Electron Transport Improvement of Perovskite Solar Cells via a ZIF-8

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Electron Transport Improvement of Perovskite Solar Cell via ZIF-8 Derived Porous Carbon Skeleton Zhixin Zhang, Xinshu Luo, Bin Wang, and Jingbo Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00098 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Energy Materials

Electron Transport Improvement of Perovskite Solar Cell via ZIF-8 Derived Porous Carbon Skeleton Zhixin Zhang, Xinshu Luo, Bin Wang*, Jingbo Zhang*

Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin 300387, China



Corresponding authors.

Jing Bo Zhang College of Chemistry, Tianjin Normal University, Tianjin 300387, China [email protected]

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract:

To improve electron transport rate of perovskite solar cell, ZIF-8

derived porous carbon skeleton layer is prepared by carbonizing the ZIF-8 thin film on conducting glass as the electron transport skeleton of perovskite solar cell. Polyvinyl pyrrolidone is added during the synthesis of ZIF-8 to reduce the particle size of ZIF-8 and decrease the carbonization temperature below 600°C. The porous structure of ZIF-8 is mainly reserved at the optimized carbonization temperature. Then TiO2 nanoparticles are deposited on the surface of porous carbon skeleton to form an electron transport layer of perovskite solar cell with the structure of FTO/ZIF-8 derived porous carbon layer/TiO2/Perovskite/Spiro-OMeTAD/Au. Due to the good conductivity of the ZIF-8 derived porous carbon skeleton, the photogenerated electron transport rate of perovskite solar cell is increased. At the same time, the porous structure of ZIF-8 derived carbon layer increases the contact area between the perovskite layer and the TiO2 layer to favor separation of photogenerated charges. Therefore, the light-to-electric conversion efficiency of CH3NH3PbI3 perovskite solar cell is enhanced from 14.25% to 17.32%. Keywords：Metal organic frameworks; Porous carbon skeleton; Good conductivity; Polyvinyl pyrrolidone; Increase of contact area; Electron transport; Perovskite solar cell


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1. Introduction

Global energy demand is continually increasing and fossil fuels are reduced. It is urgent to exploit inexpensive, clean and renewable energy sources. Up to now, photovoltaic devices with high efficiency, low cost and large scale fabrication are being fabricated, which is an excellent mean to alleviate this problem. Recently, organic-inorganic hybrid perovskite solar cell with high efficiency and low cost, has attracted a research upsurge in the field of energy conversion. Organic-inorganic hybrid perovskites, such as CH3NH3PbX3 (X = I, Br, Cl), have become attractive light-absorbing materials for low cost and high efficiency solar cells resulting in a rapid increase in the conversion efficiency over last 9 years. The theoretical photoelectric conversion efficiency of perovskite solar cells was calculated as 26%, which is close to the level of monocrystalline silicon solar cell (25.6%) [1,2]. The newly reported photoelectric conversion efficiency of perovskite solar cell reached 23.7% [3], which is higher than that of polycrystalline silicon solar cell (18%). It has a very broad market application prospect [4]. For most of perovskite solar cells, the porous titanium metal oxide thin film with submicron thick was used for deposition of perovskite. Semiconductor electron transport materials should match their energy level with that of other layers in solar cell and have high carrier mobility. TiO2 nanoparticles powder is widely used as an electron transport material in perovskite solar cells [5,6]. It has large specific surface area to provide the directional growth space for perovskite thin film, which facilitates loading of perovskite materials with enough amounts. Meanwhile, TiO2



nanoporous layer can fully contact with perovskite materials to ensure the effective injection and separation of photogenerated electrons and holes [7,8]. In order to further improve photoelectric conversion efficiency of perovskite solar cell, nanoscale TiO2 materials with high specific surface area, low defect, appropriate aperture are needed to be prepared for more photosensitizer loading, resulting in a higher photocurrent [9]. Electron transport materials, which extract photo-induced electrons from perovskite and transport them to electrodes, play an important role in high-performance perovskite solar cells [10,11]. However, TiO2 as the most commonly used electron transport material, has low electron mobility, therefore SnO2 and ZnO with good optical performance, high carrier mobility and n-type metal oxide semiconductor band matching, were also used to make the electron transport layer of perovskite solar cells [12,13]. The graphene nanosheet/nano-TiO2 composite was used as the electron transport layer, because graphene has high conductivity and suitable work function (between FTO and TiO2). It provides a high-speed channel for electron transport and collection, significantly reducing the series resistance of the cell. Therefore, the short-circuit photocurrent and fill factor are significantly improved [14]. Based on the discussion as mentioned above, three-dimensional porous material with high specific surface area and electrical conductivity, easily prepared with easy chemical reactants and easily attached to the conducting glass surface, should be the ideal electron transport material. Then TiO2 deposited on the surface of the porous


