Green Antiâsolvent Processed Efficient Flexible Perovskite Solar Cells

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Green Anti-solvent Processed Efficient Flexible Perovskite Solar Cells Deyu Xin, Zenghua Wang, Min Zhang, Xiaojia Zheng, Yong Qin, Jianguo Zhu, and Wen-Hua Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06190 • Publication Date (Web): 27 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Sustainable Chemistry & Engineering

Green Anti−solvent Processed Efficient Flexible Perovskite Solar Cells Deyu Xin

a,b,†,

Zenghua Wang

a,c,†,

Min Zhang a, Xiaojia Zheng

a,*,

Yong Qin c, Jianguo Zhu

b,*,

Wen−Hua Zhang a,* a Sichuan Research Center of New Materials, Institute of Chemical Materials, China Academy of Engineering Physics, 596 Yinhe Road, Shuangliu, Chengdu 610200, China b Department of Materials Science, Sichuan University, Chengdu 610064, China c Institute of Nanoscience and Nanotechnology, School of Physical Science and Technology, Lanzhou University, 222 South Tianshui Road, Lanzhou, Gansu 730000, China Corresponding Author * Email: [email protected] (X. Z.) * Email: [email protected] (J. Z.) * Email: [email protected] (W.−H. Z.) † These

authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT

Flexible perovskite solar cells (PSCs) possess compatible features with low − cost, high throughput production approaches, showing great potentials in wearable, portable, flyable and deployable applications. However, flexible PSCs with superior efficiency are commonly fabricated with anti−solvent assisted crystallization (ASAC) method, which uses highly toxic solvents as the processed solvents. In this work, we use environmentally benign anti−solvents for the fabrication of flexible PSCs. Benefitting from the low−temperature, solution processability of high quality SnO2 electron transport layer (ETL) with superior electron extraction and transfer features, flexible PSCs show an impressive PCE over 17% under reverse scan. This, to the best of our knowledge, is among the highest performances for flexible PSCs, and is the first report for flexible PSCs fabricated by green anti−solvent processed techniques. These results provide new opportunities for environmental−friendly manufacturing of high performance flexible PSCs.

KEYWORDS: green solvent, flexible, perovskite solar cell, charge recombination, tert−Butanol


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INTRODUCTION In the past few years, organometal halide perovskites have emerged as a promising candidate for photovoltaic (PV) application due to their suitable band gap (Eg), high absorption coefficient, low trap states, long and balanced carrier diffusion length etc.1-5 The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased rapidly from 3.8% to 23.3% in a short time, making them the forefront of PV research activity.6-10 In addition to their high PCE, the good solubility due to their ionic nature enables versatile low−temperature solution processes, including spin coating, blade coating, slot−die coating, screen printing etc., most of which are scalable and compatible with roll−to−roll (R2R) large−scale manufacturing processes.11-12 These features provide an exceptional opportunity for fabricating lightweight flexible PSCs through high throughput production approaches, which can reduce the manufacturing cost significantly. Considering the low−cost and lightweight flexible substrates are mostly polymer−based materials at present (such as polyethylene terephthalate, PET), which typically are not compatible with high temperature fabrication process (higher than150 °C). Therefore, a challenge for obtaining efficient flexible PSCs is that all of the components in the cell must be deposited and treated at low temperature (usually less than 120 oC). Most of polymer and small−molecule charge transport materials, such as of PEDOT:PSS, phenyl−C61−butyric acid methyl ester (PCBM) and C60, can be fabricated at low temperature, which has been widely used for flexible PSCs.13-15 Besides organic charge transport materials, inorganic metal oxide can provide better stability and lower cost, which are successfully applied in rigid glass based PSCs, and PCEs beyond 21% have been already achieved by both low−temperature processed TiO2 and SnO2 electron transport layers (ETLs).16-17 However, record efficiency for flexible PSCs is 18.4%


