Oligomer acceptors achieve high stability and high efficiency organic solar cells

On November 11, 2022, Beijing time, the team of Professor Huang Fei and Professor Li Ning of South China University of Technology published a research result entitled “Stable and Efficient Organic Solar Cells Using Oligomer Acceptors” in the journal Nature Energy. In this work, it is reported that organic solar cells with high efficiency and excellent stability can be constructed by synthesizing oligomer acceptors with defined chemical structures and regulating the thermodynamic properties and molecular packing behavior of organic optoelectronic materials.

The corresponding authors of the paper are Li Ning and Huang Fei, and the first authors are Liang Youcai and Zhang Difei.

Organic solar cells (OSCs) have received extensive attention from researchers around the world in the past two decades because of their advantages of flexibility, light weight and translucency, as the next generation of photovoltaic technology in portable energy, power generation exterior wall glass and photovoltaic greenhouse integration and other applications. At present, organic solar cells based on Y series small molecule receptors (SMAs) have achieved a photoelectric conversion efficiency (PCE) of more than 19%, but the low thermal transition temperature of such materials makes the morphology of the active layer unstable under light or heat, resulting in a greater impact on the stability of the corresponding devices, which is one of the biggest bottlenecks for the commercial application of organic solar cells. In order to solve the problem of unstable micromorphology of the active layer, many researchers at home and abroad adopt the strategy of polymerization of small molecule receptors, and the obtained polymer receptors can not only maintain the efficiency of small molecule receptors, but also show better photostability or thermal stability. However, the performance of such polymer acceptors has a strong dependence on molecular weight distribution and polymerization batches, which is not conducive to large-scale material production and technical application, so it is an unresolved design problem that organic solar cells have both high efficiency and excellent stability.

Figure 1: Material characterization of acceptors.

In this work, the team of Professor Huang Fei and Professor Li Ning of South China University of Technology proposed an oligomerization strategy of non-fullerene acceptor (NFA) to synthesize a series of Y series oligomers with defined chemical structures, which realized the effective regulation of thermodynamic properties, crystallization properties and molecular packing of materials. By comparing with the precursor small molecule OY1 and the reference polymer PY, it is found that the trimer OY3 can not only inherit the excellent photoelectric properties of small molecules and polymers, but also overcome their respective shortcomings, and break the limitation of the balance between photoelectric conversion efficiency and stability.

Figure 2: Device performance for OY1-OY4 and PY.

First, the researchers synthesized the dimers of the Y series by using the unbalanced ingredient ratio one-pot method, and then separated and purified them separately. The results show that the number of structural units of the materials has little effect on the photophysical properties of the materials and the efficiency of the devices. However, after in-depth research, it was found that the oligomer acceptor has a higher cold crystallization temperature and tighter molecular packing properties; The topography characterization of GIWAXS and spectroscopic analysis also showed that OY3-based blended films had a better packing orientation than those based on OY1 or PY, which was conducive to the formation of stable micromorphology in the active layer.

Figure 3: Molecular packing properties and photophysical properties of OY1, OY3 and PY.

In the aging experiment, OY3 can maintain excellent thermal stability and light stability, and OY3-based organic solar cells can still maintain an initial efficiency of more than 90% after 1000 hours of working under light. Thanks to the higher thermal transition temperature and more ordered micromorphology of OY3, its T80 stability can reach 25,000 hours after extrapolation, which is equivalent to more than 16 years of use in Guangzhou. This result is the first organic solar cell reported in the art to combine high efficiency (>15%) and high service life (> 15 years).

Figure 4: Thermal and light stability of organic solar cells.

This study shows that the Y series oligomer acceptors can not only inherit the excellent photoelectric properties of small molecules and polymers, but also solve the key problems of low thermal transition temperature and polymer batch repeatability of small molecule acceptors, and put forward a new molecular design idea for commercial application in this field. (Source: Web of Science)

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