The Influence of Thickness and Spectral Properties of Green Color-Emitting Polymer Thin Films on Their Implementation in Wearable PLED Applications

Abstract

A systematic investigation of optical, electrochemical, photophysical, and electrooptical properties of printable green color-emitting polymer (poly(9,9-dioctylfluorene-alt-bithiophene)) (F8T2) and spiro-copolymer (SPG-01T) was conducted to explore their potentiality as an emissive layer for wearable polymer light-emitting diode (PLED) applications. We compared the two photoactive polymers in terms of their spectral characteristics and color purity, as these are the most critical factors for wearable lighting sources and optical sensors. Low-cost, solution-based methods and facile architecture were applied to produce rigid and flexible light-emitting devices with high luminance efficiencies. Emission bandwidths, color coordinates, operational characteristics, and luminance were also derived to evaluate the device's stability. The tuning of emission's spectral features by layer thickness variation was realized and was correlated with the interplay between H-aggregates and J-aggregates formations for both conjugated polymers. Finally, we applied the functional green light-emitting PLED devices based on the two studied materials for the detection of Rhodamine 6G. It was determined that the optical detection of the R6G photoluminescence is heavily influenced by the emission spectrum characteristics of the PLED and changes in the thickness of the active layer.

Keywords: PLED; flexible electronics; optical sensor; rhodamine 6G; solution process.

Scheme 1

(a) The architecture of the fabricated devices, (b) spin-coated PLED (up) and slot-die-coated stripes (down).

Figure 1

(a) The UV-Vis absorption and photoluminescence spectra of the F8T2 and SPG-01T thin films, (b) the respective chromaticity coordinates.

Figure 2

(a) Cyclic voltammetry of both green color-emitting polymers. Energy level diagram of proposed architecture based on (b) F8T2 and (c) SPG-01T as emitting material.

Figure 3

The real ε₁(Ε) (a) and imaginary part ε₂(Ε) (b) of the dielectric function ε(Ε) of the representative spin-coated (SC) and slot-die-coated (SD) F8T2 and SPG-01T thin films, calculated using the best-fit parameters derived by SE, (c) dependence of thickness on rotational speed and the flow rate for the produced SC and SD samples, respectively, and (d) a comparison of the E_g^TL and E_g^Tauc values for the F8T2 and SPG-01T layers with various thicknesses. The inset depicts the E_g^Tauc value variations for different thicknesses of each polymer and fabrication method.

Figure 4

The electroluminescence spectra of the spin-coated and slot-die-coated (a) F8T2-based and (b) SPG-01T-based PLED devices with sequential thickness decrease. (c) The relative intensity of the emission peaks corresponding to different electronic transitions for the sequential thickness decrease (arrows indicate the trend of decreasing thickness). (d) The dependence of peak intensity ratios of I_0–0/(I_0–1 + I_0–2) transitions on the thickness and the EL FWHM for the fabricated devices. (e) The photos of rigid and flexible PLED devices during operation.

Figure 5

Current density-voltage (J-V) and luminance-voltage (L-V) characteristic curves of the selected slot-die-coated (SD) and spin-coated (SC) PLEDs with sequential thicknesses of (a) F8T2 and (b) SPG-01T.

Figure 6

The evolution of EL Chromaticity Coordinates of the selected SD and SC PLEDs with sequential thicknesses of (a) F8T2 and (b) SPG-01T by increasing voltage. The horizontal dotted lines denote the respective (x, y) coordinates of BT709 recommended standard green CIE coordinates. (c) The chromaticity coordinates at 10 V operational voltage of the produced F8T2 and SPG-01T PLEDs.

Figure 7

The UV-Vis absorption of R6G and the electroluminescence spectra of (a) F8T2 and (b) SPG-01T-based spin-coated PLEDs with different thicknesses of EMLs. The PL spectrum of R6G and the measured spectrum after its illumination by the (c) F8T2 and (d) SPG-01T-based spin-coated PLEDs with different thicknesses of EMLs.