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Polarized Thin Layer Deposited Electrochemically on AluminumDoped Zinc Oxide as a Cathode Interlayer for Highly Efficient Organic Electronics Rong Wang,†,§ Li Nian,‡,§ Liang Yao,† Linlin Liu,‡ Zengqi Xie,*,‡ and Yuguang Ma*,†,‡ †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China

‡

S Supporting Information *

ABSTRACT: Herein, we demonstrated a polarized thin film (PBzP2C4) electrochemically deposited from phosphonate and a carbazole-difunctionalized conjugated molecule as an aluminum-doped zinc oxide (AZO) modifier for highperformance inverted organic solar cells (OSCs)/polymer light-emitting diodes (PLEDs). The PBzP2C4 film showed a controllable thickness and fully covered the surface of AZO, resulting in a smooth and uniform electrode. PBzP2C4 modification reduced the WF of AZO and highly improved the electron extraction/injection in inverted OSCs/PLEDs. As a result, a maximum power conversion efficiency of 10.35% was achieved for inverted OSCs with PTB7-Th:PC71BM as the active layer, and a maximum luminous efficiency of 21.4 cd A−1 was obtained for inverted PLEDs based on P-PPV. KEYWORDS: electrochemical deposition, cathode interlayer, organic solar cells, polymer light-emitting diodes, high performance

O

barrier between the lowest unoccupied molecular orbital (LUMO) of the light-emitting material and conduction bands of ZnO (AZO). Also, because of the difficulty of electron injection and resulting unbalance of electrons and holes in the emitting layers, carrier recombination and exciton formation take place very close to the cathode, which induce excitions trapped and quenched by oxygen vacancy defects.7,14 Therefore, decreasing the surface defects and reducing the incompatibility at the inorganic/organic interface are essentially important for further improvement of the device performance of both OSCs and PLEDs. An effective approach is to modify the ZnO (AZO) surface with a polarized organic interlayer, such as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), 1 5 poly(ethylenimine) (PEI),14,16 polyethylenimine ethoxylated (PEIE),18 etc. The polarized interlayer is a film containing a strong polar group that exhibits certain polarity, which can form interfacial dipoles and influence the work function (WF) of the electrode. These kinds of materials can modify the surface defects effectively and realize efficient electron extraction/ injection due to the formation of interfacial dipoles. However, because of the low electron mobility/conductivity or even insulating nature, the thickness for these polarized interlayers

rganic electronics, especially organic solar cells (OSCs) and polymer light-emitting diodes (PLEDs), have attracted much attention in the past decades because of their various advantages such as low cost, light weight, flexibility, and potential for development in fast roll-to-roll (R2R) production.1−4 For the production of efficient organic electronics, the cathode interlayer between the active layer and cathode plays a key role because of its fundamental importance to the electron extraction/injection process in OSCs/PLEDs.5−8 Zinc oxide (ZnO) and aluminum-doped ZnO (AZO) are promising electron extraction/injection cathode interlayer materials because of their low cost, high electron mobility, and excellent optical transparency. However, high densities of surface defects, like oxygen vacancy, that may trap electrons commonly occur during the fabrication process.9−11 Meanwhile, the easy formation of pinholes in the solution-processed thin films of ZnO (AZO) is also problematic for the serious leak current in devices.11−14 In addition, inefficient electron extraction/ injection and inherent incompatibility between the inorganic metal oxides and the organic active layer are other penalties. For OSCs, despite the fact that a deep valence band of ZnO (AZO) can effectively prevent hole carrier transport into the cathode, the devices usually show low fill factor (FF) and low power conversion efficiency (PCE) most likely due to electron trapping by the large number of surface defects on ZnO (AZO) and the leak current induced by the pinhole defects.7,11,12 For PLEDs, electron injection is poor because of the larger energy © XXXX American Chemical Society

Received: July 17, 2016 Accepted: September 28, 2016

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DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic of the Chemical Structure of BzP2C4 and Electrochemical Deposition of BzP2C4 on AZO

