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

Abstract

We have utilized a plethora of transient and steady state optical and magneto-optical spectroscopies in a broad spectral range (0.25-2.5 eV) for elucidating the primary and long-lived photoexcitations in a low band-gap π-conjugated donor-acceptor (DA-) copolymer used for efficient photovoltaic solar cells. We show that both singlet excitons (SE) and intrachain triplettriplet (TT) pairs are photogenerated in the DA-copolymer chains. From the picosecond transient magnetic field response of these species we conclude that the SE and TT spin states are coupled. The TT decomposition into two intrachain geminate triplet excitons maintains spin coherence and thus their spin-entanglement lasts into the microsecond time domain. * Correspondence to: val@physics.utah.edu 2The field of 'photoexcitations in π-conjugated polymers' has been debated since as early as 1980 with trans polyacetylene. 1 The debate heated up when the nature of the primary photoexcitations, namely free carriers vs. excitons was considered. 2 This debate took a new twist when the exciton dissociation in polymer/fullerene blends was discussed, 3,4 since this process has bearing on potential applications in organic photovoltaic solar cells. 5Recently the organic solar cell quantum efficiency has dramatically increased when the low band-gap (E g~1 .4 eV) π-conjugated donor-acceptor (DA-) copolymers were introduced. 6,7 However the nature of the primary photoexcitations in these materials has not been well characterized, probably because of limited available ultrafast spectroscopies in the mid-IR spectral range. 8The DA-copolymer chains contain, by definition, two different organic moieties with different electron affinities (see Fig. 1(a) inset) that play the role of electron donor (D) and electron acceptor (A). [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. 9-12 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. If this is true also in DA-copolymers, then the TE energy, E T (=E g -Δ ST =1.4 to 1.6-0.7) is ~0.7-0.9 eV. This would lead to 'resonant condition' between the lowest SE (at E S =E g ≈1.4-1.6 eV) and the intrachain triplet-triplet (TT)-pair state at energy 2E T ≈1.4-1.8eV. This may have strong influence on the photophysics of the DA-copolymers, since SE-TT coupling may occur. For investigating a possible SE-TT resonant interaction in DA-copolymers having E s ≈2E T , we have used, for the first time, the transient magneto photoinduced absorption (t-MPA) technique in the time domain from 0.2 picosecond (ps) to milliseconds, which is the magnetic field effect (M-) of the transient photo-induced absorption (t-PA) spectrum. We indeed measured spin-entangl...

Optical studies of native defects in π-conjugated donor–acceptor copolymers

“…In this case, the first zero crossing energy at 2.08 eV is therefore assigned to the average singlet exciton energy, E SE of the copolymer. 39,40 The second EA feature is a derivative-like of the phonon replica 41 (0-1 transition) $180 meV higher than E SE . The third EA feature is an induced absorption band at 2.64 eV, which does not have any relation with the absorption derivative spectrum.…”

“…We therefore assign it as due to the forbidden exciton, m 1 A g state in the singlet manifold that becomes partially allowed due to the symmetry breaking caused by the applied electric field in the PBnDT-FTAZ film. 39,41 The energy difference, D SE ¼ E mAg À E SE ¼ 0.56 eV, is traditionally taken to be the lower bound of the exciton binding energy, E b in p-conjugated polymers; 41 and we adopt this interpretation here for PBnDT-FTAZ. We note that E b of $0.56 eV is substantially lower than E b values in traditional p-conjugated polymers such as Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]…”

Electronic and vibrational spectroscopy studies of PffBT4T π-conjugated donor–acceptor copolymer

“…We, therefore, assign it, due to the forbidden exciton, m 1 A g state in the singlet manifold that becomes partially allowed due to the symmetry breaking caused by the applied electric field in the PffBT4T film. 30,37 The energy difference, Δ SE ¼ E mAg − E SE ¼ 0.5 eV, is traditionally taken to be the lower bound of the exciton binding energy, E b , in π-conjugated polymers; 37 and we adopt this interpretation here for PffBT4T. We note that E b of ∼0.5 eV is much lower than E b values in traditional polymers, such as MEH-PPV and P3HT (∼0.9 eV).…”

“…2(b)]; it is therefore caused by Stark shift of the lowest SE with a zero-crossing energy at 1.75 eV, which is therefore assigned to E SE of the copolymer. 30,36 (ii) The second EA feature is a derivative-like of the phonon replica 37 [0 to 1 transition in Fig. 2(b), ∼180 meV higher than E SE ].…”

“…22 This is most likely because of a possible resonance between the energies of the TT state and charge-transfer exciton in this copolymer. 30 To characterize the strongly coupled vibrational modes of the copolymer that influence the polaronic excitations, we measured the RRS and PA spectra in films of pristine PffBT4T and PffBT4T/PCBM blends, respectively. We show that the main charge excitation in the PffBT4T chains is the polaron having two characteristic absorption bands below the gap.…”