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Effect of the vapor diffusion and improved light harvesting for Perovskite-Cu2ZnSnS4 hybridized solar cells
Organic Electronics 59 (2018) 190–195
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Effect of the vapor diffusion and improved light harvesting for PerovskiteCu2ZnSnS4 hybridized solar cells
T
Shih-Jen Lina, Jyh-Ming Tinga,∗, Chia-Tsung Hungb, Yaw-Shyan Fub,∗∗ a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Cu2ZnSnS4 (CZTS) Grain growth Grain boundaries
In this study, a novel hybridized perovskite-Cu2ZnSnS4 solar cell has been demonstrated. The CH3NH3PbI3 layer was synthesized using a facile one-step solution process under ambient conditions. The precursor solution consists of an equimolar methylammonium ion [MA+] and [PbI3−] anion in heptane solvent. The vapor-assisted crystal growth at the grain boundaries occur due to the methylamine (CH3NH2, MA). Moreover, 1 wt% Cu2ZnSnS4 (CZTS) nanoparticles (NPs) was added to the solution to enhance not only the absorption from visible to infrared regions and but also the transfer of the photogenerated charge carriers, leading to reduced charge recombination. The device structure was glass/ITO/1 wt% CZTS NPs hybrid poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS)/CH3NH3PbI3 (MAPbI3)/C60/BCP/Al. The obtained MAPbI3 perovskitebased solar cell shows a short-circuit current density of 19.00 mA/cm2, an open-circuit voltage of 0.84 V, a fill factor of 0.47, and a power conversion efficiency (PCE) of 7.55%.
1. Introduction In recent years, inorganic−organic perovskite solar cells have been highly considered due to their long electron-hole diffusion lengths, low temperatures processing, and outstanding photoelectric properties [1–3]. Organic halide perovskites have ABX3 crystal structures (A = organic cation, alkali; B = Ge, Sn, Pb and X = halide), where the band gap can be adjusted through varying material compositions. The typical CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, CH3NH3PbI3-xClx, CH3NH3PbBr3-xClx, and CH3NH3PbBr3-xIx Perovskites have been extensively investigated in the field of photodetectors [4–6]. In 2009, Miyasaka's group used MAPbI3 perovskites for dye-sensitized solar cells for the first time [7]. Recent reports have shown higher efficiencies for various device structures. Both single and multiple solution processes have been developed, including solid-state structures with n-i-p mesoscopic TiO2 [8,9], super-meso structures Al2O3 scaffold [10], and nickel oxide-based structures with a p-i-n planar heterojunction (PHJ) architecture [11–13]. Furthermore, through a vapor-assisted solution-process deposition technique, in-situ chemical vapor deposition enables a flatter morphology and improved grain size via gas-solid crystallization [14,15]. Lv's et al. employed ternary CuInS2 QDs as a hole transport materials (HTM), where their results strongly suggested that a wide band gap of metal chalcogenide HTM is more efficient with regard to
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J.-M. Ting), [email protected] (Y.-S. Fu).
∗∗
https://doi.org/10.1016/j.orgel.2018.05.010 Received 17 April 2018; Received in revised form 8 May 2018; Accepted 8 May 2018 1566-1199/ © 2018 Published by Elsevier B.V.
achieving a high open circuit voltage (Voc) [16]. These various types of metal chalcogenide thin film solar cells based on the copper-chalcopyrite family of semiconductors: CuIn1−xGaxSe2 (CIGS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe). Among them, CZTS is the most appropriate for thin film solar cells because non-toxic, earth-abundant, and inexpensive. Moreover, CZTS also has a high absorption coefficient greater than 10−4 cm2, which making it suitable as a solar-cell absorber. Many studies have shown improvement photocatalytic performance of the open-circuit voltage of solar cell by adjusting the band gap and recombination rate [17,18]. As an example, outstanding open circuit voltage of 1.353 V via a tandem device architecture has been achieved [19]. However, perovskite materials are unstable in polar solvents. Thus, providing more stable constituents of HTM for perovskite-based photovoltaics is needed [20]. Pérez-Prieto et al. reported the highly dispersible and photostable lead perovskite-octadecylammonium bromide (PODA) by using both toluene and (dimethylformamide) DMF [21]. The perovskite could stable both in colloidal solution in toluene and solid in DMF + toluene. On the other hand, Oleh Vybornyi et al. synthesized perovskite nanocrystal (NCs) with replacing DMF using 1-octadecene (ODE) with the presence of oleylamine and oleic acidv [22]. Therefore, herein, we report high crystalline MAPbI3 films prepared by a one-step methylamine (MA) vapor-assisted solution-precipitation process with polar-free solvents.
Organic Electronics 59 (2018) 190–195
S.-J. Lin et al.
Fig. 2. XRD patterns of as-synthesized (a) kesterite CZTS NPs, tetragonal MAPbI3 NCs with and without MA vapor-assisted treatment, (b) peaks right shift through MA vapor treatment.