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structure also has a higher specific surface area, so as to load more photosensitizers and allow more electrons to reach the electrode substrate quickly. Metal organic frameworks (MOFs), porous materials combined by metal ions and organic ligands, have some excellent advantages such as high specific surface area, ordered porous structure, design of the structure and so on [15,16]. Zeolitic imidazolate frameworks (ZIFs) are a kind of typical MOFs, which are constructed from metal ions like Zn2+、 Fe2+ 、 Cu2+ or Co2+ bridged by the imidazolate ligands. ZIFs are mainly used in fields related to porous solids, such as ion exchange, storage of adsorptive gases, purification and separation, sensors, catalysis and magnetic supercapacitors. ZIF-8, a porous material coordinated by Zn metal ions, is chemically robust, cheap and thermally stable. More importantly, ZIF-8 can be deposited on the substrates such as glass slides, silicon wafers, porous titania and alumina to form a thin film [17]. ZIF-8 of materials were used in dye-sensitized solar cells, lithium batteries and perovskite solar cells [18-20]. Here, ZIF-8 is introduced into the perovskite solar cell structure as the part of an electron transport layer. ZIF-8 powders with smaller nanoparticles were prepared by the normal temperature solution method using polyvinylpyrrolidone (PVP) to adjust the particle size [21]. The ZIF-8 thin layers prepared on the fluorine-doped tin oxide (FTO) conducting glass substrate by the spin-coating method were carbonized to form ZIF-8 derived porous carbon layer (DPCL). A thin layer of TiO2 was then deposited on the surface of porous carbon as an electron transport layer. At last, in the

ambient

air,

the

perovskite

solar

cell


with

a

structure

of


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FTO/DPCL/TiO2/Perovskite/Spiro-OMeTAD/Au was assembled. Because the highest temperature that FTO can afford is 600°C, PVP was added during the preparing process of ZIF-8 to decrease the carbonization temperature of ZIF-8. The carbonization temperature range is from 450°C to 600°C. The porous carbon structure formed by carbonizing ZIF-8 at 550°C can make the electrons generated by the perovskite thin film reach the conductive glass quickly through TiO2, which effectively improves the light-to-electric conversion efficiency of the cell to 17.32%. 2. Experimental section 2.1 Materials Zn(NO3)2, 2-methylimidazole (MIm), polyvinyl pyrrolidone (PVP), methanol, isopropyl alcohol, triton X-100, TiCl4 solution, HCl and Zn powder were purchased from Alfa Aesar, Inc., China. PbI2 was purchased from TCI, Japan. DMSO acetonitrile, chlorobenzene, ethylacetate and DMF were purchased from Aladdin. TiO2

(Dyesol,

18

NR-D),

4-tert-butylpyridine,

CH3NH3I,

2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene(Spiro-OMeTA D) and lithium bis(trifluoromethylsulfonyl)imide were purchased from Xi’an Polymer Light Technology Co., China. All the chemicals were analytical grade and used directly without further purification. FTO conducting glass (10 Ω·sq-1) was purchased from Nippon Sheet Glass Co., Ltd, Japan. 2.2 Synthesis and carbonization of ZIF-8.


