Photovoltaics: capturing hot electrons.


Meeting the clean-energy demand of the twenty-first century has been the major driving force behind improving the efficiency of photovoltaic cells1,2. The present silicon solar cells operate in the range 15–25% depending on the scale and operating conditions. They absorb nearly 60% of solar light and their small bandgap (1.1 eV) enables photons with low energy (for example, red light) to create the necessary electron–hole pairs that generate photocurrent. However, the absorption of high-energy photons (blue light) results in ‘hot’ electrons, which are both electronically excited and thermally activated, and lose most of their energy as heat without contributing to electric power. Therefore a hurdle to boosting energy-conversion efficiency in photovoltaic solar cells — and not just those made from silicon — is to capture the excess energy of the thermally unrelaxed electrons before it is lost as heat. In Science, a team led by David J. Norris, Eray S. Aydil and X.-Y. Zhu3 now report the transfer of hot electrons from excited lead selenide (PbSe) nanocrystals (quantum dots) to titanium dioxide. Like silicon, PbSe is a small-bandgap material that can harvest photons in the red and infrared without being able to capture the energy of hot electrons. Following the high-energy excitation of PbSe quantum dots, the hot electrons typically relax to the conduction band-edge within a subpicosecond timescale, losing energy in the process. To extract the electrons before relaxation this competing process needs to be overcome either by increasing the rate of hot-electron extraction or slowing down the cooling (the energy diagram in the inset of Fig. 1b illustrates the two competing pathways). The challenge is therefore to create an electron donor–acceptor system that incorporates one or both of these concepts, making the choice of materials vitally important. In recent years several semiconductor quantum dots (for example, CdS, CdSe, PbS, PbSe) have been successfully employed in the operation of solar cells. Moreoever, they offer the advantage of slowing the relaxation time of hot charge carriers. This is because in quantum dots the quasi-continuous conduction and valance energy-bands of a bulk semiconductor are replaced by a discrete electronic structure. The spacing between these discrete energy levels is such that the possible relaxation processes that can occur are limited — creating what is known as a phonon bottleneck — and allowing only slower multiphonon emission. Specifically choosing PbSe quantum dots offers certain advantages over other materials, for example the large Bohr radius (46 nm) of PbSe allows the synthesis of a wide array of PbSe particles in the quantum size regime. Also, using PbSe quantum dots not only allows them to achieve strong quantum confinement, but helps electronic coupling to nearby electronaccepting materials because their electronic wavefunction extends in space beyond the quantum dot particle. Furthermore, the high density of states present in TiO2 makes it a good choice of electron acceptor. Such pHOTOvOLTAICS

DOI: 10.1038/nchem.814

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Cite this paper

@article{Kamat2010PhotovoltaicsCH, title={Photovoltaics: capturing hot electrons.}, author={Prashant V Kamat}, journal={Nature chemistry}, year={2010}, volume={2 10}, pages={809-10} }