Effect of localized UV irradiation on the crystallinity and electrical properties of dip-coated polythiophene thin films

Abstract

Ensuring high performance in polymer devices requires conjugated polymers with interchain π-π stacking interactions via van der Waals forces, which can induce structural changes in the polymer thin film. Here, we present a systematic study of using simple localized UV irradiation to overcome the low crystallinity and poor charge carrier transport in dip-coated poly(3-hexylthiophene) (P3HT) thin films, which are consequences of the limited selection of solvents compatible with the dip-coating process. UV irradiation for only a few minutes effectively promoted P3HT chain self-assembly and association in the solution state. Brief UV irradiation of a P3HT solution led to well-ordered molecular structures in the resultant P3HT films dip-coated using a low boiling point solvent with rapid solvent evaporation. In addition, the position at which UV light was irradiated on the dip-coating solutions was varied, and the effects of the irradiation position and time on the crystallinity and electrical properties of the resultant P3HT thin films were investigated.

Fig. 1. (a) UV-vis absorption spectra of dip-coated P3HT films using a solution irradiated with UV light for various times. (b) Normalized UV-vis absorption spectra of a dip-coated P3HT film using a solution irradiated with UV light for various times; the spectra are normalized to the A_0–2 transition (λ = 522 nm). The inset shows a magnification of the A_0–0 transition. (c) The ratio of the intensities of the A_0–0 and A_0–2 transitions in the absorption spectra of P3HT films. (d) Thickness of the P3HT films prepared with solutions irradiated for different times. The inset photos in (c) show the color change of the solutions irradiated with UV light for various times (0–10 min).

Fig. 2. Tapping-mode AFM phase images of the P3HT films prepared with solutions irradiated with UV light for various times: (a) 0, (b) 1, (c) 5, (d) 7, and (e) 10 min. The insets show height images of the films. The surface profiles were extracted from the height image. (f) The roughness of the P3HT films as a function of UV irradiation time.

Fig. 3. The out-of-plane GIXD patterns of the P3HT films prepared with solutions irradiated with UV light for different times, as plotted on (a) linear and (b) logarithmic axes. The inset shows a magnification of the (010) peak. (c) Structural change of the P3HT chain from benzoidal to quinoidal in the P3HT molecular structure under UV irradiation.

Fig. 4. (a) Plots of the drain current versus the gate voltage at a fixed drain voltage (V_D = −60 V) on both linear (left axis) and log(right axis) scales for P3HT films prepared with solutions irradiated with UV light for different times. (b) Average field-effect mobilities (left axis) and on–off ratio (right axis) of the OFETs fabricated using P3HT films irradiated with UV light for different times.

Fig. 5. (a) UV-vis absorption spectra of dip-coated P3HT film prepared using solutions irradiated with UV light at various positions. (b) Normalized UV-vis absorption spectra of P3HT films dip-coated using solutions irradiated with UV light at different positions; the spectra were normalized to the A_0–2 transition (λ = 522 nm). (c) The ratio of the intensities of the A_0–0 and A_0–2 transitions (left axis) and the thickness (right axis) of P3HT films prepared with solutions irradiated at different positions. (d) Photograph showing the color change of the P3HT solution according to the UV irradiation position.

Scheme 1. Schematic showing experimental process and improved crystallinity of the P3HT film formed with UV irradiation of the top region during the dip-coating process (film T).

Fig. 6. Tapping-mode AFM phase images of (a) a pristine P3HT film (P) and films prepared with solutions irradiated with UV light at different positions: (b) whole (W), (c) top (T), and (d) bottom (B). The surface profiles were extracted from the height image. The insets show height images of the P3HT films. The out-of-plane GIXD patterns of the P, W, T, and B P3HT films plotted on (e) linear and (f) logarithmic axes. The inset shows a magnification of the (010) peak.

Fig. 7. (a) Plots of the drain current versus the gate voltage at a fixed drain voltage (V_D = −60 V) on both linear (left axis) and logarithmic (right axis) scales for OFETs fabricated using P3HT films prepared from solutions irradiated with UV light at different positions. (b) Average field-effect mobilities (left axis) and on–off ratio (right axis) of the OFETs fabricated using P3HT films prepared from solutions irradiated with UV light at different positions.