Figure 1. Morphological and electronic properties of the surface of ITO/AZO/PBzP2C4: (a) AFM height image (2.5 μm × 2.5 μm) of ITO/AZO/ PBzP2C4; (b) UPS spectra of the ITO/AZO and ITO/AZO/PBzP2C4 substrates.

thin film fully covers the surface of AZO, resulting in a smooth and uniform electrode. Meanwhile, the PBzP2C4 film reduced the WF of AZO. In OSCs, the PBzP2C4 film not only decreases electron trapping but also enhances the built-in electric field, while in PLEDs, it reduces both the electron injection barrier and electroluminescence quenching. By using AZO/PBzP2C4 as the cathode interlayer, a maximum power conversion efficiency (PCEmax) of 10.35% was achieved for inverted OSCs with PTB7-Th:PC71BM (the chemical structure is shown in Figure S1) as the active layer and a maximum luminous efficiency (LEmax) of 21.4 cd A−1 was obtained for inverted PLEDs using P-PPV (the chemical structure iss shown in Figure S1) as the light-emitting layer. The chemical structure of the precursor (BzP2C4) for electrochemical deposition is shown in Scheme 1. The electrondeficient 2,1,3-benzothiadiazole in a conjugated backbone is beneficial to electron transportation. The peripheral carbazole unit is a highly electroactive group with various advantages such as low oxidation potential, efficient coupling, and structurally well-defined coupling products, which facilitates the formation of smooth surface morphology through electrochemical crosslinking reactions.21−24 Pioneering studies showed that the P O functional group remarkably reduces the WF of the electrode by formation of an interfacial dipole at the semiconductor/ electrode interface and realizes efficient electron injection in organic electronics.25−27 Our electrochemically deposited thin film of PBzP2C4 was fabricated on an AZO-modified indium−tin oxide (ITO; designated as ITO/AZO) electrode, like our previously reported method,21−24 as shown in Scheme 1 and Figure S2a.

must be kept below 10 nm to achieve high device performance, which is difficult in order to be compatible with the R2R coating process.13,16,18,19 It is highly desirable to develop a new deposition technique that allows the formation of uniform ultrathin films while still being suitable to large-scale production for mending the surface defects of ZnO and AZO. Previous reports have proven that electrochemical deposition is an efficient method to produce conducting polymer films with low cost and highly controllable properties.20−22 The advantages of electrochemical deposition include several aspects. First, in the electrochemical deposition process, a coupling reaction occurs preferentially at sites where it is deficient, which suggests that full surface coverage of the electrode can be easily obtained. Second, the thickness of deposited films can be precisely controlled by the electrochemical deposition parameters (e.g., solution concentration, scanning voltage region, scanning cycle, etc.); thus, it is a promising method to fabricate uniform ultrathin films in largescale production. Third, electrochemically deposited films are usually chemically cross-linked and insoluble in common solvents, which favors the fabrication of multilayer devices. Therefore, we suppose that electrochemical deposition would be an excellent technology to fabricate uniform ultrathin polarized organic films and effectively modify the surface defects on the electrode. In this work, we demonstrated a polarized thin film electrochemically deposited from monomeric phosphonate and a carbazole-difunctionalized conjugated molecule (designated as PBzP2C4), as an AZO modifier for high-performance inverted OSCs/PLEDs. The electrochemically deposited B

DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Energy-level diagram of the components of the inverted OSCs based on AZO or AZO/PBzP2C4 interlayers. The energy level of ITO/ AZO or ITO/AZO/PBzP2C4 was determined from UPS results, and those of the other materials were taken from the literature: PTB7-Th, PC71BM and MoO3,17 and Al.9 (b) J−V characteristics of the OSCs under 1000 W m−2 AM 1.5G illumination. (c) EQE curves.

