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Perovskite solar cells were fabricated and characterized. X-ray diffraction analysis and transmission electron microscopy were used for investigation of the devices. The structure analysis by them showed structural transformation of the crystal structure of the perovskite, which indicated that a cubic-tetragonal crystal system depended on the annealing condition. The photovoltaic properties of the cells also depended on the structures. Metal doping and halogen doping to the perovskite and TiO2 were also investigated. The results showed an increase in the efficiencies of the devices, due to the structural change of the perovskite compound layers.
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
Masahito Zushi
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
Kohei Suzuki
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
Yuya Ohishi
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
Taisuke Matsumoto
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
Atsushi Suzuki
Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan
*Address all correspondence to: oku@mat.usp.ac.jp
1. Introduction
Various organic-inorganic hybrid solar cells with perovskite-type pigments have been broadly studied recently [1–4]. Organic solar cells with a CH3NH3PbI3 compound that has a perovskite structure have high conversion efficiencies [5–7]. Since achieving a photoconversion efficiency of 15% [8], higher efficiencies have been reported for various device structures and processes [9–11], and the photoconversion efficiency increased up to ca. 20% [12–18]. The solar cell properties depend on the crystal structures of the perovskite phase, electron transport layers, hole transport layers (HTLs), nanoporous layers, and fabrication process. Especially, the energy band gaps and carrier transport of the perovskite compounds are dependent on the crystal structures [19], and further analyses of the structures and properties are imperative.
In this article, fabrication and characterization of perovskite-type solar cells are reviewed and summarized. Various perovskite compounds, such as CH3NH3PbI3, [HC(NH2)2]PbI3, and CsSnI3, are expected for solar cell materials. Since these perovskite-type materials often have nanostructures in the solar cell devices, information on the crystal structures, fabrication, and characterization would be useful for fabrication of the perovskite-type crystals. Transmission electron microscopy, electron diffraction, and high-resolution electron microscopy are powerful tools for structure analysis of solar cells [20] and perovskite-type structures in atomic scale [21–23].
The crystals of CH3NH3PbX3 (X = Cl, Br, or I) have perovskite structures and provide structural transitions upon heating [24–26]. The crystal structures of cubic, tetragonal, and orthorhombic CH3NH3PbI3 are shown in Figure 1(a)–(c), respectively. Space group is Pm-3m, and the lattice constant a = 6.391 Å at 330 K for cubic CH3NH3PbI3 [27]. Hydrogen positions in the orthorhombic CH3NH3PbI3 were also determined at 4 K by neutron diffraction [28], as shown in Figure 1(d). Although the crystals of perovskite CH3NH3PbX3 provide a cubic system as the high-temperature phase, the CH3NH3+ ions are polar and have a symmetry of C3v. This results in formation of cubic phase with disordering [27]. Besides the CH3NH3+ions, disordering of the halogen ions is also observed in the cubic perovskite phase, as indicated in Figure 1(a). Site occupancies of I were 1/4, and those of C and N were 1/12, respectively. The CH3NH3 ion occupies 12 equivalent orientations of the C2 axis, and hydrogen atoms have two kinds of configurations on the C2 axis. Therefore, the total degree of freedom is 24 [26].
Figure 1.
Structure models of CH3NH3PbI3 with (a) cubic, (b) tetragonal and (c) orthorhombic structures, and (d) orthorhombic CH3NH3PbI3 with hydrogen positions.
In addition to the CH3NH3PbI3 (MAPbI3), [HC(NH2)2]PbI3 (formamidinium lead iodide, FAPbI3) provided high conversion efficiencies [17, 18]. Structure parameters, including hydrogen positions, were also determined at 298 K by neutron diffraction [29], and the structure model with the lattice constant a = 6.3620 Å is shown in Figure 2(a).
Figure 2.
Structure models of (a) [HC(NH2)2]PbI3, (b) CsSnI3, and (c) CsGeI3 with cubic structures.
CH3NH3 ions can be substituted by other elements such as Cs. The structure models of CsSnI3 and CsGeI3 for high-temperature phase are shown in Figure 2(b) and (c), respectively [30–32]. Space group is Pm-3m (Z = 1), and a = 6.219 Å at 446 K for CsSnI3, a = 6.05 Å at 573 K for CsGeI3, respectively. Solar cells with F-doped CsSnI2.95F0.05 provided a photoconversion efficiency of 8.5% [6].
