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Fullerene-Free Organic Solar Cells with Efficiency Over 12% Based on EDTA-ZnO Hybrid Cathode Interlayer Xiaodong Li, Xiaohui Liu, Wenjun Zhang, Hai-Qiao Wang, and Junfeng Fang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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Chemistry of Materials

Fullerene-Free Organic Solar Cells with Efficiency Over 12% Based on EDTA-ZnO Hybrid Cathode Interlayer Xiaodong Li, Xiaohui Liu, Wenjun Zhang, Hai-Qiao Wang and Junfeng Fang* Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China; E-mail: [email protected]

ABSTRACT: Interlayer materials play an important role in the rapid development of organic solar cells (OSCs). State-ofthe-art interlayers in fullerene OSCs may be not the best choice for fullerene-free OSCs, thus new interlayers need to be developed. Here, we report high performance fullerene-free OSCs with efficiency over 12% by using low temperature (150 o C) solution processed EDTA-ZnO hybrid interlayer. The performance improvement can be attributed to the dual nature of EDTA. First, EDTA itself shows good ability in interface modification as an interlayer, leading to 10% device efficiency. Second, when combined with ZnO, EDTA can passivize defects in ZnO film due to its chelation function, leading to balanced hole and electron mobility in OSCs. As a result, high internal quantum efficiency (IQE) approaching 100% is achieved in the range from 480 nm to 720 nm. The remarkable IQE means that in OSCs with EDTA-ZnO, every absorbed photon can generate a separated pair of charged carriers and that almost all photo-generated carriers can be collected at electrodes. Importantly, the devices with EDTA-ZnO show good reproducibility and efficiency of 8.81% can be obtained even in 1 cm2 area devices.

Organic solar cells (OSCs) have attracted much attention due to their advantages of flexibility, low cost and large scale fabrication through roll-to-roll printing.1-2 Typically, OSCs consist of a bulk-heterojunction of two matching materials that work as electron donor and acceptor respectively.3 For electron acceptor, fullerene derivatives have been intensively investigated over the past few decades4 and power conversion efficiency (PCE) over 10% can be obtained.5-8 Meanwhile, fullerene-free acceptors are also advancing, driven by the need to find alternative ones that overcome the poor absorption property, limited bandgap adjustability and high synthetic cost of fullerene acceptors.9-11 In 2015, a landmark fullerene-free acceptor (ITIC) was developed and exhibited PCE up to 6.8%, which was very close to that of fullerene based OSCs.12 Through chemical structure optimization, the PCE can be further improved to 9.6% by employing ITIC-Th acceptor.13 Subsequently,

many efforts have been done on the synthesis of fullerene-free acceptors14-20 and corresponding matching donors14, 21-24, pushing efficiency to 9-11%. Recently, device efficiency of 12% can be achieved by energy level modulation of fullerene-free acceptors. 25-26 Besides the synthesis of photo-active materials, interlayer modification is also crucially important due to its role in solving the intrinsic limitation of mismatched energy level and unbalanced carriers mobility in OSCs.27 Different to fullerene acceptors (PC61BM, PC71BM), top-level fullerene-free acceptors usually exhibit up-shifted lowest unoccupied molecular orbital (LUMO) of ~4.0 eV (PC71BM: ~4.3 eV).25-26 Furthermore, the electrons mobility of fullerene-free acceptors (10-4~10-6 cm2V-1s-1)11, 13, 24, 28 is about two orders lower than fullerene acceptors (10-2~10-4 cm2V-1s-1).4, 29 As a result, the interlayers previously designed for fullerene OSCs may be not the best choice in fullerene-free OSCs. For example, in

