Visualizing excitations at buried heterojunctions in organic semiconductor blends

Jakowetz, Andreas C.; Böhm, Marcus L.; Sadhanala, Aditya; Huettner, Sven; Rao, Akshay; Friend, Richard H.

doi:10.1038/nmat4865

Cited by 110 publications

(119 citation statements)

References 42 publications

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“…Based on the existing data, we cannot differentiate between these two different mechanisms. The timescale of conversion from ICPs to free charges is estimated to be around 400 ps, which is in striking contrast to sub‐ps free carrier formation observed in most polymer–fullerene blends and some other NF materials . This highlights the conceptually different molecular mechanism of charge generation in high‐performance NF acceptors such as NFBDT and NCBDT.…”

Section: Methodsmentioning

confidence: 90%

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

et al. 2017

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Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

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Section: Methodsmentioning

confidence: 90%

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

et al. 2017

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“…The coexistence of neat and mixed donoracceptor domains is thought to be beneficial as it could provide an energy cascade [4,5] for charge separation, [6,7] followed by charge transport in the neat phases. Such interpenetrating donoracceptor mixtures form complicated multilength scale morphologies, often involving several phases such as ordered/ disordered donor, mixed donor-acceptor and ordered/disordered acceptor.…”

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confidence: 99%

Charge Transport in Pure and Mixed Phases in Organic Solar Cells

Melianas

Pranculis

Spoltore

et al. 2017

Advanced Energy Materials

116

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mixture of electron donating and accepting materials-a bulk heterojunction (BHJ). Such interpenetrating donoracceptor mixtures form complicated multilength scale morphologies, often involving several phases such as ordered/ disordered donor, mixed donor-acceptor and ordered/disordered acceptor. The coexistence of neat and mixed donoracceptor domains is thought to be beneficial as it could provide an energy cascade [4,5] for charge separation, [6,7] followed by charge transport in the neat phases. However, the neat regions may be discontinuous, forcing charges to cross mixed domains multiple times during extraction. If the mixed regions are severely disordered or contain less than the percolation threshold of required material, charge transport is thought to deteriorate significantly, limiting device performance. [8] The effect of donor-acceptor mixing (or phase purity) on charge transport has been previously addressed using microstructural characterization, [9,10] steady-state, [11][12][13] and/or timeresolved [14][15][16] mobility measurements on BHJs with a varying donor-acceptor stoichiometry and/or processing conditions. Although such studies have revealed important trends, they remain mostly semiquantitative-the question of how pure the neat domains have to be and how detrimental domain discontinuities or donor-acceptor mixing are in relation to charge transport kinetics remains to a large extent unanswered. Not knowing which important charge transport features to optimize, limits the development of next generations of organic optoelectronic devices.Here, we address this by measuring photo-generated charge motion from the first hopping events (with sub-picosecond time resolution) to full extraction in complete solar cell devices based on coevaporated bulk heterojunctions of α-sexithiophene (α-6T) and buckminsterfullerene (C 60 ). We carefully vary the molar fraction of α-6T in C 60 from homogeneously diluted (<10% molar), to a point where α-6T begins to form isolated aggregates (>10%-25% molar) or is strongly aggregated (50% molar). We thus vary the distance between isolated α-6T sites and the level of disruption of the C 60 phase in a controlled manner-the α-6T:C 60 system may be viewed as a model for the mixed donor-acceptor phase in OPV. C 60 was chosen as the model acceptor since its use in organic electronics is In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor-acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor-acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor-acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured i...

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“…Critical to photovoltaic operation is the donor/acceptor (D/A) interfaces, where photogenerated excitons, namely, Coulombically bounded electron‐hole pairs are dissociated and separated into free charges . Although it is still a challenge to directly probe the heterogeneous, nanoscale, disordered, and buried interfaces at the atomistic level by optical X‐ray and photoelectron techniques, there is a general consensus that tuning the D/A interfaces is essential to maximize exciton dissociation (ED) and minimize charge recombination (CR) for achieving high power conversion efficiencies (PCEs) …”

mentioning

confidence: 99%

Atomistic Insight Into Donor/Acceptor Interfaces in High‐Efficiency Nonfullerene Organic Solar Cells

Han

Guo

et al. 2018

Solar RRL

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Donor/acceptor (D/A) interfaces play a crucial role in photoelectric conversion for organic solar cells. However, it is impossible to experimentally probe D/A interfaces at the atomistic level to date, in particular for organic solar cells based on nonfullerene acceptors due to their anisotropic structures. In this work, we have investigated the interfacial structures of a representative D-A copolymer donor PBDB-T with a well-known A-D-A structured nonfullerene acceptor ITIC, in comparison with a fullerene acceptor PC 71 BM, by means of atomistic simulations. It is found that owing to different side-chain steric hindrance between the polymer A and D units, both acceptors are more likely to approach the polymer A units, and more apparently for ITIC in consideration of the size and shape matching between the acceptors and the polymer A and D units. Importantly, docking of ITIC with polymer occurs mainly through local π-π interaction between the terminal moieties of ITIC and the A units of the polymer, and such interfacial structures are favorable for efficient exciton dissociation. Our work sheds light on the impact of sidechain nature and location as well as acceptor structures on D/A interfaces and charge-transfer dynamics, which will be very helpful for further improving the performance of organic photovoltaics.

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Visualizing excitations at buried heterojunctions in organic semiconductor blends

Cited by 110 publications

References 42 publications

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Charge Transport in Pure and Mixed Phases in Organic Solar Cells

Atomistic Insight Into Donor/Acceptor Interfaces in High‐Efficiency Nonfullerene Organic Solar Cells

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