The Effects of Improved Photoelectric Properties of PEDOT:PSS by

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The effects of improved photoelectric properties of PEDOT:PSS by two-step treatments on the performance of polymer solar cells based on PTB7-Th: PC71BM Ling Zhao, Suling Zhao, Zheng Xu, Di Huang, Jiao Zhao, Yang Li, and Xurong Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09561 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 18, 2015

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ACS Applied Materials & Interfaces

The effects of improved photoelectric properties of PEDOT:PSS by two-step treatments on the performance of polymer solar cells based on PTB7-Th: PC71BM Ling Zhao,†,‡ Suling Zhao,*, † , ‡ Zheng Xu, †,‡ Di Huang, †,‡ Jiao Zhao,†,‡ Yang Li,†,‡ and Xurong Xu†,‡ †

Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University),

Ministry of Education, Beijing, 100044, China ‡

Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing, 100044, China

ABSTRACT: In this paper, we present a smart two-step treated method to simultaneously improve the work function, conductivity and transmittance of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). With the two-step treated PEDOT:PSS, the short circuit current density (JSC) of polymer solar cells (PSCs) based on PTB7-Th:PC71BM increases from 13.41 to 22.10 mA/cm2, and the power conversion efficiency (PCE) increases from 7.35% to 9.82% with 33% improvement. The underlying mechanisms on performance improvement of PSCs can be summarized as follows: (1) two-step treated PEDOT:PSS with the improved work function and the conductivity, which contributes significantly to the charge collection of PSCs; (2) two-step treated PEDOT:PSS with higher transmittance, which is benefited to the light absorption of the active layer in PSCs.

KEYWORDS: PEDOT:PSS, two-step treated method, work function, conductivity, transmittance

INTRODUCTION Polymer bulk heterojunction (BHJ) solar cells, a kind of green energy source, have attracted more and more attention due to their unique advantages, such as environmentally friendly, low processing cost, flexible, light weight and large area.1-4

ACS Paragon Plus Environment


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The power conversion efficiency (PCE) of BHJ based polymer solar cells (PSC) has exceeded 9%, with sophisticated modification over energy gap and energy levels of donor and acceptor materials, morphology control of BHJ layers, as well as interfacial buffer layers for charge carriers collection.5-9 In addition to the properties of the BHJ layer, the device performance of the PSC is deeply dependent upon the properties of interfacial buffer layers between the BHJ layer and the two electrodes. The desired interfacial buffer layers should (1) promote Ohmic contact formation between electrodes and the BHJ layer; (2) possess appropriate work function to minimize the energy losses; (3) have sufficient conductivity to reduce resistive losses; (4) have low light absorption in the Vis-NIR wavelengths to minimize optical losses.10 Therefore, controlling these parameters of the interfacial buffer layer is essential for the PCE improvement of PSCs. In the conventional PSC architecture in which ITO is used as the anode, a thin hole extraction layer (HEL) of conductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) is often applied to planarize the ITO electrode and to improve its work function for effective hole extraction.11 Although PEDOT:PSS is a widely used HEL in PSCs, its relative low transmittance and conductivity, and the mismatch between the work function of PEDOT:PSS and highest occupied molecular orbital (HOMO) of the donor material limit the further improvements of performance in PSCs. Some researchers have paid attention to optimize the rule of PEDOT:PSS in PSCs. Chih-Wei Chu reported that the transmittance of PEDOT:PSS increased after methanol treatment by the dip method.12 It was also reported that the conductivity of PEDOT:PSS could be changed by doping different organic solvents, such as glycerol, dimethyl sulfoxide (DMSO) and Ethylene glycol (EG) and.13,14 While comparing to pristine PEDOT:PSS, solvent treated PEDOT:PSS typically shows decreased work function values,15-17 which could increase the charge extraction barrier. Lee reported that

the

addition

of

tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer (PFI) increased the work function of PEDOT:PSS from 4.86 eV to 5.39 eV while the weight ratio of PFI to PEDOT:PSS was 0.209, and the short circuit current density


