Earth-abundant and environment friendly organic–inorganic hybrid tetrachloroferrate salt CH₃NH₃FeCl₄: structure, adsorption properties and photoelectric behavior

Abstract

Organic–inorganic hybrid-based lead perovskites show inherent and unavoidable problems such as structural instability and toxicity. Therefore, developing low-cost and environment-friendly organic–inorganic hybrid materials is extremely urgent. In this study, we prepared earth-abundant and environment-friendly organic–inorganic hybrid tetrachloroferrate salt CH₃NH₃FeCl₄ (MAFeCl₄) for optoelectronic applications. The single crystal diffraction data are assigned to the orthorhombic MAFeCl₄ (Pnma space group), with parameters a = 11.453 (5) Å, b = 7.332 (3) Å, c = 10.107 (5) Å, α = 90.000, β = 90.000, and γ = 90.000. The band gap of MAFeCl₄ is approximately 2.15 eV. Moreover, three-emission luminescence (398, 432 and 664 nm) was observed. To the best of our knowledge, this is the first study involving the investigation of the structure, adsorption properties and photoelectric behavior of MAFeCl₄. A low cost photodetector based on the MAFeCl₄ thin film is efficient under different monochromatic light from 330 nm to 410 nm with different chopping frequencies (1.33 Hz to 40 Hz). The photoelectric conversion efficiency based on FTO/TiO₂/MAFeCl₄/carbon electrode device reaches 0.054% (V_oc = 319 mV, J_sc = 0.375 mA cm⁻², and fill factor = 0.45) under AM1.5, 100 mW cm⁻² simulated illumination. Our findings will attract attention from the magnetic, piezoelectric and photoelectronic research fields.

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Introduction

As one type of organic–inorganic hybrid material (OIHM), organic–inorganic hybrid perovskite (OIHP) has received widespread attention in the scientific community and in the industrial world. Among OIHPs, organic–inorganic hybrid lead perovskite exhibits very advantageous performances in photovoltaics,¹, lasers,² photodetectors³ and so on. Recently, it has been reported that the power conversion efficiency (PCE) of solid thin film solar cells using organic-inorganic hybrid lead perovskite as a light absorbing material is up to 22%,⁴ suggesting that they could become promising commercial solar cells. In addition, the preparation of perovskite by solvothermal methods greatly reduces the cost of mass production. It is well known that a high-quality photoelectric device not only requires high performance and low cost, but also should have good stability and be environmentally friendly. However, there are some unavoidable problems in organic–inorganic hybrid lead perovskites. On the one hand, the cost of lead halide is high. On the other hand, the toxicity of lead in organic–inorganic hybrid lead perovskites is troubling.⁵ Therefore, developing low-cost and environment-friendly organic–inorganic hybrid materials is extremely urgent. Recently, Pb-free and IVA-metal (Tin or Germanium)-based OIHPs have been studied. Organic-inorganic hybrid tin materials were synthesized using different halide sources to directly synthesize the nanostructure or by applying a postsynthetic anion exchange reaction.⁶ Organic–inorganic hybrid germanium materials, AGeI₃ (A = Cs, organic cation), with second harmonic generation (SHG) properties were synthesized by replacing the inorganic cation in the A position of the perovskite unit.⁷ Non-IVA-metal [single metal(+2) or mixed metal(+1, +3)]-based OIHP have also been developed. The organic–inorganic hybrid double-perovskite structure material Cs₂AgBiBr₆ with a long room-temperature fundamental photoluminescence (PL) lifetime was formed by incorporating nontoxic Bi³⁺ into the perovskite lattice.⁸ Layered (CH₃NH₃)₂MnCl₄ (MA₂MnCl₄) and amorphous manganese MAMnI₃ were exploited for optoelectronic applications.^9,10 A lead-free highly stable C₆H₄NH₂CuBr₂I compound that exhibits extraordinary hydrophobic behaviour was developed.¹¹ However, the efficiency and cost of the abovementioned materials are not ideal, and the stability is not very good.

