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Phys. Status Solidi A 209, No. 7, 1363–1368 (2012) / DOI 10.1002/pssa.201228040
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Graphene oxide/PEDOT:PSS and reduced graphene oxide/PEDOT:PSS hole extraction layers in organic photovoltaic cells Yensil Park, Kyoung Soon Choi, and Soo Young Kim*
School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Heukseok-Dong, Dongjak-gu, Seoul 156-756, Korea Received 16 January 2012, revised 19 March 2012, accepted 22 March 2012 Published online 25 April 2012 Keywords graphene oxide, hole extraction layers, organic photovoltaic cells * Corresponding
author: e-mail
[email protected], Phone: 82-2-820-5875, Fax: 82-2-824-3495
A comparison was made between the use of graphene oxide (GO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and reduced graphene oxide (rGO)/PEDOT:PSS as a hole extraction layer (HEL) in organic photovoltaic (OPV) cells. Hydrazine hydrate (HYD) and the thermal method were adopted to change the GO to rGO. The OPV cell with the GO (2 nm)/PEDOT:PSS HEL exhibits a power conversion efficiency (PCE) as high as 3.53% under 100 mW/cm2 illumination and air mass conditions, which is higher than that
of the OPV cell without the HEL, viz. 1.78%. The device with the PEDOT:PSS/GO HEL shows a similar PCE of 3.48%. However, the PCE of the OPV cell with the rGO/PEDOT:PSS HEL is not high as those of the cells with the HYD-rGO/ PEDOT:PSS and T-rGO/PEDOT:PSS, viz. 3.3 and 3.37%, respectively. The work function of GO was 4.7 eV, but those of HYD-rGO and T-rGO were 4.2 and 4.5 eV, respectively, suggesting that the decrease of the barrier height between GO and active materials is higher than that in rGO case.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Organic photovoltaic cells (OPVs) have become more efficient these days, with a power conversion efficiency (PCE) of over 7% being reached, but there is still a long way to go before they can compete with inorganic ones [1]. Much research has been carried out in various ways to improve the efficiency of the OPVs, including the use of a tandem cell structure, developing new organic compounds, applying anti-reflection coatings, and so on [2–4]. One of the ways to improve the efficiency of OPVs is to reduce the barrier height at the interface of the electrode/organic conjugated polymers. Poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the material most commonly used as a hole extraction layer (HEL). However, PEDOT:PSS also has some drawbacks which reduce the efficiency of the device, such as its unstable interface with indium tin oxide (ITO), inhomogeneous conductivity, and acidity which etches the ITO electrode [5–8]. These issues illustrate the need for a chemically stable and mechanically uniform material to be used as the HEL to replace PEDOT:PSS in OPVs. Therefore, many attempts have been made using metal oxides, selfassembled monolayers, and p-type polymer blends [9–19].
Graphene is a two-dimensional single atomic material which is constructed with sp2 hybridized carbon atoms in hexagonal form [20]. It has exceptional electronic, optical, thermal, and mechanical properties [21–24]. Graphene oxide (GO) has oxygen functional groups on the graphene plane. Although, reduced graphene oxide (rGO), which is made by the reduction of GO, has worse properties than graphene, it has been greatly researched because of its low cost and potential for large-scale production. OPVs with a GO HEL layer having a conventional and inverted structure have been reported [25, 26]. rGO has been used as the electrode of OPVs instead of an ITO electrode. However, the performance of the devices