How we work
The long-term goal with our research is to develop thin film solar cells that can be used for renewable electricity production in a large scale. However, going from cells in cm2-size to an industrial manufacturing of m2-size modules without losing too much in efficiency is a great challenge. To succeed, we need processes that are robust, possible to use industrially and cost effective. It is also important to make solar cells with high stability. The solar cell modules are usually sold with a warranty of at least 20 years. We investigate the stability with accelerated life time tests in our climate chambers and on our roof testing site for outdoor climate.
We work with a baseline philosophy where every step in the process is carried out in a specific way, like following a recipe in a cookbook. This way our experiments are repeatable, and with demands on the robustness, scalability and cost effectiveness for every process step we assure that our work is significant for industrial applications. The baseline process for CIGS-solar cells is described below:
The substrates used in the baseline process are either 12.5 x 12.5 cm (2 mm thick) or 10 x 10 cm (1 mm thick) low-iron soda lime glass (SLG). After deposition of CIGS the substrates are usually cut into four parts where every 5 x 5 cm device contains 32 0.5 cm2 cells. Before the back contact is deposited the glasses are cleaned. First, the samples are immersed into a tank with deionized water and detergent. The tank is heated to 60°C and put in an ultrasonic bath. After this, the samples are rinsed in four steps in deionized water, and finally the substrates are dried by spinning in a dryer.
- Back contact
The Mo-contact is deposited with sputtering from a Mo-target. The substrate passes in front of the target for deposition and gets a film thickness of about 350 nm.
There are several methods that we use for deposition of the CIGS layers. One of these are co-evaporation. For this method, the system first has been pumped with turbo-molecular pumps to about one ten-thousandth of a Pa (normal air pressure is around 100 000 Pa). The substrates are mounted vertically and inserted from a load-lock to a carousel in the deposition chamber, facing inwards towards the metal sources. The substrates are then moved through a heating zone, a deposition zone and a cooling zone which takes 60 minutes in total. There are three metal sources in the middle of the chamber with gallium, copper and indium. The selenium is evaporated in excess from a source at the bottom of the chamber. With constant temperatures and similar source filling heights the composition for the CIGS-layer is very similar for all samples and runs, and the thickness is about 1700 nm. In the process, an almost linear gradient of Ga is also formed where the concentration increases from the front contact to the back contact. This gives rise to a small electrical field that pushes the electrons and thereby decreases the recombination at the back contact.
- Buffer layer
The buffer layer is deposited as soon as possible after the samples are brought out in air, usually within 5 min. This is to minimize oxidation and other unwanted reactions with the CIGS-layer that starts to occur as soon as it has been exposed to air. The short air exposure is especially important for alternative buffer layers that are deposited with atomic layer deposition (ALD) which does not include etching of the surface such as for chemical bath deposition (CBD) due to the ammonia.
CdS as a buffer layer is deposited with CBD in a solution of ammonia, thiourea and cadmium acetate. The substrates are immersed into the solution at room temperature and then heated to 60°C in a water bath. The deposition takes around 8 min and after this the samples are directly moved to deionized water to stop the growth process. This gives a thickness of 50 nm on CIGS.
ZnSnO is an alternative buffer layer which is deposited with ALD. The process takes place in 120°C with diethyl zinc, tetrakis tin and deionized water. Both water and diethyl zinc are brought into the chamber at room temperature, whereas the tin compound is heated in a water bath to 40°C to achieve a suitable vapor pressure. Nitrogen gas is used as a carrier gas. The substrates are loaded in the reactor 30 min before the deposition to stabilize the temperature, and the films are then deposited in pulses from the compounds. The buffer layer will then have a thickness of around 10-20 nm.
- Front contact
The front contact is deposited with radio frequency sputtering, which is used when the target that is sputtered is non-conductive. The substrate are stationary at deposition and deposited with one layer of nondoped ZnO (i-ZnO) followed by one layer of Al-doped ZnO (ZnO:Al). The layer of i-ZnO is improving the electrical properties of solar cells with CdS as buffer layer, while it is ZnO:Al which is working as a front contact. Argon is used as an inert gas of low pressure for the sputtering. The thickness of i-ZnO is typically 90 nm if deposited and measured on the glass substrate, and for ZnO:Al around 350 nm.
- Grid deposition
To facilitate the current collection from the front contact, a metal grid is deposited as extra contact. This consists of three layers; Ni/Al/Ni, deposited with evaporation where the grid pattern is defined by a mask. The two nickel layers prevent reaction between the aluminum and the oxygen in the front contact and from the air. For this evaporation, an electron beam is used to heat up the evaporation source. The total thickness is about 3000 nm.
To define the cell areas to 0.5 cm2, mechanical patterning of the cells is used. We also have the possibility to manufacture modules of series connected solar cells with patterning. In this case, the Mo-back contact is first scribed to individual cells with laser, which is called P1-scribing. After depositing the CIGS-layer, the buffer layer and the layer of i-ZnO, a line is scribed down to the back contact close to the P1-lines with a stylus, which is called P2. When the front contact of ZnO:Al has been deposited, P3-lines are scribed next to the P2-lines which isolate the front contact electrically by scribing down to the Mo-contact using a stylus.