**3. Enlargement of cell**

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 spincoating, 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-

**Figure 7.** (a) High-resolution TEM image and (b) structure model of CH3NH3PbI3. (c) Lattice image of TiO2.

illumination.

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

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,

coating, which would result in the highest efficiency.

224 Nanostructured Solar Cells

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 spincoating 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.

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

**Figure 9.** (a) Photograph of perovskite solar cell measuring 70 mm × 70 mm. (b) Schematic illustration of arrangement

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 spincoating 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

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

electrodes on the substrate are shown in **Figure 9(a)** and **(b)**, respectively.

of Au electrodes on the substrate.

226 Nanostructured Solar Cells

efficiency of the solar cells.

**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.
