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Graphene is of great interest for future applications in organic photovoltaics (OPVs) due to its high three-dimensional aspect ratio, large specific surface area, remarkable optical transmittance, extraordinary thermal response, excellent electron/hole transport properties, superior mechanical stiffness and flexibility. Graphene-based functional materials can be used as transparent window/counter electrodes, interface layers, hole/electron transport materials and can also function as buffer layers to retard charge recombination in OPVs. Future work would focus on the following aspects: (a) design and preparation of novel graphene-based functional materials with good stability, high transparency and excellent conductivity for OPVs; (b) development of the new approaches that constitute a significant advance toward the production of graphene-based transparent conductive electrodes in OPVs; (c) evaluation on the long terms stability of devices with GO based modifying layers; (d) delicate control of unique graphene nanostructures; and (e) interface engineering of the graphene in terms of modifying its work function and surface free energy.
©2012 Optical Society of America
1. Introduction
As the thinnest material ever known in the universe, one-atom-thick two-dimensional graphene, which exhibits high thermal conductivity (~5 × 103 W m−1K−1 at room temperature), high charge/hole mobility (2 × 105 cm2V−1s−1) and charge carrier concentration of up to 1013 cm−2, large specific Brunauer-Emmett-Teller surface area (theoretical value: 2630 m2g−1; experimental value measured by nitrogen adsorption at 77 K: 640 m2/g), and an unusual half-integer quantum Hall effect for both electron and hole carriers in grapheme, has attracted tremendous amount of attention [1–5] in both materials science and condensed-matter physics since its successful isolation a few years ago. The chemistry of graphene reported in the literatures mainly concerns the chemistry of an electrically insulating graphene oxide (GO) with chemically reactive oxygen functionality, including carboxylic acid groups at the edges of GO, and epoxy and hydroxyl groups on the basal planes [3]. Both small molecules and polymers have been covalently attached to GO’s highly reactive oxygen functionalities, or non-covalently attached on the graphitic surfaces of chemically modified graphenes, for potential use in polymer composites, paper-like materials, sensors, photovoltaic applications, and drug-delivery systems. The reduction of GO, by which the graphene intrinsic structure along with outstanding electrical conductivity and electron/hole mobilities can be to a large extent restored, has been recognized to be the most efficient method to produce large amounts of reduced graphene oxide (RGO) [6–8].
Despite being only one atom thick, graphene is found to absorb a significant (α = 2.3%) fraction of incident white light. Graphene is therotically a zero-bandgap semiconductor with a tiny overlap between valence and conduction bands, this leading to an excellent electrical conductivity, and posing a major challenge for graphene electronics. Yong et al. have been the first to predict the efficiency of graphene-based organic photovoltaics (OPVs) which show the potential for single-cell efficiencies exceeding 12% (and 24% in a stacked structure) [9]. Their experimental results suggest that the tunable bandgap and band-position features of graphene, as well as its superior thermal stability and mechanical integrity, render it a credible material for the construction of next-generation low-cost, high-efficiency, thermally stable, environmentally friendly, and lightweight flexible solar-energy-conversion devices. For these reasons, this paper surveys recent research advances in the area of graphene-based OPVs.
2. Graphene-based organic polymer solar cells
Although the π-conjugated polymers process many advantages such as comparatively easy manufacture of thin film devices by spin coating or screen- printing technologies, larger optical absorption coefficients exceeding 105 cm−1, and low cost preparation, the largest disadvantages are their low charge carriers mobility (10−3 ~10−4 cm2/Vs), low exciton diffusion lengths (~10 nm), and narrow absorption bands (the UV/Vis absorption spectra of the conjugated polymer-based device only covers about 10-15% of the solar light spectrum). These factors limit the further improvement of conjugated polymers-based photovoltaic cells. On the other hands, the most intriguing features of graphene are its exceptional electronic quality and the outstanding electron/hole transport properties of individual graphene crystallites. These outstanding properties make graphene and its derivatives the promising candidates required for the practical OPVs. A possible way for improvement of energy conversion efficiency is thus to combine the outstanding properties of conjugated polymers and graphene.
