Recent Advances of Solution-Processed Heterojunction Oxide Thin-Film Transistors

Thin-film transistors (TFTs) made of metal oxide semiconductors are now increasingly used in flat-panel displays. Metal oxides are mainly fabricated via vacuum-based technologies, but solution approaches are of great interest due to the advantages of low-cost and high-throughput manufacturing. Unfortunately, solution-processed oxide TFTs suffer from relatively poor electrical performance, hindering further development. Recent studies suggest that this issue could be solved by introducing a novel heterojunction strategy. This article reviews the recent advances in solution-processed heterojunction oxide TFTs, with a specific focus on the latest developments over the past five years. Two of the most prominent advantages of heterostructure oxide TFTs are discussed, namely electrical-property modulation and mobility enhancement by forming 2D electron gas. It is expected that this review will manifest the strong potential of solution-based heterojunction oxide TFTs towards high performance and large-scale electronics.


Introduction
Today, there is a growing demand for flat-panel displays with higher resolution, larger screen sizes, better viewing, and lower power consumption, pushing traditional amorphous silicon thin-film transistor (TFT) technology to its limits [1][2][3]. TFTs made of metal oxide semiconductors hold great promise in future display technology, owing to their high mobility, good transparency, and scalability [4][5][6][7]. Commercial metal oxides are grown via physical vapor deposition technologies, but solution-based approaches have been attracting particular attention recently [5,[8][9][10][11]. Compared with conventional vacuum-based technologies, the solution approaches have additional advantages, including cost effectiveness, atmospheric fabrication, higher throughput, and material composition that is easy to tune [12][13][14][15]. Ways to reduce defect states and improve electrical performance and stability are an urgent challenge for solution-based metal oxide TFTs [16,17]. Various approaches have been taken to solve the above challenge, such as doping, modification of components, addition of additives, and novel post-treatments [9,[18][19][20]. However, electron transport properties are still hindered by these defect-prone oxides [21][22][23][24][25].
A notable strategy has been recently developed to enhance the electrical performance of solution-derived oxide TFTs by utilizing heterojunction channels [26]. The schematic of the heterojunction oxide TFTs is demonstrated in Figure 1. It is revealed that heterostructures could modulate electrical performance by taking advantage of both the front channel (providing high mobility) and the back channel (maintaining low off current) [27,28]. More importantly, some recent studies argue that the presence of a 2D electron gas system formed at the carefully engineered oxide heterointerface can greatly improve device mobility [24,[29][30][31][32][33]. In this review, we summarize the recent progress of solution-processed heterostructure oxide TFTs. The heterojunction channel strategy could address the shortcomings of single-layer devices, providing a new route for future TFT technology development [34][35][36].
Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 13 A notable strategy has been recently developed to enhance the electrical performance of solution-derived oxide TFTs by utilizing heterojunction channels [26]. The schematic of the heterojunction oxide TFTs is demonstrated in Figure 1. It is revealed that heterostructures could modulate electrical performance by taking advantage of both the front channel (providing high mobility) and the back channel (maintaining low off current) [27,28]. More importantly, some recent studies argue that the presence of a 2D electron gas system formed at the carefully engineered oxide heterointerface can greatly improve device mobility [24,[29][30][31][32][33]. In this review, we summarize the recent progress of solution-processed heterostructure oxide TFTs. The heterojunction channel strategy could address the shortcomings of single-layer devices, providing a new route for future TFT technology development [34][35][36].

