Plasmon based Ultrathin Photovoltaics
A fundamental challenge for solar cells is to absorb as much sunlight as possible while simultaneously minimizing recombination, where electrons lose their excess energy instead of contributing to the generation of electricity. Recombination losses are suppressed when the solar cell material is made thinner, but with conventional technology it is on the other hand not possible to absorb enough sunlight in ultrathin layers with thicknesses on the nanoscale. We therefore investigate the possibility for efficient solar energy conversion by means of plasmonic nanostructures, with characteristic thicknesses around 10 nanometer. Through the dramatic thickness reduction that this represents compared to conventional solar cells, the recombination losses can be reduced while resources and costs are saved.
Plasmonic technology is based on the use of surface plasmon resonances. These resonances may be excited in nanostructured metals upon interaction with light or another electrical perturbation. Metals are characterized by the atoms sharing of their outermost electrons in an electron cloud, distributed within the periodic lattice of the atomic nuclei. The “free” electrons of the cloud respond collectively to external fields, rather than as individual particles. Light on the other hand consist of electromagnetic radiation, which is a wave-train of coupled electric and magnetic fields. When small metal particles are irradiated by light, the electromagnetic field interacts with the free electron cloud such that it is displaced from the atomic nuclei. A restoring force is then created between the negatively charged cloud and the positively charged nuclei. The light causes the electron cloud to oscillate, so that it is swinging back and forth around its equilibrium position. For some frequencies (or equivalently wavelengths), the light field is in resonance with the electron cloud-nuclei oscillatory system, causing the amplitude of the oscillations to become very large. This corresponds to the excitation of a localized surface plasmon resonance. In the simplest case all parts of the electron cloud moves in the same direction at a given time, which is called a dipolar mode. For shorter wavelengths more complex movement patterns may be created, corresponding to higher modes of the resonance.
For solar cells, plasmon resonances in small metal particles may be used to trap light and transfer the energy to very thin semiconductor layers, generating free charge carriers that can contribute to the production of electricity. In this research, we are especially interested in energy transfer which occurs by means of the strong electromagnetic field that surrounds a nanoparticle when a plasmon resonance is excited. Via this near-field, plasmon energy can be transferred to nearby semiconductor material and excite charge carriers there. In both theoretical and experimental work, a high potential for efficient solar energy conversion in ultrathin semiconductor layers has been demonstrated for this opto-electronic effect, although friction-like losses are introduced with the metal nanoparticles. The greatest challenge is now to accomplish efficient charge separation and contacting of the optimized metal-semiconductor nanostructures.
In our work towards an efficient, ultrathin solar cell architecture, numerical calculations are combined with experimental work to evaluate different geometries and material, from both optical and electronic viewpoints. The sample preparation is primarily based on self-assembly and nanolithography, along with atomic layer deposition (ALD) and other methods for thin film deposition. Spectroscopic ellipsometry is a key tool for the optical characterization.
Our research strives to contribute to the creation of a new class of solar cells with ultrathin absorber layers, approaching the general lower limit for thickness as established by the basic physics involved and the conditions for efficient solar energy conversion.
Contact: Carl Hägglund
Do you want to read more about the research on plasmon-based solar cells? Here you can find a popular science summary of one of our articles.