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Amphiphilic Diblock Fullerene Derivatives as Cathode Interfacial Layer for Organic Solar Cells Jikang Liu, Junli Li, Xiangfu Liu, Fu Li, and Guoli Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18331 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Amphiphilic Diblock Fullerene Derivatives as Cathode Interfacial Layer for Organic Solar Cells

Jikang Liu, Junli Li, Xiangfu Liu, Fu Li, Guoli Tu*

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, P. R. China

ABSTRACT

A

new

amphiphilic

diblock

fullerene

derivative

[6,6]-Phenyl-C61-butyricacid-4-(9,9,9',9'-tetrakis(3-bromopropyl)-9H,9'H-[2,2'-bifluor en]-7-yl)phenol-(N,N,N-trimethylpropan-1-aminium)

bromide

(C60-4TPB),

was

synthesized and applied in Organic Solar Cells (OSCs). Solvent annealing by toluene could obvious induce the self-assembly of the C60-4TPB layer, which can be tested by the measurements of the water contact angle. After the treatment of toluene, a vertical like arrangement in the ultrathin layer of C60-4TPB molecular will be formed between electron-collecting zinc oxide (ZnO) layers and the active layer (blend system of PTB7/PC71BM), leading to the improvement of the interfacial compatibility between active layer and ZnO layer. On the top surface of the C60-4TPB layer, the C60 molecules can be expected to induce the enrichment of PC71BM and block the hole, resulting further increasing the open-circuit voltage (VOC) and fill factor (FF). After

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spin-coated the C60-4TPB solutions onto the ZnO layer with a concentration of 0.5mg/ml in dimethyl sulfoxide (DMSO), obvious improved performances were obtained with power conversion efficiency (PCE) of 8.07%, which can be attribuited to the optimized interface morphology between hydrophilic ZnO and hydrophobic PTB7:PC71BM.

KEYWORDS ： organic solar cells, amphiphilic diblock, solvent annealing, self-assembly, water contact angle, interfacial compatibility, interface morphology

1. INTRODUCTION Due to the advantages of low cost, light weight, ﬂexibility, large-area fabrication and semi-transparency et.al, OSCs has been a promising technology for clean and renewable energy conversion.1-5 In order to improve the PCE, much attentions have been spend on interface control, material design, self-assembly of donor and acceptor phases and device fabrication.6-8 And the PCE of organic solar cells have reached over 13%.9-10 One of the strategies is to develop new donor or acceptor materials to enhance the short-circuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF).11,12 On the other hand, the approaches during the device fabrications, such as incorporation of additives, controlling the growth rate of films and interface modifications, have also been well studied and exhibited extremely important influences.13-14 The device geometry and interface properties are verified to be two main critical factors toward high performance OSCs.15-18 Interface modifications by


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placing ultra-thin interlayers between active layer and electrodes, which may result in significant lower interface barrier and faster charge transporting, has been demonstrated to be an effective approach for OSCs.19-22 As an electron transporting layer, ZnO offers the opportunities to prepare high transparency and atomically smooth surfaces, especially in the inverted devices. The work function of ZnO is about -4.3 eV, which shows obvious charge injection barriers with most of the organic acceptors in the active layers.23-24 Therefore, the tuning the work function of ZnO is an important process in the preparation for better device. Based on their advantage of tunable functions through facile modification of molecular structures, excellent electrical and electronic properties, organic interfacial materials exhibit promising interface modification ability for metal or metal oxide electrodes to enhance the device performance of OSCs25-26. They can also provide good opportunities to fabricate multilayer organic optoelectronic devices without interface mixing during the solution processing. In the past few years, the application of water/alcohol soluble small molecules (WSCSs) for the modification of ZnO has attracted much attentions and been proved to be an efficient way of improving the device performances of OSCs. Fullerene derivatives have high electron affinity, isotropic charge transport with good mobility and unique spherical geometry, which have been the most used acceptors for OSCs.27-29 The fullerene derivatives are also ideal candidates for cathode interface materials in OSCs.30-34 Based on the fullerene, the cathode interface materials will possess proper electron mobility and the



capability of tuning electrode work functions to improve electron extraction and photocurrent generation. In this work, we developed a new amphiphilic diblock fullerene derivative (C60-4TPB) with ammonium groups on side chain of fluorene block (Figure 1). This diblock molecule shows selected solubility and can support mutil-layer OSCs structures by solutions processing. Due to its rigid amphiphilic design, the solvent annealing will be applied and expected to induce the self-assembly of the interface layer, which can help to investigate the influence of morphology in the interface layer on the performances of OSCs. The existence of large amount of fullerene in the cathode interface layer are expected to form a multi transmission channels for electrons and decrease the hole quenching at the cathode for the polymer: fullerene solar cells (Figure 1). When applied in the OSCs with inverted structure of ITO/ZnO/interface layer/PTB7:PC71BM/Ca/Ag, it was found that the PCE of the device could be obvious enhanced to 8.07% from the PCE of 7.34% in the control device.


