Low-Temperature Solution-Processed Zinc Tin Oxide Film as a

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Low-temperature solution-processed zinc tin oxide film as a cathode interlayer for organic solar cells Jiajun Wei, Zhigang Yin, Shan-Ci Chen, and Qingdong Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13724 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Low-Temperature Solution-Processed Zinc Tin Oxide Film as a Cathode Interlayer for Organic Solar Cells Jiajun Wei,†,‡ Zhigang Yin,†,‡ Shan-Ci Chen,† and Qingdong Zheng†,* †

State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

KEYWORDS: Interface engineering, Efficiency, Electron transport, Solution-processed, Organic solar cells

ACS Paragon Plus Environment

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ABSTRACT

In this study, Sn-doped ZnO (ZTO) is prepared by a sol-gel method, and employed as an electron transport material for organic solar cells (OSCs). After Sn modification, the fabricated ZTO films exhibited better charge transport properties and smoother surface morphology, especially for those processed at a low temperature of 120 ºC. By incorporating the high-temperature (200 º

C) processed ZTO films, inverted OSCs showed the highest power conversion efficiency (PCE)

of 9.32%, which is higher than those based on the same temperature processed ZnO films. For the devices based on the low-temperature processed ZTO, a high PCE over 9.0% with long-term stability was achieved which is much better than those based on the same temperature processed ZnO (8.46% PCE). Here the ZTO films can be fabricated without high-temperature annealing, demonstrating their great potential as electron transport layers for efficient flexible OSCs.

Introduction Organic solar cells (OSCs) have attracted extensive attention for their advantages such as low cost, light weight and large area solution-processability.1-2 So far, tremendous advances have been made in regards to the enhancements of device PCE as well as device stability.1-7 Apart from exploring and designing new light-harvesting materials, interface engineering between the active layers and electrodes of OSCs, which plays an important role in charge extraction and transfer process, is indispensable for high performance OSCs.5-7 Interfacial materials with desired charge selectivity and universality for different active materials have been studied in the


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past decade, in order to tune the energy level alignment for an ohmic contact and to improve the film morphology of active layer deposited on them.8 Transition metal oxides have been used as efficient interfacial materials for OSCs owing to their good transparency, high charge mobility, and environmental stability.9-13 In particular, with the advantages of low work function (WF) and easy synthesis, Zinc oxide (ZnO) films prepared by the sol-gel method are one of the best electron-transporting layers (ETLs) for high performance OSCs.4, 14-16 An ohmic contact can be formed by the insertion of ZnO film between indium tin oxide (ITO) and the active layer, which decreases the energy barrier and facilitates the electron transfer. On the other hand, high work function metals (e. g. Ag) are used as a top anode for the hole collection, which greatly improves the device stability. However, the presence of defects with the absorbed oxygen17 and inhomogeneous spatial distribution of nanoparticles18 restrict the performance of the devices based on ZnO films. Meanwhile, high-temperature annealing is usually necessary to form highly crystalline ZnO films and to increase the charge mobility as revealed in the study of field-effect transistors.19-20 To prepare devices on flexible substrates and to achieve large-area industrial production, low-temperature fabrication of ETLs is needed. Sun et al. fabricated ZnO films by a sol-gel method at relatively low annealing temperatures (below 200 oC).21 When the processing temperature dropped to 150 or 130 oC, the electron mobility of ZnO film decreased, and the resulting OSCs showed performance degradation.21 Incomplete ligand removal could be a major disadvantage for low-temperature processed ZnO films,22 which significantly influences the film quality and electrical properties. To enhance the electron mobility and tune the energy level of ZnO, several strategies on interface engineering have been carried out, such as introducing another ETL between the pristine ZnO layer and the active layer to form a bilayer, or modifying ZnO films by doping


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methods.22-26 It is known that by Sn doping, the electron mobility of ZnO films can be greatly improved when a high annealing temperature above 400 oC was adopted in the film deposition.2730

Although the zinc tin oxide (ZTO) films have often been used for the application of organic

field effect transistor (OFET),27-30 the use of ZTO films as an ETL for OSCs is seldom explored probably due to the requirement of high annealing temperature for the ZTO films.31 One example regarding the OSC application of ZTO films was from Oo et al. who utilized high temperature (500 oC) processed ZTO films as ETLs for OSCs with a best PCE of 3.05%.31 For practical applications, low-temperature annealed ZTO films are needed, especially for flexible OSCs on plastic substrates. However, sol-gel ZTO layers suitable for OSCs without high temperature annealing have not been disclosed until now. In this work, a small portion of Sn doped ZnO films are prepared by employing a sol-gel method to enhance the electron transport as well as to tune the WF of the films. The ZTO films were fabricated in mild condition without any vacuum instrument or complex preparation method, and the resulting OSCs showed a highest PCE of 9.32%. When annealed at a low temperature of 120 oC, the films maintained good uniformity and high electron mobility. In contrast to the pristine ZnO films whose properties are quite sensitive to the annealing temperature, the ZTO films annealed at 120 oC displayed a similar energy level alignment and film morphology compared to those annealed at 200 oC. As a result, the corresponding OSCs with ZTO films showed higher PCEs (with the highest PCE of 9.02%) compared to those with ZnO films (highest PCE of 8.46%). Furthermore, the ZTO-based devices exhibited long-term stability in air which can be ascribed to the high quality ZTO films at the cathode and high WF MoO3 at the anode. The results demonstrate that ZTO film is a promising ETL suitable for largearea, low-temperature processed flexible devices.