Vacuum-Processed Heterojunction Oxide TFTs
Before reviewing solution-processed heterojunction oxide TFTs, we would like to make a short introduction on vacuum-based heterojunction oxide devices. In 2008, Kim et al. produced InSnO/GaInZnO (ITO/GIZO) heterojunction TFTs by magnetron sputtering, with a high mobility of 104 cm 2 /Vs, a suitable threshold voltage (Vth) of 0.5 V, and a low Vth shift of 0.75 V for 4 h under 10 V bias voltage [37]. They found that the lower layer of highly conductive oxides could provide high mobility for the TFTs, while the upper layer of oxides with lower carrier concentration could adjust the threshold voltage. This new structure provides a new way to adjust the performance of TFTs. Subsequently, a number of scientists have studied and produced various excellent heterojunction oxide TFTs. In 2014, Chen et al. prepared InSnO/SnZnO (ITO/TZO) TFT on a glass substrate by taking advantage of ITO's higher carrier concentration and TZO's ability to control the charge conductance, and they obtained a high mobility of 105 cm 2 /Vs [38]. In 2016, Cong et al. built quasi-double-channel (QDC) AlSnZnO (ATZO) TFTs with a superior mobility of 108 cm 2 /Vs and an on/off ratio of 10 9 [36]. In 2019, He et al. prepared InGaZnO/In2O3 (IGZO/In2O3) TFTs by magnetron sputtering at room temperature, exhibiting high mobility (64.4 cm 2 /Vs) and high on/off ratio (10 7 ), with large enhancement compared with single-layer IGZO and In2O3 TFTs [39]. They attributed this improvement to the defect self-compensation mechanism between the two layers. In 2019, Furuta et al. prepared IGZO/IGZO TFTs with a mobility of 24.7 cm 2 /Vs and an on/off ratio of 10 7 [40]. Table 1 summarizes the recent progress in vacuum-processed heterojunction oxide TFTs. It can be observed that heterojunction oxide TFTs show excellent electrical properties, which are much better than those of the traditional single-layer device.

Vacuum-Processed Heterojunction Oxide TFTs
Before reviewing solution-processed heterojunction oxide TFTs, we would like to make a short introduction on vacuum-based heterojunction oxide devices. In 2008, Kim et al. produced InSnO/GaInZnO (ITO/GIZO) heterojunction TFTs by magnetron sputtering, with a high mobility of 104 cm 2 /Vs, a suitable threshold voltage (V th ) of 0.5 V, and a low V th shift of 0.75 V for 4 h under 10 V bias voltage [37]. They found that the lower layer of highly conductive oxides could provide high mobility for the TFTs, while the upper layer of oxides with lower carrier concentration could adjust the threshold voltage. This new structure provides a new way to adjust the performance of TFTs. Subsequently, a number of scientists have studied and produced various excellent heterojunction oxide TFTs. In 2014, Chen et al. prepared InSnO/SnZnO (ITO/TZO) TFT on a glass substrate by taking advantage of ITO's higher carrier concentration and TZO's ability to control the charge conductance, and they obtained a high mobility of 105 cm 2 /Vs [38]. In 2016, Cong et al. built quasi-double-channel (QDC) AlSnZnO (ATZO) TFTs with a superior mobility of 108 cm 2 /Vs and an on/off ratio of 10 9 [36]. In 2019, He et al. prepared InGaZnO/In 2 O 3 (IGZO/In 2 O 3 ) TFTs by magnetron sputtering at room temperature, exhibiting high mobility (64.4 cm 2 /Vs) and high on/off ratio (10 7 ), with large enhancement compared with single-layer IGZO and In 2 O 3 TFTs [39]. They attributed this improvement to the defect self-compensation mechanism between the two layers. In 2019, Furuta et al. prepared IGZO/IGZO TFTs with a mobility of 24.7 cm 2 /Vs and an on/off ratio of 10 7 [40]. Table 1 summarizes the recent progress in vacuum-processed heterojunction oxide TFTs. It can be observed that heterojunction oxide TFTs show excellent electrical properties, which are much better than those of the traditional single-layer device.

Solution-Processed Heterojunction Oxide TFTs
Compared with conventional vapor-based techniques, solution processing (such as spin-coating, spraying, and printing) allows for the design and fabrication of novel oxide TFTs in a low-cost and straightforward fashion [43][44][45]. Many researchers have begun to study solution-grown heterojunction oxide TFTs. Table 2 and Figure 2 summarize the recent advances in solution-processed heterojunction oxide TFTs. For heterojunction oxide TFTs, we discuss two of the most prominent advantages, namely electrical-property modulation and mobility enhancement by forming 2D electron gas.
Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 13 straightforward fashion [43][44][45]. Many researchers have begun to study solution-grown heterojunction oxide TFTs. Table 2 and Figure 2 summarize the recent advances in solution-processed heterojunction oxide TFTs. For heterojunction oxide TFTs, we discuss two of the most prominent advantages, namely electrical-property modulation and mobility enhancement by forming 2D electron gas.

