Solar Cell Contacts Using Nano-Sized Dispersions
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competitive with C-based fuel) is $ 1/Watt for new panels. Today, commercial solar panels cost - $4/Watt. Reducing the cost to $ 1/Watt, and thereby bringing solar energy into the electrical energy generation marketplace, can be realized by one of two ways: 1. Reduction of the costs to produce product; 2. Increase in the efficiency of the product. We are evaluating the use of nano-sized dispersions for the spray deposition of contacts to solar cells. We anticipate that the technique should be useful for both cost reduction and efficiency maximization. Potentially, production costs can be markedly decreased as the capital investment of spray deposition (i.e., $1OKs) is orders of magnitude lower than for vacuum systems (i.e., $1Ms). In addition, the use of nano-sized dispersions may provide a number of advantages: (1) improved alloying owing to improved contact areas; (2) the ability to direct write hundred-micron features (e.g., ink jet); and, (3) the ability to easily change the stoichiometry of the sprayed film by modifying the dispersion composition.
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Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
In this paper we present results on two efforts to improve the large area back contacts on solar cells by spray depositing nano-sized dispersions. In each example, the goal is to produce a highly-doped p+ layer on a p-type semiconductor which, in turn, can be easily contacted for current extraction. Towards increased efficiency via improved alloying, we use nano-Al as an alternative to micron-Al for p+ contact to Si. Towards cost reduction, we use spray deposition as an alternative to sputtering for the application of Te as a contact to CdTe solar cells. In each case, our initial results are very encouraging. EXPERIMENTAL AND RESULTS Solar cells are multilayer devices consisting typically of an opaque electrode, a junction region where the photons are absorbed and carriers generated, and a transparent electrode. For
example, Figure la shows a typical Si solar cell and Figure lb a CdTe solar cell. For Si, one side is p-doped and the other n-doped to form a homojunction. Contact is normally made to the pand n-type sides by alloying Al and Ag metals to form p+ and n+ contacts, respectively. These Al and Ag contacts act as both contacts and electrodes, with sunlight passing between Ag grid lines. For CdTe, the incident sunlight passes through a glass superstrate that is coated with a transparent conductor, a heterojunction partner (i.e., n-type CdS) and then a photon absorbing ptype CdTe layer. Contact is made to the back-side of the CdTe layer with one of many p+ materials (e.g., Hg(Cu)Te [1,2], Sb-Te [2,3], or Te [4]). A metal electrode (e.g., Ag or Al) is placed on top of the p+ contact layer.
Al back contact P
SiW fe
Electrode •
m
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Back Contact: CdTe CdS
nSnO2 Ag grid front contact
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a.
b.
Figure 1. Pictorial representations of (a) a Si solar cell and (b) a CdTe solar cell. Nano-AI Back Contacts To Silicon Nano-Al was obtained from the Argonide Corporation
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