Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH₃CH₂NH₃)PbI₃

Abstract

A new nanocrystalline sensitizer with the chemical formula (CH₃CH₂NH₃)PbI₃ is synthesized by reacting ethylammonium iodide with lead iodide, and its crystal structure and photovoltaic property are investigated. X-ray diffraction analysis confirms orthorhombic crystal phase with a = 8.7419(2) Å, b = 8.14745(10) Å, and c = 30.3096(6) Å, which can be described as 2 H perovskite structure. Ultraviolet photoelectron spectroscopy and UV-visible spectroscopy determine the valence band position at 5.6 eV versus vacuum and the optical bandgap of ca. 2.2 eV. A spin coating of the CH₃CH₂NH₃I and PbI₂ mixed solution on a TiO₂ film yields ca. 1.8-nm-diameter (CH₃CH₂NH₃)PbI₃ dots on the TiO₂ surface. The (CH₃CH₂NH₃)PbI₃-sensitized solar cell with iodide-based redox electrolyte demonstrates the conversion efficiency of 2.4% under AM 1.5 G one sun (100 mW/cm²) illumination.

Background

Semiconductor nanocrystals have received much attention due to quantum confinement effect, in which the continuous optical transitions between the electronic bands in the bulk crystals become discrete in the nanocrystals and thereby the properties of the nano-sized materials become size-dependent [1–3]. The size-dependent optical properties of semiconductor n anoparticles have been widely applied in displays [4], biomedical imaging sensors [5], and photovoltaic solar cells [6]. In the case of solar cell application, semiconductor nanomaterials can be used as a light-absorbing material (photosensitizer) in either a solid-state pn junction structure or a photoelectrochemical junction type [7]. Dispersion of semiconductor nanocrystal on a high-surface-area n-type or p-type support is an effective method to utilize it as a photosensitizer. For this reason, semiconductor (or quantum dot)-sensitized solar cell has recently attracted a lot of interest [8, 9]. As photosensitizers in the semiconductor-sensitized solar cell, metal chalcogenides have been mostly studied, where Sb₂S₃-sensitized solar cell demonstrated a conversion efficiency as high as 6.18% at simulated one sun (100 mW/cm²) illumination [10]. Recently, a conversion efficiency of 6.54% at one sun was reported based on perovskite semiconductor (CH₃NH₃)PbI₃[11], where (CH₃NH₃)PbI₃ was found to form in situ on a nanocrystalline TiO₂ surface from spin coating of the CH₃NH₃I and PbI₂ mixed solution. Moreover, an organic–inorganic hybrid perovskite structure has advantage over other crystal structures as for the sensitizer since it has high light absorption property and thermal stability as well. Since the perovskite ABX₃ structure was known to be stabilized depending on the ionic radii of A and B cations in relation with tolerance factor [12, 13], it can be possible to tailor a new perovskite-type semiconductor sensitizer by substituting methylammonium cation in the cuboctahedral A site with longer alkyl-chain ammonium cations. Change in the A-site cation is expected to tune the bandgap energy of alkylammonium lead iodide perovskite sensitizer due to change in chemical boding nature. Here, we report for the first time on the synthesis and structural analysis of (CH₃CH₂NH₃)PbI₃. Valence band position and optical bandgap are evaluated by ultraviolet photoelectron spectroscopy (UPS) and UV-visible (UV–vis) spectroscopy, respectively. Photovoltaic performance of a (CH₃CH₂NH₃)PbI₃-sensitized solar cell is investigated in the presence of an iodide-based redox electrolyte.

