CityU realizes a new mechanism for high-efficiency organic solar cells

On November 15, 2022, Beijing time, Professor Ren Guangyu’s team from City University of Hong Kong and Professor Zhang Chunfeng’s team from Nanjing University published an article entitled “Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture” in Nature Energy Research results.

This achievement reports how to realize the three-wire state regulation in the same material system through the physical method of device engineering, thereby suppressing the bimolecular composite loss of free carriers, increasing the photocurrent without sacrificing the photovoltage, and achieving a photoelectric conversion efficiency of more than 19%. This photogenerated carrier dynamics process mechanism was verified by ultrafast spectroscopy experiments and molecular dynamics calculations, revealing the correlation between trilinear excitons and device performance in organic solar cells, and providing a new perspective direction for further understanding the structure-activity relationship and efficiency optimization strategies of high-performance organic solar cells. The corresponding authors are Ren Guangyu, Zhang Chunfeng, and Francis (Ray) Lin; The first authors are Jiang Kui, Zhang Jie, Zhong Cheng.

Since the invention of the first organic solar cells (OPVs) in 1980, OPVs dominated by bulk heterojunction structures (BHJ) have achieved rapid development. In recent years, the discovery of two star molecules, ITIC and Y6, has greatly promoted the rapid improvement of OPV photoelectric conversion efficiency. These all show that the development of materials determines the future of organic photovoltaics, and the physical working mechanism behind it is constantly improved and updated with the changes of materials.

For a long time, the formation of three-wire excitons (T1) in organic photovoltaics has had a significant impact on the performance of organic photovoltaic devices, but the influence mechanism behind it is not particularly clear. In 2013, British scientist Richard Friend proposed to achieve T1 suppression by controlling the carrier wavelet function delocalization, and again proposed in 2021 that the change of T1 is directly related to the open-circuit voltage loss (Voc loss). However, these studies are based on different material systems, and by comparing T1 to form fewer material systems, it is proved that inhibiting the formation of T1 improves the performance of organic photovoltaics. Therefore, there has always been a question: in different material systems, it is normal for different materials to have different properties and properties, such as the amount of T1, the change of photovoltage, so the direct relationship between T1 and photovoltage is not very clear, so how to achieve the regulation of T1 under the same material system and whether the relationship between T1 and Voc can be directly observed has never been studied and verified.

Recently, Professor Ren Guangyu’s team based on the new discovery of the relationship between layer-by-layer deposition device preparation technology and material properties in the early stage of the research group (Nat. Commun. 2021, 12, 468), by selecting a donor polymer D18 with strong aggregation properties and Y6 series small molecule acceptors, theoretical calculation simulates the interaction behavior between molecules, precise thin film property regulation has achieved a photoelectric conversion efficiency of more than 19%, and has been certified in Newport photovoltaic laboratory in the United States, and at the same time cooperated with the team of Professor Zhang Chunfeng of Nanjing University to achieve ultrafast spectral characterization technology. The kinetic behavior of excitons and carriers in the traditional BHJ structure formed by layer-by-layer deposition was analyzed. This work provides strong support for understanding the working mechanism of organic solar cells with high-efficiency PMHJ structure.

Organic solar cells that reduce exciton binding energy can help realize the microstructure of PMHJ。 Through theoretical calculations and ultrafast spectral analysis, the research group found that there is a delocalized exciton singlet state (DSE) in both the donor polymer and the acceptor small molecule, which reduces the exciton binding energy that needs to be overcome to achieve charge separation, and at the same time, ultrafast spectroscopic characterization confirms that the DSE in the acceptor small molecule film can transition to the charge separation state, that is, the exciton separation process does not have to be achieved by forming a charge transfer state (CT) at the acceptor (D/A) interface. This mechanism ensures that the small D/A interface in the PMHJ crystal structure will not become a limiting factor for exciton dissociation to generate free carriers.

Figure 1: Exciton transfer mechanism and ultrafast spectra of molecules and theoretical calculations

meantimeThe PMHJ microstructure can inhibit the production of T1。 Through ultrafast spectroscopic observation, the research team found that the production of T1 can be seen in the crystal structure of BHJ and PMHJ, but the PMHJ structure obviously shows a weaker T1 signal, and a weaker T1 signal means less free carrier bimolecular recombination, which benefits from the less D/A contact area in the PMHJ structure, which limits the chance of meeting recombinant excitons in photogenerated electrons in the acceptor and photogenerated holes in the donor at the D/A interface.

Figure 2: Planar hybrid heterojunction inhibits the formation of trilinear states

The PMHJ structure realizes high-current and high-efficiency OPVs。 Since the PMHJ structure inhibits the bimolecular recombination loss of photogenerated carriers, it can achieve faster carrier transfer and higher photogenerated carrier concentration, thereby effectively increasing the short-circuit current (Jsc). This is a perfect match for the device results, and it is also found that Voc in BHJ and PMHJ devices of the same material system is almost identical, indicating that no direct correlation between T1 formation or inhibition and Voc has been observed in the D18 and Y6 series acceptor material combinations. This provides us with a new perspective to understand the mechanism between T1 and Voc, and provides ideas for further reducing the balance coordination between Voc and Jsc to achieve higher energy conversion efficiency.

Figure 3: Device performance vs. photovoltage loss analysis

(Source: Science Network)

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