High-Efficiency and Stable Organic Solar Cells Enabled by Dual

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High-Efficiency and Stable Organic Solar Cells Enabled by Dual Cathode Buffer Layers Zhaoxiang Huai, Lixin Wang, Yansheng Sun, Rui Fan, Shahua Huang, Xiaohui Zhao, Xiaowei Li, Guangsheng Fu, and Shaopeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15240 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

High-Efficiency and Stable Organic Solar Cells Enabled by Dual Cathode Buffer Layers Zhaoxiang Huai†, Lixin Wang*†, Yansheng Sun†, Rui Fan†, Shahua Huang†, Xiaohui Zhao*‡, Xiaowei Li‡, Guangsheng Fu‡ and Shaopeng Yang*†‡ †Hebei Key Laboratory of Optic-electronic Information Materials, Hebei University, Baoding 071002, P. R. China ‡College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China KEYWORDS: cathode interlayer; alcohol soluble materials; bathocuproine; stability; organic solar cell

ABSTRACT: Various cathode interface materials have been used in organic solar cells (OSCs) to realize high performance. However, most cathode interface materials have their respective weaknesses in maximizing the efficiency or stability of OSCs. Herein, three kinds of alcohol soluble cathode interfacial materials are combined with bathocuproine (BCP) to serve as multifunctional bilayer cathode buffers for the regular OSCs, and thus greatly enhanced power conversion efficiencies (PCEs) over 10.11%, and significantly improved device stability have been achieved. By utilizing double interlayers, both light absorption and light distribution in active layer

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are improved. Furthermore, double interlayers offer favorable energy-level alignment, alcohol treatment and duplicate protection of active layer, resulting in significantly reduced leakage current, suppressed recombination and efficient charge collection. The improved device stability is related to the blocking effect of the complex formed between BCP and the metal electrode, and the additional protection effect of the underlying alcohol soluble materials. In view of the universal use of alcohol soluble organic electrolyte as cathode buffer layers, and by courtesy of the superiority of the double cathode layers relative to the monolayer controls, the double interlayer strategy demonstrated here opens a new way to fully exploiting the potential of OSCs and is believed to be extended to a wider application.

1. INTRODUCTION Organic solar cells (OSC) have received extensive research efforts due to their advantages of synthetic variability, light-weight, low-cost and easy fabrication1–3. In recent years, single junction OSCs with power conversion efficiencies (PCEs) >10% have been widely reported4–9. A variety of donor and acceptor materials as the light absorber have been synthesized to harvest sun light as much as possible. Among them, the narrow bandgap polymer of PTB7-Th has been widely used as the donor in OSCs to achieve high performance.4,5,7,8,10,11 Solvent additives are commonly used to improve the morphology of active layer. However, residual additive in the active layer may cause undesirable vertical phase separation or surface segregation of specific components12. In addition, Bertrand et al. have shown that the widely used solvent additive of DIO may accelerate photo-degradation of PTB7-Th13. To solve these problems, it has been demonstrated that removal of the residual solvent additive can be achieved by treatment


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with polar solvents such as methanol and ethanol, which are usually used as the solvent of alcohol soluble cathode interface materials13–16. Therefore, using alcohol soluble interfacial materials is a dual-functional strategy, which not only help active layer form ohmic contact with electrode, but also improve morphology of the active layer. A lot of alcohol soluble conjugated polymers have been synthesized to achieve high device performance, such as poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)17,18and its derivatives. The other kind of alcohol soluble cathode interfacial material is small molecules, among which perylene diimide derivatives (PDIN/PDINO) have been successfully used in OSCs19. In addition to organic alcohol soluble cathode interface materials, an ethanol soluble metal complex of zirconium acetylacetonate (ZrAcac) was designed as an effective cathode interface layer (CIL), and shows enhanced performance for various bulk hetero-junctions (BHJs)20. Besides alcohol soluble cathode interface layer (alco-CIL) materials, sparingly soluble and thermally evaporated small-molecule CILs also function well in many cases21 Typically, bathocuproine (BCP) that possesses a large bandgap and a deep HOMO energy level can be used to block excitons generated in active layer and prevent excitons quenching at the interface of the cathode/active-layer22. It has also been reported that BCP can act as an optical spacer to tune the optical field in active layer.23,24 Additionally, BCP can work as a diffusion barrier for metal ions.23,25 However, aforementioned cathode interface materials all have their weakness when used alone in OSCs. PFN and ZrAcac are usually combined with Al to serve as efficient cathode for OSCs, however, it is detrimental to the long-term device stability due to the chemical reaction of low work function Al cathode with air. Furthermore, low conductivity of PFN and PDIN results in difficulty in obtaining thick film to protect the underlying active layer and to utilize the roll-to-roll


