First a-SiC : H photovoltaic-powered monolithic tandem electrochromic smart window device

We report on the first monolithic, amorphous-silicon-based, photovoltaic-po wered electrochromic window coating. The coating employs a wide band gap a- Si1-xCx:H n-i-p photovoltaic (PV) cell as a semitransparent power supply, a nd an LiyWO3/LiAlF4/V2O5 electrochromic (EC) device as an optical-transmitt ance modulator. The EC device is deposited directly on top of a PV device t hat coats a glass substrate. The a-Si1-xCx:H PV cell has a Tauc gap of 2.2 eV and a transmittance of 80% over a large portion of the visible light spe ctrum. We reduced the thickness of the device to about 600 Angstrom while m aintaining a 1-sun open-circuit voltage of 0.9 V and short-circuit current of 2 mA/cm(2). By employing the LiAlF4 as the Li+ ion electrolyte, the para sitic electronic current through the device has been significantly reduced (< 10 mu A/cm(2) under 1 V bleaching voltage). By properly controlling y an d the thickness of each layer, the coloration and bleaching voltage of the EC device could be adjusted within the range of - 0.6 to - 1.3 V (coloring) and 0.1-0.6 V (bleaching) for compatibility with the underlying PV cell. O ur prototype 16 cm(2) PV/EC device modulates the transmittance by more than 60% over a large portion of the visible spectrum. Its color is pale yellow at bleached state and dark blue at colored state. The coloring and bleachi ng times of the electrochromic device are approximately 2 min under normal operating conditions ( +/- 1 V). The device is hermetically sealed for a lo ng lifetime. (C) 1999 Elsevier Science B.V. All rights reserved.