Fig. 1. Schematic illustration of (a) MAPbI3 was synthesized in glass petri dish 9.5 cm in diameter; and, (b) the basic device architecture of the glass/ITO/1 wt % Cu2ZnSnS4 hybrid PEDOT:PSS/ MAPbI3 with MA vapor-assisted treatment/ C60/BCP/Al layers.
2.2. Synthesis of MAPbI3 NCs MAPbI3 was synthesized by mixing 3 mL 0.1 M MA+ precursor which was diluted with n-Heptane (C-ECHO, > 99%) and 1 mL 0.1 M PbI3−. Meanwhile, the rubyred CH3NH3PbI3 NCs were precipitated from the reaction in Fig. 1(a).
We demonstrated that MA vapor-enhanced grains growth and crystal size. Moreover, the addition of CZTS NPs hybrid with PEDOT:PSS has been demonstrated to not only form p-type HTM but also broadened the infrared region for a PHJ perovskite device.
2.3. Synthesis of Cu2ZnSnS4 NPs 2. Materials and methods +
2.1. Materials and preparation of the MA , PbI3
−
The CZTS NPs were fabricated via a conventional hydrothermal synthesis method as reported in our previous work [23]. Briefly, a certain amount of thiourea (NH2CSNH2, Riedel-de Haën, 99%) was dissolved in DI-water. Subsequently, zinc chloride (ZnCl2, Merck, 98%), stannous chloride dehydrate (SnCl2·2H2O, Shimakyu, 98%), and drop of the diethylenediamine (C4H13N3, Panerac quimica sa, 98%) as chelating regent, finally copper (II) chloride anhydrous (CuCl2, Choneye pure chemicals Co., Ltd., 98%) were added in the solution. The obtained mixture was then transferred in to autoclave and the temperature was set at 180 °C for 72 h.
precursors
All chemicals were of analytical grade and used as received without further purification. The methylamine vapor was obtained from methylamine solution (CH3NH2, SHOWA, 40%). The vapor was distilled in vacuum condition at 40 °C and injected into oleic Acid (OA, SHOWA, 90%) to form an anhydrous vapor-liquid salt as an [MA+][OA−] precursor. The Lead (II) Iodide (PbI2, SHOWA, 98%) was dispersed in 1Octadecene (ODE, Acros, 90%) and stirred for 30 min at 150 °C, after which it was purified in a vacuum oven at 100 °C to remove moisture. Finally, oleylamine (OAm, across, approximate C18-content 80–90%) with OA as the surfactant was poured into the mixture until PbI2 dissolved completely to form a transparent precursor [PbI3−][OAm+].
2.4. Device system The obtained active area is 0.06 cm2 of pre-patterned glass/indiumtin-oxide (ITO) (15Ω/sq) was sequentially cleaned with detergent, distilled water, acetone and isopropyl alcohol 30 min, respectively by 191
Organic Electronics 59 (2018) 190–195
S.-J. Lin et al.
Fig. 3. Surface morphologies of MAPbI3 NCs (a) without MA vapor-assisted, (b) with MA vapor-assisted, (c) cross-section SEM morphology of as-synthesized MAPbI3 NCs with MA vapor-assisted, TEM analysis of (d) and the lower-right inset is the pertinent nanobeam pattern, (e) high-resolution TEM image, and (f) SAED micrographs.
ultrasonic treatment. Then, the water drops were removed by N2 purge. Likewise, the carbon contamination was removed by ultraviolet (UV)/ ozone surface treatment 30 min. Poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS, Uni-Onward Co., Ltd., Taiwan) both with and without the addition of 1 wt% CZTS NPs were filtered with 0.45 μm PVDF membrane filter (Toyo Roshi Kaisha Ltd., Japan) via a spin-coated onto the ITO glass substrate at 3000 rpm for 30s. Then followed by annealed in air at 150 °C for 15 min. The CH3NH3PbI3 perovskite precursors were then synthesized on the glass/ITO/ PEDOT:PSS placed in glass petri dish 9.5 cm in diameter under open air ambient conditions at room temperature for 18 h. The perovskite films were then rinsed by n-Heptane and n-Hexane (Fisher Scientific UK, > 99%) to remove the byproduct and heat at 80 °Cwith n-Hexane for purification in closed vial glass. Finally, the sample glass/ITO/ PEDOT:PSS/ CH3NH3PbI3 was loaded into the closed vial glass with full of methylamine vapor to treat 3 s for recrystallization. Afterward, the samples were transferred to a thermal evaporator to deposit the fullerene (C60, 30 nm, > 99.0%, Solenne), bathocuproine (BCP, 10 nm, Sigma-Aldrich), and Al electrode (100 nm) in sequence under a vacuum of 6 × 10−6 torr as shown in Fig. 1(b).
