Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C<sub>60</sub> Organic Solar Cell Blends

Moore, Gareth John; Causa, Martina; Hardigree, Josué F. Martínez; Karuthedath, Safakath; Ramírez, Iván; Jungbluth, Anna; Laquai, Frédéric; Riede, Moritz; Banerji, Natalie

doi:10.1021/acs.jpclett.0c01495

Cited by 20 publications

(19 citation statements)

References 55 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Hence, the solar sell efficiency is primarily dependent on the hole transfer from the excited acceptor to the CT state. [45] The driving force for the hole transfer, or A* → CT reaction, has three distinct contributions…”

Section: Formation Of the Charge Transfer Statementioning

confidence: 99%

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

Markina

Lin

Liu

et al. 2021

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

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...

show abstract

Section: Formation Of the Charge Transfer Statementioning

confidence: 99%

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

Markina

Lin

Liu

et al. 2021

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

show abstract

“…Critically, this high charge mobility may enable charges to be extracted before triplets can have a significant impact in device performance. [82]…”

Section: Effects Of Oxygenmentioning

confidence: 99%

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

Marín-Beloqui

Toolan

Panjwani

et al. 2021

Advanced Energy Materials

View full text Add to dashboard Cite

understood. Triplet behavior in organic solar cells is one of the most understudied aspects. The reason of the relatively low number of studies is that triplet formation is typically considered a loss pathway, and thus associated with systems with low device efficiencies. Indeed, many previous studies indicated that triplets were formed only when charge separation did not occur or was very inefficient. [3][4][5][6] As such, triplets were considered irrelevant for the highest efficiency OPV devices. However, it has recently been demonstrated that OPV blends with non-fullerene acceptors (NFAs), the main driver of the current record OPV efficiencies, can show pronounced triplet formation and yet still provide high efficiencies. [7] In the benchmark PM6:Y6 blend, for example, NFA triplet exciton formation accounts for 90% of charge recombination at open circuit, leading to a 60 mV reduction of the open circuit voltage. As such, it is important to elucidate triplet pathways in organic solar cells, in particular how they affect charge photogeneration and recombination. Triplets can be generated via several different pathways in organic solar cells. [8,9] First, the photogenerated singlet exciton can undergo intersystem crossing (ISC). Second, triplets can form via charge transfer (CT) states at the donor/acceptor interface. These CT states can undergo rapid spin-mixing due to the low exchange energy between 3 CT and 1 CT states, often Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non-fullerene acceptors. Specifically, grazing incidence wide-angle X-ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet-charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet-charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non-fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA.

show abstract

“…The donor-to-acceptor energy transfer is long-range and is faster than the electron transfer which relies on many charge transfer reactions. Hence, the solar sell efficiency is primarily dependent on the hole transfer from the excited acceptor to the CT state [45].…”

Section: Formation Of the Charge Transfer Statementioning

confidence: 99%

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

Markina,

Lin,

Liu

et al. 2022

Preprint

View full text Add to dashboard Cite

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.

show abstract

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C₆₀ Organic Solar Cell Blends

Cited by 20 publications

References 55 publications

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

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

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

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

Contact Info

Product

Resources

About

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends

Cited by 20 publications

References 55 publications

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

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

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

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

Contact Info

Product

Resources

About

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C₆₀ Organic Solar Cell Blends