Modification of the Highly Conductive PEDOT:PSS Layer for Use in

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Modification of the highly conductive PEDOT:PSS layer for use in silver nanogrid electrodes for flexible inverted polymer solar cells Jie Wang, Fei Fei, Qun Luo, Shuhong Nie, Na Wu, Xiaolian Chen, Wenming Su, Yuanjie Li, and Chang-Qi Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16341 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

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ACS Applied Materials & Interfaces

Modification of the highly conductive PEDOT:PSS layer for use in silver nanogrid electrodes for flexible inverted polymer solar cells Jie Wang,†,‡ Fei Fei, ‡ Qun Luo, *,‡ Shuhong Nie, ‡ Na Wu, ‡ Xiaolian Chen, ‡ Wenming Su,*, ‡ Yuanjie Li, *,† Chang-Qi Ma,‡ †

School of Electronic and Information Engineering, Xi’an Jiaotong University ‡

Printable Electronics Research Center, Suzhou Institute of Nano-Tech and

Nano-Bionics, Chinese Academy of Sciences (CAS), Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou, 215123, P. R. China. Email: [email protected]; [email protected]; [email protected] Tel: +86-512-6287-2730, Fax: +86-512-6260-3079 KEYWORDS: Light soaking; Interfacial modification; PEDOT:PSS/ZnO; Flexible inverted organic solar cells; Work function regulation

ACS Paragon Plus Environment


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ABSTRACT: Silver nano-grid based flexible transparent electrode is recognized as the most promising alternative to ITO electrode for organic electronics, owing to its low production cost and excellent flexibility. Typically, high conductive thin film coating layer, such as high conductive PEDOT:PSS (HC-PEDOT:PSS) is usually deposited onto the Ag-grid electrode to smooth the surface and to minimize the sheet resistance. In this paper, we found

that

inverted

flexible

polymer

solar

cells

with

structure

of

Ag-grid/HC-PEDOT:PSS/ZnO/photoactive layer/MoO3/Al generally exhibits strong S-shaped J-V curves, which could be eliminated by light-soaking treatment. Kelvin probe force microscope (KPFM) measurement proved that a large work function (WF) difference (0.70 eV) between HC-PEDOT:PSS and ZnO is the main reason for the formation of S-shape. White light soaking on the Ag-grid/HC-PEDOT:PSS gradually decreased the WF of HC-PEDOT:PSS from 5.10 eV to 4.60 eV, leading to a reduced WF difference between HC-PEDOT:PSS and ZnO from 0.70 to 0.38 eV. Such a WF difference decreasing was believed to be the working mechanism for the light soaking effect in this flexible device. Based on this finding, the HC-PEDOT:PSS solution was then modified by doping with polyethylenimine (PEI) and ammonia aqueous. The modified PEDOT:PSS film is characteristic of adjustable work function through varying PEI doping concentrations. By using such a modified PEDOT:PSS layer, “light soaking”-free flexible inverted polymer solar cell with a power conversion efficiency of 6.58% was achieved for PTB7-Th:PC71BM cells. The current work provides a useful guideline for interfacial modification for Ag-grid based flexible electrode.


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1. INTRODUCTION: Polymer solar cells are considered as promising alternatives to low-cost solar cell technology due to their advantages of low-temperature-solution processing, light weight and flexibility.

1-3

The organic solar cell has presented great potential in space

application 4, and wearable electronics 5. Recently, ITO-free flexible electrodes based on oxide-

