2-1 Syntheses
The synthetic route of FBTCN, DTCTiFBT, and DTCToFBT is illustrated in Scheme 2-1. FBTBr was adopted as the starting material and synthesized according to the previous literature15. FBTBr underwent Palladium-catalyzed cyanation to give the key intermediate FBTCN in 39% yield. Stille coupling of 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)thiophene (dtat-tin) and FBTCN afforded DTCTiFBT in 77% yield. For DTCToFBT, dtat-tin was reacted with FBTBr via Stille coupling to afford DTToFBTBr in 67% yield, which subsequently underwent Palladium-catalyzed cyanation to give DTCToFBT in 89% yield. From infrared (IR) spectra analyses, DTCTiFBT and DTCToFBT exhibit distinct absorption of nitrile group in 2222 and 2227 cm-1 respectively, while DTToFBTBr shows no absorption in 2210–2260 cm-1.25 Their chemical structures were further confirmed via nuclear magnetic resonance spectroscopy (NMR) and mass spectroscopy.
Scheme 2-1. Synthetic route for FBTCN, DTCTiFBT, and DTCToFBT.
2-2 Optical Properties
Optical properties of DTCTiFBT and DTCToFBT are shown in Figure 2-1.
Their corresponding properties are summarized in Table 2-1. In solution, both molecules show slightly bathochromic shifts in absorption compared to DTCTB due to the introduction of a fluorine atom. All molecules perform most intensive absorption band located in 500–700 nm and extinction coefficient around 23000 M-1cm-1, which could be attributed to charge transfer transition from the electron-donating amine moieties to the electron withdrawing benzothiodiazole moieties. In vacuum deposited thin films, all molecules exhibit further bathochromic shifts in their absorption maxima in the same trend as solution state.
(a) (b)
Figure 2-1. Absorption spectra of DTCTiFBT (triangles) and DTCToFBT (diamonds) in (a) CH2Cl2 solutions and (b) vacuum deposited thin films.
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Table 2-1. Photophysical and electrochemical parameters for DTCTiFBT,
Cyclic voltammograms of DTCTiFBT and DTCToFBT are shown in Figure 2-2.
Both molecules exhibit same oxidation and reduction potential in solution state. The energy level of HOMO is -5.49 eV and the energy level of LUMO is -3.66 eV, which are both lower than those of DTCTB. Moreover, the LUMO energy level is lowered more than the HOMO energy level, resulting in slightly narrowed energy bandgap.
The lowered HOMO and the bathochromic absorption are beneficial for pursuing high Voc and Jsc respectively, which showed our molecular design is promising for efficient OPV devices.
Figure 2-2. Cyclic voltammograms of DTCTiFBT and DTCToFBT.
2-4 Theoretical Calculations
DFT calculations of optimized geometries for DTCTiFBT and DTCToFBT are illustrated in Figure 2-3. The dihedral angles between thiophene (T) and benzothiodiazole (BT) units are 3.5o for DTCTiFBT, 4.2o for DTCToFBT, and 7.0o for DTCTB. The coplanarity of these molecules is presumably due to van der Waals (vdW) forces between the inward hydrogen atom on BT unit and the sulfur atom on T unit, as well as between the inward nitrogen atom on BT unit and the outward hydrogen atom on T unit. The coplanarity of DTCTiFBT is more strengthened due to the substitution of inward electron-deficient fluorine atom, and thus the stronger interaction with the electron-rich sulfur atom. The speculation was confirmed by calculating the distance between the atoms fore-mentioned via space-filling model, which reveals contact between those atoms mentioned. Such coplanarity enhances effective conjugation length and facilitates the intramolecular charge transfer from the electron donating group to the electron withdrawing group, resulting in bathochromic shift and high extinction coefficient. The coplanarity also provides benign intermolecular – interaction, which is beneficial for close packing in solid film.
Front Flank DTCTiFBT
DTCToFBT
Figure 2-3. DFT-optimized geometries for DTCTiFBT and DTCToFBT.
The dipole moments of DTCTiFBT, DTCToFBT, and DTCTB are summarized in Table 2-2. The dipole moment for all six dyes in this thesis point from electron donating amines toward electron withdrawing BT and cyano groups. As compared to DTCTB, DTCTiFBT shows a smaller dipole moment at the ground state, which is cancelled out by the inward direction of the fluorine atom. However, the outward fluorine atom on DTCToFBT lines same direction with the molecular dipole moment, resulting in an increment. At the first excited state (S1), because of further charge separation caused by charge transfer, all molecules show larger dipole moment in same trend, giving a positive Δμge. For the overall dipole moment overlap between S0
and S1, since μgand μe point to nearly the same direction, μtr is very close to μg for each molecule. We anticipate that a higher μe is beneficial for charge separation and thus the incident photo-to-light conversion efficiency.
Table 2-2. Dipole moment parameters for DTCTiFBT, DTCToFBT, and DTCTB. overall dipole moment overlap between S0 and S1.
Table 2-3. TD-DFT calculated oscillator strengths, absorption wavelengths, molecular orbital compositions, and transition characters for DTCTiFBT and DTCToFBT.
dyes electronic transition, f
λexp /λcalc
(nm) MO Composition character DTCTiFBT S0 S1, 0.7495 574/496 5% HOMO −1 LUMO
92% HOMO LUMO CT DTCToFBT S0 S1, 0.8416 569/489 4% HOMO −1 LUMO
92% HOMO LUMO CT
Since the fact that (1) the charge transfer intensity of DTCTiFBT and DTCToFBT are higher than –* band in intensity theoretically and experimentally,
and (2) the main transition of both molecules are from HOMO to LUMO (Table 2-3), we selectively reported isodensity surface plots of the HOMOs and LUMOs of both molecules. As shown in Figure 2-4, the HOMOs of the molecules are mainly populated at ditolylamine and thiophene fragments, whereas the LUMOs are mainly localized at benzothiadiazole and cyano units. The benign separation between HOMO and LUMO facilitates charge transfer absorption, which is beneficial for utilizing the most intense solar irradiance region.