with the rGO electrode was very poor compared to that with the ITO electrode [27–29]. It is expected that the use of GO beneath PEDOT:PSS could protect ITO from in diffusion and acid etching. Furthermore, the work function of graphene, 4.6 eV, is adequate to extract holes from the active layer [30]. Therefore, we tried to use both GO (or rGO) and PEDOT:PSS as HELs in OPVs. In this study, we investigated the effect of GO or rGO beneath PEDOT:PSS as the HEL of OPVs to modify the metal anode contact. A simple spin-coating process was used ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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to modify the ITO anode contact with the GO and rGO used as a HEL. The properties of GO and rGO were examined by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and ultra-violet photoemission spectroscopy (UPS). After GO or rGO was inserted beneath PEDOT:PSS as the HEL in the OPV cells, the performances of the OPVs were measured. It is shown that GO is more suitable as a HEL in OPV cells than rGO. 2 Experimental 2.1 Preparation of GO GO was prepared by the modified Hummers method [31]. Briefly, 2 g of graphite powder (Alfa Aesar graphite powder, universal grade, 200 mesh, 99.9995%) was stirred with 2 g of NaNO3 and 100 mL of concentrated H2SO4 for 1 day in an ice water bath. Afterwards, 12 g of KMnO4 was gradually added. Once it was mixed well, the ice bath was removed and the solution was stirred at 35 8C until a highly viscous liquid was obtained. After adding 100 mL of pure water, the suspension was heated in a water bath at 98 8C for 15 min. Then, it was further treated with warm water and H2O2 in sequence. The mixture was centrifuged at 6000 rpm and washed with HCl and water. The centrifuging and washing processes were repeated a few times. Finally, GO was dried at 50 8C for 24 h in a vacuum oven. 2.2 Reduction of GO GO prepared from natural graphite powder was exfoliated under ultrasonication. The brown dispersion of GO sheets was then centrifuged at 8000 rpm for 10 min to remove the unexfoliated GO (WiseSpin1 CF-10, Daihan Scientific). For the reduction of GO, the hydrazine hydrate (HYD) method and thermal treatment were adopted [32, 33]. In the case of HYD reduction, the GO thin films coated on the substrates were reduced inside a glass Petri-dish containing HYD at 80 8C for 2 h (HYD-rGO). In the case of thermal reduction, the GO thin films coated on the substrates were annealed at 200 8C in air ambient for 2 h (T-rGO). 2.3 Fabrication of OPVs A patterned ITO glass was used as the starting substrate. The substrates were cleaned in sequence with acetone, isopropyl alcohol, and de-ionized water and then dried with high purity nitrogen gas. The substrates were treated with ultra-violet ozone (UVO) for 15 min. Three types of samples were prepared, as shown in Fig. 1. In the case of Device I, the GO solution was first spincoated on the ITO substrate and then the PEDOT:PSS. The thickness of GO was measured to be 2 nm by atomic force microscopy (AFM). In the case of Device II, PEDOT:PSS was spin-coated on the ITO substrate and then the GO solution. In the case of Device III, the GO solution was spincoated on the ITO substrate and the HYD or thermal reduction was carried out using the method mentioned above. Then, PEDOT:PSS was spin-coated. Devices with rGO deposited on top of the PEDOT:PSS layer were not studied due to the low stability of the organic polymer against heating or HYD treatment. ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1 (online colour at: www.pss-a.com) Structures of OPVs with (a) HEL of GO/PEDOT:PSS, (b) HEL of PEDOT:PSS/GO, and (c) HEL of rGO/PEDOT:PSS.
