Precise Modification of Electron Linkers for Improving the Performance of All-Polymer Solar Cells

Organic solar cells (OSCs), which can be applied to flexible wearable electronic devices, indoor light-harvesting applications, building-integrated photovoltaics, and the internet of things, have attracted particular attention. Recently, breakthroughs in material design and optimized device techniques have driven the efficiency of OSCs exceeding 19% when using the non-fullerene small molecule acceptors and polymer donors system.

In addition to the readily tunable structural, optical, and electrochemical properties, all-polymer organic solar cells (all-OSCs), based on polymer donors and polymer acceptors, have unique advantages, such as excellent stability and robustness compared with their counterparts based on fullerenes or small molecules non-fullerenes.

The PCE of all-OSCs has been rapidly increased from 11% to more than 15% through the strategy of "polymerized small-molecule acceptors". (Joule 2020, 4, 1070-1086; Adv. Funct. Mater. 2021, 2010411.) However, device stability often shows unsatisfactory results. This is mainly due to the poor phase separation between the donor and the acceptor in the bulk heterojunction, which leads to the degradation of the device performance.

In general, many factors are affected the device stability, such as molecular structure, donor/acceptor (D/A) miscibility, light, temperature, and mechanical stress, and so on. Therefore, significant efforts have been devoted to overcoming these factors to reach the stability of commercial applications. Among them, designing materials may be the most effective way to balance the efficiency and morphology (phase separation or donor/acceptor miscibility). In other words, the molecular miscibility and intermolecular interactions of OSCs are important considerations for ensuring the high performance of the device.

This study is led by Prof. Jie Min (The Institute for Advanced Studies Wuhan University). Bearing in mind that the promising advantages of the ladder-type D-A-D fused cores, Prof. Min group designed and synthesized a new series of fused-ring conducted polymer acceptors, namely PY-X(O, S, Se), using a highly efficient Y5-C20-derivative polymer acceptor changed the electron linkers (furan(O), thiophene(S), and selenophene(Se)).

By blending the different polymer acceptors with the polymer donor PBDB-T, PBDB-T:PY-Se system with remarkable D/A compatibility showed maximum performance with an efficiency of 15.48%, which is much higher than those of PBDB-T:PY-O (9.80%) and PBDB-T:PY-S (14.16%) devices, supported by the optimized bulk microstructure with respect to its physical mechanisms in parallel.

Systematic investigation shows that electron linker engineering significantly affects the physicochemical properties, intermolecular interactions, and charge transport properties of polymer acceptors.

Compared with the other two polymer acceptor materials, PY-Se exhibits higher crystallinity, which can be attributed to the stronger intermolecular interaction between PY-Se molecules. Next, to further investigate the influence of electron linker engineering, the photovoltaic performance of the relative devices was comprehensively studied.

When PBDB-T as the polymer donor matched the different acceptor, PBDB-T:PY-Se devices possessed a much higher PCE of 15.48% compared to PY-O- (9.80%) and PY-S-based devices (14.16%). Meantime, it was found that electron linkers can regulate the intermolecular arrangement and crystallinity, thus affecting the blend morphology and device efficiency.

The relationship between intermolecular interactions and phase separation in blends is generally concerned, but the effects of D/A miscibility, as well as their intermolecular interactions on relevant stability issues, are neglected in some cases. Further, the PY-Se-based blend displayed much higher storage stability and light-soaking stability than those of the other two systems.

In addition, an all-polymer system is considered to be one of the most potential application materials for wearable electronic devices. Therefore, the researchers tested its mechanical properties. Compared with the small-molecule non-fullerene system, the all-polymer system has better mechanical properties, which can be mainly attributed to the entanglement of polymer chains.

As a result, this strategy of precise modification of electron linkers can be a practical way to simultaneously actualize molecular crystallinity and phase miscibility for improving the performance of all-polymer solar cells, showing practical significance.

Source: https://phys.org/

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