Patent Application
20160126483
This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2014-0152799 filed on Nov. 5, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
The present disclosure relates to a ZSO-based perovskite solar cell and a method for preparing the same.
Organolead halide-based hybrid solar cells (or perovskite solar cells) are being spotlighted a lot in recent years as a new highly efficient solid-state thin film solar cell. The structure of the first perovskite solar cell stems from the dye-sensitized solar cell (DSSC), where the ruthenium-based or organic dye sensitizers have been replaced with organolead halide perovskites such as CH3NH3PbI3 (methylammonium lead iodide; MALI). Afterward, some structural variations such as the mesoscopic heterojunction structure, planar heterojunction structure, incorporation of p-/n-type organic semiconductors, and so forth have been reported.
Most of the solar cells with planar structures are fabricated through either a solution process or thermal evaporation. In spite of these structural variations, the sensitized solar cell that incorporates a mesoscopic TiO2 electron-transporting layer is still widely investigated.
Although development of a photoanode which exhibits faster charge injection from the sensitizer and electron diffusion through the nanoparticle layer than the TiO2 photoanode while allowing for easy control of optoelectronic properties through composition change and impurity addition is required, no effective research result has been reported yet.
The present disclosure is directed to providing a photoanode which exhibits faster charge injection from a sensitizer and electron diffusion through a nanoparticle layer than a TiO2 photoanode while allowing for easy control of optoelectronic properties through composition change.
In an aspect, the present disclosure provides a solar cell including: (a) a transparent substrate; (b) a first Zn2SnO4 layer formed on the transparent substrate; (c) a second Zn2SnO4 layer formed on the first Zn2SnO4 layer; (d) a CH3NH3PbI3 layer formed on the second Zn2SnO4 layer; and (e) a hole-transporting material layer formed on the CH3NH3PbI3 layer.
In another aspect, the present disclosure provides a method for preparing a solar cell, including: (A) forming a first Zn2SnO4 layer on a transparent substrate; (B) forming a second Zn2SnO4 layer on the first Zn2SnO4 layer; (C) forming a CH3NH3PbI3 layer on the second Zn2SnO4 layer; and (D) forming a hole-transporting material layer on the CH3NH3PbI3 layer.
In accordance with the present disclosure, a photoanode which exhibits faster charge injection from a sensitizer and electron diffusion through a nanoparticle layer than a TiO2 photoanode while allowing for easy control of optoelectronic properties through composition change and impurity addition can be prepared.
Hereinafter, various aspects and exemplary embodiments of the present disclosure will be described in more detail.
In an aspect, the present disclosure provides a solar cell including: (a) a transparent substrate; (b) a first Zn2SnO4 layer formed on the transparent substrate; (c) a second Zn2SnO4 layer formed on the first Zn2SnO4 layer; (d) a CH3NH3PbI3 layer formed on the second Zn2SnO4 layer; and (e) a hole-transporting material layer formed on the CH3NH3PbI3 layer.
That is to say, according to an aspect of the present disclosure, the conventional TiO2 layer is replaced by a compact first Zn2SnO4 layer and a porous second Zn2SnO4 layer which are adjacent to each other. The first Zn2SnO4 layer formed instead of the TiO2 layer may prevent charge recombination from an FTO substrate and minimize loss of incident sunlight. And, due to the second Zn2SnO4 layer formed instead of the TiO2 layer, a photoanode which exhibits faster charge injection from a sensitizer and electron diffusion through a nanoparticle layer may be obtained and photogenerated charges may be collected faster because of faster photoresponse saturation.
In addition, it is advantageous in that optoelectronic properties can be controlled easily through composition change and impurity addition.
In an exemplary embodiment, the first Zn2SnO4 layer has a compact (dense) structure. Specifically, it has a compact structure with a porosity of 0-5%, specifically 0-3%, more specifically 0-1%. The second Zn2SnO4 layer has a porous structure. Specifically, it has a porosity of 50-70% and an average pore size of 10-100 nm.
It was confirmed that, when the first Zn2SO4 layer has a porosity within the above range, the components on top of the FTO substrate may be physically separated, recombination resulting from reverse transportation of electrons from the FTO substrate toward the cell may be prevented, and loss of incident light may be reduced significantly.
