Received: 11 May 2018
Revised: 5 July 2018
Accepted: 19 July 2018
DOI: 10.1002/jccs.201800173
ARTICLE
Perovskite solar cells using TiO2 layers coated with metal-organic framework material ZIF-8 Ho-Yang Chung | Chia-Her Lin | Samikannu Prabu | Hong-Wen Wang Department of Chemistry, Chung-Yuan Christian University, Taoyuan, Zhongli, Taiwan, R.O.C Correspondence Hong-Wen Wang, Department of Chemistry, Chung-Yuan Christian University, Taoyuan, Zhongli 320, Taiwan, R.O.C. Email:
[email protected] Funding information Ministry of Science and Technology, Taiwan, Grant/Award Number: MOST 105-2119-M033-003
A metal-organic framework of ZIF-8 is used as an interlayer between the mesoporous TiO2 and the perovskite layer in perovskite solar cells (PSCs). The ZIF-8 solution is dried on the mesoporous TiO2 layer, which can then act as an additional light absorbing layer at the short-wavelength range for the solar cells, leading to improved performance of the cell. After an immersion time of 1–5 min, a very thin ZIF-8 film is formed on TiO2. An enhancement in the incident photon to current conversion efficiency (IPCE) is achieved for PSCs using the ZIF-8-coated TiO2 layer. The conversion efficiency of PSCs is improved from 9.6 to 12.0% when ZIF-8 is present. UV–vis and photoluminescence (PL) spectra are used to investigate the beneficial effect of ZIF-8/TiO2 hybrid layers. KEYWORDS
MOF, perovskite solar cells, ZIF-8
1 | INTRODUCTION Perovskite solar cells (PSCs), mostly based on methylammonium lead iodide (CH3NH3PbI3), have shown remarkable progress in the past 5 years. A power conversion efficiency (PCE) exceeding 22.1%[1] has been reported. In all PSCs studies, fabrication techniques and morphology control are the most critical issues to realize the high-quality perovskite films needed for high performance. Metal-organic frameworks (MOFs) are a kind of coordination polymers with defined crystallinity and porosity.[2] MOFs have attracted much attention in recent years due to their distinct advantages of high surface area and controllable porous structures. These features of MOFs provide an excellent platform for many applications such as catalysis, gas storage and separation, drug delivery, sensing, and so on. MOFs have been used as photovoltaic materials in dye-sensitized solar cells (DSSCs) as a coating layer on TiO2[3] or ZnO.[4] An MOF was employed by Vinogradov[5] to form a TiO2-MOF composite for PSCs, but only 6.4% efficiency was obtained. Chang et al.[6] reported that a significant enhancement in the morphology and crystallinity of the perovskite thin film
could be achieved by adding microporous nanocrystals of MOF-525 into the perovskite solution. A 12.0% efficiency was achieved compared to that of pristine PSCs (10.1%). Li et al.[7] introduced a 3D MOF [In2(phen)3Cl6]CH3CN2H2O (In2) into the hole transport material of PSCs, by which the PCE was enhanced from 12.8 to 15.8%. Jin et al.[8] employed a 2D MOF Cu-BHT (BHT = benzenehexathiol), a solution-processed transparent electrode with high conductivity and transmittance, for three classic photovoltaic solar cells (PSCs, quantum dot SCs, and organic SCs). The performances of all three devices were comparable to those with an indium tin oxide (ITO) electrode, indicating the great potential of the Cu-BHT electrode in future low-cost and flexible optoelectronics. ZIF-8 is a representative MOF material that is built from the supramolecular self-assembly of Zn2+ and 2-methyl imidazole and has a specific surface area of >1,400 m2/g.[9] This material is chemically robust and thermally stable with large cavities (11.6 Å) and small pore diameters (3.4 Å), which favor the penetration of the electrolytes. ZIF-8 films can be grown on many kinds of substrates, including glass slide, silicon wafer,[10] porous titania,[11] and α-alumina.[12]
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim J Chin Chem Soc. 2018;1–6.
