Ambipolar Transport in Methylammonium Lead Iodide Thin Film Transistors

Abstract

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.

1. Introduction

Hybrid organic-inorganic perovskites are emerging materials with remarkable optoelectronic properties [1]. Among them, methylammonium lead iodide (CH₃NH₃PbI₃) has proven to be an exceptional light harvester for solar cells, recording power conversion efficiencies of over 20% in just a few years [2,3,4,5,6]. Despite low-temperature (<150 °C) processing, the density of electronic defect states in CH₃NH₃PbI₃ (MAPbI₃) is surprisingly low [7,8,9,10,11,12,13]. This combined with other unique properties such as high photoluminescence quantum yield, tunable bandgap, and a high absorption coefficient, are the reasons for the success of the MAPbI₃ in photovoltaics (PV) [1,14]. In addition to PV applications, MAPbI₃ is also appealing for several other applications, including light emitting diodes (LEDs) [8,10,11,15], thin-film transistors (TFTs) [16,17,18,19,20,21] and light emitting transistors (LETs) [2].

For large-area TFT applications, MAPbI₃ 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, MAPbI₃ 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, MAPbI₃ 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/PbI₂) and thermal annealing conditions, thus resulting in preferential n-type or p-type transport characteristics [30]. Ambipolar transport 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 (AlO_x) gate-insulator (GI) to decrease the driving voltages.

2. 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 AlO_x layer spin coated from solution in ambient air (relative humidity = 50 ± 5, Temperature = 25 °C ± 2 °C). We prepared the AlO_x 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/AlO_x/indium zinc oxide (IZO) structures (400 µm × 400 µm), the capacitance of the AlOx layer per unit area (C_OX) is estimated to be approximately 200 nF/cm². We define the source and drain electrodes by patterning a 60 nm thick IZO layer deposited at 280 °C by sputtering. We spin-coated 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 AlO_x 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 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 μ_FE and threshold voltage (V_TH) values from the slopes and intercepts of the linear regression lines of the I_D^1/2(V_GS) plots in the saturation region (V_DS > V_GS – V_TH), according to the equation, I_D = (W/L)*C_OX*μ_FE*(V_GS – V_TH)². Here, W, L, V_GS, V_DS, and I_D denote the channel width, channel length, gate voltage, drain voltage, and drain current, respectively. We derived the μ_FE and V_TH from the |I_DS|^1/2(V_GS) plots. We estimated the subthreshold V_GS swing (SS) by (d log (I_D)/d V_GS)⁻¹.

3. 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,h) and XRD analysis.

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 AlO_x 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 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].

We observed clear room temperature ambipolar transport in the MAPbI₃ 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 MAPbI₃ 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.

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 MAPbI₃ 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/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.

4. 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 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 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 (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 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 improves lateral conduction and minimizes the vacancy-mediated migration of ions in the MAPbI₃ film. This could be the reason for the high I_e 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 |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).

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 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/PbI₂ ratio of 1.2. The MAI/PbI₂ 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.

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 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-MAPbI₃ 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/PbI₂ 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/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 Ѱ_M is much larger than Ѱ_S. On the other hand, the film processed with MAI/PbI₂ = 1.2 is almost p-type but the contact in also schottky, given that Ѱ_M should be much larger than Ѱ_S to achieve an ohmic contact for hole transport (Figure 12).

The larger R_CON 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 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 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.

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/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.

5. 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.

6. Patents

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