Thin film technology could turn most of a building into a solar collector. Phil Kreveld reports.

Thin film photovoltaic (PV) panels are making their presence felt in Europe but are yet to have any substantial effect in our market.

The technology behind the new panels is well established and offers important advantages, but in Australia we are wedded to thick silicon crystalline and amorphous panels.

There is a little ‘movement at the station’, in that the soon to be disbanded Australian Renewable Energy Agency (ARENA) has been working with BlueScope Steel to develop roofing with integral thin film silicon technology.

ARENA says the experimental building material has been designed with thermal ducting to supplement air-conditioning. However, this technology may well stall, as it was developed with a grant of $2.3 million dollars from ARENA.

Thin film technology comes in various guises, the experimental version from BlueScope being a silicon crystalline thin panel structure.

On the world stage, and very much in Germany, thin film developments are used along with conventional panels.

One increasingly important area is CIS thin film technology, which is well established and poised to eventually take over the market in silicon-based solar panels. CIS stands for ‘cadmium indium selenide’ (gallium is also employed, so you will also see the technology referred to as CIGS).

Unlike silicon-based technology, CIS thin film panel structures involve a far more efficient production process: manufacturing energy input is 150kWh/m² for CIS and 550kWh/m² for thick silicon PV. In addition, CIS is easy to integrate into building materials, such as PV glazing.

The new application of PV cells goes by the intials BIPV (building-integrated photovoltaics).

PV glazing comprises mosaics – alternating PV cells and transparent areas. In production, the connection between individual cells takes place as the active materials are deposited on a glass substrate. Thin film panels weigh about 3kg/m².

The efficiency of CIGS PV glass in standard test conditions ranges between 9% and 13%. However, this standard rating offers only an approximation of performance in an installation. Panels are rated allowing an irradiance of 1000W/m² and an air mass factor of 1.5 (when the sun is directly overhead at the equator, the air mass is 1.0).

Thermal gain is a problem with glazing, as PV panel efficiency drops with increasing temperature. Therefore convection cooling must be allowed for in this case.

The German company Solarion makes roofing shingles measuring 1300mm x 630mm that incorporate CIGS thin film cells. The shingles are laminated onto coloured aluminium.

Soltecture, another German company, makes industrial facade elements of various sizes that incorporate CIS technology.

Practical examples abound in Germany, but one of the earliest BIPV examples is the Co-operative Insurance Society Tower in the UK city of Manchester. Deteriorated exterior mosaic cladding was replaced with 575.5kW of blue (PV) cells to provide about 180,000kWh of electricity per year.

There is a basic difference in the physics of CIS PV panels with respect to the band gaps of poly-crystalline (amorphous) and crystalline panels.

To review the photovoltaic process. Light quanta (photons) need sufficient energy to raise electrons from the valence band to the conduction band. Electrons in the valence band are more or less tightly held in the structure of PV panel cell semiconductor material.

The lower the band gap – the difference between the upper energy of the valence band electrons and the lower energy limit of conduction electrons – the less energy is required in the photons to produce useful conduction electrons.

The energy of photons is inversely proportional to their wavelength, packing more energy towards the blue end of the visible spectrum and less towards red and beyond to infra-red.

However, the band gap energy difference is not the whole story, as electrons have properties including momentum (think of a spinning top).

For silicon cells, electrons in the conduction band and those in the valence band have opposite momentum, thus requiring additional energy to create a conduction electron. This material is labelled ‘indirect band gap’, and the additional energy brings fresh conduction electrons to the same momentum as the others. (See Figure 1.)

The accompanying diagram illustrates what is going on.

The top image shows the situation for indirect band gap material, for which the minimum energy of the conduction band is ‘out of phase’ with the maximum energy of the valence band. The lower image shows that the conduction energy is minimum when the valence energy is at a maximum.

What happens physically for the indirect case is that the valence electron, newly elevated to an ‘almost’ conduction electron, needs a further energy kick from the vibrating crystallographic lattice.

The CIS/CIGS technology offers intrinsically superior quantum efficiency (production of electron-hole pairs for a given ‘shower’ of light photons) compared with crystalline and amorphous silicon panels.

But by virtue of the much thinner structure imposed by the production process, there is an escape factor that results in lower overall efficiencies.

A typical silicon panel might have an energy conversion efficiency of 15%, and thin film panels might reach 12%. The German companies Shell Solar (CIS) and Wurth (CIGS report efficiency of 13% and 13.1% respectively for their panels.

The monolithic production process for thin film panels, light-capturing surface treatment and ‘dark side’ reflection layers provide a significant advantage in hazy and cloudy weather. Their output of diurnal kWh is often superior to conventional panels, thereby cutting payback periods by 30% or more.

Long term durability of CIS/CIGS thin film panels is crucial, particularly if they are used as building cladding.

Proper encapsulation is obviously important. However, due to the strong ion-binding properties of CIS semiconductor compounds, the energy of chemical bonds between the constituent elements leads to high chemical and thermal stability.

The risk of performance degradation over time is reduced, as the energy of photons in the solar spectrum is lower than the compound binding energies, thus avoiding solar irradiation damage.

The large panel surface areas encountered in building cladding will influence the type of inverter to be used.

Admittedly, economies are gained by using a central inverter. However, when variable shading is taken into account (as panel surface areas increase), the business of maximum power point tracking becomes horribly complicated, if not impossible.

Deploying micro inverters for individual panel strings offers a simpler solution, and a good level of redundancy. With micro inverters, solar modules are installed in ‘parallel’ with only an AC connection (no DC), so issues with any one module will not affect the rest of the array.

So, where to from here? The mooted relaxation of the Renewable Energy Target is a worrying matter if our appetite for new technology is to be whetted.

There are more than 1.1 million solar PV installations, covering about 10% of Australian roofs. Solar accounts for about 5% of the total generation capacity of 56GW and more than 2% of energy.

Solar and wind have already had a big effect on network-supplied energy: annual demand is down from more than 220TWh some years ago to just over 200TWh.