Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

Wu, Jiaying; Lee, Jinho; Chin, Yi‐Chun; Yao, Huifeng; Cha, Hyojung; Luke, Joel; Hou, Jianhui; Kim, Jinhyun; Durrant, James R.

doi:10.1039/d0ee01338b

Cited by 196 publications

(238 citation statements)

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“…Fitting the kinetics gives a bimolecular rate constant on the order of 10 −11 cm 3 s −1 ( Supplementary Fig. 6b), which is of similar magnitude or slower than that observed in typical bulk heterojunction OSCs [31][32][33] . This fluence dependence therefore suggests that this PIA at 780 nm originates from free charge carriers rather than being caused by the formation of CT or triplet states 30,[34][35][36] .…”

Section: Resultssupporting

confidence: 52%

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

et al. 2020

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Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.

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Section: Resultssupporting

confidence: 52%

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

et al. 2020

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“…These values are in agreement with recent publication by Durrant and co-workers measuring low charge trapping (in thin junctions). [49] We note that we have previously reported the higher values of σ EA ¼ 71 meV and σ IE ¼ 83 meV. [9] The lower energetic disorder values reported here most likely arise from a higher purity and less defects in PM6 and Y6 batches we have used in this work, which may be a possible reason for higher device performance than other similar works.…”

Section: Resultssupporting

confidence: 48%

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

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Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non‐fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (≈400 nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5% CN compared with 0.5% CN. This correlates with improved electron and hole mobilities and a 50% suppressed bimolecular recombination, such that the non‐Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA‐based OSCs.

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“…For example, the in‐situ accelerated photoirradiation study of the Y6, ITIC, and O‐IDTBR materials under N 2 exhibited that both IDTBR and ITIC showed significant changes in their molecular vibrational spectra, while no major changes in the vibrational spectra of Y6 indicating its superior photochemical stability. [ 221 ] The global potential energy minimum for Y6 molecule from DFT calculation appears only at dihedral angle of 0°, strongly favorable with respect to the local minimum at 180°, which is higher in energy by 0.4 eV ( Figure 16 ). Conversely, for ITIC and IDTBR the potential energy for 0° and 180° configurations are equivalent.…”

Section: Toward Superior Stability Of Nfa‐based Oscsmentioning

confidence: 99%

“…Even if the record‐performing NFA of Y6 family still do not provide exceptional operational stability in devices, [ 260 ] recent evidences demonstrate its highly rigid structure controlled by outer core alkyl chains conformational locking. [ 221 ] This suggests an outlook of further tuning of the A–D–A–D–A molecular structure, for a thorough understanding of the role of side chains not only in boosting efficiency, [ 41 ] but also in ensuring long device lifetime. Since the alkene linkage between the donor–acceptor building blocks present in the most relevant NFA structures (IDTBR, ITIC, Y6) has been pointed out as the starting point of chemical degradation, [ 176 ] an important challenge for molecular engineering is the development of new synthetic strategies for conjugated NFA structures possibly free from this weak linkage or with powerful noncovalent bonds to stabilize it.…”

Section: Key Challengesmentioning

confidence: 99%

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Recent Progress and Challenges toward Highly Stable Nonfullerene Acceptor‐Based Organic Solar Cells

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Advanced Energy Materials

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Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

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Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

Abstract:
In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly...

Cited by 196 publications

References 69 publications

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

Recent Progress and Challenges toward Highly Stable Nonfullerene Acceptor‐Based Organic Solar Cells

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Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

Abstract: In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly...

Cited by 196 publications

References 69 publications

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

Recent Progress and Challenges toward Highly Stable Nonfullerene Acceptor‐Based Organic Solar Cells

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Abstract:
In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly...