Efficient, stable and flexible perovskite solar cells using two-step solution-processed SnO2 layers as electron-transport-material

Efficient, stable and flexible perovskite solar cells using two-step solution-processed SnO2 layers as electron-transport-material

Organic Electronics 58 (2018) 126–132 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel ...

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Organic Electronics 58 (2018) 126–132

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Efficient, stable and flexible perovskite solar cells using two-step solutionprocessed SnO2 layers as electron-transport-material

T

Xincan Qiua, Bingchu Yanga,∗, Hui Chena, Gang Liua, Yuquan Liua, Yongbo Yuana, Han Huanga, Haipeng Xiea, Dongmei Niua, Yongli Gaoa,b, Conghua Zhoua,∗∗ a

Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan Province, 410083, PR China b Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Flexible perovskite solar cells Tin oxide Electron-transport-material Wettability Crystallinity Charge extraction

Flexible perovskite solar cells were prepared using two-step solution-processed SnO2 layers as electron-transportmaterial. The first layer was constructed by hydrothermal derived SnO2 nanoparticles, and the second layer was in-situ formed from hydrolysis and oxidization of SnCl2. It was observed that the two-step formed SnO2 layers could efficiently improve the device performance of the flexible perovskite solar cells. Without such layers, no photovoltaic effect was observed due to poor wettability of substrate. After SnO2 was deposited, power conversion begins due to improved crystallinity of perovskite layer. Compared with one-step processed SnO2 layer, the two-step processed SnO2 layers could take the advantages of accelerated charge extraction, lowered contact resistance, as well as reduced charge recombination, which help to upgrade open circuit voltage, short circuit current, fill factor and power conversion efficiency of the flexible devices. Device efficiency of 15.21% (AM 1.5 G, 100 mW/cm2) was obtained for the flexible devices, with prolonged shelf-stability in addition. After being stored in dark for 111 days (relative humidity of 35–55%, no encapsulation was used), device could keep 85% of starting efficiency. Meanwhile, specific power (ratio between power and device weight) of 0.87 kW/kg was obtained, as well as sound fatigue ability against bending. The solution based two-step formation process thus provides a reliable strategy for future application of flexible perovskite solar cells.

1. Introduction Organometal trihalide perovskite solar cells (PSCs) have attracted ever-increasing attention since the pioneered work in 2009 [1–8]. Right now this new type of photovoltaic device has achieved power conversion efficiency (PCE) higher than 20%, even up to the record of 22.7% (certified at AM 1.5 G, 100 mW/cm2) [9]. However, these devices were fabricated mainly on glass based transparent & conductive substrates. Glass is fragile and heavy, thus hindering the application of PSCs. As a result, it is appealing to make the devices on flexible transparent & conductive substrates. In fact, flexible PSCs have been tried immediately after PSCs were proposed. For example, in 2013, Kumar et al. prepared flexible PSCs on indium-tin oxide (ITO) based plastic substrate where they modified the substrate by zinc oxide nanorods and obtained PCE of 2.62% [10]. In 2015, Li et al. prepare flexible PSCs on silvermesh based substrates and obtained PCE of 14.0% [11]. While in 2018, Han et al. prepared flexible PSCs on thin metal folium [12]. Amongst the flexible substrates, the mostly used one is ITO-coated plastic, like ∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (C. Zhou).

∗∗

https://doi.org/10.1016/j.orgel.2018.04.010 Received 10 March 2018; Received in revised form 3 April 2018; Accepted 3 April 2018 Available online 04 April 2018 1566-1199/ © 2018 Published by Elsevier B.V.

ITO/PEN (polyethylene 2,6-naphthalate), but the surface chemistry of ITO deposited on plastic is pristinely different from that of on glass. For example, it is possible to achieve uniformly formed perovskite layer on glass substrates coated by either FTO (F: SnO2) [13] or ITO [14]. But when it is done on ITO/PEN, less success was achieved due to poor wettability, as will be shown later. As a result, modification is needed to the substrate surface. Indeed, such modification is usually included in studies about electron-transport-material (ETM), which could be coated by chemical bath deposition (CBD) [10], atomic layer deposition (ALD) [15,16] or pulse laser deposition (PLD) [17]. However, one question remains, or how does the thin layer of ETM affect the coarsening dynamics of the perovskite layer and hence the device performance. Recently, this topic was discussed by Dou et al. [18], but more studies are needed. SnO2 is an important ETM applied in PSCs [17,19]. Here in the article, flexible PSCs were prepared on ITO/PEN using two-step formed SnO2 layers as ETM, with the first layer being prepared from hydrothermal reacted SnO2 nanoparticles (SnO2 NPs hereafter), and the

