5.3 Results and discussion
5.3.4 Structures and properties of the D/A blends, charge transfer
As mentioned in the Methodology section, structures of composite systems P1- P5/PCBMare obtained at CPCM-M062X/6-311G(d,p) level. All the optimized structures are shown in Figure C4 (see Appendix C). As observed in these two figures, structures ofπ-bridged donor/PCBM composites are very different than the structure ofP1/PCBM. The optimized vertical distances between the donor
0 50000 100000 150000 200000 250000 300000 350000 400000
300 400 500 600 700 800 900 1000 1100 1200
ε (M−1 cm−1 )
Wavelength (nm)
P1 P2 P3 P4 P5
Figure 5.4: Simulated absorption spectra of P1-P5/PCBMcomposite sys- tems at the TD-CPCM-CAMB3LYP/6-311G(d,p) level. Half-width at half-
maximum height is 2500 cm−1.
oligomers and PCBM are 3.20 ˚A, 3.40 ˚A, 3.50 ˚A, 3.52 ˚A and 3.18 ˚A for P1- P5/PCBM, respectively.
Simulated absorption spectra of the five blend structures are shown in Figure 5.4. Each spectrum shows only one peak with a very largefoscand thefoscvalues of these peaks forP2-P5/PCBM blends are much larger than that ofP1/PCBM.
λmax values of P1-P5/PCBM composite systems are 656, 618, 629, 616 and 618 nm, respectively. This shows that while the λmax values of the newly designed composites are close to each other, those are blue-shifted with respect to that of P1/PCBM. Excitation energies, corresponding oscillator strengths,lCTand types of excited-states of the composite systems P1/PCBM and P2-P5/PCBM are tabulated in Table 5.6. The table shows that all the above mentioned peaks in the absorption spectra correspond to S0 → S1 transitions. It is notable that the oscillator strengths of first excitations are greatly enhanced upon incorporation of π bridges. Excited states can be assigned either as locally-excited (LE) or ICT states. In our case, CDD plots are used to assign the character of the states.
As an example, CDD plots for three different excited states of P1/PCBM com- posite are shown in Figure 5.5. This figure shows that the electrons and holes are localized either on P1 or on PCBM, for S1 and S8 states, respectively and
(a)
(b)
(c)
Figure 5.5: Charge density difference plots forS1(a),S5(b), andS8(c) excited states of the P1/PCBM composite. Blue (red) stands for depletion (accu- mulation) of negative charges. While S1 and S8 are LE states, S5 is a CT state. These results are obtained at TD-CPCM-CAMB3LYP/6-311G(d,p) level
of theory.
on the donor and acceptor parts, respectively and this state at 530 nm is iden- tified as the first ICT state in case of P1/PCBM composite. The fosc for S0→ S5 is quite small. Similar to the results of P1/PCBM, first excited states in the other four composites are also LE states. As seen in the Figure 5.6, S6 state for P2/PCBM, S6 state for P3/PCBM, S9 state for P4/PCBM and S11 for P5/PCBM are the first ICT states. These ICT states appear at 526 nm, 540 nm, 498 nm, 475 nm, respectively, for P2-P5/PCBM. As shown in the Table 5.6, lCT values for the above charge-transfer states ofP1-P5/PCBMare 3.47 ˚A, 4.36 ˚A, 8.23 ˚A, 5.43 ˚A and 3.22 ˚A, respectively. Above lCT values are relatively larger than the values of various low-lying LE states inferring that electrons and holes are clearly separated in the excited states, and that there are greater extents of charge transfers. In particular, lCT for the first ICT state of thienothiophene based composite is very large. It is to be noted from the table that there are other excited states for which lCT values are still larger but those are still LE states.
P2
P3
P4
P5
Figure 5.6: Charge density difference plots for the S6, S6, S9 and S11 of the P2-P5/PCBM interfacial systems. The mentioned states are the first ICT
states.
Hence, lCT values alone cannot correctly identify an ICT state. Rather a com- binations of CDD and lCT values provide a correct description of type of excited states. In addition to CDD plots, transition density matrix (TDM) analysis is also effective in understanding the charge transfer and characterizing the excited states. Diagonal and off-diagonal elements of the TDM matrix reveal local and charge-transfer characters, respectively. As shown in the Figure5.7, theS1 andS8 states involve electron-hole correlations within local sites, i.e. donor and acceptor parts, respectively, only. On the other hand, the results for S5 state shows that there is electron-hole coherence between the donorP1 andPCBM. Similar plots for two locally excited and the first charge transfer states of other composites are shown in Figures C5 and C6 (see Appendix C).
