Impact of grain size on the optoelectronic performance of 2D Ruddlesden–Popper perovskite-based photodetectors

Abstract

Two-dimensional (2D) Ruddlesden–Popper perovskites have been spotlighted as photodetectors due to their tunable bandgap and high photoluminescence quantum efficiency. The performance of photodetectors is affected by grain boundaries that can trap charge carriers and thereby degrade charge transport in perovskites. To fabricate high-performance photodetectors, it is important to control the grain boundaries of the perovskite film. Here, we use a hot casting method to synthesize highly-crystalline, large-grained films of 2D Ruddlesden–Popper perovskites (C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇ for use in improving the efficiency of photodetectors. Increasing the grain size of the perovskite film alleviates the effects of grain boundaries. To control the grain size of the film, the composition of the precursor mixture N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is varied. As the DMSO content increases, the grain size of the film increases from the nanometer to the micrometer scale. Photodetectors fabricated using films with large grains show high photocurrent and on/off ratio compared to those with small grains. This method to synthesize 2D Ruddlesden–Popper perovskite films with tailored grain sizes can facilitate the fabrication of highly-efficient optoelectronic devices.

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Introduction

Organic–inorganic hybrid perovskites (OIPs) including three-dimensional (3D) or layered two-dimensional (2D) structures have excellent charge carrier transport, a long electron–hole diffusion length, and a tunable bandgap, due to which they have applications in optoelectronics.^1–7 Development to implement the superior optoelectronic properties of OIPs has led to the design of various applications such as solar cells, photodetectors,^8,9 lasers,^10,11 and photo supercapacitors.^12,13 Besides, to improve the optical properties of OIPs, several methods including the doping of small molecules in OIPs, the use of OIP quantum dots, and the introduction of multilayer structures have been proposed.^14–16 In addition, by lowering the dimensions from 3D to 2D, 2D Ruddlesden–Popper layered perovskite (RPLP) materials have attracted considerable attention for OIP-based optoelectronic applications.^17–21 2D RPLP materials have weak ionic bonding, so they can be grown easily using vapor transport or solution processes.²² Moreover, functionalization with appropriate functional organic groups during synthesis yields RPLP films that have tunable structures and properties; improved optoelectronic properties and environmental stability compared to those of 3D OIPs.^23–27

To improve the optical properties such as photocurrent and power conversion efficiency (PCE) of an OIP-based optoelectronic devices, the grain size of the film should be optimized.^28,29 Increasing the grain size in OIP-based polycrystalline films alleviates the effects of grain boundaries that can trap the charge carriers and thereby degrade the charge transport in OIPs.³⁰ In 2D RPLP films, which consist of an inorganic layer sandwiched between organic spacers, the influence of grain size and crystal orientation is important because they affect the anisotropic charge transport in the films.³¹ Charge transport between grains in 2D RPLP films can be enhanced by increasing the grain size and thereby reducing the number of grain boundaries. Therefore, various methods have been investigated to increase the grain size in OIP-based films such as hot-casting,^32,33 solvent engineering,³⁴ and thermal nanoimprint technology.^35,36 Among them, crystal growth depends on the substrate temperature; so, the hot-casting method allows the grain size to reach the millimeter scale.³² Furthermore, the grain size can be controlled by choosing an appropriate solvent. In (CH₃(CH₂)₂NH₃)₂(CH₃NH₃)₄Pb₅I₁₆ films, an optimized mixture of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) could increase the grain size from nanoscale to microscale, which resulted in solar cells having a high PCE.³⁷ Although several studies on controlling the grain size have been conducted in solar cells, it is also essential to investigate the effect of grain size on photodetectors to improve the device performance.

In this study, we synthesize (C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇ (BA₂MAPb₂I₇, where BA = butylammonium; MA = methylammonium) films using a hot-casting method and solvent engineering to achieve a highly sensitive photodetector. We quantify the effect of grain sizes on the optical response of the photodetector using polar solvents DMSO and DMF, either pure or mixed. The improvement in optoelectronic performance can be obtained by increasing the grain size of the film with a reduced number of grain boundaries. This method to synthesize BA₂MAPb₂I₇ films can suggest a way to control the grain size and improve the device performance in optoelectronic applications.