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(R2R) technology. As for BCP, charge transport occurs through the damage-induced traps generated during the deposition of metal cathode. The residence of these traps in deep level induces an electric filed that is opposite to the filed induces by applied voltage, resulting in increased recombination at the interface of active layer and BCP.26 To solve these problems, double CILs strategy was supposed to eliminate the weakness of single CIL. However, most efforts are focused on the cathode in inverted OSCs, in which various materials are used to modify the contact of metal oxides and active layer.8,27–30 The application of a double CILs strategy in conventional OSCs is rare.31 Recently, we reported a high efficiency non-fullerene organic solar cell based on a cathode structure of PDIN/BCP/Ag.32 However, the mechanism of the superior device performance induced by doubleCILs has not been thoroughly explored. In this work, we evaporate a thin layer of BCP on top of three different kinds of alco-CILs (PDIN, PFN and ZrAcac) to form the multifunctional doubleCILs as an active layer protector, an interfacial modulator, an optical spacer, and a diffusion barrier. Furthermore, the morphology of active layer can also be modified with alcohol when spinning coating alco-CILs. The OSCs incorporating double CILs show simultaneous improvement in JSC and FF, resulting PCEs over 10% for devices with PDIN/BCP/Ag and PFN/BCP/Ag, and PCE over 9.5% for devices with ZrAcac/BCP/Ag, all of which are significantly higher than those values of the controls that is below 8.5%. Additionally, the conventional devices with double CILs show significantly improved stability in N2 due to the role of metal ions diffusion barrier of BCP. 2. Results and Discussion 2.1. Device Performance


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A commonly used polymer, PTB7-Th, is chosen as the donor and PC71BM as the acceptor to fabricate single junction device with conventional structure. In previously reported PTB7Th:PC71BM and other OSCs, PDIN, PFN and ZrAcac (denoted as alco-CIL) have been successfully used as cathode interlayer to modify the contact of cathode Al with active layer, and high PCEs over 8% were achieved.20,33,34 However, their environmental stability is poor due to the chemical reaction of cathode Al.35 To improve the environmental stability of PTB7-Th:PC71BM OSCs, we first simply replace Al with more air-stable cathode Ag. Therefore, devices of glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PDIN(or PFN, ZrAcaca)/Al(or Ag) are fabricate, and devices with only Al and Ag cathode are fabricated as well. The optimal thickness of PDIN, PFN and ZrAcac are 10nm, 3nm and 11nm, respectively. The current density-voltage (J-V) characteristics are plotted in Figure 1a and 1b, and the detailed photovoltaic parameters are listed in Table 1. Interestingly, the reference Al-only device exhibit a PCE of 7.22% with a VOC of 0.82V, which is contrast to low VOC in previous reports.34,36 This high V OC indicates that low-vacuumdeposited ( ≈10-4 Pa) Al itself can form ohmic contact with PTB7-Th:PC71BM without any modified layer due to its low WF (≈3.4eV).37,38 Devices with PDIN/Al, PFN/Al and ZrAcac/Al show improved and similar performance with PCEs 8.25%, 8.17% and 8.35%, and these results are comparable to previous reports.33,34,39 When Al was replaced by Ag, the single Ag device exhibit poor photovoltaic performance with the V OC of 0.33V and FF of 29.19%. It is well known that the VOC of BHJ OSCs can achieve maximum only if both anode and cathode form ohmic contact with BHJ.40 Such low VOC for Ag only device may be attributed to the Schottky contact between BHJ and Ag because of the high WF (≈4.6eV) of Ag. After adding alcohol soluble CILs between BHJ and Ag, only PDIN/Ag device exhibit comparable performance with Al based devices. The photovoltaic parameters of PFN/Ag and ZrAcac/Ag devices all get improved, but are


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still lower than those of PDIN/Ag device. Interestingly, despite relatively large difference of V OC and FF of these three alco-CIL/Ag devices, the JSC are almost comparable. This suggests that the field is sufficiently strong to extract the generated charges under short circuit condition, while it is not for the Ag only device. 4

(b)

0

Current Density (mA/cm2)


(a)

Al PDIN/Al PFN/Al ZrAcac/Al

-4 -8 -12 -16 -20

(c)

4 0

4

Ag PDIN/Ag PFN/Ag ZrAcac/Ag

0 -4 -8 -12 -16 -20

0.0


0.2

0.4 Voltage (V)

0.6

0.8

0.0

(d)

0.2

0.4 Voltage (V)

0.6

0.8

80 70 60

BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

-4 -8

EQE (%)

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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-12

50

PDIN/Al PFN/Al ZrAcac/Al BCP/Ag PFN/BCP/Ag PDIN/BCP/Ag ZrAcac/BCP/Ag

40 30 20

-16

10 0

-20 0.0

0.2

0.4 Voltage (V)

0.6

0.8

400

500

600

700

800

Wavelength (nm)

Figure 1. (a)-(c) J-V characteristic curves of the PTB7-Th:PC71BM OSCs with different cathode interfacial layers under the illumination of AM 1.5G irradiation (100 mW cm-2). (d) EQE spectral of PTB7-Th:PC71BM OSCs with different cathodes. Considering the high WF of Ag and the WF of PFN (or ZrAcac) modified Ag may still not low enough to form ohmic contact with BHJ, a thin layer of BCP that can largely lower the WF of various metal was introduced, and is evaporated on alco-CILs before Ag. During the deposition of


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Ag on BCP, BCP can interact with Ag and generate gap states near Fermi level to facilitate charge transport and collection.41 To investigate the influence of alco-CIL/BCP/Ag on the performance of OPVs, devices glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PDIN(or PFN, ZrAcaca)/BCP(4 nm)/Ag were fabricated (as shown in Figure 2a). In addition, device with BCP/Ag is also fabricated as reference. The chemical structures of PDIN, PFN, ZrAcac and BCP are depicted in Figure 2b. (a)

A

(b)

