Graphene Oxide by UV-Ozone Treatment as an Efficient Hole

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Graphene Oxide by UV-Ozone Treatment as Efficient Hole Extraction Layer for Highly Efficient and Stable Polymer Solar Cells Yingdong Xia, Yufeng Pan, Haijuan Zhang, Jian Qiu, Yiting Zheng, YongHua Chen, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05422 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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ACS Applied Materials & Interfaces

Graphene Oxide by UV-Ozone Treatment as Efficient Hole Extraction Layer for Highly Efficient and Stable Polymer Solar Cells

Yingdong Xia,† Yufeng Pan,† Haijuan Zhang,† Jian Qiu,‡ Yiting Zheng,§ Yonghua Chen,†,* Wei Huang†, ∥,*

†

Key Laboratory of Flexible Electronics (KLOFE) & Institution of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, Jiangsu, 211816, P. R. China

‡

2011 college, Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, Jiangsu, 211816, P. R. China §

College of Chemistry and Molecular Engineering, Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, Jiangsu, 211816, P. R. China ∥

Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. Email: [email protected]; [email protected]

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Abstract Hole extraction layer has significant impact on achieving high-efficiency polymer solar cells (PSCs). Here, we report an efficient approach to by direct UV-Ozone treatment larger device performance enhancement employing grapheme oxide (GO). The dramatically performance enhancement of PSCs with P3HT:PCBM blend as an active layer was demonstrated by UV-Ozone treated GO for 30 min: best PCE of 4.18%, FF of 0.63, Jsc of 10.94 mA cm-2, and Voc of 0.61 V, which is significantly higher than the untreated GO (1.82%) and highly comparable PEDOT:PSS-based PSCs (3.73%). In addition, PSCs with UV-Ozone treated GO showed a longer stability than the PSCs with PEDOT:PSS. The significant enhancement of PCE of PSCs can be attributed the fact that ozone molecules can oxidized GO into CO2 and leave highly conductive grapheme particles. We suggest that this simple UV-Ozone treatment can provide an efficient method for highly efficient GO hole extraction in high-performance PSCs. KEYWORDS: graphene oxide, UV-Ozone, hole extraction layer, stability, polymer solar cells.


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1. Introduction Polymer solar cells (PSC) have attracted significant attention owing to their low cost, light weight, solution processable, and flexibility.1-4 Since the first report by Tang et al from 1986,5 power conversion efficiencies (PCE) over 12% have been achieved by improving the fabrication techniques and device structures engineering.6 In a PSC device, hole extraction layer (HEL) is necessary to be placed on or below a conductivity anode (e.g., indium tin oxide, ITO) to avoid the current leakage caused by direct contact with the electrodes of the donor and acceptor phases.7-10 A conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is commonly employed as a HEL. Although high-performance devices have been achieved, PEDOT:PSS is highly acidic (pH=1, easy to corrode ITO) with hygroscopic and inhomogeneous electrical properties, which results in poor stability.11 The other extensive employed HELs are wide band gap, mechanical strength, and chemical stable inorganic materials with high work function to replace PEDOT:PSS, such as V2O5, WO3, MoO3, and NiO (the most effective).12-14 However, high-cost vacuum techniques have been used for fabricating uniform films, which cannot complete with low-cost roll-to-roll techniques. Although solutionprocessable metal oxides by sol-gel methods have been successfully demonstrated,14-18 the high annealing temperature (> 200 °C) for the fully stoichiometric metal oxides limits their application. Therefore, development of cost-insensitive and simply solution processable HELs is highly desired. Very recently, grapheme oxide (GO) and reduced GO as efficient HELs have been emerged for high-performance PSCs.19-25 GO are highly oxidized two dimensional nanosize graphitic domains as the random diblock copolymer. The sp2 conjugation of the hexagonal grapheme lattice is disrupted since sp3 hybridized is the most of carbon atoms bonded with oxygen, where 3 ACS Paragon Plus Environment


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the substantial sp3 fraction in GO makes it an insulating material. Accordingly, the thickness of GO is important to the GO-based device performance.19 Although the reduction of GO can be used as efficient HELs by chemical processing method using a toxic hydrazine reagent or by ultra-high vacuum high-temperature (up to 1050 °C) approach under Ar and H2,20 the complicated, high-temperature, and poisonous processes are violation of the simple and low-cost production of PSCs. In this paper, we report an effective strategy for the fabrication of efficient GO HELs directly treated by UV-Ozone (Figure 1). We observed that the ozone treated GO contribute to larger enhanced device performance than that used the pristine GO without changing the transporting characteristics. In particular, dramatically performance enhancement of PSCs was achieved by UV-Ozone treated GO for 30 min: best PCE of 4.18%, FF of 0.63, Jsc of 10.94 mA cm-2, and Voc of 0.61 V, which is significantly higher than the untreated GO (1.82%) and highly comparable PEDOT:PSS-based PSCs (3.73%). Moreover, PSCs with UV-Ozone treated GO showed a longer stability than the PSCs with PEDOT:PSS HEL. The structural changes of the GO during exposure time for UV-ozone treatment were significantly characterized. The significant enhancement of PCE of PSCs can be attributed the fact that ozone molecules can partially oxidized GO into CO2 and leave grapheme particles, making it highly conductive. We suggest that this simple UV-Ozone treatment can provide an efficient method for highly efficient GO hole extraction in high-performance PSCs.

2. Experimental Section The GO was synthesized by the modified Hummer’s method.26 The GO solution was prepared by dispersed GO into Dimethylformamide (DMF) with different concentration from 0.25 mg ml-


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1

to 2 mg ml-1. The GO layer was obtained by spin-coating from its DMF solution at 2000 rpm

for 1 min. The film was either annealing at 150 °C for 30 minutes or UV-Ozone treatment (UVO3 Cleaner-40 W, SunMonde Inc. Shanghai) for 10-40 minutes. The device fabrication and measurements can be found in ref. 23 in detail.

3. Results and discussion Figure 1 shows the device architecture and a schematic diagram of the UV-ozone treatment method. GO was prepared by a modified Hummers’ method.26 Several concentrations of GO at Dimethylformamide (DMF) (0.25 mg ml-1 to 2 mg ml-1) were prepared for fabrication of GO thin films. The active layer consisted of a common used blend of P3HT:PCBM (P3HT: poly(3hexylthiophene) and PCBM: [6,6]-phenyl-C61-butyric acid methyl ester).23, 27, 28 A PEDOT:PSSbased device was also fabricated as a reference.

Figure 1. Device structure and the molecular structure of P3HT and PCBM. Figure 2a shows the device performance of PSCs with different concentration of GO HELs annealed at 150 °C and treated by UV-ozone, respectively. Table 1 summarizes the device 5 ACS Paragon Plus Environment


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performance. The device with annealed GO HELs show very poor performance, as shown in Table 1 and Figure 2, which were gradually decreased with increasing the concentration of GO. This can be attributed to the insulating property of GO especially with high concentration.19 Figure S1 shows the comparison of device performance after annealing with different concentration of GO (Supporting Information, Figure S1). The best performance was achieved at 0.5 mg ml-1 with PCE of 1.82% (Voc of 0.54 V, FF of 0.53, Jsc of 6.41 mA cm-2). However, the device performance exhibited remarkable enhancement after simple UV-Ozone treatment regardless of the concentrations of GO. The Vocs, Jscs, FFs, and PCEs were increased up to 0.610.62, 7.34-10.94, 0.61-0.64, and 2.78-4.18%, respectively (Table 1 and Figure S2, Supporting Information). The device with 1.5 mg ml-1 GO gave the best device performance with Voc of 0.61 V, Jsc of 10.94 mA cm-2, FF of 0.63, and PCE of 4.18%, which is higher than the PEDOT:PSSbased device (Table 2). Figure S3 (Supporting Information) gave the statistics of device performance. The performance was gradually increased with increasing the concentration then seriously decreased at higher concentration of GO (>1.5 mg ml-1). It is different from the device with annealed GO HELs, where the best performance was obtained at 0.5 mg ml-1. Although the higher concentration of GO (1.5 mg ml-1) was used in UV-Ozone treatment methods, the device performance was obviously better than that with a lower concentration (0.5 mg ml-1) of GO treated by annealing, demonstrating that the conductivity of GO could be changed after UVOzone treatment. Moreover, the treatment time by UV-Ozone was carefully optimized at a centration of 1.5 mg ml-1 GO from 10 min to 40 min (Figure 2b and Table 2). The device showed significantly improved performance after just 10 min treatment while decreased at a long-term exposure time higher than 30 min. The reduced performance could be owing to the significant morphology evolution and GO film damage by UV-Ozone treatment for < and >30


