Bilayer SnO2 as Electron Transport Layer for Highly Efficient

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Bilayer SnO2 as electron transport layer for highly efficient perovskite solar cells Haimang Yi, Dian Wang, Md Arafat Mahmud, Faiazul Haque, Mushfika B Upama, Cheng Xu, Leiping Duan, and Ashraf Uddin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01076 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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ACS Applied Energy Materials

Bilayer SnO2 as Electron Transport Layer for Highly R

R

Efficient Perovskite Solar Cells

Haimang Yi *,† , Dian Wang † , Md Arafat Mahmud † , Faiazul Haque † , Mushfika Baishakhi Upama P

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†,

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P

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Cheng Xu † , Leiping Duan † and Ashraf Uddin *,† P

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AUTHOR ADDRESS †School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Corresponding Author *E-mail: [email protected] . (H.Y.) U

U

*E-mail: [email protected]. (A.U.)

ACS Paragon Plus Environment

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Abstract Tin Oxide (SnO 2 ) has been reported as a promising electron transport layer (ETL) for planar R

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heterojunction perovskite solar cells (PSCs). This work reports a low temperature solutionprocessed bilayer SnO 2 as an efficient ETL in gas-quenched planar-heterojunction R

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methylammonium lead iodide (MAPbI 3 ) perovskite solar cells. SnO 2 nanoparticles were R

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employed to fill the pin-holes of sol-gel SnO 2 layer and form a smooth and compact bilayer R

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structure. The PCE of bilayer devices has increased by 30% compared with sol-gel reference device and the J sc , V oc and FF has been improved simultaneously. The superior performance of R

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bilayer SnO 2 is attributed to the reduced current leakage, enhanced electron extraction R

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characteristics, and mitigated the trap-assisted interfacial recombination, via X-Ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and space-charge limited current–voltage (SCLC) analysis.

KEYWORDS: Bilayer electron transport layer, Perovskite solar cell, Tin oxide, Fill Factor, Low temperature processed


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1.

Introduction

Perovskite solar cells have drawn enormous attentions in recent years. With the incredible improvement of power conversion efficiency, perovskite solar cells (PSCs) have become one of the most promising photovoltaic technologies to industry. In 2009, Kojima et al first incorporated methylammonium lead halides (MAPbI 3 and MAPbBr 3 ) as light harvester in dye-sensitized solar R

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cells (DSSCs) achieving 3.8% power conversion efficiency.1 At present, laboratory scale cells P

with the certified highest power conversion efficiency (PCE) of 23.3% have been achieved. 2 It has been realized that perovskite absorber materials possess ambipolar property which transport both holes and electrons.3-5 In conventional structure, the electron transport layer (ETL) modifies the interfaces and prevent recombination, thus plays an important role in PSC device performance. Titanium oxide (TiO 2 ) has been widely adopted as the electron transport layer in conventional R

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PSCs. In fact, most of the highest performance PSCs have the compact and mesoporous TiO 2 R

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system (FTO/c-TiO 2 /meso-TiO 2 /Perovskite/Spiro-OMeTAD/Au).6-7 The main reasons for using R

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TiO 2 as ETL are its favorable conduction band (CB) edge in relation to perovskite absorber as R

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well as deep valence band (VB) position, ensuring good electron transport and hole blocking abilities.8 However, severe hysteresis phenomena, i.e. mismatch of forward and reverse scanned current density-voltage (J-V) curves, are observed in TiO 2 based perovskite solar cells, due to R

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charge accumulation at the interfaces.9-11 Besides, the mesoporous TiO 2 is sensitive to UV light R

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which raises an issue on long term stability.11-12 Accordingly, other inorganic metal oxides have drawn a wide research attention to potentially replace the mesoporous TiO 2 structure. Among R

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them, tin oxide (SnO 2 ) is a one of the suitable candidates for planar heterojunction structure. SnO 2 R

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has wider bandgap, 3.6 eV to 4.1 eV and deeper conduction band than TiO 2 , which ideally could R

form a better ohmic contact and facilitate better electron charge transport.

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13-14

Moreover, SnO 2 R

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has a higher electron mobility as well as a well-matched band alignment with perovskite absorber.15-17 More importantly, SnO 2 ETL can be deposited in low temperature solutionR

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processed method, which does not require the high sintered temperature as compact TiO 2 ETL.16, R

18-19

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It is worth noticing that Singh, et al has recently developed a low temperature processed TiO2

nanoparticles as ETL in MAPbI3 PSCs with PCE of 17.43%.20 In 2015, Ke, et al. firstly reported SnO 2 as efficient electron transport layer in planar heterojunction PSCs by a low temperature R

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solution-processed method and SnO 2 ETL by such sol-gel method had been widely used in PSCs R

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thereafter.16, 21-24 In 2017, Yang, et al. have achieved 19.73% on MAPbI 3 device by tuning the R

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concentration of SnO 2 quantum dot.25 Recently, Jiang, et al. showed solution-processed SnO 2 R

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nanoparticles as promising ETL for highly efficient PSCs, with efficiency over 20%.26-27 Although the single layer SnO 2 ETL seems promising in the planar structured PSC devices, it can be deemed R

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as arguable whether a single, thin layer ETL is efficient enough to block the hole transportation or the leakage of current would form. Other studies intended to incorporate SnO 2 with other inorganic R

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ETL such as ZnO and TiO 2 to form a bilayer structure and obtain highly efficient perovskite R

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devices.28-31 Mahmud et al. showed that bilayer ZnO ETL exhibits better performance with PSC than single layer ZnO ETL and this demonstrates an intriguing prospect for bilayer SnO 2 ETL.32 R

