16 research outputs found
Ultrashort and metastable doping of the ZnO surface by photoexcited defects
Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defect or dopant levels creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extend of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al, Phys. Rev. B, 91, 235313 (2015)]. The transient dopants exhibit a multi-step decay ranging from hundred’s of picoseconds to 77 μs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we described previously [Gierster et al. Nat. Commun. 12, 978 (2021)]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to μs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in literature [Madel et al., J. Appl. Phys. 121, 124301 (2017)]. In agreement with theory on PPC [Lany and Zunger, Phys. Rev. B 72, 035215 (2005)], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties such as the electrical conductivity from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects
Uncovering the (un-)occupied electronic structure of a buried hybrid interface
The energy level alignment at organic/inorganic (o/i) semiconductor
interfaces is crucial for any light-emitting or -harvesting functionality.
Essential is the access to both occupied and unoccupied electronic states
directly at the interface, which is often deeply buried underneath thick
organic films and challenging to characterize. We use several complementary
experimental techniques to determine the electronic structure of
p-quinquephenyl pyridine (5P-Py) adsorbed on ZnO(10-10). The parent anchoring
group, pyridine, significantly lowers the work function by up to 2.9 eV and
causes an occupied in-gap state (IGS) directly below the Fermi level
. Adsorption of upright-standing 5P-Py also leads to a strong work
function reduction of up to 2.1 eV and to a similar IGS. The latter is then
used as an initial state for the transient population of three normally
unoccupied molecular levels through optical excitation and, due to its
localization right at the o/i interface, provides interfacial sensitivity, even
for thick 5P-Py films. We observe two final states above the vacuum level and
one bound state at around 2 eV above , which we attribute to the
5P-Py LUMO. By the separate study of anchoring group and organic dye combined
with the exploitation of the occupied IGS for selective interfacial
photoexcitation this work provides a new pathway for characterizing the
electronic structure at buried o/i interfaces
Photoexcited organic molecules en route to highly efficient autoionization
The conversion of optical and electrical energy in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2,7-ditertbutyl-9,9-spirobiuorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron (2PPE) and optical spectroscopy. We find that, after initial ultrafast relaxation on fs and ps timescales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (rDex = 1.3 nm) and exhibits a long decay time of 0.1 s. Because of this long lifetime a photostationary state with average T-T distances below 2 nm is established at excitation densities in the 1020cm-2 s-1 range. This large density enables decay by T-T annihilation (TTA) mediating autoionization despite an extremely low TTA rate of kTTA = 4.5 10-26 m3s-1. The large external quantum efficiency of the autoionization process (up to 15 %) and photocurrent densities in the mA cm-2 range offer great potential for light-harvesting applications