Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Abstract

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron-hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through electron and hole interactions with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron-hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the nearterm future for both fundamental research and the development of working devices which exploit MEG. MEG E g E hνAbsorption of a single photon with energy in excess of two times the band gap, Eg, produces multiple excitons at the band edge. TEM picture of typical PbSe nanocrystals.

Step-like increase of quantum yield of 1.5 μm Er-related emission in SiO2 doped with Si nanocrystals

“…In addition, we also note that for the sample with the smaller linewidth of the emission band (E PL % 1.55 eV, d NC % 3 nm, indicating a more narrow NC size distribution), the EQY dependence has a step-like character; we recall that a step-like behavior has been previously observed for carbon nanotubes 31 and Si NCs, 32 and was also theoretically modeled for the multiple exciton generation in semiconductor nanocrystals. 33 The steps are not so evident for the sample with the larger linewidth of the emission band (E PL % 1.37 eV, d NC % 4 nm, indicating a broader NC size distribution). As discussed before, the step-like increase of the EQY smears out for the broader size distribution.…”

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…(1) is only valid for samples with an absorbance length, , for the pump wavelength which is long compared to the beam path through the sample, . 42 Moreover, as discussed previously, 10 for ≪ the value of ∆ / observed will be reduced, since the volume of excited sample through which the probe beam passes is decreased as the pump beam is completely absorbed close to the front facet of the cuvette. A dilemma arises because the magnitude of ∆ / is also proportional to the absorbance at the probe wavelength, A, i.e.…”

Developing InP-based solar cells: Time-resolved terahertz measurements of photoconductivity and carrier multiplication efficiencies

“…The threshold for carrier multiplication in the InP quantum dot sample was measured to be 2.1 E g [5], which is the theoretical threshold for the process as required by the conservation of energy [12].…”