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In particular, we point out that the effect of a sodium-free glass is twofold. On the one hand the amount of Na⁺available for PID is reduced. On the other hand, as mentioned, thanks to a higher glass resistivity the leakage current is reduced and so is the electric field in the SiNx

layer. In this case, even if the sodium resulting from the contamination of the solar cell surface was enough for causing the PID effect, the reduced electric field ~E would limit the drift of such ions towards the interface with the bulk silicon (see Figure 3.3).

The role of the encapsulant material is also fundamental. Encapsulant materials with high resistivity such as polyolefin or ionomer have been shown to prevent or significantly delay the occurrence of PID compared to standard EVAs ([94], [9]). We address this aspect in Chapter 6, where we evaluate the PID resistance of different combinations of materials in the module (cell, encapsulant, and rear cover).

PID can be critical for many of the solar parks built in the early phase of PV deployment (2000–

2012), however it can still occur in modern modules when lower grade materials are used in PV manufacturing (e.g. encapsulants), a choice often driven by the need for cheap solar electricity.

3.2.5 Role of the climatic conditions

PID is strongly dependent on the climatic conditions in which modules operate. High temper-atures accelerate the phenomenon due for example to reduced resistivity of the glass and the encapsulant material. As discussed in Section 3.2.1, high relative humidity conditions, rain events, or the presence of dew over the module surface also lead to harsher degradation by making the glass surface equipotential to the frame. We address these points in Chapter 4, where we investigate with accelerated testing the relationship between power loss and the environmental stress factors. In Chapter 5 we then simulate the PID for modules installed in sites with different climatic conditions.

3.3 The physical mechanism of performance regeneration

A peculiarity of PID with respect to other degradation modes is that it is reversible. The module shunt resistance and thus the power can indeed recover, partly or completely, under certain weather conditions. The reversibility holds for the PID of the shunting type (the focus of our thesis) but does not apply to the other degradation modes related to high potential such as the corrosion of the metallic fingers.

Some studies on samples exposed outdoors (see [81] among others) showed that regeneration (or recovery) can happen:

• at night, when the cells’ operating voltage is null. We refer to this case as regeneration in the dark or thermal recovery;

• during hot and dry days (regeneration during light exposure), when the low conductivity of the glass surface limits the leakage currents and the high potential difference remains limited to the cells in the perimeter of the module.

Power recovery can also be induced by applying a positive voltage bias between the cells and the grounded frame during the night by means of dedicated electrical devices [130]. In the remainder of this section we describe these three kinds of power recovery from PID.

3.3.1 Regeneration in the dark

At night, in the absence of a voltage, regeneration is purely a diffusion mechanism, therefore driven by temperature. This process of thermal recovery was extensively examined in the last few years, with major contributions especially by Lausch and Naumann from Fraunhofer CSP.

By means of microscopic investigations, they proved that thermal recovery is due to diffusion of the Na⁺out of the stacking faults [91]. In the absence of a bias voltage, indeed, the Na⁺ current towards the cell surface stops and the Na atoms diffuse from the stacking fault back into the oxide (SiO_x) layer where now the Na concentration is lower (see Figure 3.7). Once in the silicon oxide layer SiOx, Na spreads laterally. Diffusion of Na into the SiN layer can also take place (and Na converts to Na⁺again), even if the diffusivity of Na in SiN is lower than in the SiO_x. The thermal recovery process was also studied through accelerated laboratory testing, where previously degraded modules are tested in conditions of regeneration. For instance, in [93] an equation for regeneration in the dark, with no bias voltage, as a function of the temperature is proposed.