Experimental section

Materials

Butylammonium iodide (BAI) and methylammonium iodide (MAI) were purchased from Greatcell Solar. Lead bromide (PbI₂, 99% purity), N,N-dimethylformamide (DMF, ≥99.8% purity), and dimethyl sulfoxide (DMSO, ≥99.9% purity) were purchased from Sigma-Aldrich. Si wafers with 100 nm thick thermally-grown SiO₂ were used as the substrate. All materials were used without further purification.

Device fabrication

Before device fabrication, the SiO₂/Si substrate was cleaned sequentially with ethanol, acetone, and deionized water. The BA₂MAPb₂I₇ precursor solution was composed of PbI₂, BAI, and MAI (2 : 2 : 1) in solvents (DMF and DMSO) in different volume ratios [P_DMSO = DMSO/(DMSO + DMF)] of 1 : 0 (P_DMSO = 0), 7 : 3 (P_DMSO = 0.3), 1 : 1 (P_DMSO = 0.5), 3 : 7 (P_DMSO = 0.7) and 0 : 1 (P_DMSO = 1). During hot-casting, the substrate was heated to 200 °C and the precursor solution was kept on a hot plate at 70 °C.³² During spin coating, the hot substrate was transferred to the spin coater chuck; the transfer time was about 1–2 s. The precursor solution was then spun at 1250 rpm for 60 s in a nitrogen atmosphere. The resulting films were allowed to cool down to room temperature, and then planar photodetectors were fabricated by thermally evaporating 50 nm thick Au electrodes through a shadow mask. The size of Au electrodes was 0.1 mm² and the distance between electrodes was 100 μm.

Measurement and characterization

The crystal structure was measured using an X-ray diffractometer (XRD; Rigaku D/MAX-2500) with Cu Kα radiation at a step size of 0.02°. The morphological images of the surface and cross-section were captured using a high-resolution field-emission scanning-electron microscope (SEM; JEOL JSM-7800F Prime) with a 10 kV acceleration voltage and an atomic-force microscope (AFM; VEECO Dimension 3100). Photoluminescence (PL) spectra were measured using a spectrometer (Horiba LabRAM HR-800). Time-resolved photoluminescence (TRPL) was measured using a time-correlated single-photon counting system (HAMAMATSU C11367-31). Current–voltage (I–V) characteristics were measured using a semiconductor parameter analyzer (Keithley 4200A-SCS) in a vacuum probe station. The optical response measurements were made by applying monochromatic light from a xenon lamp (Newport 71SI00059) under ambient conditions. Grain sizes G were calculated using the ASTM E112 intercept method.³²G was calculated as G = 6.643856 log₁₀ (P_L) −3.288, where P_L is the number of grain boundary intersections per unit length of the test line and was determined using P_L = P_i/(L/M), where P_i is the total number of intercepts of all test lines, L is the total length of test lines, and M is the magnification.

