Organic–inorganic hybrid CH₃NH₃PbI₃ perovskite materials as channels in thin-film field-effect transistors

Abstract

Organic–inorganic hybrid perovskite materials promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials, which make organic–inorganic hybrid perovskite materials good substitutes in all the applications put forth for organic materials and extend their application to higher speed devices than is presently possible with either a-Si or organic semiconductors. Recent reports have shown high carrier mobility and long electron–hole diffusion lengths of organic–inorganic hybrid perovskite materials. We have demonstrated a thin-film field-effect transistor with an organic–inorganic hybrid CH₃NH₃PbI₃ material as the semiconducting channel based on these advantages via a low-temperature vapor-assisted solution process. The obvious electrical field effect is obtained in organic–inorganic hybrid CH₃NH₃PbI₃ perovskite TFTs with a field-effect mobility of 396.2 cm² V⁻¹ s⁻¹, current modulation greater than 10⁴, sub-threshold current of 0.4035 V per decade and threshold voltage of −3.501 V.

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1. Introduction

In recent years, display devices have become common due to the development of mobile phones, flat-screen TVs, personal computer monitors, etc. The essential element for such devices is a thin-film transistor (TFT).

TFTs are usually manufactured using organic materials or inorganic materials as channels. Organic thin-film transistors (OTFTs) offer many advantages such as processability and mechanical flexibility over inorganic technologies^2–4 and have been developed for use in a wide range of applications including displays,^5,6 sensors,^7,8 and radio frequency identification.^9,10 However, the mobilities of organic TFTs are usually lower than 1 cm² V⁻¹ s⁻¹. Although many methods have been developed to enhance the mobilities of organic TFTs, most mobilities reported for organic TFTs are lower than 10 cm² V⁻¹ s⁻¹.¹¹ The weak van der Waals interaction bonding neighboring molecules limits the further improvement of their carrier mobility. Therefore, the low carrier mobility of organic TFTs limits their device-switching speeds and their range of potential applications.

As to the inorganic TFTs, the polycrystalline silicon (poly-Si) (50–400 cm² V⁻¹ s⁻¹) TFT has a mobility one or two magnitudes higher than that of the amorphous silicon (a-Si:H) TFT (1 cm² V⁻¹ s⁻¹) and organic TFT. However, the fabrication of this material is complicated and needs a temperature higher than 500 °C (even up to 900 °C) for crystallization, which is not suitable for flexible electronics application. Alternative semiconducting materials for thin field-effect transistors (TFTs), which have mobility at least comparable to that of amorphous silicon (a-Si) and may also be easily processed with low-cost techniques, are required to enable new opportunities for display and storage technologies.

Recently, the amorphous metal-oxide show many advantageous properties, such as the high mobility, optical transparency, and low process temperature. However, mobilities of these TFTs are usually lower than 10 cm² V⁻¹ s⁻¹. In order to achieve a high mobility, it has to be annealed at a temperature higher than about 300 °C, which is a hurdle for the plastic film substrate.^12–19 Advanced materials and processing techniques developed in recent years, such as nanowires, single-walled carbon nanotubes (SWCNTs) and grapheme, which have been fabricated into TFTs for flexible electronic applications. These methods usually need complicated process procedures or high temperatures^19–24 even if the device performance could be enhanced. Despite great efforts being exerted in this field, so far no one has succeeded in demonstrating a good performance TFT with relatively simple and real low-temperature processing suitable for high quality, low-cost and flexible displays.

In the present work, we have proposed a novel TFT that has an organic–inorganic hybrid perovskite as channel layer. This material is crystallized from organic halide and metal halide salts to form crystals in the ABX₃ structure, where A is the organic cation, B is the metal cation and X is the halide anion (see Fig. 1). The inorganic component forms an extended framework bound by strong covalent or ionic (or both) interactions to provide high carrier mobility. The organic component facilitates the self-assembly of these materials, enabling hybrids to be deposited by the same simple, low-cost, low-temperature processes as the organic materials. The organic component is also used to tailor the electronic properties of the inorganic framework. Engineering the organic–inorganic hybrid perovskite on the molecular scale may be done to maximize carrier mobility. The combination of high carrier mobility and ease of processing may make organic–inorganic hybrid perovskite materials good substitutes in all the applications put forth for organic materials. The potentially higher carrier mobility of organic–inorganic hybrid perovskite materials may extend their application to higher speed devices than is presently possible with either a-Si or organic semiconductors. As we know, the organic–inorganic hybrid perovskite materials have been successfully used in solar cells,^25–27 which the power conversion efficiency (PCE) has been up to 20.1% (ref. 28) from 3.8% (ref. 29) due to distinct merits of organic–inorganic hybrid perovskite materials.^1,29–32 Thus the remarkable properties of organic–inorganic hybrid perovskite materials illustrate that they are suitable for the application in TFTs.

