Correlation of electronic and microscopic properties of TiOx/Al-based electron-selective contacts in silicon solar cells
Valeriya Titova1,3, Christoph Flathmann2, Michael Seibt2, Jan Schmidt1,3
1Institute for Solar Energy Research Hamelin (ISFH), Emmerthal, Germany
/2University of Goettingen, 4th Physical Institute Solids and Nanostructures, Goettingen, Germany
/3Institute of Solid-State Physics, Leibniz University Hannover, Hannover, Germany

Contacts to crystalline silicon (c-Si) based on ultrathin layers of silicon oxide (SiOy) and titanium oxide (TiOx) have recently been shown to be highly selective to electrons if aluminum (Al) is used as top metal. Such SiOy/TiOx/Al electron-selective rear contacts result in an excellent passivation and a low contact resistance (and hence an excellent selectivity) after annealing at 350°C, whereas before annealing very high contact resistances are measured combined with a poor surface passivation. In this contribution, we examine the SiOy/TiOx/Al stack as implemented at the rear of our solar cells using Transmission Electron Microscopy (TEM)-based techniques and correlate electronic properties, deduced from measurements performed on solar cells, with the structural and compositional properties of the SiOy/TiOx/Al layer stack. We show that during annealing at 350°C, a significant interdiffusion mainly affecting the aluminum and to a lesser extend the oxygen and silicon distribution takes place within the layer stack, which seems to be the decisive prerequisite for the excellent electron conduction of the stack after annealing.