Low-Temperature All-Solution-Processed Transparent Silver

Dec 12, 2016 - We present a kind of all-solution-processed transparent conductive film comprising of silver nanowire (AgNW), polyvinyl butyral (PVB), ...
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Low-Temperature All-Solution-Processed Transparent Silver Nanowire-Polymer/AZO Nanoparticles Composite Electrodes for Efficient ITO-Free Polymer Solar Cells Xiaoqin Zhang,†,‡ Jiang Wu,† Jiantai Wang,†,‡ Qingqing Yang,†,‡ Baohua Zhang,† and Zhiyuan Xie*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: We present a kind of all-solution-processed transparent conductive film comprising of silver nanowire (AgNW), polyvinyl butyral (PVB), and Aldoped ZnO nanoparticles (AZO NPs) composite (APA) by layer-by-layer bladecoating on glass substrate at low temperature. This kind of transparent APA film exhibits high transmittance at a wide range of 400−700 nm. The sheet resistance of the APA film can be as low as 21 Ω sq−1 with transmittance over 94% at 550 nm. The introduction of PVB significantly improves the APA composite adhesion to glass substrate. The overlaid coating of AZO NPs not only reduces the sheet resistance but also improves the ambient and thermal stability of the APA film. This highly conductive and transparent APA film on glass substrate is employed as the bottom electrode to fabricate high-efficiency polymer solar cells (PSCs). A power conversion efficiency of 8.98% is achieved for the PBDTTT-EFT:PC71BM PSCs employing the APA composite as transparent bottom electrode, close to 9.54% of the control device fabricated on the commercial indium tin oxide substrate. As it can be easily prepared with all-solution-processed blade-coating method at low temperature, this kind of AgNWbased composite film is promising to integrate with roll-to-roll manufacturing of flexible PSCs. KEYWORDS: silver nanowires, composite, transparent electrode, solution processing, adhesion, polymer solar cells

1. INTRODUCTION Polymer solar cells (PSCs) featured with lightweight, low cost, and flexibility possess a wide range of potential application, as they can be integrated in windows, foldable curtains, buildings, clothes, etc. Highly conductive indium tin oxide (ITO) has been widely used as the transparent bottom electrode in PSCs. However, its high cost and mechanical brittleness limit its practical application in PSCs, especially in flexible PSCs.1−3 It is imperative to develop solution-processable ITO-free transparent electrode to accommodate the roll-to-roll manufacturing of PSCs.4−6 Various kinds of transparent conductive materials have been reported such as carbon nanotubes,7−11 graphene,12 conductive polymers,13−15 aluminum-doped zinc oxide (AZO),16 and nanoscale metal materials including nanowires,17−20 ultrathin films, and nanogrids.21−25 Metal nanowires are one of the promising transparent electrode materials due to high flexibility, high conductivity, and solution processability.26−28 Solution-processed silver nanowire (AgNW) films have been readily prepared via various methods such as spin coating,27 drop casting,29 rod coating,30 and air spraying,31 and they have been applied as the transparent electrodes in PSCs.32−36 The sheet resistance of solution-processed AgNW networks is mainly dominated by contact resistance of internanowire junctions. Some effective approaches have been © XXXX American Chemical Society

developed to reduce the junction resistance between AgNWs including thermo,37 electrochemical or nanoplasmonic welding,38−40 high-force pressing,29,41 and nanoscale joule heating.36 However, some methods are not suitable for large-scale AgNW substrate. The stability and adhension of AgNWs to the substrate should also be considered, since the pristine AgNWs are prone to be oxidized,42−44 and the solution-processed AgNW networks on glass substrate can be easily detached due to poor adhension.45 Highly transparent and low-resistance film composed of AgNWs and conductive metal oxides have been successfully developed to serve as transparent electrode for thin-film solar cells.34,46,47 Moon et al. has first reported highly transparent and conducting ZnO/AgNW/ZnO multilayer electrode prepared by magnetron sputtering at room temperature.34 The ZnO/AgNW/ZnO multilayered composite film exhibited high electrical conductivity and excellent thermal stability. To avoid vacuum deposition, Moon et al. have further prepared all-solution-processed transparent AgNW/metal oxide composite via sequential spin coating of conductive metal oxides and AgNW.48 The composite film exhibited high Received: September 21, 2016 Accepted: December 1, 2016