metal-

nanotubes,10-12

oxide

multi-layers

graphene,13-15

configuration,6-7

highly

Ag

conductive

nanowires,8-9

carbon

poly(3,4-ethylenedioxy

thiophene):poly(styrenesulfonate) (HC-PEDOT:PSS),16-17 and metal grids

2-3, 18-24

have

been reported for use in polymer solar cells, among which, the Ag-grid electrode is the most potential electrode for large-area flexible device due to its excellent conductivity, high transparence, and highly compatibility with roll-to-roll printing process.18, 25-26 Recently, fully-printed flexible Ag-grid transparent electrode was developed for use in organic electronics, where silver grids are imbedded in PET substrates in the form of a hexagonal pattern.27 Note that the width of the Ag-lines is only 2-3 micrometers, and the distance between silver lines is more than 100 µm, these Ag grids are invisible to naked eyes. The Ag-grid electrodes are prepared by filling the hexagonal patterns on PET substrate with silver ink, leaving grooves with height over 100 nm after the solidification of the silver lines.2-3 For organic electronic devices, the functional layer thicknesses are typically less than 100 nm, the grooves in some cases would limit the device performance. To solve this problem, a thick layer of highly conductive PEDOT:PSS (HC-PEDOT:PSS, e.g. PH1000) is usually coated onto the Ag-grid electrodes to smooth the surface and to minimize the sheet resistance, and finally to achieve high performance.2-3 The cooperation of HC-PEDOT:PSS onto Ag-grid, on the other hand,



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introduces new interfaces between HC-PEDOT:PSS and the photoactive layer (or the electrode buffer layer) in the inverted polymer solar cells. Although solar cells using this Ag-grid/HC-PEDOT:PSS electrode have been reported in the literatures,2-3 the influence of the HC-PEDOT:PSS layer on the interfacial electronical properties and the consequent polymer solar cell performance was not reported. On the side of organic solar cells with inverted structure, a well-know “light soaking” effect that related with low electron extraction efficiency has been widely reported in the n-type metal oxides electron employed devices

28

Through using the chemical doped

metal oxides, e. g. ZnO:Al, 28-29 TiO2:F, 30 TiOx:N, 31 or surface UV irradiation and polar solvent treatment, 32 this problem can be improved or solved. Furthermore, A. Sundqvist et al. proposed that the low built-in potential at ITO/metal oxide interface was the underlying reason of electron extraction barrier. 33 Nevertheless, almost these references reporting the “light soaking” focused on the rigid inverted organic solar cells with ITO as bottom electrode. No systematical investigation is carried out on the “light soaking” effect of the ITO-free flexible organic solar cells. Herein, we will use this Ag-grid/HC-PEDOT:PSS composite flexible electrode as cathode in inverted polymer solar cells, and systematically study the influence of the HC-PEDOT:PSS layer on the interfacial properties as well as the device performance. Typical “S-shape” kinked J-V curves are measured in the Ag-grid/HC-PEDOT:PSS based inverted polymer solar cells, which was ascribed to the mismatched work functions (WF) between HC-PEDOT:PSS and ZnO layers. Light induced WF difference reduction was found in the HC-PEDOT:PSS/ZnO interface, which lead to the


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elimination of S-shaped J-V curves under light illumination. By successful tuning the WF of HC-PEDOT:PSS film through chemical doping with ammonia: PEI (polyethylenimine) composite, the modified Ag-grid/PEDOT:PSS electrode can be used in polymer solar cells directly without undesired S-shaped J-V curves. The current work provides a helpful guideline for the interfacial modification on the metal-grid based flexible electrodes.

2. EXPERIMETNAL SECTION: 2.1 Materials: High conductive poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PH1000 and FCE) and PEDOT:PSS Clevios PVP AI 4083 were purchased from Heraeus Precious Meatals GmbH & Co. KG. Regioregular poly(3-hexylthiophene), phenyl-C61-butyric

acid

methyl

ester

(PC61BM),

and

poly[(ethylhexyl-thiophenyl]-benzodithiophene-(ethylhexy)-thienothiophene] (PTB7-Th) were purchased from Solarmer Energy, Inc. (Beijing). Branched polyethylenimine (PEI, Mn = 2.5×104 g/mol) was purchased from Sigma-Aldrich. Ammonia (40% in deionized water) was purchased from China National Pharmaceutical Group Corporation. 2.2 The preparation of ZnO inks: ZnO colloidal particles were synthesized according to the route as reported by Beek et al.34 The ZnO colloidal particles were precipitated 3 times in methanol and finally dispersed in acetone through ultrasonic dispersion. 2.3 The preparation of FCE: Ammonia: PEI composite inks: Branched PEI was dissolved in deionized water through continuous stirring overnight at room temperature. Then, the FCE: Ammonia: PEI composite inks were obtained through mixing FCE,



