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Abstract
Graphene is an ideal substitute for indium tin oxide electrode in organic photovoltaic (OPV) devices, due to its outstanding electrical, optical, chemical and mechanical properties. However, the graphene electrode suffers from work function mismatch with common hole injection layer and intrinsic hydrophobic property. Here, CuxO is proposed to modify monolayer graphene in order to increase the work function of graphene (from 4.45 to 4.76 eV) and decrease the water contact angle (from 88° to 59°). Then, the OPV devices based on the CuxO modified graphene anode are fabricated successfully, and power conversion efficiency (PCE) is enhanced from 4.00 ± 0.44 to 5.23 ± 0.47%. Furthermore, the ternary blended polymer solar cell is fabricated by adding a small molecular material 1, 2, 5-thiadiazole-fused 12-ring polyaromatic hydrocarbon into the active layer, and the PCE is improved to 6.03 ± 0.53%, due to the enhanced absorption and depressed recombination inside the active layer.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene as a one-atom thick planar sheet with hexagonal arrangement of carbon atoms has gained much attention due to its flexibility and high optical transmittance in the visible and near infrared region [1, 2]. It has been considered to be an ideal substitute for ITO and been extensively studied in recent years [3–7]. However, the performances were usually sacrificed in those devices based on graphene electrode due to some intrinsic drawbacks of graphene, e.g., high contact resistance, large surface roughness, low work function and limited surface wettability [1, 3, 5, 8].
So far, acid [9–13], organic materials [2, 11, 14, 21] and metal chloride [15–20] have been widely used to modify graphene. Beyond those, metal oxide is another type of interfacial modification material, which is beneficial for improving work function and electrical conductivity of graphene [7]. For instance, Wu reported MoOx-modified graphene electrode and successfully enhanced the PCE with simultaneous increase in conductivity and work function and decrease in surface roughness of graphene [5]. Besides, Kuruvila and associates demonstrated that the performance of device was improved, along with the V2O5 and WO3 modification [7]. By now, CuxO has not been reported as an interfacial modification material for graphene although it is an intrinsic p-type semiconductor material with low cost and nontoxic property [22].
In this paper, CuxO was used to modify graphene by depositing Cu on graphene through thermal evaporation, followed by an UV/ozone treatment. Subsequently, organic solar cells (OPVs) with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5–b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]: [6,6]-phenyl-C71-butyric acid methyl ester (PCE-10:PC71BM) active layer were fabricated on the CuxO-modified graphene sheets, with their PCEs significantly improved from 4.00 ± 0.44 to 5.23 ± 0.47% due to improved work function and surface hydrophilicity. Furthermore, a small molecular material 1, 2, 5-thiadiazole-fused 12-ring polyaromatic hydrocarbon (DNNBT-C12) was introduced into the PCE-10:PC71BM active layer to form a ternary blended polymer solar cell, and a higher PCE of 6.03 ± 0.53% was achieved.