A

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

and final product was dispersed again in ethanol at a concentration of 15 mg mL−1. 2.3. Fabrication and Characterization of AgNW-PVB/AZO Composite Electrode. The initial AgNW dispersion was diluted to 5 mg mL−1 with ethanol. Then the PVB solution in ethanol (50 mg mL−1) was added to AgNW dispersion with a volume ratio of 1:7 to form the AgNW ink. The AgNW ink was deposited onto the precleaned glass substrate via doctor-blade coating at 35 mm s−1 and baked for 5 min at 50 °C to remove the solvent. The AZO NPs ink was deposited onto the AgNW-coated substrate at 10 mm s−1 and baked for 5 min at 50 °C. One to four layers of AZO NPs were coated on the AgNW layer. Sheet resistance values of various kinds of conductive films were measured with a KDY-1 instrument. Transmittance of the various films was measured using a SolidSpec-3700 UV−vis−NIR spectro-photometer equipped with an integrated sphere. The atomic force microscopy (AFM) images were obtained using a SPI3800N AFM instrument (Seiko Instrument Inc.) in tapping mode under ambient condition. Scanning electron microscope (SEM) images were obtained on Philips XL-30 ESEM FEG SEM FEI. 2.4. Fabrication of PSCs Employing APA Composite as Transparent Electrode. The PSCs were fabricated on the APA composite film-coated glass substrate. The control device on the ITO glass substrate with a sheet resistance of 10 Ω sq−1 was also fabricated for comparison. ITO glass substrate was subject to routine cleaning process and the UV-ozone treatment for 25 min before use. For the inverted PSCs, a 25 nm thick ZnO layer was first spin-coated on the ITO or APA substrate as the cathode buffer layer. The 80 nm thick PCDTBT:PC71BM (1:4 in weight) active layer was subsequently deposited via spin-coating from a 1,2-dichlorobenzene solution consisting of 3.5 mg mL−1 PCDTBT and 14 mg mL−1 PC71BM at 600 rpm for 3 min. The devices were completed by thermally depositing a MoO3 (12 nm)/Al (100 nm) anode under a base pressure below 4 × 10−4 Pa. For the conventional PSCs, a 40 nm thick PEDOT:PSS layer was spin coated on the APA or ITO glass substrates and then was annealed at 140 °C for 1 h. Then the PBDTTTEFT:PC71BM active layer was spin-coated on the PEDOT:PSS layer from its chlorobenzene (CB) solution containing 3% 1,8-diiodoctane in volume at 900 rpm for 90 s. Methanol was used to spin-rinse the active layer at 4000 rpm for 30 s. A 25 nm thick ZnO layer was then spin-coated on the active layer as the cathode buffer layer, and the Al cathode (100 nm) was thermally deposited in a vacuum chamber at 4 × 10−4 Pa. The active area for each cell defined by overlap of the top Al and the bottom electrodes was 12.56 mm2. A Newport solar simulator was used to provide 100 mW cm−2 AM1.5G simulated solar light, and the light intensity was determined by a calibrated silicon diode with KG-5 visible color filter. Current density−voltage (J−V) curves were recorded with a Keithley 2400 source meter. The external quantum efficiency (EQE) measurement was performed on Enlitech QE-R spectral response measurement system. The photocurrent mapping images of the devices were obtained using an MP 15 Mapping System for Photovoltaic Cell (Lasertec) under ambient condition.

