Hydroxyl-Terminated CuInS2-Based Quantum Dots - ACS Publications

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Hydroxyl-Terminated CuInS2 based Quantum Dots: Potential Cathode Interfacial Modifiers for Efficient Inverted Polymer Solar Cells Hui Chen, Pengjie Chao, Dengbao Han, Huan Wang, JingSheng Miao, Haizheng Zhong, Hong Meng, and Feng He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16305 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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ACS Applied Materials & Interfaces

Hydroxyl-Terminated CuInS2 based Quantum Dots: Potential

Cathode

Interfacial

Modifiers

for

Efficient Inverted Polymer Solar Cells Hui Chen,† Pengjie Chao,†, § Dengbao Han,‡ Huan Wang,† Jingsheng Miao,§Haizheng Zhong,*‡ Hong Meng,§ Feng He*†

† Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China ‡ Beijing Key Laboratory of Nanophotonics and Ultraﬁne Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Beijing 100081, China §School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (FH); [email protected] (HZ)

ACS Paragon Plus Environment

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ABSTRACT

The use of interfacial modifiers on cathode or anode layers can effectively reduce the recombination loss and thus have potential to enhance the device performance of polymer solar cells. In this work, we demonstrated that hydroxyl-terminated CuInS2 based quantum dots could be potential cathode interfacial modifiers on ZnO layer for inverted polymer solar cells. By casting a thin film of CuInS2 based quantum dots onto ZnO layer, the controlled devices show obvious enhancements of open-circuit voltage, short-circuit current, and fill factor. With an optimized interfacial layer with ~7 nm thickness, an improvement of power conversion efficiency up to 16% is obtained and the optimized power conversion efficiency of PTB7-based polymer solar cells approaches to 8.51%. Detailed analysis shows that the performance enhancement can be explained to the improved light absorption, modified work function, reduced surface roughness as well as the increased electron transfer of ZnO cathode interlayer.

Keywords: CuInS2, quantum dots, ZnO, interfacial modifier, polymer solar cells


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1. INTRODUCTION Solution-processed thin film polymer solar cells (PSCs) based on blended donor and acceptor have been of great interests on account of their plenty advantages containing low manufacturing cost and mechanical flexibility of devices.1-7 After about 20 years efforts, the power conversion efficiency (PCE) of PSCs has increased up to ~12% by optimizing the materials, morphology and interface.8,9 In comparison with conventional PSCs, inverted devices based on metal oxides possesses unique advantages toward commercialization including increased air stability and enhanced photon collection efficiency.10,11 Sol-gel processed ZnO layer is widely investigated as cathode interlayer in inverted PSCs owing to their good transparency in visible spectral, high electron mobility, appropriate the energy levels as well as easy solution processing.12-14 Due to the poor contact at the organic/inorganic interface, the intrinsic surface defects are likely to introduce charge trapping and carrier recombination, which result in unsatisfied devices with limited short current density and fill factors.15,16 To overcome these problems, surface modification of ZnO layer is a feasible way to achieve efficient devices. Self-assemble monolayer of fullerene derivatives,17 Al doping,18,19 porphyrin small molecules,20 hydrogen plasma treatment,21 water/alcohol soluble conjugated polymers22 and perylenebisimde23 have been successfully applied for modifying the surface of ZnO layer, boosting the PCE of inverted PSCs. In this work, we demonstrate that hydroxyl-terminated CuInS2 based quantum dots (CIS-QDs) could be potential interfacial modifiers on ZnO layer to enhance the performance of inverted PSCs. Among the family members of colloidal QDs, CIS-QDs are less toxic materials with superior optical properties that are suitable for light harvesting applications.24 They have been successfully utilized as solar harvesters and counter electrodes to construct efficient quantum dots based solar cells.25-28 The use of CIS-QDs as alternative electron acceptors to PCBM for polymer solar cells


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was also investigated.29,30 These progresses suggest that CIS-QDs could be functional materials to absorb light and transport carriers. To our knowledge, no report concerns on the use CIS-QDs as interfacial modifiers in PSC. We recently report the fabrication of hydroxyl-terminated CIS-QDs through in situ ligand exchange of 6-mercaptohexanol (MCH) for efficient light emitting diodes.31 Herein, we illustrated that hydroxyl-terminated CIS-QDs with valance band of -3.6 eV could be used as cathode interfacial modifiers for ZnO interlayer to improve the performance of PSCs. By casting a thin film of CIS-QDs onto ZnO layer, the controlled devices show obvious enhancements of open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and an improvement of PCE up to 16% is obtained. 2. RESULTS AND DISCUSSION Figure 1 schematically illustrates the use of CIS-QDs as interfacial modifiers in inverted PSC devices. The inverted devices were fabricated from the blend PTB7 and PC71BM (PTB7: Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]]; PC71BM: [6,6]-phenyl C71 butyric acid methyl-ester) by adapting an inverted device structure of ITO/ZnO/ /PTB7:PC71BM/MoO3/Ag. As shown in the inset of figure 1, the as-fabricated CIS-QDs have an average diameter of ~3.2 nm and photoluminescence peak of 545 nm, and can be dissolved into methanol for device fabrications. By casting a thin layer of CIS-QDs on the ZnO cathode interlayer, the device performance can be significantly enhanced.


