PPLO is the smallest cell

Research team develops solar cell with record efficiency

Record: Scientists from the Institute for Solar Energy Research (ISFH) and the Institute for Materials and Components in Electronics (MBE) at Leibniz Universit├Ąt Hannover have succeeded in developing a new type of solar cell that works significantly more efficiently with an efficiency of 26.1 percent than the previous solar cells based on the boron-doped silicon commonly used in industry. This development could help to reduce the costs of generating electricity using photovoltaics in the long term. In addition, such efficient solar cells are interesting for applications with limited space, e.g. on the roof of electric vehicles. The research project 26+ was funded for three years by the Federal Ministry for Economic Affairs and Energy (BMWI) with three million euros and will run until June 30, 2018.

The maximum possible efficiency of a solar cell made of silicon, which is particularly important for the application, is 29.5 percent. The practical realization of an efficiency of more than 26 percent with boron-doped silicon was previously considered to be almost unattainable. The basis of the newly developed crystalline silicon solar cell is a contact that was also created at the ISFH and the MBE. A solar cell consists essentially of pure silicon, where light is captured, which generates positive and negative charge carriers. So that they can be used as electricity, the various charge carriers have to be dissipated via differently treated areas of the silicon. For this you need contacts made of metal through which the charge carriers are passed on. At this point - at the transition from metal to silicon and vice versa - there have so far been high losses of charge carriers. This is where the newly developed so-called POLO contacts come into play.

POLO stands for "polycrystalline silicon on oxide" and describes the layers used. In order to extract the positive charge carriers on one side and the negative charge carriers on the other side of the solar cell, a further layer of silicon was used for the POLO contacts, but one with a different structure than inside the cell. The polycrystalline, conductive silicon was applied as a thin layer on a wafer-thin film of silicon oxide. This insulating intermediate layer passivates the contacts.

Tiny pores are created in the passivation underneath by heating to temperatures around 1000 degrees Celsius. The diameter of the pores is in the nanometer range. Through them, the charge carriers can now be passed on with almost no loss via the polycrystalline silicon layer and the metal contacts. The losses could thus be reduced significantly. The so-called POLO contacts therefore combine efficient current extraction from the solar cell with excellent passivation.

In the current research project 26+, it has now been possible to integrate the POLO contacts into a solar cell in such a way that an efficiency of 26.1 percent was achieved. "By using laser processes, we have also paved the way for industrial applications," explains project manager Dr. Felix Haase from ISFH. "The result shows that photovoltaics research in Germany is still among the best in the world and can make significant contributions to reducing electricity generation costs and opening up new fields of application for photovoltaics," says Prof. Dr. Robby Peibst, who heads the working group at the ISFH and holds a junior professorship at the MBE.


Note to the editor:

For further information please contact Prof. Dr. Robby Peibst, working group leader at the ISFH and MBE at Leibniz Universit├Ąt Hannover, on +49 5151 999 100 or by email at [email protected]