2. Experimental details
Graphene films were bought from Hangzhou Gelanfeng nanotechnology Co. Ltd., which were monolayer with a thickness of 0.35 nm and synthesized with a chemical vapor deposition (CVD) process using Cu foils as substrates [26]. Poly(methyl methacrylate) (PMMA) anisole solution was spin-coated onto the graphene-covered Cu foil as a supporting layer, and then the PMMA-coated Cu foil was annealed at 120 °C for 5 min. Subsequently, the bottom Cu foil was etched in a 0.1 M ammonium persulfate ((NH4)2S2O8) solution [21]. After 24 h, the graphene-PMMA stack was lifted from the initial substrate, followed by three times washing with deionized water, and then the graphene-PMMA film was transferred onto the target glass substrate. After that, the sample was put on a hot plate and heated at 160 °C for 15 min for improving the bonding formation between graphene and the substrates. Finally, the PMMA was dissolved into warm acetone for few hours. In order to modify the graphene, a series of ultrathin Cu films were thermally deposited on the glass-supported graphene sheets by loading them into a high vacuum chamber. Thereafter, a 7-min UV/ozone treatment was performed for the CuxO formation. The oxidation processes of Cu under the UV/ozone ambient may follow the principles of
To fabricate the OPVs, a 200 μL PEDOT:PSS solution was dropped and spin-coated onto the graphene/CuxO anode at a speed of 2000 rpm for 60 s to form a 40-nm PEDOT:PSS film. Then it was dried at 120 °C for 30 min. After that, the blended film of PCE-10:PC71BM was prepared by dissolving PCE-10 (10 mg) and PC71BM (15 mg) in dichlorobenzene (1 mL), followed by a continuous stirring for 72 h at room temperature in a glove box. Then the active layer was spin-coated onto the PEDOT:PSS-covered graphene/CuxO at a speed of 1000 rpm for 60 s and dried in a glove box at room temperature for 30 min. Finally, LiF and Al were sequentially evaporated in vacuum under a pressure below 1.9 × 10−3 Pa. The active area of the device was 0.1 cm2. OPVs based on ITO and graphene without CuxO treatment were also prepared for comparison. For the ternary blended OPV, small molecule of DNNBT-C12 with a molecular weight of 1374.15 was doped into the PCE-10:PC71BM blend with a doping concentration of 4 wt%.
The sheet resistance, optical transmittance and Raman spectra of graphene were measured by the 4-probe resistivity measurement system (RTS-9, China), the PerkinElmer Lambda 650 UV−vis spectrophotometer and the confocal Raman microscope (InVia, Renishaw), respectively. Ultraviolet photoelectron spectroscopy (UPS) was carried out by using a He discharged lamp (He I 21.22 eV, Kratos Axis Supra). The measurements on the current density-voltage characteristics were operated with a Keithley 2400 sourcemeter under 100 mW cm−2 illumination (AM 1.5 G, Oriel Sol3A, 300 W) in a glove box with N2 atmosphere (H2O < 0.1 ppm and O2< 1 ppm) without further encapsulation. External quantum efficiency (EQE) measurements were performed by measuring the short-circuit current with spectrally resolved monochromatic beam and locked-in amplifier.
3. Results and discussion
The sheet resistance and transmissivity of the graphene and CuxO-modified graphene were measured. As shown in Fig. 1, the increasing CuxO thickness on graphene not only increased the sheet resistance of graphene but also decreased its transmissivity due to intrinsic low conductivity and relatively low transparency for the CuxO semiconductor. Deposition of a 4-nm CuxO thin layer onto the graphene sheet induced a 0.5-fold enhancement on sheet resistance and an average 6% decrease in film transmissivity at 550 nm. But fortunately, the CuxO modification improved hydrophilicity of graphene, as confirmed by the homogenous films of poly(3,4-ethylene dioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) in Fig. 2 and decreased water contact angle (from 88° to 59°) of graphene [Fig. 3(a)].
Fig. 1 (a) Sheet resistance and (b) transmissivity curves of graphene modified with different thicknesses of CuxO.
Fig. 3 (a) Raman spectra of graphene modified with different thicknesses of CuxO, (b) G and (c) 2D peak distribution histogram extracted from Raman spectra. Inset: Contact angle of graphene.
Raman spectroscopy was conducted to investigate the effect of CuxO on the quality and doping situations of graphene. As shown in Fig. 3(a), G and 2D peaks of graphene films without modification are around 1588 and 2683 cm−1, with an intensity ratio of G to 2D band (IG/I2D) of 0.29, demonstrating a monolayer graphene used in this work. There are no obvious D band (around 1350 cm−1) associated with defects, indicating the film integrity and no more defects induced by CuxO. Figures 3(b) and 3(c) show that G and 2D peaks of graphene film shift from 1588 and 2683 cm−1 to 1599 and 2693 cm−1, respectively, at the deposition of 4.0-nm thick CuxO. The blue shifts of the G and 2D peaks after the CuxO treatment are attributed to the p-doping effect of CuxO on the graphene [21].
