Carbon Quantum Dots Electron Extraction

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Solution-Processable ZnO/Carbon Quantum Dots Electron Extraction Layer for Highly Efficient Polymer Solar Cells Ruqin Zhang, Min Zhao, Zhongqiang Wang, Zongtao Wang, Bo Zhao, Yanqin Miao, Yingjuan Zhou, Hua Wang, Yuying Hao, Guo Chen, and Furong Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17969 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

Solution-Processable ZnO/Carbon Quantum Dots Electron Extraction Layer for Highly Efficient Polymer Solar Cells Ruqin Zhang†, Min Zhao*,†, Zhongqiang Wang*,†, Zongtao Wang †, Bo Zhao†, Yanqin Miao†, Yingjuan Zhou†, Hua Wang†, Yuying Hao†, Guo Chen§ and Furong Zhu*,‡ †

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of

Education, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China ‡

Department of Physics, Institute of Advanced Materials, and Institute of Research and

Continuing Education (Shenzhen), Hong Kong Baptist University, Kowloon Tong, Hong Kong, China §

Key Laboratory of Advanced Display and System Applications, Ministry of Education,

Shanghai University, Yanchang Road 149, Shanghai, 200072, China KEYWORDS ：EEL, polymer solar cells, bilayer, ZnO, C-QDs ABSTRACT: In this work, we report the effort to develop high efficiency inverted polymer solar cells (PSCs) by applying a solution processable bilayer ZnO/carbon quantum dots (C-QDs) electron extraction layer (EEL). It is shown that the use of the bilayer EEL helps to suppress the exciton quenching by passivating the ZnO surface defects in the EEL, leading to an enhanced

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exciton dissociation, reduced charge recombination and more efficient charge extraction probability, and thereby achieving high power conversion efficiency (PCE). The inverted PSCs, based on the blend of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diylalt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} and [6,6]-phenyl C71butyric acid methyl ester, possess a significant improvement in PCE of ~9.64%, which is >27% higher than that of a control cell (~7.59%). The use of a bilayer ZnO/C-QDs EEL offers a promising approach for attaining high efficiency inverted PSCs.

INTRODUCTION With the increasing attention to solve the energy supplies without adversely affecting the environment, the development of clean and sustainable energy has become one of the most focusing research subjects. Polymer solar cells (PSCs) have attracted worldwide attention and are considered as one of the promising renewable energy technologies due to the unique advantages of high absorption characteristics in the visible-light region, and solution-fabrication capability for large-area low-cost flexible PSCs.1-5 The single junction PSCs with a power conversion efficiency (PCE) of >14% have been reported due to the continued progresses made in the development of low bandgap polymer donors, non-fullerene donors and electrode modification materials, improved understanding of charge transport properties and device design.6,7 Apart from the progresses in the development of low bandgap polymer for high efficiency PSCs, omnidirectional and broadband light absorption enhancement can be realized in PSCs with 2-D photonic structures, achieved from the combined effects of light scatting, excitation of surface plasmon polaritons, waveguide modes and their mutual coupling.8,9 The interfacial engineering, such as anode and cathode modification, plays a critical role in


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determining PCE and the stability of the devices. A comprehensive study of the stability of PSCs was conducted using a combination of optical admittance and transient photocurrent (TPC) analyses.10,11 The charge recombination and charge extraction properties in the PSCs were investigated using TPC measurements. The results revealed that the imbalanced charge mobility in the bulk heterojunction (BHJ) is one of the degradation mechanisms.12 The localized failure in the active area and the oxidation process that occurred at the BHJ/electrode interface, because of the presence of environmental oxygen and moisture, serving as the degradation pathways in the PSCs were identified.13 The improved interfacial contact at the electrode/BHJ interface benefits the efficient operation of PSCs. A 20% increase in the PCE of the PSCs with an optimal oxide hole/electron interlayer was attained as compared to that in an optimized control cell without an oxide interlayer.14 In general, air-sensitive materials are used as the electron extraction layer in the conventional structure devices. Hence, it is hard to fabricate PSCs without an appropriate protection in air.13 To solve this issue, the inverted cell structure is adopted, which reverses the cell configuration by adopting a top high work function metal anode or a bilayer MoO3/metal top anode, and a low work function interlayer-modified ITO front transparent cathode. In comparison with the conventional cell structure, the origin of high performing inverted PSCs is mainly due to the suppression of bimolecular recombination and the enhancement of charge extraction probability, enabled by a greater effective internal electric field in the inverted cells.15 In addition, inverted PSCs had advantages of slow degradation process compared to that of the cells with a conventional configuration, avoiding the oxidation of top cathode due to the moisture and oxygen encroachment, preventing the deterioration of the ITO and physical degradation of active layers. 16,17