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ZIF-8 was prepared according to the reported method [22]. In the typical synthesis processes, under stirring, 3.5 g MIm and 0.2 g PVP were added in 70 mL methanol and 1.5 g Zn(NO3)2·6H2O were dissolved in 70 mL methanol. Then the former was slowly added to the latter under stirring, and the mixed solution was stirred for 12 h. The synthesized powders were aged in methanol for 12 h. After centrifugation, the as-prepared ZIF-8 was washed with methanol for several times, and then dried at 100°C in a vacuum for 24 h. 0.05 g ZIF-8 powders, 500 μL triton-X-100 and 3 mL isopropyl alcohol were mixed and grinded for 6 h in the ball mill to form ZIF-8 paste. FTO conductive glass substrate was patterned by etching with 4 M HCl and Zn powder [23]. The patterned FTO is firstly soaked in soapy water for 5 min, cleaned in an ultrasonic bath with water, acetone and ethanol for 15 min, respectively. After dried under air, FTO was sintered at 500°C for 10 min in air to remove residual organics. The ZIF-8 paste was then spin-coated on FTO substrate at 4000 r·min-1 for 20 s, which was heated at 125°C for 10 min in a tube furnace under N2. To convert ZIF-8 into carbon, the ZIF-8 thin film was sintered in a tube furnace at different temperatures for 3 h with a rapid heating rate of 2°C·min-1 under N2. The final sample was denoted as C450, C500, C550 and C600 corresponding to the carbonization temperature of 450°C, 500°C, 550°C and 600°C, respectively. 2.3 Fabrication of perovskite solar cells The diluted 18 nm TiO2 particle paste in ethanol was spin-coated on the ZIF-8 derived porous carbon layer at 4000 r·min-1 for 20 s. The weight ratio of paste and



ethanol is 1:3.5. After dried at 125°C for 30 min in a tube furnace under N2, the thin films were sintered at 500°C for 30 min. Then the thin films were treated by dipping them in TiCl4 (25 mM) solution for 30 min at 70°C, followed by rinsing with deionized water and then annealed at 450°C for 30 min [24]. The perovskite thin film was deposited in air by spin-coating the perovskite precursor solution onto the surface of TiO2 thin film. The precursor solution with the concentration of 1.2 M was prepared by dissolving a stoichiometric amount of PbI2 and CH3NH3I (molar ratio of 1.05:1) in a mixed solvent of DMSO and DMF (volume ratio of 4:1). The spin-coating procedure include two steps, first at 4000 r·min-1 for 10 s, then 350 μL ethyl acetate was gently dropped on the spinning substrate at 4000 r·min-1 for 20 s to form smooth perovskite films. The CH3NH3PbI3 layer was then annealed at 100°C for 10 min. After cooling down to room temperature, the hole transporting material of Spiro-OMeTAD was deposited on the perovskite layer by spin-coating at 4000 r·min-1 for 20 s. The deposition solution was prepared by mixing 72.3 mg Spiro-OMeTAD, 28.8 μL 4-tert-butylpyridine, 17.5 μL 520 mg∙mL-1 lithium bis(trifluoromethylsulfonyl) imide acetonitrile solution in 1 mL of chlorobenzene [25]. Finally, an Au electrode with thickness of 100 nm was thermally evaporated to form the back contact in a high vacuum chamber. 2.4 Characterizations of the prepared layers The morphologies of the porous carbon layers and the perovskite layer on nanoporous TiO2 thin film were observed using a scanning electron microscope (SEM, FEI Nova Nano SEM 230, 15 kV). The microstructures of the porous carbon


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frameworks were determined by high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20). The specific surface area of the powders was measured by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption and desorption isotherms on a Micromeritics ASAP 2020 instrument at 77 K. The pore size and size distribution were calculated by the Barrett-Joyner-Halenda method. The crystalline structure of the samples was characterized by an X-ray powder diffraction meter (XRD, Rigaku D/max-2500, Cu Kα). The photoluminescence (PL) spectra were measured by a fluorescence spectrophotometer (HITACHI F-5000) and the excitation wavelength is 520 nm. The Raman spectra were analyzed by Labram HR Evolution (HORIBA Scientific). The absorption spectra of perovskite thin films were measured using a spectrophotometer (UV-2600) at room temperature. Thermogravimetric analysis (TGA) was carried out on a TGA instrument (Perkin-Elmer Pyris-1) with a calibrated platinum pan, and the temperature was increased from 90°C to 800°C at 10°C·min-1. Infrared spectra were acquired from a KBr pellet of ZIF-8 using a Fourier transform infrared (FTIR) spectrophotometer (Nicolet 360). The chemical composition and the valence states of the samples were analyzed on an X-ray photoelectron spectrometer (XPS, AXIS ULTRA Al Kα). J-V curves of perovskite solar cells were measured in air and under illumination with a solar simulator (Newport 94023 A) at 1 sun (AM 1.5, 100 mW·cm-2). And the active area is 0.20 cm2. Electrochemical impedance spectroscopy (EIS) were measured in the darkness with a frequency response tracer (Solartron 1260) and a potentiostat (Solartron 1287). The frequency range of EIS is from 105 Hz to 10-1 Hz. Tafel