Results and discussion

A 2D RPLP with the chemical formula (RNH₃)₂(CH₃NH₃)_n−1MX_3n+1, where RNH₃ is a long chain organic cation, M is a metal cation, and X is a halide anion, was selected to evaluate the morphology, texture, and performance of the photodetector. To increase the grain size and improve the crystallinity of the 2D RPLP films, we combined solvent engineering with the hot casting method (Fig. 1a).³² The morphology and grain size of BA₂MAPb₂I₇ films can be influenced by crystallization kinetics such as the rate of solvent evaporation. LaMer's graph is used to explain the nucleation and growth processes, which are related to the concentration change of the solution over time (Fig. 1b).³⁸ As the solvent evaporates, the solution concentration reaches the saturation concentration C_s; then, when the solution concentration exceeds the critical value C_c, nuclei start to form and then grow by consuming the solute. Slow evaporation of the solvent leads to rapid growth of the nuclei due to fast consumption of the solute. This hypothesis suggests that the increase in the grain size of BA₂MAPb₂I₇ films after the addition of DMSO to DMF occurs because the mixture evaporates more slowly than pure DMF. The crystallization process of BA₂MAPb₂I₇ films was suggested using pure DMF solution and the mixture of DMF/DMSO solution by considering LaMer's graph (Fig. 1c). The vapor pressure of the solvent influences its evaporation rate, and this change can affect the morphology and grain size of the film. As reported previously, DMF has a higher vapor pressure than DMSO,³⁹ so DMF evaporates quickly during the spin coating process. When the precursor concentration exceeds C_c, the BA₂MAPb₂I₇ film obtained using DMF solution nucleates and grows quickly because of the fast evaporation of DMF.⁴⁰ However, DMSO has a lower vapor pressure (0.42 mmHg) than DMF (2.7 mmHg).⁴¹ Adding DMSO to DMF impedes the evaporation of the solvent during the spin-coating process, so the solvent evaporates slowly and thereby extends the time required for the precursor concentration to reach C_c. Also, considering the large grain size of the film that was obtained using P_DMSO = 0.7, the nucleation period (t₁′–t₂′) of the film that was synthesized from the DMF and DMSO mixture may be longer than the nucleation period (t₁–t₂) of the film that was obtained from pure DMF. The XRD patterns of films that had been synthesized from different P_DMSO showed (0k0) diffraction peaks (Fig. S1, ESI†). As the ratio of the DMSO increased, the intensity of (020) increased. These observations indicate that the addition of DMSO improved the crystal quality of the film. The improvement of crystallinity is thought to be related to the perovskite crystallization process. DMSO has a higher boiling point (189 °C) and a lower vapor pressure than DMF that has a lower boiling point of 152 °C and a higher vapor pressure.⁴¹ These characteristics induce the differences in the evaporation rates of solvents that can affect the crystallization rate and film quality during the spin-coating process.³⁹ Due to these differences between DMSO and DMF, the crystallization kinetics are slower in the OIP film obtained using DMSO than those in the film obtained using DMF.³⁷ As a result, the film formed using DMSO has better crystallinity than the film formed using DMF.

Fig. 1 (a) Schematic illustration of the BA₂MAPb₂I₇ thin film deposition process using hot-casting and solvent engineering. (b) LaMer's diagram and (c) modified LaMer's diagram for BA₂MAPb₂I₇ film formation using pure DMF (P_DMSO = 0) and a mixture of DMF and DMSO. C_s and C_c are the saturation concentration and critical concentration for nucleation, respectively.

This phenomenon also affected the film morphology. Top-view SEM images of the films obtained using P_DMSO = 0, 0.3, 0.5, 0.7, and 1 were taken (Fig. 2); as P_DMSO increased, the grain size of the resulting BA₂MAPb₂I₇ film increased (Fig. 2a–e), from the nanometer scale at P_DMSO = 0 to the micrometer scale at P_DMSO = 1. We also calculated the grain size of the film utilizing the ASTM intercept method.³² This method has been explained in the experimental section in detail. The average grain size of the film increased as P_DMSO increased, i.e., from 0.6 ± 0.2 μm at P_DMSO = 0 to 1.7 ± 0.7 μm at P_DMSO = 0.3, 4.4 ± 0.5 μm at P_DMSO = 0.5, 7.5 ± 1.2 μm at P_DMSO = 0.7, and 14.1 ± 1.9 μm at P_DMSO = 1 (Fig. 2f). Also, the BA₂MAPb₂I₇ film obtained using P_DMSO = 1 was uniformly formed with a thickness of about 350 nm (Fig. S2, ESI†). In addition, we used AFM to assess the quality of the BA₂MAPb₂I₇ film. AFM (scan size = 10 μm × 10 μm) images showed that the film obtained using DMF (P_DMSO = 0) had an average root-mean-squared roughness of R_rms = 24.5 nm, whereas the film obtained using DMSO (P_DMSO = 1) had R_rms = 17.8 nm (Fig. S3, ESI†).