Fig. 4. AFM images demonstrating the change in surface roughness of 108.52, 102.23, and 6.51 nm for the samples prepared (a) glass/ITO/PEDOT:PSS/ MAPbI3, (b) glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3, and (c) glass/ ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted.
transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan). For Atomic Force Microscopic (AFM) investigation was carried out on a MultiMode AFM with Nanoscope-V controller (Bruker, Santa Barbara, CA, USA). Rectangular silicon nitride cantilevers with a force constant of 0.4Nm−1 and an average resonance frequency of 130 kHz was used for imaging with PeakForce-Quantitative Nanomechanical Mapping (PFQNM) mode in air (ScanAsyst-Air-HR, Bruker AFM Probes
2.5. Characterizations The crystalline structures of the powders were examined using X-ray diffraction (XRD Bruker D8 advance, Billerica, MA, USA). The morphology was examined by field emission scanning electron microscopy (FESEM, JEOL JSM-6701F, Tokyo, Japan) and the microstructure with 192
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Fig. 5. (a) UV-vis-NIR absorbance spectrum, (b) PL spectra of as-synthesized 1 wt% CZTS NPS hybrid PEDOT:PSS, and (c) the energy-band diagram of a glass/ITO/ 1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted/C60/BCP/Al structure.
even at room temperature. This means that the reaction path could not completely reach MAPbI3 with [MA+] as shown impurity phase at 12.60° that corresponds to the PbI2 complex. More importantly, MAPbI3 films indicates a better crystallinity of grains with fewer defects after MA treatment, which promotes the formation of (110) and (220) planes on the substrate [14]. The morphology of MAPbI3 NCs was investigated by SEM and TEM as shown in Fig. 3. The shapes of as-deposited MAPbI3 NCs with a majority of regular NCs were obtained; however, the surface morphology exhibited faceted surface voids. Fig. 3b shows the morphology of the sample, which is prepared with MA vapor-assisted diffusion. As can be seen, MAPbI3 NCs is well grown in between the grains boundaries. The upper-middle inset shows that the roughness decreased progressively from the mist transition to mirror images of as-prepared Pt coating for SEM analysis. Fig. 3c shows the cross-section view of the as-deposited MAPbI3 film after MA-vapor assisted diffusion. The film appears uniform and adherent on top of 1 wt% CZTS NCs hybrid PEDOT:PSS layer and the thickness of the resulting film is approximately 380–400 nm. It may be indicative a cluster by cluster mechanism, where colloids nucleate in the solution and then an ion-by-ion growth mechanism leads to form a compact and flat polycrystalline thin film. Fig. 3d shows the TEM images of the MAPbI3 NCs with MA vaporassisted treatment. It can be seen that the shapes are cubic-like with an average grain size of around 20–30 nm. The nanobeam diffraction pattern (Inset image in Fig. 3d) for the MAPbI3 NCs shows the crystal plane (220), which is in agreement with the XRD results. As shown in the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3e, the interplannar lattice spacing of 0.31 nm corresponds to the (220) planes, and the select area electron (SAED) image matches the tetragonal MAPbI3 structure, with spots corresponding to the (110) and (220) planes in Fig. 3f [5]. Fig. 4 shows the surface roughness of the films determined by AFM for a scanning area 30 × 30 μm. The results show that the roughness ranges from 108.52, 102.23 to 6.51 nm, for glass/ITO/PEDOT:PSS/MAPbI3, glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3, and glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/
Americas, Camarillo, CA, USA). The optical properties were investigated using the UV–Vis absorption spectrum (PerkinElmer Lambda 7500, Waltham, MA, USA). The PL spectra were obtained with a LS-55 fluorescence spectrometer (PerkinElmer, Waltham, MA, USA). The current density–voltage (J–V) curves were measured in a nitrogen-filled glove box using a Keithley 2400 Source Meter under standard 1 sun AM 1.5G simulated irradiation (100 mW cm−2) from a Newport 91160A 300W Solar Simulator (Class A). The light intensity for the illumination can be tuned by placing a neutral density filter in front of the device during measurement. The scan rate was 0.1 V s−1, and the simulated solar irradiance was corrected by a Schott visible-color glass-filtered (KG5 color filter) Si diode (Hamamatsu S1133). A xenon lamp (Newport, 300 W), monochromatic equipment (Newport Cornerstone 260) and Keithley 2401 instrument were employed to measure the external quantum efficiency (EQE).
