Correlation between microstructure and charge transport in poly(2,5-dimethoxy-$p$-phenylenevinylene) thin films

We report a study of thin films of poly(2,5-dimethoxy-$p$-phenylenevinylene) (PDMeOPV) prepared by a precursor route. Conversion at two different temperatures, namely, 120 and $185°\mathrm{C}$, produces partially and fully converted films. We study the structural, optical, and charge transport characteristics of these samples in order to relate transport properties to microstructure. Micro-Raman mapping and photoluminescence (PL) imaging reveal the existence of coarse, depth-averaged domains of around $50\mu \mathrm{m}$ in lateral extent, with more pronounced contrast for conversion at the higher temperature. The contrast in both micro-Raman and PL maps can be attributed to fluctuations in film density. Spectroscopic ellipsometry studies of the films indicate that the average film density is approximately 15% higher for conversion at the higher temperature. Time-of-flight photocurrent transients, recorded here in PDMeOPV films, are typically dispersive but yield hole mobilities in excess of ${10}^{-4}{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at modest applied fields $(\sim 1.2\times {10}^5\mathrm{V}∕\mathrm{cm})$ in the fully converted films. To our knowledge, these are amongst the highest reported mobility values for a poly($p$-phenylenevinylene) derivative. Fully converted films, while yielding higher hole mobilities, exhibit a stronger dependence on electric field than partially converted ones. The higher mobility can be attributed to the almost complete conversion of the flexible saturated subunits within precursor chains to conjugated vinylene moieties at elevated temperature. This results in a correspondingly higher packing density, an improvement in intrachain transport, and a reduction in the smallest interchain hopping distance. We suggest that the stronger electric field dependence is due to the increasing influence of intermolecular electrostatic interactions with decreasing interchain separation. We propose that a greater proportion of chains in the fully converted films packs in a three-dimensional, interdigitated arrangement similar to that described previously for crystalline samples of PDMeOPV [J. H. F. Martens et al., Synth. Met. 55, 449 (1993)].

Figure 1

Conversion routes to partially and fully converted films of poly(2,5-dimethoxy-$p$-phenylenevinylene) (PDMeOPV).

Figure 2

(a) Typical room-temperature TOF photocurrent transients (at $E=2.4\times {10}^5\mathrm{V}∕\mathrm{cm}$) for fully (filled circles) and partially (empty circles) converted freestanding PDMeOPV films $(d=1.2\mu \mathrm{m})$. Note that the data are displayed on a double-logarithmic plot. The corresponding transit times $t_{\mathit{tr}}$ are marked on the plot: $t_{\mathit{tr}}$ is reduced from $37\text{to}10\mu \mathrm{s}$, respectively, on going from the partially to the fully converted sample. (b) Electric field dependence of the room-temperature hole mobility in partially (empty circles) and fully converted PDMeOPV free standing films $(d=1.2\mu \mathrm{m})$. The data are plotted as a function of the square root of the external electric field. Each data point was determined from the intersection of the asymptotes to the corresponding double-logarithmic photocurrent transient plots. (c) Temperature and electric field dependences of hole mobility for the fully (upper panel) and partially (lower panel) converted films.

Figure 3

(a) Representative Raman spectra $({\lambda }_{\mathrm{ex}}=780\mathrm{nm})$ showing the dominant backbone modes discussed in the main text. The spectra were taken in a bright (empty circles) and a dark (filled circles) region of the map of the fully converted PDMeOPV TOF sample [see Fig. 5a]. (b) Raman spectra showing the principal Raman modes for both a fully (solid line) and a partially (empty circles) converted film. The spectra have been normalized to the ring-stretching mode $(\sim 1581{\mathrm{cm}}^{-1})$ intensity. The observed softening of the backbone modes and weakening of the vinylene $\mathrm{C}\mathrm{C}$ stretch intensity $\left(1621{\mathrm{cm}}^{-1}\right)$ in fully converted samples is consistent with an increase in effective conjugation length.

Figure 4

$500\mu \mathrm{m}$ length scans for the fully converted PDMeOPV TOF sample $(d=1.2\mu \mathrm{m})$ showing (a) the ratio of the intensity of the $\approx 1280{\mathrm{cm}}^{-1}$ backbone mode to that of the $\approx 1581{\mathrm{cm}}^{-1}$ quadrant ring-stretch mode, (b) the peak wave number of the quadrant ring-stretch mode, and (c) the FWHM of the quadrant ring-stretch mode.

Figure 5

(Color online) $250\times 250\mu {\mathrm{m}}^2$ Raman maps $({\lambda }_{\mathrm{ex}}=780\mathrm{nm})$ displaying the ratio of the intensities of the 1280 and $1581{\mathrm{cm}}^{-1}$ modes (see main text for details) for (a) fully converted and (b) partially converted PDMeOPV TOF samples $(d=1.2\mu \mathrm{m})$. Lighter shades (higher ratios) correspond to a higher packing density.

Figure 6

UV-visible absorption and photoluminescence spectra for fully converted (solid line) and partially converted (dashed line, displaced vertically for clarity) spin-coated PDMeOPV films. Absorption spectra were obtained for films $(d=90\mathrm{nm})$ spin coated onto fused quartz Spectrosil B substrates. The PL spectra correspond to relatively thick films $(d=1.2\mu \mathrm{m})$ spin coated onto KBr substrates. FWHM values for the PL spectra are 0.30 and $0.39\mathrm{eV}$ for the fully and partially converted samples, respectively.

Figure 7

$238\times 238\mu {\mathrm{m}}^2$ PL micrographs displaying the ratio of the PL intensity detected at $660\mathrm{nm}$ to that detected at $600\mathrm{nm}$ for (a) fully converted and (b) partially converted PDMeOPV films spin coated onto silicon substrates. The samples were spin coated from the same solution used to prepare the TOF devices, and they were excited in a region close to the substrate.

Figure 8

Representative PL spectra (not normalized) collected at two different locations on a fully converted PDMeOPV sample spin coated onto a silicon substrate. The spectra were taken at locations corresponding to the bright (full line) and dark (dashed line) regions of the PL map shown in Fig. 7a. The excitation wavelength was $488\mathrm{nm}$. A standard (excited over an area of $4{\mathrm{mm}}^2$) PL spectrum for a fully converted film, spin coated onto a Spectrosil B fused quartz substrate, is also shown but offset vertically for clarity.