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Figure 1. Chemical structures of amphiphilic diblock fullerene derivative C60-4TPB and the structure of the inverted device.

2. EXPERIMENTAI 2.1 Materials [6,6]-Phenyl-C61-butyricacidmethylester

(PC61BM)

was

purchased

from

Luminescence Technology Corp. 1,2-Dichlorobenzene (ODCB) was purchased from Aladdin Industrial Corp (99.0% purity). Other reagents and solvents were purchased from Sinopharm Chemical Reagent and used without further purification unless noted. 2.2 Synthesis The synthetic scheme of C60-4TPB was showed in Scheme 1. Some of the 1H NMR spectrums of the intermediate products were showed in Figure S1. The length sizes of



the hydrophilic and hydrophobic parts are similar, and the chain (CH2)3 in PCBM made it possible to rotate during the solvent annealing.

Scheme 1. Synthetic scheme of C60-4TPB.

2.3 Chemical properties 2.3.1 The characterization of nuclear magnetic The 1H NMR spectrums of 4TPB and C60-4TPB were showed in Figure S2. Both 4TPB and C60-4TPB showed good solubility in DMSO. During the synthesis of C60-4TPB, most of the excess amount of 4TPB and PC61BA could be removed by water and 1,2-dichlorobenzene respectively after the processing of centrifugation. Then the processing of dialysis in water, methanol and 1,2-dichlorobenzene could further remove 4TPB and PC61BA in the product for C60-4TPB, which has almost no


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solubility both in methanol and 1,2-dichlorobenzene. As shown in Figure S2(a), the δ=2.882-2.895 ppm were corresponding to chemical shift of the methyl（CH3） on quaternary ammonium salt, which were the characteristic pecks of 4TPB. The δ=1.197-1.277, 2.125-2.140 and 3.158-3.311 ppm were corresponding to chemical shift of the methylene (CH2) on alkyl chain. This means 4TPE has been obtained successfully. In Figure S2(b), the pecks on the hydrophilic part of C60-4TPB were all marked, especially the methyl（CH3）on quaternary ammonium salt. In the strong polarity solvent of dimethyl sulfoxide, it is difficult to distinguish the hydrogen atom (H) on benzene ring in PC61BM. The multi peaks range from 6 ppm to 9 ppm can be attributed to the hydrogen atoms on benzene of the C60-4TPB. Preparation of the ZnO solution: The zinc acetate dihydrate (Zn(CH3COO)2·2H2O, Aldrich, 99.9%, 0.9878 g) was dissolved in 2-methoxyethanol(15ml), then stirred at 70 °C for one hour, 2-aminoethanol (NH2CH2CH2OH, aldrich, 99.5%, 270ul) was added to the above reaction mixture stirring for another two hours at 70 °C. Finally, the new reaction mixture was allowed to stir at room temperature overnight.35 2.3.2 The UV-visible absorption in films To confirm the structure of C60-4TPB further, the UV-visible absorption spectra of the 4-TPB, PC61BA and C60-4TPB were compared in Figure S3. A sharp absorption caused by the conjugate structure appeared at 351nm in the absorption of 4-TPB. There were two peaks from the absorption of PC61BA (330 and 435 nm), which were the characteristic absorption peak of PC61BA. In the absorption of C60-4TPB, we attributed the wide peak ranged from 290 nm to 390 nm to the strong intra-molecule



