This document discusses several emerging solar window technologies, including organic solar concentrators, luminescent solar concentrators, dye-sensitized solar cells, and honeycomb patterned thin film devices. Organic and luminescent solar concentrators use plastic or glass sheets coated with dyes to absorb sunlight and guide it to solar cells along the edges through total internal reflection. Dye-sensitized solar cells can be produced through low-cost printing but rely on expensive materials. Honeycomb patterned thin films capture some light through a hexagonal pattern while remaining mostly transparent. Standalone windows could power devices directly without conversion losses. However, degradation and material costs present challenges to commercializing these technologies.

1. A window on the
future of solar glazing
Gavin D. J. Harper
g.harper@glyndwr.ac.uk
@gavindjharper
www.gavindjharper.com
http://orcid.org/0000-0002-4691-6642
Welsh Energy Sector Training (WEST) Conference,
Liberty Stadium,
Swansea, Wales,
16th September 2014

2. Solar Concentrators
• Solar concentrators collect sunlight from a very wide area,
and concentrate it down to a much smaller area.
• A smaller quantity of photovoltaic material can be
located at the smaller area.
• This makes more efficient use of the photovoltaic material.
• This could potentially lead to cost reductions in photovoltaic
devices.
• There are “large scale” solar concentrator technologies –
e.g. “mirrors in the desert”, but technologists are also
investigating whether the principle could apply on a
smaller scale for BIPV.

3. Organic Solar
Concentrators
(OSC’s)
• A variation on this
technology
developed at MIT
is known as
“luminescent
solar
concentrators”
(LSC’s)

4. Organic Solar Concentrators
• OSC’s consist of a sheet of plastic,
surrounded by photovoltaic devices on
their edges.
• The plastic is “sprayed” with a dye.
• The combination of dye and plastic act
as a “waveguide”.
• A waveguide is a device which captures
light and directs it along a path to a
particular location.
• The edges of the sheet appear bright as the
light is concentrated.
• It is this concentrated light that the
photovoltaic device captures.

5. Organic Solar Concentrators
• OSC’s consist of a sheet of plastic, surrounded by
photovoltaic devices on their edges.
• The plastic is “sprayed” with a dye.
• The combination of dye and plastic act as a
“waveguide”.
• A waveguide is a device which captures light and directs it
along a path to a particular location.
• The edges of the sheet appear bright as the light is
concentrated.
• It is this concentrated light that the photovoltaic device
captures.

6. Organic Solar Concentrators
• Light hits the plastic, the dye absorbs the light.
• The energy is thereby transferred to the dye, causing the
electrons in those molecules to jump to a higher energy level.
• When the electrons fall back to a lower energy level, the dye
molecules release that energy into the plastic sheet, where it
gets stuck.
• The light can’t escape the plastic, this is known as total internal
reflection.
• (This is the same principle used to transmit data using light over fibre
optic cables).
• It just bounces around in the material, ultimately making its
way to the outer surface. At the outer surface, the solar cells
are waiting to absorb the light and generate electricity.

7. Organic Solar Concentrators
• Approximately 80% of the re-emitted
photons are trapped within the
waveguide by total internal reflection for
ultimate collection by a PV device
mounted on the substrate edges.
• Photon loss (dashed lines) occurs via
non-trapped emission or absorption by
other dyes.
• Light transmitted through the first OSC
can be captured and collected by a
second OSC whose dyes absorb and
emit light at lower energies for electrical
conversion at a second, lower bandgap
PV device.
• Alternatively, the bottom OSC can be
replaced by a low-cost PV cell or used
Image & Text from:
M.J. Currie, J.K. Mapel, T.D. Heidel, S.
Goffri, M.A. Baldo
http://softsemi.mit.edu/Research/p
hotovoltaic-devices/organic-solar-concentrators

8. Drawbacks to OSC’s
• While the light energy bounces around in the plastic, it
sometimes gets reabsorbed into the dye molecules and
ends up emitted as heat. This energy, then, never makes it
to the solar cells.

9. Luminescent Solar Concentrators
• Luminescent Solar Concentrators are
an evolution of the Organic Solar
Concentrator.
• The plastic of an Organic Solar
Concentrator is replaced with a
sheet of glass coated with a dye.
• A type of aluminum called tris(8-
hydroxyquinoline) is added to the
dye molecules.
• These aluminum molecules cause
the dyes to emit light waves at
frequencies the dyes can't absorb.
• This stops light loss through re-absorption
as the light makes its way
to the solar cells at the
concentrators edge.
An image of a Luminescent Solar Concentrator under test.
Image: Viktoria Levchenko
http://www.researchgate.net/profile/Levchenko_Viktoria/publications

10. Device Duraability
• At the moment, this technology is one to consider for the
future.
• The challenge is that the dyes used within the device are
unstable and over a period of three months or so degrade.
• Work is ongoing to improve the performance of these
devices.

11. Pythagoras Solar Windows
Image from: Pythagoras Solar, www.pythagorassolar.com

12. Pythagoras Solar Windows
Image from: Pythagoras Solar, www.pythagorassolar.com

13. Pythagoras Solar Windows
• Stacked its solar cells.
• Appears like venetian blinds inside a window pane, so you
can still see the view while generating electricity.

14. Solar
Windows
Images from:
Pythagoras Solar

15. Dye Sensitised
Solar Cells
The modern version of a
dye solar cell, also known
as the Grätzel cell, was
originally co-invented in
1988 by Brian O'Regan
and Michael Grätzel at
UC Berkeley

16. Dye Sensitised Solar Cells
• Simple to make using conventional roll-printing techniques
• This could allow for “continuous” rather than “batch” production.
• Semi-flexible and semi-transparent which offers a variety
of uses not applicable to glass-based systems
• Utilises many low cost materials.
• HOWEVER, uses small amounts of platinum and ruthenium which
are expensive and have proven very hard to eliminate from the
process.
• Challenges with dye stability / degradation mechanisms.
• European Photovoltaic Roadmap suggests that these
degradation mechanisms can be overcome and DSC’s will
make a significant contribution to the solar generation mix by
2020

17. Honeycomb Patterned Thin Film
Devices
• Honeycomb patterned thin film devices capture some
sunlight from PV material deposited in a “honeycomb”
pattern, but allow light to pass through the middle of the
hexagons.
• The material blends “Fullerenes” (carbon) and
semiconductor materials.
Images Brookhaven / Los Alamos National Laboratory

18. Honeycomb Patterned Thin Film
Devices
• “The material stays transparent because the polymer chains
pack densely only at the edges of the hexagons, while
remaining loosely packed and spread very thin across the
centers…The densely packed edges strongly absorb light
and may also facilitate conducting electricity…while the
centers do not absorb much light and are relatively
transparent.”
• “Combining these traits and achieving large-scale
patterning could enable a wide range of practical
applications”
Lead scientist Mircea Cotlet, Brookhaven’s Center for Functional Nanomaterials

19. Standalone Window for Low Voltage
DC
• Developed by Nihon
Telecommunication System Inc.
• ‘Stand Alone’ does not require
interconnection with circuits in
building.
• Growing use of low voltage DC in
consumer electronic devices.
• Avoids the losses associated with
converting DC-AC with an inverter,
and then back from AC-DC.

20. Standalone
Window for
Low Voltage
DC
• Many portable
electronic devices
have converged
around USB as a
charging standard.

21. If you found any of this interesting…
Please stay in touch
Gavin Harper
g.harper@glyndwr.ac.uk
www.gavindharper.com
http://www.cser.org.uk/
@gavindjharper
@CSER_PV
@LCRI_WEST
https://www.westproject.org.uk/