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There is no report on low temperature solution-processed bilayer SnO 2 to combine the advantages R

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of both methods and the underline differences of both methods, sol-gel and nanoparticles have not yet discussed. In this study, we have presented a systematic comparative study of single and bilayer SnO 2 ETL R

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along with gas quenched MAPbI 3 PSC. Two types of SnO 2 ETLs, formed by sol-gel method and R

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nanoparticle precursor, were parallelly compared and SnO 2 nanoparticle layer exhibits better R

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electrical property than sol-gel SnO 2 due to the higher crystallinity. The sol-gel SnO 2 and the R

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nano-SnO 2 are denoted as S-SnO 2 and N-SnO 2 respectively. After incorporating a thin layer NR

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SnO 2 scaffold on top of S-SnO 2 , a uniform and pin-hole free bilayer ETL has been formed R

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(denoted as B-SnO 2 ). The bilayer SnO 2 structure showed an enhanced average fill factor (FF) R

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(71.25%) compared with single layer S-SnO 2 reference device (57.6%). The average PCE of R

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bilayer devices have been boosted to 16.84%. The best performing PSC devices yield a PCE of 17.61%. To gain a deeper understanding of the device performance, a series of measurement and characterization methods have been adopted. Our research reveals that the bi-layered ETL is superior to single layered ones owning to enhanced FF.

2. Experimental section 1.1.

Fabrication process

Patterned ITO glass substrates (12 mm × 12 mm) were purchased from Lumtech with the sheet resistance of 15 Ω/□. The substrates were sequentially washed with Hellmanex III, deionized water, acetone and isopropanol in 10 min cycle. For ETL, sol-gel SnO 2 precursor solution was R

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prepared by dissolving 52.59 mg Tin (IV) chloride pentahydrate (SnCl 4 ·5H 2 O) into 1 ml R

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isopropanol and stirred for 12 hours. To form a compact S-SnO 2 film, the sol-gel solution was R

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spin-coated on clean ITO substrate at 4000 rpm for 30 seconds inside the N 2 filled glovebox and R

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then annealed at 200 ºC for 60 min at ambient condition. For N-SnO 2 films, SnO 2 colloid R

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dispersion (Alfa Aesar) was diluted into 1% weight ratio with deionized water. The nanoparticle dispersion was then spin-coated on UVO-treated substrate at 3000 rpm for 30 seconds and then annealed at 150 ºC for 30 min in ambient condition. The methylammonium lead iodide perovskite (CH 3 NH 3 PbI 3 ) films were prepared by one-step method with gas quenching method. 461 mg lead R

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(II) iodide (PbI 2 ) and 159 mg methylammonium iodide (MAI) were dissolved and then stirred in R

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1 ml N,N-Dimethylformamide (DMF) for 12 hours. The solution was spin coated on the ITO substrates with different SnO 2 ETLs at a spin rate of 2500 rpm for 30 seconds. The nitrogen gas R

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was applied on the substrates during the spin coating. Then the substrates were transferred onto a hotplate inside the N 2 filled glove box and annealed at 100 ºC for 10 min, resulted in 400 nm thick R

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film. For the hole transport layer (HTL), a precursor solution of 72.3 mg/mL of 2,2′,7,7′tetrakis[N,Ndi(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) in chlorobenzene was prepared, with 28.8μL of 4-tertbutylpyridine and 17.5μL of Li-TFSI (520 mg/mL in acetonitrile) as doping to increase its p-type conductivity. The HTL precursor solution was spincoated at a spin-rate of 3000 rpm for 30 seconds in ambient condition. To complete the device, a 100 nm Ag layer was evaporated on top of hole transport layer (evaporation rate: 2 Å /s, vacuum level: 1 × 10−6 mbar). The overall device structure is ITO/sol-gel SnO 2 /nano-SnO 2 /MAPbI 3 /SpiroR

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OMeTAD/Ag, as shown in Figure 1 (a) and the active device area is 4.5 mm2.

1.2.

Characterization process

The current density – voltage (J-V) measurements were conducted on a I-V testing system equipped with Keithley 2400 source meter from PV Measurements, Inc. under 1 sun intensity (100 mW/cm2) with AM 1.5G filter. External quantum efficiency (EQE) measurement has been carried on PV Measurements QEX7 Spectral Response System. The optical characterization was performed on a UV−vis−NIR spectrometer (PerkinElmer−Lambda 950). Ultraviolet Photoelectron Spectroscopy (UPS) was carried out on ESCALAB 250Xi, Thermo Scientific, UK with a He I source (energy 21.2 eV). ESCALAB250Xi (Thermo Scientific Inc.) was used to perform X-Ray


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Photoelectron Spectroscopy (XPS) measurement using mono-chromated AlKα (hυ=1486.68 eV) anode (120 W, 13.8 kV, 8.7 mA) with background pressure at 2×10−9 mbar. The surface topology images were captured by NanoSEM 450 (scanning electron microscopy) fitted with a retractable annular backscattered electron detector as well as a Bruker SDD-EDS detector. The surface roughness was detected by Bruker Dimension ICON SPM (atomic force microscopy) on contact mode with scan size: 5 µm × 5 µm and samples/line: 512. X-ray diffraction (XRD) characterization was performed on PANalytical Empyrean Thin-Film XRD machine with CuKα radiation by stepscanning with a step size of 0.02°. The impedance analysis was performed on Autolab PGSTAT30 analyzer inside N 2 filled glovebox. The frequency analyzer module is in the frequency range R

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of 106–100 Hz and AC oscillating amplitude was as low as 20 mV (rms) to maintain the linearity of the response.

