Two-Photon Photoemission Study of Competing Auger and Surface-Mediated Relaxation of Hot Electrons in CdSe Quantum Dot Solids

Solids composed of colloidal quantum dots hold promise for third generation highly efficient thin-film photovoltaic cells. The presence of well-separated conduction electron states opens the possibility for an energy-selective collection of hot and equilibrated carriers, pushing the efficiency above the one-band gap limit. However, in order to reach this goal the decay of hot carriers within a band must be better understood and prevented, eventually. Here, we present a two-photon photoemission study of the 1P(e)-> 1S(e) intraband relaxation dynamics in a CdSe quantum dot solid that mimics the active layer in a photovoltaic cell. We observe fast hot electron relaxation from the 1P(e) to the 1S(e) state on a femtosecond-scale by Auger-type energy donation to the hole. However, if the oleic acid capping is exchanged for hexanedithiol capping, fast deep hole trapping competes efficiently with this relaxation pathway, blocking the Auger-type electron-hole energy exchange. A slower decay becomes then visible; we provide evidence that this is a multistep process involving the surface.