Enhancement in the performance of organic photovoltaic devices with pristine graphene

Materials Letters 99 (2013) 72–75

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Enhancement in the performance of organic photovoltaic devices with pristine graphene Fei Yu, Vikram K. Kuppa n School of Energy, Environmental, Biological & Medical Engineering, 866 ERC, University of Cincinnati, Cincinnati, OH 45221-0012, United States

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 November 2012 Accepted 18 February 2013 Available online 26 February 2013

We investigate organic bulk heterojunction (BHJ) devices that employ graphene nanosheets in the active layer. Solar cells based on P3HT:PCBM:graphene show device physics significantly different from traditional BHJs, and display a monotonic increase in performance with graphene concentration. The efficiency increases threefold upon addition of graphene due to the higher photocurrent. External quantum efficiency measurements demonstrate significant changes in morphology for devices with graphene. Incorporating such high aspect ratio fillers enables an alternate paradigm for polymer-based solar cells, with much higher concentrations of conjugated polymer, thereby enhancing the capture of solar energy. & 2013 Elsevier B.V. All rights reserved.

Keywords: Polymer solar cells P3HT Charge transport Graphene Efficiency

1. Introduction The ability of organic photovoltaic (OPV) devices to effectively convert incident solar radiation into electricity is controlled not only by the inherent material properties such as work functions, but is also strongly governed by the morphology of the constituent material blends. The present generation of organic solar cells is based on the formation of bulk heterojunctions (BHJ) between conjugated polymers and functionalized fullerene molecules [1–4]. Although polymer/fullerene BHJs show the highest efficiency in OPV cells with efficiencies approaching 5% [5], their scope for improvement is limited since a considerable fraction of the cell material (fullerene) is devoted not to harnessing solar radiation, but to the transport of charges. Reducing the filler content is of paramount importance, since this would enable the more effective capture of a larger fraction of incident energy and boosting efficiency. Graphene is an allotrope of carbon with sp2 bonds in the form of a two-dimensional honeycomb lattice formed by two interpenetrating triangular sublattices. The graphene sheet consists of a percolating set of p-conjugated bonds, and charge transport within this phase is highly facilitated because of the delocalization of the electron (or hole) wavefunction [6]. Graphene thus has excellent carrier mobilities between 15,000 cm2 V  1 s  1 and 200,000 cm2 V  1 s  1, depending on processing and environmental factors [7]. There have been a few attempts at using graphene as a component of the active layer in OPVs. Liu et al. fabricated

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Corresponding author. Tel.: þ1 5135562059. E-mail address: [email protected] (V.K. Kuppa).

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BHJ devices based on poly-3-hexylthiophene (P3HT) and functionalized graphene oxide (GO) compatible with organic solvents [8]. The best performance of 1.1% efficiency was obtained in cells with approximately 10% of graphene by weight, although it is surprising that this level of performance was obtained by using graphene oxide, with its substantially lower charge mobility. Yang et al. [9] showed that nanocomposites consisting of P3HT and reduced graphene oxide (RGO) could successfully be prepared due to the favorable p2p interactions between the thiophene rings and graphene. Their systems also exhibited pronounced photoluminescence (PL) quenching, demonstrating exciton dissociation. Hill et al. [10] also used graphene oxide in solar cells, but their cell performances were poor, with efficiencies of about three orders of magnitude lower than that of devices utilizing (6,6)-phenyl-C61butyric acid methyl ester (PCBM). Yu et al. employed graphene in OPV devices in two ways: first, by chemically grafting chains of P3HT onto reduced graphene oxide [11], bilayer cells with P3HT showed efficiencies of about 0.6%; second, a lithiation reaction was used to covalently attach monosubstituted fullerene (C60) onto graphene nanosheets [12], which were then dispersed in solvent at a concentration of about 12 wt% to spin-coat BHJ solar cells with 1.2% efficiency. Other efforts on using conjugated carbon-based nanomaterials include cells fabricated with nanotubes, with both single-walled [13–16] and multi-walled carbon nanotubes [17] being added in various concentrations to P3HT/ PCBM devices. In each of these cases, although improvements in efficiency were observed, the filler constituted about half of the weight of the active layer, thus doing little to address the issue of harnessing more solar energy. Almost all P3HT-based devices fabricated till date [18–21] have utilized a 1:1 blend with PCBM, since a large amount of the

F. Yu, V.K. Kuppa / Materials Letters 99 (2013) 72–75

PCBM is essential for percolation [22] so as to obtain uninterrupted pathways for electron transport and maximum efficiency. It has been shown that effective exciton dissociation is achieved at PCBM concentrations as low as 10% by weight, as indicated by complete photoluminescence quenching [23,24]. However, a photocurrent increase is not established until the PCBM phase percolates, which is obtained at a 50% weight fraction. Hence, the ‘‘base’’ blend used in this research is a 10:1 by weight ratio of P3HT and PCBM. Our strategy is aimed at reducing the fraction of the optically irrelevant acceptor material present in the device, and increasing the fraction of the light-sensitized conjugated polymer. This approach is facilitated by the use of pristine graphene (PG) which is an excellent conductor, and which acts as a charge transport material. GO and RGO which have been used in previous studies are readily processed in solution and easily dispersed. However devices with such modified graphene sheets only display modest efficiencies due to the comparatively poor conductivity caused by the sp3 hybridization on the functionalized carbon atoms. Our approach is also successful due to the high aspect ratio of the PG platelets: it has been mathematically demonstrated that the percolation of fillers in a matrix can be achieved at much lower weight fractions if the aspect ratio of the fillers is high [25]. Our results clearly demonstrate that adding minute concentrations of pristine graphene (PG) improve device performance dramatically over those containing P3HT and PCBM alone. Coupled with the excellent optical properties of single sheets and the very high charge mobilities, solar cells fabricated with PG show over a threefold increase in power conversion efficiency as discussed below. Although demonstrated here for P3HT/PCBM systems, the approach is potentially universal, and could for example be used to overcome the low mobility hindering the efficiency of all-polymer solar cells [26].

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3. Results and discussion The current–voltage curves of devices with and without graphene are shown in Fig. 1. Device characteristics, including open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and efficiency (Z) values are extracted and shown in Table 1. Devices incorporating PG show slightly decreased Voc compared to the P3HT:PCBM only device due to the work function difference between graphene [27] and PCBM. Hence, the formation of multiple heterojunctions modify the open-circuit voltage, as has been shown for other blend materials [28], including those devices fabricated with carbon nanotubes [14]. We note here that there are three distinct interfaces possible in the device: between P3HT and PCBM, between PCBM and PG, and between P3HT and PG. Our previous results on P3HT blended with graphene shows significant photoluminescence quenching [24], demonstrating that the P3HT/PG interface plays a role in exciton dissociation. The substantially greater Jsc in Table 1 also indicates that PG enhances the movement of free charges through the active layer. Hence, the much higher charge mobility in graphene leads to significantly better current densities in the cell. The FF values in Table 1 show a steady decrease with graphene concentration. The decreased FF, as well as the shape of J–V curves, indicates the morphological change when PG is incorporated into the active layer. Since the 2D and the orientation of PG flakes were not accurately engineered in our research, adding the more electric conducting PG will inevitably lead to more textured top surface of the active layer as well as increased shunt paths through the active layer, both causing decreased FF (and possibly decreased Voc). However, the lower FF also indicates that there is room for improvement of the device parameters and efficiency, and that dramatically better performances are possible by subtle manipulation of the structure of the active layer. The overall

2. Methods

J [mA/cm2]

2

PG monolayer flakes were purchased from Graphene Supermarket (1 mg/L solution suspended in ethanol). P3HT and PCBM were purchased from Reike Metals and Sigma-Aldrich respectively, and were used as received. PG was transferred into dichlorobenzene (DCB) by mixing 10 mL of the PG/ethanol solution with 1 mL DCB. The ethanol was then evaporated in a nitrogen atmosphere by heating to 80 1C, forming a suspension of 0.01 mg/mL PG in DCB. Solutions of P3HT, PCBM and PG were prepared in the requisite concentrations for device fabrication. All solutions were sonicated for 2 h and kept stirred until device fabrication to prevent aggregation of graphene sheets. A 40 nm thick Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, from H. C. Starck) was spin coated onto patterned and cleaned ITO-coated glass slides, and was dried at 120 1C for 10 min in nitrogen. The active layer was deposited by spincoating (800 rpm, 90 s) using different ratios of P3HT:PCBM:PG solution in a nitrogen filled glove box, and was allowed to dry overnight at room temperature. The top electrode was then deposited by evaporating 0.6 nm thick LiF and 200 nm aluminum on top of the active layer. The shape and position of top electrode were defined by a shadow mask. Six individual cells were fabricated on each slide, and the active area of each cell was 3 mm  3 mm. Post-production annealing was carried out in a nitrogen filled glove box at 120 1C for 10 min. Fabrication procedures described above were followed for all cells and active layer compositions. Devices were characterized in dark and under illumination by a Newport 150 W solar simulator with AM 1.5G spectrum at 100 mW/cm2 intensity. Current–voltage curves were recorded using a Keithley 2400 Source Meter unit.

20:2:0 20:2:0.005 20:2:0.01

0

-2

-4

-6 -0.5

0

0.5

1

voltage [V] Fig. 1. Current–voltage curves of various devices under standard (AM1.5G) illumination. Legend indicates composition ratios by weight of P3HT, PCBM, and graphene.

Table 1 Device parameters and performance as a function of composition. The first column indicates ratio by weight of the different components (P3HT:PCBM:graphene) in the devices. Composition

Voc (V)

Jsc (mA/cm2)

FF

Z (%)

20:2:0 20:2:0.005 20:2:0.01

0.6 0.52 0.52

0.8 2.7 3.6

0.42 0.385 0.36

0.2 0.55 0.7

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power-conversion efficiency of the devices is significantly improved upon the addition of graphene. A P3HT:PCBM blend device at a 20:2 concentration ratio displays an efficiency of about 0.2%, while the addition of only small amounts of graphene (concentration ratio 20:2:0.01 for P3HT:PCBM:graphene) leads to better performance of about 0.7% efficiency. Hence, although the introduction of graphene sheets may have a slightly negative effect on the Voc and FF, the subsequent improvement in charge mobility and increase in Jsc compensates more than for these drawbacks, and leads to almost threefold increase in efficiency, as shown in Table 1. Standard BHJs with 50/50 composition of P3HT and PCBM have also been prepared and studied, and an increase in performance is seen here as well. A complete analysis and details of these devices will be revealed in a subsequent publication. Fig. 2 shows the external quantum efficiency (EQE) of devices with different compositions, measured using a Cornerstone 130 monochromator. The EQE is indicative of the structure of the active layer: altering the nanoscale morphology influences exciton dissociation and charge transport, as well as the electronic structure, resulting in different J–V signals upon illumination by monochromatic light of varying wavelengths [29,20]. Our results clearly demonstrate that when graphene is incorporated in the device, the EQE peak increases from about 10% for the P3HT:PCBM only device to about 14% for the 20:2:0.005 device and to more than 18% for the 20:2:0.01 device. A slight redshift of the EQE peak at 360 nm was observed when graphene is added, and there is a significant increase of EQE in the 400–600 nm range for 20:2:0.01 device. These changes in the EQE are evidences of substantial modifications of the physical structure of polymer– nanoparticle–nanosheet (P3HT–PCBM–PG) ternary blends, as compared with the binary polymer–nanoparticle (P3HT–PCBM) blends. To study the charge transport properties of the cells, Jsc values were recorded under different illumination intensities and are shown in Fig. 3. The scaling of the short circuit current with intensity is a rich source of information regarding the physical mechanisms operative in the device. Free charges generated in the active layer in OPV devices from dissociated excitons are susceptible to recombination processes. The relevant recombination mechanism is determined by the exponent a in the relationship lnðJ sc Þ2lnðIntensityÞa [30–34]. Geminate or monomolecular recombination occurs when an electron–hole pair belonging to

20 20:2:0 20:2:0.005 20:2:0.01

EQE (%)

15

10

5

20:2:0 20:2:0.005 20:2:0.01

1

ln [ Jsc (mA/cm2)]