transmittance of 93.4% at 550 nm with low sheet resistance of 11.3 Ω sq−1. However, the newly developed combustion-based sol−gel chemical approach must be annealed at 200 °C to allow dense and uniform composite electrode. AgNW composite with sol−gel processed SnOx has been prepared at room temperature by Zilberberg et al. and has been used as the transparent top electrode in PSCs.1 PSCs, especially flexible PSCs, require transparent conductive bottom electrode that allows large-scale solution-processing at relatively low annealing temperature and has good adhension to the substrate. Inspired by the above achievements, we present a kind of allsolution-processed transparent conductive film comprising of AgNW, polymer, and AZO nanoparticles (APA) composite prepared with blade-coating method at low annealing temperature and successfully employ it as the transparent bottom electrode to fabricate PSCs. This kind of transparent APA film exhibits high transmittance at a wide range of visible region. The sheet resistance of the APA film can be as low as 21 Ω sq−1 with transmittance over 94% at 550 nm. Introduction of polymer PVB in AgNW ink improves the adhesion of the resulting APA composite film to glass substrate. The overlaid AZO nanoparticles (NPs) on AgNW networks not only significantly reduce the sheet resistance by pushing close contact of AgNW networks but also improve the ambient and thermal stability of the resulting APA film. This highly conductive and transparent APA film on glass substrate is employed as the bottom electrode to fabricate high-efficiency PSCs. A PCE of 8.98% has been achieved for the PBDTTTEFT:PC71BM PSCs employing the APA composite electrode, close to 9.54% of the control device fabricated on the commercial ITO substrate. As its facile preparation with allsolution-processed blade-coating method at low temperature, this kind of AgNW-based composite film is compatible with roll-to-roll manufacturing of flexible PSCs.

2. EXPERIMENTAL SECTION 2.1. Materials. AgNWs were obtained from Blue Nano Inc. (the initial concentration is 10 mg mL−1, the diameter is ca. 35 nm, the length is longer than 20 μm). Poly[N-9″-heptadecanyl-2,7-carbazolealt-5,5-(4′,7′-dithienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with a weight-average molecular weight of 28 000 and a polydispersity index (PDI) of 1.8 was synthesized in our laboratory. Poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2− 6-diyl] (PBDTTT-EFT) was purchased from 1-Materials Inc. [6,6]Phenyl C71-butyric acid methyl ester (PC71BM, >99%) was purchased from American Dye Source Inc. The aqueous solution of PEDOT:PSS (Clevios P VP AI 4083) was purchased from Heraeus and filtered with a 0.45 mm polytetrafluoroethylene filter before use. ZnO NPs with a diameter of 5 nm were prepared according to the synthesis route reported by Beek et al.49 Polyvinyl butyral (PVB) of aircraft grade was purchased from Sinopharm Chemical Reagent Co. Ltd. Zn(CH3COO)2·H2O was used as received. Al(NO3)3·9H2O was of 99.99% metal basis and was obtained from Aladdin. 2.2. Preparation of Al-Doped ZnO Nanoparticles. Al-doped ZnO NPs with diameters of 4−8 nm were prepared according to the method reported by Zhang et al.50 The metal salt Zn(CH3COO)2· H2O was dissolved in 100 mL of ethanol with a concentration of 0.01 M, and then 0.0375 g of Al(NO3)3·9H2O was added into the solution. The solution was stirred at 80 °C for 30 min. Tetramethylammonium hydroxide was added into the solution, and pH value was adjusted to ∼10. The mixture was stirred at room temperature for 1 h. The mixture was exposed to high-intensity ultrasonic irradiation (6 mm diameter Ti-horn, 300 W, 20 kHz) at room temperature under ambient condition for 45 min. At last, the supernatant was removed,

3. RESULTS AND DISCUSSION Scheme 1 illustrates the layer-by-layer doctor-blade coating process to prepare the APA composite films. AgNW ink was involved with a small amount of PVB to improve its solution processability and the adhesion of the APA to the substrate. The solution-processable AZO NPs solution was used to cover the underlying AgNW networks due to its high conductivity.51,52 Figure 1 shows AFM topographic and cross-section SEM images of the pristine AgNW, AgNW-PVB, and APA composite (three-layer AZO) films prepared with doctor-blade coating on glass substrates, respectively. As shown in Figure 1a,d, it can be seen that the pristine AgNWs coated with doctor-blading are loosely stacked on the glass substrate to form randomly interconnected networks. This would not only lead to poor adhesion of AgNWs to the glass substrate but also B

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2 shows the transmittance spectra of the pristine AgNW, AgNW-PVB, and APA films covered with different

