Methylammonium Chloride Additive in Lead Iodide Optimizing the Crystallization Process for Efficient Perovskite Solar Cells

Wang, Hongqiao; Zhang, Jingquan

doi:https://doi.org/10.1155/2022/5288400

International Journal of Photoenergy

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 5288400 | https://doi.org/10.1155/2022/5288400

Methylammonium Chloride Additive in Lead Iodide Optimizing the Crystallization Process for Efficient Perovskite Solar Cells

Hongqiao Wang¹and Jingquan Zhang^1,2

Academic Editor: Jurgen Hupkes

Received06 Apr 2022

Accepted07 Sept 2022

Published29 Sept 2022

Abstract

Perovskite solar cells (PSCs) were fabricated using a two-step spin-coating method with MACl added to the inorganic layer. The properties of the perovskite films were characterized by SEM, XRD, PL, UV-vis spectra, etc. The morphology of the PbI₂ film was significantly changed, and the formation of MACl-related intermediate phase was observed at the grain boundaries. The grain size and crystallinity of the perovskite film increased, and the morphology at grain boundaries was optimized, while the composition of perovskite remained unchanged. The introduction of MACl improved the open circuit voltage () and fill factor () of PSCs, and the optimal device efficiency reached 21.59%. This paper presents a new approach using additives in inorganic layers to optimize the crystallization process for efficient PSCs.

1. Introduction

Since first reported in 2009, PSCs have made great progress with highest photoelectric conversion efficiency (PCE) of 25.7% [1–12]. Additive engineering plays an important role in achieving highly efficient and stable PSCs. MABr coherently incorporates into the lattice of MAPbI₃, where Br⁻ and I⁻ ions can be combined in any ratio with band gap changing continuously [13, 14], while MACl does not match the lattice of MAPbI₃ [15, 16]. As a result, Cl⁻ ions retain at the surface of perovskite rather than incorporate into the lattice [17–19]. Nevertheless, the introduction of MACl will significantly improve the perovskite film morphology, which may arise from the sublimation of MACl or the loss of intermediate phase formed by MACl at a relatively low temperature [20–23]. Recently, Zuo et al. reported annealing MAPbI₃ film in a MACl vapor atmosphere, leading to significantly increased grain size and improved film quality, which confirmed this conjecture from the side [24].

Compared to MAPbI₃, FAPbI₃ is thermally more stable, with more ideal band gap for high performance PSC. However, phase transition occurs in FAPbI₃ below 150°C, where black α-phase becomes photoinactive yellow δ-phase, causing stability issues [8, 25]. Kim et al. reported α-phase FAPbI₃ film before annealing by using MACl additive and observed improved film morphology with increased grain size [26].

In the two-step method, two thin film crystallization processes are carried out sequentially, i.e., spin-coating PbI₂ with short time annealing first, then spin-coating FAI with long time annealing, which is different from the one-step method. PbI₂ crystallizes in the first process, and through chemical reaction, perovskite crystallizes in the second [25, 27]. Traditionally, MACl is added to the organic halide precursor (FAI) only [23, 28]. Here, we use MACl as an additive in the inorganic halide precursor (PbI₂). MACl enlarges PbI₂ grains and forms an intermediate phase scattered distributing on the surface of PbI₂. As a result, perovskite films are prepared with better morphology, larger grains, higher crystallinity, less defects at grain boundary, longer carrier lifetime, and band gap unchanged. Finally, we fabricate MACl optimized PSC with PCE of 21.59%.

2. Materials and Methods

2.1. Materials

SnO₂ colloidal solution (15 wt% in water) was purchased from Alfa Aesar. N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropanol (IPA), chlorobenzene (CB), acetonitrile (ACN), 4-tert-Butylpyridine (tBP), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)tri[bis(trifluoromethane)sulfonimide] (FK209 Co(III)), and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were purchased from Sigma-Aldrich. PbI₂ (99.99%) was purchased from TCI. Formamidinium iodide (FAI) and methylammonium bromide (MABr) were purchased from GreatCell Solar (Australia). PbCl₂, CsI, and methylammonium chloride (MACl) were purchased from Xi’an Polymer Light Technology in China. Spiro-OMeTAD (99.86%) and ITO (7 Ω/sq, cm²) were purchased from Youxuan Technology in Liaoning, China.

