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DOI: 10.1039/C6RA17197D (Communication) RSC Adv., 2016, 6, 85026-85029

Insight into the effect of ion source for the solution processing of perovskite films†

Chongwen Li‡ ab, Zaiwei Wang‡b, Yue Changb, Yingxia Zongb, Fuxiang Jib, Bo Zhangb, Haiyan Li*a and Shuping Pang*b
aInstitute of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, P. R. China. E-mail: lihy@ouc.edu.cn
bQingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China. E-mail: pangsp@qibet.ac.cn

Received 5th July 2016 , Accepted 3rd August 2016

First published on 26th August 2016

Abstract

We find that there exists an ion exchange phenomenon in the precursor solutions for perovskite films. Despite the different ion introduction routes, the samples fabricated from precursor solutions with same ion molar ratio finally present similar properties. This finding illuminates a wide selectivity of reactants for the preparation of perovskite films.

Perovskite solar cells (PSCs), in which the organolead halide perovskites act as the light-absorbing materials, are deemed to be a promising alternative to traditional solar cells.1,2 The organic–inorganic light absorbing materials adopt an ABX3 formula, where A is an organic cation (CH3NH3+, NH2CH[double bond, length as m-dash]NH2+), B is a metal ion (Pb2+, Sn2+), and X is a halide anion (I−, Cl− or Br−).3–6 Not just acceptable for their suitable band gap, high absorption coefficient and long hole/electron diffusion length, they have also been regarded as dramatic photovoltaic materials for their solution-processability and low cost of raw materials. Since Miyasaka et al.'s pioneering work on adopting perovskite as a light-absorbing material in solar cells showed an efficiency of 3.8%,7 an efficiency of over 22% has already been reported recently.8 The increasing efficiency of PSCs is closely attributed to the development of perovskite film fabrication processes.9 Among traditional fabrication processes, one-step precursor solution deposition is undoubtedly the most convenient potential method for commercial application in the future.9 However, the prepared perovskite thin films are rough and multiply defective.10–13 In the MAPbI3 precursor solutions, colloidal clusters such as MAI–PbI2–xDMF and MAI–PbI2–xDMSO formed by strong coordination bonds between solvent, organic components and the lead atom, can serve as heterogeneous nucleation sites to assemble into bulk structures due to the presence of strong van der Waals bonding between them.14,15 Therefore, an increase in the number of MAPbI3 nuclei14,16,17 together with a reduction in their van der Waals bonding are essential to enhancing the film quality. In this regard, one strategy is to introduce excess organic components, such as FACl, MACl, and NH4Cl, into the precursor solutions.18–23 Meanwhile, it was recently discovered that the use of lead acetate, mixed lead iodide and lead chloride could also improve the film uniformity and density.24–27 It seems that both the hetero-ions such as Cl− and Ac− favour the formation of perovskite films with better coverage, but the effect of their sources on film morphology, crystallinity and device efficiency has not been systematically studied.28

Herein, we introduce excess NH4+ and Ac− ions into MAPbI3 precursor solutions through different introduction routes, as listed in Table 1. NH4Ac[thin space (1/6-em)]:[thin space (1/6-em)]MAI[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 (S-NH4Ac), MAAc[thin space (1/6-em)]:[thin space (1/6-em)]NH4I[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 (S-MAAc) and MAI[thin space (1/6-em)]:[thin space (1/6-em)]NH4I[thin space (1/6-em)]:[thin space (1/6-em)]PbAc2[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5 (S-PbAc2), each have the same NH4+[thin space (1/6-em)]:[thin space (1/6-em)]Ac−[thin space (1/6-em)]:[thin space (1/6-em)]MA+[thin space (1/6-em)]:[thin space (1/6-em)]Pb2−[thin space (1/6-em)]:[thin space (1/6-em)]I− ion ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3. Based on these three kinds of solution, the perovskite films were fabricated and their corresponding morphology, crystallinity, photophysical property and photovoltaic performance were studied sequentially.

