Interface study of insertion layers in organic semiconductor devices

Abstract

Inserting an ultra-thin interlayer has been an important means in modifying the performance of organic semiconductor devices. Using photoemission and inverse photoemission spectroscopy (UPS, XPS and IPES), we have investigated the electronic structure of a number of insertion layers widely used in organic semiconductor devices. We found that inserting alkali metal compound thin layers such as LiF between the electron transport layer (ETL) and the cathode can induce energy level shift in the ETL that reduces the electron injection barrier. The reduction is attributed to the release of the alkali metal that n-doped the ETL, and as such it depends on the cathode material deposited on top of the insertion layer. For thin metal oxide insertion layers, such as MoO 3 , between the anode and the hole transport layer (HTL), reduction of the hole injection barrier is also observed. This reduction, however, is due to the large workfunction of the oxide that subsequently moves the highest occupied molecular orbital (HOMO) toward the anode Fermi level. Effects of other insertion layers, such as metal insertion layer in organic bistable device (OBD) and organic insertion layer in bipolar organic thin film transistors (OTFT) will also be discussed.

On the Growth of Evaporated LiF on P3HT and PCBM

“…Although such a layer thickness of the alkali halide is insufficient to form a closed layer, the deposited layer significantly enhances the device performance, − and depending on the interlayer thickness and salt choice, the efficiency can be doubled. − This interlayer is of particular interest as it is effective in both OPVs and organic light-emitting diodes, yet the devices function in the exact opposite manner. , The role and function of the salt layer on a molecular level has not yet been agreed upon, but there are several proposals in the literature. These proposals include doping effects, ,,,− surface plasmon resonance generation, electrode workfunction modification via dipole formation, − and preservation of the electronic properties of the organic layer upon electrode deposition. ,,,, Ohmic contact formation has also been proposed for the case of LiF. ,,, It is known that components of the layers forming in these devices can interdiffuse at elevated temperatures encountered in device manufacture and operation, but the molecular distribution occurring between the organic and inorganic layers has not been fully examined . Such diffusion of components across interfaces will influence the function of the OPV, but it has not yet been taken into account for modeling photovoltaic performance.…”

“…Ideally, the concentration depth profiles of the components across the interface should also be determined to check for diffusion. There are many reports in the literature that have determined changes of the electronic structure for the near-surface area upon deposition of LiF onto organic interface materials ,,,− and the LiF layer formation on various organic layers, ,,,, but none as yet have addressed the changes in the electronic structure of the salt/organic layer interface upon salt deposition, whilst monitoring the chemical changes with salt deposition and also determining the vertical distribution of the salt.…”

Quantum Chemical Insight into the LiF Interlayer Effects in Organic Electronics: Reactions between Al Atom and LiF Clusters

“…Despite possessing a bright future, organic electronic devices still have some obstacles to overcome before their extensive applications in competitive market products. , Among these aspects, the issue on charge carrier transfer across the organic layer (OL)/electrode interfaces is of high concern for an efficient organic electronic device. − To polish the interface electron process across OL/cathode, lithium fluoride (LiF) and other alkali halides (AHs) were introduced into organic light emitting diodes (OLEDs) , and organic photovoltaic cells (OPVCs) , as interlayers. The enhancement effect of LiF on device performances has been explained by several mechanisms, mainly including tunneling effect, , doping effect induced by chemical reactions, − interface dipole formation, − and band bending, , where confilicting views were often presented, and it remains to be fully understood. − …”