2.2. Device Fabrication

ITO glass substrates were soaked in deionized water diluted detergent for 3 hours then cleaned by deionized water and ethanol in an ultrasonic cleaner for 30 min, respectively. The ITO glasses were dried by a nitrogen gun and treated with UV-ozone for 15 min before use. 0.5 mL of SnO₂ colloidal solution (15 wt%) was diluted in 1.5 mL of deionized water. SnO₂-based electron transport layer was spin-coated on the precleaned ITO glasses at 4000 rpm for 20 s, followed by annealing at 180°C for 20 min in air. The substrates were then treated with UV-ozone for 12 min before transferring into the glovebox. For the fabrication of perovskite films, a typical two-step sequential deposition method was deployed here. First, 80 μL of PbI₂ precursor solution (600 mg PbI₂, 17.4 mg CsI, and 9 mg PbCl₂ dissolved in 0.9 mL of DMF and 0.1 mL of DMSO) was spin-coated on SnO₂ films at 1500 rpm for 30 s. We added 2 mg/5 mg/10 mg MACl to PbI₂ mixture separately before dissolving. The as-prepared FAI/MABr/MACl mixed solution (60 mg FAI, 6 mg MABr, and 6 mg MACl dissolved in 1 mL IPA) was spin-coated on the PbI₂ film at 1700 rpm for 30 s. Afterward, the films were annealed at 150°C for 15 min in a glovebox filled with N₂ gas. After cooling to room temperature, a spiro solution, which consists of 73 mg spiro-OMeTAD dissolved in 1 mL CB, added with 30 μL 4-tert-Butylpyridine, 18 μL Li-TFSI solution (520 mg Li-TSFI in 1 mL of ACN), and 29 μL FK209 Co(III) solution (300 mg FK209 Co(III) in 1 mL of ACN) was deposited on perovskite films at 4000 rpm for 30 s. Finally, 100 nm of Au was thermally evaporated as the electrode under a high vacuum using an electron-beam thermal evaporation system.

2.3. Characterization

The SEM images were obtained by Phenom Pro (Phenom-World B.V.) and the XRD patterns were characterized by DX-2700 (HaoYuana Instrument) using a Cu Kα. The steady-state PL and time-resolved PL decay were measured by FLS980 (Edinburgh Instruments) with a xenon light at the wavelength of 600 nm and a 655 nm laser excitation source. The J-V curves were characterized under simulated air mass (AM) 1.5G illumination using a solar simulator (Enlitech SS-F5-3A, 100 mW cm⁻²) and a Keithley 2400 SourceMeter. The solar cells were measured under reverse scan (+1.2 V to −0.1 V) and forward scan (−0.1 V to +1.2 V) with a step of 0.01 V and a delay time of 100 ms.

3. Results and Discussion

3.1. Characterization of Lead Iodide and Perovskite Thin Films

The surface morphology of PbI₂ and perovskite films can be directly observed by the high-resolution SEM. Figure 1 shows the effect of adding different concentrations of MACl on the morphology of the PbI₂ layer: (a) without MACl, (b) 2 g/L MACl, (c) 5 g/L MACl, and (d) 10 g/L MACl. The grains at the bottom of PbI₂ are dense and are loose and small at the surface. After spin-coating the PbI₂ precursor solution, PbI₂ thin film was annealed on a hot plate at 70°C for 13 s. Due to the low annealing temperature, short time and heating from the bottom to the top, PbI₂ grains on the surface were not fully grown. In the cases added with MACl, spindle-shaped grains appeared on the surface of PbI₂ films. With the addition of MACl increasing, size of grains on the surface of PbI₂ increased, and more spindle-shaped grains formed and became thicker. It is especially obvious on the surface of the PbI₂ film added with 10 g/L MACl. The small grains of PbI₂ that were originally granular and dispersed grew into large grains with interconnected and indistinguishable boundaries. Many spindle-shaped grains were attached or embedded in PbI₂ grains.