Table 1 Compositions of three different precursor solutions
S-NH4Ac S-MAAc S-PbAc2
NH4Ac[thin space (1/6-em)]:[thin space (1/6-em)]MAI[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 NH4I[thin space (1/6-em)]:[thin space (1/6-em)]MAAc[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 MAI[thin space (1/6-em)]:[thin space (1/6-em)]NH4I[thin space (1/6-em)]:[thin space (1/6-em)]PbAc2[thin space (1/6-em)]:[thin space (1/6-em)]PbI2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5

We first employ the typical scanning electron microscopy (SEM) images of the top view of perovskite films prepared from different precursor solutions. Fig. 1(A) shows the full-coverage film morphology for S-NH4Ac thin film, and no clear crystal boundary can be observed. When the ion introduction routes changed to S-MAAc and S-PbAc2, the overall appearances of the thin films shown in Fig. 1(B) and (C) exhibit similar degree of coverage and grain size of perovskite. As shown in Fig. 1(D), all the UV-vis (UV) spectra of the films present absorption onsets at ∼780 nm, which represents a general feature of MAPbI3. X-ray diffraction (XRD) measurements were performed on these films to gain an understanding of the crystal structure. As can be seen in Fig. 1(E), in all cases, pure MAPbI3 perovskite films were obtained and their equivalent relative intensities of all the diffraction peaks indicate their coincident lattice orientation. The full width at half maxima (FWHM) of the (110) peak of the S-NH4Ac, S-MAAc and S-PbAc2 thin films were subsequently measured. It was calculated that their crystal sizes are 49.07, 45.85 and 47.14 nm, respectively, according to the Scherrer formulate of D = Kγ/B[thin space (1/6-em)]cos[thin space (1/6-em)]θ.


image file: c6ra17197d-f1.tif
Fig. 1 (A–C) SEM images of perovskite films made from S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions. (A) MAI, PbI2 and NH4Ac (1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio) or S-NH4Ac; (B) NH4I, PbI2 and MAAc (1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio) or S-MAAc; (C) NH4I, MAI, PbAc2 and PbI2 (1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5 molar ratio) or S-PbAc2. (D and E) UV-vis absorption spectra and XRD patterns of perovskite films made from S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions, respectively.

For a deeper understanding of the perovskite formation process, we investigated the coordination states of the S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions and their reactants with the help of Fourier-transform infrared spectroscopy (FTIR). FTIR spectra of reactants (NH4Ac, MAAc, MAI, NH4I, PbAc2 and PbAc2), and S-NH4Ac, S-MAAc and S-PbAc2 thin films before and after annealing are shown in Fig. 2(A and B). For the unannealed S-NH4Ac thin film, the stretching vibration of N–H (ν(N–H)) in NH4+ or MA+ appears at 3189.9 cm−1, the ν(C[double bond, length as m-dash]O) in Ac− appears at 1641.4 cm−1 and the appearance at wavenumber 1466.8 cm−1 corresponds to the bending vibration of C–H in MA+ (Fig. 2(A)). The ν(C[double bond, length as m-dash]O) in Ac− of the NH4Ac reactant appears at 1549.21 cm−1 compared to the wavenumber of 1641.4 cm−1 in the unannealed S-NH4Ac thin film, which may be on account of the coordination reaction between PbI2 and MAI. Similarly, a coordination reaction happens in unannealed S-MAAc and S-PbAc2 thin films, leading to the wavenumbers of ν(C[double bond, length as m-dash]O) (1581.2 cm−1) and ν(C[double bond, length as m-dash]O) (1514.5 cm−1) in Ac− of MAAc and PbAc2 reactants shifting to 1641.8 and 1641.2 cm−1, respectively, after coordination (Fig. 2(B)). The same wavenumber for ν(C[double bond, length as m-dash]O) indicates that all three systems form a similar intermediate phase in the fresh thin films. With subsequent annealing treatment, the vibration signal of C[double bond, length as m-dash]O in Ac− disappears with the volatilization of the NH4Ac additive from the films.


image file: c6ra17197d-f2.tif
Fig. 2 (A) FTIR spectra of perovskite films made from S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions before and after annealing treatment. (B) Valleys of the stretching vibration of C[double bond, length as m-dash]O (ν(C[double bond, length as m-dash]O)) in the corresponding spectra and FTIR spectra of reactants.

Furthermore, we traced back the formation process to the original solution state. As reported before,15 the perovskite precursor solutions are proved not to be real solutions, but colloidal dispersions in a mother solution and the colloidal precursors play a crucial role in film quality and solar cell performance. To confirm the colloidal state in these three NH4Ac additive precursor solutions, we here employed dynamic light scattering to verify the size distributions of the colloidal particle. As shown in Fig. 3(A), we calculated the sizes of the compounds in S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions to be 785.9 nm, 786 nm and 791.9 nm, respectively, with negligible difference. To gain an understanding of the structural information, we compared the UV spectra for all the colloidal precursor solutions. As shown in Fig. 3(B), the absorption plateau edge of MAPbI3 colloidal precursor locates at 477 nm, and there is a distinct red shift to 482 nm for S-NH4Ac, S-MAAc and S-PbAc2 colloidal precursors. The shift may come from the improvement in coordination degree of compounds in the MAPbI3 colloidal precursor. A more convincing interpretation here is that the solutions with an additive may form a mixed full-coordination compound of [PbI6−xAcx]4−; this structure will lead to a tuning of the band gap of the colloidal compound, and the absorption band edge is therefore shifted. Based on this mechanism, there exists a conspicuous ion exchange process in the solutions. Reactant ions dispersed in the solvent will exchange with others and finally reach a relatively stable state. So despite the different ion introduction routes, the same solution states and similar initial film morphology will result.


