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CdS as electron transport layer
Solar Energy 191 (2019) 647–653
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Improvement of planar perovskite solar cells by using solution processed SnO2/CdS as electron transport layer
T
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Fateme Mohamadkhania, Sirus Javadpoura, , Nima Taghaviniab,c a
Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz 7134851154, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran c Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Planar perovskite solar cell Electron transport layer SnO2 CdS nanoparticle Electron extraction
The efficiency of planar perovskite solar cells (PSCs) with SnO2 as electron transport layer is already more than 19% achieved under controlled atmosphere. PSCs with solution processed SnO2 show high hysteresis and low fill factor. One way to improve the planar PSCs is using buffer layer between electron transport layer and perovskite to enhance the photo-electron extraction process. In this study, SnO2 and SnO2/CdS layers were fabricated by solution process using a suspension including CdS nanoparticles synthesized via a simple solution route. Then planar PSCs with the structure of Glass/FTO/ETL/Perovskite/Sprio-OMeTAD/Au were fabricated in ambient air condition using SnO2 and SnO2/CdS as ETL. It is shown that a thin interface layer of CdS nanoparticles on top of SnO2 layer consistently improves the electron transporting properties of SnO2 layer. Mott-Schottky analysis shows a gradual change of electron affinity takes place by deposition of CdS nano particles. CdS interface layer can act as an intermediate step to facilitate electron transfer from perovskite layer to SnO2. The hysteresis index reduces from 0.17 to 0.05 and the efficiency improves from 15.0% to 17.18%. Impedance spectroscopy indicates that interface resistance is reduced by incorporating CdS nanoparticles.
1. Introduction In the past few years, perovskite solar cells (PSCs) have attracted much attention due to high efficiency and low-cost fabrication process (Christians et al., 2015; Lee et al., 2012; Singh and Miyasaka, 2018; Snaith, 2013; Yang et al., 2017.) The perovskite absorber possesses properties such as proper band gap, high absorption coefficient and high charge carrier diffusion length (Snaith, 2013). In a typical planar PSC configuration, absorber layer is sandwiched between electron transporting layer (ETL) and hole transporting layer (HTL) (Correa Baena et al., 2015). The most commonly used ETL and HTL are Tittanium oxide (TiO2) and Spiro-OMeTAD, respectively (Snaith, 2013). TiO2 is usually used as a two-layer structure of dense/mesoporous films (Im et al., 2014; Singh and Miyasaka, 2018; Zhen et al., 2019). However, Another inorganic n-type semiconductor that can be a good alternative for TiO2 is tin oxide (SnO2) (Guo et al., 2018), (Song et al., 2015) which does not need mesoporous layer in order to show high efficiency PSC. SnO2 has some advantages such as high electron mobility(Roose et al., 2016), wide band gap (3.6–3.8 eV) and high transparency in visible range spectrum(Jiang et al., 2017). Nevertheless, the deposition of high quality SnO2 ETL is not straightforward and usually needs some sort of modification in order to
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attain high device efficiency. Different strategies have been reported; Xiong et al. used Mg doped quantum dot (QD) SnO2 as ETL (Xiong et al., 2018). They have shown that the high temperature Mg doped QD SnO2 shows better transparency than spin coated SnO2 and TiO2 layer. Li doped SnO2 demonstrated lower conduction band energy and higher conductivity than un-doped, thus better electron transfer may occur from perovskite to ETL (Park et al., 2016). It was shown that chemical bath deposited Nb doped SnO2 can act as a promising ETL with lower hysteresis (Halvani Anaraki et al., 2018). PSCs with SnO2 ETL usually show low fill factor and high hysteresis. This problem can be improved by using an interface layer between ETL and perovskite layer to reduce the interface trap density and facilitate photo-electron extraction process. Tittanium(IV) chloride (TiCl4) treated SnO2 nano particles can act as a mesoporous scaffold in PSC and comparable with PSCs with TiO2 ETL (Li et al., 2015). By TiCl4 treatment, the recombination is significantly retarded so the open circuit voltage (VOC) increases in comparison to bare SnO2. ZnO/SnO2 double layer as ETL can increase the VOC from 1.07 V to 1.15 V because of higher Fermi level and conduction band level of ZnO/SnO2 than SnO2 (Wang et al., 2018). Chemical bath deposited SnO2 on top of the spin coated SnO2 has also improved the coverage of ETL on FTO and therefor can enhance FF and the efficiency of PSC (Anaraki et al.,
Corresponding author.
https://doi.org/10.1016/j.solener.2019.08.067 Received 31 May 2019; Received in revised form 25 August 2019; Accepted 27 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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on top of the ETLs via one step deposition method using chlorobenzene as anti-solvent (details of the deposition of perovskite can be find elsewhere.(Saliba et al., 2018)). Precursor solution for the perovskite was prepared by dissolving PbI2 (1.5 mmol, TCI), PbBr2 (1.5 mmol, IRASOL), Formamidinium Iodide (FAI) (1 mmol, dyesol) and Methylammunium Iodide (MABr) (1 mmol, dyesol) in mixture of anhydrous DMF: DMSO (4:1; v: v) (1 ml, Merck). Then CsI solution (50 µl) (CsI (1.5 mmol, Sigma, 99) dissolved in DMSO (1 ml)) was added to 950 µl of aforementioned solution. The perovskite precursor solution was spin coated on top of the ETL layers at 1000 rpm for 10 s and 6000 rpm for 20 s, followed by the injection of chlorobenzene at last 6 s. Then, the samples were immediately transferred to the hotplate at 100 °C for 60 min at ambient air. Spiro-OMeTAD precursor solution was prepared by dissolving of spiro-OMeTAD (72.3 mg, Borun, 99.8) in chlorobenzene (1 ml, Sigma) mixed with dopants; 4-tert-butylpyridine (28.8 µl, Sigma, 96) and bis (trifluoromethylsulfonyl)imide lithium salt (17.5 ml from solution of 520 mg/ml bis(trifluoromethylsulfonyl)imide lithium salt (99.95, Sigma) dissolved in acetonitrile (Merck)). This precursor solution was deposited on perovskite absorber layers by spin coating at 5000 rpm for 30 s. Finally, 70 nm thin film of gold was evaporated at high vacuum on the cells as top contact metal electrode.