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Very recently, ZIF-8 films were successfully prepared on a flexible nylon substrate via a contra-diffusion synthesis approach.[13] Hupp et al. confirmed that the thickness of the ZIF-8 film increased linearly with the growth time.[10] The relationship between the overlayer thickness and the value of Voc could be investigated by simply controlling the reaction time. The results indicated that ZIF-8 could be used as a novel shell material to enhance the Voc of DSSCs. However, very little progress on this subject has been reported since then; nor has any report appeared regarding the use of ZIF-8 in PSCs. Only very recently MOFs have been introduced as an interfacial layer in PSCs.[14] An interface modified with the MOF ZIF-8 efficiently enhanced the perovskite’s crystallinity and grain size, which improved the photovoltaic performance of the PSCs up to a PCE of 16.99%. In this study, a ZIF-8 solution was used to coat the mesoporous TiO2 electrode for PSCs. The enhancements in conversion efficiency are characterized and discussed.
2 | EX PER IM ENT AL PROC ED UR E 2.1 | Preparation of the TiO2 substrates Fluorine-doped tin oxide (FTO)-coated glass substrates were cut into 2 cm × 2 cm size, cleaned using ethanol and acetone in an ultrasonic bath for 30 min, and washed using deionized (DI) water for another 30 min three times. The substrates then were washed with acetone and ethanol and cleaned in an oxygen plasma for 5 min before use. All reagents and solvents were purchased from commercial sources and used without further purification unless otherwise noted. All synthetic procedures and device fabrications were carried out under a nitrogen atmosphere. About 2 cm × 3.2 mm area on the FTO-coated glass substrates was protected using a tape before spin-coating. A 0.2 M titanium tetra-isopropoxide (TTIP) in an HCl/ethanol solution (0.02 M HCl in EtOH) was spin-coated on the substrates at 2,000 rpm for 60 s. A ~50-nm-thick TiO2 compact layer (c-TiO2) was deposited on the FTO coating, whose thickness estimated by a fieldemission scanning electron microscope (FESEM). After spin coating, the substrates were left at 100 C for 10 min and then left to cool down to room temperature. A mesoporous TiO2 layer (m-TiO2) was deposited by spin coating for 10 s at 2,000 rpm using a commercial paste containing 20-nm anatase particles (RL-002 Ruilong, Taiwan) diluted in ethanol to achieve a 250-nm-thick layer. After spin coating, the substrate was immediately dried at 100 C for 10 min and then sintered again at 525 C for 30 min in air. 2.2 | Preparation of ZIF-8 solution Zn(NO3)26H2O (50 nM) was dissolved in DI water. This solution is referred to hereafter as solution A. Another solution containing 100 mM NaCOOH and 100 mM
2-methylimidazole in methanol was also prepared by heating and stirring at 50 C, and then cooled down to room temperature. This solution is denoted as solution B hereafter. When the two solutions were mixed together, ZIF-8 crystals gradually formed, depending on the duration. For coating the ZIF-8 solution on TiO2 substrates, the TiO2 substrates were immersed in solution A for 5 min, and then solution B was added to solution A. The TiO2 substrates were taken out from the mixed solution after a duration of 1–5 min. Then the film was rinsed with methanol and dried at 70 C. The powder left in the solution was centrifuged, rinsed with methanol, and dried at 70 C for X-ray diffraction (XRD) characterization. 2.3 | Preparation of PSCs The perovskite films were deposited from a precursor solution containing 1:1 methylammonium iodide and lead iodide in an anhydrous solution of 1:1 dimethylsulfoxide (DMSO) and γ-GBL. One hundred microliters of the perovskite solution was spin-coated on the TiO2/FTO substrate in two steps, at 1,000 and 5,000 rpm for 10 and 20 s, respectively. During the second step, toluene (100 μL) was poured on the spinning substrate at the 17th second. The substrates were then annealed at 100 C for 10 min in a nitrogen-filled glove box. After annealing the perovskite, the substrates were cooled down for a few minutes. A 63 mM spirofluorenelinked methoxy triphenylamine (spiro-OMeTAD, Merck) solution was spun on to the surface of perovskite at 2,000 rpm for 30 s. The spiro-OMeTAD solution was prepared as described in the literature.[8] Briefly, 80 mg spiroOMeTAD was added to 28.8 μL 4-tert-butylpyridine and 17.6 μL Li-TFSI (520 mg/mL in acetonitrile), and dissolved in 0.99 mL chlorobenzene. Finally, 100 nm of silver (Ag) was deposited on top of spiro-OMeTAD by thermal evaporation under a high vacuum, using a shadow mask to pattern the electrode. 2.4 | Characterization The morphology and sectional view of PSCs were observed by an FESEM (JEOL JSM-7600F) equipped with an energydispersive spectroscopy attachment (EDS, Oxford, 80 mm2). The crystalline phase and structure of ZIF-8 were analyzed using a PANalytical PW3040/60 X'Pert Pro X-ray diffractometer with a Cu target and Ni filter at the scanning rate of 4 / min from 2θ = 10 to 80 . Photoluminescence (PL) data were obtained using a Hitachi F-7000 instrument, in which the specimens were illuminated using a 246 nm light source. UV–vis spectra of ZIF-8/TiO2 were obtained using a Shimadzu UV2550 spectrophotometer, in the range 190–800 nm. The photovoltaic characteristics of the PSC devices were measured from an illuminated area of 0.30 cm2 by an electrochemical analyzer (CHI 6173B, CH Instruments Co.) under a standard AM 1.5 sunlight illumination (XES-151S, San-Ei, Japan) with a
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ZIF-8 exp. ZIF-8 cal.
10
20
30
Average performance of PSCs for immersion time of 1–5 min and dried at 70 C (nine test spots)
TABLE 1
40
50
o
2θ ( ) FIGURE 1
Experimental and calculated XRD patterns of ZIF-8 2
100 mW/cm light source. The incident photon to current conversion efficiency (IPCE) was measured in the range 350–800 nm using an IPCE system (Enlitech, Taiwan).
3 | R E S U L T S AN D D I S C U S S I O N
Immersion time (min)
Jsc (mA/cm2)
Voc (V)
0
19.1
0.887
1
19.3
2
18.3
3
η (%)
FF
8.1 1.06
0.51
0.862
8.5 1.76
0.51
0.951
10.1 1.91
0.58
19.7
0.846
8.9 2.68
0.52
4
18.7
0.902
7.9 0.57
0.47
5
19.5
0.863
7.3 2.50
0.43
*FF=Fill Factor.
Best performance of PSCs for immersion times of 1–5 min and dried at 70 C
TABLE 2
Immersion time (min)
Jsc (mA/cm2)
Voc (V)
η (%)
FF
0
19.5
0.931
9.6
0.53
1
19.6
0.973
11.3
0.59
2
19.8
0.972
12.0
0.62
3
20.6
0.961
11.9
0.60
4
20.3
0.936
10.4
0.54
5
17.7
0.847
8.6
0.54
*FF=Fill Factor.
3.1 | Conventional PSCs with ZIF-8 layer ZIF-8 powder was formed after solution A and solution B were mixed. As the XRD pattern in Figure 1 shows the pattern of crystal phase of ZIF-8 powder collected from the solution fitted well the calculated pattern. The morphology of the ZIF-8 crystals is shown in Figure 2a. The ZIF-8 samples display spherical, rhombic dodecahedral, or cubic shapes with truncated edges in sizes ranging from 90 to 200 nm. However, the deposited ZIF-8 film on TiO2 films is
FIGURE 2
very thin because of the short reaction time even when the concentration was 10 times higher than that in the literature.[6] No peaks could be indexed in the X-ray pattern of the ZIF-8 on TiO2 films, indicating that the thin layer of ZIF-8 on TiO2 was too thin to show any significant peak on the XRD pattern. As shown in Figure 2b, the ZIF-8 layer cannot be easily identified in the sectional view of the PSC. The EDS of a single ZIF-8 crystal on c-TiO2 layer shows a
(a) ZIF-8 crystals, (b) section-view of PSC devices, (c) ZIF-8 crystals on compact layer, (d) EDS on (c)
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100
With ZIF-8 layer Without ZIF-8 layer
90 80 70
IPCE ( )
clear Zn signal, as shown in Figure 2c,d. It is believed that only a thin ZIF-8 layer (
