EVOLVING SOLAR PANEL TECHNOLOGIES - A BRIEF INTRODUCTION

All solar (PV) cells use the same basic construction. PV cells are made of special materials called semiconductors.

Light contains a particulate matter called Photons. When light strikes a PV cell, a portion of the photons is absorbed within the semiconductor material. The photons knock electrons loose, allowing them to flow freely. PV cells all have one or more electric fields that act to force the freed electrons to flow in a certain direction. This flow of electrons is known as "current", and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's Voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

For years, companies have been looking for a way to reduce manufacturing costs associated with traditional photovoltaic (PV) technologies. As the industry continues to evolve, several types of solar technologies have been and continue to be introduced.

"First generation" systems include silicon solar cells. They are made from a single silicon crystal (mono-crystalline), or cut from a block of silicon that is made up of many crystals (multi-crystalline - shown below).

Thin-film technologies are usually classified as either a "second generation" or "third generation" panel technologies, and have significantly reduced manufacturing costs as compared to the "first generation" PV systems.

Today, "first generation" solar panels still provide the most power per square foot of surface area, but their physical characteristics may make them difficult to use in some applications. Even so, rigid silicon-based solar panels are widely considered to be the best technology today, and they are also still the most expensive type of Photovoltaic technology to manufacture. Efficiencies for rigid first generation panels are today average around 20%, and have set the standard for other PV technologies.

"Second generation" thin-film solar cells are less expensive to produce than traditional silicon solar cells as they require a decreased amount of materials for construction. The thin-film PV cells are, just as the name implies, a physically thin technology, that has been applied to photovoltaics. They are only slightly less efficient than other types but do require more surface area to generate the same amount of power.

Additionally, thin-film solar panels often have the benefit of being "foldable" or "rollable", making them easy to store and transport. Their physical attributes make them ideal in applications where only low power is required, and extreme portability is a concern.

For a more detailed look at how photovoltaic cells operate, please click here.

For a brief introduction to the Solar Stik's™ panel technology, please visit the Synopsis of the Solar Stik™ page.

WHAT IS A "THIN-FILM SOLAR PANEL"?

There are many types of thin-film solar technologies available in the Portable Power market. The two most common types of technologies are Amorphous Silicon and C.I.G.S. (Copper Indium Gallium Selenide)

While each of these will provide the user with a reliable power source, the advantages of either technology are often best realized by choosing the right one for the intended operating environment.

"Thin-film" refers to the construction of a VERY thin layer of a substance on a substrate. Thin is a relative term. In comparison to the first silicon crystal solar cells produced, current thin film cells are 1% of the the thickness. Naturally these are of great interest to the solar manufacturing industry, as many more cells can be made with far less material. With single crystal or multi-crystal cells the materials were up to 40% of the unit cost. Thin-film technology slash that dramatically. There are a variety of materials that are used in the production of thin-film solar cells.

THIN-FILM SOLAR PANEL TYPES

Amorphous Silicon:

Amorphous silicon is the oldest thin-film technology and is therefore the best developed. Amorphous silicon refers to silicon atoms that can be laid down on a backing substance but do not need to be set into a grid in order to display semiconducting behavior. This allows it to be deposited onto large areas with relative ease. Amorphous silicon does not conduct as well as crystalline silicon because the connections between the silicon atoms are not as consistent and so electron flow is more interrupted. Still, an amorphous silicon solar cell can achieve an initial efficiency of around 15%, sometimes more, so they are excellent value compared to the thick silicon wafer.

Amorphous silicon can be placed on a wide variety of substrate materials, making them more adaptable to varying field applications. For example, a polymer plastic can be used, which is more flexible than a metal substrate. This means that the panel may be "rolled" up, whereas a panel that uses a metal substrate would not be rollable.

C.I.G.S. Thin Film Technology:

CIGS is short for Copper Indium Gallium Selenide (C.I.G.S.), which are the elements used to make the photoelectric layer in this type of thin film solar cell. The principles behind the operation of the CIGS solar cell is the same as that of the silicon solar energy cell.

Copper acts to receive electrons, making it the same as the positive silicon layer (P-type silicon). Selenium provides extra electrons to act in the same way as the negative silicon layer (N-type silicon).

These materials can be placed onto a variety of substrates including thin flexible steel, glass, and various polymers. At present the most widely used substrate is flexible steel as it is the most resistant to the high temperatures needed for the process of laying down the PV elements onto the backing sheet.

CIGS technology is improving steadily. Current CIGS thin film solar cells with a steel backing sheet having laboratory efficiencies of up to 17%.

LOW-LIGHT PERFORMANCE

Compared to a crystalline silicon panel, an amorphous silicon panel of comparable size produces less power but performs better at lower light intensities, making it a good choice for environments where interrupted sunlight is the norm. The CIGS panel has better performance than the amorphous silicon in terms of maximum output, but worse low light performance.

THE EFFECTS OF GROUND-PLACEMENT

Flexible panels, like rigid panels, require the same ingredients for maximum power production. These ingredients are:

1.  DIrect Sunlight

2.  Cool Operating Temperatures

3.  MPPT charge controls

A thin-film array is typically deployed on the ground due to their physical size.  Ground placement usually means that the PV panels face three negative effects:

1.  Indirect Sunlight

2.  High Heat

3.  Panel Surface Buildup (dirt, grass, dust)

If the operator is using thin-film panels, it is a good idea to account for a 40% daily loss of the PV array's rated power output due to these factors.  The easiest solution to these proclivities are to deploy extra panels.

