Ambipolar Transport in Methylammonium Lead Iodide Thin Film Transistors

We report clear room temperature ambipolar transport in ambient-air processed methylammonium lead iodide (MAPbI3) thin-film transistors (TFTs) with aluminum oxide gate-insulators and indium-zinc-oxide source/drain electrodes. The high ionicity of the MAPbI3 leads to p-type and n-type self-doping, and depending on the applied bias we show that simultaneous or selective transport of electrons and/or holes is possible in a single MAPbI3 TFT. The electron transport (n-type), however, is slightly more pronounced than the hole transport (p-type), and the respective channel resistances range from 5–11 and 44–55 MΩ/μm. Both p-type and n-type TFTs show good on-state characteristics for low driving voltages. It is also shown here that the on-state current of the n-type and p-type TFTs is highest in the slightly PbI2-rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by varying precursor ratio.

For large-area TFT applications, MAPbI 3 enables low manufacturing costs, as it can be synthesized from simple precursors through solution processing or vapor deposition methods [17][18][19][20][21][22]. Additionally, MAPbI 3 possesses high carrier mobility and a bandgap in the near-infrared energies, which can be easily tuned with chemical composition [23][24][25][26][27]. The ability to be fabricated from solution and its potential for high field-effect mobility (µ FE ) has been particularly interesting in flexible, printable or wearable devices, where the development of such a semiconductor is key [4].
Due to the existence of both the inorganic and organic components, MAPbI 3 has the potential for ambipolar transport, where simultaneous or selective transport of electrons and/or holes is possible. Theoretical calculations have revealed that charged MA, Pb, and I vacancies have low formation energies [28,29], suggesting that the high ionicity of this material may lead to n-type or p-type self-doping. Furthermore, previous studies have shown that carrier concentration can vary over orders of magnitude, depending on the precursor ratio (MAI/PbI 2 ) and thermal annealing conditions, thus enables the design of circuits with low-power dissipation and good noise margins, similar to those encountered in complementary metal-oxide semiconductor (CMOS) logic circuits. However, nearly all MAPbI3 TFTs in previous studies have been shown to transport only electrons (n-type) [17,18,31,32] with the exception of those measured at and below 198 K with high driving voltages (over 100 V) and/or doped with cesium (Cs) and formamidinium (FA) [2,33].
Here we investigate the room temperature (RT) ambipolar transport in the MAPbI3 TFTs by correlating charge carrier transport with the TFTs' current-voltage (I-V) characteristics. In contrast to previous reports, we process the MAPbI3 in ambient air for room temperature stability and use a thin (32 nm) UV ozone-treated aluminum oxide (AlOx) gate-insulator (GI) to decrease the driving voltages.

Materials and Methods
We fabricated the MAPbI3 TFTs on glass with the inverted coplanar structure (Figure 1). The gate electrode is a 40-nm-thick Mo layer deposited by sputtering at 280 °C. For the gate insulator (G. I), we used a 40-nm-thick AlOx layer spin coated from solution in ambient air (relative humidity = 50 ± 5, Temperature = 25 °C ± 2 °C). We prepared the AlOx solution by mixing aluminium chloride (AlCl3) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd. ), acetonitrile (35%) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.), and ethyleneglycol (65%) (0.2 M) (Merck Korea Ltd./Sigma-AldrichKorea Ltd.). The solution is spin coated at 2000 rotations per min (rpm) for 30 s. Two layers were used to achieve the thickness of 40 nm and curing at 250 °C for five minutes was performed to evaporate the solvent, followed by 5 min UV ozone treatment. For film densification, we performed a final anneal for 2 h at 350 °C. Using Mo/AlOx/indium zinc oxide (IZO) structures (400 μm × 400 μm), the capacitance of the AlOx layer per unit area (COX) is estimated to be approximately 200 nF/cm 2 . We define the source and drain electrodes by patterning a 60 nm thick IZO layer deposited at 280 °C by sputtering. We spincoated a positive photoresist (PR) (Sigma-Aldrich, Inc., Steinheim, Germany) layer on top of the source and drain electrodes and patterned it to form "banks" into which we spin coated the MAPbI3. The banks were necessary to avoid the use of photolithography processes directly on the hybrid perovskite because the MAPbI3 is vulnerable to wet processes. After the formation of the banks, we subjected the samples to UV-ozone treatment (AHTECH LTS., Gyeonggi-do, South Korea) for 15 min before depositing the MAPbI3 in ambient air (at 25 °C and relative humidity of ~52%). The UV-ozone treatment modifies the surface of the AlOx G.I. for better MAPbI3 adhesion. To make the MAPbI3 solution, we dissolved precursors CH3NH3I (99.999%) (Greatcell