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polarization curves (TPC) of the symmetrical cells fabricated with two identical electrodes

were

intensity-modulated

measured

with

photocurrent

a

potentiostat

spectroscopy

(Solartron (IMPS)

1287). and

The

incident

photon-to-current efficiency (IPCE) measurements were performed on the Zahner Zennium electrochemical workstation. For the IMPS characterization, a blue LED light (λ = 594 nm) was used to provide a sinusoidal optical perturbation signal, whose amplitude was 10% of the background light intensity with the frequency ranging from 0.5 Hz to 1.0 MHz. 3. Results and discussion

3.1 Characterization of ZIF-8 prepared with PVP XRD patterns of ZIF-8 powders prepared with and without addition of PVP were shown in Fig. 1a. The peaks located at 7.60°, 10.57°, 12.85°, 14.80°, 16.60°, 18.12°, 19.70°, 24.60°, 25.75°, 26.78°, 29.74°, 30.66° and 32.36°, respectively. They correspond to the diffraction angles of (011), (002), (112), (022), (013), (222), (114), (233), (224), (134), (044), (244) and (235) crystal planes of ZIF-8 [19,21]. No difference can be found between two patterns, which means that addition of PVP does not affect the crystalline structure of ZIF-8.


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Intensity (a.u.)

a

ZIF-8+PVP

ZIF-8

5

10

15

20

25

30

35

1500

1000

40

2θ (Degree)

b ZIF-8+PVP Tramsmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZIF-8

4000

3500

3000

2500

2000

Wavenumber (cm-1)

500

Fig. 1 XRD patterns (a) and FTIR spectra (b) of ZIF-8 prepared without and with PVP.



The FTIR peaks of ZIF-8 powders prepared with and without addition of PVP are almost same and in agreement with the reported peaks as shown in Fig. 1b [21]. The peaks at 3133 cm-1 and 2932 cm-1 represent the aromatic and aliphatic C-H stretch of the imidazole ring, respectively. The out of plane and in plane bending of the imidanzole ring are located between 692-744 cm-1 and 950-1177 cm-1, respectively. The peak at 427 cm-1 is due to the stretching of the Zn-N band. The strong peaks in the region of 1300-1450 cm-1 represent the C-N absorption band. The N2 adsorption isotherm (Fig. S1) of ZIF-8 powders prepared with PVP shows typical type I isotherm, which matches well with the microporous structure of ZIF-8. A narrow pore size distribution (1-1.5 nm) was shown in inset in Fig. S1. The BET surface area is 1503.83 m²·g-1, micropore area is 1470.05 m²·g-1, external surface area is 33.77 m²·g-1, and micropore volume is 0.69 cm³·g-1. Such porous structure and high specific surface area have certain advantages for the deposition of TiO2 nanoparticles. The mean size of ZIF-8 particles is less than 100 nm, and the size distribution is homogeneous, which can be confirmed from SEM images as shown in Fig. S2. It has been reported that the particle shape of ZIF-8 can be adjusted by adding PVP [21]. However, we found addition of PVP during synthesis of ZIF-8 could make the ZIF-8 particle size be smaller, which is conducive to our subsequent research work. The synthesized smaller ZIF-8 particles are more easily ground to form slurry and spun onto FTO substrate to form homogeneous thin films.


