Excited state dynamics and exciton diffusion in triphenylamine/dicyanovinyl push–pull small molecule for organic optoelectronics

Triphenylamine-based small push–pull molecules have recently attracted substantial research attention due to their unique optoelectronic properties. Here, we investigate the excited state de-excitation dynamics and exciton diffusion in TPA-T-DCV-Ph-F small molecule, having simple chemical structure with asymmetrical architecture and end-capped with electron-withdrawing p-fluorodicyanovinyl group. The excited state lifetime in diluted solutions (0.04 ns in toluene and 0.4 ns in chloroform) are found to be surprisingly shorter compared to the solid state (3 ns in PMMA matrix). Time-dependent density functional theory indicates that this behavior originates from non-radiative relaxation of the excited state through a conical intersection between the ground and singlet excited state potential energy surfaces. Exciton diffusion length of ~ 16 nm in solution processed films was retrieved by employing time-resolved photoluminescence volume quenching measurements with Monte Carlo simulations. As means of investigating the device performance of TPA-T-DCV-Ph-F, we manufactured solution and vacuum processed bulk heterojunction solar cells that yielded efficiencies of ~ 1.5% and ~ 3.7%, respectively. Our findings demonstrate that the short lifetime in solutions does not hinder per se long exciton diffusion length in films thereby granting applications of TPA-T-DCV-Ph-F and similar push–pull molecules in vacuum and solution processable devices.


Section 1. Synthetic procedures {5-[4-(diphenylamino)phenyl]-2-thienyl}(4-fluorophenyl)methanone (2). 2.5 M solution of n-
butyllithium (4.77 mL, 10 mmol) in hexane was added dropwise to a solution of compound 1 (2.5 g, 10 mmol) in 70 mL of dry THF at -78 ºC. Afterwards the reaction mixture was stirred for 60 min at -78 ºC and then 4-fluorobenzoyl chloride (1.21 g, 10 mmol) was added in one portion. The reaction mixture was stirred for 1 hour at -78 ºC, then the cooling bath was removed, and the stirring was continued for 1 hour. After completion of the reaction, 150 mL of diethyl ether, 100 mL of water and 5.6 mL of 1 N HCl were added to the reaction mixture. The organic phase was separated, washed with water, dried over sodium sulfate and filtered. The solvent was evaporated in vacuum and the residue was dried at 1 Torr. The product was purified by column chromatography on silica gel (eluent toluene:hexane = 1:1) to give pure compound 2 (1.65 g, 50%) as a green solid. 1
After completeness of the reaction, the pyridine was evaporated in vacuum and the residue was dried at 1 Torr. This crude product was purified by column chromatography on silica gel (eluent dichloromethane:hexane = 1:1). Further purification included precipitation of the product from its THF solution with toluene and hexane to give pure product as a red solid (1.0 g, 80%  Sigma-Aldrich Co. and used without further purification. Diphenyl[4-(2-thienyl)phenyl]amine (1) was prepared as described in reference 1 . Pyridine, THF, dichloromethane and hexane were dried, purified according to the known techniques and then used as the solvents. All reactions, unless stated otherwise, were carried out under inert atmosphere.
1Н NMR spectra were recorded at a Bruker WP-250 SY spectrometer, working at a frequency of 250.13 MHz and using CDCl3 signal (7.25 ppm) and DMSO-d6 (2.50 ppm) as the internal standard. 13C NMR spectra were recorded using a Bruker Avance II 300 spectrometer at 75 MHz. In the case of 1Н NMR spectroscopy, the compounds to be analyzed were taken in the form of 1% solutions in CDCl3 or DMSO-d6. In the case of 13C NMR spectroscopy, the compounds to be analyzed were taken in the form of 5% solutions in CDCl3 or DMSO-d6. The spectra were then processed on the computer using the ACD Labs software.
Mass-spectra (MALDI) were registered on the Autoflex II Bruker (resolution FWHM 18000), equipped with a nitrogen laser (work wavelength 337 nm) and time-of-flight mass-detector working in reflections mode. The accelerating voltage was 20 kV. Samples were applied to a polished stainless steel substrate.
Elemental analysis of C, N and H elements was carried out using CHN automatic analyzer CE 1106 (Italy).
The settling titration using BaCl2 was applied to analyze sulfur. Experimental error for elemental analysis is 0.30-0.50%. The Knövenagel condensation was carried out in the microwave "Discovery", (CEM corporation, USA), using a standard method with the open vessel option, 50 watts. In the case of column chromatography, silica gel 60 ("Merck") was taken.
DSC analysis of the samples was carried out by a DSC-822е (Mettler-Toledo, Switzerland) at a heating rate 20°C min -1 in argon. TGA was done by a Derivatograph-C instrument (MOM, Hungary), at a heating rate 10°C min -1 in air and argon.
Solubility of oligomer was measured using its saturated solution in ODCB, which was prepared by stirring of an excess of solid material in the solvent. For this purpose, materials were added in small portions to 1 ml of pure solvent. As prepared, the saturated solutions were filtered through 0.25-mm PTFE syringe filters 7 and the solvent was evaporated using a rotary evaporator. Afterwards the residue was dried in vacuum at 130°C until it achieved constant weight, which was used to calculate the exact solubility value.  Differential scanning calorimetry scans of TPA-T-DCV-Ph-F at the first heating (red), cooling, second heating (green).

