Carbon nanotubes in photovoltaics

Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. A promising low cost alternative to silicon solar cells, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.Cite journal | first = B. J. | last = Landi | coauthors = Raffaelle, R.P.; Castro, S.L.; Bailey, S.G., | date = March 2005 | title = Single-wall carbon nanotube-polymer solar cells | journal = Progress in Photovoltaics: Research and Applications | volume = 13 | issue = 2 | pages = 165-172 ]

Carbon nanotube composites in the photoactive layer

Combining the physical and chemical characteristics of conjugated polymers with the high conductivity along the tube axis of carbon nanotubes (CNTs) provides a great deal of incentive to disperse CNTs into the photoactive layer in order to obtain more efficient OPV devices. The interpenetrating bulk donor–acceptor heterojunction in these devices can achieve charge separation and collection because of the existence of a bicontinuous network. Along this network, electrons and holes can travel toward their respective contacts through the electron acceptor and the polymer hole donor. Photovoltaic efficiency enhancement is proposed to be due to the introduction of internal polymer/nanotube junctions within the polymer matrix. The high electric field at these junctions can split up the excitons, while the SWNT can act as a pathway for the electrons.Cite journal | first = E. | last = Kymakis | coauthors = Alexandrou, I.; Amaratunga, G.A.J., | date = February 2003 | title = High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites | journal = Progress in Photovoltaics: Research and Applications | volume = 93 | issue = 3 | pages = 1764-1768 ]

The dispersion of CNTs in a solution of an electron donating conjugated polymer is perhaps the most common strategy to implement CNT materials into OPVs. Generally poly(3-hexylthiophene) (P3HT) or poly(3-octylthiophene) (P3OT) are used for this purpose. These blends are then spin coated onto a transparent conductive electrode with thicknesses that vary from 60 to 120 nm. These conductive electrodes are usually glass covered with indium tin oxide (ITO) and a 40 nm sublayer of (poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(styrenesulfonate) (PSS). PEDOT and PSS help to smooth the ITO surface, decreasing the density of pinholes and stifling current leakage that occurs along shunting paths. Through thermal evaporation or sputter coating, a 20 to 70 nm thick layer of aluminum and sometimes an intermediate layer of lithium fluoride are then applied onto the photoactive material. Multiple research investigations with both multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) integrated into the photoactive material have been completed.Cite journal | first = Hiroki | last = Ago | coauthors = Petritsch, Klaus; Shaffer, Milo S.P.; Windle, Alan H.; Friend, Richard H., | date = October 1999 | title = Composites of carbon nanotubes and conjugated polymers for photovoltaic devices | journal = Advanced Materials | volume = 11 | issue = 15 | pages = 1281-1285] Cite journal | first = A. J. | last = Miller | coauthors = Hatton, R.A.; Silva, S.R.P., | date = September 2006 | title = Water-soluble multiwall-carbon-nanotube-polythiophene composite for bilayer photovoltaics | journal = Applied Physics Letters | volume = 89 | issue = 12 | pages = 123115-1-3] Cite journal | first = E. | last = Kymakis | coauthors = Amaratunga, G.A.J., | date = January 2002 | title = Single-wall carbon nanotube/conjugated polymer photovoltaic devices | journal = Applied Physics Letters | volume = 80 | issue = 1 | pages = 112-114] Cite journal | first = R. P. | last = Raffaelle | coauthors = Landi, B.J.; Castro, S.L.; Ruf, H.J.; Evans, C.M.; Bailey, S.G., | date = May 2005 | title = CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells | journal = Solar Energy Materials and Solar Cells | volume = 87 | issue = 1-4 | pages = 733-746] Cite journal | first = S. | last = Kazaoui | coauthors = ); Minami, N.; Nalini, B.; Kim, Y.; Hara, K., | date = October 2005 | title = Near-infrared photoconductive and photovoltaic devices using single-wall carbon nanotubes in conductive polymer films | journal = Journal of Applied Physics | volume = 98 | issue = 8 | pages = 84314-1-6 ]