Scheme 1. All-Solution-Processed Procedure of the Transparent and Conductive APA Composite Film on Glass Substrate Prepared via Multilayer Doctor-Blade Coating

Figure 2. Transmittance spectra of various Ag NWs-based films as well as the commercial ITO glass substrate.

a large series resistance due to loose contact among AgNWs. With incorporating polymer PVB into AgNW ink to fabricate the AgNW film, the AgNW networks adhered closely to the glass substrate, and the stacking among individual AgNWs is also improved as shown in Figure 1b,e. As discussed later, the improved contact among individual AgNWs reduces dramatically the junction resistance. However, the AgNW-PVB film is still very rough and not suitable to serve as transparent bottom electrode. Figure 1c,f shows the AFM image and cross-section SEM image of the AgNW-PVB film covered with three-layer AZO prepared with blade coating. It can be seen that the bladecoated AZO NPs are well filled into the spaces among AgNWs and that the resulted APA film is very smooth with roughness down to ca. 5 nm. More details about the morphology changes of this kind of APA composite films covered with different number of AZO layers are shown in Figures S1 and S2. More AZO layers result in more smooth film, and the AgNW networks are fully embedded within the AZO matrix. The APA composite films are highly transparent, and the photographs of patterned APA composite on glass substrate are shown in Figure S3.

number of AZO layers as well as commercial ITO on glass. Introduction of PVB in AgNW ink does not affect the transmittance of the resulted AgNW film in whole visible region. With increasing the number of AZO layers, the transmittance below 450 nm is decreased. The APA composite films exhibit high transmittance of 85−95% at a range of 400− 700 nm. The exact values of transmittance of various films at 550 nm are given in Table 1. The sheet resistance values (RS) of the various AgNW-based films were measured, and the results were also shown in Table 1. The pristine AgNW film demonstrates higher RS than 800 Ω sq−1 due to loosely stacking and high contact resistance between adjacent AgNWs, which is consistent with the reported results.26,47 With incorporating small amount of PVB in AgNW ink, the RS of the resulted AgNW-PVB film is reduced to ∼87 Ω sq−1. Small amount of insulating PVB does not increase the RS of the AgNW-PVB film, since it mainly lies between the AgNW networks and the glass substrate. Furthermore, PVB is favorable to improve close stacking of AgNWs due to its entanglement with AgNWs when

Figure 1. AFM topographic and cross-section SEM images of the various films prepared with doctor-blade coating on glass substrate. (a, d) Pristine AgNW, (b, e) AgNW-PVB, and (c, f) APA composite (three-layer AZO) films. (insets) The corresponding SEM images in plain view. C

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Electrical and Optical Properties of the Pristine AgNW Film and the APA Composite Films Covered with Different Layers of AZO Nanoparticles electrode pristine AgNW AP (without AZO) APA (onelayer AZO) APA (twolayer AZO) APA (threelayer AZO) APA (fourlayer AZO)

sheet resistancea (Ω·□−1)

transmittance (%) (@ 550 nm)

figure of merit (@550 nm)

840.49

95.75

10.22

86.90

96.12

72.28

34.27

95.37

229.32

25.50

94.68

266.76

21.09

94.77

328.33

20.08

93.33

267.32

Figure 3. Rs changes of the APA composite and AgNW/AZO films against taping cycles during the tape test using 3 M Scotch tape.

