Fabrication and Characterization of CH3NH3PbI3 Perovskite Solar Cells Added with Polysilanes
Effects of polysilane additions on CH3NH3PbI3 perovskite solar cells were investigated. Photovoltaic cells were fabricated by a spin-coating method using perovskite precursor solutions with polymethyl phenylsilane, polyphenylsilane, or decaphenyl cyclopentasilane (DPPS), and the microstructures were examined by X-ray diffraction and optical microscopy. Open-circuit voltages were increased by introducing these polysilanes, and short-circuit current density was increased by the DPPS addition, which resulted in the improvement of the photoconversion efficiencies to 10.46%. The incident photon-to-current conversion efficiencies were also increased in the range of 400~750 nm. Microstructure analysis indicated the formation of a dense interfacial structure by grain growth and increase of surface coverage of the perovskite layer with DPPS, and the formation of PbI2 was suppressed, leading to the improvement of photovoltaic properties.
Inorganic-organic solar cells have been developed and studied as next-generation systems. Since organic thin-film solar cells have the advantages of low cost, flexibility, and being lightweight, the photovoltaic and optical properties of the organic solar cells have been studied [1–7]. Polysilane is a p-type semiconductor and has been used as an electrical conductive material and photovoltaic systems [7–14]. Polysilanes are known as σ-conjugate polymers, and their hole mobility is 10−4 cm2 V−1 s−1 . Although the polysilanes could be applied to p-type semiconductors on organic thin-film solar cells, few studies on polysilane-based solar cells have been reported [8, 14, 15].
Recently, inorganic-organic CH3NH3PbI3-based perovskite solar cells have been widely studied because of its easy fabrication process and the high conversion efficiencies compared to the common organic solar cells [16–19]. After a conversion efficiency reached 15% in 2013 , higher efficiencies have been achieved for various processes and device structures, and the conversion efficiency increased above 20% [21–25]. The photovoltaic properties of the perovskite solar cells are fairly dependent on compositions and the crystal structures of the perovskite compounds. Halogen atom doping, such as bromine (Br) or chlorine (Cl), at the iodine (I) sites of the perovskite crystals have been investigated. The doped Cl ions could lengthen the diffusion length of excitons, which resulted in the improvement in the conversion efficiency [26–30]. In addition, studies on metal atom doping, such as tin (Sn) [31, 32], germanium [33, 34], antimony (Sb) [35–40], copper (Cu) [41–44], arsenic (As) , or thallium (Tl) , at the lead (Pb) sites have been performed. The optical absorption ranges of the perovskite solar cells were expanded by Sn or Tl doping [31, 32, 34], and the photoconversion efficiencies were improved by Sb-, Cu-, or As doping. Further doping of cesium , rubidium , or formamidinium (NH=CHNH3) [21, 22] also improved the conversion efficiencies of the perovskite solar cells. Detailed studies on the elemental doping at the I, Pb, and/or CH3NH3 sites are fascinating for effects on the photovoltaic properties and microstructures, and tolerance factors are also valuable to estimate stabilities of the perovskite crystals .
Carrier transport layers are also important for the improvement of the perovskite solar cells. Commercial 2,2,7,7-tetrakis[N,N-di-pmethoxyphenylamino]-9,9-spirobifluorene (spiro-OMeTAD) has been normally used as the hole transport layers. However, spiro-OMeTAD is an expensive organic compound and not so much stable at elevated temperatures, which is one of the crucial issues for the development of the perovskite-type solar cells. In our previous works, several polysilanes have been applied for perovskite solar cells . The conversion efficiencies of CH3NH3PbI3-based solar cells using polysilane-doped hole transport layers were improved compared with those of a conventional CH3NH3PbI3-based solar cells using spiro-OMeTAD.
The purpose of the present work is to investigate photovoltaic properties of CH3NH3PbI3-based solar cells introduced with various polysilanes in the perovskite precursor solutions. Effects of polysilane addition on the photovoltaic properties were investigated by current density-voltage curves and incident photon-to-current conversion efficiencies. Microstructures of the thin films were investigated by X-ray diffraction (XRD) and optical microscopy.