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Figure 1. Schematic illustration of the use of CIS-QDs as interfacial modifiers in inverted PSC devices. Figure 2 shows steady-state J-V curves of devices with and without CIS-QDs as cathode interlayer and Table 1 provides a comparison of the devices performance with different thickness of CIS-QDs layer using an inverted structure of ITO/ZnO/CIS-QDs/PTB7:PC71BM/MoO3/Ag, the detailed J-V curves of devices with different thickness of CIS-QDs are shown in Figure S1. The performance of devices including series resistance RS and shunt resistance RSH was regulated by the thickness of CIS-QDs modifier and result in an optimized thickness of ~7 nm. The device with ZnO interlayer showed a moderate performance with PCE of 7.33%. The device with casting CISQDs (~7 nm) interlayer shows an obvious improvement with PCE of 8.51%. The enhanced PCE is due to significant improvement in Jsc from 14.58 to 15.65 mA cm-2, and also slightly increased Voc and FF in the CIS-QDs involved devices. The variation of RS and RSH also supports the observed phenomena. The reduced RS from 7.41 Ω cm2 to 5.66 Ω cm2 indicates the ZnO/CIS-QDs has higher electrical conductivity than primal ZnO interlayer, bringing about significant improvement in Jsc.32 The enhanced RSH correlates to the preferable interfacial contact between cathode


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interlayer and active layer,33 implying that increased Voc and FF. While, a thicker CIS-QDs (16nm) modifier with poor electrical conductivity on account of inferior interfacial contact generates an unsatisfied performance with PCE of 7.49%, Voc of 0.75 V, Jsc of 14.92 mA cm-2, and FF of 66.86 %, respectively.

Figure 2. J-V curves of PSCs devices with CIS-QDs as cathode modifiers (~7 nm) under 100 mW cm-2 AM 1.5 G irradiation.


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Table 1. The performance parameters of PSC devices with different cathode interlayer under 100 mW cm-2 AM 1.5G irradiation. Cathode interlayer

VOC (V)

JSC (mA cm2)

FF (%)

PCE(%)

RS (Ω cm2)

RSH (Ω cm2)

ZnO

0.73

14.58

69.29

7.33(7.21±0.04)a

7.41

522.23

ZnO/CIS QDs (4 nm)

0.74

15.21

70.81

8.11(8.01±0.04)

5.95

1271.56

ZnO/CIS QDs (7 nm)

0.75

15.65

72.03

8.51(8.31±0.05)

5.66

2336.92

ZnO/CIS QDs (11 nm)

0.75

15.33

70.07

8.09(7.88±0.04)

6.13

874.41

ZnO/CIS QDs (16 nm)

0.75

14.92

66.86

7.49(7.27±0.04)

7.35

609.75

a

Average value of PCE ± standard deviation for 30 different devices.

As it is known, the Voc of a PSC device was mainly dominated by the difference between the highest occupied molecular orbital (HOMO) level of donor and lowest unoccupied molecular orbital (LUMO) level of acceptor.34 Moreover, the interfacial contact barrier also influences the Voc and other performance parameters of PSCs. To clarify the influence of CIS-QDs modifier on the device performance, the work function (WF) of ZnO and ZnO/CIS-QDs thin film on surface of ITO was measured by using ultraviolet photoelectron spectroscopy (UPS) measurements.35 The UPS spectra of ZnO with varied thickness of CIS-QDs modifier are displayed in Figure 3a and Figure S2. As distinct shown in Figure 3a, the extracted WF values of ZnO and ZnO/CIS-QDs (7 nm) are 4.2 eV and 4.0 eV, respectively. To gain an insight into charge transportation in the PSCs, the energy level structure of device based on ZnO/CIS-QDs interlayer is illustrated in Figure 3b.


