Antisolvent diethyl ether as additive to enhance the performance of perovskite solar cells

Accepted Manuscript Antisolvent diethyl ether as additive to enhance the performance of perovskite solar cells

Huiping Wang, Wenjin Zeng, Ruidong Xia PII: DOI: Reference:

S0040-6090(18)30513-3 doi:10.1016/j.tsf.2018.07.041 TSF 36798

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

10 February 2018 22 June 2018 29 July 2018

Please cite this article as: Huiping Wang, Wenjin Zeng, Ruidong Xia , Antisolvent diethyl ether as additive to enhance the performance of perovskite solar cells. Tsf (2018), doi:10.1016/j.tsf.2018.07.041

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ACCEPTED MANUSCRIPT Antisolvent diethyl ether as additive to enhance the performance of perovskite solar cells Huiping Wang†, Wenjin Zeng†, Ruidong Xia* Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory

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for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic

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Innovation Center for Advanced Materials (SICAM), School of Materials Science and

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Engineering, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, P. R. China

W. Zeng and H. Wang have equally contributed to this work.

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†

* Corresponding author. Email address: [email protected] (R. Xia)

ACCEPTED MANUSCRIPT ABSTRACT Optimizing the grain size of the perovskite crystals is a significant procedure for improving the device performance of the perovskite solar cells. This study applies a common antisolvent diethyl ether as solvent additive for MAPbI3 precursor solution.

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It reveals that the addition of diethyl ether can effectively influence the crystallization

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process. The grain size of MAPbI3 crystals increases with the increasing of diethyl

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ether ratio. As a result, light absorption property of the perovskite films are also modified corresponding to the quality of the perovskite layer. With an optimized ratio

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of 4% for diethyl ether, a maximum power conversion efficiency of 15.09% was

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achieved, which is improved by more than 20% compared to the control device. Keywords: Solar cells; Perovskite; Diethyl ether; Solvent additive; Crystal size;

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Antisolvent

ACCEPTED MANUSCRIPT 1. Introduction Perovskite (PVSK) solar cells have attained rapid progress during the past decades. The power conversion efficiency (PCE) of PVSK solar cells increased from 3.9% to the current word record of 22.7% [1–7]. Since the first report on solvent engineering

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of PVSK [8], techniques based on solvent engineering have become the most popular

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and efficient methods to achieve high-performance PVSK. The quality of the

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perovskite layer is the most crucial factor in determining the performance of PVSK. Delicate control of the grain size of the perovskite layer is an effective route to

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achieve high-quality perovskite layer [9]. N,N-dimethylformamide (DMF) and

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dimethyl sulphoxide (DMSO) are typically used as a mixture solvent for MAPbI3 precursors due to their synergistic effect. DMF is a strong polar solvent suitable for

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the dissolution of MAI and PbI2, while DMSO has strong coordination effect with

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Pb2+ which can retard the crystallization of PbI2 and improve the quality of the perovskite films [10,11].Some other solvents with weak polarity have also been

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investigated, such as ethanol, acetonitrile, isopropanol, etc [12–14] . Such weakly

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polar solvents have weak coordination ability with the perovskite precursors which can affect the growth of the perovskite crystals. At the same time, they are poor solvent for the lead ions. Therefore, it is critical to optimize the blend ratio of the weakly polar solvent in the matrix solution. Diethyl ether, a common non-polar solvent, now has been widely used as an efficient antisolvent for the perovskite crystallization since first reported in [15]. In this study, diethyl ether was applied as solvent additive instead of antisolvent into

ACCEPTED MANUSCRIPT the matrix of MAPbI3 precursor solution. Its effect on the crystallization process of MAPbI3 perovskite is investigated, as well as its subsequent enhancement on the light absorption property and device performance of MAPbI3-based PVSK. 2. Experimental Section

iodide (PbI2), and DMSO, DMF,

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Methylamine iodide (MAI), lead (II)

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2.1 Materials

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[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), bathocuproine (BCP) were purchased from Xi’an Polymer Light Technology Co. and Sigma-Aldrich respectively,

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as used without further purification. Indium-tin oxide (ITO) coated glass was used as

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the device substrate, purchased from China Southern Glass Holding Co Ltd with a surface resistance of 10 Ω/ sq. The matrix solvent is DMF and DMSO, with a molar

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ratio of 4:1. As for additive-modified devices by antisolvent, diethyl ether was added

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to the matrix solvent of DMF: DMSO at different molar ratio of 3%, 4%, 5% and 10%. MAI and PbI2 were dissolved in the solvent mixture with a molar ratio of 1:1, with the

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ultimate concentration kept constant at 1.5×10-3 mol/mL referred to [16]. The

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MAI-PbI2 solution was stored in the glovebox under dry nitrogen atmosphere before spin-coating. Nickel oxide (NiOx) nanoparticles were synthesized following the procedure introduced in [17]. 2.2 Device preparation All the device structure is of the same configuration, ITO/NiOx(40 nm)/Perovskite layer(800 nm)/PCBM(50 nm)/BCP(10 nm)/Ag(100 nm). The difference is the additive ratio of diethyl ether in the perovskite solution. For simplicity, the control

