Efficient Perovskite Solar Cells Based on Multilayer Transparent

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Efficient Perovskite Solar Cells Based on Multilayer Transparent Electrodes Through Morphology Control Xue Liu, Xiaoyang Guo, Zhihong Gan, Nan Zhang, and Xingyuan Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10288 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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The Journal of Physical Chemistry

Efficient Perovskite Solar Cells Based on Multilayer Transparent Electrodes Through Morphology Control Xue Liu,†,‡ Xiaoyang Guo,*,† Zhihong Gan, † Nan Zhang, † Xingyuan Liu,*,† †

State Key Laboratory of Luminescence and Applications, Changchun Institute of

Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: A multilayer transparent electrode WO3/Ag/WO3 (WAW) has been introduced into perovskite solar cells (PSCs). It is found that the substrate has obvious affection on the perovskite morphology and crystallization, thus power conversion efficiency (PCE) of the PSCs. The precursor composition and its affection on the morphology, crystal and device properties of the perovskite films based on WAW and ITO electrodes have been investigated in detail. When the CH3NH3I (MAI):PbI2 molar ratio is 1.04:1, the perovskite film shows flat and dense morphology formed by the complete reaction of MAI and PbI2, and PSC device shows the maximum PCE value of 9.73%, comparable with the controlled device with the MAI:PbI2 molar ratio of 1:1 based on ITO electrode (10.51%). Meanwhile, the flexible PSC based on WAW transparent electrode has also been fabricated, which exhibit a PCE of 8.04%, indicating WAW multilayer transparent electrodes have the potential application in PSCs, especially in flexible PSCs. 1

ACS Paragon Plus Environment


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INTRODUCTION Organic-inorganic hybrid perovskite materials CH3NH3PbX3 (MAPbX3, X=Cl, Br, I) have drawn great attention owing to the merits of proper bandgap, high absorption coefficient, high carrier mobility, long carrier diffusion length and lifetime,1-4 making them as the promising photovoltaic materials for the next-generation solar cells. During the past six years, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased at an unexpected rate from 3.8% to 22.1%,5-14 the highest PCE of 22.1% has been certified by NREL, approaching to state-of-the-art copper indium gallium diselenide (CIGS) solar cells, and exhibiting their commercial application prospect. To date, there are two main architectures in PSCs. One is the mesoporous PSCs, which originated from dye-sensitized solar cell (DSSC). Since 2012, the first solid state perovskite sensitized solar cell is reported by Park et al.,15 the mesoporous PSCs became popular, which commonly using a fluorine-doped tin oxide (FTO) transparent electrode covered a mesoporous metal oxide layer (e.g., TiO2 and Al2O3) as the scaffold for depositing perovskites.16 Although the PCEs of mesoporous PSCs have reached over 20%,13-14, 17-18 the high temperature annealing processing hindered the fabrication of large-area and flexible PSCs. The other architecture is planar structure, which is similar to the polymer solar cells, popular constructing on an indium tin oxide (ITO) transparent electrode, together with the perovskite layer sandwiched between a hole-transporting layer and an electron-transporting layer.6, 2


9, 11, 19

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Comparing with the mesoporous PSCs, planar structure can be fabricated under lower temperature, facilitating for flexible PSCs. However, the most commonly used ITO electrode is brittle and high-temperature processed, which limit its utility for flexible devices. Furthermore, the reservation of indium is very limited, the extensive use of ITO means the indium may be consumed in the near future.20 Therefore, design and fabrication of novel transparent electrodes for PSCs, especially for flexible PSCs, will have great significance on the development and applications of PSCs. In the past few years, several alternatives to ITO for PSCs have been reported, such as AZO,21-23 graphene,24 oxide Ni/Au,25 carbon nanotube26 and conducting polymers.27-28 However, these alternative electrodes suffer from either large sheet resistance or high-temperature processing, which degenerated the PCE of PSCs or hindered the applications in flexible PSCs. As one of the alternative electrodes, dielectric-metal-dielectric (DMD) multilayer transparent electrodes possess high transmittance and low sheet resistance, which can be achieved through choosing the dielectric material with high refractive index and adjusting the thickness of each layer in the DMD structure. To the best of our knowledge, the multilayer transparent electrodes with DMD structure have been successfully applied in the organic photovoltaic cells (OSCs) and organic light emitting diodes (OLEDs),29-41 such as InZnSnOx/Ag/InZnSnOx,

