Efficient organic photovoltaic cells on a single layer graphene transparent conductive electrode using MoO_x as an interfacial layer

Abstract

The large surface roughness, low work function and high cost of transparent electrodes using multilayer graphene films can limit their application in organic photovoltaic (OPV) cells. Here, we develop single layer graphene (SLG) films as transparent anodes for OPV cells that contain light-absorbing layers comprised of the evaporable molecular organic semiconductor materials, zinc phthalocyanine (ZnPc)/fullerene (C60), as well as a molybdenum oxide (MoO_x) interfacial layer. In addition to an increase in the optical transmittance, the SLG anodes had a significant decrease in surface roughness compared to two and four layer graphene (TLG and FLG) anodes fabricated by multiple transfer and stacking of SLGs. Importantly, the introduction of a MoO_x interfacial layer not only reduced the energy barrier between the graphene anode and the active layer, but also decreased the resistance of the SLG by nearly ten times. The OPV cells with the structure of polyethylene terephthalate/SLG/MoO_x/CuI/ZnPc/C60/bathocuproine/Al were flexible, and had a power conversion efficiency of up to 0.84%, which was only 17.6% lower than the devices with an equivalent structure but prepared on commercial indium tin oxide anodes. Furthermore, the devices with the SLG anode were 50% and 86.7% higher in efficiency than the cells with the TLG and FLG anodes. These results show the potential of SLG electrodes for flexible and wearable OPV cells as well as other organic optoelectronic devices.

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Introduction

Organic photovoltaic (OPV) cells have attracted substantial scientific and commercial interest due to their light weight, compatibility with flexible substrates, suitability for roll-to-roll mass production and potentially low cost.^1–3 A critical component of the OPV cell is the transparent conductive electrode (TCE), which acts as a window for light to pass into the device as well as the means of charge extraction. As such the TCE plays a critical role in device performance. To date, transparent indium tin oxide (ITO) is the most commonly used TCE in OPV cells. However, the rapid reduction of indium reserves, the highly brittle nature of ITO, and the effect of indium ion diffusion into the organic semiconductor layers indicate that alternative TCEs need to be developed if OPV technology is going to reach its potential.^4–6

Graphene, a single layer of sp²-hybridized carbon atoms bonded together in a hexagonal “honeycomb” lattice, has remarkable electrical conductivity, optical transmittance, mechanical flexibility, and chemical stability. These properties and the successful synthesis of large-area graphene by chemical vapor deposition (CVD) have led to growing interest in its application for OPV cells as an alternative to ITO as the TCE.^7–9 However, the CVD-grown single layer graphene (SLG) has a high sheet resistance, typically ∼600–1000 Ω sq⁻¹, with 97% transmittance, which in both cases is much higher than that of ITO (∼10 Ω sq⁻¹ with 85% transmittance), and hence is generally thought as unsuitable for OPV applications.⁹ Therefore, the early demonstration of OPV cells with graphene electrodes usually used multilayer graphene films that were produced by multiple transfer and stacking of CVD-grown SLGs to reduce the sheet resistance.^10–12 However, the multiple transfer and stacking processes have a number of drawbacks. First, they are time-consuming and greatly increase the production cost of graphene TCEs. Second, the light transmittance of the multilayer graphene TCEs is reduced since each additional layer absorbs ∼3% of the incident light.¹³ Third, multiple transfer and stacking can lead to a significant increase of the surface roughness of the graphene TCE. That is, the density and surface feature heights due to wrinkles and polymer residues such as poly(methyl methacrylate) (PMMA), which originate from the growth and transfer processing, are increased.^14,15 A large surface roughness can give rise to a high leakage current, act as potential shunt pathways, and even cause electrical shorts between the graphene and the metal electrodes through the organic semiconductor layer. Furthermore, residues such as PMMA are electrical insulators and hence may inhibit charge extraction, and if in a particulate form can decrease the device uniformity.¹⁶ Thus, there is strong impetus to improve the performance of SLG films to avoid the problems of the multilayer graphene electrodes.

