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 (ΔVon). 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 (ΔVon 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 (ΔVon 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/Tb4f n—O2p6 to Pr/Tb 4f n+1—O2p5 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 4f 7 with a isotropic half-full configuration compared to that of Pr 4f 1, which would make it easier for the Tb4+ to absorb the visible light than the Pr4+.

Addressing the Conflict between Mobility and Stability in Oxide Thin‐film Transistors

“…In the bilayer structure, the photo‐induced electrons in the CTL quickly move into the CRL, and then can promote the neutralization of ionized V O with aid of the Pr dopant in the CRL. [ 65 , 66 ] Consequently, the total number of ionized V O in the bilayer decreases, leading to reduced photocurrent of the bilayer TFTs. It may be an average effect.…”

Enhancement of electrical stability of metal oxide thin-film transistors against various stresses

“…Furthermore, the transition of V O states to V O 2+ shallow states induced by illumination would result in the generation of additional delocalized electrons in the CBM. 53 Especially in ambient atmosphere, light exposure would also lead to the oxygen desorption at the back-channel surface of AOS, resulting in the addition of free electrons in the channel. 95 Therefore, negative bias illumination stress (NBIS) reduced the threshold voltage values needed to achieve the same drain current and caused a negative shift in the threshold voltage values (Fig.…”

“…Furthermore, recently, it has been reported that doping certain metal cations can suppress the activation of V O . 53 Specifically, doping certain metal cations such as tetravalent lanthanides can prevent the activation of V O under illumination stress by converting incoming illumination to non-radiative transition due to charge transfer transition. 54 Thus, choosing an appropriate amount of dopants can result in AOS TFTs with high stability.…”