ZnO/Ag/ZnO,

MoO3/Ag/V2O5,

MoO3/Ag/WO3,

V2O5/Ag/V2O5 etc.. These device properties are comparable to those of the devices based on ITO electrodes, indicating their potential application in photoelectric devices. 3



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According to our previous researches, WO3/Ag/WO3 (WAW),32-33 NiO/Ag/NiO,34 and SnOx/Ag/SnOx/Bi2O335 multilayer transparent electrodes have been studied, which exhibit excellent photoelectric properties and flexibility, the rigid and flexible OSCs based on these transparent electrodes revealed good device performance compared with ITO based devices. However, the DMD multilayer transparent electrodes have seldom been employed in PSCs, especially in flexible PSCs. It is known that morphology of perovskite films is significant for the device performance,42-43 which can be optimized through controlling the composition of precursors and their crystallization process.44-45 Therefore, study on the morphology of perovskite films grown on the DMD multilayer transparent electrodes will have profound influence on the improvement of the performance of the DMD based PSCs. Herein, WAW multilayer transparent electrodes have been introduced into the PSCs to replace ITO acted as the transparent electrodes. It is found that the substrate has obvious affection on the perovskite morphology and crystallization, thus PSC performance. The precursor composition and its affection on the morphology, crystal and device properties of the perovskite films based on WAW and ITO electrodes have been investigated in detail. The optimized molar ratio of MAI and PbI2 on WAW is 1.04:1, while the optimized molar ratio is 1:1 on ITO substrate. The optimized PSC based on WAW electrode shows a PCE of 9.73%, which is approaching the PCE of the ITO based PSC (10.51%). The flexible PSC based on WAW has also been fabricated, which shows a desired PCE of 8.04%, indicating WAW multilayer transparent electrodes have the potential application in PSCs, especially in flexible 4


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PSCs.

Figure 1. (a) Transmittance spectra of glass/WAW and glass/ITO. (b) Cross-section. 5



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SEM image of the PSC based on WAW electrode. (c) Cross-section SEM image of the PSC based on ITO electrode. (d) Corresponding energy level of the PSC devices.

EXPERIMENTAL SECTION WAW electrode fabrication and characterization. Patterned WAW electrodes were deposited on the pre-cleaned glass or PET substrates by electron beam evaporation through shadow masks. WO3 (35 nm), Ag (10 nm) and WO3 (35 nm) were deposited sequentially at room temperature by electron beam evaporation, which had been optimized in our previous studies (Figure S1).33 The evaporation rates of WO3 and Ag were 0.1 and 1 nm s-1, respectively, which were monitored in situ with a thin film deposition controller (MDC-360C). Transmittance spectra were obtained by using a Shimadzu UV-3101PC spectrophotometer. The sheet resistance was measured using a four-probe method. Figure 1a shows the transmittance spectra of WAW and ITO films, the average transmittance of WAW is 82.22% over the wavelength range of 400-800 nm, which is a little lower than that of ITO (83.64%) at the same range. The sheet resistences of two kinds of electrodes are also proximate, about 15 Ω sq−1. PSC

fabrication

and

characterization.

The

device

structure

is

WAW(ITO)/PEDOT:PSS/MAPbI3/C60/Bphen/Ag. Figure 1b and c show the cross-section scanning electron microscopy (SEM) images of PSCs based on WAW and ITO, respectively. PSCs in this work were fabricated by using one-step solution deposition. MAI and PbI2 with the different molar ratios of 1:1, 1.02:1, 1.04:1 and 1.06:1 were dissolved in anhydrous N,N-dimethylformamide (DMF), and stirred for 6


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several hours in a N2 filled glove box. Before depositing perovskite layers, a hole transporting layer of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS; Clevios P AI 4083) filtered through 0.22um filters was spin-coated on substrates at a speed of 4000 rpm for 40s, and then baked at 140°C for 10 min. Afterwards, the substrates were transferred into the glove box, and a 500 nm perovskite film was deposited through spin-coating the precursor solution and during this spin-coating processing, 300 µL chlorobenzene was dropped onto the substrate at about 6th second to accelerate the perovskite crystal formation. Subsequently, the film was annealed on a hot plate at 100°C for 10 min.46-47 Finally, 40 nm C60, 8 nm Bphen and 80 nm Ag were thermally deposited in vacuum at a pressure of 3.0×10-4 Pa. The active area of the PSC devices was 0.12 cm2. All measurements were carried out in air without encapsulation. The X-ray diffraction (XRD) patterns were taken on a Rigaku D/Max-2500 diffractometer (Cu Kα, λ= 1.54 Å). SEM images were taken using a Hitachi S4800 microscope. The absorption spectra were measured by a Shimadzu UV-3101 spectrophotometer. Atomic force microscope (AFM) measurements were performed on a Shimadzu SPM-9700 (Shimadzu Corp., Japan) in tapping mode. The contact angle was measured using the image of 3µL MAPbI3 droplet on the substrate. The current density-voltage (J-V) characteristics of the PSCs were measured using a computer-controlled Keithley 2611 source meter under AM 1.5G illumination from a calibrated solar simulator with an irradiation intensity of 100 mW·cm-2. External quantum efficiency (EQE) measurements were performed with a lock-in amplifier at a 7



