Construction of Layered Structure of Anion-Cations to Tune the Work

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Construction of Layered Structure of Anion-Cations to Tune the Work Function of AZO for Inverted Polymer Solar Cells Weitao Ma, Yinqi Luo, Li Nian, Jianqiao Wang, Xinbo Wen, Linlin Liu, Muddasir Hanif, Zengqi Xie, and Yuguang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16653 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

Construction of Layered Structure of Anion-Cations to Tune the Work Function of AZO for Inverted Polymer Solar Cells Weitao Ma,† Yinqi Luo,† Li Nian,† Jianqiao Wang, Xinbo Wen, Linlin Liu, Muddasir Hanif, Zengqi Xie*, Yuguang Ma Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. †

These authors contributed equally to this work.

KEYWORDS: perylene bisimide, cathode interlayer, interface dipole, polymer solar cells, electrochemistry

ABSTRACT: Suitable work function of cathode in polymer solar cells (PSCs) is of essential importance for the efficient electron extraction and collection to boost the power conversion efficiency (PCE). Herein, we report a facile and efficient method to tune the surface work function of aluminum doped zinc oxide (AZO) through building of a definite interfacial dipole, which is realized by construction of a layered structure of positive and negative ionized species. A cross-linked perylene bisimide (poly-PBI) thin film is deposited onto AZO surface firstly; and then it is reduced to radical anion state (poly-PBI•-) in an electrochemical cell, using

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tetraoctylammonium (TOA+), a bulky cation, as counter ion. Owning to the huge volume of TOA+, it is absorbed on the surface of the cross-linked PBI•- thin film through Coulomb force, and thus a definite interface dipole is formed between the two ionized layers. Due to the definite interface dipole, the surface work function of the electrode modified with ionized layers is decreased dramatically to 3.9 eV, which is much lower than that of the electrode modified with neutral PBI layer (4.5 eV). By using this novel cathode interlayer with definite interface dipole in PSCs, a significantly increased open circuit voltage (VOC) is obtained. The results indicate it is a facile and unique method by construction of definite interface dipole to tune the surface work function of electrode for the application in organic electronic devices.

INTRODUCTION

Organic photovoltaics have garnered great attention and extensive research interest due to their potential to be a renewable energy alternative to the inorganic counterpart.1-4 Owing to some undesirable issues like application of air-sensitive metals as cathode for traditional device geometry, enormous efforts have been done on device with inverted architecture (i-PSCs).1,2 In particular, interface engineering plays critical roles to the whole device performance, i.e. a vital modification of the cathode in i-PSCs may facilitate the electron extraction and collection.2-6 Up to now, various outstanding electron-transport layers (ETLs) have been successfully applied in iPSCs.1 Several inorganic metal salts and metal oxides such as LiF, ZnO etc, have been proved effectively improving the electron extraction efficiency.5-8 Although these inorganic ETLs show intrinsic merits such as high electron mobility, wide bandgap to block hole and transparency etc, the contact between the inorganic interlayer and the organic active layer is not ideal and need to be enhanced.9 Whilst a series of polymeric surfactants, including PEIE10, PFN11, PEI12, PEO13,


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WPF-oxy-F14, and small molecules like PDIN15 have also been successfully applied as ETLs. These organic/polymeric ETLs increase the interface contact with the active layers; however, the intrinsic low conductivity of these materials limits the thickness of the interlayer usually less than teens of nanometer, which greatly increases the difficulty of obtaining large-scale and pinhole-free film by fast role-to-role technique. In addition, numerous organic-inorganic composite materials, such as self-assembled monolayer (SAM) on ZnO as bilayer cathode modifier, have also been successfully applied as ETLs to enhance the electron collection ability in device.16-19 This kind of organic-inorganic composite interlayer not only improved the poor contact between inorganic ZnO and organic active layer but also evaded the possible leak current from incomplete coverage (pinholes) of the very thin organic interlayer. An ideal directional cathode interfacial dipole have proven to be admirable to constitute a superposition with the pristine build-in electric field originated from the asymmetric contact of the electrodes to reinforce the build-in potential, and thus effectively promote the electron extraction process from the active layer.11,15,16 Application of the organic/polymeric ETL materials with polar units in device usually produce interfacial dipole by partially electron transfer from the organic/polymeric electron-rich species to the electrode, for the cases of both SAM decorating the surface of ZnO16,17,21 and polar molecules/polymers on ITO10-14 as mentioned above. Such interfacial dipole is capable of inducing a vacuum-level shift and thus modify the work function (WF) of the electrode.11,15,16 For example, the application of PFN in both conventional PSCs and i-PSCs the WF of the cathode was efficiently tuned and the devices showed simultaneously enhancement of open circuit voltage (VOC), short circuit current (JSC), and fill factor (FF).11 For the cases of polar solvent treated ZnO surface, some indispensable polar group was adopted to induce the formation of dipole through the interaction with "ripple-