(a)

(b)

(c)

(d)

The composition of the spindle-shaped grains may be MACl or an intermediate phase formed by the reaction of MACl and PbI₂. The intermediate phase is distributed on the surface of the PbI₂ layer and may serve as the nucleation site of perovskite grains during subsequent spin-coating of the organic layer, promoting the formation of the perovskite phase [29–32]. The MACl component in it, sublimed in the subsequent annealing process, may optimize the perovskite film formation process and increase the grain size, which was confirmed in the subsequent experiments.

Figure 2 shows the effect of adding different concentrations of MACl on the surface morphology of the perovskite films prepared by the two-step method: (a) without MACl, (b) 2 g/L MACl, (c) 5 g/L MACl, and (d) 10 g/L MACl. The grain size of the formed perovskite film increases gradually with the increase of the additional amount of MACl. In Figure 2(a), small grains with discontinuous and broken edges exist, and some tiny irregular grains accumulate at the grain boundaries, which can introduce defects. After the addition of MACl, this problem was significantly optimized. On the surface of the perovskite film with 10 g/L MACl added, the grains showed a regular geometric shape, and the grain boundaries were tightly packed. At the same time, large grains mean smaller grain boundary density and therefore less grain boundary defects. The introduction of MACl improves the quality of perovskite thin films by affecting the morphology of the PbI₂ layer and the crystallization process of the perovskite layer, which increases the perovskite grain size and optimizes the morphology at grain boundary.

(a)

(b)

(c)

(d)

We performed XRD experiment to test the crystal structure of the perovskite films, as shown in Figure 3. Symbol ‘^#’ marks the PbI₂ phase, which corresponds to the (001) crystal plane of PbI₂, and the ‘’ marks the perovskite phase, corresponding to the (101, 202) planes of the perovskite. The intensity of the diffraction peaks of the perovskite phase increases with the increase of the additional amount of MACl. With 10 g/L MACl addition, the diffraction peak intensity of the perovskite phase is significantly enhanced compared with that of without MACl addition, which indicates that the addition of MACl in the PbI₂ layer promotes the formation of the perovskite phase, which is also consistent with the phenomenon that perovskite grains increase after adding MACl showed in the SEM image, indicating that the crystallinity of perovskite improved. The position of the diffraction peak remains unchanged, so the interplanar spacing remains unchanged according to the Bragg formula . No diffraction peaks appeared or disappeared, indicating that the crystal structure and phase were remained. The diffraction peak intensity of PbI₂ phase remained basically the same after adding 2 g/L MACl, increased after adding 5 g/L MACl, and decreased but still existed after adding 10 g/L MACl. This is due to the presence of excess PbI₂ in the perovskite film prepared with this precursor ratio, which is beneficial for reducing defects and improving device performance. On the one hand, it was observed from the SEM that the surface morphology of the PbI₂ layer was changed after the addition of MACl, and there was an intermediate phase formed on the surface of the PbI₂ layer, that is, the influence of MACl mainly affected the surface of the PbI₂ layer, which explained that PbI₂ still exists in the finally formed perovskite film. On the other hand, the intermediate phase on the surface of PbI₂ can act as a nucleation center in the subsequent reaction with the organic layer, and sublimation of MACl can occur during the second step of annealing, which improves the perovskite film formation process and increases the perovskite grain size. Therefore, the diffraction peak intensity of the perovskite phase increases, and the position of the peak does not change.

Figure 4 shows (a) UV-vis transmission spectra and (b) Tauc curves of perovskite films after adding different concentrations of MACl. The transmittance curves of the perovskite films added with different concentrations of MACl basically overlap. According to the Tauc formula, for a direct semiconductor, , where is the incident photon energy, is a constant, and is the absorption coefficient. Transmittance , where is the intensity of incident light, is the intensity of out light after absorption by the film, and is the film thickness. In the Tauc plot in Figure 4, the intercept of the curve on the horizontal axis is the optical band gap of the film. From the Tauc plot, within the error range, the optical band gap of the perovskite films prepared by adding different concentrations of MACl to the PbI₂ layer remains unchanged, indicating that the chlorine atoms do not enter the perovskite lattice, or the amount of entering is very small. This result is similar to the addition of MACl to the organic layer and the addition of MACl to the one-step precursor solution reported in literature, which may be attributed to the lower sublimation temperature of MACl and the large difference in the radii of chloride and iodide ions, which leads to lattice mismatch.