effect between PC61BA and 4TPB. Compared with the characteristic absorption peak of PC61BA and 4TPB, the absorption spectra of the C60-4TPB is also an evidence to confirm the structure of C60-4TPB. 2.3.3 The solvent annealing on silicon wafer To prepare thin films of C60-4TPB on silicon wafer (Si), C60-4TPB was dissolved in DMSO with a concentration of 2 mg/ml. The silicon wafer with a size of 12.7 x 12.7 x 0.5 mm were cleaned by sequentially ultrasonication in methanol, deionized water, ODCB and DMSO for 30 minutes respectively and then dried at 120℃ overnight. After cleaning the silicon wafer with a stream of air or nitrogen and treating the surface with UV ozone for 30 minutes, the C60-4TPB layer was spin-coated onto the surfaces at the speed of 2000 rpm for 60s, and then the silicon wafers coated with C60-4TPB layer were placed in vacuum until the film dry completely. Some of the obtained films were annealed by methanol and toluene for 24 hours respectively. Measurements of the water contact angle images (Figure S4) showed that after solvent annealing, the C60-4TPB molecular exhibited a self-assembly process on silicon wafer. The contact angle of water were 46, 67 and 72 degree on the devices annealed by methanol, the reference without annealing and annealed by toluene respectively, which originated from the unique amphiphilic structure of the C60-4TPB. 2.3.4 The characterization of infrared spectra The infrared spectra of PC61BA, 4TPB and C60-4TPB were showed in Figure S5. The peaks ranged from 3700cm-1 to 2800cm-1 were originated from the stretching vibration of X-H (X referred the N and C). And the peaks ranged from1300cm-1 to


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1050cm-1 were attributed to the stretching vibration of C-O-C. The weak absorption band from 800cm-1 to 700cm-1 was caused by the rocking vibration of (CH2)n. The peaks mentioned above suggested that the structures of N-H, C-H, C-O-C and (CH2)n in C60-4TPB.. Compared with the infrared spectra of PC61BA and 4TPB, the infrared spectra is a further confirmation of the structure of C60-4TPB. 3. RESULT AND DISCUSSION 3.1 Device fabrication The new fullerene derivative C60-4TPB was dissolved in DMSO with different concentrations of 0.25mg/ml, 0.50mg/ml, 1.0mg/ml and 2.0mg/ml. Then the C60-4TPB films were prepared by spin-coating on silicon wafer (3000 rpm, 120 s) with the solutions of C60-4TPB mentioned above. The samples were dried in the air. AFM images of the sample Si/C60-4TPB(x) (x=0, 0.25, 0.50, 1.0 and 2.0 mg/ml) were showed in Figure 2a. Smooth top surfaces were observed by AFM for the C60-4TPB layers with the surface roughness in the range of 0.429–0.881 nm. Compared with the smoothly surface of silicon wafer with the surface roughness of 0.292 nm, there was a mount of small holes with an average depth of 4nm distributed on the surface of the sample Si/C60-4TPB(0.25 mg/ml, with the surface roughness of 0.881 nm), as shown in Figure 2a1-a2. When the concentration of C60-4TPB increased to 0.5mg/ml, the sample Si/C60-4TPB (0.50 mg/ml) had a more smoothly surface with a surface roughness of 0.429 nm, the number and the depth of small holes were also decreased obviously. A few protrusions appeared on the surface of this sample, which can be attributed to the aggregation of C60-4TPB. With the further increasing the



concentrations of C60-4TPB, the number of the protrusions on the surfaces increased obviously as showed in Figure 2a4 -a5, leading to a rough surface with the surface roughness of 0.444 and 0.538 nm respectively. According to the AFM results, it was clearly that both the low connection (0.25 mg/ml) and high connection (1.0 mg/ml and 2.0 mg/ml) of C60-4TPB would form a rough interface layer. So the C60-4TPB layer from solution of 0.5 mg/ml with the thickness about 4nm was chosen for the following tests. The measurements of the water contact angles on silicon wafer (shown in Figure 2b) showed that when increasing connection of C60-4TPB, the contact angle of water also increased from 41 to 51.5 degree (Figure 2b2-b5). Another information was that the contact angle of water on the samples almost unchanged when the C60-4TPB increased concentrations from 1.0 mg/ml to 2.0 mg/ml. With annealing by toluene for 24 hours, the contact angle of water on the samples all increased (Figure 2b6-b9). This means the surface of Si/C60-4TPB layer have caused cause the self-assembly and more C60 molecules congested on the top surface of the C60-4TPB layer.


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Figure 2. (a) AFM images (2 µm x 2 µm) of the films of Si/C60-4TPB(x): (a1)x=0, (a2)x=0.25 mg/ml, (a3)x=0.50 mg/ml, (a4) 1.0 mg/ml, (a5) x=2.0 mg/ml, and



(b)photographs of water droplets on the surfaces Si/C60-4TPB(x): (b1)x=0, (b2)x=0.25 mg/ml, (b3)x=0.50 mg/ml, (b4)x=1.0 mg/ml, (b5)x=2.0 mg/ml (2-5 without toluene annealing), (b6)x=0.25 mg/ml, (b7)x=0.50 mg/ml, (b8)x=1.0 mg/ml, (b9)x=2.0 (6-9 with toluene annealing for 24 hours).

3.2 The measurement of C60-4TPB self-assembly on ITO/ZnO Indium tin oxide(ITO) conductive glass (19 Ω/cm2) were cleaned by sequentially ultrasonication in detergent, deionized water, acetone and isopropyl, then blew by nitrogen and dried in oven at 80℃ overnight. After treating the ITO substrates by plasma for 5 minutes, the ZnO layer was prepared on it by spin-coating at 4000rpm for 60s and annealed at 200 °C for one hour. The C60-4TPB layers were obtained by spin-coating on top of ZnO (2000 rpm, 120s) with the different concentrations (0.25 mg/ml, 0.50 mg/ml, 1.0 mg/ml and 2.0mg/ml) and annealed at 90 °C for 10 min. Then the thin film of C60-4TPB layers were annealed by toluene for 24 hours. Rough surfaces were observed by AFM with the surface roughness in the range of 2.45–2.67 nm (as showed in Figure 3a). Compared with Figure 3a1, 3a2 and 3a3, it was obvious that the number of the small holes on the surface was decreased with the increasing of the connection of C60-4TPB, and the surface of ZnO was well covered by the C60-4TPB layer. For the sample without toluene treatment, the C60-4TPB was homogeneous and there is no obvious aggregations. After the toluene treatment, plenty of protrusions can be observed, indicating that the aggregations on the top surface of the C60-4TPB layers. In this work, thermally annealed C60-4TPB layers annealed with and without


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toluene solvent were referred to as ‘SA C60-4TPB’ and ‘TA C60-4TPB’ respectively. To confirm that annealing by toluene could enhance the crystallinity of the C60-4TPB, X-ray diffraction (XRD) measurements of the samples ITO/ZnO/C60-4TPB(x) (x=0, 0.25, 0.50, 1.0 and 2.0 mg/ml) with and without toluene treatment were carried out, as shown in Figure S6. Compared with the weak diffraction peak in the TA C60-4TPB sample, the samples of ITO/ZnO/C60-4TPB(x) (x=0.50, 1.0 and 2.0 mg/ml) exhibited obvious diffraction peak at 30 degree appearing in the XRD spectra. However, the diffraction peak was almost unchanged in the ITO/ZnO/C60-4TPB (0.25 mg/ml). The increased diffraction peak can support the conclusion of enhanced crystallization in the SA C60-4TPB samples. This also means that the ZnO layer almost covered by C60-4TPB with concentrations over 0.5 mg/ml. The work function of the samples ITO/ZnO/C60-4TPB(x) (x=0, 0.25, 0.50, 1.0 and 2.0 mg/ml) after solvent annealing by Kelvin probe measurement and the photographs of water droplets on the surfaces were also showed in Table.S1 and Figure 3b respectively.

After

treated

with

toluene,

the

work

function

of

sample

ITO/ZnO/C60-4TPB (x) were decreased about -0.05-0.10 eV, which was cause by the increased crystallization of fullerene derivative. The measurements of the water contact angles (shown in Figure 3b) showed that when increasing connection of C60-4TPB, the contact angle of water also increased from 42 to 58.5 degree. Another interesting result was that the contact angle of water on the samples almost unchanged when the C60-4TPB increased concentrations from 0.5 mg/ml to 2.0 mg/ml. This means the surface of ZnO layer is covered well with C60-4TPB when the



concentration is 0.5mg/ml, thus leading to the unchanged contact angles. After the annealing of toluene, the ionic groups on the side chains could endow C60-4TPB with strong interface-modification functions, and the C60 molecules on the top surface of the C60-4TPB layer can be expected to induce the enrichment of PC71BM and block the hole, resulting the improvement of device performance.


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Figure 3. AFM images (2 um x 2 um) of the films (a) and photographs of water droplets on the surfaces (b) of devices ITO/ZnO/C60-4TPB(x): (n1)x=0, (n2)x=0.25 mg/ml, (n3)x=0.50 mg/ml, (n4)x=1.0 mg/ml, (n5)x=2.0 mg/ml after annealing by toluene for 24 hours (n=a, b).

Ultraviolet photo-electron spectroscopy (UPS) was employed to clarify the energy level of the ZnO and ZnO/C60-4TPB (2.0mg/ml), as shown in Figure S7. The HOMO level energies are defined according to the equation.36 = ℎ − −

where hν was the incident photon energy of 21.2 eV, and was gained from the high binding energy cutoff of a spectrum,37 and was delivered from the

right panel. The HOMO energies for ZnO and ZnO/C60-4TPB were -7.68 eV and -7.13 eV, respectively. According to these HOMO energy levels and the optical band gap obtained from the UV-vis absorption spectrum, the LUMO energy levels were estimated to be -4.31 eV for ZnO and -3.84 eV for ZnO/C60-4TPB, as summarized in Table 1. The band gap for C60-4TPB was obtained from UV-vis absorption spectrum of C60-4TPB, which was 2.93 eV (as showed in Figure S8). The transmission spectra of the device ITO/ZnO/C60-4TPB (x) (x=0, 0.25, 0.50, 1.0 and 2.0 mg/ml) were showed in Figure S9. The value of transmission increased obvious with the connection ranged from 0 mg/ml to 2.0 mg/ml, which was caused by the addition depth of the C60-4TPB layer. While the HOMO and LUMO levels of PC71BM were



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-6.10eV and -3.70eV,38-39 so the ZnO/C60-4TPB had a more suitable level to PC71BM than bare ZnO. Table.1.Energy levels of ZnO and ZnO/C60-4TPB Buffer layer

Eg

HOMO (UPS)

LUMO (Eg)

ZnO

3.37

-7.68

-4.31

ZnO/ C60-4TPB

3.29

-7.13

-3.84

To explore the relationship between fullerene interfacial materials and the performance of photovoltaic devices, the OSCs were fabricated with the device configuration of ITO/ZnO/C60-4TPB (with or without)/BHJ/MoO3/Ag using PTB7:PC71BM blend. The blended solution PTB7‫׃‬PC71BM was spin coated on the top at 1500 rpm for 60 s(100 nm) and

anode MoO3(10 nm)/Ag(100 nm) was

prepared by thermal evaporation. We speculated that an A1-D-A2 triad type of complex is formed when C60-4TPB assembled onto ZnO, fullerene (A1)-ammonium group (D)-ZnO (A2).40-41 In this structure, the excited charge transfer can be either in forward (D-A2) or backward (A1-D) direction, based on the control of the fullerene energy level that is higher or lower than that of ZnO. As in Figure 4, the C60-4TPB layer could enhance the electron collection efficiency of the inverted devices by smoothening and passivating the ZnO surface.42 By introducing the ultra-thin C60-4TPB layer (about 4 nm), PCBM in C60-4TPB structure could be used as the electron

transport

layer

because

of

the

well-known

transport/hole-blocking capability active layer to the ZnO.43-44


efficient

electron

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Figure 4. Schematic of electronic transmission and vertical phase separation of the PTB7:PC71BM blend in device: (a) without annealing by toluene, (b) with layer annealing by toluene.

A series of devices ITO/ZnO/C60-4TPB(x)/PTB7:PC71BM/MoO3/Ag (x=0.25, 0.50, 1.0 and 2.0 mg/ml) were prepared to study the influences of the cathode interface of C60-4TPB in an inverted structure, as summarized in Table 2. The reference devices ITO/ZnO/C60-4TPB(x)/PTB7:PC71BM/MoO3/Ag without annealing by toluene were also shown in Figure 4. Compared with the Jsc of 16.41mA/cm2, FF of 62.08% and PCE of 7.34% in the reference device based on bare ZnO, the toluene-annealed device of ITO/ZnO/C60-4TPB(0.5mg/ml)/PTB7:PC71BM/ MoO3/Ag showed an average PCE of 8.07% with better parameters of Jsc of 16.84 mA/cm2, FF of 65.94%. The difference of the performance between devices with TA C60-4TPB and SA C60-4TPB relying on series connections of C60-4TPB and the J-V characteristics of the three ACS Paragon Plus Environment


