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.
Keywords:(CH3CH2NH3)PbI3; 2H perovskite; Dye-sensitized solar cell; Nanodot; Sensitizer
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 , biomedical imaging sensors , and photovoltaic solar cells . 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 . 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 . Recently, a conversion efficiency of 6.54% at one sun was reported based on perovskite semiconductor (CH3NH3)PbI3, 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.
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 . 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 H2PtCl6xH2O 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  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 hk0, h = 2n for h00, and k = 2n for 0k0) 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  and refined by the Rietveld method with the FULLPROF program . 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 (CH3CH2NH3)PbI3. 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 (CH3CH2NH3)PbI3
Table 2. Unit cell, positional, and thermal parameters for (CH3CH2NH3)PbI3
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 (CH3CH2NH3)PbI3-deposited TiO2nanoparticles. 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 (CH3CH2NH3)PbI3-deposited TiO2film 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 . The optical absorption coefficient (α) can be calculated using reflectance data according to the Kubelka-Munk equation , , where R is the reflectance data. The incident photon energy (hν) and the optical bandgap energy (Eg) are related to the transformed Kubelka-Munk function, , 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 Eg of the bare TiO2 determined based on indirect transition is 3.1 eV, which is well consistent with the data reported elsewhere . 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 Eg 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 (EVB) of (CH3CH2NH3)PbI3 is determined to be 5.6 eV with respect to vacuum level. Therefore, from the Eg and the EVB values, conduction band energy (ECB) is estimated to be 3.4 eV, which is 0.8 eV higher than that of the ECB for TiO2 (4.2 eV versus vacuum).
Figure 4 . Diffuse reflectance spectra, UPS spectrum, and schematic energy profile for (CH3CH2NH3)PbI3. 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 Eg of ca. 2.2 eV.
Figure 5 . Photocurrent density-voltage curve of the (CH3CH2NH3)PbI3-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.
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%.
The authors declare that they have no competing interests
J-HI carried out the synthesis of perovskite materials and the fabrication of solar cell devices. JC and S-JK carried out the X-ray diffraction measurement and structure analysis. N-GP contributed to the conception and design of experiments, data interpretation, and writing of the manuscript. All authors read and approved the final manuscript.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea under contract nos. 2011–0016441, 2011–0030359, and R31-2008-10029 (WCU program) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy under contract no. 20103020010010.
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