Not too long after the first report on carbon nanotubes in 1991 by IIjima that the excellent mechanical and electrical properties of carbon nanotubes were identified. Single wall carbon nanotubes (SWCNTs) have a current conductivity of 1 to 3 × 106 S/m and mobility of 100,000 cm2 V−1 s−1.75, 76 However, a random network of CNTs in films have the highest reported conductivity of 6600 S cm−1 and mobilities in the 1 to 10 cm2 V−2 range as a result of large junction (surface to surface) resistance.75 A wide set of advanced applications have been envisioned for CNTs but their commercial use has been hindered by several processing issues. Such processing issues, particularly purification of CNTs and their dispersion, have proven to be major challenges. Bulk CNTs comprises a mixture of semiconducting (1/3) and metallic (2/3) forms causing non-ohmic contacts in films. In addition, their high surface energy renders them very susceptible to bundling. Although CNTs can be dispersed in various solvents using different dispersing agents such as surfactants, they tend to re-bundle once the dispersing agents are removed. A common method of deposition of CNTs involves: dispersion in a solvent with the use of dispersing agents, vacuum filtration, removal of surfactants by repeated washing of the filtrate CNTs, and finally transferring the CNT filtrate to a substrate of choice with the use of e.g. a (patterned or un patterned) polydimethylsiloxane (PDMS) stamp. Such a transfer method was employed first by Zhou et al. to deposit SWCNTs on PET achieving a sheet resistance of 120 Ω□−1 with a transmission similar to ITO in the visible region.77 Several in-depth reviews on CNT properties and processing, and their varied applications in solar energy conversion can be found elsewhere.75, 78-80
CNTs as a potential transparent conductor was identified by Lee et al. when they used a 1000 Å thin SWCNT film with a 60% transmission as a transparent p-contact in GaN LED.81 It was, however, Wu et al. who is given credit for highlighting the potential of SWCNTs as transparent conductors. For a 50 nm thin film of p-doped SWNT film, a sheet resistance of 30 Ω□−1 with a transmission of >70% over visible region of the light spectrum was reported.82 Such properties of SWCNT films have set a benchmark as subsequent reports have been mostly unable to achieve similar results (see Table I). A large number of parameters (largely uncontrollable) affect CNT conductivity such as purity, lattice perfection, bundle size, wall number, metal/semiconductor ratio, diameter, length, and doping level which vary among studies. Tenent et al. reported a sheet resistance of 60 Ω□−1 for a 40 nm thick film using less defect prone SWCNTs that are grown using laser vaporization and are p-doped.87 Note that sheet resistance of CNT films is dominated by junction resistance and the use of doping by acid treatments have been shown to cause a threefold decrease in junction resistance and a 30% increase in the nanotube conductivity when compared with pristine untreated samples90 (Figure 7). So far, spray deposition is the most up-scalable low cost technique reported for fabrication of SWCNT transparent conductors. Using this method, Tenent et al. reported a sheet resistance of 110 Ω□−1 (pristine SWCNT) and 37 Ω□−1 (doped SWCNT) with transmission of 78 and 76% (550 nm) respectively (Figure 7).87 The large spread in the properties of SWCNT films (Table I) is indicative of the processing and purification challenges that continue to delay the advancement of SWCNT transparent conductors from proof-of-principle to actual applications.
Figure 7. a) Comparison of transmission spectra for ITO and SWCNT (with and without PEDOT : PSS). Inset shows TEM image of ultrasonically sprayed films; b) Plot of measured film conductivity as a function of the inverse of mean junction resistance; c) Ultrasonically spray deposited large area (6 × 6 inch2) SWCNTs film.87 Reprint permissions: (a and b) Reprinted from Refs.88 and90, with permission from ©American Institute of Physics. c) Reprinted from Ref.87, with permission from ©John Wiley and Sons.