Therefore, it is very necessary to develop new types of low-cost and environment-friendly organic–inorganic hybrid materials. In this study, we prepared CH₃NH₃FeCl₄ (MAFeCl₄) single crystals and films (see Experiment section). The crystal structure, luminescence, band gap, adsorption properties and photoelectric behavior were studied.

Experimental

Preparation of CH₃NH₃FeCl₄ single crystal

162.5 mg FeCl₃ and 67.5 mg CH₃NH₃Cl were added to a mixed solvent (3 mL methanol, 3 mL ethanol and 2 mL 35–36% HCl). The above solution was heated to 80 °C for 2 h. Then, the temperature was reduced to 40 °C over 20 h, following which a yellow-brown single crystal was obtained.

Single crystal X-ray diffraction (XRD) characterization

Data were collected on a Bruker SMART CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and expanded via difference Fourier techniques with the SHELXL-97 program.

Preparation of the CH₃NH₃FeCl₄ optoelectronic device

Patterned FTO-coated glass substrates with sheet resistances of 15 Ω sq⁻¹ were coated with a TiO₂ compact layer by spin-coating the TiO₂ organic sol at 3000 rpm for 30 s, followed by drying at 450 °C for 30 min following our previous literature report.^9,10 The MAFeCl₄ layer was prepared by spin coating 162.5 mg of FeCl₃ and 67.5 mg of the CH₃NH₃Cl mixed precursor at 3000 rpm for 30 s, followed by gradual heating to 70 °C; the layer was baked at this temperature for 30 min. Finally, the carbon electrodes were prepared following our previous literature protocol;^9,10 the MAFeCl₄ layer was coated with conductive-carbon paste by the doctor-blade method using adhesive tapes for creating patterns and spaces, followed by drying at 70 °C for 40 min. The size of the FTO/TiO₂/MAFeCl₄/carbon electrode device was 1 cm².

Characterization of the CH₃NH₃FeCl₄ optoelectronic device

Nanostructures of MAFeCl₄ films were characterized by scanning electron microscopy (SEM, Hitachi SEM S-4800). UV-vis absorption spectra and diffuse reflectance spectra were obtained using a UV-vis-NIR spectrometer (PerkinElmer, λ, 750S). The photocurrent–time performance of FTO/TiO₂/MAFeCl₄/carbon electrode devices was studied using an electrochemical workstation system (CHI760, Chenhua, and Shanghai) equipped with a solar simulator (IV5, PV Measurements, Inc., United States). The photocurrent–time with incident photon (wavelength) and frequency response was measured by an electrochemical workstation system (CHI760, Chenhua, and Shanghai) equipped with a quantum efficiency/spectral response (SR)/incident photon to current conversion efficiency (IPCE) measurement system (QEX10, PV Measurements, Inc., United States). The photocurrents of the devices were measured under a bias voltage of 0.00 V.

Results and discussion

The color of the MAFeCl₄ single crystal is yellow-brown. The single crystal structure was solved by a direct method and was extended using the SHELXL-97 program using differential Fourier technology. The single crystal diffraction data were assigned to the orthorhombic MAFeCl₄ (Pnma space group), with parameters a = 11.453 (5) Å, b = 7.332 (3) Å, c = 10.107 (5) Å, α = 90.000, β = 90.000, and γ = 90.000. The schematic diagram of the unit cell structure is shown in Fig. 1. As seen from the projection down to the ac-plane (Fig. 1), tetrachloroferrate FeCl₄⁻ ions are distributed in the unit cell. Due to the four Cl ligands and the d⁵ of Fe³⁺, FeCl₄⁻ ions form regular tetrahedron structures. Thus, there should be a residual electron on the remaining d-orbit. MA⁺ is distributed around the FeCl₄⁻ ions. There is a relatively short hydrogen bond (NH⋯Cl, 2.797 Å) between the MA⁺ groups and the FeCl₄⁻ ions. Accordingly, the shortest Cl⋯Cl distances between the neighboring FeCl₄⁻ ions is 4.133 Å, which is much longer than the van der Waals distance (3.90 Å). These structural features suggest stronger s–d interactions between the FeCl₄⁻ ions and the MA⁺ groups. The detailed crystal data and structural refinement of the MAFeCl₄ single crystal can be observed in Tables 1, 2, S1, and S2.† The structure of the MAFeCl₄ single crystal is different from that of the classical perovskite structure (ABX₃). First, the crystal structure of MAFeCl₄ is no longer the perovskite structure. Second, the valence of Fe in MAFeCl₄ is +3, which is different from that of Pb in MAPbI₃. Third, the band gap of MAFeCl₄ is different from that of MAPbI₃. Thus, the performance of the MAFeCl₄ single crystal is also different from that of the classic perovskite.