The introduction of an interfacial layer between the electrodes and the organic semiconductor layers has been shown to improve the power conversion efficiency (PCE) of OPV cells by optimizing the electrical contacts leading to a reduction in charge recombination at the electrodes and hence improving charge collection.^17,18 Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most widely used interfacial layer material for TCEs. However, the hydrophobic nature of the graphene surface makes it difficult to deposit PEDOT:PSS uniformly over its surface.¹⁴ Moreover, the work function of PEDOT:PSS (∼5.2 eV) is relatively low, and its acidic nature may cause the devices to have reliability issues.^19,20 Molybdenum oxide (MoO_x, 2 < x < 3) has shown promising results as an interfacial layer for graphene TCEs in optoelectronic devices.^14,21,22 It can be processed by evaporation and depending on the conditions can have a work function as high as ∼6.4 eV, although it is sensitive to moisture.²³ More importantly, MoO_x can also act as a p-type dopant to decrease the resistance of graphene TCEs.^24,25 Therefore, MoO_x has potential to simultaneously enhance charge extraction and increase the electrical conductivity of graphene TCEs.

Here, we report the first use of CVD-grown SLG films as transparent anodes for OPV cells with evaporable molecular organic semiconductor light-absorbing layers, namely a bilayer structure of zinc phthalocyanine (ZnPc)/fullerene (C60). A MoO_x layer was thermally deposited onto the SLG TCE to form an interfacial layer to simultaneously increase the conductivity of the graphene and reduce the energy barrier between the graphene anode and the active layer. In addition to an optical transmittance increase, the SLG anode has shown a significant decrease in surface roughness compared to two and four layer graphene (TLG and FLG) TCEs fabricated by the multiple transfer and stacking of CVD-grown SLGs. More importantly, the introduction of the MoO_x interfacial layer caused the SLG anode to have a significantly reduced resistance, which is closer to that of the TLG and FLG anodes. In addition to excellent flexibility, the OPV cells with SLG anodes on plastic substrates (∼1000 Ω sq⁻¹) had an open circuit voltage (V_OC) of 0.44 V, short circuit current density (J_SC) of 5.2 mA cm⁻², fill factor (FF) of 0.37, and a PCE (η) of 0.84%. The PCE was only 17.6% lower than that of devices with the same organic semiconductor layers but using the more conductive ITO as the anode (15 Ω sq⁻¹). In addition, the PCE of devices with SLG anodes was 50% and 86.7% higher than that of the devices using TLG and FLG anodes, respectively.

Experimental

Fabrication of graphene TCEs and OPV cells

Fig. 1 shows the fabrication process of the OPV cells by using graphene transparent anodes and a MoO_x interfacial layer. First, an SLG film (Fig. S1†) was synthesized by CVD on a copper foil.²⁶ It was then transferred onto a poly(ethylene terephthalate) (PET) substrate with dimensions of 2.5 × 2.5 cm². The SLG TCE was obtained by etching away the Cu foil in an aqueous solution of FeCl₃ (0.03 g mL⁻¹) with spin-coated PMMA as a protecting layer, which was subsequently dissolved in warm acetone.^27–29 TLG and FLG TCEs were obtained by transfer and stacking two and four layers of the SLG films, one by one, onto a PET substrate, respectively. Subsequently, the TCE was patterned by mechanically rubbing away the peripheral area along the two opposite sides. After patterning, the graphene anode was loaded into a high vacuum chamber to deposit two Ag contact electrodes, a 30 nm MoO_x interfacial layer, followed sequentially by a 5 nm CuI buffer layer, 25 nm ZnPc and 35 nm C60 light-absorbing layers, a 10 nm bathocuproine (BCP) hole blocking layer, and an Al cathode (PET/graphene/MoO_x/CuI/ZuPc/C60/BCP/Al). Each substrate contained 6 devices, and each device had a 0.2 cm² active area defined by the cathode. For comparison, OPV cells were also fabricated on an ITO (Xin Yan Ltd) TCE on glass. Before device fabrication, the ITO on glass was sonicated sequentially in Alconox detergent, deionized water, acetone, and 2-propanol, and dried under a nitrogen gas flow.

Fig. 1 Fabrication of OPV cells comprised of an SLG anode and a MoO_x interfacial layer. (a) A CVD-grown graphene film on a PET substrate formed by transfer of a graphene film grown on a Cu foil, (b) a patterned graphene TCE, (c) Ag contact electrodes with MoO_x deposited on the graphene film, (d) OPV cells made by sequential deposition of a CuI buffer layer, evaporable molecular organic semiconductor light-absorbing layers, a BCP hole blocking layer and an Al cathode.