3. Result and discussion 3.1.

Photovoltaic Performance

Sol-gel SnO 2 , nano-SnO 2 and bilayer SnO 2 films were used as ETLs along with conventional R

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structured MAPbI 3 perovskites (PSCs). Since the thickness of the electrical transport layer can R

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play an important role in planar heterojunction PSCs, we fabricated devices with different ETL thickness (sol-gel SnO 2 , nano-SnO 2 and bilayer SnO 2 ) to obtain the optimal value. According to R

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Figure S1, 0.15M SnO 2 sol-gel precursor and 1% w/v SnO 2 nanoparticle dispersion give the best R

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performance in their kinds. The thicknesses for sol-gel SnO 2 , nano-SnO 2 are 35 nm and 15 nm R

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respectively. When combined these two layers, the performance of devices is dramatically improved (as seen on Table S1). Different thicknesses were also optimized which reconfirmed the optimized data and the best performance could be achieved on 35 nm sol-gel SnO 2 /15 nm nanoR

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SnO 2 (0.15M/1%). the following discussion will be based on the optimal results. R

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The current density – voltage (J-V) curves of perovskite solar cell devices with three different ETLs (S-SnO 2 , N-SnO 2 and bilayer SnO 2 ) have been illustrated in Figure 2 (a). Table 1 summaries R

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the average value of 10 devices’ performance parameters as well as the best photovoltaic performance of PSC devices, including V oc , J sc , FF, PCE, R s and R sh . According to Figure 2 (a), R

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inserting a thin layer of SnO 2 nanoparticle scaffold on top of the sol-gel SnO 2 significantly R

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enhanced the device performance. Current density – voltage (J-V) characteristics of devices in both FB-SC (forward bias to short circuit) and SC-FB (short circuit to forward bias) directions have been measured at a scan rate of 0.05 V s−1 in Figure S2. The hysteresis index is calculated by using the following equation: 33

HI 

J FB  SC (Voc / 2)  J SC  FB (Voc / 2) J FB  SC (Voc / 2)

(2)

J FB  SC (Voc / 2) and J SC  FB (Voc / 2) denote current densities at an applied bias of half the

where

(Voc / 2) in the FB-SC and SC-FB directions, respectively. The calculated HI

open circuit voltage

for S-SnO 2 , N-SnO 2 and B-SnO 2 PSC devices are 0.082, 0.037 and 0.040 respectively. Although R

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the HI of B-SnO 2 and N-SnO 2 devices are much lower than that of S-SnO 2 , the rather large R

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hysteresis is probably due to the poor perovskite crystal property by gas quenching deposition.34 The stabilized current density and PCE of the fabricated PSCs at maximum power point (MPP) were also measured, where B-SnO 2 and N-SnO 2 PSCs demonstrated more stabilized output than R

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S-SnO 2 reference device, as shown in Figure 2 (b). As comprehended from Table 1, bilayer SnO 2 R

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PSCs exhibit nearly 30% improvement on PCE compared with S-SnO 2 devices and 20% R

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improvement than single layered N-SnO 2 PSCs. The average V oc value for bilayer SnO 2 is 1.05V, R

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which is slightly higher than that of single layer ETL (1.03V). Figure 3 (a) illustrates the absorption spectrum of MAPbI 3 perovskites deposited on different R

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SnO 2 ETLs. Despite the slight blue shift of absorption peak observed in B-SnO 2 devices compared R

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with S-SnO 2 and N-SnO 2 ones, the absorption pattern of perovskite did not change much. R

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Therefore, the increase of J sc in bilayer SnO 2 devices was partially influenced by the quality of R

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perovskite layer, but mainly attributed from the enhancement of the electrical property. The EQE spectrum in Figure 3 (b), also confirmed the enhancement in short circuit current in bilayer ETL based PSC compared to PSC incorporating mono layer ETL. The integrated short-circuit current density (J sc ) of S-SnO 2 , N-SnO 2 and B-SnO 2 PSCs are 20.57, 21.20 and 21.64 mA/cm2 R

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respectively, which matched closely to the value obtained from J-V measurements. The difference between EQE and J-V measurement could be traced from several sources, such as inconsistency in solar simulator and tungsten lamp and characterization time span.35 Most prominently, the bilayer devices exhibit 24% higher FF compared to S-SnO 2 reference devices (FF of bilayer SnO 2 : R

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71.25%, S-SnO 2 : 57.6%, N-SnO 2 : 61.44%). As observed from Table 1, bilayer SnO 2 devices R

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demonstrate about 55% drop on serious resistance, R s value (6.42 Ω·cm2), and 37% rise on shunt R

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resistance, R sh value (2651 Ω·cm2) compared with S-SnO 2 reference device (14.11 Ω·cm2 and R

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1932 Ω·cm2 respectively), which conforms to the enhanced charge extraction and transfer property demonstrated by the bilayer ETL device. To further explore the improved electrical properties for PSCs with a bilayer SnO 2 interlayer, R

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dark J-V characteristic measurements were also conducted. As seen on Figure 4, B-SnO 2 PSC R

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devices displayed the significantly lower leakage current density under reverse bias, which was only one-fifth compared with that of single layer ETL PSCs. Since V oc can be expressed as 36 R

Voc 

R

kT J sc ln( ) q Jo

(3)

where, k, T, and q stand for the Boltzmann constant, absolute temperature in Kelvin, and elementary charge, respectively, lower J 0 and higher J sc in B-SnO 2 PSCs lead to a rise in V oc R

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value, which explains why the bilayer device has the higher value of open circuit voltage. Moreover, the steep slope of J-V curves of B-SnO 2 devices at high bias voltage region (> 0.75 V) R

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indicates low series resistance, which implicates towards improved n-type conductivity and enhanced charge extraction property at perovskite/ETL interface.37 The results obtained from dark IV characterization are in consistence with the data obtained in Table 1.