Methods

The semiconductor sensitizer of (CH₃CH₂NH₃)PbI₃ was prepared by direct deposition of the γ-butyrolactone (Aldrich, Sigma-Aldrich Corporation, St. Louis, MO, USA) solution with equimolar CH₃CH₂NH₃I and PbI₂ on a nanocrystalline TiO₂ surface. CH₃CH₂NH₃I was synthesized by reacting 18.2 mL of ethylamine (2.0 M in methanol, Aldrich) and 10 mL of hydroiodic acid (57 wt.% in water, Aldrich) in a 250-mL round-bottomed flask at 0°C for 2 h. The precipitate was collected by evaporation at 80°C for 1 h, which is followed by washing three times with diethyl ether and then finally dried at 100°C in a vacuum oven for 24 h. The synthesized CH₃CH₂NH₃I powder was mixed with PbI₂ (Aldrich) at a 1:1 mole ratio in γ-butyrolactone at 80°C for 2 h, which was used as a coating solution for the in situ formation of (CH₃CH₂NH₃)PbI₃ on the TiO₂ surface. The concentration of the coating solution was 42.17 wt.%, which contains 2.234 g of CH₃CH₂NH₃I (12.9 mmol) and 6.016 g of PbI₂ (12.9 mmol) in 10 mL of γ-butyrolactone.

Nanocrystalline TiO₂ particles were prepared by hydrothermal method at 230°C, and non-aqueous TiO₂ paste was prepared according to the method reported elsewhere [14]. Fluorine-doped tin oxide (FTO) conductive glass (TEC-8, 8 Ω/sq, Pilkington, St Helens, UK) was pre-treated with 0.1 M Ti(IV) bis(ethyl acetoacetato)-diisopropoxide (Aldrich) in 1-butanol (Aldrich) solution, in which the nanocrystalline TiO₂ paste was deposited and heated at 550°C for 1 h. The thicknesses of the annealed TiO₂ films were determined by an alpha-step IQ surface profiler (KLA-Tencor Corporation, Milpitas, CA, USA). The perovskite coating solution was spread on the annealed TiO₂ film (38.46 μL/cm²) and was spun for 10 s at a speed of 2,000 rpm in ambient atmosphere. The perovskite (CH₃CH₂NH₃)PbI₃ formed on the TiO₂ surface was dried at 100°C for 15 min. Pt counter electrode was prepared by spreading a droplet of 7 mM H₂PtCl₆x H₂O in 2-propanol on a FTO substrate and heated at 400°C for 20 min in air. The (CH₃CH₂NH₃)PbI₃-sensitized TiO₂ working electrode and the counter electrode were sandwiched using 25-μm-thick Surlyn (SX1170-25, Solaronix SA, Aubonne, Switzerland). The redox electrolyte was prepared by dissolving 0.9 M LiI (Aldrich), 0.45 M I₂ (Aldrich), 0.5 M tert-butylpyridine(Aldrich), and 0.05 M urea (Aldrich) in ethyl acetate (Aldrich), which was introduced into the space of the sealed electrodes prior to measurement.

Powder X-ray diffraction (XRD) profiles were recorded on a Rigaku D/MAX-2200/PC diffractometer (Tokyo, Japan) using graphite-monochromated CuKα radiation (λ = 1.5418 Å). Data were collected over the 2θ range from 5° to 100° for 4 s in each 0.02° step at ambient temperature. The TREOR software [15] was used for indexing and determining the lattice parameters. For XRD measurement, (CH₃CH₂NH₃)PbI₃ powder was obtained by drying the solution of the equimolar mixture of CH₃CH₂NH₃I and PbI₂ at 100°C. Photocurrent and voltage were measured from a solar simulator equipped with a 450-W xenon lamp (6279NS, Newport Corporation, Irvine, CA, USA) and a Keithley 2400 source meter (Cleveland, OH, USA). Light intensity was adjusted with the NREL-calibrated Si solar cell having KG-2 filter for approximating one-sun light intensity (100 mW/cm²). While measuring current and voltage, the cell was covered with a black mask having an aperture, where the aperture area was slightly smaller than the active area. Distribution of perovskite (CH₃CH₂NH₃)PbI₃ in the TiO₂ film was investigated by a distribution mapping technique using an energy-dispersive X-ray spectroscope (EDS) combined with a field-emission scanning electron microscope (FE-SEM, Jeol JSM 6700 F). X-ray energies corresponding to Ti, Pb, and I were collected as the SEM scanned the electron beam over the surface and cross-sectional area in the TiO₂ film. The X-ray data were synchronized with the SEM image, and an elemental mapping was created showing the presence of the selected element throughout the selected area. Transmission electron microscope (TEM) image was investigated using high-resolution TEM (HR-TEM, Jeol, JEM-2100 F) at an acceleration voltage of 200 kV. The UV–vis reflectance spectra of the powdered (CH₃CH₂NH₃)PbI₃, the (CH₃CH₂NH₃)PbI₃-adsorbed TiO₂ nanoparticle, and the bare TiO₂ particle were recorded using a UV/VIS/NIR spectrophotometer (Lambda 950 model, PerkinElmer, Waltham, MA, USA) in a wavelength of 200 to 1,100 nm. UPS equipped with He-I source (hν = 21.22 eV) (AXIS Nova, Kratos Analytical Ltd., Manchester, UK) was used to determine the valence band energy of (CH₃CH₂NH₃)PbI₃.