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(Jsc) increased from 14.1 mA/cm2 to 16.7 mA/cm2.18 How to improve the conductivity or work function of PEDOT:PSS by doping different solvent additives has been widely investigated.19,20 If the work function, the conductivity and the transmittance of PEDOT:PSS can be improved together, the performance of PSCs will be enhanced greatly. In this paper, the hole extraction layer

PEDOT:PSS with two-step treatment by using PFI and DMSO was designed to improve the work function and conductivity of PEDOT:PSS for further extracting hole from the BHJ layer, as well as to enhance the transmittance of PEDOT:PSS for minimizing the optical losses. With two-step treated PEDOT:PSS, the JSC of PSCs increased from 13.41 to 22.10 mA/cm2. As a result, the PCE of corresponding PSCs increased from 7.35% to 9.82%, which thanks for the improved properties of PEDOT:PSS cooperative treated by PFI and DMSO.

EXPERIMENTAL SECTION The used materials were bought from different companies. PEDOT:PSS (Clevios P VP.AI

4083)

was

supplied

by

Heraeus

Germany.

5wt%

tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (a perfluorinated ionomer, PFI) and 1,8-diodooctane (98.0% purity) were from Sigma-Aldrich

Co.

chlorobenzene

(CB)

Dimethyl sulfoxide with

99.0%

(DMSO)

purity

with

were

99.0% purity

from

J&K

and

Scientific.

poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]

dithiophene-

co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]

[6,6]-phenyl

C71butyric

acid

methyl

(PTB7-Th),

ester

Poly(9,9-bis(3’-(N,N-dimethyl)-propyl-2,7-fluorene)-

(PC71BM)

and

alt-2,7-(9,9-dioctylfluorene))

(PFN-P1) were from 1-Material Inc. PSCs were prepared on glass/ITO substrates. The treating process of ITO refers to reference 4. A HEL of PEDOT:PSS doped with different volume of PFI named as HEL1 (12:1), HEL2 (8:1) and HEL3 (4:1), respectively, was spin coated onto the treated ITO substrates from an aqueous solution. Then the obtained HEL films with the thickness about 70 nm were annealed on a hot plate in the ambient atmosphere at



150゜C for 10 min. After that, different PEDOT:PSS:PFI layers were fluxed by DMSO, then annealed on a hot plate in the ambient atmosphere at 150゜C for 10 min. The remainder thickness of all fluxed HELs is about 20 nm according to the measurement. Then the PTB7-Th:PC71BM solution was spin-coated to form the BHJ layer on the top of HEL. Then the solution of PFN was spin-coated on the BHJ layer to form the electron transport layer. Subsequently, the aluminum cathode was deposited via thermally evaporation method. The photoactive area of the cells is 0.04 cm2, which is defined by the vertical overlap of ITO anode and Al cathode. Figure 1 shows the structure and the energy level diagram of devices. The work function and the conductivity of HELs were detected by Photoelectron Spectrometer AC-2 and four-point probe technique CRESBOX respectively. The thickness of HELs was tested by KLA Tencor P6. The morphology of films was investigated by atomic force microscopy (AFM) using a multimode Nanoscope IIIa operated in tapping mode. The absorption spectra and transmittance spectra of films were measured by a Shimadzu UV-3101 PC spectrometer. The current density– voltage (J–V) of devices was measured by the Keithley 4200 semiconductor characterization system and ABET Sun 2000 solar simulator. All the measurements were performed at room temperature in air without any device encapsulation. -3.81eV

Al

PFN Active layer HEL ITO glass substrate

-4.70eV ITO

PC71BM

Al

PFN

-4.20eV

PTB7-Th

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-4.30eV

Al

HEL -5.38eV -6.10eV

Figure 1. Structure and energy level diagram of devices.

