Maximum photovoltaic module efficiency requires that incident sunlight is not reflected en route to the absorber layer …
by Dan M.J. Doble, John W. Graff, Fraunhofer Center For Sustainable Energy Systems, Massachusetts Institute of Technology, Cambridge MA USA
Maximum photovoltaic module efficiency requires that incident sunlight is not reflected en route to the absorber layer, and that light that does enter this layer is not subsequently reflected back out, or transmitted through the cell.
A variety of anti-reflection technologies have been deployed in PV modules and the topic remains the subject of active research. Anti-reflection technologies can be broadly split into two categories: 1) Anti-reflective coatings (ARCs) reduce reflection at interfaces above the light absorbing layer of cells; and 2) Texturized surfaces serve the dual purpose of increasing light transmittance and also trapping light within the absorber layer. Often the most effective strategies employ a combination of these techniques.
Figure 1. Reducing reflection by a) exploiting interference effects with a single-layer ARC; b) a textured surface for reduced reflection.
The choice of appropriate anti-reflection technology is a complex optimization of factors that includes more than just performance, but also reliability, manufacturability, and ultimately cost.
The Fresnel equations describe what happens when light travels between two optical media of differing refractive index. Two possibilities are allowed, transmission and reflection, and their ratio depends upon the index of refraction of the materials and the angle of incidence.
An interface with a large mismatch in refractive index will result in strong Fresnel reflection. All of the common cell absorber materials (silicon, cadmium telluride, and copper indium gallium diselenide) exhibit relatively high refractive index. In the case of bare silicon, for example, this causes over 30% of the incident light to be reflected with the remainder transmitted through the interface.
Traditional ARCs are employed to reduce reflection by exploiting interference effects. Consider a thin coating on top of a silicon wafer (Fig. 1a). Incident light partially reflects at the first interface, and partially transmits. The same thing happens at the second interface. A series of multiple reflections takes place within the ARC layer, which add together to form a composite reflected and transmitted wave.
It can be shown mathematically that reflection is minimized when the optical thickness is one quarter of the wavelength of incident light and when the refractive index of the intermediate layer is
A single layer cannot be a quarter wavelength over the entire spectral or angular range of interest, and this means that best performance is only achieved at one wavelength and angle. To determine the optimal thickness, the spectral response of the cell and the spectral content of the incident radiation must be considered, as well as the expected incident angle distribution. These factors themselves are often approximations.
The availability of materials can be an additional constraint; materials with the necessary properties that can be economically deposited may not exist, or good candidates may not exhibit sufficient durability. These problems can be partially overcome by applying a multi-layer ARC structure with more traditional materials.
An alternate method to achieve an ARC structure features a continuous and gradual transition in refractive index between the materials either side. This graded index approach can be approximated by multiple discrete layers, each with slightly different index than the preceding layer. As the effective index is an average of the parent bulk material and air, recent developments in materials with nano-scale porosity have enabled a wide range of available refractive index. Solutions for both cells and module glass have been applied that utilize this technique .
Another method to reduce reflection losses involves larger scale texturing of a surface. This approach increases the transmission at the interface by allowing light that is initially reflected to try again (Fig. 1b). When combined with a reflective surface on the back of the cell, texturing can significantly increase optical path length in the semiconductor, and therefore the probability of absorption. In this way, the path length of light in crystalline silicon cells can be increased by a factor of 6 or more.
The geometric structure of the surface texture is very important for optimizing transmission of incoming light and the trapping of internally reflected light. Inverted pyramid structures have been shown to be particularly effective for this purpose.
Modules and cells
While the semiconductor-air interface receives quite a bit of attention, in practice nearly all PV modules consist of two interfaces for which Fresnel reflection is the greatest concern: first, between air and the glass sheet, and second, as light enters the high-refractive index cell structure absorber. Reflection at both of these interfaces must be addressed in order to maximize performance.
An example of an air-glass ARC successfully used in modules features alternating layers of physical vapor deposited (PVD) silicon nitride and silicon oxide. In addition to reduced reflectance, this hard coating stack can improve the durability of the modules to weather, abrasion, and pollution. Also, the coating reduces transmission of infrared light below the band-gap of most absorber materials, which can increase module efficiency due to reduced operating temperature .
Texturing techniques have also been applied to modules, using glass cutting and grinding processes to produce a variety of different patterns. The greatest improvements are reported for inverted pyramid structures, with an efficiency gain of 3% under standard conditions and 10% for light that is incident at 70°. Of course, there is the risk that such features may trap dust and rainwater in certain locations, resulting in soiling of the module and power loss. For this reason, grooved patterning is also available, which allows the free flow of water from the module surface.
Reflection from the second interface (at the absorber surface) is managed with similar techniques, though the index contrast is typically more significant. In the case of amorphous silicon thin film cells, the surface of the overlying transparent conducting oxide is usually textured, which ensures a textured surface to the subsequently deposited amorphous silicon. For crystalline silicon cells, silicon nitride coatings formed by chemical vapor deposition (CVD) are commonly used as single layer ARCs. It is interesting to note that due to the high index of the encapsulant, thin films of materials with higher refractive index, such as titanium dioxide, actually produce lower reflectance. However, manufacturing issues favor silicon nitride, as well as the additional benefit of supplying excess hydrogen, a proven bulk-passivation agent.
Visual observation will show that cells using a single layer silicon nitride ARC are shiny blue or purple, indicating that although reflection is greatly reduced, there is nevertheless reflection at some wavelengths. In advanced crystalline silicon solar cells, surface texturing is often combined with an ARC to give an exceptionally low total reflectivity and black appearance.
Some recent innovations in antireflection take inspiration from nature. When examined under an SEM, the surface of the cornea on a moth’s eye is seen to have a textured surface in which features are which are roughly 200nm in height and spaced in a hexagonal pattern with centers approximately 300nm apart. This sub-wavelength profile is equivalent to a graded index, which reflects very little light because there are no abrupt index changes. The result is exceptionally low reflectance and high transmission of light from any direction, which is important for the moth’s night vision capability as well as reducing its visibility to predators. Similar nanoscale texturization has been experimentally applied to surfaces used in photovoltaics via holographic lithography processing, sol-gel liquid processing, and plasma etching techniques .
The choice of anti-reflection technology depends upon many factors. In addition to the gain in light transmission, and hence improved efficiency, the cost, manufacturability, side benefits and environmental stability are all critically important. Looking to the future and the anticipated reduced cost of solar photovoltaics, any further improvements in anti-reflection technology will have to be balanced against cost. Whereas there is obviously much need for improvement in module efficiency, there will be progressively diminishing returns from further improvements in anti-reflection. It may be that in the future, the most significant innovations in anti-reflection will not be those that result in the transmission of incrementally more light, but rather those that can be manufactured on a massive scale at a significantly reduced cost.
Dan M.J. Doble received his PhD in chemistry from the U. of Nottingham in the UK. He is group leader, photovoltaics, at the Fraunhofer Center For Sustainable Energy Systems, Massachusetts Institute of Technology, Cambridge, MA, USA; [email protected]; www.fraunhofer.org. John W. Graff received his PhD in electrical engineering from Boston U. He is group leader for energy device prototyping at the Fraunhofer Center For Sustainable Energy Systems, MIT.