Like any other electronic device, semiconductors require an electrical current generated by an electric voltage, which arises following the movement of charge carriers. Charge carrier transport determines the electric current flow within semiconductors. The use of vacuum deposition methods has been shown to consistently maintain exceptional film quality and charge transport characteristics in semiconductors.
Researchers in a September 2018 APL Materials study investigated whether solution-processed semiconductors can feature similar characteristics.
Understanding Differences in Material Characteristics
Recent efforts have been made to replace vacuum deposited transport layers in organic light emitting devices (OLEDs) with those that have been produced by solution processed techniques. This trend arises following several years of research that has been focused on determining the differences that exist when semiconductor devices, such as OLEDs, are developed by vacuum deposited methods compared to solution processed techniques.
Since these studies have shown that the current-voltage characteristics in the produced semiconductor materials are not similar, the researchers in the 2018 APL Materials study were interested in looking deeper into this area by investigating the differences that may exist in the hole transport of these materials.
In the current study, two amorphous hole-transport materials including N-N’-di(1-naphthyl)-N-N’-diphenyl-(1-1’-biphenyl)-4,4’-diamine (α-NPD) and 2,2’,7,7’-tetrakis(N-N-diphenylamino)-9,9-spirobifluorene (spiro-TAD) were used to investigate hole transport in both solution processed and vacuum deposited films. Both the α-NPD and spiro-TAD films were deposited onto a 55 nanometer (nm) thick poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film by either spin coating, which is a type of solution process, or vacuum deposition. It is important to note that both the α-NPD and spiro-TAD molecules were originally designed for vacuum deposition purposes only.
For films that were developed by spin coating, three different solvents including chloroform, chlorobenzene or toluene were cast in a nitrogen atmosphere and then annealed prior to deposition. Vacuum deposition methods occurred at a rate of 1-2 Å/s at a pressure of 1 x 10-7 mbar. Drift-diffusion numerical simulations were used to model charge carrier transport characteristics.
The researchers determined that, for both vacuum deposition and solution processed conditions, the hole transport mobility in both the α-NPD and spiro-TAD reached equal values. In terms of surface morphology, the spin-coated spiro-TAD films appeared to exhibit smoother surfaces and a more uniform appearance compared to those developed by thermal evaporation techniques.
Furthermore, the charge carrier mobility in both vacuum deposited and solution processed materials appeared to exhibit a temperature, carrier density, and electric field dependence.
The wide range of similarities that were found between the α-NPD and spiro-TAD materials produced by both solution processed and vacuum deposition methods demonstrated that, in terms of charge transport, solution processing can be a practical alternative to vacuum deposition methods.
- Mangalore, D. K., Blom, P. W. M., & Wetzelaer, G. A. H. (2018). Hole-transport comparison between solution-processed and vacuum-deposited organic semiconductors. APL Materials 7. DOI: 10.1063/1.5058686.
The research in the study discussed here was conducted at the Max Planck Institute for Polymer Research in Mainz, Germany.