RESULTS AND DISCUSSION Figure 2 shows the J-V characteristics of devices with different HELs, i.e., the pristine PEDOT:PSS HEL of 70 nm, two-step treated PEDOT:PSS HELs about 20 nm, and also a pristine PEDOT:PSS HEL with the thickness of 20 nm as a reference. And


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the detailed performance parameters of devices are summarized in Table 1. Apparently, the PCE of PSCs with two-step treated PEDOT:PSS is higher than that of PSCs with pristine PEDOT:PSS. And the champion PCE of 10.10% was obtained from the PSCs with HEL2. The JSC increases from 14.31 of the device with 70nm pristine PEDOT:PSS to 19.68, 22.10 and 19.83 mA/cm2 for devices with HEL1, HEL2, HEL3, respectively. A JSC over 20 mA/cm2 was realized in such kind devices. While the open circuit voltage (VOC) of all of the devices kept as a constant and the fill factor (FF) decreased almost 10%. So the enhancement of performance could be ascribed to the increased JSC. Since all the devices were prepared under the same experimental conditions, these changes in the performance are obviously caused by the various treatments of the PEDOT:PSS layers. Moreover, the shunt resistance (Rsh) of devices based on HELs are much lower than that of device based on pristine PEDOT:PSS, as shown in table 1. This phenomenon indicates a higher charge carrier recombination which goes against the improvement of FF. Meanwhile the smaller series resistance (Rs) signifies a lower resistance of the semiconductor bulk resistance and a better metal/semiconductor interfaces induced by HELs.21

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 PEDOT:PSS (70 nm) PEDOT:PSS (20 nm) HEL1 HEL2 HEL3

-5 -10 -15 -20 -25 0.0

0.2

0.4

0.6

0.8

Voltage (V)

Figure 2. J-V characteristics of devices based on pristine PEDOT:PSS and two-step treated PEDOT:PSS. Table 1 Performance parameters of PSCs with different hole extraction layers under AM 1.5 light power of 100 mW/cm2. JSC

VOC

FF

PCEave

PCEmax


Rs

Rsh


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(mA/cm2)

(V)

(%)

(%)

(%)

(Ω·cm2)

(Ω·cm2)

PEDOT:PSS (70nm)

14.31

0.80

64.20

7.35

7.46

8.35

694.44

PEDOT:PSS (20nm)

15.10

0.80

64.96

7.85

7.91

6.18

862.07

HEL1

19.68

0.80

54.07

8.62

8.81

6.39

176.68

HEL2

22.10

0.80

55.51

9.82

10.10

5.89

156.25

HEL3

19.83

0.80

52.65

8.35

8.50

7.64

155.52

The averaged PCE values are calculated based on 30 cells.

In order to investigate the effect of two-step treated PEDOT:PSS HELs on JSC, the work function and the conductivity of different HELs was measured and shown in Table 2. And with the volume ratio of PFI increased, the work function HELs increased from 5.21 eV (pristine PEDOT:PSS) to 5.28, 5.37 and 5.58 eV for HEL1, HEL2 and HEL3, respectively. The increase of work function could be attributed to the enrichment of PFI on the film surface which induced by self-organization of PFI.18 The work function of HEL2 is 5.37 eV, which can match well with the HOMO energy level of the polymer donor PTB7-Th (5.38 eV) comparing to HEL1 (5.28 eV) and HEL3 (5.58 eV). And the energy level match could reduce the charge collection barrier from PTB7-Th to electrode. All this well explain the reason of device with HEL2 performs better than the device with HEL1 or HEL3. Table 2 The work function and conductivity of pristine and two-step treated PEDOT:PSS. HEL

Conductivity (s/cm)

Work function (eV)

-4

5.21

HEL1 (PEDOT:PSS:PFI(12:1)/DMSO)