Solar, Queanbeyan, Australia) and PbI2 (99.999%, Sigma-Aldrich) (1:1 molar ratio) (manufacturer, city, Country) in N, N dimethylformamide anhydrous, DMF (99.8%) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.), and added 50 μL of hydriodic acid (57 wt%) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.). We used MAI:PbI2 ratios of 0.5:1, 0.8:1, 1:1, and 1.2:1 and stirred the solutions for six hours at 60°C before spin-coating in ambient air. The spin-coating speed and time were respectively 6000 rpm and 70 s, achieving a thickness of ~130 nm for the MAPbI3. During the After the formation of the banks, we subjected the samples to UV-ozone treatment (AHTECH LTS., Gyeonggi-do, South Korea) for 15 min before depositing the MAPbI 3 in ambient air (at 25 • C and relative humidity of~52%). The UV-ozone treatment modifies the surface of the AlO x G.I. for better MAPbI 3 adhesion. To make the MAPbI 3 solution, we dissolved precursors CH 3 NH 3 I (99.999%) (Greatcell Solar, Queanbeyan, Australia) and PbI 2 (99.999%, Sigma-Aldrich) (1:1 molar ratio) (manufacturer, city, Country) in N, N dimethylformamide anhydrous, DMF (99.8%) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.), and added 50 µL of hydriodic acid (57 wt%) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.). We used MAI:PbI 2 ratios of 0.5:1, 0.8:1, 1:1, and 1.2:1 and stirred the solutions for six hours at 60 • C before spin-coating in ambient air. The spin-coating speed and time were respectively 6000 rpm and 70 s, achieving a thickness of~130 nm for the MAPbI 3 . During the spin-coating process, we added 150 µL of toluene (99.8%, Sigma-Aldrich) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.) as an anti-solvent. The toluene reduces the crystallization time by rapidly reducing the solubility of the MAPbI 3 , thereby promoting fast nucleation and growth of crystals in the film.
As the MAPbI 3 solutions sunk into the holes in the PR layer (Figure 1), it was not necessary to pattern the MAPbI 3 films through photolithography. Note that direct photolithography on these hybrid materials may lead to contamination. Finally, we annealed the samples at 115 • C for 10 min on a hotplate to finish the crystallization process. We analyzed the physical properties of the films using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, INCORPORATED, PHI 5000 VersaProbe, Kanagawa, Japan), atomic force microscope (AFM) images (Park Systems, XE-100, Suwon, Korea), X-ray diffraction (XRD) (Malvern Panalytical Ltd, X'Pert PRO, Royston, UK) and scanning electron microscope (SEM) images (Hitachi High Technologies, S-4700, Tokyo, Japan ). We respectively derived the µ

Results
Consistent with previous reports [34], the ambient air-processed MAPbI 3 films consisted of a network of thick dendritic bundles and the thickness of the bundles increased with increasing MAI/PbI 2 ratio ( Figure 2). This dendritic structure is common to films exposed to humidity during processing and it indicates anisotropic growth, where faster growth occurs along energetically favorable crystallographic directions [34]. Despite the poor film coverage, dendritic bundles are known for their good film conductivity and are thus desirable for lateral conduction in solution processed semiconductors [34]. Each of the bundles exhibited good crystallinity as seen from the high resolution SEM images (Figure 2b,d,f,h) and XRD analysis. spin-coating process, we added 150 μL of toluene (99.8%, Sigma-Aldrich) (Merck Korea Ltd./Sigma-Aldrich Korea Ltd.) as an anti-solvent. The toluene reduces the crystallization time by rapidly reducing the solubility of the MAPbI3, thereby promoting fast nucleation and growth of crystals in the film. As the MAPbI3 solutions sunk into the holes in the PR layer (Figure 1), it was not necessary to pattern the MAPbI3 films through photolithography. Note that direct photolithography on these hybrid materials may lead to contamination. Finally, we annealed the samples at 115 °C for 10 min on a hotplate to finish the crystallization process. We analyzed the physical properties of the films using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, INCORPORATED ,PHI 5000 VersaProbe, Kanagawa, Japan), atomic force microscope (AFM) images (Park Systems, XE-100, Suwon, Korea), X-ray diffraction (XRD) (Malvern Panalytical Ltd, X'Pert PRO, Royston, UK) and scanning electron microscope (SEM) images (Hitachi High Technologies, S-4700, Tokyo, Japan ). We respectively derived the μFE and threshold voltage (VTH) values from the slopes and intercepts of the linear regression lines of the ID 1/2 (VGS) plots in the saturation region (VDS > VGS -VTH), according to the equation, ID = (W/L)*COX*μFE*(VGS -VTH) 2 . Here, W, L, VGS, VDS, and ID denote the channel width, channel length, gate voltage, drain voltage, and drain current, respectively. We derived the μFE and VTH from the |IDS| 1/2 (VGS) plots. We estimated the subthreshold VGS swing (SS) by (d log (ID)/d VGS) −1 .