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ammonia, and PEI solution together with volume ratio of 4:1:1. 2.4 Characterization of the films: The UV-vis absorption spectra were recorded by the Lamada 750 UV/vis/NIR spectrophotometer (PerkinElmer). The film thickness was measured using the AlphaStep profilometer (Veeo, Dektak 150). The work function of the composite films was record by the Kelvin probe force microscopy (KPFM, Dimension 3100) in air using a highly ordered pyrolytic graphite (HOPG) as reference. The conductivity of FCE: Ammonia: PEI composite film (ρ) was calculated using the following equation: ρ = 1/(Rs·d), where Rs was the sheet resistance that recorded by four-point probe, and d was the film thickness. 2.5 Polymer solar cell fabrication and characterization: Organic solar cells were fabricated with indium tin oxide (ITO) coated glasses or PET/Ag-grid as bottom electrode. First, patterned ITO substrates were subsequently ultrasonic cleaned in acetone, deionized water, and isopropanol, and followed by O2 plasma treatment. The flexible substrate was treated with O2 plasma for 3 min at 100 W without washing. FCE or FCE:Ammonia:PEI composite films were spin coated onto the ITO- or Ag-grid-based cathode and followed by annealing in N2 glove box at 120 ºC for 10 min. For the FCE/PEI stack layer, the PEI layer was deposited through spin coating the PEI solution (solved in methoxyethanol with weight concentration of 0.4 wt%) at 5000 rpm for 1 min, and then annealed at 120 oC for 10 min. Then 30 nm ZnO layer was deposited through spin coating at 2000 rpm for 1 min. For P3HT:PC61BM heterojunction solar cells, the blend solution composed of P3HT and PC61BM (1:1,

w/w dissolved in

1,2-dichlorobenzene) were spin coated at 600 rpm for 60 s. Subsequently, the active


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layers were sequentially solvent annealed and thermal annealed for 1.5 h and 10 min, respectively. For the PTB7-Th:PC71BM device, the active layer was obtained through spin-coating the PTB7-Th:PC71BM blend solution (1:1.5, w/w dissolved in chlorobenzene with polymer concentration of 7 mg/mL, and with 3% volume ratio of DIO as additive) at 1300 rpm for 1 min. Finally, a 10 nm MoO3 and 100 nm Al electrode were evaporated at pressure about 8×10-5 Pa to complete the device fabrication. The current density-voltage (J-V) characteristics was recorded by Keithley 2400 source meter in nitrogen glove box. The simulated solar illumination was provided by white light from halogentungsten lamp after filtering.35 External quantum efficiencies (EQE) were carried out using a homemade IPCE testing system that containing a 150 W tungsten halogen lamp (Osram 64642) probe light, a monochromator (Zolix, Omni-λ300), an optical chopper, and an I-V converter.

3. RESULTS AND DISSCUSSION: 3.1 Characterization of Ag-grid/HC-PEDOT:PSS electrodes As mentioned above, a layer of highly conductive PEDOT:PSS (HC-PEDOT:PSS) is usually coated onto the Ag-grid electrode to smooth the surface and reduce sheet resistance. In this work, two different types of HC-PEDOT:PSS, named PH1000 and FCE 36 were chosen for this purpose, and the conductivity and transparency were firstly investigated. Figure 1a depicts the transmittance spectra of PH1000 and FCE with a layer thickness of 155 nm. As can be seen here, the PH1000 film shows low transmittance of 75-90% over 400-1100 nm, whereas the FCE shows a high



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transmittance around 95% over 400-1100 nm. In the meanwhile, the FCE films show much higher conductivity than PH1000 (Figure 1b). Knowing that the grooves on Ag-grid electrode can be as high as 100 nm, FCE should be better for use in Ag-grid electrode. Therefore, we will focus our study on the influence of FCE on polymer solar cell performance in this manuscript.