2. Materials and Methods
Figure 1 shows a schematic illustration of fabricated device structure of the present photovoltaic devices. The detailed fabrication process is described in our previous reports [47–49], except for the process of polysilane doping into CH3NH3PbI3 solutions. Fluorine-doped tin oxide- (FTO-) coated glass substrates were cleaned in an ultrasonic bath containing acetone and methanol. After rinsing with ultrapure water and drying under nitrogen gas, the substrates were treated with an ultraviolet-ozone cleaner (Asumi Giken Ltd., ASM 401N) for 15 min. For a compact TiO2 layer, 0.15 and 0.30 M TiOx precursor solutions were prepared from titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich, 0.055 and 0.110 mL) as a solute and 1-butanol (Nacalai Tesque, 1 mL) as a solvent. The precursor solutions were stirred for 12 h at room temperature. The 0.15 M TiOx precursor layer was spin-coated onto the cleaned FTO surface by a spin coater (Mikasa, MS-A100) at 3000 rpm for 30 s and was heated onto a hot plate (As One, ND-1) at 125°C for 5 min. Then, the 0.30 M TiOx precursor layer was spin-coated on the 0.15 M TiOx layer at 3000 rpm for 30 s. This process was repeated twice. After the spin-coating, the 0.30 M TiOx layer was heated onto the hot plate at 125°C for 5 min. The TiOx on the FTO was annealed in an electric furnace (As One, SMF-1) at 500°C for 30 min to form a compact TiO2 layer. After cooling to room temperature, a mesoporous TiO2 layer was spin-coated on the compact TiO2 layer at 5000 rpm for 30 s, where the TiO2 paste was prepared by diluting TiO2 powder (Aerosil, P-25, 100 mg) in ultrapure water and adding poly(ethylene glycol) (molecular weight: 20000, Nacalai Tesque, 10 mg), acetylacetone (Sigma-Aldrich, 10 mL), and surfactant (Sigma-Aldrich, Triton X-100, 5 mL) for 30 min. After heating at 125°C for 5 min, the samples were annealed in the electric furnace at 500°C for 30 min. For the preparation of CH3NH3PbI3 layer, CH3NH3I (Showa Chemical, 98.8 mg) and PbI2 (Sigma-Aldrich, 289.3 mg) powders were dissolved in γ-butyrolactone (Wako Pure Chemical Industries, 0.275 mL), and N,N-dimethylformamide (Sigma-Aldrich, 0.225 mL) was prepared at 60°C. The molar ratio of the solutes is 1 : 1. Three kinds of polysilanes, poly(methyl phenylsilane) (PMPS, Osaka Gas Chemicals, OGSOL SI-10-10, 3 mg), poly(phenylsilane) (PPS, Osaka Gas Chemicals, OGSOL SI-20-10, 3 mg), and decaphenylcyclopentasilane (DPPS, Osaka Gas Chemicals, OGSOL SI-30-10, 3 mg), were used as additives to the perovskite solution, and structures of these polysilanes are shown in Figure 1. In the present work, the perovskite layers are described as CH3NH3PbI3, +PMPS, +PPS, and +DPPS. The CH3NH3PbI3 solution was spin-coated on the mesoporous TiO2 layer at 2000 rpm for 60 s. The samples were annealed onto the hot plate at 100°C for 15 min to form a CH3NH3PbI3 layer. After cooling to room temperature, hole transport layers were spin-coated. For the hole transport layer, a solution of spiro-OMeTAD (Sigma-Aldrich, 36.1 mg) in chlorobenzene (Wako Pure Chemical Industries, 0.5 mL) was mixed with a solution of lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI, Tokyo Chemical Industry, 260 mg) in acetonitrile (Nacalai Tesque, 0.5 mL) for 12 h. The former solution with 4-tert-butylpyridine (Aldrich, 14.4 μL) was mixed with the Li-TFSI solution (8.8 μL) for 30 min at 70°C. All procedures for preparation of the thin films were carried out in ordinary air. Finally, gold (Au) electrodes were evaporated as top electrodes using a metal mask for the patterning.
