First Monolithic Tandem Photovoltaic-Powered Electrochromic Smart Window
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0 3+ye- +yLi÷
*-- LiyWO 3
(1)
which converts transparent W0 3 into optically absorbing LiyWO 3 and back again. The concentration of Li÷, y, determines the absorption coefficient of the LiyWO 3 film and thus the optical properties of the darkened EC device. In an EC device, an ion storage layer (e.g. V2 0 5) undergoes a complementary reaction similar to Equation (1), but there is no corresponding color change. An ion conductor layer (e.g. LiAIF 4 ) ensures that all current flow through the device must 345
Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
be carried by Li+ ions instead of electrons. The color-state of the complete device is determined by the applied electrostatic potential. 10 The monolithic, tandem PV/EC device (see Fig. 1) requires a transparent PV coating that still outputs enough voltage to drive the EC device and enough current to operate the device at a reasonable speed. For the EC device currently employed, a 25 mC/cm 2 charge is required to finish a darkening or bleaching process. To darken the window in 5 minutes requires only about 0.1 mA/cm 2 current density from the PV device. This gives room to increase the bandgap and reduce the thickness of a standard terrestial PV device in order to reach a reasonable bleached-state transparency. Although there is a considerable body of research on wide band-gap silicon carbon alloys, we believe this is the first project to utilize a semi-transparent solar cell made entirely out of a-SiC:H material.11 The main technical challenge lies in reducing the thickness to less than 100 nm for semitransparency. When semitransparent PV devices become very thin, the PV device may develop shorts more easily and render the PV/EC stack useless. This problem has made the fabrication of monolithic PV/EC devices challenging. To realize a monolithic PV/EC structure, each of the nine layers of the tandem device as shown in Fig. la, our target design, must be optimized to obtain good device performance. In Figure la, transparent conducting oxide (TCO) is used for each conducing layer to maximize the device transmittance. By connecting three conducting layers to a battery and controlling circuitry, the device could be fully controlled by the user in the manual mode or automatically controlled by external circuitry. When the device is colored under the light, the top TCO and bottom TCO are connected by an external wire. When the coloration processes finished, extra energy generated by the PV device could be stored in a rechargable battery via the middle contact. This energy could be used to bleach the EC device when necessary. Earlier, we reported side-by-side testing of prototype PV and EC devices, 12 , 13 however, this approach hid some problems we encountered when constructing the monolithic, stacked PV/EC device. Figure lb shows the device structure reported here. We used thin gold as the top electrode to save time and improve reproducibility during our preliminary fabrication processes. The middle contact was removed to avoid the PV shorting pro
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