BCP

Ag BCP

ETL

PFN

PTB7-Th:PC71BM PEDOT:PSS ZrAcac

ITO

PDIN

Figure 2. (a) Configuration of conventional structured OSCs fabricated in this work, ITO/PEDOT:PSS/PTB7-Th:PC71BM/alcohol soluble materials/BCP/Ag. (b) Chemical structures of the cathode interface materials BCP, PDIN, PFN and ZrAcac. The J-V characteristics of champion devices with alco-CIL(or without)/BCP/Ag are plotted in Figure 1c, and the detailed photovoltaic parameters are listed in Table 1. Device with BCP/Ag exhibit comparable performance with alco-CIL/Al based devices. The relatively low FF of BCP/Ag device is probably due to the recombination induced by trap states in BCP as mentioned earlier. Optimal devices based on PDIN/BCP/Ag and PFN/BCP/Ag afforded PCEs 10.05% and


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10.11%, and optimal devices based on ZrAcac/BCP exhibited a PCE 9.66%, all of which significantly outperform the mean PCEs of alco-CIL/Ag based devices, even alco-CIL/Al based devices. As clearly shown, with the incorporation of BCP between alco-CIL and Ag, the JSC and FF get promoted simultaneously compared with devices without BCP. Most importantly, the VOC increases significantly from 0.55V and 0.73V to 0.814V for PFN and ZrAcac based devices. To highlight the superior of alco-CIL/BCP/Ag, devices with alco-CIL/Al and BCP/Ag are used as reference in the rest of this work. Besides improved VOC by inserting a BCP layer in alco-CIL/Ag interface, JSC is significantly improved compared with all the other single CIL devices. Figure 1d shows the external quantum efficiency (EQE) spectral of devices with alco-CIL/Al, BCP/Ag and alco-CIL/BCP/Ag. Enhanced EQE of BCP/Ag and alco-CIL/BCP/Ag based devices present an average value of 70% in the range 450-700nm, which agree well with the increased measured J SC under AM 1.5G illumination, while it is only around 60% for devices with alco-CIL/Al. Since the enhancement of EQE is not in the whole range of wavelength, it is more like an optical change, which will be discussed later. In addition to improved J SC, the FF is also found to be around 70%, no matter what it is for devices with single interlayer. It suggests that charge transport in alco-CIL/BCP/Ag is significantly improved. It is well known that FF is associated to series resistance (Rs) and shunt resistance (Rsh). Surprisingly, an additional layer of BCP does not increase the Rs to the bulk. This suggests that only alco-CIL is not optimized for charge transport. It is probably because that alco-CILs are not dense enough to protect underlaying active layer from Ag deposition, which would create carrier traps or unfavorable interface dipole and hinder the charge transport. To verify this, device with thinner PDIN is fabricated, and the Rs increases from 6.91Ωcm2 to 8.96Ωcm2 , indicating that a enough thick interlayer is beneficial for the charge extraction. In addition, the BCP-metal complex


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is believed to be positive in the BCP/metal interface, which would enhance the internal electric field. The improved internal electric filed can facilitate the charge transport and extraction, and hence increase the FF.42 Table 1. Photovoltaic Parameters of Regular PTB7-Th:PC71BM OSCs with and without Different Cathode Interface Layer under AM 1.5G Irradiation (100 mW cm-2).

VOC (V)

JSC (mA/cm-2)

FF (%)

Average PCEa (%)

Max PCE (%)

Rs (Ωcm2)

Rsh (Ωcm2)

Al

0.820

13.86

63.58

6.83

7.22

9.28

356.30

PDIN/Al

0.820

14.72

68.82

8.25

8.31

6.91

460.07

PFN/Al

0.802

15.87

65.47

8.17

8.33

6.65

354.19

ZrAcac/Al

0.814

15.13

69.14

8.35

8.45

5.26

532.54

Ag

0.334

12.48

29.19

1.18

1.21

21.98

51.94

PDIN/Ag

0.814

16.50

65.68

8.62

8.82

4.58

582.61

PFN/Ag

0.550

15.85

52.27

4.52

4.55

8.80

488.33

ZrAcac/Ag

0.730

15.12

60.25

6.52

6.65

9.29

466.19

BCP/Ag

0.815

15.97

65.16

8.22

8.48

6.11

561.93

PDIN/BCP/Ag

0.815

17.62

70.07

9.94

10.05

4.02

1051.77

PFN/BCP/Ag

0.814

17.84

69.55

9.97

10.11

4.34

1282.88

ZrAcac/BCP/Ag

0.814

16.91

70.23

9.51

9.66

4.24

1259.14

Device

a

The average values of PCE were based on 12 individual devices.

2.2 Energy-Level Alignment In order to investigate the role of BCP in improving the photovoltaic characteristics of alcoCIL/BCP/Ag devices, energy-level alignment between BHJ (i.e., PTB7-Th:PC71BM) and cathode was studied using UPS. Figure 3a shows the UPS spectra of different cathode interlayers on