A typical fabrication process of the TiO2/CH3NH3PbI3 photovoltaic devices is described here [8, 33, 34]. Fluorine-doped tin oxide (FTO) substrates were washed in an ultrasonic cleaner using methanol and acetone, and then dried in N2 gas. Precursor solution of 0.30 M TiOx was prepared from titanium diisopropoxide bis(acetyl acetonate) with 1-butanol, and the TiOx precursor solution was spin-coated on the FTO substrate at 3000 rpm and annealed at 125°C for 5 min. This process was carried out two times, and the FTO substrate was annealed at 500°C for 30 min to form the compact TiO2 layer as an electron transport layer. After that, TiO2 paste was coated on the substrate by a spin-coating method at 5000 rpm to form a mesoporous structure. For the mesoporous TiO2 layer, TiO2 paste was arranged with TiO2 powder with poly(ethylene glycol) in ultrapure water. The solution was stirred with triton X-100 and acetylacetone for 30 min. The prepared cells were heated at 120°C, and annealed at 500°C for 30 min in air. Designed for the preparation of pigment with a perovskite structure, a solution of CH3NH3I and PbI2 with a mole ratio of 1:1 in γ-butyrolactone was mixed at 60°C. The mixture solution of CH3NH3I and PbI2 was then poured into the TiO2 mesopores by spin-coating, and annealed at 100°C. After that, the hole transport layer (HTL) was prepared by the spin coating. For preparation of the HTL, a solution of spiro-OMeTAD in chlorobenzene was mixed with a solution of lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI) in acetonitrile for 12 h. The former solution with 4-tert-butylpyridine was mixed with the Li-TFSI solution at 70°C. Finally, gold (Au) metal contacts were evaporated as top electrodes of the cell. Layered structures of the present photovoltaic cells were represented as FTO/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au, as shown in Figure 3.
Figure 3.
Schematic illustration for the fabrication of CH3NH3PbI3 photovoltaic cells.
The typical J–V characteristics of the TiO2/CH3NH3PbI3/spiro-OMeTAD photovoltaic cells under illumination are shown in Figure 4(a), which indicates an annealing effect of the CH3NH3PbI3 layer. The as-deposited CH3NH3PbI3 cell provided a conversion efficiency of 2.83%. The CH3NH3PbI3 cell annealed at 100°C for 15 min provided better photovoltaic properties compared with the as-deposited one, as shown in Figure 4(a). The highest efficiency was obtained for the annealed CH3NH3PbI3 cell, which provided a power conversion efficiency of 5.16%, a fill factor of 0.486, a short-circuit current density of 12.9 mA cm−2, and an open-circuit voltage of 0.827 V [35]. Figure 4(b) shows results of multiple spin-coating of CH3NH3PbI3, which will be described later.
Figure 4.
J–V characteristics of TiO2/CH3NH3PbI3 photovoltaic cells. (a) As-deposited and annealed samples. (b) CH3NH3PbI3 layers prepared by multiple spin-coating.
XRD patterns of CH3NH3PbI3 thin films on the glass substrate are shown in Figure 5(a). The diffraction reflections could be indexed with tetragonal and cubic structures for as-deposited and annealed films, respectively. Though the as-deposited film showed a single phase of the perovskite structure, broader diffraction reflections owing to a PbI2 phase appeared after annealing, as shown in Figure 5(a). Figure 5(b) and (c) is enlarged XRD patterns at 2θ of ~14° and ~28°, respectively. Split diffraction reflections of 002–110 and 004–220 for the as-deposited sample changed into diffraction reflections of 100 and 200 after annealing, which indicate the structural transformation from the tetragonal to cubic system. The CH3NH3PbI3 crystals have perovskite structures and provide structural transitions from a tetragonal to a cubic system upon heating at ~330 K, as shown in the structure models of Figure 1(a) and (b). For the high-temperature phase, unit cell volume of the cubic system is 261 Å3, which is bigger compared with that of the tetragonal system (246 Å3), as shown in Table 1 [35]. This might be because of both thermal expansion of the unit cell and atomic disordering of iodine in the cubic structure. As the temperature decreases, the tetragonal structure is transformed to the orthorhombic structure because of ordering of CH3NH3 ions in the unit cell [37].
Figure 5.
(a) XRD patterns of CH3NH3PbI3 thin films before and after annealing. Enlarged XRD patterns at 2θ of (b) ~14° and (c) ~28°.
Measured and reported structural parameters of CH3NH3PbI3.
V: unit cell volume; Z: number of chemical units in the unit cell.
The XRD results in Figure 5 indicated phase transformation of the CH3NH3PbI3 perovskite structure from the tetragonal to the cubic system by partial separation of PbI2 from the CH3NH3PbI3 phase at elevated temperatures, which would be related to the decrease of the unit cell volume of the perovskite structure from 248.3 to 246.8 Å3, as shown in Table 1. Besides the iodine atoms, Pb atoms may be deficient, and the occupancy of the Pb sites might be smaller than 1. It should be noted that the structural transition of the CH3NH3PbI3 from the tetragonal to cubic system here would be attributed to the formation of PbI2 by decomposition of the CH3NH3PbI3 phase, which is different from the ordinary tetragonal-cubic transition at 330 K [24, 25].
Figure 6(a) and (b) is the TEM image and the electron diffraction pattern of TiO2/CH3NH3PbI3, respectively [35]. The TEM image shows TiO2 nanoparticles with sizes of ~50 nm, and the polycrystalline CH3NH3PbI3 phase shows dark contrast, which is due to Pb having the largest atomic number in the present materials.
The electron diffraction pattern of Figure 6(b) shows the Debye-Scherrer rings from the nanocrystalline TiO2 particles, which can be indexed with the 101, 004, and 200 reflections of anatase-type TiO2. Thickness of the mesoporous TiO2 layer was found to be ~300 nm from atomic force microscopy measurements. Along with the Debye–Scherrer rings of TiO2, diffraction reflections agreeing with the CH3NH3PbI3 structure [6] were observed and indexed, as shown in Figure 6(b). Other diffraction spots, except for the Debye-Scherrer rings of TiO2, are also from the CH3NH3PbI3 nanoparticles. A structure model and its calculated electron diffraction pattern of a cubic CH3NH3PbI3 phase projected along the [210] direction are shown in Figure 6(c) and (d), respectively. The calculated electron diffraction pattern agrees well with the observed pattern of Figure 6(b).