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Experimental section Materials. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] and tin (II) chloride dihydrate [SnCl2·2H2O] purchased from Sigma-Aldrich Inc. were used to prepare the ZTO precursor solution. Poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-co-3-fluorothieno[3,4b]thiophene-2-carboxylate] (PTB7-Th) (molecular weight, Mn = 59 KDa; polydispersity index, PDI = 2.2) was synthesized in our lab according to the published method32 and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) was purchased from American Dye Source Inc. Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDB-T)

and

3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis

(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene

(ITIC)

were

purchased from the Solarmer Materials Inc. Other materials and reagents were purchased from Sigma-Aldrich Inc., Aladdin-Reagent Inc. or Adamas-beta Ltd. and used without further purification. Preparation and characterization of ZTO interlayers. The ZTO precursor solution was prepared by dissolving Zn(CH3COO)2·2H2O and SnCl2·2H2O in 2-methoxyethanol with a total metal concentration of 0.23 mol/L and stirred at room temperature overnight. The molar ratio of Sn/(Zn+Sn) was fixed at 5%. Ethanolamine was used as a stabilizing agent. The ITO/glass substrates were cleaned by sequentially ultrasonication in detergent, deionized water, acetone, and isopropyl alcohol for 30 min each and then dried at 80 °C overnight. Solution-processed ZTO films were prepared by spin-coating the precursor


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solution on the substrates at 3000 rpm for 50 s, after pretreating the substrates by UV-ozone for 15 min. Then the films were annealed at 120 °C (or 200 °C) for 60 min in air and transferred into a N2-filled glovebox. ZnO films were prepared without SnCl2·2H2O by the same procedure as that used for ZTO films. UV-vis absorption and transmission spectra were recorded by a Lambda35 spectrophotometer at room temperature. The film thicknesses were determined by a Bruker Dektak XT surface profiler. Atomic force microscopy (AFM) images were performed in air by using a Bruker Nanoscope 8.15 atomic force microscope. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were obtained by a Thermo Scientific ESCALAB 250Xi XPS/UPS system with a monochromatic Al Kα source for surface chemical analysis. The structural properties of the films (deposited on silicon substrates) were evaluated by X-ray diffractometer (XRD, Bruker, D8 Advance) using Cu Kα radiation. Electron-only devices (ITO/ZTO (ZnO) /PTB7-Th:PC71BM/Al) were fabricated to estimate the electron-transport property of ZTO films by the space charge limited current (SCLC) method. The electron mobility can be calculated by the equation:33 J = 9ε0εrµV2/8L3, where ε0 is the permittivity of free space, εr is the dielectric constant of the materials, µ is the electron mobility, V is the voltage drop across the device (V = Vapp − Vbi, where Vapp is the applied voltage to the device, Vbi is the built-in voltage due to the difference in WF of the two electrodes), L is the thickness of the active layer. Device fabrication and measurements. OSCs were fabricated with ZTO or ZnO films as ETLs. PTB7-Th:PC71BM (1:1.5 by weight) in a 25 mg/mL chlorobenzene and 1,8-diiodooctane (100:2 by volume) solution was spin-coated at


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1600 rpm for 60 s on the ETLs in the N2 filled glovebox. Then, the devices were transferred into a vacuum chamber. 10 nm of MoO3 and 100 nm of Ag were deposited continuously through shadow masks by thermal evaporation under a vacuum of ~5×10-5 Pa. The effective area of the electrodes and devices was fixed at 6 mm2. For the other active layer system, PBDB-T:ITIC (1:1 by weight) in chlorobenzene (20 mg/mL) and 1,8-diiodooctane (99.5:0.5) solution was spincoated at 2500 rpm for 60 s, and then annealed at 160 oC, 10 min.3 Solar cell characterization was performed under AM 1.5G irradiation (100 mW cm-2) by an Oriel Sol3A simulator (Newport) with a NERL-certified silicon reference cell. After encapsulation, the J-V curves were tested by using a Keithley 2440 source measurement unit. The EQE spectra were measured by using a Newport EQE measuring system. For some devices with light soaking effect, the OSCs were soaked under the simulated one sun for 20 s. For the device stability measurement, the encapsulated devices were stored and periodically tested in air under ambient conditions. The ambient conditions were the darkness, temperature of 25 oC, and relative humidity of 50%. Results and discussion Surface chemical analysis of ZTO and ZnO films. Table 1. XPS O 1s peak fitting results of different films. M-O bond a

VO b

Sample Peak position (eV)

Fractional area (%)

Peak position (eV)

Fractional area (%)

ZnO 120 oC

530.3

60

531.8

40

ZnO 200 oC

530.4

50

531.8

50

ZTO 120 oC

530.4

41

531.7

59

ZTO 200 oC

530.5

38

531.8

62


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a

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M-O bond: metal-oxygen bond; b Vo: oxygen deficiency.

Figure 1. XPS O 1s spectra of ZnO and ZTO films (a) ZnO 120 ºC, (b) ZnO 200 ºC, (c) ZTO 120 ºC, (d) ZTO 200 ºC. ZnO and ZTO films were prepared at high temperature (200 oC) and low temperature (120 oC) respectively. XPS measurements were performed to illustrate the components and surface chemical states of the ZnO and ZTO films on ITO substrates. The photoelectron binding energy scale was calibrated to the C 1s peak for the C-C bonds at 284.8 eV. The O 1s XPS spectra were fitted by the Gaussian-Lorentzian profile as shown in Figure 1, and the relative contents are summarized in Table 1. The intense peak at ~530.4 eV corresponds to the oxygen bonded to the fully coordinated metal (M-O bond), and the peak at ~531.8 eV corresponds to the oxygen deficiency (VO). It has been reported that the concentration of Vo strongly controls the electrical


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conductivity of ZnO,34 which may act as shallow donors to increase the free electron density, consequently increase the electron extraction and transportation. As illustrated in Table 1, the obviously higher concentration of VO in ZTO films indicated better performance on charge transfer, which could be obtained at both high and low processing temperatures. However, a large decrease on the concentration of VO in low-temperature processed ZnO was observed which may cause negative impacts on the electronic property. Meanwhile, Zn 2p and Sn 3d XPS spectra of the films are shown in Figure S1. The peak positions of Zn 2p do not show obvious difference for all the films. The binding energy of the Sn 3d5/2 XPS spectra peak located at ~486.5 eV for ZTO films (Figure S1), indicating that the Sn ions in the films are mainly existed as Sn4+. With the excellent electronic properties and low-temperature adaptability, ZTO films can also be deposited on flexible substrates. The XRD patterns are also illustrated in Figure S2 to study the structural properties of ZnO and ZTO films. None of these films showed obvious peaks of ZnO or SnO2 phases, indicating that the fabricated ETLs have an amorphous structure instead of a crystalline structure. The electron-transfer process between the cathode and the BHJ is greatly determined by the energy levels of the ETLs. We performed UPS measurements to investigate the electronic structures of the ZnO and ZTO films (Figure 2a). The WFs were calculated from the onset of the high binding energy side as depicted in Figure 2b. The WFs of ZTO films are similar at both annealing temperatures (~4.6 eV), while the WF of ZnO is 4.4 eV at the low annealing temperature, which is 0.1 eV higher than that of ZnO at the high annealing temperature. The relative positions of valance band maximums (VBMs) were extracted from the onset of the low binding energy side. Then the conduction band minimums (CBMs) were estimated by the VBMs and the optical gaps extracted from UV-Vis absorption spectra (Figure S3). For OSCs, the


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energy level matching between the acceptor material and the ETL is critical for the efficient extraction of electrons, which greatly influences the device performance. The energy level diagrams of ZnO and ZTO films are exhibited in Figure 2c (band bending is not shown). It is revealed that at the higher annealing temperature, the CBMs of ZnO and ZTO are both around 4.0 eV, which matches well with the lowest unoccupied molecular orbital (LUMO) of PC71BM. However, at the lower annealing temperature, the CBM of ZnO dramatically increased to 4.3 eV while the CBM of ZTO maintained at the similar value. This energy level mismatch may lead to a larger electron extraction barrier at the interface and increase the exciton recombination rate in the active layer. The results demonstrate that ZTO films can provide an improved ohmic contact between the ITO and the active layer. Furthermore, the ZTO films are relatively insensitive to the processing temperature which is beneficial for their application in flexible devices.


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Figure 2. (a) UPS spectra of ZnO and ZTO films and (b) corresponding detail view on high binding energy side. (c) The calculated energy level diagrams. Surface morphology of ZTO/ZnO films.