2016). The conductivity and electrical mobility of SnO2 ETL have been reported be improve by water vapor treatment, and therefore PSCs with flexible substrate can show the efficiency of 18.36% (Wang et al., 2017). CdS has also been studied as an ETL. Devices based on CdS ETL show reduced hysteresis and better stability in comparison to devices based on TiO2 ETL due to reduced surface defects of CdS layer (Hwang et al., 2015; Peng et al., 2016; Wessendorf et al., 2018). CdS represents much higher electron mobility than SnO2 and TiO2 (electron mobility of CdS , SnO2 and TiO2 are 350 cm2/V.S , 240 cm2/V.S and 1 cm2/V.S, respectively) (Jiang et al., 2017; Tiwana et al., 2011; Wessendorf et al., 2018). In this research, Cadmium sulfide nanoparticles (CdS NPs) suspension was synthesized and applied on top of SnO2 ETL as an interface layer. Then planar PSCs with the structure of Glass/FTO/ETL/ Perovskite/Sprio-OMeTAD/Au were fabricated in ambient air condition using SnO2 and SnO2/CdS as ETL The pristine SnO2 ETL show the efficiency as high as 15% which was improved to 17.18% by CdS interface layer. The fill factor increased and hysteresis decreased after modifying with CdS NPs. Interface resistance was studied by impedance spectroscopy to clarify the effect of CdS NPs on charge carrier transfer and recombination rate in the planar perovskite solar cells. 2. Methods 2.1. Preparation of CdS nanoparticle suspension
3. Results and discussion
CdCl2·2.5H2O (1 mmol, Sigma, 99.99) and Sulfur (2 mmol, Sigma, 99.98), were completely dissolved in butyl amine (5 ml, Merck, 99.9) in separate vials at room temperature. Then, the S containing solution was added to Cd containing solution while the mixture was stirred continuously on magnetic stirrer at room temperature. The solution was transparent initially and after several hours of stirring the solution became cloudy and a pale yellow opaque precipitate was formed, indicating the formation of CdS NPs. The precipitate was separated by centrifuging and washed by mixture of ethanol and chloroform for several times. Finally, the CdS NPs were dispersed in Dimethylformamide (DMF) (Merck, 99.8) by sonication to form a stable suspension with 10 mg/ml concentration. For structural and opto-electronic characterization, thin films of CdS NPs were deposited on glass substrate by spin coating and annealed at 250 °C under N2 atmosphere.
3.1. Characterization of CdS nanoparticles and thin films TEM image of CdS NPs fixed on grid as shown in Fig. 1a illustrates that the particle size of synthesized CdS NPs are almost less than 10 nm. No large agglomerated particles can be seen, due to the fact that the synthesized CdS NPs were well dispersed. XRD patterns of the CdS powder; as-synthesized CdS NPs and the one annealed at 250 °C are shown in Fig. 1b. XRD pattern of as-synthesized CdS NPs coating shows diffraction peaks corresponding to cubic CdS phase (JCPDS card no; 1-080-0019). Broad diffraction peaks represent the nanocrystalline structure with poor crystallinity of assynthesized CdS NPs coating. While, the XRD pattern of annealed CdS NPs coating shows diffraction peaks corresponding to hexagonal CdS phase (JCPDS card no; 00-001-0783), which shows phase transformation and relative grain growth during annealing process. UV/Vis Transmittance spectra and calculated tauc plot corresponding to the CdS layers deposited on glass substrate and annealed at 250 °C for 5 min are shown in Fig. 1c and d, respectively. The band gap of CdS layer is determined by linear extrapolation of tauc plots about 2.66 eV which is blue shifted from the band gap of bulk CdS in hexagonal phase (the band gap of bulk CdS in hexagonal phase is reported 2.57 eV (Baykul and Orhan, 2010)). The FESEM images of the FTO samples, FTO/SnO2 and FTO/SnO2/ CdS are shown in Fig. 2 a, b and c, respectively. As can be seen in FESEM images, the SnO2 and CdS NPs coatings are continuous with full coverage. From photography images inset in FESEM images, CdS NPs coated sample looks slightly yellowish. The AFM topographical images of FTO, FTO/SnO2 and FTO/SnO2/ CdS surfaces are shown in Fig. 2. According to the AFM images the surface roughness of FTO is 29.1 nm, after deposition of SnO2 layer the surface roughness decreases to 24.5 nm (for FTO/SnO2 sample). Also by deposition of CdS NPs layer the surface roughness decreases further to 21.8 nm (for FTO/SnO2/CdS reduced). It can be due to the formation of continues and few nanometres thick layers during SnO2 and CdS NPs deposition. Mott-Schottky analysis is used to calculate the carrier density and flat band potential of ETLs. The FTO/SnO2 and FTO/SnO2/CdS samples are considered as working electrode in Mott-Schottky analysis which is explained by the following equation:
2.2. Fabrication of perovskite solar cells Fluorine doped tin oxide (FTO) coated glass substrates were cut into 1.4 cm × 1.4 cm square pieces. In order to provide a metal contact position, one side of FTO substrates were etched with zinc powder and HCl (2 M), then rinsed with sufficient amount of deionized water. The FTO substrates were washed and cleaned by ultrasonic bath in different solutions; diluted HCl in ethanol, acetone, isopropyl alcohol and ethanol, respectively. Finally, the FTO substrates were heated up to 500 °C for 30 min. The FTO substrates were treated with UV/Ozone just before coating steps. SnO2 precursor solution was prepared by dissolving SnCl2·2H2O (0.1 mmol, Sigma, 99.99) in ethanol (1 ml, Merck, 99.9). The precursor solution was spin coated on clean FTO substrates at 5000 rpm, and annealed at 500 °C for 1 h. To investigate the effects of CdS NPs on the performance of solar cells, some of FTO/SnO2 layers were coated by CdS NPs. A thin layer of CdS NPs was deposited on top of the SnO2 layers via spin coating of the CdS NPs dispersed in DMF (mentioned above) at 4000 rpm for 60 s. Then the FTO/SnO2/CdS layers were annealed at different temperatures; 150, 250, 350 °C under N2 atmosphere, to form dense CdS layers and optimize the annealing temperature. A mixed cations mixed halides perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) absorber layer was deposited 648
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Fig. 1. (a) TEM image of as-synthesized CdS NPs dispersed in DMF, (b) XRD patterns of as-synthesized CdS NPs and CdS NPs annealed at 250 °C, (c) UV/Vis spectrum of CdS NPs annealed at 250 °C and (d) tauc plot extracted from UV/Vis spectra of CdS NPs annealed at 250 °C.