Figure 6.
(a) TEM image and (b) electron diffraction pattern of TiO2/CH3NH3PbI3. “P” indicates CH3NH3PbI3 perovskite phase. (c) Structure model and (d) its calculated electron diffraction pattern of cubic CH3NH3PbI3 projected along the [210] direction.
Figure 7(a) is a high-resolution TEM image of the CH3NH3PbI3 taken along the a-axis [33]. The images of thinner parts of the crystals indicate the direct projection of the crystal structure [21, 22]. The darkness and the size of the dark spots corresponding to Pb positions could be directly identified, and atomic positions of iodine (I) in the crystal indicate weak contrast, as compared with the projected atomic structure model of CH3NH3PbI3 along the [100] direction in Figure 7(b). NH3 and CH3 molecules cannot be represented as dark spots in the image, which is due to the smaller atomic number of N and C. Figure 7(c) is a high-resolution image of the surface of a TiO2 nanoparticle, which indicates {101} lattice fringes.
The J–V characteristics of the TiO2/CH3NH3PbI3/spiro-OMeTAD photovoltaic cells prepared by multiple spin-coating of CH3NH3PbI3 are shown in Figure 4(b). Figure 4(b) indicates the effect of spin-coating times of CH3NH3PbI3 on the photovoltaic properties. The highest efficiency of 6.96% was achieved for the cell coated for four times, which provided a JSC of 16.5 mA cm−2, a VOC of 0.848 V, and FF of 0.496. More spin-coating reduced the efficiencies of the cells. Although 2-step deposition [8] (spin-coating PbI2 and dipping in the CH3NH3I solution) was also performed in air, the efficiency was lower compared with that by multiple spin-coating, as observed in Figure 4(b). It is believed that the CH3NH3PbI3 phase was embedded inside pores of the mesoporous TiO2 layer during one- or two-time spin-coating. After the inside pores of the mesoporous TiO2 were completely filled with the perovskite compound, only the perovskite layer might be formed on the mesoporous TiO2 layer by four-time spin-coating, which would result in the highest efficiency.
Figure 7.
(a) High-resolution TEM image and (b) structure model of CH3NH3PbI3. (c) Lattice image of TiO2.
The IPCE spectrum of the photovoltaic cell with the TiO2/CH3NH3PbI3/spiro-OMeTAD structure exhibits photoconversion efficiencies between 300 and 800 nm, which nearly agrees with the measured energy gaps of 1.51 eV [37] for the CH3NH3PbI3 compound. This indicates that excitons might be effectively generated in the perovskite compound layers upon light illumination.
An energy level diagram of TiO2/CH3NH3PbI3 photovoltaic cells is summarized as shown in Figure 8(a). The electronic charge generation is caused by light irradiation from the FTO substrate side. The TiO2 layer receives the electrons from the CH3NH3PbI3 crystal, and the electrons are carried to the FTO. On the other hand, the holes are carried to the Au electrode through the HTL of spiro-OMeTAD. For these processes, the devices were produced in air, which would induce the reduction of device stability. Perovskite compounds with higher crystal quality would be produced in future works.
Figure 8.
(a) IPCE spectrum and (b) energy level diagram of TiO2/CH3NH3PbI3 cell. (c) Model of interfacial structure.
From the TEM results, size distributions of TiO2 nanoparticles were observed, indicating a microcrystalline structure, as shown in Figure 6(b), and there seems to be no special crystallographic relation at the interface. The interface between the TiO2 and CH3NH3PbI3 phases would not be perfectly connected over the large area. The cell prepared by four-time spin-coating provided the highest efficiency, which would have an interfacial microstructure as shown in Figure 8(b). The layer thickness of the CH3NH3PbI3 phase was too thick for the cells prepared by 10-time spin-coating, which resulted in an increase in the inner electronic resistance and decrease in the efficiency.
As a summary, the structure analysis of TiO2/CH3NH3PbI3 indicated phase transformation of the perovskite structure from the tetragonal to the cubic system by partial separation of PbI2 from the CH3NH3PbI3 compound upon annealing, which was presumed by decrease of the unit cell volume of the perovskite structure and resulted in the enhancement of photovoltaic properties of the devices. Effects of the multiple spin-coating were also investigated, which improved the efficiency when the four-time spin-coating was carried out. The improvement of the devices might attribute to the complete coverage and optimal thickness of the perovskite layer on the porous TiO2. Additionally, the lattice constants and crystallite sizes of the CH3NH3PbI3 increased and decreased, respectively, which indicates the microstructural difference of the perovskite phase between the inside of and above the porous TiO2.
Enlargement of the cell area is especially mandatory to enable the use of perovskite devices such as actual commercial solar cell panels [38]. The photovoltaic properties of perovskite-type solar cells with a substrate size of 70 mm × 70 mm were investigated [39].
