Applied Science and Convergence Technology 2018; 27(6): 169-172
Published online November 30, 2018
Copyright © The Korean Vacuum Society.
Yeonghun Yuna, Jae Yu Chob, Jaeyeong Heob,*, and Sangwook Leea,*
aSchool of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea, bDepartment of Materials Science and Engineering, and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 61186, Republic of Korea
Methylammonium lead halide (MAPbI3), an organic-inorganic hybrid perovskite material, is promising as light absorbing materials of solar cells. Nevertheless, necessity of reducing Pb contents in the perovskite materials is increasing due to the toxicity of Pb. Recently, tin-lead mixed perovskite materials have been studied intensively, by dissolving Sn and Pb sources simultaneously in one precursor solution. Here, for the first time, we report a two-step method to synthesize MASnI3 and MASnxPb(1-x)I3 films. The films were prepared via spin-coating MAI or MAPbI3 precursors on as-prepared SnS thin films. Crystal structures, phases, surface chemical states, and morphologies of the MASnI3 and MASnxPb(1-x)I3 films were investigated. Our two-step method is expected to be used not only for Sn based perovskites but also for diverse compositions of perovskite materials.
Keywords: Organic-inorganic hybrid perovskite, MASnI3, MASnxPb1-xI3, SnS, Thin film
Onrganic-inorganic hybrid perovskite materials have been attracting great attention due to superior photovoltaic properties, such as long carrier diffusion length, high absorption coefficient and tunable bandgap, and to low cost and low process temperature [1–6]. Especially, lead (Pb) based perovskite materials (APbX3, A = methylammonium (MA), formamidinium (FA) or Cs, and X = Cl, Br or I) have been researched as photodetectors [7,8], light-emitting diodes [9,10], lasers  and perovskite solar cells (PSCs) [1,12]. PSCs based on Pb containing perovskite materials have been rapidly developed from 3.8 % of power conversion efficiency (PCE) at 2009  to over than 23 % recently . However, there was always a big issue regarding harmfulness of Pb, so the lead-free perovskite materials have been drawn interests. Tin (Sn) based perovskite light absorbing materials have been one of the promising alternatives because of the lower bandgap up to the NIR range and less toxicity [15–17]. Besides, they have been studied for the tandem solar cells to achieve high efficiency because of their wide range light absorption . Nevertheless, perovskite composition incorporating only Sn had some limitations that high carrier concentration and mobility, due to behave like conductors, so it is known that not suitable for PSCs . Furthermore, under the ambient condition, the Sn2+ oxidizes to more stable Sn4+, leading to more metallic [15,16] and decomposition of ABX3 perovskite structures. To overcome these problems, Kanatzidis et al. introduced Sn-Pb mixed perovskite compositions . When Pb was added to MASnI3, electrical properties and the amount of Sn4+ were controlled and made it possible to operate the PSCs better. Also, it enabled bandgap tuning and band alignment modification in accordance with the ratio of Pb to Sn. Moreover, Hayase et al. reported that incorporation of Ge into Sn-perovskite increases PCE and the stability in the ambient condition of PSCs . However, in most cases, Sn-Pb mixed perovskite thin films were synthesized by one-step spin-coating of a precursor dissolving the Sn halide and Pb halide sources.
In this paper, MASnI3 and MASnxPb(1-x)I3 thin films were synthesized, by a two-step method; spin-coating MAI or MAPbI3 precursor on an as-prepared vapor-transport-deposited (VTD) SnS thin films. Successful synthesis of MASnI3 and MASnxPb(1-x)I3 thin films were confirmed by structural, surface chemical and morphological analyses. We found from literatures that MASnI3 and MASnxPb(1-x)I3 have slightly higher conduction band edges than electron transport materials (TiO2, [6,6] phenyl C61 butyric acid methyl ester (PCBM)) of PSCs (Fig. 1) [17,20], which would make the Sn-perovskite layers suitable for light absorption layers of PSCs. Moreover, our experimental results show SnS residue layer after the synthesis of Sn-perovskite films, implying formation of junction of MASnI3/SnS or MASnxPb(1-x)I3/SnS which would be helpful for extracting holes to a metal electrode due to slightly higher valence band edges of SnS than that of Sn-perovskites (Fig. 1).
SnS thin films were deposited by VTD using a one-zone tube furnace (S&R Korea, SRVF-LV-3B-1508). 4N SnS source powder (iTasco, LT40SNS312) was loaded in the middle of the heating zone [Fig. 2(a)]. The process pressure was set at 1.4 Torr by flowing Ar 100 at sccm, and the process temperature was set at 530 °C. The growth duration was 15 min to obtain ~500-nm-thick SnS. Further details on the growth of SnS and its properties can be found elsewhere .
MASnI3 thin film was synthesized by spin coating MAI precursor solution on the SnS thin film at 3000 rpm for 7 s. MAI precursor was prepared by dissolving the MAI (Xi’an Polymer Light Technology Corp., ≥ 99.5 %) powder in N,N-dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8 %) for 29.7 wt%. MAI coated substrates were annealed at 130 °C for 4 hr.