The ambient and thermal stability of the APA composite film were further evaluated. Figure 4a illustrates the sheet resistance changes of the pristine AgNW and APA (three-layer AZO) films on glass substrates stored in ambient condition within 15 d. The sheet resistance of the pristine AgNW film is drastically increased over 10 times within 5 d possibly due to oxidation. It has been reported that the AgNWs are prone to oxidize when exposed under ambient condition.42,53 Figure S5 shows the morphology changes of the pristine AgNW film after being exposed in air for 60 d. It can be seen that the surface of AgNWs has been severely oxidized after exposure. Nevertheless, the RS of the APA composite film increases less than 60% even after 15 d, since the AgNWs are well-encapsulated by the AZO NPs. Figure 4b illustrates the sheet resistance changes of the pristine AgNW and APA (three-layer AZO) films on glass substrates after post annealing at different temperatures for 1 h. For the pristine AgNW/glass substrate, a gradual decrease of sheet resistance was observed up to 170 °C. This is attributed to heat-induced welding of the cross-linked AgNWs that reduces the junction resistance. With further increasing the annealing temperature to 200 °C, a clear rise in sheet resistance was observed. However, the APA composite/glass substrate shows superior thermal stability, and its sheet resistance is kept at ca. 22 Ω sq−1 with annealing temperature up to 250 °C. It has been reported that the thermal stability of AgNWs can be greatly improved when it is sandwiched between the sputteringdeposited ZnO layers.34 Herein, the enhanced thermal stability of APA film may result from the overlaid AZO NPs. The SEM images shown in Figure 4c,d indicate that the pristine AgNWs are broken at high annealing temperature, while the AgNWs in APA composite film do not show distinct changes. These results indicate that the APA composite film possesses good ambient and thermal stability. The APA composite film was also deposited on PET substrate to test its flexibility. Sheet resistance changes of the APA/PET substrate and the commercial ITO/PET substrate against the bending radius are plotted in Figure S6. With decrease of the bending radius, the resistance of the ITO/PET substrate is dramatically increased. However, the resistance of the APA/PET substrate is very stable when the bending radius ranges from 22.7 to 1.0 mm, confirming its higher flexibility than ITO. The inverted PCDTBT:PC71BM PSCs are fabricated on the APA composite/glass susbstrate with a structure of APA cathode/ZnO/PCDTBT:PC71BM/MoO3/Al anode. The control devices based on the ITO/glass and pristine AgNW/glass

The values are the average of five-time measurements for each sample. a

PVB is solidified on the glass substrate. The RS of APA composite film is further decreased to as low as 21.09 Ω sq−1. This is mainly originated from a firm encapsulation of AgNW networks by the overlaid AZO NPs that press more tightly the underlying AgNWs by the electrostatic force as well as capillary force during solvent evaporation. In other words, the lateral conductivity of the APA film is determined by the AgNW networks embedded within the APA composite film. Both the incorporated PVB in AgNW solution and the overlaid AZO NPs favor to improve the stacking of AgNW networks and thus the lateral conductivity of the APA film. The improved lateral conductivity results in the remarkably decreased RS of the APA film. The quality of the various AgNW-based films are evaluated via the following Tinkham formula:5 −2 ⎛ Z σop ⎞ T = ⎜1 + 0 ⎟ 2R s σdc ⎠ ⎝

Where Z0 = 377 Ω is the impedance of free space, σop and σdc are the optical and direct-current conductivities of the film, respectively. The σdcσop−1 value is often used as a figure of merit (FOM) to compare intuitively the optoelectrical properties of transparent conductive film, and high values mean high quality. The calculated FOM values of the various AgNW-based films are summarized in Table 1. A high FOM of 328 can be achieved for the APA composite film, much higher than those of AgNW and AgNW-PVB films. The adhesion of the APA composite film to the glass substrate was tested by using 3 M scotch tape as shown in Figure S4a. It is noted that the pristine AgNW prepared on glass substrate can be easily erased with finger and that the APA composite film remains intact after 500 times of tape peel-off test as illustrated in Figure S4b. Figure 3 shows the RS changes of the AgNW/AZO and APA composite films as a function of taping cycles. The RS of the APA composite film is changed less than 3% after 500 times of tape test. However, RS of the AgNW/AZO film is increased ∼4 times after 250 cycles of test. This result indicates that the introduced PVB in AgNW ink improves the mechanical adhension of the resultant APA film to glass substrate. D

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Rs changes of the pristine AgNW and APA composite films stored in ambient condition for 15 d. (b) Rs changes of the pristine AgNW and APA composite films on glass substrates annealed at different temperatures. (c, d) The SEM images of the pristine AgNW and APA composite films annealed at 250 °C for 1 h, respectively.