The photovoltaic devices consisted of a CH3NH3PbI3 compound layer, TiO2 electron transport layers, and spiro-OMeTAD hole-transport layer, prepared by a simple spin-coating technique. The effect of the distance from the center of the cell on conversion efficiency was investigated based on light-induced J–V curves and IPCE measurements. A photograph of a perovskite solar cell measuring 70 mm × 70 mm and a schematic illustration of the arrangement of Au electrodes on the substrate are shown in Figure 9(a) and (b), respectively.
Figure 9.
(a) Photograph of perovskite solar cell measuring 70 mm × 70 mm. (b) Schematic illustration of arrangement of Au electrodes on the substrate.
The measured short-circuit current density, open-circuit voltage, fill factor, and photoconversion efficiency of the present TiO2/CH3NH3PbI3 cell as a function of the distance from the center of the cell are shown in Figure 10(a–d), respectively. The highest efficiency was obtained for the electrode at 12.7 mm from the cell center, which provided a photoconversion efficiency of 3.15%, a VOC of 0.653 V, a JSC of 13.0 mA cm−2, and a FF of 0.371. Due to the long diffusion length of exciton [40], the JSC values were nearly constant at ~12 mA cm−2 for all electrodes on the solar cell, as observed in Figure 10(a). Although the FF value slightly decreased as the distance (d) from the center of the cell increased, the deviation was small, as observed in Figure 9(c). On the other hand, the value of VOC depended fairly on the d values, as observed in Figure 10(b), which led to decreased efficiency, as shown in Figure 10(d). The dependency of VOC values on the d values might be related to the thickness of CH3NH3PbI3 layer prepared on the large substrate by the spin-coating method. The low VOC and FF values would be related to the coverage ratio of CH3NH3PbI3 at the TiO2/CH3NH3PbI3 interface, and further multiple spin-coating of CH3NH3PbI3 layers on the TiO2 mesoporous layer would improve the coverage of CH3NH3PbI3 on the TiO2 mesoporous layer, which would induce the increase in the conversion efficiency of the solar cells.
IPCE spectra of electrodes at 4.2, 12.7, and 22.8 mm from the cell center are shown in Figure 10(e). All spectra show similar changes on the wavelength, which agrees with the JSC results shown in Figure 9(a). The perovskite CH3NH3PbI3 structure showed photoconversion within the whole measurement range of 300–800 nm, which nearly agrees with the reported energy gaps for the CH3NH3PbI3 phase. Control of the energy levels of the conduction band and valence band is important for carrier transport in the cell. The conversion efficiencies obtained for the present cells are lower than the previously reported values. It might be difficult to control the uniformity of the layer thickness and interfacial structure using the spin-coating. In the present work, the samples were prepared in air, which might result in a decrease in the efficiency of the present cells, and perovskite crystals with higher quality and a uniform surface should be prepared in future works.
Figure 10.
Measured (a) short-circuit current density; (b) open-circuit voltage; (c) fill factor and (d) conversion efficiency of TiO2/CH3NH3PbI3 cell as a function of the distance from the center of the cell. (e) IPCE spectra of the same cell.
As a summary, perovskite solar cell devices with a substrate size of 70 mm were produced by a spin-coating method using a mixture solution. The photovoltaic properties of the solar cells and the size effect of the substrate were investigated by J–V and IPCE measurements, and the dependency of their conversion efficiency on the distance from the center of the cell was investigated. Nearly constant values of short-circuit current density were obtained over a large area, due to the long exciton diffusion length of the CH3NH3PbI3 compound. The open-circuit voltage fairly depended on the distance from the center of the cell, which led to a change in conversion efficiency. Optimizing the layer thickness and structure would be important for improving the performance of the devices.
The electron-transport layers (ETLs) such as TiO2 are also important for the CH3NH3PbI3-based photovoltaic devices. Here, niobium (V) ethoxide was chosen as an additional chemical for TiO2 [41]. When niobium (Nb) atoms with five valence electrons are introduced at Ti sites with four valence electrons, extra electrons are introduced in the 3d band and could work as a donor. Since the energy level of impurity in the TiO2 band gap is shallow, transparency could be conserved after the Nb doping [42–46]. Additionally, the radius of Nb ion is close to that of the Ti ion, which leads to a solid solution of titanium and niobium in the anatase-type TiO2 crystal. The TiO2 crystal added with Nb is denoted as Ti(Nb)O2 here.
The XRD patterns and crystal structure of TiO2 and Ti(Nb)O2 thin films on the FTO substrate are shown in Figure 11(a) and (b), respectively. Diffraction peaks of TiO2 101 are observed, and the intensity increased upon Nb-doping. The XRD data indicate that the d-spacing of Ti(Nb)O2 (1.802 Å) is almost the same as that of TiO2 (1.807 Å). The crystallite size seems to increase a little upon Nb addition (28 nm) to TiO2 (24 nm).
Figure 11.
(a) XRD patterns of TiO2 and Ti(Nb)O2 thin films on FTO. (b) Crystal structure of TiO2.