3. Results and discussion The XRD patterns of the kesterite CZTS and the tetragonal MAPbI3 NCs with and without MA vapor treatment are shown in Fig. 2a it shows the major CZTS diffraction peaks at 28.47°, 32.99°, 47.35°, and 56.14°, which can be indexed to the crystal planes of (112), (200), (220), and (312) of the kesterite structure (JCPDS no. 26–0575), respectively. The formation mechanism of the CZTS NCs was reported in our previous work as Cu2S + SnS2 + ZnS → Cu2ZnSnS4 [23]. The microstructure of the perovskite-phase peaks at 14.14°, 28.45°, 43.24°, and 58.92° are assigned to the (110), (220), (310), and (404) planes, of the tetragonal MAPbI3 crystal structure [5]. We thus propose the use of structural control to form single phase tetragonal MAPbI3, [MA+] [OA−] + [PbI3−][OAm+] → MAPbI3 + [OA−][OAm+]. Fig. 2b shows the effect on crystal growth for sample with and without MA vaporassisted recrystallization. It shows that the peak intensity increase and shift to the right forMA vapor-assisted. The mechanism of the formation of intermediates [OAm+]PbI3 [OA−] causes the MA is easily evaporate 193
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The CZTS NCs hybridized PEDOT:PSS exhibits an energy level of 5.10–5.20 eV, a cascade between the work function of ITO with the valence band (VB) edge level of MAPbI3 for the transport of holes. The CZTS NPs hybridized PEDOT:PSS generated excited electrons and holes from its VB to its conduction band (CB), after which the electrons generated in the CZTS conduction band jumped to the MAPbI3. Meanwhile, the energy barriers at VB and conduction band CB edge levels of MAPbI3 with BCP and CZTS NPs hybrid PEDOT:PSS, respectively, would block the transport of the holes and electrons to reach the electrodes [7]. Fig. 6a presents the schematic device structure and the current density–voltage (J–V) curves under the standard 1 sun AM 1.5G simulated solar irradiation. . The cell performance are shown in Table 1. Herein, we observe the photovoltaic parameters of a Voc of 0.82 V, a Jsc of 14.01 mA/cm2, and a FF of 0.46, corresponding to a PCE of 5.28% for the glass/ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al device, and a VOC of 0.85 V, a JSC of 18.16 mA/cm2, and a FF of 0.42, corresponding to a PCE of 6.46% for the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3/C60/BCP/Al device, and a VOC of 0.84 V, a JSC of 19.00 mA/cm2, and a FF of 0.47, corresponding to a PCE of 7.55% for the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vaporassisted/C60/BCP/Al device. It can be seen that the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted device has the highest PCE. In comparing the PCE of the device structures with factors contributing to the low PCE, we speculate that the performances are result of internal defects, such as impurity phase of intermediates [OAm+]PbI3 [OA−] could not completely reacting with MA+ and effectively form MAPbI3. This result indicates that CZTS acts as a HTM able to transport holes effectively and MA vapor-assisted exhibited a better crystallinity of grains with fewer defects. Fig. 6b shows the EQE spectrum of the cell fabricated by 1 wt% CZTS hybrid PEDOT:PSS and MAPbI3 film with MA vapor-assisted having higher EQE above 60% at approximately 520–800 nm. However, the internal defects still exist between MAPbI3 NCs of residual oxide in the open air environment during deposition process, despite MA vapor treatment. Therefore, further study into decreasing the defects and achieving a higher PCE in Perovskite-based solar cells is needed.
Fig. 6. (a) Effect of MA vapor-assisted on the current-voltage characteristics of the Perovskite solar cells, and (b) The EQE spectra under AM 1.5G solar irradiation of the cells of 1 wt% CZTS hybrid PEDOT:PSS, and MAPbI3 film with MA vapor-assisted structures.
MAPbI3 with MA vapor-assisted, respectively. The MAPbI3 with MA vapor-assisted film has the lowest roughness, reaches as low as 6.51 nm. The evolution of the MAPbI3 NCs absorption spectrum recorded when 1 wt% CZTS NPs were used is shown in Fig. 5a. It can be seen that the spectrum without hybrid CZTS NPs exhibits weak absorption in the infrared region with a wavelength longer than 825 nm. Notably, 1 wt% CZTS NPs enhanced the light absorption, and this fact can be ascribed to CZTS NPs which have the widely absorbable wavelength and high absorption coefficients [20]. The PL analysis was performed to determine the effect of the additional CZTS NPs on the recombination rate of the device., Fig. 5b shows that the PL intensity was decreased when 1 wt% of CZTS NPs was added to the PEDOT:PSS. This suggests that the electrons promoted in the conduction band of CZTS jump to the MAPbI3, thus reducing the recombination rate of the structure. On the other hand, 1 wt% CZTS hybridized PEDOT:PSS that absorbed the visible light and efficiently transferred the photogenerated charge carriers. Fig. 5c illustrates the energy levels of each layer, composed of glass/ITO/1 wt% CZTS hybridized PEDOT:PSS/ MAPbI3/C60/BCP/Al.
4. Conclusions In this paper, we fabricated MA+ and PbI3− precursors with polarfree solvents. The obtained sample show that MA vapor-assisted method increase the crystallinity and more regular morphology. Furthermore, it was found that adding 1 wt% CZTS serves not only increased the light harvesting broad light absorption region from visible to near-infrared, but also prevented the recombination of electron-hole pairs and improved the charge transfer processes. Furthermore, the enhancement photovoltaic performance of the device was improved by the addition 1 wt% CZTS NPs hybrid PEDOT:PSS with MA vapor-assisted exhibited the highest PCE. The results indicate that the solutionprecipitation technique in this work could be a promising method to achieve large-area deposition of a perovskite active layer for mass production. We believe that the grain size of the deposited film and environmental influences are still the key factors to contribute toward higher power conversion efficiency for the future advances in Perovskite-based solar cells.