It was also confirmed that, when the second Zn2SO4 layer has a porosity and an average pore size within the above ranges, photoelectrons generated from CH3NH3PbI3 are injected to the first Zn2SO4 layer nanoparticle photoelectrode and current-collecting efficiency is significantly improved because conductivity is 10 times or more greater than the existing TiO2 nanoparticle layer.
In another exemplary embodiment, the first Zn2SnO4 layer may have a thickness of 100-120 nm. It was confirmed that the remarkable effect of charge recombination prevention and electron lifetime increase is achieved only when the thickness of the compact ZSO layer is in the range of 100-120 nm and such a remarkable effect is not achieved when the thickness of the first Zn2SnO4 layer is outside the range.
In another exemplary embodiment, the second Zn2SnO4 layer may have a thickness of 250-350 nm. It was confirmed that the fill factor and open-circuit voltage decrease greatly when the thickness of the second Zn2SnO4 layer is smaller than 250 nm and that the short-circuit current decreases greatly when the thickness of the second Zn2SnO4 layer exceeds 350 nm.
In another exemplary embodiment, the transparent substrate may be selected from fluorine tin oxide (FTO), indium tin oxide (ITO), aluminum zinc oxide (AZO), niobium tin oxide (NTO) and zinc tin oxide (ZTO).
In another aspect, the present disclosure provides a method for preparing a solar cell, including: (A) forming a first Zn2SnO4 layer on a transparent substrate; (B) forming a second Zn2SnO4 layer on the first Zn2SnO4 layer; (C) forming a CH3NH3PbI3 layer on the second Zn2SnO4 layer; and (D) forming a hole-transporting material layer on the CH3NH3PbI3 layer.
In an exemplary embodiment, the step (A) may be performed by conducting first coating of a solution of ZnCl2 and SnCl2 on the transparent substrate and then conducting first annealing, thereby forming the first Zn2SnO4 layer. In particular, when the above materials are used as Zn and Sn precursors, a thin and uniform layer may be formed.
In another exemplary embodiment, a molar ratio of the Zn precursor and the Sn precursor in the solution may be 1.9-2.1. When the molar ratio of the Zn precursor and the Sn precursor in the solution is smaller than the lowest limit or exceeds the highest limit, a secondary phase may be formed in addition to the Zn2SnO4 layer.
In another exemplary embodiment, the first coating may be spin coating and the first annealing may be conducted at 300-400° C. for 5-20 minutes. When the temperature of the first annealing is below the lowest limit, anions may remain in the layer. And, when it exceeds the highest limit, surface roughness of the layer may increase or deterioration of the FTO substrate may occur due to increased crystal size. And, when the first annealing time is shorter than the lowest limit or exceeds the highest limit, anions may remain in the layer.
In another exemplary embodiment, the first coating may be conducted by performing spin coating at 2000-4000 rpm for from 20 seconds to 1 minute. When the first coating time is shorter than the lowest limit, it may be difficult to form a uniform layer. And, it may be difficult to form a uniform layer and the layer may become excessively thick when the rotation speed of the first coating is below the lowest limit, and the layer may become excessively thin or the FTO substrate below may be exposed when the rotation speed exceeds the highest limit.
In another exemplary embodiment, the step (B) may be performed by conducting second coating of a Zn2SnO4 nanoparticle on the first Zn2SnO4 layer and then conducting second annealing.
In another exemplary embodiment, the second coating may be conducted by performing spin-coating a Zn2SnO4 nanoparticle paste diluate. When the paste is not diluted, it is impossible to form a thin layer with a uniform thickness of hundreds of nanometers due to high viscosity. In addition, when doctor blade coating or screen printing is used instead of the spin coating, it is impossible to form a thin layer with a thickness of hundreds of nanometers.
In another exemplary embodiment, the diluate may be obtained by diluting a Zn2SnO4 nanoparticle paste with a terpineol solvent. In the present disclosure, an alcohol-based solvent such as terpineol, ethanol, isopropanol, etc. or a mixture solvent thereof may be used to prepare the diluate. Particularly, when terpineol is used as a solvent in particular, it is advantageous in that the functions of the additives added together may be maintained because terpineol is a constituent of the paste. However, when other solvents are used, there may occur a problem that the additives are dissolved.
