ITRPV and customers feedback show that there is a shift towards bifacial modules, however a concern on how to correctly test bifacial modules. Eternal Sun Group has together with research and development institutes like ECN looked at the complications and implications hereof.

1. Towards Developing a Standard for Testing Bifacial PV Modules:
Single-Side versus Double-Side Illumination Method I-V Measurements
Under Different Irradiance and Temperature
Stefan Roest1
, Witek Nawara1
, Bas B. Van Aken2
and Elias Garcia Goma1
1
Eternal Sun Group, Den Haag, Zuid-Holland, Wolga 11 2491 BK, The Netherlands
2
ECN Solar Energy, Petten, Noord-Holland, Westerduinweg 3 1755 LE, The Netherlands
Abstract—Industrial production of bifacial photovoltaic mod-
ules is becoming more and more cost-effective in recent years.
For this reason, the development of an agreed international
standard test that provides the guidelines to measure the
current-voltage characteristics of bifacial modules, especially
under standard test conditions, is of utmost importance [1].
In order to contribute to the international norm, the goal of
this research is to compare the two main bifacial indoor testing
methods under several irradiance and temperature conditions,
combining a flash simulator and a steady-state simulator. Among
other insights, the results successfully quantify the offset in
maximum power measurements between methods.
Index Terms—bifacial, standard, characterization, single side,
double side.
I. INTRODUCTION
Due to technological advancements regarding bifacial mod-
ules, including enhanced energy output gain and reduced costs
associated with extra processing steps, it is becoming more
and more attractive for the industry to invest in this market.
Currently, some companies already manufacture and sell
commercial bifacial PV modules, with an installed capacity
of over 120 MWp by the end of 2016 [2]. Additionally,
it is expected that bifacial modules will represent 25% of
the market share in 10 years [3]. Thus, the development of
an agreed international standard test method that allows to
benchmark different bifacial cells and modules becomes more
and more demanded by the PV community.
The main goal of such standard is to describe how to
measure and report the current-voltage characteristics of a
module with both sides being illuminated simultaneously with
an irradiance GF for the front and GR for the rear. However,
if only one solar simulator is available, the front side shall
be illuminated with an equivalent irradiance, abbreviated as
GE, greater than GF and that compensates for the absence
of GR. Furthermore, any irradiance on the non-illuminated
side should be minimised, e.g. by preventing reflection on
objects behind the module by covering the rear with a non-
reflecting plate. This method is referred in this report as single
side illumination method. Finally, when a double simulator
indoor testing setup is available, the bifacial module can be
simultaneously illuminated in front and rear side with GF and
GR, respectively, and thus its current-voltage characteristics
under bifacial operation can be measured and reported. This
Fig. 1. Schematic of the two main methods for testing bifacial
PV modules.
method is referred in this report as double side illumination
method. Both methods are represented in Fig. 1.
Previous works by Deline et al., have validated single
side method current-voltage measurements, by comparing the
maximum power measurements with outdoor experimental
data [4]. The research of Deline et al. shows that maxi-
mum power measurements employing the indoor single side
method agree within 1%-2% with outdoor bifacial data, for
a GR range up to 0.3GF. However, a comparison between
current-voltage measurements employing the indoor double
side method and the indoor single side method has not yet
been assessed in literature. The goal of this paper is to perform
such comparison in order to account and quantify for any
possible divergences on the electrical parameters that both
methods might yield.
To do so, first an assessment of the basic solar sim-
ulator spectrum and sweep time requirements for testing
high efficiency modules, which involves a big share of the
current bifacial modules, is carried out. Secondly, single side
illumination IV measurements on the front and rear side
of a bifacial module at different irradiance are performed.
Then, the technical considerations surrounding the double
simulator setup are analysed. Finally, double side illumination
measurements on the same module are performed and its
output compared with the single side illumination results.
II. EXPERIMENTAL METHOD
In order to analyze the effect of sweep time and direction
on high efficiency modules, a series of IV measurements
under STC conditions are performed on a monofacial n-type

