Spectroscopic investigation of oxygen and water induced electron trapping and charge transport instabilities in n-type polymer semiconductors

Transcription

1 Supporting Information Spectroscopic investigation of oxygen and water induced electron trapping and charge transport instabilities in n-type polymer semiconductors Riccardo Di Pietro 1, Daniele Fazzi 2, Tom B. Kehoe 1, Henning Sirringhaus* 1. 1 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, CB3 0HE Cambridge. 2 Center for NanoScience and Technology@PoliMi, Istituto Italiano di Tecnologia, via Pascoli 70/3, 20133, Milano. S1

2 P(NDI2OD-T2) Excited state calculation The excited state calculations carried out at the TDDFT level are reported here for both (NDI2OD-T2) 1 and (NDI2OD-T2) 5 oligomer with and without the presence of the oxygen molecule. TDDFT have been carried out on the relative DFT optimized molecular geometries. The M06-2X functional with the 6-311G** basis set have been used for the calculations. In the ground state calculations (as reported in the manuscript), for the evaluation of the counter poise BSSE interaction energy, the explicit singlet and triplet multiplicities of the NDI2OD-T2 oligomers and oxygen molecule have been considered respectively. In TDDFT calculations both singlet and triplet states have been computed. From the analysis of Table S1, where ground-to-excited state transition energies and oscillator strengths are reported, we can observe that the interaction between the (NDI2OD-T2) 1 and the oxygen induces a slight red shift of the low energy dipole allowed excited state, namely S 1 =2.954 ev for non interacting (NDI2OD-T2) 1 and S 2 =2.942 ev for (NDI2OD-T2) 1 -O 2 complex. From the analysis of the molecular orbitals mainly involved in the electronic transition (as reported in Table S1) we can deduce that the S 2 state of the interacting system has the same character of S 1 for the non-interacting case: S 2 is mainly described by an H L+1 (HOMO LUMO+1) transition while S 1 by an H L one. In other words, for the (NDI2OD-T2) 1 -O 2 complex the L+1 molecular orbital should be compared to the LUMO orbital of the non-interacting system because the LUMO orbital for (NDI2OD-T2) 1 -O 2 is the intra-gap molecular orbital level resulting from the interaction with the oxygen molecule. Table S1. TD-M06-2X/6-311G** calculated ground to singlet excited states for (NDI2OD-T2) 1 as alone and (NDI2OD- T2) 1 -O 2 complex. In bold are reported the most active dipole allowed transitions. (NDI2OD-T2) 1 (NDI2OD-T2) 1 -O 2 Singlet excited state Osc. Strength f / a.u. Singlet excited state Osc. Strength f / a.u. energy / ev energy / ev S 1 = (H L) S 1 = (H intra-gap level) S 2 = (H-1 L) S 2 = (H L+1) S 3 = S 3 = S 4 = S 4 = S 5 = S 5 = S 6 = (H L+2) S 6 = (H-1 L+1) S 7 = S 7 = S 8 = S 8 = (H L+2) S 9 = S 9 = S 10 = S 10 = Furthermore, the TDDFT calculations carried out on the (NDI2OD-T2) 1 -O 2 complex show that those excited states where the contribution of the O 2 localized molecular orbitals is dominant are triplet states, and are thus dipole forbidden and have small oscillator strength. For the above considerations we can conclude that the interaction with the oxygen molecule causes a reorganization of the molecular orbital energy levels (as well reported in the manuscript) and, consequently, a reorganization of the excited states energies (Table S1). Moreover, the dipole allowed excited states are those states in which the contribution of the polymer molecular orbitals is dominant while the contribution of the O 2 molecular orbitals is negligible or even not present. S2

3 Additionally we can observe from both the analysis of Table S1 and Figure S1, that a blue shift of the high energy band is also predicted. In particular, the S 2 state for (NDI2OD-T2) 1 at ev shifts to the S 6 state for (NDI2OD-T2) 1 -O 2 at ev. S 0 S 2 S 0 S 6 S 0 S 1 S 0 S 2 Figure S1: TD-(U)M06-2X/6-311G** calculated ground-to-singlet excited state transitions for (NDI2OD-T2) 1 as alone (blue bars) and interacting with one oxygen molecule ((NDI2OD-T2) 1 -O 2 red bars). The main transitions are reported as insets. Our TDDFT calculations carried out on the monomer unit of P(NDI2OD-T2) with and without the oxygen molecule, qualitatively confirm the experimental CAS spectra reported in the manuscript (see Figure 4) where the low energy feature (around 1.7 ev) red-shifts, while the high energy feature (around 3eV) blue-shifts. Due to the TDDFT limitations and us having considered only the monomer unit of the polymer, the calculated transition energies are energetically higher than the experimental values, but we can confidently assign the low energy feature of CAS spectra to the S 2 state (interacting system) and the high energy feature of CAS spectra to the S 6 state (interacting system), respectively red-shifted and blue-shifted with respect to the S 1 and S 2 states of the non-interacting unit (see Table S1). To study the effect of the polymer chain length, we considered also a longer oligomer, namely (NDI2OD-T2) 5 featuring five repeating units. Due to the high number of atoms to consider for this case, in our TDDFT calculations we considered a smaller excited states window than the previous case. In particular we considered the first 10 low lying excited states. Amongst them, for the (NDI2OD-T2) 5 -O 2 complex, there is only one singlet state (named here as S 1 ), being the others triplet low lying states (not dipole allowed) induced by the presence of the oxygen molecule. The first singlet excited state S 1 for (NDI2OD-T2) 5 -O 2 complex has an energy of 2.38 ev with an oscillator strength f=0.9 and a mainly H L+1 character. This transition has to be compared with the first dipole allowed excited state, named S 1, for the non-nteracting system, (NDI2OD-T2) 5, that has an energy of 2.52 ev, f=2.6 and a main H L character. S3

4 In Figure S2 we report the TDDFT exited state transitions for the case of (NDI2OD-T2) 5 with and without the oxygen molecule. S 0 S 1 S 0 S 1 Figure S2: TD-(U)M06-2X/6-311G** calculated ground to singlet excited states for (NDI2OD-T2) 5 alone (blue bars) and interacting with one oxygen molecule (NDI2OD-T2) 5 O 2 (red bar). Only the low energy region is reported. From the above results we can confirm the red shift of the low energy band, as observed in the CAS experiments, due to the interaction between the polymer and the oxygen. These results further support both the model used for describing the polymer-oxygen interactions and the experimental findings, as reported in the manuscript. S4

5 P(NDI2OD-T2) Optical transmission coefficient Transmission ( % ) Energy (ev) Figure S3. Optical transmission spectrum of a P(NDI2OD-T2) film (40 nm thick) on spectrosil. The spectrum is corrected for the baseline and spectrosil absorption, showing only the contribution of the polymer film in the graph. S5

6 Cartesian optimized structures Here below we report the Cartesian optimized structures for (NDI2OD-T2) 1, (NDI2OD-T2) 1 O 2, (NDI2OD-T2) 5 and (NDI2OD-T2) 5 O 2, as obtained by (U)M06-2X/6-311G** calculations. For each system we report a table with the optimized structure as tables with the following fields: atomic number x-coordinate y-coordinate z-coordinate Structure of (NDI2OD-T2) S6

7 Structure of (NDI2OD-T2) 1 -O S7

8 S8

9 Structure of (NDI2OD-T2) S9

10 S10

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12 Structure of (NDI2OD-T2) 5 -O S12

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