3.2.

Material and optical characterization of ETLs

We have investigated the transmittance spectrum of three different ETL films, as shown in Figure 5 (a), on ITO/glass substrates. In n-i-p PSC devices, the transmittance of ETL is expected to be high enough to ensure the maximum sunlight absorbed in perovskite layer. From Figure 5 (a), N-SnO 2 /ITO possessed the highest average visible transmittances (AVT) in the visible region R

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(380–780 nm), 84.57%, followed by S-SnO 2 /ITO (83.74%) and B-SnO 2 /ITO (83.51%). Our R

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observation confirmed that inserting a thin layer SnO 2 nanoparticle scaffold did not compensate R

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the optical performance much. Figure 5 (b) demonstrates the Tauc Plot extracted from absorption spectrum of ETLs on glass substrates, where the optical bandgap of B-SnO 2 (3.92eV) is higher R

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than that of single layer SnO 2 (3.90eV and 3.85eV). The band structure of semiconductors could R

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be modified by the interatomic distances and relative positions of atoms.

38-39

The difference of


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band gaps between single layer and bilayer SnO 2 reveals the change in their electronic properties. R

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Since the substrate often subjects the film to the mis-match strain in the application of semiconductor thin films, the SnO 6 octahedra in single layer SnO 2 is less ordered than that of R

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bilayer SnO 2. 38 R

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According to Table 1, B-SnO 2 device also demonstrated slightly higher open circuit voltage. R

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Previous literature suggested that the higher V OC was contributed by the higher quasi Fermi-level, R

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E F .40 The V OC of a PSC can be defined as the energetic offset between the quasi Fermi-levels of R

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ETL/cathode and HTL/anode contacts, expressed as: 41

Voc  EFn  EFp

(4)

Since the HTL films (spiro-OMeTAD) were common in all device structures, the enhancement of V oc can be attributed from the upshift of quasi Fermi-level of the bilayer SnO 2 ETL. The fermi R

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level of S-SnO 2 , N-SnO 2 and B-SnO 2 are estimated to be 4.46 eV, 4.45 eV and 4.47 eV from UPS R

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measurement (Figure 5 (c)). The slight up-shift of work function (0.01 eV/0.02 eV compared with S-SnO 2 and N-SnO 2 ) on B-SnO 2 confirms the 0.02 V improvement of open circuit voltage. R

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According to Figure 5 (d), the value of E VBM can be determined to be 3.30 eV, 3.40 eV and 3.20 R

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eV from E VBM = E cut-off – 21.2 eV (emission energy from He irradiation). Therefore, E CB can be R

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calculated as 2.99 eV, 3.19 eV and 2.92 eV from E CB = E VB – Eg = WF + E VBM - E g . R

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In order to gain further insight of elemental composition, we have conducted XPS elemental analysis of S-SnO 2 , N-SnO 2 and B-SnO 2 films. The related peak binding energy, peak full width R

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at half maximum (FWHM) amd atomic elemental percentage are listed in Table S2. According to Figure 6, all three ETL films have two Gaussian sub-peaks, O 1s A and O 1s B . The peak at lower R

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binding energy level is related to Sn-O bond and the one at higher level is related to O-H bond or oxygen vacancy.42-43 The atomic percentages of O 1s R

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for S-SnO 2 , N-SnO 2 and B-SnO 2 are R

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37.3%, 21.87% and 15.42% respectively (as shown in Table S2). The oxygen deficient regions (O 1s A ) can act as recombination centers for photogenerated electrons and holes.21 The low content R

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of hydroxylgroups in B-SnO 2 film contributed to the enhancement of FF in B-SnO 2 PSC (Table R

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1).

3.3.

Morphology and material characterization

The surface morphology of the ETLs including sol-gel SnO 2 , nanoparticle SnO 2 and bilayer R

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SnO 2 on ITO, respectively, is shown in Figure 7. As seen on Figure 7 (a, b), S-SnO 2 layer shows R

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an amorphous-like morphology. In contrast, the N-SnO 2 layer deposited on the ITO substrate R

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shows a clearer feature of aggregated nanoparticles, as shown in Figure 7 (c d). However, both single layer SnO 2 ETLs possess visible pin holes between the aggregations and particle R

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boundaries, which may lead to severe recombination resulting from direct contact between the perovskite absorber and the ITO electrode.44 For bilayer SnO 2 , as shown in Figure 7 (e, f), pinR

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holes on the surface have been eliminated ensuring a better film coverage and less recombination sites. In addition, the surface of the SnO 2 bilayer looks smoother compared with single layer ETL R

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in SEM image. This was confirmed by the AFM measurement results as depicted in Figure 8. The surface roughness data has been summarized in Table 2, where R a is the arithmetic average value R

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and R q is root mean squared value of roughness. The AFM measurement results shows that the R

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surface roughness of both single and double layered SnO 2 ETL is within 3 nm range and bilayer R

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ETLs exhibited lower surface roughness (RMS=1.69 nm) than single layer ETL (2.22 nm and 2.86


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nm for N-SnO 2 and S-SnO 2 respectively). This is because of that SnO 2 nanoparticle scaffold filled R