ACCEPTED MANUSCRIPT device and the additive-modified devices were named as Device A, B, C, D and E, corresponding to the additive ratio of 0% (control), 3%, 4% , 5% and 10%. Polished ITO glass was thoroughly cleaned before use in ultrasonic solvent bath of detergent, acetone, isopropanol and deionized water in sequence. Surface treatment of

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oxygen plasma was also performed on the ITO glass to completely remove the

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organic residues before the spin-coating process. NiOx aqueous solution after

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ultrasonic dispersion was spin-coated on the ITO glass substrate at 3000 rpm and annealed at 130 oC for 10 min to form a solid film of around 40 nm. Solution mixture

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of MAI and PbI2 was spin-coated on top of NiOx layer under the washing of toluene

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to prepare the perovskite photovoltaic layer. After the thermal annealing of the perovskite layer for 15 min, 50 nm PCBM was spin-coated from chlorobenzene

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solution acting as the electron-transport layer. The whole fabrication process was

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completed after the thermal evaporation of 10 nm BCP and 100 nm Ag on top of the PCBM layer under high vacuum of 4×10-4 Pa.

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2.3 Characterization instruments

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Photovoltaic performance of the perovskite solar cells was employed on a solar cell testing system consisting of a computer-programmed sourcemeter (Keithley 2400) and a solar simulator (AM 1.5, Newport), with the light density of 100 mW/cm2 calibrated

by

a

standard

Si

photodiode.

The

monochromatic

incident

photon-to-electron conversion efficiency (IPCE) was collected on a calibrated testing system from Enlitech Co (QE-R3018). The thickness of the organic layers was measured using a calibrated surface profiler (Alfa Step-500, Tencor). X-ray diffraction

ACCEPTED MANUSCRIPT (XRD) patterns were collected on a X-ray diffractometer (D8 Advance, Bruker Co.) using monochromatic Cu source (λκα1(Cu)= 0.15418 nm) at 5.0 KV. Top-view morphology was studied via scanning electron microscope (S-4800, Hitachi). The measurement of ultraviolet-visible spectrophotometry (UV-Vis) was carried out on the

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absorbance analysis system of PerkinElmer Co (Lambda 35).

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3. Results and discussions

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The characteristic curves of photocurrent density versus voltage (J-V) and incident-photon-to-current efficiency (IPCE) were demonstrated in Fig. 1 (a) and (b),

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with the detailed J-V parameters summarized in the Table 1. It seems that there is no

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apparent modification in VOC as indicated in Fig. 1 (a) at low blend ratio of diethyl ether, such as 3%, 4%, and 5%. Only when the blend ratio of diethyl ether further

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increased to 10% (Device E), the addition of diethyl ether leads to a significant

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reduction in VOC from 1.08 V to 0.91 V compared with the control device. When photo current density is taken into account, the enhancement on device performance is

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highly significant. The addition of diethyl ether can increase the photocurrent density

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remarkably. Especially when the ratio of diethyl ether reaches 4%, as listed in Table 1, the short-circuit current density (JSC) of Device C increases to 19.47 mA/cm2, which is higher than 16.20 mA/cm2 of Device A by 20%. As a result, the maximum PCE of 15.09% was attained in Device C. Such parameters are in accordance with the results of IPCE as indicated in Fig. 1 (b) and the theoretical value of the integrated current density JSC′, although with minor deviation in a reasonable range due to device degradation when IPCE curves are collected after J-V curves were characterized.

ACCEPTED MANUSCRIPT To correlate the morphology property with the device performance of the PVSK solar cells, top-view scanning electron microscopy (SEM) images of MAPbI3 on top were collected as indicated in Fig. 2 (a) - (e). The most obvious difference is that the grain size of MAPbI3 crystal has been changed with the addition of diethyl ether by

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comparing the SEM images, especially when the blend ratio of diethyl ether reaches 5%

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or more. At the same time, the boundary of the MAPbI3 crystals becomes blurred. It

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means that the addition of diethyl ether can effectively modify the crystallization process of MAPbI3. Diethyl ether, when acting as the antisolvent for the perovskite

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precursor solution, has been proved to assist the formation of larger grain of MAPbI3

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perovskite. It may suppress the participation of DMF to the crystallization of MAPbI3 perovskite. Therefore the obtained perovskite crystals consist of mainly MAPbI3 with

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only a minimal amount of PbI2-MAI-DMSO. The formation of PbI2-MAI-DMSO

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plays a critical role in attaining high-quality MAPbI3 film, since it can effectively decouple the process of film formation and crystallization [18].

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Also the cross-section SEM images are indicated in Fig 3 (a) - (e) correspondingly.

