Two thirds of the world’s electricity production currently comes from fossil fuels. In order to have clean energy in the future, renewable energies need to become the dominant form of electricity production. Among these, photovoltaic conversion of sunlight is the one that has undergone the most growth in the last decade. Research into photovoltaic systems is being directed towards technologies that will be competitive against traditional energy sources. Despite the emergence of new technologies based on different thin film materials, it is those based on crystalline silicon that are dominating the market. Among crystalline silicon based photovoltaic cells, heterojunctions are the ones that can achieve the highest efficiency levels. In these structures, thin layers of p-type and n-type doped amorphous silicon are deposited on both faces of the wafer to act as layers to select holes and electrons respectively. This low temperature technology is compatible with the use of thin wafers. Surface passivation is an intrinsic part of the heterojunction structure, and can cover the whole surface of the system in two-faced structures. Such simplicity contrasts with traditional high efficiency solar cells, which require higher etch rates. However, the market is still dominated by the traditional diffusion method. Reasons for the limited implantation of heterojunction technology include the need for relatively complex deposit systems, and the presence of dangerous gases.
In this project, we shall explore non-conventional heterojunction structures in a search for alternatives to amorphous silicon. There are materials that, on a silicon base, selectively extract a kind of load carrier, giving rise to layers that transport holes or electrons, which are known as selective contacts. On an absorbent layer of silicon, the deposit of complementary selective contacts on each electrode produces a solar cell. Although this is a very recent field, high efficiency systems have already been produced. The materials that can act as selective contacts are accessible and can be deposited using simple techniques like evaporation or cathodic pulverisation.
The project has also explored alternatives to the absorbent silicon layer. The cost of the silicon wafers used in the traditional technology represents around 40% of the overall cost of the module. The reduction of wafer thickness to lower the cost is a technological issue that has yet to be resolved. As an alternative, we are proposing research of the use of absorbent layers of silicon obtained from laser recrystallization of the deposited silicon, or bulk passivation of multicrystalline silicon wafers.
As well as non-conventional heterojunction cells, we also plan to develop new manufacturing processes based on laser technology. The first process will be metallisation based on the transfer of metal pastes from a carrier substrate to the cell by means of laser scripting techniques, followed by laser radiation sintering. The aim is to develop an attractive metallisation technology from an industrial viewpoint and one that is compatible with low temperature structures. We shall also study laser etching processes to manufacture Interdigitated Back-Contact cells.
This research is funded by the 2016 Call of the Spanish State Programme for Research, Development and Innovation Aimed at Society’s Challenges (MINECO) and is related with SDG 7: Ensure access to affordable, reliable, sustainable and modern energy for all. This is a coordinated project between the University of Barcelona, the Polytechnic University of Catalonia (coordinator), CIEMAT and the Polytechnic University of Madrid.