(a)

(b)

In order to study the effect of MACl on the optoelectronic properties, carrier lifetime, and transport properties of the formed perovskite films, PL and TRPL tests were performed on the perovskite films added with different concentrations of MACl, and the results are shown in Figure 5(a) steady-state PL spectrum and (b) TRPL curve (sample structure: corning glass/perovskite). As the addition of MACl increased, the positions of the PL peaks remained unchanged at 788 nm, corresponding to a band gap of 1.57 eV, which was consistent with the results calculated by the Tauc plot, indicating that the addition of MACl did not change the band gap of the perovskite film. This shows that chlorine atoms do not enter the perovskite lattice or enter in a small amount. The PL intensity gradually increased with the increase of the addition of MACl, indicating that the addition of MACl to PbI₂ may reduce the defect density, thereby weakening the nonradiative recombination, reducing the number of carriers trapped by deep-level defects, and more photoexcited carriers transition to lower energy levels by radiative recombination and releasing energy in the form of photons. According to the SEM results, as the addition of MACl increases, the grains of the formed perovskite films increase and the grain boundaries become denser, and the irregular grains at the grain boundaries decrease, so the grain boundary density decreases and grain boundary defects decrease, which together lead to the reduction of deep-level defects. That is consistent with the results of PL. TRPL tests were performed on perovskite monolayers with different additions of MACl at a measurement frequency of 0.5 MHz. And we fitted the TRPL curve in Figure 5(b) using the double exponential formula: where and represent the weights of different decay processes and and correspond to the lifetime of each process. The average lifetime is measured as

(a)

(b)

The result is showed in Table 1. As the addition of MACl increases, the lifetime of the carriers in the prepared perovskite films gradually increases, indicating that the deep-level defects in the film are reduced and the nonradiative recombination is weakened, which is consistent with the analysis of the PL results. As the addition of MACl further increases, the carrier lifetime tends to be stable, indicating that the perovskite film has been sufficiently passivated.

3.2. Device Fabrication and Performance

The perovskite solar cell devices added with different concentrations of MACl were prepared, and the device structure was ITO/SnO₂/PVSK/Spiro-OMeTAD/Au. The steady-state J-V curves of the device were tested under AM1.5 illumination, as shown in Figure 6. With the increase of MACl addition, the ,, , and of the device all improved, and the best device efficiency of 21.59% was achieved by adding 10 g/L MACl to the PbI₂ layer, corresponding to a fill factor of 77.46%, which benefited from the improved perovskite morphology, increased grain size, and reduced grain boundary defects.

The statistics of device performance with different subcells are shown in Figure 7. The introduction of MACl improves the of the cell, and the overall performance of the device with the addition of 10 g/L MACl improves. Due to the improvement of and , the of the device is improved. The optimized indicates the density of deep-level defects decreased, and the nonradiative recombination was suppressed.

(a)

(b)

(c)

(d)

4. Conclusions

We added different amount of MACl to PbI₂ precursor solution in the two-step spin-coating method. With the addition of MACl, an intermediate phase formed on the surface of PbI₂ and the surface morphology of PbI₂ significantly changed. The intermediate phase can react as nucleation site of perovskite grains or release gaseous MACl during the second annealing process, leading to larger grain size, less grain boundary defect, and better crystallinity, which was confirmed by XRD. Tauc plot and PL both demonstrate that MACl cannot incorporate into the lattice of FAPbI₃. With the addition of MACl, carrier lifetime and PL intensity increased, indicating deep-level defect decreased and nonradiative suppressed, which is due to better perovskite film morphology, larger grain size, and less broken in grain boundaries. With the addition of MACl, we fabricated PSC with better performance, especially higher and . The PCE of the best device is 21.59%, and the is 77.46%.

Data Availability

The data used to support the findings of this study are included within the article. Further data or information is available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

This work was supported by the Sichuan Provincial Project [grant number: 2021YFG0102]. The authors appreciate the supports from Sichuan University, China, for providing help during the research and preparation of the manuscript.

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Copyright © 2022 Hongqiao Wang and Jingquan Zhang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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