Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

Abstract

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.PACS numbers: 78.66. Qn, 71.20.Rv, 71.35.Cc,The primary photophysical process in polymer solar cells is photoinduced charge transfer, whereby optical excitation at the junction between a donor conjugated polymer and acceptor molecules creates a charge transfer (CT) exciton whose dissociation leads to charge carriers. The donor polymeric materials used to be homopolymers such as polythiophene which absorb in the visible range of the solar spectrum [1]. Homopolymers have recently been replaced by block copolymers whose repeat units consist of alternating donor (D) and acceptor (A) moieties [2][3][4][5][6][7][8][9][10][11]. This architecture reduces the optical gap drastically, and the DA copolymers absorb in the near infrared, where the largest fraction of the photons emitted by the Sun lie. The power conversion efficiencies (PCEs) of organic solar cells with DA copolymers as donor materials have exceeded 10% [11], and there is strong interest in the development of structure-property correlations that will facilitate further enhancement of the PCE. Clearly, this requires precise understanding of the nature of the primary photoexcitations of DA copolymers.Existing electronic structure calculations of DA copolymers are primarily based on the density functional theory (DFT) approach or its time-dependent version (TD-DFT) [12][13][14][15][16][17][18]. The motivations behind these calculations have largely been to understand the localized versus delocalized character of the excited state reached by ground state absorption. Experimentally, DA copolymers exhibit a broad low energy (LE) absorption band at ∼ 700 − 800 nm and a higher energy (HE) absorption band at ∼ 400 − 450 nm [2][3][4]. There is agreement between the computational studies that the LE band is due to CT from D to A, and the HE band is a higher π-π * excitation.Recent optical studies indicate that the above simple characterization of the LE band might be incomplete, and as in the homopolymers [19], electron correlations play a stronger role in the photophysics of the DA copolymers than envisaged within DFT approaches. Grancini et al. determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is c...

Exact wave packet dynamics of singlet fission in unsubstituted and substituted polyene chains within long-range interacting models

“…Thus we find that even in substituted polyenes, if the initial state is an S 1 state rather than other higher energy singlet state, SF is efficient. This agrees with some recent studies which suggest substitution by heteroatoms within organic chromophore 16,39,40 or copolymerising donor-acceptor moieties [41][42][43] can play an important role in tailoring candidate molecules for SF. The mixing of different eigenstates on donor-acceptor substitution, although well-known in π-conjugated carbon systems, its significance in singlet fission has not been explored.…”

Transient Magnetophotoinduced Absorption Studies of Photoexcitations in π -Conjugated Donor-Acceptor Copolymers

“…22 Consequently, from E(m 3 A g ) and PA T transition energy in the triplet manifold we can determine the energy of the lowest TE, E(1 1 B u )≈2E T , which calls for a possible interaction between the lowest SE and TT states in this copolymer. 12 This interaction may lead to the occurrence of a direct optical transition from the copolymer ground state into the TT manifold, as predicted theoretically.…”

“…1(a) inset); 14 but other DA-copolymers with different D-A moieties have similar photoexcitations. 12,15 As is clearly seen in Fig.1(a) the PDTP-DFBT backbone structure does not possess inversion symmetry in contrast to the more traditional π-conjugated polymers; this has important implications on the photoexcitation species in these materials. For example, the SE is not strictly an odd-parity state, whereas the TT is not strictly a covalent state; this effect may provide the basis of a possible interaction between these two states.…”

“…[6][7][8][9][10] This intrachain DA character leads to lower E g than that in more traditional polymers, 11 and therefore can absorb more photons from the solar spectrum that consequently increases the solar cell effciency. [9][10][11][12] However the small E g (1.4-1.6 eV) in the most efficient DAcopolymers may lead to another phenomenon that has not been explored yet. It is known that the energy difference, Δ ST between the singlet exciton (SE) and triplet exciton (TE) energies in polymers 13 is about 0.7 eV.…”

Singlet Fission: Progress and Prospects in Solar Cells

“…The intramolecular SF (iSF) process is inherently less sensitive to intermolecular interactions. While the mechanism of SF is the subject of intense study, it is generally agreed that photoexcitation generates a triplet pair 1 (TT) state either coherently or incoherently from the singlet exciton, and this state may subsequently separate into two triplets (2 × T 1 ) in an overall spin‐allowed process . The most recent mechanism studies have shown that the bound triplet pair 1 (TT) state undergoes a spatially separated triplet pair intermediate 1 (T···T) before it decouples into two independent triplets 2 × T 1 (see Figure ) .…”