In previous posts, I presented instabilities within organic and perovskite solar cells. Most are due to the presence of water and oxygen. Therefore contrary to silicon modules, third generation solar cells degrade very fast when exposed to an oxygen and moisture atmosphere. For instance, an typical organic solar cell can degrade in just a few hours, meaning that its Power Conversion Efficiency decreases.

Solutions do exist: new architectures and new active materials can be found to overcome this issue. However, it might not allow the research of higher performances, that also needs new architectures and active materials. Most of the time it is hard to combine those two properties.

Barrier Layers

For purposes of finding a compromise between high efficiency and low degradation, encapsulation of the cells is usually seen as the best opportunity. The principle is very simple: adding barrier layers for moisture and oxygen not to diffuse up to the solar module (Cros et al.). However, there are many constraints: the materials should be transparent, flexible, with low permeation rates, chemically and physically stable and compatible with the cells. It might seem antagonist, but the use of thin films is a way to overcome the issue.

Thus the protection against moisture and oxygen requires the development of protective materials with sufficiently low water and oxygen permeation. To do so, industrials like Armor or Heliatek usually laminate films on their optoelectronic devices.

Measuring permeation

Measuring permeation requires specific techniques. The principle is simple: comprising a thin film between two chambers and measuring the rate of water that goes through the film. In the upstream side chamber, we set a partial atmosphere saturated with D₂O vapour at a fixed temperature. In the downstream side chamber, we fix the partial pressure to high vacuum, maintained with a high pumping system, and detect D₂O flow with a mass spectrometer. The use of D2O is justified because the abundance of D₂O is much lower than H₂O then the measures are more precise by reducing the noise. That process has been patented

When measuring permeation, we use two values: Water Vapour Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR). For the needs of OPV encapsulation, the WVTR must be lower than 10^-3 gm^-2d^-1 and the OTR lower than 10^-3 cm³ m^{-2 d-1.}

Multilayers films

When devising transparent and flexible films, one can think about PET. Yet it is far from fulfilling the requirements I exposed with permeation. In facts, water and gazes can diffuse inside polymers. Hence it requires the addition of inorganic films to block that diffusion. However, the barrier properties are very limited inside inorganic films, due to possible defects and the fact that inorganic films are prone to present pinholes or to crack and then delaminate. For this reason, even if it might seem counter-intuitive, increasing the thickness of an inorganic film leads to higher permeability.

For this reason, [Affinito et al]{https://doi.org/10.1016/S0040-6090(96)09202-4}. had the idea to develop dense organic-inorganic multilayers barrier films. Inorganic films can present defects but the presence of organic layers induces a decorrelation of the defects. Hence gases molecules need to diffuse laterally within each organic layer to go through the next effect. That idea increases the tortuosity, resulting in a rise in the time needed for the molecules to cross the membrane (time lag), and diminishes the WVTR. This inorganic films can be deposited via plasma-enhanced chemical vapour deposition (PE-CVD).

Yet the multilayers deposited via PE-CVD are expensive to process because it requires the deposition to be made in a high vacuum. The organic layers are solution-processable and that cannot be done in a high vacuum. It means that one must process an organic layer in the air, then process an inorganic layer in high vacuum conditions, and repeat. A way to address the issue is to achieve solution-processed inorganic layers. Hence scientists need to find a solution precursor of those inorganic layers. It is a promising topic on which Dr Arnaud Morlier worked when he did his PhD at CEA.

Edge permeation

Here we described solutions for gases and water not to go through the solar cells, by stacking protective layers on top of the devices. However, edge permeation exists and it could represent more than 50% of the whole permeation. Water and gases diffusing through the interfaces cannot be neglected. For this reason, there is a need to consider the couple gas-barrier film + adhesive. It is positive to say that on the edges, the requirements are much lower: because the layer can be made larger on the edges, the required WVTR is comprised between 1 to 10 g m^-2 d^-1.

To measure properties and permeability of the edge materials, one can at first, measure permeation on thin films. Yet scientists have developed a modelisation of the optoelectronic devices, by using a calcium test. We encapsulate a piece of calcium as if was is a solar cell. Calcium is opaque and with thermal/water ageing, it can evolve to Ca(OH)₂, that is a transparent material. Measuring the transparency of the piece of calcium leads to understanding if water permeates or not.

Designing edge materials will be a key issue for the commercialisation of durable OPV materials.