Results
Consistent with previous reports [34], the ambient air-processed MAPbI3 films consisted of a network of thick dendritic bundles and the thickness of the bundles increased with increasing MAI/PbI2 ratio ( Figure 2). This dendritic structure is common to films exposed to humidity during processing and it indicates anisotropic growth, where faster growth occurs along energetically favorable crystallographic directions [34]. Despite the poor film coverage, dendritic bundles are known for their good film conductivity and are thus desirable for lateral conduction in solution processed semiconductors [34]. Each of the bundles exhibited good crystallinity as seen from the high resolution SEM images (Figure 2b,d,f, and h) and XRD analysis.   XRD measurements verified that the films were MAPbI 3 films of the intended MAI/PbI 2 ratios. The XRD patterns of all the four ambient air-processed MAPbI 3 films contained sharp peaks at 2θ values of 13.9 • , 28.2 • , and 31.7 • , the intensity of which increased with increasing MAI/PbI 2 ratio ( Figure 3). These peaks are respectively assigned to the (110), (220) and the (310) crystal planes of the MAPbI 3 and they indicate high crystallinity in tetragonal phase [35]. Consistently, the intensity of the peak at the 2θ value of 12.67 • gradually decreases with increasing MAI/PbI 2 ratio until it completely disappears when the ratio exceeds one. This is the PbI 2 peak and its absence indicates the absence of PbI 2 residues in the film [36][37][38][39][40].
Crystals 2019, 9, x FOR PEER REVIEW 4 of 13 XRD measurements verified that the films were MAPbI3 films of the intended MAI/PbI2 ratios. The XRD patterns of all the four ambient air-processed MAPbI3 films contained sharp peaks at 2θ values of 13.9°, 28.2°, and 31.7°, the intensity of which increased with increasing MAI/PbI2 ratio ( Figure 3). These peaks are respectively assigned to the (110), (220) and the (310) crystal planes of the MAPbI3 and they indicate high crystallinity in tetragonal phase [35]. Consistently, the intensity of the peak at the 2θ value of 12.67° gradually decreases with increasing MAI/PbI2 ratio until it completely disappears when the ratio exceeds one. This is the PbI2 peak and its absence indicates the absence of PbI2 residues in the film [36][37][38][39][40]. The AFM images of the AlOx and MAPbI3 film (MAI/PbI2 = 1.0) are shown in Figure 4a,b, respectively. The AlOx form a very smooth film with roughness of root mean square (rms) value approximately equal to 0.2 nm. However, the dendrites of the MAPbI3 film results in a much higher rms value of the roughness, reaching 40 nm. The AlOx is amorphous ( Figure 5a) and its atomic concentrations are shown in Figure 5b. Using a TFT structure (Figure 1), we measured the breakdown voltage of the AlOx gate insulator to be 4 MV/cm Figure 5c). These results are consistent with previous reports [41].  The AFM images of the AlO x and MAPbI 3 film (MAI/PbI 2 = 1.0) are shown in Figure 4a,b, respectively. The AlOx form a very smooth film with roughness of root mean square (rms) value approximately equal to 0.2 nm. However, the dendrites of the MAPbI 3 film results in a much higher rms value of the roughness, reaching 40 nm. The AlO x is amorphous ( Figure 5a) and its atomic concentrations are shown in Figure 5b. Using a TFT structure (Figure 1), we measured the breakdown voltage of the AlOx gate insulator to be 4 MV/cm Figure 5c). These results are consistent with previous reports [41]. XRD measurements verified that the films were MAPbI3 films of the intended MAI/PbI2 ratios. The XRD patterns of all the four ambient air-processed MAPbI3 films contained sharp peaks at 2θ values of 13.9°, 28.2°, and 31.7°, the intensity of which increased with increasing MAI/PbI2 ratio ( Figure 3). These peaks are respectively assigned to the (110), (220) and the (310) crystal planes of the MAPbI3 and they indicate high crystallinity in tetragonal phase [35]. Consistently, the intensity of the peak at the 2θ value of 12.67° gradually decreases with increasing MAI/PbI2 ratio until it completely disappears when the ratio exceeds one. This is the PbI2 peak and its absence indicates the absence of PbI2 residues in the film [36][37][38][39][40].    