**4. Electron transport layers**

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 energydispersive 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.

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

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 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

improving the performance of the devices.

The TiO2 crystal added with Nb is denoted as Ti(Nb)O2 here.

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

**4. Electron transport layers**

228 Nanostructured Solar Cells

**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

**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.


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**.

**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 niobiumbased 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 *J*SC value increased, which resulted in the increase in conversion efficiency.
