OFET Simulation¶
Organic field-effect transistors (OFET) utilize organic electronics as charge transport materials in molecular transistors.
OFET modeling in Bumblebee is performed by adding a source-drain field perpendicular to the primary gate electrodes. This allows the cross-current behavior to be modeled.
Fig. 64 Default probe-field OFET device geometry used by Bumblebee¶
The OFET device uses an organic conductor to channel carriers between the transistor source and drain. Dielectric layers are used to shield the gates.
Bumblebee uses 2 gate electrodes by default. In order to model single-gate OFETs, an insulator can be used to cut off access to one of the gates.
Fig. 65 Single-gate OFET device configuration¶
Create Materials¶
In this tutorial, we will discuss the modeling of an OFET memory device, using a charge trapping layer for polaron confinement.
Fig. 66 OFET memory device configuration (MOFET)¶
We will not focus on the role of excitons in this tutorial. For this reason, the OFET components will be treated as transport layers. (If desired, excitonic processes can be added to the OFET simulation to account for internal loss mechanisms.)
Conductive Layer¶
Pentacene is used in the conductive channel. We will use the Transport template when creating a new material.
We set a HOMO level of -6.1 eV and a LUMO level of -3.9 eV. For the excitons, we use a singlet binding energy of 1.2 eV and a triplet binding energy of 1.4 eV. Default Gaussian broadening is used for both polaron and exciton energy levels.
The electron mobility prefactor is set to 0.5 to study the effect of a slower diffusion rate of the electron species compared to the holes.
Dielectric Layer¶
SiO2 is used as a dielectric. The Transport template is used when creating this material.
We set a HOMO level of -9 eV and a LUMO level of -1 eV. For the excitons, we use a singlet binding energy of 2 eV and a triplet binding energy of 2.1 eV. Default Gaussian broadening is used for both polaron and exciton energy levels.
Both electron and hole mobility prefactors are set to 0.1 to account for polaron blocking behavior of the dielectric.
In the Advanced tab, we can specify a source-drain injection prefactor. For the dielectric, this value is adjusted to 0 to block off the transistor contacts, which are meant to connect only to the conductive layer.
Fig. 67 Source-drain injection prefactor setting in the Advanced material configuration¶
Memory Layer¶
PVN is used as the charge storing material in the memory layer. We use the Transport template to create this material.
We set a HOMO level of -6.2 eV and a LUMO level of -1.2 eV. For the excitons, we use a singlet binding energy of 1.1 eV and a triplet binding energy of 1.5 eV. Default Gaussian broadening is used for both polaron and exciton energy levels.
The source-drain injection prefactor for the memory layer is set to 0, as only the conductive layer interacts with the transistor contacts.
Create a Stack¶
We will use the anode as the active gate. Therefore, we start by placing a 15 nm SiO2 layer. This is followed by a 10 nm PVN layer and a 35 nm Pentacene layer.
Create a Parameter Set¶
We will use the OFET template when creating the parameter set.
The device voltage is set to 10 V.
The anode contact is taken as the transistor gate. We assume a Au contact and set the Fermi level to -5.1 eV.
The cathode will represent the interface with the air. We set the Fermi level to 0 eV to avoid interactions with the device.
The OFET template should have automatically enabled the Transistor module, which adds the source-drain contacts to the device. Polaron injection at the gates is also disabled. This prevents polaron hopping into the vacuum.
Note
If you want to model the OFET in a cross-current setup, with active gates, electrode injection can be re-enabled by adjusting the injection and collection prefactors in the Advance Parameters tab.
Note
The exciton module is not enabled by default in the OFET template. If you want to model excitonic processes inside the OFET, remember to manually enable this module.
The Transistor tab allows configuration of the transistor voltage. The drain-source field is initially set to 3 V. Technical parameters are provided for controlling the gate field.
Fig. 68 Transistor configuration in the parameter set¶
The single-gate transistor has zero electric field at the vacuum interface. This condition is maintained as charge carriers populate the device, resulting in a change in the electric field inside the stack.
An update is performed at a given field check interval to correct the gate field for the changing polaron distribution. By default, this is implemented as an external field correction. The field is adjusted incrementally using the field control step size. Alternatively, polaron exchange with the gate can be enabled to adjust the carrier distribution.
To avoid oscillations in the electric field, a field control margin is defined. Field updates are only performed when the gate field error exceeds this value.
The adjustment to the gate field is performed to maintain the vacuum level. The position of the vacuum is given by the field check position. For the MOFET stack, this value is set to 60 nm.
Starting the Simulation¶
Sweeps of the gate voltage can be used to obtain OFET transfer curves. For this tutorial, we perform a voltage sweep from -10 to 10 V using 11 steps.
Parameter sweeps can also be performed for the drain-source field strength to investigate the effect of cross-field interactions on the device current.
To simulate memory programming/erasure cycles, transient switches can be used to adjust the device polarity during the simulation. Consult the transient response tutorial for more details.
Simulation Output¶
Polaron mobility, channel conductivity and transfer curves are available in the OFET Report section of the Sweep Report. The response of the memory layer can be analyzed in the Transient OLED Response section of the Multibox Report.