A scanning electron microscopy (SEM) image of the Ti(Nb)O2 thin film is shown in Figure 12(a), and the image indicates several particles with sizes of ca. 1 μm on the smooth surface. Elemental mapping images of Ti and Nb using SEM with energy-dispersive X-ray spectroscopy (EDX) are shown in Figure 12(b) and (c), respectively, which indicate that Ti and Nb elements are homogeneously distributed in the films. The elemental ratio of Ti:Nb was estimated to be ~1.00:0.10 from SEM-EDX analysis. The dispersed particles observed in Figure 12(a) were found to be Nb-rich phase, as observed in Figure 12(c), which resulted in an Nb-rich (ca. 9 atomic %) composition compared with the preparation composition (ca. 5 atomic %). From XRD analysis, no diffraction peak corresponding to Nb and Nb2O5 was observed.
Figure 12.
(a) SEM image of Ti(Nb)O2 thin film. Elemental mapping of (b) Ti (Lα) and (c) Nb (Lα).
The sheet resistances of TiO2 and Ti(Nb)O2 thin films were measured to be 1.7×106 and 4.2×104 Ω/sq, respectively. The sheet resistance significantly decreased upon Nb addition. The J–V characteristics of Ti(Nb)O2/CH3NH3PbI3/spiro-OMeTAD photovoltaic cells under illumination are shown in Figure 13(a). The detailed parameters of the best device are listed in Table 2. The Ti(Nb)O2/CH3NH3PbI3 photovoltaic cell provided an η of 6.63%, a FF of 0.416, a JSC of 20.8 mA cm−2, and a VOC of 0.768 V. The JSC value was especially improved upon Nb addition, which resulted in increased conversion efficiency. The averaged efficiency (ηave) of three electrodes on the cells is 6.46%, as listed in Table 2.
Figure 13.
(a) J–V characteristics of Ti(Nb)O2/CH3NH3PbI3 photovoltaic cells. (b) Differential absorption spectra of TiO2 and Ti(Nb)O thin films.
ETL
JSC (mA cm−2)
VOC (V)
FF
η (%)
ηave (%)
TiO2
14.6
0.796
0.478
5.56
5.03
Ti(Nb)O2
20.8
0.768
0.416
6.63
6.46
Table 2.
Measured parameters of Ti(Nb)O2/CH3NH3PbI3 cells.
Figure 13(b) shows differential absorption spectra of FTO/TiO2 and FTO/Ti(Nb)O2 after subtracting the spectrum of the FTO substrate. These absorption spectra appear to be closely equal. Based on the band structure of indirect transition [60], energy gaps for TiO2 and Ti(Nb)O2 were estimated to be 3.54 and 3.52 eV from Figure 13(b), respectively, which indicate that the energy gaps are almost the same for TiO2 and Ti(Nb)O2.
Figure 14.
(a) EQE and (b) IQE spectra of Ti(Nb)O2/CH3NH3PbI3 cells.
The IPCE of the cells was also investigated, and the external quantum efficiency (EQE) and internal quantum efficiency (IQE) were measured by a spectral response system. The EQE spectra of the photovoltaic cells with the Ti(Nb)O2/CH3NH3PbI3/spiro-OMeTAD structure are shown in Figure 14(a). The perovskite CH3NH3PbI3 phase shows photoconversion efficiencies between 300 and 800 nm. By Nb addition into the TiO2 layer, the perovskite CH3NH3PbI3 structure shows high EQE values of ca. 60% at 500–600 nm and ca. 5% at 800 nm, and the EQE was 0% for ordinary TiO2 at 800 nm. The IQE spectra of Ti(Nb)O2/CH3NH3PbI3/spiro-OMeTAD cells were computed from the reflectance and EQE, as shown in Figure 14(b). The IQE of both cells increased in the range of 500–800 nm, which implies that suppression of reflection of light in the range of 500–800 nm could increase the photoconversion efficiencies of the cells. High IQE values of ~70% are seen in the range of 500–600 nm by the Nb addition in the TiO2 layer.
Two mechanisms could be considered for the decrease in the sheet resistances of TiO2 by the Nb addition. The first mechanism is niobium doping at the titanium sites in the TiO2 crystal. Owing to the XRD and differential absorption results of Figures 11(a) and 13(b), the TiO2 phase still preserved the crystal structure, energy gap, and transparency of anatase TiO2. In addition, a small amount of Nb atoms are widely distributed in the TiO2 phase, as observed by SEM-EDX of Figure 10(c) and (d), which could imply a solid solution of titanium and niobium in the TiO2 structure. The extra electron of Nb might be introduced into the 3d band of Ti and behaves as a donor [60]. The second conceivable mechanism is enhancement of carrier transport by formation of niobium-based particles in the TiO2 layer, as observed in SEM-EDX images. Nanoparticles in electron-transport and hole-transport layers could facilitate the carrier transport [47, 48], and the present niobium-based particles might contribute to the carrier transport. Both mechanisms could provide an increase in carrier concentration and transport, and an improvement of conversion efficiency through the increase in JSC.
As a summary, Ti(Nb)O2/CH3NH3PbI3-based photovoltaic devices were fabricated by a spin-coating method using a mixture solution of niobium(V) ethoxide, and the effects of Nb addition into the TiO2 layer were investigated. By adding a simple solution of niobium(V) ethoxide to the TiO2 precursor solutions, the sheet resistance of the Ti(Nb)O2 thin film decreased, and the JSC value increased, which resulted in the increase in conversion efficiency.
