Electrically controllable light trapping for self-powered switchable solar windows

Electrically controllable light trapping for self-powered switchable solar windowsPrepared by:Soumen GiriM.Tech. 1st yr.16MS60R19Materials Science CentreIIT Kharagpur

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2https://www.youtube.com/watch?v=SaZWW5nASZk

Smart glassSmart glass or switchable glass (also smart windows or switchable windows) is a type of glass whose light transmission properties are altered when voltage, light or heat is applied.

Generally, the glass changes from translucent to transparent, changing from blocking some (or all) wavelengths of light to letting light pass through.

Smart glass technologies include electrochromic, photochromic, thermochromic and Polymer Dispersed Liquid Crystal (PDLC) devices.https://en.wikipedia.org/wiki/Smart_glass#Polymer_dispersed_liquid_crystal_devices3

Polymer Dispersed Liquid Crystal(PDLC)Micro droplets of a liquid crystal material is dispersed in a polymer matrix.Incident light is transmitted through the PDLC layer when refractive index of liquid crystal droplets (n0) and refractive index of polymer matrix (np) are matched.

(a) (b)https://en.wikipedia.org/wiki/Smart_glass#Polymer_dispersed_liquid_crystal_deviceshttp://pubs.acs.org/doi/ipdf/10.1021/acsphotonics.6b00518

Fig.1: (a) PDLC layer without applied voltage Fig.1:(b) PDLC layer with applied voltage4

Basic principle of amorphous silicon (a-Si) solar cell

http://www.residentialsolar101.org/thin-film-solar-technology/Fig.2: a-Si solar cell5

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Experiment7Indium Tin Oxide (ITO) is used as a conducting material.

The a-Si layer is deposited by RF sputtering in argon atmosphere onto glass coverslips.

The PDLC mixture consists of liquid crystal E7 (Cyanobiphenyl and cyanoterphenol components) and photocurable polymer NOA65 (Norland Products) with 50:50 weight ratio.

Fig.4: (a) Photograph (OFF state). (b) Schematic of a representative device in the off (i, e, diffusive) state with no applied bias. (c) Photograph (ON state). (d) Schematic of the same device in the ON (i. e, transmissive) state with an applied voltage of 150V.8

Results and Discussion

Fig.5: (a-d) measured (circles) and calculated (shaded areas) absorption for devices with two a-Si thicknesses (13 nm (a, c) and 28 nm (b, d)) in the OFF state (a, b) and the ON state (c, d).

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Fig.6: Power generation density are shown with the self powering threshold marked as a dashed line with 13 nm and 28 nm a-Si. Power densities for 13 nm a-Si are 1.3 mW/cm (ON state) and 2 mW/cm (OFF state).

Power densities for 28 nm a-Si are 1.9 mW/cm (ON state) and 3.2 mW/cm (OFF state).

Self-powering threshold for PDLC layer is 0.79 mW/cm.

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1. Effect of a-Si thickness on power generation

The structure with no a-Si indicates maximum transmission but no power generation.

In OFF state, the power generation by a-Si solar cell is more as the incident light is scattered more by PDLC layer.

The generated power can be increased using a thicker absorbing a-Si layer .Fig.7: Transmission spectra for the PDLC switchable solar windows in the ON (circles) and OFF (triangles) states.

112. Effect of a-Si thickness on transmission

Fig.8: (a) Measured (open symbols) and calculated (solid lines) absorption for the PDLC device with 13 nm a-Si at incident angles 45 and 13 in the ON and OFF states.Optical image through the device in the ON state at (b) normal incidence and (c) 45 from normal. 123. Effect of incident angle of light on absorption

13https://mundaylab.umd.edu/wp-content/uploads/Murray_ACSPhotonics_InPress

ConclusionsThe switchable solar window devices have the potential to self power even in the low absorption state with as little as 13 nm of a-Si.

Devices with 13 nm a-Si have excellent smart window properties: high transparency and clarity (ON state) and high diffusivity (OFF state) for enhanced privacy and reduced solar heating.

Device performance can be improved for non normal incident illumination, which is typically for PV (Photovoltaics) applications.

External power can be saved.14

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