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chopping frequency of 20 Hz under illumination by monochromatic light from a Xenon lamp.

Figure 2. XRD patterns of MAPbI3 films with different MAI and PbI2 ratios deposited on (a) WAW, and (b) ITO electrodes.

RESULTS AND DISCUSSION As it is known, WO3 is widely used as a hole transporting material in OSCs and OLEDs48-50 and the WO3 based DMD transparent electrode also has a high work function of 5.16 eV.33 Figure 1d shows the energy level diagram of different materials used in the PSCs. Although the WAW electrode has a high work function which is matching with the highest occupied molecular orbital (HOMO) level of MAPbI3, the PSCs based WAW show a poor device performance (Figure S2). Therefore, a hole transporting layer PEDOT:PSS has been inserted between WAW and perovskite to prevent the possible interfacial reaction and improve perovskite film quality. In order to investigate the nature of perovskite films deposited on WAW based 8


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substrates, XRD measurements were carried out.42 Figure 2a shows the XRD patterns of perovskite films deposited on WAW based transparent electrodes by using precursor solutions with different MAI:PbI2 molar ratios. Firstly, a perovskite film with a MAI:PbI2 ratio of 1:1 (optimized on ITO electrode) was prepared on WAW electrode. Strong diffraction peaks of 14.15, 28.49, 31.99 and 40.75° are assigned to (110), (220), (310) and (224) planes, respectively, which are attributed to MAPbI3 perovskite crystals. 51-53 At the same time, a PbI2 peak at 12.57° is also appeared on the film with the MAI:PbI2 ratio of 1:1,51, 54-55 which indicates the excessive PbI2 existed in this perovskite film. So the amount of MAI was increased gradually to react with the excessive PbI2, when the ratio is increased to 1.02:1, there is still a small peak of PbI2. As further increasing the ratio to 1.04:1 and 1.06:1, the peak of PbI2 disappears completely, indicating PbI2 has been reacted completely at the MAI:PbI2 ratio of 1.04:1. For comparing, perovskite films with the same condition have been prepared on ITO based electrodes, which is shown in Figure 2b. As the MAI:PbI2 ratio of 1:1 in the perovskite film prepared on ITO based electrode is prior optimized , so there is no PbI2 peak seen at any molar ratio from 1:1 to 1.06:1.

9



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Figure 3. SEM images of MAPbI3 films deposited on WAW electrodes with different precursor molar ratios. (a) MAI:PbI2 = 1:1, (b) MAI:PbI2 = 1.02:1, (c) MAI:PbI2 = 1.04:1, (d) MAI:PbI2 = 1.06:1. SEM images of MAPbI3 films deposited on ITO electrodes with different precursor molar ratios. (e) MAI:PbI2 = 1:1, (f) MAI:PbI2 = 1.02:1, (g) MAI:PbI2 = 1.04:1, (h) MAI:PbI2 = 1.06:1.

As it is known that the performance of PSCs are sensitive to their crystal morphology, so the influence of precursor solutions with different MAI:PbI2 molar ratios on the morphology of the perovskite films deposited on different substrates were measured by SEM and shown in Figure 3. It is observed that there are some small grains on the surface of perovskite films based on WAW electrodes at the MAI:PbI2 molar ratios of 1:1 and 1.02:1 (Figure 3a and b). These small grains show a relatively high contrast in comparison with the large perovskite grains, due to they are less conductive,56 which correspond to the unreacted PbI2 phase, as proved by XRD patterns (Figure 2). When the MAI:PbI2 molar ratio is increased to 1.04:1, the small grains disappear, forming a smooth and dense perovskite film with large perovskite grain size of about 200−400 nm (Figure 3c). As the MAI:PbI2 molar ratio further increased to 1.06:1 (Figure 3d), some embossments appear on the perovskite surface, which may be attributed to the excessive MAI. Analogously, SEM images of the perovskite films deposited on ITO based transparent electrodes are shown in Figure 3e-h. When the MAI:PbI2 molar ratio is 1:1, the surface of perovskite film is flat and homogeneous with the perovskite grain size of about 100−200 nm (Figure 3e). As the 10


