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Alqahtani O, Hosseini SM, Ferron T, Murcia V, McAfee T, Vixie K, Huang F, Armin A, Shoaee S, Collins BA. Evidence That Sharp Interfaces Suppress Recombination in Thick Organic Solar Cells. ACS Appl Mater Interfaces 2021; 13:56394-56403. [PMID: 34787408 DOI: 10.1021/acsami.1c15570] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Commercialization and scale-up of organic solar cells (OSCs) using industrial solution printing require maintaining maximum performance at active-layer thicknesses >400 nm─a characteristic still not generally achieved in non-fullerene acceptor OSCs. NT812/PC71BM is a rare system, whose performance increases up to these thicknesses due to highly suppressed charge recombination relative to the classic Langevin model. The suppression in this system, however, uniquely depends on device processing, pointing toward the role of nanomorphology. We investigate the morphological origins of this suppressed recombination by combining results from a suite of X-ray techniques. We are surprised to find that while all investigated devices are composed of pure, similarly aggregated nanodomains, Langevin reduction factors can still be tuned from ∼2 to >1000. This indicates that pure aggregated phases are insufficient for non-Langevin (reduced) recombination. Instead, we find that large well-ordered conduits and, in particular, sharp interfaces between domains appear to help to keep opposite charges separated and percolation pathways clear for enhanced charge collection in thick active layers. To our knowledge, this is the first quantitative study to isolate the donor/acceptor interfacial width correlated with non-Langevin charge recombination. This new structure-property relationship will be key to successful commercialization of printed OSCs at scale.
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Affiliation(s)
- Obaid Alqahtani
- Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States
- Department of Physics, Prince Sattam Bin Abdulaziz University, Alkharj 11942, KSA
| | - Seyed Mehrdad Hosseini
- Optoelectronics of Organic Semiconductors Institute, University of Potsdam, Potsdam-Golm 14476, Germany
| | - Thomas Ferron
- Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States
| | - Victor Murcia
- Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States
| | - Terry McAfee
- Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | - Kevin Vixie
- Department of Mathematics, Washington State University, Pullman, Washington 99164, United States
| | - Fei Huang
- Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China
| | - Ardalan Armin
- Department of Physics, Swansea University, Singleton Park, Swansea, Wales SA2 8PP, U.K
| | - Safa Shoaee
- Optoelectronics of Organic Semiconductors Institute, University of Potsdam, Potsdam-Golm 14476, Germany
| | - Brian A Collins
- Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States
- Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States
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Hammer S, Zeiser C, Deutsch M, Engels B, Broch K, Pflaum J. Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices. ACS Appl Mater Interfaces 2020; 12:53547-53556. [PMID: 33167608 DOI: 10.1021/acsami.0c17152] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.
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Affiliation(s)
- Sebastian Hammer
- Experimental Physics VI, Julius Maximilians University Würzburg, Am Hubland, 97074 Würzburg, Germany
| | - Clemens Zeiser
- Institute for Applied Physics, Eberhard Karls University Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
| | - Marian Deutsch
- Institute for Physical and Theoretical Chemistry, Julius Maximilians University Würzburg, Emil-Fischer-Straße 42, 97074 Würzburg, Germany
| | - Bernd Engels
- Institute for Physical and Theoretical Chemistry, Julius Maximilians University Würzburg, Emil-Fischer-Straße 42, 97074 Würzburg, Germany
| | - Katharina Broch
- Institute for Applied Physics, Eberhard Karls University Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
| | - Jens Pflaum
- Experimental Physics VI, Julius Maximilians University Würzburg, Am Hubland, 97074 Würzburg, Germany
- Bavarian Center for Applied Energy Research, Magdalene-Schoch-Straße 3, 97074 Würzburg, Germany
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Abstract
Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small Δ EST (0-0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small Δ EST in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field.
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Affiliation(s)
- Monima Sarma
- Department of Chemistry , National Taiwan University , Taipei 10617 , Taiwan
| | - Ken-Tsung Wong
- Department of Chemistry , National Taiwan University , Taipei 10617 , Taiwan
- Institute of Atomic and Molecular Science , Academia Sinica , Taipei 10617 , Taiwan
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