Photovoltaic devices can be used to produce electrical energy from light energy, such as from the sun or a thermal source. Current photovoltaic devices include p-n junction devices based on crystalline or amorphous semiconductor materials, heterojunction devices based on crystalline or amorphous semiconductor materials, metal/semiconductor Schottky heterojunction devices, and devices based on combinations of metals, semiconductor materials, and electrolytic solutions. During operation of a current photovoltaic device, light is absorbed by a photoactive material to produce charge carriers in the form of electron-hole pairs or excitons. Electrons exit the photoactive material through one electrode, while holes exit the photoactive material through another electrode. The net effect is a flow of an electric current through the photovoltaic device driven by incident light energy, which electric current can be delivered to an external load to perform useful work. The inability to convert the total incident light energy to useful electrical energy represents a loss or inefficiency of the photovoltaic device.
Current photovoltaic devices typically suffer a number of technical limitations on the ability to efficiently convert incident light energy to useful electrical energy. A significant loss mechanism of current photovoltaic devices typically derives from a mismatch between an incident light spectrum, such as a solar spectrum, and an absorption spectrum of the photovoltaic devices. Photons with energy greater than a bandgap energy or an energy gap of a photoactive material typically lead to the production of photoexcited electron-hole pairs with excess energy. Such excess energy is typically not converted to electrical energy but is rather typically lost as heat through hot charge carrier relaxation or thermalization. Photons with energy less than the bandgap energy of the photoactive material are typically not absorbed and, thus, typically do not contribute to the conversion to electrical energy. As a result, a small range of the incident light spectrum can be efficiently converted to useful electrical energy.
Also, in accordance with junction designs of current photovoltaic devices, charge separation of electron-hole pairs is typically confined to a region around a depletion zone, which can be limited in extent to, for example, a plane in a photoactive material. Electron-hole pairs that are produced further than a diffusion or drift length from the depletion zone typically do not charge separate and, thus, typically do not contribute to the conversion to electrical energy. As a result, most electron-hole pairs that are produced in the photoactive material typically do not contribute to an electric current.
Another loss mechanism of current photovoltaic devices typically derives from recombination of photoexcited electron-hole pairs. Recombination of photoexcited electron-hole pairs reduces the number of charge carriers contributing to an electric current, thus reducing the conversion efficiency. Current photovoltaic devices can sometimes exhibit an undesirable level of recombination of photoexcited electron-hole pairs as a result of short recombination times and the presence of defects, which can serve as recombination sites or trapping sites. Since current photovoltaic devices typically rely on minority charge carrier transport, strict fabrication conditions can be required to reduce the impact of charge carrier recombination.
A further loss mechanism of current photovoltaic devices typically derives from resistive loses, such as from series and parallel resistance. As charge carriers traverse a photoactive material, the charge carriers typically encounter electrical resistance, which leads to resistive loses. Further resistive losses can derive from electrical resistance at a depletion zone that separates photoexcited charge carriers and at a contact between electrodes and the photoactive material.
It is against this background that a need arose to develop the nanostructured materials and the photovoltaic devices described herein.