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structure" ZnO surface and dramatically enhanced PCE was demonstrated.20 Despite successful examples of highly efficient i-PSCs using above polar ETLs have been reported, the detail of the interfacial dipole was still obscure and the tune of the dipole was rarely discussed for the complicated chemical and structural factors that may influence the charge distribution at the interface. For some dipole surfactant interlayer, indeed, the intensity of the interface dipole was influenced by the arrangement of the pendent side polar groups.11,13,15 In this paper, we report the construction of a definite interfacial dipole by building a layered structure of positive and negative ionized species to tune the surface work function of aluminum doped ZnO (AZO). A cross-linked perylene bisimide (poly-PBI) thin film was deposited onto AZO surface firstly (SI); and then it was reduced to radical anion state (PBI•-) using a bulky TOA+ as counter ion that was absorbed on the surface of the poly-PBI•- thin film through Coulomb force. Due to the definite interface dipole formed between the two ionized layers, the surface work function of the electrode was decreased dramatically. By using this novel cathode interlayer with definite interface dipole in i-PSCs, significantly increased PCEs were obtained in both fullerene and nonfullerene systems, which were mainly attributed to the obviously increased open circuit voltage (VOC). RESULTS AND DISCUSSION The PBI monomer (Cz4Ph2Cl4PBI) bearing four electro-cross-linkable carbazole units at two tips and four electron-deficient chlorine atoms at the bay-positions was synthesized according to our previously reported method.22 The frontier molecular orbitals of the monomer, calculated by the density functional theory (DFT) with B3LYP/6-31+G(d,p), showed two nodes at the nitrogen atoms of the imide positions, which eliminates the conjugation effect between the electron-rich


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Figure 1. (a) The molecular structure of poly-PBI. (b) Schematics of the volumes of the cations used in this work. (c) Schematic of the layered structure of poly-PBI•- and tetraoctylammonium (TOA+) that results in definite interface dipole. carbazole and electron-poor PBI units.22 These nodes preserve the intrinsic n-type properties of PBI even it was electrochemically polymerized to polymers. An AZO thin layer was deposited onto ITO-Glass (SI), on which the PBI film was deposited by electropolymerization (EP) of the Cz4Ph2Cl4PBI precursor through cyclic voltammetry (CV)


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Li+ Hybrid reduction state

Cs+ + TEA

Clear reduction state 2-

PBI

-1.0

-0.9

-0.8

Scan way

•-

+ TOA

PBI

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

Potential (V)

Figure 2. The electrochemical reduction behavior of poly-PBI film for different electrolyte. similar to our previous reports22,28,29 as shown in SI. This neutral cross-linked poly-PBI film is insoluble in common solvents, which shows reversible redox properties when applied a negative potential due to the high electron affinity of the tetra-chlorinated PBI core. In order to get insight into the electrochemical reduction performance of the porous poly-PBI film, four different kinds of cations as shown in Figure 1 were used as counter ions and different CV responses and disparate cyclic stability were observed as shown in Figure 2 and Figure S1d. For the relative small size alkali cations, the reduction behaved almost the same even with the cation changing from Li+ to Cs+. For this case, the originally staged reduction of PBI22,23 was disappeared and just one hybrid reduction state was presented as the spectraelectrochemistry proved (Figure S1), indicating the reduction of PBI was uncontrollable and an undesirable n-doping process was occurred. Moreover, the reduction state in this situation showed inferior stability which could be partially oxidized easily to neutral state in air (SI). Actually, this undesired results was predictable because of the fact that the relative smaller alkali cations could enter the internal of film easily to interact with the carbonyl groups closely24,25 and form cation-anion pair called "pin" doping effect as reported previously.26,27 However, when the relative bulky