DEGRADATION OF THIN FILM SOLAR PANELS -

"LIFE EXPECTANCY"

Like their elder counterparts, Thin-film photovoltaic technologies will produce power for many years.

It is the physical construction of a FLEXIBLE thin-film solar panel that typically limits the lifespan of the panel.  Construction factors include the integration of fabric, plastics, and other materials that will degrade with use and exposure.

For example, when used in austere environments, a CIGS panel with a heavy-duty plastic or stainless steel substrate will usually last longer that an Amorphous Silicon solar panel of the same power rating, but lighter-duty construction. It is prudent to examine the physical construction of any flexible solar panel technology before choosing it for a certain application. 

Additional factors include proper care and maintenance by the user, and the operating environment where it is used.

Flexible thin-film panels require much more care and maintenance than rigid panel technologies. It is important that the buyer beware of cheaply produced thin-film panels, as the PV technology may be well crafted, but if the panel construction is poor, the PV may become unusable in a very short period of time.

OPERATION

Connection of Additional Panels - Many thin-film panel manufacturers have "daisy chain connectors" that can be used for the connection of additional panels.  Adding panels is simple and allows the operator to scale their system up or down according to their power requirements; however, there is a limit to the scalability of a thin-film PV array.  Connecting additional panels DOES NOT always provide an increase in power as many thin-film panels use small wiring, which will limit the amount of power it can safely conduct.  Be sure to consult the thin-film panel manufacturer's literature for information regarding the addition of panels to an existing PV array.

It is also important that the same panel technologies be used in an array, especially when using MPPT charge controls.  Different panel technologies operate at different Voltages, which can negate the effect of using an MPPT charge control to obtain maximum power production.  For example, a flexible Amorphous Silicon panel SHOULD NOT  be connected into a circuit that also uses rigid mono-crystalline panels.  If it is connected, no damage will occur, however a significant degradation of the total power from the panel arrays will be the result.

Connection of Appliances - Many thin-film panels have standard 12VDC receptacles that can be used for the connection of a 12VDC equipment.  As a general rule of practice, an appliance that OPERATES on DC power should NOT be connected directly to a PV array for operation. A PV array is an unregulated power supply, which may damage an appliance if direct connection is made; HOWEVER, there are some exceptions. Nearly ANY type of rechargeable device that can be RECHARGED from a DC power source CAN be connected directly to a thin-film PV panel.  For example:  if the user needs to recharge a cell phone, camera, laptop computer, or other electronic device, 12VDC adapters are often available that allow the equipment to be recharged using a 12VDC source.  If the intended appliance does NOT have a 12VDC plug adapter available for it, then it should NOT be connected directly connected to a 12VDC power source.

Charge Controls - If a thin-film panel is connected directly to a 12VDC battery, there is a risk that the battery may receive an "overcharge", which can cause damage.  Visit the Battery School page to learn more about battery charging.  Regulating a panel's charging current is always recommended.  MPPT charge controls are the most beneficial for any battery-based solar power system.

WHICH THIN-FILM TECHNOLOGY IS THE BEST FOR PORTABLE APPLICATIONS?

There are many manufacturers of thin-film solar panels, and most sales-pitches proclaim the general operating characteristics of their particular panel.  It is often incumbent upon the user to determine which technology or package will BEST serve the user's actual intended application.

A distinct advantage that anytThin-film flexible panel has over its rigid panel sibling is that it can be transported in a much smaller package.  Thin-film panels can be folded or rolled into a compact package that is often necessary for "man-portable" applications, however, as outlined above, transport conditions should be fully considered when choosing a Thin-film panel

It is important to invest in a panel that will withstand harsh environments including heat, inclement weather, and rough physical treatment.

For "Portable Power" applications, we recommend the use of Amorphous Silicon Solar Panels manufactured by PowerFilm™ for the following reasons:

1. Ruggedness - The PowerFilm panel technologies are physically MUCH stronger than other comparative thin-film technologies. The construction of the PowerFilm™ panel is performed using a plastic substrate, yielding lighter weight and increased panel flexibility as compared to CIGS. It can handle more abuse and punishment that is often experienced during the "real-world" conditions of military, disaster-relief, and humanitarian missions.

2. Temperature Dependency - With any thin-film solar panel, heat is a major factor in the panel's ability to produce power. Compared to amorphous silicon thin film, the power degradation of CIGS is three times as high, so for every degree that the temperature of the photovoltaic cell rises, the panel loses 3 times the power output as compared to the amorphous panel. Even though a CIGS panel DOES have a slightly higher rated output per square foot of surface area than the amorphous silicon panel, the tendency for a CIGS thin-film panel to degrade its output due to cell heat negates any power gain that the CIGS panel would provide.

3. Storage Resistant - As with most mobile applications, transport conditions often expose the panel to high heat. It is not uncommon to store a thin-film panel in a dark place such as a protective case, vehicle trunk, or man-pack. Exposure to HIGH HEAT in DARK environments has a detrimental impact on the CIGS panel technologies. It is typical for a CIGS panel to require "Light Soaking" after dark, hot storage conditions before it can be used. It can often take as much as 72 hours of direct exposure to sunlight for a CIGS panel to regain its rated power generating capabilities. Unlike CIGS thin film, the amorphous silicon panel will produce its rated power immediately after exposure to high heat and dark conditions. It's ability to operate at rated capacity is NOT degraded after high heat exposure in dark environments, which is a critical factor in most mobile applications where power may be demanded at a moment's notice in austere conditions.

All flexible solar panels produce DC power and they are usually available in common Voltages such as 12 or 24 Volts DC.

Additional education & information about the Solar Stik™ System including batteries and inverters can be found in our "Education" Pages.