Table 1 Photovoltaic parameters calculated from the J-V curves in Fig. 6. Devices
Jsc (mA/cm2)
Voc (V)
FF
η(%)
ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3/C60/BCP/Al ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted/C60/BCP/Al
14.01 18.16 19.00
0.82 0.85 0.84
0.46 0.42 0.47
5.28 6.46 7.55
194
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Acknowledgement
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Letter
Effect of the vapor diffusion and improved light harvesting for PerovskiteCu2ZnSnS4 hybridized solar cells
T
Shih-Jen Lina, Jyh-Ming Tinga,∗, Chia-Tsung Hungb, Yaw-Shyan Fub,∗∗ a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Cu2ZnSnS4 (CZTS) Grain growth Grain boundaries
In this study, a novel hybridized perovskite-Cu2ZnSnS4 solar cell has been demonstrated. The CH3NH3PbI3 layer was synthesized using a facile one-step solution process under ambient conditions. The precursor solution consists of an equimolar methylammonium ion [MA+] and [PbI3−] anion in heptane solvent. The vapor-assisted crystal growth at the grain boundaries occur due to the methylamine (CH3NH2, MA). Moreover, 1 wt% Cu2ZnSnS4 (CZTS) nanoparticles (NPs) was added to the solution to enhance not only the absorption from visible to infrared regions and but also the transfer of the photogenerated charge carriers, leading to reduced charge recombination. The device structure was glass/ITO/1 wt% CZTS NPs hybrid poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS)/CH3NH3PbI3 (MAPbI3)/C60/BCP/Al. The obtained MAPbI3 perovskitebased solar cell shows a short-circuit current density of 19.00 mA/cm2, an open-circuit voltage of 0.84 V, a fill factor of 0.47, and a power conversion efficiency (PCE) of 7.55%.
1. Introduction In recent years, inorganic−organic perovskite solar cells have been highly considered due to their long electron-hole diffusion lengths, low temperatures processing, and outstanding photoelectric properties [1–3]. Organic halide perovskites have ABX3 crystal structures (A = organic cation, alkali; B = Ge, Sn, Pb and X = halide), where the band gap can be adjusted through varying material compositions. The typical CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, CH3NH3PbI3-xClx, CH3NH3PbBr3-xClx, and CH3NH3PbBr3-xIx Perovskites have been extensively investigated in the field of photodetectors [4–6]. In 2009, Miyasaka's group used MAPbI3 perovskites for dye-sensitized solar cells for the first time [7]. Recent reports have shown higher efficiencies for various device structures. Both single and multiple solution processes have been developed, including solid-state structures with n-i-p mesoscopic TiO2 [8,9], super-meso structures Al2O3 scaffold [10], and nickel oxide-based structures with a p-i-n planar heterojunction (PHJ) architecture [11–13]. Furthermore, through a vapor-assisted solution-process deposition technique, in-situ chemical vapor deposition enables a flatter morphology and improved grain size via gas-solid crystallization [14,15]. Lv's et al. employed ternary CuInS2 QDs as a hole transport materials (HTM), where their results strongly suggested that a wide band gap of metal chalcogenide HTM is more efficient with regard to
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Corresponding author. Corresponding author. E-mail addresses: [email protected] (J.-M. Ting), [email protected] (Y.-S. Fu).
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https://doi.org/10.1016/j.orgel.2018.05.010 Received 17 April 2018; Received in revised form 8 May 2018; Accepted 8 May 2018 1566-1199/ © 2018 Published by Elsevier B.V.
achieving a high open circuit voltage (Voc) [16]. These various types of metal chalcogenide thin film solar cells based on the copper-chalcopyrite family of semiconductors: CuIn1−xGaxSe2 (CIGS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe). Among them, CZTS is the most appropriate for thin film solar cells because non-toxic, earth-abundant, and inexpensive. Moreover, CZTS also has a high absorption coefficient greater than 10−4 cm2, which making it suitable as a solar-cell absorber. Many studies have shown improvement photocatalytic performance of the open-circuit voltage of solar cell by adjusting the band gap and recombination rate [17,18]. As an example, outstanding open circuit voltage of 1.353 V via a tandem device architecture has been achieved [19]. However, perovskite materials are unstable in polar solvents. Thus, providing more stable constituents of HTM for perovskite-based photovoltaics is needed [20]. Pérez-Prieto et al. reported the highly dispersible and photostable lead perovskite-octadecylammonium bromide (PODA) by using both toluene and (dimethylformamide) DMF [21]. The perovskite could stable both in colloidal solution in toluene and solid in DMF + toluene. On the other hand, Oleh Vybornyi et al. synthesized perovskite nanocrystal (NCs) with replacing DMF using 1-octadecene (ODE) with the presence of oleylamine and oleic acidv [22]. Therefore, herein, we report high crystalline MAPbI3 films prepared by a one-step methylamine (MA) vapor-assisted solution-precipitation process with polar-free solvents.
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Fig. 2. XRD patterns of as-synthesized (a) kesterite CZTS NPs, tetragonal MAPbI3 NCs with and without MA vapor-assisted treatment, (b) peaks right shift through MA vapor treatment.
Fig. 1. Schematic illustration of (a) MAPbI3 was synthesized in glass petri dish 9.5 cm in diameter; and, (b) the basic device architecture of the glass/ITO/1 wt % Cu2ZnSnS4 hybrid PEDOT:PSS/ MAPbI3 with MA vapor-assisted treatment/ C60/BCP/Al layers.