Characterization

An Atomic Force Microscope (AFM, Veeco MultiMode NanoScope IIIa, tapping mode) was used to characterize the surface morphology of the MoO_x film on graphene. The surface roughness of graphene on PET and ITO on glass was characterized by using a WYKO NT 1000 profiler. The sheet resistance and transmittance of the graphene and ITO TCEs were measured by using a 4-probe resistivity measurement system (RTS-9, Guangzhou, China) and a UV-vis-NIR spectrometer (Varian Cary 5000), respectively. Current–voltage characterization of the OPV cells was performed by using a Keithley 2400 source measurement unit in a four-point source-sense configuration under a N₂ atmosphere in a glove box (H₂O < 0.1 ppm and O₂ < 0.1 ppm). An AM 1.5G solar simulator (ABET Technologies) at 100 mW cm⁻² intensity was used for illumination measurements. External Quantum Efficiency (EQE) measurements (Incident Photon Conversion Efficiency – IPCE) were obtained by using a QEX7 setup from PV Measurements Inc, which was operated without white light bias and chopped and locked in the small perturbation limit.

Results and discussion

We first measured the transmittance, sheet resistance and surface roughness of the SLG TCE and compared it with those of TLG, FLG and ITO TCEs. As shown in Fig. 2a, the transmittance of the SLG TCE at 550 nm wavelength was as high as 96.5%, which is higher than that of the TLG (92.7%), FLG (84.1%) and ITO (80.7%) TCEs. The presence of small islands on the surface of the SLG film leads to the transmittance being slightly lower than that of a perfect monolayer of graphene (97.7%). However, the SLG had a sheet resistance of ∼1000 Ω sq⁻¹ (Fig. 2b), which is nearly two times higher than that of the TLG anode (∼600 Ω sq⁻¹), four times higher than that of the FLG anode (∼260 Ω sq⁻¹), and 70 times higher than that of the standard commercial ITO TCE on glass (15.0 Ω sq⁻¹). Fig. 2d–f show the typical surface profiler images of the SLG, TLG and FLG films. It was found that the three graphene TCEs had no big difference in the average surface roughness (R_a), with the arithmetic average of the absolute values of the roughness profiles being about 8–12 nm, which is 3–4 times higher than that of commercial ITO on glass (∼3.3 nm, Fig. S2†). One reason for the high average roughness of the graphene anodes is that the PET substrate has a R_a of about 10 nm. However, there was a distinct difference in the maximum distance of peak to valley (where R_t is the vertical distance between the highest and the lowest points in the evaluated area) for each of the materials investigated. R_t is critical as it gives information as to whether the devices are likely to be short circuited. As shown in Fig. 2c, SLG has a similar R_t (54.3 nm) to that of commercial ITO on glass (40.1 nm), but it is much lower than that of both the TLG (264.9 nm) and FLG (349.9 nm) anodes, indicating that an increase in the number of graphene layers has a significant effect on the peak surface roughness of the graphene TCEs. It is clear that there are more red sharp peaks (indicating the highest peaks) for the FLG than the TLG anodes (Fig. 2e and f), while the SLG anode has almost no such peaks (Fig. 2d). Thus overall, with respect to transparency and surface roughness, the SLG TCE is better for devices than the multilayer graphene films, but to be used effectively the sheet resistance must be substantially reduced.

Fig. 2 (a) Transmittance, (b) sheet resistance, and (c) R_t of graphene TCEs with different numbers of layers on PET, and an ITO TCE on glass. Optical surface profiler images and the measured roughness (scale bar) of the (d) SLG, (e) TLG, and (f) FLG TCEs.

The high work function transition metal oxide, MoO_x, has been widely used as an interfacial layer in OPV cells to improve the hole extraction from the organic semiconductor layer.^30,31 Furthermore, the large work function difference between MoO_x (∼6.4 eV) and graphene (∼4.4 eV) can lead to electron transfer from graphene to MoO_x at the graphene/MoO_x interface, and hence p-doping of the graphene anode.²⁵ A 30 nm thick MoO_x was vacuum evaporated onto the SLG surface, which only caused a small decrease in the transmittance of the SLG TCE at 550 nm – from 96.5% to 91% (Fig. S3†), which is still higher than pristine FLG on PET and ITO on glass TCEs (Fig. 2a). The AFM image (Fig. 3a) shows that the MoO_x layer is uniformly deposited onto the SLG/PET. To evaluate the effect of the MoO_x interlayer on the conductivity of the SLG TCE, the electrical resistance of a SLG film with an area of 2 × 1.5 cm² was measured between two Ag contact electrodes (Fig. 1c) before and after MoO_x deposition using a multimeter in a glove box. For comparison, the electrical resistance of TLG, FLG and ITO anodes were also measured before and after MoO_x deposition. It was worthy of note that the multimeter is easily taken into the glove box to measure the electrical resistance (not sheet resistance) of the graphene TCEs, and importantly the two Ag contact electrodes precisely define the measurement area and guarantee that the same area was measured before and after MoO_x deposition. As shown in Fig. 3b, the deposition of MoO_x dramatically decreased the electrical resistance of the SLG anode, by nearly a factor of ten, (from 4430 to 539 Ω) but only halved the resistance of the TLG anodes to 593 Ω from 1015 Ω. In the case of the FLG anodes there was little difference in the resistance (∼365 Ω in both cases). Thus the graphene–MoO_x interaction has the greatest effect on the graphene TCEs with the fewest layer numbers. This would indicate that hole doping tends to occur at the MoO_x/graphene interface independent of the number of graphene layers. Consequently, the use of a MoO_x interlayer or similar is important when using SLG as the TCE in optoelectronic devices. In contrast, we did not observe any influence of a MoO_x interfacial layer on the resistance of the ITO TCE on glass.

