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Lead-free perovskite solar cells using Sb and Bi-based A3B2X9 and A3BX6 crystals with normal and inverse cell structures
© Korea Nano Technology Research Society 2017
Received: 15 June 2017
Accepted: 17 September 2017
Published: 22 September 2017
Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3 +, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.
Recent advancements of organic–inorganic perovskite (CH3NH3PbI3) thin-film solar cells, which can be fabricated by economical-promising solution process, have marked significant achievements of photoconversion efficiency (PCE) from the initial efficiency of 3.8% to over 22% [1–3]. The PCE progress has been realized due to the excellent ability of CH3NH3PbI3 about light harvesting, charge separation and charge transportation. These excellent properties also realize to fabricate perovskite solar cells in different structural designs as normal (n-i-p) mesoscopic and inverted (p-i-n) planar configurations. However, the working instability had been a profound problem [4–7]. Moreover, the toxicity of lead (Pb) shows big impact on the environment and human being, and then, the quest for such lead-free thin film solar cell has been highly demanded [8, 9]. The first lead-free perovskite solar cells were initially fabricated with the substitution of lead to tin (Sn) for the mesoscopic structure solar cells as <FTO glass/cp-TiO2/CH3NH3SnI3/Spiro-OMeTAD/Au> [10, 11]. A moderate PCE of 6% was reported, but the rapid oxidation of Sn2+ to be Sn4+ at the ambient condition arised concern. Afterward, however, it was found that tin cation shows higher toxicity than lead . Hence, another elements have been considered for the further ongoing lead-free solar cells.
In order to substitute the Pb, another potential, germanium (Ge2+) cation was implemented into the organo-metal-halide crystal as a harvesting layer, which has the same oxidation state and lower electronegativity and resulted in 0.2% PCE . As another divalent, earth abundant and non-toxic element, copper cation was utilized in mesoscopic perovskite solar cell structure with (CH3NH3)2CuClxBry as a photon harvesting layer . The low PCE of 0.017% was observed due to the low absorption coefficients, the high effective mass of holes and the low intrinsic conductivity of the employed perovskite layer.
The other viable opinion towards lead cation (Pb2+) substitution is adaptation of heterovalent (not divalent) cation into the organic-metal-halide crystals. These substitutions have to follow charge neutrality and can alter perovskite structure (ABX3) to another crystal (A: organic ion; B: metal ion; X: halide ion). Recently, trivalent cations of antimony (Sb) and bismuth (Bi) based organic-metal-halide crystals (A3B2X9 or A3BX6) got significant interest as a photon harvesting layer due to their environmentally friendly nature and available inactive outer shell s orbital [15, 16]. Öz et al. proposed (CH3NH3)3Bi2I9 based inverted solar cells as <ITO/PEDOT:PSS/(CH3NH3)3Bi2I9/PCBM/Ca/Al. However, the energy mismatch limited the charge extraction, resulting in the low PCE to 0.07%. Hebig et al. proposed <ITO/PEDOT:PSS/(CH3NH3)3Sb2I9)/PCBM/ZnO/Al> structures and reported 0.49% PCE [15, 16]. The trivalent bismuth (Bi3+) has utilized for the fabrication of (CH3NH3)3Bi2I9 based organic-metal-halide layer, which showed optimal absorption coefficient [17, 18]. This Bi-based organic-metal-halide solar cells were reported 0.33% of the low PCE, which was limited by its high exciton binding energy of 300 meV. It was founded that the morphology of (CH3NH3)3Bi2I9 was changed by the selection of electron transporting layers (ETL), and that the small amount addition of NMP (N-methyl-2-pyrrolidone) solvent could also attain to grow uniform surface morphology [18, 19].
In order to improve the Bi-based A3BX6-structured solar cells, recently, Ag and Cs were composed as A-site monovalent cations to be Cs2AgBiX6 structure, due to better optical and electrical properties and its ambient stability [20–23]. The substitution of methylammonium ion with silver can extend the dimension of all inorganic active material maintaining its 3d structure, which can be more suitable for photo harvesting due to its uniform nature. Ag found application in various structural compounds including AgBiI4 and (CH3NH3)2AgBiBr6 although, only optical properties are established and the PCE is yet to be demonstrated [24, 25]. Moreover, the Chemical materials Evaluation and REserarch BAse (CEREBA) reported the incorporation of the silver based A-site cation by replacing the methylammonium (CH3NH3 +) ion preserving the Bi and iodine (Ag3BiI6) and reported 4.3% PCE . Hence, it can be assumed that the Ag–Bi based photo absorber can perform the lead free organo-metal-halide solar cells with better PCE.