Electrical-Property Modulation
As a product of structural engineering technology, heterojunction oxide TFTs can take advantage of the excellent electrical properties of each layer [30,37]. As the front-channel layer has good conductivity, it can provide higher carrier concentration, thus forming maximum charge accumulation and finally achieving high mobility [10,41,42]. The carrier concentration in the back-channel layer is much lower than that of the front-channel layer, which leads to the difference in electron activation energy between the conduction band minimum and the Fermi energy level, forming an energy barrier at the interface [51]. Due to the existence of an energy barrier, the back-channel layer can effectively control the electron flow, thus reducing off current (I off ) and adjusting threshold voltage (V th ) [46]. A series of representative papers show that heterojunction TFTs can achieve both high mobility and I off by selecting suitable front-and back-channel materials.
In 2012, Jeong et al. adjusted the carrier concentration of the AlInZnO/InZnO (AIZO/IZO) interface and barrier height by changing the ratio of In/Zn in the IZO layer and the thickness of the IZO layer. The IZO layer could provide an enhanced mobility for the device due to its high electron concentration. Compared with the IZO layer, the AIZO layer had a larger E C -E F due to its lower carrier concentration, forming an energy barrier at the interface and reducing I off . As the thickness of the conductive IZO layer decreased, the AIZO/IZO TFTs V th shifted positive, and I off decreased from 10 −8 to 10 −11 A. With a 12-nm-thick IZO layer, they obtained a device mobility of 5.63 cm 2 /Vs and an on/off ratio of 10 6 [46]. Rim et al. boosted up the mobility of solution-processed oxide TFTs using an extremely thin layer of conductive InSnZnO (ITZO) inserted between the dielectric layer and the InGaZnO (IGZO) active layer [51]. The ITZO/IGZO TFTs have a high mobility (22.16 cm 2 /Vs) and an excellent on/off current ratio (10 7 ). As shown in Figure 3a, the mobility of ITZO/IGZO is over ten times higher than that of single-layer IGZO (from 1.56 to 22.16 cm 2 /Vs). At the front channel of ITZO, Sn 4+ replaced In 3+ to provide additional electrons to increase electron concentration, forming a highly conductive channel and providing high mobility for devices. Moreover Nam et al. prepared high-performance solution-processed indium-free ZnO/SnO2 TFTs at 300 °C by UV annealing [53]. The ZnO/SnO2 TFTs exhibited a mobility of 15.4 cm 2 /Vs, an outstanding on/off ratio of 10 8 , and superior bias stability. As shown in Figure 4a, ZnO/SnO2 bilayers are composed of a Zn-rich layer, a Zn-Sn mixed zone, and an Sn-rich layer. The Sn-rich channel has high conductivity and provides a path for rapid electronic transport. Meanwhile, Zn atoms can diffuse into the Zn-Sn mixing zone to reduce Ioff by control carrier concentration. Furthermore, due to the suppression of oxygen vacancy (Vo) defects in the bilayer film, the ZnO/SnO2 TFTs exhibited remarkable bias-stress stability (Figure 4b). Nam et al. prepared high-performance solution-processed indium-free ZnO/SnO 2 TFTs at 300 • C by UV annealing [53]. The ZnO/SnO 2 TFTs exhibited a mobility of 15.4 cm 2 /Vs, an outstanding on/off ratio of 10 8 , and superior bias stability. As shown in Figure 4a, ZnO/SnO 2 bilayers are composed of a Zn-rich layer, a Zn-Sn mixed zone, and an Sn-rich layer. The Sn-rich channel has high conductivity and provides a path for rapid electronic transport. Meanwhile, Zn atoms can diffuse into the Zn-Sn mixing zone to reduce I off by control carrier concentration. Furthermore, due to the suppression of oxygen vacancy (V o ) defects in the bilayer film, the ZnO/SnO 2 TFTs exhibited remarkable bias-stress stability (Figure 4b).

