**6. Metal doping to CH3NH3PbI3**

**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−*x*Cl*<sup>x</sup>* 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

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

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

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/

In summary, TiO2/CH3NH3PbI3−*x*Cl*x*-based photovoltaic devices were fabricated by a spincoating 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 Cldoping (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

, as shown in **Table 4**. From

tetragonal to cubic system upon heating at ~330 K [27–29].

tetragonal to cubic transformation [35].

234 Nanostructured Solar Cells

CH3NH3PbI3−*x*Cl*x*/spiro-OMeTAD cells.

CH3NH3PbI3 phase from the normal 261 Å3 to the present 244 Å3

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−xSb*x*I3/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−xSb*x*I3 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−*x*Sb*x*I3 photovoltaic cells. (b) Conversion efficiencies of CH3NH3Pb1−*x*Sb*x*I3 as a function of Sb concentration.


Preparation compositions of Sb are indicated by *x*.

**Table 5.** Measured photovoltaic parameters of TiO2/CH3NH3Pb1−xSb*x*I3 cells.

IPCE spectra of the CH3NH3PbI3 and CH3NH3Pb0.97Sb0.03I3 cells are shown in **Figure 20**. The perovskite CH3NH3Pb1−xSb*x*I3 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−xSb*x*I3 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−xSb*x*I3 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−xSb*x*I3 cells with *x* > 0.03. **Figure 21(b)** shows measured lattice constants *a* of CH3NH3Pb1−xSb*x*I3 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.

increased, the efficiencies decreased, as shown in **Figure 19(b)** and **Table 5**. An *η* of 9.7% was

**Figure 19.** (a) *J–V* characteristics of TiO2/CH3NH3Pb1−*x*Sb*x*I3 photovoltaic cells. (b) Conversion efficiencies of

IPCE spectra of the CH3NH3PbI3 and CH3NH3Pb0.97Sb0.03I3 cells are shown in **Figure 20**. The perovskite CH3NH3Pb1−xSb*x*I3 shows photoconversion efficiencies between 300 and 800 nm.

XRD patterns of CH3NH3Pb1−xSb*x*I3 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−xSb*x*I3 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−xSb*x*I3 cells with *x* > 0.03. **Figure 21(b)** shows measured lattice constants *a* of CH3NH3Pb1−xSb*x*I3 as a function of Sb concentration. A small

The IPCE was improved in the range of 350–770 nm by adding a small amount of Sb.

**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

also reported by addition of SbI3 and NH4Cl to the CH3NH3PbI3 [54].

CH3NH3Pb1−*x*Sb*x*I3 as a function of Sb concentration.

236 Nanostructured Solar Cells

Preparation compositions of Sb are indicated by *x*.

**Table 5.** Measured photovoltaic parameters of TiO2/CH3NH3Pb1−xSb*x*I3 cells.

**Figure 21.** (a) XRD patterns of CH3NH3Pb1−*x*Sb*x*I3 solar cells. (b) Lattice constants *a* of CH3NH3Pb1−*x*Sb*x*I3 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−xSb*x*I3 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−xSb*x*I3-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.