Fig. 2 Plan-view SEM images of films cast from (a) pure DMF (P_DMSO = 0) (inset: magnified image), (b) DMF : DMSO = 7 : 3 (P_DMSO = 0.3), (c) DMF : DMSO = 1 : 1 (P_DMSO = 0.5), (d) DMF : DMSO = 3 : 7 (P_DMSO = 0.7), and (e) pure DMSO (P_DMSO = 1). (f) Average grain sizes of BA₂MAPb₂I₇ films cast from P_DMSO = 0, 0.3, 0.5, 0.7, and 1.

To measure the photosensitivity of the film obtained using BA₂MAPb₂I₇, lateral photodetectors were fabricated by combining solvent engineering and hot-casting, with P_DMSO = 0, 0.3, 0.5, 0.7, or 1. The schematic illustration and optical image of the fabricated photodetector device are shown in Fig. S4 (ESI†). Irradiation using the visible light produced electron–hole pairs in the OIP. In all devices, the current increased as the voltage increased (Fig. 3a). The I–V curves of BA₂MAPb₂I₇-based photodetectors that were fabricated using P_DMSO = 0, 0.3, 0.5, 0.7, or 1 were measured in the dark and under the illumination with a wavelength of 460 nm and an intensity (L_I) of 300 μW cm⁻² (Fig. 3b). In the dark, the photocurrent of the devices was ∼1 × 10⁻¹¹ A at a voltage bias of 30 V (Fig. S5, ESI†). The relationship between the photocurrent and incident light intensity of the films was shown (Fig. 3c). As the incident light intensity increased, the photocurrent increased; this trend confirms the light-sensitive properties of BA₂MAPb₂I₇ films. The photocurrent of the devices was measured at a voltage bias of 30 V with a wavelength of 460 nm at different L_I. As P_DMSO increased, the photocurrent in the devices increased. The increase in photocurrent may have occurred because the films produced at P_DMSO = 0.7 or 1 had larger grains, and therefore fewer grain boundaries, than the film produced at P_DMSO = 0.

Fig. 3 I–V characteristics of the photodetectors based on P_DMSO = 0, 0.3, 0.5, 0.7, and 1 under irradiation with a light intensity of 300 μW cm⁻² at a wavelength of 460 nm in the dark with (a) a linear scale and (b) a semilogarithmic scale. (c) Photoresponse of the film under 460 nm light with light intensities ranging from 20 μW cm⁻² to 500 μW cm⁻².

We quantified the effect of L_I on the photoresponse of the device obtained using P_DMSO = 1 (Fig. 4a). The I–V characteristics were measured in the dark and under the illumination with a wavelength of 460 nm at 0 ≤ L_I ≤ 300 μW cm⁻² and a voltage bias of 30 V. As L_I increased, the photocurrent increased from 1 × 10⁻¹¹ A in the dark to 5 × 10⁻⁹ A at 300 μW cm⁻². An increase in L_I induced increased generation of charge carriers, which yielded an increased photocurrent. The photoswitching properties of the devices were measured at a voltage bias of 30 V with 460 nm light illumination with a fixed L_I = 300 μW cm⁻² (Fig. 4b). The photocurrent increased sharply when the light was turned on and decreased sharply when it was turned off. The on/off current ratio was ∼120. Ten cycles of photoswitching were demonstrated that the device had reliable and stable photoresponses. To confirm the feasibility of the low-voltage operation of the fabricated photodetector, we measured the photoswitching properties of a device that had P_DMSO = 1 at 5 V using 460 nm light illumination. The photocurrent increased sharply when the light was turned on and decreased when it was turned off (Fig. S6, ESI†). It showed that the photodetector devices could be operated at a low voltage. R is an important parameter to characterize sensitivity. We calculated R = (I_light − I_dark)/(L_IA), where I_light is the current under illumination, I_dark is the current in the dark, and A is the active area in the device. The active area of the light illumination was 9 × 10⁻⁴ cm². At a bias voltage of 30 V, as L_I increased from 20 μW cm⁻² to 500 μW cm⁻², R decreased from 26.3 A W⁻¹ to 1.4 A W⁻¹ (Fig. 4c). We also quantified the effect of L_I on the external quantum efficiency EQE = Rhc/(λe), where R is the responsivity of the photodetector, h is Planck constant, c is the speed of light, λ is the wavelength of incident light, and e is the electron charge.^42,43 The calculated EQE was 2.2 × 10³% at 200 μW cm⁻² (Fig. S7, ESI†). We then calculated D* = RA^0.5/(2eI_dark)^0.5. D* decreased as L_I increased (Fig. 4d), and the highest value was estimated to be 4 × 10¹² Jones (1 Jones = 1 cm Hz^0.5 W⁻¹). The photoresponse of the photodetector can be further improved by reducing the numbers of pinholes and defects, which can act as non-radiative recombination centers in OIP films.