Fig. 1 Perovskite ABX₃ crystal structure where typically A = CH₃NH₃⁺, B = Pb²⁺ and X = I⁻, Br⁻, Cl⁻, or mixtures thereof.

In this paper, we proposed a new kind of thin-film field-effect transistor using organic–inorganic hybrid perovskite CH₃NH₃PbI₃ material as the semiconducting channel.

2. Experimental section

2.1. Thin film fabrication

Here, we used a facile vapor assisted solution approach to perovskite material formations with enhanced controllability over the film quality,³³ where the inorganic framework film was formed by depositing precursor solution on the substrates, and subsequently treated with the desired organic vapor (Fig. 2). PbI₂ solution was prepared in DMF at the concentration of 345 mg ml⁻¹. The prepared PbI₂ solution was preheated at 110 °C on a hot plate, followed by spin coating on the substrates at 4000 rpm for 40 s, at the same time a dry nitrogen gas flow was blown over the surface of the PbI₂ solution during the spin-coating process in air,²⁶ then put back on the hot plate for 15 min of drying. To obtain the perovskite thin film, substrates with PbI₂ film were then put into a vacuum coating machine, then CH₃NH₃I was deposited by thermal evaporation for 0.5 h. After cooling down to room temperature, the perovskite films was annealed for 0.3 h at 100 °C.

Fig. 2 Schematic illustration of the procedure of perovskite thin film prepared via modified vapor-assisted solution process.

2.2. Thin film field-effect transistors (TFTs) fabrication

Fig. 3 shows the structure of device of our perovskite thin film field-effect transistors.³⁴ Heavily n-doped silicon wafers were used as the gate electrode. The gate dielectric layer was a 200 nm thermally grown silicon dioxide. Organic–inorganic hybrid CH₃NH₃PbI₃ as the semiconducting channel was obtained via modified vapor-assisted solution process. High work-function metal source and drain electrodes such as Ag were deposited by evaporation through a shadow mask.

Fig. 3 Device architecture. Schematic view of the perovskite TFT configuration: n⁺⁺ Si substrate, SiO₂ gate insulator, CH₃NH₃PbI₃ film as the semiconducting channel, Ag source and drain electrodes.

2.3. Analysis of characterizations on active layers and analysis of TFT devices

The crystal structures of the CH₃NH₃PbI₃ films were characterized by X-ray diffraction (XRD, Philips PANalytical X'Pert Pro) with a copper X-ray source, and the surface morphologies were observed by scanning electron microscope (SEM, Hitachi SU8010) and atomic force microscopy (AFM) (Seiko SPA-400SPM UNIT). The photoluminescence (PL) spectrum measurements were conducted by a Fluorolog-3-212 steady state spectrometer. The laser wavelength is 350 nm. The electrical performance of the TFT devices was measured with Keysight Technologies B1500A Semiconductor Device Analyzer. All devices were tested in air and at room temperature. Device performances were extracted by the standard field-effect transistor equations that apply to both organic and inorganic TFTs. The mobility of the device was extracted using the following equation:

μ = (dI_DS/dV_G)(L/V_DC_iW)

where μ is the device mobility, I_D and V_G respectively correspond to the drain current and gate voltage, L channel length, W channel width and C_i the gate insulator capacitance. The sub-threshold slope (SS) was calculated as the minimum value of the inverse slope of the log I_D versus V_G:

SS = (dV_G/dlog I_DS)

