Effects of Guanidinium and Formamidinium Addition to CH 3 NH 3 PbI 3 -Based Perovskite Solar Cells †

: Additive effects of guanidinium [C(NH 2 ) 3 , GA] iodide, formamidinium [CH(NH 2 ) 2 , FA] iodide, and guanidinium chloride to CH 3 NH 3 PbI 3 -based photovoltaic devices were investigated. Short-circuit current densities, open-circuit voltages, series resistances and shunt resistances were improved by the GA addition. The short-circuit current densities were increased by FA addition with GA, and the external quantum efficiencies increased, which resulted in suppression of pinholes in perovskite layers by the GA addition. X-ray diffraction showed that the lattice constants of the perovskite crystals increased by the GA and FA addition, and that the GA substituted partially at the CH 3 NH 3 -site.

The purpose of the present work was to investigate the effects of guanidinium iodide (GAI), guanidinium chloride (GACl) and formamidinium iodide (FAI) addition to CH3NH3PbI3-xClx-based perovskite [16].GA addition was expected to extend the carrier lifetime and to reduce the carrier recombination in the perovskite layers [15].GACl addition was expected to provide effects of both GA and Cl additions.FA addition was also expected to expand the wavelength range of light adsorption [4].The effects of these additives on the formation of perovskite compounds for the photovoltaic cells were investigated by light-induced current density-voltage (J-V) characteristics, incident photon-tocurrent conversion efficiency (IPCE), X-ray diffraction (XRD), optical microscopy, and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS).

Experimental
Details of the basic fabrication process are described in published reports [24][25][26][27][28]. Fdoped tin oxide (FTO) substrates were cleaned using an ultrasonic bath with acetone and methanol, and dried under nitrogen gas.The 0.15 and 0.30 M TiO2 precursor solutions were prepared from titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich, 0.055 and 0.11 mL) with 1-butanol (1 mL), and the 0.15 M TiO2 precursor solution was spincoated on the FTO substrate at 3000 rpm for 30 s and heated at 125 °C for 5 min in air to form a TiOx layer.The 0.30 M TiO2 precursor solution was spin-coated onto the TiOx layer at 3000 rpm for 30 s, and heated at 125 °C for 5 min.This process of coating with a 0.30 M solution was then performed two times, and the FTO substrate was annealed at 550 °C for 30 min to form a compact TiO2 layer.For the mesoporous TiO2 layer, TiO2 paste was prepared with TiO2 powder (Nippon Aerosil, P-25) with poly(ethylene glycol) (Nacalai Tesque, PEG #20000) in ultrapure water.The solution was mixed with acetylacetone (Wako Pure Chemical Industries, 10 μL) and triton X-100 (Sigma-Aldrich, 5 μL) for 30 min, and then left for 12 h to suppress the bubbles in the solution.After that, the TiO2 paste was coated onto the substrate by spin-coating at 5000 rpm for 30 s.The cells were then annealed at 120 °C for 5 min and at 550 °C for 30 min to form a mesoporous TiO2 layer.
For the preparation of perovskite compounds, a solution of CH3NH3I (Showa Chemical Co. Ltd. (Tokyo, Japan), 190.7 mg) and PbCl2 (Sigma-Aldrich (St.Louis, MO, USA), 111.2 mg) was prepared with a mole ratio of 3:1 and in N,N-dimethylformamide (Nacalai Tesque, 500 μL) with additives of GAI (10 mol%, 7.5 mg), GACl (10 mol%, 3.8 mg), and GAI + FAI (10 mol%, 7.5 and 6.9 mg).These perovskite cells are denoted +GAI, +GACl, and +GAI+FACl, respectively.The reaction mechanism proposed for the CH3NH3PbI3 perovskite is as follows: 3CH3NH3I + PbCl2 → CH3NH3PbI3 + 2CH3NH3Cl (↑).Since 2CH3NH3Cl is generated as a byproduct during the reaction, the quantity 10 mol% means 10 mol% of the produced CH3NH3PbI3.The solutions were stirred at 70 °C for 24 h.Then, the solutions of CH3NH3PbI3(Cl) were introduced into the TiO2 mesopores by the spincoating method and annealed at 150 °C for 20 min (perovskite with additive) or 140 °C for 15 min (standard) to form the perovskite layer.A hole transport layer was then prepared by spin-coating onto the perovskite layer.For the hole transport layer, a chlorobenzene solution of 0.5 mL was mixed with a solution of lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI, Tokyo Chemical Industry (Tokyo, Japan), 260 mg) in acetonitrile (Nacalai Tesque (Singapore City, Singapore), 0.5 mL) for 12 h.The former solution with 4-tert-butylpyridine (Aldrich, 14.4 μL) was mixed with the Li-TFSI solution (8.8 μL) for 30 min at 70 °C.All procedures for preparation of the thin contacts were evaporated as top metal electrodes.The layered structures of the present solar cells were denoted FTO/TiO2/perovskite/spiro-OMeTAD/Au, as shown in Figure 1.The J-V characteristics of the photovoltaic cells were measured under illumination at 100 mW cm −2 using an air mass (AM) 1.5 solar simulator (San-ei Electric, XES-301S, Osaka, Japan).
The J-V measurements were performed by source measure unit (Keysight, B2901A Precision SMU).The scan rate and sampling time were ≈0.08 V s −1 and 1 ms, respectively.Four cells were tested for each condition.The solar cells were illuminated through the sides of the FTO substrates, and the illuminated area was 0.090 cm 2 .The IPCE of the cells were also measured (Enli Technology, QE-R).The microstructures of the present cells were investigated using an X-ray diffractometer (Bruker, D2 PHASER), a transmission optical microscope (Nikon, Eclipse E600), and a scanning electron microscope (Jeol, JSM-6010PLUS/LA) equipped with EDS.

Results and Discussion
Figure 2 shows the J-V characteristics of the TiO2/perovskite/spiro-OMeTAD photovoltaic cells under illumination, which indicates the effects of the GA and FA addition [16].The measured photovoltaic parameters of the cells are summarized in Table 1.Standard CH3NH3PbI3(Cl) cells provided a power conversion efficiency (η) of 5.31%, and the averaged efficiency (ηave) of four electrodes on the cells was 4.42%, as listed in Table 1.The short-circuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF) were increased by GAI addition and the highest efficiency of 12.51% was obtained for the +GAI cell.The highest JSC and VOC were obtained for +GAI+FAI cells, which provided an η of 11.66% and ηave of 11.01%.FF values of the +GAI+FAI cells were lower than that of +GAI cells.Addition of GACl to CH3NH3PbI3(Cl) provided a higher η of 9.14% than that of the standard cells, but lower than that of +GAI cells.Table 1 shows that the series resistances (RS) and the shunt resistances (RSh) were decreased and increased by GA addition, respectively.These results lead to improvements of JSC and VOC.Further improvements could be expected by optimizing the compositions and annealing conditions.

Conclusions
Photovoltaic properties, such as JSC, VOC, RS and RSh of MAPbI3-xClx-based perovskite photovoltaic cells, were improved by GA addition.Increase of JSC and VOC by GA and FA addition implies that MA-GA-FA mixed cation-based perovskite solar cells have a potential for further improvement of the photovoltaic performances.

Figure 2 .
Figure 2. J-V characteristics of the present perovskite photovoltaic cells.

Table 1 .
Measured photovoltaic parameters of solar cells.