Zhang et al. reported polymer solar cells with gold nanoclusters decorated multi-layer graphene as transparent electrode [10]. A thin layer of ultraviolet-ozone (UVO) treated gold (Au) was introduced on multi-layer graphene (MLG) to enable the MLG as an effective anode for polymer solar cells. UVO treated Au provides favorable band alignment at the MLG/polymer interface. With proper UVO treatments on MLG/Au (2 nm), devices with open-circuit voltage (VOC) of 0.52-0.54V, short-circuit current density (JSC) of 4.5-5.5 mA/cm2, fill factor(FF) of 43-48%, and power conversion efficiency(PCE) up to 1.24% are obtained, exhibiting largely enhanced performance compared with MLG cells directly modified by poly(3,4-ethylenedioythiophene):poly(styrenesulfonate) (PEDOT:PSS) and UVO.
Wang and his associates fabricated the P3HT/PCBM-graphene photovoltaic device in which P3HT acts as the photo excited electron donors and graphene as electron acceptor [11]. An improved performance was observed due to an extension of the excitons dissociation area and to faster electron transfer through the graphene. The obtained best PCE value reached 1.4%, resulting in an increase 59% compared with the undoped graphene. And more, the same group also [12] studied the influence of PPV/ poly (3-octylthiophene) (P3OT) on the performance of P3HT:Graphene bulk heterojunction (BHJ) structure polymer solar cells. As a result, the PCE of the co-donors:SPFGO (SPFGO: solution-processable functionalized graphene oxide) BHJ solar devices with moderate doped donor amount can be influenced by the change of molecular packing structure, which can be interpreted in terms of incorporating doped donor. By using water as the solvent for the active layer, Tian et al. fabricated the “Green” polymer solar cells with the device structure of ITO (indium–tin-oxide) /PEDOT: PSS/P3KT: a-dG/ZnO/Al, in which P3KT (poly [3-(potassium-6-hexanoate) thiophene-2, 5-diyl]) acts as donor, aqueous-dispersible noncovalent functionalized graphene (a-dG) sheets as acceptor and ZnO as both electron transporting and hole blocking layer [13]. The device with 0.6 wt% a-dG gave the best performance with a PCE of 0.027%.
Graphene can also be used as a replacement for ITO for the transparent electrode of an OPV cell. As an example, the OPV device was fabricated using the configuration of graphene/PEDOT:PSS(40nm)/ P3HT:PCBM (50nm)/Al(80nm). This device displays a PCE of 3.98%, higher than that of the ITO device (3.86%) [14]. Adding a small amount of single walled carbon nanotubes (SWNTs) in the GO modifying layer can significantly improve the P3HT:PCBM photovoltaic devices’ FF [15]. Solution-processable graphene-based thin film has been used as an efficient anode interfacial layer for high-efficiency and high-stability OPVs [16]. Enhanced performance of OPVs was demonstrated by introducing RGO as an anode interfacial layer into the solar cells, and the overall photovoltaic characteristics of cells with RGO were highly comparable with those of PEDOT:PSS-based cells. The PEDOT:PSS/fluorinated graphene composite film-coated ITO electrode was used as an anode for polymer BHJ solar cells reaching an overall PCE of about 0.75% upon excitation with 100 mW cm−2 AM 1.5 white light [17].
The use of ITO and fluorine tin oxide (FTO) appears to be increasingly problematic due to the limited availability of the element Indium, the intrinsic chemical and electrical drawbacks and limited transparency in the near infrared region. In order to overcome these problems, Kalita et al. fabricated the graphene constructed carbon film (TGF) from the botanical derivative camphor by controlled pyrolysis [18]. This film shows very good transparency in a wide range of wavelengths (0.3–2 mm) in contrast to ITO. A P3HT:PCBM BHJ OPV cell, which was fabricated on a TGF electrode with transparency of 81% at 550 nm and a conductivity of 357 S cm−1, exhibited very good Voc while the Jsc and FF are affected with high sheet resistance of TGF.