3. Results and discussion

Organic–inorganic hybrid CH₃NH₃PbI₃ perovskite materials via modified vapor-assisted solution process were obtained with full surface coverage, uniform grain structure with grain size up to micrometers. The key step is film growth via in situ reaction of the as-deposited film of PbI₂ with CH₃NH₃I vapor (Fig. 2). This method is conceptually different from the current solution process and vacuum deposition by avoiding co-deposition of organic and inorganic species. In one-step deposition, the two precursor salts (metal halide and organic halide) are dissolved in an organic solvent, the mixture is spin-coated onto a substrate, and the perovskite is formed through an annealing process at around 100 °C, finally the perovskite crystals are micrometer-sized in size. However spin-coating the mixture directly onto a substrate initially resulted in a dewetting effect,³⁵ thus leading to poor surface coverage.^36,37 As to the co-deposition technique,³⁸ although nearly 100% surface coverage can be obtained, the perovskite crystals obtained are a few hundred nanometers in size, and thus smaller than the micrometer-sized platelets obtained from spin-coating. However the modified vapor-assisted solution process we used to fabricate perovskite films combine the full surface coverage with micrometer-sized crystals, which indicates high-quality films.

Meanwhile, an additional step was introduced where a dry nitrogen gas flow was blown over the surface of the PbI₂, promoting evaporation of the solvent and accelerating the supersaturation of the solution. In general, a solution precipitation process involves two steps: nucleation and crystal growth. Nuclei will form when a solution reaches supersaturation and the volume of the nuclei in the system would depend on the rate of nucleation and the degree of the supersaturation. This addition step where a dry nitrogen gas flow was blown over the surface facilitated a high degree of supersaturation of the PbI₂ in the wet film during spin-coating, resulting in a large number of nuclei which reacted with CH₃NH₃I vapor and, finally, the full coverage of the CH₃NH₃PbI₃ layer was obtained when the as-deposited film of PbI₂ completely reacted with CH₃NH₃I vapor. This process takes advantage of the kinetic reactivity of CH₃NH₃I and thermodynamic stability of perovskite during the growth process in situ and provides films with grain sizes up to microscale, full surface coverage, and small surface roughness, which is required for TFTs channel.

Firstly, to optimize the perovskite films, we researched the influence of PbI₂ solution concentration and the amount of evaporated CH₃NH₃I on perovskite films performance, respectively.

3.1. The influence of PbI₂ solution concentration on perovskite films performance

Fig. 4 shows the top-view SEM images of PbI₂ layer and perovskite films prepared with different PbI₂ solution concentration varied from 100 mg ml⁻¹ to 500 mg ml⁻¹, where the amount of CH₃NH₃I is 159 mg. And X-ray diffraction (XRD) measurements of perovskite films corresponding to different PbI₂ solution concentration were taken. As shown in Fig. 4F, a set of strong peaks at 14.08°, 28.41° and 31.85°, assigned to (110), (220) and (310) of the CH₃NH₃PbI₃ crystal,^33,39 indicate an orthorhombic crystal structure of halide perovskite with high crystallinity. Meanwhile a tiny signature peak at 12.65° is observed in Fig. 4F(c and d),⁴⁰ which indicates a low-level impurity of PbI₂. Table 1 shows the surface roughness of PbI₂ layer and perovskite films with different PbI₂ solution concentration. We have found that the PbI₂ solution concentration has main influence on the roughness and the PbI₂ residue of perovskite films rather than the grain size. As can been seen, with the PbI₂ solution concentration increasing, the surface roughness of the PbI₂ film gradually increases (see Table 1). This change benefit PbI₂ to contact MAI and convert completely to perovskite, where final perovskite films are obtained without PbI₂ residue as shown in Fig. 4F(d and e). At the same time, the sufficient reaction is also beneficial to reducing surface roughness of perovskite films, as shown in Fig. 4 and Table 1. However, when the PbI₂ solution concentration is too high, for instance, 500 mg ml⁻¹, the surface roughness of PbI₂ layer would increase, leading to the increase of the surface roughness of the resulted perovskite films. Hence, considering with the surface roughness of perovskite films and the PbI₂ residue, we suggest that the PbI₂ solution concentration of 345 mg ml⁻¹ is suitable to fabricate perovskite films.