Effects of Cl-doping CH3NH3PbI3 using a mixture solution of perovskite compounds on the microstructures and photovoltaic properties have been investigated [49]. The J–V characteristics of the TiO2/CH3NH3PbI3−xClx/spiro-OMeTAD photovoltaic cells under illumination are shown in Figure 15(a), which indicate an effect of Cl-doping to the CH3NH3PbI3 layer. Measured photovoltaic parameters of TiO2/CH3NH3PbI3−xClx cells are summarized in Table 3. The CH3NH3PbI3 cell provided a power conversion efficiency of 6.16%, and the averaged efficiency of four electrodes on the cells is 5.53%, as listed in Table 3. The highest efficiency was obtained for the CH3NH3PbI2.88Cl0.12 cell, which provided an η of 8.16%, a FF of 0.504, JSC of 18.6 mA cm−2, and a VOC of 0.869 V. As a Cl composition increased, the JSC and VOC decreased, as shown in Figure 15(b) and Table 3. Energy gaps (Eg) of CH3NH3PbI3, CH3NH3PbI2.88Cl0.12, and CH3NH3PbI1.8Cl1.2 were estimated to be 1.578, 1.590, and 1.593, respectively, from the optical absorption, which indicated the energy gap of CH3NH3PbI3 increased by the Cl-doping.
Figure 15.
(a) J–V characteristic of TiO2/CH3NH3PbI3−xClx photovoltaic cells. (b) Conversion efficiencies of the cells as a function of Cl concentration.
Preparation composition
JSC (mA cm−2)
VOC (V)
FF
η (%)
ηave (%)
CH3NH3PbI3
17.5
0.844
0.416
6.16
5.53
CH3NH3PbI2.94Cl0.06
17.7
0.871
0.487
7.53
6.02
CH3NH3PbI2.92Cl0.08
18.1
0.825
0.478
7.14
6.52
CH3NH3PbI2.88Cl0.12
18.6
0.869
0.504
8.16
7.77
CH3NH3PbI2.77Cl0.23
13.9
0.865
0.440
5.29
4.97
CH3NH3PbI2.65Cl0.35
11.7
0.709
0.347
2.87
2.51
CH3NH3PbI1.80Cl1.20
14.8
0.598
0.436
3.87
2.00
Table 3.
Measured photovoltaic parameters of TiO2/CH3NH3PbI3−xClx cells.
Figure 16.
(a) XRD patterns of CH3NH3PbI3−xClx thin films. (b) Enlarged XRD patterns at 2θ of ~28.5°.
Preparation composition
Crystal system
Lattice constant (Å)
V (Å3)
Z
V/Z (Å3)
CH3NH3PbI3
Cubic
a = 6.2524
244.42
1
244.42
CH3NH3PbI2.88Cl0.12
Pseudocubic
a = 6.2446
243.51
1
243.51
CH3NH3PbI1.80Cl1.20
Tetragonal
a = 8.8255 c = 12.6180
982.81
4
245.70
Table 4.
Measured and reported structural parameters of CH3NH3PbI3–xClx.
V: unit cell volume; Z: number of chemical units in the unit cell.
Figure 17.
(a) SEM image of TiO2/CH3NH3PbI2.88Cl0.12. Elemental mapping images of (b) Pb Mα line, (c) I Lα line, and (d) Cl Kα line.
XRD patterns of CH3NH3PbI3−xClx thin films on the FTO/TiO2 are shown in Figure 16(a). The temperature for XRD measurements was ~292 K. The diffraction peaks can be indexed by cubic and tetragonal crystal systems for CH3NH3PbI3 and CH3NH3PbI1.8Cl1.2 films, respectively. Although the deposited films are a single perovskite phase, broader diffraction peaks due to PbI2 compound appeared in the CH3NH3PbI3 film, as shown in Figure 16(a). Figure 16(b) shows enlarged XRD patterns at 2θ of ~28.5°. A diffraction peak of 200 for the CH3NH3PbI3 split into diffraction peaks of 004/220 for the CH3NH3PbI1.8Cl1.2 by the heavy Cl-doping, which indicates the structural transformation from the cubic to tetragonal crystal systems [19]. The heavy Cl-doping suppressed the formation of PbI2, and no PbCl2 was detected for the CH3NH3PbI1.8Cl1.2. For the CH3NH3PbI2.88Cl0.12, a small shoulder is observed just left of the 200 reflection as shown in Figure 16(b), which would be due to the pseudocubic structure between the cubic and tetragonal phases. The measured structural parameters of the CH3NH3PbI3−xClx are summarized in Table 4.