We observed clear room temperature ambipolar transport in the MAPbI3 TFTs. In unipolar enhancement mode TFTs, negligible ID flows across the channel when the VGS is equal to zero volts, unless the VGS is increased to a value greater than the VTH. For the MAPbI3 TFTs investigated herein, however, the ID increases rapidly with increasing |VDS| when VGS = 0 V ( Figure 6). Under p-type bias conditions [ Figure 6a], for instance, electrons are injected from the drain to the source for VDS < (VGS-VTHn). Here VTHn is the VTH for the n-type characteristics. The flow of current at VGS = 0 V is due to the presence of both hole and electron transport, and the choice of the carrier is determined by the equivalent bias conditions as shown in Figure 7.  We observed clear room temperature ambipolar transport in the MAPbI 3 TFTs. In unipolar enhancement mode TFTs, negligible I D flows across the channel when the V GS is equal to zero volts, unless the V GS is increased to a value greater than the V TH . For the MAPbI 3 TFTs investigated herein, however, the I D increases rapidly with increasing |V DS | when V GS = 0 V ( Figure 6). Under p-type bias conditions [ Figure 6a], for instance, electrons are injected from the drain to the source for V DS < (V GS -V THn ). Here V THn is the V TH for the n-type characteristics. The flow of current at V GS = 0 V is due to the presence of both hole and electron transport, and the choice of the carrier is determined by the equivalent bias conditions as shown in Figure 7. We observed clear room temperature ambipolar transport in the MAPbI3 TFTs. In unipolar enhancement mode TFTs, negligible ID flows across the channel when the VGS is equal to zero volts, unless the VGS is increased to a value greater than the VTH. For the MAPbI3 TFTs investigated herein, however, the ID increases rapidly with increasing |VDS| when VGS = 0 V ( Figure 6). Under p-type bias conditions [ Figure 6a], for instance, electrons are injected from the drain to the source for VDS < (VGS-VTHn). Here VTHn is the VTH for the n-type characteristics. The flow of current at VGS = 0 V is due to the presence of both hole and electron transport, and the choice of the carrier is determined by the equivalent bias conditions as shown in Figure 7.  When the VDS is swept from 0 to -10 V with the VGS fixed at zero volts, the equivalent bias conditions favor electron injection from the drain (see p-type characteristics in Figure 7). If the semiconductor supports bipolar transport, electron transport will dominate conduction under these bias conditions. This is the case for the MAPbI3 studied herein. The electron current under the p-type bias conditions (when VDS < (VGS -VTHn)) is similar to that of a TFT in the diode configuration (i.e., VGS = VDS. Although in saturation (VDS > VGS -VTH), the ID does not level-off but continues to exponentially increase with VDS in a manner similar to the current of a forward biased diode). As the VGS decreases from 0 to -6 V, the electron injection from the drain is shifted to smaller and smaller values of VDS because negative VGS depletes electrons. With further decrease in VGS (−8 to −10 V), holes accumulate in the channel and the injection of holes from the source to the drain is initiated. Under the hole transport, the output curves saturate at VDS = (VGS -VTHp), where VTHp is the VTH for the ptype characteristics (Figure 6a). Note that the direction of the current is the same for both the electron and hole transport (i.e., from the source to the drain). The significance of this is that the simultaneous or selective transport of electrons and/or holes is possible, depending on the bias conditions. Similarly, injection of holes from the drain is obtained under the n-type bias conditions when VGS = 0 V and VDS > (VGS -VTHp). The hole current under the n-type bias conditions is also similar to that of a TFT in the diode configuration (Figure 6b). With increasing VGS, hole injection from the drain shifts towards larger and larger values of VDS until electrons are the dominant carriers, and the output curves saturate at VDS = (VGS -VTHn).
It is interesting to see here that the ambipolar transport is apparent in all MAI/PbI2 ratios investigated (Figure 6a-h). However, the precursor composition has a clear effect on the magnitudes of each carrier transport. Hole and electron current increase with increasing MAI/PbI2 ratio, which is an indication of increased structural order in the MAPbI3 film with the increasing ratio. However, the electron transport appears to reach a peak when the MAI/PbI2 ratio 0.8, and decreases thereafter.