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3.2 Carbonization of ZIF-8 thin films TGA of ZIF-8 under inert atmosphere with a continuous temperature ramping rate of 10°C·min-1 was shown in Fig. 2. The starting temperature of framework decomposition/carbonization is ca. 450°C, which is in good agreement with early reports [26,27]. The decomposition and carbonization of ZIF-8 above 450°C can be observed through its complete transition to black, which is presumably carbon. Although the crystalline structure and molecular bonds of ZIF-8 are not affected as PVP was added during the synthesis of ZIF-8, it is very exciting that the carbonization temperature decreases as PVP was contained. As shown in Fig. 2, the component loss of ZIF-8 prepared with PVP is easier, and the carbonization temperature decreases more than 50°C. Because of the limitation of temperature that FTO conducting glass can afford, it is very meaningful for the film on FTO substrate that the carbonization temperature can be less than 600°C.

100

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

ZIF-8

ZIF-8+PVP

60

0

200

400

Temperature (C)


600

800


Fig. 2 TGA curves of ZIF-8 prepared without and with PVP. The SEM morphologies of as-prepared ZIF-8 thin film and ZIF-8 derived thin films were shown in Fig. 3. The morphologies of ZIF-8 derived carbon materials are different to that of ZIF-8 when the carbonization temperature is lower than 600°C (Fig. S2). The ZIF-8 derived carbon could basically maintain the original morphology of ZIF-8 thin film. Except, the surface of C600 was coarse and coacervate as depicted in Fig. 3e. In the high resolution image of C550 as shown in Fig. 3f, porous structure can be clearly observed. The size for some bigger holes is even more than 100 nm. Fig. S3a showed XRD patterns of the ZIF-8 derived porous carbon materials carbonized at different temperatures. Two broad peaks are located at 24° and 44°, respectively, corresponding to the (002) and (100) facets of carbon [18,28].


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a

b

c

d

e

f

Fig. 3 SEM images of ZIF-8 derived porous carbon layers carbonized at 0 °C (a), 450 °C (b), 500 °C (c), 550 °C (d, f) and 600 °C (e). Fig. 4 further showed TEM images of ZIF-8 derived carbon at 550°C. The d-value of 0.33 nm is consistent with that of the graphitic structure as shown in Fig. 4b. Raman spectra of the ZIF-8 derived carbon materials (Fig. S3b) indicated that two peaks at 1350 cm-1 and 1590 cm-1 can be ascribed to D and G bands of carbon,



respectively, the closer the ratio of IG to ID is, the higher the degree of graphitization is [29]. The porous carbon structure could favorably influence the conductivity of the samples [30].

a

b

0.33nm

Fig. 4 TEM image (a) of C550 and HRTEM images (b) particles marked with circle in (a). The surface composition and chemical states can be determined by means of XPS spectrum according to the characterizing binding energies of the different elements on material surfaces. Fig. 5 showed XPS spectra of C550. The XPS


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spectrum of the Zn 2p as shown in Fig. 5b displays a doublet at about 1022 eV and 1045 eV, corresponding to the Zn-2p3/2 and 2p1/2 core levels [31]. The C1 and C2 peaks are assigned to a sp2-hybridized graphite-like carbon structure and a sp3-hybridized carbon, respectively. Furthermore, the N 1s peaks are deconvoluted into three different types of nitrogen species: pyridinic-N (N1, 398.4 eV), pyrrolic-N

Zn-2p 1/2 Zn-2p 3/2

(N2, 399.5 eV) and graphitic-N (N3, 400.7 eV) (Fig. 5d) [32].

2p1/2

1000

800

600

Intensity (a.u.)

C-1s

400

200

2p3/2 1045.2 eV

1050

1045

Binding energy (eV)

1035

1030

1025

1020

398.4 eV

d

284.6 eV

C1s

N1

290

Intensity (a.u.)