different devices with the C60-4TPB connection of 0.5mg/ml were showed in Figure 5. These differences of the device performance can be attributed to existence of the interface layer. The ionic groups on side chain of fluorine of C60-4TPB can induce a strong interfacial dipole and accelerate the electron extracting. After annealing by toluene, a vertical like arrangement in the C60-4TPB layer will emerge due to the molecules self-assembly, locating between the electron-collecting zinc oxide (ZnO) layers and the active layer. The hydrophilic fluorene blocks with ionic groups will exhibit strong intermolecular interaction with the ZnO layer and the hydrophobic fullerene units will be rich on the top surface of the cathode interface layer and show tight contact with the organic active layer of PTB7/ PC71BM. Then the C60-4TPB layer could appropriately improve the interfacial compatibility between active layer and ZnO. The vertical phase separation in the active layer is believed to relate to the differences in the surface energy of each component. The solvent evaporation process during spin-coating allows the morphology to reach a thermodynamically favorable state via vertical phase separation. PCBM itself has a very high density of electrons, thus resulting induced dipole moments presume ably affect the intermolecular interactions between the PCBM molecules.45-46 When fullerene derivative C60-4TPB applied in inverted device, the surface layer of the PCBM could enhance the vertical phase separation of the PTB7:PC71BM blend (showed as Figure 4), more PC71BM molecular would move to the side of the C60-4TPB thin layer in the active layer. The samples with the structure of ITO/ZnO/C60-4TPB(x)/PTB7:PC71BM(x=0, 0.5mg/ml) were prepared to study the effect of C60-4TPB on the phase separation in the active


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layer. A rough surface topology was determined by AFM for active layer with the surface roughness in the range of 2.31–2.70 nm (showed in Figure 6). The sample with C60-4TPB layer annealed in toluene for 24 hours got a more smoothly active layer than the samples on the bare ZnO layer and C60-4TPB layer without annealed in toluene. This hints that less PC71BM large size aggregation on the surface of the active layer, which maybe attribute to the inducing aggregation of PC71BM onto the C60-4TPB layer by the fullerene block on its top surface. Table

2.

Photovoltaic

parameters

of

the

devices

with

ITO/ZnO/C60-4TPB(x)/PTB7:PC71BM/ MoO3/Ag (x=0.25, 0.50, 1.0 and 2.0mg/ml) x (mg/ml)

Annealing

0

Jsc (mA/cm2)

VOC (V)

FF (%)

PCE (%)

16.41

0.721

62.08

7.34

0.25

TA

16.80

0.721

60.99

7.39

0.25

SA

16.33

0.725

65.44

7.75

0.50

TA

16.56

0.727

62.23

7.50

0.50

SA

16.84

0.726

65.94

8.07

1.00

TA

17.00

0.722

62.58

7.68

1.00

SA

16.55

0.725

64.99

7.79

2.00

TA

16.84

0.728

61.66

7.56

2.00

SA

16.85

0.737

63.68

7.91



Figure 5. The device (ITO/ZnO/C60-4TPB(x)/MoO3/Ag) performance of C60-4TPB layer annealed with and without toluene solvent: (a) Current density–connection characteristics of devices. (b) Voltage–connection characteristics of devices. (c) Fill factor–connection characteristics of devices. (d) PCE–connection characteristics of devices. (e) J-V characteristics of the devices.


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Figure 6. AFM images (5 µm x 5 µm) of the films for the devices ITO/ZnO/C60-4TPB(x)/PTB7:PC71BM: (a)x=0, (b)x=0.5mg/ml, (c)x=0.50mg/ml with the C60-4TPB layer annealed by toluene for 24 hours.

4. CONCLUSIONS In summary, an amphiphilic diblock fullerene derivative C60-4TPB have been developed and successfully employed with novel self-assembly properties by solvent annealing. Due to its interface modification properties, C60-4TPB can be applied as cathode interface layers between the ZnO and active layer in the inverted organic solar cells to improve the interfacial compatibility between ZnO and organic layer. The solvent annealing was observed to increase the assembly of the fullerene block at the top surface of the C60-4TPB layer. The enriched C60 molecules are also expected to influence the distribution of PC71BM in the active layer and decrease the quenching of the hole at the cathode interface, resulting increased fill factor. For the device ITO/ZnO/C60-4TPB (0.5 mg/ml)/PTB7:PC71BM/MoO3/Ag annealed by toluene solvent, an enhanced average PCE of 8.07% with a relatively long-term stable cathode



interface were observed. The results demonstrated that C60-4TPB layer have ideal to improve the photovoltaic performance of inverted OSCs based on ZnO.

ASSOCIATED CONTENT Supporting Information Additional figures as mentioned in the text, include 1H NMR spectrum, UV-vis absorption spectrum, infrared spectra of 4TPB and C60-4TPB; contact angles films of C60-4TPB on silicon substrate; XRD patterns, work function, UPS spectra, UV-vis absorption spectrum, and transmission spectrum of SA and TA films for devices ITO/ZnO/C60-4TPB(x).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (21274048). We also thanks to the HUST Analytical and Testing Center for allowing us to use its facilities.

REFERENCES


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