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at present, which is obvious lagging behind with devices fabricated on rigid substrates.18 Therefore, more research needs to be done for improving the efficiency of flexible PSCs. Besides the performance, the environmental impact of the processing solvents should be assessed, due to a key feature for high throughput production is that it does not harm the environment from both materials and manufacturing process. Anti−solvent assisted crystallization (ASAC) approach shows great promise for high performance PSCs, and almost all currently reported high−efficiency PSCs (beyond 20%) are obtained by this method. However, the most frequently used low polarity anti−solvents solvents are high toxic toluene, chlorobenzene (CB) etc., which represent a major obstacle to large scale manufacturing.19-20 Besides ethyl acetate green solvent, methoxybenzene (PhOMe), a fragrance for cosmetics and food, also works well for obtaining perovskite films with high quality.21-26 High efficiency beyond 20% was obtained based on the PhOMe processed PSCs, which demonstrate great practical significance for developing high quality perovskites for PV devices and other optoelectronic devices. Here, we adopt the ASAC approach with PhOMe as the green anti−solvent to fabricate high performance flexible PSCs. By optimizing the charge separation/transport features in PSCs through an additive of tert−Butanol (TBA) during the fabrication of SnO2 thin film, PCEs exceeding 20% and 17% have been achieved for PSCs with rigid glass/ITO and flexible PET/ITO respectively. This study demonstrates a facile process routs for obtaining high quality ETLs, as well as a greener pathway to reduce the environmental impact and health risks for researchers.


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EXPERIMENTAL SECTION Device Fabrication: SnO2−colloid precursor (tin(IV) oxide, 15% in H2O colloidal dispersion) was purchased from Alfa Aesar. The SnO2 nanoparticles solution was diluted by H2O or H2O/TBA mixture with various volume dilution rate before use. The SnO2 electron transport layer (ETL) was spin coated on glass/ITO or PET/ITO substrates at 4000 rpm for 30s in ambient air, and then dried at 100°C in air for 15 min. After depositing the ETLs, the perovskite layer was fabricated by anti−solvent assisted crystallization (ASAC) method. Mixed−cation lead mixed−halide perovskite solution was prepared by adding FAI (1 M), PbI2 (1.1 M), MABr (0.2 M) and PbBr2 (0.22 M) into a 4 : 1 (v:v) mixture of anhydrous DMF:DMSO (Sigma−Aldrich), and stired for 24 h. Then 1.5 M stock solution of CsI (Sigma−Aldrich) in DMSO was added to the above solution just before use (5 : 95, v:v). The spin−coating procedure was done in a glove box through a two step spin coating process (10 s at 1000 rpm and 40 s at 4000 rpm). 120 μL methoxybenzene or chlorobenzene was dripped during the second step, 10 s before the end. The perovskite films were annealed at 110 °C for 20 min. Hole transport material (HTM) was deposited by spin coating at 5,000 rpm for 20 s in glove box. The HTM solution was prepared by dissolving 101 mg spiro−OMeTAD [2,2’,7,7’−tetrakis(N,N−di−p−methoxyphenylamine)−9,9−spirobifluorene]

in

1

ml

methoxybenzene, with addition of 24.5 μL Li−TFSI/acetonitrile (520 mg/mL), 49 μL [tris(2−(1H−pyrazol−1−yl)−4−tert−butylpyridine)cobalt(III)

bis(trifluoromethylsulphonyl)

imide] (FK209) /acetonitrile (300 mg/mL) and 40 μL 4−tert−butylpyridine (TBP). Finally, 80 nm of gold was deposited by thermal evaporation. Characterization:


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Scanning electron microscopy (SEM) images were obtained under an accelerating voltage of 5 kV ( SIGMA HD, ZEISS Company). UV−vis absorption spectra of all samples were recorded on Evolution™ 201 spectrophotometer (Thermo fisher scientific Corporation). Contact angle measurements were conducted on a Dataphysics OCA−20 with a drop of SnO2−colloid precursor diluted with water or TBA/water mixture (1:6, v:v). Fourier Transform Infrared Spectroscopy (FTIR) is conducted on Nicolet iS10 FT-IR Spectrometer. The steady state PL was measured with Edinburgh Instruments Ltd (FLS980). Current density−voltage measurements were recorded by applying an external potential bias to the cell while recording the generated photocurrent with a digital sourcemeter (Keithley Model 2400) under simulated one−sun AM 1.5G illumination at 100 mW cm−2 (SSF5−3A, Class AAA Solar Simulator, Enlitech) in glove box. A 50 ms scanning delay was adopted for each data point. Before test, the exact light intensity was calibrated with a KG5−filtered Si reference diode (SRC−2020−KG5−RTD, Enlitech). A metal mask of 0.09 cm2 was applied for measurement. Bend testing for flexible PSCs is measured with a bending radius of 1 cm. External quantum efficiencies were measured by a QE−R3011 Quantum Efficiency Measurement System (Enlitech) under direct current mode. Electrochemical impedance spectroscopy results were recorded by an electrochemical workstation (VMP3 Bio−logic instruments) at −0.6 V under dark condition for frequency from 100 mHz to 100 kHz with an AC amplitude of 20 mV.

RESULTS AND DISCUSSION


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Figure 1. Contact angle of SnO2−colloid precursor diluted with (a) water and (b) water/TBA mixture on glass/ITO substrates. SEM images for SnO2 films prepared by (c) SnO2 aqueous solution and (d) SnO2 aqueous solution with TBA additive. One of the advantages of PSCs is their solution processability, including all of the ETLs, perovskites and HTMs. For film deposition through solution process, the wetting behavior of the solvents on the substrate is crucial for obtaining high quality films with perfect homogenous features, and this always governs the optoelectronic properties of the functional layers in PSCs. Therefore, to vary the wetting behavior of solution precursors on the substrate is pronounced for high performance PSCs. For flexible PSCs with regular cell structure, the challenge is how to obtain homogenous ETL on the flexible substrates at low temperature (usually less than 120 oC). It has been show that ultraviolet−ozone (UVO) or O2 plasma treatment can introduce hydroxyl


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(OH) and carbonyl (C=O) groups, thereby changing the surface of ITO to a more hydrophilic character.27-28 However, sufficient length of duration (10−15 min) is usually required to fully convert the surface, and the hydrophilic feature always fades quickly, which make the fabrication process complicated. Besides changing the surface chemistry of ITO to vary the wetting behavior, changing the chemical composition of solutions can achieve the same effect. Herein, methyl groups (−CH3) are introduced in SnO2 aqueous solutions by adding TBA to vary its wetting behavior on ITO substrates for depositing ETL with high uniformity. We firstly assessed the optimized volume ratio for TBA, and found that precipitate of SnO2 from the aqueous solution could be observed when adding too much TBA. More detailed information can be found in Figure S1. Therefore, a volume dilution rate of 1:6 for TBA:H2O was used in this work. To evaluate the wetting behavior of the solvents, surface contact angle measurement was conducted (Figure 1a−b). As a control sample (without TBA additive), the contact angle is large (72.5o), suggesting an inferior wetting behavior. Correspondingly, when TBA is adding into the SnO2 aqueous solution, the surface contact angle considerably reduces to 30.4o, indicating largely improved surface wettability, which can enable a uniform film formation. The microstructure of the SnO2 films is shown in Figure 1c−d. From the scanning electron microscope (SEM) images, it can be found that TBA additive can result in a smoother and homogeneous film, which is beneficial for avoiding pin−holes in the films, thereby improving the charge transport in ETLs and benefitting the reduction of recombination process in PV devices. Residue of TBA in the SnO2 ETLs may affect its electronic properties. Fourier Transform Infrared Spectroscopy (FTIR) was conducted to check whether TBA could be residual after an annealing process at 100°C for 15 min. FTIR spectra for SnO2 films prepared by SnO2−colloid precursor diluted with water (SnO2-control) and water/TBA mixture (SnO2-TBA) are shown in Figure S2. No peak assignable


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to TBA has been found. Considering TBA is small aliphatic alcohol, it can be easily removed due to its low boiling point of 83 °C. Therefore, it is concluded that SnO2 film prepared by SnO2−colloid diluted with water/TBA mixture is mainly free of TBA.