2.2. Synthesis of MAPbI3 NCs MAPbI3 was synthesized by mixing 3 mL 0.1 M MA+ precursor which was diluted with n-Heptane (C-ECHO, > 99%) and 1 mL 0.1 M PbI3−. Meanwhile, the rubyred CH3NH3PbI3 NCs were precipitated from the reaction in Fig. 1(a).
We demonstrated that MA vapor-enhanced grains growth and crystal size. Moreover, the addition of CZTS NPs hybrid with PEDOT:PSS has been demonstrated to not only form p-type HTM but also broadened the infrared region for a PHJ perovskite device.
2.3. Synthesis of Cu2ZnSnS4 NPs 2. Materials and methods +
2.1. Materials and preparation of the MA , PbI3
−
The CZTS NPs were fabricated via a conventional hydrothermal synthesis method as reported in our previous work [23]. Briefly, a certain amount of thiourea (NH2CSNH2, Riedel-de Haën, 99%) was dissolved in DI-water. Subsequently, zinc chloride (ZnCl2, Merck, 98%), stannous chloride dehydrate (SnCl2·2H2O, Shimakyu, 98%), and drop of the diethylenediamine (C4H13N3, Panerac quimica sa, 98%) as chelating regent, finally copper (II) chloride anhydrous (CuCl2, Choneye pure chemicals Co., Ltd., 98%) were added in the solution. The obtained mixture was then transferred in to autoclave and the temperature was set at 180 °C for 72 h.
precursors
All chemicals were of analytical grade and used as received without further purification. The methylamine vapor was obtained from methylamine solution (CH3NH2, SHOWA, 40%). The vapor was distilled in vacuum condition at 40 °C and injected into oleic Acid (OA, SHOWA, 90%) to form an anhydrous vapor-liquid salt as an [MA+][OA−] precursor. The Lead (II) Iodide (PbI2, SHOWA, 98%) was dispersed in 1Octadecene (ODE, Acros, 90%) and stirred for 30 min at 150 °C, after which it was purified in a vacuum oven at 100 °C to remove moisture. Finally, oleylamine (OAm, across, approximate C18-content 80–90%) with OA as the surfactant was poured into the mixture until PbI2 dissolved completely to form a transparent precursor [PbI3−][OAm+].
2.4. Device system The obtained active area is 0.06 cm2 of pre-patterned glass/indiumtin-oxide (ITO) (15Ω/sq) was sequentially cleaned with detergent, distilled water, acetone and isopropyl alcohol 30 min, respectively by 191
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Fig. 3. Surface morphologies of MAPbI3 NCs (a) without MA vapor-assisted, (b) with MA vapor-assisted, (c) cross-section SEM morphology of as-synthesized MAPbI3 NCs with MA vapor-assisted, TEM analysis of (d) and the lower-right inset is the pertinent nanobeam pattern, (e) high-resolution TEM image, and (f) SAED micrographs.
ultrasonic treatment. Then, the water drops were removed by N2 purge. Likewise, the carbon contamination was removed by ultraviolet (UV)/ ozone surface treatment 30 min. Poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS, Uni-Onward Co., Ltd., Taiwan) both with and without the addition of 1 wt% CZTS NPs were filtered with 0.45 μm PVDF membrane filter (Toyo Roshi Kaisha Ltd., Japan) via a spin-coated onto the ITO glass substrate at 3000 rpm for 30s. Then followed by annealed in air at 150 °C for 15 min. The CH3NH3PbI3 perovskite precursors were then synthesized on the glass/ITO/ PEDOT:PSS placed in glass petri dish 9.5 cm in diameter under open air ambient conditions at room temperature for 18 h. The perovskite films were then rinsed by n-Heptane and n-Hexane (Fisher Scientific UK, > 99%) to remove the byproduct and heat at 80 °Cwith n-Hexane for purification in closed vial glass. Finally, the sample glass/ITO/ PEDOT:PSS/ CH3NH3PbI3 was loaded into the closed vial glass with full of methylamine vapor to treat 3 s for recrystallization. Afterward, the samples were transferred to a thermal evaporator to deposit the fullerene (C60, 30 nm, > 99.0%, Solenne), bathocuproine (BCP, 10 nm, Sigma-Aldrich), and Al electrode (100 nm) in sequence under a vacuum of 6 × 10−6 torr as shown in Fig. 1(b).