Figure 3. AFM height images of ZnO and ZTO films (a) ZnO 120 ºC, (b) ZnO 200 ºC, (c) ZTO 120 ºC, (d) ZTO 200 ºC. The surface morphology of the ETLs strongly affects the proper interface contact and efficient electron transfer through the interfaces. Tapping-mode AFM height images of ZTO and ZnO films on ITO were captured and depicted in Figure 3 (Figure S4 shows three-dimensional surface topography images). ZnO films showed rougher surfaces with root-mean-square (RMS) roughnesses of 2.34 and 2.03 nm at 120 and 200 oC, respectively. After adding the Sn


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component, the prepared ZTO films showed roughnesses of only 1.01 and 1.37 nm at 120 oC and at 200 oC, respectively. The small RMS values of ZTO films can be ascribed to the decreased grain sizes as shown in Figure S4. Consequently, ZTO films with smoother surface can passivate the surface defects, decrease the contact resistance and enhance the flatness of active layers spincoated on them. All of above can contribute to the reduction in current leakage of devices. Combining the appropriate energy level alignment, ZTO films can provide high-quality interface contact, which will facilitate the electron extraction and injection process, and lead to better device performance. Photovoltaic performance.

Figure 4. Schematic illustration of ZTO-based device configuration and the molecular structures of PTB7-Th and PC71BM. Inverted OSCs incorporating ZTO ETLs were fabricated by using PTB7-Th:PC71BM as the active layer. Device configuration and molecular structures of the donor and acceptor materials are shown in Figure 4. It should be noted that the device performance has been optimized through investigating the influences from the Sn/(Sn+Zn) molar ratio and the thickness of lowtemperature processed ZTO film (Table S1 and Table S2). The best performance devices were


12

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obtained from the ZTO films with the Sn/(Sn+Zn) molar ratio of 0.05 and the film thickness of 28 nm. ZnO-based devices were also prepared for comparison. The J-V characteristics under AM 1.5G irradiation (100 mW cm-2) are shown in Figure 5(a). At the 200 oC annealing temperature, ZTO-based devices showed the best PCE of 9.32%, which is slightly higher than that of ZnObased devices (9.26%). At the 120 oC annealing temperature, devices with ZTO layers still showed a high PCE over 9%, however, a large decrease in PCE is observed for the ZnO-based devices. As illustrated in Table 2, the main factor for the deteriorated performance of the devices based on low-temperature processed ZnO was the low Jsc, which can be attributed to the inferior interface morphology and electrical properties as mentioned above. Not only the poor contact at the interface resulting from the rough surface but also the energy level mismatch between the ETL and PC71BM may suppress the electron transport process and increase the carrier recombination. For the ZnO films, lower shunt resistances (Rshs) and higher series resistances (Rss) of ZnO processed at 120 ºC indicated that the quality of interlayer and proper contact between interfaces deteriorated by low-temperature annealing, thus leading to a decrease of the fill factors (FFs) of the devices based on low-temperature processed ZnO. The corresponding EQE spectra were measured and shown in Figure 5b. All the devices have high EQEs across the visible region. At the 120 ºC annealing temperature, ZTO-based devices showed obviously higher EQEs than the ZnO-based devices in the ranges of 350-450 nm and 600-700 nm, which implies the enhancement of current density for more efficient electron collection. The higher EQEs are also consistent with the higher Jsc value for the ZTO-based devices. Integrating the EQE data of the devices based on ZnO 120 ºC, ZnO 200 ºC, ZTO 120 ºC, and ZTO 200 ºC with the AM 1.5G spectrum, Jsc values of 16.44, 17.48, 17.20, and 17.28 mA cm-2 are obtained,


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respectively. The calculated Jsc values are within 2% errors compared to the measured values shown in Table 2.

Figure 5. (a) J-V characteristics of the devices with ZnO and ZTO ETLs, and (b) the corresponding EQE spectra (hollow symbols are the integrated photocurrents with the AM 1.5G spectrum). Table 2. Device characteristics of OSCs with ZnO and ZTO interlayers. Voc

Jsc

FF

PCEmax

PCEavga

Rs

Rsh

(V)

(mA cm-2)

(%)

(%)

(%)

(Ω·cm2)

(Ω·cm2)

ZnO 120 ºC

0.80

16.58

64.02

8.46

8.27 ± 0.13

8.18

781.49

ZnO 200 ºC

0.80

17.25

67.05

9.26

8.89 ± 0.18

6.56

1045.90

ETL


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ZTO 120 ºC

0.80

17.61

64.39

9.02

8.86 ± 0.11

7.28

736.94

ZTO 200 ºC

0.80

17.51

66.37

9.32

8.88 ± 0.20

6.93

962.01

a

The average PCEs were based on 10 devices. The Rs and Rsh were calculated by the inverse of the slope of the corresponding J-V curves under illumination at J = 0 and V = 0, respectively.

Dark J-V characteristics are displayed in Figure S5. As shown in leakage-dominated region (

Low-Temperature Solution-Processed Zinc Tin Oxide Film as a

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