Releasing confined holes from type-II quantum dots by inelastic scattering with hot photoelectrons
Résumé
We study a novel architecture for quantum dot (QD) solar cells. While all models and experiments on QD solar cells studied QDs located in the depletion region, we focused on the solar cell structure in which an electron-blocking barrier spatially separates type-II QD absorber from the depletion region. It is important that these QDs are still within the diffusion length from the depletion region. Inter-band QD absorption of the part of solar spectrum with the photon energy less than the energy band gap in the spacers generates a large number of localized excitons in type-II QDs solar cells. The energy of such photons is not utilized in conventional solar cells. However, the proposed architecture enables QD solar cells to benefit from additional photocurrent generated by disintegrating of localized excitons. Hereby we show that hot photoelectron collision with QDs facilitates exciton disintegration. One fifth of solar photons have energy exceeding GaAs bandgap by more than 0.5 eV. They generate hot photoelectrons that usually relax by releasing the excess energy to optical phonons in 1 ps time scale by multiple electron-phonon scattering. Our study has shown that collisions with QDs may compete with the scattering with optical phonons in GaSb/GaAs type-II QD absorber. The excess energy is transferred to confined holes. Such collisions enable confined holes to escape from QDs into mobile states in the valence band of absorber and dissociate excitons into mobile electrons and holes. Collection of these mobile carriers will increase both photocurrent and conversion efficiency limit of GaAs solar cells by estimated 50%, well above the Shockley-Queisser limit. (C) 2015 Elsevier B.V. All rights reserved.