Transparent TiO₂ and ZnO thin films on glass for UV protection of PV modules

To stabilize the global temperature and mitigate climate change, the emission of anthropogenic greenhouse gases will have to be greatly reduced. To make it possible, the energy sector will have to transfer from fossil energy to environmentally friendly and carbon neutral sources.¹

Solar energy exists in abundance. In roughly 90 minutes, the solar energy that reaches the earth equals the consumption of all human societies globally during one year.² Only a fraction of this energy is captured today, and photovoltaic (PV) modules account for a marginal part of the electricity production worldwide, around 1.8% at the end of 2016. In recent years, however, the sector has been growing exponentially at a rapid rate, which means that the ability to increase efficiency and lifespan of PV modules is interesting from an energy perspective.³

PV modules consist of a number of interconnected PV-cells, embedded in an encapsulant and a protective cover glass on the top. One of the issues facing the PV modules available today is the degradation of their encapsulant, which most often consists of ethylene vinyl acetate (EVA).

It is damaged by UV radiation with wavelengths below 350 nm. The UV radiation makes the encapsulant degrade and acquire a yellow and eventually brown hue, which reduces the efficiency of the PV modules.^4,5

Developing the cover glass has become increasingly important as the share of cost for the cover glass is high.⁶ The cover glass^7,8 has several important functionalities, e.g., providing optimal light capture, rigidity, mechanical protection, and chemical protection. Optimal light capture depends on the optical properties of the cover glass, such as absorption and reflection. The latter comprises the largest part, about 8% for a typical flat glass, which can be minimized by employing antireflective coatings.⁹

The optical properties of flat glass^10,11 are affected by the presence of iron impurities in the glass melt as the iron in the glass increases the absorption of light in the glass in the UV-Vis region of the electromagnetic spectrum. Iron can be used as a colorant of glass, giving the glass a green tint.¹² In some cases, this is a positive feature, e.g., when UV-protection is needed in beer and champagne bottles.¹³ In other cases, as with PV-modules where transparency is coveted,¹⁴ the iron in the glass is considered as a contaminant. In these cases, low-iron glass, where measures have been taken to reduce the iron in the glass, is frequently used.

In the case of cover glass for PV modules, the trend has been to use low-iron glass to increase transmitted light.⁸ A drawback to this type of glass is that a larger amount of high-energy UV radiation is transmitted, which is harmful to the encapsulation material EVA that is used in most PV modules today.¹⁵ When UV radiation below 350 nm reaches the PV module, both the semiconductor material¹⁶ and the laminate^5,17 are degraded. The degradation of the EVA laminate is the major reason for the annual degradation of 0.6–2.5%.^17,18 As a result of the UV radiation, EVA degrades and loses some of its high transmissivity as it gets a yellow/brown hue and eventually starts to delaminate, letting moisture into the PV modules, which leads to failure of the PV module.⁵

In the current study we have investigated float glass coated with ZnO and TiO2 thin films by spray pyrolysis of organometallic compounds of zinc and titanium (Table 1 and 2).

Results

Glass coated with ZnO showed a trend to shift the UV-cutoff to longer wavelength as well as lowering the optical band gap of the coated glass sample. The major reason for this is likely to be caused by tetrahedrally coordinated Fe³⁺ having an absorption peak at about 380 nm but also being sensitized by the presence of the ZnO coating. Such a trend is less clear for the samples coated with TiO₂.

Both sample series showed a significant increase in total reflection for the normal incident light due to the higher refractive index of the thin film oxide coatings (Figure 1a). However, the increase in diffuse reflection was significantly lower, less than 4% (Figure 1b); this is an advantage for application on the cover glass of PV-modules where most of the incoming light will be of diffuse character.

The coated glass showed a potential improvement in life expectancy of PV modules through a decrease of destructive UV-radiation transmission to the encapsulant up to a relative 36.0% and 54.3% for TiO₂ and ZnO coatings, respectively. Additionally, although the coated samples have shown a relative transmission reduction at the useful spectral region up to 21.8 and 12.3% for TiO₂ and ZnO coatings, respectively, the transmissivity degradation of the encapsulant should be effectively prevented.

For ZnO it is evident that the Fe³⁺ content plays an important role for the UV-blocking activity, which would be a tradeoff between limiting the glass’s iron content while still having enough UV protection. Furthermore, ZnO-coated glass also showed potential regarding down conversion of UV light to visible wavelength with peaks at 377 nm and 640 nm. Thus, ZnO is feasible to be investigated for application as coating to cover glasses of PV modules but must be optimized as there is a tradeoff between UV-blocking and transmittance in the useful spectral region for PV modules (Table 3).

Implications for PV modules

We have shown that UV blocking can be achieved with the cost of reducing the transmittance. This opens the possibility for maintaining UV protection and gaining useful energy for the PV by lowering the Fe₂O₃ content in the glass without compromising the service lifetime of the PV module. The energy balance for transmitted and useful light for PV modules will be possible to model and optimize in future studies based on information as, for instance, possible limits for Fe₂O₃ content, cost, and efficiency.

Furthermore, photon energy down-conversion, i.e., photoluminescence, can be an advantage and a route to utilizing UV light while still not exposing the PV cells to UV light. As for disadvantages, we can list higher reflectivity and scattering. If the surface coating is properly structured, it might not be a serious disadvantage or perhaps even an advantage,¹⁹ as the diffused light contains in fact more photons than the direct light of normal incidence. This is especially valid for façade-applied PV modules where there is in fact very little solar radiation of normal incidence.

Another parameter not previously mentioned is the factor of heat. A photon’s energy that is not converted into electricity is transformed into heat that in fact lowers the efficiency of the PV module. Beyond the scope of the current paper we would also like to draw the attention to making crystalline ZnO or TiO₂ coatings having similar beneficial properties but with the added value of photocatalysis^20,21 and hydrophilic behavior with UV exposure,^22,23 thereby giving PV-covered glasses reduced maintenance. Doped ZnO also offers another dimension as a transparent conductive coating offering possible IR reflection for wavelengths nonconvertible to energy for PV modules.⁸

Acknowledgments

Jakob Thyr at the Ångström Laboratory, Uppsala University is greatly acknowledged for his supervision during the photoluminescence measurements.

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This article is excerpted from “Transparent TiO₂ and ZnO thin films on glass for UV protection of PV modules,” Frontiers in Materials, October 2019 (CC BY 4.0). https://doi.org/10.3389/fmats.2019.00259