Charge Photogeneration in Donor–Acceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

Abstract

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

Enhanced Photogeneration of Polaron Pairs in Neat Semicrystalline Donor–Acceptor Copolymer Films via Direct Excitation of Interchain Aggregates

“…This rather short lifetime of the SE is in agreement with previous reports on PCPDTBT 11,36 and is characteristic of low-band-gap donor−acceptor polymers. 41 Figure 2c shows the ΔT/T spectrum measured on the same film at 300 fs delay, this time selectively exciting the π-aggregates by tuning the pump pulse to 840 nm.…”

Random Terpolymer Designed with Tunable Fluorescence Lifetime for Efficient Organic/Inorganic Hybrid Solar Cells

“…is included in almost all D-A terpolymers, 46 and the electric field plays a less important role in the light-absorbing material, the neutral exciton lifetime of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 [ ] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 decay can be obtained by following expression: 2 2 1 1 2 2 1 1 2 2 a a a a τ τ τ τ τ…”

Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO + 1 in Organic Photovoltaics

“…[1][2][3][4][5][6][7][8] With the rapidly growing interests in OPVs in both scientific and industrial communities, theoretical modeling of the electronic structure of excitons and their accompanying dynamics has been widely implemented and now plays an important role in interpreting the elementary processes or designing new materials. [5][6][7][8][9][10][11][12][13][14][15][16][17] Due to the large size of the system, the theoretical investigations are mainly based on simplified models with the common assumption that only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of each molecule participate in the exciton dynamics within the donor (D)/acceptor (A) heterojunction, i.e.…”