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The lower WF facilitates to form Ohmic contact and enhance built-in potential ascribing the reduction of Schottky barrier between ZnO layer and ITO electrode, being benefit to carrier collection efficiency and suppressing carrier recombination in interfacial regions.36

Figure 3. (a) The UPS spectra of ZnO and ZnO/CIS-QDs (7 nm) thin film, (b) the schematic energy structure of PSCs with using CIS-QDs as interfacial modifier. To further understand the improved Jsc, the absorption and transmission spectra of optimized ZnO/CIS-QDs (7 nm) thin film and controlled ZnO thin film are shown in Figure 4a and Figure S3, respectively. As can be seen from figure 4a, the absorption of ZnO/CIS-QDs was found to be improved remarkably in 300-500 nm, the visible light of this band is attributed to the presence of CIS-QDs layers. CIS-QDs are excellent light harvesting materials and significantly account for the photovoltaic response in the wavelength between 300- 500 nm under illumination due to the generated electron hole pairs of CIS-QDs and subsequent charge seperation.37 The enhanced photoconductivity could facilitate the electron extraction for PSCs,23 resulting a higher value of external quantum efficiency (EQE) in this band. The observed feature is shown in Figure 4b, the EQE value of device with CIS-QDs has vibrated shoulder between 300 and 500 nm. The


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absorption of blend films of PTB7:PC71BM on ZnO or ZnO/CIS-QDs interlayer (Figure S4) support this assumption. Furthermore, the device with CIS-QDs exhibits a slight enhancement on EQE value from 500 to 710 nm as compared with that without CIS-QDs. The theoretical Jsc estimated by integrating the product of EQE well matches those from J-V curves.

Figure 4. (a) The absorption spectra of thin films with ZnO and ZnO/CIS-QDs thin, (b) the EQE spectra of PTB7-based devices with ZnO and ZnO/CIS QDs (7nm) interfacial layer. Organic-inorganic interface contact is a key factor in determining the performance of an inverted PSCs device. Atomic force microscopy (AFM) was carried out to further investigate the influences of CIS-QDs modifiers on the organic-inorganic interface. As seen in Figure 5, pristine ZnO thin film has a rough surface with a root-mean-square (RMS) of 7.34 nm. The large roughness blocks the favorable interfacial contact between cathode interlayer and active layer, implying the surface defects and recombination centers could be formed easily in therein. After coating CIS-QDs layer, the ZnO/CIS-QDs film shows much smoother surface with a RMS roughness of 1.56 nm. AFM of active layer films on ZnO (a) or ZnO/CIS-QDs were also studied and the results are shown in Figure S5. The film on ZnO/CIS-QDs has a lower RMS roughness of 1.46 nm than that of ZnO


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(1.70 nm), which is favorable for interface contact of cathode interlayer and active layer.38 Based on the above results, we can conclude that the CIS-QDs also provides a viable approach to optimize the film morphology of ZnO interlayer and may benefit the charge transport and carrier collection.

Figure 5. The AFM of tapping mode images of ZnO(a) and ZnO/CIS-QDs(b) thin films, respectively. To get an insight into the influence of the CIS-QDs charge transportation, carrier mobility was measured by applying space charge limited current (SCLC) method.39 The carrier mobility of which influences simultaneously the drift and diffusion length is a key parameter of dynamics with device in work conditions. The electron-only J-V characteristics of the single charge carrier devices with different cathode interlayer are shown in Figure 6a. The electron mobility increased from 1.6ൈ10-4 cm2 V-1 s-1 for original ZnO layer to 5.8ൈ10-4 cm2 V-1 s-1 for the ZnO thin films with coating 7 nm CIS-QDs layer. Moreover, the higher electron mobility may also contribute to the slight increase of Voc due to reducing charge recombination.


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Figure 6. (a) The J-V curves of electron-only devices with ZnO and ZnO/CIS-QDs interfacial layer, (b) the correlation between the light intensity (I) and current density Jsc for the devices with ZnO and ZnO/CIS-QDs interfacial layer. Furthermore, the light intensity (I) depended on Jsc was measured by regulating range of illumination intensity. As presented in Figure 6b, the correlation between Jsc and I can be depicted following J sc ∝ I α , where α is the exponential factor. The value of α is more approximate 1, the bimolecular recombination can be more suppressed.41 The value of α obtained by fitting power law is 0.92 for control device, 0.96 for the device treated by the incorporation of the CIS-QDs modifier. The results give a slight hint that the devices with CIS-QDs modifiers might be an influence on the bimolecular recombination and charge collection. To illustrate the versatility of CIS-QDs as interfacial modifier on ZnO layer, the inverted PSCs based on PTB7-Th and PC71BM were also fabricated. J-V curves and extracted devices performance were shown in Figure S6 and Table S1. The optimized device with CIS-QDs exhibits an improved PCE up to 9.12% over the control device (8.26%).