By using the resultant C60-graphene hybrid as the electron acceptor in P3HT-based BHJ solar cells, Yu et al. demonstrated a 2.5-fold increase in the power conversion efficiency with an enhanced short-circuit current density and open-circuit voltage with respect to those of the C60/P3HT system under AM 1.5 illumination (100 mW/cm2) [19]. The PCE value of C60-G:P3HT device (1.22%) is relatively low compared to the widely investigated P3HT:[60]PCBM (phenyl-C61- butyric acid methyl ester) cells(5-6%), and can be further improved by optimization of the device structure. The thinnest GO thin films (thickness ~2 nm) deposited from neutral solutions are effective hole transport layers (HTLs) in OPVs [20]. The efficiency values of devices obtained with GO HTLs are comparable to devices fabricated with PEDOT:PSS, which is most commonly used as an anode interfacial layer to improve anode contact and to increase hole collection in organic electronics. However, PEDOT:PSS shows high acidity, inhomogeneous electrical properties and hygroscopic properties, giving rise to poor long-term stability. Band alignment energies obtained from optical gap and work function measurements of the GO thin films demonstrate that hole transport and electron blocking is facilitated. A semi-transparent OPV that uses a top laminated graphene electrode on an inverted-type polymer solar cell using a simultaneous thermal releasing/annealing technique was demonstrated by Lee et al [21]. This device exhibits a promising PCE of approximately 76% of that of the standard opaque device using an Ag metal electrode. The deposition of the top graphene electrode is fully compatible with the roll-to-roll manufacturing in the future market integration of low-cost plastic solar cells.
An OPV device with an ITO/PEDOT:PSS/P3HT:GO/Al structure, in which GO was used as electron acceptor, exhibited an increase in short-circuit current and conductivity but a decrease in open circuit potential [22]. The PCE values of photovoltaic devices with an ITO/PEDOT:PSS/a mixture of P3HT and GO derivative/LiF/Al structure are in the range of 0.005% and 1.1% [23,24]. Yu et al. prepared P3HT covalently grafted GO sheets (GP3HT) via esterification between the carboxylic groups in GO and CH2OH- terminated P3HT [25]. A bilayer photovoltaic device based on the solution-cast GP3HT/C60 hetero-structures showed a PCE of 0.61% under AM 1.5 illumination (100 mW/cm2). To facilitate the application of graphene in nanodevices and to effectively tune the bandgap of graphenes, a promising approach is to convert the 2D graphene sheets into 0D graphene quantum dots (GQDs). Followed this idea, Li et al. report an alternative electrochemical approach for direct preparation of functional GQDs with a uniform size of 3–5 nm, which present a green luminescence and can be retained stably in water for several months without any changes [26]. Polymer photovoltaic cell with the structure of ITO/PEDOT:PSS/P3HT:GQDs/Al exhibited a PCE of 1.28%.
1,3-dipolar cycloaddition reaction of azomethine ylide has been successfully applied to the polymeric functionalization of RGO [6–8]. Followed this approach, Li et al. synthesized a new solution-processable poly[(9-phenyl-9H-carbazole){4,7-di(thiophen-2-yl)benzo[c] [1,2,5] thiadiazole}(9,9-dihexyl-9H-fluorene)] (PCTF) covalently grafted RGO derivative (PCTF-RGO) [27]. A photovoltaic device from blend solutions with a PCTF-RGO:[70]PCBM ([6,6]-phenyl C71 butyric acid methyl ester) composition of 1:3 w/w showed a 258% increase of the short-circuit current density and a 144% increase of the power conversion efficiency with respect to the PCTF-RGO counterpart under AM 1.5G illumination (100 mW/cm2), as shown in Fig. 1 . The remarkable difference in photocurrent between the PCTF-RGO-based devices (with and without [70]PCBM) could in part be associated with the different amounts of photons absorbed by RGO and [70]PCBM. The doping of [70]PCBM into the PCTF-RGO polymer resulted in the considerable enhancement of the photocurrent.