substrate are also fabricated for comparison. The chemical structures of PCDTBT and PC71BM are shown in Figure S7, and the device architecture together with the corresponding energy level diagram is presented in Figure S8. The inverted PCDTBT:PC71BM PSCs fabricated on the various APA composite/glass substrates are first evaluated, and their illuminated J−V characteristics together with their EQE curves are shown in Figure S9. Their photovoltaic parameters are summarized in Table S1. Among these devices, the device based on the APA composite film with three AZO layers demonstrates the best photovoltaic performance. For comparison, the illuminated J−V characteristics of the devices employing the ITO, pristine AgNW, and the APA composite (three-layer AZO) are plotted in Figure 5a, and the corresponding photovoltaic parameters are summarized in Table 2. The control device fabricated on the ITO/glass substrate demonstrates a VOC of 0.88 V, a JSC of 10.17 mA cm−2, and a fill factor (FF) of 63.63%, leading to an overall PCE of 5.69%. The device fabricated on the pristine AgNW networks/glass substrate shows merely a VOC of 0.55 V, a JSC of 6.15 mA cm−2, and an FF of 26.91%, giving a PCE of 0.91%. This is possibly resulted from the high sheet resistance of loosely stacked AgNW networks and large leakage current due to the rough surface of pristine AgNW electrode. When the optimized APA composite/glass substrate is used, the resultant device demonstrates a VOC of 0.89 V, a JSC of 9.97 mA cm−2, and an FF of 61.79%, leading to an overall PCE of 5.48%, which is almost comparable to the control device fabricated on the ITO/glass substrate. The EQE spectra of the devices fabricated on the ITO and APA substrate are shown in Figure 5b. Both of the two devices show high EQE over 60% at a range of 400− 600 nm. The calculated JSC values from the EQE are wellconsistent with the measured JSC values as shown in Table 2. As the highly conductive AgNWs are randomly dispersed on the glass substrate, it is not sure whether this kind of transparent APA electrode can achieve uniform photovoltaic performance. The in situ photocurrent mapping experiment was performed to evaluate the lateral conduction ability of the transparent APA film and its charge-collection capability in

Figure 5. (a) The illuminated J−V characteristics of the inverted PCDTBT:PC71BM PSCs employing the pristine AgNW, ITO, and APA composite as the cathode, respectively. (b) EQE curves of the corresponding devices.

PSCs. Figure 6 shows the photocurrent distribution images of the inverted PCDTBT:PC71BM PSCs employing the pristine AgNW, APA composite, and ITO as the transparent cathodes, respectively. As shown in Figure 6c, the control device fabricated on the ITO/glass substrate shows more uniform E

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Table 2. Photovoltaic Parameters of the Inverted PCDTBT:PC71BM PSCs Employing the Pristine AgNW, APA Composite, or ITO as the Transparent Cathode on the Glass Substrate

a

electrode

VOC (V)

JSC (mA/cm2)

FF (%)

PCEa (%)

RS (Ω cm2)

RSH (Ω cm2)

Cal JSCb (mA/cm2)

pristine AgNW APA ITO

0.56 ± 0.05 0.88 ± 0.01 0.88 ± 0.01

5.96 ± 0.30 9.89 ± 0.33 9.95 ± 0.25

27.05 ± 2.64 61.29 ± 1.59 64.09 ± 1.32

0.89 ± 0.31 5.40 ± 0.14 5.62 ± 0.27

79.82 10.03 8.56

104.99 513.11 992.60

6.09 9.75 9.79

The values are the average of 10 devices. bThe JSC values are calculated from EQE spectra.

Figure 6. Photocurrent images of the PCDTBT:PC71BM PSCs employing different transparent electrodes: (a) pristine AgNW, (b) APA composite, and (c) ITO.

