ArticlePDF Available

Abstract and Figures

Tandem solar cells offer the potential of higher conversion efficiencies than single-junction solar cells, but incur higher fabrication costs. The question arises under which conditions a tandem solar cell becomes economically preferable to both of the single-junction sub-cells it comprises. We present an analysis based on cost and efficiency relations to answer this question for a double-junction tandem solar cell. We find that combining two ideally band-gap-matched single-junction solar cell technologies into a tandem should be a “marriage of equals”: The sub cells should be produced at similar $/W costs, both sub cell should have similar efficiencies when operated independently, and the costs to turn both cells into a system should be similar. We discuss examples of different hypothetical and actual tandem solar cell technologies and show the intricacies of imbalances in the mentioned factors. We find that tandem-solar-cell-based PV power stations for existing solar-cell technologies offer the potential to reduce levelized cost of electricity (LCOE), provided suitable top cells are developed.
Content may be subject to copyright.
Techno-economic analysis of tandem photovoltaic
systems
I. M. Peters,*S. Soa, J. Mailoa and T. Buonassisi*
Tandem solar cells oer the potential of conversion eciencies exceeding those of single-junction solar
cells, but also incur higher fabrication costs. The question arises under which conditions a tandem solar
cell becomes economically preferable to both of the single-junction sub-cells it comprises. We present
an analysis based on cost and eciency relations to answer this question for a double-junction tandem
solar cell. We nd that combining two ideally band-gap-matched single-junction solar cell technologies
into a tandem should be a marriage of equals: the sub cells should be produced at similar $ per W
costs, both sub cells should have similar eciencies when operated independently, and the costs to turn
both cells into a system should be similar. We discuss examples of dierent hypothetical and actual
tandem solar cell technologies and show the intricacies of imbalances in the mentioned factors. We nd
that tandem-solar-cell-based PV power stations for existing solar-cell technologies oer the potential to
reduce the levelized cost of electricity (LCOE), provided suitable top cells are developed.
1. Introduction
Eciencyis the technical variable that most strongly inu-
ences the cost of electricity provided by solar cell modules and
systems.
1
With established technologies like Si
24
and GaAs
46
approaching their practical eciency limits, non-concentrating
tandem solar cell
710
technology has gained renewed interest.
Tandem solar cells oer a path to increase eciencies beyond
the ShockleyQueisser limit
11
by stacking multiple junctions
made from dierent absorber materials, thus reducing ther-
malization losses.
12
Tandem technology is appealing because it
can leverage well-established technologies with AM1.5 e-
ciencies >20%. Resulting tandem eciencies exceed single-
junction eciencies by several percent absolute. Highest e-
ciencies were achieved with IIIV materials;
1315
recently reach-
ing 29.8% with GaInP on Si.
16
Pathways to practical eciencies
exceeding 33% (ref. 17) exist. Notable are also results for hybrid
organic lead halide perovskites
18,19
on silicon tandem solar
cells
20
that oer a potential path to low-cost manufacturing and
have recently exceeded 20% eciency.
21
Other thin-lm tech-
nologies like CdTe
22,23
and CIGS
24
also oer low-cost and high
eciency potential.
A tandem solar cell is economically viable if and only if the
cost of the electric power provided by the tandem is lower than
that of either the top or bottom single-junction cell operating
independently. At rst glance, tandems require only a small
additional areal cost associated with a few thin-lm layers to
achieve the aforementioned eciency gain. However, tandems
have hitherto failed to gain market traction, because the bene-
ts of eciency improvements to date have not exceeded the
cost of the additional fabrication steps, balance of systems, and
power electronics.
In this work, we conduct a techno-economic analysis with
parametric cost relations, to identify the circumstances under
which tandems are economically preferable to single-junction
constituent devices. Cost modelling is a valuable method to
determine innovation pathways to achieve cost reductions for
PV electricity.
1,2531
We use a bottom up cost-model and the
minimum sustainable price (MSP) methodology to calculate $
per W, as formulated by Doug Powell and Alan Goodrich
1
for
silicon and by Sin Cheng Siah
32
for CdTe. To calculate levelized
cost of electricity (LCOE), we use the System Advisor Model
(SAM) from the National Renewable Energy Laboratory.
33
Once baseline models are established, we explore a wider
techno-economic parameter space by developing parametric
cost relations, thus identifying under which circumstances it is
economically benecial to combine two single-junction solar
cell technologies at AM1.5 conditions into a tandem. Our
simplied parametric cost relation requires only two inputs: the
ratio of solar cell to system related costs, and the eciency of
the three solar cells (one tandem and two single-junction
devices). Denition of the cost relations is formulated, and an
eciency calculation for tandem solar cells is described in the
Methodologysection. The cost breakdown for PV modules
and PV installations is discussed in the section entitled
Breakdown of system costs. We then describe the impact of
dierent technical and economic parameters on the cost-
eectiveness of tandems and show the negative eects of
imbalances for these parameters. We nd that tandems make
Massachusetts Institute of Technology, Cambridge, MA 02139, USA. E-mail:
impeters@mit.edu; buonassisi@mit.edu
Cite this: RSC Adv.,2016,6, 66911
Received 22nd March 2016
Accepted 7th July 2016
DOI: 10.1039/c6ra07553c
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66911
RSC Advances
PAPER
economic sense only when three conditions are satised
simultaneously: (1) the band-gaps are well matched to enable
a high eciency potential, (2) the areal manufacturing costs ($
per m
2
) of top and bottom single-junction devices are similar,
and (3) both sub-cells have a similar eciency when operated
independently (marriage of equals). We conclude with
a recommendation for future research, including the need for
a wider range of low-cost ($ per m
2
), high-eciency, low-capex,
and highly reliable absorbers with band-gaps in the 1.41.9 eV
range.
2. Methodology
The presented methodology aspires to relate the cost of energy
for a tandem solar cell with the cost of energy of the two single-
junction solar cells it comprises. For this purpose we estimate
(i) the cost of a tandem solar cell system from the cost of the two
comprising single-junction solar cell systems, and (ii) the e-
ciency of a tandem solar cell from the eciencies of the two
single-junction solar cells it is made of.
2.1 Relation of single-junction solar cell and tandem solar
cell system costs
First we need to dene the term PV system. In the context of
this analysis, the term PV system is used in a broad sense and
refers to any arrangement of components that may include solar
cells, a supporting structure, electronics to convert DC elec-
tricity into AC, and other system costs. We intentionally dene
the term PV systembroadly, so it comprises both PV modules
and PV power stations, which serve as exemplary systems in our
analysis.
For any such system, it is possible to break down the system
costs C
sys
into one part that is associated with the cost of
making the solar cells C
cell
and one part associated with the cost
of making the supporting structure C
str
. Fig. 1 shows a sche-
matic sketch of this approach, with more detailed explanations
in Sections 3.1 and 3.3.
For a single-junction (sj) solar cell the break down is then
given by
C
sys,sj
¼C
cell
+C
str,sj
.(1)
In the following section, we discuss how these values are
determined for a PV module and a PV power station. For
a double junction (dj) tandem solar cell PV system, costs can be
split up in an analogous way
C
sys,tan
¼C
cell,top
+C
cell,bot
+C
str,tan
,(2)
where C
cell,top
(C
cell,bot
) is the cost to fabricate the top (bottom)
junction. In the following, we will abbreviate this term as C
top
(C
bot
). In general, the breakdown dened in (1) and (2) is
ambiguous as some elements could be counted as part of the
cell or part of the structure. In the present analysis, the break-
down will be made such that the cost for fabricating a junction
of a certain material is the same for the single-junction- and the
tandem solar-cell fabrication processes. In this way, the break-
down becomes unique and the generality of the presented
results is not aected. Dierences between the single-junction
and the tandem fabrication process are now summarized in
the costs of the supporting structure C
str
. For example, if
a tunnel junction
34
is required, the additional cost of the tunnel
junction is embedded in C
str
.
Depending on the specics of the fabrication process, C
str,sj
could dier signicantly from C
str,tan
. This is particularly the
case for PV modules and for highly integrated tandem fabrica-
tion processes. For PV power stations, the dierences will be
smaller, as many of the components will be required regardless
of the specics of the solar cell. In the following, we will assume
whichever structure cost of the two single-junction solar cells is
higher as an approximation for the structure cost of the tandem.
C
str,tan
zMax[C
str,top
,C
str,bot
]. (3)
This approximation can be turned into an equation by
introducing a factor DC,
C
str,tan
¼Max[C
str,top
,C
str,bot
]+DC.(4)
The factor DCsummarizes all dierences between the single-
junction and the tandem fabrication process. We discuss these
dierences and what values DCcan take later in this section.
Finally, we dene the relative cost-benet function as
RCsys;top;Csys;bot ¼
MinCsys;top
Ptop
;Csys;bot
Pbot Csys;tan
Ptan
MinCsys;top
Ptop
;Csys;bot
Pbot (5)
where C
sys,top
and C
sys,bot
are the costs of making PV systems
using only single-junction top or bottom cell respectively, P
top
and P
bot
are the powers generated by PV systems made from
single-junction top or bottom cell, and P
tan
is the power
generated by the tandem PV system. The function Ris unitless
and its entries are deliberately ambiguous, as the analysis
encompasses dierent types of PV systems. While Cis always in
unit of $, Pdepends on the type of system investigated. For a PV
module, Pis power in units of watts, whereas for a PV power
station, Prepresents work in units of kilowatt hours. Both
quantities are ultimately calculated from the eciency of the
Fig. 1 Schematic sketch of the denition for a PV system used in this
study.
66912 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
corresponding solar cell, but the later involves assumptions that
ultimately lead to energy-yield calculations.
35
Ris positive if the cost per output is lower for a given tandem
PV system than for each of the single-junction PV systems it
comprises hence, if the tandem PV system is economically
preferable. The value of Rstates how much the cost per output
of the tandem PV system is lower (in percent) than the cost per
output of the less expensive single-junction PV system. The term
outputcan refer to power or work, depending on the system
considered.
