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 In
2O
3 semiconductors doped with Pr
4+ or Tb
4+, which can effectively improve the NBIS stability. The differences between the Pr
4+-doped In
2O
3 (Pr:In
2O
3) and Tb
4+-doped In
2O
3 (Tb:In
2O
3) are investigated in detail. The undoped In
2O
3 TFTs with different annealing temperatures exhibit poor NBIS stability with serious turn-on voltage shift (Δ
Von). After doping with Pr
4+/Tb
4+, the TFTs show greatly improved NBIS stability. As the annealing temperature increases, the Pr:In
2O
3 TFTs have poorer NBIS stability (Δ
Von are −3.2, −4.8, and −4.8 V for annealing temperature of 300, 350, and 400 °C, respectively), while the Tb:In
2O
3 TFTs have better NBIS stability (Δ
Von 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 Pr
4+/Tb
4+:In
2O
3 TFTs is attributed to the absorption of the illuminated light by the Pr/Tb4
fn—O2
p6 to Pr/Tb 4
fn+1—O2
p5 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:In
2O
3 TFTs compared to Pr:In
2O
3 TFTs is ascribed to the smaller ion radius of Tb
4+ and the lower energy level of Tb 4
f7 with a isotropic half-full configuration compared to that of Pr 4
f1, which would make it easier for the Tb
4+ to absorb the visible light than the Pr
4+.
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