The device with rGO on top of PEDOT:PSS layer was not tried due to the weakness of PEDOT:PSS on heat or HYD. Regio-regular poly(3-hexylthiophene) (P3HT) was first dissolved in 1,2-dichlorobenzene to make a 20 mg/mL solution, followed by blending with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) at a weight ratio of 6:4. The blend was stirred for 14 h in a glove box before being spin-coated on top of the three kinds of substrate. The thickness of the active layer was measured to be 200 nm by a surface profiler. The devices were annealed on a hot plate in a glove box at 130 8C for 10 min. The cathodes were fabricated by depositing LiF (ca. 1 nm) and Al (ca. 100 nm) at a base pressure of 2 106 Torr. The active device area was ca. 0.04 cm2. 2.4 Characterization The current density–voltage (J–V) curves were measured under air ambient with glass encapsulation using a Keithley 2400 source measurement unit. The photocurrent was measured under AM1.5G 100 mW/cm2 illumination. The Raman spectra of the GO and rGO thin films were obtained with a LabRAM HR (Horiba Jobin Yvon) at an excitation wavelength of 514.54 nm. The XRD measurements were carried out on a Bruker-AXS. The AFM images were obtained with an XE100 in non-contact mode. XPS was carried out on a Sigma Probe model (ThermoVG, UK). Corrections due to charging effects were made by using C 1s as an internal reference and the Fermi edge of a gold sample. 3 Results and discussion 3.1 Materials characterization Figure 2(a) shows the images of the water dispersions of GO flakes. The concentrations of these solutions were 0.1, 0.3, 0.5, and 1 mg/mL, respectively, from left to right. Homogeneous www.pss-a.com
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Table 1 shows the change of the resistivity and transmittance values after reduction. The resistivity of the GO sample was measured to be infinity, indicating that the GO shows insulating behavior. However, the resistivity was determined to be 25 kV for the HYD-rGO sample and 12.5 kV for the T-rGO sample, suggesting that the HYD and thermal treatment were effective in reducing the GO. There is a marked transmission difference between the non-reduced and reduced materials. This darkening of the reduced material has been observed previously and suggests the partial restoration of the p-electron system in the GO [35]. The deconvoluted C 1s XPS spectra acquired from the GO, HYD-rGO, and T-rGO thin film samples are shown in Fig. 3(a). The C 1s peak of the film was separated into four components, which are one main C–C and three small C–O components, viz. C–C (284.6 eV) for the sp2 carbon in graphite and C–OH (286.0 eV), C–O–C (286.5 eV), C ¼ O (288.0 eV) [36]. In order to separate the chemical bonding states, including those in the spectra, the spectral line shape was simulated using a suitable combination of Gaussian and Lorentzian functions. For the fitting of all of the multiplets, Figure 2 (online colour at: www.pss-a.com) (a) The images of GO in water with concentrations of 0.1, 0.3, 0.5, and 1 mg/mL. (b) AFM image of GO flake. (c) XRD spectra of GO and pristine graphite. (d) Raman spectra of GO and pristine graphite.
colloidal suspensions of GO were immediately produced in water having a typical brown color, as reported elsewhere, indicating that the GO was well made. Figure 2(b) shows the AFM topography image of the GO spin-coated on the glass substrate. The lateral dimensions of the GO flakes ranged from 1 to 10 mm. The average thickness of the sample was estimated to be 2 nm with a corresponding rms roughness value of 0.97 nm. These values are lower than the rms roughness of the bare glass/ITO substrate of 3 nm, indicating that the deposition of the GO layers serves to planarize the anode surface. The XRD spectra are shown in Fig. 2(c). The pristine graphite has a peak centered at 2u ¼ 26.58 (d ¼ 0.335 nm). This peak was shifted to 2u ¼ 11.38 (d ¼ 0.782) after applying the Hummers method. This means that the graphite is exfoliated and the d-spacing increased, indicating that GO is formed. Figure 2(d) presents the Raman spectra of the GO and pristine graphite. GO has two broad peaks which are known as the G peak (1590 cm1) and D peak (1350 cm1), while the pristine graphite has a strong G peak and very weak D peak. The G peak corresponds to sp2 hybridized carbon and the D peak means that defects participate in the double resonance [34]. The G peak of GO was up-shifted compared with that of graphite. This was attributed to the presence of isolated double bonds that resonate at frequencies higher than that of the G-band of the graphite. The increase in the intensity of the D peak in GO means that the graphite developed a lot of defects consisting of –O–, –OH, and –COOH functional groups. www.pss-a.com
Table 1 The change of the sheet resistance and transmittance after reduction.
GO HYD-rGO T-rGO
sheet resistance (kV)
transmittance at 550 nm (%)
1 25 12.5
97 85 88
Figure 3 (online colour at: www.pss-a.com) The relative transmittance spectra of the ZnCdS/ZnS þ PMMA and ZnCdSe/ ZnS þ PMMA films. The transmittance of glass/ITO was set to be 100%. The light is irradiated from the opposite side of the ITO. ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4 (online colour at: www.pss-a.com) UPS spectra of GO, T-rGO, HYD-rGO, and PEDOT:PSS.
