Effects of Guanidinium and Formamidinium Addition to CH₃NH₃PbI₃-Based Perovskite Solar Cells ^†

Abstract

Additive effects of guanidinium [C(NH₂)₃, GA] iodide, formamidinium [CH(NH₂)₂, FA] iodide, and guanidinium chloride to CH₃NH₃PbI₃-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 CH₃NH₃-site.

1. Introduction

Recently, thin film solar cells with perovskite-type methylammonium trihalogenoplumbates (II) (CH₃NH₃PbI₃) compounds have been widely studied due to their easy fabrication processes and high conversion efficiencies compared to conventional organic solar cells [1,2,3,4]. It was reported that the photovoltaic properties of perovskite solar cells depended severely on the compositions and crystal structures of the perovskite compounds. Doping with elements such as cesium [5], rubidium [6,7], potassium [8,9,10], sodium [11], formamidinium (CH(NH₂)₂, FA) [10,12], ethylammonium (CH₃CH₂NH₃, EA) [13], or guanidinium (C(NH₂)₃, GA) [14,15,16] at the methylammonium (CH₃NH₃, MA) sites improved the conversion efficiencies. Studies on doping with halogen atoms, such as chlorine (Cl) [17,18,19,20,21,22,23] or bromine (Br) [6], at the iodine (I) sites of the perovskite crystals have also been reported. The doped Cl ions were found to lengthen the diffusion length of excitons, which resulted in improvement of the conversion efficiency [17,19]. Various elemental substituted perovskite compounds have been reported [24].

The purpose of the present work was to investigate the effects of guanidinium iodide (GAI), guanidinium chloride (GACl) and formamidinium iodide (FAI) addition to CH₃NH₃PbI_3-xCl_x-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-to-current conversion efficiency (IPCE), X-ray diffraction (XRD), optical microscopy, and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS).

2. Experimental

Details of the basic fabrication process are described in published reports [24,25,26,27,28]. F-doped 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 spin-coated on the FTO substrate at 3000 rpm for 30 s and heated at 125 °C for 5 min in air to form a TiO_x layer. The 0.30 M TiO₂ precursor solution was spin-coated onto the TiO_x 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 spin-coating 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 cm². 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.

3. 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. Measured photovoltaic parameters of solar cells.

Device	JSC (mA cm−2)	VOC (V)	FF	η (%)	ηave (%)	RS (Ω cm2)	RSh (Ω cm2)	Eg (eV)
Standard	19.0	0.823	0.339	5.31	4.42	9.64	70	1.55
+GAI	20.9	0.972	0.616	12.51	11.20	7.05	1599	1.53
+GACl	19.1	0.875	0.548	9.14	7.30	8.20	290	1.53
+GAI+FAI	21.3	0.999	0.547	11.66	11.01	8.92	667	1.54

. Standard CH₃NH₃PbI₃(Cl) cells provided a power conversion efficiency (η) of 5.31%, and the averaged efficiency (η_ave) of four electrodes on the cells was 4.42%, as listed in Table 1. The short-circuit current density (J_SC), open-circuit voltage (V_OC) and fill factor (FF) were increased by GAI addition and the highest efficiency of 12.51% was obtained for the +GAI cell. The highest J_SC and V_OC were obtained for +GAI+FAI cells, which provided an η of 11.66% and η_ave 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 (R_S) and the shunt resistances (R_Sh) were decreased and increased by GA addition, respectively. These results lead to improvements of J_SC and V_OC. Further improvements could be expected by optimizing the compositions and annealing conditions.

4. Conclusions

Photovoltaic properties, such as J_SC, V_OC, R_S and R_Sh of MAPbI_3-xCl_x-based perovskite photovoltaic cells, were improved by GA addition. Increase of J_SC and V_OC 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.