Mobility Enhancement by Forming 2D Electron Gas
In addition to electrical-performance modulation, some recent studies suggest that well-designed heterojunction oxides could greatly boost mobility by forming 2D electron gas at the interface [52]. Through careful interface engineering, electron transfer and confinement at the heterointerface can occur because of a large conduction band offset between the two layers, resulting in the formation of 2D electron gas in the interface [55]. The formation of 2D electron gas enables the realization of TFTs with mobilities close to the theoretical limit set by phonon scattering in the absence of impurity scattering [30]. In this situation, the mobility of the heterojunction device is often several times or even ten times higher than that of the single-layer device. Additionally, the enhanced electron mobility is accompanied by a marked change in the charge transport mechanism. Through fitting the transfer curves and analyzing the temperature dependence of mobility, it was revealed that heterojunction TFTs exhibited band-like electron transport, while the single-layer device showed trap-limited conduction [55]. It should be mentioned that the transfer curve of the heterojunction TFTs shifted to the negative direction compared with the single-layer device, due to the formation of 2D electron gas. Nam et al. prepared high-performance solution-processed indium-free ZnO/SnO2 TFTs at 300 °C by UV annealing [53]. The ZnO/SnO2 TFTs exhibited a mobility of 15.4 cm 2 /Vs, an outstanding on/off ratio of 10 8 , and superior bias stability. As shown in Figure 4a, ZnO/SnO2 bilayers are composed of a Zn-rich layer, a Zn-Sn mixed zone, and an Sn-rich layer. The Sn-rich channel has high conductivity and provides a path for rapid electronic transport. Meanwhile, Zn atoms can diffuse into the Zn-Sn mixing zone to reduce Ioff by control carrier concentration. Furthermore, due to the suppression of oxygen vacancy (Vo) defects in the bilayer film, the ZnO/SnO2 TFTs exhibited remarkable bias-stress stability (Figure 4b). Reproduced with permission [53]. Copyright 2016, Royal Society of Chemistry.

Mobility Enhancement by Forming 2D Electron Gas
In addition to electrical-performance modulation, some recent studies suggest that welldesigned heterojunction oxides could greatly boost mobility by forming 2D electron gas at the interface [52]. Through careful interface engineering, electron transfer and confinement at the heterointerface can occur because of a large conduction band offset between the two layers, resulting in the formation of 2D electron gas in the interface [55]. The formation of 2D electron gas enables the realization of TFTs with mobilities close to the theoretical limit set by phonon scattering in the absence of impurity scattering [30]. In this situation, the mobility of the heterojunction device is often several times or even ten times higher than that of the single-layer device. Additionally, the enhanced electron mobility is accompanied by a marked change in the charge transport mechanism. Through fitting the transfer curves and analyzing the temperature dependence of mobility, it was revealed that heterojunction TFTs exhibited band-like electron transport, while the single-layer device showed  [55]. The mobility of In 2 O 3 /ZnO TFTs (45 cm 2 /Vs) was 2 to 100 times greater than that of single-layer In 2 O 3 and ZnO devices. According to X-ray photoelectron spectroscopy, optical absorption, and Kelvin probe measurements, the In 2 O 3 /ZnO interface has a large conduction band offset (0.36 eV), which makes the electron transfer from the ZnO layer to In 2 O 3 and forms 2D electron gas. 2D electron gas greatly increases the concentration of free electrons in the In 2 O 3 layer of the crystal. The electron transport mechanism of In 2 O 3 /ZnO TFTs was changed from trap-limited conduction to percolation conduction. This marked improvement originated from the presence of 2D electron gas formed at the atomically sharp heterointerface induced by the large conduction band offset between In 2 O 3 and ZnO. The In 2 O 3 /ZnO TFTs developed in this work not only surpassed the performance of single-layer In 2 O 3 and ZnO TFTs but also compared favorably to state-of-the-art vacuum-processed devices. Lin et al. used solution-grown In 2 O 3 , Ga 2 O 3 , and ZnO to construct heterojunction and quasi-superlattice (QSL) TFTs (Figure 5a) [52]. By carefully optimizing the structure, QSLs with smooth interfaces and surfaces could be realized (Figure 5b). As shown in Figure 5c, it was proved that single-layer metal oxide TFTs were dominated by trap-limited conduction (TLC), while QSL-I/III were dominated by percolation conduction (PC). The change of electron transport mode led to a great increase in electron mobility (from 4 to 30 cm 2 /Vs).
vacuum-processed devices. Lin et al. used solution-grown In2O3, Ga2O3, and ZnO to construct heterojunction and quasi-superlattice (QSL) TFTs (Figure 5a) [52]. By carefully optimizing the structure, QSLs with smooth interfaces and surfaces could be realized (Figure 5b). As shown in Figure  5c, it was proved that single-layer metal oxide TFTs were dominated by trap-limited conduction (TLC), while QSL-I/III were dominated by percolation conduction (PC). The change of electron transport mode led to a great increase in electron mobility (from 4 to 30 cm 2 /Vs). Later studies showed that layer configuration and annealing temperature greatly affect heterojunction device performance. Khim explored the effect of layer configuration on electron transport in heterojunction transistors composed of ZnO and In2O3 [57]. They found that depositing In2O3 first followed by ZnO resulted in a smooth interface, while reversing the layer order yielded poor interface roughness. Tetzner et al. studied the influence of annealing temperature on morphology, chemical state, and electrical performance of solution-based heterostructure In2O3/ZnO TFTs [54]. It was found that the annealing temperature changed surface roughness and atomic diffusion at the interface of In2O3/ZnO. At the annealing temperature of 400 °C, the In2O3/ZnO TFTs showed an optimized mobility of 48 cm 2 /Vs and an on/off current ratio of ~10 4 .
