Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Li, Xian'e; Zhang, Qilun; Xu, Ye; Zhang, Rui; Wang, Chuanfei; Zhang, Huotian; Fabiano, Simone; Liu, Xianjie; Hou, Jianhui; Gao, Feng; Fahlman, Mats

doi:10.1038/s41467-022-29702-w

Cited by 59 publications

(52 citation statements)

References 41 publications

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“…The RSOXS measurement thus provides additional evidence of hole transport in Y6 and explains why PTB7‐Th:Y6 QHJ outperforms PM6:Y6 QHJ. As hinted at in the TA measurement, interfacial effects such as dipolar interactions, [ 52 ] will affect recombination kinetics. Band‐bending induced by quadrupolar fields [ 32 ] could possibly also enhance charge transport by physically/energetically separating electrons and holes within the Y6 phase itself.…”

Section: Resultsmentioning

confidence: 99%

Quasi‐Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

et al. 2022

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as printing fabrication, light weight, flexibility, low toxicity, and short energy payback time. The fused-ring electron acceptors (FREAs) pioneered by the Zhan group have broken through the bottleneck of fullerene acceptors, [1][2][3] and OSCs have achieved revolutionary breakthrough recently. [1][2][3][4][5][6] To date, power conversion efficiencies (PCEs) of FREA-based OSCs have reached 18-19%. [7][8][9][10] Among the diverse FREAs, Y6 (chemical structure shown in Figure S1, Supporting Information) and its derivatives have been widely studied recently due to their high photovoltaic performance. [11][12][13][14] Since most organic semiconductors have low dielectric constants (ε ≈ 3-4), [15] Frenkel excitons with high binding energies (E B ) rather than free charges are generated intrinsically upon photoexcitation. Donor (D)/acceptor (A) interfaces that can provide a driving force for exciton dissociation are essential. [16,17] According to the general consensus developed in fullerene-based OSCs, a bulk heterojunction (BHJ) with D/A phase separation size of around 10-20 nm was the optimal morphology for efficient exciton dissociation and charge transport. [18] Accordingly, most high-efficiency optimized OSCs have In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency.Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

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Section: Resultsmentioning

confidence: 99%

Quasi‐Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

et al. 2022

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“…It is striking that the EL emission intensity of the PHJ device is two orders of magnitude higher than that of the BHJ device (Figure S12c,d, Supporting Information), resulting in an EQE EL of 5.3 × 10 −3 % for the PHJ device (Figure S12e, Supporting Information). This further demonstrates that, in IT4F/PM6 heterojunctions (a representative small highest occupied molecular orbital (HOMO) offset system), [ 27 ] holes injected into the PM6 side can be easily transferred to the IT4F layer where they recombine with a relatively high‐radiative yield. This dominance of charge recombination within the IT4F layer of the PHJ devices is most likely the reason for the only modest increase in charge carrier lifetimes in the PHJ relative to the BHJ observed in our TPV analyses, despite the large reduction in interfacial surface area.…”

Section: Resultsmentioning

confidence: 89%

Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non‐Fullerene Acceptor

Lee

Dong

Pacalaj

et al. 2022

Adv Funct Materials

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While non‐fullerene acceptors (NFAs) have recently been demonstrated to exhibit long‐range exciton diffusion, most organic photovoltaic and photodetector studies still focus on blended polymer: NFA systems. Herein, a 40 nm exciton diffusion length for IT4F excitons is determined, and it is demonstrated that sharp interface, planar heterojunction (PHJ) IT4F/PM6 devices with the IT4F layer thickness matched to this diffusion length yield optimized photovoltaic and photodetector performance. The PHJ devices yield an enhanced device open‐circuit voltage relative to bulk heterojunction (BHJ) devices, associated with suppressed bimolecular recombination losses. The PHJ architecture also results in a ≈100‐fold increase in electroluminescence (EL) quantum efficiency relative to the BHJ device, correlated with a shift from charge transfer state EL for the BHJ to IT4F exciton dominated EL for the PHJ, attributed to significant hole injection from PM6 into IT4F. Of particular note, the PHJ architecture is shown to suppress dark leakage current, resulting in 83 times higher photodetector detectivity at −2 V bias than the equivalent BHJ device.

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“…36 The reason for this is not clear yet, but we emphasize here that the ionization energy from UPS is very sensitive to the electrostatics at the surface, which will most likely be different in neat and blend films. 91 Therefore, we point out that experiments on neat films have to be taken with caution when they are used to explain the energetics of blend films. To actually access the HOMO energies of the individual components in blend films, we subtract the (scaled) reference valence band spectrum of neat PM6 from the blend spectrum (see Fig.…”

Section: Comparison Of Energy Levels Homo Offsets and Transport Gapsmentioning

confidence: 98%

Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance

et al. 2022

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Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Cited by 59 publications

References 41 publications

Quasi‐Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

Quasi‐Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non‐Fullerene Acceptor

Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance

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