Table 1. Device Performances of OSCs with Different Cathode Interlayers under 1000 W m−2 AM Illuminationa cathode Interlayer

Voc [V]

Jsc [mA cm−2]

FF [%]

PCEb [%]

AZO AZO/PBzP2C4

0.80 ± 0.01 0.80 ± 0.01

16.34 ± 0.19 17.39 ± 0.17

66.58 ± 0.22 73.54 ± 0.24

8.61 ± 0.17(8.84) 10.14 ± 0.16(10.35)

a

The device structure is ITO/cathode interlayer/PTB7-Th:PC71BM (100 nm)/MoO3 (10 nm)/Al (100 nm). Statistical data were achieved from seven independent devices. bThe maxima PCEs are given in parentheses.

(Figure S5). The more hydrophobic surface of AZO/PBzP2C4 could improve the contact between the organic active layer and the interlayer in the device. Because the WF of the interlayer has a great influence on the device performance, ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic properties of AZO and AZO/PBzP2C4 on ITO substrates, as shown in Figure 1b. The AZO/PBzP2C4 film showed a reduced WF compared with that of the AZO film (from 4.1 eV for AZO to 3.73 eV for AZO/PBzP2C4). The 0.37 eV reduction by PBzP2C4 modification was attributed to the interface dipole caused by the high polar phosphonate group in PBzP2C4. The low WF is beneficial for enhancement of the built-in voltage in OSCs and also reduction of the electron-injection barrier in PLEDs, which is crucial for producing efficient inverted organic electronic devices. The AZO/PBzP2C4 film was used as the cathode interlayer for the fabrication of inverted OSCs with a device configuration of ITO/AZO (30 nm)/PBzP2C4 (5 nm)/PTB7-Th:PC71BM (100 nm)/MoO3 (10 nm)/Al (100 nm), where PTB7-Th and PC71BM act as the electron donor and acceptor, respectively. PTB7-Th was chose to enable high-performance devices due to its narrow band gap (1.58 eV).28 As a reference, devices were

Importantly, a linear relationship between the thickness of the electrochemically deposited film and the scanning circles was found (Figure S2b). The thickness of the film deposited with 40 scanning cycles was 20 ± 2 nm; therefore, the average thickness increase from one scanning cycle was about 0.5 nm, indicating a totally controllable film thickness. The structure information on the PBzP2C4 film was characterized by Fourier transform infrared (FTIR) spectroscopy, which proved the formation of dimeric carbazoles during the electrochemical deposition process (Figure S3). The surface morphology of AZO/PBzP2C4 and the AZO film was investigated by atomic force microscopy (AFM), as shown in Figures 1a and S4. The formation of an ultrathin PBzP2C4 film (5 nm) on the surface of AZO decreased the surface roughness (the root-mean-square roughness decreased from 1.32 nm for AZO to 1.19 nm for AZO/PBzP2C4) and enhanced the surface uniformity. These results suggest that the electrochemical deposition of BzP2C4 is an effective method to modify the surface of AZO, with the understanding that the deposition occurred prior to the highly conductive sites such as the pinhole, as shown in Scheme 1. The increased water contact angle from 60.5° for AZO to 74.0° for AZO/PBzP2C4 also proved the full surface coverage of PBzP2C4 on the AZO film C

DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Electron-transition characteristics of the AZO and AZO/PBzP2C4 interlayers with an ITO/Al (40 nm)/AZO (30 nm)/PBzP2C4 (0 or 5 nm)/PTB7-Th:PC71BM (100 nm)/Ca (20 nm)/Al (100) device configuration. (b) Change in the voltage (ΔV) as a function of time for the OSCs in the TPV measurement. (c) TPC as a function of time for the OSCs in the TPC measurement. The device configuration is ITO/AZO (30 nm)/PBzP2C4 (0 or 5 nm)/PTB7-Th:PC71BM (100 nm)/MoO3 (10 nm)/Al (100 nm).