Fig. 4. AFM images demonstrating the change in surface roughness of 108.52, 102.23, and 6.51 nm for the samples prepared (a) glass/ITO/PEDOT:PSS/ MAPbI3, (b) glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3, and (c) glass/ ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted.
transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan). For Atomic Force Microscopic (AFM) investigation was carried out on a MultiMode AFM with Nanoscope-V controller (Bruker, Santa Barbara, CA, USA). Rectangular silicon nitride cantilevers with a force constant of 0.4Nm−1 and an average resonance frequency of 130 kHz was used for imaging with PeakForce-Quantitative Nanomechanical Mapping (PFQNM) mode in air (ScanAsyst-Air-HR, Bruker AFM Probes
2.5. Characterizations The crystalline structures of the powders were examined using X-ray diffraction (XRD Bruker D8 advance, Billerica, MA, USA). The morphology was examined by field emission scanning electron microscopy (FESEM, JEOL JSM-6701F, Tokyo, Japan) and the microstructure with 192
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Fig. 5. (a) UV-vis-NIR absorbance spectrum, (b) PL spectra of as-synthesized 1 wt% CZTS NPS hybrid PEDOT:PSS, and (c) the energy-band diagram of a glass/ITO/ 1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted/C60/BCP/Al structure.
even at room temperature. This means that the reaction path could not completely reach MAPbI3 with [MA+] as shown impurity phase at 12.60° that corresponds to the PbI2 complex. More importantly, MAPbI3 films indicates a better crystallinity of grains with fewer defects after MA treatment, which promotes the formation of (110) and (220) planes on the substrate [14]. The morphology of MAPbI3 NCs was investigated by SEM and TEM as shown in Fig. 3. The shapes of as-deposited MAPbI3 NCs with a majority of regular NCs were obtained; however, the surface morphology exhibited faceted surface voids. Fig. 3b shows the morphology of the sample, which is prepared with MA vapor-assisted diffusion. As can be seen, MAPbI3 NCs is well grown in between the grains boundaries. The upper-middle inset shows that the roughness decreased progressively from the mist transition to mirror images of as-prepared Pt coating for SEM analysis. Fig. 3c shows the cross-section view of the as-deposited MAPbI3 film after MA-vapor assisted diffusion. The film appears uniform and adherent on top of 1 wt% CZTS NCs hybrid PEDOT:PSS layer and the thickness of the resulting film is approximately 380–400 nm. It may be indicative a cluster by cluster mechanism, where colloids nucleate in the solution and then an ion-by-ion growth mechanism leads to form a compact and flat polycrystalline thin film. Fig. 3d shows the TEM images of the MAPbI3 NCs with MA vaporassisted treatment. It can be seen that the shapes are cubic-like with an average grain size of around 20–30 nm. The nanobeam diffraction pattern (Inset image in Fig. 3d) for the MAPbI3 NCs shows the crystal plane (220), which is in agreement with the XRD results. As shown in the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3e, the interplannar lattice spacing of 0.31 nm corresponds to the (220) planes, and the select area electron (SAED) image matches the tetragonal MAPbI3 structure, with spots corresponding to the (110) and (220) planes in Fig. 3f [5]. Fig. 4 shows the surface roughness of the films determined by AFM for a scanning area 30 × 30 μm. The results show that the roughness ranges from 108.52, 102.23 to 6.51 nm, for glass/ITO/PEDOT:PSS/MAPbI3, glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3, and glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/
Americas, Camarillo, CA, USA). The optical properties were investigated using the UV–Vis absorption spectrum (PerkinElmer Lambda 7500, Waltham, MA, USA). The PL spectra were obtained with a LS-55 fluorescence spectrometer (PerkinElmer, Waltham, MA, USA). The current density–voltage (J–V) curves were measured in a nitrogen-filled glove box using a Keithley 2400 Source Meter under standard 1 sun AM 1.5G simulated irradiation (100 mW cm−2) from a Newport 91160A 300W Solar Simulator (Class A). The light intensity for the illumination can be tuned by placing a neutral density filter in front of the device during measurement. The scan rate was 0.1 V s−1, and the simulated solar irradiance was corrected by a Schott visible-color glass-filtered (KG5 color filter) Si diode (Hamamatsu S1133). A xenon lamp (Newport, 300 W), monochromatic equipment (Newport Cornerstone 260) and Keithley 2401 instrument were employed to measure the external quantum efficiency (EQE).