-2

3.65×10

5.28


3.38×10-2

5.37


-2

5.58

PEDOT:PSS (pristine, 70 nm)

9.64×10

4.44×10

From table 2, we can see that the conductivity of two-step treated PEDOT:PSS increases from 9.64×10-4 S/cm (pristine PEDOT:PSS) to 3.65×10-2, 3.38×10-2 and 4.44×10-2 S/cm for HEL1, HEL2 and HEL3, respectively. The underlying mechanisms on conductivity improvement of PEDOT:PSS can be summarized as follows: (1) DMSO-fluxing treatment induces strong screening effect between


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counterions and charge carriers, which reduces the coulombic interaction between positively charged PEDOT and negatively charged PSS dopants;22,23 (2) the excess insulator PSS was washed away from the film surface;24 (3) in addition, the HEL thickness decrease from 70±2 to 20±2 nm is also conducive to the increase of conductivity. The higher conductivity is, the easier charge collection is. Figure 3a shows the transmittance of pristine and two-step treated PEDOT:PSS films on the substrate of quartz substrate. After the two-step treatment, the transmittance of PEDOT:PSS films increase obviously. The transmittance of two-step treated PEDOT:PSS films exhibits the highest value around 350 nm in all HELs. The thickness of pristine PEDOT:PSS films were controlled by tuned the spin speed. After doped with PFI, the transmittance of 70 nm PEDOT:PSS:PFI film enhances more than the 70 nm pristine PEHOT:PSS film, even as high as that of the 20 nm pristine PEDOT:PSS film. It is concluded that the enhanced transmittance of PEHOT:PSS:PFI is not due to the thickness thinning but due to the effect of PFI. The doping PFI helps the self-assembly of PEDOT:PSS during the process of film forming, which results in the enhanced transmittance. After fluxed by DMSO, even the thickness of the corresponding three HELs doped with PFI decrease to 20 nm, their transmittance are higher than that of the pristine 20nm PEDOT:PSS film and 70 nm PEDOT:PSS:PFI film. It indicates the combined effect of PFI and DMSO on the properties of PEDOT:PSS. Therefore, the enhancement of transmittance could attributed to the following aspects: (1) the self-assembly PEDOT:PSS doped with PFI; (2) the decrease of thickness of HELs; (3) more importantly, the two-step treatment, it not only wash away PSS, but also improve the crystallization as shown in figure 4. To further investigate the optical effect of two-step treated PEDOT:PSS HELs, the absorption spectrum of PTB7-Th:PC71BM films on the pristine and two-step treated PEDOT:PSS HELs was measured and shown in Figure 3b. Obviously, the absorbance of PTB7-Th:PC71BM films on two-step treated PEDOT:PSS HELs are significantly higher than that of the film on the pristine PEDOT:PSS HEL, which can be attributed to the enhanced transmittance of two-step treated PEDOT:PSS HELs. The more the



light is absorbed, the more the excitons are generated. This leads to the enhanced JSC of devices with two-step treated PEDOT:PSS HELs.

100

0.6

Absorbance (arb.unites)

(a)

99

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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98 97 PEDOT:PSS (70nm) PEDOT:PSS:PFI (70nm) PEDOT:PSS (20nm) HEL1 HEL2 HEL3