N1s

287.8 eV

292

1040

Binding energy (eV)

C1

c

294

1022.3 eV

Zn2p

O-1s Sn-2p 3/2 Sn-2p 5/2 N-1s 1200

b

ZIF-8 DPCL

Intensity (a.u.)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C2

288

286

284

282

280

278

400.7 eV N3

399.5 eV N2

410

408

Binding energy (eV)

406

404

402

400

398

396

Binding energy (eV)

Fig. 5 XPS spectra of C550 (a), Zn 2p (b), C1 (sp2) and C2 (sp3) (c), N 1s (d). 3.3 Photovoltaic performance of perovskite solar cells based on porous carbon layer


394

392


Perovskite solar cells based on the porous carbon layer were fabricated with the structure of FTO/PCL/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au as shown in Fig. 6a. The schematic energy diagram of the ZIF-8 DPCL based device was shown in Fig. 6b. As the provskite is irradiated, the electrons are excited to the conduction band of provskite, and then injected into the CB of TiO2 and carbon, finally collected at the FTO anode (work function: 4.60 eV) [33]. According to the band position in Fig. 6b, ZIF-8 derived porous carbon will be beneficial to electron injection during photoexcitation [34]. Due to high conductivity of the carbon layer and thin thickness of the TiO2 layer, the porous carbon structure coated by a thin TiO2 layer as an electron transport layer of perovskite solar cell can increase the electron transport rate, thus improve light-to-electric conversion efficiency of the cell.

a


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b

Fig. 6 The structure schematic of perovskite solar cells with and without the ZIF-8 derived porous carbon layer (a) and the schematic energy diagram of the ZIF-8 DPCL based device (b). XRD of perovskite was illustrated in Fig. S4. The peaks at 14.2°, 28.5° and 31.8°, correspond to the (110), (220) and (330) reflections of terahedral perovskite crystal structure, respectively, demonstrating the formation of perovskite. Two peaks at 12.5° and 25.8° are assigned to PbI2. It was reported that small amount of PbI2 could suppress the charge recombination and enhance the performance of perovskite solar cells [25]. The top view of the perovskite thin film deposited on the TiO2 electron transport layer and the cross sectional SEM images of the whole perovskite solar cell were presented in Fig. S5. The corresponding images without the derived porous carbon layer were also shown in Fig. S5 to demonstrate the effect of the derived carbon layer on the morphologies of perovskite layer. The mean size of grains distributed in the perovskite thin film is approximate 450 nm. And the grain size of perovskite is slightly increased due to the introduction of the porous



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carbon layer as shown in Fig. S5b. The cross sectional SEM image of the full cell reveals its uniform architecture. The thickness of the perovskite capping layer is about 400 nm, and the thickness of the TiO2 coated on porous carbon layer is about 520 nm. It can be seen from Fig. S5c that CH3NH3PbI3 was successfully infiltrated into the pores of the TiO2 coated on porous carbon layer. Thus, the thickness of the photo-active film is enough to gain a relatively high photocurrent value. From the cross-section image of the cell structure without ZIF-8 DPCL as shown in Fig. S5d, the thickness of the perovskite capping layer is about 390 nm, and the thickness of the TiO2 layer is about 380 nm. The UV-Vis absorption spectra of perovskite thin film on TiO2 coated on porous carbon layer was shown in Fig. S6. The introduction of ZIF-8 derived layer increases the optical absorption of (CH3NH3)PbI3 in the visible light range. Bandgap for the (CH3NH3)PbI3 deposited on the TiO2 thin film is determined to be 1.57 eV from the extrapolation of the liner part in Tauc plot (Fig. S6b), which also indicates that the optical absorption in the perovskite sensitizer occurs via a direct transition [2,4]. When the ZIF-8 derived carbon layer was introduced, the electron transfer from MAPbI3 to TiO2 is enhanced significantly according

to

the

stronger

photoluminescence

quenching

in

the

FTO/PCL/TiO2/MAPbI3 film as shown in Fig. S6a, which can be ascribed to the outstanding electron extraction ability of ZIF-8 derived carbon layer. In order to further study the effect of the ZIF-8 derived carbon layer on photovoltaic performance of perovskite solar cell, we assembled a solar cell without the ZIF-8 derived carbon layer for comparison. Fig. 7 showed the J-V curves of the


Page 21 of 34

cells without and with the ZIF-8 derived carbon layers carbonized at different temperatures. The corresponding photovoltaic parameters such as short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (ff) and light-to-electric conversion efficiency (η) were collected in Table 1. η of solar cell without the ZIF-8 derived porous carbon layer is 14.25%. By contrast, perovskite solar cell based on the C550 layer gets 17.32% of η, being better than that of the cells with the derived porous carbon layer prepared at other temperatures and without the ZIF-8 derived layer. We can see less hysteresis of perovskite solar cell with ZIF-8 DPCL as in shown in Fig.S7. The performance of the cell based on the sorely ZIF-8 derived porous carbon layer without depositing the TiO2 nanoparticle is bad as shown in Fig.S8, which indicate the deposition of TiO2 nanocrystalline layer is necessary to prevent direct contact of perovskite and hole transport material with FTO, because the derived carbon layer is porous.