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out the pin-holes and smoothed the surface of the S-SnO 2 underlayer. The pinhole-free and smooth R

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contact between SnO 2 ETL and perovskite absorber suggests reduced defects, therefore less R

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recombination sites 45, which reconfirms the higher current density and FF obtained in Table 1. MAPbI 3 perovskite films were deposited on top of the S-SnO 2 , N-SnO 2 and B-SnO 2 ETLs via R

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gas-assisted method. To investigate the perovskite film quality grown on different SnO 2 ETLs, we R

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have performed morphology and crystal characterization. The SEM and AFM images of perovskites have been shown in Figure S3 and S4. Perovskite deposited on B-SnO 2 demonstrated R

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a smoother surface with less pin-hole, as shown in Table S3. Figure 9 (a) depicts the XRD patterns of corresponding perovskite layers, where the major characteristic peaks (110), (220), (310), (224), and (314) have been clearly identified 46-47. According to Figure 9 (b), perovskite films grown on N-SnO 2 and B-SnO 2 ETL have relative higher intensity in major peaks, predominately on (110) R

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at the diffraction angle of ~14º. The higher peak intensity represents the better crystallinity of MAPbI 3 perovskite films.48-49 R

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Figure 9 (c, d, e) illustrates the crystal size, microstrain and dislocation density of perovskite films from XRD spectral fitting and corresponding numerical values are listed in Table S4. Crystal size is calculated from Scherrer equation50-51:

D

K  cos 

(5)

Where D is the mean size of the ordered crystalline domains, K is a dimensionless shape factor, λ is the X-ray wavelength, 𝛽 is full-width at half maximum (FWHM) and 𝜃 is Bragg diffraction angle. As seen on Figure 9 (c), perovskte on bilayer SnO 2 possesses the largest crystal size, ~55 R

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nm in (110) orientation, and relatively larger value pertaining to (220) and (314) orientations. Figure 9 (d) summarizes the calculated value of microstrain, which is expressed as


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

 4  tan( )

Page 14 of 36

(6)

Where 𝛽, 𝜃 refer to microstrain, full-width at half maximum (FWHM) and Bragg diffraction angle, respectively. The microstrain refers to the quantitative measure of the local deviation of atoms with reference to their equilibrium position at the crystalline lattice, caused by the presence of porosities, point defects or dislocation at the grain boundaries. 48, 52 Therefore, for perovskite on bilayer SnO 2 , the orientation of atoms has been better arranged where the grain boundary defects R

R

are reduced and thus the microstrain in this nanocrystalline perovskite system is reduced as shown in Figure 9 (d). Moreover, in line with the reduced value of microstrain, the dislocation density has been relatively reduced in bilayer system as shown in Figure 9 (e). The dislocation density, which attributes to the mismatch in crystalline structure, can be mathematically expressed by the following equation: 53 

n d2

(7)

Where 𝛿, n and d refer to dislocation density, mathematical factor and crystal dimension (as calculated from Scherrer equation), respectively. Thus, it is apparent from the spectral XRD analysis that, perovskite film on bilayer SnO 2 demonstrates higher crystallinity, larger crystallite R

R

size, better crystalline orientation and smaller dislocation density in contrast with perovskite on single layer SnO 2 . R

3.4.

R

Charge transport characterization

Electrochemical impedance spectroscopy (EIS) measurement has been conducted to gain further insights into the charge transfer properties and the trap-assisted recombination phenomena of the fabricated devices.40, 54 Impedance analysis suggests that the thin mesoporous layer is an “active


14

Page 15 of 36 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


scaffold” selectively collecting electrons from perovskite and can effectively suppress recombination.55 Figure 10 represents the Nyquist plot and corresponding equivalent circuits. The equivalent circuit consists of two parallel resistance-capacitance components, R C -C C and R REC R

R

R

R

R

R

C μ , along with a series resistance, R SE . Briefly, R SE is associated with series resistance owing to R

R

R

R

R

R

metal and wire connection, while R C and R REC are the interfacial contact resistance originating R

R

R

R

from perovskite/ETL or perovskite/HTL interface and recombination resistance respectively. C μ R

R

refers to the chemical or bulk capacitance and C C is the chemical capacitance.56 R

R

Table 3 summarizes the fitting values of different electronic parameters from Nyquist plots of S-SnO 2 , N-SnO 2 and bilayer SnO 2 PSCs at 0.9V bias voltage. According to the table, the contact R

R

R

R

R

R

resistance R C for double layer SnO 2 PSC is 4.46 Ω·cm2, which is significantly lower than that of R

R

R

R

single layer SnO 2 PSC (15.93 Ω·cm2 for N-SnO 2 PSC and 28.26 Ω·cm2 for S-SnO 2 PSC), and R

R

R

R

R

R

follows the trend of series resistance obtained from Table 1. Since the HTL layer (spiro-OMeTAD) remains constant for all PSCs, the variation in contact resistance between the two devices is expected to originate from perovskite/ETL interface.57 Hence, the lower R C value of bilayer SnO 2 R

R

R

R

PSC reveals the enhanced charge extraction phenomena from perovskite to neighboring bilayer ETL. The recombination resistance, R REC is related to the recombination sites, where the higher R

R

value represents the less chance of carriers to be recombined. N-SnO 2 devices demonstrated the R

R

largest R REC value, which was 46% higher than that in S-SnO 2 reference devices. For bilayer SnO 2 R

R

R

R

R

R

device, the R REC is 206.55 Ω·cm2, only 10% lower than that of N-SnO 2 PSC, which did not R