2.2 Relation of single-junction solar cell and tandem solar
cell eciencies
In a second step, we need to relate the power or work generated
or done by the tandem solar cell PV system to that of the single-
junction solar cells it comprises. Work and power can both be
calculated from the solar cell power-conversion eciency. As
arst-order approximation, we use solar-cell eciencies as
proxies for power and work. Consequently, we need to relate the
eciencies of the tandem solar cell h
tan
and the eciencies of
the two single-junction solar cells h
top
and h
bot
. Note that when
mentioning a single-junction solar cell eciency, we always
refer to the eciency generated by this solar cell on its own and
under standard testing conditions.
The radiative eciency limit of a single-junction solar cell
h
SQ
was determined by Shockley and Queisser.
11
In this limit,
the eciency is completely determined by the band-gap E
g
of
the material used. The limiting eciency of a double-junction
solar cell can be calculated by a modication of this
approach
7
and is completely determined by the band-gaps of
the two sub-cells, E
g,top
and E
g,bot
. The tandem solar cell is,
however, not uniquely dened by the two band-gaps. It also
dependsonhowthetwosub-cellsareintegrated.Mainly,
there are two possible congurations:atwo-andafour-
terminal conguration. In the four-terminal conguration,
both sub-cells are contacted independently. Eciencies for
top and bottom cell are calculated separately and are added to
obtain the tandem eciency. In the ideal case, all solar
photonswithenergiesuptohy<E
g,top
are utilized by the top
cell, which is identical to the top cell being operated as
a single-junction cell. The bottom cell eciency is calculated
with an altered spectrum, containing all photons with ener-
gies E
g,top
<hy<E
g,bot
.
In the two-terminal conguration, the sub cells are mono-
lithically integrated and electrically separated by a tunnel
junction. The sub-cell with lower current generation limits the
current of the tandem device. If the bottom cell is limiting,
current generation in the top cell, j
top
can be reduced, within
limits, by thinning
36,37
or by area adjustment.
38
Thus
jtan ¼8
<
:
jtop if jtop \jbot
1
2jtop þjbotif jtop .jbot
;(6)
with j
tan
(j
bot
) the current generated by the tandem (bottom)
solar cell. The voltages generated by the two sub-cells are added
to obtain the tandem solar cell eciency.
The eciency of a non-ideal single-junction solar cell h
sj
can
be dened by two parameters: the band-gap of the material used
and the fraction f
SQ
of the radiative limit at which the solar cell
is operating. Thus,
h
sj
(E
g
,f
SQ
)¼h
SQ
(E
g
)f
SQ
.(7)
The eciencies of two non-ideal single-junction solar cells
can then be linked to the eciency of a tandem solar cell. The
tandem solar cell eciency is given as a function of E
g,top
,
f
SQ,top
,E
g,bot
and f
SQ,bot
. The scaling factors f
SQ,top
(f
SQ,bot
)
provide no freedom of breaking down losses into parts corre-
sponding to current, voltage and ll factor. As dierent distri-
butions into these parts are possible, the tandem solar cell
eciency is not uniquely given if the single-junction eciencies
are known. Furthermore, the approach does not take into
account any additional losses that occur by tandem integration.
The used approach can, therefore, only serve as a rst-order
approximation. The approximation works the better, the
closer to the radiative limit the two single-junction cells operate.
For a more detailed analysis, a complete device simulation is
required.
The broad view presented in this analysis necessarily ignores
some aspects that will aect the comparison of single junction
and multi junction solar cells. This topic has been discussed in
the context of PV LCOE.
39
Examples for such factors are dier-
ences in the energy yields of single junction and multi-junction
solar cells
35
and degradation.
2.3 Cost regimes
An example of R(C
1
,C
2
) is plotted in Fig. 2; we will refer to this
type of plot as cost-regime plot. Parameters used in the plot
Fig. 2 Exemplary cost-regime plot. The tandem solar cell comprises
a top cell with a band-gap of E
g,top
¼1.74 eV, and a single-junction
eciency of h
top
¼21.8% (f
SQ,top
¼76%). The bottom solar cell (E
g,bot
¼
1.124 eV) has a single-junction eciency of h
bot
¼21.8% (f
SQ,bot
¼
64%). The calculated tandem eciency is h
tan
¼32.7% in the four-
terminal conguration. The two-terminal eciency would be h
tan
¼
32.3%. Structure costs for all three solar cells are equal (C
sys,top
¼
C
sys,bot
¼C
sys,tan
).
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66913
Paper RSC Advances
are stated in the gure caption. The used parameters do not
correspond to any fabricated solar cell system; they were chosen
because they represent an ideal example of a tandem solar cell
from a cost relation point of view. It will be discussed in the
following sections, in what sense this example can be consid-
ered ideal. Axes in the cost-regime plot are given as the ratio of
solar cell C
cell
to supporting structure C
str
cost. A ratio of one
signies equal cost shares and corresponds to 50% contribution
of each cost to the system cost C
sys
.
The plot in Fig. 2 marks three regimes, separated by the
black and white lines. In each regime either of the three solar
cell systems, using only the top cell (upper le), using only the
bottom (lower right) cell and using the tandem (lower le
corner) has the lowest cost per output. The lines mark the
conditions under which two of the three solar cell systems have
the same cost per output. The triple point marks the condition
for which all three solar cell systems are at the same cost per
output. In the depicted ideal case the triple point is located at (1,
1).
The white line additionally marks the gradient and the ridge
in relative cost-benet. Approaching the extension of the white
line within the tandem regime is, therefore, desirable. From
this we draw the rst conclusion about under which conditions
two solar cells should be combined into a tandem: both solar
cells, operated in a system independently, should be at a similar
cost per output level.
Another point of interest is the maximum relative cost
benet (MRCB). The MRCB is located numerically by analysing
the cost-relations plot. It will later be used to compare impacts
of dierent parameters on the nancial competitiveness of the
tandem PV system. In Fig. 2, the MRCB is located in the origin.
However, this is not generally the case.
3. Breakdown of system costs
As indicated earlier, the term systemin this work is used in
a broad sense. The presented method for analysing cost regimes
is valid for any arrangement that includes solar cells and sup-
porting structural components. We will in the following discuss
two classes of systems of interest: PV modules and PV power
stations. These examples address the economic interest of
dierent trades: module manufacturers for which we will use
the MSP as a gure of merit, and PV installers or yieldcos for
which we will use the total system costs and LCOE as a gure of
merit. As these two system types include dierent components,
also the breakdown of system costs will be dierent. We will
show how such a break down can be made and indicate what
values for C
cell
/C
str
can be expected in each case. Note that the
presented analysis uses data for cases within the United States,
and numbers would have to be adapted for other locations.
3.1 PV modules
We analyse two PV module fabrication processes: mono-
crystalline silicon and CdTe, as an example for a thin-lm
technology. Bottom-up cost analyses for these processes are
presented in ref. 1 and 32. Table 1 shows a cost breakdown for
these fabrication processes in three categories: feedstock &
absorber, absorber to cell and cell to module. Each of those
categories can be further broken down into material, labour,
electricity, maintenance and capex related costs (not shown).
For CdTe we included two examples: one process for a module
with frame and one process for a frameless module. C
cell
includes feedstock & absorber and absorber to cell costs. C
str
includes all cell to module costs. As indicated earlier, we per-
formed the break down such that C
cell
would be similar in
a tandem fabrication process and all variations are subsumed
under C
str
. One example for which this issue this has an eect is
the glass substrate for the thin-lm solar cell. Usually, the glass
would be considered part of the absorber to cell process. Here,
we consider it as part of the cell to module process. This has two
reasons: (i) in a silicon PV module, glass is used as a cover and
we wanted the two processes to be as close as possible, and (ii)
in a tandem solar cell, one of the cells would be deposited on an
existing junction and not on glass. The results of our analysis
are summarized in Table 1. The rst column for each tech-
nology states the fabrication cost, the second column the MSP.
Using this cost breakdown, we can project the costs for
a hypothetical tandem fabrication by combining dierent
materials. In Table 2 we show the combination for a hypothet-
ical thin-lm on silicon tandem PV module, and a hypothetical
frameless thin-lm on thin-lm tandem PV module. Initially we
assume that all processes steps are similar to single junction,
i.e.,DC¼0. The table again states fabrication costs and MSP.
Table 1 Cost breakdown for dierent single-junction solar cell module technologies
CdTe single-junction,
frameless
CdTe single-junction, with
frame Silicon single junction
Cost
[$ per m
2
]
MSP
[$ per m
2
]
Cost
[$ per m
2
]
MSP
[$ per m
2
]
Cost
[$ per m
2
]
MSP
[$ per m
2
]
Feedstock & absorber C
cell
17.98 30.77 17.98 30.77 42.08 56.16
Absorber to cell C
cell
17.76 26.22 17.76 26.22 27.12 39.64
Cell to module C
str
21.43 25.29 25.39 29.93 33.81 42.94
Total 57.16 82.27 61.13 86.92 103.01 138.74
C
cell
/C
str
1.67 2.25 1.41 1.90 2.04 2.23
66914 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
3.2 Discussion of DC
In the breakdown shown in Table 2, single-junction and tandem
solar cell PV module fabrication processes use the exact same
process steps. While feedstock and absorber costs should
remain largely unaected, dierences can be anticipated in the
absorber to cell process in actual fabrication. These dierences
will depend on the tandem solar cell architecture. In the
following we will give a brief discussion on DCfor a two-
terminal and a four-terminal tandem PV module.
Two-terminal conguration. In the two-terminal congura-
tion, the cell process can be assumed to be highly integrated.
The top thin-lm solar cell is deposited onto the silicon- or the
thin-lm bottom cell. The cells are electrically connected by
a tunnel junction. Consequently, the rear contact of the top cell
and the front contact of the bottom cell become obsolete. We
estimate the corresponding savings to be up to 5 $ per m
2
.
