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Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes

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Science  31 Jan 2014:
Vol. 343, Issue 6170, pp. 512-516
DOI: 10.1126/science.1246249

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  1. Fig. 1 Schematics of interfacial photophysical processes in OPVs (and molecules studied).

    (A) Overview of charge photogeneration at a heterojunction. Light absorption generates excitons in the bulk (1a) and at interfaces (1b). When next to an interface, excitons undergo rapid charge transfer into an electron-hole pair (2), generating a dipole-like electric field in its surroundings (Embedded Image). The electron and hole then separate further and form free charges (3). (B) Stark shift of the absorption spectrum (S) due to an electric field Embedded Image (dark blue) and the resulting electro-absorption signature (red) calculated from the difference between the shifted and unshifted spectra. The EA amplitude is proportional to Embedded Image. (C) Chemical structure of PC61BM (gray) and PCDTBT (red). (D) Chemical structure of PC71BM (gray) and p-DTS(FBTTh2)2 (blue). (E) Absorption spectra of the molecules presented in (D).

  2. Fig. 2 Transient absorption spectra of excited states in p-DTS(FBTTh2)2:PC71BM films.

    (A and B) Visible and near-infrared measurements of a 60:40 film prepared identically to the active layer of an efficient OPV device. The samples were excited with 700-nm laser pulses at a fluence of 2 μJ/cm2. (C) Measurement of a 90:10 film, where PC71BM aggregation is prevented and charge separation is inefficient. (D) TA time slices of the 100:0, 90:10, and 60:40 films normalized at 640 nm. The steady-state EA signature, obtained from a device using a steady-state electro-absorption measurement (CW), is proportional to the first derivative of the absorption spectrum.

  3. Fig. 3 Excited-state and EA dynamics of p-DTS(FBTTh2)2:PC71BM and PCDTBT:PC61BM blends.

    (A) Spectral signatures present in the p-DTS(FBTTh2)2:PC71BM (60:40) blend. The signals are obtained from a global fit spanning the entire visible range (see text for details). The data set spans 30 to 2500 fs, and the sample was excited with broadband (525 to 625 nm) laser pulses compressed to 30 fs at a fluence of 8 μJ/cm2. The reference EA is obtained independently (see Fig. 2D). (B) Dynamics of the S1, hole, and EA amplitudes obtained by fitting the data set with a linear combination of the excited-state signatures shown in (A). The yellow zone indicates the region over which artefacts prevent accurate measurement of the EA. (C and D) Spectral signatures and their associated dynamics for three different compositions of PCDTBT:PC61BM blends (see supplementary materials for details of the analysis). The samples were excited at a fluence of 2 μJ/cm2, and the reference EA spectrum was measured on a device. (E) Total energy stored in the electric field per electron-hole pair (at room temperature and 4 K). The EA is converted to a spatially integrated electric field square using a calibration from steady-state measurements (Embedded Image, where CEA is the calibration constant for a given system (see supplementary materials for details). The time-resolved energy per hole (Embedded Image) is obtained assuming that half of the field is in the donor phase. The low temperature (4 K) measurement grows continuously up to 1 ns, where it saturates at ~200 meV (see supplementary materials).

  4. Fig. 4 Model of initially accessible electronic states in fullerene derivatives and calculated electron-hole separation distance.

    (A) Excited states before and immediately after charge transfer. When excitons (S1) dissociate at interfaces with aggregated PC61/71BM, the isoenergetic charge transfer places the electrons in delocalized band states, where they undergo wave-like propagation within the PC61/71BM aggregate. In this model, the hole at the interface induces a well of depth W, reducing the width of the band from its bulk value B to ~B-W. This system can sustain two typical electron wave functions represented as ϕ1,2. The electron is either trapped at the interface (ϕ1) or propagating through the band (ϕ2). (B) Calculation of electron-hole separation dynamics per charge pair for (i) injection of a fully coherent electron wave packet; (ii) tunneling of the electron into delocalized states (Fermi golden rule, FGR); and (iii) tunneling of the electron into localized states (i.e., Marcus-type electron transfer). The multiple lines represent different values of disorder and couplings, spanning 100 to 200 meV and 30 to 50 meV, respectively (31, 32). r0 is the separation distance of the initial next-neighbor charge-transfer state [typically 1.5 nm (9)], and l is the length of the PC61/71BM aggregate over which the wave function is delocalized (5.25 nm for the calculation shown).