96 95 94 93 300

400

500

600

700

800

(b) 0.5 0.4

PEDOT:PSS+PTB7-Th:PC71BM HEL1+PTB7-Th:PC71BM HEL2+PTB7-Th:PC71BM

0.3

HEL3+PTB7-Th:PC71BM

0.2 0.1 0.0 300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 3. (a) Transmission spectrum of different hole extraction layer, (b) absorption spectrum of PTB7-Th:PC71BM films atop pristine and two-step treated PEDOT:PSS. All the films were prepared on quartz substrate. Figure 4 shows surface topographic AFM images of PEDOT:PSS films with different treatments: (a) pristine PEDOT:PSS, (b) HEL1, (c) HEL2, (d) HEL3. As shown in figure 4a, the surface morphology of pristine PEDOT:PSS film is very smooth, and its root mean square roughness (Rms) is only 1.28 nm. However, the AFM results indicate that PEDOT:PSS films with two-step treatments change greatly to 1.95 nm, 2.04 nm and 2.24 nm for HEL1, HEL2 and HEL3, respectively. The increasing surface roughness shows the improved crystallization of PEDOT:PSS. It is also observed that small crystalized particles distribute uniformly in the surface of treated films, which will results in the higher conductivity of treated PEDOT:PSS films than that of pristine film. In addition, the increased surface roughness will increase the interface area between the BHJ layer and HEL, providing more routes for holes extraction.25,26 However the enhanced surface roughness of HEL also induced the large leakage current and sacrificed the device FF,16 as shown in table 1. It is reported that the morphology of BHJ layer could be tuned by controlling the surface properties of HEL.27 The Rms of the PTB7-Th:PC71BM films increased from 2.79 nm atop pristine PEDOT:PSS to 3.39 nm atop HEL1, 3.34 nm atop HEL2 and 3.43 nm atop HEL3, respectively, as shown in the Figure S2. The optimized surface


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morphology and phase separation of PTB7-Th:PC71BM induced by HELs may be conducive to exciton dissociation and charge carrier transport.

Figure 4. Surface topographic AFM images of pristine and two-step treated PEDOT:PSS: (a) pristine PEDOT:PSS, (b) HEL1, (c) HEL2, (d) HEL3. To further understand the effect of two-step treatment, the champion cells using HEL2 and the pristine PEDOT:PSS HEL were investigated systematically. Figure 5a shows the effect of the HEL2 on the photocurrent density (Jph). The value of Jph is determined by the equation Jph=JL−JD, where JL is the current density under illumination, JD is the current density in the dark. The effective voltage (Veff) is determined by the equation Veff=Vo−Va, where Va and Vo are the applied voltage and the voltage when Jph equals zero, respectively. The Jph first increases linearly with Veff, and then reaches a saturated level at sufficiently high values of Veff.28 Obviously, the saturation photocurrent density (Jsat) of PSCs with HEL2 is higher than that of PSCs with the pristine PEDOT:PSS HEL. The Jsat is given by Jsat=qGmaxL, where q is the electronic charge, Gmax is maximum exciton generation rates and L is the thickness of



BHJ layer (L=80 nm). Consequently, the value of Gmax of PSC with HEL2 is higher than that of PSC with the pristine PEDOT:PSS HEL. Because the value of Gmax is governed only by the light absorption,29,30 the enhanced value suggests that the application of HEL2 increases the degree of light absorption in the PSC, as shown in Figure 3b. In order to further confirm the effect of HEL2, the space charge limited current (SCLC) approach was introduced to study the charge carrier transport in PTB7-Th:PC71BM.31,32 According to the Mott-Gurney law, the current density is given by:

J = 9ε ε μV ⁄8L (1) where ε is the permittivity of free-space, ε is the relative dielectric constant of the BHJ layer, μ is the hole mobility，V is the voltage drop across the device, and L is the thickness of the BHJ layer.33 The J–V characteristics of the hole-only devices, fabricated with the structures of ITO/HEL/PTB7-Th:PC71BM(80nm)/Au(80nm), are shown in Figure 5b. The results show that the hole-mobility μ increases from 9.98×10-5 cm2/Vs (pristine PEDOT:PSS) to 1.62×10-4 cm2/Vs (HEL2). It means that the hole, injected from PTB7-Th, could be easily extracted for device based on HEL2. The enhanced μ of hole-only device with HEL2 further illustrates the positive effect of enhancement of work function and improvement of conductivity for hole extraction. 2 Current density (A/cm )

(a) 1

10

Jph (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PEDOT:PSS HEL2

0

10

1 (b)