While the tunnel junction is an added feature of the tandem
solar cell, other features of the single-junction solar cell become
also obsolete. Among them is the antireection (AR) coating of
the bottom cell. We hypothesize that the tunnel junction
deposition replaces the AR coating deposition and that the
tunnel junction can be integrated in fabrication with the front
passivation of the bottom cell and the rear passivation of the top
cell. As a result, we don't assume any additional cost for the
tunnel junction.
A further factor to consider is fabrication yield. As the inte-
grated fabrication process requires more fabrication steps, yield
will likely decrease. A decreased yield would result in higher
cost. Following the processes described in ref. 1 and 32, a yield
reduction of 1% absolute for the tandem PV module was
assumed.
Further potential costs could be related to the higher power
generated by the tandem solar cell PV module. This could have
implications on the modularization process; dierent wiring or
dierent junction boxes could be required. This is not consid-
ered here.
Following the given discussion, we expect DCfor the 2-
terminal conguration to be up to 5 $ per m
2
. This corre-
sponds to 15% of the cell to module costs for a silicon PV
module and 23% for a thin-lm PV module.
Four-terminal conguration. In the four-terminal congu-
ration, the two sub cells are fabricated and operated indepen-
dently. We will assume here that they are mechanically stacked.
Fabrication procedure and yield remain largely unaected;
though, the four-terminal tandem requires some changes in the
design of top and bottom cell.
As the top cell needs a translucent rear contact, the full-area
metal contact needs to be replaced by a transparent contact and
a rear AR coating. AR coatings can potentially be deposited on
both sides of the cell at the same time. The full area metal
contact could be replaced by a metal grid. Ensuing costs are
marginal and we neglect additional cost for these changes.
Mechanical stacking will, most likely, require an additional
polymer layer between the two sub-cells. An air layer is,
however, possible. An additional EVA layer would come at an
additional cost of 1.8 $ per m
2
.
Potential design changes in the bottom cell include AR
coating thickness and junction prole. Foreseeable changes in
process costs are small and are ignored here.
Independent contacts for the two sub cells require inde-
pendent circuitry, an additional junction box and additional
cables. We estimate ensuing costs at $7.5 per module (4.6 $ per
m
2
). Integration could reduce this cost and a customized
junction box and cable design could be envisioned that includes
contacts for both cell types and could reduce this cost factor.
Considering these changes, DCfor the 4-terminal congu-
ration would be 6.4 $ per m
2
, corresponding to 19% of the cell to
module cost for a silicon module and 30% for a (frameless) thin-
lm module.
Relative changes in the cell to module process correspond
directly to changes in MSP. We have plotted the impact of
a change in DCof 25% relative on the cost regime in Fig. 3.
Note that this analysis was conducted for a specic case and
location and that all results are likely to vary as a result of the
variation in PV module manufacturing by geography/
manufacturer.
40,41
3.3 PV power stations
The cost breakdown for PV power stations requires a general-
ization of the approach used for PV modules. The cost of a PV
power station includes more cost components than that of a PV
module, including costs for PV module, racks, mounting,
wiring, land, permits, labor, and inverters. Replacing single-
junction solar cells in a PV power station by more ecient
tandem solar cells results in the station generating more power.
Consequently, all components that scale with a higher power
Table 2 Cost breakdown for two hypothetical tandem module technologies
TF on silicon tandem TF on TF tandem (frameless)
Cost [$ per m
2
] MSP [$ per m
2
] Cost [$ per m
2
] MSP [$ per m
2
]
Feedstock & absorber (top) 17.98 30.77 17.98 30.77
Absorber to cell (top) 17.76 26.22 17.76 26.22
Feedstock & absorber (bot) 42.08 56.16 17.98 30.77
Absorber to cell (bot) 27.12 39.64 17.76 26.22
Cell to module 33.81 42.94 25.39 25.29
Total 138.76 195.72 92.90 139.25
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66915
Paper RSC Advances
output for a constant area or number of PV panels need to be
considered as a part of C
cell
. Apart from the solar cells, this
includes especially inverters and cables. C
str
includes all other
components. We used ref. 1 and 32 again for the solar cell and
module costs and material published within the solar advisory
model (SAM) from ref. 33 to establish C
str
and C
cell
.AsC
str
changes with the size of the installation, we have considered
three dierent station sizes for each residential scale and utility
scale. Additionally, for each solar cell technology, CdTe and Si,
we considered three dierent eciencies. These eciencies will
later be used to investigate the impact of dierent parameters
on the cost regimes. The results of the calculations are
summarized in Table 3.
Comparing Tables 1 and 3, the signicant dierence
between C
cell
/C
str
for PV modules and power stations becomes
clear. Whereas in a PV module this ratio is between 1.4 and 2.3,
for power stations the ratio is between 0.06 and 0.5. From this
result it can be concluded that, whether a tandem solar cell is
economically attractive, will be judged dierently by a module
manufacturer and a system installer or yieldco. The smaller
ratios for power stations show that tandem solar cell technology
will become interesting to installers before it will become
interesting to manufacturers.
It also becomes obvious that the main share of the cost in
a PV power station is not related to solar cells. This has impli-
cations on DC. It can be argued that fabrication dierences
between two- and four-terminal solar cells are exhausted at the
module level. For example, as cables and inverters scale with
power, wiring in four-terminal tandems could be imagined that
connects dierent inverters to dierent cells. Therefore, most of
C
str
becomes independent of the solar cell technology and the
relative dierences become much smaller. We estimated DCat
3% of C
str
at the power station level.
4. Impact of dierent parameters on
cost relations
4.1 Impact of dierent pairings of band-gaps (E
g
)
The combination of band-gaps of top and bottom cell deter-
mines the limiting eciency for the tandem solar cell. This
limit is dierent for the two- and four-terminal conguration, as
the former requires current matching. We calculated the
limiting eciencies for a range of dierent band-gap pairs
according to the method described in the methodology section
for the two- and four-terminal conguration. The results of this
calculation are shown in Fig. 4 and are plotted as contour lines.
We also calculated which band-gap pairing gives the highest
eciency for a given top cell/bottom cell band-gap (blue dotted
line). For each band-gap pairing, we calculated the MRCB,
which is represented by the color code. Finally, we calculated
the band-gap pairing that gives the highest MRCB for a given
bottom/top cell band-gap (black and red line, respectively).
Generally, the band-gap pairings that result in the highest
eciency also result in the highest MRCB. This is strictly true
for the two-terminal conguration. For the four-terminal
conguration, there is an ambiguity for small bottom-cell
Fig. 3 Eect of a relative change in DCon the cost regimes. The black
lines border conditions in which the tandem solar cell provides
a positive cost benet. A change in DCscales the corresponding area
and moves the triple point along the (white) line for equal cost per
output of the top and bottom cell.
Table 3 Cost breakdown for dierent single-junction solar power stations. The 1 kW residential and the 600 MW utility scale system mark the
smallest and largest value in each row
Cell type h[%] MSP [$ per W]
C
cell
/C
str
Residential Utility
1 kW 5 kW 13.5 kW 50 MW 186.7 MW 600 MW
CdTe 14.0 0.62 0.077 0.168 0.206 0.313 0.332 0.338
16.4 0.53 0.069 0.160 0.202 0.319 0.331 0.337
20.8 0.40 0.059 0.148 0.197 0.308 0.337 0.345
Si 12.2 1.14 0.139 0.283 0.339 0.499 0.524 0.531
16.4 0.85 0.114 0.260 0.326 0.488 0.521 0.530
24.7 0.56 0.084 0.224 0.301 0.467 0.514 0.527
66916 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
band-gaps (E
g,bot
< 1 eV). The result depends on whether in the
calculation the top- or the bottom-cell band-gap is xed.
The reason for the peculiar behavior of the four-terminal
conguration lies in the relative nature of the metric. A single-
junction solar cell with either a very small or a very large
band-gap generates a very small eciency on its own. In the
four-terminal conguration, the eciencies of a top cell with
a large band-gap and a bottom cell with a small band-gap nearly
add up. The relative cost benet compared to either of the
single-junction solar cells is then very large, but the absolute
price per output is very high, so that these combinations are not
actually desirable. To avoid these solutions, it is necessary to
look at both, the E
g,top
that results in the highest MRCB for
axed E
g,bot
(black line) and the E
g,bot
that results in the highest
MRCB for a xed E
g,top
(red line).
To illustrate the eect of dierent band-gap pairings on cost
regimes, we compare a close to ideal band-gap pairing on
silicon (E
g,bot
¼1.124 eV, E
g,top
¼1.74 eV) to a non-ideal one
(E
g,bot
¼1.124 eV, E
g,top
¼1.42 eV). The results are shown in
Fig. 5. For the close to ideal band-gap pairing (Fig. 5a), we used
a top cell eciency of h
top
¼20.8%, corresponding to a value of
f
SQ,top
¼72%. This eciency was chosen with the current record
eciency for GaInP
42
in mind. The band-gap of GaInP can be
tuned by a variation of In and P content. The current record
eciencies were achieved with a material that had a slightly
higher band-gap (1.81 eV). A silicon solar cell with the same cell
quality (f
SQ,bot
¼72%) is at h
bot
¼24.7%, which is about 1%
below world record.
43
The calculated tandem solar cell eciency
for this combination would be h
tan
¼33.2% (four terminal) or
h
tan
¼32.6% (two terminal).
Fig. 4 Maximum relative cost benet as a function of top cell band-gap E
g,top
and bottom cell band-gap E
g,bot
for the four-terminal (a) and two-
terminal (b) conguration. The contours show the limiting eciencies for the corresponding band-gap combination. The blue dotted line
represents the band-gap pairing with the highest eciency for a given E
g,top
,E
g,bot
. The black line represents the band-gap pairing with the
highest relative cost benet for a given E
g,bot
. For the four-terminal tandem, we also show the band-gap pairing with the highest relative cost
benet for a given E
g,top
(red line).
Fig. 5 Illustration of the impact of ideal and non-ideal band-gap combination on the cost regimes. A close to ideal band-gap pairing is shown on
the left (a), a non-ideal band-gap pairing on the right (b). The blue line at the top of each graph represents the C
cell
/C
str
value for a silicon PV
module (Table 1), the blue band at the bottom represents the range of C
cell
/C
str
values for dierent silicon PV power stations (compare Table 3).