Figure 1. (a) Transmittance spectra of PH1000 (square) and FCE (cycle) films with a layer thickness of 155 nm; (b) Conductivity (solid symbols and dash lines) and transmittance (at 550 nm, empty symbols and lines) of PH1000 (square) and FCE (cycle) films on ITO glass.

3.2 Photovoltaic performance of Ag-grid/HC-PEDOT:PSS based polymer solar cells Inverted

polymer

solar

cells

with

device

architecture

of

PET/Ag-grid/HC-PEDOT:PSS/ZnO/P3HT:PC61BM/MoO3/Al were fabricated, where Ag-grid/HC-PEDOT:PSS served as the cathode, ZnO and MoO3 layers were used as the cathode and anode buffer layers (see Figure 2 for the device structure). Figure 3a and b depicts the J-V curves of the Ag-grid/PH 1000 and Ag-grid/FCE based solar cells under


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light illumination, respectively. The corresponding performance parameters were listed in Table 1, and all the statistical data of VOC, JSC, FF and PCE could be seen in Table S1. As can be seen here, for both PH1000 and FCE electrode based devices, typical S-shaped J-V curves were measured, which led to a poor power conversion efficiency (PCE) of 1.78% with a VOC of 0.61 V, JSC of 8.11 mA/cm2, and FF of 36% for the PH1000 based device, and a PCE of 1.65% with a VOC of 0.58 V, JSC of 8.63 mA/cm2, and FF of 33% for the FCE based device, respectively (Table 1). The low device performance was mainly due to the low FF, which can be clearly seen from the S-shaped J-V curves. Interestingly, such undesired kinked J-V curves can be improved by white light soaking (Figure 3), correspondingly lead to device performance enhancement. Reasonable PCE of 2.47% with a VOC of 0.61 V, JSC of 8.44 mA/cm2, and FF of 48% for the PH1000 based device, and a PCE of 2.74% with a VOC of 0.60 V, JSC of 8.61 mA/cm2, and FF of 53% for the FCE based device were obtained after white light illumination for 15 min. Such a device performance improvement was mainly due to the improvement of FF during light illumination (Figure 4). Besides, we also took interesting into the light-soaking reversibility (Figure S1). The normal devices after light illumination have been stored in N2-golve box or in air in the dark for different time. From Figure S1, one can see that normal J-V curve was measured if the device was stored in dark inside N2-filled glovebox, while S-shaped J-V appeared again if the device was stored in ambient.



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Figure 2. (a) The molecular structure of HC-PEDOT:PSS (FCE, PH1000), PEI, P3HT, PTB7-Th, PC61BM and PC71BM. (b) The device structure of the flexible device.

Figure 3. J-V curves evolution of (a) Ag-grid/PH1000, and (b) Ag-grid/FCE based flexible P3HT:PC61BM cells after light illumination for different time.

Figure 4. Evolution of the photovoltaic parameters of the flexible polymer solar cells during light illumination for different time.


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Similar dynamic change of photovoltaic performance parameters was also reported in various ITO based inverted polymer solar cells.28,

31

And this was ascribed to the

increase in conductivity of metal oxides by the release of oxygen from the metal oxide nanoparticle through light illumination,37-39 or due to incomplete interfacial recombination of charge accumulation.40-42 As shown in Table 1 and Figure S1 (supporting information), the ITO based inverted solar cells with a structure of Glass/ITO/ZnO/P3HT:PC61BM/MoO3/Al fabricated in our lab did not show any S-shaped J-V curves or light soaking effect (Table 1, Entry 5 and 6, and Figure S2 in the supporting information). It indicates that the S-shaped J-V curves of the flexible solar cells cannot be ascribed to the ZnO layer or the ZnO/P3HT:PC61BM interface. On the other hand, insertion of a FCE layer between ITO and ZnO, i.e. device with a structure of ITO/FCE/ZnO/P3HT:PC61BM/MoO3/Al led to a typical S-shaped J-V curves and an obvious “light soaking” effect (Table 1, Entry 7 and 8, and Figure S3 in the supporting information). Besides the P3HT:PC61BM devices, we also found typical “light soaking” effect in Ag-grid/FCE based PTB7-Th:PC71BM devices (Figure S4). It reflects the generality of the light-soaking issue in the Ag-grid/HC-PEDOT:PSS based flexible inverted polymer solar cells. All these experiment results confirmed that the undesired S-shaped