Results and discussion

Figure 1a shows the XRD pattern of the synthesized (CH₃CH₂NH₃)PbI₃. All reflections are indexed by an orthorhombic unit cell with a = 8.7419(2) Å, b = 8.14745(10) Å, c = 30.3096(6) Å. Reflection conditions (h + k = 2n for hk 0, h = 2n for h 00, and k = 2n for 0k 0) observed in the XRD pattern indicate that possible space groups are P2₁mn and Pmmn (Table 1). By assuming a centrosymmetric space group Pmmn, the structural parameters for heavy atoms such as Pb and I are determined by applying the direct method using the EXPO software [16] and refined by the Rietveld method with the FULLPROF program [17]. Table 2 shows the atomic coordinates, isotropic temperature factors, and agreement factors. The structural information about C, N, and H atoms could not be obtained due to the low resolution of the laboratory XRD equipment. As shown in Figure 1b, the structure of (CH₃CH₂NH₃)PbI₃ can be described as a 2 H perovskite type which consists of infinite chains of face-sharing (PbI₆) octahedra running along the b-axis of the unit cell. These chains are separated from one another by ethylammonium ions.

Figure 1

Powder XRD pattern (a) and crystal structure (b) of (CH ₃ CH ₂ NH ₃ )PbI ₃ . Red and purple spheres represent Pb and I ions, respectively.

Table 1 Miller indices ( hkl ), spacing of lattice plane ( d ), and XRD peak intensity ( I ) of (CH ₃ CH ₂ NH ₃ )PbI ₃

Table 2 Unit cell, positional, and thermal parameters for (CH ₃ CH ₂ NH ₃ )PbI ₃

Figure 2 shows TEM image of the (CH₃CH₂NH₃)PbI₃ deposited on TiO₂ nanoparticles, where the (CH₃CH₂NH₃)PbI₃ dots are clearly seen and sparsely distributed on the TiO₂ surface. This indicates that spin coating of the solution containing CH₃CH₂NH₃I and PbI₂ leads to (CH₃CH₂NH₃)PbI₃ dots on the TiO₂ surface. The average size of the deposited (CH₃CH₂NH₃)PbI₃ is estimated to be about 1.8 nm in diameter.

Figure 2

TEM image of the (CH ₃ CH ₂ NH ₃ )PbI ₃ -deposited TiO ₂ nanoparticles. Arrows indicate (CH₃CH₂NH₃)PbI₃ nanodots.

Figure 3 shows cross-sectional EDS mapping, where Pb and I are well distributed three-dimensionally in the mesoporous TiO₂ film. Atomic percentages from EDS elemental analysis are found to be 1.66% and 4.74% for Pb and I, respectively, which indicates that the ratio of Pb to I is close to 1:3.

Figure 3

Cross-sectional SEM micrographs of (CH ₃ CH ₂ NH ₃ )PbI ₃ -deposited TiO ₂ film and EDS maps for titanium, lead, and iodine.