Eb is the energy to be overcome for the dissociation of an exciton to an electron and a hole at the interface. IP, EA and Eg,Blend1 values are tabulated in Table C1. Smaller the Eb value, better is the possibility of charge separation. Eb values for (P1) -(P5) are shown in Table 5.7. Our calculated E s vary between
Table 5.6: Excitation energies (Eg,Blend), oscillator strengths (fosc), charge transfer lengths (lCT) and types of excited states of theP1-P5/PCBMblend systems. Eg,Blends are in eV. The values inside the parentheses in the third column are the wavelengths in nm corresponding to the Eg,Blends in eV. These
results are obtained at TD-CPCM-CAMB3LYP/6-311G(d,p) level.
Oligomer/PCBM States Eg,Blend(eV) fosc lCT(˚A) Type of excited state
P1
S1 1.88(656) 2.6979 2.422 LE
S2 2.16(572) 0.2603 3.462 LE
S3 2.26(547) 0.0031 2.026 LE
S4 2.29(539) 0.0064 2.439 LE
S5 2.33(530) 0.0785 3.474 ICT
S6 2.34(527) 0.1084 3.100 ICT
S7 2.36(524) 0.5404 2.774 LE
S8 2.42(511) 0.0002 0.988 LE
S9 2.43(509) 0.0010 1.262 LE
S10 2.48(498) 0.0085 3.083 ICT
P2
S1 2.00(618) 4.696 3.223 LE
S2 2.10(588) 0.085 4.147 LE
S3 2.21(560) 0.456 4.056 LE
S4 2.17(545) 0.001 1.312 LE
S5 2.31(536) 0.002 1.921 LE
S6 2.35(526) 0.012 4.366 ICT
S7 2.37(522) 0.010 4.267 ICT
S8 2.41(514) 0.000 0.786 LE
S9 2.42(510) 0.000 0.829 LE
S10 2.45(505) 0.804 15.065 LE
P3
S1 1.96(629) 4.748 9.809 LE
S2 2.11( 586) 1.621 15.277 LE
S3 2.18( 567) 2.344 13.910 LE
S4 2.24(553) 0.007 1.355 LE
S5 2.27(544) 0.000 1.492 LE
S6 2.29( 540) 0.012 8.231 ICT
S7 2.36(524) 0.151 10.237 ICT
S8 2.37(522) 0.630 19.212 LE
S9 2.41(513) 0.000 3.961 LE
S10 2.42(511) 0.006 3.845 LE
P4
S1 2.01(616) 7.233 3.389 LE
S2 2.07(597) 0.157 7.214 LE
S3 2.26(546) 0.046 4.981 LE
S4 2.27(543) 0.003 0.990 LE
S5 2.31(534) 0.001 1.084 LE
S6 2.40(515) 0.001 1.097 LE
S7 2.41(512) 0.001 0.568 LE
S8 2.43(509) 0.738 16.631 LE
S9 2.48(498) 0.001 5.433 ICT
S10 2.50(495) 0.001 5.259 ICT
P5
S1 2.00(618) 6.820 3.491 LE
S2 2.09(590) 0.255 4.661 LE
S3 2.21(559) 0.419 5.067 LE
S4 2.27(545) 0.002 0.945 LE
S5 2.31(536) 0.001 1.358 LE
S6 2.40(514) 0.519 13.966 LE
S7 2.41(513) 0.008 0.805 LE
S8 2.42(512) 0.001 0.651 LE
S9 2.55(484) 0.001 1.061 LE
S10 2.58(478) 0.001 2.440 LE
0.08-0.41. While P2 possess the smallest Eb largest Eb occurs for P5. Hence, dissociation of exciton to free charge carriers is difficult in P5.
1 50 100 150 200 Atom Number 50
100 150 200
Atom Number 0 0.005 0.01 0.015 0.02
1 50 100 150 200 Atom Number
0 0.001 0.002
1 50 100 150 200 Atom Number
0 0.002 0.004 0.006
Figure 5.7: Two-dimensional site representation of the transition density matrix (TDM) for S1, S5 and S8 excited states (from left to right) of the P1/PCBM composite system. Both X and Y-axes represent the atom num- bers. Atom numbers 1-148 are for the donor P1 and from 148 onwards are for the acceptor PCBM. Hydrogen atoms are ignored here. These results are
obtained at TD-CPCM-CAMB3LYP/6-311G(d,p) level of theory.