Perovskite Solar Cell (PSC) Tutorial Part C: Exploring the device structure

🎯 Learning objectives

Open the Layer editor and identify the key fields that define each layer (name, thickness, optical material, type).
Explain the difference between Active, Contact, and Other layers, and decide which setting is appropriate for the layers in a typical perovskite stack (FTO, TiO₂, Perovskite, Spiro, Au).
Understand why the Perovskite absorber is always set Active, and why in this tutorial TiO₂ and Spiro are also made Active so that transport properties can be analysed.
Adjust the thickness of the perovskite active layer and reason about the trade-off between stronger optical absorption and increased recombination losses.
Run a set of simulations to explore how device performance metrics (PCE, J_SC, V_OC, FF) vary with perovskite thickness.
Recognise cases where additional active layers may be required — for example, when simulating blocking contacts that cause S-shaped JV curves, or when modelling emissive devices such as OLEDs.

📦 Prerequisites

OghmaNano installed and working.
Basic familiarity with solar-cell concepts.
Completion of Part A: Quick start – simulate your first perovskite.

⏱ Estimated time

Part A: 5-10 min
Part B: 10-15 min
Part C: 15-20 min
Part D: 15-20 min
Part E: 10-15 min
Part F: 10-15 min
In total around an 1 hr depending on much you explore.

Before starting this section, complete Part A and Part B.

1. Editing device layers

Main OghmaNano window with the Device structure panel; the Layer editor button is highlighted. — Open the *Layer editor* from the **Device structure** panel to view and edit the stack.

Layer editor table showing FTO, TiO₂, Perovskite (perovskites/std_perovskite), Spiro, and Au with thickness and layer type. — The perovskite device stack used in this tutorial: **FTO (50 nm)** / **TiO₂ (200 nm)** / **Perovskite (400 nm; `perovskites/std_perovskite`)** / **Spiro (200 nm)** / **Au (100 nm)**. Set the perovskite as an *Active* layer so optical/electrical generation is calculated in it.

Perovskite solar cells are built as a stack of thin layers—some collect or transport charge, others absorb light. In OghmaNano the stack is edited with the Layer editor, accessible from the main window under the Device structure tab (see ??). The editor itself is shown in ??.

Each row in the editor corresponds to a layer with editable fields: Layer name, Thickness, Optical material, and Layer type (e.g. Active, Contact, Other). In this tutorial the active absorber is std_perovskite (a literature-averaged MAPbI₃ dataset).

Thickness is a key design parameter. Very thin absorbers (~50 nm) minimise transport distance but miss some light; thicker films (~400 nm) harvest more photons but increase the path that carriers must travel, raising the chance of recombination before collection. Performance therefore peaks at an intermediate thickness rather than increasing indefinitely—this trade-off is central to perovskite device design.

3. More about the Layer editor

The Layer editor is where you define the stack used by the simulator. Each row is a layer and each column controls how that layer behaves optically and electrically.

Layer name – a human-readable label (e.g. FTO, TiO₂, Perovskite, Spiro, Au). The name does not affect the physics.
Thickness – the physical thickness of the layer (units shown in the table, typically nm).
Optical material – the n/k dataset used for optics (chosen from the databases). This is independent of the electrical model.
Layer type – tells the solver how to treat the layer:
- Active: full drift–diffusion solved; carriers can be generated and recombine.
- Contact: an electrode where boundary conditions (work function, recombination velocity) are applied.
- Other: passive/transport layers; included in optics and electrostatics but without full generation–recombination physics.

4. Which layers should be active?

In principle, only layers that support both electrons and holes and where photogeneration or recombination occurs need to be marked as Active. In a minimal perovskite cell this would mean just the Perovskite absorber, while transport layers (e.g. TiO₂ and Spiro) would typically be left as passive layers. Contacts such as FTO and Au are never active, since they act only as electrodes.

However, in this tutorial the TiO₂ (electron transport layer) and Spiro (hole transport layer) have also been marked as Active. This setup allows you to investigate how carriers move through these transport layers, rather than treating them as idealised resistors. This distinction is important: it lets you capture transport-related losses in perovskites, whereas in OPV tutorials we normally keep transport layers as Other to simplify the physics.

As a general rule, only set extra layers as active when you are specifically interested in their transport properties or in phenomena like blocking contacts or S-shaped JV curves. Otherwise, keeping the number of active layers to a minimum will make simulations faster and results easier to interpret.

📝 Questions (Part C)

Which column in the Layer editor sets a layer to Active, Contact, or Other?

💡 Show answer

The Layer type column. This dropdown lets you select whether a layer is treated as Active, Contact, or Other.
In the FTO / TiO₂ / Perovskite / Spiro / Au stack, why are TiO₂ and Spiro also marked Active in this tutorial?

💡 Show answer

Because we want to study charge transport in the transport layers, not just in the absorber. Setting TiO₂ and Spiro as Active means the solver calculates drift–diffusion physics inside them, capturing effects such as transport bottlenecks or recombination at interfaces. In OPVs, these layers are usually left as Other to keep the model simpler.
What is the trade-off when increasing the Perovskite thickness (optical absorption vs transport/recombination)?

💡 Show answer

A thicker absorber captures more light and increases J_SC, but carriers must travel further, which raises recombination losses. The efficiency peaks at an intermediate thickness that balances absorption and carrier extraction.
How would you modify layer types to intentionally explore S-shaped JV behaviour caused by imperfect contacts?

💡 Show answer

Set one of the transport layers (e.g. TiO₂ for electrons or Spiro for holes) to Active in the Layer editor, and then deliberately reduce the carrier mobility for that layer. For example, lowering the electron mobility in TiO₂ or the hole mobility in Spiro-OMeTAD will restrict carrier transport at that interface. The solver will then show carrier build-up and injection barriers, which can produce the characteristic S-shaped JV curve associated with blocking contacts.
Where do you find summary performance values (J_SC, V_OC, FF, PCE) for each run?

💡 Show answer

Key metrics are written to sim_info.dat. The full JV curve data are in jv.csv.
Give an example of a device class where more than one active layer might be appropriate (hint: emissive devices).

💡 Show answer

OLEDs (organic LEDs). Multiple layers can host electrons and holes and take part in recombination, so they may need to be treated as active simultaneously.