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tetraethylammonium (TEA+) was adopted, a mixed reduction state of PBI radical anion and dianion was presented for its CV curve. Interestingly, if the cation bulk were increased from TEA+ to tetraoctylammonium (TOA+), two clear reduction peaks were obtained corresponding to two separate reduction state and so did its corresponding spectraelectrochemistry as proved by us recently,22 indicating the successful control for the reduction process of poly-PBI film. Moreover, when using TOA+ as the counter ion, the stability of PBI anion and dianion was sharply increased compared with the situation with Li+, Cs+, TEA+ (Figure S5). Thus, it was reasonable to conclude that the TOA+ was relatively difficult to enter the bulk of the film and mainly distributed on the poly-PBI•- surface. For this case, we speculated that the bulky cation TOA+ maintained a certain degree distance with the charged carbonyl groups of poly-PBI•-, a physical process (by Coulomb force) that was responsible for the stepwise reduction process and its outstanding stability (Figure S5b). Thus, we could accurately control the reduction state of poly-PBI by deliberately selecting the corresponding TOA+ cation to construct a layered zwitterion interlayer where a surface permanent dipole pointing outwards from AZO/poly- PBI•- and ending to TOA+ thin layers arised spontaneously (Figure 1c), rather than electrochemically doping as reported previously.29 Considering the excellent stability of PBI•- form and the definite layered structure of the zwitter-ion, in the following experiment an electrochemically fabricated dipole interfacial layer (poly-PBI•--TOA+) as shown in Figure 1c was investigated in detail, which was constructed by reduction of poly-PBI on AZO through potentiostatic method using TOA+ as the counter cation in supporting electrolyte (Figure S2f). We performed Electron Paramagnetic Resonance (EPR) measurement (details in SI) on the electrochemically reduced film of poly-PBI•--TOA+ (Figure 3a). One symmetric spectrum with a g-factor of 2.003 was detected with no hyperfine spectrum consistent with previous


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(a)

3

(b )

poly-PBI

Absor ban ce

1 0

poly-PBI

0.15

•+ poly-PBI -TOA

2

In tensity

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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•+ poly-PBI -TOA

0.10

0.05

-1 -2

0.00 3420

3440

3460

3480

3500

400

600

800

1000

W avelength (nm )

Magnetic fie ld [G]

Figure 3. EPR spectrum (a) and absorption spectra (b) of poly-PBI film and ionized poly-PBI•-TOA+ film. observation,30 which proves the presence of PBI•-. This observation verified the feasibility to achieve the poly-PBI•- by using the bulky TOA+ cation. Besides, the UV-Vis absorption bands also transformed from visible region to near-infrared region (Figure 3b), which also verified the formation of PBI radical anion after reduction of the film.22,23 It was worth mentioning that this spectral transformation is a benefit for the i-PSCs device because it does not interfere with the absorption of the active layer. In addition, the long-term stability of the encapsulated film of poly-PBI•--TOA+ was monitored by UV-Vis absorption under ambient conditions. Even after the film was stored in air under daylight for one month, the absorption curve is identical to that of the initial stage, which clearly demonstrated the inert behavior of the zwitter-ion film that is very important for the practical applications. Indeed, the unti-oxidation ability of poly-PBI•--TOA+ film in air at room temperature was observed to be greatly enhanced when compared with the films containing alkali cations, which is attributed to the protection of TOA+ layer on the surface of the film that isolated PBI radical anions to water and oxygen molecules in circumstance.


