**2. Basic device structures**

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

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*-3*m*, 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

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

**Figure 1.** Structure models of CH3NH3PbI3 with (a) cubic, (b) tetragonal and (c) orthorhombic structures, and (d) ortho-

in formation of cubic phase with disordering [27]. Besides the CH3NH3

configurations on the C2 axis. Therefore, the total degree of freedom is 24 [26].

<sup>+</sup> ions are polar and have a symmetry of C3v. This results

+

ions, disordering of

scale [21–23].

218 Nanostructured Solar Cells

high-temperature phase, the CH3NH3

rhombic CH3NH3PbI3 with hydrogen positions.

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 TiO*x* was prepared from titanium diisopropoxide bis(acetyl acetonate) with 1-butanol, and the TiO*<sup>x</sup>* 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-

**Figure 4.** *J–V* characteristics of TiO2/CH3NH3PbI3 photovoltaic cells. (a) As-deposited and annealed samples. (b) CH3NH3PbI3 layers prepared by multiple spin-coating.

circuit voltage of 0.827 V [35]. **Figure 4(b)** shows results of multiple spin-coating of CH3NH3PbI3, which will be described later.

the present photovoltaic cells were represented as FTO/TiO2/CH3NH3PbI3/Spiro-

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-

**Figure 4.** *J–V* characteristics of TiO2/CH3NH3PbI3 photovoltaic cells. (a) As-deposited and annealed samples. (b)

CH3NH3PbI3 layers prepared by multiple spin-coating.

**Figure 3.** Schematic illustration for the fabrication of CH3NH3PbI3 photovoltaic cells.

OMeTAD/Au, as shown in **Figure 3**.

220 Nanostructured Solar Cells

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 2*θ* 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

**Figure 5.** (a) XRD patterns of CH3NH3PbI3 thin films before and after annealing. Enlarged XRD patterns at 2*θ* of (b) ~14° and (c) ~28°.

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 hightemperature 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].


**Table 1.** Measured and reported structural parameters of CH3NH3PbI3.

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)**.

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-

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

*c* = 12.6453

*c* = 12.685

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

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

**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

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

Annealed Cubic *a* = 6.2724 246.78 1 246.78

Ref. [27] (330 K) Cubic *a* = 6.391 261.0 1 261.0

, which is bigger compared

**)** *Z V***/***Z* **(Å3**

993.10 4 248.27

982.33 4 245.6

, as shown in **Table 1**. Besides

**)**

), as shown in **Table 1** [35]. This might be because of

temperature phase, unit cell volume of the cubic system is 261 Å3

structure because of ordering of CH3NH3 ions in the unit cell [37].

**Samples Crystal system Lattice constants (Å)** *V* **(Å3**

with that of the tetragonal system (246 Å3

222 Nanostructured Solar Cells

As-deposited Tetragonal *a* = 8.8620

Ref. [36] (220 K) Tetragonal *a* = 8.800

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

largest atomic number in the present materials.

[24, 25].

**Table 1.** Measured and reported structural parameters of CH3NH3PbI3.

unit cell volume of the perovskite structure from 248.3 to 246.8 Å3

**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 highresolution 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 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 spincoating, 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 spincoating 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.