Fig. 4 SEM top-view images of PbI₂-coated films (a–e) and CH₃NH₃PbI₃ films (A–E): PbI₂ solution concentration varied from 100 mg ml⁻¹ (a and A), 250 mg ml⁻¹ (b and B), 285 mg ml⁻¹ (c and C), 345 mg ml⁻¹ (d and D), 500 mg ml⁻¹ (e and E); (F) X-ray diffraction pattern of CH₃NH₃PbI₃ films: PbI₂ solution concentration varied from 100 mg ml⁻¹ (a), 250 mg ml⁻¹ (b), 285 mg ml⁻¹ (c), 345 mg ml⁻¹ (d), to 500 mg ml⁻¹ (e).

Table 1 Parameters derived from AFM measurements corresponding to Fig. 4a–E

Roughness surface RMS (nm)	100 mg ml^−1	250 mg ml^−1	285 mg ml^−1	345 mg ml^−1	500 mg ml^−1
PbI2 films	7.184 × 10^−1	2.029 × 10^0	1.255 × 10^0	1.852 × 10^0	6.035 × 10^0
CH3NH3PbI3 films	1.352 × 10^1	1.460 × 10^1	1.128 × 10^1	1.948 × 10^0	1.083 × 10^1

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3.2. The influence of amount of CH₃NH₃I evaporation on perovskite films performance

In order to further optimize CH₃NH₃PbI₃ films to obtain larger crystals, we prepared three types of CH₃NH₃PbI₃ thin films: type A, the amount of CH₃NH₃I precursor used in thermal evaporation process was 120 mg; type B, the amount of CH₃NH₃I precursor used in thermal evaporation process was 200 mg; type C, the amount of CH₃NH₃I precursor used in thermal evaporation process was 280 mg. Fig. 5 present scanning electron microscope (SEM) images of CH₃NH₃PbI₃ films interfaced to SiO₂ substrate.

Fig. 5 Scanning electron microscopy (SEM) images of the CH₃NH₃PbI₃ film: the amount of CH₃NH₃I precursor in thermal evaporation process is 120 mg (a), 200 mg (b), 280 mg (c), respectively.

For type A (Fig. 5a), the grain size is the biggest, but there are many voids among adjacent grains. Although the CH₃NH₃PbI₃ film is full surface coverage in type C (Fig. 5c), the grain sizes are only a few hundred nanometers. The grain sizes and the voids among adjacent grains are relevant to the density of nuclei which grows into grain. When the amount of CH₃NH₃I is low, the density of nuclei is low, resulting in large grain size, while due to the lack of crystal nuclei, many voids between (or in) the grains come up, leading to the poor surface coverage. On the contrary, excessive amount of CH₃NH₃I could bring high density of nuclei and cause small grain size. For type B, the CH₃NH₃PbI₃ film has full surface coverage and small surface roughness in type B (Fig. 5b). Meanwhile, the grains as large as 3 μm were obtained. Full surface coverage and huge grains indicate the few grain boundaries and then a low density of trap states, which is crucial to the TFTs devices performance.

In order to further verify the crystallization of perovskite CH₃NH₃PbI₃ films obtained via vapor-assisted solution process, X-ray diffraction (XRD) measurements were taken. As shown in Fig. 6A. We can see the tiny signature peak at 12.65° is all observed in three types,⁴⁰ and the emergence of a tiny signature peak at 12.65° is relative to the amount of CH₃NH₃I precursor, because PbI₂ could not sufficiently react with CH₃NH₃I, when the amount of CH₃NH₃I precursor was low. However when the amount of CH₃NH₃I precursor was excessive, nuclei of CH₃NH₃PbI₃ quickly appeared which resulted high density of nuclei of CH₃NH₃PbI₃, then compact CH₃NH₃PbI₃ film was obtained on the surface of PbI₂, which hindered PbI₂ further reaction with CH₃NH₃I and resulted in PbI₂ residue. As can be observed (Fig. 6A), type B has almost no PbI₂ residue. At the same time, a representative ultraviolet-visible photoluminescence (PL) spectrum of CH₃NH₃PbI₃ thin film (sample B) is shown in Fig. 6B, which consists with previous reports that the PL is right at the band edge. Consequently, it is indicated that the amount of CH₃NH₃I have an obvious influence on crystallization of perovskite CH₃NH₃PbI₃ films, then effects perovskite TFT performance.

Fig. 6 (A) X-ray diffraction pattern of the CH₃NH₃PbI₃ film: the amount of CH₃NH₃I precursor in thermal evaporation process is 120 mg (a), 200 mg (b), 280 mg (c), respectively. (B) A PL spectra for the CH₃NH₃PbI₃ thin film of sample B.