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Indeed, the stability of the bilayered film was rather good even when the temperature was as high as 60oC (See SI). The Kelvin Probe method was used to measure the work function (WF) of poly-PBI on AZO and its reduced state with different counter cations, and the results were summarized in Table S1. The poly-PBI neutral film on AZO showed a WF of 4.5 eV, which is slightly higher than that of the AZO film with a typical value of 4.4 eV. For the reduced film on AZO containing alkali cations, the surface WF turn out to be almost the same as that of the poly-PBI neutral film on AZO, and the film containing TEA+ showed a WF of 4.3 eV, slightly lower than that of neutral film. The bulk doping of these samples might be the reason for the minor change of the surface WF. But for the poly-PBI•--TOA+ film on AZO, it showed a much lower WF of 3.9 eV. Considering the structure difference between the bulk doping sample and the bilayer “surface doping”, we attributed the obviously decreased WF to the definite interface dipole. The decreased WF of the composite electrode is suitable to be used as cathode in i-PSCs that will be discussed in next section. Besides, the surface morphology of poly-PBI•--TOA+ film on AZO was as smooth as that of neutral poly-PBI, and both showed RMS as low as about 2 nm, as revealed by AFM images as shown in Figure S3. And the water contact-angle experiment showed increased hydrophobicity (Figure S4). Thus, series appealing features, such as good compatibility with solvent processability, the elimination of absorption in visible area, intimate contact with organic active layer, especially appropriate work function, inspired us to use AZO/poly-PBI•--TOA+ as a ETL in i-PSCs

devices.

The

device

structure

is

ITO/AZO(40nm)/poly-PBI•--TOA+(2-5

nm)/PTB7:PC71BM (90nm)/MoO3(10nm)/Al(100nm). Here, the device with neutral poly-PBI to replace poly-PBI•--TOA+ layer was fabricated for comparison. The current density-voltage (J-V)


9


100

0

(b)

poly-PBI poly-PBI•--TOA+

-3

poly-PBI

90

•+ poly-PBI -TOA

80

EQE (%)

(a)

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

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-6 -9

70 60 50 40

-12

30 20

-15

10 -18

0

0.0

0.2

0.4

0.6

0.8

300

400

500 600 700 Wavelength (nm)

Voltage (V)

800

Figure 4. (a) J-V characteristics of the i-PSCs with two different types cathode interlayers. (b) EQE spectra. Table 1. The i-PSCs device performance of different interlayers. The device configuration is ITO/cathode interlayer/PTB7:PC71BM/MoO3/Al. Average values are given based on 18 devices.

Cathode interlayer

VOC (V)

JSC (mA cm-2)

FF (%)

PCE (%)

a

Rs (Ω cm2)

b

Rsh(Ω cm2)

AZO/poly-PBI

0.56f0.01

16.84f0.17

53.02f0.21

5.00f0.23

4.55

186.72

AZO/poly-PBI•--TOA+

0.75f0.01

17.01f0.25

72.34f0.24

9.23f0.19

2.45

333.34

a

b

Series resistance Rs was defined from the J-V curves of the best devices at V=VOC. Shunt resistance Rsh was defined from the J-V curves of the best devices at V=0.

characteristics of the inverted cells under AM 1.5G irradiation at 1000 W/m2 are shown in Figure 4a. The extracted device performance metrics, including Rs and shunt resistance (Rsh), are summarized in Table 1. The device with AZO/poly-PBI cathode interlayer gave a PCE of 5% with an open-circuit voltage (VOC) of 0.56 V, a short-circuit current density (JSC) of 16.84, and a fill factor (FF) of 53.02%. While the device with AZO/poly-PBI•--TOA+ cathode interlayer showed much better performance with a PCE of 9.23% (VOC of 0.75 V, JSC of 17.01 mA cm-2, FF of 72.34%). Such a significant improvement in the PCE is mainly induced by the simultaneous increase of VOC and FF and slightly increased JSC. The obviously increased VOC for sure is


