25 November 2012 Octavi E. Semonin, Joseph M. Luther, and Matthew C. Beard
Beard et al. discuss the current status of research efforts towards utilizing the unique properties of colloidal quantum dots for solar photon conversion.
Colloidal quantum-confined semiconductor nanostructures are an emerging class of functional material that are being developed for novel solar energy conversion strategies. One of the largest losses in a bulk or thin film solar cell occurs within a few picoseconds after the photon is absorbed, as photons with energy larger than the semiconductor bandgap produce chargecarriers with excess kinetic energy, which is then dissipated via phonon emission. Semiconductor nanostructures, where at least one dimension is small enough to produce quantum confinement effects, provide new pathways for controlling energy flow and therefore have the potential to increase the efficiency of the primary photoconversion step. In this review, we provide the current status of research efforts towards utilizing the unique properties of colloidal quantum dots (nanocrystals confined in three dimensions) in prototype solar cells and demonstrate that these unique systems have the potential to bypass the Shockley-Queisser single-junction limit for solar photon conversion.
Nanomaterials form a flexible material platform that has great promise for providing new ways to approach solar energy conversion. The synthesis, investigation, and utilization of these novel nanostructures lie at the interface between chemistry, physics, materials science, and engineering. The chemistry community is providing simple and safe solution phase syntheses that yield monodisperse, passivated nanocrystals (NCs) of high optoelectronic quality with a growing degree of control over composition, shape, and structure.
These novel structures provide physicists and materials scientists with new avenues towards controlling energy flow. One of the largest scientific challenges regarding solar energy conversion is increasing the efficiency of the primary photoconversion process. In recent years we have studied the process of multiple exciton generation (MEG), where a photon bearing at least twice the energy of the bandgap can produce two or more electron-hole pairs and thereby bypass some wasteful heat production. 1,2
Third generation photovoltaics and multiple exciton generation
Traditional solar cells only harvest a fixed amount of energy from any given solar photon. However, the solar spectrum consists of photons with energies spanning 0.4 eV to 4.0 eV (see Fig. 1). The band-gap of the semiconductor determines how much solar energy can be converted to electrical power: photons with energy less than the bandgap are not absorbed, while photons with energy greater than the bandgap lose excess energy unnecessarily via emission of phonons (thermalization). Fig. 1 shows that the available free energy from an ideal present day single junction cell is about 33 %, while another 33 % is lost to thermalization and the remaining third is divided up between photons not absorbed and unavoidable thermodynamic losses. Those losses are associated with extracting photoexcited electrons at the contacts prior to radiative recombination.
Future directions and challenges
Surpassing the SQ limit for single junction solar cells is both a scientific and technological challenge and the use of semiconductor NCs to enhance the primary photoconversion process is a promising avenue towards such a goal. The MEG result is remarkable not only as a conclusive demonstration of MEG, but also as a demonstration that the ‘extra’ carriers can be collected in a suitable quantum dot solar cell. Thus, one of the tenets of the SQ limit, that high-energy photons only produce one electron-hole pair in a semiconductor, can be bypassed. However, the present day MEG solar cell only benefits by about 4 % in its photocurrent from collection of multiple excited carriers per photon. The challenge now is to further improve the MEG efficiency, as well as to continue to improve the fundamental QD film and device architecture. One avenue of future research is to explore MEG in a variety of shapes, compositions, and structures. Quantum wires or rods (QRs) with two-dimensional confinement, and quantum platelets (QPs) with one-dimensional confinement both are relatively unexplored, and QRs have already given some promising results that show further enhancement of MEG.
Other approaches to high-efficiency devices, most notably multijunction solar cells, are very promising avenues as well. However, in order for any of these approaches to be effective, a fundamentally well-controlled material is essential. As Kramer and Sargent propose, carrier mobility, trap density, and trap level position are useful metrics to use in this vein. We would also add that the carrier lifetime (which is intrinsically related to trap density) is an accessible and important parameter to monitor in this endeavor.
The diffusion length, determined from the product of lifetime and mobility, will determine how thick efficient cells can be made before recombination dominates. This length, presently around 100 nm, must be extended to the length scales of optical and NIR absorption lengths (about one micron).
We are thankful for the support of the division of Chemical, Geoscience and Biosciences, within the office of Basic Energy Sciences, office of Science, US Department of Energy for the work on the photophysics and chemistry of quantum dots. Our work on quantum dot solar cells was supported as part of the Center for Advanced Solar Photophysics an Energy Frontier Research Center within the office of Basic Energy Sciences, Office of Sciences, US DOE. Funding was provided to NREL under contract number DE-AC36-086038308 with DOE.
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