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Rijal K, Fuller N, Rudayni F, Zhang N, Zuo X, Berrie CL, Yip HL, Chan WL. Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2024; 36:e2400578. [PMID: 38762779 DOI: 10.1002/adma.202400578] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/11/2024] [Revised: 04/22/2024] [Indexed: 05/20/2024]
Abstract
Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force.
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Affiliation(s)
- Kushal Rijal
- Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA
| | - Neno Fuller
- Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA
| | - Fatimah Rudayni
- Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA
- Department of Physics, Jazan University, Jazan, 45142, Saudi Arabia
| | - Nan Zhang
- Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, 999077, Hong Kong
| | - Xiaobing Zuo
- X-ray Science Division, Argonne National Laboratory, Lemont, IL, 60439, USA
| | - Cindy L Berrie
- Department of Chemistry, University of Kansas, Lawrence, KS, 66045, USA
| | - Hin-Lap Yip
- Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, 999077, Hong Kong
- School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, 999077, Hong Kong
- Hong Kong Institute for Clean Energy, City University of Hong Kong, Tat Chee Avenue, Kowloon, 999077, Hong Kong
- Center of Super-Diamond and Advanced Films, City University of Hong Kong, Tat Chee Avenue, Kowloon, 999077, Hong Kong
| | - Wai-Lun Chan
- Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA
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Jindal V, Aldahdooh MKR, Gomez ED, Janik MJ, Milner ST. Tight-binding model predicts exciton energetics and structure for photovoltaic molecules. Phys Chem Chem Phys 2024; 26:15472-15483. [PMID: 38751347 DOI: 10.1039/d4cp00554f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/30/2024]
Abstract
Conjugated molecules and polymers are being designed as acceptor and donor materials for organic photovoltaic (OPV) cells. OPV performance depends on generation of free charge carriers through dissociation of excitons, which are electron-hole pairs created when a photon is absorbed. Here, we develop a tight-binding model to describe excitons on homo-oligomers, alternating co-oligomers, and a non-fullerene acceptor - IDTBR. We parameterize our model using density functional theory (DFT) energies of neutral, anion, cation, and excited states of constituent moieties. A symmetric molecule like IDTBR has two ends where an exciton can sit; but the product wavefunction approximation for the exciton breaks symmetry. So, we introduce a tight-binding model with full correlation between electron and hole, which allows the exciton to coherently explore both ends of the molecule. Our approach predicts optical singlet excitation energies for oligomers of varying length as well as IDTBR in good agreement with time-dependent DFT and spectroscopic results.
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Affiliation(s)
- Vishal Jindal
- Department of Chemical Engineering, The Pennsylvania State University, USA.
| | | | - Enrique D Gomez
- Department of Chemical Engineering, The Pennsylvania State University, USA.
- Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
| | - Michael J Janik
- Department of Chemical Engineering, The Pennsylvania State University, USA.
| | - Scott T Milner
- Department of Chemical Engineering, The Pennsylvania State University, USA.
- Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
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Oh CM, Lee J, Park SH, Hwang IW. Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2021; 250:119227. [PMID: 33248892 DOI: 10.1016/j.saa.2020.119227] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/10/2020] [Revised: 11/03/2020] [Accepted: 11/10/2020] [Indexed: 06/12/2023]
Abstract
To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3-6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).
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Affiliation(s)
- Chang-Mok Oh
- Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea
| | - Jihoon Lee
- Department of Physics, Pukyong National University, Busan 48513, Republic of Korea
| | - Sung Heum Park
- Department of Physics, Pukyong National University, Busan 48513, Republic of Korea
| | - In-Wook Hwang
- Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea.
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Huang J, Mo Y, Yao Y. Charge-transfer state dynamics in all-polymer solar cells: formation, dissociation and decoherence. Phys Chem Chem Phys 2019; 21:2755-2763. [PMID: 30666324 DOI: 10.1039/c8cp06467a] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
All-polymer solar cells have made substantial achievements in recent years, offering numerous unsettled subjects for mechanical researchers. In order to quantitatively study the influence of the molecular electrostatic potential on the charge generation proposed by the experimenter, we simulate the ultrafast dynamics of the charge-transfer (CT) state at the interface between two polymer chains, which are respectively regarded as the donor and acceptor in all-polymer solar cells. The formation of a stable CT state is found to be sensitive to the distance between two oppositely charged polarons and the relevant critical electrostatic potential is thus quantified, which is in good agreement with experiments. In order to get insight into the dependence of the dissociation of the CT state on the width of the interfacial layer, two quantities are calculated: one is the Coulomb capture radius between the two polarons and the other is the quantum trace distance which serves as the fingerprint of the quantum coherence between them. The dissociation of the CT state is found to take place within an ultrafast timescale for an optimum interfacial width. The classical spatial distance and the quantum trace distance manifest a converging trend, suggesting a decoherence scenario for the charge separation in all-polymer solar cells.
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Affiliation(s)
- Jiaqing Huang
- Department of Physics and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China.
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