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attributed to the decreased WF of AZO/poly-PBI•--TOA+ that enhanced the build-in potential as given in the following section. Besides, the slightly increased JSC might attribute to the reduced Rs (from 4.55 to 2.45 Ω cm2) and enhanced Rsh (from 186.72 to 333.34 Ω cm2) of the AZO/poly-PBI•--TOA+ based device, which is also responsible for the enhancement of FF. The external quantum efficiency (EQE) spectra of i-PSCs in Figure 4b support the increase in JSC, and the calculated JSC obtained by integration of the EQE curves showed less than 2% mismatch compared with JSC values obtained from the J-V curves. To investigate the substantial enhancement of VOC of AZO/poly-PBI•--TOA+ based device, we explored the influence imposed by TOA salt to the device with the device configuration of ITO/AZO(40nm)/TOA salt/PTB7:PC71BM (90nm)/MoO3(10nm)/Al(100nm) (SI). The results showed that the device using AZO/TOA salt interlayer showed slightly lower performance than that of solo AZO interlayer based device (Table S3). This result indicated that the TOA salt had negligible influence in the interface electron process. Thus, the improved VOC of the AZO/polyPBI•--TOA+ based device could be mainly attributed to the reduction of the WF which was caused by the interfacial dipole. The decreased WF enables preferable matching between the cathode and the LUMO level of PC71BM (Figure 5a), which allows a better acquisition the VOC from the achievable maximum magnitude derived from the difference between the HOMO of the donor polymer and the LUMO of acceptor fullerene.16,31 To further clarify the substantial influence of the cathode interlayers on the VOC, capacitance-voltage characteristics were obtained from the devices based on different ETLs, and then Mott-Schottky (MS) analysis was performed (Figure 5b). The MS curve exhibited a linear region at moderate bias region related to the formation of a Schottky contact,31-33 allowing extraction of the built-in potential (VBI) by extending the fitting line to the intercept on the bias axis directly.31 The density of impurity (N)


11

5 4

•+ poly-PBI -TOA poly-PBI

-2

14

-2

C /10 F cm

(b)

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3 2 1 0 -0.8

-0.4

0.0

0.4

0.8

1.2

Bias /V

(c)

(d)

30

+ poly-PBI -TOA poly-PBI

26

Isc (Normalized)

28

1/2

24

1/2

•+ poly-PBI -TOA poly-PBI

1.0

•

J (A /m)

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

4


22 20 18 16

0.8 0.6 0.4 0.2

14 12

0.0 1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Effictive Voltage (V)

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Times (µs)

Figure 5. (a) Energy level diagram of the components of the inverted PSCs device based two different ETLs. (b) Mott-Schottky analysis for two different ETL based device. (c) J1/2-V characteristics of the electron-only devices with two different interlayers based on PTB7:PC71BM blend. Electron-only device configuration: Al/AZO(40nm)/poly-PBI or polyPBI•--TOA+(3nm)/PTB7:PC71BM(90nm)/CsF(10nm)/Al(100nm). (d) TPC as a function of time for the PSCs in the TPC measurement. was derived from the linear slope by the MS relation C-2=(2/qƐN)(VBI -V), where q is the elementary charge and Ɛ≈Ɛ0 was the permittivity of the blend (dielectric permittivity of 3 has been assumed). By using these parameters, the depletion width corresponding to zero applied


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bias was calculated by the equation w=(2VBI/qN)1/2. MS analysis data were all summarized in Table 2 and Table S2. It showed that the depletion width was almost the same for two different ETLs based devices. Above all, when the neutral poly-PBI was replaced by poly-PBI•--TOA+, the built-in potential VBI and N were both increased from 0.64 V to 0.87 V and 4.61 ×1016 cm-3 to 7.84×1016 cm-3. Thus the increased VOC for poly-PBI•--TOA+ based device could preferably be explained by the enhancement of VBI. And the enhancement of N may responsible for the slightly increase of JSC. Similar with the previously results reported by Jen et al that an enhanced VOC was obtained when a C60 surfactant was used to modify the cathode,31 we can also attribute to an interface dipole induced decrease of WF at the interface for the enhancement of the VBI. These results, in turn, also confirmed our expectation that dipole was vital for the regulation of electrode WF. For the case of poly-PBI•-TOA+, an interfacial dipole arise from negative PBI•- to positive TOA+ which is absent for the poly-PBI. Because the strong electron affinity and lower LUMO energy level of the neutral poly-PBI, an energy barrier may form between active layer and electrode. To better understand the improvement of JSC, electron-only devices with configuration of Al/interlayer/PTB7:PC71BM/CsF/Al were fabricated. Then SCLC model was conducted as shown in Figure 5c. The results showed that the electron-current densities of the device based on poly-PBI•--TOA+ (3.57×10-5cm2V-1S-1) were obviously higher than that of the device based on poly-PBI (2.72×10-5cm2V-1S-1). This results confirmed the improvement in the electron transport properties of the AZO/poly-PBI•--TOA+ layer, which can also account for the observed lower Rs values and higher FF values as mentioned above. Therefore, the introduction of poly-PBI•--TOA+ dipole interlayer to AZO surface can also promote interfacial electron conductivity related to the


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Table 2. Summary of the built-in potential VBI, dopant concentration N, and depletion width w of the organic/cathode Schottky contact from Mott-Schottky analysis. MS analysis was performed under ambient circumstance in the dark condition using devices prepared above with an active area of 16 mm2. The device configuration is ITO/cathode interlayer/PTB7:PC71BM/MoO3/Al.