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Figure 2. (a) Transmittance spectra of glass/ITO substrate and glass/ITO/SnO2 films and (b) photoluminescence spectrum of perovskites deposited on various substrates. Optical transmission can influence the absorption of incident photons for perovskite layer, and thus affect the short circuit current density (Jsc) of the device. Figure 2a shows the optical transmission spectra of the glass/ITO and glass/ITO/SnO2 samples. It is interesting to found that the transparency around 400 nm is obviously enhanced after SnO2 deposition, which maybe


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aroused by the reduction of reflection. The improved transmission implies that SnO2 film would allow more photon flux to reach the perovskite light absorber, thereby generating more charge carriers. Next, we fabricated the perovskite films on the SnO2−control and SnO2−TBA layers by ASAC approach with PhOMe anti-solvent. The microstructure of the perovskite thin films is shown in Figure S3. No obvious difference is found between them due to similar substrates and same fabrication procedure. Steady−state photoluminescence (PL) was conducted to gain more insight information on the charge transfer/recombination properties at perovskite/ETL interface. Figure 2b presents the steady−state PL spectra of perovskites on various substrates under same experimental condition. PL intensity of the perovskites is significantly quenched when SnO2 existing, demonstrating that SnO2 can result in effective electron extraction. The weaker PL intensity for SnO2−TBA indicating a better electron extraction and transport features. We also notice that the PL peaks show blue shift when SnO2 existing, indicating a passivation of surface trap−states of perovskites by SnO2. Conductivity of ETLs is another important factor for high performance solar cells in considering their critical impacts on serious resistance and charge transport characteristics. Pinholes in the SnO2 film may introduce high electron scattering during the transport process, thereby increasing the resistance. The current-voltage (I-V) characteristics and the calculated resistivity for SnO2−control and SnO2−TBA are shown in Figure S4. From the experimental results, we can see that the resistivity for SnO2−TBA is slightly lower than SnO2−control, which means lower electron transfer resistance in it. The increased conductivity for SnO2−TBA should be in favor of improving the performance of PSCs.


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Figure 3. (a) Cross−sectional SEM image of the PSC based on SnO2−TBA. (b) J−V curves, (c) EQE spectra and (d) electrochemical impedance spectroscopy of PSCs based on SnO2−Control and SnO2−TBA. Devices were fabricated with PhOMe anti-solvent. Solar cells were fabricated to evaluate the electron extraction and transport features for SnO2 prepared via different routes. The regular planar structure PSCs in this study were structured as ITO/SnO2/perovskites/Spiro−OMeTAD/Au, as shown in Figure 3a. Figure 3b presents the current density–voltage (J−V) curves of the PSCs measured in reverse scan mode (from open circuit voltage (Voc) to Jsc) under AM 1.5G illumination (air mass 1.5 global) for PSCs fabricated with PhOMe anti-solvent. PSC based on SnO2−TBA exhibits a Voc of 1.15 V, a Jsc of 22.29 mA cm−2, and a fill factor (FF) of 80.01%, producing a PCE of 20.51%. In contrast,