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amount of MAI increasing, more and more embossments are emerging on the surface of perovskite films, which are ascribed to the increasing MAI phase. Whatever, comparing with the optimized perovskite film deposited on ITO based transparent electrode, the optimized perovskite film deposited on WAW based transparent electrode reveals flat and dense film with larger grain size, which is propitious to reduce charge trapping and recombination at grain boundaries during fabricating high performance PSCs.

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Figure 4. Contact angle of (a) 3µL CH3NH3PbI3 droplet on WAW/PEDOT:PSS and (b) 3µL CH3NH3PbI3 droplet on ITO/PEDOT:PSS. AFM images of (c) WAW, (d) WAW/PEDOT:PSS, (e) ITO and (f) ITO/PEDOT:PSS, respectively.

In order to further reveal the reason for the formation of the lager perovskite grain 12


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size deposited on WAW based electrode, contact angle measurement has been carried out and shown in Figure 4a and b. The contact angle of 3µL MAPbI3 droplet on WAW/PEDOT:PSS and ITO/PEDOT:PSS are 14.98 º and 7.69 º, respectively. Although the two MAPbI3 droplets are all dropped on the surfaces of PEDOT:PSS layers, the two PEDOT:PSS layers are deposited on two different substrates, which have different morphology and surface roughness. Figure 4c-f show the AFM images of different substrates. Comparing with WAW and WAW/PEDOT:PSS substrates, ITO and ITO/PEDOT:PSS substrates show larger root mean square roughness (RMS) of 3.23 nm and 2.22 nm, respectively, resulting in a smaller contact angle as seen in Figure 4b.57 And the different contact angle and surface roughness maybe further result in different interaction strength between the precursors and the substrate, which may further affect the spin-coating process, and the optimized precursor composition in the film.58 According to the classical theory, the nucleation of crystallization is related to the surface free energy of the substrate, which is associated with the contact angle, as described in the following expression: ΔGheterogeneous=ΔGhomogeneous×f (θ) where f (θ)=(2-3cosθ+cos3θ)/4, ΔGheterogeneous and ΔGhomogeneous are the free energy needed for heterogeneous and homogeneous nucleation, respectively, and θ is the contact angle between solid and liquid interfaces.59 Smaller contact angle will decrease f (θ), and lower the nucleation barrier, thus the nucleation and growth of crystals will be more easily. Conversely, larger contact angle will increase the nucleation barrier and hinder nucleation. Accordingly, comparing with the 13



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WAW/PEDOT:PSS substrate, the ITO/PEDOT:PSS substrate will be more beneficial to perovskite nucleation and growth, resulting in more crystal nucleus and smaller grain size as shown in Figure 3.

Figure 5. J-V curves of PSCs based on (a) WAW, (b) ITO. (c) The dependence of PCE, FF, Voc, and Jsc of the PSCs on the MAI:PbI2 molar ratio.

14


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Table 1. Performance of PSCs based on different electrodes with different MAI:PbI2 molar ratio . Electrode

MAI:PbI2

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

JEQE (mA/cm2)

WAW

1:1

0.96 ± 0.01

14.01 ± 0.24

0.60 ± 0.02

8.07 ± 0.50

13.40

1.02:1

0.97 ± 0.01

15.00 ± 0.32

0.61 ± 0.01

8.90 ± 0.41

14.40

1.04:1

0.97 ± 0.02

15.64 ± 0.36

0.64 ± 0.01

9.73 ± 0.57

15.35

1.06:1

0.97 ± 0.01

15.37 ± 0.28

0.60 ± 0.01

8.90 ± 0.46

14.90

1:1

0.95 ± 0.01

16.88 ± 0.37

0.66 ± 0.02

10.51 ± 0.75

16.40

1.02:1

0.96 ± 0.02

15.72 ± 0.26

0.66 ± 0.01

9.96 ± 0.53

15.11

1.04:1

0.96 ± 0.02

15.58 ± 0.26

0.59 ± 0.02

8.78 ± 0.69

15.00

1.06:1

0.94 ± 0.01

15.57 ± 0.24

0.56 ± 0.02

8.21 ± 0.50

14.90

ITO

Figure 6. EQE spectra for the devices based on (a) WAW and (b) ITO electrodes.