1 2 kT ⎞ ⎛ = ⎜V − Vf − ⎟ C2 qεND A2 ⎝ q ⎠
PbI2 doesn't have deleterious effect on the device performance (Jacobsson et al., 2016). The band gap of the perovskite measured by tauc plot extracted from UV/Vis spectrometry is about 1.62 eV that is agree with other reports (Fig. S2c and d) (Saliba et al., 2016). From all of results mentioned above, stacked structure and band alignment of the layers in the planar PSCs investigated in this research can be schematically visualized like Fig. 3. Fermi level of SnO2/CdS layer insert between the conduction band of SnO2 and perovskite layer and can be act as an intermediate step that facilitates electron transfer from perovskite to SnO2 layer.
(1)
In which C is capacitance, q is the elementary charge, ɛ is the dielectric constant, ND is the doping density, V is applied potential, Vf is the flat band potential and A is the surface area of the interface (Cardon and Gomes, 1978). Mott-Schottky plot is 1/C2 vs applied voltage (V) thus the flat band potential can be achieved by linear extrapolation of linear portion of the plot and the slope of the linear portion deducted the carrier density of the material (Haghighi et al., 2018; Meissner et al., 1988). The Vf corresponds to a situation that there is no band bending in the equilibrium Fermi level arising from the electrode material and the electrolyte contact (Feng et al., 2009; Bakr et al., 2018). As can be seen in the Mott-Schottky plot of FTO/SnO2 and FTO/ SnO2/CdS demonstrated in Fig. 2e and f, the positive slope of curves represents n-type semiconducting materials. By utilizing the best fit in linear portion of Mott-Schottky plots, the flat band potential and carrier density of SnO2/CdS and SnO2 are calculated about 4.18 eV and 8.4E + 18 cm−3, 4.46 eV and 5.2E18 cm−3, respectively. The more carrier concentration demonstrate the better diode behaviour. The flat band potential of FTO calculated about −4.55 eV from Mott-Schottky plot. The XRD patterns of FTO/SnO2, FTO/SnO2/CdS shown in Fig. S1a and b, respectively, which are in good agreement with JCPDS card no; 01-077-0451. After loading CdS NPs on the surface of SnO2, no peaks of CdS can be observed, although, the peaks weakened and broadened. Exactly, because of CdS NPs deposited in very thin thickness and it is difficult to detect. From transmission spectra and tauc plot of SnO2 layer in Fig. S1, it can be deducted that the band gap is calculated about 3.65 eV. The top view FESEM image from the Perovskite absorber layer is shown in Fig. S2 a. Micron sized uniform grain structure of the perovskite absorber layer represents the high quality of the absorber layer. In XRD pattern of perovskite layer in Fig. S2 b, the peaks at 12.78°, 14.1° are characteristic peaks of PbI2 and perovskite, respectively (Hu et al., 2017). On the base of other groups researches, existence a little
3.2. Characterization of planar perovskite solar cells with SnO2 and SnO2/ CdS as ETL To investigate the effect of CdS NPs on the performance of PSCs, devices were fabricated with SnO2 and SnO2/CdS as ETL. After CdS NPs deposition on SnO2 layer, to find the better annealing temperature 5 categories of ETLs including SnO2, SnO2/CdS-150, SnO2/CdS-250 (SnO2/CdS) and SnO2/CdS-350 (that means after CdS NPs deposition, the SnO2/CdS ETLs annealed at 150 °C, 250 °C, 350 °C, respectively) were investigated. J-V measurement of the solar cells were done at room temperature in air under AM1.5G simulated sunlight. The best performing devices investigated in this research are devices with SnO2/CdS ETLs annealed at 250 °C. Higher annealing temperature is likely causing better crystallinity of the CdS film, as well as sintering of NPs into more continuous films, demonstrating better electronic properties (Liu et al., 2010). The J-V plots of PSCs with FTO/SnO2/CdS (FTO/SnO2/CdS-250) and FTO/SnO2 as ETL are shown in Fig. 4a and b and J-V plots of PSCs with ETLs annealed at other temperatures are shown in Fig. S3 a and b. All of steps related to preparation of solution precursor and deposition of perovskite layer were done at ambient air. The performance characteristics of PSCs are shown in Table 1 (the performance characteristics of PSCs with FTO/SnO2/CdS-150 and FTO/ SnO2/CdS-350 ETLs are shown in Table S1). By CdS NPs deposition on the surface of SnO2 ETLs and annealing at 250 °C, the efficiency of the 649
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(a)
(b)
(d)
RMS= 29.1nm
(e)
FTO
0
0.4
0.2
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
0.8
FTO/SnO 2
(h)
0.6 0.4 0.2 0.0
RMS= 21.8nm
200nm
A2/C 2(mF -2.cm2)
(g)
(f)
200nm
A2/C 2(mF -2.cm2)
A2/C 2(mF -2.cm2)
RMS= 24.5nm
0
0.6
0.0
91 nm
105nm
200nm
0
1μm
1 μm
1 μm
127nm
(c)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
6 5
(i)
FTO/SnO 2/CdS
4 3 2 1 0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Fig. 2. (a, b, c) Top view FESEM images of FTO, FTO/SnO2 and FTO/SnO2/CdS samples, d, e, f) AFM surface topography images of FTO, FTO/SnO2 and FTO/SnO2/ CdS layers, (g, h, i) Electrochemical Mott-Schottky plots of FTO, FTO/SnO2 and FTO/SnO2/CdS layers, respectively.
lowest to highest hysteresis. The most significant effect caused by CdS NPs interface layer is decreasing the HI from 0.17 to 0.05 for devices with FTO/SnO2 and FTO/SnO2/CdS ETLs, respectively, which can be attributed to the fact that CdS NPs can passive the trap states made by surface defects in SnO2 and decrease the recombination at the ETL/ perovskite interface. PL measurement of the perovskite film deposited on SnO2 and SnO2/CdS ETLs are shown in Fig. 4c. The higher the charge of carrier injection, the higher the PL quenching of perovskite film, so these ETLs can be appropriate for the electron extraction from perovskite absorber.
solar cells is increased from 15% to 17.18%. Actually, the best performing device shows 17.18%, 0.723, 1.03 V, 23.08 mA/cm2 in efficiency, FF, Voc and Jsc, respectively. Hysteresis is a usual problem in perovskite solar cells. The hysteresis Index (HI) is defined by:
Hysteresis Index =
Jscan − (0.8VOC ) − Jscan + (0.8VOC ) Jscan − (0.8VOC )
(2)
where Jscan−(0.8Voc) is the photocurrent density at 0.8 Voc in reverse bias while Jscan+(0.8Voc) is the photocurrent density at 0.8 Voc in forward bias (Kim et al., 2018). HI varies from 0 to 1 corresponding to
Fig. 3. Schematics of a and (b) band alignment of layers in planar perovskite solar cells with SnO2 and SnO2/CdS ETLs, respectively (The energy bands of perovskite and spiro-OMeTAD is from literature (Byranvand et al., 2018). For conversion of reference energy levels to vacuum level, the absolute electrode potential of Ag/AgCl reference electrode is considered 4.66 eV (Trasatti, 1986)). The scheme is based on the electrochemically measured flat band potential levels of FTO, SnO2 and SnO2/CdS.