The large conduction band offset between the two layers is one of the key points to induce 2D electron gas for mobility improvement. A doping strategy has been adopted to enlarge the conduction band offset between the two layers. Khim and co-workers reported the controlled growth of In2O3/Li-ZnO TFTs by modulation doping [29]. It was revealed that Li addition in ZnO led to ntype doping and allowed for the accurate tuning of its Fermi energy. Therefore, doping of Li could precisely regulate ΔEF between In2O3 and Li-ZnO and change the conduction band offset between the two layers ( Figure 6). When the doping amount of Li was 20%, the mobility of In2O3/Li-ZnO Later studies showed that layer configuration and annealing temperature greatly affect heterojunction device performance. Khim explored the effect of layer configuration on electron transport in heterojunction transistors composed of ZnO and In 2 O 3 [57]. They found that depositing In 2 O 3 first followed by ZnO resulted in a smooth interface, while reversing the layer order yielded poor interface roughness. Tetzner et al. studied the influence of annealing temperature on morphology, chemical state, and electrical performance of solution-based heterostructure In 2 O 3 /ZnO TFTs [54]. It was found that the annealing temperature changed surface roughness and atomic diffusion at the interface of In 2 O 3 /ZnO. At the annealing temperature of 400 • C, the In 2 O 3 /ZnO TFTs showed an optimized mobility of 48 cm 2 /Vs and an on/off current ratio of~10 4 .
The large conduction band offset between the two layers is one of the key points to induce 2D electron gas for mobility improvement. A doping strategy has been adopted to enlarge the conduction band offset between the two layers. Khim and co-workers reported the controlled growth of In 2 O 3 /Li-ZnO TFTs by modulation doping [29]. It was revealed that Li addition in ZnO led to n-type doping and allowed for the accurate tuning of its Fermi energy. Therefore, doping of Li could precisely regulate ∆E F between In 2 O 3 and Li-ZnO and change the conduction band offset between the two layers ( Figure 6). When the doping amount of Li was 20%, the mobility of In 2 O 3 /Li-ZnO heterojunction TFTs reached the maximum value of 11.4 cm 2 /Vs and the on/off current ratio of~10 5 . Chen et al. demonstrated high performance In 2 O 3 /In 2 O 3 :polyethylenimine (PEI) heterostructure TFTs [56]. The 2D electron gas was achieved by creating a band offset between In 2 O 3 and In 2 O 3 :PEI via work function tuning of the PEI-doping ratio. The resulting device exhibited a mobility of 10 cm 2 /Vs on SiO 2 gate dielectric. Similarly, Liu et al. took In 2 O 3 as the front channel and combined it with the back-channel AlInO to construct heterojunction transistors. By adjusting the thickness of AlInO and the doping amount of Al, AlInO (30%)/In 2 O 3 heterostructure TFTs with a high mobility of 40 cm 2 /Vs, a threshold slope of 0.7 V/dec, and an on/off ratio of 10 7 could be realized [58]. [56]. The 2D electron gas was achieved by creating a band offset between In2O3 and In2O3:PEI via work function tuning of the PEI-doping ratio. The resulting device exhibited a mobility of 10 cm 2 /Vs on SiO2 gate dielectric. Similarly, Liu et al. took In2O3 as the front channel and combined it with the back-channel AlInO to construct heterojunction transistors. By adjusting the thickness of AlInO and the doping amount of Al, AlInO (30%)/In2O3 heterostructure TFTs with a high mobility of 40 cm 2 /Vs, a threshold slope of 0.7 V/dec, and an on/off ratio of 10 7 could be realized [58].

