Up/Down Conversion
Schematic diagram of (a) down-conversion; and (b)
up-conversion systems.
In previous years the combination of solar cells
with luminescent up- and/or down-converters have both been demonstrated
in theoretical studies by Centre researchers to be very promising
approaches to reach very high conversion efficiencies. These ideas
are also well established as new third-generation-concepts within
the PV community.
For down-conversion, the luminescent
converter is located on the front surface of a solar cell, which
has a band-gap energy Eg. High-energy photons with energy >2Eg
are absorbed by the converter and efficiently down-converted into
two lower energy photons with energy >Eg, which can both be
absorbed by the solar cell. (A geometry with the down-converter
on the rear surface of the cell gives a slightly higher efficiency
but requires limited width conduction and valence bands in the
cell, which is rather unrealistic). For up-conversion,
the converter is located on the rear of a bifacial cell. It absorbs
low energy photons transmitted by the cell and re-emits photons
above the band gap of the cell. In both cases the solar cell and
the converter are electronically isolated from each other.
Previously published theoretical analysis, based
on detailed balance calculations, was extended to realistic Air
Mass (AM) spectra. This revealed that a solar cell with a band-gap
energy of 2 eV and with an optimised up-converter attached to its
rear surface can reach an efficiency of up to 50.7% for
a non-concentrated AM1.5 spectrum.
By variation of the incident spectrum from AM1 to
AM10 it could also be shown that the up-conversion system has a
significantly better spectral robustness than a series connected
triple tandem solar cell. This is a fundamental advantage that
is particularly important for any terrestrial application. The
up-conversion system is also preferable to anon-series connected
tandem stacked cell as the latter requires six contacts and external
voltage matching whilst the up-converter only needs two external
contacts.
One of the appealing aspects and advantages of these
approaches is that they can be applied to existing solar cells
and that therefore experimental work can be carried out with relatively
uncomplicated structures. In initial proof-of-concept studies,
the focus was on the up-conversion system, mainly due to the availability
of efficient luminescent phosphors.
Up-conversion phosphors (NaYF4:20% Er3+), kindly
provided by collaborators from the university of Bern, Switzerland,
were adhered to the rear surface of bifacial buried contact solar
cells.
There was a small enhancement in photoresponse, with
spectral absorption features characteristic of the particular phosphor
used, giving clear proof that the enhanced photo response of the
cell is due to up-conversion. Furthermore, an external
quantum efficiency (EQE) exceeding 1% could be demonstrated,
a very promising result, especially given that it was obtained
with a test structure that was far from being optimised.
There are several paths towards significantly higher
EQEs, including: optimisation of the phosphor concentration; optimisation
of the thickness and the homogeneity of the phosphor layer; and
the use of more efficient bifacial cells with a better red response
(which will be provided by the Fraunhofer Institute für Solare
Energiesysteme). Significantly higher up-conversion efficiencies
may thus be achieved in the near term, which will then be in a
range, where real efficiency improvements of the solar cells are
within reach. |
