Thermophotonics
Thermophotovoltaics involves the photovoltaic conversion
by a receiver cell of radiation from an emitter, which could be
heated by various sources including sunlight. A prime difference
from normal solar photovoltaics is that emitted energy unable to
be used by the receiver can, in principle, be recycled allowing
high conversion efficiency. An extension of this concept called ‘thermophotonics’ has
been developed at the Centre, where the emitter is “active”,
namely a heated diode. This increases the rate of energy transfer
for a given emitter temperature and concentrates emission in an
energy range more suited for conversion by the receiver.
Figure 1: (a) In a thermophotovoltaic system,
a heated emitter radiates towards a PV cell attached to a load.
Below bandgap radiation is reflected back to the emitter by the
filter. b) In a thermophotonic system, a heated light-emitting
diode radiates towards a PV cell connected to a load. Forward
biasing the diode results in narrow band radiation without a
filter.
A thermophotonic system consists of a heated forward
biased light-emitting diode (LED), and an unheated solar cell attached
to a load. In order to achieve net conversion of heat to electricity,
a very high external quantum efficiency (EQE) LED is required,
so that the LED cools when a voltage is applied.The EQE required
depends on the applied voltage V via EQEreq > qV/(Eg+kT),
where q is the electronic charge, Eg is the bandgap of the LED,
k is Boltzmann’s constant and T is the temperature of the
LED. As a first step towards this goal, we have been aiming to
achieve cooling of an optically pumped GaAs double heterostructure.
In this scheme laser light at the bandgap energy Eg is used to
excite electron-hole pairs, which then thermalise with the lattice
and recombine, emitting light with energy Eg + kT. Thus the required
EQE is Eg/(Eg+kT) ≈ 98% for GaAs with Eg = 1.4 eV.
For our experiments, InGaP/i-GaAs/InGaP double heterostructures
were epitaxially grown over a 50 nm AlAs release layer on a semi-insulating
GaAs substrate by metal organic chemical vapour deposition (MOCVD).The
In0.49Ga0.51P cladding layers were undoped
and had a thickness of 500 nm. The undoped active layer had a thickness
of 800 nm.
Figure 2: An ELO film mounted on a sapphire
(or ZnSe) substrate. Coupling to a high refractive index hemisphere
was used to increase the EQE in our measurements.
In order to avoid absorption of photoluminescence,
the double heterostructure film was separated from the GaAs substrate
by the epitaxial lift-off (ELO) technique. A 1mm diameter mesa
structure was applied to the sample by standard photolithgraphy.
The light extraction scheme used is shown above. Measurements were
conducted with and without a ZnSe dome.
The photoluminescence EQEs of the samples were measured
using a calibrated photoluminescence measurement inside an integrating
sphere, and an independent combined thermal and photoluminescence
measurement. The thermal technique was developed at the Centre
and involves simultaneous measurement of photoluminescence and
temperature as a function of laser power. It can be used to measure
the EQE of undoped samples to a high degree of accuracy. In the
photoluminescence experiment, an EQE of 90% was measured for an
undoped planar sample without dome. This result was close to the
value of 92 ± 3% measured with the combined thermal and
photoluminescence measurement. The slightly lower value obtained
by the integrating sphere method was because the different pumping
powers used in these two measurements resulted in slightly different
injection levels. Using the calibrated photoluminescence measurements
we observed that a ZnSe dome applied on top of a sample mounted
on a ZnSe plate enhanced the EQE from 90% up to 96%, very close
to the 98% value required to achieve cooling. |