Figure 17(a) is a SEM image of TiO2/CH3NH3PbI2.88Cl0.12, and the image shows particles with sizes of ca. 10 μm. Mapping images of Pb, I, and Cl elements by SEM equipped with EDX are shown in Figure 17(b–d), respectively. These mapping images of elements indicate that the dispersed particles observed in Figure 17(a) correspond to the perovskite CH3NH3PbI3−xClx phase. The composition ratio of Pb:I:Cl was 1.00:2.70:0.11, which was calculated from their EDX spectra using each element’s line after background correction by normalizing the spectrum peaks on the atomic concentration of Pb element. The present result indicates that iodine atoms would be deficient comparing with the starting composition of CH3NH3PbI2.88Cl0.12, and the deficient I might increase the hole concentration. The CH3NH3PbI3 crystals have perovskite structures, and provide structural transitions from tetragonal to cubic system upon heating at ~330 K [27–29].
The XRD results in Figure 16 indicated phase transformation of the CH3NH3PbI3 perovskite structure from tetragonal to cubic system by partial separation of PbI2 from CH3NH3PbI3 phase through the annealing [35], which is related to decrease in the unit cell volume of the cubic CH3NH3PbI3 phase from the normal 261 Å3 to the present 244 Å3, as shown in Table 4. From the SEM-EDX results, the site occupancies of I atom might be smaller than 1, which would also decrease the cell volume. The conversion efficiencies were reported to be increased by the tetragonal to cubic transformation [35].
The X-ray diffraction pattern indicates division of diffraction peaks from C200 to T004/T220 by means of heavy Cl-doping. This designates reduction of the symmetry of the crystal structures from the cubic to tetragonal system, which resulted in decrease of the photoconversion efficiencies. Once a small amount of Cl was added in the CH3NH3PbI3 phase, the cubic structure was still preserved as the pseudocubic phase. The doped Cl atoms would lengthen diffusion length of excitons [7, 40], which would result in the increase of the efficiencies.
EQE spectra of the photovoltaic cell with the TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD structure are shown in Figure 18(a). The perovskite CH3NH3PbI3 phase shows photoconversion efficiencies between 300 and 800 nm. In the present work, the energy gap of the CH3NH3PbI3 phase increased from 1.578 to 1.590 eV by Cl-doping, which could contribute to the increase in open-circuit voltage. IQE spectra of TiO2/CH3NH3PbI3 and TiO2/CH3NH3PbI2.92Cl0.08 were computed from EQE spectra and reflectance, as shown in Figure 18(b). The IQE of both cells increased in the wavelength range of 500–800 nm, and this indicates that improvement of the optical absorption in that range might improve the photoconversion efficiencies of TiO2/CH3NH3PbI3−xClx/spiro-OMeTAD cells.
In summary, TiO2/CH3NH3PbI3−xClx-based photovoltaic devices were fabricated by a spin-coating method using a mixture solution, and effects of PbCl2 addition to the perovskite CH3NH3PbI3 precursor solutions on the photovoltaic properties were investigated. The microstructure analysis showed phase transformation of the perovskite structure from cubic to tetragonal system by heavy Cl-doping to the CH3NH3PbI3 phase. A small amount of Cl-doping (CH3NH3PbI2.9Cl0.1) at iodine sites increased the efficiencies up to ~8%, and it might be owing to conservation of the cubic perovskite structure and to extension of diffusion length of excitons and energy gap. Both the EQE and IQE increased in the range of 300–800 nm by means of a small amount of Cl-doping, and the IQE data designate that the inhibition of the optical reflection in the wavelength range of 500–800 nm might improve the photoconversion efficiencies further.
Figure 18.
(a) EQE and (b) IQE spectra of CH3NH3PbI3 and CH3NH3PbI2.92Cl0.08 cells.
The properties of solar cells are dependent on the atomic compositions and the crystal structures of perovskite CH3NH3PbI3 compounds. Metal atom and halogen doping such as tin (Sn) and chlorine (Cl)/bromine (Br) at the Pb and I sites, respectively, in the CH3NH3PbI3 structure have been investigated [12–14, 50–52]. Particularly, researches of the metal element doping at Pb sites are fascinating in the view of Pb-free devices and influence on the photovoltaic properties.
The objective here is to investigate photovoltaic properties and microstructures of photovoltaic devices with perovskite-type CH3NH3Pb1−xSbxI3 compounds, prepared by a spin-coating technique in ordinary air. Antimony (Sb) is an element in the group 15 and might work as electronic carriers at the Pb sites in the group 14. Effects of SbI3 addition to a CH3NH3PbI3 mixed solution on the microstructures and photovoltaic properties were investigated [53, 54].
The J–V characteristics of the TiO2/CH3NH3Pb1−xSbxI3/spiro-OMeTAD photovoltaic cells under illumination are shown in Figure 19(a), which indicate an effect of Sb addition to CH3NH3PbI3. The measured photovoltaic parameters of TiO2/CH3NH3Pb1−xSbxI3 cells are summarized in Table 5.
The CH3NH3PbI3 cell provided a power conversion efficiency of 6.56%, and the averaged efficiency of four electrodes on the cells is 6.37%, as listed in Table 5. The highest efficiency was obtained for the CH3NH3Pb0.97Sb0.03I3 cell, which provided an η of 9.07%, a FF of 0.560, a JSC of 19.2 mA cm–2, and a VOC of 0.843V. As the x value (preparation composition of Sb) increased, the efficiencies decreased, as shown in Figure 19(b) and Table 5. An η of 9.7% was also reported by addition of SbI3 and NH4Cl to the CH3NH3PbI3 [54].