J-V

curves

of

the

flexible

solar cells are

rather

due

to

HC-PEDOT:PSS/ZnO interface than the ZnO layer or ZnO/P3HT:PC61BM interface.


the


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Table 1 Device performance of the flexible device as made and after light soaking for 15 min. Cathode Light VOC PCE Entry JSC FF Aver. 2 a soaking (V) (mA/cm ) (%) (%) PCE (%)b 1 Ag-grid/PH1000/ZnO N 0.61 8.11 36 1.78 1.21±0.59 2 Y 0.61 8.44 48 2.47 2.45±0.23 3 Ag-grid/FCE/ZnO N 0.58 8.63 33 1.65 1.24±0.47 4 Y 0.60 8.61 53 2.74 2.73±0.26 5 ITO/ZnO N 0.61 8.19 61 3.05 3.01±0.06 6 Y 0.61 8.11 62 3.07 3.01±0.06 7 ITO/FCE/ZnO N 0.56 8.65 29 1.40 1.01±0.31 8 Y 0.60 9.28 55 3.06 2.84±0.22 a

Current density determined from EQE spectra. b. Standard deviation was obtained from 8 individual

devices.

To further understand the interfacial contact property between HC-PEDOT:PSS and ZnO, a model device with a structure of ITO/FCE/ZnO/Al was fabricated. Figure S5 showed the J-V curves of this device after light illumination for different time. As we can see here that the original device showed a diode behavior, indicating a Schottky contact at the FCE/ZnO interface. After 15 min white soaking, a typical ohmic contact behavior was measured for this device. Meanwhile, note that no contact improvement was detected when the device was stored in dark (supporting information Figure S6). This result is in good correspondence with the evolution of the J-V characteristics of the Ag-grid/FCE and ITO/FCE based inverted devices, which indicates that the “light soaking” issue was dominantly related to the FCE/ZnO contact. 3.3 Work Function Changes of the PEDOT:PSS and ZnO layer under light illumination To further understand the mechanism of light-soaking effect, the changes of the work


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function of FCE and ZnO layer under continuous illumination were investigated. The work function of FCE and ZnO were monitored by a Kelvin probe in air immediately after been irradiated for a certain time under a solar simulator. Using the Kelvin probe to acquire surface work function, the tip and sample were electrically connected, and a dc voltage is applied to the tip. The dc voltage will adjust automatically to nullify the electronic force. Since we can know the dc voltage and tip work function, the work function of sample can be calculated according to the following equation: dc = ∅ ∅

, where dc, ∅ , and ∅ are the dc voltage, sample surface work

function, and the tip work function respectively.

43

Figure 5 depicts the WF changes

both of FCE and ZnO films, and the WF difference between these two films. Here we can see the work function of the original FCE film was measured to be 5.10 eV, which is in good agreement with the reported data (4.8~5.2 eV) for the highly conductive PEDOT:PSS film.36, 44-45 In the meanwhile, the WF of ZnO film was measured to be 4.40 eV, which is also similar to the value reported in the literatures.46 Therefore, a large WF difference of 0.70 eV exists between HC-PEDOT:PSS and ZnO films, which leads to charge injection barrier at the interface. This results explain the reason of S-shaped J-V curves in the Ag-grid/HC-PEDOT:PSS based inverted solar cells, as well as the Schottky contact in the ITO/FCE/ZnO/Al device (vide supra). Upon exposure the FCE film in simulated solar illumination, surprisingly, the work function decreased gradually from 5.10 to 4.60 eV. Such a light induced WF change of FCE film was double checked in our lab and the same trend was obtained. The detail reason for the WF changes of FCE layer under light illumination is not fully understood yet. Light induced interaction



