The enhancement of polymeric solar cells through the addition of nanostructured material complexes has been investigated to facilitate exciton dissociation and carrier transport through a polymer matrix. The dispersion of single-wall carbon nanotubes (SWNTs) into poly-3-octylthiophene (P3OT) has been shown to dramatically improve both the electrical conductivity and optical absorption of the polymer in comparison to the pure polymer. The photoresponse of solar cells using P3OT doped with SWNTs is significantly improved over the undoped version under simulated air mass zero (AM0) illumination. In addition, cadmium selenide (CdSe) quantum dots (QDs) have been used by several groups to facilitate exciton dissociation and improve the efficiency of P3OT-based solar cells. Through the synthesis of QD-SWNT complexes, researchers at the NASA Glenn Research Center produced a nanostructured additive for polymeric solar cells that exhibits both a high electron affinity and high electrical conductivity. This year, researchers from Glenn and the Rochester Institute of Technology (RIT) synthesized copper-indium-sulfide- (CIS-) QD/SWNT complexes and assessed their viability as an additive in polymeric solar cells.
There are four main problems to address in improving polymeric solar cells:
An ideal nanomaterial additive for a polymeric solar cell would have a high electron affinity, low percolation threshold, high electrical conductivity, and a suitable bandgap. Although this may be achieved by a single material, another approach would be to combine nanomaterials with complementary characteristics: for example, combining appropriate covalently or noncovalently semiconducting QDs to an SWNT (ref. 1).
Energy-level (E) diagrams adjusted in relation to the vacuum level and equilibrated at the Fermi energy for a QD-SWNT-polymer solar cell showing the electronic transitions from optical absorption, exciton dissociation, and resulting carrier transport of the components (ref. 1); E1 > E2 > E3.
A possible idealized energy band diagram for a polymeric solar cell with QD-SWNT complexes is shown in the preceding diagram. The relative levels have been adjusted to illustrate a favorable relationship between the materials. Such a combination is possible for bandgaps and electron affinities that have been reported in the literature for the various constituents.
SWNTs were synthesized using analexandrite laser vaporization process (ref. 2). Raw soot was purified using conventional nitric acid and thermal oxidation steps to achieve SWNT mass fractions of >95 wt% in the overall sample; then, CIS QDs were prepared by conventional colloidal organometallic chemical bath syntheses (ref. 3). QD-SWNT complexes were formed through covalent and noncovalent attachment schemes involving carboxylic acid-functionalized SWNTs with CIS-mercaptoacetic acid QDs.
Left: Optical absorbance versus wavelength for P3OT doped with CIS QDs. Right: Optical absorbance of CIS–SWNT complexes.
The addition of lower bandgap QDs and SWNTS can enhance the absorption spectrum of an ordinary polymeric solar cell. The preceding graph on the left shows how the addition of CIS QDs changes the optical absorbance of P3OT. The graph to the right shows that absorbance improves dramatically when nanotubes are combined with CIS.
Photovoltaic devices of the nanomaterial-P3OT composites were made by spin coating the materials onto commercially obtained indium-tin-oxide- (ITO-) coated polyethylene terapthalate (PET) substrates that were pretreated with a thin film of polyethylenedioxythiophene (PEDOT). Thermally evaporating aluminum grid contacts were used. The next graph shows the AM0 photoresponse of a P3OT solar cell with a 50-wt% doping of CIS QDs.
Current-versus-voltage photoresponse of a 50-wt% CIS-P3OT composite solar cell under 1-sun AM0 illumination.
Covalent and noncovalent attachment schemes to SWNTS were attempted to address the deficiencies associated with using QDs alone. A dramatic improvement in the cell characteristics with the addition of SWNTs is shown in the final graph.
Current-versus-voltage photoresponse of a nanomaterial-P3OT solar cell under 1-sun AM0 illumination.
QD-SWNT complexes, which combine a high electron affinity and low percolation threshold, provide an attractive nanomaterial additive for polymeric solar cells. By using a simple covalent coupling reaction, researchers from Glenn and RIT demonstrated that such complexes are possible. Initial prototype cells using these complexes in a P3OT-based device showed enhanced photoconversion in comparison to undoped cells. The quantum confinement found in the nanomaterials described should allow for the optimization of absorption to the solar spectrum and efficient device designs that will increase performance in polymeric solar cells.
The authors acknowledge the support of NASA through grants NCC3-956 and NAG3-2484, the National Science Foundation through grant ECS-0233776, and BP Solar.
Find out more about the research of Glenn’s Photovoltaics and SpaceEnvironments Branch: http://www.grc.nasa.gov/WWW/5000/pep/photo-space/
Glenn contact:
Dr. Sheila G. Bailey, 216-433-2228, Sheila.G.Bailey@nasa.gov
Rochester Institute of Technology contact:
Dr. Ryne P. Raffaelle, 585-475-5149, rprsps@rit.edu
Authors:
Brian J. Landi, Dr. Ryne P. Raffaelle, and Dr. Sheila G. Bailey
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