Since the discovery of application of CH₃NH₃PbI₃ compounds to solar cells [1] , various types of solar cells have been fabricated and characterized [2] [3] [4] [5] . To improve the photovoltaic properties, halogen doping, such as chlorine (Cl) or bromine (Br) at the iodine (I) sites of the CH₃NH₃PbI₃ has been studied [6] [7] [8] . The doped Cl would lengthen the diffusion length of excitons, which resulted in the improvement of the efficiency [9] [10] .

Additionally, studies on elemental doping such as tin (Sn) [11] , antimony (Sb) [12] [13] , germanium (Ge) [14] [15] , thallium (Tl) [15] , or indium (In) [15] at the lead (Pb) sites have been carried out. Especially, the conversion efficiencies were improved by Sb-doping to the perovskite phase [12] [13] . To improve the photovoltaic properties, detailed searches on the metal and halogen doping at the Pb and I sites are needed.

The purpose of the present work is to investigate a co-doping effect of arsenic (As) and Cl to CH₃NH₃PbI₃ perovskite solar cells. As a group 15 element, it would be expected to improve the photovoltaic properties as previously reported Sb doping to the CH₃NH₃PbI₃ [12] [14] . Cl is expected to increase the carrier diffusion length in the perovskite phase [9] [10] , and an improvement of the morphology of the perovskite films is also expected by adding NH₄Cl [16] [17] . Devices were fabricated by a spin-coating, and the photovoltaic properties and microstructures were investigated by light-induced current density-voltage (J-V) characteristics, incident photon-to-current conversion efficiency (IPCE), scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD).

A schematic illustration of the fabrication process of the present TiO₂/ CH₃NH₃Pb(As)I₃(Cl) photovoltaic cells is shown in Figure 1. The details of the fabrication process are described in the reported papers [2] [12] [13] , except for AsI₃ addition. F-doped tin oxide (FTO) substrates were cleaned using an ultrasonic bath with acetone and methanol, and dried under nitrogen gas. 0.15 M and 0.30 M TiO₂ precursor solution was prepared from titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich, Tokyo, Japan, 0.055 mL and 0.11 mL) with 1-butanol (1 mL), and the 0.15 M TiO₂ precursor solution was casted on the FTO substrate at 3000 rpm for 30 s, and heated at 125˚C for 5 min. Then, the 0.30 M TiO₂ precursor solution was casted on the TiO_x layer at 3000 rpm for 30 s, and heated at 125˚C for 5 min. This casting process of 0.30 M solution was performed two times, and the TiO_x was sintered at 500˚C for 30 min to form the compact TiO₂ layer. After that, a mesoporous TiO₂ layer was formed on the compact TiO₂ layer by spin-coating at 5000 rpm for 30 s. For the mesoporous TiO₂ layer, the TiO₂ paste was prepared with TiO₂ powder (Nippon Aerosil, Tokyo, Japan, P-25) with poly (ethylene glycol) (Nacalai Tesque, Kyoto, Japan,