Fig. 1 (a) The position of iron in the periodic table of elements. (b and c) Schematic diagrams of the unit cell structure in ball-stick type and polyhedron type, respectively.

Table 1 Crystal data and structure refinement for CH₃NH₃FeCl₄

Compound	C·H6·Cl4·Fe·N
Chemical composition	CH3NH3FeCl4
Crystal system	Orthorhombic
Space group, Z	Pnma
Temperature/K	298(2) K
a, A	11.453(5)
b, A	7.332(3)
c, A	10.107(5)
α, deg	90
β, deg	90
γ, deg	90
Volume, A^3	848.7(7)
ρ, Mg m^−3	4, 1.798
μ, mm^−1	2.939
Total reflections, R(int)	3945/810, 0.0370
Data/restraints/parameters	810/0/42
Final R1, wR2 [I > 2γ(I)]	R1 = 0.0393, wR2 = 0.0971

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Table 2 Bond lengths [A] and angles [deg] for CH₃NH₃FeCl₄^a

Bond or angles	Lengths or [deg]
a Symmetry transformations used to generate equivalent atoms: #1 x, −y + 3/2, z.
Fe(1)–Cl(3)	2.162(2)
Fe(1)–Cl(2)	2.181(2)
Fe(1)–Cl(1)	2.1857(15)
Fe(1)–Cl(1)#1	2.1857(15)
N(1)–C(1)	1.397(9)
Cl(3)–Fe(1)–Cl(2)	110.82(11)
Cl(3)–Fe(1)–Cl(1)	110.28(6)
Cl(2)–Fe(1)–Cl(1)	108.16(5)
Cl(3)–Fe(1)-Cl(1)#1	110.28(6)
Cl(2)–Fe(1)–Cl(1)#1	108.16(6)
Cl(1)–Fe(1)–Cl(1)#1	109.07(9)

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In order to characterize the absorbance properties of MAFeCl₄, UV-Vis absorption spectra were recorded. The UV-Vis absorption spectrum of MAFeCl₄ is shown in Fig. 2a. Strong absorption from 300 nm to 568 nm is observed. The absorbance of MAFeCl₄ declined steeply from 539 nm to 582 nm, indicating the presence of the band gap. In order to characterize the reflection property of MAFeCl₄, diffuse reflection spectroscopy was performed. The diffuse reflection spectrum of MAFeCl₄ is shown in Fig. 2b. The reflectivity of MAFeCl₄ decreases from 351 nm to 530 nm. The above results are consistent with the characteristics of the UV-visible absorption of MAFeCl₄. In order to acquire the band gap of MAFeCl₄, the diffuse reflectance spectrum was converted into the Kubelka–Munk spectrum (Fig. 2c). According to the Kubelka–Munk formula, F(R) = (1 − R)²/2R, where R represents the percentage of reflection, the relationship of the incident photon energy (hv) and E_g can be expressed as the transformed Kubelka–Munk relation, [F(R)hv]^p = A(hv − E_g), where E_g is the band gap energy, A is the absorption constant, h is the Planck ’s constant, ν is the frequency of the light (s⁻¹) and P is the power index that is related to the optical absorption process. Theoretically, P is equal to 1/2 or 2 for an indirect or a direct allowed transition, respectively. The E_g of MAFeCl₄ was determined to be approximately 2.15 eV from the intercept of the linear plot on the X axis if MAFeCl₄ is regarded as a directly allowed transition. After UV-Vis absorption spectroscopy and diffuse reflection spectroscopy for MAFeCl₄ were performed, the photoluminescence (PL) spectroscopy of MAFeCl₄ was also carried out. The PL spectrum of MAFeCl₄ is shown in Fig. 2d. The emission peaks at 398, 432 and 664 nm are observed at 360 nm excitation wavelength. Thus, the PL characteristic of MAFeCl₄ is attributed to these three emission peaks. MAFeCl₄ thin films (Fig. 3a) were prepared by spin-coating a mixture of equimolar MACl and FeCl₃ in DMF solution.