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ITO/PEDOT:PSS/BHJ. Compared with reference substrate BHJ, the secondary electron cutoff shift to higher binding energy after depositing PFN and ZrAcac, indicating downward shift of the vacuum level. This suggests that charge transfer induced dipoles are formed in the BHJ/PFN and BHJ/ZrAcac interfaces, while there is no dipole in the interface of BHJ/PDIN. When additional BCP is deposited on alco-CILs, secondary electron cutoffs are all shifted to higher binding energy, indicating that an addition dipole is formed in the alco-CIL/BCP interface. Moreover, HOMOs of BHJ and different CILs on BHJ can also be obtained in Figure 3a (right panel). Note that interlayer(s) involves two interfaces in one OSC, that is BHJ/CIL and CIL/metal electrode. Without taking into account both interfaces, it is hard to give a correct energy-level alignment between BHJ and cathode.37 Therefore, the WF changes in CIL/Ag interfaces are analyzed by measuring UPS spectra of different CILs on Ag as shown in Figure 3b. Based on the secondary electron cutoff shifts, after depositing PDIN, PFN, ZrAcac and BCP on Ag, the WF are lowered from 4.7eV to 3.92eV, 4.36eV, 4.41eV and 3.55eV, respectively. According to above UPS spectra results, energy-level alignments of BHJ and cathodes are illustrated in Figure 3c. Interestingly, depositing PDIN on BHJ does not induce vacuum energy level shift. But the WF of PDIN/Ag (3.92eV) is lower than that of BHJ/PDIN (4.52eV), which would result in electrons transfer from Ag to BHJ through PDIN. Due to the electrons transfer, an additional electric field forms across the PDIN, oriented in the same favorable direction for photogenerated electron extraction as the built-in field.43 Being different from PDIN, depositing PFN on BHJ directly reduce the WF of BHJ, indicating a dipole is induced in the interface. However, since the WF of PFN modified Ag is higher than that of BHJ/PFN, electrons will transfer from BHJ to Ag after depositing Ag on PFN, which increase the WF of BHJ near the BHJ/PFN interface. This suggests that the built-in potential, which is the upper limit of the attainable V OC,44 may be


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decreased. This deduction can be confirmed by the lowered turn-on voltage determined by dark JV of PFN/Ag device compared with PFN/Al device, as shown in Figure S1. The energy-level alignment of BHJ/ZrAcac/Ag is similar to that of BHJ/PFN/Ag. Therefore, the low VOC and FF of devices with PFN/Ag and ZrAcac are attributed to low built-in potential caused by electrons transfer from Ag to BHJ because of high WF of PFN/Ag and ZrAcac/Ag. When inserting BCP between alco-CILs and Ag, energy-level alignment will be more favorable. On one hand, depositing BCP induces an additional dipole in the alco-CIL/BCP interface, which can enhance the selectivity of charge transport and electrons extraction. On the other hand, extremely low WF of Ag/BCP makes the electrons transfer from Ag to BHJ that ensures strong enough built-in potential. With these favorable energy-level alignments, the VOC, JSC and FF of devices with alco-

Fermi Level

(a) BHJ/ZrAcac/BCP BHJ/PFN/BCP BHJ/PDIN/BCP BHJ/ZrAcac BHJ/PFN BHJ/PDIN BHJ 21 20 19 18 17 16 15 4

3

2

1

Normalized Intensity (arb. units)

CIL/BCP/Ag are expected to enhance compared with devices with alco-CIL/Ag. Normalized Intensity (arb. units)

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

Binding Energy (eV)

(b) Ag/BCP

Ag/ZrAcac

Ag/PFN

Ag/PDIN

Ag 20

19

18

17

16

15

Binding Energy (eV)


11


(c)

Evac

4.52

EF

Evac

4.52

4.21 EF

0.37

0.37

1.63

BHJ

PDIN PDIN on on BHJ Ag

BHJ

Evac

4.52

EF

4.36

4.52

EF

4.52

PFN on BHJ

PFN on Ag

BHJ

0.37

1.63 2.03

PDIN BCP BCP on on on BHJ BHJ/ Ag PDIN

4.52

4.21 3.86 3.55 EF

EF 1.42 2.41

BHJ

4.11

4.41

1.07

ZrAcac ZrAcac on on BHJ Ag

Evac

4.52 3.86 3.55

0.37

1.42

Evac

0.37

BHJ

Evac

4.52 3.92

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PFN BCP BCP on on on BHJ BHJ/ Ag PFN

0.37

4.11 3.76 3.55

1.07

2.52

ZrAcac BCP BCP on on on BHJ BHJ BHJ/ Ag ZrAcac

Figure 3. (a) UPS spectra of different cathode interlayers depositing on ITO/PEDOT:PSS/PTB7Th:PC71BM. (b) UPS spectra of different cathode interlayers depositing on Ag. (c) Schematic energy-level diagram of different layers relative to Fermi level. 2.2. Light Absorption Because of optical interference between the incident light and back-reflected light,45 the spatial distribution of light intensity is not uniform in the whole OSC. When inserting a second CIL, the optical electric-field will be redistributed. As Alberto et al. have shown, the incorporation of BCP


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layer as optical spacer not only brings an electrical improvement but also an optical improvement.24 To verify if the light intensity in active layer get promoted due to the spatial redistribution of light intensity, reflectance spectrum of complete devices with different CILs is measured. As shown in Figure 4, the reflectance of devices with BCP/Ag is higher than that of devices with alco-CIL/BCP/Ag between 500-700nm, indicating those devices with alcoCIL/BCP/Ag can absorb more light in that range, which can be responsible for the enhanced JSC. It is worth noting that this difference is not due to the absorption of alco-CILs (shown in Figure S2), because there is nearly no absorption between 500-700nm for PFN and ZrAcac. Despite the different absorption of CILs, the reflectances exhibit similar profile. At this point, the reason for decreasing reflectance is either the redistribution of light intensity by inserting alcohol soluble CIL or the changed absorption of active layer by solvent-fluxing with alcohol or both of them. It has been shown that the absorption of several polymer:fullerene systems with solvent-fluxing and vacuum-drying are almost identical.46 Therefore, we conclude that the enhanced absorption is attributed to the redistribution of light intensity due to alco-CIL/BCP. 70

(a)

PDIN/Al PFN/Al ZrAcacAl BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

60 Reflectance (%)

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


50 40 30 20 10 0 300

400

500 600 Wavelength (nm)

700

800


13


24

(b) 21 Reflectance (%)

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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18 15 12 9 450

500

550 600 Wavelength (nm)

650

700

Figure 4. (a) Reflectance spectra of PTB7-Th:PC71BM OSCs with different cathodes; (b) Magnification of reflectance spectral from 450nm to 700nm. However, devices with alco-CIL/Al and alco-CIL/BCP/Ag cannot be compared directly because of the different reflectance of the applied cathodes of Ag and Al (shown in Figure S3). To remove the effect of cathode, we fabricated device with alco-CIL/Ag based devices with Ag as cathode and measured their reflectance. As expected, devices with alco-CIL/Ag cathode exhibit much higher reflectance than that with alco-CIL/Al as shown in Figure S4. Indeed, despite similar work function of PDIN treated Al/Ag/Au to form ohmic contacts with fullerene acceptor, the J SC of devices with these cathode differ with each other significantly, 19 which imply the great impact of cathode reflectance on the OSC performance. Therefore, the use of BCP in our OSCs not only contribute to electron extraction, but also help get more absorption to enhance JSC.

2.3. Charge Generation To examine if the generated excitons are effectively dissociated into free charges after absorbing light and subsequently move to the electrodes before recombination process take place,


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photocurrent density are measured in a wide reverse bias range under AM 1.5G illumination. Figure 5 plots the photocurrent density (J ph = J L - JD) versus the effective applied voltage (Veff = V0 - Vapp), where the JL and JD are the current density under illumination and in the dark, respectively, V0 is the compensation voltage at which Jph = 0, and Vapp is the applied voltage. With Veff increasing, the electric field increases, which lead more electron-hole pairs to dissociate and migrant to electrodes. Finally, J ph reaches saturation at the large Veff (> 2), where all photogenerated excitons are dissociated into free charges and all charges are collected at the electrodes

(a)

10

10

1

0.1 0.01

(b)

PDIN/Al BCP/Ag PDIN/BCP/Ag 0.1 1 Effective Voltage (V)

1

0.1 0.01

Photocurrent Density (mA/cm2)

10

Photocurrent Density (mA/cm2)

without any recombination. Photocurrent 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


PFN/Al BCP/Ag PFN/BCP/Ag 0.1 1 Effective Voltage (V)

(c)

1

0.1 0.01

ZrAcac/Al BCP/Ag ZrAcac/BCP/Ag 0.1 1 Effective Voltage (V)

A

Figure 5. Photocurrent density versus effective voltage characteristics of PTB7-Th:PC71BM based OSCs with BCP/Ag and (a) PDIN/Al and PDIN/BCP/Ag, (b) PFN/Al and PFN/BCP/Ag, and (c) ZrAcac/Al and ZrAcac/BCP/Ag. In saturation case, saturated photocurrent density (Jsat) is only limited by the amount of absorbed photons18,47 when the active layers are identical, and is independent of the mobility of electrons or holes.48 The Jsat values are 18.15, 17.8 and 17.53mA/cm2 for devices with PDIN/BCP/Ag, PFN/BCP/Ag and ZrAcac/BCP/Ag, which are slightly larger than that (17.22 mA/cm2) of device with BCP/Ag and distinctly larger than that (16.00, 15.90 and 15.36 mA/cm2 ) of devices with PDIN/Al, PFN/Al and ZrAcac/Al. Larger Jsat imply that devices with BCP/Ag and alco-


15


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CIL/BCP/Ag can absorb more photons. Considering the active layer in all devices here are identical and have the same thickness, the photons absorbed by incident light is identical. Hence, absorbed photons flux is only influenced by the light reflected by cathode. As revealed by reflectance in previous section, light reflected by cathode is apparently enhanced by Ag compared with Al, and light intensity distribution is improved by combining alco-CILs and BCP/Ag. Therefore, it is further demonstrated that the enhanced J SC for devices with BCP/Ag and alcoCIL/BCP/Ag is due to more light absorption induced by enhanced light reflection by Ag and light intensity redistribution. At low Veff, only a fraction of photogenerated excitons will dissociate into free carriers, and the dissociation probability can be calculated from J ph/Jsat. Table 2 shows the photocurrent density of the devices at different conditions of short-circuit current, maximal power output and saturation Jph (Veff = 2V) (denoted as Jph,sc, Jph,mp and Jsat) as well as the corresponding Jph/Jsat for devices with different CILs. Specifically, at the short-circuit condition, Jph,sc/Jsat for devices with BCP/Ag, PDIN/Al and PFN/Al are lower than that for devices with alco-CIL/BCP/Ag, indicating alcoCIL/BCP/Ag can effectively suppress recombination. Interestingly, J ph/Jsat for devices with ZrAcac/Al is almost the same with that for devices with alco-CIL/BCP/Ag, suggesting efficient recombination suppression ability of ZrAcac itself, which is consistent with the highest FF among devices with single CIL. At maximal power output condition, J ph,mp/Jsat are accordance with the trend at the short current condition, except for larger difference. Table 2. Photocurrent Density at Short Current Condition, Maximal Power Output Condition and Saturation Condition (Veff = 2V) and Corresponding Jph/Jsat for Devices with Different CILs.

Device

Jph,sca (mA/cm2)

Jph,mpb (mA/cm2)

Jsat (mA/cm2)

Jph,sc/Jsat (%)

Jph,mp/Jsat (%)


16

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PDIN/Al

14.93

12.53

16.00

93

78

PFN/Al

14.97

12.54

15.90

94

79

ZrAcac/Al

14.80

12.70

15.36

96

83

BCP/Ag

15.89

13.21

17.22

92

77

PDIN/BCP/Ag

17.41

15.09

18.15

96

83

PFN/BCP/Ag

17.02

14.79

17.80

96

83

ZrAcac/BCP/Ag

16.89

14.89

17.53

96

85

a

The Jph,sc is Jph at short current condition; bThe Jph,mp is Jph at maximal power output condition.

The ratio Jph/Jsat associates with exciton dissociation efficiency and/or charge collection efficiency. It has been shown that methanol treatment could induce passivation of surface traps, increase hole mobility and reduce charge recombination, 15 and thus enhanced charge collection efficiency can be achieved, which partly explain the relatively higher J ph/Jsat at low Veff for device with alco-CIL. However, such alcohol treatment alone cannot explain the improvement of photocurrent from alco-CIL/Ag to alco-CIL/BCP/Ag. Thus, interface property must be accountable for the improved charge generation. Again, as mentioned earlier, alco-CILs are not dense enough to protect underlaying active layer from Ag deposition, which would create carrier traps and hinder the charge transport. Therefore, utilizing BCP as the second interlayer can protect the active layer to suppress recombination and thus improved charge generation.

2.4. Charge Transport To understand the effects of al-CIL/BCP/Ag on charge transport, we first measure the J-V characteristics under dark condition. As shown in Figure 6, the device with single BCP CIL shows larger leakage current than all the alco-CIL/BCP/Ag based devices during the voltages below 0.7V. This result implies that alco-CIL/BCP could protect underlying active layer and effectively


17


suppress leakage which was caused by pin holes and traps.49 The possible reason for large leakage current of device with BCP/Ag is that the damage depth (~10nm) 50 of the BCP by hot Ag atoms is larger than the thickness of BCP in our device (4nm), which induce direct contact between the photoactive donor and electrodes where high densities of carrier traps exist. 51,52 Meanwhile, PFN alone induces larger leakage than BCP, which may attributed to its thinner thickness (3nm) and incomplete/weak coverage on active layer, 37,53 inducing damages of active layer during metal cathode deposition. This is also consistent with the relatively low FF of device with PFN/Al and BCP/Ag. In addition, decreasing reverse current densities for alco-CIL/BCP/Ag devices indicate that holes injected from metal electrode are effectively suppressed, hence hole can be blocked effectively from active layer to electrode. (a)

101 100

PDIN/Al BCP/Ag PDIN/BCP/Ag

10-1 10-2 10-3 10

(b)

101 100

PFN/Al BCP/Ag PFN/BCP/Ag

10-1 10-2 10-3 -4

-4

-2

102


102



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

Page 18 of 35

10 -1

0 1 Voltage (V)

2

-2

-1

0 1 Voltage (V)

2

102 101 100

(c) ZrAcac/Al BCP/Ag ZrAcac/BCP/Ag

10-1 10-2 10-3 10-4 10-5 -2

-1

0 1 Voltage (V)

2

Figure 6. Dark J-V characteristics of PTB7-Th:PC71BM OSCs with BCP/Ag and (a) PDIN/Al and PDIN/BCP/Ag, (b) PFN/Al and PFN/BCP/Ag, and (c) ZrAcac/Al and ZrAcac/BCP/Ag. At intermediate positive voltage, the line in exponential regime is determined by two parameters: one is the ideality factor, which can be calculated from the slope of the line; another one is reverse saturation current, which can be calculated from the y-axis intercept. The ideality factor reflects the recombination behavior with respect to internal BHJ morphology. 54 As clearly shown in Figure 6, the slope of exponential regime of devices with alco-CILs are almost identical, and are slightly larger than that of device with BCP/Ag, indicating alcohol-fluxing does affect the morphology of


18

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active layer. It was demonstrated that the ideality factor obtained from dark J-V curve is determined by the dominant type of charge carrier (electron here). 55 The result indicates that electron transport can be enhanced by alcohol-fluxing due to suppressed recombination. The reverse saturation current reflects the number of charges (holes) able to overcome the energetic barrier in the reverse bias.54,56 Similarly, the reverse saturation current of devices with alco-CIL are almost identical, and are slightly lower than that of device with BCP/Ag (see Figure S5), indicating the main hole blocking CIL are alco-CIL rather than BCP. At higher voltage, dark current is limited by the series resistance. Relatively high current density of devices with alco-CIL/Al and alco-CIL/BCP/Ag are all larger than that with BCP/Ag, which is consistent with decreased series resistance (shown in Table 1) deduced from J-V curve under illumination. This result implies enhanced electron transport in BHJ and/or at interface of BHJ/CIL as aforementioned discussion. Figure 7 shows the typical optimization progress of PFN/BCP/Ag from PFN/Al and BCP/Ag.

Cathode Optimazation

BCP Al Ag PFN PTB7-Th:PC71BM Figure 7. Optimization progress of PFN/BCP/Ag (down) from PFN/Al (up left) and BCP/Ag (up right) to the PTB7-Th:PC71BM based regular OSCs. For PFN/Al and BCP/Ag cathodes, metal atoms penetrate through PFN and BCP to form direct contact with active layer, respectively. Lighter yellow reflected arrow in BCP/Ag cathode represents stronger reflectance by Ag than that


19


by Al in PFN/Al cathode. By utilizing PFN/BCP/Ag, metal atoms are effectively blocked, and more light can be absorbed compared with PFN/Al. 2.5. Device Stability Stability is a practical factor that must be considered when commercializing OSCs. Generally speaking, OSCs with a conventional architecture exhibit faster degradation than inverted ones. In this work, the stability of unencapsulated devices with alco-CIL/Al, BCP/Ag and alcoCIL/BCP/Ag under N2-filled glovebox are tested. 1.0

(a)

0.8 PDIN/Al PFN/Al ZrAcac/Al BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

0.6 0.4 0.2 0.0 0

1.0

Normalized Voc

Normalized PCE

1.0

10

20 30 Time (days)

(b)

0.9 PDIN/Al PFN/Al ZrAcac/Al BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

0.8 0.7 0.6

40

50

0

(c)

1.0

10

20 30 Time (days)

40

50

40

50

(d)

0.6

Normalized Jsc

0.8 Normalized FF

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

Page 20 of 35

PDIN/Al PFN/Al ZrAcac/Al BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

0.4 0.2 0.0

0.8

PDIN/Al PFN/Al ZrAcac/Al BCP/Ag PDIN/BCP/Ag PFN/BCP/Ag ZrAcac/BCP/Ag

0.6

0.4 0

10

20

30

40

50

0

Time (days)

10

20

30

Time (days)

Figure 8. Normalized (a) PCE, (b) JSC, (c) VOC, and (d) FF of PTB7-Th:PC71BM based regular OSCs with different cathodes as a function of storage time in N 2-filled glovebox. Devices with alco-CIL/Al degrade fast in the first 3 days, while devices with BCP (BCP/Ag or alco-CIL/BCP/Ag) remain almost unchanged (see Figure 8). Specifically, VOC for all devices keep


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almost unchanged, while FF exhibits fast decrease for devices with PFN/Al and ZrAcac/Al and JSC exhibit fast decrease for devices with PDIN/Al and PFN/Al. At this stage, since CILs in alcoCIL/BCP/Ag based devices do not induce significant decreasing performance, the degradation in single CIL based devices does not come from the chemical or physical change of alco-CILs or active layer by themselves. However, the alco-CILs or active layer may degrade by the penetration/diffusion of metal cathode. As previously reported, the optimal thickness of PDIN, PFN and ZrAcac are less than or around 10nm, which make Al penetration/diffusion into the active layer possible. This penetration/diffusion of Al into active layer can lead to recombination sites for exciton and/or charge carriers,57 especially for ultrathin PFN. This is reasonable when considering the initial FF of PFN/Al based device and its fast decreasing FF and/or J SC. To verify the negative effect of diffusion of Al, alco-CILs/Al based devices with varying alco-CILs thickness are fabricated, and their stability is tested for 1 week (see Figure S6). For devices with PDIN(22 nm)/Al and ZrAcac(23 nm)/Al, thicker film of CIL does induce better device stability, indicating the diffusion of Al to active layer dominates the degradation. Note that, although the device stability can be improved by increasing the alco-CIL thickness, it is still distinctly inferior compared with alco-CIL/BCP/Ag devices. For example, device with PDIN(22 nm)/Al only maintains 92% of its initial PCE, while device with PDIN(10 nm)/BCP/Ag maintains 98%. This suggests that it is BCP that improve the stability by blocking metal (Al or Ag). For device with PFN/Al, however, the result is contrast to that for PDIN or ZrAcac based devices. PFN/Al device with thicker PFN layer (6 nm) exhibits faster degradation than that with thinner PFN (3 nm), and J-V curve of device with thicker PFN layer shows serious “S-shape” characteristics soon while it is not in the beginning. The S-shaped J-V curve is common when the devices are fabricated with low-conductivity interlayer that is too thick. This is evidenced by about a 13-fold increase in R s


21


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from 8.4Ωcm2 to 111.4Ωcm2 after a week. Again, considering much improved stability of PFN/BCP/Ag device, it seems that the conductivity of PFN is decreased due to direct contact with Al. From this point of view, BCP can not only protect active layer by blocking metal diffusion, but also can protect alco-CIL from its direct contact with metal. As for BCP, it can serve as a ligand that forms stable complexes with metal ions, such as Cu2+and Al3+,22,58 which prevent metal diffusion to underneath layer after cathode deposition. 59 We believe that the role of BCP in our devices can also prevent Ag diffusion to active layer. In alcoCIL/BCP/Ag based devices, most of the Ag are blocked by the complex of BCP and Ag in the first place, only a small number of Ag diffuse through BCP; then alcohol soluble CILs again block Ag. As a result, almost all the Ag can be blocked by the alco-CIL/BCP and the stability of alcoCIL/BCP/Ag based devices are significantly enhanced. Afterwards until 50 days, the positive effect of BCP disappeared when it was found that V OC of device with BCP/Ag decreased linearly and distinctly, while others remained almost unchanged. Meanwhile, JSC of all devices only exhibit a slight decrease. Since the J SC does not change a lot for the BCP/Ag device, which means active layer itself does not degrade, we speculate that the decrease of VOC is due to the degrade of BCP itself. At this point, it is beneficial to insert a layer of alcohol soluble CIL, avoiding the direct contact of PTB7-Th:PC71BM and BCP, which is detrimental to BCP to maintain its role to lower the WF of Ag. Notably, one can suspect that the Ag diffusion to active layer through ultrathin BCP may influence the VOC. However, this can be ruled out considering the device with similar ultrathin layer of PFN allowing Ag diffusion. As for FF, it is more complicated. It decreases slightly faster for devices with BCP (BCP/Ag or alcoCIL/BCP/Ag) than that for devices with alco-CIL/Al, except for PDIN/BCP/Ag based device. And interestingly, the PDIN/Al based device recovers its FF from 92% in 6 days to 98% in 45 days.


22

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Taking above parameters together, PCEs are indeed maintained higher for alco-CIL/BCP/Ag based devices than that for single CIL based devices, indicating the effectiveness of our dual cathode interlayer strategy. Most surprisingly, the PDIN/BCP/Ag based device still maintain about 98% of its initial PCE after 47 days, while it is only about 78% for PDIN/Al based device and about 56% for BCP/Ag based device.

3. Conclusion In conclusion, high performance and stable OSCs are successfully fabricated with double cathode interfacial layers combining an alcohol soluble material layer and a BCP layer, resulting maximum PCEs over 10% for PTB7-Th:PC71BM devices. First of all, favorable energy-level alignment of alco-CIL/BCP/Ag ensures the efficient electrons transport and extraction. With the incorporation of BCP/Ag, large J SC values are achieved due to enhanced light absorption and improved light intensity redistribution. By applying alco-CIL/BCP/Ag, underlying active layer get protection from detrimental metal deposition, hence leakage current and recombination can be effectively suppressed. In addition, electron transport is enhanced due to decreased surface traps after alcohol fluxing. Furthermore, the stability of the devices with alco-CIL/BCP/Ag is significantly enhanced due to the metal diffusion barrier of BCP. We believe the combination of alcohol soluble CIL materials and BCP/Ag as demonstrated herein can be extended to a wider range of applications. In summary, this work provides a simple method to improve the efficiency and stability of OSCs.

4. Experimental Section


23


Page 24 of 35

4.1 Materials. PTB7-Th and PC71BM were purchased from Solarmer Material Inc. PDIN, PFN, ZrAcac and BCP were purchased from SunaTech Inc., Derthon Optoelectronic Materials Science Technology Co Ltd, Aladdin Industrial Co. and Luminescence Technology Co., respectively. 4.2 Device Fabrication and Characterization. The ITO-coated glass substrates were cleaned by sequential sonication in detergent, deionized water, acetone, isopropanol and anhydrous alcohol for 20 min at each step. Then the precleaned ITO substrates were treated with UV-Ozone for 20 min. A solution of PEDOT:PSS (Clevious PVP Al4083) was spin coated onto the ITO substrates and then annealed in air at 130 °C for 20 min. Next, the active layer (ca. 100 nm) was deposited by spin coating from a PTB7-Th:PC71BM (1:1.5 wt%, 20 mg mL-1) solution in a mixed solvent of CB:DIO (100:3 vol%) under 1200 rpm for 60 s. For device with BCP/Ag, BCP and 100nm Ag were deposited onto active layer sequentially. For devices with PDIN/Al, PFN/Al and ZrAcac/Al, 15 min after depositing active layer, PDIN (1.5 mg mL -1 in methanol), PFN (0.3 mg mL-1 in methanol) and ZrAcac (1.4 mg mL-1 in ethanol) solution were spin coated on active layer for 40 s, respectively. Finally, 100nm Al were deposited onto active layer. For devices with double CILs, alcohol soluble CILs, BCP and Ag were deposited onto active layer in the manner described in single CIL based devices. The device area was fixed at 0.1256 cm2. The thickness of the BCP was directly determined by a surface profiler (Dektak 150, Veeco). The thickness of the PDIN and PFN was determined by surface profiler, in conjunction with extrapolation from an absorbance– thickness curve that assumes a linear dependence of absorbance on film thickness. The optimal thickness of the ZrAcac is extrapolated from the literature. The current density–voltage characteristics were measured inside a N2-glovebox, using a Keithley 2400 sourcemeter under the illumination of AM 1.5G (100 mW cm−2). The EQE measurements were performed using a CrownTech quantum efficiency measurement system (QTesT 1000ADX) in air. The UPS


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measurements were performed using an ultrahigh vacuum (UHV) system (AXIS ULTRA DLD, Kratos) at a pressure of ≈3.0 × 10−8 Torr (photon excitation energy = 21.2 eV (He I), sample bias = −9 V). Reflectance spectra were recorded on a UV-vis spectroscopy (Hitachi, U-3900H).

ASSOCIATED CONTENT Supporting Information. Dark J-V characteristics of PTB7-Th:PC71BM OSCs with PFN/Al and PFN/Ag. Absorption of the film of PDIN, PFN and ZrAcac, reflectance of the film of Ag (100 nm) and Al (100 nm), reflectance spectra of PTB7-Th:PC71BM OSCs with alcohol soluble interlayers (PDIN, PFN and ZrAcac) and Al or Ag, dark J-V characteristics of PTB7-Th:PC71BM OSCs with different interlayers. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Co-corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was sponsored by the Natural Science Foundation of Hebei Province, China (E2017201188), the Applied Basic Research Program of Hebei Province, China (14964306D), and the Fund of Hebei Science and Technology Bureau (No. 16211241). REFERENCES (1)

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Table of Contents

Cathode Optimazation

Al PFN

BCP Ag PTB7-Th:PC71BM

PCE=10.11%


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High-Efficiency and Stable Organic Solar Cells Enabled by Dual

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