PEG #20000) in ultrapure water. The solution was mixed with acetylacetone (Wako Pure Chemical Industries, Osaka, Japan, 10 µL) and triton X-100 (Sigma-Aldrich, Tokyo, Japan, 5 µL) for 30 min, and was left for 12 h to suppress the bubbles in the solution. The cells were annealed at 120˚C for 5 min and at 500˚C for 30 min to form the mesoporous TiO₂ layer. A solution of CH₃NH₃I (Showa Chemical Co., Ltd., Tokyo, Japan, 98.8 mg), PbI₂ (Sigma-Aldrich, Tokyo, Japan), NH₄Cl (Wako Pure Chemicals Industries, Ltd., Osaka, Japan), and AsI₃ (Sigma-Aldrich) with a desired mole ratio in γ-butyrolactone (Nacalai Tesque, 350 µL) and N,N-dimethylformamide (DMF, Sigma-Aldrich, 150 µL), was mixed at 60˚C. DMF and NH₄Cl were added to γ-butyrolactone to improve photovoltaic properties [13] [16] [17] [18] . The detailed preparation compositions of TiO₂/CH₃NH₃Pb(As)I₃(Cl) cells with different additives are listed in Table 1. The solution of CH₃NH₃Pb(As)I₃(Cl) was then introduced into the TiO₂ mesopores by a spin-coating and an air flow method at 50˚C, and annealed at 100˚C for 15 min. Then, a hole transport layer (HTL) was prepared by spin-coating. As the HTL, a solution of 2,2’,7,7’-tetrakis [N, Ndi (p-methoxyphenyl) amino]-9,9’-spirobifluorene (spiro-OMeTAD, Wako Pure Chemical Industries, 36.1 mg) in chlorobenzene (Wako Pure Chemical Industries, 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, 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 were carried out in air. Finally, gold (Au) metal contacts were evaporated as top electrodes. Layered structures of the present photovoltaic cells are denoted as FTO/TiO₂/CH₃NH₃Pb(As)I₃(Cl)/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 by using an AM 1.5 solar simulator (San-ei Electric, Osaka, Japan, XES-301S). The solar cells were illuminated through the side of the FTO substrates, and the illuminated area was 0.090 cm². The IPCE of the cells were also investigated (Enli Technology, Kaohsiung, QE-R). The microstructures of the thin films were investigated by using an X-ray diffractometer (Bruker, Kanagawa, Japan, D2 PHASER) and a scanning electron microscope (Jeol, Tokyo, Japan, JSM-6010PLUS/LA) equipped with EDS.

The J-V characteristics of the TiO₂/CH₃NH₃Pb(As)I₃(Cl)/spiro-OMeTAD

photovoltaic cells under illumination are shown in Figure 2. The measured photovoltaic parameters of TiO₂/CH₃NH₃Pb(As)I₃(Cl) cells are also summarized in Table 2. The CH₃NH₃PbI₃ cell provided a power conversion efficiency (η) of 0.249%, and the averaged efficiency (η_ave) of four electrodes on the cells is 0.171%. A short-circuit current density (J_SC) increased up to 11.1 mA cm^?2 by addition of AsI₃, which would imply an increase of carrier concentration. The highest efficiency was obtained for a cell added with [AsI₃ + NH₄Cl], which provided an η of 6.31%, a fill factor (FF) of 0.563, a J_SC of 13.9 mA cm^?2, and an open-circuit voltage (V_OC) of 0.807 V. A J_SC value for the AsI₃-doped sample increased further by adding NH₄Cl, as observed in Figure 2.

IPCE spectra of the CH₃NH₃Pb(As)I₃(Cl) cells are shown in Figure 3. The perovskite CH₃NH₃Pb(As)I₃(Cl) shows photoconversion efficiencies between 300 nm and 790 nm, which corresponds to an energy gap of 1.57 eV. The IPCE was improved in the range of 350 - 750 nm by adding As and Cl.

Figure 4(a) shows XRD patterns of CH₃NH₃Pb(As)I₃(Cl) cells on the FTO/ TiO₂. The observed diffraction peaks and reported values are summarized as Table 3. The diffraction peaks can be indexed by a tetragonal crystal system (I4/mcm), as shown in Figure 4(b). A tetragonal structure is sometimes included in the cubic perovskite phase [8] [19] , and they indicates that the fabricated films are a single perovskite structure. In addition to the perovskite phase, diffraction peaks of attributed to PbI₂ appeared in the CH₃NH₃PbI₃(Cl) films, as

shown in Figure 4(a), which indicates that the As addition suppressed the formation of PbI₂.

Figure 5(a) is a SEM image of TiO₂/CH₃NH₃Pb(As)I₃ cell with an additive of AsI₃, and the particle sizes are approximately 2 - 3 µm. Elemental mapping images of Pb, As, I, C, and N by SEM-EDX are shown in Figure 5(b)-(f), respectively. The elemental mapping images indicate the particles observed in Figure 5(a) would correspond to the CH₃NH₃PbI₃ phase.