The lower boundary corresponds to a small residential system, the upper boundary to a large utility scale system. $ per W values show at what
MSP levels the module would be.
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66917
Paper RSC Advances
The non-ideal case corresponds to a GaAs on Si tandem solar
cell.
35
GaAs on silicon was chosen as an example as these
materials represent mature technologies with a clear path
toward 30% at panel tandem eciency.
44
For consistency, we
also used a top cell with f
SQ,top
¼72%, which corresponds to h
top
¼24.4% eciency, about 4% below world record
45
but still at
a competitive level.
46
The calculated combined eciency for this
tandem solar cell is h
tan
¼30.7% (four terminal) and h
tan
¼
28.5% (two terminal). The two-terminal eciency takes
a stronger penalty due to the non-ideal band-gap pairing.
Fig. 5 also shows $ per W values that are obtained from MSP
calculations. Values from Table 1 were used with C
str,top
¼
C
str,bot
¼C
str,tan
¼42.94 $ per m
2
. As the cost regimes depends
on h
top
and h
bot
, which are dierent in both cases, the lines for
equal cost per output for top and bottom cell are not diagonal
anymore. Looking at C
cell
/C
str
for a silicon PV module (blue
dotted line), there is no path with current technology in which
the tandem is preferable, regardless of the band-gap of the IIIV
top cell. At the power-station level, an ideal band-gap combi-
nation has the possibility for an economically attractive
tandem, provided C
top
/C
str
is smaller than 0.5 for a utility-scale
system or smaller than 0.4 for a residential system.
For the non-ideal band-gap combination of GaAs and silicon,
current technologies are not cost-advantageous as a tandem,
even for utility-scale stations. For residential power stations,
tandems become preferable, provided C
top
/C
str
is smaller than
0.3. In either case, IIIV solar cells would have to become very
inexpensive. A rough estimate for a split according to (1) and (2)
can be made using.
47
At least with current fabrication proce-
dures, IIIV technology is at a signicantly higher cost level than
Si. Technological solutions for reducing cost have been sug-
gested, e.g., in ref. 48 and 49. Further opportunities for GaAs-on-
silicon solar cells lie in areas where the solar cell contributes an
even smaller fraction of the entire system costs or where other
factors, like weight or size, are important. Examples are
outdoor, concentrator
50
and space applications.
51
From this analysis we conclude that ideal band-gap pairings
are very desirable for the commercialization of tandem solar
cells. This is especially true for tandem solar cells in a two-
terminal conguration, as these cells take a larger eciency
hit if they are not current matched.
4.2 Impact of dierent pairings of solar cell qualities f
SQ
As a measure for the quality of a solar cell we use the fraction of
the radiative limit f
SQ
at which the cells operate. To gauge the
impact of dierent cell qualities on the cost-benet, we assess
the MRCB as a function of f
SQ,top
and f
SQ,bot
. The results of this
calculation are shown in Fig. 6. We use a close to ideal band-gap
pairing with a silicon bottom cell (E
g,bot
¼1.124 eV, E
g,top
¼1.74
eV). Results are similar for other ideal band-gap pairings,
though absolute eciency numbers change.
The combination of f
SQ,top
and f
SQ,bot
that yields the highest
improvement of tandem solar cell eciency over the
comprising single-junction solar cell eciency for a given f
SQ,top
or f
SQ,bot
coincides with the combination that yields the highest
MRCB. The ideal combination of f
SQ,top
and f
SQ,bot
is given if h
top
¼h
bot
. This not only true for the shown band-gap combination
but holds generally. The slope of the line in Fig. 6 is given by the
ratio of the radiative limits of bottom and top cell h
SQ,bot
/h
SQ,top
.
To illustrate the eect of ideal and non-ideal cell quality
pairings, we show the cost relations for a perovskite on silicon
tandem solar cell with two dierent silicon bottom cells. The
result of this calculation is shown in Fig. 7.
Perovskite on silicon solar cells are of interest as perovskite
solar cells have achieved high eciencies up to 21%,
21
are
potentially inexpensive, and can be fabricated with band-gaps
that are close to the ideal pairing with silicon.
In the presented calculation (Fig. 7a) we have used parame-
ters taken from an in-house study.
52
The top cell has an e-
ciency of h
top
¼12.0% (f
SQ,top
¼41%) at E
g,top
¼1.7 eV band-
gap. The bottom cell had a similar eciency of h
bot
¼12.2%
(f
SQ,bot
¼38%) at E
g,top
¼1.124 eV. The calculated tandem solar
cell eciency is 17.8% (comparing to 17.0% for a 1.6 eV top cell
in ref. 24; a dierent band-gap was used in order to vary only
one parameter at a time). While the achieved eciencies are
low, they provide a good case study for similar eciencies. It
can be seen that the regime in which the tandem is preferable is
comparably large. At the module level, there is, also here, no
scenario in which the tandem is preferable to either top or
bottom cell alone. On the power station level, the tandem
becomes preferable if C
cell
/C
str
for the perovskite solar cell are
below 0.7 for utility-scale stations and 0.5 for residential-scale
stations. Given the currently unknown but potentially low
fabrication cost of perovskite solar cells, these goals seem
achievable. The shown $ per W values in Fig. 7a are comparably
high because of the low eciencies of the comprising solar
cells. Increasing h
top
and h
bot
simultaneously will reduce those
Fig. 6 Surplus tandem solar cell eciency (h
tan
max[h
top
,h
bot
]) over
the higher of the comprising single-junction solar cell eciencies as
a function of f
SQ,top
and f
SQ,bot
. The top cell has a band-gap of E
g,top
¼
1.74 eV, and a radiative eciency of h
SQ,top
¼28.4%. The bottom cell
has a band-gap of E
g,bot
¼1.124 eV and a radiative eciency of h
SQ,bot
¼33.0%. The tandem solar cell eciency is shown by the contour
lines. The maximum eciency is h
SQ,tan
¼44.5%. Also shown are lines
representing the highest eciency benet (blue, segmented) and the
highest cost benet for a given f
SQ,top
or f
SQ,bot
(black). Note that these
lines coincide. They also mark the case for which h
top
¼h
bot
.
66918 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
numbers while not changing the rest of the plot. Of course,
resolving the reliability issue is a precondition for commer-
cialization at any scale.
In Fig. 7b we show the eect of replacing the 12.2% bottom
cell by a 20.5% bottom cell (f
SQ,bot
¼60%). The calculated
tandem solar cell eciency in this case is h
tan
¼21.7% and is
still higher than the single-junction eciency of the silicon
solar cell. However, the eciency benet of the tandem solar
cell has reduced from 5.6% for case (a) to 1.2% in case (b). As
a consequence, the top cell has to be fabricated for virtually zero
cost (C
cell
/C
str
< 0.05) for the tandem solar cell to become pref-
erable. This case is of interest because it is frequently argued
that an existing solar cell system could be improved by adding
an inexpensive top cell. Our analysis does not support this
argument but indicates that both sub cells should operate at
similar eciencies to achieve a cost benet from the tandem.
C
str
for silicon were taken from Table 1 for PV modules
(C
str,top
¼C
str,bot
¼C
str,tan
¼42.94 $ per m
2
) and from Table 3 for
power stations. At the current stage of technology, C
str
for
perovskite solar cells is unknown. Structure costs for perovskite
solar cells can be anticipated to be lower than those for silicon,
especially for modules, yet reliability concerns may require use
of additional encapsulants. Dierences were omitted as the
intent of the example was to illustrate the eect of an imbalance
in cell quality. The impact of dierent system costs is discussed
in the next section.
It is worth mentioning here again that the presented analysis
only determines which of the three options, a system made only
from the top cell, a system made only from the bottom cell or
a system made from the tandem solar cell is at the lowest cost
per output level. Comparison to other solar cells requires
comparing $ per W or ¢ per kW per h values for the respective
systems.
4.3 Impact of dierent pairings of structure costs C
str
A third important factor that we want to highlight in this study
is the impact of dierences in structure costs. C
str
for a module
is given by the costs of cell to module processing. As shown in
Table 1, the dierence in costs required to make a module out
of a fabricated solar cell can be as high as 50% relative when
comparing a frameless thin-lm PV module with a silicon wafer
PV module. As discussed earlier, these dierences are strongly
attenuated when looking at PV power stations. Dierences in
C
str
due to dierences in technology here are in the range of
10% relative. Variations in structure costs due to the size of the
PV power station, its location and the cost of permits has
a much stronger impact. To illustrate the impact of dierences
in C
str
, we calculated the MRCB as a function of C
str,bot
and
C
str,top
using eqn (4) for C
str,tan
and DC¼0. The results are
shown Fig. 8.
Fig. 7 Illustration of the impact of similar and dierent single-junction solar cell eciencies on the cost regimes. A similar eciency pairing is
shown on the left (a). A pairing with dierent eciencies is shown on the right (b). C
cell
/C
str
for a silicon PV module and PV power stations are
again indicated.
Fig. 8 Maximum relative cost benet as a function of C
str,bot
and
C
str,top
. The top cell has a band-gap of E
g,top
¼1.74 eV, and a radiative
eciency of h
top
¼28.4% (f
SQ,top
¼1). The bottom cell has a band-gap
of E
g,bot
¼1.124 eV and a radiative eciency of h
bot
¼33.0% (f
SQ,bot
¼
1). The resulting tandem solar cell eciency is h
tan
¼44.5%. Also
shown is the line representing the highest MRCB for a given C
str,bot
or
C
str,top
.
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66919
Paper RSC Advances
As the two sub-cells contribute to the tandem with dierent
eciencies, the highest cost benet is not a diagonal. The
system costs enter the calculation twice, consequently the
position of the highest MRCB has a slope of ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
htop=hbot
q.