Discussion
A distinct characteristic of the MAPbI3 TFTs studied herein is the low driving voltages and stable room temperature operation. While the former is due to the thin high-k AlOx gate insulator, the latter is related to the ambient air processing. Ambient oxygen has been reported to have a passivation effect on the defects in perovskite materials [34,42,43]. It is also important to note that the ambipolar transport in the MAPbI3 is unbalanced. For MAI/PbI2 = 0.5, the electron current is about twenty times larger than the hole current (Figure 6a,b). Although the current of both carriers increases with increasing ratio, the electron current (Ie) increases much faster compared to the hole current (Ih), such that the Ie becomes about one hundred times larger than the Ih when the ratio reaches 0.8 (Figure 6e,f). However, the electron current appears to saturate, while the hole current continues to increase, when the ratio exceeds 0.8, such that the electron current becomes only fifty times larger than the hole current when the ratio reached 1.2.
We have recently shown that that small amounts of residual PbI2 (as in the case of MAI/PbI2 = 0.8) may boost electron transport and minimize ion migration by passivating ion vacancy defects in grain boundaries [44]. These ion vacancy defects are pathways for ion migration and their passivation When the V DS is swept from 0 to −10 V with the V GS fixed at zero volts, the equivalent bias conditions favor electron injection from the drain (see p-type characteristics in Figure 7). If the semiconductor supports bipolar transport, electron transport will dominate conduction under these bias conditions. This is the case for the MAPbI 3 studied herein. The electron current under the p-type bias conditions (when V DS < (V GS -V THn )) is similar to that of a TFT in the diode configuration (i.e., V GS = V DS . Although in saturation (V DS > V GS -V TH ), the I D does not level-off but continues to exponentially increase with V DS in a manner similar to the current of a forward biased diode). As the V GS decreases from 0 to -6 V, the electron injection from the drain is shifted to smaller and smaller values of V DS because negative V GS depletes electrons. With further decrease in V GS (−8 to −10 V), holes accumulate in the channel and the injection of holes from the source to the drain is initiated. Under the hole transport, the output curves saturate at V DS = (V GS -V THp ), where V THp is the V TH for the p-type characteristics (Figure 6a). Note that the direction of the current is the same for both the electron and hole transport (i.e., from the source to the drain). The significance of this is that the simultaneous or selective transport of electrons and/or holes is possible, depending on the bias conditions. Similarly, injection of holes from the drain is obtained under the n-type bias conditions when V GS = 0 V and V DS > (V GS -V THp ). The hole current under the n-type bias conditions is also similar to that of a TFT in the diode configuration (Figure 6b). With increasing V GS , hole injection from the drain shifts towards larger and larger values of V DS until electrons are the dominant carriers, and the output curves saturate at V DS = (V GS -V THn ).
It is interesting to see here that the ambipolar transport is apparent in all MAI/PbI 2 ratios investigated (Figure 6a-h). However, the precursor composition has a clear effect on the magnitudes of each carrier transport. Hole and electron current increase with increasing MAI/PbI 2 ratio, which is an indication of increased structural order in the MAPbI 3 film with the increasing ratio. However, the electron transport appears to reach a peak when the MAI/PbI 2 ratio 0.8, and decreases thereafter.

Discussion
A distinct characteristic of the MAPbI 3 TFTs studied herein is the low driving voltages and stable room temperature operation. While the former is due to the thin high-k AlO x gate insulator, the latter is related to the ambient air processing. Ambient oxygen has been reported to have a passivation effect on the defects in perovskite materials [34,42,43]. It is also important to note that the ambipolar transport in the MAPbI 3 is unbalanced. For MAI/PbI 2 = 0.5, the electron current is about twenty times larger than the hole current (Figure 6a,b). Although the current of both carriers increases with increasing ratio, the electron current (I e ) increases much faster compared to the hole current (I h ), such that the I e becomes about one hundred times larger than the I h when the ratio reaches 0.8 (Figure 6e,f). However, the electron current appears to saturate, while the hole current continues to increase, when the ratio exceeds 0.8, such that the electron current becomes only fifty times larger than the hole current when the ratio reached 1.2.
We have recently shown that that small amounts of residual PbI 2 (as in the case of MAI/PbI 2 = 0.8) may boost electron transport and minimize ion migration by passivating ion vacancy defects in grain boundaries [44]. These ion vacancy defects are pathways for ion migration and their passivation improves lateral conduction and minimizes the vacancy-mediated migration of ions in the MAPbI 3 film. This could be the reason for the high I e exhibited by the TFTs with MAI/PbI 2 = 0.8.