Enhancements of more than two orders of magnitude have been observed in the photocurrent from adding SWCNTs to the P3OT matrix. Improvements were speculated to be due to charge separation at polymer–SWCNT connections and more efficient electron transport through the SWCNTs. However, a rather low power conversion efficiency of 0.04% under 100 mW cm-2 white illumination was observed for the device suggesting incomplete exciton dissociation at low CNT concentrations of 1.0% wt. Because the lengths of the SWCNTs were similar to the thickness of photovoltaic films, doping a higher percentage of SWCNTs into the polymer matrix was believed to cause short circuits. To supply additional dissociation sites, other researchers have physically blended functionalized MWCNTs into P3HT polymer to create a P3HT-MWNT with fullerene C60 double-layered device.Cite journal | first = Basudev | last = Pradhan | coauthors = Batabyal, Sudip K.; Pal, Amlan J., | date = February 2006 | title = Functionalized carbon nanotubes in donor/acceptor-type photovoltaic devices | journal = Applied Physics Letters | volume = 88 | issue = 9 | pages = 093106] However, the power efficiency was still relatively low at 0.01% under 100 mW cm-2 white illumination. Weak exciton diffusion toward the donor–acceptor interface in the bilayer structure may have been the cause in addition to the fullerene C60 layer possibly experiencing poor electron transport.

More recently, a polymer photovoltaic device from C60-modified SWCNTs and P3HT has been fabricated.Cite journal | first = Cheng | last = Li | coauthors = Chen, Yuhong; Wang, Yubing; Iqbal, Zafar; Chhowalla, Manish; Mitra, Somenath, | date = 2007 | title = A fullerene-single wall carbon nanotube complex for polymer bulk heterojunction photovoltaic cells | journal =: Journal of Materials Chemistry | volume = 17 | issue = 23 | pages = 2406-2411] Microwave irradiating a mixture of SWCNT–water solution and C60 solution in toluene was the first step in making these polymer-SWCNT composites. Conjugated polymer P3HT was then added resulting in a power conversion efficiency of 0.57% under simulated solar irradiation (95 mW cm-2). It was concluded that improved short circuit current density was a direct result of the addition of SWCNTs into the composite causing faster electron transport via the network of SWCNTs. It was also concluded that the morphology change led to an improved the fill factor. Overall, the main result was improved power conversion efficiency with the addition of SWCNTs, compared to cells without SWCNTs; however, further optimization was thought to be possible.

Additionally, it has been found that heating to the point beyond the glass transition temperature of either P3HT or P3OT after construction can be beneficial for manipulating the phase separation of the blend.10 This heating also affects the ordering of the polymeric chains because the polymers are microcrystalline systems and it improves charge transfer, charge transport, and charge collection throughout the OPV device. The hole mobility and power efficiency of the polymer-CNT device also increased significantly as a result of this ordering.Cite journal | first = D. | last = Chirvase | coauthors = Parisi, J.; Hummelen, J.C.; Dyakonov, V., | date = September 2004 | title = Influence of nanomorphology on the photovoltaic action of polymer-fullerene composites | journal = Nanotechnology | volume = 15 | issue = 9 | pages = 1317-1323]

Emerging as another valuable approach for deposition, the use of tetraoctylammonium bromide in tetrahydrofuran has also been the subject of investigation to assist in suspension by exposing SWCNTs to an electrophoretic field.Cite journal | first = Said | last = Barazzouk | coauthors = Hotchandani, Surat; Vinodgopal, K.; Kamat, Prashant V., | date = November 2004 | title = Single-wall carbon nanotube films for photocurrent generation. A prompt response to visible-light irradiation | journal = Journal of Physical Chemistry B | volume = 108 | issue = 44 | pages = 17015-17018] In fact, photoconversion efficiencies of 1.5% and 1.3% were achieved when SWCNTs were deposited in combination with light harvesting CdS quantum dots and porphyrins, respectively.Cite journal | first = Istvan | last = Robel | coauthors = Bunker, Bruce A.; Kamat, Prashant V., | date = October 2005 | title = Single-walled carbon nanotube-CdS nanocomposites as light-harvesting assemblies: Photoinduced charge-transfer interactions | journal = Advanced Materials | volume = 17 | issue = 20 | pages = 2458-2463]

Among the best power conversions achieved to date using CNTs were obtained by depositing a SWCNT layer between the ITO and the PEDOT : PSS or between the PEDOT : PSS and the photoactive blend in a modified ITO/PEDOT : PSS/ P3HT : (6,6)-phenyl-C61-butyric acid methyl ester (PCBM)/Al solar cell. By dip-coating from a hydrophilic suspension, SWCNT were deposited after an initially exposing the surface to an argon plasma to achieve a power conversion efficiency of 4.9%, compared to 4% without CNTs.Cite journal | first = Sumit | last = Chaudhary | coauthors = Lu, Haiwei; Muller, Astrid M.; Bardeen, Christopher J.; Ozkan, Mihrimah, | date = July 2007 | title = Hierarchical placement and associated optoelectronic impact of carbon nanotubes in polymer-fullerene solar cells | journal = Nano Letters | volume = 7 | issue = 7 | pages = 1973-1979]

However, even though CNTs have shown potential in the photoactive layer, they have not resulted in a solar cell with a power conversion efficiency greater than the best tandem organic cells (6.5% efficiency).Cite journal | first = Young Kim | last = Jin | coauthors = Lee, Kwanghee; Coates, Nelson E.; Moses, Daniel; Nguyen, Thuc-Quyen; Dante, Mark; Heeger, Alan J.,| date = July 2007 | title = Efficient tandem polymer solar cells fabricated by all-solution processing | journal = Science | pages = 222-225 ] But, it has been shown in most of the previous investigations that the control over a uniform blending of the electron donating conjugated polymer and the electron accepting CNT is one of the most difficult as well as crucial aspects in creating efficient photocurrent collection in CNT-based OPV devices. Therefore, using CNTs in the photoactive layer of OPV devices is still in the initial research stages and there is still room for novel methods to better take advantage of the beneficial properties of CNTs.

Carbon nanotubes as a transparent electrode

ITO is currently the most popular material used for the transparent electrodes in OPV devices; however, it has a number of deficiencies. For one, it is not very compatible with polymeric substrates due to its high deposition temperature of around 600^o C. Traditional ITO also has unfavorable mechanical properties such as being relatively fragile. In addition, the combination of costly layer deposition in vacuum and a limited supply of indium results in high quality ITO transparent electrodes being very expensive. Therefore, developing and commercializing a replacement for ITO is a major focus of OPV research and development.

Conductive CNT coatings have recently become a prospective substitute based on wide range of methods including spraying, spin coating, casting, layer-by-layer, and Langmuir–Blodgett deposition. Cite journal | first = Miguel | last = Contreras | coauthors = Barnes, Teresa; Van De Lagemaat, Jao; Rumbles, Garry; Coutts, Timothy J.; Weeks, Chris; Glatkowski, Paul; Levitsky, Igor; Peltola, Jorma; Britz, David A.,| date = September 2007 | title = Replacement of transparent conductive oxides by single-wall carbon nanotubes in Cu(In,Ga)Se2-based solar cells | journal = Journal of Physical Chemistry C | volume = 111 | issue = 38 | pages = 14045-14048] Cite journal | first = T. V. | last = Sreekumar | coauthors = Tao Liu; Kumar, S.; Ericson, L.M.; Hauge, R.H.; Smalley, R.E., | date = January 2003 | title = Single-wall carbon nanotube films | journal = Chemistry of Materials | volume = 15 | issue = 1 | pages = 175-178] Cite journal | first = Xiaolin | last = Li | coauthors = Zhang, Li; Wang, Xinran; Shimoyama, Iwao; Sun, Xiaoming; Seo, Won-K; Dai, Hongjie, | date = April 2007 | title = Langmuir-Blodgett assembly of densely aligned single-walled carbon nanotubes from bulk materials | journal = Journal of the American Chemical Society | volume = 129 | issue = 16 | pages = 4890-4891] The transfer from a filter membrane to the transparent support using a solvent or in the form of an adhesive film is another method for attaining flexible and optically transparent CNT films. Cite journal | first = Zhuangchun | last = Wu | coauthors = Chen, Zhihong; Du, Xu; Logan, Jonathan M.; Sippel, Jennifer; Nikolou, Maria; Kamaras, Katalin; Reynolds, John R.; Tanner, David B.; Hebard, Arthur F.; Rinzler, Andrew G., | date = August 2004 | title = Transparent, conductive carbon nanotube films | journal = Science | pages = 1273-1276] Other research efforts have shown that films made of arc-discharge CNT can result in a high conductivity and transparency. Cite journal | first = Daihua | last = Zhang | coauthors = Ryu, Koungmin; Liu, Xiaolei; Polikarpov, Evgueni; Ly, James; Tompson, Mark E.; Zhou, Chongwu, | date = September 2006 | title = Transparent, conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes | journal = Nano Letters | volume = 6 | issue = 9 | pages = 1880-1886] Furthermore, the work function of SWCNT networks is in the 4.8 to 4.9 eV range (compared to ITO which has a lower work function of 4.7 eV) leading to the expectation that the SWCNT work function should be high enough to assure efficient hole collection. Cite journal | first = J. | last = van de Lagemaat | coauthors = Barnes, T.M.; Rumbles, G.; Shaheen, S.E.; Coutts, T.J.; Weeks, C.; Levitsky, I.; Peltola, J.; Glatkowski, P., | date = June 2006 | title = Organic solar cells with carbon nanotubes replacing In2O3:Sn as the transparent electrode | journal = Applied Physics Letters | volume = 88 | issue = 23 | pages = 233503-1-3] Another benefit is that SWCNT films exhibit a high optical transparency in a broad spectral range from the UV-visual far into the near IR range. Only a few materials retain reasonable transparency in the infrared spectrum while maintaining transparency in the visible part of the spectrum as well as acceptable overall electrical conductivity. SWCNT films are highly flexible, do not creep, do not crack after bending, theoretically have high thermal conductivities to tolerate heat dissipation, and have high radiation resistance. However, the electrical sheet resistance of ITO is an order of magnitude less than the sheet resistance measured for SWCNT films. Nonetheless, initial research studies demonstrate SWCNT thin films can be used as conducting, transparent electrodes for hole collection in OPV devices with efficiencies between 1% and 2.5% confirming that they are comparable to devices fabricated using ITO. Cite journal | first = M. W. | last = Rowell | coauthors = Topinka, M.A.; McGehee, M.D.; Prall, H.-J.; Dennler, G.; Sariciftci, N.S.; Liangbing Hu; Gruner, G., | date = June 2006 | title = Organic solar cells with carbon nanotube network electrodes | journal = Applied Physics Letters | volume = 88 | issue = 23 | pages = 233506-1-3] Thus, possibilities exist for advancing this research to develop CNT-based transparent electrodes that exceed the performance of traditional ITO materials.

CNTs in dye-sensitized solar cells

Due to the simple fabrication process, low production cost, and high efficiency, there is significant interest in dye-sensitized solar cells (DSSCs). Thus, improving DSSC efficiency has been the subject of a variety of research investigations because it has the potential to be manufactured economically enough to compete with other solar cell technologies. Titanium dioxide nanoparticles have been widely used as a working electrode for DSSCs because they provide a high efficiency, more than any other metal oxide semiconductor investigated.Cite journal | first = Shlomit | last = Chappel | coauthors = Chen, Si-Guang; Zaban, Arie, | date = April 2002 | title = TiO2-coated nanoporous SnO2 electrodes for dye-sensitized solar cells | journal = Langmuir | volume = 18 | issue = 8 | pages = 3336-3342] Yet the highest conversion efficiency under air mass (AM) 1.5 (100mWcm−2) irradiation reported for this device to date is about 11%.Cite journal | first = Zhang | last = Zhipan | coauthors = Ito, S.; O'Regan, B.; Daibin Kuang; Zakeeruddin, S.M.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M.K.; Pechy, P.; Humphry-Baker, R.; Koyanagi, T.; Mizuno, T.; Gratzel, M., | date = 2007 | title = The electronic role of the TiO2 light-scattering layer in dye-sensitized solar cells | journal = Zeitschrift fur Physikalische Chemie | volume = 221 | issue = 3 | pages = 319-327] Despite this initial success, the effort to further enhance efficiency has not produced any major results.Cite journal | first = Tae Young | last = Lee | coauthors = Alegaonkar, P.S.; Yoo, Ji-Beom, | date = April 2007 | title = Fabrication of dye sensitized solar cell using TiO2 coated carbon nanotubes | journal = Thin Solid Films | volume = 515 | issue = 12 | pages = 5131-5135] The transport of electrons across the particle network has been a key problem in achieving higher photoconversion efficiency in nanostructured electrodes. Because electrons encounter many grain boundaries during the transit and experience a random path, the probability of their recombination with oxidized sensitizer is increased.Cite journal | first = Anusorn | last = Kongkanand | coauthors = Dominguez, Rebeca Martinez; Kamat, Prashant V., | date = March 2007 | title = Single wall carbon nanotube scaffolds for photoelectrochemical solar cells. Capture and transport of photogenerated electrons | journal = Nano Letters | volume = 7 | issue = 3 | pages = 676-680] Therefore, it is not adequate to enlarge the oxide electrode surface area to increase efficiency because photo-generated charge recombination should be prevented. Promoting electron transfer through film electrodes and blocking interface states lying below the edge of the conduction band are some of the non-CNT based strategies to enhance efficiency that have been employed.

With recent progress in CNT development and fabrication, there is promise to use various CNT based nanocomposites and nanostructures to direct the flow of photogenerated electrons and assist in charge injection and extraction. To assist the electron transport to the collecting electrode surface in a DSSC, a popular concept is to utilize CNT networks as support to anchor light harvesting semiconductor particles. Research efforts along these lines include organizing CdS quantum dots on SWCNTs. Charge injection from excited CdS into SWCNTs was documented upon excitation of CdS nanoparticles. Other varieties of semiconductor particles including CdSe and CdTe can induce charge-transfer processes under visible light irradiation when attached to CNTs.Cite journal | first = M. | last = Olek | coauthors = ); Busgen, T.; Hilgendorff, M.; Giersig, M., | date = July 2006 | title = Quantum dot modified multiwall carbon nanotubes | journal = Journal of Physical Chemistry B | volume = 110 | issue = 26 | pages = 12901-12904] Including porphyrin and C60 fullerene, organization of photoactive donor polymer and acceptor fullerene on electrode surfaces has also been shown to offer considerable improvement in the photoconversion efficiency of solar cells.Cite journal | first = Taku | last = Hasobe | coauthors = Fukuzumi, Shunichi; Kamat, Prashant V., | date = December 2006 | title = Organized assemblies of single wall carbon nanotubes and porphyrin for photochemical solar cells: Charge injection from excited porphyrin into single-walled carbon nanotubes | journal = Journal of Physical Chemistry B | volume = 110 | issue = 50 | pages = 25477-25484] Therefore, there is an opportunity to facilitate electron transport and increase the photoconversion efficiency of DSSCs utilizing the electron-accepting ability of semiconducting SWCNTs.

Other researchers fabricated DSSCs using the sol-gel method to obtain titanium dioxide coated MWCNTs for use as an electrode. Because pristine MWCNTs have a hydrophobic surface and poor dispersion stability, pretreatment was necessary for this application. A relatively low destruction method for removing impurities, H₂O₂ treatment was used to generate carboxylic acid groups by oxidation of MWCNTs. Another positive aspect was the fact that the reaction gases including CO₂ and H₂O were non-toxic and could be released safely during the oxidation process. As a result of treatment, H₂O₂ exposed MWCNTs have a hydrophilic surface and the carboxylic acid groups on the surface have polar covalent bonding. Also, the negatively charged surface of the MWCNTs improved the stability of dispersion. By then entirely surrounding the MWCNTs with titanium dioxide nanoparticles using the sol-gel method, an increase in the conversion efficiency of about 50% compared to a conventional titanium dioxide cell was achieved. The enhanced interconnectivity between the titanium dioxide particles and the MWCNTs in the porous titanium dioxide film was concluded to be the cause of the improvement in short circuit current density. Here again, the addition of MWCNTs was thought to provide more efficient electron transfer through film in the DSSC.

References

ee also

*Optical properties of carbon nanotubes
*Carbon nanotube
*Selective chemistry of SWNT
*Allotropes of carbon