also fabricated with AZO as the cathode interlayer. The energy level diagram of the devices is given in Figure 2a. The current density−voltage (J−V) characteristics of the inverted OSCs under air mass (AM) 1.5G irradiation at 1000 W m−2 and the corresponding external quantum efficiency (EQE) are given in Figure 2b,c; the extracted device performance metrics are given in Table 1. Devices with a AZO/PBzP2C4 cathode interlayer exhibited a PCEmax of 10.35% with an open-circuit voltage (Voc) of 0.80 V, a short-circuit current density (Jsc) of 17.53 mA cm−2, and a FF of 73.80%, which were significantly higher than the values obtained from the control device using AZO as the cathode interlayer, which showed a PCEmax of 8.84% (Voc of 0.80 V, Jsc of 16.54 mA cm−2, and FF of 66.81%). A significant positive effect of an AZO/PBzP2C4-based device is that Jsc and FF are highly enhanced. The integrated Jsc values from EQE spectra were 17.11 and 16.23 mA cm−2 for AZO/PBzP2C4and AZO-based devices, respectively, which agree well with the J−V measurements (the error is within 3%). The above device performance is comparable to the best results reported in the literature in which AZO/carbon dots,10 ZnO/PBI-H,12 or PFN28 were used as the cathode layers. We further investigated the influence of the thickness of the PBzP2C4 layer on the device performance, and the results are given in Figure S6 and Table S1. To find out the reason for the significantly improved device performance of OSCs, electron-only devices with an Al/AZO/

PBzP2C4 (0 or 5 nm)/PTB7-Th:PC71BM/Ca/Al configuration were fabricated to investigate the influence of PBzP2C4 modification on the electron-transport property (Figure 3a). The electron current densities increased significantly for the AZO/PBzP2C4-based device compared with the AZO-based device, indicating much better electron-transport property. The main reasons for this improvement should be the low WF, which reduced the electron-injection barrier from the cathode interlayer to the active layer (as shown in Figure 2a), and the more hydrophobic surface, which improved the contact between the active layer and interlayer. Transient photovoltage (TPV) and transient photocurrent (TPC) measurements were used to study the chargerecombination dynamics and charge-extraction process in inverted OSCs. From TPV analysis (Figure 3b), the chargecarrier lifetime increased from 3.15 μs for the AZO-based device to 5.30 μs for the AZO/PBzP2C4-based device, indicating a reduced recombination process for the AZO/ PBzP2C4-based device. From TPC analysis (Figure 3c), the charge-extraction time of the AZO/PBzP2C4-based device was reduced by half compared to the AZO-based device (0.43 μs for the AZO-based device and 0.21 μs for the AZO/PBzP2C4based device), which was consistent with the much improved electron-transport property of the AZO/PBzP2C4-based device. The longer carrier lifetime and increased charge extraction rate caused by PBzP2C4 modification contributed D

DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Energy-level diagram of the components of the inverted PLEDs based on an AZO or AZO/PBzP2C4 interlayer. The energy level of PPPV was taken from ref 30. (b) J−V−L curves. (c) LE−J curves. (d) Electron transition characteristics of AZO and AZO/PBzP2C4 interlayers. The device configurations are ITO/AZO (30 nm)/PBzP2C4 (0 or 5 nm)/P-PPV (80 nm)/CsF (1 nm)/Al (100 nm).

improvement) and LEmax (a 10-fold improvement) compared to the AZO-based PLED. The corresponding electroluminescent (EL) spectra of inverted PLEDs with different cathode interlayers are shown in Figure S7, in which both devices have similar EL spectra. This result was much higher than that of other inverted devices based on P-PPV using ZnO/gold nanoparticles (10.8 cd A−1)30 and PFN-OX (14.8 cd A−1)8 as the cathode interlayer and reached a top level in inverted PLEDs. Besides, it was even comparable to those devices with conventional structure,29 demonstrating its great potential as a cathode interlayer in PLEDs. The low Vturn‑on was attributed to the low WF by PBzP2C4 modification that effectively reduced the electron-injection barrier between the LUMO of P-PPV and the conduction band of AZO, as shown in Figure 4a. Also, the above information suggested that the surface defects were effectively modified by PBzP2C4 deposition; therefore, it dramatically decreased the electron trapping and exciton quenching at the AZO/P-PPV interface, which have a large contribution to the significantly improved luminance and luminous efficiency. Similarly, we also investigated the electron-injection properties of the different interlayers by fabricating electron-only devices with an ITO/ interlayer/P-PPV/CsF/Al configuration. As shown in Figure 4d, the electron current densities increased significantly for the AZO/PBzP2C4-based device compared with the AZO-based device, indicating a much better electron-transport property, which was consistent with the results in the OSCs discussed above. Therefore, the low WF, reduced surface defects,