Download figure to PowerPoint
Table I. Compilation of Selected Few Results of Properties of Carbon Nanotube-Based Transparent Conductors and the Photovoltaic Parameters of their Corresponding Organic Solar Cells
|Device||Treatment of SWCNT||Active area (mm2)||Rs of graphene film (Ω□−1)||Transmittance (%)||Jsc (mA cm[minus]2)||Voc (V)||FF (%)||PCE (%)||Refs.|
|Glass/SWCNT/PEDOT : PSS/P3HT:PCBM/Ga : In||None||7||282||85 (650 nm)||6.65||0.5||30||0.99||83|
|PET/SWCNT/PEDOT : PSS/P3HT:PCBM/Al||Surfactant||4||200||NA||7.8||0.61||52||2.5||84|
|Glass/SWCNT/PEDOT : PSS/P3HT:PCBM/Al||Acid||10||50||70 (650 nm)||9.2||0.56||29||1.50||85|
|Glass/MWCNT mat/PEDOT : PSS/||NA||9||588||50||5.5||0.49||49||1.32||86|
|Glass/SWCNT/PEDOT : PSS/P3HT:PCBM/Ca/Al||Cellulose + acid||3||60||70 (400–1800 nm)||11.5||0.58||48||3.1||87|
|Glass/SWCNT/P3HT : PCBM/Ca/Al||Cellouse + acid||NA||24||NA||11.4||0.54||55.4||3.37||88|
|Glass/SWCNT/P3HT : PCBM/LiF/Al||Acid||NA||51||69||11.1||0.59||54.2||3.6||89|
Apart from processing challenges, the roughness of the CNTs thin films and their adhesion to the substrates has been equally impeding factors for the efficacy in organic solar cells. Shunts due to roughness of the SWCNT surface is circumvented by using 0.5 to 1 μm thick active layer83, 85 which ultimately undermined the efficiency of these devices. PEDOT: PSS was used as a planarization layer and a three-fold increase in device performance was observed.85 Rowell et al. showed significant improvement in roughness (10 nm over 25 μm2 scan area) using a PDMS based transferring method for surfactant assisted dispersion of un-doped SWCNTs. The film had a transmission of 85% and a sheet resistance of 200 Ω□−1.84 Ultimately, the improved film properties were reflected in the PSCs (PCE: 2.5%) which was comparable to ITO-based reference devices (PCE: 3%).
Apart from SWCNTs, multiwalled carbon nanotube (MWCNT) sheets or mats were incorporated in PSCs86, 91 soon after their simple processing possibility was demonstrated.92 These sheets were strong, highly transparent, and conductive. However, the rough topology required planarization with PEDOT : PSS ultimately resulting in a 50% light transmission to the photoactive layer. Nevertheless, a PCE of 1.32% was achieved in a normal device structure.86
All the earlier reported devices have an active area of <1 cm2 of area and used thin film fabrication techniques that are not readily scalable. In regard to processing of large area CNT thin films, progress has been slow and so far spraying techniques are the most scalable technique investigated.87, 88, 93 Tenent et al. dispersed SWCNTs in aqueous solvents with high molecular weight (∼90,000 MW) cellulose (sodium carboxymethylcellulose, CMC) as dispersing agent.87 With the use of ultrasonic spraying, the CMC-based dispersion is deposited over large areas (6 × 6 inches) (Figure 7). Finally, the film is exposed to nitric acid to remove CMC while simultaneously functionalizing or doping the nanotubes. Such a film is found to be highly homogenous (rms roughness of 3 nm scanned over 100 μm2 area) with superior electrical conductivity and optical transmission and resulted in PCE of 3.1% in PSCs, albeit slightly lower than ITO-based reference devices (PCE: 3.6%). Later, the same group avoided the use of PEDOT : PSS as planarization and hole transport layer and instead used 900 nm thick active layer and observed a PCE of 3.7%. The SWCNT film had a sheet resistance of 30 Ω□−1.88 It was hence concluded that SWCNTs can replace both ITO and hole transport PEDOT : PSS buffer layers and that these devices show much higher Jsc as a result of higher transmission beyond the visible region (above theoretical predictions of Jsc based on visible transmission) (Figure 7). Similarly, Kim et al. reported similar SWCNTs deposition technique to Tennent et al. with the difference that surfactants are used instead of CMC and deposition is done by pressure driven spray coating. Kim et al. evaluated the effect of several surfactants94 and compared patterning of large area through lithography and contact stencil.89 The dip-coating of bare substrate in 1% solution of 3-aminopropyltriethoxy silane in deionized water is found to improve adhesion of SWCNTs onto the substrates due to the formation of cross-linked siloxane on the surface of substrates. The final optimized device had a sheet resistance 51 Ω□−1 with a transmission of 69% (at 550 nm) and resulted in PSCs with PCEs of 3.6% and 2.6%, respectively on glass and PET substrates. This is the highest reported performance of PSCs with SWCNTs transparent conductors.
From all aforementioned reports that incorporated SWCNT-based transparent conductors in PSCs, two issues clearly emerge: firstly, nearly all devices are studied in a normal geometry and secondly, the processing choices are very limited for large area substrates. Spray coating seems to be the only technique that allows for easy processing on large area substrates. There is a long way before SWCNTs transparent conductors can their make way into R2R processing. The proof-of principle has been demonstrated and the task ahead needs to show its flexibility and feasibility with inverted devices and performance with respect to PSCs stability if any. For example, Tenent et al. observed an increase in sheet resistance of spray coated films from 150 to 240 Ω□−1 over a 10-day period.87 Such effects are deteriorating for PSCs and require further investigations.
The metamorphosis of graphene from the theoretical world to physical reality in 2004 has presented the possibility of a number of advanced applications: transparent conductors being one of them. With less than 0.1% reflectance and 2.3% absorbance for every single graphene sheet, the theoretical transmission limit of a single layer graphene sheet is 97.7% and the corresponding sheet resistance for undoped graphene sheet is 6 kΩ□−1.95, 96 A higher value of transmission (99%) and lower sheet resistance (30 Ω□−1) than the theoretically predicted values have been reported in experimental results and are attributed to the presence of defects, increasing stacking of graphene sheets, and extrinsic doping97, 98 Sheet resistance of graphene decreases rapidly with increasing stacking of graphene sheets and with doping, however, at the expense of optical transmission. In the case of sheet resistance of 30 Ω□−1, the corresponding transmission was 90% which is nonetheless comparable to ITO. Apart from sheet resistance and transmission, several other properties such as the high chemical and thermal stability, high charge carrier mobility (200 cm2 V−1 s−1), high current carrying capacity (3 × 108 A cm−2), high stretchability, and low contact resistance with organic materials renders graphene a very favourable alternative to ITO.99, 100 Translating theoretical wonders of graphene into real applications has been met with several challenges.
The processing of high quality graphene remains the biggest challenge. High quality graphene is either micromechanically cleaved or grown by chemical vapor deposition; both of which are not low cost and large scale compatible. In principle, a monolayer of graphene possesses ballistic charge transport due to delocalization of electrons over the complete sheet, however, in practice defects are introduced during growth and processing of graphene. Such defects, for example, lattice defects, grain boundaries, and oxidative traps due to functionalization results in high sheet resistance of graphene.95, 101 As a result, graphene films must be made thicker than a monolayer to attain practical sheet resistance. Defects are more prominent in graphene films processed by solution based methods such as liquid phase cleaving with ultra-sonication or by reduction of graphene oxide. Although these techniques provide lower cost alternatives to processing of graphene, however, graphene produced by such methods exhibit poor properties with sheet resistance in the kΩ□−1 range due to structural defects and poor interlayer contact as a result of vigorous exfoliation and reduction processes.102 Several reviews on the properties and processing of graphene are present elsewhere97, 100, 103 and a recent review elaborates on the application of graphene as electrodes in electrical and optical devices.104 Henceforth, we briefly present the use of graphene as transparent conductors in organic solar cells.