the full-width-at-half-maximums were fixed accordingly. It is shown that the ratio of C–C bonds in the C 1s spectra increased from 53% (GO) to 77.8% (HYD-rGO) and 74.2% (T-rGO). Furthermore, the atomic ratio (O 1s/C 1s) decreased from 0.53 for GO to 0.50 for HYD-rGO and 0.39 for T-rGO. Therefore, it is considered that the reduction of the GO sheets was well performed. Figure 3(b) presents the XRD patterns observed for the GO, HYD-rGO, and T-rGO samples. The peak in GO was observed at 2u ¼ 11.38. After applying the reduction treatment to GO, the main peak shifted to approximately 268, which is typical of graphitic substances, indicating that the recovery of the sp2 bonds took place. However, the peak observed in the XRD pattern of HYD-rGO is very broad and a remnant of the peak at lower angle is observed in the case of T-rGO, meaning that the reduction process is not perfect. Figure 4 shows the UPS spectra of the GO, PEDOT:PSS, HYD-rGO, and T-rGO samples. The onset of the secondary electron binding energy was determined by extrapolating the two solid lines from the background and straight onset in the UPS spectra. The work function was calculated by subtracting the onset of the secondary electron binding energy from the He I excitation energy (21.2 eV). The work function of PEDOT:PSS is calculated to be 5.0 eV, which means that it is suitable to use as an interlayer. GO has a work function of 4.7 eV. After reducing the GO to rGO, its work function decreases to 4.2 eV for HYD-rGO and 4.5 eV for T-rGO. 3.2 Device characterization Figure 5 shows the typical J–V curves of the OPVs without a HEL and with the GO, HYD-rGO, T-rGO, and PEDOT:PSS HELs. In the plot, open-circuit voltage (VOC) ¼ 562.8 mV, short-circuit current density (JSC) ¼ 8.53 mA/cm2, fill factor (FF) ¼ 36.45%, and PCE ¼ 1.78% for the device without the interlayer. However, the introduction of GO, HYD-rGO, T-rGO, or PEDOT:PSS between the ITO anode and ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5 (online colour at: www.pss-a.com) J–V characteristics of the devices without an interlayer and with interlayers of GO, HYDrGO, T-rGO, and PEDOT:PSS
P3HT:PCBM active layer increases the response metrics. The device with the GO interlayer exhibits values of VOC ¼ 613.1 mV, JSC ¼ 8.33 mA/cm2, FF ¼ 51.78%, and PCE ¼ 2.5%. When GO was inserted, JSC was slightly decreased, but VOC and FF were increased, indicating that GO definitely acted as a HEL. The OPVs with the PEDOT:PSS and GO HELs have the same VOC, but the one with the PEDOT:PSS interlayer has higher JSC, FF, and PCE values of 8.73 mA/cm2, 65.08%, and 3.63%, respectively. This result indicates that GO can act as a HEL in OPVs but that PEDOT:PSS is nevertheless a better interlayer. The PCE values of the devices with the HYD-rGO and T-rGO interlayers were 1.8 and 1.9%, respectively. It seems that the effect of HYD-rGO and T-rGO as a HEL in the OPVs is not huge compared with that of the GO interlayer. Both the GO (or rGO) and PEDOT:PSS interlayers were used as HELs in OPVs, as shown in Fig. 6. The PCE values were 3.53% for the GO/PEDOT:PSS HEL and 3.48% for the PEDOT:PSS/GO HEL, indicating that the performance of the OPV is same within the error range, regardless of the location of GO. In the case where the GO in the HEL is substituted with HYD-rGO and T-rGO, the PCE decreased to 3.3 and 3.37%, respectively. Although, the PCE values of all of the samples using the double HELs were lower than that of the sample using a PEDOT:PSS single HEL, the performance of the OPVs using GO is better than that using rGO. 3.3 Discussions Based on these experimental observations, the effect of using GO/PEDOT:PSS or rGO/ PEDOT:PSS as a HEL on the performance of OPV cells can be explained as follows. GO was synthesized well using the modified Hummer’s method. The XPS data showed that the ratio of C–C bonds in the C 1s spectra increased from 53% (GO) to 77.8% (HYD-rGO) and 74.2% (T-rGO). The XRD data showed that the peak at 2u ¼ 11.38 was shifted to approximately 268 after reduction, indicating that the www.pss-a.com
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that of the ones using rGO. It is considered that the energy level alignment of GO between ITO and P3HT is more suitable for extracting the holes from the active layer in the OPV cells compared with that of HYD-rGO and T-rGO.