Figure 3.7: Schematic drawing of a p-type c-Si cell cross-section and illustration of the physical mechanism of thermal recovery from PID. The Na atoms diffuse out of the stacking faults back into the oxide (SiO_x) layer, where they spread laterally. They may also diffuse into the SiN layer (and convert to Na⁺again), a slower process. The white arrows indicate the paths of the Na⁺ and Na atoms (green circles). Image reproduced from [91]

3.3. The physical mechanism of performance regeneration

3.3.2 Regeneration during light exposure

The mechanism of recovery that occurs during the day, when the cells are exposed to negative voltages towards ground, is less known. In [82], the authors show that regeneration can be affected by irradiance. We further analyze the regeneration mechanism in Chapter 4 by reproducing in the laboratory the power recovery under irradiance and with applied negative voltage. The complex interaction between degradation and regeneration still requires further investigation, and one aim of our work is to improve the understanding of this aspect and of how to model it in simulating the performance of PV modules outdoors.

3.3.3 Methods to recover PID affected modules in the field

In addition to the previous power recovery processes, which occur during natural operating conditions, methods to artificially recover modules affected by PID in a PV plant exist.

• One method consists in applying a high positive voltage (e.g. +1000 V) to the modules (see e.g. [130]). As proposed for solar cells in [91], the process that allows for power recovery can be explained with the out-diffusion model also in this case. Indeed, in correspondence to the positive voltage applied, an electric field results inside the SiNx

layer and is such that the Na⁺accumulated at the interface between SiN_xand bulk Si (i.e. in the SiO_x) are driven out of the Si bulk, towards the cell surface. The reduction of Na⁺concentration at the interface inverts the concentration gradient between the interface and the stacking faults decorated by Na, which now starts to diffuse out of the stacking faults. With respect to a purely thermal recovery, the process of out-diffusion with a reverse bias applied is accelerated because a greater concentration gradient of Na is induced.

Some industrial products (“offset kits”) are available on the market use this principle and apply a positive voltage to the full module string during the night (e.g. iLUMEN’s PID box [60] or Pidbull [128]). The module power is thus recovered during the night, thereby limiting the impact of PID.

• Another method to recover degrade modules or strings is to proper ground the strings:

by grounding the negative pole of the inverter all the modules would be exposed to a positive potential relative to ground (see Figure 3.4). However, this is not possible when the inverter is transformerless, a topology that is increasingly employed in the PV industry.

The effectiveness of these methods was analyzed in some recent studies. For example, in [47]

the authors compared the recovery ability of PID-affected strings of modules treated with the first and second method. They found that the grounding solution proved less efficient than the use of anti-PID devices. All the modules treated with the anti-PID device showed a power recovery over the 70 days of test, even if in some cases the level of recovery was lower than

expected and in partial contradiction with the device supplier announcements. In particular, the recovery for the severely degraded modules was very slow, which highlights the importance of an accurate monitoring of PV plants for an early detection of PID.

A drawback for the use of such anti-PID devices is primarily the addition of costs. Secondly, it has been pointed out that applying a high voltage to modules always induces leakage currents that may cause other degradation modes in the modules, such as electrochemical corrosion [38].

In this work, we are interested in studying and modeling the natural evolution of PID. The use of these recovery devices will therefore not be considered. We will on the contrary focus on the thermally-driven power recovery that occurs during the night (Section 4.2.2) and at daytime (Section 3.3.2). From the methodology that we propose in Chapters 4 and 5, starting from indoor accelerated tests on a given module type it is indeed possible to predict the effect of PID on the module in normal operating conditions in the field. In the planning phase, these predictions should allow an investor to evaluate whether a given PV module type is suitable to resist PID for a specific climate condition. The need of recovery devices would increase the OPEX and therefore the LCOE of the PV plant. Additionally, no information about the performance and safety of these kits on the long-term is generally available. Further experience is thus required before envisaging a large-scale adoption of these devices. Moreover, as mentioned, if PID is detected in field modules at an advanced state, the use of recovery devices might not be sufficient to recover the module performance to its initial level. The use of modules resistant to PID is therefore recommended, and in Chapter 6 we treat this topic in detail by identifying combinations of materials that allow us to manufacture modules with good resistance against PID.