0.1

0.01

PEDOT:PSS HEL2

1E-3 -1

0

10

10

Voltage (V)

Veff (V)

1

Figure 5. (a) Plots of Jph-Veff for devices based on pristine PEDOT:PSS and HEL2 (b) J–V characteristics of ITO/HEL/PTB7-Th:PC71BM(80 nm)/Au (80 nm). The stability measurements of devices based on pristine and two-step treated


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PEDOT:PSS are shown in Figure 6. All the measurements were carried out in air without any encapsulation, but all devices were stored in a high purity nitrogen-filled glove box. In figure 6a, the Voc of the device based on pristine PEDOT:PSS kept almost constant, while the Jsc, FF and PCE of device based on pristine PEDOT:PSS degraded rapidly, the Jsc, FF and PCE decreased to 67.20%, 73.82% and 49.62% of their initial values, respectively. If the lifetime of a device is defined as the duration of its survival to within 50% of its initial PCE value, the lifetime of devices based on pristine PEDOT:PSS was less than 120 hours. For device based on two-step treated PEDOT:PSS HEL2, the Voc kept almost constant, while the Jsc, FF and PCE decreased to 74.27%, 78.74% and 58.52% of their initial values, respectively. By the same criterion, the lifetime of device based on HEL2 was more than 240 hours. Obviously, the life of device based on HEL2 was double that of device based on pristine PEDOT:PSS. Since the structure of the two devices is the same except PEDOT:PSS layers with/without two-step treatment, the improved stability of device based on

Normalized device performance

HEL2 could be attributed to the two-step treatment. Normalized device performance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.9 0.8 0.7

(a) Jsc Voc FF PCE

0.6 0.5

1.0 0.9 0.8 0.7

(b) Jsc Voc FF PCE

0.6 0.5 0.4

0.4 0

20

40

60

80

100

120

0

40

80

Time (h)

120

160

200

240

Time (h)

Figure. 6 Normalized device performance (Jsc, Voc, FF, and PCE) as a function of storage time for devices based on pristine and two-step treated PEDOT:PSS, (a) pristine PEDOT:PSS, (b) two-step treated PEDOT:PSS HEL2.

CONCLUSIONS In summary, a simple two-step treated method was introduced to improve the photoelectric properties of PEDOT:PSS synchronously. Further studies confirmed the improvement of photoelectric properties of PEDOT:PSS: (1) the hole extraction



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barrier decreased from 170 to 10 mV; (2) the conductivity increased from 9.64×10-4 to 3.38×10-2 s/cm; (3) the transmittance of two-step treated PEDOT:PSS was improved significantly which induced the increase of absorption of the BHJ layer. The PCE of PSC with two-step treated PEDOT:PSS as HEL increased from 7.35% to 9.82%, which was attributed to the increased JSC from 13.41 to 22.10 mA/cm2. We attribute the increase of JSC to the following reasons: (1) the enhanced hole mobility which induced by the optimized work function and the improved conductivity; (2) the increased photocurrent caused by the increase of transmittance for light absorption. Furthermore, this two-step treated method may provide a new sight to improve the performance of PSCs by controlling the photoelectrical properties of hole extraction layer.

ASSOCIATED CONTENT Supporting Information Supplementary information available: Chemical structures of PEDOT:PSS, PFI and DMSO, Atomic force microscopy (AFM) height and phase diagram of PTB7-Th:PC71BM films atop pristine PEDOT:PSS HEL and two-step treated PEDOT:PSS HELs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected], Tel(Fax):86-10-51684462.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors express the thanks to the National Natural Science Foundation of China (61575019 and 51272022), the Specialized Research Fund for the Doctoral


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Program of Higher Education of China (20120009130005), and the Fundamental Research Funds for the Central Universities, China (2012JBZ001).