To highlight the impact of C
str
, we investigate a module and
a PV power station made of hypothetical thin-lm on silicon
tandem solar cells and made of hypothetical thin-lm on thin-
lm tandem solar cells. In both cases we use a top cell eciency
of h
top
¼14.0% (E
g,top
¼1.74 eV, f
SQ,top
¼49%), a bottom cell
eciency of h
bot
¼16.4% (E
g,bot
¼1.124 eV, f
SQ,bot
¼49%),
which results in a tandem eciency of h
tan
¼21.8% for two-
terminal and 22.2% for the four-terminal conguration. These
numbers were chosen having recent eciencies of commer-
cially available CdTe and screen-printed multicrystalline silicon
solar cells in mind (we are aware that both technologies are
capable of higher eciencies but believe that the ratio of both
eciencies is representative, which is the key variable in the
analysis). The cost breakdown of the corresponding modules is
shown in Table 2. Cost regimes are shown in Fig. 9.
As Fig. 9 shows, dierences in C
str
result in a reduction of
size of the tandem regime. The further the size is reduced, the
larger the dierence is. As shown in Fig. 9a and b, the cost
structures of the thin-lm and silicon solar cells make the
tandem PV module a non-preferred option. Only fundamental
reduction in the cost of solar cell fabrication, to about a third of
the current cost, can change this. However, when looking at PV
power stations, (Fig. 9a and c) tandems made out of these
hypothetical solar cells are preferable to their single-junction
counterparts. As C
str
for power stations is very similar (1.61 $
per W for thin-lm and 1.73 $ per W for silicon), there is little
dierence between the cost regimes for the two tandems. For
a 50 MW utility scale station, the dierence is 7.5% relative and
varies between 7% for a large utility scale station and 11% for
a small residential station.
We calculated the LCOE for dierent power station sizes
using.
33
We used default parameters, neglected degradation,
and avoided use of advanced nancial mechanisms. The
reference location was Boston, MA. We calculated the LCOE for
all power stations from Fig. 9a and c. These include two resi-
dential and three utility-scale stations of dierent sizes. Results
are shown in Table 4. The LCOE calculations conrm that the
hypothetical tandem PV power stations operate at a lower LCOE
than either comprising single-junction technology. The relative
improvement is between 5% and 7.5% relative to each single-
junction technology.
Fig. 9 Cost regimes for dierent PV systems: (a) the cases for both PV modules and PV power stations made from a hypothetical pair of thin-lm
solar cells. C
str
are identical for top and bottom cell and are 29.9 $ per m
2
(see Table 1) for a module and 1.61 $ per W for a reference power
station.
20
(b) The case for a hypothetical thin-lm on silicon tandem PV module. C
str
for top and bottom cells dier signicantly. (c) The case for
a hypothetical thin-lm on silicon tandem PV power station. C
str
for top and bottom cell were chosen for a reference (50 MW, utility) power
station. C
cell
/C
str
values for modules (blue segmented line) and dierent size PV power stations (the blue band again marks the range for stations
of dierent size as specied in Table 3) are indicated.
Table 4 LCOE calculations for the dierent solar cell technologies discussed in this section
Cell type h[%] MSP [$ per W]
Nom. LCOE [¢ per kW per h]
Residential Utility
5 kW 13.5 kW 50 MW 186.7 MW 600 MW
TF top 14 $0.62 11.31 10.02 11.89 11.54 11.45
TF bot 16.4 $0.53 10.51 9.18 10.82 10.46 10.37
TF on TF 21.8 $0.66 9.88 8.63 10.31 9.95 9.86
Rel. LCOE benet [%] 5.99 5.99 4.71 4.88 4.92
TF top 14 $0.62 11.31 10.02 11.89 11.54 11.45
Si bot 16.4 $0.85 11.35 10.02 12.12 11.76 11.67
TF on Si 21.8 $0.90 10.52 9.26 11.28 10.93 10.84
Rel. LCOE benet [%] 6.98 7.58 5.13 5.29 5.33
66920 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
5. Summary and discussion
In this work, we aspired to answer the following question: given
two single-junction solar cell technologies A and B, under which
circumstances is it economically benecial to combine these
technologies into a tandem solar cell C? To answer this ques-
tion, we rst related the costs and eciencies of the two single-
junction technologies to the tandem. Tandem costs were esti-
mated by breaking down the costs of the fabrication processes
for the single-junction solar cells and constructing the costs for
fabricating the tandem solar cell from this breakdown. Two
types of PV systems were considered: PV modules and PV power
stations. The ratio between the cost to make the solar cell and
the cost to make the supporting structure C
cell
/C
str
is a key
parameter in this study; we calculated this parameter for
a range of dierent modules and power stations. Tandem e-
ciencies were estimated via the radiative limit and the fraction
of the radiative limit at which each single-junction solar cell
operates.
Using these numbers, we introduced the cost-regime plot,
which shows under which conditions a PV system using either
of the three solar cell technologies A, B, or C generates power or
work at the lowest cost. The plot also shows the relative cost-
benet of C towards A and B. Generally, we nd that tandems
become the more favourable the smaller the fraction of the solar
cell-related cost in the entire system is. As a rule of thumb,
tandem solar cells can become attractive if C
cell
/C
str
for A and B
are below 50%.
We investigated the impact of dierent parameters on the
cost regimes using the described methodology. Our results
suggest that a tandem solar cell should be a marriage of
equals. By this we mean that:
(i) The tandem solar cell should be made using two solar-cell
technologies that, when operated independently as single-
junction solar cells, should be at a similar cost per unit
output power or energy. Matching this condition ensures that
the tandem solar cell has the highest cost-benet over the
comprising single-junction solar cells. This was discussed in the
context of Fig. 2.
(ii) The tandem solar cell should be made from two materials
that form an ideal band-gap pairing. Matching this condition
ensures that the tandem solar cell has the highest potential
eciency benet over the comprising single-junction solar
cells. We have illustrated this with the example of dierent
hypothetical IIIV on silicon tandem solar cells (Fig. 5).
(iii) The tandem solar cell should be made from two solar
cells that, when operated independently as single-junction solar
cells, should be at a similar eciency. Matching this condition
ensures that the tandem solar cell has the highest eciency
surplus over the comprising single-junction solar cells. This
condition is dierent from (ii) as it addresses non-idealities and
losses in the cell architecture. We have illustrated this with the
example of dierent perovskite on silicon tandem solar cells
(Fig. 7).
(iv) The tandem PV system should be made from two tech-
nologies with similar costs for supporting structure. This
condition is more relevant for PV modules than for PV power
stations. Matching this condition ensures that the tandem PV
system has the highest cost benet compared to PV systems
made of the comprising single-junction solar cells. We have
illustrated this with dierent hypothetical thin-lm on thin-lm
and thin-lm on silicon tandem solar cells (Fig. 9).
Moreover, we compared dierent tandem solar cell archi-
tectures (two terminal and four terminal). The two-terminal
architecture oers the greater potential for reducing the
number of fabrication steps and material, while the four-
terminal tandem allows greater exibility in terms of band-
gap pairings and, hence, material choice. Whereas we nd
some dierences on the module level that favour the two-
terminal architecture, we don't see a clear trend toward either
architecture on the power station level. A ner analysis of energy
yield will be able to oer a clearer picture here.
A further conclusion of this work is that a module manu-
facturer and a PV installer or a yieldco will give dierent
answers to the posed question. The structural cost component
C
str
inamoduleismuchsmallerthaninaPVpowerstation.In
none of the investigated examples did we nd a case in which
a tandem PV module could be produced at a lower $ per W
level than either of the comprising single-junction PV
modules. Solar cell fabrication costs in current technology
would have to be reduced signicantly in order to achieve this.
ForPVpowerstations,ontheotherhand,thecurrentcost
structure of silicon and thin-lm solar cells make tandems an
attractive option with lower LCOE values for the tandem than
for either single junction cell (provided cells with suitable
characteristics are available). This dierence creates a conun-
drum; an installer may be interested in tandem PV modules
while a module producer may want to avoid a higher-cost
product. Stable market conditions and clear regulations can
help solve this conundrum, as they will allow module
producers to foresee the value of tandem PV modules to the
customer and charge a premium on high eciency solar cell
technologies.
Finally we want to give suggestions for future research
directions that are motivated by this study:
(i) Established single-junction solar cells target band-gaps
between 1.0 eV and 1.4 eV band-gap, as these band-gaps merit
the highest single-junction eciencies. This band-gap range is
also suitable for the bottom cell in a double-junction tandem.
However, top cells with appropriate band-gaps (1.62.0 eV) and
eciencies (>20%) are far less developed and available. We,
therefore, recommend research for the development of suitable
top cells. We believe that high lifetimes are a prerequisite for
this.
53
(ii) Additionally, suitable deposition techniques for top cells
are required, as fabrication should be as integrated as possible.
We therefore also recommend research on equipment design to
accelerate throughput without sacricing quality.
(iii) While this point has been made before, it is worth
repeating that in today's PV power stations the solar cell
generates 100% of the power, yet the contribution to the total
system cost is small. Under this condition, increasing the solar
cell eciency, even at increased cost, is worthwhile. However,
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66921
Paper RSC Advances
there is an equal or even larger opportunity in reducing LCOE by
reducing system related costs, which requires eorts on the
technological as well as on the policy side.
Acknowledgements
We thank Dirk Weiss (First Solar) and BJ Stanbery for very
helpful discussion and suggestions. Research presented in this
work was nancially supported by the Department of Energy
under Award Number DE-EE0006707 and by the National
Research Foundation Singapore through the Singapore MIT
Alliance for Research and Technology's Low Energy Electronic
Systems research program.
References
1 D. M. Powell, T. M. Winkler, A. Goodrich and T. Buonassisi,
Modeling the Cost and Minimum Sustainable Price of
Crystalline Silicon Photovoltaic Manufacturing in the
United States, IEEE J. Photovolt., 2013, 3, 662668.
