispersity index (PDI) are also comparable, that is, 35.11 kDa/1.46 and 26.93 kDa/1.51 for P75 and PBDB–T, respectively, which minimize the impact of molecular weight on the performance. [ 47 ]…”