Figure 6(a) is a SEM image of CH₃NH₃Pb(As)I₃(Cl) cell. By adding NH₄Cl to the CH₃NH₃Pb(As)I₃, the surface morphology was drastically changed, and dense packing of particles is observed. Elemental mapping images of Pb, As, I, C, N, and Cl are shown in Figure 6(b)-(g), respectively. Figure 6 indicates the

perovskite CH₃NH₃PbI₃ phase is dispersed homogeneously on the device surface. Table 4 shows a composition ratio of Pb, As, I, Cl and C:N calculated from the EDX spectrum. Basic compositions were calculated only on the Pb, As, I and Cl elements, and iodine is the highest percentage in these elements. To investigate the C:N ratio in the perovskite crystal, the C:N composition ratio was also calculated separately. This result indicates that I would be deficient from the starting composition of CH₃NH₃Pb(As)I₃(Cl), and the deficient I might increase the hole concentration.

An energy level diagram of TiO₂/CH₃NH₃Pb(As)I₃(Cl) photovoltaic cells is proposed as shown in Figure 7. The electronic charge generation is caused by light irradiation from the bottom of FTO substrate side. The TiO₂ layer receives the electrons from the CH₃NH₃Pb(As)I₃(Cl) layer, and the electrons are transported to the FTO. The holes are transported to an Au electrode through spiro-OMeTAD.

Three mechanisms can be considered for the improvement of the photoconversion efficiencies [12] [13] . The first is as follows: I^- ions would be attracted at the I sites by As³⁺ with more ionic valence compared with that of Pb²⁺, which resulted in the suppression of PbI₂ elimination from CH₃NH₃PbI₃. This would improve the TiO₂/CH₃NH₃PbI₃ interfacial structure, which improve the V_OC values. As the amount of arsenic increases, the lattice constants would decrease by a smaller ionic size of As³⁺ (0.76 Å) compared with that of Pb²⁺ (1.49 Å).

The second mechanism is as follows: when a small amount of Cl was doped in the CH₃NH₃PbI₃, diffusion length of excitons would be lengthened [9] [10] , which results in the increase of the J_SC values.

The third is as follows: by adding AsI₃ with NH₄Cl to the CH₃NH₃PbI₃, the homogeneous surface and interface were formed, which improved the photovoltaic properties, especially the FF values. In addition, the doping effects of As and Cl would also contribute the improvement of the J_SC values. However, further studies are needed for the photovoltaic mechanism.

CH₃NH₃PbI₃ perovskite solar cells co-doped with As and Cl were fabricated and characterized. Results of J-V characteristics and IPCE showed that the photovoltaic properties of the perovskite solar cells were improved by adding a small amount of AsI₃ and NH₄Cl to perovskite precursor solutions. XRD indicated suppression of PbI₂ formation by AsI₃ addition, and I⁻ ions would be attracted at the I sites by As³⁺ with more ionic valence compared with that of Pb²⁺, which led to the suppression of PbI₂ elimination from CH₃NH₃PbI₃, and this would improve the TiO₂/CH₃NH₃PbI₃ interfacial structure. The microstructure structure analysis also indicated formation of a homogeneous microstructure by adding AsI₃ with NH₄Cl, which would improve the FF values. SEM-EDS analysis showed the deficiency of iodine in the perovskite phase, which would lead to higher hole concentration and increase of J_SC which improved the V_OC values. A small amount of Cl was also doped in the CH₃NH₃PbI₃ from the EDS analysis, and the diffusion length of excitons would be lengthened, which resulted in the increase of the J_SC values. From these combining effects, the photovoltaic properties of the CH₃NH₃Pb(As)I₃(Cl) cell were improved.

This work was partly supported by Satellite Cluster Program of the Japan Science and Technology Agency, and a Grant-in-Aid for Scientific Research (C) No. 25420760.

Hamatani, T., Shirahata, Y., Ohishi, Y., Fukaya, M. and Oku, T. (2017) Arsenic and Chlorine Co-Doping to CH₃NH₃PbI₃ Perovskite Solar Cells. Advances in Materials Physics and Chemistry, 7, 1-10. http://dx.doi.org/10.4236/ampc.2017.71001