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min (Supporting Information, Figure S4).29 The GO sheets are not formed a uniform film but “particles” on top of ITO both in annealing and UV-Ozone treatment, which are different from the previous reports.19,

22

The number of “particles” gradually reduced with increasing the

treatment time, indicating the UV-Ozone could destroy the GO sheets. The device, therefore, shows poor performance after treatment for 40 min. The film morphology of GO were further investigated by SEM (Supporting Information, Figure S5). It is confirmed again that there is no uniform GO film formation but big “particles” (d~10 µm) on top of ITO. Furthermore, these suggest that the efficient ratio of GO to ITO surface is formed for highly efficient extraction of holes up to 30 min ozone treatment. The large particles might be unoxidized graphene aggregates surrounded by GO, which thus increase the conductivity of GO. Therefore, we proposed that the initial 30 min UV-Ozone treatment might oxidize GO into CO2, thus exposing the highly conductive graphene particles and improving device performance. UV-Ozone further oxidizes the graphene particles after 30 min, thus forming new GO and reducing the conductivity, and deteriorates device performance.



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Figure 2. (a) Device performance comparison between GO annealing and GO UV-Ozone treatment at different concentration of GO at DMF. (b) Device performance with different treatment time by UV-Ozone and PEDOT:PSS as a reference. Table 1. Summary of device performance comparison between GO annealing and GO UVOzone treatment for 30 min at different concentration of GO at DMF. -1

Concentration (mg ml ) Treatment Voc (V) Jsc (mA cm-2)

FF

PCE (%)

Annealing

0.42

6.36

0.45

1.20

UV-Ozone

0.61

7.55

0.61

2.78

Annealing

0.54

6.41

0.53

1.82

UV-Ozone

0.60

8.61

0.61

3.17

Annealing

0.51

5.36

0.43

1.18

UV-Ozone

0.61

9.70

0.57

3.32

Annealing

0.37

5.30

0.34

0.66

UV-Ozone

0.61

8.94

0.64

3.51

0.34

5.06

0.37

0.62

0.61

10.94

0.63

4.18

0.31

4.50

0.37

0.52

0.62

7.34

0.63

2.87

0.25

0.5

0.75

1

Annealing 1.5 UV-Ozone Annealing 2 UV-Ozone


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Table 2. Summary of best-performing devices with different UV-Ozone treatment time.

Treatment time (min) Voc (V) Jsc (mA cm-2)

FF

PCE (%)

10

0.62

8.17

0.65

3.26

20

0.62

9.48

0.65

3.84

30

0.61

10.94

0.63

4.18

40

0.60

8.64

0.60

3.12

PEDOT:PSS

0.62

8.91

0.67

3.73

Since P3HT:PCBM is a prototype active layer with limited device efficiency, we also fabricated devices based on PTB7:PC71BM active layer (Supporting Information, Figure S6). The device performance increased from 7.07% for PEDOT:PSS device to 7.31% for GO device by UV Ozone treatment. The device performance is comparable to the recent publications,30-33 which demonstrates that our approach can be used in highly efficient polymer solar cell based on different type polymer donors. To analyze the GO in structural damage after annealing and UVOzone treatment, XPS, Raman, and FTIR were conducted. XPS can measure the elemental composition and chemical state of the elements in a material. Figure 3 exhibits the XPS spectra evolution of the GO from annealing to UV-Ozone exposure time for 30 min. After 30 min UVOzone treatment leads to the disappearance of C-O bond, providing a strong evidence for the GO sheets oxidized into CO2 since UV-Ozone is a very strong oxidant.20, 22 Moreover, the peak of main C1s was red-shifted from 284 to 284.2 eV and slightly narrowed, indicating no p-type doping caused by Ozone after UV-Ozone treatment for 30 min.29 Due to the fact that GO can be reduced by UV light,23 we fabricated device with GO exposed at UV light (365 nm) for 30 min (Supporting Information, Figure S7). However, the poor device performance (PCE: 1.83%) 9 ACS Paragon Plus Environment


