Organic Photovoltaics with Stacked Graphene Anodes - ACS Applied

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Organic Photovoltaics with Stacked Graphene Anodes Ehsan Keyvani-Someh, Zachariah Hennighausen, William Lee, Rachna C. K. Igwe, Mohamed E Kramdi, Swastik Kar, and Hicham Fenniri ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00020 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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ACS Applied Energy Materials

Organic Photovoltaics with Stacked Graphene Anodes Ehsan Keyvani-Someh,1 Zachariah Hennighausen,4 William Lee,1 Rachna C. K. Igwe,1 Mohamed Elamine Kramdi,1, Swastik Kar,4 and Hicham Fenniri1-3,* 1

Department of Chemical Engineering, 2Department of Bioengineering, 3Department of

Chemistry and Chemical Biology, 4Department of Physics, Northeastern University, 360 Huntington Avenue, Boston, MA 02115-5000 KEYWORDS: Organic photovoltaics, graphene, stacking, transparent electrodes, thin films

*Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Graphene has recently been used to achieve power conversion efficiencies (PCE) equal to those of ITO-based devices, although they remain a challenging and costly replacement for ITO. Herein, we employed chemical vapor deposition (CVD) to grow graphene islands and transferred them onto a transparent substrate. The resulting stacked graphene films were characterized by Raman and UV-Vis spectroscopy, and conductivity measurements. Solar cells fabricated with stacked graphene (1-4 layers)/PEDOT:PSS/P3HT:PCBM/Ca/Al architecture showed an enhancement of PCE as a function of the number of stacked layers. The highest efficiency was measured for the double-layered graphene anode because of its optimal conductance and transmittance. This work establishes that readily prepared layered graphene islands are a viable and economical substitute for ITO.


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To overcome some of the limitation of inorganic solar cells, organic photovoltaics (OPVs) have been introduced because of their mechanical flexibility, light weight, solution processability, recyclability, and affordability, which could fulfill key requirements needed for an alternative energy source to supplement or replace current electrical systems.1-4 Indium tin oxide (ITO) is the most common electrode material in OPV devices because of its high transmittance and conductivity.5,6 Since it is the costliest component of the architecture, it remains a significant hindrance to the fabrication of solar cells.

Graphene with high

conductivity, electron mobility, and transparency has all the ingredients for a viable replacement material.7,8 Although several groups reported equal or higher power conversion efficiency (PCE) for high quality graphene anodes compared to ITO-based devices, the high cost associated with producing these graphene electrodes limits their large scale adoption.9-12

To expedite the

fabrication and extend the life of graphene-based electrodes, chemical vapor deposition (CVD) was performed on a copper foil and the CVD-grown graphene was transferred onto a transparent substrate.13-16 Reported PCEs for CVD-grown graphene-based OPVs were much lower than ITObased solar cells due to the formation of graphene islands and the difficulty in preventing folding during film transfer.17,18 We reasoned that stacking multiple graphene layers would enhance the conductivity by increasing surface coverage and decreasing sheet resistance.19-23 In this report, we establish an efficient and simple stacking method of CVD-grown graphene films and investigate their effectiveness in bulk hetero-junction OPVs. Graphene fabrication. The quality of the CVD-grown graphene was examined using scanning electron microscopy (SEM) and optical microscopy. A typical SEM micrograph (Figure 1a) of a single layer of graphene (light gray areas) shows incomplete surface coverage as well as the formation of small folded domains. Although all of the CVD and transfer parameters were kept


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identical, different sheet formations were observed in each sample, while the degree of coverage and physical properties were consistent across samples. A schematic representation of overlapping islands in a two-layer stack (Figure 1b, upper islands in grey and lower islands in black). The optical image of a 2-layer graphene film (Figure 1c) shows full coverage as well as folded domains resulting in randomly-distributed thicker areas (bright spots). As anticipated, as the number of layers increases, graphene films absorb more light and the film turn darker. Spectroscopic and conductivity measurements. The effect of CVD growth conditions on graphene film thickness was examined using Raman spectroscopy

as

transmittance

and

this

parameter

affects

Figure 1. (a) SEM micrograph of a single conductivity.