To determine optical bandgap and valence band position, UV–vis reflectance and UPS measurements are performed. Figure 4a,b,c,d,e,f shows the diffuse reflectance spectra and the transformed Kubelka-Munk spectra for the bare TiO₂ nanoparticle, the powdered (CH₃CH₂NH₃)PbI₃, and the deposited (CH₃CH₂NH₃)PbI₃ on the TiO₂ surface. The dependence of the optical absorption coefficient with the photon energy has been known to help to study the type of transition of electrons and semiconductors' bandgap energy as well [18]. The optical absorption coefficient (α) can be calculated using reflectance data according to the Kubelka-Munk equation [19], $\text{F}\left(R\right)=\alpha =\frac{{\left(1-R\right)}^2}{2R}$, where R is the reflectance data. The incident photon energy (hν) and the optical bandgap energy (E_g) are related to the transformed Kubelka-Munk function, ${\left[\text{F}\left(R\right)\text{hν}\right]}^{\frac{1}{p}}=\text{A}\left(\text{hν}–E_{\text{g}}\right)$, where A is the constant depending on transition probability and p is the index that is related to the optical absorption process. Theoretically, p equals to 2 or ½ for an indirect or direct allowed transition, respectively. The E_g of the bare TiO₂ determined based on indirect transition is 3.1 eV, which is well consistent with the data reported elsewhere [19]. For the case of (CH₃CH₂NH₃)PbI₃, a transformed Kubelka-Munk function can be constructed by plotting [F(R)]² against the photon energy, which is indicative of direct transition. As shown in Figure 4c,d,e,f, an E_g of ca. 2.2 eV is estimated for both the powdered (CH₃CH₂NH₃)PbI₃ and the deposited one. According to UPS spectrum, the valence band energy (E_VB) of (CH₃CH₂NH₃)PbI₃ is determined to be 5.6 eV with respect to vacuum level. Therefore, from the E_g and the E_VB values, conduction band energy (E_CB) is estimated to be 3.4 eV, which is 0.8 eV higher than that of the E_CB for TiO₂ (4.2 eV versus vacuum).

Figure 4

Diffuse reflectance spectra, UPS spectrum, and schematic energy profile for (CH ₃ CH ₂ NH ₃ )PbI ₃ . Diffuse reflectance spectra and the transformed Kubelka-Munk function for (a, b) the bare TiO₂, (c, d) the powdered (CH₃CH₂NH₃)PbI₃, and (e, f) the (CH₃CH₂NH₃)PbI₃ deposited on TiO₂. (g) UPS spectrum and (h) schematic energy profile for (CH₃CH₂NH₃)PbI₃. In UPS spectrum, binding energy was adjusted with respect to He-I (21.22 eV).

Figure 5 shows the photovoltaic property of the (CH₃CH₂NH₃)PbI₃-sensitized solar cell, where I₃⁻/I⁻ redox electrolyte is employed. A photocurrent density of 5.2 mA/cm², a voltage of 0.660 V, and a fill factor of 0.704 are observed at AM 1.5 G one sun (100 mW/cm²) illumination, leading to an overall conversion efficiency of 2.4%. Incident photon-to-current conversion efficiency (IPCE) spectrum shows that the electron excitation starts to occur at around 570 nm, which is consistent with the estimated E_g of ca. 2.2 eV.

Figure 5

Photocurrent density-voltage curve of the (CH ₃ CH ₂ NH ₃ )PbI ₃ -sensitized solar cell under AM 1.5 G one-sun light intensity. The 2 H perovskite-type (CH₃CH₂NH₃)PbI₃-sensitized TiO₂ layer was heated at 100°C for 15 min. The active area and TiO₂ film thickness were 0.323 cm² and 5.4 μm, respectively. The inset shows the IPCE spectrum.

Conclusions

We synthesized a new nanocrystalline sensitizer based on organic–inorganic hybridization. The crystal structure of the synthesized (CH₃CH₂NH₃)PbI₃ was determined to be 2 H perovskite-type orthorhombic phase. The optical bandgap was estimated to be ca. 2.2 eV, and the valence band energy position was determined to be 5.6 eV based on UPS measurement. The conduction band edge position of (CH₃CH₂NH₃)PbI₃ was 0.8 eV higher than that of TiO₂, which allowed injection of photo-excited electrons from (CH₃CH₂NH₃)PbI₃ to TiO₂. Under full sun illumination, the (CH₃CH₂NH₃)PbI₃-sensitized solar cell showed an overall conversion efficiency of 2.4%.