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PSC based on SnO2−control, a Voc of 1.13 V, a Jsc of 22.12 mA cm−2, and a FF of 76.94% are obtained, yielding an overall PCE of 19.23%. These results clearly show that TBA additive during the preparation of SnO2 ETL can boost the PCE to higher values by increasing all of the Voc, Jsc, and FF parameters, which can be attributed to better electron extraction and transport features for SnO2−TBA. It was noticed that PSCs based on SnO2−TBA exhibited less hysteresis effect at the same time (Figure S5−6). In our previous work, we find that capacitive effect in PSCs dominates the formation of non−steady state photocurrent and governs the J−V hysteresis behavior.29 It has been reported that the inefficient extraction of charge carriers can result in a large interfacial electronic dipole polarization due to the accumulated electrons or holes at the perovskite/electrode interface, which can introduce a large capacitive effect in the PSCs.30-32 According to PL results, SnO2−TBA can provide better electron extraction and transfer features, thereby decreasing the capacitive effect in the PSCs, which may be responsible for the reduced hysteresis effect. From the external quantum efficiency (EQE) shown in Figure 3c, the high EQE data suggesting that solar light can be efficiently converted to charge carriers and be collected by the terminal electrode, and superior EQE for SnO2−TBA solar cell is responsible for its higher Jsc. The electrochemical impedance spectroscopy (EIS) spectra represent the intrinsic interfacial charge transfer and charge transport kinetics in PSCs. Figure 3d shows the Nyquist plots of the PSCs employing two SnO2 ETLs measured under dark condition. There are two typical arcs, where the first arcs at high frequency can be mainly attributed to the charge transport features in bulk materials and the semicircles in the low frequency range mainly represent charge recombination process at interface and Warburg ion diffusion (WS) in PSCs.33-34 The Nyquist plots are fitted by the Z−view software with an equivalent circuit diagram (Figure S7), where the


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ohmic resistance (Rs) mainly reflects the sheet resistance of the electrodes and intrinsic resistance come from the bulk materials, the transport resistance (Rtr) in the high−frequency region represents charge transport features in PSCs, and the recombination resistance (Rrec) is associated to charge recombination process in the devices.35-36 The Rtr value in PSC without TBA is ~124 KΩ, higher than the value of ~85 KΩ for the PSC based on SnO2−TBA, whilst the Rrec increase from 1.4 MΩ to 1.7 MΩ when TBA existing. Considering the same fabrication process except for SnO2, so the difference between the Rtr and Rrec should originate from the SnO2 layer and SnO2/perovskite interface. The decreased Rtr and increased Rrec for PSC based on SnO2−TBA can be associated primarily with better electron extraction and transport features, which resulting a lower recombination rates, thereby giving better performance for PSCs. For comparison, PSC was also fabricated by CB anti-solvent on SnO2−TBA ETL, and its photovoltaic performance was shown in Figure S8. PSC shows PCEs of 20.14% under reverse scan, which is slightly lower than PSC fabricated with PhOMe. These results are consistent well with solar cells fabricated on TiO2 ETLs reported in our previous work.25 Thus, PhOMe antisolvent can produce PSCs with high performance and shows great practical significance for decreasing the environmental impact of fabrication process caused by the released toxic solvent. Figure S9 presents the photovoltaic performance of PSCs based on SnO2−control using UVO treated glass/ ITO substrate. These devices show similar results with PSCs based on SnO2−TBA without UVO treatment, demonstrating that TBA additive can provide similar function as UVO treatment to obtain a good wetting behavior for uniform ETL deposition.

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Figure 4. (a) Photograph of the flexible PSCs. (b) Transmittance of the PET/ITO substrates. (c) J−V curves and (d) EQE spectra of the flexible PSCs. By taking advantage of the low temperature solution processed techniques, we next fabricate flexible PSCs utilizing PET/ITO as the conductive transparent electrode, adopting the same configuration as the rigid glass/ITO counterpart. Figure 4a shows the photograph of the flexible PSCs, and Figure 4b presents the transmittance features of the flexible PET/ITO substrate. It is clearly show that the transmittance is above 80% between 430 nm and 800nm, with maximum value around 90% for the used PET/ITO substrate. The conductivity of the PET/ITO substrate is about 17 Ω/□ from four-point probe characterization. Figure 4c shows the J−V curves and corresponding photovoltaic parameters of the flexible PSCs. For the best flexible


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PSCs, we derive values of 22.57 mA cm−2, 1.06 V, and 63.58% for Jsc, Voc, and FF, respectively, yielding a PCE of 15.21% for the forward scan. For the reverse scan, the device shows a Jsc of 22.48 mA cm−2, a Voc of 1.11 V, a FF of 68.49%, and a PCE of 17.09%. This, to the best of our knowledge, is among the highest performances for flexible PSCs, and is the first report for flexible PSCs fabricated by green anti−solvent processed techniques. EQE data for the flexible PSCs is shown in Figure 4d. When compare EQE spectra for PSCs based on glass/ITO and PET/ITO, it can be found that the EQE for PSC based on glass/ITO is higher than flexible PSC if wavelength less than 550nm, but shows lower value when light wavelength beyond 550 nm. The integrated Jsc is comparable with each other, which are consistent well with the Jsc derived from the J-V curves.