The ambipolar nature of the MAPbI 3 also shows in the TFT transfer characteristics (Figure 8). The transfer characteristics are semi-log plots of the |I D | as a function of the V GS and they describe ON and OFF (switching) characteristics of the TFTs. In unipolar p-type TFTs, for example, negative V GS pushes the fermi level (E F ) towards the valence band (E V ). The µ FE , V TH and SS values extracted from the saturation characteristics are shown in Figure 9. In the ON state, the I D due to the flow of electrons is over an order of magnitude larger than that due to the flow of holes, for all ratios (Figure 8). Furthermore, the ON/OFF ratio of the p-type transfer characteristics is about two orders of magnitude, whereas that of the n-type transfer characteristics is about five orders of magnitude. Consistently; the electron µ FE is two orders of magnitude larger than that of the holes (Figure 9a,b).
improves lateral conduction and minimizes the vacancy-mediated migration of ions in the MAPbI3 film. This could be the reason for the high Ie exhibited by the TFTs with MAI/PbI2 = 0.8.
The ambipolar nature of the MAPbI3 also shows in the TFT transfer characteristics (Figure 8). The transfer characteristics are semi-log plots of the |ID| as a function of the VGS and they describe ON and OFF (switching) characteristics of the TFTs. In unipolar p-type TFTs, for example, negative VGS pushes the fermi level (EF) towards the valence band (EV). The μFE, VTH and SS values extracted from the saturation characteristics are shown in Figure 9. In the ON state, the ID due to the flow of electrons is over an order of magnitude larger than that due to the flow of holes, for all ratios ( Figure  8). Furthermore, the ON/OFF ratio of the p-type transfer characteristics is about two orders of magnitude, whereas that of the n-type transfer characteristics is about five orders of magnitude. Consistently; the electron μFE is two orders of magnitude larger than that of the holes (Figure 9a,b).
Under the n-type bias conditions, the ID is due to holes flowing from the drain to the source, when the VGS is close to zero volts. Increasing the VGS positively results in the depletion of the holes and hence the decrease in ID. However, this off-state current is a sum of channel, gate leakage (IG) and displacement currents. As a result, negative current can be recorded at the drain when the IG dominates the three, which is the reason for the small hump in the n-type semi-log plots of |ID| (see Figure 8b,d,f,h). This hump comes from |IG| before the TFT turns ON. When the VGS is greater than seven volts, the accumulation of electrons is initiated and the ID is due to electrons flowing from the source to the drain. The concentration of these electrons increases with VGS and the opposite is true for the hole current.  Under the n-type bias conditions, the I D is due to holes flowing from the drain to the source, when the V GS is close to zero volts. Increasing the V GS positively results in the depletion of the holes and hence the decrease in I D . However, this off-state current is a sum of channel, gate leakage (I G ) and displacement currents. As a result, negative current can be recorded at the drain when the I G dominates the three, which is the reason for the small hump in the n-type semi-log plots of |I D | (see Figure 8b,d,f,h). This hump comes from |I G | before the TFT turns ON. When the V GS is greater than seven volts, the accumulation of electrons is initiated and the I D is due to electrons flowing from the source to the drain. The concentration of these electrons increases with V GS and the opposite is true for the hole current. The IG of the TFTs presented herein is quite significant, given the relatively low ON state currents of the TFTs. Figure 10 shows the transfer characteristics of TFTs with varying channel length (L) and the IG curves for the MAI/PbI2 ratio of 1.2. The MAI/PbI2 ratio of 1.2 is shown here because it achieves the highest Ih. The ID scaled proportionally with L for both the hole (Figure 10a) and electron ( Figure  10b) transport. For both p-type and n-type characteristics, the IG ranges from 0.1 to 1 nA in the ON state. The relatively high IG is due to the thin (40 nm) gate insulator and can be decreased by increasing the thickness of the gate insulator. To separate the contact resistance (RCON) from the channel resistance (RCH), we used the "channel resistance" method [45]. In this method, it is assumed that the overall device resistance (RTOT = VDS/ID) is given by the sum of the effective channel resistance, RCH*LEFF, and RCON. RCH is supposed to be gate modulated, while RCON and LEFF (the effective L) are assumed to be independent from the VGS, such that the overall resistance is given by RTOT (VGS) = RCH (VGS)*LEFF + RCON. For a given VGS, using the linear least-squares method, it is possible to determine RCH(VGS) by fitting RTOT against LMASK, as shown in Figure 11a,b). The gradient gives the RCH at each VGS (Figure 11c,d).
