🎯 Learning objectives

Set up a donor–acceptor domain (sphere in box) for exciton simulations.
Run the exciton transport solver and visualize generation and densities in slices/3D.
Interpret exciton_siminfo.dat to compute dissociation efficiency.
Understand the governing rate equations for singlets, triplets, and doped domains.
Explore how domain size, diffusion, and decay rates affect dissociation yield.

📦 Prerequisites

OghmaNano installed and working.
Basic knowledge of excitons, diffusion length, and interfacial dissociation.

⏱ Estimated time

Setup & first run: 10 min
Analysis & parameter sweeps: 20 min

Exciton Domain Simulation

This tutorial explores exciton transport and dissociation in an idealized donor–acceptor system: a donor sphere inside an acceptor box. Light absorption generates excitons in the donor, which must diffuse to the interface before recombining or dissociating. By changing parameters (lifetime, diffusion length, decay rates, domain size), you can estimate dissociation efficiency.

1. Governing equations

The exciton transport is described by coupled rate equations for singlet density \(N_S\), triplet density \(N_T\), and doped-domain populations \(N_{SD}, N_{TD}\). Work on this model is ongoing; the current implementation solves:

\[\frac{dN_{S}}{dt}=\frac{1}{4}R_{free}+\frac{1}{4}\kappa_{TT} N_{T}^{2} -(\kappa_{S}+\kappa_{ISC})N_{S} -\left(\tfrac{7}{4}\kappa_{SS} N_{S} -\kappa_{ST} N_{T}\right)N_{S}\]

\[\frac{dN_{T}}{dt}=\frac{3}{4}R_{free}+\kappa_{ISC} N_{S} + \tfrac{3}{4}\kappa_{SS} N_{S}^2\]

\[\frac{dN_{SD}}{dt}=\frac{\kappa_{FRET}}{N_{DOP}} (N_{DOP}-N_{SD}-N_{TD})N_{S} +\tfrac{1}{4}\kappa_{TTD}N_{TD}^{2} -\left(\tfrac{7}{4}\kappa_{SSD} N_{SD}+\kappa_{STD}N_{TD}\right) N_{SD}\]

These equations capture radiative/nonradiative decay, singlet–triplet conversion, triplet–triplet annihilation, and transfer processes. The dissociation efficiency is estimated by comparing integrated interfacial dissociation to total generation.

2. Geometry & setup

Create a new simulation via New simulation → Exciton domain. The default geometry is:

Sphere (donor): excitons generated inside, radius settable in the Shape editor.
Box (acceptor): the surrounding matrix.
Photon generation: applied inside the sphere from top illumination.

3. Running the simulation

Click Run. Inspect slices:

Photon generation: follows the donor sphere volume.
Exciton density: zero at the interface, maximum in the center, following diffusion-limited profiles.

4. Analyzing results

Exciton density slice through donor sphere.

To quantify results, open exciton_siminfo.dat. It lists spatially integrated generation, decay, and dissociation for each shape/layer. Compare dissociation at the interface to generation inside donor to obtain dissociation efficiency (e.g., ~89% in the base case).

5. Parameter sweeps

Increase radiative decay \(k_{\mathrm{rad}}\): lowers efficiency by annihilating excitons before reaching the interface.
Decrease donor radius: excitons more easily reach interface, efficiency rises.
Increase donor radius: excitons are lost to decay before reaching interface, efficiency drops.

Use the Electrical Parameter editor (bottom panel when Exciton simulation type is active) to adjust \(k_{PL}, \tau, L_D\) and other constants. The main drift–diffusion parameters are disabled in this mode.