In‑Situ Annealing Boosts Mobility of Spray‑Coated TIPS‑Pentacene OFETs

Background

OFETs are promising for flexible electronics, displays, RFID tags, and logic circuits [1–7]. Various deposition methods—blade coating, ink‑jet printing, gravure printing, and spray coating—have been explored to create high‑performance devices [6–16]. Spray coating is particularly attractive due to its low‑solvent requirement, high throughput, and compatibility with diverse substrates [17–19].

Recent studies have examined droplet size [16], solvent composition [20], and substrate heating [21–26] to improve film crystallinity. However, the role of in‑situ annealing during spray coating remains underexplored. Conventional solution processing often necessitates lengthy baking steps that interrupt production. Introducing a simple in‑situ annealing protocol could therefore accelerate manufacturing while preserving or enhancing device performance.

In this work, we applied in‑situ annealing at 60, 90, and 120 °C during spray deposition of TIPS‑pentacene on PMMA dielectrics. The 60 °C treatment yielded the highest mobility (0.191 cm²/Vs), attributable to improved crystallinity and molecular ordering, as confirmed by microscopy and XRD.

Methods

The device architecture is illustrated in Fig. 1(a). PMMA (100 mg/mL in anisole) was spin‑coated to a 520‑nm thickness and baked at 150 °C for 1 h. A 30‑nm TIPS‑pentacene layer (3 mg/mL in dichlorobenzene) was spray‑coated at 20 μL s^–1 from a 12‑cm nozzle height, while the substrate was held at the desired temperature. Finally, 50‑nm Au source/drain electrodes were thermally evaporated through a shadow mask (channel width/length = 1 cm / 100 μm). The TIPS‑pentacene thickness was determined by subtracting the PMMA thickness from the total stack measured with a step profiler. Electrical characteristics were recorded with a Keithley 4200 in air. Mobility was extracted from the saturation regime using the standard transfer‑curve equation. Morphology was examined by optical microscopy (U‑MSSP4, Olympus), AFM (MFP‑3D‑BIO, Asylum Research), and XRD (TD‑3500, Dandong, 30 kV, 20 mA).

a Schematic of OFET fabrication by spray coating. b, c Molecular structures of PMMA and TIPS‑pentacene. d Device architecture.

Results and Discussion

Device A was fabricated with a 120 °C post‑anneal (20 min). Devices B, C, and D employed in‑situ annealing at 60, 90, and 120 °C, respectively. Transfer and output characteristics (Fig. 2) demonstrate typical p‑type behavior. Table 1 summarizes key performance metrics.

a Transfer curves of devices A–D. b–e Output curves.

With 60 °C in‑situ annealing (Device B), mobility rose to 0.191 cm²/Vs, and the threshold voltage shifted positively from –1.7 to –0.9 V. In contrast, 90 °C and 120 °C treatments degraded performance: μ dropped to 0.04 and 0.0005 cm²/Vs, and V_TH shifted to +2.0 V. The sub‑threshold swing increased with annealing temperature, indicating higher trap densities at the dielectric/semiconductor interface [27].

Optical microscopy (Fig. 3) revealed large, elongated grains for Device B, whereas Devices C and D displayed smaller, circular grains. AFM images (Fig. 4) confirmed that the 60 °C film possessed well‑ordered grains with sparse boundaries, while higher temperatures produced densely packed, small islands that impede charge transport. XRD (Fig. 5) showed the strongest (00l) Bragg peaks for Device B, confirming superior crystallinity. The reduced peak intensity at 90 °C and 120 °C correlates with the observed performance decline.

Optical micrographs: (a) post‑annealed 120 °C; (b–d) in‑situ annealing at 60, 90, 120 °C.

AFM topography: (a) post‑annealed 120 °C; (b–d) in‑situ annealing at 60, 90, 120 °C. Insets: 1 μm scale.

Normalized XRD spectra for post‑annealed and in‑situ annealed films.

Conclusions

Spray‑coated TIPS‑pentacene OFETs fabricated with a 60 °C in‑situ anneal exhibit a four‑fold increase in mobility (0.191 cm²/Vs) compared to conventional post‑annealing. This improvement stems from enhanced crystallinity and ordered grain growth, as evidenced by optical, AFM, and XRD analyses. In‑situ annealing therefore offers a scalable, low‑cost route to high‑performance OFETs suitable for flexible electronics and other emerging applications.