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between the components within the FCE layer, or the change of surface composition of FCE film was speculated to be possible reason, since higher WF of PEDOT:PSS was reported when increasing surface PSS component. 47 In contrast, during the light soaking process, the work function of ZnO decreased slightly from 4.40 to 4.22 eV. Such a WF decrease for the ZnO film was also reported by Bao et al, and this was attributed to desorption of negatively charged oxygen molecules from ZnO surface.48 Thus with the increasing of light soaking time, the work function difference between PEDOT:PSS and ZnO gradually decreased from 0.70 eV to 0.38 eV (Figure 5a and 5b). This work function difference is similar to the difference between ITO (4.8 eV)

49

and ZnO (4.4

eV). The smaller WF difference may facilitate electron injection and extraction at the interface of ZnO and the Ag-grid/HC-PEDOT:PSS cathode. 33 With these, we clearly confirmed that the large WF difference is response for the formation of S-shaped J-V curve in the Ag-grid/HC-PEDOT:PSS flexible electrode based polymer solar cells, whereas light induced decrease in the WF difference of the PEDO:PSS and ZnO layers facilitates the electron injection and extraction at the HC-PEDOT:PSS/ZnO interface, and leads to improvement of device performance and the elimination of S-kinked J-V curves.


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Figure 5. (a) The work function of the ITO/FCE and ITO/ZnO films vs. light soaking time. (b) The energy band diagrams of the inverted solar cells as fabricated, and after light soaking for 30 min. 3.3 Light soaking- free flexible polymer solar cells based on doped PEDOT:PSS According to the above observations, we demonstrated that the large WF difference between HC-PEDOT:PSS and ZnO is the main reason for the S-shaped J-V characteristics, and light induced WF difference decrease eliminates the S-shaped J-V curves. Based on this finding, reducing the WF of the HC-PEDOT:PSS layer should be able to diminish the charge injection barrier between HC-PEDOT:PSS and ZnO layers, and consequently eliminate the S-shaped J-V curves for the flexible Ag-grid electrode based inverted polymer solar cells. It is known that non-conjugated polyimides, such as polyethylenimine (PEI) and ethoxylated polyethylenimine (PEIE) are able to substantially reduce the work function of conducting polymer and metal oxides.50-51 Chen et al. also reported the successful decrease the WF of PEDOT:PSS by depositing a thin layer of PEI on the top of it. The modified PEDOT:PSS/PEI layer can be used as electron transporting layer in polymer solar cells.

52

We therefore deposited a thin layer of PEI on the Ag-grid/FCE electrode,

and Figure S7 in supporting information shows the J-V curves of the Ag-grid/FCE/PEI based P3HT:PC61BM solar cell. As can be seen here, neither typical S-shaped J-V curve nor light soaking effect was observed for these type solar cells, demonstrating again that lowering WF of FCE would diminish the charge injection barrier between FCE and ZnO layer. However, the device performance was low, which is mainly due to the low FF of



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the devices. To know the detail reason for the low device performance, film formation of PEI on FCE surface was measured by atomic force microscope (AFM). Figure S8 shows the AFM images of FCE/PEI and the reference FCE films. As can be seen here, the pristine FCE surface is homogenous and smooth with a roughness of 1.6 nm, whereas PEI forms inhomogeneous and non-connected surface with large surface roughness of 40.0 nm. This poor film quality of PEI on FCE surface could be the main reason for the poor performance. To solve this problem, we therefore think about doping HC-PEDOT:PSS with PEI directly. However, directly mixing PEI with FCE leads to the precipitation of PEDOT:PSS from the suspension (Figure S9 in supporting information). Fortunately, by adding ammonia aqueous into the FCE suspension, PEI can be successfully mixed with the Ammonia: FCE mixture and keep stable. And the modified FCE:Ammonia:PEI mixtures also can be successfully coated onto the Ag-grid electrode. Figure 6a shows the relationship between the WF of the FCE:Ammonia:PEI composite films and the PEI concentration. Results showed that the work function of the FCE:Ammonia:PEI composite films gradually reduced from 5.10 eV for the pristine film to 4.30 eV for the doped films. To confirm such a WF decrease of the doped films could reduce the charge injection barrier between FCE and ZnO, electron only devices ITO/ FCE:Ammonia:PEI /ZnO/Al were fabricated and tested. Figure 6b depicts the J-V curves of the ITO/FCE:Ammonia:PEI devices. For comparison, J-V curve of the ITO/FCE/ZnO/Al device is also shown in this figure. It can be seen that by increasing the PEI doping concentration, the J-V curve changes from a diode behavior for the