At this infant stage, it is difficult to estimate when research on graphene as transparent conductor will bear fruit. The difficulties in low cost high quality production of graphene and the very few proof-of-concept studies on utilizing graphene as transparent conductors in organic solar cells (yet fewer in PSCs) highlights the need for more research emphasis on processing. The earlier reports on graphene as transparent conductors were adopted in dye sensitized and small molecule solar cells, however, the performance of such devices were limited (PCEs of <1%) by the high sheet resistance of the films which is often in the kΩ range.101, 105 Although it has been demonstrated that fabrication of large area (30 inch) CVD graphene on Cu substrates can give a sheet resistance as low as 30 Ω□−1 with 90% transmission,98 it appears that reproducing such results has not been possible in independent laboratories. De Arco et al. demonstrated the achievement of CVD graphene with a transmission of 72% (550 nm) and a sheet resistance of 230 Ω□−1 but employed an optimized graphene with 3.5 k Ω□−1 with 89% transmission to successfully demonstrate a small molecule solar cell with PCE of 1.18%.106 Graphene is hydrophobic and requires functionalization (e.g. UV/ozone treatment) to improve its wetting properties. Such functionalization creates defects in graphene sheets increasing its sheet resistance and therefore limiting the final performance. Noncovalent functionalization improves wetting while maintaining the structural integrity of graphene. Such a noncovalent functionalization of graphene with self-assembled pyrene butanoic acid succidymidyl ester (PBASE) improved its wetting properties towards PEDOT:PSS and resulted in a greater than two-fold increase in PCE.107 By depositing a 20 Å thin layer of MoO3 over graphene, Wang et al. demonstrated improved wetting properties of graphene toward PEDOT : PSS as well as improved work function of graphene electrodes.108 Their best device with the structure: doped-graphene/MoO3/PEDOT : PSS/P3HT : PCBM/LiF/Al resulted in a PCE of 2.5% which was 83.3% of the PCE obtained on ITO-based equivalent devices (3%). They further noted that the use of MoO3 results in superior results than the use of PBASE (PCE of 2%) for surface functionalization graphene to improve its hydrophilicity. The main contribution of this report was however in processing of graphene using a simplified PMMA-based transfer method through which they were able to produce multiple layers of graphene with the best results found with four layer graphene exhibiting a sheet resistance of 80 Ω□−1 and a transmission of 90% (550 nm). The doping of graphene with AuCl3 is reported to reduce sheet resistance of graphene by 77% with only 2% decrease in transmission.109 Park et al. noted an improvement in PCE from 1.36% for undoped graphene to 1.63% for AuCl3-doped graphene film.110 Choe et al. used high quality CVD grown multilayer graphene films (15 layers) to make P3HT:PCBM-based PSCs and obtained a PCE of 2.60% with the use of TiO2 electron selective buffer layer and graphene grown at 1000°C. A strong dependence of device performance on the growth temperature of graphene was found.111
As apparent from Table II, almost all organic solar cells studied with graphene as transparent electrode serves the objective of proof-of-concept. Sheet resistance is seldom lower than 200 Ω□−1 which may not be critical in small area devices but will prove detrimental to photovoltaic properties upon upscaling. For graphene to be a meaningful replacement to ITO, sheet resistance has to be significantly reduced with processing conditions conducive to low-cost fabrication. At this juncture, graphene represent a long term possibility.
Table II. Compilation of Selected Few Results of Properties of Graphene-Based Transparent Conductors and the Photovoltaic Parameters of their Corresponding Organic Solar Cells
|Structure||Area (mm2)||Rsh (Ω□−1)||T (%)||Jsc (mA cm−2)||Voc (V)||FF (%)||PCE (%)||Refs.|
|Quartz/graphene/CuPc/C60/BCP/Ag||0.81||5 × 103 –1 × 106||85–95||2.1||0.48||34||0.4||101|
|PET/CVD-graphene/PEDOT : PSS/CuPc/BCP/Al||0.7||3.5 × 103||89||4.73||0.48||52||1.18||106|
|Glass/CVD graphene: PBASE/PEDOT : PSS/P3HT : PCBM/LiF/Al||n.a.||1350–210||91–72||6.05||0.55||51.3||1.71||107|
|Glass/graphene/MoO3/PEDOT : PSS/P3HT : PCBM/LiF/Al||n.a||80||90||8.5||0.59||0.51||2.5||108|
|Glass/graphene/PEDOT : PSS/P3HT : PCBM/TiO2/Al||n.a.||610 ± 140||86.9 ± 1.2||9.03||0.60||48||2.60||111|
|Quartz/AlCl3-doped graphene/PEDOT : PSS/CuPc/C60/BCP/Ag||1.21||300–500||91.2–97.1||9.15||0.43||42||1.63||110|