2 Panasonic Press Release, Panasonic HIT® Solar Cell Achieves
World's Highest Energy Conversion Eciency of 25.6% at
Research Level, 10 April 2014, http://panasonic.co.jp/corp/
news/ocial.data/data.dir/2014/04/en140410-4/en140410-
4.html, accessed 24 April 2014).
3 A. Richter, M. Hermle and S. W. Glunz, Reassessment of the
Limiting Eciency for Crystalline Silicon Solar Cells, IEEE J.
Photovolt., 2013, 3, 11841191.
4 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and
E. D. Dunlop, Solar Cell Eciency Tables (Version 46),
Prog. Photovoltaics, 2015, 23, 805812.
5 B. Kayes, H. Nie, R. Twist, S. G. Spruytte, F. Reinhardt,
I. C. Kizilyalli and G. S. Higashi, 27.6% conversion eciency,
a new record for single-junction solar cells under 1 sun
illumination, Proceedings of the 37th IEEE Photovoltaic
Specialists Conference, 2011.
6 O. D. Miller, E. Yablonovitch and S. R. Kurtz, Strong Internal
and External Luminescence as Solar Cells Approach the
ShockleyQueisser Limit, IEEE J. Photovolt., 2012, 2, 303311.
7P.W
¨
urfel, There is abundant literature on tandem solar
cells, for descriptions of the concept see for example,
Physics of Solar Cells, Wiley-VCH, 2005, pp. 155160.
8 A. De Vos, Detailed balance limit of the eciency of tandem
solar cells, J. Phys. D: Appl. Phys., 1980, 13, 839846.
9 A. Marti and G. L. Araujo, Limiting eciencies for
photovoltaic energy conversion in multigap systems, Sol.
Energy Mater. Sol. Cells, 1996, 43, 203222.
10 F. Dimroth and S. Kurtz, High-Eciency Multijunction Solar
Cells, MRS Bull., 2007, 32, 230235.
11 W. Shockley and H. Queisser, Detailed Balance Limit of
Eciency of pn Junction Solar Cells, J. Appl. Phys., 1961,
32, 510519.
12 P. Verlinden, Challenges and Opportunities of High-
Performance Solar Cells and PV Modules in Large Volume
Production, Plenary Talk at the 42
nd
IEEE PVSC conference,
New Orleans, 2015.
13 T. Soga, T. Kato, M. Yang, M. Umeno and T. Jimbo, High
eciency AlGaAs/Si monolithic tandem solar cell grown by
metalorganic chemical vapor deposition, J. Appl. Phys.,
1995, 78, 41964199.
14 H. Taguchi, T. Soga and T. Jimbo, Fabrication of GaAs/Si
Tandem Solar Cell by Epitaxial Li-OTechnique, Jpn. J.
Appl. Phys., 2003, 42, 14191421.
15 S. A. Ringel, R. M. Sieg, J. A. Carlin, S. M. Ting,
E. A. Fitzgerald, M. Bulsara, and B. M. Keyes, Proceedings of
the 2
nd
World Conference and Exhibition on Photovoltaic
Solar Energy Conversion, Vienna, 1998, pp. 35943597.
16 NREL press release, NREL and CSEM Jointly Set New Eciency
Record with Dual-Junction Solar Cell, January 5 2016, http://
www.nrel.gov/news/press/2016/21613.
17 Z. Ren, et al., unpublished results.
18 M. Liu, M. B. Johnston and H. J. Snaith, Ecient planar
heterojunction perovskite solar cells by vapour deposition,
Nature, 2013, 501, 395402.
19 H. Zhou, et al., Interface engineering of highly ecient
perovskite solar cells, Science, 2014, 345, 542546.
20 S. Albrecht, et al., Monolithic perovskite/silicon-
heterojunction tandem solar cells processed at low
temperature, Energy Environ. Sci., 2016, 9,8188.
21 J. Werner, C. Weng, A. Walter, L. Fesquet, J. P. Seif, S. De
Wolf, B. Niesen and C. Ballif, Ecient Monolithic
Perovskite/Silicon Tandem Solar Cell with Cell Area > 1
cm
2
,J. Phys. Chem. Lett., 2016, 7, 161166.
22 X. Wu, et al., Advances in CdTe R&D at NREL, Conference
Paper, presented at the 2005 DOE Solar Energy Research
Technologies Program Review Meeting, Denver, USA, 2005.
23 M. Carmody, et al., Single-crystal IIVI on Si single-junction
and tandem solar cells, Appl. Phys. Lett., 2010, 96, 153502.
24 C. Bailie, et al., Semi-transparent perovskite solar cells for
tandems with silicon and CIGS, Energy Environ. Sci., 2015,
8, 956963.
25 P. A. Basore, Understanding Manufacturing Cost Inuence
on Future Trends in Silicon Photovoltaics, IEEE J.
Photovolt., 2014, 4, 14771482.
26 D. M. Powell, R. Fu, K. Horowitz, P. A. Basore,
M. Woodhouse and T. Buonassisi, The capital intensity of
photovoltaics manufacturing: barrier to scale and
opportunity for innovation, Energy Environ. Sci., 2015, 8,
33953408.
27 R. Fu, T. James and M. Woodhouse, Economic
Measurements of Polysilicon for the Photovoltaic Industry:
Market Competition and Manufacturing Competitiveness,
IEEE J. Photovolt., 2015, 5, 515524.
28 F. Fertig, S. Nold, N. W¨
ohrle, J. Greulich, I. H¨
adrich,
K. Krauß, M. Mittag, D. Biro, S. Rein and R. Preu,
Economic feasibility of bifacial silicon solar cells, Prog.
Photovoltaics, 2016, 24, 800817.
29 V. Lo, C. Landrock, B. Kaminska and E. Maine,
Manufacturing cost modeling for exible organic solar cells,
proceedings of PICMET, 2012, pp. 29512956.
30 S. Nold, N. Voigt, L. Friedrich, D. Weber, I. Haedrich,
M. Mittag, H. Wirth, B. Thaidigsmann, I. Brucker,
M. Hofmann, J. Rentsch and R. Preu, Cost Modeling of
66922 |RSC Adv.,2016,6, 6691166923 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper
Silicon Solar Cell Production Innovation along the PV Value
Chain, proceedings 27th EUPVSEC, 2012, pp. 10841090.
31 N. Espinosa and F. C. Krebs, Life cycle analysis of organic
tandem solar cells: When are they warranted?, Sol. Energy
Mater. Sol. Cells, 2014, 120, 692700.
32 S. C. Siah, Defect Engineering in Cuprous Oxide (Cu
2
O) Solar
Cells, Ph.D. thesis, MIT, 2015.
33 System Advisor Model Version 2015.1.30 (SAM 2015.1.30),
National Renewable Energy Laboratory, Golden, CO,
accessed March 12, 2015, https://sam.nrel.gov/content/
downloads.
34 L. Esaki, New phenomenon in narrow germanium pn
junction, Phys. Rev., 1958, 109, 603607.
35 H. Liu, Z. Ren, Z. Liu, A. G Aberle, T. Buonassisi and
I. M. Peters, The realistic energy yield potential of GaAs-on-
Si tandem solar cells: a theoretical case study, Opt. Express,
2015, 23, 382390.
36 W. E. McMahon, K. E. Emery, D. J. Friedman, L. Ottoson,
M. S. Young, J. S. Ward, C. M. Kramer, A. Duda and
S. Kurtz, Fill Factor as a Probe of Current-Matching for
GaInP
2
/GaAs Tandem Cells in a Concentrator System
During Outdoor Operation, Prog. Photovoltaics, 2008, 16,
213224.
37 A. Luque and S. Hegedus, Handbook of Photovoltaic Science
and Engineering, Wiley VCH e-book, 2011, 8.54.
38 A. Shah, Thin Film Silicon Solar Cells, EPFL Press, 2010, pp.
252260.
39 S. B. Darling, F. You, T. Veselka and A. Velosa, Assumptions
and the levelized cost of energy for photovoltaics, Energy
Environ. Sci., 2011, 4, 31333139.
40 D. Yue, F. You and S. B. Darling, Domestic and overseas
manufacturing scenarios of silicon-based photovoltaics:
Life cycle energy and environmental comparative analysis,
Sol. Energy, 2014, 105, 669678.
41 A. C. Goodrich, D. M. Powell, T. L. James, M. Woodhouse
and T. Buonassisi, Assessing the drivers of regional trends
in solar photovoltaic manufacturing, Energy Environ. Sci.,
2013, 6, 28112821.
42 J. F. Geisz, M. A. Steiner, I. Garc´
ıa, S. R. Kurtz and
D. J. Friedman, Enhanced external radiative eciency for
20.8% ecient single-junction GaInP solar cells, Appl.
Phys. Lett., 2013, 103, 041118.
43 K. Makuso, et al., Achievement of More Than 25%
Conversion Eciency With Crystalline Silicon
Heterojunction Solar Cell, IEEE J. Photovolt., 2014, 4, 1433
1435.
44 Z. Ren, J. P. Mailoa, Z. Liu, H. Liu, S. C. Siah, T. Buonassisi
and I. M. Peter, Numerical Analysis of Radiative
Recombination and Reabsorption in GaAs/Si Tandem, IEEE
J. Photovolt., 2015, 5, 10791086.
45 B. M. Kayes, N. Hui, R. Twist, S. G. Spruttye, F. Reinhard,
I. C. Kizilyalli and G. S. Higashi, 27.6% Conversion
eciency, a new record for single-junction solar cells under 1
sun illumination, Presented at the 37th IEEE PVSC in Seattle,
USA, 2011.
46 G. J. Bauhuis, P. Mulder, E. J. Haverkamp, J. C. C. M. Huijben
and J. J. Schermer, 26.1% thin-lm GaAs solar cell using
epitaxial li-o,Sol. Energy Mater. Sol. Cells, 2009, 93,
14881491.
47 M. Woodhouse and A. Goodrich, A Manufacturing Cost
Analysis Relevant to Single- and Dual-Junction Photovoltaic
Cells Fabricated with IIIVs and IIIVs Grown on Czochralski
Silicon, NREL/PR-6A2060126, 2013.
