2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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current topics in solid state physics
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Phys. Status Solidi C 7, No. 3–4, 1045–1048 (2010) / DOI 10.1002/pssc.200982866

Using amorphous silicon solar cells to
boost the viability of luminescent so-
lar concentrators
Daniel J. Farrell
**1
, Wilfried G. J. H. M. van Sark
*2,3
, Steven T. Velthuijsen
2
, and Ruud E. I. Schropp
2

1
Physics Department, Imperial College London, South Kensington campus, SW7 2AZ, London, U.K.
2
Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Nanophotonics – Physics of Devices, P.O. Box
80000, 3508 TA Utrecht, The Netherlands
3
Utrecht University, Copernicus Institute for Sustainable Development and Innovation, Science, Technology and Society,
Heidelberglaan 2, 3584 CS Utrecht, The Netherlands
Received 3 August 2009, revised 4 November 2009, accepted 4 November 2009
Published online 24 February 2010
PACS 84.60.Bk, 84.60.Jt

** e-mail daniel.farrell@imperial.ac.uk (http://www.imperial.ac.uk/qpv)
* Corresponding author: e-mail W.G.J.H.M.vanSark@uu.nl

We have, for the first time, designed and fabricated hydro-
genated amorphous silicon solar cells to be used in conjunc-
tion with Luminescent Solar Concentrators (LSCs). LSCs
are planar plastic sheets doped with organic dyes that ab-
sorb solar illumination and down shift the energy to nar-
rowband luminescence which is collected by solar cells at-
tached to the sheet edge.
We fabricated an LSC module with two bonded solar cells
and performed characterisation with the cells connected in
series and parallel configurations. We find that the LSC
module has an optical collection efficiency of 9.5% and an
optimum power conversion efficiency of approaching 1%
when the cells are in a parallel connection.

2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
Luminescent solar concentrators (LSCs) [1] are a pos-
sible low cost alternative for widespread deployment of
photovoltaic (PV) solar energy converters. These concen-
trators absorb direct and diffuse light in an organic dye,
which can subsequently re-emit, with a slight red-shift. A
large fraction of the emission solid angle (75%) is trapped
in the sheet due to total internal reflection and can be col-
lected at the sides of the LSC plate by one or more bonded
solar cells, Fig. 1. Of crucial importance is that the organic
dyes possess a high fluorescent quantum yield (QY). Cur-
rently, organic dyes with near unity QY are mass-produced
which span the visible spectrum, however it has not been
possible thus far to achieve high QYs for dyes with band
gaps larger than 2 eV (600 nm). This severely limits the
spectral range that can be harvested with dyes. One possi-
ble approach is to use nanocrystalline absorbers such as
quantum dots (QDs) and rods (QRs), in which the absorp-
tion edge can be tuned simply by changing the nanostruc-
ture size [2]. Although research QDs and QRs have been
fabricated with a QY approaching that of dyes this as not
yet been replicated in a commercial process.
Figure 1 Operation of the LSC: light is absorbed, re-emitted by
luminescent dyes and then, depending on emission direction, ei-
ther escapes the device (rays labels 1) or is collected at a bonded
PV cell (rays labels 2).

A silicon cell, with a band gap of 1.1 eV, while readily
available, is therefore not the optimum candidate for a dye
based LSC, because the excess energy of the luminescence
is lost via thermalisation of the carrier within the cell. Thus,

1046 D. J. Farrell et al.: Amorphous Si solar cells to boost the viability of luminescent solar concentrators
2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com
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conversion efficiencies of 3-4% have been reported for
LSCs with c-Si solar cells. Specially designed III-V cells
(such as GaAs and InGaP), with band gaps much closer to
the emission energy of the dyes have been recently used
and a record efficiency is found of 7.1% [3]. It remains to
be seen whether this can produce an economical photo-
voltaic product. This is because III-V cells are expensive
and typically only used in high concentration (1000x) sys-
tems. This is required to reduce the net system cost-per-
watt to a competitive level.

It is in this context that we propose to use hydrogen-
ated amorphous silicon cells (a-Si:H) with LSCs as a cost-
effective alternative to III-V cells. We have designed and
fabricated a-Si:H cells for attachment to the side of an LSC
plate, that are spectrally matched to the emission of the
dyes in the LSC plates (see Fig. 2).
2 Luminescent concentrator performance
2.1 Collection efficiency The collection efficiency
of a luminescent concentrator is the fraction of the entering
photons that are down-shifted, become trapped and are
successfully transported to the bonded cells. At each of
these stages photon loss mechanisms are introduced: (i)
upon absorption there is a chance that non-radiative re-
combination within the dye molecule occurs, this removes
photons from the system, (ii) when trapped, surface imper-
fections cause scattering which removes photons from
waveguide modes, (iii) during transport, non-radiative ab-
sorption can occur in the transparent host medium, as well
as radiative re-absorption in the dye. For state of the art
materials all non-radiative and surface losses are negligible.
The main loss mechanism is therefore re-absorption. This
is because the following re-emission occurs isotropically,
and after many re-absorption-re-emission events trapped
photons get redistributed in non-trapped modes and are lost
via emission through the top surface.
Figure 2 Normalised absorption and emission profile of the LSC
sample and quantum efficiency of the a-Si:H cell.
Figure 3 The incident and edge spectra in units of (a) spectral
photon rate and (b) spectral photon flux.

Before attaching cells to our LSC sample we first per-
formed the optical characterization discussed below to de-
termine the collection efficiency. The LSC sample was
purchased from Lucite (sample code 4T56), cut to dimen-
sions (LxWxD) 5x5x0.5 cm
3
and the edges polished. The
normalised absorption and emission are shown in Figure 2.
The square absorption profile indicates that the plate is
doped with multiple luminescent dyes with sequential
overlap between the absorption and emission bands. This
cascades the absorbed energy to the last dye in the chain
and is the reason only red luminescence is observed [4]. To
maximize the collection efficiency a PTFE Lambertian
back surface reflector with broadband reflectivity 98% was
placed behind the sample during measurements. Any unab-
sorbed lamp photons or luminescent photons escaping the
bottom surface were therefore reflected back into the plate.
The reflector was not index matched to the plate, this en-
sured that total internal reflection is preserved and that the
reflector only operates on light that what would otherwise
be lost.

A Steuernagel Lichttechnik class B solar simulator
with Osram HMI 575W/SE discharge tube was used to
produce uniform illumination over the sample surface.
The irradiance and photon flux in the plane of the samples
top surface and also emitted from the edge was found by
measurements with a 2.56 mm
2
Siemens BPW34 reference
photodiode in combination with an Ocean Optics HR4000
fiber optics spectrometer. The spectrometer was used to
find each spectrum in arbitrary units, which was subse-
quently scaled using reference measurements from the
photodiode taken in the same configuration. This yields an
incident irradiance of 693 W/m
2
(2.07x10
21
photons/m
2
/s)
and an escaping luminescent irradiance of 321 W/m
2