The Mechanism of the Photostability Enhancement of Thin-Film Transistors Based on Solution-Processed Oxide Semiconductors Doped with Tetravalent Lanthanides

Abstract

The applications of thin-film transistors (TFTs) based on oxide semiconductors are limited due to instability under negative bias illumination stress (NBIS). Here, we report TFTs based on solution-processed In₂O₃ semiconductors doped with Pr⁴⁺ or Tb⁴⁺, which can effectively improve the NBIS stability. The differences between the Pr⁴⁺-doped In₂O₃ (Pr:In₂O₃) and Tb⁴⁺-doped In₂O₃ (Tb:In₂O₃) are investigated in detail. The undoped In₂O₃ TFTs with different annealing temperatures exhibit poor NBIS stability with serious turn-on voltage shift (Δ<i>V_on</i>). After doping with Pr⁴⁺/Tb⁴⁺, the TFTs show greatly improved NBIS stability. As the annealing temperature increases, the Pr:In₂O₃ TFTs have poorer NBIS stability (Δ<i>V_on</i> are -3.2, -4.8, and -4.8 V for annealing temperature of 300, 350, and 400 °C, respectively), while the Tb:In₂O₃ TFTs have better NBIS stability (Δ<i>V_on</i> are -3.6, -3.6, and -1.2 V for annealing temperature of 300, 350, and 400 ℃, respectively). Further studies reveal that the improvement of the NBIS stability of the Pr⁴⁺/Tb⁴⁺:In₂O₃ TFTs is attributed to the absorption of the illuminated light by the Pr/Tb4<i>f</i>ⁿ-O2<i>p</i>⁶ to Pr/Tb 4<i>f</i>ⁿ⁺¹-O2<i>p</i>⁵ charge transfer (CT) transition and downconversion of the light to nonradiative transition with a relatively short relaxation time compared to the ionization process of the oxygen vacancies. The higher NBIS stability of Tb:In₂O₃ TFTs compared to Pr:In₂O₃ TFTs is ascribed to the smaller ion radius of Tb⁴⁺ and the lower energy level of Tb 4<i>f</i>⁷ with a isotropic half-full configuration compared to that of Pr 4<i>f</i>¹, which would make it easier for the Tb⁴⁺ to absorb the visible light than the Pr⁴⁺.