X-ray photoelectron spectroscopy (XPS) is an important testing approach to throw a clearer insight into the bonding nature of the constituent elements. In this work, the surface composition of CuxO-covered graphene was studied by XPS, with as-measured XPS spectra shown in Fig. 4(a). Figure 4(b) shows Cu 2p XPS recorded with a high-resolution mode. Binding energy of 933.0-934.0 eV for the Cu2p3/2 peak and shake-up peak of 954.0 eV are characteristic for CuO [23], while the Cu 2p3/2 binding energy at 932.5 eV and the Cu 2p1/2 one at 952.0 eV are characteristics for Cu and Cu2O coexisting [24]. Furthermore, UPS spectra of the intrinsic and the CuxO-modified graphene were also analyzed to observe the influence of CuxO on graphene’s work function (WF), as shown in Fig. 4(c). The WF of the as-modified graphene increases from 4.45 eV to 4.76 eV with the CuxO thickness up to 4 nm, but it doesn’t increase anymore with a further increase in thickness. Above enhancement on the WF of graphene indicates that CuxO-modified graphene has a potential to promote carrier extraction and therefore improve FF in the OPVs.
Fig. 4 (a) XPS; (b) Cu 2p XPS; and (c) magnified UPS spectra of graphene modified with different thicknesses of CuxO.
Based on the graphene anode modified with CuxO, OPV cells were manufactured with the device configuration of glass/anode/CuxO (0, 1.3, 2.6, 4.0 nm)/PEDOT:PSS (40 nm)/PCE-10:PC71BM (100 nm)/LiF (0.5 nm)/Al (90 nm). Schematic device structure, energy level diagram and related molecular structures are shown in Fig. 5. As shown in Fig. 6(a) and Table 1, it is found that the OPV based on 2.6-nm thick CuxO performs the best performance since there exists a trade-off between work function and transmissivity of the electrode when the CuxO thickness is increased. As the dark current curves shown in Fig. 6(b), the injection current is increased with the deposition of CuxO, demonstrating the high work function CuxO is beneficial for hole injection.
Fig. 5 (a) Device structure of OPVs with graphene anode, (b) molecular structures of PCE-10, PC71BM and DNNBT-C12 and (c) energy levels of OPVs comprised of the graphene anode with and without CuxO modification.
Fig. 6 Current-voltage characteristic curves for the OPVs under (a) AM 1.5G illumination and (b) dark conditions.
Table 1. The photovoltaic characteristics with different thicknesses of CuxO to modify the graphene anode. The statistical data were obtained from 5 groups of devices.
Figure 7 depicts the current density curves of several devices with different anodes under AM 1.5G illumination and dark conditions. Here, besides the OPVs with 2.6-nm CuxO-modified graphene, the OPVs with pure graphene without any modification and common ITO were also fabricated for comparison. Parameters including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), PCE, as-calculated Jsc from EQE (denoted as Jsccal),serial resistance (Rs), shunt resistance (Rsh) and photo-generated excitons (Gmax) were extracted and listed in Table 2. It is found that the PCE of the pristine graphene-based OPVs is 38.7% smaller than that of the ITO one. The solar cells based on the pristine graphene have relatively low Jsc (12.40 ± 1.75 mA cm−2), FF (40.3 ± 2.00%) and PCE (4.00 ± 0.44%) due to the larger sheet resistance (900 Ω sq−1) and intrinsic hydrophobicity of the graphene, resulting in a large Rs (32.3 Ω cm2) inside the OPV. In addition, the low WF of the pristine graphene also makes carrier extraction become difficulty, resulting in a low Rsh and thus a decreased PCE. In contrast, the performances of OPVs based on the CuxO-modified graphene are obviously improved, with Jsc, FF and PCE increased to 12.87 ± 1.16 mA cm−2, 51.2 ± 3.70% and 5.23 ± 0.47%.