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fullerene OSCs, PFN interlayer can lead to the record efficiency of 10.6%8, which is obviously higher than that with ZnO interlayer (~9.4%)30. While in fullerene-free OSCs, the device efficiency with PFN (10.7%) is inferior compared with ZnO (11.3%).31 Despite the successful application in fullerene-free OSCs, previously reported ZnO needed high temperature (200 oC) annealing to reduce the defects, which is energy-consuming and not compatible with flexible substrates.25 Here, we report a low temperature processed EDTA-ZnO hybrid interlayer (EDTA: ethylene diamine tetraacetic acid) that enables highly efficient fullerene-free OSCs with PCE up to 12.10%. EDTA can passivize the defects in ZnO due to its chelation function, thus a temperature as low as 150 oC is enough to form effective EDTA-ZnO interlayer. Different to previous reported PEO or PEG passivized ZnO,32-33 where the organic materials own no ability as an interlayer and thus lead to a trade-off between defects passivation and charge transport property, EDTA itself exhibits good interface modification as cathode interlayer in OSCs34 and PCE of ~10% can be achieved. On the other hand, the main challenge for EDTA interlayer lies in its low conductivity. When combined with ZnO, the film conductivity can be increased due to high conductivity of ZnO. As a result, PCE over 12% is obtained by introducing EDTA-ZnO interlayer. Besides efficiency, the OSCs with EDTA-ZnO also exhibit high internal quantum efficiency (IQE) approaching 100% in the regime from 480 nm to 720 nm. Importantly, EDTA-ZnO interlayer even works well in large area OSCs and PCE of 8.81% can be obtained with aperture area of 1 cm2. These results indicate the importance of interlayer design in fullerene-free OSCs and would speed up the commercial application of OSCs.

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Figure 1. (a) Devices configuration; (b) Chemical structure of photoactive materials; (c) The fabrication scheme for EDTAZnO.

Figure 1a shows the inverted device configuration in this work. Figure 1b provides the chemical structure of photoactive materials: donor PBDB-T and fullerene-free acceptor ITM. Figure 1c describes the fabrication scheme of EDTA-ZnO interlayer. Solution processed ZnO usually has high density defects that would act as recombination centers for photogenerated carriers.35 Different to previously reported self-assembled monolayer on ZnO where only surface defects can be passivized 35, here, EDTA is added into the precursor solution and can coordinate with ZnO during its formation, thus completely passivizing the defects in both surface and bulk. Table 1. The device parameters of OSCs with different interlayers. Interlayer

Voc

Jsc (mA/c 2 m)

FF

PCE

Rs

(V)

(%)

(%)

(Ωcm )

(Ωcm )

Bare ITO

0.566

15.50

56.3

4.94

6.2

338.8

EDTA

0.942 (0.942)

16.53

65.1

10.14

4.4

548.6

(16.24)

(63.2)

(9.66)

(7.9)

(669.8)

0.949

16.23

71.9

11.07

3.3

817.6

(0.947)

(15.83)

(70.5)

(10.56)

(4.4)

(770.4)

73.2

12.10

2.7

1106.9

(72.1)

(11.67)

(3.0)

(1002.0)

ZnO EDTAZnO

0.953 (0.949)

a

a

17.34 (17.06)

Rsh 2

2

the Jsc integrated form EQE is 16.63 mA/cm . Other values in brackets are the average parameters among 15 separated for EDTA and ZnO, 30 devices for EDTA-ZnO.


2

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Figure 2. (a) J-V curves; (b) Specific parameters of devices with EDTA-ZnO, Inset: PCE distribution among 30 separated devices; (c) External quantum efficiency (EQE), (d) Internal quantum efficiency (IQE) and reflective absorption of OSCs with EDTA-ZnO. Inset: the illustration of reflective absorption measurement setup.

J-V curves of OSCs with different interlayers are shown in Figure 2a. For devices on bare ITO, the performance is poor (PCE of 4.94%), mainly due to the direct contact between active layer and ITO electrode. When EDTA is introduced as cathode interlayer, the devices performance is greatly improved and a PCE of 10.14% can be obtained with high Voc (0.942 V) and Jsc (16.53 mA/cm2), despite the moderated FF of 65.1% (Table 1). While the FF can be improved to 71.9% for ZnO based devices but the Jsc is slightly decreased to 16.23 mA/cm2, leading to a PCE of 11.07%. When EDTA-ZnO is used as cathode interlayer, the Voc, Jsc and FF are all increased. As a result, highest PCE of 12.10% is achieved with Voc of 0.953 V, Jsc of 17.34 mA/cm2 and FF of 73.2% (Figure 2b). In addition, the devices with EDTA-ZnO show good reproducibility and an average PCE of 11.67±0.21% can be obtained among 30 separated devices (Figure 2b insert). Furthermore, the universality of EDTA-ZnO interlayer is also

verified by using other fullerene-free acceptor (ITIC). As a result, the devices with EDTAZnO also showed better efficiency of 10.8% compared with that based on ZnO interlayer (PCE of 10.1%) (Figure S1). Figure 2c shows the external quantum efficiency (EQE) of OSCs with EDTA-ZnO. The devices exhibit high EQE and the value can even reach near 90% around 580 nm. The integrated Jsc from EQE is 16.63 mA/cm2, in agreement with that obtained from J-V curves (4% mismatch). To further investigate the devices response with sunlight, internal quantum efficiency (IQE) measurement is conducted in OSCs with EDTA-ZnO. First, the absorption spectrum is measured in reflectance to detect the light absorption process in real devices (Figure 2d inset).36 Through dividing the reflectance absorption with EQE value, the IQE can be obtained. As shown in Figure 2d, the IQE of devices with EDTA-ZnO ap-



