The encapsulation of solar cells into a photovoltaic module introduces some optical loss mechanisms as shown schematically in Figure 1. Typically, the output power of the module is less than the total sum of individual cells. This difference is referred to as cell-to-module (CTM) losses. These losses typically occur due to the reflection at subsequent interfaces, namely air-glass, glass-encapsulant and encapsulant-solar cells [respectively marked as (1), (2) and (3) in Figure 1] and parasitic absorption by the front glass and encapsulant material [marked as (4) and (5)].
Figure 1: Schematic illustration of the optical light paths in a PV module.
On the positive note, encapsulation also introduces some optical gains. The fact that the refractive indices of encapsulants is higher than air leads to a significant reduction in the reflection from front surface. This provides direct optical coupling gain. Furthermore, there can be an additional cell-to-module gain by scattering of light which falls within the gap spaces between the cells (strikes the backsheet). When the angle of scattered light exceeds the total internal reflection angle for glass-air interface, it redirects it back to the cells where it can be absorbed. Therefore, the CTM loss can be reduced by carefully selecting the module materials and taking optics into account when designing the module.
- Parasitic absorption of EVA (Ethylene-vinyl acetate) at shorter wavelengths can be minimised by substituting standard EVA with downshifting EVA or high-transmittance EVA. Downshifting EVA replaces UV-absorbers in EVA with luminescent downshifting molecules which has a relatively high absorption in the ultraviolet part of the solar spectrum but subsequently emits light at a lower energy (i.e., this is why is referred to as downshifting) which can still be used by the solar cells. High-transmittance EVA, on the other hand has higher transparency for UV light which ensures that a higher light intensity reaches the solar cell.
- The optical gain can also be improved by incorporating high reflective backsheets in contrast to standard backsheet. In some cases even black backsheets are used for aesthetic reasons which is not optimal when trying to minimize the CTM loss. As this strategy allows for the harvesting of light which is normally not impinging on the solar cell this strategy is a potential CTM gain.
- The ribbons used for electrical interconnection of the solar cells are opaque and consequently reduce the light absorption in the solar cell. However, using light-trapping ribbons (LTR) or ribbons with light trapping films (LTF) can increase the amount of light absorbed by the PV module. Both types ribbons tend to reflect the light at angles higher than total internal reflection angle at glass-air interface. This reflected light is then directed back to the solar cells with a second chance of absorption. As this mechanism is not available when measuring a solar cell in air, this mechanism can actually result in a CTM gain.
Mismatch Losses
When cells with different electrical parameters are connected in a module, the total generated power can be less than the power achieved by summing the output power of individual cells. The most common loss in performance of a module arises from the incorporation of cells with dissimilar I-V characteristics. Mismatch losses occur due to a mismatch between output currents of the solar cells in the PV module as module current is limited by the current of the lowest-current cell when using series interconnection. The voltage of the module is determined by the sum of the individual voltages of the solar cells and thus a variance in the voltage does not result in a cell-to-module loss. However, as every solar cell is measured at the end of the solar cell manufacturing process and grouped according to their electrical properties, this mismatch loss is typically not very large.
Hot-Spot Losses
In some cases, the operation of the module can result in conditions that result in the overheating of a module in a local spot. This is typically the result of a mismatch in solar cell current induced by local shading or a damaged solar cell. As the unaffected solar cells still want to maintain a high current, they force the shaded or damaged solar cell to operate in reverse bias and typically the current in a reverse biased solar cell is very localised. This results in very high localised current densities and consequently high temperatures. These local high temperatures can severely damage the PV module. For this reason, all PV modules contain bypass diodes and all solar cells are tested for their reverse bias current densities (if these currents are too high, these solar cells will not be processed into PV modules).
Snail Trails
After a few years of module operation, dark lines, which resembles the random movement of a snail have been observed in the panels and are called snail trails as show in Figure 2. Snail trails can be a consequence of multiple factors; however, its actual cause is still unclear. It is usually attributed to a multimode failure involving a compromised backsheet and a crack in a solar cell. The compromised backsheet allows moisture to enter the PV module which can reach the front of the solar cell via the the crack in the solar cell. This moisture then reacts with the silver and the EVA resulting in a so-called snail trail. Fortunately, snail trail do not significantly affect the PV module performance but are obviously not desired from an aesthetics point of view.
Figure 2: Photograph of snail trails on a PV module.
Soiling Losses and EVA browning
The loss in output power resulting due to the accumulation of snow, dust, dirt and other particles that cover the surface of PV modules are termed as soiling loss. Since the performance of a module depends on the amount of sunlight reaching the cells, these small environmental particles directly a result in a reduction in the power output. Considering the immense value of the maintenance of modules after installation, the PV industry is coming up with multiple solutions to keep the modules clean for longer times. Another reason for a reduced PV module performance after prolonged operation is the deterioration of the EVA. Some EVA compositions are not completely UV stable, and this results in the browning of the EVA. This is not only an aesthetic problem but also resulted in a decreased in the module performance due to an increase in the absorption in the EVA film as shown in Figure 4.
Figure 3: Wavelength-dependent transmission of virgin EVA, yellow EVA, and brown EVA.