The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology, especially in areas such as photovoltaics, photocatalysis, and photosynthesis. A usual assumption is that absorption of a single photon by a semiconductor or a molecule produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat by exciting molecular or lattice vibrations (phonons). Under this assumption, the maximum power-conversion efficiency of solar cells is limited by ~31% (the Shockley-Queisser limit). In principle, one can surpass this limit using carrier multiplication, a process in which absorption of a single photon produces not one but multiple excitons. Carrier multiplication is inefficient in bulk semiconductors because of restrictions imposed by energy and momentum conservation and extremely fast energy losses due to phonon emission. In 2004, we discovered that the efficiency of carrier multiplication can be greatly increased using nanosized semiconductor particles known as semiconductor nanocrystals or nanocrystal quantum dots (Schaller & Klimov, Phys. Rev. Lett. 2004). While the exact mechanism for this enhancement is still a subject of debate, high multiexciton yields in nanocrystals are likely due to factors such as close proximity between interacting charges, reduced dielectric screening, and relaxation in translation-momentum conservation. In my presentation, I will review the current status of carrier multiplication research and discuss some of the existing controversies especially concerning experimental measurements of multiexciton yields and mechanisms for multiexciton generation under conditions of extreme quantum confinement. I will also discuss challenges for practical implementations of this effect in solar-energy conversion technologies.


Thursday, September 10, 15:00. Blue Lecture room