Fig. 1 (a) Molecular structure of PCFT-RGO; (b) The device structure; and (c) Current density-voltage characteristics (J-V) of the PCTF-RGO-based photovoltaic device with and without [70]PCBM under a simulated AM1.5G 100 mW illumination.
Soluble reduced graphene films as the transparent electrode were incorporated in P3HT-[60]PCBM photovoltaic devices (Fig. 2 ). The photovoltaic devices obtained from the graphene films showed lower performance than the reference devices with ITO, due to the higher sheet resistance (2 kΩ/sq) and the poor hydrophilicity of the spin coated graphene films [28]. By the multi-transfer process of chemical vapor deposition (CVD)-grown graphene on glass substrate, Choi et al. obtained multilayer graphene (MLG) electrodes prepared at room temperature [29]. The as-fabricated MLG/PEDOT:PSS/ P3HT:PCBM/Ca/Al device showed a PCE of 1.17% indicating that MLG films are a promising indium-free transparent electrode substitute for the conventional ITO electrode for use in cost-efficient OPVs. Choe et al. also reported the results of applying MLG films as transparent conductive electrodes in OPVs [30]. The performance of OPVs with 1000°C-grown MLG films was found to be the best with a PCE of 1.3%. The PCE was further enhanced when a hole-blockingTiOX layer was inserted in the device structure, resulting in a PCE of 2.6%.
Fig. 2 (a) The illuminated current–voltage (I–V) curves of the ITO/PEDOT:PSS/ P3HT:PCBM/Al and Graphene/PEDOT:PSS/P3HT:PCBM/Al photovoltaic cells; (b) Schematic of the photovoltaic devices.
The standard electron-blocking layer, PEDOT:PSS, is likely a major factor limiting the OPV device durability and possibly performance. A single layer of electronically tuned GO is an effective replacement for PEDOT:PSS [31]. It significantly enhances device durability while concurrently templating a performance-optimal active layer π-stacked face-on microstructure. Such GO-based BHJ polymer solar cells exhibit PCEs as high as 7.5% while providing a 5 × enhancement in thermal aging lifetime and a 20 × enhancement in humid ambient lifetime versus analogous PEDOT:PSS- based devices.
3. Graphene-based hybrid solar cells
As low cost alternatives for photovoltaic technologies for renewable energy harvesting, hybrid nanostructure heterojunction solar cells have been recognized to be very promising [32]. By using RGO as the transparent conductive electrode and ordered ZnO nanostructures as the acceptor material, Yang et al. fabricated heterojunction solar cells with the configuration of RGO/ZnO/P3HT/ PEDOT:PSS/Au [33]. These devices show the best solar energy harvesting efficiency due to the large interfacial area of the ZnO-nanotubes for charge separation and RGO’s high electrode conductance for current collection. The highest PCE they achieved is 0.46%. Huang and his associates reported the use of low-cost ambient pressure CVD graphene films as front electrodes for CdTe solar cells [34]. The device with an glass/graphene/ZnO/CdS/CdTe/ (graphite paste) structure shows a photovoltaic efficiency of 4.17%, providing encouraging evidence that the graphene transparent conducting films may be used as a new low cost front electrode material for thin film photovoltaic devices. The same group also successfully incorporated Cu-nanowire-doped graphene (Cu NWs/graphene) as the back contact in thin-film CdTe solar cells [35]. The back contact reported in this study possesses a high electrical conductivity of 16.7 S cm− 1 and a carrier mobility of 16.2 cm2 V−1s−1. The obtained PCE is up to 12.1%, higher than that of cells with traditional back contacts using Cu-particle-doped graphite (10.5%) or Cu thin films (9.1%).
Using Au (5 nm)/graphene combined layers as the Schottky contact electrodes to the single nanowires (NWs) or nanobelts (NBs), the high-performance single CdS NW (NB) Schottky junction solar cells were fabricated [36]. Typical as-fabricated NW solar cell shows photovoltaic behavior with an Voc of ~0.15 V, a short circuit current of ~275.0 pA, and a PCE of ~1.65%. Ye et al. developed a simple and scalable graphene patterning method using electron-beam or ultraviolet lithography followed by a lift-off process, and applied this method to fabricate CdSe nanobelt/graphene Schottky junction solar cells [37]. An energy conversion efficiency of 1.25% was achieved.