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Greater grain size is observed as the doping concentration of diethyl ether increasing, while the layer thickness of the perovskite layers appear no obvious difference when the doping concentration of diethyl ether changes. As for the morphology, the layer with higher fill factor typically carries a more uniform morphology. In the samples with high doping ratio, such as 5% and 10%, obvious pinholes were observed in the cross-section SEM images, which are related to the poorer fill factor. The corresponding XRD spectra is indicated in Fig. 4, in which the diffraction peaks

ACCEPTED MANUSCRIPT located at the diffraction angles (2θ) of 13.89 °, 28.32 °and 38.75 ° were respectively indexed as (110), (220) and (310) crystal plane of the MAPbI3. The reaction of MAI with PbI2 is complete since there is no diffraction peak for PbI2 residue reflected on the XRD spectra. Based on the diffraction peak at 13.89o, which represents (110)

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plane of MAPbI3 crystal, the grain size can be estimated to be 91.6 nm, 91.2 nm, 98.8

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nm, 122.8 nm and 128.0 nm corresponding to the blend ratio of diethyl ether at 0%,

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3%, 4%, 5% and 10% according to the Debye-Scherrer formula referring to the calculation method in [19, 20], i. e. D = Kλ/β·cosθ, where D represents the grain size,

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K is constant (value 0.89 if β is defined as the full width at half maximum of the XRD

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peaks), λ is the wavelength of the incident X-ray (value 0.154 nm), β is defined as the full width at half maximum of the XRD peak in radian unit and θ is the diffraction

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angle, with 2θ of 13.89o taken for our calculation.

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Although there is no apparent trend in the relative XRD intensity as a function of the diethyl ether concentration, the absorbance intensity of the perovskite layer increases

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initially and attains the maximum performance as the ratio of diethyl ether reaches 4%

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with the grain size of MAPbI3 increasing, as indicated in Fig. 5. However when the ratio further increases, the excessive addition of diethyl ether reduces the light absorption of the perovskite layer instead, which can lead to the reduction of photo-generated current density as shown in the characteristic curves of J-V in Fig. 1. Such phenomenon can be explained by the increased pinholes in the MAPbI3 film when excessive diethyl ether is added as indicated in Fig. 2. Such pinholes will act as defects to the perovskite photovoltaic layer and lead to the loss of photo-generated

ACCEPTED MANUSCRIPT carriers. 4. Conclusions In summary, we have presented an effective method to improve the performance of perovskite solar cells based on MAPbI3 by applying antisolvent diethyl ether as

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solvent additive in the perovskite precursor solution. SEM characterization reveals

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that the addition of diethyl ether can affect the crystal process of MAPbI3, leading to

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the grain size of the perovskite layer increasing. The increased grain size may result in the enhancement of the light absorption of the perovskite layer if the blend ratio of

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diethyl ether is optimized, accompanied with the increase of photo-generated current

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density of the perovskite layer. In our study, 4% is the optimized ratio for the addition of diethyl ether into MAPbI3-based perovskite solar cells, in which the PCE of the

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Acknowledgements

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devices increased by 20%.

The authors would like to thank the financial support of the Chinese 973 Project

61376023),

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(61504066,

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(Grant No. 2015CB932203), National Natural Science Foundation of China

(BK20150838).

Natural

Science

Foundation

of

Jiangsu

Province

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Fig. 1. The characteristic curves of (a) photocurrent density-voltage (J-V) and (b) IPCE of the perovskite solar cells with different blend ratio of diethyl ether as additive.

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Fig. 2. Top-view scanning SEM images of the perovskite layers with different blend ratio of diethyl ether as additive, (a) Device A (0%), (b) Device B (3%), (c) Device C (4%), (d) Device D (5%) and (e) Device E (10%).

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Fig. 3. Cross-section SEM images of the perovskite layers with different blend ratio

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of diethyl ether as additive, (a) Device A (0%), (b) Device B (3%), (c) Device C (4%),

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(d) Device D (5%) and (e) Device E (10%).

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Fig. 4. XRD patterns for the perovskite layers with different blend ratio of diethyl

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ether as additive.

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Fig. 5. UV-Vis absorbance of the perovskite layer with different blend ratio of diethyl

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ether as additive.

ACCEPTED MANUSCRIPT Table 1 Device performance of the perovskite solar cells with different blend ratio of diethyl ether as additive JSC (mA/cm2)

JSC′ (mA/cm2)*

FF (%)

PCE (%)

Device A (0%)

1.08

16.20

16.12

75.66

13.28

Device B (3%)

1.07

17.40

16.45

70.98

13.24

Device C (4%)

1.10

19.47

18.78

70.48

15.09

Device D (5%)

1.06

19.20

14.27

Device E (10%)

0.91

15.91

13.65

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VOC (V)

Blend Ratio

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61.67

12.60

64.37

9.29

* JSC is the theoretical value of the current density integrated from the IPCE curves in

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Fig 1 (b).

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Highlights > Antisolvent diethyl ether is used as the solvent additive in perovskite precursor solution. > The addition of diethyl ether increases the perovskite grain size significantly. > Power conversion efficiency is enhanced by 20% for MAPbI3 perovskite solar cells.