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pristine FCE based device to a typically ohmic contact J-V characteristics gradually. This J-V characteristic of ITO/ FCE:Ammonia:PEI /ZnO/Al is in good correspondence with J-V characteristics of the ITO/FCE:Ammonia:PEI/ZnO/P3HT:PC61BM/MoO3/Al device under light illumination (vide infra). Inverted P3HT:PC61BM solar cells using FCE:Ammonia:PEI as modification layer of ITO electrode were then fabricated and tested. Figure 6c shows the J-V characteristics of these cells. Again, one can clearly see that the S-shaped J-V characteristics gradually eliminated with the increase of PEI concentration, which is very similar to the “light soaking effect” of the pristine FCE based device. Then the PEI doping concentration dependent device performance shows that device with 10.0 mg/mL PEI doped FCE electrode showed a normal J-V curve with a VOC of 0.59 V, JSC of 7.56 mA/cm2, FF of 59%, and PCE of 2.63 %, which is reasonable for P3HT:PC61BM based device. All these results unambiguously confirmed that WF of the FCE layer was decreased by using PEI doping, which eliminated the charge injection barrier between FCE and ZnO, and consequently improved the device performance. The influence of sole ammonia on WF of FCE and photovoltaic performance were also studied. Results showed that the FCE:Ammonia film has a WF of 4.98 eV, which is slightly decreased when compared to that of pristine FCE film (5.10 eV). Slightly improved J-V curves were measured for the FCE:Ammonia based cells (Figure S10) due to weak doping effect of ammonia. However restricted by high volatility of ammonia, the doping concentration of ammonia within the FCE films should be very low after thermal annealing. Thus, sole ammonia doping can’t significantly eliminate the



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S-shaped J-V characteristics and improve the device performance. Spin coating ammonia directly onto the FCE layer was also investigated. However such a surface deposition of ammonia could not totally eliminate the S-shaped J-V characteristics either (Figure S11), which could be due to the same reason as mentioned above.

Figure 6 (a) The work function of FCE:Ammonia:PEI composite films vs. PEI concentration. (b) J-V characteristics of the ITO/FCE/ZnO/Al and ITO/FCE:Ammonia:PEI /ZnO/Al devices. (c) J-V characteristics of solar cells with structure of ITO/ FCE:Ammonia:PEI /ZnO/P3HT:PC61BM/MoO3/Al. Finally, with this PEI doped FCE in hand, Ag-grid/ FCE:Ammonia:PEI /ZnO/ PTB7-Th:PC71BM/MoO3/Al flexible devices were fabricated. Figure 7 shows the J-V curves of the PTB7-Th:PC71BM device with pure FCE and FCE:Ammonia:PEI as modification layer of PET/Ag-grid electrode. The J-V curve of the PTB7-Th:PC71BM device with pristine FCE exhibited typical S shape, while the device with PEI doped FCE did not show S-shaped J-V characteristics. A reasonable power conversion


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efficiency with max PCE of 6.58% and average PCE about 6.00 % calculated over 20 individual devices were obtained. For the optimized device, the optimized performance parameters VOC, JSC, FF, PCE were 0.78 V, 14.29 mA/cm2, 59%, and 6.58%, respectively. As shown by Figure 7 (b) the performance histogram of the FCE:Ammonia:PEI involved flexible device, we can see 12 in 20 devices give efficiency above 6.0%. From the normal J-V curves of this device, it can confirm the elimination of charge injection barrier between FCE and ZnO layer. But the insertion of PEI modified FCE has lower conductivity relative to the FCE film (see supporting information Figure S12). It might be mostly responsible for the low JSC of the FCE:Ammonia:PEI employing device than the pristine FCE involved device. Nevertheless, this results demonstrated that FCE:Ammonia:PEI modified electrode could significantly eliminate the “light-soaking” effect of the flexible organic solar cells.