An approach to assist charge extraction out of the P3HT:PCBM blend in the inverted hybrid photovoltaic cells was demonstrated by using graphene nanoflakes (GNFs) [38]. A significant improvement in device performance by up to 100% is obtained. Instead of a continuous film, incomplete coverage of the ZnO-nanorod growth surface by GNFs is able to lead to large increments in photocurrent and open circuit voltage. A facile method for assembling graphene–CdSe nanocomposites (NPs) on the flexible substrate by electrophoretic deposition [39] was used to fabricate quantum dot sensitized solar cells, among which the best PCE and IPCE (incident photon to current conversion efficiency) values were found to be 0.6% and 17%, respectively. Zhang et al. combined two planar nanostructures, graphene and CdSe nanobelts, to construct Schottky junction solar cells with Voc of about 0.5 V and cell efficiencies on the order of 0.1% [40]. The graphene–CdSe nanobelt solar cells show a great flexibility in creating diverse device architectures, and might be scaled up for cell integration based on assembled nanobelt arrays and patterned graphene (or carbon nanotube) films.
The integration of Cu+ species with the exfoliated GO sheets yielded a simple and facile route to design an RGO-Cu2S composite electrode. By designing RGO-Cu2S composite, Radich et al. have succeeded in shuttling electrons through the RGO sheets and polysulfideactive Cu2S more efficiently than Pt electrode, improving the fill factor by ~75% [41]. A sandwich CdSe quantum dot sensitized solar cell constructed using the optimized RGO-Cu2S composite counter electrode exhibited an unsurpassed PCE of 4.4%.
Graphene quantum dots (GQDs), which could be a cost-effective, environmentally friendly, and more stable material for photovoltaics than current organic materials, blended with regioregular poly(3-hexylthiophene-2,5-diyl) or MEH-PPV polymer results in a significant improvement in the OPV characteristics as compared to graphene sheets (GSs) blended conjugated polymers [42]. As shown in Fig. 3 , for the OPV device having the structure ITO/PEDOT:PSS/P3HT: aniline(ANI)-GQDs/ LiF/Al, when the content of ANI-GQDs in P3HT is 1 wt.%, maximum values of η = 1.14%, Voc = 0.61V, Jsc = 3.51 mA cm−2, and FF = 0.53 were obtained. The FF of the P3HT/1wt.%ANI-GQDs hetrojunction device is much higher (0.53) as compared to ~0.33 for GSs, and 0.28 for 10 wt%ANI-GQDs.
Fig. 3 (upper) J-V characteristics of the photovoltaic devices based on ANI-GQDs with different GQDs content and ANI-GS (under optimized condition) annealed at 160°C for 10 min, in AM1.5G 100mW illumination; (down) AFM images of (a) P3HT/ANI-GSs, (b) P3HT/ANI-GQDs, and (c) MEH-PPV/methylene blue-GQDs.
The OPVs with hole transport layers of GO, NiOx and GO/NiOx studied in Ryu’s work exhibit the conversion efficiency of 2.33%, 3.10% and3.48%, respectively [43]. The cell efficiency is correlated with the matching of energy levels between ITO, hole transport layer and P3HT and thus a well-matched stack layer of ITO/GO/NiOx/ P3HT:PCBM/LiF/Al shows the best cell efficiency of 3.48% with the JSC of 8.71mA/cm2, VOC of 0.602V and FF of 66.44%.