DTCTiFBT DTCToFBT
Figure 2-4. Isodensity surface plots of the HOMO (green) and LUMO (red) of DTCTiFBT and DTCToFBT.
2-5 Crystal Structures and Packings
The crystal of DTCTiFBT was obtained via biphasic diffusion between dichloromethane and methanol for X-ray analysis, and its crystal packing is illustrated in Figure 2-5. DTCTiFBT packs in parallel fashion in dimer with an average distance in the range of 3.34–3.45 Å , indicating benign intermolecular −interactions.
However, the dimer lines in same direction with head to head orientation, thus creating net dipole moment. The uncancelled dipolarity could be involved in controlling charge transfer pathways, resulting in energetic disorder in OSCs.27 In crystal packing, DTCTiFBT exhibits nearly perpendicular arrangement between neighboring molecules (the ditolylamine moiety was omitted for clarity.) Though no obvious conduction channel for charge carrier is found, the dipoles cancel in larger domain as illustrated. Moreover, a higher FF of 0.60 was reached for DTCTiFBT/C60 OPV device, suggesting better morphologies in blending layer.
The dihedral angle between T and BT unit is 7.4o for DTCTiFBT, which is less than 7.7o for DTCTB.26 Such observation is similar to the theoretical calculation in Section 2-4.
The crystals of DTCToFBT were obtained via biphasic diffusion between
chloroform or dichloromethane and methanol or pentane; however, they could not yield results under X-ray analysis, probably due to the soft and needle-like crystal nature.
Figure 2-5. Crystal structures of DTCTiFBT in molecular packing.
2-6 Thermal Properties
Thermal stabilities and morphological properties of DTCTiFBT and DTCToFBT were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Their corresponding data are shown in Fig.
SB1 – Fig. SB4. The decomposition temperatures (Td) (referring to 5% weight loss) are 261–292 oC for these molecules. Thermal stability of materials is crucial for device fabrication under vacuum deposition. Although DTCToFBT shows lowest Td
and Tm among six molecules characterized in this thesis, it maintained the quality of vacuum deposited films and afforded efficient OPVs. To sum up, our D-A-A type molecular configuration could provide benign thermal stability for vacuum fabrication.
Table 2-4. Thermal parameters for DTCTiFBT, DTCToFBT, and DTCTB.
dyes Td (oC) Tm (oC)
DTCTiFBT 290 211
DTCToFBT 261 192
DTCTB26 292 250
2-7 Photovoltaic and Electrochemical Impedance Characteristics
All molecules were subject to vacuum deposition with the OPV structure: ITO / MoO3 (20 nm) / dye: C60 (x nm) / C60 (y nm) / bathocuproine (BCP) (7 nm) / Al (100 nm). MoO3 and BCP were employed as a hole transporting layer and an electron transporting layer, respectively. The active layers comprised of D:A blend films and a thin neat electron acceptor film. The optimized device of DTCTB was repeated to eliminate factors of different time and cooperative partners from Dr. Hao-Chun Ting’s result. The photovoltaic and electrochemical impedance characteristics were measured under simulated AM 1.5 G (100 mW cm-2) illumination. The ratio of dye: C60 was varied from 1:1.6 to 1:2.6 via altering evaporation rate (Å /s) of dyes and C60, and the total thickness of active layers (dye: C60 / C60) were varied from 60 nm to 90 nm, in search of best parameters for device performance. The optimized conditions and device performances are summarized in Table 2-5, and their current density-voltage (J-V) curves along with external quantum efficiency (EQE) spectra are shown in Figure 2-6. It is reasonable that the dye: C60 ratio is 1:2.2 for all thiophene-based devices. Since we only did little change to each molecular backbone, the best mixing ratio along with the morphology in each D:A blend film are similar. Among all, DTCToFBT/C60 OPV cell exhibited a Voc of 0.83 V, a Jsc of 6.85 mA/cm2, and a FF of 0.58, achieving a PCE of 3.28%, which outperformed counterpart DTCTiFBT and non-fluorinated DTCTB. Besides, DTCTiFBT/C60 OPV cell exhibited a Voc of 0.82 V, a Jsc of 6.33 mA/cm2, and a higher FF of 0.60, achieving a PCE of 3.11%, which
still outperformed DTCTB. The lowered HOMO energy levels for DTCTiFBT and DTCToFBT account for the higher Voc, and the bathochromic absorption resulted in the higher Jsc. As for EQE analyses, responses centered at 380–410 nm are contributed mainly by C60 were 55.8%, 52.6%, and 29.0% for DTCTiFBT, DTCToFBT, and DTCTB, respectively; responses centered at 585–598 nm mainly from dyes are 35.2%, 40.9%, and 30.9%, respectively. The former EQE response is stronger than the latter response for DTCTiFBT and DTCToFBT, due to the higher mixing ratio of C60. We anticipate that raising the ratio of an electron donor will improve the 500–700 nm light utilizing efficiency. Further ratio and morphology control in D:A blend film are essential to enhance the performance of each electron donor in this chapter.
Table 2-5. Photovoltaic parameters for DTCTiFBT, DTCToFBT, and DTCTB with OPV structure: ITO / MoO3 / dye: C60 (x nm) / C60 (y nm) / BCP / Al. DTCToFBT (diamonds), and DTCTB (pentagons) for C60-based OPV.
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