Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface

Abstract

We investigate the prototypical hybrid interface formed between PTCDA and conductive ndoped ZnO films by means of complementary optical and electronic spectroscopic techniques.We demonstrate that shallow donors in the vicinity of the ZnO surface cause an integer charge transfer to PTCDA, which is clearly restricted to the first monolayer. By means of DFT calculations, we show that the experimental signatures of the anionic PTCDA species can be understood in terms of strong hybridization with localized states (the shallow donors) in the substrate and charge back-donation, resulting in an effectively integer charge transfer across the interface. Charge transfer is thus not merely a question of locating the Fermi level above the PTCDA electron-transport level, but requires rather an atomistic understanding of the interfacial interactions. The study reveals that defect sites and dopants can have a significant influence on the specifics of interfacial coupling and thus on carrier injection or extraction.

Band Bending Engineering at Organic/Inorganic Interfaces Using Organic Self‐Assembled Monolayers

“…in ref. [6], is straightforward but would unduly complicate the calculations and the interpretation of the results.…”

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).…”

“…24,25 No evidence of surface photovoltage effects or radiation damage was observed across the range of excitation intensities used (250 -500 pJ / pulse).…”

“…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.…”

“…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.…”