Conclusions and Outlooks
Great progress has been made in the past few years in solution-processed heterojunction oxide TFTs. A detailed review of this topic has been presented, with special attention on the latest developments over the past 5 years. It was revealed that heterojunction channels could overcome the disadvantages of single-layer structures. By using this novel strategy, solution-based oxide transistors with high mobility (~50 cm 2 /Vs) and operational stability could be realized, competing with or even surpassing vacuum-grown counterparts.
In terms of future research directions, several key issues need to be addressed. First, the reported high mobility heterojunction oxide TFTs often suffered from negative threshold voltages, high offstate currents, or poor stability, which have a negative impact on commercial applications. The negative threshold voltage and high off-state current of heterojunction oxide TFTs are closely related to the formation of 2D electron gas. Lee et al. constructed corrugated structure ITZO/IGZO TFTs with both high mobility (51 cm 2 /Vs) and low off-state current [30]. The thin ITZO/IGZO portion increased the overall resistivity of the current path, effectively reducing the off-state current. The thick ITZO/IGZO could provide free electrons to form a high-speed electronic channel, highly improving the electron mobility. By using this new corrugated heterojunction, solution-based oxide TFTs with high mobility and low off current could be realized. Unfortunately, this could increase the complexity of the process. Lin et al. reported solution-processed ZnO/ZnO-NPs/PS/In2O3 multilayer TFTs with high electron mobility (50 cm 2 /Vs) and prolonged operational stability [31]. Insertion of the ozonetreated polystyrene interlayer could passivate electron traps in the channel, leading to high mobility and excellent operational stability. However, continued work should be carried out. It should be mentioned that well-optimized single-layer vacuum-based metal oxide TFTs show high performance and stability. Secondly, previous studies argued that the mobility enhancement is attributed to the 2D electron gas that formed at the heterogeneous interface. However, the traditional 2D electron gas usually exists in high-quality epitaxial heterojunction systems (such as AlGaAs/GaAs

Conclusions and Outlooks
Great progress has been made in the past few years in solution-processed heterojunction oxide TFTs. A detailed review of this topic has been presented, with special attention on the latest developments over the past 5 years. It was revealed that heterojunction channels could overcome the disadvantages of single-layer structures. By using this novel strategy, solution-based oxide transistors with high mobility (~50 cm 2 /Vs) and operational stability could be realized, competing with or even surpassing vacuum-grown counterparts.
In terms of future research directions, several key issues need to be addressed. First, the reported high mobility heterojunction oxide TFTs often suffered from negative threshold voltages, high off-state currents, or poor stability, which have a negative impact on commercial applications. The negative threshold voltage and high off-state current of heterojunction oxide TFTs are closely related to the formation of 2D electron gas. Lee et al. constructed corrugated structure ITZO/IGZO TFTs with both high mobility (51 cm 2 /Vs) and low off-state current [30]. The thin ITZO/IGZO portion increased the overall resistivity of the current path, effectively reducing the off-state current. The thick ITZO/IGZO could provide free electrons to form a high-speed electronic channel, highly improving the electron mobility. By using this new corrugated heterojunction, solution-based oxide TFTs with high mobility and low off current could be realized. Unfortunately, this could increase the complexity of the process. Lin et al. reported solution-processed ZnO/ZnO-NPs/PS/In 2 O 3 multilayer TFTs with high electron mobility (50 cm 2 /Vs) and prolonged operational stability [31]. Insertion of the ozone-treated polystyrene interlayer could passivate electron traps in the channel, leading to high mobility and excellent operational stability. However, continued work should be carried out. It should be mentioned that well-optimized single-layer vacuum-based metal oxide TFTs show high performance and stability. Secondly, previous studies argued that the mobility enhancement is attributed to the 2D electron gas that formed at the heterogeneous interface. However, the traditional 2D electron gas usually exists in high-quality epitaxial heterojunction systems (such as AlGaAs/GaAs heterojunctions). The oxide thin films prepared by the solution method are usually polycrystalline or amorphous, and the 2D electron gas formation mechanism is still unclear. Thirdly, the carrier transport properties in heterojunction oxide need further investigation. A deep understanding of the heterojunction electron transport can promote better design of high-performance heterojunction oxide devices. Fourthly, the previous research mainly focused on the In 2 O 3 /ZnO system; it is necessary to extend this to other multicomponent oxide semiconductor heterostructures (such as the IGZO system) for further device performance improvement. We believe that by addressing the issues presented above, solution-processed heterojunction oxide TFTs will show great promise in future large-area and high-performance electronics.