25

Current density (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15

Without C450 C500 C550 C600

10 5 0

0

200

400

600

800

1000

1200

Voltage (mV)

Fig. 7 J-V curves of perovskite solar cells based on the TiO2 electron transport layers without and with ZIF-8 derived porous carbon layers carbonized at different



Page 22 of 34

temperatures measured with a mask. Table.1 Photovoltaic performance parameters of perovskite solar cells without and with the ZIF-8 derived porous carbon layers carbonized at different temperatures. Cells

Jsc (mA·cm-2)

Voc (V)

ff

η (%)

Without

20.61

0.98

0.65

14.25

C450

20.41

1.01

0.64

14.83

C500

21.75

1.03

0.69

16.15

C550

22.13

1.06

0.72

17.32

C600

20.21

1.00

0.63

12.87

3.4 Photovoltaic performance enhancement mechanism due to porous carbon layer It is worth noting that the perovskite solar cell composed of the C550 layer has significantly improved Jsc and ff values compared with that without one. The improved photovoltaic parameters may be due to the high-speed channel for electrons formed by the ZIF-8 derived carbon layer as illustrated in Fig. 6a, enabling electrons to reach FTO quickly through TiO2 and thus improving the ability to extract charge carriers from the active layer. Conductivity is used to describe the degree of charge flow parameters in material. We further tested Tafel curves of FTO and ZIF-8 derived porous carbon layer coated on FTO to demonstrate the conductivity of the derived carbon [35]. As shown in Fig. 8a, when the applied voltage is 0.5 V, the current density value of thin film electrodes without and with ZIF-8 carbonized at different temperatures is 3.0×10-2 mA·cm-2 (Without), 2.7×10-2 mA·cm-2 (C450), 7.6×10-1 mA·cm-2 (C500), 9.5×10-1 mA·cm-2 (C550) and 1.5×10-2 mA·cm-2 (C600), respectively. When the voltage remains constant, the conductance


Page 23 of 34

is proportional to the DC current in the circuit. Therefore, the porous conducting framework obviously improves the conductivity of nanocrystalline thin film [36]. The electron extraction efficiency of perovskite solar cells was first evaluated by steady state photoluminescence measurement. As shown in Fig. 8b, the carbonized ZIF-8 thin film was introduced to result in strong photoluminescence quenching of perovskite. The decrease in intensity is closely related to more efficient electron extraction from perovskite [12,37]. The photoluminescence quenching due to TiO2 coated on the ZIF-8 derived carbon thin layer can explain the injection efficiency improvement of electrons in the contact interface of perovskite and TiO2. 1.5 1.0

log(J) (log(mAcm-2))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

0.5 0.0 -0.5 Without C450 C500 C550 C600

-1.0 -1.5 -2.0 -2.5 -0.6

-0.4

-0.2

0.0

0.2

V (V)


0.4

0.6


b

Perovskite C450/TiO2/Perovskite C500/TiO2/Perovskite C550/TiO2/Perovskite C600/TiO2/Perovskite

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

700

720

740

760

780

800

820

840

Wavelength (nm)

Fig. 8 Tafel polarization curves (a) of symmetrical cells with two identical electrodes and photoluminescence spectra (b) of perovskite thin films on different substrates. The excitation wavelength is 520 nm. EIS is usually employed to study the dynamics of charge transfer and recombination in perovskite solar cells [23,24]. Fig. 9a showed EIS of perovskite solar cells based on different ZIF-8 derived porous carbon layers measured in the dark, and an equivalent circuit to fit EIS was shown inset. The fitting parameters were summarized in Table 2. When the ZIF-8 derived carbon thin layers are introduced, the recombination resistance (Rrec), as shown in the low frequency response, increases from 4152 Ω·cm2 to 6051 Ω·cm2 for the cell with the C550 layer. The corresponding Crec for the best cell based on the C550 layer is increased from 1.91×10-7 F to 6.96×10-6 F, which indicates that MAPbI3 has more interface with TiO2 thin layer due to the porous carbonized ZIF-8 thin layer [38,39]. These results