Fig. 2 (a–c) UV-Vis absorption spectrum, diffuse reflection spectrum and Kubelka–Munk spectrum of MAFeCl₄, respectively; (d) photoluminescence spectrum of MAFeCl₄ at 360 nm excitation wavelength.

Fig. 3 (a) Picture of the MAFeCl₄ thin film on the TiO₂ layer. (b) Calculated powder XRD profiles from the MAFeCl₄ single crystal and the MAFeCl₄ thin film on the TiO₂ layer. (c) SEM images of MAFeCl₄ on the TiO₂ layer. (d) Amplifying cross-sectional SEM images of the MAFeCl₄ device. (FTO/TiO₂/MAFeCl₄).

In order to determine the structure of the film, we recorded the XRD profiles of the MAFeCl₄ thin film. The diffraction XRD results of the MAFeCl₄ thin film on TiO₂ are shown in Fig. 3b. The diffraction peaks of MAFeCl₄ films on TiO₂ were basically consistent with the diffraction peaks calculated from the MAFeCl₄ single crystal. The corresponding crystal faces of each diffraction peak are shown in Fig. 3b. Strong peaks at 11.48°, 14.18°, 26.62°, and 37.91° are assigned to the (101), (011), (212), and (123) planes, respectively. In terms of the intensity of the peaks, the intensity of (101) peak was clearly stronger than that of the other peaks. These results suggest that the surface of the TiO₂ electron collecting layer induces the growth of MAFeCl₄ along the (101) plane. In order to study the morphology of the MAFeCl₄ thin film, we carried out surface SEM analysis. The SEM test result is shown in Fig. 3c. It can be clearly seen that non-uniform-size nanoparticles were formed on the surface of the TiO₂ layer. The sizes of the nanoparticles are about 50 to 90 nm. In order to test the uniformity and thickness of the FTO/TiO₂/MAFeCl₄ device, we tested the cross-sectional SEM image. The result is shown in Fig. 3d. The MAFeCl₄ thin film is uniformly covered on the TiO₂ film. The thickness of the MAFeCl₄ film ranges from 59 nm to 84 nm. In addition, the thickness of the electron collection layer (TiO₂) is uniform (about 59 nm).

The photoresponse of MAFeCl₄ has not been studied. Thus, photoresponsive devices based on MAFeCl₄ were fabricated. We prepared a carbon electrode as the back electrode on the MAFeCl₄ thin film by a doctor-blade, and evaluated the photoresponse by recording the photocurrent density–time characteristics. The I_ON/I_OFF photocurrent density response of the MAFeCl₄ device is clearly observed under ON/OFF illumination of AM 1.5 (100 mW cm⁻²) using a solar simulator (Fig. 4b). As time progressed, the photocurrent density increased. In order to determine the time required for the photocurrent density could attain stability, the photocurrent density–time characteristics of the FTO/TiO₂/MAFeCl₄/carbon electrode device were investigated under continuous illumination of AM 1.5 (100 mW cm⁻²) using a solar simulator (Fig. 4c). An evident photoresponse with maximum photocurrent density of 1352 nA cm⁻² at 14.4 seconds was observed. The photocurrent density can stabilize after 54 seconds of illumination. In order to investigate the monochromatic photocurrent-response of the MAFeCl₄ device, the monochromatic photocurrent response at different wavelengths with a flash frequency of 1.33 Hz was measured. The results are shown in Fig. 4. The I_ON/I_OFF photocurrent density response of the MAFeCl₄ device is not observed at 300 nm (Fig. 4d). At a wavelength of 330 nm, the I_ON/I_OFF photocurrent density response is significantly enhanced (Fig. 4e). The I_ON/I_OFF photocurrent response is also observed at a wavelength of 400 nm (Fig. 4f). At 410 nm, the I_ON/I_OFF photocurrent response is significantly weaker, but still measurable (Fig. S1†). At 430 nm and 450 nm, the I_ON/I_OFF photocurrent response was ineffective (Fig. S2 and S3†). The reason for this ineffective I_ON/I_OFF photocurrent response from 400 to 558 nm might be due to the unreasonable device structure or the energy level mismatch. In the future, we will optimize the device structure and energy level of the HTM material to generate I_ON/I_OFF photocurrent responses from 400 to 558 nm. The photoelectric conversion efficiency based on the FTO/TiO₂/MAFeCl₄/carbon electrode device reaches 0.054% (V_oc = 319 mV, J_sc = 0.375 mA cm⁻², and fill factor = 0.45) under AM1.5, 100 mW cm⁻² simulated illumination (Fig. S4 and Table S3†).