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Figure 5. Bending fatigue test for flexible PSCs. (a) Photograph of the flexible PSCs under various bending radii. Normalized (b) PCE, (c) Voc, (d) Jsc and (e) FF as a function of bending cycles under various bending radii for the flexible PSCs. Reliability assessment is critical for the application of flexible PSCs. Bending fatigue properties of flexible PSCs are shown in Figure 5. We test the stability of the flexible PSCs under three bending radii (1cm, 0.8 cm and 0.6cm), and the corresponding change of PV


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parameters with bending cycles are shown in Figure 5b-e. After 100 bending cycles, no obvious PCE loss was found with a bending radius of 1cm, and 85% of the PCE could be maintained under a bending radius of 0.6 cm. Though obvious reduction on the performance of flexible PSCs was found under small bending radius, it is worth to point that the PCE of the PSC after 100 bending cycles with a radius of 0.6 cm can be recovered to its initial value following a stress relaxation overnight. This PCE self-recovered phenomenon was also found in light-stability test before.16,

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Besides good flex resistance properties, the flexible PSCs also exhibited good

stability under dark storage. PSCs showed slightly decrease in Jsc, an increase in Voc after 2000 h dark storage in ambient air (~20% RH) without encapsulation (Figure S10), keeping similar PCE as its original efficiency. The changes in Jsc and Voc may be resulted from the oxidation of spiro−OMeTAD during storge.38-39 These results indicate that the great durability of flexible PSCs is suitable for future practical applications.

CONCLUSION In summary, we have successfully developed a robust fabrication process by adding TBA in the aqueous solution for obtaining high quality SnO2 ETL to avoid the complicated UVO or O2 plasma treatment of the substrates. The SnO2−TBA shows better electron extraction and transport features, which is favorable for suppressing the recombination in the PSCs. As a result, the PSCs fabricated by green anti−solvent assisted approach show a high PCE of over 20%, surpassing that of the control device using a regular SnO2 ETL, and comparable with PSCs fabricated by high toxic chlorobenzene anti−solvent. Benefitting from the low−temperature, solution processability of high quality SnO2−TBA, flexible PSCs with regular cell structure can


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be fabricated and shows an impressive PCE over 17% under reverse scan, which is among the state−of−the−art values reported for flexible regular planar structure PSCs. Moreover, the greener fabrication process can dramatically reduce the environmental impact and health risks for researchers. This study not only demonstrates facile process routs for fabricating high quality SnO2 ETL, but also a greener pathway toward high−performance flexible PSCs.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Figures S1−S10, showing the photographs of SnO2 solutions, FTIR spectra, SEM images for perovskite films, I-V characteristics and corresponding resistivity for SnO2 ETLs, J−V curves for PSCs fabricated with different substrates or anti-solvents, equivalent circuit diagram used to fit the Nyquist plots, and stability test of flexible PSCs (PDF).

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (X. Z.). * Email: [email protected] (J. Z.) * Email: [email protected] (W.−H. Z.) Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. NSFC21773218) and Sichuan Province (Nos. 2017GZ0052, 2018JY0206, 2018RZ0119). X.Z. also acknowledges the financial support from Institute of Chemical Materials, China Academy of Engineering Physics under Contract No.0207. REFERENCES [1].

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TOC

25

PCE = 17.09%

20 15 Au Pe HT ro M Sn vsk i IT O2 te PE O T

Current density (mA cm-2)

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10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Efficient flexible perovskite solar cells are fabricated through green anti−solvent processed techniques, whicn can dramatically reduce the environmental impact by replacing the toxic antisolvents.


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Green Antiâsolvent Processed Efficient Flexible Perovskite Solar Cells

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