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α = 0.796

0 α = 0.823 -1 α = 0.798

-2

3

4

5

ln [ Intensity (mW/cm2)] Fig. 3. Device J–V response under different intensities. Plot shows ln(Jsc) vs ln(Intensity) measured for cells with different PG ratio. Lines are the result of linear fit and slope of line are marked, showing value of recombination exponent a. Black circles: 20:2:0; Red squares: 20:2:0.005; Green triangles: 20:2:0.01. Curves are shifted slightly (20:2:0 curve is shifted down by 0.5) to avoid overlaps and more clearly show the intensity dependence. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

the same exciton recombines after dissociation of their bound state, and is distinguished by an a value of 1.0, since the number of free charges produced is directly proportional to the incident energy. Non-geminate or bimolecular recombination is observed when electrons and holes belonging to different excitons (perhaps generated at different places in the film) combine to release energy, and is characterized by an exponent of 0.5. Bimolecular recombination is indicative of the efficient separation of exciton pairs, and typically occurs at sites that are spatially distal from the origin of the charge transfer reaction. Hence, evidence of bimolecular recombination reveals the efficiency of charge transport in the materials constituting the active layer. A linear fit of the data points in the plot of ln(Jsc) vs ln(Intensity), as shown in Fig. 3 for different devices, shows recombination exponents that are dependent on device compositions. For the P3HT:PCBM only device, a ¼ 0:823. When graphene is added, a decreases to 0.798 for the 20:2:0.005 device and 0.796 for the 20:2:0.01 device. Although all three sets of devices show both monomolecular and bimolecular recombinations, they clearly indicate the different degrees to which the various recombination mechanisms are operative. The lower exponent of the graphene-based cells demonstrates larger contribution from bimolecular recombination in the active layer. These measurements have been repeated several times, and the exponents are always lower when graphene is added to the active layer in 20:2 devices. We postulate that this change in slope originates in the fast transport of free charges away from their sites of creation, with a recombination occurring between non-geminate pairs at regions far from the polymer/fullerene interface. These results confirm the efficacy of graphene in rapidly transporting charges in solar cells.

4. Conclusions 0

300

400

500

600

700

800

wavelength (nm) Fig. 2. External Quantum Efficiency (EQE) of devices with different P3HT:PCBM:PG ratios. Black circles: 20:2:0; Red squares: 20:2:0.005; Green triangles: 20:2:0.01. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

Our results demonstrate the extent to which the introduction of graphene influences the device physics in polymer-based solar cells and leads to significantly better performance. Even with the unoptimized cell parameters shown here (as indicated by low FF values), the effect of graphene is evident in the increased current densities obtained. The presence of approximately 10% of the

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PCBM is necessary to ensure that sufficient interfacial area is present to promote exciton dissociation. Furthermore, the unfunctionalized surface of pristine graphene is also crucial in applications where charge transport is of consequence, since modifying the surface to allow miscibility in solvents and promoting dispersion are invariably detrimental to electronic properties. The high short circuit current values obtained in our cells indicate that, due to the graphene nanosheets, efficient transport of charges well beyond that obtained in P3HT:PCBM films with similar compositions is obtained. Consequently, the formation of interconnected channels for charge conduction is assisted. The resulting enhanced mobility aids speedy transport of electrons and holes with fewer losses, higher short-circuit current, and greater power conversion efficiency. The increase in performance, albeit well below the highest efficiency achieved in OPVs, is nevertheless significant. These encouraging data reveal the promise of pristine graphene as an acceptor. Precise engineering of the dimensions and distribution of graphene platelets can potentially lead to a new paradigm for polymer-based OPV devices, one that is a modification of the traditional bulk heterojunction concept. The exceptional conductivity of graphene is a critical parameter that permits the fabrication of polymer-based solar cells with much greater concentrations of the conjugated polymer. Graphene is present in minute amounts, but its large aspect ratio is such that percolation of charge conducting pathways is obtained even at these very small concentrations. References [1] Sariciftci NS, Hummelen JC, Heeger AJ, Wudl F. Science 1992;258(5087): 1474–6. [2] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Science 1995;270(5243): 1789–91. [3] Deibel C, Dyakonov V. Rep Prog Phys 2010;73(9):096401. [4] Vakhshouri K, Kesava SV, Kozub DR, Gomez ED. Mater Lett 2013;90(0): 97–102.

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