Fig. 4 (a) I–V characteristics of the device that was fabricated using P_DMSO = 1 in the dark and at different light intensities under 460 nm light illumination ranging from 50 μW cm⁻² to 300 μW cm⁻². (b) Photoswitching at a light intensity of 300 μW cm⁻². (c) Photocurrent and responsivity as a function of light intensity. (d) Detectivity as a function of light intensity.

We compared the absorption spectra and PL of the BA₂MAPb₂I₇ films obtained using different P_DMSO (Fig. 5). The BA₂MAPb₂I₇ films that were synthesized with different P_DMSO showed band edges at ∼602 nm (Fig. 5a); the corresponding optical bandgap was 2.05 eV, which is consistent with previous reports.^9,44 The morphological change and the different grain sizes induced different optoelectronic properties such as PL. The films formed using P_DMSO = 0.7 or 1 showed good crystallinity with large grain sizes and therefore had a higher PL intensity than that of the films formed using P_DMSO = 0, 0.3, or 0.5 (Fig. 5b). The TRPL of BA₂MAPb₂I₇ films with different P_DMSO was measured to confirm the carrier lifetime of the films on the substrate, which could provide evidence that the quality of the film was improved by using DMSO (Fig. 5c). The PL decay of the films was fitted using a bi-exponential decay model.^45,46 The fast decay component τ₁ is related to charge carrier trapping, and the slow decay component τ₂ indicates radiative recombination of free carriers.^47,48 The film formed at P_DMSO = 1 showed a higher PL lifetime (τ₁ = 467 ns and τ₂ = 1278 ns) than the film formed at P_DMSO = 0 (τ₁ = 360 ns and τ₂ = 978 ns). The large grain size of the film with a reduced trap density may increase the PL lifetime.

Fig. 5 (a) Absorbance, (b) photoluminescence (PL), and (c) time-resolved PL (TRPL) spectra of the films cast from P_DMSO = 0, 0.3, 0.5, 0.7, and 1.

Conclusions

We demonstrated the control of grain size in BA₂MAPb₂I₇ films by combining solvent engineering and the hot casting method. Introducing DMSO into DMF increased the grain size of BA₂MAPb₂I₇ films and improved the quality of the film. As the grain size of the film increased, the photocurrent and on/off ratio increased. The photodetector that was fabricated using pure DMSO exhibited the largest grain size and the highest photocurrent, R, and D*. The improved device performance using the film that had a large grain size could be explained by a decrease in the number of carrier recombination sites, because of a reduced number of grain boundaries and a decrease of defect density. The proposed method to control the grain size of films may enable the development of high-quality perovskite films that can be applied to advanced optoelectronics.

Author contributions

J.-S. L. conceived and directed the research. J.-S. L. and B. H. designed and planned the experiments. B. H. and Y. P. performed the experiments and acquired the data. B. H., Y. P., and J.-S. L. wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF-2016M3D1A1027665 and NRF-2019R1A2C2084114). This work was also supported by the Industrial Strategic Technology Development Program (20003968) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). In addition, this work was partially supported by the Brain Korea 21 PLUS project (Center for Creative Industrial Materials).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc04350h

‡ These authors contributed equally.

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