Dielectric Surface-Controlled Low-Voltage Organic Transistors via n-Alkyl Phosphonic Acid Self-Assembled Monolayers on High-k Metal Oxide

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO2) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO2 with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO2 hybrid dielectrics, pentacene-based OTFTs operate under −2.0 V with low hysteresis, on-off current ratios above 1 × 106, threshold voltages below −0.6 V, subthreshold slopes as low as 100 mV dec−1, and field-effect mobilities as high as 1.8 cm2 V−1 s−1.