NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 66 of 144
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two. There are at least three causes for this difficulty: (1)
the spectral range of 1-1.4 eV is very narrow, (2) the
spectrum of the simulator's xenon lamp has multiple
spikes in this spectral region, and (3) for terrestrial spectra,
there are two large atmospheric-absorption notches in the
1.0-1.4 eV spectral range. In addition, variations in the
band gap of the GaInNAs alloy and drifts in the spectrum
of the simulator can cause significant variation in the
spectral correction factor, necessitating its measurement
for every sample. We conclude that attempts to measure
the currents for these devices by using the conventional
expedient of a reference cell are likely to result in
significant errors if a xenon arc lamp is the light source.
0.0 0.1 0.2 0.3 0.4
Figure 3. Measured IV curves at -I sun AMl.5 Direct
illumination for several of the prototype devices of Fig. 2. The
devices were covered by a GaAs filter and the currents were
calibrated as described in the text. The corresponding dark IV
curve for PA280#7 is shown also.
a 0.25 -
cr 0.10 - Pg7#
f ' - - - PA280#7
1.0 1.1 1.2 1.3 1.4
photon energy (eV)
Figure 4. Measured internal QE curves for several of the
prototype devices of Fig. 2.
Table HI. GaAs-filtered AMI.5D device performance parameters
Vee, .s, and F from the IV curves of Figs. 3 and 6(a); band
gaps E. from the QE curves of Figs. 4 and 6(b); and, in the last
two columns, the currents calculated from the internal QE for the
AMI.5D and AMO solarsectra.
Device Voc Jsc FF Eg Jsc Jsc
(mV) (mNdn2) (%) (eV) (mNcn) (mNanr)
ext. QE in.QE int. QE
AIMI.5D AMI.5D AMO
PA277#7 353 1.8 61 1.052 2.5 2.7
PA280#7 435 1.8 66 1.078 2.6 2.7
PA293#7 435 1.8 - 64 1.095 2.6 2.7
PA377#3 294 6.5 59 1.025 9.2 10.2
- -- PA277#7
- - - PA280#7
4. CONVENTIONAL PROTOTYPES: RESULTS
The short-circuit current (J.), open-circuit voltage
(Voc), and fill factor (FF) from these IV curves are
summarized in Table II (the parameters for device
PA377#3 will be discussed below). The IV curves are
shown in Fig. 3. The corresponding dark-IV curve for the
n/p heterojunction device, PA280, is shown also. A
comparison of the light and dark IV curves for this device
shows that the apparent shunt resistance visible in the light
IV curve can be attributed to field-aided collection, as the
dark IV curve is not shunted. The V c expected for an
ideal device in this band-gap range would be -0.6-0.7 V,
so that the V c of the prototypes are within -0.2 V of the
ideal. The nonideality of the FFs appears to result from
various causes including field-aided collection and the low
J,, which is at least a factor of three less than expected.
The problems with J., will be discussed below.
The crucial test of whether the devices convert
photons down to the desired energy of 1 eV is whether the
devices show photoresponse down to these energies.
Figure 4 shows the measured internal QE curves for the
devices whose IV curves are shown in Fig. 3. All device
QE data presented in this paper were taken at short circuit,
i.e., zero applied voltage bias, at very low light levels of
<10-2 suns. The band edges, tabulated in Table II, are
roughly 1 eV, as desired for the third junction in the three-
or four-junction ultrahigh-efficiency device designs of
Fig. 1. However, while the QE curves confirm the devices'
conversion of light down to -1 eV, the curves also indicate
a serious deficiency in these devices: the QEs are a factor
of 3-5 lower than the near-unity QE for a good-quality
GaAs cell, resulting in the low Ju values measured from
the IV curves for these devices.
5. CONVENTIONAL PROTOTYPES: DISCUSSION
In order to study the QE problem in more detail. a
series of GaInNAs epilayers were grown on GaAs
substrates under a wide range of growth conditions, at
various thicknesses, both without and with back-surface
A A Si
105 101 101 108
carrier concentration (cm')
Figure S. The QE of a variety of GalnNAs epilayers at 200.meV
above the band edge, as a function of canier concentration. All
data shown are from layers with band gaps Eg<l.15 eV.
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/66/: accessed October 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.