Figure 7. (a) J-V curves of the PET/Ag-grid/FCE/ZnO/PTB7-Th:PC71BM/MoO3/Al, and PET/Ag-grid/ FCE:Ammonia:PEI /ZnO/PTB7-Th:PC71BM/MoO3/Al. (b) The performance histogram of the PET/Ag/ FCE:Ammonia:PEI /ZnO/PTB7-Th:PC71BM/MoO3/Al device.



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4. CONCLUSIONS In this work, we systematically investigated the influence of the highly conductive PEDOT:PSS on the J-V characteristics of the flexible inverted polymer solar cells. Results confirmed that the work function mismatch between FCE and ZnO is the dominating reason of the S-shaped J-V curves. Light soaking on the HC-PEDOT:PSS films lowered the WF of the PEDOT:PSS film, and consequently diminished the undesired S-shape. Further, we demonstrated that work function of FCE can be decreased from 5.10 eV for the pristine FCE film to 4.30 eV. Finally, this ternary composite modification layer was used in the flexible PTB7-Th:PC71BM inverted solar cells, a reasonable device performance of with PCE 6.58% is obtained without light-soaking issue.

ASSOCIATED CONTENT

Supporting Information. Figure S1. J-V characteristics of the inverted solar cells after storage

in

dark

for

10

hrs.

ITO/ZnO/P3HT:PC61BM/MoO3/Al

Figure

device

S2.

under

J-V

characteristics

illumination.

Figure

of

the

S3.

J-V

characteristics of the ITO/FCE/ZnO/P3HT:PC61BM/MoO3/Al devices. Figure S4. Evolution of J-V characteristics of the Ag grid/FCE/ZnO/PTB7-Th:PC71BM /MoO3/Al devices under illumination. Figure S5. J-V characteristics of the ITO/FCE/ZnO/Al device vs. aging time under continuous illumination. Figure S6. J-V characteristics of the ITO/FCE/ZnO/Al device vs. aging time which were stored in dark. Figure S7. J-V


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characteristics of the device of Ag grid /FCE/PEI/ZnO/P3HT:PC61BM/MoO3/Al. Figure S8. AFM images of (a) the ITO/FCE and (b) ITO/FCE/PEI film. Figure S9. The photograph of the (a) fresh FCE, (b) FCE:PEI (PEI: 1.0 mg/mL), (c) FCE:PEI (PEI: 10.0 mg/mL)

films,

and

(d)

FCE:Ammonia:PEI

(PEI:

1.0

mg/mL),

and

(e)

FCE:Ammonia:PEI (PEI: 10.0 mg/mL) composite inks after storage in ambient for 16 months. Figure S10. J-V characteristics of the ITO/FCE/ZnO/P3HT:PC61BM/MoO3/Al and ITO/FCE:Ammonia/ZnO/P3HT:PC61BM/MoO3/Al devices. Figure S11. J-V characteristics of the ITO/FCE/Ammonia/ZnO/P3HT:PC61BM/MoO3/Al. Figure S12. The conductivity of the FCE:Ammonia:PEI composite film vs. PEI concentration. Table S1. Device performance of the flexible devices as made and after light soaking for 15 min. This supporting information is available free of charge on the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (61306073), Ministry of Science and Technology of China (No 2016YFA0200700),



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Jiangsu Provincial Natural Science Foundation (BK20130346, BE2015071), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA09020201).

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Table of Contents Graphic


Modification of the Highly Conductive PEDOT:PSS Layer for Use in

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