R

R

R

compromise the overall performance considering the 72% drop on R C value. R

R

To investigate further into the PV performance between single layer (S-SnO 2 or N-SnO 2 ) and R

R

R

R

bilayer SnO 2 based PSC, we have completed the Mott-Schottky (MS) and C-V characterization of R

R


15


Page 16 of 36

representative devices. Figure 11 illustrates that no depletion layer capacitance C dl can be R

R

identified, providing a direct transition between geometrical capacitance C g region and R

R

accumulation capacitance C s region. The direct transition provides that the perovskite layers on R

R

both single layer and bilayer SnO 2 are highly crystallized and demonstrate defect density less than R

R

1017 cm-3, so that no distinguishable C dl is exhibited in MS curve.58 Therefore, we cannot R

R

determine the flat-band potential from MS analysis. However, as shown in CV curve, double layer SnO 2 demonstrates the lowest capacitance value in the high bias accumulation capacitance (C s ) R

R

R

R

region, followed by N-SnO 2 and S-SnO 2 . The accumulation capacitance is related to the ionic R

R

R

R

accumulation mechanism occurring at the perovskite/ETL interface59-60. Since C s arises from the R

R

Fermi level modulation of minority carriers, it has a detrimental effect on the device performance.58 Therefore, bilayer SnO 2 PSC with lower accumulation capacitance exhibits enhanced device R

R

performance, which is well in line with previous results. Furthermore,

we

have

fabricated

electron

only

devices

with

the

structure

of

ITO/SnO 2 /MAPbI 3 /PCBM/Ag. The dark current-voltage (I-V) curves of electron only devices is R

R

R

R

shown in Figure 12. the curves are divided into three regions. In the range of low voltage, the current increase linearly with the increase of voltage, representing an ohmic response ( I  V ). At intermediate voltages, the current exhibited a rapid non-linear increase ( I

 V n , n  3 ), indicating

the trap-filled limit (TFL), where all available trap states are filled by the trap filled limit carriers.6164

The voltage between the ohmic contact and trap-filled regime is known as trap-filled limit

voltage V TFL . For quantitative analysis, the electron trap state density is evaluated from trap-filled R

R

limited voltage (V TFL ) using equation:65 R

R


16

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VTFL

qnt L2  2 r  0

(8)

where n t is the trap-state density, L is the thickness of active layer, R

R

of active layer (  r of MAPbI 3 is 28.8)62 and R

R

0

 r is the relative permittivity

is the vacuum permittivity. The measured V TFL R

R

of S-SnO 2 , N-SnO 2 and B-SnO 2 electron-only devices are 0.63V, 0.44V and 0.28V respectively. R

R

R

R

R

R

Therefore, the obtained electron trap densities, n t are 1.64 1016 cm-3, 1.14 1016 cm-3 and R

R

7.28  1015 cm-3. Therefore, the reduced n t of bilayer SnO 2 device confirms the enhancement of R

R

R

R

film quality with less trap sites and reduced defect centers. At high voltage regime, current increases quadratically ( I  V 2 ). From this region, the charge carrier mobility can be calculated by using the equation as follows: 66

9 V2 J  ( ) r  0  3 8 L

(9)

 is the mobility. The electron mobilities of devices with different ETLs have been

where

calculated from Figure 12 (d) by extracting J-V2 slope. The obtained

 for S-SnO 2 , N-SnO 2 and R

R

R

R

B-SnO 2 are 7.64 104 , 1.21103 and 1.74 103 cm 2 /v  s respectively. Compared with S-SnO 2 R

R

R

R

and N-SnO 2 , B-SnO 2 exhibited much higher electron mobility owning to its significantly lower R

R

R

R

electron trap density. The results of space-charge limited current–voltage (SCLC) measurement reconfirmed the previous results obtained from XPS measurement as well as EIS characteristics, proving the enhanced electrical property of bilayer SnO 2 . R

R


17


Page 18 of 36

Conclusion In summary, we have reported a bilayer SnO 2 as ETL in MAPbI 3 -based perovskite solar cell. R

R

R

R

We also compared the performance of low temperature processed sol-gel SnO 2 and nano-SnO 2 R

R

R

R

single layer. With the aid of thin layer SnO 2 nanoparticle scaffold, the average PCE of bilayer R

R

SnO 2 PSC has been boosted from 12.97% to 16.84%, mainly attributed from the 23.7% R

R

enhancement of FF. According to the morphology characterization, SnO 2 nanoparticles fill the R

R

visible pin holes of compact sol-gel SnO 2 layer, replicating this smooth, void-free ETL surface. R

R

The XPS measurement also confirms the enhanced film property of B-SnO 2 due to reduced oxygen R

R

deficient regions. Perovskite film deposited on such bilayer ETL shows improved crystallinity and less dislocation, confirmed by X-ray diffraction analysis. The electrical impedance spectroscopy characterization confirms that the interfacial contact resistance and charge accumulation capacitance at perovskite/ETL interface reduce vastly with the incorporation of a B-SnO 2 ETL. R

R

Besides, B-SnO 2 demonstrated lower electron trap density and higher electron mobility according R

R

to SCLC calculation. Thus, the reported bilayer SnO 2 ETL demonstrates promising prospects R

R

towards highly efficient PSC with the potential of the roll-to-roll and flexible application in lowtemperature route.