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Technologies) and X-ray diffraction (XRD, D8, Advance). Selected area electron diffraction (SAED) was performed along with TEM. X-ray photoelectron spectroscopy (XPS) was measured (SPECS XR-MF) with a monochromatized Al source (hv = 1486.6 eV). Sheet resistance of ITO/ PEN film was measured using four-point probe method (SDY-4D). Absorption spectra of PVSK films were measured by UV–vis spectrophotometer (TU-1800). Electrochemical impedance spectroscopy (EIS) was recorded by electrochemical workstation (CHI 660D) with frequency from 0.1 Hz to 1 MHz and perturbation voltage of 5 mV. The obtained spectra were fitted using homemade software. Current-voltage characteristics were recorded by a digital source meter (model 2400, Keithley) under simulated illumination (Newport 91160 S, AM1.5G). The intensity was 100 mW/cm2 calibrated by standard silicon cell (SRC1000-TC-QZ-N, Oriel). External quantum efficiency (EQE) was performed on spectrum performance testing system (7-SCSpec) with AC mode. Transient photocurrent (TPC) spectroscopy was recorded by an in-house-built system consisting of oscilloscope (DOS-X 3104 A, Agilent) and Nitrogen laser (337 nm, NL 100, Stanford Research System). Impedance of 50 Ω was used and the test was performed in dark. Shelf-stability of flexible PSCs was tested by storing the devices in ambient air without encapsulation. Relative humidity (RH) was recorded along with test. All of the tests were done in open air except for TEM, SAED, SEM as well as XRD.

second layer from hydrolysis and oxidization of SnCl2 solution. Effect of the two-step formed SnO2 layers on crystallinity of perovskite, as well as power conversion properties of the flexible PSCs are systematically studied. 2. Experimental section 2.1. Materials and regents 2,2′,7,7′-tetrakis [N,N-di (4-methoxyphenyl)amino]-9,9′-spiro-bifluorene (Spiro-OMeTAD), lead iodide (PbI2, 99%), formamidinium iodide (FAI), methylammonium bromine (MABr), methylammonium chlorine (MACl), N,N–dimethylformamide (DMF, 99.9%), isopropyl alcohol (IPA, 99%), chlorobenzene (CB, 99.8%), 4-tert-butylpyridine (4tBP, 96%), Tin(II) chloride dehydrate (SnCl2·2H2O, 98%), acetone (99%), ethanol (99%) were all used as received, without further purification. Deionized water was prepared in laboratory. 2.2. Synthesis of SnO2 NPs and preparation SnCl2 solution SnO2 NPs were synthesized by hydrothermal reaction. Typically and at first, 0.34 g tin chloride pentahydrate (SnCl4·5H2O, Mw = 350.60 g/ mol, 99%) were dissolved in mixed solution (50 mL, volume ratio between deionized (DI) water and anhydrous ethanol is 1:1). Then the solution was transferred into an autoclave (100 mL in capacity) and heated in oven. The obtained solution was centrifuged and washed by anhydrous ethanol, and ultrasonically dispersed after which SnO2 sols was obtained, with concentration of about 0.019 mol/L. SnCl2 solution was prepared by dissolving SnCl2·2H2O in anhydrous ethanol with concentration of about 0.025 mol/L.

3. Results and discussion 3.1. Two-step solution processed SnO2 layers SnO2 NPs were synthesized by hydrothermal reaction like that described in literature [20–22]. Particle sizes is tuned by varying temperature and reaction period. Fig. 1(a) shows typical TEM image of SnO2 NPs reacted at 150 °C for 24 h. Uniform distribution is depicted which centers at 5.16 nm according to the inset. Selected area electron diffraction (SAED) image and enlarged image is shown in Fig. 1(b) and (c) respectively, while more images can be found in Fig. S1. X-ray diffraction (XRD) patterns show that tetragonal phase was harvested [19,23], coinciding well with SAED. Average crystalline size of the NPs is calculated using Sherrer's equation:

2.3. Assembly of flexible PSCs Before deposition of SnO2 layer, flexible substrate of ITO/PEN (Peccell, 60 Ω/sq) were ultrasonically cleaned in DI, IPA each for 20 min, then dried in oven, and further treated by UV/Ozone in air for 20 min. The two-step formation of SnO2 layers is performed as following. The first layer is prepared by spin-coating SnO2 sols (2000 rpm, 30 s) on ITO/PEN, being followed by annealing at 100 °C for 20 min; while the second layer was coated by spin-coating SnCl2 solution (3000 rpm, 30 s) on top, and annealed at 100 °C for 20 min to let hydrolysis. The obtained film was also treated by UV/Ozone in air for 20 min. Both of the two steps were performed in open air. Perovskite film was prepared by conventional two-step method [19]. At first, PbI2 solution (1.2 mol/L, dissolved in anhydrous DMF) was spin coated (3000 rpm, 30 s) onto SnO2 layers. Then, mixture solution of FAI:MABr:MACl (60 mg:6 mg:6 mg, dissolved in 1 mL IPA) were spin-coated on top (3000 rpm, 30 s). The two-layered films were annealed in glove box at 150 °C for 20 min, after that a layer of co-doped perovskite films ((FAPbI3)0.97 (MAPbBr3)0.03) was formed. For sake of simplification, this film is noted as PVSK in following discussions. After that, SpiroOMeTAD solution was coated (3000 rpm, 30 s) on top of the perovskite film and dried at 100 °C for 20 min. The solution was prepared by dissolving 72.5 mg spiro-OMeTAD in 1 mL CB, 17.5 μL bis (trifluoromethylsulfonyl)-imide lithium salt (LiTFSI, 520 mg in 1 mL acetonitrile) along with, 28.8 μL 4-tBP were added. Finally, 100 nm-thick Au film was thermally evaporated (0.6 Å/s) on top to prepare the top electrode. Active area of the device was 0.12 cm2 according to designated pattern.

D=

kλ , β cos θ

(1)

where k is the Scherrer constant and equaling to 0.89; β is the fullwidth at half magnitude (FWHM); D is the average crystallite size (nm); λ is 0.154 nm. Four kinds of crystalline size are obtained for the SnO2 NPs (Fig. 1(d)). Also noting that, “Tyndall effect” was observed when particle size was small in the SnO2 sols, but it faked as particle size increases, due to heavier light scattering. The two-step forming procedures are shown in Fig. 2 (e) and (f). For the first step (step 1), SnO2 NPs are coated on ITO/PEN; while for the second layer, SnCl2 solution is coated on top (step 2), followed by hydrolysis and oxidization process due to H2O/O2 in air and also the UV/ Ozone treatment. XPS study shows that (Fig. S2), the formed SnO2 contains small amount of Cl, which is from either HCl (byproduct of hydrolysis reaction) or residential SnCl2 (due to incomplete hydrolysis). The two-step formation changes the surface of ITO/PEN in two aspects. The first is the wettability. As shown in top row of Fig. 2, poor wettability is observed in case of ITO/PEN, though it has been treated by UV/Ozone for 20 min. This is different from ITO coated on glass. After deposition of SnO2 (both steps 1 and 2), wettability is obviously improved. The other aspect lies on surface roughness [24,25], this could be reflected from the four typical AFM images (Fig. 2 (g, h, i, j)). From Fig. 2(k), one can see that with RMS increases monotonously, from 2.28 nm for bare ITO to 37.3 nm for that coated by 10 nm SnO2 NPs. Anyhow, after the second SnO2 layer is formed, roughness decreases. More AFM images are referred to Fig. S3. After formation of SnO2 layers, perovskite (PVSK) is coated on top as described before. The

2.4. Materials characterization and device performance evaluation Morphological and crystallographic properties of SnO2 NPs and perovskite films were characterized by transmission electron microscopy (TEM, JEOL JEM-2100 F), scanning electron microscopy (FESEM, JSM-6490 L V), atomic force microscopy (AFM 5500, Agilent 127

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Fig. 1. Morphological and crystallographic properties of SnO2 NPs. (a) Typical TEM image (×100 000) of SnO2 NPs synthesized by hydrothermal reaction (150 °C/24 h) (inset shows corresponding size distribution). (b) Selected area electron diffraction (SAED) of SnO2 NPs fetched from (a). (c) High-resolution TEM image of single SnO2 crystallite. (d) Xray diffraction patterns of SnO2 NPs. (e) Relationship between average crystallite size of SnO2 NPs and reaction parameter (inset shows photo-image of SnO2 sols under illumination of laser).

obtained device holds structure like that depicted schematically in Fig. 2 (l).

depicted in Fig. 3. It could be found that, without ETM, there is no photovoltaic effect due to short-circuit. After formation of ETM, performance arises and evolves with the forming procedure. Fig. 3(b) shows all of the four performance parameters, open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) along with power conversion efficiency (PCE). They come out with quite similar