48 K. Schmieder, et al.,Analysis of GaAs Solar Cells at High
MOCVD Growth Rates, Presented at the IEEE PVSC, Denver,
USA, 2014, pp. 21302133.
49 G. J. Hayes and B. M. Clemens, Laser lioof gallium
arsenide thin lms, MRS Commun., 2015, 5,15.
50 R. M. Swanson, The Promise of Concentrators, Prog.
Photovoltaics, 2000, 8,93111.
51 S. N. Fatemi, H. E. Pollard, Q. H. Hong and P. R. Sharps,
Solar array trades between very high-eciency multi-junction
and Si space solar cells, Presented at the 28th IEEE PVSC,
Anchorage, USA, 2000, pp. 10831086.
52 J. P. Mailoa, C. D. Bailie, E. C. Johlin, E. T. Hoke, A. J. Akey,
W. H. Nguyen, M. D. McGehee and T. Buonassisi, A 2-
terminal perovskite/silicon multijunction solar cell enabled
by a silicon tunnel junction, Appl. Phys. Lett., 2015, 106,
121105.
53 R. E. Brandt, V. Stevanovic, D. S. Ginley and T. Buonassisi,
Identifying defect-tolerant semiconductors with high
minority-carrier lifetimes: beyond hybrid lead halide
perovskites, MRS Commun., 2015, 5, 265275.
This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6, 6691166923 | 66923
Paper RSC Advances
... It was found that all the TSCs offer significantly lower LCOE than conventional PERC solar cells. Peter et al. [226] performed a techno-economic analysis with parametric cost relations to identify the conditions under which TSCs are economically superior to the SJ constituent sub-cells, and found that TSC PV power stations operate at a lower LCOE compared with their SJ counterparts. Li et al. [227] performed a detailed cost analysis of PK/Si and PK/PK-TSCs with an accuracy of 0.01 c/kWh ( Fig. 20a-b). ...
Article
Perovskite (PK)-based tandem solar cells (TSCs) are an emergent photovoltaic (PV) technology with potential to surpass the Shockley–Queisser theoretical limit of efficiency (η) of single-junction (SJ) silicon solar cells. The promising efficiency of PK/Si-TSCs > 29% indicates the potential of next-generation PV technology as efficiencies of approximately 45% could be achieved through complete optimization of the optical and electrical parameters of PK/Si-TSCs. To establish a technological roadmap for future research, it is necessary to review and discuss the future technological advancements and economic prospects concerning PK-based TSCs, as well as determine the economic impact. Here, the merits and demerits of different configurations of PK-based-TSCs are reviewed to determine the best design strategies for optical/electrical connection and light distribution between the top and bottom sub-cells. The current status and recent advances in both two- and four-terminal PK/Si tandems are discussed, along with the technological approaches employed and the maximum η achieved. Additionally, the three-terminal tandem configuration and how it solves the current mismatch in the two-terminal configuration and the optical losses in four-terminal architecture are summarized. Future technological developments and economic prospects of PK-based TSCs are discussed. The PK/Si heterojunction (SHJ)-TSC-based PVs were found to cause similar or less environmental impact than c-Si PVs, along with the possibility of further improving the environmental outcome by replacing the SHJ with SJ-Si (p-n homojunction) bottom sub-cell. Additionally, by further reducing the levelized cost of electricity, TSCs fabricated with mc-Si are likely to have strong economic prospects.
... The critical challenge is that these multi-junction devices are cost-effective only when both photo-absorbers have a similar cost per unit output power. 16 As claimed by Zhengshan et al., the top cell with a cost below 100 $ m −2 and efficiency above 20% is essential for having a lower LCOE (levelized cost of electricity) than a single-junction photo-device. 17 Among various multi-junction devices for the SRFB demonstrated so far, 9,11,18,19 a perovskite/c-Si tandem device demonstrated by Li et al. 9 is considered to have the potential to meet this requirement. ...
Article
Full-text available
Solar-rechargeable redox flow batteries (SRFBs) offer feasible solar energy storage with high flexibility in redox couples and storage capacity. Unlike traditional redox flow batteries, homemade flow cell reactors are commonly used in most solar-rechargeable redox flow batteries integrated with photoelectrochemical devices as it provides high system flexibility. This perspective article discusses current trends of the architectural and material characteristics of state-of-the-art photoelectrochemical flow cells for SRFB applications. Key design aspects and guidelines to build a photoelectrochemical flow cell, considering practical operating conditions, are proposed in this perspective.
... eV lower band gap bottom cell with 1.7-1.8 eV wide band gap top cell has a potential to achieve >30% power conversion efficiency [7]. Highly efficient low band gap semiconductors such as Si, CuInSe 2 (CISe), and low Ga Cu(In,Ga)Se 2 (CIGSe) are perfectly suitable for bottom cells. ...
Article
Full-text available
Cu(In,Ga)S2 is a promising semiconductor that offers excellent prospects for photovoltaics. However, the performance has been plagued mostly due to large photovoltage deficit. Here, we investigate defects and optoelectronic properties of 1.7-eV band gap near-stoichiometric Cu(In,Ga)S2 (CIGS). We have estimated quasi-Fermi-level splitting of 921 meV from steady state photoluminescence (PL) measurements at 1 sun. Detailed analysis of temperature and excitation dependent PL reveals the behavior of a strongly compensated semiconductor. We show that spatially varying energetic disorder, described by electrostatic potential fluctuations, causes band-tail recombination and strongly affects the carrier recombination in compensated CIGS. Apart from band-tail transitions, we also observe two deep defects at about 0.3 and 0.45 eV below the band edge. The defects are also discerned by admittance measurements. Temperature dependent current-voltage measurements show that the open-circuit voltage is further limited by interface recombination. Thus, the performance improvement lies in the mitigation of deep defect states and absorber/buffer interface optimization.
... The main drawback of III-V multi-junction cells remains their cost, which can be 2 to 3 orders of magnitude higher compared to terrestrial c-Si photovoltaics (PV) [8]. Indeed, the terrestrial silicon PV industry has now reached a high degree of industrial maturity (100 GW annual market) and very low prices (2019 modules ~ 0.2€/Wp [9]). ...
Article
— In this study, we evaluate the potential of III-V//Si solar cell technology for space applications. We present experimental results on wafer bonded 2-terminal III-V//Si solar cells degradation under 1 MeV electrons irradiation. Beginning-Of-Life (BOL) and End Of Life (EOL) electrical performances were measured under Normal Irradiance (100% AM0) and Room Temperature (300K) conditions (NIRT). No degradation of the wafer bonding interface was detected. The impact of electron irradiation on effective diffusion length in the silicon bottom cell was calculated using the Internal Quantum Efficiency model for different solar cells architectures. Improvement paths towards higher EOL III-V//Si solar cells are discussed.
Article
With high efficiency and fast processing, metal halide perovskite (PK) solar cells promise a new paradigm for low-cost solar power. In addition to single junction device performance that is near...
Article
An ideal catalytic interface for photoelectrodes that enables high efficiency and long‐term stability remains one of the keys to unlocking high‐performance solar water splitting. Here, fully decoupled catalytic interfaces realized using surface‐structured cocatalyst foils are demonstrated, allowing optimized photoabsorbers to be combined with high‐performance earth‐abundant cocatalysts. Since many earth‐abundant cocatalysts are deposited via solution‐based methods, deposition on chemical‐sensitive photoabsorbers is a significant challenge. By synthesizing cocatalyst foils prior to device fabrication, photoabsorbers are completely isolated from corrosive chemical environments and are provided with outstanding protection during operation. Si and GaAs photoelectrodes prepared using Ni‐based cocatalyst foils achieve excellent half‐cell efficiencies and generate stable photocurrents for over 5 days. Furthermore, a GaAs artificial leaf achieves a solar‐to‐hydrogen efficiency of 13.6% and maintains an efficiency of over 10% for longer than nine days, an accomplishment that has not been previously reported for an immersed solar water splitting system. These results, together with theoretical calculations of other photoelectrode systems, demonstrate that cocatalyst foils offer a very attractive method for fabricating high‐performance solar water splitting systems. Although solar water splitting efficiencies continue to rise, long‐term stability remains a key challenge. In this work, earth‐abundant cocatalyst foils are used to isolate semiconductors from harsh chemical environments during both device fabrication and operation. A GaAs artificial leaf fabricated using cocatalyst foils maintains a solar‐to‐hydrogen efficiency of over 10% for longer than nine days.
Article
Meeting the ambitious challenge of net-zero greenhouse gas emissions by 2050 and holding the average increase in global temperature below 1.5 °C necessitate the upscaling of readily available renewable energy sources, especially solar photovoltaics. Since the window of time to achieve this goal is closing fast, it is of paramount importance that we accelerate the decarbonization of the global energy system by increasing the power output of solar cells through advancing their power conversion efficiencies toward and beyond the Shockley–Queisser limit. In this Perspective, we describe how the integration of perovskites into the well-established silicon production infrastructure to form perovskite/silicon tandem photovoltaics can raise the rate of solar deployment. We present a holistic analysis of the technology from different perspectives, such as materials science, manufacturing, sustainability, and business, which highlights how the pairing of perovskite and silicon is advantageous at many different levels of consideration. Altogether, perovskite/silicon tandems deliver a technological disruption in efficiency while maintaining compatibility with the present photovoltaics industry, making it the fastest route to enhance the silicon market and rapidly address climate change.
Article
In recent years, perovskite/silicon tandem solar cells (PK/c‐Si tandem) have demonstrated high power conversion efficiency (PCE) and demonstrated great application potential in photovoltaic (PV) systems. However, the PCE of PK/c‐Si tandem devices is still below the theoretical limit. From a broader perspective, their poor stability and difficulty in large‐area realization are crucial barriers for commercial viability. In this report, the detailed constraints facing high PCE of tandem devices and the corresponding solutions are discussed. The authors propose that the main obstacle comes from the limitation of the perovskite top cell. However, careful understanding of the optical and electrical properties of each functional layer is expected to be the core process to further promote efficiency. Regarding the environmental and intrinsic instability issues, encapsulation is considered to be the most effective method to address environmental instability. Preventing ion migration is one of the fundamental methods to eliminate intrinsic instability. It is believed that low dimensional perovskite materials will become a competitive solution to simultaneously solve these two instabilities. Finally, some suggestions for reducing costs and preparation of PK/c‐Si tandem on a large‐scale are also discussed which provides guidance for further boosting the development of PK/c‐Si tandem.