Figure 6 (online colour at: www.pss-a.com) J–V characteristics of the devices without an interlayer and with interlayers of GO/PEDOT:PSS, PEDOT:PSS/GO, HYD-rGO/PEDOT:PSS, and T-rGO/PEDOT:PSS.
recovery of the sp2 bonds took place. The resistance decreased from infinity to 25 kV for HYD-rGO and to 12.5 kV for T-rGO. The transmittance at 550 nm also decreased from 97 to 85% for HYD-rGO and to 88% for T-rGO, due to the partial restoration of the p-electron system in the GO. Therefore, it is considered that the reduction of GO was well performed. The PCE value of the device with the GO HEL was increased from 1.77 to 2.5%. However, HYD-rGO and T-rGO were not effective as a HEL layer, even though their resistances are lower than that of GO. From the UPS data, the work functions of GO, HYD-rGO, and T-rGO were calculated to be 4.7, 4.2, and 4.5 eV, respectively. It is reported that NH2 functional groups have lone pair electrons, and these groups exhibit electrondonating characteristics [16]. Furthermore, negatively charged oxygen atoms could repel the free electrons in the conduction band, increasing the work function of ITO [37]. Therefore, it is thought that the lone pair electrons of the NH2 component in HYD-rGO and thermal desorption of oxygen in T-rGO induced extra electrons in rGO, which raise the Fermi level from the Dirac point, decreasing the work function. When excitons are generated in the active area, holes diffuse to the ITO layer. Because the HOMO levels of P3HT and ITO are 5.2 and 4.4– 4.7 eV, respectively, a better device can be obtained when a HEL with a high work function is adopted. Also, this is why the PEDOT:PSS interlayer gave the best results. The device with the GO interlayer shows a lower PCE than the one with PEDOT:PSS, but it is better than the one with no interlayer, indicating that GO has the potential to be used as an interlayer. When both the GO (or rGO) and PEDOT:PSS interlayers were used as HELs in the OPVs, the PCE value increased regardless of the location of GO. Even though the PCE values of all of the samples using double HELs were lower than that of the sample using the PEDOT:PSS single HEL, the performance of the OPVs using GO was better than www.pss-a.com
4 Conclusions We investigated the effect of inserting GO or rGO beneath PEDOT:PSS on the performance of OPVs. HYD and the thermal method were adopted to change the GO to rGO. The resistance and transmittance decreased from infinity and 97% to 25 kV and 85% for HYD-rGO and 12.5 kV and 88% for T-rGO, respectively. The plane distance was shortened and the ratio of carbon was increased after reduction. The OPV cell with the GO/PEDOT:PSS HEL exhibits a PCE value as high as 3.53%, which is higher than that of the OPV cell without a HEL, viz. 1.78%. The device with the PEDOT:PSS/GO HEL shows a similar PCE of 3.48%. However, the PCE of the OPV cell with the rGO/PEDOT:PSS HEL is not high as those of the cells with HYD-rGO/PEDOT:PSS and T-rGO/PEDOT:PSS HELs, viz. 3.3 and 3.37%, respectively. The work function of GO was measured to be 4.7 eV, but those of HYD-rGO and T-rGO were 4.2 and 4.5 eV, respectively. It is suggested that the decrease of the barrier height between GO and P3HT:PCBM is higher than that in rGO case. Therefore, it is considered that the work function and transmittance of the HEL is one of the important factors determining the properties of the OPV. Acknowledgements This research was supported in part by the Seoul R&BD program (ST100040M093171), in part by the Basic Science Research Program and (2011-0008994), and Midcareer Research Program (2011-0028752) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology, and in part by the Center for Green Airport Pavement Technology (CGAPT) of Chung-Ang University.
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