**5. Halogen doping to CH3NH3PbI3**

addition, which resulted in increased conversion efficiency. The averaged efficiency (*ηave*) of

**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

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

**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

three electrodes on the cells is 6.46%, as listed in **Table 2**.

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.

**Table 2.** Measured parameters of Ti(Nb)O2/CH3NH3PbI3 cells.

230 Nanostructured Solar Cells

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−*x*Cl*x*/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−*x*Cl*<sup>x</sup>* 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.


**Table 3.** Measured photovoltaic parameters of TiO2/CH3NH3PbI3−*x*Cl*x* cells.

**Figure 15.** (a) *J–V* characteristic of TiO2/CH3NH3PbI3−*x*Cl*x* photovoltaic cells. (b) Conversion efficiencies of the cells as a function of Cl concentration.

**Figure 16.** (a) XRD patterns of CH3NH3PbI3−*x*Cl*x* thin films. (b) Enlarged XRD patterns at 2*θ* of ~28.5°.


V: unit cell volume; Z: number of chemical units in the unit cell.

**Figure 15.** (a) *J–V* characteristic of TiO2/CH3NH3PbI3−*x*Cl*x* photovoltaic cells. (b) Conversion efficiencies of the cells as a

**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−*x*Cl*x* cells.

**Figure 16.** (a) XRD patterns of CH3NH3PbI3−*x*Cl*x* thin films. (b) Enlarged XRD patterns at 2*θ* of ~28.5°.

function of Cl concentration.

232 Nanostructured Solar Cells

**Table 4.** Measured and reported structural parameters of CH3NH3PbI3–*x*Cl*x*.

**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−*x*Cl*x* 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 2*θ* 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 Cldoping 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−*x*Cl*x* 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−*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 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−*x*Cl*x*/spiro-OMeTAD cells.

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 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.
