Since the discovery of application of CH3NH3PbI3 compounds to solar cells  , various types of solar cells have been fabricated and characterized     . 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    . The doped Cl would lengthen the diffusion length of excitons, which resulted in the improvement of the efficiency   .
Additionally, studies on elemental doping such as tin (Sn)  , antimony (Sb)   , germanium (Ge)   , thallium (Tl)  , or indium (In)  at the lead (Pb) sites have been carried out. Especially, the conversion efficiencies were improved by Sb-doping to the perovskite phase   . 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   . Cl is expected to increase the carrier diffusion length in the perovskite phase   , and an improvement of the morphology of the perovskite films is also expected by adding NH4Cl   . 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).
2. Experimental Procedure
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    , 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 TiOx 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 TiOx 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,
Figure 1. Schematic illustration for the fabrication of CH3NH3Pb(As)I3(Cl) photovoltaic cells.
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     . 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.
3. Results and Discussion
The J-V characteristics of the TiO2/CH3NH3Pb(As)I3(Cl)/spiro-OMeTAD
Table 1. Preparation composition of TiO2/CH3NH3Pb1-xAsxI3Cl3-y cells with different additives.
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 (ηave) of four electrodes on the cells is 0.171%. A short-circuit current density (JSC) 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 JSC of 13.9 mA cm?2, and an open-circuit voltage (VOC) of 0.807 V. A JSC 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   , 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
Figure 2. J-V characteristic of TiO2/CH3NH3Pb1-xAsxI3Cl3-y photovoltaic cells.
Table 2. Measured photovoltaic parameters of TiO2/CH3NH3Pb1-xAsxI3Cl3-y cells.
Figure 3. IPCE spectra of TiO2/CH3NH3 Pb1-xAsxI3Cl3-y cells.
Table 3. Measured photovoltaic parameters of TiO2/CH3NH3Pb1-xAsxI3Cl3-y cells.
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
Figure 4. (a) XRD patterns of TiO2/CH3NH3Pb1-xAsxI3Cl3-y cells. (b) Structure model of tetragonal CH3NH3Pb(As)I3(Cl). Indices are based on a tetragonal system.
Figure 5. (a) SEM image of CH3NH3Pb(As)I3 cell with an additive of AsI3. Elemental mapping images of (b) Pb M line, (c) As L line, (d) I L line, (e) C K line, and (f) N K line.
Figure 6. (a) SEM image of CH3NH3Pb(As)I3(Cl) cell with additives of AsI3 with NH4Cl. Elemental mapping images of (b) Pb M line, (c) As L line, (d) I L line, (e) C K line, (f) N K line, and (g) Cl K line.
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.
Table 4. Measured compositions of TiO2/CH3NH3Pb(As)I3(Cl) cell. Only C:N composition is calculated separately.
Figure 7. Energy level diagram of CH3NH3Pb(As)I3(Cl) cells.
Three mechanisms can be considered for the improvement of the photoconversion efficiencies   . 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 VOC 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   , which results in the increase of the JSC 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 JSC 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 JSC which improved the VOC 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 JSC 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.