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Fig. 4. (a) J-V measurement of PSC with the structure of: FTO/SnO2/perovskite/spiro-OMeTAD/Au (SnO2) with cross section view FESEM image inset, (b) J-V measurement of PSC with the structure of: FTO/SnO2/CdS/perovskite/spiro-OMeTAD/Au (SnO2/CdS), (c) photoluminescence of perovskite layer deposited on SLG, SLG/SnO2 and SLG/SnO2/ CdS(SLG for soda lime glass), (d) IPCE spectra of the PSCs with the structure of FTO/SnO2/perovskite/ spiro-OMeTAD/Au (black line) and FTO/SnO2/CdS/ perovskite/spiro-OMeTAD/Au (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(Juarez-Perez et al., 2014). Since the main difference between devices is ETL, the variation in the resistance is associated to the electron transfer resistance at the ETL/perovskite interface. CdS NPs deposition at the interface of ETL/ perovskite can help to reduce the resistance against to the charge transfer from perovskite layer to SnO2 ETL observable in dark and light condition. The arc in low frequency region can be seen in light condition attributed to the slow process which is proposed to be associated with ion migration, ion intercalation or ferroelectric effect of perovskite layer that activated at light condition (Dualeh et al., 2014; Gonzalezpedro et al., 2014). Hysteresis behaviour in planar PSCs under illumination is due to charge carrier accumulation at the ETL/perovskite interface (Yadav et al., 2018), therefore by reducing the charge accumulation at the ETL/perovskite interface by using SnO2/CdS as ETL the hysteresis behaviour of PSCs are improved.
Table 1 The performance characteristics of PSCs with the structure of FTO/SnO2/perovskite/spiro-OMeTAD/Au and FTO/SnO2/CdS/perovskite/spiro-OMeTAD/ Au. ETL
Scan direction
Jsc(mA/cm2)
Voc(V)
FF
Efficiency (%)
HI
SnO2
Reverse Forward
21.32 21.35
1.034 1.010
0.68 0.57
15 12.29
0.17
SnO2/CdS
Reverse Forward
23.08 23.01
1.030 1.010
0.723 0.703
17.18 16.34
0.05
However, the perovskite layer deposited on SnO2/CdS shows more quenched PL than perovskite layer deposited on SnO2. Fig. 4d shows the IPCE spectra of the best performing solar cells with FTO/SnO2 and FTO/SnO2/CdS ETLs. As can be seen in the IPCE plot, the photon conversion increases by implementing the CdS NPs at the interface of SnO2 and perovskite. The absorption range of the solar cells starts from 760 nm in two devices therefore it can be deducted that CdS NPs don’t vary the absorption edge of perovskite absorber layer. The integrated JSC from IPCE diagram is 19.19 and 21.38 mA/cm2 for the PSCs with SnO2 and SnO2/CdS as ETLs, respectively, which is in good agreement with JSC from J-V diagrams. The electronic transfer phenomena at the interfaces were studied using EIS measurements at Voc bias. Measurements were done at dark and at 100 W/m2 illumination for best performing devices observed in Fig. 5a and b. The Nyquist plot can be used to distinguish the mechanisms of charge separation and recombination at the interfaces (Juarez-Perez et al., 2014; Zarazua et al., 2016). The equivalent circuit is shown in Fig. 5c and d includes series resistors related to the wirings and bulk resistivity, and two parallel resistors related to the carrier recombination resistance and electron transfer resistance parallel to chemical capacitances (Guillén et al., 2014; Liu et al., 2017). The resistance values of PSCs with various ETLs deducted from Nyquist plot are represented in table S2. As can be seen in the Nyquist plot, in light condition, there are two features in high and low frequency. The semicircle in high frequency is associated with the electron transfer through the interface of ETL/perovskite or perovskite/HTL
4. Conclusion In this study, the dispersion including CdS NPs was synthesized via a simple solution route and deposited on SnO2 coated FTO substrates. Structural analysis showed that the CdS nanoparticles formed few nanometer thick continuous coating on top of the SnO2 layer. Electrochemical Mott-Schottky analysis performed on the SnO2 and CdS surfaces confirmed that CdS creates a gradual change of electron affinity. Therefore, the CdS layer can act as an intermediate electronic buffer layer between SnO2 and Perovskite absorber layer to facilitate charge extraction and electron transport from absorber layer. It was found that the CdS layer enhanced PL quenching of perovskite layer, indicating better electron transfer from perovskite layer. The planar PSCs were fabricated with the structure of Glass/FTO/ ETL/Perovskite/Sprio-OMeTAD/Au using solution processed SnO2 and SnO2/CdS as ETL at ambient air condition. The efficiency of planar PSCs fabricated using SnO2 and SnO2/CdS as ETL was increased from 15.0% to 17.78% and hysteresis index were decreased from 0.17 to 0.05, respectively. EIS measurement demonstrated lower charge transfer resistance for PSCs with SnO2/CdS compared to SnO2 as ETL thus CdS NPs suppressed the recombination at ETL perovskite interface 651
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Fig. 5. (a, b) Nyquist plots of PSCs with the structure of FTO/SnO2/perovskite/spiro-OMeTAD/Au and FTO/SnO2/CdS/perovskite/spiro(SnO2) OMeTAD/Au (SnO2/CdS) measured at dark and 100 W/m2 illumination, (c, d) equivalent circuits of PSCs with the structure of FTO/SnO2/perovskite/ spiro-OMeTAD/Au (SnO2) and FTO/SnO2/CdS/perovskite/spiro-OMeTAD/Au (SnO2/CdS) measured at dark and illumination.
and lead to lower hysteresis.
