Simulating Bulk-Heterojunction Morphologies with 2D Slices

🎯 Learning objectives

Import a BHJ morphology and convert it to a simulation mesh.
Apply mesh reduction to obtain a numerically efficient triangulation.
Build a two-material active layer with electron-/hole-selective transport.
Run a 2D drift–diffusion simulation and inspect JV and carrier maps.
Explore morphology variants and voltage-dependent carrier distributions.

Prerequisites

OghmaNano installed with access to the Shape database.
Basic familiarity with the Layers editor and Optical/Drift–Diffusion ribbons.
Optional: an external morphology image/volume you wish to import.

Workflow overview

Create a new simulation via New → Simulations → Morphology.
Build a three-layer stack: top contact / active layer (two materials) / bottom contact.
Load or select a morphology from the Shape database and generate a mesh.
Reduce the mesh to a practical triangle count (node/triangle reduction).
Assign materials and transport asymmetry to form an effective BHJ.
Run the 2D electrical simulation and analyze JV.dat and snapshots.

Example fine and coarse BHJ morphologies used to generate 2D slices — Fine vs. coarse BHJ morphologies used to generate 2D electrical slices.

Bulk-heterojunction (BHJ) devices are often modeled as 1D effective-medium stacks. That approach is fast and excellent for many OPV use-cases. In this tutorial, we deliberately keep part of the lateral structure by taking 2D slices through a 3D BHJ morphology and solving a 2D drift–diffusion problem. You’ll learn how to import a morphology, discretize it efficiently, assign materials and mobilities, and run JV and field-map analyses.

1) Device setup & active-layer composition

Layers editor showing top/bottom contacts and a two-material active layer — Three-layer stack with a two-material active layer.

Create or open the supplied BHJ morphology example. In the Layers editor, set three layers:

Top contact (hole- or electron-selective, depending on convention).
Active layer composed of two interpenetrating materials:
- Polymer (red): high hole mobility, low electron mobility.
- Complement (light-blue matrix): high electron mobility, low hole mobility.
Bottom contact (the “other” selective contact).

This asymmetry creates an effective BHJ: electrons prefer the blue network, holes prefer the red network. Bandgaps and interfacial energetics can be refined later; here we demonstrate the concept with transport asymmetry.

2) Load morphology & generate a mesh

Shape database entry for BHJ morphology with threshold and mesh options — Shape database: thresholding and mesh generation.

Open the Database tab → Shape database and choose a morphology (e.g., Morphology 1 for a fine structure or Morphology 3 for a coarse structure). If importing an external image/volume:

Use Load image and apply a Threshold filter to obtain a binary phase map.
Ensure the object is closed (no holes to the exterior) to form a well-defined domain.
Click Build mesh to create the initial triangulation.

3) Reduce triangles for efficiency

The raw mesh can contain tens of thousands of tiny triangles, which is inefficient. Use Node/Triangle reduction (e.g., Reduce → Build mesh) to remove redundant elements, especially in flat/featureless regions. The tool performs multiple passes until no further reduction is possible, yielding a compact mesh that solves much faster while preserving morphology.

4) Assign materials & mobilities

Material parameter panel highlighting electron and hole mobilities for the two phases — Set asymmetric mobilities to mimic donor/acceptor networks.

In the material parameter panels:

Polymer (red): high hole mobility, low electron mobility.
Matrix (light-blue): high electron mobility, low hole mobility.

Ensure the red phase touches the hole-selective contact and the blue phase touches the electron-selective contact. If needed, slightly embed phases into the respective electrodes to guarantee percolating contact.

5) Run the 2D drift–diffusion simulation

Run the electrical simulation (dark or illuminated). Under illumination you should see a sensible JV curve (e.g., JV.dat with current densities on the order of 10² A·m⁻² and a realistic V_OC), depending on your parameters. Use the Snapshots folder to inspect:

Generation (G) heatmaps: where photons generate carriers in the polymer phase.
Free electron/hole densities: 2D maps that reveal transport pathways.
Voltage stepping: scroll through bias to study how densities evolve with field.

6) Analysis ideas

Compare fine vs. coarse morphologies (Morphology 1–3) on J_SC, V_OC, and FF.
Vary mobility contrast to explore percolation limits.
Add excitons and SRH traps if you want to study recombination pathways in detail.
Map carrier overlap regions to infer recombination “hot spots.”

Advanced options

Excitons: enable in the physics panel to include generation–dissociation kinetics.
Bandgap & offsets: refine energetic alignment for more realistic JV.
Custom morphologies: paint/edit in the Shape database or import new images/volumes.

Notes & provenance

The example morphologies shown here are inspired by numerical phase-field constructions (fine → coarse tunability). We slice the 3D structures to obtain 2D electrical problems for speed and clarity. For deeper mathematical background on morphology generation and preconditioning strategies, see the Chemnitz group’s phase-field work (acknowledgement to “Martin & colleagues”).