Page 25 of 34

show that the ZIF-8 derived thin layer introduced in the cells favors electron transfer from perovskite and transport in the TiO2 layer towards FTO, and thus effectively reduces the charge recombination, which is one of the reasons for the performance improvement of this kind of cells [37].

3500

a

3000


-Z'' (Ω cm-2)

2500 2000 1500

Rs

1000 500 0

0

Rct

Rrec

Cct

Crec

1000 2000 3000 4000 5000 6000 7000 8000

Z' (Ω cm-2)

6 5


b

4 3

I'' (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1 0 -1 -2 -2

-1

0

1

2

3

4

5

6

I' (A)

Fig. 9 EIS (a) and IMPS (b) of perovskite solar cells without and with ZIF-8 derived



Page 26 of 34

porous carbon layers carbonized at different temperatures. The equivalent circuit to fit EIS is inset in (a). Table 2. Parameters obtained from EIS and IMPS of perovskite solar cells without and with the ZIF-8 derived porous carbon layers carbonized at different temperatures. Cells

Rs(Ω·c

Rct(Ω·cm2)

Rrec(Ω·cm2)

Cct (F)

Crec (F)

τ (s)

m2) Without

23.5

915

4152

7.07×10-6

1.91×10-7

2.76×10-3

C450

29.6

1213

3563

3.05×10-6

2.54×10-7

2.35×10-3

C500

22.4

1021

5944

2.35×10-6

2.04×10-7

1.71×10-4

C550

20.4

782

6051

1.01×10-7

6.96×10-6

1.11×10-4

C600

30.1

1135

5012

5.01×10-6

2.42×10-7

3.20×10-4

IMPS was extensively used to investigate carrier transport in dye-sensitized solar cell. It was recently used by Guillen et al. on mesostructured perovskite solar cells [39]. The semicircle at the low frequency region of IMPS directly relates with the electron transport in TiO2, where the electron transporting time (τ) can be calculated from the equation of τ=1/2πf. Clearly, as shown in Fig. 9b and Table 2, the perovskite solar cell based on the ZIF-8 derived porous carbon layer has shorter τ than the cell without one, suggesting the enhanced electron transport in the TiO2 electron transport layer, due to the carbonized skeleton structure to provide a high-speed channel for electrons to reach the base of FTO quickly as illuminated in Fig. 6a. Compared with the cell without the carbonized ZIF-8 layer, perovskite solar


Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cell based on the ZIF-8 derived layer has higher electron transfer efficiency and lower recombination rate. 4. Conclusions The ZIF-8 derived porous carbon skeleton was prepared by carbonizing ZIF-8 coated on FTO substrate. The derived porous carbon was buried under the TiO2 thin layer to be used as the electron transport layer for perovskite solar cell. The photovoltaic performance of perovskite solar cell based on the porous carbon layer was improved obviously. The derived carbon layer can be used as the high-speed channel of electron transport from the TiO2 thin layer to FTO substrate. In addition, the specific surface area of subsequent deposited TiO2 thin layer relatively increase, which increases the contact interface area between the TiO2 layer and the perovskite layer. Therefore, the performance enhancement of the perovskite solar cells was attributed to the specific surface area increase of the TiO2 thin layer and improvement of the electron transport through the ZIF-8 derived porous carbon layer. By contrast, perovskite solar cell based on the ZIF-8 derived porous carbon thin layer gets 17.32% of conversion efficiency, being better than the cell without the derived layer. Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Grant Nos. 21273160), the Nature Science Foundation of Tianjin (Grant No. 14JCYBJC18000) and the Program for Innovative Research Team in University of Tianjin (TD13-5074).



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Electron Transport Improvement of Perovskite Solar Cells via a ZIF-8

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