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Energy Environ. Sci. 2012, 5, 9662. (13) Eom, S. H.; Senthilarasu, S.; Uthirakumar, P.; Yoon, S. C.; Lim, J.; Lee, C.; Lim, H. S.; Lee, J.; Lee, S.-H., Polymer Solar Cells Based on Inkjet-Printed PEDOT:PSS Layer. Org. Electron. 2009, 10, 536-542. (14) Thomas, J. P.; Zhao, L.; McGillivray, D.; Leung, K. T., High-Efficiency Hybrid Solar Cells by Nanostructural Modification in PEDOT:PSS with Co-Solvent Addition.

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with Very High Efficiency. Adv. Funct. Mater. 2014, 24, 4978-4985. (20) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J., A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Science 2012, 336, 327-332. (21) Cui, Y.; Huang, D.; Li, Y.; Huang, W.; Liang, Z.; Xu, Z.; Zhao, S., Aluminium Nanoparticles Synthesized by a Novel Wet Chemical Method and Used to Enhance the Performance of Polymer Solar Cells by the Plasmonic Effect. J. Phys. Chem. C 2015, 3, 4099-4103. (22) Chen, J.-G.; Wei, H.-Y.; Ho, K.-C., Using Modified Poly (3, 4-ethylene dioxythiophene): Poly (styrene sulfonate) Film as a Counter Electrode in Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2007, 91, 1472-1477. (23) Kim, J.; Jung, J.; Lee, D.; Joo, J., Enhancement of Electrical Conductivity of Poly (3, 4-ethylenedioxythiophene)/Poly (4-styrenesulfonate) by a Change of Solvents. Synthetic Metals 2002, 126, 311-316. (24) Palumbiny, C. M.; Heller, C.; Schaffer, C. J.; Körstgens, V.; Santoro, G.; Roth, S. V.; Müller-Buschbaum, P., Molecular Reorientation and Structural Changes in Cosolvent-Treated Highly Conductive PEDOT:PSS Electrodes for Flexible Indium Tin Oxide-Free Organic Electronics. J. Phys. Chem. C 2014, 118, 13598-13606. (25) Hsu, M.-H.; Yu, P.; Huang, J.-H.; Chang, C.-H.; Wu, C.-W.; Cheng, Y.-C.; Chu, C.-W., Balanced Carrier Transport in Organic Solar Cells Employing Embedded Indium-Tin-Oxide Nanoelectrodes. Appl. Phys. Lett. 2011, 98, 073308. (26) Fung, D. D.; Qiao, L.; Choy, W. C.; Wang, C.; Wei, E.; Xie, F.; He, S., Optical and Electrical Properties of Efficiency Enhanced Polymer Solar Cells with Au Nanoparticles in a PEDOT–PSS Layer. J. Mater. Chem. 2011, 21, 16349-16356. (27) Yip, H.-L.; Jen, A. K. Y., Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994. (28) Chen, F.-C.; Wu, J.-L.; Lee, C.-L.; Hong, Y.; Kuo, C.-H.; Huang, M. H., Plasmonic-Enhanced

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2 Current density (mA/cm )

100 99

Transmittance (%)

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98 97 PEDOT:PSS HEL2

96 95 94

HEL HEL2

PEDOT:PSS (pristine) (PEDOT:PSS:PFI(8:1)/DMSO)

Conductivity (s/cm)

Work function (eV)

9.64×10-4 3.38×10-2

5.21 5.37

93 300

0 2

-5

JSC (mA/cm )

VOC (V)

FF (%)

PCEave (%)

PEDOT:PSS

14.31

0.80

64.20

7.35

7.46

HEL2

22.10

0.80

55.51

9.82

PCEmax (%) 10.10

PEDOT:PSS HEL2

-10 -15 -20 -25

400

500

600

700

800

0.0

Wavelength (nm)

0.2

0.4

Voltage (V)


0.6

0.8

The Effects of Improved Photoelectric Properties of PEDOT:PSS by

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