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Improvement of planar perovskite solar cells by using solution processed SnO2/CdS as electron transport layer
T
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Fateme Mohamadkhania, Sirus Javadpoura, , Nima Taghaviniab,c a
Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz 7134851154, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran c Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Planar perovskite solar cell Electron transport layer SnO2 CdS nanoparticle Electron extraction
The efficiency of planar perovskite solar cells (PSCs) with SnO2 as electron transport layer is already more than 19% achieved under controlled atmosphere. PSCs with solution processed SnO2 show high hysteresis and low fill factor. One way to improve the planar PSCs is using buffer layer between electron transport layer and perovskite to enhance the photo-electron extraction process. In this study, SnO2 and SnO2/CdS layers were fabricated by solution process using a suspension including CdS nanoparticles synthesized via a simple solution route. Then planar PSCs with the structure of Glass/FTO/ETL/Perovskite/Sprio-OMeTAD/Au were fabricated in ambient air condition using SnO2 and SnO2/CdS as ETL. It is shown that a thin interface layer of CdS nanoparticles on top of SnO2 layer consistently improves the electron transporting properties of SnO2 layer. Mott-Schottky analysis shows a gradual change of electron affinity takes place by deposition of CdS nano particles. CdS interface layer can act as an intermediate step to facilitate electron transfer from perovskite layer to SnO2. The hysteresis index reduces from 0.17 to 0.05 and the efficiency improves from 15.0% to 17.18%. Impedance spectroscopy indicates that interface resistance is reduced by incorporating CdS nanoparticles.
1. Introduction In the past few years, perovskite solar cells (PSCs) have attracted much attention due to high efficiency and low-cost fabrication process (Christians et al., 2015; Lee et al., 2012; Singh and Miyasaka, 2018; Snaith, 2013; Yang et al., 2017.) The perovskite absorber possesses properties such as proper band gap, high absorption coefficient and high charge carrier diffusion length (Snaith, 2013). In a typical planar PSC configuration, absorber layer is sandwiched between electron transporting layer (ETL) and hole transporting layer (HTL) (Correa Baena et al., 2015). The most commonly used ETL and HTL are Tittanium oxide (TiO2) and Spiro-OMeTAD, respectively (Snaith, 2013). TiO2 is usually used as a two-layer structure of dense/mesoporous films (Im et al., 2014; Singh and Miyasaka, 2018; Zhen et al., 2019). However, Another inorganic n-type semiconductor that can be a good alternative for TiO2 is tin oxide (SnO2) (Guo et al., 2018), (Song et al., 2015) which does not need mesoporous layer in order to show high efficiency PSC. SnO2 has some advantages such as high electron mobility(Roose et al., 2016), wide band gap (3.6–3.8 eV) and high transparency in visible range spectrum(Jiang et al., 2017). Nevertheless, the deposition of high quality SnO2 ETL is not straightforward and usually needs some sort of modification in order to
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attain high device efficiency. Different strategies have been reported; Xiong et al. used Mg doped quantum dot (QD) SnO2 as ETL (Xiong et al., 2018). They have shown that the high temperature Mg doped QD SnO2 shows better transparency than spin coated SnO2 and TiO2 layer. Li doped SnO2 demonstrated lower conduction band energy and higher conductivity than un-doped, thus better electron transfer may occur from perovskite to ETL (Park et al., 2016). It was shown that chemical bath deposited Nb doped SnO2 can act as a promising ETL with lower hysteresis (Halvani Anaraki et al., 2018). PSCs with SnO2 ETL usually show low fill factor and high hysteresis. This problem can be improved by using an interface layer between ETL and perovskite layer to reduce the interface trap density and facilitate photo-electron extraction process. Tittanium(IV) chloride (TiCl4) treated SnO2 nano particles can act as a mesoporous scaffold in PSC and comparable with PSCs with TiO2 ETL (Li et al., 2015). By TiCl4 treatment, the recombination is significantly retarded so the open circuit voltage (VOC) increases in comparison to bare SnO2. ZnO/SnO2 double layer as ETL can increase the VOC from 1.07 V to 1.15 V because of higher Fermi level and conduction band level of ZnO/SnO2 than SnO2 (Wang et al., 2018). Chemical bath deposited SnO2 on top of the spin coated SnO2 has also improved the coverage of ETL on FTO and therefor can enhance FF and the efficiency of PSC (Anaraki et al.,
Corresponding author.
https://doi.org/10.1016/j.solener.2019.08.067 Received 31 May 2019; Received in revised form 25 August 2019; Accepted 27 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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on top of the ETLs via one step deposition method using chlorobenzene as anti-solvent (details of the deposition of perovskite can be find elsewhere.(Saliba et al., 2018)). Precursor solution for the perovskite was prepared by dissolving PbI2 (1.5 mmol, TCI), PbBr2 (1.5 mmol, IRASOL), Formamidinium Iodide (FAI) (1 mmol, dyesol) and Methylammunium Iodide (MABr) (1 mmol, dyesol) in mixture of anhydrous DMF: DMSO (4:1; v: v) (1 ml, Merck). Then CsI solution (50 µl) (CsI (1.5 mmol, Sigma, 99) dissolved in DMSO (1 ml)) was added to 950 µl of aforementioned solution. The perovskite precursor solution was spin coated on top of the ETL layers at 1000 rpm for 10 s and 6000 rpm for 20 s, followed by the injection of chlorobenzene at last 6 s. Then, the samples were immediately transferred to the hotplate at 100 °C for 60 min at ambient air. Spiro-OMeTAD precursor solution was prepared by dissolving of spiro-OMeTAD (72.3 mg, Borun, 99.8) in chlorobenzene (1 ml, Sigma) mixed with dopants; 4-tert-butylpyridine (28.8 µl, Sigma, 96) and bis (trifluoromethylsulfonyl)imide lithium salt (17.5 ml from solution of 520 mg/ml bis(trifluoromethylsulfonyl)imide lithium salt (99.95, Sigma) dissolved in acetonitrile (Merck)). This precursor solution was deposited on perovskite absorber layers by spin coating at 5000 rpm for 30 s. Finally, 70 nm thin film of gold was evaporated at high vacuum on the cells as top contact metal electrode.