Chapter
Clean and green energy sources are the need of the hour, as global warming is drastically changing the weather conditions around the world. Renewable or nonconventional energy sources have become the ultimate solution to clean and sustainable energy requirements. The Sun is a source of abundant, clean, and sustainable energy that offers a practical strategy to reduce CO2 emissions. Solar cells are one of the most suitable methods of harvesting solar energy in a sustainable way. Three generations of solar cells have been evolved to harvest sunlight as efficiently as possible. Modified third-generation solar cells, for example, tandem and/or organic–inorganic configurations, are emerging as fourth-generation solar cells to maximize their economic efficiency. This chapter comprehensively covers the basic concepts, performance, and challenges associated with third-generation solar cells. The third generation of solar cells includes organic solar cells, dye-sensitized solar cells, quantum dot solar cells, and perovskite solar cells. We also briefly discuss the rational design of efficient solar devices constructed from advanced materials such as three-dimensional graphene, doped polymers and nanostructured ternary metal sulfides. Limitations in the efficiency of solar cell devices, challenges to improved performance of third-generation solar cells, and future perspectives of photovoltaic technology toward emerging tandem, multijunction, and plasmonic solar cell architectures are also briefly described.
Article
Luminescent solar concentrators (LSCs) collect and concentrate the solar irradiation to generate electricity through the photovoltaic effect. LSCs have attracted attention due to their concentration factors, their ability to provide wavelength‐to‐bandgap matched photons, and because of their high functionality under both diffuse and direct illumination. This paper assesses luminescent solar concentrators, in which the inclusion of a tunable photoluminescence optical layer is analyzed. Three main goals are achieved. First of all, we achieved for the first time an optical window based on a randomly oriented six‐dye system that exchanges energy sequentially between them (Förster resonance energy transfer [FRET]), which is interesting from a scientific point of view. Second, the present work achieved a broad absorption spectrum by combining six dyes, which is interesting from a practical point of view. Third, although we can achieve a similar broad absorption with a reduced number of dyes, this strategy allows to tune both the absorption of the optical window and the emission with high selectivity (by removing dyes) in order to match the emission of the luminescent system to the bandgap of the PV technology used in the LSC, whose efficiency is increased by 26.9% with respect to the reference prototype. Luminescent Solar Concentrator with a tunable optical window layer, based on a randomly oriented six‐dye system that exchanges energy sequentially between them (FRET – Förster Resonance Energy Transfer). This design allows to tune both the absorption of the optical window and the emission with high selectivity (by removing dyes) in order to match the emission of the luminescent system to the bandgap of the PV technology used in the LSC, whose efficiency is increased by 26.9%.
Article
Full-text available
Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in-depth numerical device simulation and advanced cost modelling. As references, Al-BSF (aluminium back-surface field) and PERC (passivated emitter and rear) cells with local rear-side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single-sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double-sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost-competitive under monofacial operation compared with an 18% efficient (≙ pMPP = 18 mW/cm2) multicrystalline silicon (mc-Si) Al-BSF cell and a 19% mc-Si PERC cell for maximum output power densities of pMPP ≥ 17.3 mW/cm2 and pMPP ≥ 18.1 mW/cm2, respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al-BSF and PERC cells. For the PERT cell, corresponding values are pMPP ≥ 21.7 mW/cm2 and pMPP ≥ 22.7 mW/cm2, respectively, assuming the current price offset (≈50%, at the time of October 2014) of n-type Czochralski-grown silicon (Cz-Si) compared with mc-Si wafers. The material price offset of n-type to p-type Cz-Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm2, which correlates to a conversion efficiency difference of 1%abs for monofacial illumination with 1 sun. From p-type mc-Si to p-type Cz-Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm2 for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear-side illumination intensities for arbitrary front-side efficiencies ηfront by means of the performed numerical simulations. For a BOSCO cell with ηfront = 18%, minimum rear-side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n-type Cz-Si PERT cell with ηfront = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n-type to p-type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright
Data
Full-text available
Monolithic perovskite/crystalline silicon tandem solar cells hold great promise for further performance improvement of well-established silicon photovoltaics; however, monolithic tandem integration is challenging, evidenced by the modest performances and small-area devices reported so far. Here we present first a low-temperature process for semitransparent perovskite solar cells, yielding efficiencies of up to 14.5%. Then, we implement this process to fabricate monolithic perovskite/silicon heterojunction tandem solar cells yielding efficiencies of up to 21.2 and 19.2% for cell areas of 0.17 and 1.22 cm2, respectively. Both efficiencies are well above those of the involved subcells. These single-junction perovskite and tandem solar cells are hysteresis-free and demonstrate steady performance under maximum power point tracking for several minutes. Finally, we present the effects of varying the intermediate recombination layer and hole transport layer thicknesses on tandem cell photocurrent generation, experimentally and by transfer matrix simulations.
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since January 2016 are reviewed. Copyright © 2016 John Wiley & Sons, Ltd.
Article
This paper addresses the issue of why concentrator systems have not gained a significant market share. The history of concentrator development is reviewed, and the status of existing concentrator efforts outlined. A critical look at the requirements to propel concentrators to a prominent market role in large-scale power production is presented. Various concentrator and flat-plate PV system approaches are compared by computing the expected cost of energy, and conclusions are drawn as to what the best course of action will be. Concentrator systems are projected to be the lowest-cost, lowest-risk PV option for medium and large PV power plants. Copyrights (C) 2000 John Wiley & Sons, Ltd.
Article
Monolithic perovskite/crystalline silicon tandem solar cells hold great promise for further performance improvement of well-established silicon photovoltaics. However, monolithic tandem integration is challenging, evidenced by the modest performances and small-area devices reported so far. Here, we present first a low-temperature process for semitransparent perovskite solar cells, yielding efficiencies of up to 14.5%. Then, we implement this process to fabricate monolithic perovskite/silicon heterojunction tandem solar cells yielding efficiencies of up to 21.2% and 19.2% for cell areas of 0.17 cm2 and 1.22 cm2, respectively. Both efficiencies are well above those of the involved sub-cells. These single-junction perovskite and tandem solar cells are hysteresis-free and demonstrate steady performance under maximum power point tracking for several minutes. Finally, we present the effects of varying the intermediate recombination layer and hole transport layer thicknesses on tandem cell photocurrent generation, experimentally and by transfer matrix simulations.
Article
Amorphous silicon thin films were first deposited by plasma-enhanced chemical vapor deposition (PECVD). The amorphous silicon layers deposited from silane by PECVD could be doped by adding to the plasma discharge either phosphine to form n-type layers, or diborane to form p-type layers. The conductivity of these thin amorphous silicon layers could be increased by several orders of magnitude. Amorphous silicon solar cells at first found only niche applications, especially as the power source for electronic calculators. For 15 years or so, they have been increasingly used for electricity generation: they seem particularly well-suited for wide applications in building-integrated photovoltaics. One of their main advantages is that they are available in the form of monolithically integrated large-area modules. In amorphous silicon thin films, both the bond angles and the bond lengths vary in a random fashion: There is a whole distribution of values. If the amorphous silicon layer has just a low "amount of disorder," then the distributions for bond angles and bond lengths will be very narrow.
Conference Paper
Single junction GaAs solar cells grown by MOCVD are fabricated over a range of growth rates targeting up to 56 μm/hr in order to evaluate the effect on photovoltaic device performance. MOCVD recipe conditions are provided. Dopant incorporation efficiency is found to increase at high growth rates, potentially due to reduced Zn desorption as the time required to deposit a monolayer of GaAs is reduced. Device results are characterized by light and dark-IV as well as external quantum efficiency and verified against bulk minority carrier lifetime data from time-resolved photoluminescence. High growth rate solar cells degrade less than 4% relative to baseline devices with Voc and Jsc losses of 1% and 3%, respectively. The comparison suggests that both bulk Shockley Read Hall (SRH) lifetime and surface recombination velocity (SRV) are affected by growth rate and contribute to a reduction in performance.
Thesis
This thesis is focused on the development of a cuprous oxide (Cu₂O) thin-film (TF) solar cell that is fabricated by manufacturing-friendly methods such as electro-deposition, sputtering and atomic layer deposition. Due to its bandgap of close to 2 eV, it has the potential of being applied as top cell in a tandem configuration. Firstly, I perform bottom-up cost and price analysis to investigate the economic feasibility of TF and c-Si based tandem photovoltaic modules. Next, I investigate the formation of good ohmic back contacts on Cu₂O absorber layer and demonstrate that low contact resistivity can be achieved with a variety of metals on heavily doped Cu₂O films by forming a tunnel junction. Then, I apply synchrotron-based X-ray absorption spectroscopy (XAS) to characterize two front contact buffer materials: amorphous Zn-Sn-O (a-ZTO) and Sndoped Ga₂O₃. I elucidate a fundamental loss mechanism in the amorphous Zn-Sn-O (a-ZTO) electron-blocking layer that has origin in local structural disorder and establish the structure-process- property relationship of a-ZTO so that the front buffer layer can be optimized for photovoltaics. Then, I investigate the doping mechanism of Sn dopant atoms in TFs and single crystalline Ga₂O₃:Sn by revealing the doping mechanism so that Ga₂O₃:Sn can be optimized for photovoltaics. Lastly. I apply bulk defect engineering to manipulate the intrinsic point defect structure of Cu₂O towards improved device performance. The key results will inform the processing conditions for improving mobility and minority carrier lifetime in Cu₂O. Keywords - Earth-abundant, thin-film solar cells, tandem, defect engineering, cost modeling, synchrotron.