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Passivation Layers for Solar Cells

Fig.1
Fig. 1: Measured effective density of fixed charge carriers in n- and p-type Si passivated with Al2O3 and a thin interface layer of HfO2 or SiO2

Highly efficient silicon solar cells require excellent surface passivation. Al2O3 emerged as high quality passivation material for p-type silicon during the last decade. At NaMLab, homogeneous Al2O3 layers are further developed to multi-oxide nanolaminates with tailored material properties:
(I) Engineering of interface charge carriers. Various types of dielectrics tend to form different amounts of fixed charges at the interface. Al2O3 contains negative fixed charges with a density in the order of 1012 cm‑2, while the number of fixed charges in HfO2 is one order of magnitude lower. SiO2 exhibits have a similar density as the ones of Al2O3, despite this they are of positive nature. One can make use of these particular features. By employing multi-oxide stacks, it is possible to influence the number of charges, which act at the interface to Silicon effectively. Fig. 1 shows the adjustability of the fixed charges between 3x1012 cm-2 and almost zero by adding a thin interfacial layer of HfO2. A SiO2 interface takes a step further. It offers the option to change the type of the fixed charges from a negative to a positive value continuously. The carrier lifetime is not affected by the HfO2 layer, but changes with the thickness of the SiO2 interlayer (see Fig. 2).

Fig. 2
Fig. 2: Carrier lifetimes of different HfO2/Al2O3 and SiO2/Al2O3 nanolaminates for different Interface thicknesses.

 

(II) Conductive passivation layers. State-of-the-art Al2O3 passivation layers are insulating. Electrical contacts to the silicon substrate are usually realized by metal point contacts, which are one of the major recombination loss factors in the solar cell. These losses can be reduced with planar conductive passivation. A combination of Al2O3 with materials of lower bandgap such as TiO2 is a possible route to follow.  The Al2O3-TiO2 stack is evaluated in double and multilayer stacks. The crucial parameter of these stacks is the thickness of the Al2O3 interface layer, because it acts as a tunnel barrier for the charge carriers. Figure 3 shows the current densities and the carrier lifetimes as a function of the thickness of the Al2O3. Very promising performance is achieved with a simple double layer nanolaminate of 5 nm Al2O3 interface layer and a 15 nm TiO2 capping layer. These results prove the potential of pure dielectric stacks for the application as conductive passivation layers.

fig. 3
Fig. 3: Current density and carrier lifetimes of different Al2O3/TiO2 nanolaminates for conductive surface passivation. Best performance is achieved by a double layer stack with 5 nm Al2O3 interface layer.

 

The applied atomic layer deposition technique provides highly accurate growth control in the sub-nm range. It opens the possibility to tailor material properties of Al2O3-based nanolaminates for novel functionalities in future solar cell concepts.

 

Contact: Dr. Matthias Grube

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NaMLab gGmbH
Nöthnitzer Str. 64 a
01187 Dresden
Germany 

Phone: +49.351.21.24.990-00
Fax: +49.351.475.83.900

info (at) namlab.com

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