The - characteristics of the photovoltaic devices were measured using a solar simulator composed of light source (San-ei Electric, XES-301S), where solar irradiance of 100 mW cm−2 was calibrated before measurements. The measurements were performed by source measure unit (Keysight, B2901A Precision SMU). The scan rate and sampling time were ~0.08 V s−1 and 1 ms, respectively. The photovoltaic devices were irradiated through the backside of FTO-coated glass substrate. The effective area of the devices was 0.090 cm2. IPCE spectra of the devices were also collected using total quantum efficiency solutions for solar cells (Enli Technology, QE-R). The microstructures of the CH3NH3PbI3 films were investigated using an X-ray diffractometer (Bruker, D2 PHASER) and an optical microscope (Nikon Eclipse E600). All measurements were performed at room temperature in ambient air.
3. Results and Discussion
Figure 2 shows the - characteristics of the TiO2/CH3NH3PbI3(polysilane)/spiro-OMeTAD photovoltaic cells under illumination, which indicate the effects of the polysilane addition. Forward and reverse scans are indicated in the figure as indicated by arrows, and the measured photovoltaic parameters (reverse scan) of the present CH3NH3PbI3(polysilane) cells are summarized in Table 1. A small hysteresis of reverse and forward scans was observed for the cells. Ordinary CH3NH3PbI3 cells provided a power conversion efficiency () of 5.82%, as listed in Table 1. By adding the present three polysilanes, open-circuit voltages () were increased, as observed in Figure 2. When the PMPS or PPS was added to the perovskite, short-circuit current density () was decreased. On the other hand, by adding the DPPS, the value and shunt resistance () were fairly increased, and the series resistance () was decreased compared with the PMPS and PPS, which resulted in the improvement of the conversion efficiency to 10.46%. The fill factor (FF) is almost the same as that of the ordinary cell.
Figure 3 shows IPCE spectra of the CH3NH3PbI3(polysilane) devices. The present cells show photoconversion efficiencies between 320 and 800 nm, which corresponds to an energy gap of 1.55 eV for the CH3NH3PbI3 . The IPCE was improved in the range of 400~750 nm by adding DPPS to the perovskite layer, leading to the increase of values, as shown in Figure 2.
Optical microscopic images of ordinary CH3NH3PbI3 cell are shown in Figure 4(a), and microparticles with sizes of 5~20 μm are observed. When the polysilanes were added to the perovskite phase, microparticles with homogeneous size distribution were formed, as shown in Figures 4(b)–4(d). In particular, by the DPPS addition, the crystal growth of the perovskite phase was observed, and the distances between the particles were decreased, leading formation of more dense microstructures, as observed in Figure 4(d).
Figure 5 shows XRD patterns of the present polysilane-added CH3NH3PbI3 solar cells. When the CH3NH3PbI3 has a tetragonal structure, an XRD peak corresponding to cubic 200 should be divided into tetragonal 004 and 220, as previously reported in . The calculated XRD pattern also indicates splitting of diffraction reflections of 200 to 004/220 , and therefore the diffraction reflections can be indexed by a cubic crystal system ( ) for the present perovskite thin films. The diffraction peaks indicated by FTO and TiO2 are from the FTO substrate and TiO2 mesoporous layer, respectively. For the standard CH3NH3PbI3 cell, a diffraction reflection of PbI2 is observed, as indicated by an arrow. When the polysilanes were added to the perovskite phase, formation of the PbI2 was suppressed. In addition, peak intensities of 100 of the perovskite phase increased with adding the polysilanes, which indicated crystal growth of the perovskite phase. It is considered that the crystal growth and PbI2 suppression might be related with affinity between the γ-butyrolactone, N,N-dimethylformamide, CH3NH3I, PbI2, and polysilanes. X-ray diffraction angles of the perovskite compounds were calibrated based on the 111 reflection of Au  of the cell electrodes. The XRD reflections due to the perovskite compounds were measured and indexed to determine the lattice distances. Then, the lattice constants of the perovskite crystals were determined from the lattice distances and indices by using a least squares method (CellCalc program ), and they are listed in Table 2. The differences of lattice constants are small and are almost constant for these cells.