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In the inverted PSCs, air-stable metal-oxide material-based electron extraction layer (EEL), such as ZnO, CsOx, Nb2O5 and TiOx, has been widely used14,15,18,19 Till now, a ZnO EEL is widely used in the inverted PSCs because of its promising electron extraction property and high transparency desired for efficient operation of the inverted cells. On the other hand, ZnO is an environmentally friendly material and can be processed by solution processable or sol-gel fabrication processes.13,20 However, sol-gel processed ZnO EEL has some limitations for application in highly efficient inverted PSCs. ZnO EEL prepared by the sol-gel process often contains unavoidable defects formed in the interior and on the surface of the film, acting as the electron trap sites and thereby contributing to the severe charge carrier recombination loss in PSCs.14,21 In addition, the incompatibility between the organic materials and the inorganic materials also results in a poor interfacial contact at the ZnO EEL/BHJ interface, leading to a large series resistance (Rs), a weak electronic coupling and an increased charge recombination.22 Many efforts have been made to facilitate efficient electron extraction, and modify the ZnO/BHJ interface to suppress trap-assisted recombination.14,15, 23,24 Organic/ZnO double-layer structure is developed, which prevents direct contact of the ZnO film with the active layers, passivates the surface defects of ZnO, and tunes the work function of ZnO.25,26 Different interfacial

materials,

such

as

self-assembled

monolayers,

water/alcohol

soluble

polymers/polyelectrolytes and ionic liquids, have been used to form bilayer EEL for application in PSCs. In contrast to the performance of PSCs with a pristine ZnO EEL, PSCs with a bilayer organic/ZnO EEL displayed an impressive enhancement in the cell performance, demonstrating 10-40% increase in PCE. Nevertheless, the enhancement in the performance of the PSCs is closely associated with the thickness and surface electronic properties of the interlayer. Thus,


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high quality and chemical stable solution-processable charge extraction materials are the perquisite for attaining high performing PSCs. Recently, the use of charge extraction interlayer, based on a series of carbon-based materials (graphene, carbon nanotubes and graphene quantum dots), for high performance PSCs has been reported. In comparison with the graphene and carbon nanotubes-based interlayer materials, charge extraction layer fabricated with the carbon quantum dots (C-QDs) has attracted increasing attention due to its technology significance and environmental benefits. The C-QDs show good water-solubility, tunable electrical and optical properties, as well as wavelength-dependent luminescence emission, resulting in wide application in optoelectronic devices. Absorption enhancement in PSCs by incorporating C-QDs in the BHJ has been reported, demonstrating the improved device performance due to the down-shifting effect of C-QDs.27-29 The use of C-QDsbased interlayer for application in PSCs has received some attentions, however, the effect of CQDs-based EEL on highly efficient inverted PSCs has not yet been systemically studied. In this study, a bilayer ZnO/C-QDs EEL, made with water-soluble C-QDs and sol-gel processed ZnO, has been developed for use in inverted PSCs, achieving significant enhancement in PCE. Compare to the cell with a pristine ZnO EEL, the use of an optimized bilayer ZnO/CQDs EEL has advantages to increase the carrier collection probability, reduce charge recombination loss and suppress exciton quenching in the inverted PSCs. As a result, the inverted PSCs, based on the blend of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), achieving PCE of 9.64%, which is >27% higher than that of the PTB7: PC71BM-based control cell made with a pristine ZnO EEL (7.59%). The suitability of a bilayer ZnO/C-QDs EEL was also examined for


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application in PSCs based on the blend system of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]phenyl-C60-butyric acid methyl ester (PC61BM), possess a significant improvement in PCE of ~4.85%, which is >28% higher than that of a control cell with a pristine ZnO EEL (~3.78%). EXPERIMENTAL DETAILS ZnO precursor preparation. The ZnO precursor was prepared by dissolving 0.836 g anhydrous zinc acetate (Zn(CH3COO)2, Energy Chemical, 99.5%) and 0.28 g ethanolamine (NH2CH2CH2OH, Aldrich, 99.9%) in 10 mL 2-methoxyethanol (CH3OCH2CH2OH, Energy Chemical, 99%). The solution was stirred overnight for the hydrolysis reaction in air. Fabrication of inverted PSCs. MoO3, P3HT and PC61BM were purchased from Rieke Company and used as received without further modification. PTB7 and PC71BM were obtained from 1-Material of U.S. and Luminescence Technology Corporation of Taiwan, respectively. Inverted PSCs were fabricated using pre-patterned ITO/glass substrates with a sheet resistance of 15 Ω/square. The ITO/glass substrates were cleaned by ultrasonication sequentially with detergent, acetone and isopropanol each for 30 min, and subsequently dried overnight at 60℃ in oven. ZnO precursor solution was spin-coated on the UV ozone-treated ITO substrates and then thermally annealed at 200 °C for 1 h in air. The C-QDs aqueous solution with fixed concentration was spin coated onto the surface of ZnO at different rotation speeds. The samples were quickly transferred to a N2-protected glovebox. Then, a 200 nm thick blend layer was spincoating on top of ZnO or ZnO/C-QDs EEL, using a solution with a solution with a ratio of PTB7 to PC71BM (1:1.5) in chlorobenzene and 1,8-diiodoctane (97:3 by volume) with a total concentration of 25 mg mL-1, or a ratio of P3HT to PC61BM (1:1 wt%) in o-dichlorobenzene (oDCB), with a total concentration of 40 mg mL-1. After spin coating, the P3HT:PC61BM based samples were slowly dried in a glass petri dish following with a thermal annealing at 120 °C for