Section 2. Photoluminescence maps and dynamical stokes shift (of solutions and matrix)
The photoluminescence (PL) maps of TPA-T-DCV-Ph-F in toluene solution, chloroform solution and in PMMA matrix are depicted in Figure S5a, and the respective dynamical Stokes shifts are shown in Figure   S5b. The dynamic Stokes shift of 40 meV in toluene is lower compared to the 100 meV in chloroform, which further demonstrates that with the increase in solvent polarity PL shifts to longer wavelengths. In the solid state (i.e., in the PMMA matrix) the dynamic Stokes shift is negligible (~ 0 eV) due to the lack of intermolecular interactions.   Figure S7 shows the stitched PL maps for the fast (< 2 ns) and slower dynamics (< 14 ns). Both PL maps were spectral integrated (in 2.2-1.5 eV range) and the resulting transients were stitched to obtain the final transient presented in the manuscript. The same procedure was applied for all the mixed films.

Section 5. Average separation between quenchers
The average separation between quenchers was calculated as: where for TPA-T-DCV-Ph-F, ρ ~ 1.17 g˖cm -3 is the density, ~ 497 g˖mol -1 is the molecular mass and 0.89 nm is the size of the molecule. is the Avogadro constant, and is the quencher molar ratio with respect to TPA-T-DCV-Ph-F.

Section 6. Monte-Carlo Simulations
The MC simulations schematics is depicted in Figure S8. The exciton lifetime, hopping distance and quencher content were used as the inputs parameters as shown in Table S2. The distance between two grid points (hopping distance) was equal to the size of the molecule ~ 0.89 nm averaged over three dimensions (see Section 5 for calculations). Each grid point was assigned an energy generated from a Gaussian distribution function centered at zero: where is the grid point energy, σ is the energetic disorder (standard deviation) 3 .
At the beginning of the simulation, 5000 excitons with a finite lifetime ( ) were placed randomly at different grids points. The exciton hopping probability was set at unity if the initial grid point has an energy ( ) higher than the final grid point energy ( ), while for the opposite case ( < ) the probability was determined from a Boltzmann distribution with kT equal to ~ 26 meV (room temperature): At each time step, every quenched exciton is eliminated from the simulation. The differential form of the Einstein-Smoluchowski relation for random walk was used to obtain the dependence of the exciton diffusion coefficient on time from the known exciton displacement as: where < 2 ( ) > is the average square of the excitons displacement, is the diffusion time.  Table S2. Monte-Carlo Simulations parameters.

Description Parameter Value
Cartesian lattice grid X-Y-Z 500-500-500 Lifetime-1 fraction  The photovoltaic performances of the devices were determined in the glovebox from J-V profiles obtained with a Keithley 2400 source meter. The devices were exposed to AM 1.5 irradiation provided by a Sun 2000 solar simulator (ABET Tech.). 1 sun (100 mW cm -2 ) irradiation was calibrated using a reference Si solar cell (Rera System).  Figure S9 shows the AFM images of evaporated TPA-T-DCV-Ph-F pristine films, evidencing a very flat topography consistently with a barely crystallinity of the material turning amorphous after annealing at 100°C. Additionally, the AFM image of solution processed blend film ( Figure S10) also showed smooth surface.