Two assumed mechanisms could be considered for the improvement in the photoconversion efficiencies. The first mechanism is the increase and homogenization of the grain sizes of the perovskite crystals by adding polysilanes as observed optical microscopy images in Figure 4. This microstructure promotes the formation of dense interfacial structures and increase of surface coverage. When the grain sizes increase, grain boundary areas that cause carrier scattering decrease, which leads to the increase of the current density . The present homogeneous structure could also suppress the formation of PbI2, as observed in Figure 5. For the cells with PMPS and PPS, the perovskite domains are isolated, and the cell performances are severely affected by the coverage. Then, the pin holes provide short circuits between n-type semiconductor TiO2 and p-type semiconductor spiro-MeOTAD, which resulted in the lower photovoltaic performances.
The second mechanism is the electronic structure change by the polysilanes. By adding polysilanes, the open-circuit voltages increased, as shown in Figure 2. From IPCE spectra in Figure 3, the bandgap energy of the perovskite crystals with PMPS and PPS increased from 1.55 eV to 1.57 eV, which leads to the increase of the values. This also corresponds to the decrease of lattice constants of cubic perovskite by addition of polysilanes. By adding DPPS, on the other hand, the bandgap energy seems to be almost the same as that of the standard perovskite crystal. Since the DPPS has more phenyl groups at the side of Si atoms compared with PMPS and PPS, carrier transport after the carrier separation might be facilitated by π-electrons on the phenyl groups, which affected the increase of IPCE in the range of 400~750 nm and the values. The existence of the phenyl groups might also reduce the electrical resistance of the DPPS-added CH3NH3PbI3, which would result in the increase of the values.
Polymer-templated nucleation and growth had been reported as a method for crystal engineering of perovskites . Poly(methyl methacrylate) (PMMA) provided heterogeneous nucleation which would be orders of magnitude faster than the homogeneous nucleation due to the lowering of the nucleation free energy barrier . The PMMA also seems to slow down the growth rate of perovskite crystals by forming an intermediate adduct with PbI2, which would provide the randomly formed nuclei, and would also lead to smooth thin films with few defects and large oriented perovskite grains [54–56]. The main improvement by the addition of the DPPS is an increase of the film coverage due to enlargement of the perovskite domains as observed in the optical micrograph of Figure 4(d). Since the DPPS has more phenyl groups at the side of Si atoms compared with PMPS and PPS, molecular interactions created by the phenyl groups would affect the nucleation and crystal growth. Introduction of quantum dots such as PbS  or SnS  could also control the energy band structure. Further studies are needed for the growth of the perovskite phase and the control of the energy bands.
CH3NH3PbI3 perovskite solar cells added with polysilanes were fabricated and characterized. Effects of PMPS, PPS, or DPPD addition to perovskite precursor solutions on the photovoltaic properties and the microstructures were investigated. The values were increased by applying these PMPS, PPS, or DPPD, and the IPCE was also increased by the DPPS addition in the range of 400~750 nm, leading to the improvement of the values. Formation of PbI2 was also suppressed by the polysilane addition. Microstructure observation of the perovskite layer with DPPS indicated increase of surface coverage to form a dense interfacial structure by grain growth, which resulted in the improvement of conversion efficiencies above 10%. Lots of phenyl group in the DPPS might contribute to the carrier transport in the perovskite layers.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was partly supported by the Satellite Cluster Program of the Japan Science and Technology Agency (JST).
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