2. Fig. 2. Single side illumination measurement setup.
Fig. 3. Detail of a junction box shading two cells of the bifacial
module.
heterojunction solar module using a flash solar simulator. The
sweep time is increased in steps of 10 ms from 10 to 230 ms.
Forward and reverse sweep are also considered. Regarding
spectrum, the spectral response of the front side of a n-type
PERT bifacial cell is carried out, and based on it, a suitable
simulator spectrum is discussed.
For the single side illumination measurement, a tabletop
flash simulator SPI-SUN 3500 is employed, as shown in Fig.
2. The device under test employed in the measurements is a
60 cell n-type PERT mono-crystalline bifacial PV module.
In more detail, the module is glass-glass and frameless.
Additionally, the cells are series connected and the module
has three by-pass diodes, embedded in three junction boxes,
dividing the module in three sets of two strings each. It
has to be noted, however, that the junction boxes partially
shade the cells of the short-edge from the rear side, as shown
in Fig. 3. During these measurements, the module is kept
at room temperature (25 ◦
C ± 2 ◦
C), which is measured
with a thermocouple attached on top of the glass of the non-
illuminated side, very close to a cell from the edge, but not
shading it. If the measurement is not exactly at 25 ◦
C, it is
Fig. 4. Double side illumination measurement setup.
corrected to it following the guidelines of IEC 60891 [5].
The double side illumination setup is mounted by rolling a
steady-state solar simulator LA150200 on top of the tabletop
flash simulator, as illustrated in Fig. 4. In order to analyze the
technical considerations of using two different light sources, a
series of parameters are analyzed. Firstly, a uniformity map of
the test area is carried out for both sides using a reference cell
placed in each of the sixty spots where the cells of the module
would be, forming a six per ten uniformity map. Secondly,
the spectral response of the two reference cells employed for
recording front and rear irradiance is analysed. Afterwards,
the reflections induced by the albedo of the top simulator are
analyzed by comparing single side measurements with and
without the steady state simulator on top of the setup.
Finally, the double side illumination measurements are
carried out with the same module used for single side. In order
to reproduce similar conditions to those found in real life, the
irradiance of the LA150200, which illuminates the rear side,
is set to its two lower values, 115 W/m2
and 200 W/m2
.
In order to compare the double side measurements to the
single side measurements, GE is calculated as shown in (1), by
using the lower value between the short-circuit current ISC and
the maximum power PMAX bifaciality coefficients, φIsc and
φP max, respectively. These coefficients are the ratio between
ISC or PMAX generated by the module when illuminated at
standard test conditions (STC) on the rear side compared to
the front side, as shown in (2) an (3).
GE = GF + Min(φIsc, φP M AX) ∗ GR (1)
φIsc =
ISC,rear
ISC,f ront
(2)
φP max =
PM AX,rear
PM AX,f ront
(3)
Finally, it is to be noted that when illuminating the rear
side with low irradiance from the steady state simulator,

3. Fig. 5. Relative power deviation as a function of sweep length
for a heterojunction monofacial module.
the bifacial module would raise its temperature from room
temperature to 30 ◦
C. For a better comparison of methods,
also this measurements are corrected to room temperature
following the guidelines of IEC 60891 [5].
III. RESULTS AND DISCUSSION
A. Technical Considerations for High Efficiency Module
Measurements
Fig. 5 shows the relative deviation in maximum power
of a high efficiency module depending on whether the IV
sweep is done forward or backwards. As it can be observed,
a greater sweep time reduces the gap and gets closer to the
true maximum power. Therefore, the upcoming single and
double side measurements will be performed at the highest
sweep time possible in the single and double side setup.
Fig. 6 presents the spectral response of a cell of the same
type as the ones used in the bifacial module under test in
this paper. It can be observed that this type of cell is able to
convert light from 300 to 1200 nm, thus a suitable simulator
with same light spectrum shall be employed for accurate
measurements, covering this entire wavelength range, as it
is shown in Fig. 7.
B. Single Side Characterization
IV curve measurements were taken for front and for rear
illumination under each irradiance setting. For all the settings
the general behaviour of the curves was the same. Therefore,
when discussing IV curves only one irradiance setting is
taken. Fig. 8 shows the IV curves of the PV module when
illuminated separately from the front and from the rear side
at the same intensity, in this case, 800 W/m2
. As it can
be observed, the front side generates higher ISC and greater
PMAX, while the VOC for both situations remains very close.
The main difference, however, comes in the FF. Looking at
the shape of the IV curves, the rear side presents a plateau
caused by the shading of the junction boxes shown in Fig.
3. In fact, a change in height in the plateau can be observed,
meaning that one of the strings is being slightly more shaded.
The reason behind this drop in ISC but not in PMAX, and
Fig. 6. Normalised spextral response of a n-type mono-
crystalline solar cell used in the bifacial module.
Fig. 7. Bins showing nominal AM 1.5 spectrum, limits of
class A (±25%) and A+ (±12.5%) spectrum, and the nominal
spectrum of a simulator that complies class A+ (PASS).
therefore change in FF, is that the ISC of each string in the
module will be limited to that of the lowest performing cell,
and so will the maximum power point current IMPP; however,
in the PMAX region the maximum power point voltage VMPP
will shift to the right and PMAX will not be greatly affected.
Besides shading, other possible causes of difference in IV
curve shape between front and rear can be, although to a lesser
extent, a higher mismatch between cells when irradiated from
the rear side, since in manufacturing they are normally only
binned according to front performance.
Fig. 9 shows ISC versus irradiance for front and rear
illumination. As it can be observed, the ISC evolution versus
irradiance is linear for both front and rear, however they
do increase at different rates, proportional to their spectral
response. As a result, φIsc remains constant as 0.785 ± 0.005
through the entire range of irradiance measurements, as shown
in Fig. 10, where it can be seen that φP max exhibits similar