Fig. 3 (a) AFM image of the MoO_x (30 nm) film on SLG/PET, and (b) electrical resistance of graphene films with different numbers of layers on PET and ITO on glass before and after MoO_x deposition. The resistance was measured between two Ag contacts on a film with a defined area of 2 × 1.5 cm².

OPV cells using the SLG anode and the MoO_x interfacial layer with the structure of PET/SLG/MoO_x (30 nm)/CuI (5 nm)/ZnPc (25 nm)/C60 (35 nm)/BCP (10 nm)/Al (80 nm) were prepared and compared to the identical devices made with TLG, FLG, and ITO TCEs. A schematic device structure and the corresponding energy levels are shown in Fig. 4a and b, respectively, and Fig. 4c is the picture of an OPV cell fabricated on the SLG anode. Each cell had an effective area of 0.2 cm² and showed excellent flexibility. The CuI (5 nm) was used as a template for the growth of face-on ZnPc (as opposed to edge-on) to achieve a higher V_OC.^32–34 It is worth noting that a CuI layer also reduced the resistance of the SLG anode, but the effect was smaller than that measured when MoO_x was used. The measured electrical resistance of the SLG anode covered with a thin CuI layer was 876 Ω, much lower than that of the pristine SLG (4430 Ω), but 1.6 times higher than that of the MoO_x/SLG anode (539 Ω). More importantly, deposition of the CuI layer had no significant effect on the resistance of MoO_x/graphene anodes.

Fig. 4 (a) Schematic of the OPV cell structure, (b) energy levels (it should be noted that, as these are measured by different techniques the values should only be used as a guide), (c) photo of flexible OPV cells comprised of the SLG anode and the MoO_x interfacial layer. (d) Light and (e) dark (J–V) characteristics of the OPV cells fabricated with different anodes.

Fig. 4d and e depict the light and dark current density versus voltage (J–V) characteristics of the OPV cells with the different anodes, and the device performance, including V_OC, J_SC, FF, η, the serial (R_s) and shunt (R_sh) resistances, are summarized in Table 1. It was found that the PCE of the best device fabricated on the SLG anode was 17.6% smaller than that of cells with commercial ITO on glass anodes. The hero cells with ITO on glass had a higher V_OC of 0.48 V, J_SC of 4.98 mA cm⁻², FF of 0.43, and η of 1.02%. However, the devices fabricated on the SLG anodes exhibited a higher V_OC (0.44 V), J_SC (5.2 mA cm⁻²), FF (0.37) and η (0.84%) than that of the devices comprised of the TLG and FLG anodes, despite a relatively large R_s. Since the devices were fabricated at the same time, the better photovoltaic performance of the SLG anode over TLG and FLG can be attributed to the following three reasons: first, the MoO_x doping reduced the difference in the R_s between the different graphene anodes – the R_s of the device with the SLG anode was only 4% and 17% larger than the devices with the TLG and FLG anodes, respectively. Second, the SLG anode had a higher transmittance, which enabled more light to be absorbed by the organic semiconductor layers within the device. Fig. S4† shows the monochromatic IPCE of the OPV cells fabricated on graphene anodes with different numbers of layers, where the peaks centered at wavelengths of λ = 620 nm and λ = 710 nm correspond to ZnPc absorption and the peak centered at λ = 440 nm corresponds to the C60 absorption. As expected, the IPCE in both the ZnPc and C60 absorption regions increases with the decrease of the number of graphene layers because of the higher transmittance of the anode. As a result, the OPV cells fabricated on the SLG anodes have a higher η than those of the devices with the TLG and FLG anodes. Third, a higher R_sh means less charge recombination and/or leakage current, which to some extent explains the lower V_OC of the TLG and FLG cells. We found that the devices fabricated on the SLG anodes had a higher R_sh (around 981 Ω cm²), and that the R_sh decreased with the increasing number of layers of graphene. This can be understood in terms of the difference in surface roughness of the graphene anodes with the different number of layers. The SLG anode had the lowest number of large peaks and hence the smallest probability of forming shorting pathways, and hence the highest shunt resistance and the lowest leakage current. In contrast, the lower R_sh of the devices fabricated on the TLG and FLG anodes was caused by the larger number of tall peaks, which led to an increase in the dark current density and poorer diode behavior (Fig. 4e), including a lower V_OC. These results suggest that the SLG TCE with the MoO_x interfacial layer is better than the multilayer graphene anodes for the development of OPV cells.