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suggests UV light is not the main contribution to the GO but UV-Ozone. We further investigate the GO between annealing and UV-Ozone by Raman (Supporting Information, Figure S8) and FTIR (Supporting Information, Figure S9) spectra since the two analyses can detect several micrometers inside the samples. Raman is commonly used to offer a fingerprint of molecules by vibration

or

rotational

while

FTIR

can

be

used

to

obtain

an infrared spectrum of absorption or emission of materials. The ratio of D band to G band is a little increased (in the allowable error) from Raman results, which is apparently smaller than previous reports where the ratio is higher than 1.34 This indicates that the GO were not totally conversed to graphene. Furthermore, the same FTIR spectra between annealing GO and UVOzone GO demonstrates that no structural damage or change inside the GO. These two further confirmed that the GO were oxidized into CO2 and leave high conductive graphene particles.

Figure 3. High-resolution XPS C1s spectra GO under (a) annealing and (b) UV-Ozone treatment. We further fabricated a device with a GO film on top of ITO, but the device performance was very poor even though after UV-Ozone treatment for 30 min (Supporting Information, Figure


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S10), demonstrating again that the “particles” play an important role in device performance. The holes can be extracted from active layer to ITO electrode by the high conductive GO (Figure 1). Furthermore, the “particles” could further improve the holes extraction due to increasing the specific surface area at GO/P3HT:PCBM interface. The surface doping of P3HT by the high conductive GO layer could be possible.23, 35 However, the scatter effect can be ruled out for the lower absorption of P3HT:PCBM active layer on top of GO than that on top of PEDOT:PSS (Supporting Information, Figure S11). Figure 4a shows the Cole-Cole plot of the PSCs using HELs by different UV-Ozone treatment time in the dark, which reflected the resistance of the devices.36, 37 The device with annealing GO shows highest resistance compared to the devices with UV-Ozone GO and PEDOT:PSS, which is consistent to the very poor device performance and indicate again the insulating property of GO. The device with UV-Ozone GO for 30 min gives the lowest resistance, which is even lower than PEDOT:PSS device. The most efficient hole extraction, therefore, can be expected in the device with UV-Ozone GO for 30 min. Moreover, due to the nature of acid and hygroscope, the PEDOT:PSS conventional HEL suffers from device instability.11 The PCE of the PEDOT:PSS-based device reduces to 77% of its initial value after storage in an Ar-filled glovebox for 30 days, as shown in Figure 4b, while the GO device still keeps 90% of the original value, suggesting that GO HEL treated by UV-Ozone gave more stable PSCs.



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Figure 4. (a) Impedance spectra of different devices in the dark. (b) PCE decay of 10 unencapsulated PEDOT:PSS and GO devices stored in a Ar-filled glovebox.

4. Conclusions In summary, we have systematically investigated the conventional P3HT:PCBM PSCs incorporating GO HEL treated by UV-Ozone. The PCE of the device using GO without UVOzone treatment show very poor performance (1.82%). However, the PCE significantly increased by 2.3 times (4.18%) together with remarkably improved Voc, Jsc and FF by UV-Ozone treatment due to partial reduced GO. Furthermore, the PSCs employed the GO HEL outperform their counterparts based on conventional PEDOT:PSS HEL (3.73%). Our research employing GO as HEL could have the potential in improving device stability. Moreover, the GO with UV ozone treatment as HEL may be used in current highly efficient non-fullerene organic/polymer solar cells for further improvement of stability and performance. The excellent device performance of GO by simple UV-Ozone treatment allow for a large room for the development of novel treatment approach for carbon nanomaterials for high-performance and stable PSCs.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional results from SEM, FTIR, Raman, absorption, and microscope characterization studies, and J-V characteristics and performance statistics of PSCs Acknowledgements This work was financially supported by the National Basic Research Program of ChinaFundamental Studies of Perovskite Solar Cells (2015CB932200), the Natural Science Foundation of China (51035063), Natural Science Foundation of Jiangsu Province, China (55135039, 55135040), Jiangsu Specially-Appointed Professor program (Grant No. 54907024), and Startup from Nanjing Tech University (3983500160, 3983500151, and 44235022).



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Graphene Oxide by UV-Ozone Treatment as an Efficient Hole

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