In another exemplary embodiment, the second annealing may be conducted at 450-550° C. for from 15 minutes to 1 hour. When the temperature of the second annealing is below the lowest limit, organic materials included in the coating solution may remain in the layer. And, when it exceeds the highest limit, disruption of the pore structure or deterioration of the FTO substrate may occur due to increased nanoparticle size. When the second annealing time is shorter than the lowest limit, organic materials included in the coating solution may remain in the layer. And, when the time exceeds the highest limit, disruption of the pore structure or deterioration of the FTO substrate may occur due to increased nanoparticle size.
In another exemplary embodiment, the step (C) may be performed by (C′) coating a PbI2 solution on the second Zn2SnO4 layer and conducting third annealing and then (C″) immersing in a CH3NH3I solution and conducting fourth annealing.
In another exemplary embodiment, the coating of the PbI2 solution in the step (C′) may be performed by applying the PbI2 solution and then conducting spin coating after waiting for from 10 seconds to 1 minute, and the immersion in the CH3NH3I solution in the step (C″) may be performed by immersing in the CH3NH3I solution and then waiting for from 20 seconds to 1 minute. When the above-described time passes after the PbI2 solution has been applied, the remaining of pores after the layer has been formed may be prevented as the mesopores of the second Zn2SnO4 layer are filled. And, when the above-described time passes after the immersion in the CH3NH3I solution, a uniform CH3NH3PbI3 layer may be formed. When the time is shorter than the lowest limit, unreacted PbI2 may remain. And, when it exceeds the highest limit, the formed CH3NH3PbI3 may be dissolved again in the solvent.
In another exemplary embodiment, the spin coating may be conducted at 6000-7000 rpm for 20-40 seconds. When the rotation speed of the spin coating is below the lowest limit, PbI2 may remain even after the immersion in the CH3NH3I solution because the PbI2 layer becomes excessively thick. And, when it exceeds the highest limit, the mesopores of the oxide layer may not be formed completely or the upper layer may not be formed adequately. When the spin coating time is shorter than the lowest limit, a nonuniform PbI2 layer may be formed.
In another exemplary embodiment, the third annealing may be conducted at 70-90° C. for 15-30 minutes, and the fourth annealing may be conducted at 70-90° C. for 15-30 minutes. When the third annealing temperature is below the lowest limit, crystallization of PbI2 may be insufficient. And, when it exceeds the highest limit, PbI2 may remain even after the immersion in the CH3NH3I solution due to excessive crystallization of PbI2. When the fourth annealing temperature is below the lowest limit, crystallization of CH3NH3PbI3 may be insufficient. And, when it exceeds the highest limit, phase separation of CH3NH3PbI3 may occur.
In another exemplary embodiment, the PbI2 solution may be a solution wherein PbI2 is dissolved in DMF, and the CH3NH3I solution may be a solution wherein CH3NH3I is dissolved in isopropanol. To prepare the PbI2 solution, dimethylformamide (DMF), γ-butyrolactone (GBL), dimethyl sulfoxide (DMSO) and a mixture solvent thereof may be used. In particular, when DMF is used as a solvent, it is easy to form a uniform layer. And, in order to prepare the CH3NH3I solution, an alcohol-based solvent such as isopropanol, ethanol, etc. or a mixture solvent thereof may be used. In particular, when isopropanol is used as a solvent, it is advantageous in that a uniform reaction can be achieved and redissolution after the CH3NH3PbI3 layer has been formed is relatively slow.
In another exemplary embodiment, the step (D) may be performed by spin-coating a spiro-OMeTAD solution on the CH3NH3PbI3 layer.
The spin coating may be performed at 3000-5000 rpm for 20-40 seconds.
Hereinafter, the present disclosure will be described in more detail through examples. However, the scope and content of the present disclosure should not be interpreted as being reduced or limited by the examples. It will be obvious that those of ordinary skill can easily carry out the present disclosure based on the disclosure of the present disclosure including the examples even when specific experimental results are not provided and that such changes and modifications are included in the scope of the appended claims.