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3. CONCLUSION In conclusion, we found that the device performance of inverted PSCs can be improved via using CIS-QDs as interfacial modifiers on ZnO layer. Compared with the control PSCs, an improvement up to 16% is achieved for the devices using CIS-QDs as interfacial modifiers and the PCE approached to 9.12%. By analyzing the working function, device performance of inverted PSC device, the improvement can be explained to the enhanced optical absorption, decrease work function, smooth surface roughness, and increased electron mobility. Therefore, our findings demonstrate CuInS2 based QDs could be potential interfacial modifiers for PSCs and may be extended to other organic electronics. 4. EXPERIMENTAL SECTION 4.1 Materials Hydroxyl-terminated CIS-QDs were synthesized following our previous report.31 PTB7 and PTB7-Th were synthesized by the published report.42 PC71BM was purchased from American Dye Source. Inc. All the purchased materials were used without further purification. 4.2 Thin Film and Device Fabrication We used the acetone, detergent, deionized water and isopropyl alcohol to sonicate the ITO substrates in sequence and followed by drying at 90 ℃ for 12 hours in vacuum oven. ZnO precursor was obtained according to previous literature.43 About 40 nm ZnO layer was spin-coated on ITO substrates and heated at 200 ℃ for 1 hour. CIS-QDs was dispersed in methanol with concentrations of 0.5, 1.0, 1.5, 2.0 mg/mL, corresponding to the thickness of 4, 7, 11, 16 nm, respectively; the CIS-QDs modifier was obtained on the top of previous ZnO interlayer by spin coating with spin-speed of 3000 rpm/min. The device structures of ITO/ZnO/CIS-QDs/PTB7:PC71BM/MoO3/Ag

and

ITO/ZnO/PTB7:PC71BM/MoO3/Ag

was

fabricated. The PTB7 and PC71BM with a blend ratio of 1:1.5 were dissolved in chlorobenzene


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(CB) including 3% 1,8-diiodooctane (DIO), the total solids concentration was 25 mg mL-1 and stirred overnight. The active layer was controlled at ~90 nm by varying the concentration of solution. Then, we transferred the prepared active films into a vacuum at 3 hours for removing the residual DIO. Afterwards, 10 nm molybdenum oxide (MoO3) anode interlayer and 100 nm Ag electrode were deposited by vapor deposition through a customized mask in a vacuum chamber with a pressure of approximately 1ൈ10-4 Pa. The devices without encapsulation were tested in closed glove box with being full of N2 gas. 4.3 Characterizations The optical absorption of the thin films was measured by using a UV-Vis spectrophotometer (UV3600, SHIMADZU) in the room temperature. Transmission electron microscope (TEM) image of the CIS-QDs was determined using a JEOL-JEM 2100F microscope operating at 200 kV. The film morphology was conducted on by atomic force microscopy (AFM, Veeco Metrology Group/Digital Instruments) operating in tapping mode. The thickness of CISQDs thin film was tested spectroscopic ellipsometer (HORIBA, iHR320), other thin film thickness were obtained by surface profilemeter (Tencor, Alpha-500). The Thermo Scientific ESSCALAB 250Xi with UPS system was used to measure the work functions of ZnO and ZnO/CIS-QDs interlayer. The J-Vcurves were determined on a Keithley 2400 under AM 1.5 solar simulator. The correlation between Jsc and light intensity were recorded by modulating the light intensity with a group of neutral density filters (NDF). The EQE was recorded by using an integrated EQE measurement system with si-photodetector. The electron motility was determined by space charge 9 V2 limited current (SCLC) model according to the Mott-Gurney law: J = ε 0ε r µ 3 . The electron8 L only device is ITO/ZnO/CIS-QDs/PTB7:PC71BM/Ca/Al, the device structure for control electrononly device is ITO/ZnO/PTB7:PC71BM/Ca/Al.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detail JV curves, transmission, absorption, UPS spectra, AFM images, table of performance parameters (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (FH). *E-mail: [email protected] (HZ). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the SUSTech (FRG-SUSTC1501A-18), the Recruitment Program of Global Youth Experts of China, the National Basic Research Program of China (2013CB834805), National Natural Science Foundation of China (21573018), the Shenzhen fundamental research programs (JCYJ20150630145302237), the Shenzhen Key Lab funding (ZDSYS201505291525382), and the Shenzhen peacock program (KQTD20140630110339343). REFERENCES


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Hydroxyl-Terminated CuInS2-Based Quantum Dots - ACS Publications

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