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proaches 100% around 580 nm and stays near 90% throughout the visible spectrum range from 480 nm to 720 nm. Such a high IQE is remarkable, indicating that almost every absorbed photo can generate a pair of separated carriers which can be further collected at electrodes.37

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from the C-H stretching vibration of EDTA molecule.38 Note that the peak of C=O stretching vibration (~1695 cm-1) in free EDTA disappears in EDTA-ZnO. And an extra new peak around 1605 cm-1 appears in EDTA-ZnO, which can be attributed to the stretching vibration of O-C-O.39 The transformation from C=O to O-C-O indicates the chelation function between EDTA and ZnO through carboxyl groups.39 Figure 3b shows the survey spectra of X-ray photoelectron spectroscopy (XPS) in EDTA, ZnO and EDTA-ZnO films. The most significant change is the enhancement of N peak due to EDTA introduction (Figure 3b inset). In addition, the Zn 2p3/2 peak in EDTA-ZnO locates at 1022.1 eV (Figure 3c), which is ~0.3 eV shift to lower binding energy compared with that in ZnO (1022.4 eV). The shift means that more Zn atoms are bound to O atoms and that the oxygen-deficient defects are effectively passivized in EDTA-ZnO.35, 40 These results, together with the FTIR analyses, indicate that the carboxyl groups in EDTA can coordinate with ZnO, thus passivizing the defects. Furthermore, we investigate the EDTA distribution in bulk EDTA-ZnO film through quantitative depth-profile study (Figure 3d). The C/Zn and N/Zn atom ratio is higher at surface than that in bulk, indicating the surface enrichment of EDTA molecule, which is beneficial to improve the interface contact between organic photoactive layer and inorganic ZnO interlayer.41 Except surface, the C/Zn and N/Zn atom ratio is stable with the prolonged sputter time, indicating the uniform distribution of EDTA in bulk, which is important for the completely passivation of ZnO defects.30, 35

Figure 3. (a) FTIR spectra of EDTA and EDTA-ZnO; (b) XPS survey spectra of EDTA, ZnO and EDTA-ZnO, inset shows the enlarged N spectra; (c) XPS spectra of Zn 2p3/2 in ZnO and EDTA-ZnO; (d) Depth profiles of the atom ratios C/Zn and N/Zn in EDTA-ZnO film.

Fourier Transform Infrared (FTIR) spectra are conducted to further investigate the interaction between EDTA and ZnO (Figure 3a). In EDTA-ZnO, the peak around 2900 cm-1 is ACS Paragon Plus Environment

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around 465 nm and 546 nm are greatly reduced, indicating the effective passivation of defects by EDTA.35, 41, 44 Furthermore, the time-resolved PL spectra (Figure 4d) show that the exciton lifetime of IT-M on ITO/EDTA-ZnO is 8.32 ns, while the exciton lifetime on ITO/EDTA and ITO/ZnO is 6.97 ns, 6.10 ns respectively. The longer exciton lifetime indicates that the photo-generated excitons own more chance to transfer to the interface between donor and acceptor, forming free carriers, which is in good agreement with the high Jsc and IQE in OSCs with EDTA-ZnO interlayer. Figure 4. (a) UPS data; (b) Energy level illustration at cathode interface; (c) Normalized Photoluminescence spectra of ZnO and EDTA-ZnO coated on ITO (excited at 310 nm); (d) Timeresolved photoluminescence spectra of IT-M acceptor coated on ITO/EDTA, ITO/ZnO and ITO/EDTA-ZnO (excited at 590 nm, emitting at 790 nm).