Fig. 4 (a) Schematics of the FTO/TiO₂/MAFeCl₄/carbon electrode device. (b) Photocurrent density–time characteristics of the FTO/TiO₂/MAFeCl₄/carbon electrode device under on/off illumination of AM 1.5 (100 mW cm⁻²) using a solar simulator. (c) Photocurrent density–time characteristics of the FTO/TiO₂/MAFeCl₄/carbon electrode device under continuous illumination of AM 1.5 (100 mW cm⁻²) using a solar simulator. (d–f) Photocurrent density–time characteristics of the FTO/TiO₂/MAFeCl₄/carbon electrode device under different wavelengths (300, 330 and 400 nm, respectively) with a flashlight frequency of 1.33 Hz.

The frequency of the writing-light is significant for the efficiency of an optical-logic device. The photoelectric responses of the MAFeCl₄ device were investigated under 330 nm illumination with different chopping-frequencies (1.33 Hz, 40 Hz, 60 Hz, and 80 Hz). The results are shown in Fig. S5.† The best photoelectric response of the MAFeCl₄ device is obtained at 1.33 Hz (Fig. S5a†). A stable photoelectric response is also observed at 40 Hz and 60 Hz (Fig. S5b and S5c†), which becomes weaker at 80 Hz (Fig. S5d†). Thus, the photocurrent–density response device is ineffective at 80 Hz. The effective photocurrent-response of the MAFeCl₄ device at high frequencies was probed to enhance the efficiency of the MAFeCl₄ optical-logic device. Our findings will be of interest in the magnetic, piezoelectric and photoelectronic research fields.

Conclusions

We prepared earth-abundant and environmentally friendly organic–inorganic CH₃NH₃FeCl₄ (MAFeCl₄) and studied its crystal structure, luminescence, band gap, adsorption properties and photoelectric behavior. The band gap of the orthorhombic MAFeCl₄ was determined to be approximately 2.15 eV. A low cost photodetector based on the MAFeCl₄ thin film is efficient under different monochromatic light ranging from 330 nm to 410 nm with different chopping frequencies (1.33 Hz to 40 Hz). Although the photoelectric conversion efficiency (0.054%) based on the FTO/TiO₂/MAFeCl₄/carbon electrode device is low, there will be significant room for improvement.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the Shandong Province Natural Science Foundation (Grant No. ZR2016BQ20; BS2015NJ013; ZR2014BQ010; ZR2016BQ21), the Colleges and Universities in Shandong Province science and technology projects (Grant No. J16LC05), the Science and Technology Innovation Foundation for the University or College Students (Grant No. 26312160502), the National Science and Technology Innovation Foundation for the University or College Students (Grant No. 201510447021), the Research Fund for the Doctoral Program of Liaocheng University (Grant No. 31805), the National Natural Science Foundation of China (Grant No. 21503104; 21601078, 21401095) and the National Basic Research Program of China (Grant No. 2011CBA00701).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1585495. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ra03498b

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