Defect‐Driven Interfacial Electronic Structures at an Organic/Metal‐Oxide Semiconductor Heterojunction

Abstract

The electronic structure of the hybrid interface between ZnO and the prototypical organic semiconductor PTCDI is investigated via a combination of ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) and density functional theory (DFT) calculations. The interfacial electronic interactions lead to a large interface dipole due to substantial charge transfer from ZnO to 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which can be properly described only when accounting for surface defects that confer ZnO its n-type properties.

Spectroscopy and control of near-surface defects in conductive thin film ZnO

“…The workfunction ߔ = 3.7(1) eV and valence band onset of 3.5(1) eV with respect to E F are consistent with previous work on similar thin ZnO films and are in good agreement with the strongly n-type nature of the films (carrier concentration of 10 19 cm -3 ). 5,10,24,25 Given a bandgap of E g = 3.4 eV, the valence band onset suggests that these films are close to or in the degenerate doping regime. In addition to the spectroscopic features typically observed for UPS of ZnO, we also find a high density of tail states extending past the valence band onset deep into the bandgap (Figure 2c).…”

“…Together with shallow donor defects it confers the low workfunction of 3.7(1) to these films, as discussed in several recent combined computational / spectroscopic studies. 5,10 In contrast, pristine single crystalline ZnO surfaces have workfunctions around 4.5 eV and do not show significant band tailing into the gap. 29 The close-up view of the gap region (Figure 2c) also shows that no density of states can be detected within ∼1 eV of E F in UPS, despite the fact that shallow donors must be present to account for the n-type character of ZnO.…”

“…Such molecules have been suggested previously to be able to accept electrons from shallow donor states in ZnO, resulting in the formation of a hybrid interface state observed already at sub-monolayer coverages. 5,24,25,27 Therefore, incremental coverages of C 60 , from sub-monolayer to bulk, were deposited and monitored by both UPS and 2PPE. Upon deposition of 1 MLE, the workfunction increases by 0.6(1) eV, further increasing by a total of 0.9(1) eV over 6 MLE.…”

“…1,4 Recent results indicate that the fundamental interactions that determine energy level alignment at the organic / inorganic interface are considerably different from the paradigmatic organic / metal interface. [5][6][7][8] In particular, though defects and dopants are generally thought to convey the n-type conductivity observed in ZnO, their precise role and nature is at present still controversial and unclear. [9][10][11][12] ZnO native point defects have been extensively studied as a means of understanding the intrinsic n-type nature of bulk ZnO.…”

“…Only a few studies have investigated the role of surface defects on the electronic structure, and an understanding of the impact of surface and near-surface defects on hybrid organic / inorganic semiconductor interfacial electronic structure and charge transfer is only starting to emerge. 5,10,[24][25][26][27][28] Most of these studies have relied on indirect methods for experimental detection of near-surface gap states, somewhat impeding a full understanding of the importance of gap states on the interfacial electronic structure of highly conductive ZnO surfaces functionalized by organic semiconductor adsorbates. 5,24,26 Here we show for the first time that two-photon photoemission (2PPE) spectroscopy can be used on highly conductive, polycrystalline thin ZnO films fabricated by plasma-enhanced atomic layer deposition (peALD) to directly reveal a density of states (DOS) located within the ZnO band gap.…”