2016). The conductivity and electrical mobility of SnO2 ETL have been reported be improve by water vapor treatment, and therefore PSCs with flexible substrate can show the efficiency of 18.36% (Wang et al., 2017). CdS has also been studied as an ETL. Devices based on CdS ETL show reduced hysteresis and better stability in comparison to devices based on TiO2 ETL due to reduced surface defects of CdS layer (Hwang et al., 2015; Peng et al., 2016; Wessendorf et al., 2018). CdS represents much higher electron mobility than SnO2 and TiO2 (electron mobility of CdS , SnO2 and TiO2 are 350 cm2/V.S , 240 cm2/V.S and 1 cm2/V.S, respectively) (Jiang et al., 2017; Tiwana et al., 2011; Wessendorf et al., 2018). In this research, Cadmium sulfide nanoparticles (CdS NPs) suspension was synthesized and applied on top of SnO2 ETL as an interface layer. Then planar PSCs with the structure of Glass/FTO/ETL/ Perovskite/Sprio-OMeTAD/Au were fabricated in ambient air condition using SnO2 and SnO2/CdS as ETL The pristine SnO2 ETL show the efficiency as high as 15% which was improved to 17.18% by CdS interface layer. The fill factor increased and hysteresis decreased after modifying with CdS NPs. Interface resistance was studied by impedance spectroscopy to clarify the effect of CdS NPs on charge carrier transfer and recombination rate in the planar perovskite solar cells. 2. Methods 2.1. Preparation of CdS nanoparticle suspension
3. Results and discussion
CdCl2·2.5H2O (1 mmol, Sigma, 99.99) and Sulfur (2 mmol, Sigma, 99.98), were completely dissolved in butyl amine (5 ml, Merck, 99.9) in separate vials at room temperature. Then, the S containing solution was added to Cd containing solution while the mixture was stirred continuously on magnetic stirrer at room temperature. The solution was transparent initially and after several hours of stirring the solution became cloudy and a pale yellow opaque precipitate was formed, indicating the formation of CdS NPs. The precipitate was separated by centrifuging and washed by mixture of ethanol and chloroform for several times. Finally, the CdS NPs were dispersed in Dimethylformamide (DMF) (Merck, 99.8) by sonication to form a stable suspension with 10 mg/ml concentration. For structural and opto-electronic characterization, thin films of CdS NPs were deposited on glass substrate by spin coating and annealed at 250 °C under N2 atmosphere.
3.1. Characterization of CdS nanoparticles and thin films TEM image of CdS NPs fixed on grid as shown in Fig. 1a illustrates that the particle size of synthesized CdS NPs are almost less than 10 nm. No large agglomerated particles can be seen, due to the fact that the synthesized CdS NPs were well dispersed. XRD patterns of the CdS powder; as-synthesized CdS NPs and the one annealed at 250 °C are shown in Fig. 1b. XRD pattern of as-synthesized CdS NPs coating shows diffraction peaks corresponding to cubic CdS phase (JCPDS card no; 1-080-0019). Broad diffraction peaks represent the nanocrystalline structure with poor crystallinity of assynthesized CdS NPs coating. While, the XRD pattern of annealed CdS NPs coating shows diffraction peaks corresponding to hexagonal CdS phase (JCPDS card no; 00-001-0783), which shows phase transformation and relative grain growth during annealing process. UV/Vis Transmittance spectra and calculated tauc plot corresponding to the CdS layers deposited on glass substrate and annealed at 250 °C for 5 min are shown in Fig. 1c and d, respectively. The band gap of CdS layer is determined by linear extrapolation of tauc plots about 2.66 eV which is blue shifted from the band gap of bulk CdS in hexagonal phase (the band gap of bulk CdS in hexagonal phase is reported 2.57 eV (Baykul and Orhan, 2010)). The FESEM images of the FTO samples, FTO/SnO2 and FTO/SnO2/ CdS are shown in Fig. 2 a, b and c, respectively. As can be seen in FESEM images, the SnO2 and CdS NPs coatings are continuous with full coverage. From photography images inset in FESEM images, CdS NPs coated sample looks slightly yellowish. The AFM topographical images of FTO, FTO/SnO2 and FTO/SnO2/ CdS surfaces are shown in Fig. 2. According to the AFM images the surface roughness of FTO is 29.1 nm, after deposition of SnO2 layer the surface roughness decreases to 24.5 nm (for FTO/SnO2 sample). Also by deposition of CdS NPs layer the surface roughness decreases further to 21.8 nm (for FTO/SnO2/CdS reduced). It can be due to the formation of continues and few nanometres thick layers during SnO2 and CdS NPs deposition. Mott-Schottky analysis is used to calculate the carrier density and flat band potential of ETLs. The FTO/SnO2 and FTO/SnO2/CdS samples are considered as working electrode in Mott-Schottky analysis which is explained by the following equation:
2.2. Fabrication of perovskite solar cells Fluorine doped tin oxide (FTO) coated glass substrates were cut into 1.4 cm × 1.4 cm square pieces. In order to provide a metal contact position, one side of FTO substrates were etched with zinc powder and HCl (2 M), then rinsed with sufficient amount of deionized water. The FTO substrates were washed and cleaned by ultrasonic bath in different solutions; diluted HCl in ethanol, acetone, isopropyl alcohol and ethanol, respectively. Finally, the FTO substrates were heated up to 500 °C for 30 min. The FTO substrates were treated with UV/Ozone just before coating steps. SnO2 precursor solution was prepared by dissolving SnCl2·2H2O (0.1 mmol, Sigma, 99.99) in ethanol (1 ml, Merck, 99.9). The precursor solution was spin coated on clean FTO substrates at 5000 rpm, and annealed at 500 °C for 1 h. To investigate the effects of CdS NPs on the performance of solar cells, some of FTO/SnO2 layers were coated by CdS NPs. A thin layer of CdS NPs was deposited on top of the SnO2 layers via spin coating of the CdS NPs dispersed in DMF (mentioned above) at 4000 rpm for 60 s. Then the FTO/SnO2/CdS layers were annealed at different temperatures; 150, 250, 350 °C under N2 atmosphere, to form dense CdS layers and optimize the annealing temperature. A mixed cations mixed halides perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) absorber layer was deposited 648
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Fig. 1. (a) TEM image of as-synthesized CdS NPs dispersed in DMF, (b) XRD patterns of as-synthesized CdS NPs and CdS NPs annealed at 250 °C, (c) UV/Vis spectrum of CdS NPs annealed at 250 °C and (d) tauc plot extracted from UV/Vis spectra of CdS NPs annealed at 250 °C.