3.2. Power conversion of the flexible PSCs Effects of the two-step formed ETM on device performance are

Fig. 2. Schematic of two-step formation of SnO2 layer on ITO/PEN: (e) step 1 and (f) step 2. Photo-images taken from the wettability test (using DMF as solvent): (a) bare FTO/Glass, (b) bare ITO/PEN, ITO/PEN coated with SnO2 layer from (c) step 1 and (d) steps 1&2. (g–j) Typical AFM images of ITO/PEN coated with one-step or two-step formed of SnO2 layers with different particle size: (g) & (h) 5 nm, (i) & (j) 8.5 nm. (k) Evolution of RMS of the ITO/PEN coated with two-step formed SnO2. (l) Schematic structure of the planar flexible PSCs. 128

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Fig. 3. (a) Typical current density-voltage curves of flexible PSCs using two-step processed SnO2 as ETM. (b) Performance parameters of flexible PSCs fabricated from one-step and two-step processed SnO2 films. (c) Maximum power point tracking plots of the champion device under bias of 0.815 V. (d) External quantum efficiency (EQE) spectrum of the champion device (black) and integrated short-circuit current density (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

spectrum (Fig. S5) by mono exponential function of I (t ) ∼ Ae−t / τ , like that done in literature [29]. As shown in Fig. 5(a), it is sensitive to the formation process. When only step 1 is applied, it turns smaller as particle size increase, reaching the smallest at 5 nm, then becomes larger again. But after step 2 is added, it decreases in all of the five cases. The accelerated charge extraction is ascribed to two reasons. The first is the improved crystallinity of PVSK, and the second relies on the enhanced contact between PVSK crystallites between the ETM. This could be induced by comparison between the RMS of ETM (seen from Fig. 3). As ETM is flattened, compact PVSK film could be obtained, as a result, contact could be improved between PVSK and the substrate, which is beneficial for the accelerated charge extraction and thus leads to shortened charge-extraction time (τ). The accelerated charge extraction is also accompanied by the increased film conductivity. Seen from Fig. 5(b) one can find that, series resistance (RS, fitted from IS spectra in Fig. S6) follows exactly the same trend as the charge-extraction time. The lowered RS is also ascribed to two aspects. The first is the improved film conductance of ETM itself. SnO2 is a kind of N-type semiconductor, and small SnO2 NPs in size would be more likely to be doped by oxygen vacancies. Another reason also arises from the flattened surface, which is beneficial for mobility due to reduced scattering. In addition, the improved contact between PVSK crystallites and ETM is also favorable to smaller RS. The decreased RS is favorable to FF of devices [8,30], coinciding with the evolvement of FF shown in Fig. 3(b). Inserting ETM also reduces recombination in the devices. This could be reflected from the dark current-voltage curves (Fig. 5(c)). And again, the two-step formed ETM comes out with weaker recombination than one step formation (for example, only step 1). The retarded recombination is also contributed by two aspects. The first is the improved quality of PVSK film (compactness), which reduces shunt resistance between hole/electron extraction layers. And the other is the passivated surface defects of SnO2 NPs, which could also prevent recombination. Merits of the two-step formation of ETM on enhanced hydrophilicity and decreased roughness of the flexible ITO/PEN, and hence the improved crystallinity of PVSK as well as the improved contact between PVSK and substrate are shown schematically in Fig. 5 (d) and (e).

trends. For cases of step 1, as particle size increases, they increase at first, reaching top at particle size of 5 nm, but then drops. Then after step 2 is added, they are upgraded in all. As a result, both of the two steps are beneficial to device performance. Champion device is obtained with combination of step 1 (SnO2 NPs, 5 nm) and step 2 (SnCl2), VOC of 1.015 V, JSC of 21.023 mA/cm2, FF of 71.29% and PCE of 15.21% is achieved under reverse scan (Fig. 3(a)). Maximum power point tracking (MPPT) is done at bias of 0.815 V for the champion device (Fig. 3(c)). PCE of 15.08% is obtained or 99.14% of that calculated by JV scanning. EQE spectrum is recorded (Fig. 3(d)), integrated JSC of 20.39 mA/cm2 (following spectrum of AM 1.5 G) is obtained, which is 97% of the measured JSC. From Fig. 3(d), one could also find that, to further upgrade device performance energy harvest in long wavelength (500–800 nm). Cause of improved performance is explored from two aspects. The first is the coarsening dynamics of PVSK crystallites. As shown in Fig. 4(a), for PVSK film deposited on bare ITO/PEN, uneven surface is observed; slip-like domains are seen in the film. According literature [26–28], these slip domains are very likely due to the unreacted PbI2. This could also be reflected by XRD pattern (Fig. 4(e)). Peaks of PbI2 are even higher that to PVSK, showing that PVSK could hardly be uniformly formed on bare ITO/PEN. After ETM is formed (only step 2, hydrolysis of SnCl2), PVSK film becomes uniform, though it contains less PbI2 (Fig. 4(e)). The crystallinity of PVSK could be further improved when step 2 is added in the formation of ETM. The improved crystallinity could also be reflected by the UV–vis absorption spectra (Fig. 4(f)). Besides, compared to cases of step 1, PVSK films tend to be more uniform and compact when they are coated on ETM prepared from two steps procedures. Quality of the PVSK film is also sensitive to roughness of substrate. When larger SnO2 particles (10 nm) were used, PVSK films became uneven again. More SEM images of the PVSK films could be referred to Fig. S4. Another contribution to device performance of flexible PSCs is the accelerated interfacial charge extraction process and retarded recombination, which has been verified by transient photocurrent spectrum (TPC), impedance spectroscopy (IS) as well as dark current measurement. Charge-extraction time (τ) is estimated by fitting TPC 129

