| 1. |
- Li, Xian'e, 1993-
(författare)
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Interfaces in Organic Solar Cells
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic semiconductors (OSs), the promising candidates for the next generation electronic devices, have shown their advantages of light weight, flexibility, semi-transparency, tunable optical gaps, and energy levels, in the development history of over 70 years. OSs have been widely used in versatile applications such as organic light-emitting diodes, organic field-effect transistors, organic photodetectors, and organic solar cells (OSCs) which are mainly studied in this thesis. Nowadays, OSCs have been developed rapidly and reached a new era of high efficiencies with new records close to 20%, since the development of non-fullerene-based donor (D) -acceptor (A) systems. Despite the impressive progress in the field of OSCs, fundamental understandings on the interface energetics are lagging far behind the development of materials and device engineering, which dampens the further material design, device optimization, and scalable production.Interface energetics take charge of many key electronic processes in organic electronic devices, such as charge injection or extraction at the electrode/ OS interface, charge generation or recombination at the D/A interface, and ambient optoelectronic stability resulted from the OS/air interface, all of which significantly affect the device performance. In this thesis, we systematically study the energy level alignment (ELA) at interfaces of inorganic/organic, organic/organic, and organic/air in OSCs, correlate the investigated energetic landscape with the device performance, and provide new understandings on the optoelectronic process in organic electronic devices.Firstly, we determine the pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) through ultraviolet photoelectron spectroscopy, to provide the critical characteristics of the material for ELA prediction at either electrode/OS or D/A interfaces. The ELA for inorganic/organic interfaces follows the predicted behavior based on integer charge transfer model, but for organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level (VL) shift. To explore the origin of the extra VL shift, we map the ELA at a range of D/NFA interfaces by fabricating and characterizing D-A bilayer heterojunctions monolayer-by-monolayer with the Langmuir-Schäfer technique. We find that the abrupt and significant VL shifts at the D-A interfaces are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of D-NFA systems. Furthermore, we investigate the influence of H2O and O2 molecules from ambient air on the work functions (WFs) of OS films. We find that OS films generally show higher WFs measured in ambient air, but lower WFs measured in high vacuum, compared to the WFs measured in ultrahigh vacuum. Two mechanisms are proposed to explain this phenomenon: (1) Competition between p-doping induced by O2 or H2O/O2 complexes, and n-doping induced by H2O clusters; (2) Polar H2O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. Finally, we fabricate the charge-transport-layer (CTL) free OSCs based on a newly developed NFA molecule with minimum performance degradation. Based on the determined D-A composition and ELA at the Anode/OS and the Cathode/OS interface, we propose several interface design rules for the efficient CTL-free devices, shedding new light on the simplified device structure for achieving more efficient optoelectronic applications.
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| 2. |
- Li, Xiane, et al.
(författare)
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Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells
- 2022
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Ingår i: Nature Communications. - : Nature Portfolio. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Energy level alignment (ELA) at donor-acceptor heterojunctions is of vital importance yet largely undetermined in organic solar cells. Here, authors determine the heterojunction ELA with (mono) layer-by-layer precision to understand the co-existence of efficient charge. Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems.
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| 3. |
- Li, Xiane, et al.
(författare)
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Pinning energies of organic semiconductors in high-efficiency organic solar cells
- 2023
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Ingår i: JOURNAL OF SEMICONDUCTORS. - : IOP Publishing Ltd. - 1674-4926 .- 2058-6140. ; 44:3
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Tidskriftsartikel (refereegranskat)abstract
- With the emergence of new materials for high-efficiency organic solar cells (OSCs), understanding and finetuning the interface energetics become increasingly important. Precise determination of the so-called pinning energies, one of the critical characteristics of the material to predict the energy level alignment (ELA) at either electrode/organic or organic/organic interfaces, are urgently needed for the new materials. Here, pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) are measured through ultraviolet photoelectron spectroscopy. The positive pinning energies of the studied donors and the negative pinning energies of NFAs are in the same energy range of 4.3-4.6 eV, which follows the design rules developed for fullerene-based OSCs. The ELA for metal/organic and inorganic/organic interfaces follows the predicted behavior for all of the materials studied. For organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level shift, which has consequences for OSC device performance.
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