2 Results and discussion
2.1 A3B2X9 crystals with methyl ammonium cation for the A site
Figure 2b shows UV–vis spectra and photographs of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, and (CH3NH3)3SbBiI9 thin films. The (CH3NH3)3Bi2I9 film absorbs with the peak appearing around 500 nm. On the other hand, The UV–vis spectrum of (CH3NH3)3Sb2I9 shows a shoulder peak around 460 nm and no clear peak. The casted (CH3NH3)3SbBiI9 film shows higher absorption around 500 nm than (CH3NH3)3Bi2I9 and (CH3NH3)3Sb2I9 films. The exact absorption coefficients were not shown in the figure due to the inhomogeneous structures of films, which will be shown later.
Elemental distribution (atomic %) of the individual elements observed on particular spot on (CH3NH3)3Bi2I9/TiO2 film shown in Fig. 4a
Elemental distribution (atomic %) of the individual elements observed on a particular spot on (CH3NH3)3Bi2I9/NiO film shown in Fig. 4b
Elemental distribution (atomic %) of the individual elements observed on particular spot on (CH3NH3)3Sb2I9/TiO2 film shown in Fig. 4c
Elemental distribution (atomic %) of the individual elements observed on a particular spot on (CH3NH3)3Sb2I9/NiO substrate shown in Fig. 4d
Elemental distribution (atomic %) of the individual elements observed on (CH3NH3)3SbBiI9/TiO2 substrate shown in Fig. 4e
Elemental distribution (atomic %) of the individual elements observed on (CH3NH3)3SbBiI9/NiO substrate shown in Fig. 4f
Photovoltaic performance of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9 photon harvesting layer based mesoscopic normal and inverted planar architecture solar cells
2.2 A3BX6 crystals with Ag or Cu cation for the A site
Elemental distribution (atomic %) of the individual elements observed on a particular spot on Ag3BiI6/TiO2 substrate shown in Fig. 8a
Elemental distribution (atomic %) of the individual elements observed on a particular spot on Ag3BiI6/NiO substrate shown in Fig. 8b
Elemental distribution (atomic %) of the individual elements observed on Ag3BiI3(SCN)3/TiO2 substrate shown in Fig. 8c
Elemental distribution (atomic %) of the individual elements observed on Ag3BiI3(SCN)3/NiO substrate shown in Fig. 8d
Elemental distribution (atomic %) of the individual elements observed on Cu3BiI6/TiO2 substrate shown in Fig. 8e
Elemental distribution (atomic %) of the individual elements observed on Cu3BiI6/NiO substrate shown in Fig. 8f
Photovoltaic performance of Ag3BiI6, Ag3BiI3(SCN)3 photon harvesting layer based mesoscopic normal and inverted planar architecture solar cells (Fig. 8g)
In summary, we fabricated the lead-free hybrid solar cells in standard mesoscopic and inverted architecture incorporating various solution processed photon harvesting layer of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6. The best PCE results was obtained using <FTO/cp-TiO2/mp-TiO2/Ag3BiI6/spiro-OMeTAD/Au> structure with 1.08% of power conversion efficiency (PCE). The grown morphology of A3B2X9 and A3BX6 crystals can be different on the substrates (porous TiO2 or planar NiO layers). The unevenness can be the hindrance for the improvement of photovoltaic effects. Hence, it is advisable to use lead-free element which can show uniform coverage of the substrate.
Other interesting findings in this report were the prohibition of short circuiting of solar cells using Sb deposition on NiO layer and the diminishment of photo I–V hysteresis using porous-TiO2/Cu3BiI6 combination, which can be considered for the further progresses.
4 Experimental details
All chemicals were of reagent grade quality and used without any further processing. Antimony iodide (SbI3) was purchased from Yanagishima Pharmaceutical Co. Ltd. Bismuth iodide (BiI3), Silver iodide (AgI), Nickel (II) acetylacetonate, Phenyl-C61-butyric acid methyl ester (PCBM) and bathocuproine (BCP) were purchased from Aldrich. Copper(I) iodide (CuI) from Kanto Chemical Ltd., Silver thiocyanate (AgSCN) from Wako Pure Chemical Industries Ltd., and Methylammonium iodide was purchased from TCI respectively. Dimethyl sulfoxide (DMSO), Chlorobenzene, and Methanol were purchased from Wako. γ-Butyrolactone (GBL) and Acetonitrile were procured from Chameleon Reagent and Kanto Chemical Co. Inc. respectively.
FTO glass (TEC-15, t = 2.1 mm), purchased from Pilkington were cut in an appropriate size and cleaned ultrasonically with detergent water, distilled water and ethanol, respectively each for 15 min. The cleaned FTO glass was passed through UV-O3 treatment for 10 min to remove the organic impurities. To fabricate the standard mesoscopic n-i-p solar cell, diluted TAA solution (Titanium di isopropoxide bis(acetylacetonate)/Sigma-Aldrich) of (200 µL in 7.5 mL ethanol) was aerosol spray coated maintaining the substrate temperature 500 °C. After arriving the ambient temperature the coated FTO glass was passed through the UV-O3 treatment and, diluted PST-30NRD TiO2 solution (1:3.5 wt/wt) was coated by spinning process at 5000 rpm for 30 s. The mesoporous TiO2 coated substrate was dried out at 120 °C for 5 min and again baked at 500 °C for 30 min in a furnace.