photocurrent density within the active area. The cell can be considered as parallel connection of infinite tiny cells. Since the ITO is a conductive film with more uniform conductivity, the same characteristics of these tiny cells in parallel connection will lead to a uniform photocurrent distribution. However, in the case of the device fabricated on the pristine AgNW/glass substrate, the device shows nonuniform photocurrent density within the active area (Figure 6a) due to different photovoltaic characteristics of the comprised tiny cells. By using the APA composite to replace the pristine AgNW as transparent cathode, the photocurrent uniformity of the device is greatly improved as illustrated in Figure 6b. This indicates that the overlaid AZO on the AgNW networks not only improves the film conductivity by pushing compact stacking of AgNWs but also enhances the uniformity of conductivity by filling the space between AgNWs with AZO NPs. The overlaid AZO on the AgNWs also favors to suppress leakage current and improve charge collection. The prepared transparent APA composite film on glass substrate is also used as the anode to fabricate conventional PSCs with PBDTTT-EFT:PC71BM as the active layer. The chemical structure of PBDTTT-EFT and the device configuration are shown in Figure S7 and Figure S10, respectively. The illuminated J−V characteristics and EQE spectra of the conventional PSCs employing the APA composite and ITO as transparent anode are shown in Figure 7. The photovoltaic parameters of the corresponding devices are summarized in Table S2. As shown in Figure 7a, the control device using ITO anode exhibits a VOC of 0.78 V, a JSC of 19.46 mA cm−2, and an FF of 62.74%, leading to a PCE of 9.54%. The PBDTTTEFT:PC71BM device using transparent APA composite anode demonstrates a VOC of 0.77 V, a JSC of 18.87 mA cm−2, and an FF of 61.83%, giving an overall PCE of 8.98%. The EQE curves in Figure 7b show that both of the two devices exhibit high quantum efficiencies. The device with transparent APA composite anode shows a flat EQE curve of ∼70% at a range of 350−750 nm, while the control device employing ITO anode shows a little higher quantum efficiencies but with different profiles. This may be originated from different transmittance profiles between ITO and APA composite film. The devices employing this kind of low-temperature solution-

Figure 7. (a) The illuminated J−V characteristics of the conventional PBDTTT-EFT:PC71BM PSCs employing the APA composite and ITO as transparent electrode. (b) The EQE curves of the corresponding PSCs.

processed transparent APA electrode show comparable photovoltaic performance to the ITO-based devices.

4. CONCLUSIONS In summary, a kind of all-solution-processed transparent and conductive AgNW-based composite film has been prepared via layer-by-layer doctor-blade coating on glass substrate at low F

DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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temperature and has been successfully applied as the transparent bottom electrode to fabricate high-efficiency polymer solar cells. This kind of AgNW-based composite film exhibits high transmittance at visible region and low sheet resistance. Polymer PVB is deliberately incorporated to improve the adhesion of the AgNW composite film to glass substrate. The overlaid coating of AZO NPs on AgNW networks not only significantly reduces the sheet resistance but also improves the ambient and thermal stability of the resulted AgNW-based film. This highly conductive and transparent AgNW-based composite film on glass substrate was employed as the bottom electrode to fabricate polymer solar cells, and a power conversion efficiency of 8.98% was achieved for the PBDTTT-EFT:PC71BM PSCs, which is close to 9.54% of the control device fabricated on the ITO/glass substrate. As its facile preparation with all-solution-processed blade-coating method at low temperature, this kind of AgNW-based composite film is compatible with roll-to-roll manufacturing of flexible PSCs.

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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.6b11978. AFM topographic images of various AgNW-based transparent conductive films, cross-section SEM images of various AgNW-based films, photographs of the patterned AgNW-based transparent electrodes on glass substrates, the photograph of tape test using 3 M Scotch tape, photographs of pristine AgNW destroyed by finger friction and APA composite electrode after 500 cycles of tape test, SEM images of pristine AgNWs before and after exposure for two months under ambient conditions, Rs changes of the APA/PET and the ITO/PET substrates during the bending test, chemical structures of PCDTBT, PBDTTT-EFT, and PC71BM, device architecture and energy level diagram of the PCDTBT:PC71BM PSCs, J−V characteristics and EQE of inverted PSCs fabricated on various APA composite films, device architecture and energy level diagram of the conventional PBDTTT-EFT:PC71BM PSCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 431 85262819. E-mail: [email protected]. ORCID

Zhiyuan Xie: 0000-0001-6081-2033 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2014CB643504), the National Natural Science Foundation of China (51325303, 51273193, 21334006), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200).

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REFERENCES

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DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b11978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX