Harvesting energy directly from the abundant resource of solar radiation through the use of solar cells is increasingly becoming a major component of future global energy production. Other renewable energy sources, like wind and hydroelectric power, can require large scale infrastructure. Solar energy, on the other hand, only needs solar cells and sunshine. Technologically feasible solutions are available today for solar electricity generation. They are predominantly based on the use of silicon conversion cells. The most efficient cells, however, use relatively expensive high-quality single-crystal or amorphous silicon wafers. Unless there are major breakthroughs, current silicon-based thin-film technologies may be reaching their limit in terms of their ratio of cost to efficiency.
Organic photovoltaics (OPVs) are made of polymers and have the advantage that they can be painted on a surface, such as on the outside walls of a building or on rooftops. Accordingly, there is a great deal of interest in putting them to use in large-scale applications. Compared with existing technologies, OPVs promise moderate power conversion efficiencies. By the same token, they have the very attractive feature that they can be made by highly scalable, high-speed coating and printing processes such as spray painting and inkjet printing to cover large areas on flexible plastic substrates. They provide a low-cost alternative for the future.
In an OPV, solar radiation is harnessed in an unusual way. Incoming radiation excites the photoactive polymer, which functions atomically as a loosely bounded electron-hole pair, referred to as an exciton. The key to OPV technology is the mechanism of effective separation and transport of the electrons and holes (charge carriers). Otherwise, energy is wasted. Examining certain classes of molecules can help in understanding the mechanism's importance.
Spherical fullerenes or C60 (also known as buckyballs) are allotropes (different forms) of carbon that are capable of trapping electrons. They can be used in OPVs for separating charges to prevent recombination of electrons and holes. However, the allotropes are neither good conductors of electricity nor optimal for charge transport. A single-wall carbon nanotube (SWNT), a cylindrical variation of a fullerene, offers a solution owing to its shape. SWNTs have a nanometer-scale diameter and exhibit ballistic electrical conductivity (many times better than copper) that can serve as tiny wires.
The key component of the OPVs developed in our group is a C60-SWNT complex. The SWNTs offers superior electron transport properties, and the spherical C60, with its large surface-to-volume ratio, is extremely efficient at separating photogenerated charge carriers. The charge partitioning at the polymer/C60 interface is followed by efficient electron transport through the nanotubes. Together, these lead to higher quantum efficiencies.
Recently, we developed the chemistry related to the synthesis of the C60-SWNT complex and the associated OPV fabrication technology.1,2 Figure 1(a) shows in schematic form nanotubes decorated with clusters of C60 molecules and the mechanism of charge transport. Figure 1(b) is a photograph of a solar cell made by coating a flexible plastic substrate. Figure 1(c) presents a scanning electron microscope image of the SWNT-C60 complex. The surface of the tubes is dotted with clusters of C60.
Adding SWNTs to a photoactive coating improves the performance of OPVs. The coating is composed of a conducting polymer: poly(3-hexylthiophene). We tested both the C60-SWNT complex and the pristine C60 in our lab under simulated AM1.5-G solar irradiation at 95 mW/cm2. When the SWNTs were introduced into the photoactive composite layer via binding with C60, the short circuit current and fill factor improved significantly with power conversion efficiency, by as much as 78%.
In photovoltaic cells without SWNTs and after charge separation at the polymer/C60 interface, electrons can move toward the cathode only by hopping between C60 molecules. In contrast, SWNTs can form a network throughout the composite layer and provide a direct pathway for enhanced electron transport. Electrons captured by C60 molecules or clusters are transferred to SWNTs for rapid current flow. The C60-SWNT composite appears to be an excellent candidate for constructing low-cost OPVs. C60 is significantly less expensive than other fullerene derivatives, and only a small amount of the more expensive SWNT is needed in the photoactive composite. Further optimization of material synthesis and device fabrication is necessary to optimize the performance of our solar cell.
This work was supported at New Jersey Institute of Technology by the US Army Armament Research, Development, and Engineering Center.
Somenath Mitra chairs the Department of Chemistry and Environmental Science at the New Jersey Institute of Technology (NJIT). His research interests include organic photovoltaics, nanotechnology, and sensor development.
Cheng Li is a research fellow. He completed his PhD in materials science and engineering at NJIT in 2003 before joining Somenath Mitra's group. His research interests include device physics of organic solar cells, organic thin-film transistors, and thin-film sensors for damage detection.