Figure 19.
(a) J–V characteristics of TiO2/CH3NH3Pb1−xSbxI3 photovoltaic cells. (b) Conversion efficiencies of CH3NH3Pb1−xSbxI3 as a function of Sb concentration.
Sb (x)
JSC (mA cm−2)
VOC (V)
FF
η (%)
ηave (%)
0.00
17.0
0.758
0.509
6.56
6.37
0.01
16.0
0.789
0.534
6.74
6.41
0.02
16.9
0.792
0.518
6.94
6.72
0.03
19.2
0.843
0.560
9.07
8.47
0.05
15.7
0.755
0.575
6.82
5.61
0.07
14.7
0.692
0.502
5.11
4.07
0.10
12.1
0.630
0.476
3.63
3.27
0.15
13.1
0.570
0.402
3.00
2.85
Table 5.
Measured photovoltaic parameters of TiO2/CH3NH3Pb1−xSbxI3 cells.
Preparation compositions of Sb are indicated by x.
IPCE spectra of the CH3NH3PbI3 and CH3NH3Pb0.97Sb0.03I3 cells are shown in Figure 20. The perovskite CH3NH3Pb1−xSbxI3 shows photoconversion efficiencies between 300 and 800 nm. The IPCE was improved in the range of 350–770 nm by adding a small amount of Sb.
XRD patterns of CH3NH3Pb1−xSbxI3 cells on the FTO/TiO2 are shown in Figure 21(a). The diffraction peaks can be indexed by a cubic crystal system (Pm3m) for the CH3NH3Pb1−xSbxI3 thin films. Although the deposited films are a single perovskite structure, broader diffraction peaks due to the PbI2 compound appeared in the CH3NH3PbI3 film, as shown in Figure 21(a). The Sb addition suppressed the formation of PbI2, and most of PbI2 was not detected for the CH3NH3Pb1−xSbxI3 cells with x > 0.03. Figure 21(b) shows measured lattice constants a of CH3NH3Pb1−xSbxI3 as a function of Sb concentration. A small increase in lattice constants a is observed for x = 0.03 and 0.05, and further addition of Sb decreases the lattice constants, which seems to be a significant difference from the error bar. The XRD result of CH3NH3PbI3 in Figure 21(a) showed the existence of PbI2 after annealing at 100°C for 15 min. This would indicate partial separation of PbI2 from CH3NH3PbI3 after annealing, which also might correspond to the smaller lattice constant a (6.266 Å) of the cubic perovskite structure, compared with that (6.391 Å) of CH3NH3PbI3 single crystal reported in Ref. [27].
Figure 20.
IPCE spectra of CH3NH3PbI3 and CH3NH3Pb0.97Sb0.03I3 cells.
Figure 21.
(a) XRD patterns of CH3NH3Pb1−xSbxI3 solar cells. (b) Lattice constants a of CH3NH3Pb1−xSbxI3 as a function of Sb concentration.
Increase in the photoconversion efficiencies could be explained by two mechanisms. The first mechanism is Sb doping effect at the Pb atom sites. The ionic valence of Sb is three, and it is higher compared with that of Pb2+. Then, the excess charge of Sb3+ might work as carriers in the CH3NH3Pb1−xSbxI3 crystal, and the JSC values were improved. The second mechanism is described as follows: I– ions might be attracted at the I sites by Sb3+ with more ionic valence compared with that of Pb2+, which resulted in the suppression of PbI2 elimination from CH3NH3PbI3 and in the increase of lattice constants a of CH3NH3PbI3. The suppression of PbI2 would improve the interfacial structure of TiO2/CH3NH3PbI3, which might result in improvement of VOC. The lattice constants are expected to be decreased by an increase in the amount of Sb with an ionic size smaller than Pb. Other elemental dopings such as Ge, Tl, and In at the Pb sites were also reported [55, 56].
In summary, TiO2/CH3NH3Pb1−xSbxI3-based photovoltaic devices were fabricated, and the effects of SbI3 addition to the perovskite CH3NH3PbI3 precursor solutions on the photovoltaic properties were investigated. The microstructures of the devices indicated that the lattice constant of CH3NH3Pb1−xSbxI3 increased a little, and that the formation of PbI2 was inhibited by the addition of a small amount of Sb, which led to the improvement of the conversion efficiencies to ~9%. The IPCE also increased in the range of 350–770 nm by the addition of Sb.
Various TiO2/CH3NH3PbI3-based photovoltaic devices were fabricated and characterized. Especially, effects of metal doping and halogen doping to the perovskite and TiO2 were investigated. Microstructure analysis indicated the changes of the perovskite structure, which resulted in the improvement of photovoltaic properties of the devices. Various elemental dopings to the perovskite structure could be studied further both by experiments and theoretical calculations as follows: Cs, Rb, and K doping to the CH3NH3 positions for stability of the structure; Ge, Sn, and Sb doping to the Pb positions for improvement of the semiconducting properties; Cl, Br, and F doping to the I positions for enhancement of carrier mobility.
The authors like to acknowledge T. Iwata, Y. Imanishi, M. Kanayama, T. Kida, H. Okada, and T. Akiyama for experimental help and support on the perovskite solar cells. 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) 25420760.
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