ASSOCIATED CONTENT Supporting Information Available J-V parameters and curves for PSC with different thickness of ETLs. J-V parameters of FB-SC and SC-FB direction for S-SnO 2 , N-SnO 2 and B-SnO 2 devices. SEM/AFM images and XRD data R

R

R

R

R

R

of perovskites deposited on three ETL films. XPS spectral fitting data for S-SnO 2 , N-SnO 2 and BR

R

R

R

SnO 2 films. R

R


18

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Acknowledgements The author appreciates the financial support from Australian Government Research Training Program Scholarship. The authors would also like to acknowledge the endless support from the staffs of Photovoltaic and Renewable Energy Engineering School, Electron Microscope Unit (EMU) and Solid State and Elemental Analysis Unit under Mark Wainwright Analytical Center, UNSW.

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63. Xiao, K.; Cui, C.; Wang, P.; Lin, P.; Qiang, Y.; Xu, L.; Xie, J.; Yang, Z.; Zhu, X.; Yu, X.; Yang, D., Amine Treatment Induced Perovskite Nanowire Network in Perovskite Solar Cells: Efficient Surface Passivation and Carrier Transport. Nanotechnology 2018, 29 (6), 065401. 64. Li, M.; Li, B.; Cao, G.; Tian, J., Monolithic MAPbI3 Films for High-efficiency Solar Cells Via Coordination and A Heat Assisted Process. J. Mater. Chem. A 2017, 5 (40), 21313-21319. 65. Bube, R. H., Trap Density Determination by Space‐Charge‐Limited Currents. J. Appl. Phys. 1962, 33 (5), 1733-1737. 66. Murgatroyd, P. N., Theory of Space-charge-limited Current Enhanced by Frenkel Effect. J. Phys. D: Appl. Phys. 1970, 3 (2), 151.


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Page 24 of 36

Figures: (a)

(b)

(c)

Figure 1 (a) Schematic diagram, (b) cross-section SEM and (c) energy band diagram of planar Perovskite solar cell with bilayer SnO 2 ETL. R

R


24

(a) 24

(b) 26

26

24

24

22

22

20

20

18

18

16

16

14

14

12

12

20

16 PCE (%)

Current Density (mA/cm2)

12

10 8

10

8

0 0.0

0.1

0.2

0.3

efficiency (S-SnO2 PSC) efficiency (N-SnO2 PSC) efficiency (B-SnO2 PSC) current density (S-SnO2 PSC) current density (N-SnO2 PSC) current density (B-SnO2 PSC)

6

S-SnO2 PSC N-SnO2 PSC B-SnO2 PSC

4

4 2 0.4

0.5

0.6

0.7

0.8

0.9

1.0

0

1.1

0

20

40

60

80

Voltage (V)

100

120

140

160

180

8 6 4 2 0 200

Time (s)

Figure 2 (a) Current density – voltage (J-V) curves and (b) Stabilized current density and PCE as a function of time (200 seconds) of perovskite devices with different ETLs (S-SnO 2 , N-SnO 2 an R

R

R

R

B-SnO 2 ) R

R

(b)100

(a) 100

90 80

80

EQE (%)

70

Absorption (%)

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


Jsc(mA/cm2)

Page 25 of 36

60

40

60 50 40 30

20

0 300

400

500

600

S-SnO2 PSC N-SnO2 PSC B-SnO2 PSC

20

Perovskite on S-SnO2 Perovskite on N-SnO2 Perovskite on B-SnO2

10 700

800

900

0 300 350 400 450 500 550 600 650 700 750 800 850 900

1000

Wavelength (nm)

Wavelength (nm)

Figure 3 (a) Absorption spectrum of perovskite layers on different ETLs and (b) External quantum efficiency (EQE) spectrum of S-SnO 2 , N-SnO 2 an B-SnO 2 PSCs R

R

R

R

R

R


25


10 1

Current Density (mA/cm2)

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 26 of 36

0.1 0.01 0.001 1E-4 S-SnO2 PSC N-SnO2 PSC B-SnO2 PSC

1E-5 1E-6 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Applied Voltage (V)

Figure 4 Dark IV curves of S-SnO 2 , N-SnO 2 and B-SnO 2 PSCs R

R

R

R

R

R


26

Page 27 of 36

(a) 100

(b) S-SnO2/Glass N-SnO2/Glass B-SnO2/Glass

60

(h)2

Transmittance (%)

80

40

S-SnO2/ITO glass N-SnO2/ITO glass B-SnO2/ITO glass

20

0 300

400

500

600

700

800

900

1000

2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2

Wavelength (nm)

hv (eV)

(c)

(d) S-SnO2 N-SnO2 B-SnO2

4.2

4.3

Intensity (a.u.)

S-SnO2 N-SnO2 B-SnO2

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


4.4

4.5

4.6

4.7

15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0

4.8

Kinetic Energy (eV)

Kinetic Energy (eV)

Figure 5 (a) Transmittance spectrum of S-SnO 2 , N-SnO 2 and B-SnO 2 thin films on ITO glass; (b) R

R

R

R

R

R

Tauc Plot of S-SnO 2 , N-SnO 2 and B-SnO 2 thin films on glass; (c) Fermi edge region and (d) cutR

R

R

R

R

R

off energy region of S-SnO 2 , N-SnO 2 and B-SnO 2 on glass from UPS measurement R

R

R

R

R

R


27


Page 28 of 36

(a)

(b)

(c)

Figure 6 Deconvoluted Gaussian subpeaks and envelop curve of high resolution O1s XPS spectra for (a) S-SnO 2 and (b) N-SnO 2 and (c) B-SnO 2 ETL films R

R

R

R

R

R


28

Page 29 of 36 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


Figure 7 Top view Scanning Electron Microscopy (SEM) images of (a, b) S-SnO 2 ; (c, d) N-SnO 2 R

R

R

R

and (e, f) B-SnO 2 films deposited on ITO substrates. R

R


29


Page 30 of 36

(a)

(b)

(c)

Figure 8 Two and three-dimensional atomic force microscopy (AFM) images of (a) S-SnO 2 ; (b) R

R

N-SnO 2 and (c) B-SnO 2 deposited on ITO substrates R

R

R

R


30

Page 31 of 36

(a)

20

25

(224)

Intensity (a.u)

15

*

30 35 2 (degree)

40

45

50

55

60



50

Crystallite size (nm)

8000

Peak Intensity (counts.)