In OSCs, Ohmic contact is critically important for the achievement of high Voc.42 To form Ohmic contact at cathode interface, the work function (WF) of cathode should be lower than but close to the LUMO level of acceptors.42-43 In this work, the LUMO level of acceptor (IT-M) is 3.98 eV. And the work function (WF) of ITO cathode is measured through ultraviolet photoelectron spectroscopy (UPS) as shown in Figure 4a. Bare ITO shows high WF of 4.63 eV, which is far away from the LUMO of IT-M, thus leading to much low Voc. When interlayer is introduced, the WF of ITO starts to decrease. As shown in Figure 4b, from bare ITO, EDTA, ZnO to EDTA-ZnO, the WF of ITO cathode gradually up-shifts, closer and closer to the LUMO of ITM, which agrees with the gradually increased Voc in OSCs. Photoluminescence spectra (PL) are conducted to investigate the passivation effect of EDTA on ZnO as shown in Figure 4c. For ZnO, in addition to the band-to-band emission at 375 nm, there are another two broad peaks around 465 nm and 546 nm, which have been known as the evidence of defects in ZnO films.35, 44 While for EDTA-ZnO, the PL peaks

Figure 5. (a) J-V curves of electron-only devices (ITO/ interlayer/PBDB-T:IT-M/Mg/Al); (b) J-V curves of hole-only devic1/2 es (ITO/MoO3/PBDB-T:IT-M/MoO3/Al). Insets are the J ~V characteristics of devices.

Electron-only devices (ITO/interlayer/ PBDB-T:IT-M/Mg/Al) are fabricated to test electron mobility of the whole devices (Figure



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5a). The electron mobility (µe) in devices with EDTA is just 0.72*10-4 cm2V-1s-1, which may be caused by the low conductivity of EDTA interlayer. Note that the devices with EDTA-ZnO show the highest µe of 2.95*10-4 cm2V-1s-1, even 49% higher than that of devices with ZnO (1.98*10-4 cm2V-1s-1). As known, balanced electron and hole mobility is critically important for carriers transport in OSCs. So the hole mobility (µh) is also measured with devices configuration of ITO/MoO3/PBDB-T:IT-M/ MoO3/Al. As a result, the hole mobility is 3.17*10-4 cm2V-1s-1 (Figure 5b). The value of µh/µe is 4.40, 1.60 and 1.07 for devices with EDTA, ZnO and EDTA-ZnO, respectively, indicating the more and more balanced carriers mobility. The more balanced carriers mobility means the less carriers recombination, thus leading to high Jsc and FF in OSCs with EDTAZnO interlayer.

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(0.06 cm2). And we believe through next optimization in photoactive layer, the device efficiency may be further improved, demonstrating EDTA-ZnO to be a promising interlayer for large area, scalable OSCs. In conclusion, high performance fullerenefree OSCs were fabricated with efficiency over 12% by using new EDTA-ZnO hybrid cathode interlayer. Compared with ZnO, EDTA-ZnO exhibit decreased work function (4.05 eV), which is much close to the LUMO of IT-M acceptor (3.98 eV), thus leading to increased Voc. In addition, EDTA can passivize the defects in ZnO by its chelating function, leading to longer exactions lifetime in IT-M, which is beneficial to the achievement of high Jsc. On the other hand, the electrons mobility is also improved in devices with EDTA-ZnO, realizing balanced hole and electron mobility, thus improving devices FF. As a result, the device efficiency is increased from 11.07% with ZnO to 12.10% with EDTA-ZnO. Importantly, high internal quantum efficiency near 100% can be obtained by using EDTA-ZnO interlayer. EDTA-ZnO also works well in large area OSCs and PCE of 8.81% is achieved with 1 cm2 aperture area. Our study highlights the promising potential of EDTA-ZnO interlayer in fullerene-free OSCs and implies the huge prospect of this interlayer in large area OSCs. ASSOCIATED CONTENT

2

Figure 6. J-V curves of OSCs with 1 cm aperture area based on EDTA-ZnO interlayer. Inset: the photograph of the OSCs

In addition, we also fabricated large area devices with 1 cm2 by using EDTA-ZnO as cathode interlayer. As a result, an exciting PCE of 8.81% can be obtained (Figure 6), which is among the highest PCE for OSCs with 1 cm2 aperture area.11, 23, 45 Compared with small area devices (0.06 cm2), the efficiency loss is mainly caused by the decreased FF (from 73.2% to 59.2%). It should be noted that the 1 cm2 area devices are fabricated by using the optimized conditions for small area devices

Supporting Information. Experimental details, Transmittance of interlayer coated ITO, statistical analysis of device performance, XPS peak fitting and SEM images are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. (Junfeng Fang)

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The Project was supported by National Natural Science Foundation of China (61474125), National Youth Top-notch Talent Support Program and Zhejiang Provincial Natural Science Foundation of China (LR14E030002).

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