Subject Number :S-16-2012
Support Type : Common use (including technical support necessary for the training),
Proposal Title (English) : Dopant Effect of Lead(II) Thiocyanate (Pb(SCN)2) for FA0.9Cs0.1PbI3 Perovskite Solar Cells
Username (English) : M. W. Lin1, M. H. Li2, H. W. Shiu1, Y. L. Lai1, T. Ohigashi3, N. Kosugi3, P. Chen2, and Y. J. Hsu1,2
Affiliation (English) : 1National Synchrotron Radiation Research Center, 2 Department of Photonics, National Cheng Kung University 3Institute for Molecular Science,

Thiocyante (Pb(SCN)2), as a dopant in perovskite FA0.9Cs0.1PbI3 (FA: HC(NH)2), and mesoscopic titania (mp-TiO2) as scaffold/electron-transport layer show high power conversion efficiencies (PCEs) up to 15.1%. Herein we studied the origin of such high PCEs in terms of morphology and chemical structure of mp-TiO2/Pb(SCN)2 doped- FA0.9Cs0.1PbI3 heterojunction by scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM).
In SEM studies (Figure 1), the Pb(SCN)2 doped- FA0.9Cs0.1PbI3 displays much larger perovskite crystalline than FA0.9Cs0.1PbI3. More than 70% of the larger crystals in 5 % Pb(SCN)2 doped- FA0.9Cs0.1PbI3 crystals exhibit a size distribution in the range of 0.8-1 m size. Increasing Pb(SCN)2 dopant to 10% in FA0.9Cs0.1PbI3, the size of perovskite crystals decrease to the 0.5 m and display homogeneous distribution. The large crystals in the perovskite bulk film are beneficial for charge transport in the perovskite,1 that is possible to explain 5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 has best PECs than the others.
To further examine the effects of adding Pb(SCN)2 on the crystalline morphology, chemical maps of N K-edge and Ti L-edge were acquired to obtain the distributions of micro-aggregates of SiN/mp-TiO2/5% and 10% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 by STXM. Figure 2 displays the STXM images of SiN/mp-TiO2/5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 (Fig. 2(a)), and SiN/mp-TiO2/10% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 (Fig. 2(b)) which are optical density (OD) maps obtained at 403.5 eV for N K-edge and 457 eV for Ti L-edge, the characteristic absorption peaks of FA0.9Cs0.1PbI3 and mp-TiO2, respectively. The N mapping on above samples show similar distribution in morphology and size to SEM results. The 5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 shows grain size of ~1 m, and decrease to 0.5 m when 10% Pb(SCN)2 doped. However, we notice some additional line shape and large aggregation as shown in red color in the composite images, that is determined as TiO2 nanoparticles which are in a uniform diameter of 2–5 μm and appear to be more uniformly dispersed in the sample of 5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3. It suggests 5% Pb(SCN)2 dopant has effective charge transfer between perovskite and TiO2. Fig. 2(c) and (d) show the micro-scpectra of N K-edge and Ti L-edge extracted from 5% and 10% Pb(SCN)2 doped- FA0.9Cs0.1PbI3 film. We notice that both N and Ti spectra exhibit dissimilar in peak ratio and shape between 5% and 10% Pb(SCN)2 doped sample. It implies that the interaction between mp-TiO2 and perovskite may be play a key role in determining the PCEs performance.
In conclusion, STXM results clear show that morphology variation attributed by TiO2 and Pb(SCN)2, and also illustrate that an important chemical interaction between mp-TiO2 and perovskite.

(a) (b) (c)

Fig. 1. The SEM image of (a) FA0.9Cs0.1PbI3, (b) 5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3, and (c) 10% Pb(SCN)2 doped- FA0.9Cs0.1PbI3.
(a) (b)

（d） (e)

Fig. 2. OD mapping of Ti L-edge (red) and N K-edge (green) of (a) SiN/mp-TiO2/5% Pb(SCN)2 doped- FA0.9Cs0.1PbI3, and (b) SiN/mp-TiO2/10% Pb(SCN)2 doped- FA0.9Cs0.1PbI3. (c) and (d) micro-spectra of N K-edge and Ti L-edge extracted from (a) and (b), respectively.

[1] Z. Xiao, et al., Adv. Mater., 26, 6503 (2014).
[2] M.-W. Lin, et al., Adv. Mater. Interfaces, 3, 1600135 (2016).