To make the perovskite layer, the substrate was treated for 5 min for UV-O3 treatment and transferred inside N2 filled glove box. To prepare the (CH3NH3)3Sb2I9 film, SbI3 and methylammonium iodide was mixed in DMSO:GBL (1:1) solvent and 0.25 M concentration of (CH3NH3)3Sb2I9 was maintained by keeping the molar ratio as CH3NH3I:SbI3 = 3:2 molar ratio. To prepare the (CH3NH3)3Bi2I9 film, BiI3 and methylammonium iodide was mixed to maintain the 0.25 M concentration of (CH3NH3)3Bi2I9 in DMSO:GBL (1:1) by keeping the molar ratio as CH3NH3I:BiI3 = 3:2. To prepare the (CH3NH3)3SbBiI9 film, SbI3, BiI3 and methylammonium iodide was mixed to maintain the 0.25 M concentration of (CH3NH3)3SbBiI9 in DMSO:GBL (1:1) by keeping the molar ratio as CH3NH3I:SbI3:BiI3 = 3:1:1. Semiconductor Ag3BiI6 layer overcoated TiO2 mp substrate was prepared by making a 0.5 M solution of AgI and BiI3 in DMSO solvent with 0.5 M concentration of Ag3BiI6 by keeping the molar ratio as AgI:BiI3 = 3:1. To fabricate and explore the photon harvesting properties of Ag3BiI3(SCN)3, BiI3 and Silver thiocyanate (AgSCN) was mixed in 1:3 molar ratio to maintain the 0.5 M concentration in DMSO solvent. Copper based photon harvesting layer (Cu3BiI6) was prepared by mixing the CuI and BiI3 in DMSO solvent with 0.4 M concentration of Cu3BiI6 by keeping the molar ratio as CuI:BiI3 = 3:1.
In all cases, the solution temperature was maintained at 80 °C and the heated solution was spun on mp TiO2 substrate with 2000 rpm for 30 s and preserved on a hot plate at 80 °C for 30 min to grow the respective photo absorber layer.
To collect the holes smoothly a hole selective layer spiro-OMeTAD (2,2,7,7-Tetrakis(N,Ndi-p-methoxy phenylamine)-9,9-spirobifluorene)of 28.9 mg in 400 µL chlorobenzene, with additives [11.5 µL t-butyl pyridine (Sigma-Aldrich), 7 µL Li-TFSI (520 mg in 1 mL acetonitrile) and 8.8 µL Co Complex (40 mg in 0.1 mL acetonitrile)] was spun on a cold substrate of lead-free prepared semiconducting layer at 4000 rpm for 20 s and stored for drying under dark. Finally, gold layer was thermally evaporated and deposited to maintain 80 nm thickness, which works as a metal back contact layer.
The inverted structure hybrid solar cell was fabricated by coating Nickel (II) acetylacetonate in acetonitrile (0.04 M) solution by spray process while maintaining the UV-O3 treated FTO glass temperature at 500 °C. After reaching the ambient temperature the coated NiO substrate was transferred to N2 filled glove box for lead-free semiconductor layer preparation. The preparation of individual photo harvesting layer was followed as described previously. On top of the lead-free photo absorber layer, a 20 mg/mL PCBM in chlorobenzene was spun at 1000 rpm for 60 s and kept under ambient N2 for 30 min to dry it out. Subsequently, a BCP solution of 1 mg/mL in methanol was spun at 1000 rpm for 60 s. Finally, a silver metal contact was prepared by thermally evaporation process maintaining its thickness 80 nm.
The I–V characteristics were measured by an AM 1.5G solar simulator equipped with a 500 W Xe lamp (YSS-80A, Yamashita Denso) by placing a black 0.09 cm2 mask area. A reference Si photodiode (Bunkou keiki co. ltd., Japan) was used to calibrate the power of the solar simulator light. The I–V characteristics were obtained by applying an external bias to the fabricated solar cell and measurement of generated photocurrent was performed with a DC voltage current source (Agilent, B2901A). SEM images were obtained by HITACHI Microscope TE3030 and EDX analysis was performed with Oxford Instruments, X-stream-2.
HM, HS performed the experiments. AKB, HM, NS analysed the data. AKB, HK, NS, and SI designed the experiments. AKB, HM, HS, HK, SK, NS, YS, MI, YN, KY, TM, TU, and HI provided an equal contribution in useful discussion and modification. AKB and SI wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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Funding and Acknowledgements
We would like to express sincere thanks to the Advanced Low Carbon Technology Research and Development Program (ALCA-JST), Japan.
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