Corrosion in crystalline silicon photovoltaic modules and the influence on performance

The work presented in this thesis comprises research into degradation paths that cause corrosion of different components of solar photovoltaic (PV) cells and quantifies the impact of corrosion on the energy yield of PV modules.

PV modules are exposed to different climatic conditions when they are installed in the field. This exposure causes various degradation modes which affect their reliability to produce electricity and their overall durability. Harsh environments can decrease the lifetime of a PV module below 25 years, which is the threshold lifetime suggested for all climates by most PV manufacturers. Crucially, this is also the expected period over which the cost of generated electricity is most often based yet has not been validated given the relative youth of the PV market. Reduction of the PV module lifetime through degradation and failures can significantly affect the financial and technical viability of solar PV as a source of clean electricity and more robust predictions of module lifetime are urgently required.

One very significant cause of failure is corrosion. Environmental moisture penetrates the PV laminates and reacts with the different polymers and metallic parts of the construction, accelerated by higher levels of temperature. From hydrolysis of the most commonly used encapsulant (Ethylene Vinyl Acetate, EVA), acetic acid is produced and attacks further the cell electrical contact metallisation, cell interconnection ribbons and back contact of the PV cells. Corrosion affects mainly the series resistance (Rs) of a PV module, causing severe decrease of the PV electrical power output, and is currently understood to be the second highest cause of energy yield loss of systems installed in the last 10 years.

The main areas requiring research have been identified as: determination of the temporal evolution of the distribution of moisture content within a PV module subjected to a given environment; full understanding of the chemical reactions taking place and separation of the impact of the different component degradation on the PV performance of a PV module; investigation of the corrosion impact on the performance of PV modules exposed to outdoor conditions, for a non-tropical climate.

The paths that the moisture follows within a PV module have been already investigated through theoretical simulations. However, these simulations currently lack rigorous experimental validation, that includes accurate values of moisture concentration accumulated within a PV module. To this end, a method that measures the equilibrium moisture content absorbed by the polymers contained in a PV module is applied, accompanied by a non-destructive method for the quantification of the moisture presence within a PV module. The values that result from the moisture measurements are used for verification of theoretical simulations implemented with COMSOL Multiphysics. Values of moisture concentration at different positions within a PV module are needed for understanding of the corrosion of the different metallic components contained in a PV module.

Published studies of the various mechanisms of corrosion of different components of a PV cell exist but stop short of relating this to power output. This is addressed here, in which a method that separates the impact of each mechanism on the degradation of the electrical power output is presented, thus allowing prediction of specific failure modes for a given bill of materials and operating environment. Half encapsulated PV cells (with either front or rear side exposed) are immersed in acetic acid baths or stored in an environment of high level of relative humidity and temperature (damp heat conditions). The results are compared to the findings of fully encapsulated PV modules aged under the same damp heat (DH) conditions. Both electrical and material characterisation are involved for identification of the different mechanisms. Results show that the most severe degradation is caused by accumulation of acetic acid on the front side of the solar cells. Although aluminium reacts severely with moisture, the aluminium back contact corrosion was found to be a negligible failure mode for fully encapsulated PV modules. Additionally, the impact of different back sheets on corrosion is studied, which again seems to be negligible, as the moisture accumulated at the front side of a PV cell is mainly affected by the diffusion coefficient of the encapsulant.

Finally, the impact of corrosion on PV modules installed in the field is poorly addressed in current literature, especially for PV modules exposed to non-tropical climates. The simulations available are not based on the physics of the degradation mode, but are empirical relationships whose parameters are extracted by fitting to indoor performance data. This approach makes these methods unreliable for outdoor predictions. Moreover, they are not validated against real outdoor data. The research in this work has achieved a method to partially understand this effect, as it is very difficult to separate the electrical signatures of different failure modes that occur simultaneously. The method involves the estimation of the series resistance evolution of PV modules operating at Loughborough University for 7 years, combined with visual inspection. The results show that seven years is a short period to observe significant corrosion in a temperate climate, but the method applied is adequate for its detection.