1 2 kT ⎞ ⎛ = ⎜V − Vf − ⎟ C2 qεND A2 ⎝ q ⎠
PbI2 doesn't have deleterious effect on the device performance (Jacobsson et al., 2016). The band gap of the perovskite measured by tauc plot extracted from UV/Vis spectrometry is about 1.62 eV that is agree with other reports (Fig. S2c and d) (Saliba et al., 2016). From all of results mentioned above, stacked structure and band alignment of the layers in the planar PSCs investigated in this research can be schematically visualized like Fig. 3. Fermi level of SnO2/CdS layer insert between the conduction band of SnO2 and perovskite layer and can be act as an intermediate step that facilitates electron transfer from perovskite to SnO2 layer.
(1)
In which C is capacitance, q is the elementary charge, ɛ is the dielectric constant, ND is the doping density, V is applied potential, Vf is the flat band potential and A is the surface area of the interface (Cardon and Gomes, 1978). Mott-Schottky plot is 1/C2 vs applied voltage (V) thus the flat band potential can be achieved by linear extrapolation of linear portion of the plot and the slope of the linear portion deducted the carrier density of the material (Haghighi et al., 2018; Meissner et al., 1988). The Vf corresponds to a situation that there is no band bending in the equilibrium Fermi level arising from the electrode material and the electrolyte contact (Feng et al., 2009; Bakr et al., 2018). As can be seen in the Mott-Schottky plot of FTO/SnO2 and FTO/ SnO2/CdS demonstrated in Fig. 2e and f, the positive slope of curves represents n-type semiconducting materials. By utilizing the best fit in linear portion of Mott-Schottky plots, the flat band potential and carrier density of SnO2/CdS and SnO2 are calculated about 4.18 eV and 8.4E + 18 cm−3, 4.46 eV and 5.2E18 cm−3, respectively. The more carrier concentration demonstrate the better diode behaviour. The flat band potential of FTO calculated about −4.55 eV from Mott-Schottky plot. The XRD patterns of FTO/SnO2, FTO/SnO2/CdS shown in Fig. S1a and b, respectively, which are in good agreement with JCPDS card no; 01-077-0451. After loading CdS NPs on the surface of SnO2, no peaks of CdS can be observed, although, the peaks weakened and broadened. Exactly, because of CdS NPs deposited in very thin thickness and it is difficult to detect. From transmission spectra and tauc plot of SnO2 layer in Fig. S1, it can be deducted that the band gap is calculated about 3.65 eV. The top view FESEM image from the Perovskite absorber layer is shown in Fig. S2 a. Micron sized uniform grain structure of the perovskite absorber layer represents the high quality of the absorber layer. In XRD pattern of perovskite layer in Fig. S2 b, the peaks at 12.78°, 14.1° are characteristic peaks of PbI2 and perovskite, respectively (Hu et al., 2017). On the base of other groups researches, existence a little
3.2. Characterization of planar perovskite solar cells with SnO2 and SnO2/ CdS as ETL To investigate the effect of CdS NPs on the performance of PSCs, devices were fabricated with SnO2 and SnO2/CdS as ETL. After CdS NPs deposition on SnO2 layer, to find the better annealing temperature 5 categories of ETLs including SnO2, SnO2/CdS-150, SnO2/CdS-250 (SnO2/CdS) and SnO2/CdS-350 (that means after CdS NPs deposition, the SnO2/CdS ETLs annealed at 150 °C, 250 °C, 350 °C, respectively) were investigated. J-V measurement of the solar cells were done at room temperature in air under AM1.5G simulated sunlight. The best performing devices investigated in this research are devices with SnO2/CdS ETLs annealed at 250 °C. Higher annealing temperature is likely causing better crystallinity of the CdS film, as well as sintering of NPs into more continuous films, demonstrating better electronic properties (Liu et al., 2010). The J-V plots of PSCs with FTO/SnO2/CdS (FTO/SnO2/CdS-250) and FTO/SnO2 as ETL are shown in Fig. 4a and b and J-V plots of PSCs with ETLs annealed at other temperatures are shown in Fig. S3 a and b. All of steps related to preparation of solution precursor and deposition of perovskite layer were done at ambient air. The performance characteristics of PSCs are shown in Table 1 (the performance characteristics of PSCs with FTO/SnO2/CdS-150 and FTO/ SnO2/CdS-350 ETLs are shown in Table S1). By CdS NPs deposition on the surface of SnO2 ETLs and annealing at 250 °C, the efficiency of the 649
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(a)
(b)
(d)
RMS= 29.1nm
(e)
FTO
0
0.4
0.2
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
0.8
FTO/SnO 2
(h)
0.6 0.4 0.2 0.0
RMS= 21.8nm
200nm
A2/C 2(mF -2.cm2)
(g)
(f)
200nm
A2/C 2(mF -2.cm2)
A2/C 2(mF -2.cm2)
RMS= 24.5nm
0
0.6
0.0
91 nm
105nm
200nm
0
1μm
1 μm
1 μm
127nm
(c)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
6 5
(i)
FTO/SnO 2/CdS
4 3 2 1 0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Fig. 2. (a, b, c) Top view FESEM images of FTO, FTO/SnO2 and FTO/SnO2/CdS samples, d, e, f) AFM surface topography images of FTO, FTO/SnO2 and FTO/SnO2/ CdS layers, (g, h, i) Electrochemical Mott-Schottky plots of FTO, FTO/SnO2 and FTO/SnO2/CdS layers, respectively.
lowest to highest hysteresis. The most significant effect caused by CdS NPs interface layer is decreasing the HI from 0.17 to 0.05 for devices with FTO/SnO2 and FTO/SnO2/CdS ETLs, respectively, which can be attributed to the fact that CdS NPs can passive the trap states made by surface defects in SnO2 and decrease the recombination at the ETL/ perovskite interface. PL measurement of the perovskite film deposited on SnO2 and SnO2/CdS ETLs are shown in Fig. 4c. The higher the charge of carrier injection, the higher the PL quenching of perovskite film, so these ETLs can be appropriate for the electron extraction from perovskite absorber.
solar cells is increased from 15% to 17.18%. Actually, the best performing device shows 17.18%, 0.723, 1.03 V, 23.08 mA/cm2 in efficiency, FF, Voc and Jsc, respectively. Hysteresis is a usual problem in perovskite solar cells. The hysteresis Index (HI) is defined by:
Hysteresis Index =
Jscan − (0.8VOC ) − Jscan + (0.8VOC ) Jscan − (0.8VOC )
(2)
where Jscan−(0.8Voc) is the photocurrent density at 0.8 Voc in reverse bias while Jscan+(0.8Voc) is the photocurrent density at 0.8 Voc in forward bias (Kim et al., 2018). HI varies from 0 to 1 corresponding to
Fig. 3. Schematics of a and (b) band alignment of layers in planar perovskite solar cells with SnO2 and SnO2/CdS ETLs, respectively (The energy bands of perovskite and spiro-OMeTAD is from literature (Byranvand et al., 2018). For conversion of reference energy levels to vacuum level, the absolute electrode potential of Ag/AgCl reference electrode is considered 4.66 eV (Trasatti, 1986)). The scheme is based on the electrochemically measured flat band potential levels of FTO, SnO2 and SnO2/CdS.
