Increasing the efficiency of CIGS thin-film solar cells through alkali treatment
Today, CIGS thin-film technology offers competitive efficiencies both in the laboratory on a small scale and in production on a large scale. This is clearly demonstrated by solar cells with a record efficiency of 23.4 per cent as well as modules with 17.6 per cent efficiency. The aim of current research is to develop new highly efficient processes for the production of CIGS thin-film solar cells and modules. This is intended to increase the efficiency of solar cells and modules and improve the stability and lifetime of thin-film technology.
Potentials of CIGS thin-film technology
The semiconductor layer of a thin-film module is about 2-3 micrometres thick, approximately 100 times thinner than that of a crystalline silicon solar cell. This means that significantly less material is required in CIGS thin-film modules. Provided that the appropriate carrier material is chosen, it is possible to produce lightweight and even flexible solar modules.
The name-giving semiconductor CIGS consists of the elements copper, indium, gallium and selenium (Cu(In,Ga)Se2) and is also known as an absorber. CIGS solar cells consist of several function layers which are deposited onto a glass or foil carrier. It starts with the element molybdenum, which serves as the back contact of the cell, followed by the semiconductor CIGS. The top side of this is covered with a transparent zinc oxide layer, which is doped with aluminium. This transparent layer is called a "window". Between the window and the absorber, the semiconductor, there is a buffer of zinc oxide sulphide (Zn(O,S)).
Within the CISHTec research project, short for "New highly efficient processes for production", scientists are developing new production processes. They are also investigating their influence on the long-term stability of the modules. Central development goals are high quality, low manufacturing costs and an efficiency of 18 per cent for 30 by 30 square centimetre modules.
Optimising function layers and reducing costs
Scientists have further developed the CIGS material system in the laboratory under optimum conditions. They aim to increase cell efficiency to 24 per cent. This requires not only improvements to the absorber but also to all the layers involved, such as rear and front contact, and the buffer. This involved varying the material composition of the function layers and optimising the necessary processes.
The absorber is produced through co-evaporation of the metals copper, indium, gallium and selenium under vacuum conditions. For CIGS solar cells with high efficiency, the metals are evaporated one after the other in a defined sequence and combination. The time this method requires has a significant impact on costs. A new industry-oriented and highly variable CIGS continuous plant now makes it possible to evaporate and therefore grow the approximately 2 micrometre thick CIGS layers almost as fast as in industrial production. The continuous process plant was developed in an earlier project, VariFast CIGS, and was used within the CISHTec project. An efficiency for CIGS solar cells of 17 per cent has already been achieved even with this rapid growth of CIGS layers. This result shows the high electro-optical quality of the absorber.
Alkali elements are very important for high efficiencies of CIGS solar cells. Even the smallest amounts of sodium contribute to a significant increase in efficiency. Within the research project, the researchers have very successfully varied the influence of substrate temperature, substance quantity and duration of aftertreatment on a laboratory plant in a number of experimental series. In addition to sodium, they investigated the other alkali elements potassium, rubidium and caesium. It was possible to successfully transfer the alkali treatment from a laboratory plant to the industry-relevant continuous process, thereby increasing the efficiency of the cells to 21.1 per cent (in-house measurement). A special mass-spectrometric method (ToF-SIMS) was very helpful for the accompanying investigations. This technique made it possible to show how rubidium is distributed in the absorber.
Simulating long-term behaviour in the laboratory
How do different process and material variations over a period of 25 years affect the long-term behaviour of CIGS modules? Investigating this question requires a link between experiments in the laboratory and the actual situation in the field. For example, high voltages in the CIGS module in the field can cause the so-called PID effect (potential induced degradation). The scientists have reproduced this effect in the laboratory and related it to behaviour in the field. The results have shown that a test lasting more than 250 hours in the laboratory is sufficient to represent the stresses in 25 years under real conditions.
Last updated: 03.08.2020