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

Abstract

We report on the first monolithic, amorphous-silicon-based, photovoltaic-powered electrochromic window coating. The coating employs a wide band gap a-Si 1− x C x : H n–i–p photovoltaic (PV) cell as a semitransparent power supply, and an Li y WO 3/LiAlF 4/V 2O 5 electrochromic (EC) device as an optical-transmittance modulator. The EC device is deposited directly on top of a PV device that coats a glass substrate. The a-Si 1− x C x : H PV cell has a Tauc gap of 2.2 eV and a transmittance of 80% over a large portion of the visible light spectrum. We reduced the thickness of the device to about 600 Å while maintaining a 1-sun open-circuit voltage of 0.9 V and short-circuit current of 2 mA/cm 2. By employing the LiAlF 4 as the Li + ion electrolyte, the parasitic electronic current through the device has been significantly reduced (<10 μA/cm 2 under 1 V bleaching voltage). By properly controlling y and 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. Our 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 bleaching times of the electrochromic device are approximately 2 min under normal operating conditions (±1 V). The device is hermetically sealed for a long lifetime.