The RCH ranges from 5-11 and 44-55 MΩ/μm, respectively for the electron and hole transport (Figure 11c,d). The RCON is approximated to be 59 and 245 MΩ, respectively for the electron and hole transport. The large RCON, low ON state current and low μFE exhibited by these TFTs could be related to the existence of an injection barrier (ϕB) at the S/D-to-MAPbI3 contact (schottky contact) ( Figure  12). A consequent of this is the current crowding that occurs in the output characteristics of the TFTs The I G of the TFTs presented herein is quite significant, given the relatively low ON state currents of the TFTs. Figure 10 shows the transfer characteristics of TFTs with varying channel length (L) and the I G curves for the MAI/PbI 2 ratio of 1.2. The MAI/PbI 2 ratio of 1.2 is shown here because it achieves the highest I h . The I D scaled proportionally with L for both the hole (Figure 10a) and electron (Figure 10b) transport. For both p-type and n-type characteristics, the I G ranges from 0.1 to 1 nA in the ON state. The relatively high I G is due to the thin (40 nm) gate insulator and can be decreased by increasing the thickness of the gate insulator. The IG of the TFTs presented herein is quite significant, given the relatively low ON state currents of the TFTs. Figure 10 shows the transfer characteristics of TFTs with varying channel length (L) and the IG curves for the MAI/PbI2 ratio of 1.2. The MAI/PbI2 ratio of 1.2 is shown here because it achieves the highest Ih. The ID scaled proportionally with L for both the hole (Figure 10a) and electron ( Figure  10b) transport. For both p-type and n-type characteristics, the IG ranges from 0.1 to 1 nA in the ON state. The relatively high IG is due to the thin (40 nm) gate insulator and can be decreased by increasing the thickness of the gate insulator. To separate the contact resistance (RCON) from the channel resistance (RCH), we used the "channel resistance" method [45]. In this method, it is assumed that the overall device resistance (RTOT = VDS/ID) is given by the sum of the effective channel resistance, RCH*LEFF, and RCON. RCH is supposed to be gate modulated, while RCON and LEFF (the effective L) are assumed to be independent from the VGS, such that the overall resistance is given by RTOT (VGS) = RCH (VGS)*LEFF + RCON. For a given VGS, using the linear least-squares method, it is possible to determine RCH(VGS) by fitting RTOT against LMASK, as shown in Figure 11a,b). The gradient gives the RCH at each VGS (Figure 11c,d).
The RCH ranges from 5-11 and 44-55 MΩ/μm, respectively for the electron and hole transport (Figure 11c,d). The RCON is approximated to be 59 and 245 MΩ, respectively for the electron and hole transport. The large RCON, low ON state current and low μFE exhibited by these TFTs could be related to the existence of an injection barrier (ϕB) at the S/D-to-MAPbI3 contact (schottky contact) ( Figure  12). A consequent of this is the current crowding that occurs in the output characteristics of the TFTs To separate the contact resistance (R CON ) from the channel resistance (R CH ), we used the "channel resistance" method [45]. In this method, it is assumed that the overall device resistance (R TOT = V DS /I D ) is given by the sum of the effective channel resistance, R CH *L EFF , and R CON . R CH is supposed to be gate modulated, while R CON and L EFF (the effective L) are assumed to be independent from the V GS , such that the overall resistance is given by R TOT (V GS ) = R CH (V GS )*L EFF + R CON . For a given V GS , using the linear least-squares method, it is possible to determine R CH (V GS ) by fitting R TOT against L MASK , as shown in Figure 11a,b). The gradient gives the R CH at each V GS (Figure 11c,d).