As

the

graphene layer showing incomplete surface

transmittance decreases, less light can reach the coverage. (b) Schematic representation of 2 active layer of the solar cell, resulting in lower overlapping graphene layers. (c) Optical image of 2 overlapping graphene layers on efficiency. On the other hand, the stacking glass substrate. The inset shows single (black increases surface coverage, which is essential for arrows) and double layers (white arrows). The generating a conductive layer.22 Thus, striking a bright spots result from partially folded graphene islands. balance between transparency and surface coverage


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is crucial.26 The ratio of 2D to G bands in the Raman spectra is a direct measure of the number of graphene layers in the film (Figure 2a). As the intensity and width of these characteristic vibrations changes with film thickness (i.e. peaks broaden/weaken with the number of stacked graphene layers), we can directly and quantitatively assess the success of our CVDgenerated graphene transfer using standard Raman spectroscopy (Figure 2a).24 The absorbance of the samples was measured by UV-Vis spectroscopy (Figure 2b) and the transmittance was found to remain constant across

the

entire

visible

spectrum.

Transparency values went from 93.8 ± 5.9% (1 layer), 89.29 ± 3.4% (2 layers), 75.53 ±

Figure 2. (a) Raman spectra of graphene based on the number of transfers. As the number of the

3.3% (3 layers), 61.9 ± 5.6% (4 layers). In layers increases, 2D/G bands ratio decreases. (b) addition,

the

absorption

hexylthiophene-2,5-diyl)

of

Poly

(P3HT),

(3- Transmittance of stacked graphene samples across the

the visible spectrum. (c) Average sheet resistance of stacked graphene decreases most significantly

electron donor used in our studies, is in the after the second transfer. range of 400 to 700 nm where stacked graphene shows high transparency.25 Finally, The resistance of the samples was measured by calibrated 2-point probe. Graphene samples showed


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Figure 3. a) Fabricated organic solar cell, from anode to cathode: graphene anode on a glass substrate, PEDOT:PSS (hole injection layer), P3HT:PCBM (photoactive layer), calcium (electron transport layer), and aluminum cathode.

b) Current density versus Voltage of

graphene-based OPVs. ohmic behavior through the entire sweep range. The average sheet resistance (Rs) was measured for the stacked graphene (Figure 2c) and was found to decrease as the number of layers increased (1 layer: 49.35 KΩ/sq, 2 layers: 28.99 KΩ/sq, 3 layers: 24.16 KΩ/sq, 4 layers: 19.5 KΩ/sq). Our results confirm that sheet resistance does not decrease significantly after the second deposition cycle and therefore the 2-layers film appeared to be an optimal compromise between good transparency and sheet resistance. OPV fabrication and characterization. Organic solar cells with the following architecture graphene/PEDOT:PSS/P3HT:PCBM/Ca/Al as well as the corresponding control devices with ITO as transparent anode were fabricated as shown in Figure 3a, and their PCE’s were measured. Figure 3b shows I-V measurements of the fabricated solar cells with graphene anodes and the corresponding data is summarized in Table 1. The PCE of the single layer graphene samples were found to be ca. 10 fold lower compared to ITO-based samples as a result of lower


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short circuit current. The significantly lower current density of graphene compared to ITO results in less efficient charge transfer from the active layer to graphene and increased charge recombination, which leads to lower current generation. For the same reasons, the open circuit voltage is also lower for all graphene-based OPVs (ca. 59-73% of Voc for ITO-based devices). However, stacking increased the Voc, FF, and Jsc. Overlapping graphene islands therefore provide better coverage on the anode side, which promotes hole extraction. Table 1. PCE parameters for graphene and ITO based OPVs Anode

Voc

Jsc

FF

PCE

Single transfer

0.33 ± 0.07

4.09 ± 1.30

0.25 ± 0.03

0.34 ± 0.04

Double transfer

0.39 ± 0.03

5.21 ± 0.70

0.42 ± 0.03

0.89 ± 0.07

Triple transfer

0.40 ± 0.03

5.00 ± 1.30

0.41 ± 0.05

0.82 ± 0.10

Quadruple transfer

0.41 ± 0.20

4.90 ± 1.00

0.36 ± 0.02

0.74 ± 0.04

ITO

0.56 ± 0.01

11.6 ± 1.00

0.54 ± 0.02

3.56 ± 0.30

The highest PCE achieved for graphene cells was obtained for a 2-layer stacked graphene film (Table 1). For 3 and 4 graphene layers, the PCE is slightly lower and it is anticipated that the trend will continue. As stacking more than 2 layers is not significantly improving PCE, increasing the number of layers would negatively impact the transmittance which would have a larger effect on lowering the PCE.27 We anticipate however that improved graphene preparation and device fabrication conditions (controlled atmosphere, better surface coverage and overlap between graphene layers) would significantly improve the PCE. Our studies have shown that stacked graphene could be a cost-effective and viable replacement for ITO. Indeed, for every 1 W of power produced with an ITO-based device, commercial graphene-based devices and stacked graphene-based devices of the same cost would produce ca.


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0.02 W and 1.5 W, respectively (Table S1, supporting information). When estimating our costs, we factored-in materials, equipment, and labor costs, and then added a 10% overhead for unexpected spending (e.g. calibrations, instrumental errors). The main reasons for the costeffectiveness of our devices relative to commercial graphene devices stems from our ability to relax our design requirements in three key areas: (a) graphene single layer coverage does not need to be extensive. While commercial graphene has 95% monolayer coverage, our devices require ca. 85% monolayer coverage, (b) polycrystalline graphene is acceptable for our OPVs, and (c) nucleation sites with bi- or tri-layer graphene are also acceptable. Following these relaxed criteria, our straightforward and inexpensive procedure allowed us to fabricate devices with high reproducibility. In summary, CVD-grown graphene was assembled on glass to generate a conducting anode layer as a potential replacement for ITO in OPVs. Stacking graphene layers resulted in improved conductance. PCE was optimal for 2-layer graphene films, as more layers resulted in decreased transmittance and did not impart significantly better conductance. The graphene anodes produced with our method are more than 80 times more cost-effective (PCE/cost) than commercially available graphene and 50% less costly than ITO-based devices (Table S2). In addition, ITO’s brittle nature does not allow for the fabrication of flexible solar cells, which can be solved using graphene.28,29 Finally, we envision that the use of Langmuir-Blodgett technology to generate continuous graphene films from graphene islands would further improve the conductivity of the graphene anodes and the PCE of the resulting OPVs; work to demonstrate this approach is in progress in our laboratories. Supporting Information. Optical microscopy images, fabrication procedures, cost analysis. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author Hicham Fenniri: [email protected] ACKNOWLEDGMENT We acknowledge the support of Northeastern University and the National Science Foundation (NSF ECCS CAREER 1351424 to SK). ABBREVIATIONS ITO, Indium tin oxide; OPV, Organic photovoltaic; PCE, Power conversion efficiency; CVD, Chemical

vapor

deposition;

UV-Vis,

Ultraviolet-visible;

PEDOT:PSS,

poly

(3,4-

ethylenedioxythiophene) poly styrene sulfonate ; P3HT, Poly (3-hexylthiophene-2,5-diyl)

;

PCBM, Phenyl-C61-butyric acid methyl ester ; Al, Aluminum; SEM, Scanning electron microscopy; Rs, Sheet resistance; FF, Fill factor; Voc, Open circuit voltage; Jsc, Short circuit current density; HIL, Hole injection layer; ETL, Electron transport layer, Cu, Copper; PMMA, Poly methyl methacrylate. REFERENCES (1)

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and Platinum-Free Counter Electrode for Green Mesoscopic Photovoltaics through Graphene Electrode and Graphene Composite Catalysts: Interfacial Compatibility. ACS Applied Materials and Interfaces 2016, 8, 5314–5319.


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Table of Content. Fabrication of multi-layer graphene. CVD-grown graphene on Cu was transferred to a transparent substrate such as glass or PET. By repeating the transfer and deposition of graphene on top of another graphene layer, multi-layer graphene was fabricated.


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Organic Photovoltaics with Stacked Graphene Anodes - ACS Applied

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