to the improved FF value and thus the high performance of the AZO/PBzP2C4-based device. We also fabricated inverted PLED utilizing an AZO/ PBzP2C4 film as the cathode interlayer. The inverted PLEDs were made with the following device structure: ITO/AZO (30 nm)/PBzP2C4 (5 nm)/P-PPV (80 nm)/MoO3 (10 nm)/Al (100 nm), where P-PPV acts as a light-emitting layer. P-PPV is a classical green-emitting material.8,29,30 As a reference, devices were also fabricated with AZO as the cathode interlayer. Figure 4a shows the energy-level diagrams of these devices, and parts b and c of Figure 4 show the current density−voltage−luminance (J−V−L) and luminance efficiency−current density (LE−J) characteristics of inverted PLEDs with different cathode interlayers. The extracted device performance metrics are given in Table 2. The device based on the AZO cathode layer gave a turn-on voltage (Vturn‑on) of 3.75 V, a maximum luminance (Lmax) of 7600 cd m−2, and a LEmax of 1.9 cd A−1. When AZO was modified with PBzP2C4, the PLED exhibited a turn-on voltage of 2.75 V, a Lmax of 50400 cd m−2, and a LEmax of 21.4 cd A−1, which showed a lower Vturn‑on (decreased by 1 V) and a remarkable improvement in L max (a 5-fold Table 2. Performance of P-PPV-Based PLEDs with Different Cathode Interlayers cathode interlayer

Vturn‑on [V]

Lmax [cd m−2]

LEmax [cd A−1]

AZO AZO/PBzP2C4

3.75 2.75

7600 50400

1.98 21.4 E

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enhanced electron injection, and improved interfacial compatibility are all attributed to the high-performance devices. In conclusion, we have demonstrated an electrochemically deposited polarized thin layer (PBzP2C4) as the AZO modifier for high-performance inverted organic electronics. Smooth and high surface coverage of PBzP2C4 thin films on AZO was fabricated by optimizing the electrochemical parameters. Also, a highly controllable thickness was achieved. The PBzP2C4 modification reduced the WF of AZO and highly improved the electron extraction/injection in inverted OSCs/PLEDs. As a result, the PCEmax was increased from 8.84% for the AZO-based device to 10.35% for the AZO/PBzP2C4-based device for inverted OSCs based on the PTB7-Th:PC71BM system, mainly because of the highly enhanced Jsc and FF. For PLEDs, LEmax increased dramatically from 1.9 cd A−1 for the device using an AZO interlayer to 21.4 cd A−1 for the device using an AZO/ PBzP2C4 interlayer, based on a classical green-light-emitting layer of P-PPV. These results indicated that electrochemical deposition technology provides a new solution for the preparation of efficient interlayers for organic electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08710. Detailed experimental procedures, chemical structures of PTB7-Th, PC71BM, and P-PPV, FTIR of BzP2C4 and PBzP2C4 thin films, AFM image of AZO, water-contactangle images, and EL spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-20-22237035. Fax: +8620-87110606. *E-mail: [email protected]. Tel: +86-20-22237035. Fax: +8620-87110606. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by National Natural Science Foundation of China (Grants 21334002, 51373054, 51473052, 51573055, and 51521002), the National Basic Research Program of China (973 Program; Grants 2014CB643504 and 2015CB655003), and the Fundamental Research Funds for the Central Universities and Introduced Innovative R&D Team of Guangdong (Grant 201101C0105067115).

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REFERENCES

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DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

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DOI: 10.1021/acsami.6b08710 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX