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Doping ZnO with Water/Alcohol-Soluble Small Molecules as Electron Transport Layers for Inverted Polymer Solar Cells Chang Liu, Lin Zhang, Liangang Xiao, Xiaobin Peng, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10264 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Doping ZnO with Water/Alcohol-Soluble Small Molecules as Electron Transport Layers for Inverted Polymer Solar Cells Chang Liu, Lin Zhang, Liangang Xiao, Xiaobin Peng*, Yong Cao State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China. Email: [email protected]

Abstract By doping ZnO with porphyrin small molecules (FNEZnP-OE and FNEZnP-T) as cathode electron transport layers (ETLs), the inverted polymer solar cells (i-PSC) with

PTB7:PC71BM

(PTB7:

polythieno[3,4-b]-thiophene-co-benzodithiophene,

PC71BM: [6, 6]-phenyl-C71-butyric acid methyl ester) as the active materials exhibit enhanced device performance. While the power conversion efficiency (PCE) of the PSCs with pure ZnO ETL is 7.52%, that of the devices with FNEZnP-T-doped ZnO ETL shows a slightly improved PCE of 8.09%, and that of the PSCs with FNEZnP-OE-doped ZnO ETL is further enhanced up to 9.24% with an over 20% improvement compared to that with pure ZnO ETL. The better performance is contributed by the better interfacial contact and reduced work function induced by 9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorenes

and

3,4-bis-(2-(2-methoxy-ethoxy)-ethoxy)-phenyls in the porphyrin small molecules. More importantly, the PCE is still higher than 8% even when the thickness of FNEZnP-OE-doped ZnO ETL is up to 110 nm, which are important criteria for eventually making organic photovoltaic modules with roll-to-roll coat processing.

Keywords: inverted solar cells; porphyrin; small molecule; interlayer; ZnO

Introduction 1

ACS Paragon Plus Environment


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Polymer solar cells (PSCs) are very promising in renewable energy due to their advantages of high flexibility, light-weight and potentially low cost.1-7 In order to improve device performance of PSCs, electrode transport layers (ETLs) are widely employed because they can reduce contact resistance and inhibit interfacial charge recombination.8-13 Since the prolonged exposure of a conventional device to air can lead to an oxidation of the air sensitive metal cathode, thus leading to the degradation of device performance,14-16 an inversion of the device architecture (inverted PSC) has been demonstrated to be an effective approach to solve this problem.5, 17-21 Among a number of inorganic interlayers effective in improving electron extraction properties of inverted PSCs in recent years,19, 22-24 ZnO is an often used one due to its air-stability, high transparency to visible light, and tunable electrical optical properties.3, 16, 25-29 However, ZnO usually exhibits relatively poor interfacial contact with the hydrophobic organic photoactive layer in organic solar cells due to the hydrophilic feature of ZnO, resulting in charge trapping in ZnO surface.9-10, 30-32 To overcome this issue,

Yang

et

al.

fabricated

ZnO

and

PFN

(poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluore ne)]) double interfacial layers to improve the power conversion efficiency (PCE) of inverted devices from 6.1% to 8.4%.20 However, fabricating the double layers needs an additional process after the ZnO layer is made, complicating the processing. An alternative approach is to blend ZnO with organic materials with polar functional groups, which can interact electrostatically with ZnO, as ETLs for organic solar cells.27 33 In this study, we synthesized two porphyrin small molecules FNEZnP-OE and FNEZnP-T (Figure 1), both with amine substituents but FNEZnP-OE with polar 3,4-bis-[2-(2-methoxy-ethoxy)-ethoxy]-phenyls and FNEZnP-T with alkyl tert-butyl substituents, and doped them into sol-gel-derived ZnO films as interfacial layers in inverted PSCs to re-engineer the interfaces. The modified ZnO films show decreased work functions and increased electron transportation, which can enhance charge extraction efficiency and decrease recombination losses. While the i-PSCs based on PTB7

and

PC71BM

(PTB7:

polythieno[3,4-b]-thiophene-co-benzodithiophene, 2


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PC71BM: [6, 6]-phenyl-C71-butyric acid methyl ester) as the active materials and pure ZnO as the ETL show a Voc of 0.74 V, a Jsc of 14.70 mAcm-2, a FF of 69.15%, and a PCE of 7.52%, those with ZnO:FNEZnP-T cathode interlayers exhibit a slightly enhanced PCE of 8.09% (Voc = 0.75 V, Jsc = 15.20 mA cm-2, and FF = 70.94%). contributed by the four polar 2-(2-methoxy-ethoxy)-ethoxy substituents, the PSCs using ZnO:FNEZnP-OE as the interlayers afford a PCE up to 9.24%, which is more than 20% enhancement compared to the devices with pure ZnO ETL. More even when the ETL of ZnO:FNEZnP-OE is up to 110 nm thick, the PCE of the is still higher than 8%.

a) O

O

O O

N

O

O

N

N

N Zn N

N

N O

O

N

O O

O

O

FNEZnP-OE

Figure 1. Molecular structures of a) FNEZnP-OE, b) PTB7 and d) FNEZnP-T, and c) device structure.

Materials: The two porphyrin small molecules FNEZnP-OE and FNEZnP-T were synthesized as the procedures shown in Scheme S1. Toluene, zinc acetate, 2-aminoethanol, 2-methoxyethanol and PC71BM were purchased from Sigma Aldrich. 3



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PTB7 was purchased from 1-materials Corp. All reagents, unless otherwise specified, were used without further purification. Experimental Methods: Sol-gel-derived ZnO films were prepared according to the reported method by dissolving 1.0 g zinc acetate and 277 ml 2-aminoethanol into 10 ml of 2-methoxyethanol then hydrolyzing for 6 h at 50 °C.19 FNEZnP-OE or FNEZnP-T was then dissolved into 1ml of the precursor solution of ZnO with the weight ratio of 1:100 to zinc acetate. Patterned indium tin oxide (ITO)-coated glasses with a sheet resistance of 15-20 ohm/square were cleaned by a surfactant scrub and then underwent a wet-cleaning process inside an ultrasonic bath, beginning with deionized water followed by acetone and then alcohol for 20 minutes in each progress. Subsequently, the ITO glasses were dried at 75 oC for 60 minutes. The solutions for ETLs were spin-coated onto ITO glasses and then annealed at 200 °C in air to obtain an electronic transport layer and then transferred to a N2-glovebox. The active layers were prepared from PTB7:PC71BM (1:1.5, w/w) solutions in chlorobenzene/1,8-diiodoctane (100:3 v/v) solution (total concentration, 25 mg/mL), which were stirred overnight at 60 °C in a N2 glove box before they were spin-casted on the top of cathode interlayer at 1400 rpm for 40 s. At a base pressure of 3×10−4 Pa, a 10 nm MoO3 was thermally evaporated at an evaporation rate of 0.1 Å/s. Finally, an 80 nm aluminum layer was evaporated with a shadow mask. The overlapping area between the cathode and anode defined a pixel size of 0.16 cm2. All the fabrication processes were carried out inside a controlled atmosphere of a nitrogen drybox (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen and moisture.

Results and discussion: In solutions and films, the two porphyrins exhibit almost the same absorption spectra with Soret and Q bands at 470 and 683 nm, respectively (Figure 2), indicating that the substituents at the phenyl groups almost can’t affect the π-electron 4


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of the conjugated backbones. And the transmittance of the porphyrin-doped ZnO film (Figure 2b) is very high and quite similar to that of pure ZnO film, due to the very low doping percentages of the porphyrins, allowing high photo flux to reach to the bulk heterojunction (BHJ) active layers. b)

a)

100 1.0

Transmittence (%)

Normalized absorption

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


FNEZnP-OE FNEZnP-T

0.5

80

ZnO:FNEZnP-OE ZnO:FNEZnP-T ZnO

60

40 0.0

300

400

500

600

700

800

20 300

900

400

Wavelength (nm)

500

600

700

Wavelength (nm)

Figure 2. (a) UV-visible absorption spectra of FNEZnP-OE and FNEZnP-T of thin films, and (b) UV-visible transmittance spectra of ITO/interlayer thin films.

Considering that the surface wettability of a cathode interlayer in an inverted PSC can determine its interface contact with the active layer, we measured the contact angles of water droplets on pure and doped ZnO films. As shown in Figure 3, the contact angles of water on ZnO, ZnO:FNEZnP-T and ZnO:FNEZnP-OE films were 32.0°, 39.6°, 42.4° respectively, indicating that doped ZnO films are slightly more hydrophobic, which can contact with the organic photoactive layers more efficiently when they were sandwiched.

5



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b)

a)

c)

Figure 3. Photographs of water droplets on the surfaces of (a) ITO/ZnO, (b)

ITO/ZnO:FNEZnP-T and (c) ITO/ZnO:FNEZnP-OE films.

Scanning Kelvin probe microscopy (SKPM) was employed to probe the electronic properties of the pure and doped ZnO-modified ITO electrodes. While the work function (WF) of ZnO modified ITO shows a value of ~4.44 eV, it decreases to ~4.28 eV and further to ~4.16 eV for ZnO:FNEZnP-T and ZnO:FNEZnP-OE modified ITO electrodes, respectively. The lower WF indicates better electron injection ability, which can increase the built-in field to enhance charge extraction efficiency and reduce recombination losses. Furthermore, the small amount of FNEZnP-OE changed the X-ray photoelectron spectroscopic (XPS) signals of N, C and O elements of ZnO films to some extend as shown in Figure S4, further supporting the doping of ZnO by the the porphyrin molecules. 6


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Figure 4 and Table 1 shows the photovoltaic performances of the i-PSCs based

on PTB7 as the active materials with pure and doped ZnO ETLs. The reference devices based on pure ZnO ETL shows an open circuit voltage (Voc) of 0.74 V, a short-circuit current density (Jsc) of 14.70 mAcm-2, a fill factor (FF) of 69.15%, and a PCE of 7.52%, which are comparable to the reported values of the BHJ i-PSC devices fabricated under similar conditions.26,33 While the devices with ZnO:FNEZnP-T ETL exhibit a slightly improved PCE of 8.09% (Voc = 0.75 V, Jsc = 15.20 mA cm-2 and FF = 70.94%), the devices with ZnO:FNEZnP-OE as cathode interlayer display remarkably enhanced performance with the PCE up to 9.24% with a Voc, a Jsc, and a FF of 0.75 V, 17.09 mA cm−2, and 72.13%, respectively. Furthermore, when PTB7-TH was used as the donor material, the PCE increased from 8.53% (pure ZnO ETL) to 9.65% (ZnO:FNEZnP-OE ETL), which is 13% PCE enhancement (Table S2), indicating the porphyrin doped ZnO ETL may effective to improve the performance of other donor material-based solar cells. Since it has been reported by Moon et al. that PFN can form chemical bonding with ZnO,26 it is reasonable to assume that the amine groups of FNEZnP-OE and FNEZnP-T can interact with ZnO because they have the same amine groups as PFN, explaining why both FNEZnP-OE and FNEZnP-T doped ZnO interfacial layers can enhance all the three photovoltaic parameters than pure ZnO one. It is noted that the PSCs with FNEZnP-OE doped ZnO ETLs show the better PCE compared to FNEZnP-T doped ones, which can be ascribed to the larger dipole of 2-(2-methoxy-ethoxy)-ethoxy substituents in FNEZnP-OE, leading to better Ohmic contact. Therefore, the electrons can be transported from the active layers to the cathode more efficiently.5, 7, 32, 34

Table 1. Photovoltaic performances of inverted solar cells using different cathode

interlayer (device configuration: ITO/cathode interlayer/PTB7:PC71BM/MoO3/Al, under irradiation of AM1.5G at100 mW/cm2, a and b: the best and the average values of PCEs, respectively). 7



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Jsc (mAcm-2)

Voc (V)

FF(%)

PCE(%)

ZnO

14.70

0.74

69.15

7.52a(7.41±0.11)b

ZnO:FNEZnP-T

15.20

0.75

70.94

8.09a(8.06±0.03)b

ZnO:FNEZnP-OE

17.09

0.75

72.13

9.24a(9.01±0.23)b

Cathode interlayer

Figure 4 shows the J-V and external quantum efficiency (EQE) curves of the

inverted PSCs with different cathode interlayers. All the devices exhibit similar EQE curve shapes, but the EQE values of the devices with ZnO:FNEZnP-OE and ZnO:FNEZnP-T ETLs are higher than those of the device with pure ZnO over the whole photocurrent response region and the EQE values of the devices with ZnO:FNEZnP-OE is remarkably improved in the wavelength range from 330 to 700 nm with EQE values of nearly 80% from 600 to 700 nm. The Jsc values calculated from EQE spectra are 14.08, 14.75 and 16.31 mA cm-2 for the devices with ZnO, ZnO:FNEZnP-T and ZnO:FNEZnP-OE interfacial layers, respectively, which are within 5% errors compared to the measured ones. The increased EQE values after the doping of ZnO with FNEZnP-OE can be contributed to the combination effects of the better contact with the active layers (as shown by the contact angle measurments), the reduced work function and the facilitated electron transportation of the ZnO:FNEZnP-OE interfacial layers. The reduced work function from ~4.44 to ~4.16 eV can enhance electron injection ability, which can increase the built-in field to enhance charge extraction efficiency and reduce

recombination

losses.

Furthermore,

the

enhanced

charge

mobility

demonstrated below can also effectively decrease the accumulation of carrier in the interface to reduce the combination of carrier, and further enhanced the EQE. Compared with the EQE curve based on ZnO:FNEZnP-T ETL, that based on ZnO:FNEZnP-OE shows further enhancement in the whole region, demonstrating that 2-(2-methoxy-ethoxy)-ethoxy groups also play very important roles to the enhanced performance along with the 9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene groups. 8


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a)

b) 3

80

0

60

-3 -6

ZnO ZnO:FNEZnP-OE ZnO:FNEZnP-T

-9

EQE (%)

Current density (mA/cm 2)

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


40

ZnO ZnO:FNEZnP-OE ZnO:FNEZnP-T

-12

20 -15 -18 -0.2

0.0

0.2

0.4

0.6

0 300

0.8

450

Voltage (V)

600

750

Wavelength (nm)

Figure 4. (a) The J-V and (b) EQE curves of the PTB7:PC71BM based PSCs with

different interlayers.

To study the effects of the porphyrins on the electron transport properties of ZnO ETLs,

electron-only

devices

(device

structure:

ITO/Al/interlayer(40

nm)/PTB7:PC71BM/Ca/Al) with pure ZnO, ZnO:FNEZnP-T and ZnO:FNEZnP-OE interlayers were fabricated. And the space charge limited current (SCLC) electron mobilities (Figure 5) are measured to be 1.21×10-4, 4.32×10-4 and 1.55×10-3 cm2 V-1 s-1 for the devices with pure ZnO, ZnO:FNEZnP-T and ZnO:FNEZnP-OE ETLs, respectively. It is noteworthy that the FNEZnP-OE doped ZnO interlayer shows 10-folds enhancement of electron mobility compared with the ZnO interlayer, which is attributed to the better interfacial contact between the photoactive layer and the interlayer, reduced series resistance from 5.80 Ω cm2 to 3.50 Ω cm2 and increased shunt resistance from 553.53 Ω cm2 to 1017.81 Ω cm2. On the other hand, the electron mobility of the ZnO:FNEZnP-T of 4.32×10-4 cm2 V-1 s-1 is almost 4-folds compared with that of ZnO. The doped ZnO reduces the electron-hole recombination within the devices so that charges can move more effectively from the photoactive layer to the electrodes, leading to the Jsc, FF and PCE enhancement.

9



70

60

ZnO ZnO:FNEZnP-T ZnO:FNEZnP-OE

50

J1/2 (A1/2/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

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40

30

20

10 2.4

3.2

Voltage(V)

Figure 5. J1/2~V curves of the electron-only devices for ITO/Al/interlayer (40

nm)/PTB7:PC71BM/Ca/Al, in which the interlayer indicates ZnO, ZnO:FNEZnP-OE and ZnO:FNEZnP-T.

Considering the high electron mobility of ZnO:FNEZnP-OE, we fabricated the PSCs with different thickness of ZnO:FNEZnP-OE ETLs. As shown in the Table 2, even when the thickness of ZnO:FNEZnP-OE ETL is up to 110 nm, the PCE of the PSCs is still as high as up to 8.14%, indicating that the PSC performance are not so sensitive to the thickness of ZnO:FNEZnP-OE ETL, which are important criteria for eventually making organic photovoltaic modules with roll-to-roll coating processing.

Table 2. Photovoltaic performances of inverted solar cells with different thickness of

ZnO:FNEZnP-OE cathode interlayer. Speed(rpm) Thickness(nm) Jsc (mAcm-2)

Voc (V)

FF(%)

PCE(%)

3000

39

17.09

0.75

72.13

9.24

2000

49

15.35

0.76

71.18

8.41

1400

60

15.14

0.76

70.86

8.15

800

110

14.80

0.77

71.31

8.14

Conclusion

The performance of the inverted polymer solar cells based on PTB7:PC71BM as the active materials is improved by the ZnO ETLs doped with porphyrin small 10


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molecules (FNEZnP-OE and FNEZnP-T). Especially, the PCE of the PSCs with FNEZnP-OE-doped ZnO ETL is significantly enhanced up to 9.24%, which is more than 20% improvement compared to that with pure ZnO ETL. The better performance is contributed by the better interfacial contact and reduced work function induced by the amine groups at the fluorenes and the polar 2-(2-methoxy-ethoxy)-ethoxyl substituents at the phenyls. More importantly, the PCE is still higher than 8% even when the ETL is up to 110 nm, which is important criteria for eventually making organic photovoltaic modules with roll-to-roll coat processing.

Acknowledgements

This work was financially supported by the grants from International Science and Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), and the National Natural Science Foundation of China (51473053, 51073060).

Supporting Information. Synthesis of FNEZnP-T and FNEZnP-OE, dark currents of

inverted solar cells, characteristics of the i-PSCs with different concentration of FNEZnP-OE doped in ZnO, thermal stability of FNEZnP-OE, X-Ray Photoelectron Spectroscopy (XPS) of ZnO, ZnO:FNEZnP-OE.

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Graphic Abstract:

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Alcohol-Soluble Small Molecules as Electron

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