To make the MASnxPb(1-x)I3 thin film, MAPbI3 precursor was spin coated on the SnS thin film at 4000 rpm for 20 s. MAI, PbI2 (Tokyo Chemical Industry, 99.99 %), dimethyl sulfoxide (DMSO, Sigma-Aldrich, anhydrous, ≥ 99.9 %) was mixed in DMF as molar ratio 1:1:1 for 26 wt% to prepare the MAPbI3 precursor solution. During the spin coating, 0.5 ml of diethyl ether (Sigma-Aldrich, anhydrous, ≥ 99.7%) was dripped to the substrate. Then, the substrate was sequentially annealed at 65 °C for 1 min and at 130 °C for 10 min.
Crystal structure of thin films were investigated by X-ray diffraction (XRD, X’Pert PRO, PANalytical). The top-view morphology and cross-sectional structure with thickness were characterized using field-emission scanning electron microscopy (FE-SEM, JSM-6710F, JEOL). The chemical state and elemental analysis were performed by X-ray photoelectron spectroscopy (XPS, Theta Probe AR-XPS System, Thermo Fischer Scientific).
First, SnS thin film was synthesized on a FTO/glass substrate for structural characterization. Space group Pbnm orthorhombic SnS was identified by XRD pattern (Fig. 2(b), ICDD 039-0354) and there were no any secondary phases. Dense and pinhole-less film was formed as shown in cross-sectional SEM image [Fig. 2(c)]. Using this SnS thin films, the MASnI3 and MASnxPb(1-x)I3 thin films were fabricated.
MASnI3 thin film was synthesized via the two-step method, by spin-coating the MAI solution on the SnS thin film, followed by annealing at 130 °C for 4 hr. Figure 3(a) shows XRD patterns of MAI coated SnS thin films. New peaks such as at 2θ = 14.1 ° and 24.8 ° are assigned to (001) and (002) crystal planes of cubic structured MASnI3. In addition, unreacted residual SnS is found in XRD patterns. MAI is considered to be fully used to make MASnI3 because MAI peak is not observed at all. Cross-sectional and top-view SEM images are presented in Figs. 3(b) and 3(c). The cross-sectional image shows a dense and pinhole-free film, while top-view image shows a lot of pores with a rough surface. This implies that the reaction of MAI and SnS was started from the top of SnS and leading non-uniform film morphology.
The Sn-Pb mixed perovskites were synthesized via similar method using an SnS thin film; MAPbI3 precursor solution was spin-coated on the SnS thin film with anti-solvent dripping. Figure 4(a) shows the XRD pattern of MAPbI3 coated SnS thin film. There were several peaks corresponding to Sn-Pb mixed perovskite and SnS residue. To further investigate the phase of the Sn-Pb mixed perovskites film, the main peak around 2
XPS measurement was carried out to examine presence of Sn and S (Fig. 5) at the surface of the Sn-Pb mixed perovskite. As shown in Figs. 5(a) and 5(b), C 1s and Pb 4f binding energies are similar with previous report . Also, Sn is clearly observed from the surface, as shown in Fig. 5(c). Based on the XRD and XPS results, formation of MASnxPb(1-x)I3 even in the top of the perovskite film is assured, implying diffusion of Sn ion from the bottom SnS layer to the top of the post-coated (MAI-PbI2 mixed) layer. Sulfur is not detected as shown in Fig. 5(d). Therefore, it can be deduced that sulfur, a by-product, is evaporated out from the film or remaining at the bottom of the film, but not doped into the perovskite lattice, because of absence of S at the surface. It is more likely that S is evaporated, because we couldn’t find any thin layer, which can be assigned to sulfur layer, from the cross-sectional SEM images [Figs. 3(c) and 4(d)]. The morphology of mixed perovskite layer is shown in Figs. 4(c) and 4(d). Pinhole or pore is not observed. The film has a dense and smooth surface with ~200 nm sized grains, and uniform thickness, which is comparable to film quality of MAPbI3 thin film.
MASnI3 and MASnxPb(1-x)I3 thin films were synthesized via the two-step method by spin-coating MAI or MAPbI3 precursor solutions on SnS thin films. Based on the peak shift of XRD patterns and the presence of tin on XPS analysis, the synthesis of MASnxPb(1-x)I3 is demonstrated. The final morphology of MASnI3 and MASnxPb(1-x)I3 was identified using SEM measurement. MASnI3 thin film shows a rough surface with pores, while MASnxPb(1-x)I3 thin film shows a uniform, smooth and dense morphology, indicating successful synthesis of good quality MASnxPb(1-x)I3 thin film. Our two-step method is expected to be used not only for Sn-based perovskites but also for diverse compositions of perovskite materials.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2016R1C1B2013087, NRF-2018R1A2B6002268).