3.3. The performances of TFT with CH₃NH₃PbI₃ as the channel

Then TFTs with CH₃NH₃PbI₃ film as channel layer have been fabricated. This TFTs have a channel length L = 0.5 mm and channel width W = 5 mm, defined by Ag source and drain electrodes. The plot of drain current, I_D, versus gate voltage, V_G, is shown as a function of the applied same drain voltage V_ds = 2 V (see Fig. 7a) for TFTs with different types of CH₃NH₃PbI₃ channel layer. And obvious electrical field effect is all observed in three types TFTs.

Fig. 7 The plot of drain current I_D versus gate voltage V_G (a) as a function of the amount of CH₃NH₃I precursor for the CH₃NH₃PbI₃ film thin-film transistor at V_ds = 2 V; (b) as a function of drain voltage V_ds for the CH₃NH₃PbI₃ film thin-film transistor in type B.

The relevant parameters derived from I_D–V_G measurements corresponding to Fig. 7a are summarized in Table 2. From the plot of I_D versus V_G (Fig. 7a) used to calculate current modulation (I_ON/I_OFF) and field-effect mobility, μ, in the linear regime. We can see that the performance of TFT in type B is the best, which the field-effect mobility is 566 cm² V⁻¹ s⁻¹ for a ±25 V sweep of V_G at V_ds = 2 V with I_ON/I_OFF > 10⁴ (Fig. 7a). Simultaneously the sub-threshold current and threshold voltage is the smallest in type B, which value were 0.4035 V per decade, −3.501 V, respectively, which indicates the best device performance (see Table 2) among these three types TFTs. Considering that the un-patterned TFTs show overestimation of field effect mobility due to the peripheral current. Thus, a 30% reduction of the μ should be considered. Additionally, patterning the semiconductor to the active device region also reduces leakage through the insulator contributing to I_OFF, increasing I_ON/I_OFF to at least 10⁴. There is no obvious gate induced leakage current in the organic inorganic hybrid perovskite TFTs, which may be related to the special state density distribution of organic–inorganic hybrid perovskite materials. This is still under study.

Table 2 Parameters derived from I_D–V_G measurements corresponding to Fig. 7a

CH3NH3PbI3 dosage	ION/IOFF	Field-effect mobility (cm^2 V^−1 s^−1)	Sub-threshold current (V per decade)	Threshold voltage (V)
120 mg	22.14	6.41 × 10^1	2.08	−4.47
200 mg	2.30 × 10^3	5.66 × 10^2	0.40	−3.50
280 mg	6.08	1.02 × 10^2	8.66	−7.83

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4. Conclusions

In summary we demonstrated a thin-film field-effect transistor having an organic–inorganic hybrid CH₃NH₃PbI₃ material as the semiconducting channel by low-temperature vapor-assisted solution process and made a preliminary optimization of the dosage of the precursors. Finally, we obtained an organic–inorganic hybrid CH₃NH₃PbI₃ TFT with field-effect mobility of 396.2 cm² V⁻¹ s⁻¹, current modulation greater than 10⁴, sub-threshold current of 0.4035 V per decade and threshold voltage of −3.501 V. The mobility of organic–inorganic hybrid CH₃NH₃PbI₃ TFTs is much higher than that of any other TFTs, such as Si-based TFTs, the metal-oxide TFTs and the best organic TFTs. Meanwhile, this material that can be processed by low temperature, which suggest that organic–inorganic hybrid TFTs may be suitable for applications that require low cost, low temperature, a large area, and the flexibility of plastic substrates. Furthermore, semiconducting organic–inorganic hybrid perovskite materials may be designed with a wide range of organic and inorganic components for use in TFTs and other flexible electronics (for example CH₃NH₃Pb(I_1−xBr_x)₃). So it is believed that organic–inorganic hybrid perovskite TFTs performance can be further improved by material engineering and optimization of device structure.

Acknowledgements

The work reported here was supported by National Natural Science Foundation of China (Project No. 61076006), National Natural Science Foundation of China (Zhang; Project No. 61377031) and the Flat-Panel Display Special Project of China's 863 Plan (Project No. 2008AA03A335).

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