Photophysics of charge transfer in a polyfluorene/violanthrone blend

We present a study of the photophysical and photovoltaic properties of blends of violanthrone in poly[9,$9\text{−}\mathrm{bis}$(2-ethylhexyl)-fluorene-2,$7\text{−}\mathrm{diyl}$] $\left({\mathrm{PF}}_{2∕6}\right)$. Photoluminescence quenching and photocurrent measurements show moderate efficiencies for charge generation, characteristic of such polymer/dye blends. Pump-probe measurements on blend films suggest that while $\sim 47\%$ of the total exciton population dissociates within $4\mathrm{ps}$ of photoexcitation, only $\sim 32\%$ subsequently results in the formation of dye anions. We attribute the discrepancy to the likely formation of complex species with long lifetimes, such as stabilized interface charge pairs or exciplexes. This conclusion is supported by the appearance of a long lifetime component of $2.4\mathrm{ns}$ in the dynamics of the photoinduced absorption signal associated to polarons in photoinduced absorption bands centered at $560\mathrm{nm}$.

Figure 1

Absorption and PL spectra of thin films of ${\mathrm{PF}}_{2∕6}$ (bold line) and violanthrone dispersed in PMMA matrix (0.5:100 weight) (dashed line). The structures of the materials are shown above their corresponding spectra.

Figure 2

PL spectra for blends of violanthrone and ${\mathrm{PF}}_{2∕6}$ with different weight ratios. Excitation was with a $354\mathrm{nm}$ $\mathrm{He}\text{−}\mathrm{Cd}$ laser line. The PL spectra of a $1\mu \mathrm{m}$ film of violanthrone dispersed in PMMA with a weight ratio 0.5:100 excited at $531\mathrm{nm}$ (dotted line) and ${\mathrm{PF}}_{2∕6}$ (open circles) are also shown for comparison. The spectra were normalized to the fraction of the light absorbed by each sample. The enhancement over $650\mathrm{nm}$ is an artefact induced by the $708\mathrm{nm}$ component of the laser line.

Figure 3

A log-log plot of the dependence of PLQE (filled squares) and EQE at 0 V on the violanthrone concentration. The experimental error in the PLQE measurements was estimated as 10% of the absolute PLQE value. The error bars for the EQE data were estimated from the deviations in photocurrent values measured on six different devices.

Figure 4

Schematic of a ${\mathrm{PF}}_{2∕6}$/violanthrone heterojunction interface. The energy level offsets were deduced from cyclic voltammetry measurements reported elsewhere (Ref. 13).

Figure 5

The EQE spectra of devices containing ${\mathrm{PF}}_{2∕6}$ (open diamonds), 6:1 violanthrone in ${\mathrm{PF}}_{2∕6}$ (open squares), and the same composition subsequently annealed at $200º\mathrm{C}$ in air for $5\min$ (open circles). The spectrum corresponding to ${\mathrm{PF}}_{2∕6}$ was magnified tenfold for ease of comparison. The absorption spectrum of the 6:1 blend film (dotted line) is also shown. Inset figures show from left to right the morphologies corresponding to spun and annealed films, respectively.

Figure 6

Absorption spectra of violanthrone before (solid line) and after (dashed line) oxidation in solution. Oxidation was achieved by adding ${\mathrm{FeCl}}_3$ to the solution. The additional long-wavelength $(620–780\mathrm{nm})$ band is assigned to violanthrone anion species.

Figure 7

(a) Differential transmission spectra recorded for a spin-coated 0.5:100 violanthrone in PMMA film at $1\mathrm{ps}$ delay (filled circles), $40\mathrm{ps}$ (filled triangles), and $400\mathrm{ps}$ (filled diamonds). The absorption spectrum of the film is also shown, offset for clarity (dotted line). (b) Differential transmission spectra of a spin-coated ${\mathrm{PF}}_{2∕6}$ film at the same time delays: $1\mathrm{ps}$ (filled circles), $40\mathrm{ps}$ (filled triangles), $400\mathrm{ps}$ (filled diamonds).

Figure 8

Differential transmission spectra of a 10:100 violanthrone: ${\mathrm{PF}}_{2∕6}$ blend at 1 ps, (dotted line, filled triangles), and $4\mathrm{ps}$ delay (bold line, filled diamonds). The lower inset shows the absorption spectrum of the film.

Figure 9

Differential transmission dynamics at $560\mathrm{nm}$ for a 10:100 violanthrone: ${\mathrm{PF}}_{2∕6}$ blend film (filled circles) and for ${\mathrm{PF}}_{2∕6}$ (filled squares). Comparison between these data suggests a longer-lived decay in the blend. The upper inset shows the short-time rise of polaron population in the blend. This was obtained by subtracting the contribution associated to singlet and polaron-pair decay to the $560\mathrm{nm}$ blend dynamics.