image file: c6ra17197d-f3.tif
Fig. 3 (A) Colloidal size distribution of MAPbI3 and S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions. (B) UV spectra of MAPbI3 and S-NH4Ac, S-MAAc and S-PbAc2 perovskite precursor solutions.

To investigate the optoelectronic properties of these films, we fabricated solar cells based on the S-NH4Ac, S-MAAc and S-PbAc2 perovskite films with porous TiO2 and Spiro-OMeTAD as the electron and hole transport layers, respectively. We measured these devices under AM 1.5G simulated light with an intensity of 100 mW cm−2, and the corresponding current density–voltage (J–V) curves are presented in Fig. 4. S-NH4Ac cells demonstrate a short-circuit photocurrent density (Jsc) of 19.08 mA cm−2, open-circuit voltage (Voc) of 0.942 V, fill factor (FF) of 57.22% and PCE of 10.28% when scanned from forward bias (FB) to short circuit (SC), and a Jsc of 18.94 mA cm−2, Voc of 0.918 V, FF of 53.49% and PCE of 9.30% when scanned from SC to FB. Their corresponding EQE and calculated Jsc are shown in Fig. S1 and Table S2.† PCE statistics of PSCs fabricated from S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions are listed in Table S3.† The cells shows a relatively low hysteresis with almost the same efficiency value, which further implies the same quality of the perovskite films prepared with these different precursor routes.


image file: c6ra17197d-f4.tif
Fig. 4 J–V curves of devices made from S-NH4Ac, S-MAAc and S-PbAc2 precursor solutions with forward and reverse scan.

In order to prove the wide adaptability of the ion change in the solution, another three samples were fabricated from MAPbI3 solutions with excess NH4+ and Cl− ions (Table S2†). All these films fabricated from different precursor routes also show similar properties, as presented in Fig. S2–S5.† Notably, these three samples show relatively poor coverage and lower photoelectric parameters than those of the Ac− systems, which indicates that the Ac− ions do better work than Cl− ions in forming more uniform perovskite films.

To visualize the chemical reaction process, in Fig. 5 we show an illustration from the beginning of the reactants' addition to the end of annealed thin films being prepared from S-NH4Ac, S-MAAc and S-PbAc2 solutions. At the initial stage, reactants are mixed in the established molar ratio. At this stage, reactants experience a disparate chemical environment, and the chemical bonds between different ions are not exactly the same. Following stirring of the precursor solutions, reactants start to dissolve in DMF whose polarity is strong enough to break the chemical bonds, and then the ions will experience a rearrangement process. Namely, PbI2 and MAI mixture in solution is assumed to form a kind of low coordinated framework which shows a relatively large colloidal particle size, and after the mixture of organic components there will exist a further coordination of [PbIx]2−x and Ac− to form a full coordination compound Pb[I6−xAcx]4− in all the solutions. Once the precursor solution has been spin coated on the substrate, redundant DMF is removed by the spin coater and afterwards with subsequently annealing treatment, the NH4Ac additive starts to evaporate and the identical perovskite structure will be formed.


image file: c6ra17197d-f5.tif
Fig. 5 Schematic illustration of the fabrication stages of S-NH4Ac, S-MAAc and S-PbAc2 thin films.

Conclusions

In conclusion, we systematically investigated the influence of different ion introduction routes of NH4+ and Ac− on the properties of the MAPbI3 precursor solutions and thin films, finding that although the routes changed, the as-fabricated perovskite films deliver similar morphology, photophysical property and crystallinity. A deeper revelation of the mechanism of the different ions introduction routes was studied by tracing back to the precursor solution state to analyse the ions' behaviour; this shows that the colloidal sizes of the particles in the S-NH4Ac, S-MAAc and S-PbAc2 solutions remain the same, which may result from their ion exchange process and their similar stable coordination state in the precursor solutions. Our work illuminates the wide selectivity of reactants for the preparation of high-quality perovskite films and devices both for laboratory fabrication and large-scale production.

Acknowledgements

Financial supports for this work from the Youth Innovation Promotion Association of CAS (2015167) and National Natural Science Foundation of China (51303164, 51302279) are gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17197d
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2016
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