Fig. 7 Current density-voltage characteristics under (a) AM 1.5G illumination and (b) dark conditions, (c) EQE and (d) photocurrent density-effective voltage curves for our OPVs.
Table 2. The photovoltaic characteristics with different anodes. The statistical data came from 5 groups of devices.
The impressive improvement can be attributed to the following several aspects: Firstly, the CuxO on the graphene surface is beneficial for the improvement of graphene’s hydrophilicity. The improved hydrophilicity allows PEDOT:PSS to homogenously spread on the graphene/CuxO surface, which results in better planarization of the film surface and facilitation of hole injection/extraction. Secondly, the WF of graphene is increased from 4.45 to 4.76 eV after CuxO modification, which is beneficial for decreasing the work function mismatch between graphene and PEDOT:PSS. Thus, charge extraction at the PEDOT:PSS/graphene interface is improved accompanied with the increasing Rsh, generating a higher FF [25]. A higher Rsh of 509.6 Ω cm2 in the CuxO-modified OPVs indicates less recombination and sufficient carrier dissociation. Note that the measured Jsc values for all devices are in well agreement with the ones calculated from the EQE spectra (Table 2).
However, the graphene/CuxO-based device in Table 2 has a significantly lower PCE than the ITO-based one. As previously reported by us, a small molecular material DNNBT-C12 could not only enhance the absorption of PCE-10:PC71BM but also reduce recombination inside the active layer [21]. Accordingly, DNNBT-C12 was doped into the PCE-10:PC71BM active layer to form a ternary blended polymer solar cell in this work. With the use of DNNBT-C12 (Fig. 7), Jsc is increased from measured and calculated values of 12.87 ± 1.16 and 13.09 mA cm−2 to 13.26 ± 1.61 and 13.89 mA cm−2 due to a complementary absorption of DNNBT-C12 (mainly in the range of 400-600 nm) with the active layer of PCE-10:PC71BM, which is demonstrated by the EQE curves in Fig. 7(c). A significant increase in FF from 51.2 ± 3.70% to 60.5 ± 4.23% is originated from an enlarged recombination resistance with the doping of DNNBT-C12 into the active layer, which contributed to the reduced recombination of photo-generated excitons [21]. Increases in both Jsc and FF ultimately result in a high PCE of 6.03 ± 0.53% in our ternary blended OPV.
4. Conclusions
In summary, metal oxide CuxO was successfully deposited onto graphene with an initial thermal evaporation of Cu and then an UV/ozone treatment. Then, the PCE-10:PC71BM based OPVs were fabricated on these CuxO-modified graphene sheets and the PCEs were significantly improved from 4.00 ± 0.44 to 5.23 ± 0.47% due to the increased carrier extraction by enhancing graphene’s work function from 4.45 to 4.76 eV. In addition, the improved hydrophilicity of graphene after CuxO modification enhanced the formation of the uniform polymer films on graphene. Furthermore, a small molecular material DNNBT-C12 was added into the active layer to form ternary blended OPVs and the PCE was further increased to 6.03 ± 0.53% due to the improved light absorption of the active layer and a larger recombination resistance in devices. This work provides with an efficient interfacial modification technique of graphene and a solution to enhance optoelectronic conversion efficiency in OPVs with graphene anodes.
Funding
National Foundation for Science and Technology Development (973 project, Grant No. 2015CB932203); National Key Research and Development Program of China (Grant No. 2017YFB0404501); National Natural Science Foundation of China (Grant Nos. 61274065, 61205195, 61705111, 61704091, 51173081, 21404058, and 61136003); Science Fund for Distinguished Young Scholars of Jiangsu Province of China (Grant No. BK20160039); Natural Science Foundation of Jiangsu Province (Grant Nos. BM2012010 and BK20170899); Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. YX030002); the Jiangsu National Synergetic Innovation Center for Advanced Materials; Synergetic Innovation Center for Organic Electronics and Information Displays; Open Foundation from Jilin University (Grant No. IOSKL2017KF04).
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