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Fig. 4. (a) J-V measurement of PSC with the structure of: FTO/SnO2/perovskite/spiro-OMeTAD/Au (SnO2) with cross section view FESEM image inset, (b) J-V measurement of PSC with the structure of: FTO/SnO2/CdS/perovskite/spiro-OMeTAD/Au (SnO2/CdS), (c) photoluminescence of perovskite layer deposited on SLG, SLG/SnO2 and SLG/SnO2/ CdS(SLG for soda lime glass), (d) IPCE spectra of the PSCs with the structure of FTO/SnO2/perovskite/ spiro-OMeTAD/Au (black line) and FTO/SnO2/CdS/ perovskite/spiro-OMeTAD/Au (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(Juarez-Perez et al., 2014). Since the main difference between devices is ETL, the variation in the resistance is associated to the electron transfer resistance at the ETL/perovskite interface. CdS NPs deposition at the interface of ETL/ perovskite can help to reduce the resistance against to the charge transfer from perovskite layer to SnO2 ETL observable in dark and light condition. The arc in low frequency region can be seen in light condition attributed to the slow process which is proposed to be associated with ion migration, ion intercalation or ferroelectric effect of perovskite layer that activated at light condition (Dualeh et al., 2014; Gonzalezpedro et al., 2014). Hysteresis behaviour in planar PSCs under illumination is due to charge carrier accumulation at the ETL/perovskite interface (Yadav et al., 2018), therefore by reducing the charge accumulation at the ETL/perovskite interface by using SnO2/CdS as ETL the hysteresis behaviour of PSCs are improved.
Table 1 The performance characteristics of PSCs with the structure of FTO/SnO2/perovskite/spiro-OMeTAD/Au and FTO/SnO2/CdS/perovskite/spiro-OMeTAD/ Au. ETL
Scan direction
Jsc(mA/cm2)
Voc(V)
FF
Efficiency (%)
HI
SnO2
Reverse Forward
21.32 21.35
1.034 1.010
0.68 0.57
15 12.29
0.17
SnO2/CdS
Reverse Forward
23.08 23.01
1.030 1.010
0.723 0.703
17.18 16.34
0.05
However, the perovskite layer deposited on SnO2/CdS shows more quenched PL than perovskite layer deposited on SnO2. Fig. 4d shows the IPCE spectra of the best performing solar cells with FTO/SnO2 and FTO/SnO2/CdS ETLs. As can be seen in the IPCE plot, the photon conversion increases by implementing the CdS NPs at the interface of SnO2 and perovskite. The absorption range of the solar cells starts from 760 nm in two devices therefore it can be deducted that CdS NPs don’t vary the absorption edge of perovskite absorber layer. The integrated JSC from IPCE diagram is 19.19 and 21.38 mA/cm2 for the PSCs with SnO2 and SnO2/CdS as ETLs, respectively, which is in good agreement with JSC from J-V diagrams. The electronic transfer phenomena at the interfaces were studied using EIS measurements at Voc bias. Measurements were done at dark and at 100 W/m2 illumination for best performing devices observed in Fig. 5a and b. The Nyquist plot can be used to distinguish the mechanisms of charge separation and recombination at the interfaces (Juarez-Perez et al., 2014; Zarazua et al., 2016). The equivalent circuit is shown in Fig. 5c and d includes series resistors related to the wirings and bulk resistivity, and two parallel resistors related to the carrier recombination resistance and electron transfer resistance parallel to chemical capacitances (Guillén et al., 2014; Liu et al., 2017). The resistance values of PSCs with various ETLs deducted from Nyquist plot are represented in table S2. As can be seen in the Nyquist plot, in light condition, there are two features in high and low frequency. The semicircle in high frequency is associated with the electron transfer through the interface of ETL/perovskite or perovskite/HTL
4. Conclusion In this study, the dispersion including CdS NPs was synthesized via a simple solution route and deposited on SnO2 coated FTO substrates. Structural analysis showed that the CdS nanoparticles formed few nanometer thick continuous coating on top of the SnO2 layer. Electrochemical Mott-Schottky analysis performed on the SnO2 and CdS surfaces confirmed that CdS creates a gradual change of electron affinity. Therefore, the CdS layer can act as an intermediate electronic buffer layer between SnO2 and Perovskite absorber layer to facilitate charge extraction and electron transport from absorber layer. It was found that the CdS layer enhanced PL quenching of perovskite layer, indicating better electron transfer from perovskite layer. The planar PSCs were fabricated with the structure of Glass/FTO/ ETL/Perovskite/Sprio-OMeTAD/Au using solution processed SnO2 and SnO2/CdS as ETL at ambient air condition. The efficiency of planar PSCs fabricated using SnO2 and SnO2/CdS as ETL was increased from 15.0% to 17.78% and hysteresis index were decreased from 0.17 to 0.05, respectively. EIS measurement demonstrated lower charge transfer resistance for PSCs with SnO2/CdS compared to SnO2 as ETL thus CdS NPs suppressed the recombination at ETL perovskite interface 651
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Fig. 5. (a, b) Nyquist plots of PSCs with the structure of FTO/SnO2/perovskite/spiro-OMeTAD/Au and FTO/SnO2/CdS/perovskite/spiro(SnO2) OMeTAD/Au (SnO2/CdS) measured at dark and 100 W/m2 illumination, (c, d) equivalent circuits of PSCs with the structure of FTO/SnO2/perovskite/ spiro-OMeTAD/Au (SnO2) and FTO/SnO2/CdS/perovskite/spiro-OMeTAD/Au (SnO2/CdS) measured at dark and illumination.
and lead to lower hysteresis.
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