Summary: Power conversion efficiency is one of the most important parameters that can directly affect the overall cost of a PV installation. In...
Summary: Power conversion efficiency is one of the most important parameters that can directly affect the overall cost of a PV installation. In the Shockley-Queisser detailed balanced analysis, the power conversion efficiency is limited to ~33% for a single light absorbing layer. The major energy loss occurs from the excess photon energy, energy greater than the semiconductor bandgap, which is lost as heat through electron-phonon coupling and subsequent phonon relaxation and energy dissipation. Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by channeling the excess photon energy to produce multiple electron-hole pairs, thereby, increasing the thermodynamically allowed power conversion efficiency to ~44% for unconcentrated light. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron-phonon coupling that cools hot-excitons in bulk semiconductors. We have demonstrated that MEG in PbSe QDs is about two times as efficient at producing multiple electron-hole pairs than bulk PbSe. Thin films of electronically coupled PbSe QDs have shown promise in simple photon-to-electron conversion architectures with power conversion efficiencies above 5%. We recently reported an enhancement in the photocurrent resulting from MEG in PbSe QD-based solar cells. We find that the external quantum efficiency (spectrally resolved ratio of collected charge carriers to incident photons) peaked at 114% in the best devices measured, with an internal quantum efficiency of 130%. These results demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells. We compare our results to transient absorption measurements and find reasonable agreement. If the MEG efficiency can be further enhanced, and charge-separation and transport can be optimized within QD-films, then QD solar cells can lead to third-generation solar energy conversion technologies.
Matthew C. Beard is a physical chemist (PhD - Yale University) whose interest spans a large range of important physical chemistry problems relating to photoconversion. This includes charge transport in nanoparticle arrays and organic semiconductors, charge and energy transfer, and size-dependent phenomena in quantum sized-materials. He is currently investigating carrier generation and charge transport in assemblies of semiconductor nanocrystals. Dr. Beard received his Ph.D from Yale University in 2002 where he pioneered time-resolved THz spectroscopy for studying charge carrier generation in a variety of nanoscale systems. He joined NREL in 2004 in the chemical and materials science center working with Arthur J. Nozik. The Center for Advanced Solar Photophysics (CASP) is an energy research frontier center funded by the Basic Energy Sciences division of the Department of Energy and is jointly led by NREL and LANL. CASP’s mission is to explore and exploit the unique advantages of nanostructured materials and solution-based fabrication methods to enable the low-cost, high-efficiency solar cells of tomorrow.