One of the challenges in the integration of graphene in OPV is the incompatibility between graphene and PEDOT:PSS hole transport layer (HTL), which significantly increases device failure rate. Instead of the conventional PEDOT:PSS HTL, an alternative MoO3 HTL is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS [44]. The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices. The factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of wellmatching counter electrodes) will provide better understanding in utilizing graphene films as transparent conducting electrodes in future solar cell applications. The unique advantages of graphene in terms of its flexibility, chemical robustness, and roll-to-roll processability can open a wide range of applications in flexible solar cell panels and display electronics that is not amenable to ITO. The device efficiency of graphene-anode-based devices (2.5%) modified by MoO3/PEDOT:PSS is very close to that of ITO-based devices (3%) [45]. By engineering the interface between the graphene film and the HTL, 83.3% of the PCE of control devices based on ITO as anode was achieved. Using MoO3-modified graphene as intermediate layer, a high Voc of 1V and a high Jsc of 11.6 mA cm−2 could be obtained in series and parallel connection, respectively, in contrast to a Voc of 0.58 V and Jsc of 7.6 mA cm−2 in single photovoltaic cell [46].
Semitransparent inverted OPVs based on P3HT:PCBM were fabricated with single-layer graphene as the top electrode (Fig. 4 ) [47]. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. All of the devices showed higher efficiency from the graphene than ITO side due to the better transmittance of the graphene electrodes. The conductance of the graphene electrode was improved by doping the graphene film with Au nanoparticles and PEDOT:PSS, which resulted in an increase of conductance for more than 400%. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm2 illuminated from graphene electrode under the AM1.5 solar simulator. The efficiency decreased with the increase of the active area due to the increased series resistance and the decreased edge effect in the device.
Fig. 4 (a) Schematic diagram of a semitransparent OPV with the structure glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/graphene;(b)band structure of the organic solar cell; (c)semitransparent solar cell with graphene top electrode; (d)Work functions of grapheme/ PEDOT:PSS electrodes prepared at different processing conditions; (e) the two approaches for the fabrication of graphene/PEDOT:PSS top electrodes in OPVs; (f) J-V characteristics measured from both sides of semitransparent OPVs with pristine or Au-doped graphene top electrode under solar simulator; and (g) external quantum efficiency measured from both sides of the organic solar cell with Au-doped graphene/PEDOT:PSS top electrode. The open and solid symbols correspond to the results characterized from ITO and graphene sides, respectively.
Flexible, chemically derived RGO film was transferred onto polyethylene terephthalate (PET) first and then used as the transparent, conductive electrode for OPV devices with the configuration of RGO/PEDOT:PSS/P3HT:PCBM/TiO2 /Al [48]. A high dependence of device performance on the sheet resistance of RGO film, which determines the charge transport efficiency, was observed if the optical transmittance of RGO film was above 65%. The highest PCE value obtained in the flexible RGO/PET-based OPV devices is 0.78%. These devices show an excellent stability after applying the bending induced tension stress, and their performance can be well maintained even after bending a thousand times.
4. Outlook and perspective
During the past few years results for organic photovoltaic materials have been disappointing, but now the PCE value has surpassed 8%. Relatively, most of the graphene-based OPVs are less efficient due to the high sheet resistance and poor hydrophilicity of the spin coated graphene films, the increase of roughness with the number of stacked layers in the film preparation process, the possible charge recombination at the interface of graphene/electrolyte in the graphene/TiO2 and graphene/QDs-based OPVs, and others. However, as indicated by Gomez De Arco, what graphene-based OPVs lack in efficiency, they can potentially more than make for in lower price and, greater physical flexibility [49]. Graphene and its derivatives can be used as transparent window/counter electrodes, interface layers, hole/electron transport materials, superior mechanical stiffness and flexibility, and also can function as buffer layers to retard charge recombination in the OPVs. Future work would be focused on the following aspects: (1) design and preparation of novel graphene-based functional materials with good stability, high transparency and excellent conductivity for OPVs; (2) development of the new approaches that constitute a significant advance toward the production of graphene-based transparent conductive electrodes in OPVs; (3) evaluation on the long terms stability of devices with GO based modifying layers; (4) delicate control of unique graphene nanostructures (including the graphene size, film morphology, energy band structure, annealing conditions, and others); and (5) Interface engineering of the graphene in terms of modifying its work function and surface free energy.
Acknowledgments
The authors are grateful for the financial support of the Fundamental Research Funds for the Central Universities, the Shanghai Eastern Scholarship program, and the Shanghai Leading Talents program.
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