4. Fig. 8. Front and rear IV measurements of a bifacial module
under 800 W/m2
.
Fig. 9. ISC versus irradiance for front and rear IV measure-
ments on a bifacial module.
behaviour, although it does slightly increase towards higher
irradiance. Logically it has a higher value than φIsc, since
the FF of the module when irradiated from the rear is higher.
Fig. 11 presents the FF evolution versus irradiance, also for
front and for rear illumination. As it is observed, an offset
occurs at all irradiance, which is in agreement with single
side measurements literature [6].
Since φIsc is lower than φP max at all irradiance levels,
it is employed in (1). However, it is to be expected that
if GE is calculated to match ISC, there will be an induced
error on PMAX when emplying the single side illumination
measurement. In a similar way, if φP max was to be employed
in (1), the methods will yield a difference in ISC.
C. Technical Considerations for Double Side Method
Fig. 12 shows the uniformity map of the front side simula-
tor for 1000 W/m2
setting. It can be observed that the test area
stays within a non-uniformity of 1.8%, thus falling in the class
A classification. Similarly, Fig. 13 shows the uniformity map
for the top simulator, which irradiates the rear side, for the
Fig. 10. Bifaciality of PMAX and ISC versus irradiance for front
and rear IV measurements on a bifacial module.
Fig. 11. FF versus irradiance for front and rear IV measure-
ments on a bifacial module.
200 W/m2
setting. The non-uniformity in this case is 4.3%,
which falls within class B classification. Previous research by
Van Aken et al. [7] has shown that this level of non-uniformity
on the rear side has little effect on PMAX determination.
Regarding the spectral differences of both light sources,
two reference cells of the same type are employed for
measuring front and rear side illumination, thus it is assumed
that no correction is required for the amount of irradiance
incident on the front side relative to the rear side.
Finally, reflections due to having a rear simulator sur-
face with higher albedo than the anti-reflective single side
background have to be taken into account. Fig. 14 compares
ISC versus irradiance for single side measurements with and
without rear simulator surface albedo. As it can be noted,
ISC is raised by the rear albedo, which is equivalent to an
increment of 2.8% of front irradiance. On the other hand, Fig.
15 compares FF under the same conditions. It is seen that FF
overlaps in both situations, therefore the effect or reflection
is little and only impacts ISC. As a consequence, while doing

5. Fig. 12. Uniformity map of the 1000 W/m2
setting of the flash
simulator. Units in W/m2
.
Fig. 13. Uniformity map of the 200 W/m2
setting of the flash
simulator. Units in W/m2
.
double side illumination measurements, this reflection can be
either corrected using an initial current offset or accounted
for by adjusting the total irradiance incident on the module.
In the present paper, the second approach is employed.
D. Double Side Characterization
Fig. 16 shows a comparison between single side measure-
ments and double side measurements with 800 W/m2
front
irradiance plus variable rear irradiance, which is converted to
single side irradiance via (1). The FF for double side illumi-
nation IV measurements is found in between the boundaries
set by the front and the rear measurements, getting closer to
the limits depending on how big the share of front or rear
irradiance is compared to the other. Fig. 17 shows how, as
it was expected, if (1) for comparing single and double side
is applied using φIsc, the PMAX of the single side method is
underestimated compared to double side illumination, which
represents the true bifacial conditions. By extrapolating the
double side measurements, under a single side irradiance of
1000 W/m2
, which is equivalent to 800 W/m2
front plus 255
Fig. 14. ISC as a function of irradiance comparing front mea-
surements with and without the albedo of the rear simulator.
Fig. 15. FF as a function of irradiance comparing front mea-
surements with and without the albedo of the rear simulator.
W/m2
rear in double side illumination, there is a difference
of 4.3 W, which represents 1.6% PMAX underestimation from
its true bifacial output.
IV. CONCLUSION
It has been shown that sweep time and extended spectrum
do play a key role to accuratelly predict the electrical pa-
rameters of high efficiency modules, such as heterojunction
and n-type mono-crystalline, including bifacial. Secondly, it
has been demonstrated that, if subjected to shading or other
causes that can induce current mismatch between cells, the
difference between φIsc and φP max will most likely increase.
It is also found that while φIsc remains constant through
irradiance, φP max slightly increases. Furthermore, a fast and
modular setup has been developed and characterized in order
to perform double side illumination measurements, employing
a steady state simulator and a long pulse flash simulator.
Finally, the double side measurements have shown that the
current approach of single side method related to double side

6. Fig. 16. FF as a function of irradiance comparing front, rear
and double, with 800 W/m2
front irradiance, measurements.
Fig. 17. PMAX as a function of irradiance comparing front, and
double, with 800 W/m2
front irradiance, measurements.
method via φIsc can yield significative differences in PMAX
prediction depending on the conditions, up to 1.6% in PMAX
for the case studied. This is due to the difference in value
between φIsc and φP max.
ACKNOWLEDGMENT
Thanks to ECN for their collaboration and to Tempress for
supplying the module.
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