Francois Pichot, Wei Gao, Brian Gregg, Suzanne Ferrere, Se-Hee Lee, John Bullock, Yueqin Xu, J. Roland Pitts, C. Ed Tracy, Scott Morrison, Satyen Deb, David Benson, Howard Branz
This paper summarizes efforts at the National Renewable Energy Laboratory to develop self-powered electrochromic window coatings that could be used for economical retrofit to existing building windows. The self-power is provided in either of two ways in separate approaches to the electrochromic (EC) design: by very thin, nearly transparent a-silicon PV cells or by dye-sensitized titania half-cells built into the multi-layer structure of the window coating. The goal of both designs is the ability to incorporate the self-powered EC coating into a laminated flexible polymer film. This film could then be retrofitted to a building window by bonding it to the interior surface and attaching it to local electronic controls that also derive their power from the windows that they control. Laboratory-scale prototype self-powered EC window coatings have been fabricated using both approaches. These are described and the remaining development challenges are discussed.
Wide-gap, transparent amorphous silicon based photovoltaics can be integrated with electrochromic materials to produce a self-powered 'smart' window coating. Existing electrochromic window designs require an external electrical connection, which may be economically unfeasible. This problem is solved by the tandem photovoltaic-electrochromic (PV/EC) device, in which a wide-gap amorphous silicon-based alloy (a-SiC:H) photovoltaic device is deposited together with an electrochromic optical transmittance modulator in a monolithic device on a single substrate. In this paper, we discuss our proposed monolithic photovoltaic-electrochromic device. We also present studies of transparent, wide-gap (Tauc gap of 1.8 to 2.2 eV) amorphous silicon-carbon thin films and p-i-n devices designed for use in the photovoltaic-electrochromic device. The photovoltaic cells in the PV-EC can operate at low current (< 1 mA/cm2) because a total injected charge of only 60 (mu) C/cm2 will darken the EC layer to a visible transmission of 5%, but they will need a high open-circuit voltage (> 1.0 V) and high transparency (approximately equals 70%). We describe our progress toward these design targets.
Recently, Rauh and Cogan presented a model for the thermodynamic equilibrium optical and electrical properties of electrochromic optical modulation devices. Such an empirically based, predictive model is very useful in the engineering design of practical electrochromic devices. The existing model, however, requires considerable computation to model the effects of changes in important device parameters such as layer thicknesses and total mobile ion charge. We present a simple, graphical technique for evaluating the equilibrium electrochemical interaction of two electrochromic layers at any applied voltage based on independent, empirical, back-EMF measurements of the materials. Our technique allows extremely rapid evaluation of different candidate materials, layer thicknesses, and total ion charges. We present the results of the model for Li-based electrochromic devices employing WO and VO layers with an emphasis on design for low coloration voltage.
Conference Committee Involvement (6)
Thin Films for Solar and Energy Technology VIII
28 August 2016 | San Diego, California, United States
Thin Films for Solar and Energy Technology VII
9 August 2015 | San Diego, California, United States
Thin Films for Solar and Energy Technology VI
17 August 2014 | San Diego, California, United States
Thin Film Solar Technology V
25 August 2013 | San Diego, California, United States
Thin Film Solar Technology IV
12 August 2012 | San Diego, California, United States
Thin Film Solar Technology III
21 August 2011 | San Diego, California, United States
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