*

(c)

10000

6000

4000

2000

0

(314)

(310)

(220)

(110)

Perovskite/S-SnO2 Perovskite/N-SnO2 Perovskite/B-SnO2

*

10

(b)

40

30

20

10

(110)

(220)

(310)

(224)

0

(314)

0.006

(220)

(310)

(224)

(314)

(e) 1.8x1011


1.6x1011 1.4x1011

Dislocation Density

0.005

(110)

Perovskite Chracteristic Peak


(d)

0.004

Microstrain

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


0.003

0.002


1.2x1011 1.0x1011 8.0x1010 6.0x1010 4.0x1010

0.001 2.0x1010 0.000

(110)

(220)

(310)

(224)

(314)

0.0

(110)

(220)

(310)

(224)

(314)



Figure 9 (a) XRD patterns of MAPbI 3 perovskite films deposited on top of S-/N-/B-SnO 2 ETLs R

R

R

R

(the * signs denote the peaks for ITO/glass substrate); (b)Peak Intensity (c) crystallite size (d) microstrain and (e) dislocation density of perovskite from the XRD spectral peak analysis


31


4500 4000

RSE

RC

RREC

CC

Cµ

3500 3000

-Z'' ()

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 32 of 36

2500 2000 1500 1000 S-SnO2 PSC N-SnO2 PSC B-SnO2 PSC

500 0 0

1000

2000

3000

4000

5000

6000

Z' ()

Figure 10 Dark Nyquist Plot under 0.9V bias of perovskite devices with S-SnO 2 , N-SnO 2 and BR

R

R

R

SnO 2 as ETLs R

R

Figure 11 Mott Schotteky curve and C-V curve of perovskite devices with S-SnO 2 , N-SnO 2 and R

R

R

R

B-SnO 2 as ETLs R

R


32

Page 33 of 36 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)

(b)

(c)

(d)(d)

Figure 12 Current density–voltage characteristics of devices with ITO/ SnO 2 /perovskite/ R

R

PCBM/Ag (electron-only devices): (a) S-SnO 2 , (b) N-SnO 2 and (c) B-SnO 2 ; (d) J-V2 plot of high R

R

R

R

R

R

voltage regime to extract the space charge limited current (SCLC) electron mobility.


33


Page 34 of 36

Tables: Table 1 Device performance parameters of perovskite devices with sol-gel SnO 2 , nano-SnO 2 and bilayer R

R

R

R

SnO 2 ETLs, under one sun illumination (AM 1.5G, 100mW·cm-2) R

R

V oc (V) R

R

J sc (mA/cm2 ) R

FF (%)

PCE (%)

R s (Ω·cm2) R

R

R

S-SnO 2 PSC R

R

N-SnO 2 PSC R

B-SnO 2 PSC

R

R

R

Average

1.03 0.029

Best

1.05

Average

1.03 0.035

Best

1.04

Average

1.05 0.017

Best

1.07

± 21.81 0.68

± 57.60 4.50

± 12.97 1.16

22.94

61.48

14.75

± 22.24 0.67

± 61.44 1.57

± 14.05 1.08

23.23

64.57

15.66

± 22.49 0.89

± 71.25 1.41

± 16.84 0.53

22.66

72.72

17.61

R sh (Ω·cm2) R

R

± 14.11 ± 3.51 1932 ± 705 10.89

2353

± 13.38 ± 1.81 2427 ± 598 9.63 ± 6.42 ± 0.73 5.58

3366 2651 ± 355 2304


34

Page 35 of 36 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


Table 2 The profile surface roughness paramaeters of sol-gel SnO 2 , nano-SnO 2 and bilayer SnO 2 films R

R

R

R

R

R

calculated from surface topography imaging with AFM. R a represents arithmetical mean deviation R

R

of the assessed profile, while R q represents root mean squared value. R

ETLs

R

Ra

R q (RMS)

S-SnO 2

2.24 nm

2.86 nm

N-SnO 2

1.78 nm

2.22 nm

B-SnO 2

1.33 nm

1.69 nm

R

R

R

R

R

R

R

R

Table 3 Fitted values of different electronic parameters from Nyquist plots of sol-gel SnO 2 , nano-SnO 2 R

R

R

R

and bilayer SnO 2 PSCs at a bias of 900 mV under dark. R

Device

R

R SE RC R REC CC Cµ 2 2 2 2 (Ω·cm ) (Ω·cm ) (Ω·cm ) (nF/cm ) (nF/cm2) R

S-SnO 2

R

R

R

R

R

R

R

R

R

0.80

28.26

156.6

355.56

200.22

N-SnO 2

1.15

15.93

228.6

473.33

178.67

B-SnO 2

3.05

4.46

206.55

397.78

162.89

R

R

R

R

R

R


35


Page 36 of 36

TOC For Table of Contents Only:


36

Bilayer SnO2 as Electron Transport Layer for Highly Efficient

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