PSCs based on WAW multilayer transparent electrodes with different MAI:PbI2 molar ratio were prepared and ITO based devices were also fabricated for comparison. Figure 5a and b give the J-V curves of the PSCs based on WAW and ITO, respectively. The detail device parameters are listed in Table 1 and the variation tendency has 15



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been shown in Figure 5c. As shown in Figure 5a and c, the devices based on WAW transparent electrodes show constant open-circuit voltage (Voc) with the gradually increased MAI, while short-circuit current density (Jsc) and fill factor (FF) improve firstly and then deteriorate, reaching a highest Jsc of 15.64 mA/cm2, and a highest FF of 0.64 at the MAI:PbI2 molar ratio of 1.04:1, resulting in a maximum PCE of 9.73%. These results demonstrate that the device performance has close relationship with the morphology and crystal properties.42, 60 The flat and dense perovskite film formed by the complete action of MAI and PbI2 at the MAI:PbI2 molar ratio of 1.04:1 (Figure 2 and Figure 3) enables a good carrier-transporting pathway and a minimum leakage current, corresponding to the increased Jsc and FF, thus the maximum PCE. Among the reference devices based on ITO electrodes, seen Figure 4b and c, the device with the MAI:PbI2 molar ratio of 1:1 shows the highest PCE of 10.51%, together with a Voc of 0.95 V, a Jsc of 16.88 mA/cm2 and a FF of 0.66. As the MAI:PbI2 molar ratio increasing, the Jsc and FF of the PSCs dropped markedly, accordingly with a decreasing PCE. These results above are highly consistent with the results of XRD patterns and SEM images mentioned before. The EQE spectra of these PSCs are shown in Figure 6, and the Jsc calculated by EQE spectra are also listed in Table 1, which are consistent with the values measured by J-V test. Additionally, both of the devices based on WAW and ITO electrodes show almost no hysteresis effect. (Figure S3)

16


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Figure 7. J-V curve for flexible PSC based on the WAW transparent electrode under 100 mW/cm2 AM 1.5G illumination.

During our previous studies, WAW was used as a flexible transparent electrode in OSCs due to its excellent stability towards bending-induced tension stress.33 In this work, we also try to introduce WAW as a flexible transparent electrode into the PSCs. Figure 7 shows the J-V curve of flexible PSC based on the WAW electrode with the optimized MAI:PbI2 molar ratio of 1.04:1 under 100 mW/cm2 AM 1.5G illumination. The flexible PSC shows a Voc of 0.90 V, a Jsc of 15.00 mA/cm2, a FF of 0.59 and a PCE is 8.04%, indicating WAW transparent electrodes have the potential application in flexible PSCs. CONCLUSIONS We have fabricated and investigated PSCs based on WAW multilayer transparent electrode. During the experiments, it is found that different contact angles and roughnesses of different substrates have a great influence on the optimized precursor 17



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composition and morphology and crystal properties of MAPbI3 films. When the MAI:PbI2 molar ratio is 1.04:1, the perovskite film based on WAW shows a flat and dense film with larger grain size, while on the ITO based substrate, the optimized MAI:PbI2 molar ratio is changed to 1:1, and shows smaller grain size. The optimized morphology and crystal properties bring the best device performance, the maximum PCE of the PSC based on WAW is 9.73%, while the PCE of the controlled device based on ITO is 10.51%. Moreover, flexible PSC based on WAW transparent electrode has also been fabricated, which exhibits a PCE of 8.04%, indicating WAW multilayer transparent electrodes have the potential application in PSCs, especially in flexible PSCs.

ASSOCIATED CONTENT Supporting Information Sheet resistance and average transmittance of WAW electrodes as a function of Ag thickness, calculated transmittance of WAW films, J-V of the device based on WAW without PEDOT:PSS, hysteresis effect measurements, absorbance of MAPbI3 films. This material is available free of charge via the Internet http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. 18


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Notes The authors declare no competing financial interest.’ ACKNOWLEDGMENT This work is supported by the CAS Innovation Program, the National Natural Science Foundation of China No. 61106057, 6140031454 and 51503196, and the Jilin Province

Science

and

Technology

Research

Project No.

20140520119JH,

20150101039JC, 20160520176JH and 20160520092JH, and project supported by State Key Laboratory of Luminescence and Applications.

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Efficient Perovskite Solar Cells Based on Multilayer Transparent

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