In the solar-cell community, scientists are increasingly focusing on polymer, or plastic, devices. Most of the attention goes to hybrid approaches in which photoactive nanomaterials are introduced into polymer-based, thin-film photovoltaic devices (see Figure 1).1–5 Nanomaterials have at least one dimension that is measured on the nanoscale, or 10-9m. In these hybrid solar cells, inorganic-semiconductor nanomaterials get dispersed in an organic-polymer matrix. This approach provides efficient, lightweight, robust, flexible, and potentially inexpensive energy from the sun.
Figure 1. This thin-film hybrid solar cell incorporates an inorganic-crystalline nanomaterial in an organic-polymer matrix.
In the late 1970s, researchers started developing polymer solar cells essentially as soon as conducting polymers emerged.6 Unfortunately, the electrical and optical properties associated with these polymers do not compare to their crystalline-semiconductor materials traditionally used in solar cells. Consequently, polymer-based devices have performed poorly, especially in terms of efficiency. Fortunately, a variety of new nanomaterials have been producing remarkable improvements in these devices.7–12 So far, nanoscale crystalline semiconductors, such as cadmium selenide, and nanostructured forms of carbon—such as Buckminster fullerenes and single wall carbon nanotubes—have produced the best results. Hybrid solar cells now deliver one-quarter of the efficiencies of silicon devices, which remain the most prevalent type of solar cells used today.
In part, researchers have improved the performance of hybrid solar cells by ‘tuning’ the properties of the nanomaterial for a particular application, such as absorbing sunlight. When the dimensions of a material become comparable to the spatial extent of the electrons that occupy it, the materials start to exhibit quantum-confinement effects. Simply speaking, they possess size-dependent optical and electrical properties.13,14Figure 2 shows a series of nanocrystalline-semiconductor suspensions under ultraviolet illumination with the only variation between samples being the size of the particles. As the particle size is reduced, the light being emitted (characteristic of the semiconducting bandgap, or the wavelengths at which the crystal can absorb light) shifts to higher energy. These materials are commonly referred to as ‘quantum dots,’ indicating that they are quantum confined in three dimensions. (A material with quantum confinement in two or one dimension is a quantum well or quantum wire, respectively).
Figure 2. Suspensions of cadmium-selenide quantum dots demonstrate quantum confinement, or size effects, via their photoluminescence under ultraviolet excitation. The wavelength of light being emitted is inversely related to the size of the cadmium-selenide nanocrystals.
Many of the nanomaterials that are being investigated for use in these polymeric-photovoltaic devices actually serve multiple roles. Non-hybrid polymer devices must rely solely on the conversion of solar photons with energies above the conducting polymer-energy bandgaps (typically greater than 2eV, which is not well suited to our solar spectrum). The nanomaterials used in the hybrid approaches generally exhibit optical absorption below the conducting-polymer bandgap, and therefore allow these composite devices to absorb a much larger portion of the solar spectrum. In addition, the nanomaterials can also play a role in liberating and transporting the potential charge carriers that are created through the absorption by the polymer host.
Hybrid solar cells might also exploit some of the other results of quantum confinement that have been demonstrated in some semiconducting–quantum dot systems. For example, my research with colleagues shows that certain nanocrystals absorb photons in the lower-energy region of the solar spectrum and ‘up-convert’, or add them together, to produce photons with energies above the polymer bandgap. Conversely, other quantum dots absorb one high-energy photon and convert it to a number of lower-energy conduction electrons rather than one electron with a lot of wasted heat.15 Finally, some researchers hope to couple single walled–carbon nanotubes or other forms of nanostructured carbon to various semiconducting quantum dots to produce a nanomaterial additive that can address many of the short-comings associated with basic-polymeric solar cells.16
In conclusion, polymeric solar cells provide a concrete example of nanotechnology—or, more appropriately, a nanomaterials approach—improving a device. Also, this hybrid approach to solar-cell development emerged very rapidly in comparison to some other photovoltaic technologies. Futhermore, the ultimate potential of these approaches can be hard to quantify, thus making predictions about the future very difficult. The results achieved to date, however, reveal that hybrids must be considered a contender for the future photovoltaics marketplace.