3. Results and discussion The XRD patterns of the kesterite CZTS and the tetragonal MAPbI3 NCs with and without MA vapor treatment are shown in Fig. 2a it shows the major CZTS diffraction peaks at 28.47°, 32.99°, 47.35°, and 56.14°, which can be indexed to the crystal planes of (112), (200), (220), and (312) of the kesterite structure (JCPDS no. 26–0575), respectively. The formation mechanism of the CZTS NCs was reported in our previous work as Cu2S + SnS2 + ZnS → Cu2ZnSnS4 [23]. The microstructure of the perovskite-phase peaks at 14.14°, 28.45°, 43.24°, and 58.92° are assigned to the (110), (220), (310), and (404) planes, of the tetragonal MAPbI3 crystal structure [5]. We thus propose the use of structural control to form single phase tetragonal MAPbI3, [MA+] [OA−] + [PbI3−][OAm+] → MAPbI3 + [OA−][OAm+]. Fig. 2b shows the effect on crystal growth for sample with and without MA vaporassisted recrystallization. It shows that the peak intensity increase and shift to the right forMA vapor-assisted. The mechanism of the formation of intermediates [OAm+]PbI3 [OA−] causes the MA is easily evaporate 193
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The CZTS NCs hybridized PEDOT:PSS exhibits an energy level of 5.10–5.20 eV, a cascade between the work function of ITO with the valence band (VB) edge level of MAPbI3 for the transport of holes. The CZTS NPs hybridized PEDOT:PSS generated excited electrons and holes from its VB to its conduction band (CB), after which the electrons generated in the CZTS conduction band jumped to the MAPbI3. Meanwhile, the energy barriers at VB and conduction band CB edge levels of MAPbI3 with BCP and CZTS NPs hybrid PEDOT:PSS, respectively, would block the transport of the holes and electrons to reach the electrodes [7]. Fig. 6a presents the schematic device structure and the current density–voltage (J–V) curves under the standard 1 sun AM 1.5G simulated solar irradiation. . The cell performance are shown in Table 1. Herein, we observe the photovoltaic parameters of a Voc of 0.82 V, a Jsc of 14.01 mA/cm2, and a FF of 0.46, corresponding to a PCE of 5.28% for the glass/ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al device, and a VOC of 0.85 V, a JSC of 18.16 mA/cm2, and a FF of 0.42, corresponding to a PCE of 6.46% for the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3/C60/BCP/Al device, and a VOC of 0.84 V, a JSC of 19.00 mA/cm2, and a FF of 0.47, corresponding to a PCE of 7.55% for the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vaporassisted/C60/BCP/Al device. It can be seen that the glass/ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted device has the highest PCE. In comparing the PCE of the device structures with factors contributing to the low PCE, we speculate that the performances are result of internal defects, such as impurity phase of intermediates [OAm+]PbI3 [OA−] could not completely reacting with MA+ and effectively form MAPbI3. This result indicates that CZTS acts as a HTM able to transport holes effectively and MA vapor-assisted exhibited a better crystallinity of grains with fewer defects. Fig. 6b shows the EQE spectrum of the cell fabricated by 1 wt% CZTS hybrid PEDOT:PSS and MAPbI3 film with MA vapor-assisted having higher EQE above 60% at approximately 520–800 nm. However, the internal defects still exist between MAPbI3 NCs of residual oxide in the open air environment during deposition process, despite MA vapor treatment. Therefore, further study into decreasing the defects and achieving a higher PCE in Perovskite-based solar cells is needed.
Fig. 6. (a) Effect of MA vapor-assisted on the current-voltage characteristics of the Perovskite solar cells, and (b) The EQE spectra under AM 1.5G solar irradiation of the cells of 1 wt% CZTS hybrid PEDOT:PSS, and MAPbI3 film with MA vapor-assisted structures.
MAPbI3 with MA vapor-assisted, respectively. The MAPbI3 with MA vapor-assisted film has the lowest roughness, reaches as low as 6.51 nm. The evolution of the MAPbI3 NCs absorption spectrum recorded when 1 wt% CZTS NPs were used is shown in Fig. 5a. It can be seen that the spectrum without hybrid CZTS NPs exhibits weak absorption in the infrared region with a wavelength longer than 825 nm. Notably, 1 wt% CZTS NPs enhanced the light absorption, and this fact can be ascribed to CZTS NPs which have the widely absorbable wavelength and high absorption coefficients [20]. The PL analysis was performed to determine the effect of the additional CZTS NPs on the recombination rate of the device., Fig. 5b shows that the PL intensity was decreased when 1 wt% of CZTS NPs was added to the PEDOT:PSS. This suggests that the electrons promoted in the conduction band of CZTS jump to the MAPbI3, thus reducing the recombination rate of the structure. On the other hand, 1 wt% CZTS hybridized PEDOT:PSS that absorbed the visible light and efficiently transferred the photogenerated charge carriers. Fig. 5c illustrates the energy levels of each layer, composed of glass/ITO/1 wt% CZTS hybridized PEDOT:PSS/ MAPbI3/C60/BCP/Al.
4. Conclusions In this paper, we fabricated MA+ and PbI3− precursors with polarfree solvents. The obtained sample show that MA vapor-assisted method increase the crystallinity and more regular morphology. Furthermore, it was found that adding 1 wt% CZTS serves not only increased the light harvesting broad light absorption region from visible to near-infrared, but also prevented the recombination of electron-hole pairs and improved the charge transfer processes. Furthermore, the enhancement photovoltaic performance of the device was improved by the addition 1 wt% CZTS NPs hybrid PEDOT:PSS with MA vapor-assisted exhibited the highest PCE. The results indicate that the solutionprecipitation technique in this work could be a promising method to achieve large-area deposition of a perovskite active layer for mass production. We believe that the grain size of the deposited film and environmental influences are still the key factors to contribute toward higher power conversion efficiency for the future advances in Perovskite-based solar cells.
Table 1 Photovoltaic parameters calculated from the J-V curves in Fig. 6. Devices
Jsc (mA/cm2)
Voc (V)
FF
η(%)
ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3/C60/BCP/Al ITO/1 wt% CZTS hybrid PEDOT:PSS/MAPbI3 with MA vapor-assisted/C60/BCP/Al
14.01 18.16 19.00
0.82 0.85 0.84
0.46 0.42 0.47
5.28 6.46 7.55
194
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Acknowledgement
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