The R CH ranges from 5-11 and 44-55 MΩ/µm, respectively for the electron and hole transport (Figure 11c,d). The R CON is approximated to be 59 and 245 MΩ, respectively for the electron and Crystals 2019, 9, 539 9 of 13 hole transport. The large R CON , low ON state current and low µ FE exhibited by these TFTs could be related to the existence of an injection barrier (ϕ B ) at the S/D-to-MAPbI 3 contact (schottky contact) ( Figure 12). A consequent of this is the current crowding that occurs in the output characteristics of the TFTs ( Figure 6). Consistent with previous reports [30], the effect of the MAI/PbI 2 ratio on the electrical properties of the TFTs indicate a shifting of the Fermi level (E F ) towards the valence band (E V ) with increasing MAI/PbI 2 ratio (Figure 12). For MAI/PbI 2 = 0.5, for instance, the film is n-type but highly resistive because of the excess PbI 2 . Furthermore, the IZO-MAPbI 3 contact is schottky, given that Ψ M is much larger than Ψ S . On the other hand, the film processed with MAI/PbI 2 = 1.2 is almost p-type but the contact in also schottky, given that Ψ M should be much larger than Ψ S to achieve an ohmic contact for hole transport (Figure 12). increasing MAI/PbI2 ratio ( Figure 12). For MAI/PbI2 = 0.5, for instance, the film is n-type but highly resistive because of the excess PbI2. Furthermore, the IZO-MAPbI3 contact is schottky, given that ѰM is much larger than ѰS. On the other hand, the film processed with MAI/PbI2 = 1.2 is almost p-type but the contact in also schottky, given that ѰM should be much larger than ѰS to achieve an ohmic contact for hole transport (Figure 12). The larger RCON for hole, as opposed to electron transport, corresponds to a larger voltage drop at the S/D-to-MAPbI3 contact, which may be the reason for the lower ON state current and the smaller μFE values. The large RCON is the reason for the strong dependency of the μFE on L for the hole transport ( Figure 9a). A possible origin for the large variation in the other TFT parameters (Figure 9) could be ion migration in the MAPbI3 film [44]. As ion migration causes hysteresis in the current-voltage characteristics, the extracted TFT parameters may vary depending on the bias history of the devices. In addition, the devices presented herein do not have a passivation layer to protect the MAPbI3 film from environmental contamination. The variation in extracted parameters could thus be due to environmental changes. However, it is interesting to see that the TFTs are stable and work very well in ambient air, despite the absence of a passivation layer. The larger R CON for hole, as opposed to electron transport, corresponds to a larger voltage drop at the S/D-to-MAPbI 3 contact, which may be the reason for the lower ON state current and the smaller µ FE values. The large R CON is the reason for the strong dependency of the µ FE on L for the hole transport ( Figure 9a). A possible origin for the large variation in the other TFT parameters (Figure 9) could be ion migration in the MAPbI 3 film [44]. As ion migration causes hysteresis in the current-voltage characteristics, the extracted TFT parameters may vary depending on the bias history of the devices. In addition, the devices presented herein do not have a passivation layer to protect the MAPbI 3 film from environmental contamination. The variation in extracted parameters could thus be due to environmental changes. However, it is interesting to see that the TFTs are stable and work very well in ambient air, despite the absence of a passivation layer.
It is also important to note that the achievement of an ohmic contact for one carrier type consequently achieves a schottky contact for the other, given that ohmic contacts are achieved with a high metal workfunction for the hole transport and a low metal workfunction for the electron transport. Another important factor to consider for the ambipolar transport of the TFTs is the initial position of the E F , which may be determined by the MAI/PbI 2 ratio and fabrication process conditions ( Figure 12). Variation in carrier concentration by varying the precursor ratio have also been previously reported [30]. Therefore, balanced carrier transport may be achieved by varying the precursor ratio and by proper source/drain electrode selection for each carrier type. Crystals 2019, 9, x FOR PEER REVIEW 10 of 13 It is also important to note that the achievement of an ohmic contact for one carrier type consequently achieves a schottky contact for the other, given that ohmic contacts are achieved with a high metal workfunction for the hole transport and a low metal workfunction for the electron transport. Another important factor to consider for the ambipolar transport of the TFTs is the initial position of the EF, which may be determined by the MAI/PbI2 ratio and fabrication process conditions ( Figure 12). Variation in carrier concentration by varying the precursor ratio have also been previously reported [30]. Therefore, balanced carrier transport may be achieved by varying the precursor ratio and by proper source/drain electrode selection for each carrier type.

Conclusions
The MAPbI3 TFTs investigated herein exhibited ambipolar transport, where simultaneous or selective transport of electrons and/or holes was possible in a single device, depending on the applied bias. However, the electron transport (n-type) was slightly more pronounced than the hole transport (p-type), and the on-state current of the n-type and p-type TFTs was highest in the slightly PbI2-rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by precursor ratio. Furthermore, the use of different types of source/drain electrodes for each carrier type is necessary to balance the ambipolar transport.

Conclusions
The MAPbI 3 TFTs investigated herein exhibited ambipolar transport, where simultaneous or selective transport of electrons and/or holes was possible in a single device, depending on the applied bias. However, the electron transport (n-type) was slightly more pronounced than the hole transport (p-type), and the on-state current of the n-type and p-type TFTs was highest in the slightly PbI 2 -rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by precursor ratio. Furthermore, the use of different types of source/drain electrodes for each carrier type is necessary to balance the ambipolar transport.

Patents
CMOS DEVICE USING AMBIPOLAR ORGANIC-INORGANIC HYBRID PEROVSKITE AND METHOD OF MANUFACTURING THE SAME.