Thin Film Solar Panels

Introduction

As the name suggests, thin film PV employs a very thin layer of semiconductor – usually just a couple of microns thick – in place of a traditional silicon wafer. Simpler to manufacture, thin film solar panels make more efficient use of raw materials and energy and results in both lower costs and a smaller manufacturing carbon footprint.

There are three types of thin film product:

• thin film PV modules (panels);
• thin film solar glass;
• thin film membranes.

This page concerns thin film modules and thin film glass. Product specifications for thin film solar glass are also available on our solar glass page.

A typical thin film solar panel consists of the semiconductor and several other thin films bonded to a sheet of glass, covered by another sheet of glass and sealed in with an industrial laminate. Some companies and researchers are focused on the development of alternative formats, including flexible thin film solar modules.

A thin film membrane has solar cells embedded in a flexible membrane.

Why thin film?

• Better low light performance. Thin film performs better in lower light, and is recommended for applications with a significant proportion of diffuse light (produced by reflection and light scattering), and in cloudy or dull weather climates.  It may also be beneficial for use in shaded environments. The performance variation is due to the range of wavelengths absorbed by the different glazing types. The sun’s spectrum ranges from short wavelength UV light to long wavelength infrared light. In contrast to crystalline cells, which absorb primarily long wavelength radiation (red light spectrum), thin film solar cells can absorb a wide spectral range, in particular thin film can absorb the short wavelength blue light on cloudier days or when the sun is low in the sky.
• Better ‘extreme’ angle performance. Thin film performs well at the more extreme angles to the sun and thus works well for vertical surfaces and northerly aspects, allowing north facing facades of buildings to be included in the BIPV project.
• Performance in shaded environments. Shading can significantly affect the yield of a PV system. In the built environment, shading can be caused by trees, neighbouring buildings, dirt or even the mounting system. Shading may change over time due to plant growth, new buildings or dirt build-up. 

Such sources of shade can be minimised by careful planning in order to maximise the incident solar radiation. Simulations of the daily and yearly path of the shadows can be carried out to enable the position of the solar modules and the orientation and structure of the building to be optimised accordingly. If shading cannot be completely avoided, its effects can be mitigated by using the appropriate module technology, the module design and the electrical connection of the modules best suited to the environment. 

A key difference between crystalline silicon modules of thin film technologies is their shade resistance due to string length. Thin film modules are designed to be high voltage and so are linked in parallel rather than in series in long strings. This means that if one module is shaded the rest of the system is not affected. Each module is therefore optimised. This effect can be replicated on crystalline modules with the use of a product such as SolarEdge, with one power optimiser per module, however this typically adds 10-15% to project costs.

• Thin film modules, in particular transparent or double-glazed modules can cope much better with temperature build-up, even where no ventilation is possible.
• The standard ‘rating’ of panels is made at 25°C. In reality, even in the UK, the module surface temperature can reach 55°C – 70°C in summer, reducing yield by 15% + for silicon cell, compared to 5% + for thin film.
• Performance degradation with increasing temperature. One feature of solar PV panels is the degradation in performance that occurs with increasing temperature.  With silicon cells, performance drops off at a rate of 0.5% per Kelvin; the equivalent thin film drop-off rate is around 0.17% per Kelvin.
• Good structural strength. Tough rigid thin film panels typically exhibit better structural strength.
•  Outstanding aesthetics.

Thin film solar PV: the techy stuff!

Thin film panels perform much better than ordinary panels at high temperatures. The ‘rated output’ of a panel is actually quite misleading because it is established at ‘standard test conditions’ when the cell operating temperature is at 25°C. In reality cell operating temperatures are much higher than 25°C, even when the outside air temperature is 20°C.

So at 20°C outside air temperature an ordinary panel will under-perform its “rated output” by about 8.8%. By comparison a thin film panel will under-perform its “rated output” by 5.85%.

Read on if you’re a techy...

The “rated output” of a panel is determined at a cell temperature of 25°C, (Standard Test Conditions (STC) meaning solar irradiance of 1,000 W/m², zero angle of incidence, solar spectrum of 1.5 air mass and 25°C cell temperature).

In fact at an outside air temperature of 20°C, the cell temperature will be much higher than 25°C, and the output will be lower as a result (output decreases with increasing temperature).

The extent to which the output will fall is determined by the “Nominal Operating Cell Temperature” (NOCT) and the Maximum Power Temperature Coefficient.

The NOCT is defined as:

• the cell operating temperature at “real world” irradiance of 800 W/m², ambient air temperature of 20°C, wind speed of 1 m/s, module tilt angle 45°C.

The Maximum Power Temperature Coefficient is essentially the change in power output due to increases in cell temperature above 25°C.

For example, a St Gobain thin film module has a NOCT of 40°C and loses 0.39% of its power output for every 1°C increase in cell operating temperature above 25°C.

So at 20°C outside air temperature and 40°C cell operating temperature, it will under-perform its “rated output” by about 5.85% (=0.39% x (40°C – 25°C)).

In contrast, an ET module has a NOCT of 45°C and loses 0..44% of its power output for every 1°C increase in cell operating temperature above 25°C.

So at 20°C outside air temperature and 45°C cell operating temperature, it will under-perform its “rated output” by about 8.8% (=0.44% x (45°C – 25°C)).