VBI (V)

N (1016cm-3)

w (nm)

AZO/poly-PBI

0.64

4.61

67.8

AZO/poly-PBI•--TOA+

0.87

7.84

66.7

Cathode interlayer

contact resistance, which was beneficial to electron extraction and reduce the recombination losses in device.9,28 Finally, transient photocurrent (TPC) was performed at 0 bias for two kinds devices based different ETLs as shown in Figure 5d. The results showed that the decay time of device based on poly-PBI•--TOA+ were slightly less than that of the device based on poly-PBI, which might be caused by the increased VBI. In order to test the widely applicable of our definite dipole interlayer in various active layers in the devices, we also fabricated i-PSCs using a famous non-fullerene ITIC as electron acceptor and an efficient semiconducting polymer PBDB-T as the electron donating polymer (SI) and similar enhanced device performance was obtained (Figure S8). The device showed average PCE of 10.22% (see Table S4), which was much superior than that of the device with the same active layer but neutral poly-PBI interlayer (PCE of 7.37%). The enhanced device performance was also mainly attributed to the increased VOC originated from the definite interface dipole. Previously, the ionized species in the interfacial layer was regarded to be inferior for the stability of the device.34 The stability of the devices based on poly-PBI•--TOA+ under storage in N2 atmosphere was primarily tested (see Figure S9), and the results indicated that the PCE was decreased to be around 80% relative to the initial value after 40 hours but it was kept almost


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unchanged in the following days. The rather good stability is probably attributed to the huge cations that cannot diffuse to the bulky of the active layer. CONCLUSIONS In summary, a bilayered anion-cation structure was successfully constructed by electrochemical reduction of a cross-linked PBI thin film using bulky TOA+ as the counter cation. Unlike small alkali cations, the bulky TOA+ could not enter the bulk of the cross-linked film but distribute on its surface, which isolated the PBI•- to the air condition; thus the poly-PBI•--TOA+ film on AZO showed outstanding stability. In addition, a definite interface dipole formed between the PBI•layer and TOA+ layer, which effectively reduced the WF of the electrode. By using this novel cathode interlayer, the devices based on both fullerene system and non-fullerene system showed obviously increased open circuit voltage, and the average PCE values of 9.23% and 10.22% were obtained in these devices respectively. This study provided a universal method to efficiently tune the WF of electrode, which is of essential importance for organic electronic device. ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, chemical structure of PTB7, PC71BM and PBDB-T, ITIC and electrochemical experiments, spectraelectrochemistry, AFM morphology of interlayers, water contact angle images et. al. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +86-20-87110606; Tel: +86-20-22237035. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for the supports from National Natural Science Foundation of China (5171101197, 51573055, 21334002, 21733005, 51521002), the National Basic Research Program of China (973 Program) (2014CB643504), Fundamental Research Funds for the Central Universities and Key Program of Guangzhou Scientific Research Special Project (201707020024). REFERENCES (1) 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, 5994-6011. (2) He Z. C.; Zhong C. M.; Su S. J.; Xu M.; Wu H. B.; Cao Y. Enhanced PowerConversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nature Photonics 2012, 6, 591-595. (3) Stubhan T.; Salinas M.; Ebel A.; Krebs F. C.; Hirsch A.; Halik M.; Brabec C. J. Increasing the Fill Factor of Inverted P3HT:PCBM Solar Cells Through Surface Modification of Al-Doped ZnO via Phosphonic Acid-Anchored C60 SAMs. Adv. Energy Mater. 2012, 2, 532-535. (4) Hains W.; Chen H. -Y.; Reilly T. H.; Gregg B. A. Cross-Linked Perylene DiimideBased n-Type Interfacial Layer for Inverted Organic Photovoltaic Devices. ACS Appl. Mater. Interfaces 2011, 3, 4381-4387.


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TOC GRAPHICS


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Construction of Layered Structure of Anion-Cations to Tune the Work

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