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15 min. A 5 nm thick MoO3 was deposited on the blend layer at an evaporation rate of 0.2 Å/s in vacuum chamber. Finally, an 80 nm-thick Al electrode was coated on the functional layer through a shadow mask, which defines an active area of 0.12 cm2. All solvents and materials were utilized as purchased without purification. Newport solar simulator system was used to measure current density-voltage (J-V) characteristics, PCE and external quantum efficiency (EQE). The J-V characteristics of the PSCs were measured using a calibrated AM 1.5 G illumination, with light intensity of 100 mW cm-2. A calibrated monosilicon diode was fixed as a reference exhibiting a response at 300-800 nm. A Hitachi F-7000 spectrofluorophotometer was utilized for the photoluminescence (PL) measurement of the ZnO and ZnO/C-QDs samples. RESULTS AND DISCUSSION The bilayer ZnO/C-QDs EEL was formed by spinning coating the aqueous solution with an optimal C-QDs concentration of 20 mg ml-1 on ZnO surface. Both the pristine ZnO EEL and bilayer ZnO/C-QDs EELs were used in inverted PSCs with the same device structure of ITO/EEL/ PTB7:PC71BM or P3HT:PC61BM/MoO3/Al (Figure 1a). As shown in Supporting Information Figure S1 and Figure S2, the thickness of the ZnO layer was adjusted over the thickness range from 15 to 60 nm. A 40 nm-thick ZnO layer was optimized for making high performance PSCs. Hence, a 40 nm-thick ZnO EEL was used in the cell fabrication. The molecular structures of the P3HT, PTB7, PC61BM and PC71BM molecules are shown in Figure 1b.


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Figure 1. a) A scheme cross-sectional view of an inverted PSC, b) molecular structures of P3HT, PTB7, PC61BM and PC71BM molecules. The thickness of the C-QDs interlayer was controlled for optimizing the cell performance. The rotation speed of C-QDs interlayer was varied from 2000 to 7000 rpm. The optimal reference device with a 40 nm-thick pristine ZnO EEL was made for comparison study. The J-V characteristics measured for the inverted PSCs and the reference cell under light and in the dark state are shown in Figure 2. A summary of the corresponding device parameters obtained for the reference cells and the PSCs prepared using the blend systems of PTB7:PC71BM and P3HT:PC61BM is listed in Table 1. For PTB7:PC71BM-based PSCs, the reference device displayed an open circuit voltage (Voc) of 0.73 V, a Jsc of 16.56 mA cm-2, a fill factor (FF) of 63.0% and a PCE of 7.59%. An obvious increase in the Jsc of the PSCs with a bilayer ZnO/CQDs EEL is observed as compared to the one measured for the reference cell, e.g., increased for the cells with a bilayer ZnO/C-QDs EEL having C-QDs interlayer prepared over the rotation speed from 2000 to 6000 rpm, and then decreased at high rotation speed of > 6000 rpm. The results reveal that the optimal device performance was obtained for C-QDs interlayer prepared at a rotation speed of 6000 rpm, achieving a Voc of 0.75 V, a Jsc of 17.45 mA cm-2, a FF of 66.4% and a PCE of 9.64%. The Jsc thus obtained the optimal PSC with a bilayer ZnO/C-QDs EEL is 18% higher than that of the reference cell (15.56 mA cm-2), leading to a 27% increase in PCE


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compared to that of a reference cell (7.59%). The dark J-V characteristics of the cells are plotted in Figure 2b, revealing that the use of a bilayer ZnO/C-QDs EEL enables a lower leakage current, and thereby a reduced Rs and a higher Rsh in the cells, as shown in Table 1.

Figure 2. a) Illuminated and b) dark J–V characteristics measured for a reference cell and a set of structurally identical PSCs with a bilayer ZnO/C-QDs EEL, having C-QDs interlayers fabricated over a rotation speed range from 2000 to 7000 rpm. The EQE spectra, measured for a reference cell and the inverted PSCs with a bilayer ZnO/CQDs EEL, are shown in Figure S3. The difference between the Jsc extracted from J–V characteristics and the one calculated using the EQE spectra is

Carbon Quantum Dots Electron Extraction

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