Charge Photogeneration in Non‐Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

Saladina, Maria; Marqués, Pablo Simón; Markina, A.; Karuthedath, Safakath; Wöpke, Christopher; Göhler, Clemens; Chen, Yue; Allain, Magali; Blanchard, Philippe; Cabanetos, Clément; Andrienko, Denis; Laquai, Frédéric; Gorenflot, Julien; Deibel, Carsten

doi:10.1002/adfm.202007479

Cited by 41 publications

(36 citation statements)

References 53 publications

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“…One can see that for B < 0.5 eV dissociation energies are too high and the CT state dissociation cannot be activated thermally. [47] We can therefore conclude that bias potentials on the order of 0.5 eV are required to dissociate CT state, in line with the results reported for one of the best performing NFA, Y6. [38]…”

Section: Dissociation Of the Charge Transfer Statesupporting

confidence: 87%

Chemical Design Rules for Non‐Fullerene Acceptors in Organic Solar Cells

Markina

Lin

Liu

et al. 2021

Advanced Energy Materials

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while fullerene-based OSCs are only 10% efficient. [13] To achieve this milestone, various design strategies have been explored, for example, modification/manipulation of the Y6 acceptor side chain design, [7] the use of ternary mixtures with a vertical phase distribution, [9] the chemical modification via chlorination [8] or a variation of a fused-ring acceptor block of the donor polymer. [10] Currently, NFAs match their inorganic counterparts in terms of current generation, but are lacking with regard to their open-circuit voltage. [14] Efficiency losses can be traced back to energy losses during the photon to free charge conversion, and are in generally lower than in the fullerene-based cells. [15][16][17] Free charge generation in organic solar cells is comprised of two steps. During the first step, a photogenerated exciton dissociates at the donor-acceptor interface into an interfacial charge transfer (CT) state. During this process, the ionization energy or electron affinity offset at the heterojunction provides the driving force for the hole or electron transfer. It is known that this offset should exceed a threshold value in order to enable efficient dissociation of the excited state. [18][19][20] For NFAs, only ionization energy offsets are relevant, because of the fast energy transfer from donors to acceptors. [20] During the second step of charge separation, the interfacial CT state dissociates into a pair of free charges, or the charge separated (CS) state. This dissociation is expected to be an endothermic process, and the exact mechanism behind the driving force for this process is still under debate. [21][22][23][24][25][26] It is, however, one of the key processes in OSCs, since the energetics and dynamics of the dissociating CT state determines the open circuit voltage of organic heterojunctions. [25,[27][28][29] Both steps involved in the free charge generation can be optimized by an appropriate design of the donor-acceptor pair. The main difficulty in formulating generic chemical design rules for OSC materials is that any changes to the chemical structure simultaneously modify the open-circuit voltage, V oc , the short-circuit current, J sc , and the fill factor of the solar cell. [30][31][32][33][34][35] Without knowing how these changes correlate with each other, it is impossible to formulate clear design rules and hence speed up the discovery of efficient donor-acceptor combinations.In this work, we identify the microscopic origin of such correlations and propose clear chemical design rules for NFAs. Efficiencies of organic solar cells have practicallydoubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. It is shown that an acceptor-donor-acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state...

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Section: Dissociation Of the Charge Transfer Statesupporting

confidence: 87%

Chemical Design Rules for Non‐Fullerene Acceptors in Organic Solar Cells

Markina

Lin

Liu

et al. 2021

Advanced Energy Materials

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“…A phenomenological fit with 6), combined with the electrostatic interfacial bias, B(d) = B exp (−[(d + d 0 )/w] 3 ), is shown for w = 2 nm, d 0 = 1 nm and a range of bias potentials. One can see that for B < 0.5 eV dissociation energies are too high and the CT state dissociation cannot be activated thermally [47]. We can therefore conclude that bias potentials on the order of 0.5 eV are required to dissociate CT state, in line with the results reported for one of the best performing NFA, Y6 [38].…”

Section: Dissociation Of the Charge Transfer Statesupporting

confidence: 84%

Chemical design rules for non-fullerene acceptors in organic solar cells

Markina,

Lin,

Liu

et al. 2022

Preprint

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Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. We propose such rules by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. We show that an acceptor-donor-acceptor molecular architecture, and molecular alignment parallel to the interface, result in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, we suggest that the molecular quadrupole moment of ca 75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency.

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“…It has been suggested that the 2D assembly of 6TIC has a distinctive advantage of acquiring large intramolecular polarization volume (optical frequency dielectric functions). [ 31 ] Moreover, the intermolecular polarization volume can be directly correlated to the degree and structure of the molecular packing, [ 32 ] which can be greatly enhanced for the NFAs in highly ordered states exhibiting large orbital overlap among molecules in excited states. Thus, these molecular and crystalline characteristics can be reasonably correlated to the spontaneous charge generation observed in this research.…”

Section: Resultsmentioning

confidence: 99%

The Molecular Ordering and Double‐Channel Carrier Generation of Nonfullerene Photovoltaics within Multi‐Length‐Scale Morphology

Chen

et al. 2022

Advanced Materials

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and low energy loss that deliver high opencircuit voltage (V OC ) and short-circuit current (J SC ) in solar cell devices. [1,2] An ideal BHJ thin film requires a bicontinuous network morphology of tens of nanometers of domain size to ensure efficient exciton splitting and charge transport, [3] leading to an improved fill factor (FF). The crystallization of NFA molecule and conjugated polymer in mixture plays an important role in determining the thin-film morphology, [4,5] where the π-π stacking is considered as the primary driving force. [6] This process results in crystallites buried in donor-acceptor mixture, [7] which significantly influences the optoelectronic process in OPV.NFA molecules crystallize through backbone and side-chain interactions. The classic fused ring NFAs arouse great interest currently, [8] whose single crystal is taken as a platform to investigate the specific intermolecular interactions that govern the crystallization. NFA molecules are much larger compared with conventional organic semiconductors such as pentacene or its soluble analogs, which leads toThe success of nonfullerene acceptor (NFA) solar cells lies in their unique physical properties beyond the extended absorption and suitable energy levels. The current study investigates the morphology and photophysical behavior of PBDB-T donor blending with ITIC, 4TIC, and 6TIC acceptors. Single-crystal study shows that the π-π stacking and side-chain interaction dictate molecular assembly, which can be carried to blended films, forming a multi-length-scale morphology. Spontaneous carrier generation is seen in ITIC, 4TIC, and 6TIC neat films and their blended thin films using the PBDB-T donor, providing a new avenue of zero-energy-loss carrier formation. The molecular packing associated with specific contacts and geometry is key in influencing the photophysics, as demonstrated by the charge transfer and carrier lifetime results. The 2D layer of 6TIC facilitates the exciton-to-polaron conversion, and the largest photogenerated polaron yield is obtained. The new mechanism, together with the highly efficient blending region carrier generation, has the prospect of the fundamental advantage for NFA solar cells, from molecular assembly to thin-film morphology.

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Charge Photogeneration in Non‐Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

Cited by 41 publications

References 53 publications

Chemical Design Rules for Non‐Fullerene Acceptors in Organic Solar Cells

Chemical Design Rules for Non‐Fullerene Acceptors in Organic Solar Cells

Chemical design rules for non-fullerene acceptors in organic solar cells

The Molecular Ordering and Double‐Channel Carrier Generation of Nonfullerene Photovoltaics within Multi‐Length‐Scale Morphology

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