Materials
2,2′,7,7′-Tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) was purchased from Merck and CH3NH3I was synthesized from methylamine and hydroiodic acid according to the literature. All other chemicals were purchased from Sigma Aldrich and used as received unless stated otherwise.
F-doped SnO2 (FTO) substrates (TEC8, Pilkington) were patterned using a laser scriber (ML20-PL-R, Kortherm Science) and cleaned by sonication in ethanol and isopropanol, followed by UV-ozone treatment before coating a compact layer.
For preparation of a ZSO-based solar cell, a compact ZSO layer was formed on the patterned FTO substrate by spin-coating a solution of ZnCl2 and SnCl2 (Zn/Sn ratio=2) at room temperature at 3000 rpm for 30 seconds, followed by annealing at 350° C. for 10 minutes.
Then, a synthesized ZSO nanoparticle paste was diluted with terpineol and spin-coated on thereon, followed by annealing at 500° C. for 30 minutes. The thickness of the mesoscopic ZSO layer could be controlled by changing the speed of spin coating.
A PbI2 solution (462 mg/cc in N,N-dimethylformamide; DMF) was spread on the ms-ZSO layer, infiltrated to the mesopores for 20 seconds and then spin-coated at 6500 rpm for 30 seconds, followed by annealing at 80° C. on a hot plate. Then, the formed PbI2/ms-oxide layer was immersed in a CH3NH3I solution (10 mg/cc in isopropanol) for 30 seconds to form a MALI/ms-oxide layer, followed by annealing at 80° C. on a hot plate.
Then, a spiro-OMeTAD solution prepared according to the literature with slight modification (i.e., exclusion of the Co complex) was spin-coated thereon at 4000 rpm for 30 seconds to form a hole-transporting material (HTM) layer with a thickness of about 110 nm.
For preparation of a TiO2-based perovskite solar cell, a compact TiO2 (c-TiO2) layer was formed by spin-coating a titanium diisopropoxide bis(acetylacetonate) solution (75 wt % in isopropanol) mixed with n-butanol (1:11 in volume ratio) on the patterned FTO substrate at 500 rpm for 5 seconds, at 1000 rpm for 5 seconds and at 2000 rpm for 40 seconds, in sequence, followed by annealing at 470° C. for 30 minutes.
Then, a commercially available TiO2 paste (Dyesol 18NRT, Dyesol) was diluted with ethanol (2:7 weight ratio) and spin-coated, followed by annealing at 500° C. for 30 minutes. The thickness of the mesoscopic TiO2 layer (ms-TiO2) could be controlled to 300 nm by adjusting the spin coating conditions. The whole processes after the formation of the mesoscopic oxide layer were performed in a fumed hood under an ambient atmosphere.
A PbI2 solution (462 mg/cc in N,N-dimethylformamide; DMF) was spread on the ms-TiO2 layer, infiltrated to the mesopores for 20 seconds and then spin-coated at 6500 rpm for 30 seconds, followed by annealing at 80° C. on a hot plate. Then, the formed PbI2/ms-oxide layer was immersed in a CH3NH3I solution (10 mg/cc in isopropanol) for 30 seconds to form a MALI/ms-oxide layer, followed by annealing at 80° C. on a hot plate.
Then, a spiro-OMeTAD solution prepared according to the literature with slight modification (i.e., exclusion of the Co complex) was spin-coated thereon at 4000 rpm for 30 seconds to form a hole-transporting material (HTM) layer with a thickness of about 110 nm.
The samples prepared in Example and Comparative Example were transferred to a thermal evaporation chamber (SJH-2A, Ultech) and a gold electrode with a thickness of about 100 nm was deposited using a mask for electrode patterning. The active area of each solar cell was measured to be generally 0.15-0.2 cm2 using an optical microscope.
