Wide GAP a-SiC:H Alloys for Novel Photovoltaic-Electrochromic Window Coatings
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EC thin films that exhibit reversible changes in optical properties with low applied voltages 5. Current design proposals for EC window coatings call for a connection to an external power supply. For retrofit application to existing windows (a much larger market than new window installation), this external power supply entails either a prohibitively large battery cost or significant cost in building re-wiring. In this paper, we describe progress toward an EC window coating powered by an integral thin film photovoltaic (PV) cell6 . In this tandem PV-EC device, a wide-gap amorphous silicon-carbon alloy PV device is deposited together with an EC optical transmittance modulator in a monolithic device on a single substrate. First, we discuss the integration of EC devices with PV power sources, and the resulting requirements on the PV cell. Next, we describe our investigations into amorphous silicon-carbon alloys (a-SiC:H) for the PV cell. Finally', we discuss the results and indicate the progress required for successful integration of the EC and PV devices. PHOTOVOLTAIC-POWERED ELECTROCHROMIC DEVICES Electrochromic device operation Electrochromism in transition metal oxides and EC devices has been extensively studied since their discovery in 19737. The fundamental mechanism of the change in optical properties of the EC material, however, is still much debated 8 -10 . For device engineering, however, a simple 589 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
phenomenological model suffices. The features of the EC material (e.g. W0 3 ) that are important for device design are (1) the ability to form a reversible, insertion phase compound with small mobile ions (e.g. LixWO 3 ), and (2) that each ion inserted produces an optical absorption center in the visible1 1. Features (1) and (2) together mean that, by inserting and extracting Li+, the optical absorbance of a W0 3 film can be continuously and reversibly varied. The lower half of Figure 1 shows the EC device structure used in our prototype PV-EC device. The EC layer (W0 3) is joined by a Li+ conductive layer to another, complementary, EC layer (V 20 5). Transport of Li+ between the two EC layers by the reversible reaction: WO 3 + xe- + xLi÷ --
Lý WO3
(1)
converts transparent W0 3 into opaque LixWO 3 , and vice versa. The Li+ chemical potential in the
W0 3 and V2 0 5 layers determines the coloration state. A voltage bias applied to the contact layers
will shift the chemical potential balance and drive the above reaction between the transparent and opaque states. In the PV-EC device, this potential is produced by the PV cell integrated with the EC device. Design Targets for Transparent Photovoltaics
The PV cell for the PV-EC tandem device has unusual design requirements compared to typical PV applications. These may be summarized as follows: (1) the cell should be transparent to most of the visible spectrum; (2) the open-circuit voltage (Voc) must be sufficient to overcome whatever barriers to Li+ transport exist in the EC device and large
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