Charge-transfer electronic states in organic solar cells

Coropceanu, Veaceslav; Chen, Xiankai; Wang, Tonghui; Zheng, Zilong; Brédas, Jean Luc

doi:10.1038/s41578-019-0137-9

Cited by 249 publications

(277 citation statements)

References 199 publications

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“…As was shown recently, σ T arises from contributions related to both dynamic disorder and static disorder. [ 59–62 ] The dynamic disorder is associated with electron‐vibration interactions that results in a time‐dependent variation in D 1 ‐state energies with a standard deviation σ D ; the static disorder is related to the amorphous and glassy nature of the TTM‐3NCz/CBP film, which leads to a time‐independent variation in the D 1 ‐state energy with a standard deviation σ S . In the case of Gaussian distributions, the total standard deviation σ T can be expressed as

σ_{T} = \sqrt{σ_{D}^{2} + σ_{S}^{2}}

.…”

Section: Resultsmentioning

confidence: 99%

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

Abroshan

Coropceanu

Brédas

2020

Adv Funct Materials

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Radical‐carrying organic molecules have received significant attention to bypass the issue related to harvesting triplet excitons in current light‐emitting materials. While the computational efforts conducted so far have treated these radical emitters as isolated entities, in actual devices, they are embedded in a host matrix and subject to emitter–host interactions. Here, by combining molecular dynamics simulations and density functional theory calculations, the impact of the host matrix on the optoelectronic performance of radical emitters is evaluated, taking as a representative example the (4‐ncarbazolyl‐2,6‐dichlorophenyl)bis(2,4,6‐trichlorophenyl)‐methyl (TTM‐3NCz) radical emitter dispersed in a 4,4‐bis(carbazol‐9‐yl)biphenyl (CBP) host. A morphological analysis shows that steric effects around the radical centers, carried by the TTM electron‐poor moieties of the emitters, disfavor π–π interactions with the host molecules, which leads to random intermolecular orientations around the TTM moieties. The 3NCz electron‐rich moieties of the emitters, however, have much lesser spatial hindrance for intermolecular π–π stacking, which modulates the structural and electronic properties of the emitters in the host matrix. The influence of dynamic and static disorders on the radiative and nonradiative recombination processes is also investigated and it is found that the rates of nonradiative recombination are small, which opens the way to 100% internal quantum efficiency for the doublet‐based emission process.

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σ_{T} = \sqrt{σ_{D}^{2} + σ_{S}^{2}}

.…”

Section: Resultsmentioning

confidence: 99%

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

Abroshan

Coropceanu

Brédas

2020

Adv Funct Materials

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“…[ 13–19 ] It is interesting to note that efficient NFAs could exhibit relatively low energy loss and fast charge separation under small driving forces, compared with fullerene‐based acceptors. [ 20–22 ] Although some progresses have been made, energy loss in OPVs based on NFAs is still obviously large, compared to Si and perovskite counterparts. [ 23 ] A common way to lower the energy loss of OPV is to narrow the energy level offsets between p‐type donor and n‐type acceptor in BHJ, but this inevitably affects the charge separation efficiency in BHJ.…”

Section: Figurementioning

confidence: 99%

Asymmetric Electron Acceptors for High‐Efficiency and Low‐Energy‐Loss Organic Photovoltaics

et al. 2020

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Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP‐S1 and BTP‐S2, are constructed by introducing halogenated indandione (A1) and 3‐dicyanomethylene‐1‐indanone (A2) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well‐known NFA Y6, but at different terminals. Such asymmetric NFAs with A1‐D‐A2 structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP‐S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 × 10−2%, one order of magnitude higher than devices based on symmetric Y6 (4.4 × 10−3%), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP‐S1 and BTP‐S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP‐S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6‐based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP‐S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single‐junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy.

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“…[164,165] The low external quantum efficiency of electroluminescence (EQE EL ) of donor and acceptor BHJ solar cells is assigned to the large nonradiative CT state decay, primarily responsible for the open-circuit voltage losses of these devices. [15,[166][167][168][169][170][171][172] A complete evaluation of the open-circuit voltage losses in OPV considers both radiative and nonradiative recombination losses as well as the conversion of excitons in the pristine BHJ components into delocalized CT states. Efficient conversion from the CT state to free charges can be related to a minimum Gibbs free energy offset between the CT and the local excitation (LE) (ΔG LE-CT ) of D or A, which has been empirically calculated to be around 0.3 eV.…”

Section: Ct State and Nonradiative Voltage Lossesmentioning

confidence: 99%

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

et al. 2020

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Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction‐based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar‐cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

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Charge-transfer electronic states in organic solar cells

Cited by 249 publications

References 199 publications

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

Asymmetric Electron Acceptors for High‐Efficiency and Low‐Energy‐Loss Organic Photovoltaics

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

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