Table 1 Summary of device performance of the cells with graphene or ITO anodes including V_OC, J_SC, FF, η, R_s and R_sh under simulated AM 1.5G illumination conditions (parameters in parentheses are for the best cells)

TCE	V OC (V)	J SC (mA cm^−2)	FF	η (%)	R s (Ω cm^2)	R sh (Ω cm^2)
SLG	0.42 ± 0.02 (0.44)	4.98 ± 0.22 (5.20)	0.34 ± 0.03 (0.37)	0.78 ± 0.06 (0.84)	319 ± 38 (281)	948 ± 33 (981)
TLG	0.35 ± 0.01 (0.36)	4.56 ± 0.24 (4.80)	0.31 ± 0.01 (0.32)	0.50 ± 0.06 (0.56)	302 ± 32 (270)	617 ± 27 (644)
FLG	0.29 ± 0.01 (0.30)	4.50 ± 0.14 (4.64)	0.31 ± 0.01 (0.32)	0.41 ± 0.04 (0.45)	253 ± 13 (240)	496 ± 12 (508)
ITO	0.46 ± 0.02 (0.48)	4.77 ± 0.21 (4.98)	0.40 ± 0.03 (0.43)	0.98 ± 0.04 (1.02)	195 ± 10 (185)	1568 ± 33 (1601)

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Finally, to further validate the importance of the MoO_x interlayer on device performance, we fabricated devices on pristine SLG anodes. As shown in Fig. S5 and Table S1,† the devices without the MoO_x interfacial layer showed a poorer η of 0.13%, which is 80% lower than that of the devices with the MoO_x layer, thus demonstrating the importance of MoO_x in terms of work function and p-doping of the SLG anode.

Conclusions

OPV cells with evaporable molecular organic semiconductor light-absorbing layers, namely a bilayer structure of ZnPc/C60 were prepared on an SLG anode. A MoO_x layer was deposited on the SLG, which had the effect of simultaneously increasing the conductivity of the SLG and reducing the energy barrier for charge extraction from the organic semiconductor light-absorbing layers. In addition to the best optical transmittance, the SLG anode had a significantly lower level of surface roughness compared to the TLG and FLG anodes. The hero OPV cell comprised of the SLG anode (∼1000 Ω sq⁻¹) showed excellent flexibility, and had a V_OC of 0.44 V, J_SC of 5.2 mA cm⁻², FF of 0.37, and η of 0.84%, which is close to that of the devices containing commercial ITO anodes (15 Ω sq⁻¹), and significantly higher than the devices with the TLG and FLG anodes. These results provide a pathway to flexible graphene-based TCEs for wearable optoelectronics.

Acknowledgements

This work was supported in part by the National Science Foundation of China (No. 51572265, 51325205, 51521091), the Ministry of Science and Technology of China (No. 2016YFA0200101), and the Chinese Academy of Sciences (KGZD-EW-303-1, KGZD-EW-303-3). This program has also been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) Australian Centre for Advanced Photovoltaics. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. PLB is a UQ Vice Chancellor's Research Focused Fellow. This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q) – a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia's researchers. The authors thank Dr T. Ma for his kind help for structure characterization of the single layer graphene.

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Footnotes

† Electronic supplementary information (ESI) available: Structural characterization of an SLG, optical profiler image of an ITO TCE on a glass substrate, transmittance of an SLG TCE before and after MoO_x deposition, IPCE of the OPV cells fabricated on graphene anodes, and photovoltaic performance of OPV cells fabricated on an SLG anode with and without a MoO_x interfacial layer. See DOI: 10.1039/c6nr06942h

‡ These authors contributed equally to this work.

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