The microstructure and crystallographic structure of the samples prepared in Example and Comparative Example were analyzed by filed-emission scanning electron microscopy (FE-SEM; S-4200, Hitachi) and X-ray diffraction (XRD; D-max 2500/server, Rigaku). Optical properties were characterized by UV-Vis spectroscopy (Lambda 35, Perkin Elmer). Spectral photoresponses were measured using an incident photon-to-current conversion efficiency measurement system (PV Measurements) and the current density-voltage curves of the solar cells with the portion other than the active area masked could be obtained using a solar simulator (Peccell Technology, 100 mW/cm2, AM 1.5) and a potentiostat (CHI 608C, CH Instruments). The light intensity of the solar simulator was calibrated using a reference cell (PV Measurements). Time constants for electron transport were measured with a weak laser pulse at 532 nm superimposed on a relatively large bias illumination at 680 nm using a transient photocurrent-voltage measurement setup. The transient photocurrent response was recorded at different frequencies under a 550 nm monochromatic beam.
Structural Observation with Preparation Steps
The perovskite solar cells were fabricated via the two-step process as described above. Specifically, the ZSO or TiO2 compact layer was deposited on the patterned FTO substrate by spin coating and then the diluted ZSO or TiO2 nanoparticle paste was spin-coated to form the mesoscopic ZSO or TiO2 layer. After annealing the compact/mesoscopic oxide layers, the PbI2/oxide layer was prepared by spin-coating the solution of PbI2 in DMF, followed by immersing in the methylammonium iodide solution to form the methylammonium lead iodide/oxide film. Then, the hole-transporting layer (spiro-OMeTAD) and a current-collecting layer (Au) were deposited by spin coating and thermal evaporation, respectively. Crystallographic structures and cross-sectional SEM images at various preparation steps are shown in
It was also found out through experiments that the presence of the compact ZSO layer as opposed to the compact TiO2 layer is crucial for having the ZSO-based perovskite solar cell work properly.
Blocking Effect Depending on Thickness of ZSO Compact Layer
In addition to the significantly retarded back-electron transfer, a significantly increased electron lifetime was observed only when the thickness of the compact ZSO layer was in the range from 100 nm to 120 nm, although the data were not presented.
Control of Thickness of ZSO Mesoscopic Layer
Effect of Thickness of ZSO Mesoscopic Layer
Based on the result of investigating the electron-blocking effect with various compact ZSO layer thicknesses, the effect of the mesoscopic ZSO layer thickness was investigated with the compact ZSO layer fixed at 110 nm. As a result, it was found out that the fill factor and the open-circuit voltage decrease greatly when the thickness is smaller than 250 nm and that the short-circuit current decreases greatly when the thickness exceeds 350 nm.
Current Density-Voltage Curve of ZSO Substrate Perovskite Solar Cell
The short-circuit current, the open-circuit voltage, the fill factor and the conversion efficiency were 13.78 mA/cm2, 0.83V, 61.4% and 7.02%, respectively. When compared with the TiO2-based solar cell with similar geometry and synthetic procedure (JSC≈20 mA/cm2, VOC≈1 V, FF≈70%, η≈14%), the short-circuit current, the open-circuit voltage and the fill factor are smaller by 31%, 17% and 12%, respectively, resulting in 50% decrease in the conversion efficiency.
The smaller short-circuit current can be explained by the spectral photoresponse as shown in the EQE curve in
Comparison of J-V Curves of Perovskite Solar Cells Having TiO2 and ZSO Compact Layers
Comparison of Transmittance of Perovskite Solar Cells with Different ZSO Compact Layer Thicknesses
Comparison of Photoresponse of Perovskite Solar Cells Having TiO2 and ZSO Compact Layers
As described above, the open-circuit voltage and fill factor increased with the increasing thickness of the mesoscopic ZSO layer, whereas the short-circuit current decreased with the increasing thickness except for the thinnest one. As a result, the perovskite solar cell having the 300-nm thick mesoscopic ZSO layer showed the highest conversion efficiency of 7.02%, which can be further improved by optimizing the fabrication process. In particular, the ZSO-based perovskite solar cell exhibited faster electron diffusion by 10 times and superior charge-collecting capability compared to the TiO2-based solar cell with similar solar cell performance. Accordingly, ZSO is very promising as an alternative to the commonly used TiO2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2014-0152799 | Nov 2014 | KR | national |
