Solar cells convert sunlight into electrical energy. Light that is incident on (in most cases) the silicon wafer – the so-called absorber – is captured and releases negative and positive charge carriers within the material. These are separated in an electrical field and flow to the front and rear sides of the respective wafers. There they are collected by metallic bus bars – one for the positive and one for the negative charges. The absorber (or the “electrical field” within the absorber) as well as the “front and rear contacts” can be found in every solar cell. Once the cells are produced, the individual solar cells are electrically connected to one another and laminated between two glass panes or between a glass pane and a rear film.
This therefore creates a “solar module” in which the solar cells are protected against rain, hail and UV radiation for at least 20 years. Solar modules typically have a surface area between 0.7 and 1.6 m², but smaller or larger sizes can be produced if required. The materials researchers at ZSW are specialised in CIGS thin-film technology and the development of new material systems.
In addition to crystalline silicon, three alternative absorber materials are used for manufacturing solar cells: amorphous silicon (a-Si) or a combination of amorphous and microcrystalline silicon (a-Si/μc- Si), the compound semiconductor cadmium telluride (CdTe) or a compound semiconductor made of copper, indium, gallium and selenium (CIS or CIGS for short). Solar cells made from the three aforementioned materials are called thin-film solar cells because the absorbers are only a few micrometres thick. Only 0.2 kg of the semiconductor materials is required as the absorber for modules with an output of 1 kW. These absorbers are not self-supporting like silicon wafers but are deposited on substrates, which are mostly glass panes. The manufacture of thin-film modules therefore differs fundamentally from the manufacture of silicon-based technology. Solar modules with already interconnected cells are processed instead of individual cells. The contact surfaces, absorber and additional intermediate layers are deposited on large glass panes in integrated processes. The layers are then laminated together under a second glass panel to form a finished solar module. All three aforementioned thin-film technologies have reached industrial maturity. In 2014, the total global production of photovoltaic modules with a-Si, CdTe and CIGS absorbers amounted to 3,144 MW, which comprised 8% of the total annual production of solar modules.
Today, CIS or CIGS technology is the thin-film technology with the highest levels of cell efficiency. ZSW used to be the record holder several times, last in 2016 with a record of 22.6%. The record stands now at 22.9 % (Solar Frontier). With this value, CIGS has the best qualifications for further strong market growth. This is real thin-film technology (the total thickness of all the layers is just a few thousandths of a millimetre on substrates made of window glass or metal or plastic films), and is mostly based on processes which have already proved their worth in architectural glass coatings. This results in a low level of material and energy use during production. In principle, this makes energy payback times of less than a year possible.
While the Mo and ZnO contact layers are usually produced using cathode sputtering or the CVD process (chemical vapour deposition), different methods are used for CIGS semiconductor deposition. For example, CIGS can be produced directly using co-evaporation under increased coating temperatures. Up to now the highest levels of efficiency have been achieved at ZSW using this method. Alternatively, Cu, In, Se or S can also be applied as preliminary layers in a cost-effective way by using print and galvanic processes as well as cathode sputtering. The layers are then crystallised under increased substrate temperatures in the Se or S atmosphere.
The image shows the basic layer structure of a solar cell made of Cu(In,Ga)Se2 in an electron microscope cross-section.
The series connection of the individual cells is a key step on the path to creating entire modules from cells, which ZSW accomplishes using monolithic, integrated series connection. Individual layers are separated and overlapped during the manufacturing process to produce series-connected cell stripes.
The principle is illustrated here. The respective individual layers are separated either mechanically or with a laser. Monolithic series connection represents a significant advantage that thin-film technology has over crystalline solar modules, since silicon wafers no longer have to be processed individually to create cells that then have to be subsequently interconnected to form a module using contact strips. All processes take place directly on the final substrate size and can be fully automated from the beginning.
Once the raw module is finished, the edges are usually de-coated (removal of the active layers in the boundary area of the module) to prevent possible short circuits between individual cells, for example across coated glass edges, prior to further processing. This is followed by contacting using metal contact strips, and encapsulation. This last step ensures that solar modules can withstand extreme climatic conditions for several decades.
A widely used standard encapsulation material is EVA (ethylene vinyl acetate). Additional encapsulation films are also being used and tested at ZSW. In particular in regards to flexible encapsulation, there still is a considerable need for research since the requirements are even higher here than for glass modules. One considerable challenge is to provide protection against the ingress of moisture. The encapsulants and the test modules created with them are characterised in terms of their processability, UV stability, heat-moisture stability, etc.
At present, ZSW holds the efficiency world record for CIGS thin-film solar cells on a laboratory scale with 22.6% (see press release of June 2016). This is the highest level of efficiency ever achieved by any thin-film technology.
Here is an overview of the current maximum efficiencies of our solar cells and modules on glass substrates:
Researchers are currently working on an improved material system for this layer in order to improve the efficiency of the cell as a whole but also to avoid having to use cadmium in the buffer layer of the thin-film cell. At ZSW, the intermediate layer system consisting of cadmium sulphide and zinc oxide has been replaced by a combination of zinc-oxide sulphide and zinc magnesium oxide. The new combination promises a higher light performance than the material used in current CIGS cells.
A cell efficiency of 21.0% was achieved with the new buffer layer (CBD Zn(S,O)).
The image demonstrates electricity generation in a cell with a Zn(S,O) buffer in the short wavelength range, which, however, is being overcompensated by voltage loss.