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Fig. 4. Typical scanning electron microscopy (SEM) images of PVSK films deposited on: (a) bare ITO, (b) SnO2 (step 1, 5 nm SnO2 NPs), (c) SnO2 (step 2, SnCl2) and (d) SnO2 (step 1, 5 nm SnO2 NPs & step 2, SnCl2). (e) XRD patterns of PVSK films coated on bare ITO/PEN and SnO2. (f) UV–vis absorption spectra for PVSK films (inset is magnifiedregion between 580 nm and 740 nm).

carbon film [2,3,20,35,36]. It is also noted that there is a risk of shelfstability that electrodes of device might be brushed off by frequent test, thus conductive silver paste is preferred [37]. Similar to that done before [11], by calculating the ratio between the power and device weight, specific power of 0.87 kW/kg is obtained, which is appealing for portable usages.

3.3. Fatigue properties and shelf-stability of the flexible PSCs Fatigue property of the flexible PSCs is tested through a “curing” manner, where the device is cured along the outside of cylinder as shown in Fig. 6(d). Effect of radii and curing time on device performance is clearly depicted in Fig. 6(a). When curing radii is relatively larger (8 mm), the performance shrinks slowly with curing times. As the radii decreases to 2 mm, the performance drops quickly. It relates closely to ITO/PEN itself [11,31]. As shown in Fig. 6(b), there exists obvious increase in sheet resistance of the flexible substrate, especially for those cured at lower radii (for example, 2 mm). As a result, there is an urgent need to improve the fatigue property of the flexible bottom electrode. Shelf-stability of the flexible PSC is also tested, without any encapsulation [32,33]. As shown in Fig. 6(c), when they were stored in ambient environment (room temperature, relative humidity RH between 35% and 55%), device performance (PCE) could retain 85% of the start value after storage of 111 days, while 74% after even 152 days. The prolonged stability is due to three facts. One is the PVSK film itself. The second is the inert photo-catalysis property of SnO2 [34]. And the last is the use of metal gold film which is more stable than silver. Anyhow, for mass production, gold is not preferred. The top electrode would be better replaced by cheaper materials, while preserve good conductivity and sound stability against oxidization, for example,

4. Conclusion Finally it comes to show that, the two-step formed SnO2 layers have not only improved the crystallinity of PVSK film on flexible substrate of ITO/PEN, but also accelerated charge extraction process between PVSK and the bottom electrode. The formation procedure is solution-based and low temperature compatible, thus providing a reliable strategy for future application in flexible PSCs. Acknowledgements X. Qiu thanks the financial support by the Fundamental Research Funds for the Central Universities of Central South University (2017zzts326). B. Yang thanks support of NSFC (No. 61172047). Y. Yuan thanks support from NSFC (No.51673218). Y. Gao thanks support from NSFC (No.11334014), and the support from NSF (CBET-1437656). 130

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Fig. 5. Effect of formation of SnO2 layers on (a) transient time and (b) charge transfer resistance across PVSK and bottom electrode, and (c) dark current-voltage curves of flexible PSCs. (d) Schematic for the effect of wettability on formation of PVSK crystallites. (e) Schematic for the effect of roughness on contact between PVSK crystallites and substrate.

Fig. 6. Effect of curing radii on (a) performance parameters of the flexible PSCs, and (b) sheet resistance ITO/PEN. (c) Shelf-stability of as-prepared flexible PSCs (without encapsulation). (d) Typical photograph of the curing test at radii of 2 mm and corresponding mimic picture. 131

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C. Zhou thanks support of NSFC (No. 61774170). [19]

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.orgel.2018.04.010.

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